In compressed air energy storage, off-peak power is taken from the grid and is used to pump air into a sealed underground reservoir to a high pressure. The pressurized air is then kept underground for peak use. When needed, this high pressure can drive turbines as the air in the reservoir is slowly heated and released; the resulting power produced may be used at peak hours.
More often, the compressed air is mixed with natural gas and they are burnt together, in the same fashion as in a conventional turbine plant. This method is actually more efficient as the compressed air will lose less energy.
There are many geologic formations that can be used in this scheme. These include, depleted gas reservoirs (most economical), naturally occurring aquifers, solution-mined salt caverns and constructed rock caverns. In general, rock caverns are about 60% more expensive to mine than salt caverns for CAES purposes. This is because underground rock caverns are created by excavating solid rock formations, whereas salt caverns are created by solution mining of salt formations.
Aquifer storage is by far the least expensive method and is therefore used in most of the current locations. The other approach to compressed air storage is called CAS, compressed air storage in vessels. In a CAS system, air is stored in fabricated high-pressure tanks. However, the current technology is not advanced enough to manufacture these high-pressure tanks at a feasible cost. The scales proposed are also relatively small compared to CAES systems.
There are five above ground components required by a basic CAES installation:
1. The motor/generator which employs clutches to provide for alternate engagement to the compressor or turbine trains.
2. The air compressor which may require two or more stages, intercoolers and aftercoolers to achieve economy of compression, and reduce the moisture content.
3. The recuperator, turbine train, high and low pressure turbines.
4. Equipment control center for operating the combustion turbine, compressor, and auxiliaries, and to regulate and control changeover from generation mode to storage mode.
5. Auxiliary equipment consisting of fuel storage and handling, and mechanical and electrical systems to support various heat exchangers required.
A CAES operates by means of large electric motor driven compressors that store energy in the form of compressed air in the reservoir. The compression is done outside periods of peak demand. As part of the compression process, the air is cooled prior to injection to make the best possible use of the storage space available. The air is then pressurized to about 75 bar.
To return electricity to the customers, air is extracted from the reservoir. It is first preheated in the recuperator. The recuperator reuses the energy extracted by the compressor coolers. The heated air is then mixed with small quantities of oil or gas, which is burnt in the combustor. The hot gas from the combustor is expanded in the turbine to generate electricity.
In a normal gas turbine, as much as two thirds of the energy produced by the power turbine is consumed by the compressor. This means that a 300 MW plant actually produces 100 MW of net output while 200 MW is consumed by the compressor. If compression is carried out at a different time to when power generation is needed, that is, by CAES, the peak power which the gas turbine can generate is increased by this amount. Hence additional power production capacity is not required.
The turbine work that conventionally would drive the compressor in a combustion turbine is directed to the generator, increasing the output. In fact the power output from each turbine shaft will be nearly three times the capacity of a comparable simple cycle combustion turbine.
CAES systems can be used on very large scales. Unlike other systems considered large-scale, CAES is ready to be used with entire power plants. Apart from the hydro-pump, no other storage method has a storage capacity as high as CAES. Typical capacities for a CAES system are around 50-300 MW. The storage period is also the longest due to the fact that its losses are very small. A CAES system can be used to store energy for more than a year.
Fast start-up is also an advantage of CAES. A CAES plant can provide a start-up time of about 9 minutes for an emergency start, and about 12 minutes under normal conditions. By comparison, conventional combustion turbine peaking plants typically require 20 to 30 minutes for a normal start-up.
If a natural geological formation is used, rather than cavern air storage (CAS), CAES has the advantage that it doesn't involve huge, costly installations of creating the cavern in a salt dome. A depleted natural gas reservoir already contains the space required, in porous rock. Moreover, a CAES project used in conjunction with a gas turbine requires 66% less natural gas to create the same amount of power. Accordingly the emission of green house gases is substantially lower than in normal gas plants.
Princeton Natural Gas, LLC is a bonded registered operator in the state of California. The Company is engaged in the business of defining and acquiring suitable depleted gas reservoirs for natural gas and CAES (compressed air energy storage). Princeton is currently in the progress of developing a gas storage field in the Sacramento basin and has secured additional sites in California that are ideal locations to build CAES combined with natural gas storage. We are seeking to create strategic alliances with other gas storage developers, wind producers and private equity sources for the burgeoning opportunities in the area of power storage.