This invention relates to a method of utilizing a compressed air energy storage system. In more detail, the invention relates to a method of utilizing a compressed air energy storage system (CAES) employing two or more storage reservoirs. The invention also relates to a compressed air energy storage system in which two or more storage reservoirs are utilized effectively and efficiently.
The electric utility industry has as its main objective the supply of power at the lowest possible cost. This purpose has led to the development of large sophisticated nuclear and fossil-fuel-fired steam generating plants. For both technical and economical reasons, these plants should be operated at a steady load. However, to meet daily and seasonal fluctuations in power demand, the industry uses so-called peaker units. The most common form of these units are gas turbine systems that use premium fuels such as natural gas and oil.
Because of the limited supply of oil and natural gas in this country and current problems in the supply of petroleum fuel from foreign sources, premium fuel has become very expensive and the long-term supply is uncertain. Therefore, electric utilities have been exploring better ways of utilizing, or even eliminating, the use of premium fuels for peaker units and the possibility of operating their large power plants at steady or constant load. These considerations have led to the investigation of energy storage systems.
Studies conducted by electric utilities indicate that CAES power plants are attractive for consideration as an energy storage system. A typical CAES plant can be divided into two basic parts: aboveground power generating equipment and underground air storage reservoir. The aboveground equipment consists of a gas turbine system and air compressors placed on either side of a motor/generator. In contrast to conventional peaker units, the gas turbine system and compressor system are uncoupled; each system operates independently.
The purpose of this arrangement is twofold. Firstly, the compressor system is driven with cheap off-peak power from an electric power grid and the energy is stored as compressed gas in an air storage reservoir. Secondly, during peak hours the compressed air stored in the reservoir is utilized to run the turbines at their full power. This is in contrast to a conventional gas turbine peaker unit wherein the total power output is reduced by one to two thirds. The reason for this reduction in power is because that portion of the system's power is used internally to operate compressors which are needed to supply compressed air to the turbine. By utilizing a CAES system fuel savings on the order of 50% to 60% are possible.
Because of the requirements for storing large amounts of high-pressure compressed air (e.g., 10.sup.7 to 10.sup.8 ft.sup.3 at 50 atmospheres for a typical 200 MW plant), it is known that underground air reservoirs are an economic necessity.
There are four types of underground reservoirs that are suitable for the storage of compressed air. They are: depleted petroleum fields, aquifers, mined rock cavities, and solution-mined salt cavities. Suitable sitings for CAES systems can be found in many parts of the United States.
The simplest type of air storage reservoir is a fixed-volume underground cavern. This type of reservoir, however, has two drawbacks. First, storage of a large amount of "cushion air" is required because air has to be supplied to the gas turbine system at a constant inlet pressure. Second, a throttling process has to be used in between the cavern and the gas turbine inlet during power generation to provide this constant inlet pressure.
One way to reduce the inefficiency and cost due to throttling might be to operate the turbine system with a variable inlet pressure. Unfortunately, this would result in severe control problems in maintaining the desired output of the gas turbine system. Also it is not certain that the net efficiency would be significantly better since the gas turbine system would be operated at nonideal conditions most of the time. For certain types of caverns the pressure of the air in the cavern can be kept constant by employing an upper water reservoir connected to the cavern with a water compensation leg. While constant pressure storage of this type is applicable for installations in hard rock, it is not suitable for salt cavern reservoirs because of problems of cavern erosion and salt carryover into the upper reservoir.
Regardless of the type, the new reservoir capital cost and associated indirect costs are major expenses in constructing a CAES system. Any significant reduction in reservoir size for equivalent CAES system performance would therefore imply a significant saving in capital costs.
According to the present invention the air in one storage reservoir is throttled down to the constant inlet pressure of a gas turbine system until the pressure in the reservoir drops to the constant inlet pressure of the gas turbine system. The air pressure in a second air storage reservoir is then used in the gas turbine system and, rather than using a throttle, a variable geometry air ejector--that is, an air ejector in which total flow rate and entrainment ratio can be varied--is used to draw air out of the first air storage reservoir, the constant inlet pressure of the gas turbine system being maintained by adjustment of the variable geometry air ejector. More efficient utilization of the total air supply in the air storage sytem is thereby attained.