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Can you store energy efficiently using low pressure compressed air? |
I know there is a bit of heat /cold transference any thing else. I'm thinking like for on a bike.Compress for braking or use the pressure for helping up hill. I've read about some compressed air trains years ago but they were loaded up with air first I think. Any ideas? Your comment about "heat /cold transference" is the real problem. It tends to make compressed air storage vary inefficient. As air is compressed, its temperature rises. This means it takes more pressure and thus more work to compress it. The rise in temperature can be very large. Just compressing air to 100 psi can raise the temperature by over 400F. Large compressers use intercooling to overcome this, the air is compressed in stages and is cooled in between stages to improve efficiency. You will end up losing about one half your energy during compression. On expansion, the air cools so you get less work out of it than you might think. The final issue is that the energy per unit volume of air is relatively low so you need quite a large heavy tank to to hold any significant amount of energy. If you want to work out the numbers yourself, the compression and expansion of air is called a polytropic process which is governed by: (T2/T1)^(1/(k-1)) = (v1/v2) = (p2/p1)^(1/k) Where T, p and v are the temperature, volume, and pressure of air. The subscripts 1 and 2 are for before and after compression or expansion and k is the specific heats ratio (1.4 for air). This assumes adiabatic conditions, meaning no heat is transferred during compression or expansion. This is normally the case. Heat transfer is between stages. You can use this to figure how hot or cold your air will get and integrate the relation to see how much work you can store in a tank of any given volume. I don't know how efficient compressed air is in terms of a ratio, but it is used in many things from power tools to door hinges to paintball guns. The heat transfer is not really much of an issue because heat is not what is relied upon for using air as an energy source. The energy comes from the fact that the air molecules are tightly packed into a contained area and want to get out and expand. Once the valve is opened the air rushes out releasing kinetic energy which can be used to turn turbines, propel objects or even the container from which it is released. I think it might work for your bike's brakes but probably not very practical for getting you uphill. Personally, I would go with a fluid-type braking system as it transfers energy to the brakes a lot better due to it's higher resistance to compression (like cars). |
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