Economic and engineering evaluations of energy-storage potential in certain USA aquifers are being conducted. Particularly detailed evaluations have been made for compressed air energy storage (CAES), using off-peak power from nearby nuclear power stations. In ''current studies, we are examining thermal energy storage and utilization prospects, using solar power or, alternatively, off-peak, low-cost nuclear power as the heat source. Economic and energy-saving aspects of selected combinations of sources, storage media, and uses are ranked with respect to various selection criteria.
The current U.S. interest in underground storage of energy grows out of a number of environm7ntal, economic, and political factors. Environmental considerations create particular difficulties for nuclear power and coal. Economic and political problems are clouding the prospects for continued supply of energy needs from outside sources of petroleum. According to the National Academy of Sciences, nuclear-fusion, solar, and geothermal energy remain at present in a primitive state of development. This and other conclusions of a four-year study are set forth in a report "Energy in Transition, 1985–2010," recently published by the Academy''s Committee on Nuclear and Alternative Energy Systems. The study also concluded that conservation and energy efficiency are essential elements of the national energy policy. Efficiency can be realized through generation of energy at a time when surplus capacity is available and storing it for use when demand exceeds capacity. One such procedure involves storage of energy in aquifers, which constitute convenient natural near-surface reservoirs for fluid storage and recovery.
Although aquifer storage has been primarily employed for natural gas, storage of compressed air for use in the generation of electrical energy is in an active stage of development. Also, thermal storage, by means of hot or cold water injection, is under serious consideration, with initiation of experimental studies funded by the U.S. government anticipated at several locations in 1980. Texas A&M University and Auburn University are operating small thermal storage projects at present. Porosity and permeability are critical rock characteristics in storage considerations. Porosity is a measure of the volume available for fluid storage, expressed in percent of the total volume of solids plus voids for a representative portion of the storage medium. Permeability is a measure of the ability of fluids to flow through the aquifer. It is usually expressed in terms of darcys or millidarcys (mD), with the higher numbers indicating greater permeability -- that is, greater ease with which a given rock will accept, transmit, and deliver fluids. For example, natural gas is stored in aquifers having permeabilities from 7 mD up to about 7,000 mD. For compressed air storage, reservoirs with very high permeability are desirable -- several thousand millidarcys, if possible.
Two aquifers in southern Illinois have been acquired for energy-storage purposes by the URS Corporation. These are anticlinal structures in Devonian-age carbonate rocks, at depths of about 600 ft, as shown in Fig. 1. Characteristics, as determined from well logs and other geologic data, are set forth in Table 1.