Storing energy in compressed air

The standard approach

Compressed-air energy storage (CAES) uses off peak electricity to compress air into either an underground structure (e.g., a cavern, aquifer, or abandoned mine) or an above ground system of tanks or pipes.

The compressed air is then mixed with natural gas, burned, and expanded in a modified gas turbine. In a conventional gas turbine, roughly two thirds of the power produced is consumed in pressurizing the air before combustion. (???) CAES systems produce the same amount of electric power as a conventional gas turbine power plant using less than 40% of the fuel.

Recent advancements in the technology include above-ground storage in empty natural gas tanks and ‘mini-CAES’, a transportable technology that can be installed at or near individual loads (e.g., on urban rooftops).

The first commercial CAES was a 290-MW unit built in Hundorf, Germany in 1978. The second commercial CAES was a 110-MW unit built in McIntosh, Alabama in 1991. Several more CAES plants are in various stages of the planning and permitting process.

Research is also ongoing to develop adiabatic CAES systems in which the heat of compression is stored and reused to heat the compressed air before expansion thus eliminating the use of natural gas in the system.

Updated April 2010. Source - Electricity Storage Assn.

Current Developers/Suppliers

Above ground isothermal CAES

A newer approach

The problem with storing compressed air is that compression heats the air. Heat can be lost, and the high temperature is harder to store.

Both LightSail and SustainX have systems vaguely described on their websites. It seems that water is sprayed into the cylinders and evaporated, thus absorbing latent heat without raising the temperature.

On decompression, the water vapour will condense, thus giving off the latent heat again.

So they are storing most of their energy in compressed water vapour.

Lightsail claim 90% round trip efficiency. They have another twist to the heat:

"Heat from compression is stored or routed to nearby buildings, providing heating. During expansion, heat is extracted from storage, or buildings providing air conditioning. This dramatically increases building energy efficiency."

The description of the process is vague, however these look like air compressors.

The compressed air/vapour can be stored in pipes above ground.

Storage pipes can be built as racks in standard container size.

Underwater compressed air

The Hydrostor system efficiently converts electrical energy to compressed air via an advanced adiabatic compression system. This air is then sent to a series of flexible accumulators located 50-500 meters below the surface of a body of water. Once in the accumulators, the energy can be stored until required by the grid. When the energy is required, the weight of the water pushes the air back to the surface where our system directs it through an expander driving a generator thus supplying energy to the grid and completing the storage cycle. This rapidly deployed, low cost system uses non toxic substances with minimal environmental disturbances while offering 70% round trip efficiencies.  

The heat from compression is stored and used to re-heat the air on exit. http://hydrostor.ca/technology/

Underwater concrete spheres

Another system patented by MIT is to store the air in large concrete spheres with walls thick enough to tether floating wind turbines.

More

In a turbine

In a gas turbine air is compressed, then mixed with flammable gas or vapour and ignited. The combustion gases expand and then turn blades on the other side of the turbine producing enough force to compress the incoming air, and with the extra force, turn the generator. This is called the Brayton cycle.

By compressing air in the off period, it saves energy and reduces the need for the turbine to compress the incoming air.

Other ways to expand the air is by using the heat given off when the air is compressed. Or to use solar thermal mirrors in a CSP tower. 

A mechanical drivetrain utilizes an electric machine and a crankshaft…This efficient mechanical link powers a two-stage, mixed-phase (water-in-air) heat-transfer process within pneumatic cylinders. During piston strokes, water is sprayed into the air-filled chamber of each cylinder, allowing heat to be transferred from water to air during expansion or from air to water during compression. The same ICAES power unit provides both isothermal compression and expansion, eliminating the cost of separate compressor and expander subsystems.
Read more at http://cleantechnica.com/2013/09/23/sustainx-completes-new-compressed-ai...

Compressed air powered vehicles

To propel the vehicle, compressed air from the tanks is injected into a small chamber, where it expands and cools. This expansion drives a downstroke of the piston. But as the ambient temperature begins to reheat the air in the first chamber, that air is forced into a second neighboring chamber, where it expands again to drive an upstroke. Using ambient heat helps capture more of the energy in the compressed air, ultimately improving the efficiency and expanding the range of MDI's Air Car. And compared with four-stroke combustion engines, in which half of the strokes are wasted to pull air and fuel into the chamber, the air engine makes use of every stroke.

MDI claim their compressed air engine is 90% efficient.

The efficiency would be improved if the heat wasted in compressing the air was recovered as hot water etc. 

If the air was compressed during periods of cheap electricity it would help to even out fluctuations.

There are plans to produce hybrid car using compressed air tank instead of a battery.

VW compressed air car

Ref

Tools and torpedoes

Compressed air for motors is nothing new. It is used in workshops to run tools for a variety of reasons. One being to cut the risk of fire in flammable areas.

Fork lifts and other industrial vehicles are often powered by compressed air.

It is also used to power torpedoes. Burning fuel in the air makes them go even faster.