Compressed Air Energy Storage (CAES)

Compressed Air Energy Storage (CAES) traditionally involves compressing air using off peak electricity in large underground caverns (Figure 1). When electricity is needed, the compressed air is released to operate a turbine driven generator. The first commercial CAES unit was developed in Huntorf, Germany in 1978 (Baxter, 2006). This 290 MW CAES unit was originally used for spinning reserve and load following. In recent years, it has been used to level out variable power from wind turbines (EPRI, 2003).

Figure 1 showing a Compressed Air Energy Storage (CAES) facility that uses an underground cavern to store compressed air (ClimateTechWiki, 2006)

The compression and expansion of air is a thermodynamic process that often requires energy input, traditionally in the form of natural gas. When the air is compressed the energy added to the air produces heat. To maximize the amount of air that can be compressed into the compression chamber, this heat is removed from the air before it is compressed into the chamber. This heat has traditionally been released to the atmosphere using cooling towers or some other means of heat exchange (Figure 1). During discharge, when the air is released to spin the turbines, which in turn spins the generator, the expanding air absorbs heat. This causes a drop in temperature that can be enough to damage the turbine machinery. For this reason, in traditional CAES units, the expanding air is heated using natural gas. This heating has the additional benefit of increasing the pressure of the expanding gas thereby increasing the power output of the generators. Compared to PHS and battery energy storage systems, CAES units that consume natural gas in the generation phase are not technically “pure” energy storage. It should be noted that the power output of the gas fired expansion turbine in a CAES plant produces two to three times more power than a simple-cycle combustion turbine plant using the same amount of fuel (Baxter, 2006).

               Aside from the increased power output, CAES units differ from gas turbine power generation plants in three important ways. The first difference is that the compressor and expander turbine train can be operated independently. In a gas turbine generator the compressor consumes roughly 66% of the turbine’s power. Without this parasitic compressor load, the CAES turbine can achieve the aforementioned two to three times greater efficiency. The CEAS unit also has great flexibility with regards to when compression and expansion can take place. The turbine can be kept at a constant rate of discharge while the compressor can turn on and off based on available power. This allows a CAES unit to provide constant power when combined with variable renewable generation (Baxter, 2006).

The second difference is that because the compressed air is stored at a controlled temperature, CAES units do not experience the derating of output power experienced by gas turbines when the ambient air temperature rises in the summer. The third difference is that traditional CEAS units must be located above geologic formations that are large enough to hold the required volume of compressed air. Gas turbine generators have much greater flexibility with regards to their installation location (EPRI, 2003).

               Traditional CAES units are characterized by very large power and energy ratings (100’s of MW up to 1 GW, with hours of discharge time). The power rating scales with the pressure in the storage chamber and the power rating of the turbine machinery while the energy rating scales with the volume of the storage tank. The round trip efficiency of CAES ranges between 75% and 80% with costs estimated at $450/kW. Energy costs which are a function of fuel costs used to heat the expanding air, and plant maintenance costs are commonly lower than traditional electricity generation costs. These units have long lifespans with a very high number of discharge cycles and short response times. Though they are appropriate for grid scale energy applications (peak shaving, renewable capacity firming, and arbitrage) they can also be used for power quality regulation and load following (Baxter, 2006).

Works Cited

Baxter, R. (2006). Energy Storage; A Nontechnical Guide. Tulsa, Oklahoma: PennWell Corporation.

ClimateTechWiki. (2006). Energy Storage: Compressed Air (CAES). Retrieved from ClimateTech Wiki: http://climatetechwiki.org/technology/jiqweb-caes#References

EPRI. (2003). EPRI-DOE Handbook of Energy Storage for Transmission & Distribution Applications. Washington DC: EPRI, Palo Alto, CA, and the U.S. Department of Energy.