Total Pageviews

Friday, October 19, 2007

Excess Energy - what to do

If we continue to install wind turbines, solar panels, tidal generators wave generators and hydro dams, we will find ourselves more and more often in the beatific state of generating more power than we know what to do with. In fact, for the effective uptake and utilization of renewables, it is essential that we install enough capacity that this occurs often. There will be times when a nor-wester is blowing, the sun is shining ( they occur together here in NZ), a spring tide is running and the reservoirs are bursting from recent rains. What should we do. We could feather the wind turbines, let the tidal generators free-wheel, and allow the excess water to flow over the spill-way without going through the turbines but this would be an unforgivable waste. With the advent of excess power the possibility opens up to use demand balancing of our grid rather than supply balancing.  What is demand balancing.

Line Signals
We've long had a system in New Zealand of heating our water at night. In the evening, at a given time, the generating company sends a signal (ripple) down the power lines. If you are set up for it, this turns on your water heater. Because you are using power when demand is low, you get a better rate. In the morning at a set time, a second signal turns off your water heater. This is the horse and buggy demand-balancing-system. We could have the space shuttle.

Instead of sending the signal at a given time, it could be sent when power generation exceeds demand. Even better there is nothing to stop the power company from sending a number of different "turn on" and "turn off" signals. The company could send Priority 1 when there is a little excess power, priority 2 when the uptake by priority 1 isn't sufficient to balance the supply and Priority 3 when it needs to find customers for even more excess power. The customer chooses (dials) which priority they want for a given function. Of course, the lower the priority (priority 3 in this example) the cheaper the rate. The price of power-on-demand stays the same at all times.  For instance, your lighting circuits which you can access by turning on a switch cost the full day time rate no matter when you use them.  The special rates are only for functions such as water heating which the power company can turn on and off to balance load with supply.

And with the advent of cheap mobile phone technology there is the option of sending the turn-on, turn-off signals by a built in, dedicated phone chip rather than through the electrical lines. The decision which way to go is purely technological/economic.

The flip side of such a system is less demand in times of low power production. If you already have a tank of hot water or if your electric car is already charged up, if your hydrogen tank is already full, you won't be demanding power when it is in short supply.

So, what are some uses of power-when-available rather than power-on-demand.

Your Electric Car
The electric vehicle charging points at your place of work will be on this system. You may have enough power in your batteries to get home after work but, given a choice, you would rather have your car fully charged. You select the most conservative, least expensive option on the dial on the office plug-in point (3) and swipe in your credit card. During the day, if the lowest priority signal is sent, your car gets some extra charge at the best rate. If not you charge your car when you get home utilizing the night rate.

On the other hand, if you arrived at work without enough power to get home, you might choose the less conservative option (1)or even the most expensive "charge-now" option and pay a little more to have your car charged. You might even choose "charge now" for $10, which would be enough to get home where you could access a more favourable night rate. Car charging is only one options for balancing demand to match existing generation.

Pumped storage
Another system which is used by some generation companies is pumped storage. When excess power is available, water is pumped into a reservoir to be used for "peak shaving" when power demand is high. This seems counter-intuitive, since, as everyone knows, no system is 100% efficient. You lose power at each stage. You are probably lucky to get back 75% of the power you use to pump the water. The reason the system is feasible is financial. To build a separate power plant that is on standby most of the time is expensive, especially when you factor costs such as the interest on the loan to build the plant. Such a plant is not generating most of the time so the return on the investment is poor. It turns out that in some cases, even with the inevitable power loss of pumped storage, it is financially more favourable to use pumped storage for peak shaving rather than building another power plant. With excess (cheap) power, pumped storage is likely to be even more attractive for some power companies.

Production of Hydrogen
Hydrogen has long been touted as the fuel of the future. It is of course not an energy source. There are no underground pools of Hydrogen we can tap as we do with oil. However it has some very attractive features as an energy-transfer mechanism. It can be used to fuel a special "battery" called a fuel cell and can be used in internal and external combustion engines. It can also be used in the kitchen in place of propane. It is produced by electrolysis and produces Oxygen as a by-product.  Oxygen, in a commercial operation, has a market for medical purposes, for welding, and for steel production. In external and internal combustion engines, hydrogen is attractive as it produces no air pollution.

Besides powering fuel cells, and internal or external combustion engines Hydrogen can be used to reduce metal ores in place of coke. It can also be combined with coal to make petrol and diesel. In this later application, there is still a carbon footprint as some fossil fuel is being used but it is much reduced over the use of pure coal and it produces a liquid fuel which is useful for transport.

Arguably, hydrogen is best use in static facilities rather than as a transportation fuel. This is because it takes a lot of energy to compress or liquefy hydrogen for use in a vehicle. Also, tanks to store compressed or liquefied hydrogen are heavy and expensive and the energy density of liquefied or compressed hydrogen is not great compared with conventional liquid fuels. In a static facility there is another way of storing hydrogen.

As a boy in Vancouver, I remember the huge tanks used to store producer-gas. For readers too young to know about producer gas, it is a nasty mix of hydrogen, methane and carbon monoxide which is produced by passing a stream of steam through burning coke or coal. If you have a gas leak in your home, the carbon monoxide in producer gas will kill you long before a similar gas leak of propane would smothered you. The producer gas was piped from the storage tanks to businesses and domestic locations around Vancouver. So how did the tanks work.

The storage tanks resembled the tanks you see in petrol refineries but they were open-topped and contain water. A second open-bottomed tank, slightly smaller in diameter, was floated inside the main tank. The gas was let into the bottom of the tank and as it flowed in, the inner tank floated higher and higher. Gas pressure is determined by how much the inner tank weighs and by how much extra weight is put on it. Such a system is only suitable for a static application but is perfectly amenable to small scale domestic use if electricity can be accessed at a suitable price to produce the hydrogen (priority 2 or 3 in our example). You can have a hydrogen storage tank as small or as large as you want at your home or  business.

A problem with hydrogen is that the hydrogen molecule is very small. It will get through the smallest gap in a joint and hydrogen even soaks into some substances and actually leaks out through the material itself. However technical fixes have been found for these problems.

Incidentally this property of Hydrogen is leading to a new storage method. Hydrogen is adsorbed by certain metal alloys. It is adsorbed so efficiently that in, say, a diving tank full of the alloy, you can store more hydrogen than would be the case if you compressed the hydrogen to 200 atmospheres into the same tank. Moreover, the storage takes place at very modest temperatures and pressures. Heat is given out when the hydrogen is absorbed and heat must be supplied to release the hydrogen, so there are some energy costs. It is possible that this storage method may make hydrogen practical for vehicles.

So hydrogen is an attractive option for using excess power when power is cheap. The hydrogen then represents an energy store which can be used when renewables are at an ebb. For some reason, possibly because of the Hindenburg, Hydrogen is considered a dangerous fuel. In actual fact it is far safer than any of the liquid fuels or any of the gaseous fuels with a vapour heavier than air. This includes all of the alkanes except methane. Ethane has a vapour of almost equal density to that of air and all the higher alkanes such as propane, butane etc. have vapours heavier than air. If there is a hydrogen leak, the hydrogen dissipates upwards and removes itself from the hydrogen source. The rest of the gaseous and liquid fuels flow down and across the ground looking for a spark. If Hydrogen ignites, you have a fire ball which rapidly rises upwards and is gone. Gaseous fuels spread their fire on the ground as far as they have dispersed and liquid fuels stay on the ground, igniting everything flammable in their path.

Stored heat
In one house, we had a box full of large bricks. It was wired into the circuit that heats the water when the electric company sent a signal down the line. By opening or closing a couple of vents, you could either keep the heat in the box or let it out.   As it is set up at present, this happens at a fixed time in the evening and turns off at a fixed time before the morning peak. Once we have truly smart grids, this could be turned on with priority 1, 2 or 3 as we decide. This is a particularly good option for the power companies since, unlike a washing machine, it doesn't have to continue on its cycle once it has started. Stored heating can be switched on and off to instantly balance power supply with power demand. You might need even smarter grids to facilitate both types of demand.

Such a storage system would be even more effective using eutectic salts.

Have you seen the tiles that they put on the outside of the shuttle. They are a wisp of frozen smoke and can be comfortably held while an acetylene flame plays on the other side. With Insulation like this and a simple thermostatically controlled flap system, the stored heat can be used as it is needed with hardly any leakage when it is not needed.

Domestic regeneration
A further possibility for balancing power is re-generation by the domestic consumer. If there is a high demand, the consumer with an electric car or a home hydrogen system could be putting power back into the grid when yet another signal is sent down the line. A family on vacation, for instance, could leave their electric car and their hydrogen system plugged in with the switch set to "supply"(4). The unit would be programed to receive power when it is least expensive and send it back at times of highest demand. Over their vacation, their house hydrogen system and/or electric car would generate a small income for them.

Home Appliances
Even  home  equipment such as your dish washer, clothes washer and so forth can have a dial in which you choose operate now or any of the lower priority possibilities. Such a feature can be built in at the factory as long as the power companies of the world agree on a standard protocol for the signalling function. a slight complication is that once your washing machine is turned on, you don't want it to stop until the cycle is completed so you may be paying for part of the cycle at a higher rate than you dialled. Stored heat, power to your electric car and to your hydrogen system don't have this limitation.

A main criticism of renewable energy is that it is pulsating and unpredictable. There is certainly some truth in this although not as much as it appears at first glance. For instance, as solar panels become common all over the country, places in the sun will balance places with cloud cover. The same applies to wind power. As fronts move from South to North along New Zealand, a pulse of wind generated electricity moves with it to be distributed by our power grid. Hydro is the ideal power source to instantly balance any shortfalls and New Zealand is rich in Hydro resources. On top of this any system which stores excess energy in times of high generation, as mentioned above, and makes it available in times of low generation is of value in balancing supply and demand.

Here in New Zealand in our present (2008) la Nina climate an interesting fact has come to light. Our wind generation is somewhat lower than average while our sun hours are greater. At present, solar electric is insignificant as a power source but as more solar comes on line, it appears that solar will help to balance wind. This would not necessarily be the case in all countries.

In the end, as our fossil energy runs out, we may even have to take a look at our tendency to be control freaks and accept that we can not always have energy exactly when we want it. Where I live we have now being living with solar water heating for half a year and while we almost always have hot water, three completely cloudy days leaves the tank cold. We find we are now much more aware of the weather and we never leave the hot water running while we do the dishes. Perhaps living with renewable energy will make us all a little more aware of our environment and our impact on it.


ngogerty said...

thanks for the post. Sending a signal down the line for the water heaters is smart. Distributed storage and efficiency is going to become more interesting going forward. Maybe someday they will make PV cell with a built in ultra-capacitors in the backing (surface area is the function for capacity), that is my thought on an approach to storage and utilization :) Have to wait for the cost to drop.

Indiantiger said...

Great comment. The way that we need to view this is that the new infrastrucutre we put in will avoid new generation and other infrastructure. In fact this smart grid infrastructure is the cheapest way to achieve high reliability.