Compressed Air as an Energy Source

Is your next car going to be powered by compressed air? Probably not, but there are groups working with compressed air as an energy source for the usual transport suspects, i.e., cars and bikes, and also for energy storage in general. Though the energy density of compressed air is poor relative to fossil fuels, it scores well in terms of efficiency – if, and it’s a big if, you can raise its pressure to store energy and lower its pressure to recover energy at constant temperature (isothermal process) and with no heat transfer to the surroundings (adiabatic process).

Air CompressorAir Compressor

Principle

Anyone who owns a bicycle knows that when you compress air it heats up – the objective being to pump the tire up as quickly as possible, without thinking about efficiency. When efficiency is important, which it is for energy storage, the aim is to convert the kinetic energy of the compressor into air pressure, not temperature rise, and not heat up the surroundings. Then to recover the energy you release the air and try to limit the temperature fall.

Vehicles

For vehicles, compressed air is cited as an alternative to battery-powered electrical vehicles. However, the major disadvantage of a compressed air system is the weight of the air storage tank. There has been mention of using carbon fiber storage tanks but that’s still in research mode, so at present tanks tend to be made of high-strength steel – think scuba tanks – thick and heavy.

Energy Storage

One of the problems with solar and wind electricity power generation is that they are intermittently available and not always matched to demand. For instance, solar panels create electricity during the day and nothing at night, but heating and light on winter evenings is a must in cold regions outside the tropics. Various forms of power generation smoothing are currently used. The basic requirement is to use electricity during off-peak demand to store energy to be released later at peak demand. Here compressed air storage and release is being cited as a possible solution. Imagine a large storage tank that can be pressurized during the day and released during the evening. Careful management of heat transfer is critical to be competitive with other alternatives.

An efficient compressed air energy storage and recovery system will capture and store the heat during the compression phase, e.g., heating water to produce steam. Then during the expansion phase the system will reintroduce the stored heat to limit the temperature drop in the air. This way you can approach the isothermal and adiabatic conditions for maximum efficiency.

Research and Development

The problem with compressed air energy storage and recovery systems is that the research can be dangerous (high pressures) and expensive (storage tanks and compressors). Clearly this is an ideal candidate for virtual testing, at least in the early concept design phase. And, you guessed it, Computational Fluid Dynamics (CFD) is a great match for this effort – being able to simulate air flow and heat transfer through pipes and valves, the constituents of a compressed air system.

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