Prospects for Grid-Scale Seasonal Energy Storage
In many countries, the market demand for electrical energy is cyclical on a daily as well as on a seasonal basis. Numerous technologies already exist that can divert large amounts of overnight off-peak electrical power into short-term storage systems. Weather patterns alter demand for electric on seasonal basis in countries that experience major changes in ambient temperature, when the demand for energy to operate air conditioning systems during the summer months escalates.
Nations and regions that undergo major seasonal climatic changes usually have ample generation capacity during the cooler months. That capacity could manifest in as additional winter rainfall to operate power dams, higher velocity winds to drive wind turbines and even more favorable ocean waves and ocean tidal currents that may serve as sources of renewable energy. Having access to grid-scale seasonal storage capacity would make more productive use of such available generation capacity.
While there is potential for seasonal energy storage between Lake Ontario and Lake Erie as well as between the Qattara Land Depression in northern Egypt and the Mediterranean, there are numerous political and economic challenges to be address and managed in the course of developing such large-scale pumped hydraulic storage. The option to adapt compressed air energy storage (CAES) or some variation of it to grid-scale seasonal storage could offer a less complicated route by which to introduce and develop such energy storage. Research and exploration undertaken by the natural gas industry already provides one possible CAES option.
The largest salt domes in the world measure up to a mile in diameter by up to 30,000-feet in vertical height. When flushed of rock salt, the emptied salt domes can hold natural gas at some 100-times atmospheric pressure. There is scope to adapt a large emptied salt dome to compressed air energy storage. A dome of 4000-feet in diameter by some 20,000-feet in vertical height can provide some 2.345 x 1011 ft3 of storage volume for air at 1500-psia at 5.4713-lb/ft3 of density with the dome top located some 3000-ft below ground surface. Earth has a density of 110-lb/ft3 and at 3000-feet depth will exert a downward pressure on the dome roof of 2290-psia. Air pressure may be allowed to drop to 1200-psia or 4.377-lb/ft3 of density during operation.
Air may be released from the emptied salt dome over 4-months duration (154-days) at a rate of 17,629-ft3/sec and involve a change of density of 1.094-lb/ft3. The air may pass through a 2-stage power turbine with a pressure ratio of 9:1 per stage and involve computer controlled 2-stage (reheat) combustion. Maximum combustion temperature using natural gas may be set at 1540 F with temperature drop of 870 F across each stage of the power turbine. The peak continuous power level would be 17,629 x 1.094 (lb/sec) x 0.24 (Cp) x 870 x 3600/2545 x .745/1000 (conversion to MW) x .81 (efficiency) = 6000MW.
There is the potential to raise seasonal power output by pumping the same cavern to 1500-psia allowing pressure to drop to 1000-psia. Air density would decrease from 5.4713-lb/ft3 to 3.64-lb/ft3 over a season and releasing 1.82374-lb/ft3 through the 2-stage power turbines operating at 8:1 pressure ratio at a rate of 17,629-ft3/sec. The denser air would undergo a temperature drop of 820 F across each turbine. Continuous power level would be 17,629 x 1.82374 (lb/sec) x 0.24 (Cp) x 820 x 3600/2545 x .745/1000 (conversion to MW) x .81 (efficiency) = 11,000MW over 154-days duration.
The power system would include a regenerative heat exchanger to preheat air flowing toward the high-pressure turbine. Exhaust heat may be used to preheat water used in a downstream steam power system, desalinate seawater or generate steam for a water-based steam-vacuum district cooling system. The turbo-compressor system may involve a 3-stage system with a 5:1 pressure ratio per stage plus after-coolers. The heat of compression may be put to productive use as seawater desalination, district heating or to preheat water being used in a steam-based power system. Seawater may be used to condense the combustion exhaust and yield potable quality water.
Compressed Air-over-water Energy Storage:
There may be a limited number of sufficiently large salt domes that can offer seasonal CAES capacity. It may be feasible to combine compressed air with pumped hydraulic storage to provide seasonal storage capacity using emptied salt domes of sufficient diameter and limited vertical height. The hybrid concept involves using the change of pressure with water depth and will involve using smaller salt domes, available caverns or specially excavated caverns at suitable locations.
The top of a salt dome may be located some 4000-feet below ground level and some 3000-ft below maritime sea level (land elevation 1000-ft above sea level). The ground above the dome of 110-lb/ft3 will exert a downward pressure of 4000 x 110/144 = 3055-psia. The dome may measure 2000-ft diameter and use 3000-ft of its vertical height of 6000-ft to 10,000-ft for energy storage. Modern drilling technology would be able to drill a borehole into the salt dome and the ocean at a depth of 6000-feet below sea level, with pressure of 6000 x 64/144 = 2670-psia.
A 3-stage air turbo-compressor with 6:1 pressure ratio per stage would be able to generate pressure of over 2700-psia, sufficient to pneumatically pump water from the salt dome. The air density in the dome would rise to 10.9-lb/ft3 in a storage volume of 1.047197 x 1010 ft3. The air would be released over a period of 154-days at a rate of 787-ft3/sec or 8578.68-lb/sec and pass through a 2-stage power turbine with 9.5:1 pressure ratio per stage.
Heating and reheating would be achieved by either natural gas combustion (night) or concentrated solar thermal energy (day). Temperature drop per stage would be 870 F. A regenerative heat exchanger would transfer exhaust heat into the incoming air. Power level would be 8578.68-lb/sec x 3600/2545 x 0.24 x 870 x (2-stages) x 81% efficiency x 0.745/1000 = 3000MW for a period of up to 4-months. A dome of 3000-ft in diameter (1500-ft radius) and 3000-ft in height (2.474 x 1010 ft3 volume) allocated to air-over-seawater pumped storage could generate 6500MW over the same duration.
On an even larger scale, it would be possible to generate some 11,000MW over a season using the top 3000-ft in height of a dome of 4000-feet in diameter. Such an option may be considered if a dome of such diameter and vertical height of under 10,000-ft were located near an oceanic coast or next to a lake. While such dimensions would limit the scope and magnitude of using the dome for seasonal CAES operation, the location and the dimensions would enhance the attractiveness of adapting such a dome for seasonal air-over-water energy storage operation.
The development of a grid-scale compressed-air-over-seawater energy storage system would require the development of special separator technology that would prevent or minimize the diffusion of air into seawater. A layer of oil floating on the water has long been shown to separate water and air and could prevent the compressed air from diffusing into water. It may also be possible to install giant inflatable plastic or polymer bags into the cavern to separate air and water. There has been research into submerging air-inflatable bags pumped with compressed air into seawater as a form of small-scale energy storage.
While there is potential to keep a small-scale air-inflatable bag submerged under seawater, keeping larger air bags submerged at greater depths becomes more challenging. Placing multiple large air-inflatable bags inside a cavern located in impermeable rock offers the option of large-scale and seasonal energy storage. While much of the technology needed to operate an air-pumped hydraulic energy storage system is already long proven, there is a need to develop additional technology to make a mega air-over-water seasonal energy storage concept more workable.
Compound Pumped Hydraulic CAES:
There are salt domes and earth caverns that have roofs located at over 5000-ft below sea level where hydraulic pressure would exceed 2250-psia. A modified air-over-hydraulic energy storage system may be applied to empty salt domes of limited vertical height, sufficient diameter and roofs at extreme depths. For a seasonal height fluctuation in seawater in the cavern, hydraulic pressure would rise to 3600-psia at 8000-ft of depth.
Except that a compound pumped hydraulic CAES could reduce pressure levels inside the cavern. Such an installation would place Kaplan pumping turbines at a depth of 2000-ft below sea level. The pumping action of the pumping turbines would reduce hydraulic and air pressure levels of between 2250-psia and 3600-psia within the empty salt dome to between 1335-psia and 2670-psia, pressure levels usually found at depths between 3000-ft and 6000-ft.
The 5000-ft of depth of earth above the dome would exert a downward pressure of some 3800-psia. For 1000-ft elevation above sea level, some 6000-ft of earth located above the salt dome would exert a downward pressure of over 4500-psia on the dome roof. The pressure from above on the dome roof would balance pressure inside the empty salt dome and offer a measure of safety at the surface while assuring the viable long-term operation of the emptied salt dome for seasonal storage.
The CAES component of a compound pumped hydraulic CAES installation would generate near equal output over the same duration as the equivalent size of empty salt dome adapted to the same operation at higher elevation. Except that the Kaplan pumping turbines would generate additional output over the same time duration. The compound system would use salt domes that may be deemed too small for pure CAES operation and where natural gas may be stored in nearby empty salt domes.
Raising Seasonal CAES Productivity:
Intercoolers placed downstream of each turbo-compressor may extract heat that may be used productively in such areas as district heating, preheating water for a steam-based thermal power station or desalinating seawater. The exhaust heat from the power turbines may be put to similar productive use during the hot summer months and also be able to energize a steam-vacuum cooling system for district cooling.
The cool incoming seawater may be cooler that the summer air and may serve as a heat sink for a district cooling system. During winter, the warm water being pumped out may serve as a heat source for a heat-pumped district heating system. There is potential at some locations to use dry cooling or available seawater to condense potable water from the power turbine's combustion exhaust gas.
Much of the technology needed to develop grid-scale, seasonal energy storage systems is well proven. While seasonal pumped hydraulic storage is technically possible in Lake Erie and Niagara Falls, there are political factors that would likely delay the development of such a project. However, a few giant salt domes that may be available at some locations that may be adapted for seasonal CAES operation. Smaller salt domes with limited vertical height may be adapted for air-over-hydraulic energy storage.
Groups of salt domes usually occur in close proximity to each other allowing an adjacent emptied salt dome to store compressed natural gas to sustain the summer operation of the seasonal CAES installation. By operating air turbo-compressors on other energy sources during the winter months, the CAES and air-over-water energy storage systems would save 40% to 50% of the natural gas that gas turbine engines would otherwise consume. Both variations of the seasonal energy storage concept would also benefit from economy of scale.
Pneumatic-over-hydraulic energy storage systems in the form of hydraulic accumulators are well proven in many applications. Researchers have more recently expanded on the concept and demonstrated the operation and potential viability of compressed-air-over-seawater energy storage systems involving airbags submerged in deep seawater and pumped with air. There is market application for large, grid-scale versions of such technology adapted to seasonal energy storage operation at coastal locations in many countries. Seasonal CAES and air-over-water CAES technology promises to be viable and productive.