For some time now I have been worried about the present batch of “Alternative Energies”, their biggest problems are to do with efficiency and their ability to deliver energy when it is needed rather than just when it is available. Great savings can be made in energy efficiency in order to reduce our need for energy but fundamentally in order to achieve a low-carbon existence we need ways to make “Alternative Energies” work for us, and by “Alternative Energies” I mean taking advantage of natural sustainable sources of energy such as wind, wave and solar power. Making best use of these sources is even more important since the German Government decided to shut down all of it’s nuclear power generation earlier than planned, because now European fuel prices have to rise dramatically because Germany will now be vastly more dependent on Fossil Fuels until they can fill the gap with viable alternatives.
Currently the way we store energy if there is an excess in the grid is to convert the excess electricity into potential or kinetic energy until it is needed again later. There are many water storage facilities in the UK which pump water up-hill to large reservoirs in a technique called “Pumped Storage Hydroelectricity“. By pumping the water up-hill when you have excess energy you can then let it come back down again and recovery the energy with hydroelectric turbines. Each time you do something like this you waste some of the energy because of energy conversion inefficiencies.
Wind energy is interesting, when the wind blows we get a fair amount of energy returned by the gigantic wind turbine. The most you can ever capture from a wind turbine is 59% of the available wind energy passing through, this is a fact of physics proved by Albert Betz in 1919. However that is the upper limit, in reality there is conversion from kinetic energy (the motion of the wind) to electrical energy and such conversions always result in a loss of efficiency in gears, dynamos and power couplings. Because this energy is available “When The Wind Blows” and at no other time there have been issues where the National Grid has had to shut down turbines because they weren’t needed and this is a great waste of their potential.
Solar energy is another area of great interest to many people and I struggle to get excited about what should be a great source of energy because everyone gets excited about Photovoltaic (PV) energy which uses chemically doped materials to directly convert sunlight into electrical current. The reason I struggle to get excited is that PV isn’t very efficient, typically high quality solar panels are about 14-17% efficient and that really isn’t very much. Also solar PV cells need various exotic chemicals in their production of which only a portion is recycled and they aren’t exactly “low carbon” in their transport around the world. Solar energy is logically only available during the hours of sunlight and again, logically, is subject to the intensity of the sun in the location.
In an “Off Grid” environment, where a home owner has no access to mains electricity from the grid, it is quite common to store energy in batteries so that the peak energy availability can be disbursed over a longer period. Not everyone has access to a source of large quantities of water and a reservoir pond (or two) to store it in. Batteries are great for our mobile phones, they store energy in chemical form for good periods of time and release it on demand. Some batteries can release their energy quickly or some can release it slowly over long periods of time. But fundamentally batteries are flawed because they depend on harsh chemical processes which break down the components over time and can result in failure of the cell. Also you can only really discharge a deep cycle battery to 70-80% before you start causing premature damage to the battery cell, thus you need to be careful with your management of supply and demand.
Some time ago I started to wonder: why don’t we store more energy as directly coupled kinetic or potential mechanical energy? Wind farms, for example, I wondered if it wouldn’t be a good idea to install giant clock springs under them (or in their stems) so that we could regulate the release of all of that good mechanical energy. Now, giant clock springs sound silly at first, but actually many companies use kinetic energy storage as a power backup medium. In computer data centres, when you have a power failure it takes time to start the local on-site diesel generators and you need something to keep all the equipment going until the generator is up to speed. Some companies use giant banks of batteries which they carefully maintain and monitor, but I have seen a few UPS failures and they get rather messy and expensive. Plus batteries can release hydrogen gas which could cause harm to operatives working in the UPS battery room. The alternative that some companies really do use is to use a motor to spin a giant “fly-wheel” on a very efficient bearing, when the power fails that mass still has a great deal of momentum, and as the motor is no longer supplying force to keep it spinning it can be used as a generator to take that kinetic energy and turn it back into electricity. There can be enough energy in the momentum of a large enough mass to keep a data centre alive until the generator is ready to take the strain. This spinning mass technique however somewhat depends on the problem that you can’t store such kinetic energy for long periods, the friction of the bearings causes momentum to be lost over time and affects efficiency but it is great for short-term non-toxic energy storage. Some buses around the world are now using spinning masses as a means of kinetic energy recovery in breaking and they can then use that energy to help move the bus away from the stop before the engine takes over again, a nice and clean “Start-Stop” technique.
This application in buses and the idea of the hydroelectric storage leads me to another angle. The disadvantage of water as an energy store is partly because it can’t be compressed, it takes up a great deal of space and the disadvantage of kinetic energy is that the spinning mass can’t spin forever. Well, what about storing energy in a static way, under compression which can be quickly released on demand. This leads us neatly to: Compressed Air Energy Storage. Now of course I don’t declare to be the first to propose such an idea, because it is already in industrial use around the world to a limited degree. But what I would like to do is highlight the concept because it deserves more attention and also because I think it might have some interesting applications as a battery replacement technology.
In an off-grid situation we could see a tank being placed in an out-building which has a store of highly compressed air, this is generated through wind, solar or other inconsistent energy supply. In addition I think that some kind of Sterling Engine arrangement could supply the mechanical work for solar energy without needing to waste energy on conversion to and from electricity just to achieve compression. What about automotive situations? Many companies are installing very expensive and potentially unreliable batteries in cars, what about compressed air tanks which could be used as a kind of compressed air transmission instead of a gearbox? Directly drive the gears with the compressed air perhaps? Just put a 600CC compressor in and regenerative breaking, should have a snappy little number!
