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Power system reserve - No need to build wind back-up

WORLWIDE: Addition of wind generation to a large power system does not require construction of back-up generating capacity.

Smart systems… Advances in technology are improving demand management and reducing the cost of operation (pic:Siemens)


  • Back-up is always provided for all power plants on a system, with or without wind
  • There is no need to build back-up for wind; existing power plants in any system provide the required back-up for all plants
  • Wind generation displaces fossil-fuel generation and some of those plants can be taken out of operation
  • As the penetration of wind increases in any power system, the volume of capacity that is operated at part load, ready to ramp up or down according to peaks in demand or unexpected generation shortfalls, increases slightly to maintain a consistent probability of security of supply
  • Since the volume of extra reserve when adding wind is modest so is the additional cost. Savings from wind replacing other generation are likely to more than cover that extra cost
  • The emissions saved by wind displacing fossil-fuel generation are far greater than any extra emissions from increased spinning reserve.

Power stations already in place can more than adequately maintain a reliable supply of electricity as the proportion of wind power grows.


Modern power systems with a mix of generation technologies include a "plant margin", power generating capacity over and above that needed to serve peak demand, whether wind is providing 0%, 10% or 40% of electricity, or more. Construction of back-up thermal plants to cater for days of low or no wind, which would be an expensive undertaking, is unnecessary. Existing plants are already available to pick up the slack.

Three basic principles apply for establishing the plant margin required on a power system when wind is added. The British power system makes for a typical example of how this works. It is a large, integrated system with a mix of thermal generation provided by coal, nuclear and gas power plants, plus about 13% of renewables, about 1.4% of which is hydro.

First, an all-thermal electricity system with a peak demand of 50GW requires about 60GW of installed capacity to keep the lights on with a high degree of reliability whatever the variations in demand and supply. The extra 10GW is the plant margin.

Second, add 30GW of wind to that power system (displacing about 25% of the electricity generated annually by thermal plants) and the reliability of the system improves because the plant margin increases. There is no need to build any extra power plants. Wind power generation is available most of the year and the plant margin already in place is sufficient to maintain the system's high reliability, even on windless days.

Third, the 30GW wind power addition actually allows for about 3GW of the thermal plants to be taken out of the system, without reducing reliability. Moreover, with wind power feeding into the system, less fossil fuel is burned, saving emissions and extending the life of thermal power stations by postponing expenditure on replacement plants.

The same three principles apply to integration of any type of new capacity into a power system, including nuclear capacity, which provides about 20% of electricity in Britain. When half of Britain's nuclear capacity goes offline for long periods (as it did in winter 2008/09), the loss does not trigger a requirement for construction of more capacity to maintain a high probability of reliable supply. The overall plant margin for the entire system does the job.

All about probability

Power system operators excel in the fine art of probability statistics. It is the theory behind the practice that keeps the lights on, whether or not wind is part of the generation mix.

All thermal plants, whether fossil-fuel or nuclear powered, can and do fail, or are unable to function at expected capacity for any number of reasons, including line outages and general service, maintenance and repairs. Sufficient plant margin on a power system is a necessity for maintaining security of supply for any combination of technologies and eventualities. Every addition of new generating capacity requires a reworking of the statistical probability for supply failure and the requirement for the size of the plant margin.

A power system based overwhelmingly on renewable energy can be operated with the same probability of security of supply as an all-thermal system. The world's wind resource alone is sufficient to meet demand many times over. Indeed, in parts of the world where geothermal or hydro power is available to complement variable wind, a high level of supply security is achievable for a renewables-based power system at only modest extra cost, even with extremely high penetrations of wind. Likewise, the extra cost of maintaining a low probability of supply failure when adding large volumes of wind to a thermal power system is a small fraction of overall cost.

Thermal economics pressured

The main contributor to the overall cost of running a system with rising levels of wind supply (see page 37) is the need to operate larger volumes of generating capacity on part load than in an all-thermal system. An economic penalty for doing so is unavoidable: the low utilisation of the capacity of any power plant to generate electricity extends loan payback periods, which must stay within a timeframe acceptable to commercial lenders. The capacity factor of a power station operating for long periods on part load risks dropping to a level where commercial viability is compromised. Whether "capacity payments" could be offered to thermal generators to compensate for part-load operation without further skewing the market in their favour is under discussion.

In Denmark, the modelling of a national power system with sufficient wind capacity to provide enough power to cover 100% of demand over the course of a year was so economically convincing long term that power system operators, utilities and the government signed up to a strategy that will make Denmark fossil-fuel free by 2050. The model demonstrates that security of supply can be maintained with 70% of electricity requirement met by wind plants and 30% by the country's thermal mix - without the use of transmission links to neighbouring countries. In practice, by 2050 Denmark's links to other power systems will be far stronger than today, providing plenty of scope for renewables generation to equal 100% of demand over the course of a year.

Implementation of the energy efficiency and renewables activities to reach Denmark's 2050 goal to be fossil-free started in March 2012. Among these are experiments with feathering blades on wind turbines so they, too, can be operated at part load until required to ramp up to cover rising demand. In this way, wind power is able to play an active role in the reserve market. In Britain, all but 2GW of the country's 8.5GW of wind capacity is attached to the high-voltage network and National Grid classifies about half of it as flexible (able to reduce or increase production on request), thus potentially reducing the need for thermal plant reserve. As IT technology allows for increasing flexibility of both generation and demand, the requirement for reserve power from thermal sources can continue to fall.

Nuclear propaganda and wind

Wind power is often erroneously believed to add a high degree of uncertainty to electricity supply, particularly compared with nuclear. For this reason, when wind is meeting some of the load, politicians and their advisers are inclined to include the additional cost of maintaining security of supply specifically to the cost of wind generation. They do not, however, add the same applicable cost for backing up nuclear, or any other power-producing technologies, to their specific generation costs. As a result, comparisons of the cost of wind with nuclear and fossil generation are made on a basis that skews the truth unfavourably against wind. Yet the potential for instantaneous loss of a large nuclear reactor must be catered for, also economically.

Tried and tested statistical models inform power-system operators of the volume of back-up and standby generation required to make a blackout extremely unlikely when a 1.2GW nuclear block suddenly trips. A nuclear outage often causes the most grief on a system: several hundred megawatts can and do go offline instantaneously. Typically, it can take three months to put a tripped nuclear reactor back online. In Britain, sudden trips of the interconnector that provides mainly nuclear power from France are notorious for causing power system headaches. Yet the associated cost is not specifically added to each nuclear kilowatt-hour.

In practice, nuclear is intermittent. Either off or on, unplanned or not. Wind is far more predictable, and production forecasts are improving steadily. Compared with a nuclear trip, or even a gas-plant failure, managing dispersed volumes of wind generation is much easier for a system operator. Groups of wind turbines seldom, if ever, drop offline at the same time. Changes in production are relatively gentle and the variability is largely predictable. Wind is not difficult to manage, particularly given advances in weather and wind output forecasting.

Better technology, falling cost

Ever-evolving technology is driving down the system costs of providing reliable supplies of electricity, somewhat mitigating the long-term trend in rising fuel prices. The addition of renewables also mitigates those higher prices. The application of advanced IT to power system operation and demand management — (colloquially referred to as the smart grid) — may also reduce cost. The advent of micro IT monitoring of demand eases the management of variable supplies of renewables. More flexible, sophisticated wind turbine control also helps.

Current knowledge of wind management is most evolved in places such as Denmark, Spain, and Texas, where controlling seriously large volumes of wind on a daily basis is rapidly advancing the science of wind integration. Experience is proving that maintaining security of supply with 50% wind penetration is certainly well within the bounds of economic responsibility. Denmark's decision to supply 50% of electricity from the wind by 2020 is economically driven, in the expectation of lower overall future cost of electricity for the consumer, as much as it is environmentally driven. Fifty per cent is not a top limit by any means.

For a power system, the addition of wind has economic benefits as well as economic penalties: dispersing generating capacity throughout the system avoids sending it over long distances and in this way reduces the losses of electricity on the wires; a power station made up of many small units does not drop offline in its entirety when one unit fails; service and maintenance is carried out on one unit at a time, avoiding the cost associated with taking a whole power plant offline. Indeed, studies indicate that the economic benefits of wind on a power system could well balance the extra cost it contributes to maintaining security of supply, certainly at the levels of wind penetration seen today.

To prove the theories in practice, a good body of experience needs to be gathered over time of modern (flexible) power system operation with large proportions of (variable amounts of) energy from modern, controllable, wind turbines. In countries with most experience of power systems with high penetrations of wind, the limited cumulative evidence so far is already so convincing that governments faced with rising electricity costs from thermal generation are deciding to hedge their bets with wind. Denmark and Germany are the best examples.

In the west of Denmark, there are periods of several hours each year when wind-power production covers 100% of electricity requirements. Over the year 35% of electricity in western Denmark is wind based. The entire country now meets 30% of its demand with wind energy. The claims made at regular intervals by anti-wind power protagonists, particularly in the UK, that wind power in Denmark is loss-making due to large giveaways of surplus energy, are pure nonsense; as Danish electricity statistics reveal, very seldom is electricity with wind in the mix given away and total exports of electricity with a high wind content bring steady revenues through the year that more than compensate for any occasional loss.

Geek-speak confusion

Commercially, the everyday business of supplying back-up to a power system is complex, with its own weird and wonderful vocabulary and geek-speak. The many categories of reserve come with their own names, some self-explanatory, like intra-day (within a day), others, like spinning reserve (a power supply that can adjust output to balance supply and demand), only fully understood by experts. Traders also have their own language when buying and selling reserve within these categories under the duress of market forces: as demand for reserve goes up and down, so does it price.

The welter of unfamiliar words frequently gives rise to misunderstanding. But the everyday business of energy trading - managing and pricing the various levels of system reserve — does not belie the basic principles behind structuring a power system for reliable supply, with or without wind. Keeping the lights on when receiving electricity from a power mix with large volumes of wind energy is perfectly feasible and does not cost an arm and a leg.

Congestion on the grid is another cause of confusion. If a mix of power stations is producing more electricity than the capacity of the local wires to carry it, that is a problem of local congestion, not of excess power on the system as a whole. The problem is fixed by upgrading grid hardware as necessary, as it would be upgraded for the addition of any power plant. The cost of doing so is a minor part of consumer bills.

The technical challenges of wind integration are well understood, as are the business challenges. The economics of running power systems with high levels of wind generation are predictable as well as manageable and, over time, the cost may well be more than compensated for by savings in fossil-fuel purchases and new nuclear build. The market structures decided by governments now need to catch up with the new understanding.



None other than the Daily Telegraph, a UK newspaper known in the wind business for its regular rages against "ineffectual" wind turbines, recently reported that "no new fossil-fuel power station has been built to provide back-up for wind farms, and none is in prospect". That revelation, at least to the newspaper, made its appearance rather oddly at the end of a wildlife column that otherwise discussed the ins and outs of badger culling.

Light seems to have dawned for wildlife columnist Geoffrey Lean in the unlikely setting of the Hay literature and arts festival, held in Wales in May. According to Lean: "It has become an article of popular faith that building wind farms also involves constructing fossil-fuelled power stations for back-up when the weather is calm. As a result, some opponents go on to say, wind turbines do little or nothing to cut carbon dioxide emissions."

Explosive information

Lean, on learning of the fallacy of this particular "popular faith" from Richard Smith of National Grid, who reportedly spoke on the subject at the festival, Lean tells his readers that this everyday fact of power system operation is "explosive" and "surprising" information.

Smith said that the volume of electricity used to back up wind generation over an 18-month period ending September 2012 was a tenth of what was needed to back up conventional power stations and less than one-thousandth of the total output of the wind turbines in that period.

"Wind saved nearly 11 million tonnes of carbon dioxide over that 18 months; standby burning of fossil fuels only reduced this by 8800 tonnes, or 0.081%," wrote the admittedly surprised Lean.

For the record, Lean got his facts and figures slightly wrong. What National Grid has reported is that wind generation of 23.7TWh in that period fell short of forecast generation by 22GWh, with other generation closing the gap. Neither is it true that "wind farms don't need fossil-fuel back-up", as the newspaper headline on the small item claimed.

All power plants require back-up, but no plants are built to provide dedicated back-up for any specific generation technology.

Lyn Harrison is contributing editor to Windpower Monthly

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