This invention concerns the generation of electricity from coal and other fuels which are relatively inexpensive because of their contents of sulfur, ash and/or moisture, and particularly the economical supply of incremental amounts of electricity during recurring periods of peak demand, utilizing a facility which stores compressed air, or other gaseous mixture, during off-peak periods.
It is well known to transfer heat of combustion of a fuel to water boiling under pressure so as to produce steam which is subsequently expanded through a turbine which produces mechanical energy which, in turn, is converted to electricity by a rotating generator or alternator. It is usual to utilize some of the combustion heat to increase the steam temperature before expansion (superheat) and common to similarly reheat it at one or more intermediate stages in the expansion. Because steam can be recondensed to water having a low vapor pressure (compared with atmospheric pressure) it is common practice to augment the energy output of the expansion by exhausting to a sub-atmospheric pressure (vacuum).
Several other methods of obtaining mechanical energy for conversion to electricity are in common use, including hydraulic and gas turbines and internal combustion engines. In general, a long-term steady demand for electricity (base load) justifies a large investment in reliable, efficient generating capacity.
It is characteristic of most large electrical supply and distribution systems, particularly those serving diversified consumers, that there is an appreciable daily, and often seasonal, variation in the total amount required. It is costly and wasteful to provide for the short term peaks in demand (which is sometimes called "peak shaving") with the same expensive efficient generating facilities justified for the base load demand.
Electric utilities and their suppliers have devised numerous alternative methods of generating supplemental peak load electricity at less incremental expense than would be required to enlarge base load capacity. Among these are older, less efficient boilers and turbines, diesel and gas turbine powered generators and pumped hydroelectric, using an elevated water reservoir which is filled by electric powered water pumps during off-peak periods.
Somewhat analogous to pumped hydroelectric is a relatively new method known as Compressed Air Energy Storage (CAES). Instead of pumping water, surplus off-peak electricity goes to a motof-generator (operating in motor mode), driving compressors which deliver atmospheric air into a storage facility where it accumulates under pressure until needed for peak shaving. Then it is heated by direct firing of oil or gas and expanded through turbines which furnish mechanical energy to the motor-generator (in generator mode), which feeds the necessary supplemental electricity into the distribution grid for the duration of peak demand.
Sites suitable for sufficiently large elevated reservoirs are relatively scarce, limiting the opportunities to utilize the pumped hydroelectric method. On the other hand, compressed air can be stored in caverns hollowed out of hard rock, solution mined cavities in salt domes and even certain aquifers (porous underground formations normally containing water). One or another of these options which, for convenience, I will lump together as storage caverns, are available to most utilities. Although operating experience with the CAES method is so far limited to one installation in West Germany, another was under contract in Illinois. A spokesman has predicted that CAES could account for half of the industry's energy storage by the year 2020.
There is, as yet, little experience to establish the feasible and economic storage pressure range of future CAES facilities. That at Huntorf, W. Germany is charged at 1000 pounds per square inch (psi) whereas that in Illinois was to be charged at 815 psi, according to published reports. It seems reasonable to expect that the range may broaden with future experience to perhaps 500 to 1500 psi. The salt cavern at Huntorf has a constant capacity and therefore varies in pressure during its operating cycle. The hard rock cavern in Illinois, on the other hand, was to be hydraulically compensated so as to store air at essentially constant pressure.
In addition to the storage cavern, a CAES facility requires an expensive multi-stage air compressor, a multi-stage gas turbine with combustion chambers, a motor-generator, compressor intercoolers and aftercooler (or heat storage), fuel, cooling water and other auxiliary systems. Besides capital, overhead and personnel costs CAES, as heretofore known, has a low energy efficiency. Although sometime described as surplus, the electricity used to charge the cavern consumed about 3 times its energy equivalent in fuel. Before it becomes electricity again it goes through a chain of conversions: to mechanical energy, to pressure energy and waste heat, to heated air, to mechanical energy and, finally, electricity. Each step compounds the inefficiency--to an overall efficiency level which varies from case to case but is unlikely to be much about 10 percent.
Copending Ser. No. 261,143. now U.S. Pat. No. 4,380,960, (the parent case) describes several embodiments of a continuous supercritical wet combustion process which is capable of recovering the dry heating values of wet, high ash or sulfurous fuels without polluting the atmosphere. Alkali and elevated pressures of oxygen and water vapor are employed to convert carbon at relatively low temperatures, at which there is virtually no production of sulfur or nitrogen oxides.
While, as a minimum, water vapor pressures may be as low as 3 atmospheres, or about 45 psi, system pressures with sulfurous fuels are usually about 100 psi and may be as high as 5000 psi. Generally, multi-stage compressors are employed to charge combustion air, driven by multi-stage turbines which recover pressure energy from flue gas. These turbines and compressors are similar to those which comprise main components of a CAES facility.
U.S. Pat. No. 4,377,066 describes a continuous supercritical wet combustion process similar to that of the parent case, except that combustion reactions take place in a bed of fluidized solids, which may also be combined advantageously with a CAES facility.
Individual fans of axial flow compressors and individual wheels of centrifugal compressors, together with their companion fixed elements, are sometimes called "stages". A number of these "stages" are frequently arranged in series within a single casing and it is not unusual to arrange two or more such casings in series. It is normally necessary or economical to cool the partially compressed gas between cases, to avoid excessive temperatures and to minimize horsepower consumption in the succeeding case. For purposes of this description, one of a series (or train) of compressor, or turbine, cases (each containing a plurality of fans or wheels) will be referred to as a stage.
The number of fans or wheels per case varies between manufacturers and may also be influenced by the economics of a particular installation. Usually, a case, or stage in my terminology, will deliver a compression ratio in the range of 4 to 7 . Gas turbine engines usually have only one such stage. The known pressurized fluidized bed combustions (PFBC's) may employ 1 to 3 stages whereas the supercritical wet combustion processes of the parent case and U.S. Pat. No. 4,377,066 may utilize 1 to 5 stages, usually 2 to 4.
Experience with air storage caverns is so far limited to compressor trains of 3 stages but it is quite possible that future installations may extend the range to 2 to 4 stages.