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Friday, March 7, 2008

Intelligent Energy Systems

Excess Energy - what to do

The first of our pieces from contributors we take a look at a view on Energy from New Zealand

If we continue to install wind turbines, solar panels, tidal 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. 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 it seems like such a waste. With the advent of excess power the possibility opens up to use demand balancing of our grid rather than supply 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 down the 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 take up by priority 1 isn't sufficient to balance the supply and Priority 3 when generation is really humming along. 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 sort of loads that these power options would be useful for would be pumped storage, heating your water, charging your electric car, and generating Hydrogen for later power use.

The flip side of such a system is less demand in times of lower power production. If you already have a tank of hot water or if your electric car is already charged up, you won't be using power when it is in short supply. The vehicle charging points at your place of work could 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 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 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 favorable night rate. This is only one option for balancing demand against available 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 60% of the power you used to pump the water. The reason the system is feasible is largely 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, 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. Firstly it can be used to fuel a special "battery" called a fuel cell. Hydrogen is particularly attractive in this regard since hydrogen fuel cells operate at room temperature. These fuel cells are pretty efficient and you get a large portion of the energy back that you used to split the water molecule. You also get very pure Oxygen as a by-product of the electrolysis process, which, in a commercial operation, has a market for medical purposes, for welding, and for steel production.

Besides powering fuel cells, hydrogen can be used in internal or external combustion engines and 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 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, though 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. 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 those of you too young to remember, producer gas is a nasty mix of hydrogen, methane and carbon monoxide which is produced by passing a stream of steam through burning coke or coal. Have a gas leak in your home and the carbon monoxide in producer gas will kill you long before a similar gas leak of propane would have 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 resemble the tanks you see in petrol refineries but they are open-topped and contain water. A second open-bottomed tank, slightly smaller in diameter, is floated inside the main tank. The gas is let into the bottom of the tank and as it flows in, the inner tank floats 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 3in our example)

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 leak out through the material itself. However technical fixes have been found for these problems.

This property of Hydrogen is leading to a new way of storing it. Hydrogen is adsorbed by certain metal alloys. It is absorbed 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. See:

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.

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". The unit would be programed to receive power when it is at the lowest rate and send it back at times of highest demand. Over their vacation, their house and/or electric car would generate a small income for them.

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 store excess energy in times of high generation, as mentioned above, and makes it available in times of low generation is of value.

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.

Hugh Williams