Energy storage as an essential part of sustainable energy systems

A review on applied energy storage technologies

Marco Semadeni

CEPE Working Paper No. 24 May 2003 CEPE ETH Zentrum, WEC CH-8092 Zürich

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1. Water for Thermal Energy Storage ..................................................................26 a. Thermal storage tanks ...............................................................................26 b. Large underground thermal energy storage systems ................................27 2. Latent Heat / Phase Change Systems ...............................................................28 3. Other thermal storage systems...........................................................................30


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Abstract Energy supply is an intricate task that provides a reliable energy service to consumers throughout the year. Import dependencies, seasonal differences in energy supply and use, and daily fluctuations in consumption require a sophisticated management of energy resources and conversion, or energy distribution and resource intermittency in order to guarantee continuous energy services throughout all sectors. Therein, energy storage plays a critical role. Energy storage balances the daily fluctuations and seasonal differences of energy resource availability, which results from physical, economical or geo-political constraints. A strongly variable energy demand through day and night also requires energy to be stored in adequate amounts. In particular, short- and mid-term storage levels out or buffers energy output gaps or overflows. Energy is mostly stored in between conversion steps from primary to secondary energy and secondary to final energy. Often rechargeable systems are used to refill the storage capacity when energy demand is low and energy services are not needed. Primary storage such as large crude oil and natural gas storage tanks are essential for the functioning of a country's energy system. Storage of water in reservoirs behind dams is valuable for selling hydropower electricity at the right time or in seasons of increased demand. Secondary or final storage systems, for instance in tanks or in batteries, are crucial for emergency situations, uninterrupted industrial production, long-distance mobility or to secure energy services at home. Storage systems are engineered to hold adequate amounts of mechanical, thermophysical, electro-chemical or chemical energy for prolonged periods of time. Energy storage systems should be quickly chargeable and should have a large energy storage capacity, but at the same time should also have high rates of recovery and high yields of energy regain. Final energy in factories or households is often stored in tanks as chemical energy in the form of heating oil or natural gas. Thermo-physical energy in the form of steam, hot or cold water, or thermo-oils is also used. For some special applications or for safety reasons energy may be stored electrochemically in batteries or physically in the form of pressurized air. Other storage systems are related to electricity and apply mechanical storage in the form of spinning turbines or flywheels, physical storage in the form of water in reservoirs in highland terrains, or electrostatic storage in super-capacitors. Research is extensive in the area of energy storage since an increase of new renewable energy technologies such as wind and solar is expected to increase fluctuations and deviations from grid parameters. These need too be balanced out using reserve power capacities, grid level power storage capabilities, distributed generation units connected to the grid, and possibly appropriate new grid architectures.

Introduction Several different types of energy storage systems are used for primary. which still plays a major role in fuel consumption. gas and electricity can result in a country’s increasing dependence on foreign gas. The proper functioning of an energy market is dependent on the available market chain. and final energy. or storage tanks for refined petroleum oils. useful energy can also be stored using mechanical or physical storage systems. and variable prices in energy markets. incentives for energy storage include the moderation of increasingly volatile energy prices. which is expected to largely substitute oil in the near future. This involves local storage of electricity. or in piles of specific energy carriers like biomass or coal. thermo-physical or thermo-chemical systems. Real time pricing in liberalized energy markets is expected to result in additional local storage within buildings and “intelligent” household appliances.Introduction 5 A. Increasing oil prices however reveal structural weaknesses in energy supply systems with the increasing energy dependence on oil. If demand or system status changes. which may require. among other things. The development of inexpensive storage options promise to cope with imbalances between demand and capacity. secondary. and a market is developing for “premium power” and customers dependent on properly functioning microprocessor systems. For instance. imbalances in a domestic energy system for heat. Secondary and final storage involves silos or piles of briquettes or biomass chips. The conversion of these forms of energy into useful end-use energy including electricity provides for immediate energy services.and oil companies and expensive trans-boundary high-voltage electricity transmission lines. An uninterrupted availability of clean energy products at the marketplace at a price that is affordable for both private and industrial consumers is important for the wellbeing of all citizens. Primary energy is predominantly stored chemically in oil or gas tanks. Distributed power generation and storage interconnected with the distribution grid can provide the needed power quality. Besides energy supply reliability. new renewable energy technologies such as wind and solar increase variations in the grid and therefore must by integrated properly into the existing electricity system. Adequate storage facilities must be available not only for crude oil but also for natural gas. Strategic petroleum reserves are an important regulatory aspect of energy supply systems in many countries. installation of grid level power storage capabilities. These address compulsory storage ratios negotiated between importing companies and the government. which consists of supply. . or chemical storage systems. Storage concepts must be well developed for an appropriate incorporation of storage technology into energy systems. In general. the spread of information technology brings new requirements for the transmission and distribution of electricity. the ongoing liberalization or deregulation of energy markets could make small-scale and large-scale storage capabilities increasingly beneficial because of the emerging competition in the energy market. With electricity. Uncertainties in grid reliabilities shift the emphasis in energy storage from producers to distributors in order to guarantee energy service. industrial and commercial customers increasingly demand power quality and reliability. For instance. storage and demand. At the same time. Energy systems rapidly change under the influence of energy policies. electrochemical and electrostatic systems. fuels and gases.

These custom power systems need to be electronically controlled within a complex grid containing multiple generating units. recharging batteries or fuel cells. storage capabilities. the development of storage systems can diminish dependency on oil and natural gas imports in the short to mid-term. super-conducting magnetic energy storage (SMES). reduce energy price volatility. Other examples are new hybrid systems. energy storage will increase system reliability. according to the laws of thermodynamics. for instance. for electricity. corrosion-. On the other hand. . To achieve the full potential of energy systems. for instance new distribution systems. and flywheel energy storage for power quality and reliability (Yeager et al. In both small uninterruptible power supply (UPS) systems for personal computers and in large pumped storage projects. photovoltaic panels or fuel cells for diverse industrial. commercial or residential applications. thermal and chemical functional materials for thermal storage.Introduction 6 In summary. Failure to develop adequate storage measures along with energy systems. some of the loss can be regained using optimized end-use technologies and control devices. help to integrate intermittent renewable resources into the grid. and. new regulatory standards for efficiency and environmental compatibility request ever faster technical or technological innovations. which also allow energy consumers to time energy purchases and shift from high to low rate periods. These include. or insulation and cooling media. all storage systems are accompanied by a loss in efficiency. In addition. ensure power quality.and impact-resistant materials. such as heat-. and facilitating high-temperature superconductivity have great potential to satisfy future heat and electricity storage requirements (Dresselhaus and Thomas 2001). abrasion. possibly leading to higher future costs for infrastructure and energy services. At utility sites however. safe on-vehicle storage and infrastructure components to assure fuel availability at filling stations. both independent and grid-based power distribution systems rely on the availability of different storage technologies. which efficiently satisfy on-demand mobility. In the future. underground storage technologies including SMES may be possible. which combine micro-turbines. Although storage can improve the capacity and service of energy systems. 1998). Innovations to store hydrogen. will pose a threat to the quality of energy services. It requires additional storage systems such as batteries. which provide large-scale instantaneous response are required.

and peaking in mobile systems when short-time power demand is high. smaller scale applications of electricity storage provide power for mobile systems or upgrade power quality and reliability (including emergency supply). while the cost of energy deals with the storage capacity of the system (Table 1).rwenet. Power must be deliverable in time frames of hours and days or months to balance seasonal changes. They are suitable for improving power quality. Both the capital costs of power and energy are important in evaluating the economics of storage systems. integration into emergency backup systems. releasing high power in a short time frame. Typically the capital cost component of power is related to the output capacity. Electricity Storage Apart from the use of secondary batteries. Large-scale electricity storage systems are suitable to balance differences between power supply and demand. The following storage technologies are often used for largescale electricity storage systems: • Pumped storage hydropower reservoirs • Compressed air energy storage • Large-scale secondary batteries and large fuel cells Other. Although still in the testing phase. The following technologies are applicable: • Batteries and fuel cells • Super-conducting (magnetic) energy storage (SMES) • Super-capacitors and ultra-capacitors • Flywheel energy storage Batteries are often used in mobile applications and systems where reliability with low maintenance is important. RWE Net . Capacitor and flywheel energy storage have rather different discharge characteristics. Electricity storage for these applications has varied capacity and time-scale characteristics. http://www. large-scale electricity storage systems can supply regulating power for proper transmission and distribution of electricity through the grid. and for continuous operation at various power ranges. 1 Pre-qualification Regulatory Energy. electricity storage is predominantly carried out in form of mechanical energy.Introduction 7 B. fuel cells are suitable for providing power over long periods of time. Large-scale electricity storage systems supply peak-load power using previously stored base-load electricity. Sometimes such power levels must be sustained for hours. As part of the ancillary services. or make it possible to integrate renewable energy technologies characterized by intermittent resource availability (e. solar and wind).g. minimal power must be available – a few MW within seconds and ten times as much within minutes1. Their release of power is instantaneous. Batteries can provide relatively prolonged power as compared to the other technologies. In addition.

The reservoir of the plant stores excess water from water-rich.e. usually at night. Extra water is pumped back up to the reservoir using base-load electricity when electricity demand is low. Hydropower Storage An important application of hydropower storage plants is to balance seasonal differences in electricity demand. the increase of electricity generation from renewable energy sources such as wind and solar requires additional storage due to the intermittency of its generation. Chester. Today. i. The use of large storage volumes is often controversial because of considerable environmental impact including the issue of pumping water from catchment areas faraway from the natural catchment areas of the river in order to enhance inflow to the reservoir. This efficient storage of potential energy allows hydropower storage schemes a broader range of energy benefits than pure run-of-river schemes. i. "Proceedings of the International Conference of Electrical Energy Storage Systems: Applications and Technologies. Electricity storage system Compressed air energy storage in tanks Underground compressed air energy storage Large scale batteries Pumped storage hydropower Super-conducting magnetic energy storage Flywheel energy storage Power capacity costs [US$/kW/a] 120 90 70 50 40 30 Energy capacity costs [US$/kWh/a] 100-1500 5-100 10-700 3-90 9-150 80-1200 Source: EESAT. It produces electricity preferentially during periods of higher load. dammed lakes. Storage hydropower plants Hydropower storage plants accumulate the natural inflow of water into reservoirs. Mechanical systems a. and the run-of-river power generated downstream utilizes part of the same water used upstream for the hydropower storage plant. and is then used for electricity generation during peak-load periods. Annual costs assume a discount rate of 9% and a 10-year life cycle. annual or multiannual storage capacity. UK. Hydropower plants are used as a part of larger parks of hydropower schemes to follow demand along the load curves. Reservoirs can provide a seasonal." International conference Electrical Energy Storage Systems. 1. in the upper reaches of a river where steep inclines favor the utilization of the water heads between the reservoir intake and the powerhouse to generate electricity. lower-demand seasons.Electricity Storage 8 Table 1: Estimates of power capacity cost and energy capacity cost for different storage systems. Reservoirs at the upper watershed regulate the river downstream. . which will typically flow more evenly throughout the year. higher demand periods. Pumped-storage hydropower plants usually follow a different strategy. 323. which is then used to generate electricity during water-poor.

Capital costs are likely to be in line with those for a conventional project. It is therefore important to identify cost-effective sites with higher water head ranges.800 MW. and relatively steep topography. i. The payback time will depend on the difference in value of peak and off-peak power. to pump water back to reservoirs during periods of low demand and using the additional storage volume for grid regulation and balancing stochastic output are interesting scenarios for a future sustainable energy system. The effective efficiency is determined by the roundtrip cycle efficiency. between $1000/kW and $3000/kW. A pumped storage plant uses two reservoirs. The energy that can be extracted from a hydropower plant depends on both the volume of water available and the head of water that can be exploited. smaller volumes of water provide the same level of energy storage. the availability of suitable sites in the future . Because of the possible significant environmental impact of these schemes. Economic performance is highly dependent on the price of the cheapest electricity produced by other plants and the variation in daily load. With higher heads. The result is a maximum efficiency of around 85% for pumped storage hydropower plants. and smaller size water passages can be used for the same level of power generation. pumped storage is considered to be one of the most efficient ways to store and regain electricity.4 kWh of energy is necessary for pumping assuming the same architecture for both operation modes. normally varying between 300 m and 800 m. A pumped storage project will provide the most efficient and cheapest operation when it can provide a high water head and volume between its two reservoirs. designs of pumped storage are optimized for overall efficiency and dynamic response. a pure pumped storage plant uses less than 5% of inflow from upper watersheds. and another to collect water back into the upper basin using surplus base-load electricity during off-peak hours. such a system will be well suited to cope with their intermittency of output. however its economic performance will always be strongly dependent on the price of the cheapest electricity produced by other plants and the variation in daily load. not a renewable energy source (IEA 2001). The global pumped storage hydropower capacity is estimated at 82. Pumped storage hydropower The most widespread large-scale electricity storage technology is pumped storage hydropower. and is therefore an energy system storage component.e. an upper storage basin providing the head to drive the hydropower turbines. equipment and construction are particularly important in view of the high costs associated with pumped-storage systems. The stored energy is only utilized for electricity generation during daily peak-load. Still. thus reducing capital costs.” for instance from surplus wind power. This will allow the greatest amount of energy to be stored in the smallest volume of water resulting in smaller pumps and turbine. Using “green electricity. which calculates the cycle with the turbine operated both as a “generator “ and as a “pump”. Hence. The effective efficiency is between 6570%. Shorter water passages will reduce the need for surge tanks to control transient flow conditions. In general.Electricity Storage 9 ii. Hydraulic turbines have a very high efficiency of more than 95% while pumps are less efficient. and for each kWh electricity produced. Appropriate location. pumpedstorage sites with high heads and short water passages are more desirable (IEA 2001). By definition. As the share of renewable energy grows. some 1.

the moment of inertia of the flywheel. into rotation. Since the centrifugal forces are equal to Mω2r = Iω2/r. For materials such as fused silica the specific energy stored in watt-hours per kilogram (Wh/kg) is more than 20 times better than steelbased flywheels and about 40 times better than lead-acid electric vehicle batteries (Table 2). The charging of a flywheel is based on putting heavy symmetrical circumferenctial masses. commercial facilities. J. John Wiley & Sons. fused silica Flywheel. uninterruptible electric power for communications networks. J. and conversion efficiency not included Source: Ristinen. carbon fiber Flywheel. originally made of steel. is the product of the total masses M and the square of their mean distance r from the center. and smaller systems are now under demonstration for mobile units.Electricity Storage 10 will limit growth in pumped hydro capacity. Energy and the Environment. higher stored energy requires stronger flywheel materials to withstand the rotational forces. A. radians per second) and the further away from the center the flywheel masses are placed. the relationship being 1/2Iω2. b. computers. Table 2: Energy densities of storage media Media Hydrogen Gasoline Flywheel. The latest modern flywheel energy storage systems make use of advanced composite materials and state-of-the-art active bearing technologies. where I. Much larger systems are currently being developed in various research laboratories. Very heavy flywheel systems have been used for many years in electricity plants. steel Lead-acid battery a Wh/kg 38000 13000 870 215 48 25 a weight of motor and generator. Single flywheel units are typically 5kWh-100kWh and can be cascaded to achieve utility-scale power and energy ratings. The masses can rotate at about 50. which is to be followed by a 20 kWh unit.000 revolutions per minute almost without friction around a titanium axle using magnetic bearings. R. high-quality. New York.0 MWh) still has some unsolved technological problems (Collinson 2001).1-10. for a given flywheel diameter and thickness. the role of pumpedstorage plants is likely to increase in the longer term. (1999). the Internet. Medium-scale storage (0. industrial manufacturing. a 2 kWh composite flywheel system was introduced and flywheels with increased power ranging up to 4 kWh should now be available. Plans include production of an 8 kWh model. . Flywheels Today’s flywheel energy storage systems can provide highly reliable. particularly in Japan and the USA (Collinson 2001). The higher the angular velocity of rotation (ω. The rotating masses are used in conjunction with a dynamo to generate electricity on demand. the more energy is stored. and distributed generation applications. Initial capital costs for construction will also increase due to environmental obligations. and Kraushaar. Recently. Even so..

which was built at Huntorf in Germany. Release of the pressurized air is subsequently used to generate electricity. A 1050 MW project has apparently been proposed in the Donbass region. The annual investment costs for a CAES are estimated between $90/kW/yr and $120/kW/yr. salt cavern. The pressure in the alternative systems will vary when adding or releasing air. are achieved. Compressed Air In compressed air energy storage (CAES) air is compressed and stored under pressure. Modern flywheels can store much more electricity than contemporary metal hydride or lead-acid batteries of similar weight or volume. and have shown similar performance to that of electric vehicles with supercapacitor or modern battery energy storage. i. . including a short range of only around 10 km have limited their applicability. Flywheels for vehicles Electric buses or trains could use rapidly-rotating flywheels for energy storage instead of secondary batteries. A small-scale application could for example be a flywheel running on frictionless bearings by means of magnetic fields that are generated by the superconducting materials. setting up a constant pressure storage system. Today’s high performance materials enable the design of electric vehicles with flywheel systems that can be comparable to petroleum-powered vehicles. can be used. both incorporating flywheels or supercapacitors as acceleration and braking energy recovery systems (Aceves and Smith 1996). on the Russia-Ukraine border. i. CAES technology was promoted in the latter half of the 1980s in the US. The most important part of the CAES plant is the storage facility for compressed air. either created by water-bearing aquifers or as a result of oil and gas extraction. Ceramic superconducting materials could also be applied in flywheel energy storage. Similar acceleration. One of the goals is to develop a zero-emission vehicle which combines hydrogen lean-burn spark-ignited engines with a high fuel economy or hydrogen fuel cells. The plant entered service in May 1991 and has since generated over 55 GWh during peak demand periods. Urban transit buses and military vehicles have used stored inertial energy as early as the 1970s. uniform cash flow. Engine and flywheel sizing requirements must be analyzed to obtain the desired level of performance. In a CAES plant compressed air is used to drive the compressor of the gas turbine. This plant operated for 10 years with 90% availability and 99% reliability. However. Usually a man-made rock cavern. Aquifers in particular can be very attractive as storage media because the compressed air will displace water. including regenerative braking. The same year Italy tested a 25 MW installation. speed and range per unit weight of energy stored. Some hybrid vehicles use a flywheel for energy storage. some impractical aspects.e. c. The Alabama Electric Co-operative built a 110 MW commercial project. or porous rock. The largest CAES plant was 290 MW. which makes up 50-60% of the total energy consumed by the gas turbine system (Collinson 2001).Electricity Storage 11 Several companies are experimenting with pairing micro-turbines with a fuel cell or flywheel system as a complete hybrid backup power system. most efficiently in conjunction with a gas turbine. Flywheels are environmentally friendly and have a much longer lifetime than electrochemical systems. depending on air storage type (EESAT 1998). Although the German utility decommissioned the plant.

the development of large-scale CAES is limited by the availability of suitable sites. 2. This approach has traditionally been used for smallscale applications. a. As a result. respectively. . Another system with an output of 10 MW and a four-hour grid capacity was constructed in California during the 1980s. The partial chemical reactions taking place in the different compartments are called reduction and oxidation reactions. Utility electricity storage requires a battery system that can be repeatedly charged and discharged.e. these correspond to necessary initial investments between $580/kWe and $770/kWe. or so-called cell reaction. Costs and performance of new smaller-scale storage systems must be competitive to hydro-storage or systems using metal hydrides for hydrogen storage (compare Table 1). either taking up electrons (positive electrode) or releasing them (negative electrode). As with pumped storage capacity. Energy Storage in Batteries Batteries use the energy involved in overall chemical reactions. Electro-chemical systems The oldest and best-established way of storing electricity is in the form of chemical energy in batteries.3 MWh and an output of 8. the overall chemical reaction. Only recently have large-scale applications of battery storage become of interest (Collinson 2001). there the use of secondary batteries does involve some technical problems. and electronically conducting bridges in an external circuit. Electrochemical storage is characterized as the ability to convert chemical binding energy directly to electricity. For instance. current research is focused on the development of systems with man-made storage tanks. batteries also use the principle of chemical bond formation to store energy. i. The process can be reversed for secondary (or rechargeable) batteries or accumulators in order to recharge the storage media.Electricity Storage 12 With a 9% discount rate and a 10-year life cycle.6 MW was constructed in Berlin to operate the city’s island electrical system. Since their cells will slowly selfdischarge. As with the energy stored in fossil fuels in form of chemical bonds formed originally via photosynthesis. these only deliver heat. Under general spontaneous exothermal reaction conditions. which results in a decreasing storage capacity and ability to be recharged. There are many more battery energy storage systems in operation today. instead of heat only. a large-scale battery storage plant with a capacity of 9. after the Second World War. The battery cell can now release an electric current. Smaller-scale storage options probably become more important in the future due to increasing capacity of intermittent renewable energy sources such as wind and solar. batteries are mostly suitable for electricity storage only for limited periods of time. they can significantly improve the stability of an electricity network. However. Because of the compartmentalization and controlled conditions inside a battery cell. is split into partial electrochemical reactions in two compartments connected through ionically conducting bridges in an electrolyte. Because batteries can release their power virtually instantaneously. It consisted of 8200 small lead-acid cells. like the lead-acid battery used in cars or previously in portable computers. exchange of electrons between the compartments. They also age.

trams.Electricity Storage 13 The performance of a battery is characterized by thermodynamic and kinetic parameters that depend on the materials and media used. electric vehicles. trams. Conventional lead-(sulfuric) acid batteries store around 20 to 50 Wh/kg. buses 90-100 . mostly immobilized 20-55 Nickel-hydroxide hydrogen gas Nickel-hydroxide metal hydrides c aqueous electrolyte. To achieve a high specific energy (Wh/kg). Of course. the speed and quality of recharge is often important for efficient and attractive applications of battery systems. high load capacity. Thermodynamically. In the following section. rechargeable high temperature system. rechargeable sealed (valveregulated) rechargeable Storage capacity [Wh/kg] a 20-50 Application starter battery for cars.000 Wh/kg of stored enthaply. and on the battery design. onboard electronics. rechargeable sealed (valveregulated). represented by a difference in reaction Gibbs energy. the recharging process can be inhibited or stopped completely after a few recharge cycles. large chemical binding energy differences between the reaction partners. rechargeable 50-60 50-80 90-120 Electric vehicles. Based on the solubility of reaction products or possible side reactions inside the cell. and ships (also unsealed nonmaintenance-free) battery system mostly for small equipment and some for propulsion. batteries for fork-lifts. pressurized. immobilized aqueous electrolyte immobilized Sodium sulfur d Sodium nickelchloride d solid electrolyte (βalumina) at temperatures of 300-400 °C solid electrolyte (βalumina) at temperatures of 300-400 °C Sealed. Besides the energy storage capacity of a cell. and is usually in the 25-55% range with modern IC engine and gas turbine-combined cycle technologies. submarine. rechargeable high temperature system. buses Electric vehicles. which is low compared to petroleum fuel whose storage capacity corresponds to 10. conversion of this enthalpy to work takes place at less than the theoretical Carnot efficiency. System Lead-acid Electrolyte aqueous electrolyte (sulfuric acid). a short overview of different battery systems is given. works at low temperatures. Table 3: Different battery systems used in applications today. the electrochemical reaction partners or electrodes should be as light as possible. mostly immobilized Type often sealed (valveregulated). appears as a cell voltage. Significant improvements in overall delivered Wh/kg have been achieved with nickel/metal hydride and lithium-ion batteries as compared to older lead and nickel/cadmium batteries (Table 3). and on-site applications battery system for many satellites can replace nickel cadmium batteries Nickel-hydroxide cadmium aqueous electrolyte. rel.

but it is inconceivable to provide electricity from batteries to supply a city.. Battery power is a flexible way to supply energy although it has limited applicability. their reduction of gaseous emissions and their independence of the nature of the primary energy carrier. 170 when applied different battery system architecture. Preuss. or cyclic operation with multiple synchronized missions. car starter. The electric secondary battery is therefore well suited for certain applications involving instant. and clean mobility environment. high voltage (4V). b a In addition to the continuously improving efficiency of batteries. i. H. Fischer. Franke. Battery systems for different applications Battery powered motive systems for indoor use are a major area of application that take an increasing part in organized transport systems in automated industries. W. their other potential advantages are quietness. reactive substrates stored outside cell c some nickel alkaline electrolyte batteries also work at low temperatures d sodium-sulfur and sodium nickel chloride have been abandoned for traction applications Source: Kiehne... Sassmannshausen. and Willmes. exchange of zinc electrode after a few cycles ca. capacitive operation with one full discharge/recharge cycle per mission. H.e.. Electric batteries store direct current (DC) and can be easily integrated into domestic DC networks. .. For internal transport tasks. rechargeable rechargeable mechanically rechargeable. lithium manganese oxide iron . adapted including additional applications). tests for some electric vehicles tests for some electric vehicles tests for some electric vehicles aqueous electrolyte alkaline or neutral electrolyte 65-70 ca. A. and combinations thereof. specific energy and specific power) or durability (cycle lifetime. Wehrle.. Will.e. The battery system provides flexible power for the traction vehicle. Stahl. freedom from maintenance... lighting and ignition (SLI) batteries provide electricity instantly at the turn of a key. W. but it is unsuited for those on a large scale. i. U. (2000). H. which is equipped with a recharging unit. Battery operation must be designed to be application-specific and must consider factors influencing cost. rel. 100 battery system for small applications. G. maintenance). E. D. high load capacity. Köthe.. Batterien: Grundlagen und Theorie aktueller technischer Stand und Entwicklungstendenzen. For example.. relatively low-voltage power. Berndt. König. P.. G. The battery system design must include the energy supply system for recharge and is dependent on the kind of vehicle operation needed. Expert Verlag.-C. An important parameter thereby is the charging (or load) factor that relates the necessary quantity of electricity in Ampere-hours (Ah) for the battery to become fully charged to the corresponding quantity released in the previous cycle.e.Electricity Storage 14 Lithium-ion polymer. The time it takes to recharge a battery is strongly dependent on operating temperature. electric traction equipment also provides a low noise. H. i. i. which often limit energy and power per unit weight and volume (energy and power density.or aluminum disulfide Zinc bromineb Zinc air (oxygen from air) b organic electrolyte at room temperature rel. lithium cobalt oxide. A good example is the fork-lift truck. low emission. K. Renningen (with permission.

For example. and battery recharging is far slower than filling a tank. which has a heat content of 13. We should note that in the last 100 years batteries have improved only by about a factor of two (Ristinen and Kraushaar 1999). and much lighter and non-toxic battery materials must be found to replace heavy metals such as lead. With the invention of the self-starter and development of a refueling infrastructure along the commercialization pathway of the car. An interesting concept now being tested is a hybrid vehicle with a small. the combustion engine using the higher energy densities of fossil fuels rather than electricity showed a far greater range per charge of energy. efficient internal combustion engine to recharge its batteries and to drive the vehicle after the electric motor has accelerated it. the IC engine can be quickly restarted after every stop and need not idle. The specific property of providing high power in a relatively short time is also necessary for starter batteries. fossil-fueled car. often as back-up safety systems to compensate for loss of electricity supply from the grid. Mobile systems Early in the 20th century vehicles using lead-acid batteries traction batteries outnumbered internal combustion (IC) powered vehicles. This car will combines high.000 Wh/kg in contrast to 25 Wh/kg of an early lead-acid battery. The average electricity consumption of powered units and the average availability of solar radiation need to be balanced using site and application specific battery systems. the battery still severely limits electric vehicle range. electric buses for public transport and electric cars are becoming more popular because for environmental reasons. A lead-acid battery of a given weight could only store electricity equivalent to 1% of its weight in gasoline. for instance for vehicle engines.pressure hydrogen gas storage with a fuel cell to achieve a peak power output of 90 horsepower and a peak torque of 140 foot- . These systems must be designed to close the supply gap for several hours and in some cases high power outputs must be available immediately or within minutes. truck and bus fleet of today with fuel cell driven vehicles is an important future task which is already in progress. Even taking into account the low energy conversion efficiency of gasoline to power. compared with the efficient end-use of electricity. Intermittent storage of solar energy is another area of application. the battery sub-system must be designed to cope with wide fluctuations of input and output currents. and because of the reserve electric motor. As back-up systems for power outages. Other immobile battery systems are used in the telecommunications business. for autonomous photovoltaic systems.Electricity Storage 15 For road traffic. No battery can have the energy density of gasoline. A possible increase in fossil fuel costs may favor electrical motive power including the application of high energy density fuel cells for cars. The problem with electric vehicles is clearly the storage of electric power. the Ford Motor company expects a fuel-cell-powered version of its Ford Focus sedan may become commercially available for use in company fleets by 2004. Battery lifetimes also require improvement. Substitution of the decentralized. These must supply a broad load profile with up to 20 A to 70 A during idling or slow drive and 300 A for up to 3 sec during the starting operation. For instance. the systems must be durable and reliable even if they are rarely put to use. thereby resulting in high fuel efficiency (Dresselhaus and Thomas 2001). Regenerative braking also helps to charge the batteries.

It is expected that mobile fuel cells for transportation will gain large market shares in the next few decades.stm . durability and the cost per weight.Electricity Storage 16 pounds. For UPS systems. 2 Darnell Group. internal combustion engines including the complete fuel cycle in each case. H2 and O2) are fed to the cells when power is needed. In addition. high reliability is required. stationary fuel cells can also be used for public mobility as centralized electrical energy supply for trams and trolleys. In contrast. Technically. However. batteries need long operating times to balance electricity supply outages of several hours.25% failure rate has been observed with lead-acid and nickel-cadmium batteries in such systems. would provide a 44% decrease in NOx emissions. and fuel cells where reactants (e. Less than a 0. and based on the 1997 Alabama (USA) generation mix. High energy density batteries Electrochemical power sources can be classified into accumulators or secondary cells having cells containing all corresponding reactive components. batteries must be able to instantaneously release very high currents over short times (minutes) until the back-up system responds. signal transmission (Telecom. From an environmental perspective. if high power storage such as SEMS and ultra-capacitors become available. Although many different high energy density battery systems are being developed. the linear extrapolation based estimate will drastically alter when installing new modern plants and technology to satisfy electricity and mobility demand.g. but a 14% increase in SO2 (Lindly and Haskew 2002). complete electrification of the vehicle fleet of gasoline-fueled. For telecom light-duty. switchgear installations. http://www. the life cycle. Worldwide Battery Pack Market Forecasts. Immobile systems For large on-site immobile battery systems mostly lead-acid or nickel based batteries are used today in power plants and solar installations. safety or emergency lighting. The most important high energy density battery systems are summarized in Table 4. light and conventional rail. nickel-cadmium or nickel-metal hydride battery. all of these criteria have yet to be simultaneously fulfilled. and air conditioning installations2.. Their battery performance is judged according to the power gain per weight and volume of the battery system. research and development of electrical vehicles with lead acid. Safety and durability are most important criteria. annunciator and traffic lights. or molten salt sodium βalumina batteries have not achieved the required reduction in weight and volume to be competitive with the conventional fossil-fueled IC engine. and high-speed trains. Inc. sealed valve-regulated lead-acid batteries are often chosen also because of their low maintenance requirements. an 18% decrease in CO2 emissions. radio) and uninterrupted power supply (UPS) systems. The vehicle has a top speed of greater than 80 miles per hour and a range of 100 miles.darnell. underground (subway). Because of the characteristics of their application.

. Class System Type Operating Temperature °C Electrolyte Energy density1) Wh/kg Power density2) W/kg Lifetime charging cycles/ years charged 750/>3 300/1 Accum ulators Lead-lead oxide system Lithium-metal sulfide (oxide) system3) Lithium-iron sulfide system Zinc-bromine system Sodium-sulfur system Sodium-nickel chloride system Pb/PbO2 Li/TiS2 <45 Ambient H2SO4 Polymer 161/35 473/100 80/160 35/400 Li/FeS2 Zn/Br2 Na/S Na/NiCl2 400 60 320 300-400 Salt melt ZnBr2+H2 O Ceramics Ceramics 650/180 430/55 758110 65082 Fuel 30/400 70/80 95/150 60/140 max. which has 1h. In recent years the fuel cell has received a great deal of attention since it offers good performances in lower temperature operation and it allows a much improved electric vehicle range. except for the Sodium-sulphur system. Important parameters are the energy and power density. H. Franke. E.Electricity Storage 17 Table 4: Properties of some secondary batteries and fuel cells. A regenerative fuel cell plant could be designed to store more than 100 megawatt-hours (MWh) of energy and provide power for hours to several thousand homes. Köthe.. Preuss. /V2O5. K.200 600 Polymer H3PO4 Carbonate melt Ceramics H2 or MeOH Natural gas Natural gas.. G. /CO2 Source: Kiehne.. and convert the stored energy back into electricity when needed.-C. W..htm .tva.. H. Fischer. P. Such plants3 would mainly 3 http://www. whereas the fuel cells store them outside the cell stack. The systems are also distinguished according to their operating temperature and electrolyte. U..9 400/2 4000/4 2000/max. Will. A. The experimental value accounts for a 5h discharging time. and Willmes. D. Wehrle. Expert Verlag. gasification Natural gas. W. H. (2000). König. 2) continuous operation/ pulse operation (30sec) 3) other combinations include /MoS2. Renningen (with permission) 1) The reactive materials are stored inside the conventional secondary cells. Stahl. G. Regenerative fuel cells can be used to store electricity in a chemical form. Sassmannshausen.. Batterien: Grundlagen und Theorie aktueller technischer Stand und Entwicklungstendenzen. Berndt. gasification H2/CO/ Air 1000 300 theoretical / experimental: The theoretical value accounts only for the weights of the reaction substrates. operation time in hours 1200 (1kW module) 16000 (several MW) 5000 (3kW module) 40000 (cell) Fuel cells Low temperature fuel cells Mid temperature fuel cells High temperature fuel cells High temperature fuel cells H2/Air H2/Air H2/Air <100 >100 . High temperature secondary batteries and fuel cells are somewhat in competition to gain future shares in the electric vehicle markets. power W/cm2 1500 150 150 1000/0.. H.. / . and lifetime..

Renewable Energy World. 2001). new developments have been achieved with one type of regenerative fuel cell. In order to find the optimal system dimensions for the specific storage application. pp.4kilowatt to 3. Witt et al. The energy density is 20 to 30 Wh per liter. the average electricity consumption and the diurnal consumption profile should be known.Electricity Storage 18 store energy during periods of low demand and produce energy during peak periods. which for instance must be adapted to the seasonal variation of solar radiation. However. such as the use of natural gas burners at night or on cloudy days (Owen 2001. which is about 30% of a normal lead acid battery. For rooftop solar power systems. Therefore redox-flow batteries are relatively large and heavy. for instance hybrid systems. The development of regenerative fuel cells is still confronted with many obstacles. are required to cope with seasonal differences. additional power installations. Technical and economic data. Table 5: Characteristics1) of Regenesys™ Power: Storage capacity: Footprint: Storage tanks: Total installed costs: Liftetime: O&M-costs: Storage time: Capital costs: Overall efficiency2) 1) 2) 15 MW maximum 120 MWh <5000 m2 2 tanks of 1800 m3 each US$ 22 million 15 years relatively small (unmanned facility) 10 hours US$185/kWh ! US$50/kWh (mature technology) 70% Renewable Energy Wold/January2000 http://www. Other very important parameters are those of the PV cells themselves.html Batteries for photovoltaic energy supply Energy storage has always been closely associated with solar installations.2 kilowatts. January 2000. Although today photovoltaics can be grid-adapted.electrosynthesis. the redoxflow battery. 85-91. which makes them only suitable for stationary applications. particularly regarding pole reversal and recharging cycles. Table 6 lists many more storage applications for PV supply technologies. . 4 Large-scale storage solution? RegenesysTM regenerative fuel cell. the problem of the intermittent nature of solar energy can be solved either through energy storage or by system back-up. according to National Power PLC are given in Table 5. including both solar heating and photovoltaic (PV) A large-scale application with a capacity of 15MW/120 MWh has been built based on the Regenesys™ regenerative fuel cell4 employing the bromide/tribromide and the polysulfide/sulfide couples in aqueous solution. Since seasonal storage is not considered for small systems. This cell stores electricity in liquid (dissolved) form and in such a way that the maximum storage capacity can easily be increased by increasing the storage capacity of the electrolyte. possible applications include battery storage systems with the option to expand the power output from a standard 2.

military KW Source: Kiehne. in order to stop the highly toxic cadmium input to the environment. which will ultimately require a change to alternative systems. Berndt. and education of the public about recycling and disposal. and certain other batteries.. P. small consumer households or businesses Stand-alone grids with electric equipment and installations Examples Solar watches and calculators Portable receivers / transmitter. Alternative secondary battery systems are lithium and nickel-metal hydride batteries. Battery Act also limits the mercury content in some consumer batteries. Power range µW mW W Typical application Integrated transistors or chips with minimal energy consumption Equipment and installation with low energy demand and only periodic use Installations for communication and measurement. e. However.. Renningen (with permission) ii. for example. that promoted the collection and recycling or proper disposal of used nickelcadmium and lead-acid batteries. König. In the U. Sassmannshausen. H. U. power tools requiring very rapid discharge. For consumer use. Still. H. K. Fischer. clear labeling of batteries. the contents of only one third of the cadmium-containing batteries find their way back to the retail shops. small sealed lead-acid batteries. D. A. Another goal is to phase out the use of mercury in batteries and provide for the efficient and cost-effective collection and recycling or proper disposal of used nickel cadmium batteries. Secondary lead smelters are the facilities . household appliances. Wehrle. especially lead-acid or nickel-cadmium batteries. nickel-cadmium batteries are now restricted to demanding markets. H. all new or alternative systems must be examined using material flows from cradle to grave.Electricity Storage 19 Table 6: System power range for PV storage applications. Expert Verlag. the recycling systems seem to fail: As shown for Germany. It is now suggested to phase out the use of nickel-cadmium batteries from a wide range of applications. For electric vehicles. has been recognized as a problem not only in the developed countries but also in developing nations. Stahl. Batterien: Grundlagen und Theorie aktueller technischer Stand und Entwicklungstendenzen. (2000). Where possible. G..S. lead is a key material of concern. W.. electricity supply on boats or in holyday houses.g.g. Köthe.g. automatic devices like ticket or vending machines. Franke. In Europe the 1992 Government Action Plan for Waste and Recycling and the 1992 Waste Management Plan for Municipalities consolidated the framework of the Environmental Protection Act 1994.... means hundreds of tonnes of toxic heavy metals are discharged yearly into the environment. nickel metal hydride is now used. e. The U. H. although throwing them into household waste has been widely banned.. But leadacid batteries are mostly being considered as a viable and cheap energy storage choices for ordinary electric vehicles.S. Potential environmental impacts from their improper disposal. Preuss. the “Battery Act”. e. and Willmes. G. radio and meteorological stations. Remote estates. E.. W. it was the 1996 Mercury-Containing and Rechargeable Battery Management Act. Will. Batteries and the Environment The increasing use of secondary batteries..-C. fire or security alarm systems Devices on buoys or for TV. electricity supply for heat pumps.. in hybrid vehicles.. Another proposed measure is to enforce dramatically lower amounts of cadmium in batteries stepwise.

Scaps are electrochemical (EC) capacitors. trucks and buses will reduce the environmental impact of the mobility today. and voltage V between them. However. -Q on the other. only if hydrogen supply and storage technology are developed and the hydrogen is generated from regenerative sources will the balance tilt significantly towards “green mobility”. at the same time permitting thousands of recharge cycles are important technological goals for the future. Dielectric capacitors can provide power densities many times more than 1 kW/kg and have very long cycle lifetimes. b. Dielectric material with a high polarizability allows a capacitor to store a large charge having applied much less voltage. which use charge as a storage mechanism. Some Scap devices are commercially available today. prolong the battery lifetime and reduce the energy requirement (or the size) of the battery. dielectric capacitors have energy densities less than 1 Wh/kg.000). and over 5 Wh/kg specific energy. with high cyclability (many thousands of cycles). In contrast.2 kW/kg and fewer cycles (generally 100-1.g. They show performance. power assist to hybrid vehicles. Supercapacitors (Scaps) are devices with the potential to achieve the necessary performance and have therefore received considerable attention in research and demonstration during the last decade. even if natural gas feeds the fuel cells in transition. In addition. leading to performances with more than 1 kW/kg specific power. and the EC capacitors provide the instantaneous power needed. e.Electricity Storage 20 where used batteries are recycled into usable lead. Lead-smelters seem still to be the key source of lead released to the environment (Maxwell and Kastenberg 1999). starting power for fuel cells. Ultra. whereas batteries can provide over 100 Wh/kg. are new passive components. A conventional capacitor with charge +Q on one plate.or Supercapacitors Capacitors are energy storage devices. whereas batteries provide less than 0. They have a huge capacitance. when EC capacitors are coupled with batteries. The applicability of dielectric capacitors is therefore clearly limited to a few specific tasks requiring high power at high voltages over very short time periods.000 times larger than conventional capacitors (typically 2000 µF). they can reduce the peak power requirement. Applications include acceleration power for electric vehicles. which is about 500. pulse power for mobile telecommunication and other electronic devices that require high power to operate. The capacitance is due to the accumulation of electronic and ionic charges at the interface between the electrodes . The development of fuel cells for cars. has a stored energy equal to 1/2 QV. which lies between that of batteries and conventional capacitors (see Subsection i). Scaps. also known as ultra-capacitors or electrochemical double-layer capacitors. which optimizes charge distributions along the atoms of the material through re-arrangement. Having storage systems available that can provide several kW/kg power output combined with several Wh/kg specific energy. Almost all capacitors use a dielectric material between the plates. The batteries provide the energy. EC capacitors can provide both power and energy. which are often made with carbon or another high surface area material as the conductor and with an aqueous or nonaqueous electrolyte. typically 1000 F (Farad = 1. electrical regenerative braking storage for electric drive systems. Super-conducting inductive systems have similar limitations (see Section 3a). for lasers.0 Coulomb or Ampere-Second per Volt). which are used in power electronic applications.

Scaps can be used as storage components in electrical systems of auto. Scaps are also well suited to be incorporated into photovoltaic powered systems to enhance the system's capability. http://www. e. ‘life of the vehicle' durability. or in hybrid systems combined with fuel cells or battery systems. Scaps can also be used in stationary industrial power applications. and consumer and industrial electronic devices. The storage capacity of Scaps is limited.maxwell. making them less suitable for large-scale electricity storage. They can be used as energy buffer bank to minimize the constraints on power supply. absorb energy disruptions efficiently and their environmentally cleanliness make them an ideal choice for many applications. Hybrid electric bus. The unit prices of large cells are expected to fall from presently $150 to less than $30. truck and buses enhancing 12. Compared to batteries. High-durability large-cell Scaps are designed specifically to meet the energy storage requirements of the transportation and industrial markets. truck and auto power trains that reduce fuel consumption and emissions by using electric power to assist acceleration and recapturing and reusing energy from braking. operate reliably in extreme temperature conditions. Low-cost Scaps are expected to become the standard option for energy storage systems of consumer electronics. solid state alternatives to batteries for short-term bridging in uninterruptible power supply (UPS) systems. their ability to discharge and recharge rapidly.g. Depending on the configuration. Scaps reduce the duty cycle of fuel cells or battery systems resulting in a considerable increase of the specific power production of these systems. as low-maintenance. Large market opportunities are therefore expected to generate significant ramp-up in volume in the next few years. Scaps cannot be used alone. based on projected manufacturing cost reductions that could be realized with new designs. industrial and transportation markets with an 80 percent cut of material costs5. hybrid vehicles. or for peak load buffering to increase efficiency and reduce the size and cost of stationary energy systems such as fuel cells. or applied for instance as load equalizer for elevators. such as steering. For conversion of mechanical functions.Electricity Storage 21 and the electrolyte. 5 Maxwell Technologies 2002. a sequential distribution of a double storage Scap system can be used both as main energy source and as energy buffer bank. braking and air conditioning and more efficient all-electric systems. wireless communications. and must be associated with a power supply system. and/or satisfy or meet regulations otherwise more difficult to achieve with conventional battery driven systems. Scap’s low cost. Such photovoltaic/supercapacitor combined power sources can operate under adverse conditions.html . Scaps have a wide scale of potential applications like memory backup-systems. Scaps are high-density energy storage cells that deliver bursts of high power on demand in applications such as automotive electrical systems and power trains.and 42-volt electrical systems for safety features.

so careful consideration must be given to the consequences of an accidental loss of coolant. while superconductivity permits a compact system design. much higher storage capacities than in conventional capacitive or inductive systems are achievable having short time releases at various voltages. with a response that is limited by the time required for conversion between DC and AC power. generation and power transmission will soon become competitive. SMES technology with 90% efficiency for storage of excess night-time electrical energy should be economically comparable 6 http://www. it is too early to judge as to whether super-conducting power storage. will require additional research and development (Larbalestier 2001). and frequency and voltage control. It offers the ideal way of storing electric power directly using supercooled inductive coils. Probably the first applications of this technology are load leveling.600 meters in diameter.g. e.18 tonnes of TNT. The efficiency of charging and discharging is very high because no energy conversion is involved. and for the same reason SMES can respond . Larger SMES systems are not yet available. Electro-magnetic systems a. are enormous.amsuper. Because there are no resistive losses. At the moment only SMES with a relative small storage capacities for power grid stabilization or industrial plants are commercial available (American Superconductor6).000 MW plant would be roughly 1. For all of these applications. Superconductors are being considered for superconducting magnetic energy storage (SMES). This SMES technology combines conventional lowtemperature material with high temperature super-conducting (HTS) materials. Although the superconductivity business is expected to be large by 2010. At present. Large-scale applications of superconductors for energy storage will depend on achieving high critical current densities (Jct) to be cost-competitive with other technologies. the costs however. high powers and enormous magnetic fields will be involved. Both physics and materials science issues are being tackled. To use super-conducting materials with higher critical temperatures for these energy-related applications. In principle.Electricity Storage 22 3. niobium–titanium alloys are used for storage at liquid helium temperatures. 2-4 K. The unit costs of power stored in a super-conducting ring decreases with increasing plant size. A super-conducting ring for a 5. the current persists indefinitely. the temperature of the much more practical and cheaper liquid nitrogen. Rapid respond within a few seconds make SMES attractive for system stabilization or primary regulation of the electricity grid. electrical energy SMES target to provide storage capacities of some 100 kWh. Research is aimed at increasing the fraction of the crosssectional area of the super-conducting wire actually carrying the high current density. 60-70K. a huge amount of energy would be released at the instant the superconductor became a normal conductor. where electric energy is stored by circulating a current in a superconducting coil. Superconductivity Superconductivity has the potential to dramatically change the generation and use of electricity. 10 MWh of stored energy is equivalent to 1. In the development of new equipment for electricity supply systems. Technological development of electrical energy storage via superconductivity will require time and large investments.

ceramic superconducting materials could be very useful in flywheel systems running on frictionless magnetic bearings made from superconducting materials. . A construction period of 4 years for a superconducting magnet. and 7 years including digging the rock base has been estimated (Masuda 1988).Electricity Storage 23 with water-storage technology with efficiencies of 65-70%. 2 million tons of structural material will be required to confine 10 million kWh of stored energy. As mentioned earlier. To show the magnitude of the technical problems involved. which is similar to that for water storage systems.

e. . Thermal Storage Systems Since ancient times the underground has been used as constant temperate zone for food storage. One of the oldest energy storage activities was gathering ice from frozen lakes and rivers. CHP units or solar power. for example from waste heat. which depends on energy density. use of solar power to reduce conventional heat sources.g. the following systems exist: • water. The benefit of long-term storage is that by balancing seasonal discrepancies in supply.g. or of constant pressure Cp are important parameters to design efficient thermal energy storage systems. The ice stored in well-insulated rooms was used to preserve food. latent heat provides large energy density) • phase change materials (salt hydrates). The transferred energy involved in temperature changes of materials is called sensible heat. The energy content E of a thermal energy storage volume V is proportional to the temperature before (T1) and after (T2) recharge. • ice (phase change at 0°C. The purpose of short-term storage generally is to store power that of longterm storage is to store energy. Since the 20’s century the underground is used for active thermal storage for cooling or heating of buildings or for industrial processing. H2O/SiO2. reflux temperatures. and condition indoor air for which mechanical refrigeration is used today.1 kcal/ kg·°C.Thermal Storage Systems 24 C. i.g. due to high energy density also suitable for long term storage. silica gel). Heat and cold storage systems are important for short-term and long-term (seasonal) storage. e. For present short-term storage. surface to volume ratio. The cost of thermal systems depends on volume and energy stored (Table 7). energy consumption can be significantly reduced. and the duration of storage. the specific heat of constant volume Cv. Depending on restrictions of volume or pressure changes in gaseous or liquid storage media. Environmental benefits include the possibility of using renewable energy sources to increase the demand coverage. • sorption systems (e. in experimental stage. Other relevant parameters for thermal systems are energy losses as a percentage of the retrieved thermal energy. insulation quality. Water has a high specific heat of 1 kcal/kg·°C as compared to steel with 0. a storage media or material specific coefficient. E = C(T2-T1)V. One of the benefits of short-term storage is to shift electric resistance heating to off-peak hours. currently in market introduction phase. The energy density (kWh/m3) depends on the temperature level stored in a media of specific heat capacity. standard medium for heat storage.e. C is the specific heat per unit volume. additional heating demand in winter or cooling demand in summer. cool drinks. while the energy involved in changing the phase of a material is called latent heat. allow phase change at other temperatures than 0°C. and in demand.

soil or groundwater systems (aquifers) are used. Also. Thermochemical couples like sulfur oxides and oxygen. although more costly. Improvements are possible in combination with heat pumps. Long-term storage is often combined with solar thermal systems. more effective storage media should be developed. or carbon oxides and hydrogen are interesting because chemical compounds having high energy densities are possible to be transported over long distances through pipes without heat loss. These systems seem most suited for homes where less cooling is required. STS. The heat coverage with solar thermal energy could technically reach 90-100%. Utrecht Centre for Energy research. with correspondingly large heat losses. Heat will than be released at the spot of demand using for instance combined heat and power (CHP) systems or fuel cells. and less by the size of the system (Table 8). Technology and Society. Large cost reductions and improvements of additives are necessary to achieve within the next decades. Technological advances of phase change materials (PCM) can increase the thermal mass per unit floor area of buildings and more specifically can improve the efficiency of thermal systems. Dept. Soil-heat exchangers integrated in pile foundation. . The corresponding net thermal energy retrieved. The recovery efficiency of heat from thermal storage depends on the reflux temperature of the heat extraction system. National Institute of Public Health and the Environment (adapted) Water as thermal storage media has a low energy density and consequently needs larger volumes. RIVM. UCE. "Daces 2050: Database Clean Energy Supply 2050. Netherlands Energy Research Foundation. the size. Especially for long-term storage the surface to volume ratios ratio should be as small as possible. and the temperature levels of the system are the most important system characteristics. Another technological straight include the development of thermochemical systems based on the reaction energies of chemical couples." Netherlands Ministry of Housing. Adsorption systems have a large potential for seasonal storage applications of solar thermal energy at the level of single homes. The energy used for circulating the storage media water through the system is determined by the permeability of the underground in use and its depth. Energy and Environment. ECN. A. The costs are strongly determined by the required pumping capacity (m3/h) and power capacity (kW). (2001). Ecofys.Thermal Storage Systems 25 Table 7: Overview current situation for short-term storage Energy density Water PCM Adsorption [kWh/m3] 50-60* 50-120 200-250 Energy losses (estimate) [%] 20% 12% 5% Cost (<10 m3) [US$/m3] 2000 2700 6800 Cost (>100 m3) [US$/m3] 700 1300 4500 Source: Collinson. for Science. Utrecht University. Spatial Planning and the Environment.

Ecofys. solar heat can cover 90-100% of required space heating in homes today. heat storage capacities range from 60 to 80 kWh/m3. e.g. Water for Thermal Energy Storage a. Underground storage systems cost around US$ 100’000 per building. reinforced concrete or wire-wound concrete tank systems. (2001). These innovative tanks are applicable for buffer and short-term storage in the range of hours and days having volumes between 5-10 m3.3 W/m2٠K) have been recently developed in Germany using fiberglass composite materials. Thermally stratified storage tanks are an effective . These large capacities are conventionally used with welded steel tanks. respectively. especially if improved building construction. STS. Low heat conducting tanks (<0. ECN. seasonal storage with volumes between 500-6000 m3. Seasonal underground storage systems can even provide coverage of up to 70% space cooling in industrial buildings having cooling capacities of 40-70 W/m2. Other thermal storage systems used for cold water storage are based on stratified storage underground tank systems combined with linear diffuser system in order to reduce inflow turbulences. and for long-term.5-50 10-500 Power capacity [kW] 5-450 100-4500 Pumping energy [%] of useful energy 5-8 5-8 Cost [US$/m3h-1] 1800-4100 1600-2700 Soil heat exchangers Groundwater system Source: Collinson. For cooling demand. Dept. Usually bigger underground hot water storage tanks are buried underneath large infrastructure components like athletic or football fields. hot water storage used with cogeneration plants. Energy and Environment. parking lots and garages having capacities of a million gallons or more. highly efficient lighting systems and electric appliances reduce cooling demand to a minimum. According to storage needs.Thermal Storage Systems 26 Table 8: Overview the current situation of seasonal storage Pumping capacity [m3/h] 0. Spatial Planning and the Environment. A. delivering thousands of ton-hours hot water storage. reducing heating demand by about 30%. underground soil or groundwater systems are very productive. Utrecht University. 1. However cost are above US$ 10. UCE. Such tanks can also be used to store cold water. Utrecht Centre for Energy research. "Daces 2050: Database Clean Energy Supply 2050. Thermal storage tanks Centralized water thermal storage is by far the most common form of thermal energy storage today. for Science. Technology and Society. In winter the same system can be used to preheat the cold air drawn in for fresh air supply in office buildings.000 per home (5000m2 floor area). National Institute of Public Health and the Environment (adapted) Technically. for storage in the range of weeks with volumes between 50-100 m3. Netherlands Energy Research Foundation. The cost of large tanks (6000m3) are presently around 100 $/m3. The costs are around US$ 300 per kW. The different temperatures contained in a thermally stratified storage tank hold the density variation of warm and cold water on top and the bottom. A combination with energy conservation strategies allows for these cooling systems to cover the entire cooling load in most office buildings. (automated) shading systems." Netherlands Ministry of Housing. RIVM.

Depending on the size of the area used. heat is normally stored at around 15-40°C. Long-term heat storage in combination with waste heat distribution. Large underground thermal energy storage systems Building. An important aspect of the short-term storage concept is to maintain thermal stratification. b. Relative new concepts of high-temperature underground thermal energy . heat/cold storage capacities of up to 5-6 GWh per year are achievable. industrial and agricultural sectors require larger thermal storage dimensions. temperatures and heat loss. soil heat exchanger) which require large storage volumes to improve storage efficiency achieving 15-30 kWh/m3 • Aquifer storage system with reduced storage volume but site specific requirements achieving 30-40 kWh/m3 • Man-made cavern storage system with confined storage volumes 40-60 kWh/m3 Early adoption of energy storage in standard project designs is essential to improve their energy efficiency. Short-term or buffer heat storage systems are ubiquitous for in-house generation of hot water in heater or boilers. and glass granulates. Decentralized thermal storage heater/boiler systems mostly use electrical night currents for electrical resistances heating. There are three forms of underground thermal storage systems (UTES) normally used: • Thermal underground storage tubes (e. for more comfortable operation 70 L/kW is recommended. It is commonly applied in combination with solar energy or waste heat reuse systems. The performance of storage-type domestic electrical water-heaters is greatly improved and energy conservation optimized. Storage temperatures are around 90°C. System parameters include various inlet and outlet locations. Cost for large systems (around 10.Thermal Storage Systems 27 technique that is widely used in energy conservation and load management applications. The thermal stratification improves the efficiency of storage tanks as heat at intermediate temperatures can be used to heat colder layers below and withdrawn without disturbing stratification too much.g. In non-sealed UTES systems. velocities. Heat storage capacities are between 30-50 kWh/m3. A minimum of 25 liters storage volume per kW boiler output is needed. The size of decentralized heat storage systems depend on the water boiler output power needed to satisfy the buildings hot water and heating demand. while cold storage areas are between 5-25°C. Designs are dependent on draw-off rates (usually 5 to 10 L/min) with the placement of heating elements and residence time being important design parameters. vacuum insulation foil. Volumes of 1000-8000 m3 have been successfully installed at several places in Germany. Future cost reductions should accomplish less than 50 $/m3. Heat conduction at the tank walls and heat loss to the ambient through the cover strongly influences the stability of thermal stratification. Optimized ratios of tank size and draw-off rates together with better thermal stratification inside the heater storage tank results in higher discharging efficiencies. The heat is stored for later use.000 m3) are around 75-150 $/m3. solar or CHP systems are also constructed using man-made water permeable gravel pits insulated and covered with heat damming materials like polypropylene foil.

at high temperature ATES consisting of vertical wells with horizontal drains. one at about 60 m for cold storage and the other at about 300 m for heat storage. and providing domestic hot water and space heating in winter. and a heat storage capacity of 5. economically. or specific processes in the agricultural and industrial sector. Glycol static ice systems cost . and having excellent heat transfer characteristics. Recently. 2. Latent heat is the energy required to change the state of a unit mass of material from solid to liquid or liquid to gas without a change in temperature. Long. Large-scale thermal storage systems must be technically. a heat/cold combined ATES has been installed in Berlin for the new governmental building complex using two separated aquifers at different levels. Other thermal energy storage systems are used where space is limiting. caverns can store heat at temperatures around 90°C. and environmentally evaluated. The storage system is coupled with a heat pump. Early adoption of cold storage as a standard design option including improvement of effective storage control and operating strategies to combine hot and cold storage should be encouraged in order to increase energy efficiency and cost effectiveness (IEA 1997).000 m3. A recent example is the cavern storage system of Lyckebo in Schweden. Chemical aspects of thermal energy storage in the aquifer must be investigated in order to solve clogging and environmental heat exchange problems.(seasonal) or short-term phase change and chemical reaction thermal energy storage are attractive for energy savings and the reduction of peak demand. water as a storage media needs large volumes to achieve adequate heat storage capacities. or hard sediment rock confining storage volumes. It has a storage volume of about 100.Thermal Storage Systems 28 storage (HT UTES) at around 70-90°C are attractive to achieve more economically efficient and environmentally benign energy systems with reliable long-term operation. with a CHP unit running on biofuels. Man-made cavern storage systems are placed in leak-tight granite. innovative and cost-effective seasonal heat/cold storage applications are attractive for a variety of building types and industrial applications. Today. Adequate scaling of ATES improves reliability and avoids or reduces potential ecological impacts. Latent Heat / Phase Change Systems Although being inexpensive. Though these advanced thermal energy storage techniques have to overcome technical and market barriers. and. Large-scale. Reaching its full operation. Having low heat conducting properties. readily available.5 GWh. Latent heat storage installations using PCMs or chemical reactions for thermal energy storage are alternative concepts for specific applications in building systems. major problems can occur at the heat exchange interface where major amounts of calcite can be formed. For example. Often phase change systems are applied which make use of latent heat. developing and testing site-specific ground-water treatment methods regarding the chemical. this ATES system should ultimately provide 16 GWh heat and about 3 GWh cooling capacities. ice is often used in glycol static ice system with phase change at 0°C and an energy density of about 90 Wh/kg for its cooling service. The oldest phase change material (PCM) in thermal storage is ice. Other types of UTES-systems are confined to aquifer storage (ATES) and duct/borehole storage (DTES) that can store waste heat from municipal facilities in summer. in case of extra load necessary in winter. Additionally. microbiological and environmental effects help to meet necessary environmental standards. gneiss.

09b 0.54a / 1.8 °C.34l Inorganic salt hydrates Organics Fatty acids Organic/inorganic mixes (Eutectics) Aromatics a Calcium chloride Sodium sulfate (Glauber's salt) Zinc nitrate Magnesium nitrate Polyglycol E400 Polyglycol E600 Octadecane Ecosane Paraffin 116 (paraffin wax) Paraffin 6403 (paraffin wax) Palmatic acid Capric acid Mystiric acid (mainly inorg.46c / 0. c 39. Table 9: Transition temperature and latent heat energy of some types of the most common groups of PCMs. However. l 49.19f / 0.16i / 0.35h 0. and most importantly show negligible supercooling tendencies.13k / 0. Eutectic PCM behave as salt hydrates but can be custom made in order to meet specific melting points.9 °C. organic PCMs are more expensive than inorganic salts but can be custom made to provide transition temperatures specific for a given application. non-corrosive. because of stopping heat . g 63. Heat storage at different temperatures can also be designed using other PCMs (Table 9). have generally higher latent heat per unit weight. for instance to absorb solar heat for air heating.6 °C. h 33.17g / 0. In combination with thermal stratification tanks.9 °C The transition temperature (melting point) and density of the solid phase are related to the specific heat and thermal conductivity of the solid and liquid phases and must be compatible with the site conditions of the application.5 °C. melt without remaining solid phase sediments.15j / 0. f 38. Compared with inorganic salts. organic PMCs are used in buildings today. The heating pipe system passes through the modules in order to exchange heat.7 °C. liquid / solid (W/mK) 0. In general.6 °C. d 95 °C. Some disadvantage of organic PCMs may be its potential flammability. but they require less space. Compounds with the lower melting points are therefore more desirable. k 83. PCM modules consist mostly of series of PCM filled plastic pipes embedded in insulation material. b 23 °C. flat PCM modules with different melting temperatures can enhance efficiency. non-hygroscopic. Group PCM type Transition temperature (°C) 27-30 32 36 89 8 22 28 37 48 62-64 63 32 54 80 Latent heat (Wh/kg) 47-53 70 41 45 28 35 68 69 58 48-53 52 42 52 Thermal conductivity.) Naphtalene 38. i 68. the costs are considerably higher than organic or inorganic PCMs. e 37 °C.5 °C.19f / 0. Nonetheless.61e 0. compatible with most building materials.4 °C. The overall temperature required to melt the PCM must be considerably less than what the output of a heating system provides. j 38.Thermal Storage Systems 29 more than cold water systems. more stable during repeated phase change cycles. they are non-toxic.49d / 0.

which were integrated in ventilation systems. waste heat of 80°C can be used to redistill the mixtures. For example. 3. Evaluation of the most suitable mixing-pairs in terms of cold/heat capacity must be carried out for each specific application. cooling is achieved in chemical heating/cooling pipe systems by the endothermic heat of mixing of solidor liquid. Similarly. This method can also be used for larger centralized thermal storage tanks. This discharging process can heat inlet air from 25°C to 100°C streaming at 6000 m3/h through 7 t of zeolites and providing a heating power of 95 kW for 14 h per day. . isobutanol/acetonitrile and 1-butanol/acetonitrile are found to produce 0. Recently. urea/water and ammonium nitrate/water are found to generate endothermic heat of 8.liquid mixable materials. releasing a humidified air stream at 40 °C.2 kW/kg as soon as the materials are mixed. in order to desorb the water molecules from the zeolite surfaces. For regeneration. Having very large surface areas. In solid-liquid pairs. The system has been successfully applied in Germany. zeolites are ideal materials that adsorb water vapor effectively releasing a lot of adsoption heat. Other thermal storage systems A very different thermal storage system is based on thermophysico-chemical properties of different chemical compounds. very innovative latent heat storage systems have been developed using molecular adsorption and desorption on chemical surfaces. During low heat demand. the charging process takes in air streams of 130°C.Thermal Storage Systems 30 conduction at surfaces and therefore improving the stability of the stratification. The storage capacity is around 150 kWh/m3 of zeolite material.8 kW/kg.

Coal Coal is a dark brown to black combustible mineral formed over millions of years through the partial decomposition of plant material subject to increased pressure and temperature under the exclusion of air. or silos. Primary energy is predominantly stored chemically in large crude oil and natural gas storage tanks. respectively.Chemical Storage Systems 31 D. The key environmental drawback of coal mining is the continuous release of CO2 due to the glow burn of coal floss. leading to selfignition. Primary coal exists in different grades called anthracite.doe. US.S. http://eia. 1. other secondary coal (world wide) are 14'601 t of Anthracite and bituminous briquettes and 11'813 of lignite briquettes In 1999. coal mining productivity has steadily increased over the last decade 7. Pollution abatement equipment is installed in modern plants to control 7 IEA World Energy Outlook 2002 . Secondary coal is produced from primary coal into metallurgical coke. DOE.5x109 tons per year (Dresselhaus and Thomas 2001). bituminous and lignite briquettes. Glow burn is impossible to control as oxygen enters the vast underground coal reservoirs during mining activities. see Table 2). Secondary or final chemical storage involves refined petroleum products kept in tanks of fuel Secondary coal. or as stockpiles or in silos of specific energy carriers like coal or biomass. coal production was about 5 billion tons worldwide (Table 10). Chemical Storage Systems The energy density achieved with chemical storage is at least a factor 100 higher than any other storage technology discussed in the previous sections (e.1x1011 tons. and households. Although production stagnated to this level. industries. World coal reserves are about 9. granulated coal and wood chips or biomass pellets are kept dry in sheds. Table 10: World coal production 1999 a Region/Country North America Central & South America Western Europe Eastern Europe & Former U. with annual consumption at about 4.g. This is the decisive advantage giving chemical storage systems unsurpassed competitiveness.S. Energy products from coal and biomass like briquettes. vehicles. anthracite. Middle East Africa Asia & Oceania World Total a b Anthracite Bituminous Lignite 100'070 0 380'612 235'399 0 0 185'329 901'410 Metallurgical Cokeb 26'185 10'694 43'499 66'454 335 5'354 214'601 367'122 Thousand Short Tons 4'768 1'086'789 28 49'457 10'308 104'661 35'576 475'221 0 1'389 2'881 324'294 298'961 1'757'120 352'522 3'798'932 Energy information Administration EIA. Coal combustion releases particulates and the chemical compounds sulfur dioxide (SO2) and nitrous oxides (NOx) responsible for the environmental acidification. bituminous and lignite with decreasing heating values. cellars.

Together with other essential equipment for coal-receiving. Crude Oil World reserves of oil are about 1. Coal stocks are kept at electric utilities (e. The coal is then taken and crushed in pulverizers into powder to feed the plant’s burners. coal transportation. World consumption is about 1. Customized solutions to fugitive coal dust problems at ground storage facilities and railroads are often required. DOE. the clean coal technology initiative has lead to pollution control and power generating processes using low sulfur sub-bituminous coal that reduced air emissions from coal burning plants including greenhouse gases 8. coal preparation.000 thousand tons in 2001).gov/ IEA World Energy Outlook 2002 .2x1010 L a day. Other coal storage facilities also consisting of coal bunkers. In the long-term. Some of the new technologies offer the potential to use high-sulfur "dirty" coals achieving comparable air emissions. US. or that more fluctuations of oil prices may occur in the future may find their arguments in possible geopolitical interference affecting fossil fuel supply or in the geological uncertainty to efficiently allocate resources in the future. The raw coal for power plants or boilers is stockpiled or stored on-site in coal silos. and space requirements for coal storage result in significantly higher capital investments. coal strait or stockpile slot systems allow coal from the different pits to be blended and provide short-term storage to accommodate peak shipping demands often performed by railway in aluminumbodied-car unit trains. The oil reserves are believed to be economically exploitable for the next 25 years and under the assumption of increasing prices up to about 2040 using improved exploitation technology (Dresselhaus and Thomas 2001). In addition. At coal-handling facilities. attractive returns on the large investments done by oil firms for exploitations today and in the near future may only be achievable if oil prices steadily rise and additional capital is made available at the right time in order to access these new deposits when needed. 2. or at coke and power plants and other industrial facilities. crushers and granulators use coal delivered from different coal pits to produce coal fragments of about 50mm in size. But capital may rather be invested into projects with quick returns. in the US 119.g. weather observation and forecasting to handle high fugitive dusting potentials.6x1014 L (1x1012 barrels) with similar amounts of undiscovered oil resources9. http://eia.Chemical Storage Systems 32 these emissions. Enhancing power plant efficiencies and capturing carbon gases achieve further CO2 emission reductions. The vast extent of the present oil production from today’s deposits is predicted to increase by at least a million barrels every year over the near future. Believes that crude oil prices will gradually increasing pushing gasoline and diesel prices up. OPEC’s present oil production and especially today’s willingness to prepare for future investments are major uncertainty variables 8 9 Energy information Administration EIA. Having adequate storage capacities help at least to balance short-term price fluctuations.doe. Environmental quality control is assured using air quality modeling and monitoring including particulate source inventories. silos. raw coal storage. and storm run-off collection and settling pond modeling.

200 1.000 209.S. lube oils. in lease tanks.700 199.eia. http://eia.Term Energy Outlook". an economic slowdown can reduce energy demand and in turn oil prices may decrease as stocks are up. other (mostly cracked) oil distillates used for heating and other residual oil stocks (light and heavy oil) are crucial for the attractiveness of the energy services based on fossil fuels (Table 11). DOE's Energy Information Administration (EIA). Also. or propane and natural gas. and stocks in pipelines. Besides the importance of primary storage for the functioning of the oil market. waxes.doe. coke. as to replenish low fuel stocks during mild winters when demand is low for heating oil. All stock levels are as of the end of the period. Stocks held at natural gas processing plants are included in “Other Oils” and in totals.doe. jet fuel. http://www. Storage is no answer for that. naphtha and other oils for petrochemical feedstock use. asphalt. diesel. SPR) Crude Oil in SPR e Total (Incl.200 48.900 400 116. SPR) a Dec 2001 (Mbbl) 312. Table 11: Example of crude oil and petroleum storage: product stocksa from the U. Note: Data may not add to total due to independent rounding.900 44. efficient secondary storage of petroleum products like gasoline. and in transit to refineries.000 144. special naphthas. stocks held under foreign or commercial storage agreements. road oil. EIA report. some simple market dynamics can be utilized by primary storage of crude oil. in pipelines.300 62. and miscellaneous oils. Does not include those held in the Strategic Petroleum Reserve(SPR) natural gas liquids and LRG’s.586..05% Sulfur (high) Residual Fuel Oil Unfinished Oils Other Oils d Total (Excl. e Crude oil stocks in the SPR include c Crude oil stocks include those domestic and Customs-cleared foreign crude oil stocks held at refineries.500 82. other hydrocarbons and oxygenates.Chemical Storage Systems 33 impacting on future price stability10. or in transit to. In the short-term.300 Product stocks include those domestic and Customs-cleared foreign stocks held at.100 550. f Distillate fuel oil stocks located in the “Northeast Heating Oil Reserve” are not included. for instance. kerosene. in Mbbl (1x106 barrels (bbl)b Year/Product Crude Oil c Total Motor Gasoline Reformulated Oxygenated Other Finished Blending Components Jet Fuel Distillate Fuel Oil f 0.400 42. refineries and bulk terminals.200 41. d Included are stocks of all other oils such as aviation gasoline. In many countries oil imports are regulated at least to some extent regarding strategic petroleum reserves (SPR). Compulsory storage ratios are stipulated 10 "Short.036. b Weekly Petroleum Status Report/Energy Information Administration EIA.100 1. These long-term uncertainties make any effort to arrive at a valid pricing system of a scarce resource like crude oil impossible (Banks 2000).05% Sulfur and under (low) Greater than 0.000 87.html . aviation gasoline blending components.

Aboveground and underground storage tanks are mostly used also for secondary storage for instance of high or low sulfur distillates. This process can lead to methane venting when the crude oil is stored in fixed roof tanks. Due to its recalcitrant properties in the environment. Thousands of oil storage tanks are distributed over the landscape of oilrich counties and store the crude oil extracted by pumps scattered throughout numerous oil fields that are sometimes as small as a few acres. The cost of replacement and remediation of contaminated groundwater and soils is estimated to go into tens of billions US$ (Blaisdell and Smallwood 1993). store the crude oil. different amounts of stock are kept by the consumer companies or. SPRs play a key role in the international energy markets stabilizing price fluctuations. about 500'000 leaking petroleum hydrocarbons tanks from secondary and final storage (mostly heating oil and gasoline tanks) have been estimated in the US during the early nineties. US total fuel distillate ending stock was about 140. From an environmental perspective. recently. and spent oil from spent-oil tanks located at a vehicle service and fueling facility. Oil tanks and the environment Tanks. At refineries such containment system also divert rainwater runoff to the refinery’s effluent treating facilities to assure no traces of chemicals or hydrocarbons enter the local waters. The emissions of gas built up in . In addition. regarding heating oil. Depending on the annual demand and capital cost of storage and the aging of oil. A fee on gasoline and other fuels at bulk distribution facilities supports the fund. a. finished products and chemicals. However. Pollution to water and soil often occurs directly through leaking oil tanks. Refined petroleum products like gasoline contain sulfur at concentrations ranging from 5 parts per million (ppm) to over 350 ppm and may contain methyl-tertiarybutyl ether (MTBE). In the US. Today. Often also large consumer companies can join the contracted storage system with the obligation not to empty their stock below a given ratio. One possibility to reduce the quantity of methane emissions is to consolidate and centralize the liquid storage facilities in the fields. also by households. The fund covers the expenses of corrective action taken in response to a release of petroleum products from a petroleum storage tank. some with capacities over 390. all storage tanks should be impermeable to volatile organic compounds (VOC) and should be equipped with pressure-vacuum valves combinable with a vapor recovery system. hydraulic fluid from systems located at a vehicle service and fueling facility.000 Mbbl in 2001 with about 40% high sulfur and 60% low sulfur distillates. as well as intermediate stocks (partially refined).000 barrels (over 16 million gallons). some of oil storage tanks have containment areas designed to hold their capacities in the event of a tank rupture. MTBE has been banned in the US to be used as gasoline additive. the petroleum storage tank remediation (PSTR) fund provides reimbursement for remediation of contamination resulting from leaking petroleum storage tanks. Reducing storage tank emissions could cost the oil industry tens of millions of dollars. Today. regulatory agencies began to develop rules that affected heavy oil storage tanks in order to meet the goals of the Clean Air Act. or ethanol (EtOH). the crude oil is processed in the fields to separate lighter hydrocarbons and produced waters.Chemical Storage Systems 34 between importing companies and the government.

Chemical Storage Systems 35 crude oil or produced water storage tanks are controlled during handling by flare combusting of the VOCs or better by tank battery vapor recovery system.4x1012 m3 per year (Dresselhaus and Thomas 2001). because of extended pipeline systems being available. Fugitive emissions cannot be completely repressed since tank opening is required for some maintenance operations. The VOC destruction or removal efficiency should be at least 90 to 95 % by weight. this cannot be exploited economically however11.4x1014 m3. Consequently. Place and volume of peak-demand is unpredictable and therefore exact timing impossible without integrated storage systems. By far the greatest volume of gas is stored in depleted gas wells or salt caverns as primary storage (Table 12). Natural Gas World reserves of natural gas are about 1. 2. This allows continuous service even when production or pipeline transportation services do not meet short time capacity needs. An air pollution control device measures the vapor destruction and removal efficiency. The consumption of natural gas varies daily and month-to-month along with changing weather conditions and with the demand for electricity generation. even a small part of the potential would already provide a very significant new source of natural gas. the risk for contamination of groundwater cannot be neglected. or in artificial underground caverns like abandoned mines or salt caverns. The degassing is controlled by liquid displacement into a vapor recovery system. LNG plays only a small role on the gas market. LNG is kept as chilled and pressurized liquid in double walled cryogenic vessels or tanks. 1998 Natural Gas: Issues and Trends . Used 11 EIA. natural gas is stored in two basic ways – compressed in tanks as liquefied natural gas (LNG) or in large underground storage facilities. In the long-term. 2001). Typically. Natural gas production does not correspond to these fluctuations. Seasonal fluctuations are as much as 50% and in the short-term more extensive variations can be observed. External or internal floating roof crude oil tank have vapor space which have to be controlled while the tank's roof is resting on the tank's inner roof supports. To enhance pipeline performance. The major transport of diesel components in rock occurs through fracture systems and diffusion of diesel through the rock. Presently. i. or fuel gas system. natural gas is moved as gas through pipeline systems. In the US however. Investigation of the environmental impact of the underground storage of diesel in igneous rock surrounding underground storage facilities showed penetration into the rock as far as 50 m from the walls of the vaults (Lorén et al. natural gas hydrates from deposits at ocean floors hold a vast tapping potential. Transportation by ships or trucks is done in its liquefied form. surplus gas is pumped during low-demand into numerous mostly underground storage facilities for use during high consumption periods.e beyond 2100. Other innovative methods to store large amounts of petroleum products like diesel have been undertaken in Europe. Underground storage systems are artificial deposits of injected natural gas in porous rocks of depleted fields or aquifers. flare. LNG requires 600 times less storage space than its gaseous form and the ability to ignite is strongly reduced. gross production of gas is about 2.

Underground storage and liberalized markets The dependence on imports and occurrence of bottlenecks within gas transmission grids requires a competitive way to cover gaps between transportation capacities and withdrawals. and corresponding storage sites for LNG.eia. During this time. They allow decreasing the total amount of investments in infrastructures.html Well functioning storage systems eliminate the need for expensive. U.S.doe. (March 2001).Chemical Storage Systems 36 as vehicle propellant in combustion engines. Underground storage facilities are localized near consumption areas. Table 12: Example of natural gas storage: primary storage in the US for the year 2000a in volumes of billion cubic feet at 1 bar pressure and at 15 degrees Celsius. excess production of natural gas is injected into approximately 410 storage reservoirs that account for almost 4 trillion cubic feet of storage capacity. natural gas is kept in tanks as compressed natural gas (CNG) on-board of vehicles and at filling stations. operational reliability. e. in order to raise the deliverability of the network. Natural Gas Monthly. using natural gas rather than petroleum and diesel for combustion engines or oil for heating purposes12.doe. Increasing nat_frame.072 EIA. This storage system allows consistent delivery of the natural gas resource to consumers having stabilized supply by using summer production levels for shortages in the winter.927 a Salt Caverns 145 Total 6. a. The liberalization of energy markets aims to separate network infrastructures and energy services. (over 15% of one year’s national gas consumption). http:// www. there will be a significant shift in consumption patterns as the population of many countries accustoms to more natural gas for the generation of electricity to power air conditioning. A gas pipeline network is composed of many supply and demand nodes. changing demographics and the deterioration of storage and pipeline systems impose technological challenges for the future of the gas markets. pipeline links that tie together the supply and demand sides.eia. Department of Energy. Traditional Storage 5. Volumetric capacity depends on head and shell thickness of the storage vessel. cooking and mobility. Innovative methods to enhance operational flexibility of the nation’s gas storage system are requested while ensuring the integrity. 2002 National Energy Technology Laboratory. additional pipeline transmission capacity that would be necessary to supply peak-demand. Supply is liberalized while buyers and sellers are allowed to use the existing gas grids operated by regulated natural monopolies. or for heating. For instance in the US.g. The use of natural gas is projected to increase at a rate of 2% per year for the next 2 decades due to ongoing decarbonization trends of fossil fuels. safety and security of the natural gas infrastructure. Present storage wells must be renewed or replaced as older storage areas experience "wear" due to the yearly cycles of injection and extraction of gas. More open gas 12 EIA. .

utilities. ISS technology innovation reduced the cost of NGV storage tanks by nearly 50%. hydrogen can be used 13 Kevin Stork. One of the technologies that can be used for conversion is the fuel cell (compare chapter B). a stateof-the-art. The ISS is easily integrated into common vehicle chassis.strength fiberglass shell containing impact-absorbing foam. well functioning gas market are security of supply. However for commercial viability cost of natural gas storage must still be significantly reduced. The intermittency of renewable resources can then be balanced by hydrogen combustion reforming water. The ISS utilizes lightweight. and trucks. weight. Hydrogen is however believed to have a great potential to be used as major energy carrier for clean energy systems in the future. Laboratory. Daimler Chrysler has tested the ISS in several sedans and reported very positive results. For instance. 4. Hydrogen Hydrogen is presently used mostly as chemical feedstock in the petrochemical industry. Natural gas and mobility While natural gas vehicles (NGVs) have penetrated some niche commercial markets for buses. and Dale Tiller. safety-certified natural gas storage unit called the Integrated Storage System (ISS) has been developed. Lincoln Composites . Johns Hopkins Applied Physics.Chemical Storage Systems 37 markets force on competition and trading. John Wozniak. The separation into two tanks. Hydrogen can be used in transportation. Its weight savings is up to 70% as compared to steel and aluminum cylinders. If combined with decarbonization technologies. Hence hydrogen is complementary and not competing with the renewable conversion technologies. Underground storage can play a major role in this gas chain. especially for remote areas. it ideally generates no emissions other than water. A society powered almost entirely by hydrogen. and install more pressure on demand balancing and price volatility. and in food. and size of existing natural gas storage cylinders. Further development of composites for cheaper and lighter weight ISS components is underway13. delivery vans. their success is still heavily constrained by the cost. is a strong vision for the future. buildings. and can be used as chemical storage compound. for instance one on the undercarriage and one located in the trunk. It would therefore strongly contribute to the reduction of energy-linked environmental impacts especially regarding anthropogenic emissions into the atmosphere. Basic conditions for a future. where improvements of natural gas services using pipelines with the capacity needed for peak demand are too expensive. and ISS does not interfere with the car’s crumple-zone. and industry. electronics and metallurgical processing industries. In the 90ties. When hydrogen is used as an energy source. Office of Transportation Technologies. high-density polyethylene to construct small diameter cylinders (called pressure cells). provide a total energy storage capacity of 45 liters of gasoline equivalents at a service pressure of 250 bar leading to an automobile driving range of over 480 km comparable to those powered by gasoline. which are subsequently encapsulated within a high. renewable resources like wind and solar power can be used to extract hydrogen from water with the only emission of oxygen. the most abundant element in the universe. b. long term planning of investments and equity of access to the grid.

Xerox Corp. 2002)15. Potential 14 15 The Clean Air Now (CAN) Solar Hydrogen Project. Making the hydrogen vision a reality in the 21st Century is the goal of many researchers at universities and industries which have started to align their efforts internationally (IEA 2000). to less carbon-intensive fuels and therefore support CO2-emission mitigation. Another demonstration of a hydrogen support system is the regenerative energy storage systems for ultra-high-flying solar aircraft.nasa. The system consists of a stand-alone electrolyzer. with compression and storage facilities could be the start for an infrastructure to support mobility based on hydrogen14. much is lost through friction (for cars about 10 kWh per 100 km) and low-efficiency energy conversion. highly abundant and its oxidation product is water. USA Therein weight is a problem aggravated by additional weight from on-board fuel storage. California.. Today. a. an effective strategy for improving urban air quality (Provenzano et al. Hydrogen storage research Hydrogen storage necessary to continuously fuel automobiles or fuel cells is a major safety problem. and only low NOx emissions (10 to 100 ppm) are exhausted by the vehicles. hydrogen is stored as a compressed gas for example in vehicles. Critical parameters for solid-state storage are hydrogen uptake capacities. a centralized hydrogen generation plant powered by solar photovoltaic. Solid-state systems bind hydrogen to a solid material like carbon. hydrogen would be ideal as a synthetic fuel because it is light. Although new battery technologies are developed. controlled hydrogen release and rapid hydrogen refilling. Hydrogen for mobility Although little energy is needed to sustain movement. Further developments are directed towards hybrid zero-emission vehicle using hydrogen lean-burning spark-ignited engines in different configurations with flywheel. b. supercapacitor or batteries as energy storage devices (Aceves and Smith 1996). and then to use a fuel cell to "regenerate" electricity at night (NASA. and is combined with an on-site storage tank for solar hydrogen. so far batteries have not shown to be an ideal option for motor vehicles. 1997). no CO and hydrocarbon emissions. If the engine operates in lean burn mode.dfrc.html . Solid-state storage systems is another. hydrogen dispensing system powered entirely by photovoltaic energy. For example. El Segundo. an environmentally benign compound. or a cryogenic liquid in physical storage systems for transport. For vehicles that store energy on board. The hydrogen fuel on the vehicles is stored in carbon composite tanks at pressures up to 250 bar providing fuel for 140 miles. Hydrogen as energy storage option for mobility is gaining more and more attention (see section b). The system is designed to store energy by converting the craft's excess solar power into hydrogen and oxygen during the day. Continued technology development and cost reduction promises to make mobility based on hydrogen competitive. safer storage technology that could potentially store more hydrogen per unit volume.Chemical Storage Systems 38 for upgrading carbon-rich fuels like biomass and some solid and liquid fossil fuels.

innovative combustion chambers. for example.g. while fertilizer production alone accounts for 35%. From an environmental perspective biomass combustion can lead to significant emissions of air pollutants if no adequate control measures are installed. application). 5. transport and preparation (wood chips as example) consume more than 75% of the biomass production energy. nitrous oxides and lots of particulates (e.g. no breakthrough is yet in sight (Dresselhaus and Thomas 2001). especially the lightweight metal alloy hydrides (Schlapbach and Züttel 2001). the main activity today is to further explore ways to reduce the costs of liquid/gaseous biofuels delivered by specific renewable energy technologies. gaseous or liquid fuels like methane. is used for space heating mostly. Fertilizing (production.Chemical Storage Systems 39 advantages of solid-state storage are the applicability under reasonable operating conditions like higher temperatures and near ambient pressures. and fiber crops. Biomass energy production depends on the availability of suitable land and water that varies by region. Despite the interest and activity in solid-state hydrogen storage research. irrigation. generate sulfur oxides. Burning wood or wood chips. Biomass is currently also converted to liquid fuels by fermentation of carbohydrates to ethanol. i. Therefore. or biooils can be produced from biomass. PM10) and many trace chemical pollutants. transport. On-board fuel tank storage system depends on the kind of vehicle and the mode of its operation. The introduction of natural gas powered vehicles benefits the efforts to develop hydrogen technology as well (Ewald 1998). and filter . As the extent of crop yield increase for food. pellets or wood chips. Besides growing and harvesting biomass. based on the experience of the industrial gas industry and of chemical process engineering on space technology different methods of hydrogen storage have been proposed and partly investigated in the past. Presently. temperature control and raw material. About 6% of the (lower) biomass heating value is used per unit production. Land requirements however remain a distinct characteristic of this renewable energy source. significant amounts of energy are consumed in producing biofuels. timber. The conversion of biomass to electricity is conceptually similar to the conversion of coal to electricity by direct combustion or gasification. both liquid biofuels (e. Gaining significant market shares would require large cost reductions and efficient biomass conversion technologies. burning briquettes. or for producing industrial process heat. biodiesel) and biogas are applicable in internal combustion engines or fuel cells.e. This chemical energy released as heat by combustion. As alternatives to petroleum-based fuels. metal hydrides are the most promising materials for hydrogen storage. Modern equipment can reduce these emissions to some degree by using optimized air to fuel ratio. Biomass Biomass fuels (biofuels) utilize the chemical energy fixed by photosynthesis and stored within plants. Only a fraction of the biomass today is harvested and used in current technologies. the land becoming available competes for settlements and corresponding infrastructure and industry. or by extraction and refining plant oils. Instead of producing heat directly. alcohols. hydrogen. Developments to improve the overall efficiency of a biomass energy system are very important tasks for the decade to come. The heat can also be converted to electricity using steam turbines. and harvest. However.

agricultural food and feed crops. although on average only 0. But a sustainable production has to be based on a highly efficient management of energy crops. It can be produced by fermentation processes. has better overall energetics and can be applied more flexibly. Annual micro-algae productivity at saturating levels of CO2 is estimated as high as 81 dry-tonnes per ha and year (36 tC ha-1 year-1). security measures to prevent (self-)ignition in the presence of heat and oxygen are important. aquatic plants. Biomass varies somewhat in both its energy and carbon content having different heating values. wood wastes and residues. Marine algae. . and 50 GtC per year at sea (Kheshgi et al.3% is stored as carbon compounds in land plants. it is more directly accessible. One major issue in secondary or final storage of biofuel is the oxidative stability of the compounds. not to forget the marketing and distribution of energy products. use of waste recovered fuels. drying. Biogas can be distributed over the gas grid or used in internal combustion engines. around 1-5 MW power plants. In general. or other bioliquids and synthesis gases (Syngas). The global plant biomass is estimated to be about 600 GtC on land. Poplar or other fast growing crops have energy contents around 15 to 20 GJ/dry-t. as energy resource exploitation is currently too expensive. animal wastes. Also biomass gasification has a cleaner technological potential for directly generating biogas. and an integrated view on the complete bio-fuel chain from growths to the logistics of harvesting. Large-scale exploitation of micro-algae is done mostly for food additives. Biogas Biogas is the most promising biofuel. and other waste materials which are logistically harvested. Cultivated macro-algae. forest residues. and delivery. Overall energy balance with regard to CO2 reduction potentials of biogas production routes or enzymatic conversions of biomass to ethanol could become significant. ranging form wood. pyrolysis or gasification techniques from many different biomass raw materials. and only about 3 GtC as marine biomass. i. 2000).Chemical Storage Systems 40 systems especially for larger. agricultural crop wastes and residues. Biofuel storage Primary biomass storage includes any organic matter available on a renewable basis. Biomass energy stored in plants Sunlight reaches the Earth’s surface with an annual average energy density of 180 W/m2. a. b. Plants on land are not the only option for renewable biomass production. like kelp. high acid numbers coupled with high viscosity numbers indicate a degraded liquid biofuel. is about 60 GtC per year on land. about 6. 2000). including kelp. collected or stored for supporting a continuous biomass resource supply chain to produce biofuels. For solid fuels. including dedicated energy crops and trees. municipal wastes. Stability test methods are developed to allow a customer to determine if the fuel will remain stable in storage over extended periods of time.7% of the solar energy input on land could be converted to chemical energy by photosynthesis. The yields seem significantly higher than for sugarcane under optimal conditions. This includes short rotation coppice. Compared to liquid biofuels. are potential renewable resources. Net primary productivity (NPP). Theoretically. are reported to have yields of up to 150 dry-tonnes ha-1 year-1 (Kheshgi et al.

8% pure ethanol. Product storage of ethanol takes place at the product terminals where gasoline and ethanol are blended. The biomass comes mostly from corn. Both techniques are also used as intermediate step in the production of methanol and ethanol. Biofuel ethanol Alcohol production through microorganisms is one of the oldest roots of biotechnology. In addition. and sludge. Biogas from 1 kg of airdried. Blending of gasoline and ethanol to produce a form of alternative transportation fuel has gained al lot of support in the U. CO2. Biomass is converted to a fermentation feedstock. the 95% ethanol must be further processed with more energy inputs to achieve 99. the blended vehicle fuel E85 contains 85% ethanol. Biogas production from biomass pyrolysis is taking place at high temperature. With E85 having less energy content than gasoline. Added to gasoline it would extend supply and also would enhance agricultural profitability while being a renewable resource. Presently. 1 liter of 95% ethanol is received after 3 distillations leaving 124 l of water behind. ii. barge or ship. barge. and in the presence of catalysts. 1 L of ethanol is gained from about 2. The tanks also function as reservoirs and must be large enough to receive the minimum shipment size without having to deplete safe working stocks. Alternatively.Chemical Storage Systems 41 agricultural plant residues. Each is stored in its respective tanks until drawn from stock. its reduction of the dependence on foreign oil is controversial. For biofuel production. which can deliver about 0. . or double membrane storage tanks holding the gas at constant pressure. For example. or ship while the ethanol arrives by truck.000. The gasoline arrives at the terminals via pipeline. tank materials must be H2S-resitient.8 kWh electric output. The estimated cost of installing a 25m-barrel tank is around US$ 450. gasification. ethanol must be isolated from the aqueous product solution by distillation. under exclusion of oxygen. which adds to the controversy in debates about the economic feasibility of large biogas production plants. However. rail. For blending. Ethanol is a regional product and is made out of agricultural produce. Gasification is possible for raw materials carrying less than 50% moisture. Since biogas contains hydrogen sulfide. challenging aspects of catalysis important for fermentation.6 kg of corn. Still. manure. gasified biomass stores about 8000 kJ energy. especially corn. Biogas storage is no different than the storage of natural gas discussed above. or pyrolysis. its fermentation yields fuel-grade ethanol. organic waste. which uses most energy in the process chain. for blending in ethanol.S. The purity of biogas as compared to natural gas is a remaining issue. gasification of biomass at high temperature yields gaseous intermediates like hydrogen and carbon monoxide used as precursors for ethanol production. The size of the ethanol tanks must be compatible with the blending program with its projected ethanol demand. where microorganisms including yeast and other bacteria produce ethanol in a relatively dilute aqueous solution. and H2 into ethanol. tanks with inflatable rooftops. require enduring research and development efforts. Larger quantities are stored in standard gas storage tanks. heat and some byproducts in an aqueous product solution. new incentives intend to “equalize” the cost comparable to conventional gasoline. anaerobic bacteria can be used to convert the CO.

a strong demand for biodiesel also as lubricant and clean out agent could add additional value to soybean production. fluorinated polypropylene. But biodiesel is a natural solvent which softens and slowly degrades certain types of elastomers and natural rubber compounds. lead. of fuel hoses and fuel pump seals. e. and the increased lubricity. The interaction between biodiesel and the accumulated tank sediments and sludge from ordinary diesel reduces the solvency problem of ordinary diesel.g. Biodiesel Biodiesel is the transformation product of long-chained fatty acids from renewable lipid sources to the corresponding mono alkyl esters. Biodiesel is typically produced through the reaction of a vegetable oil or animal fat with methanol in the presence of a catalyst to yield glycerine and methyl esters. zinc. a sustained biodiesel market would also provide direct benefits to the agricultural sector. This tendency is the most innovative characteristics usable also for oil tank clean up.Chemical Storage Systems 42 iii. biodiesel tends to freeze at higher temperatures than other types of diesel. . Interesting properties of biodiesel are the absence of sulfur or aromatics. However the major problem is its economics and the negative energetics of the biodiesel production. Copper. acceptable storage tank materials are fluorinated polyethylene. and tin parts should be replaced with aluminum or steel since these metals oxidize both diesel and biodiesel fuels and create sediments. For instance. or blended with petroleum diesel for use in compression ignition (diesel) engines. Another catalytic process is the direct reaction of a fatty acid with methanol to methyl ester. and Teflon (Howell and Weber 1996). yet. The “clean out” tendency of biodiesel carries increasing amounts of dissolved sediments into the fuel systems of the vehicle where fuel filters catch most of it. Biodiesel can be stored in standard diesel storage tanks. Its physical and chemical operation properties are similar to ordinary diesel fuel. Biodiesel blends do not separate easily in the presence of water. Similar to ethanol. Besides aluminum and steel. brass. Biodiesel is used as alternative fuel especially in agriculture either in neat form.

while energy storage and control devices allow consumers to time energy purchases and shift from high to low rate periods. Thermal storage technologies are especially useful in combination with solar energy. Over the last twenty years. New energy storage systems involving flywheels are very promising especially in combination with other storage technologies to support uninterruptible electricity supply systems. Load distribution strategies and energy end use efficiency increase customer benefits. safe and reliable to contribute to the well-functioning of energy supply markets and energy systems. Efficient electricity storage in larger quantities is only indirectly possible by using established storage techniques like pumped storage hydropower plants pumping water back to the reservoir at times of low electricity demand. for such large systems.Conclusions 43 E. Conclusions According to the thermodynamic laws of physics an efficiency loss is unavoidable for all storage systems. must continually be optimized. distributed electricity generation. Batteries remain significant for smaller appliances and applications where grid power independence is necessary. smaller scale electricity storage technologies have progressed greatly. Energy storage should be lean. The use of coal and oil however. the capacity and services of energy systems. Nonetheless. Although large thermal storage systems like HT-UTES and ATES are more efficient. or political events . This established storage technique could become even more important in the future. small-scale applications intelligently integrated into building technology have immense potential. Energy storage also increases system reliability and will be the key to balance the intermittency of new renewable energy sources in the future. economical. The transition to natural gas. the foremost limiting factor for future growth is the availability of suitable sites with low environmental impact. hydrogen and biomass as prime chemical energy storage is envisaged for the future. and heat production. poses great problems for sustainable energy due to CO2-emissions. thus improving reliability and quality. However. the development of large-scale CAES will depend on developing systems with man-made storage tanks. including load distribution or system reliability. Similarly. and waste heat distribution to provide hot water and heat for housing. Chemical storage still remains the definitive storage method with its high energy density. Interesting examples are the use of PCMs and Zeolites in heating. Supercapacitors have been developed as an additional electricity storage technology to maintain grid stability and satisfy rapid power needs. Cost reductions in the various chemical storage techniques will be crucial for applications in mobility. cooling or ventilation systems. CHP. Some redundant capacity of energy storage is important for all sectors of the socio-economy to cope with disruptions caused by various technical. Increased capacity in electricity storage including the sum of all smaller-scale technologies will be required as the level of electricity generation from intermittent renewable energy sources increases.

Berg (Factor AG. J. I would also like to thank Dr. Appleby (Texas AM University. D. Goldblatt for his readthrough. Switzerland) for his valuable comments about the fossil fuel sections and Dr. . Dr. Biolaz (Paul Scherrer Institute. M. S. Spreng (CEPE) for his encouragement and advice in putting this article together. Finally. I would like to thank Anne Semadeni-George very much for her help in editing the English language of this text. Thanks also go to Dr. D. Switzerland) for some helpful comments regarding the hydrogen and biomass sections in chapter C. USA) for invaluable comments and corrections provided for chapter B and chapter A. Dr. I am indebted to Prof.Acknowledgement 44 Acknowledgement I would like to thank Prof.

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