This invention relates generally to the storage and subsequent recovery of compressed air energy and more specifically to the retrofit of simple cycle gas turbine engines for compressed air energy storage application.
It will be appreciated that there exists a significant daily variation in the total amount of power required from a baseload power plant. However, it would be most costly and wasteful to provide for the peak demands of short duration with the same expensive, although efficient, baseload power plant machinery which is justified for baseload power generation.
In the past, power utilities have addressed the problem of providing additional non-baseload peaking power in an overall economical manner through the use of simple cycle gas turbines (SCGTs). Simple cycle gas turbines (SCGTs) are state-of-the-art turbomachinery designed for peaking duty operation. Their main elements are an air compressor, a turbine and a combustor. To meet consumer demand, SCGTs are coupled to electric generators to provide additional power to baseload power plants during peak loads.
Most recently, power utilities have addressed the aforementioned problem of supplying peaking power in an overall cost effective manner through the use of Compressed Air Energy Storage (CAES) systems. CAES plants store off-peak energy from relatively inexpensive energy sources such as coal and nuclear baseload plants by compressing air into storage devices such as underground caverns or reservoirs. Underground storage can be developed in hard rock, bedded salt, salt dome or aquifer media.
Following off-peak storage, the air is withdrawn from storage, heated, combined with fuel and expanded through expanders, i.e., turbines, to provide needed peaking or intermediate power. Since inexpensive off-peak energy is used to compress the air, the need for premium fuels, such as natural gas and imported oil, is reduced by as much as about two-thirds compared with conventional gas turbines. In such circumstances, utilization of non-peak energy for the subsequent production of peak energy is clearly desirable, especially since non-peak energy can often be obtained for far less than the cost of obtaining peak energy.
Compressors and turbines in CAES plants are each connected to a synchronous electrical machine such as a generator/motor device through respective clutches, permitting operation either solely of the compressors or solely of the turbines during appropriate selected time periods. During off-peak periods (i.e., nights and weekends), the compressor train is driven through its clutch by the generator/motor. In this scheme, the generator/motor functions as a motor, drawing power from a power grid. The compressed air is then cooled and delivered to underground storage.
During peak/intermediate periods, with the turbine clutch engaged, air is withdrawn from storage and provided to a combustor. The combustor combines the pre-heated compressed air with a fuel, such as No. 2 fuel oil, and expands the mixture of fuel and compressed air in a turbine, which provides power by driving the generator/motor. In this scheme, the generator/motor functions as a generator, providing power to a power grid. To improve the CAES heat rate, waste heat from a low pressure turbine exhaust is used to pre-heat high pressure turbine inlet air in a recuperator.
For a more complete discussion of CAES systems, see Nakhamkin, M. et al. "Compressed Air Energy Storage: Plant Integration, Turbomachinery Development", ASME International Gas Turbine Symposium and Exhibition, Beijing, Peoples' Republic of China, 1985 and Nakhamkin, M. et al. "Compressed Air Energy Storage (CAES): Overview, Performance and Cost Data for 25 MW to 220 MW Plants", Joint Power Generation Conference, Toronto, Canada 1984, both incorporated herein by reference.
Unfortunately, SCGTs require the use of significantly more fuel than CAES systems in order to provide a like quantity of peaking power. Additional considerations such as that of fuel quality required for proper operation and efficient use of previously stored energy also tend to favor CAES systems. However, numerous SCGT peaking power plants are presently in existence and do provide a means for supplying peaking power, albeit at considerable expense.
Due to the considerable expense in constructing new CAES plants it is most desirable to convert existing simple cycle gas turbine plants to CAES plants. Unfortunately, direct use of SCGTs in the compression and expansion trains of CAES systems such as those described in the cross-referenced applications presents difficulty due to SCGT maximum cycle pressures which are generally too low for typical underground storage depths and corresponding compressed air storage pressures required in typical CAES air storage and operation. An additional difficulty encountered in direct use of SCGTs in CAES systems relates to pressure losses generally occurring in underground air storage devices.