CAES Using Air Injection

ESPC developed a novel CAES plant concept based on application of Dry Air Injection (DAI) technology that reduced by more than 50% specific CAES plant costs ($/kW). This concept (see List of Publications 3, 5, 9, 22) is based on the injection of the stored air into existing combustion turbines all within their capacity limitations.

Schematic of Compressed Air Energy Storage Utilizing Dry Air Injection into a GE7FA Gas Turbine

Wind, coal or nuclear plants can be used to power the motor-driven air compressor during nighttime hours, and fill the underground storage with compressed air. The stored air is injected into the gas turbine during daytime peak electric hours.

Of the total 192 MW of power from a GE7FA gas turbine, 123 MW can be generated by stored air with an incremental heat rate of 4000 Btu/kWhr. Forty to sixty-five percent of the gas turbine power comes from the stored-compressed air. Other industrial gas turbine engines offer a similar potential for the utilization of compressed air.

Wind power suffers from the fact that its output fluctuates so greatly and in an unpredictable manner such that its power cannot be considered to be available when the system needs it. Some economic means of storing the output of wind turbines, and then utilizing that stored energy to generate electricity must be found before wind power can become a significant fraction of a grid’s generating capacity.

Compressed air energy storage (CAES) is the only proven technology to store electric energy on an industrial scale of hundreds of MW’s, Until now, CAES has suffered from the requirement of an expensive, custom-built expander-turbine to convert the stored compressed air into electricity.

Air injection for gas turbine power augmentation offers an economic alternative to custom-built CAES turbines. The power capability of a gas turbine is increased when compressed and optionally heated air is injected into the engine at a location upstream of the combustor.

Inlet guide vanes (IGV’s) may be used to reduce the airflow into gas turbine compressors and allow part-load operation. The compressor inlet airflow can be reduced by as much as 35% to 45% when IGV’s are closed. Closing the inlet guide vanes allows the maximum injection of compressed air from underground storage.

Locations suitable for underground storage are plentiful all over the US and the world, and the technology to create the storage is well established. The air compressor and the heat recovery unit for air injection are standard equipment available from various vendors. Injection into the compressor discharge plenum is possible on most industrial gas turbines.

In locations where salt deposits or aquifers are not available, compressed air can be stored in pipes. THe cost of pipe storage is higher than for underground mines, but it is still eonomic for many aqpplications.

· Reduces the CAES plant specific costs to approximately $200-250/kW (vs. $600-650/kW for the conventional concept) due to the elimination of costs and development issues/expenses associated with highly customized turbomachinery trains for the CAES plant application.

· Can meet a variety of the CAES plant capacities requirements by combining the incremental CAES plant capacities of various combustion turbines.

· Allows for the utilization of sub-surface storage systems (with a typically small storage capacity) that is sized for the incremental capacity of a combustion turbine and could be installed in its vicinity.

· Significantly reduces the project schedule avoiding the turbomachinery manufacturing time, and the time for tuning up and field adjustments.

· Provides the lowest emissions by capitalizing on the state-of the art gas turbine combustors.

· Significantly reduces risk factors by using standard combustion turbines for the power generation.

This concept, due to its lowest cost, is the best alternative for improvement of economics of wind power plants and other renewable energy sources by storing the renewable energy (whenever it is produced) and by releasing it in a controlled fashion as needed during peak hours.

The novel CAES concept is very effective in increasing efficiency (reducing fuel consumption) of existing combustion turbines which currently is particularly important in the times of high fuel costs. The incremental CAES power is produced with the heat rate of approximately 4000 Btu/kWh that is significantly lower than the heat rate of contemporary combustion turbines (of approximately 9500-10500 Btu/kWh) thus reducing the average fuel consumption per kWh produced.

Air Injection Technology