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Access to reliable mega-scale energy storage technology can offer economic advantages to renewable energy technology and also to large-scale thermal power stations. Thermal power stations can operate at higher rates of reliability when they can operate at constant temperature and at a constant output. The thermal stresses on the thermal componentry inside the power station will be greatly reduced under such operating conditions and the life expectancy of the power station can be greatly extended. Continually heating and cooling thermal componentry can produced thermal stresses than can lead to fatigue and eventually cause components to break down
Storing Energy
A variety of methods exist that allow large amounts of energy to be stored in stationery locations. One non-mobile technology that is commonly used to store energy would be "flow-batteries" or "redox-batteries". These large batteries are well suited for small-site energy storage requirements (such as stand-by power or peak power) in certain geographic locations. There are other technologies that may be more suitable to be used for large-scale energy storage at other geographic locations.
The natural gas industry has been instrumental in discovering and using cavities in the earth to store compressed natural gas as well as compressed air. There are geological formations known as salt domes that may be found deep underground around the world. The top of a salt dome may typically be found between 500-metres and 1500-metres below the earth's surface at specific locations.
Salt domes have been estimated to measure up to 1500-metres in diameter and up to 9000-metres in vertical height. Natural gas companies may spend up to 3-years to flush the rock salt out of salt domes. Small salt domes are called salt jugs and are usually found closer to the earth's surface. Natural gas is usually pumped into storage under high pressure into salt jugs and into salt domes that have been flushed of salt. Salt jugs that occur too close to the earth's surface are often unsuitable to store compressed natural gas or compressed air. The high pressure has been known to blow the top off a few salt jugs that are too close to the surface.
Salt Domes in Coastal Mountains
There are several locations around the world where mountains that are located next to an ocean have an elevation of over 1000-metres. There are mountains that exist around the world that are the result of major tectonic activity that occurred during an earlier period of the earth's history. Such tectonic activity increases the likelihood that the ceilings of salt domes could protrude into the higher elevations of a coastal mountain. It is also possible that groups of salt domes could occur in close proximity to each other in such mountains. It is statistically probable that coastal mountains could also contain salt jugs hidden deep within them.
Seismic testing would need to be undertaken on coastal mountains to pinpoint the exact locations of any hidden salt jugs and salt domes that may occur at elevations that are above sea level. A close proximity to the ocean would allow salt jugs to be completely flushed of salt (using ocean water) and salt domes to be partially flushed of salt using ocean water. The addition of extra salt into the ocean could partially offset the concern that global warming could cause polar ice to melt and thereby reduce the salinity of the world's oceans.
Hydraulic Storage Using Sea Water
There are many coastal mountains around the world that have an elevation of 1000-metres. It is possible that the top of a salt dome of 600-metres diameter could extend upward into such a mountain and reach an elevation of 300-metres below the surface of such a mountain. The volume of the hemisphere of the salt dome would be just over 56-million cubic meters. Ocean water could be pumped into the emptied hemispherical dome of such a salt dome and be temporarily stored at an elevation of over 300-metres above sea level.
The water could be released through water turbines that are made from rust-resistant alloy steel and located at sea level. The turbines and generating equipment could each operate at an efficiency of 91-percent. They could deliver some 2000-megawatts of power for a period of some 15-hours with water flowing through the turbines at 700-cubic meters per second.
Mega-Hydraulic Storage using Ocean Water
There are several locations around the world where mountains that exceed 1800-metres in elevation may be located within sixteen kilometers of ocean water. It is statistically probable that a salt dome could lie hidden deep inside such a coastal mountain. The ceiling of the dome could reach an elevation of some 1500-metres above sea level inside a mountain having an elevation of over 2000-metres. An empty salt dome of 1000-metres diameter could store 260-million cubic meters of ocean water at an elevation above 500-metres.
Water could be released through turbines located at sea level and deliver some 15,000-megawatts of power for a duration of over 15-hours. Such storage capacity would be sufficient to serve the power needs of several metropolitan areas that are connected to the same power grid. The storage capacity could allow several thermal power stations that are connected to the same power grid to transfer energy into storage during off-peak periods.
Mega-storage of energy would allow thermal power stations to operate at constant temperature and constant output for extended durations. The thermal power stations would be used more productively. During peak periods the combination of power stations and the energy being released from storage would by far exceed the generating capacity of the power stations. Mega-storage capacity can help reduce the costs of building additional new thermal power stations.
Lowland Coastal Energy Storage
Salt domes and salt jugs could occur in lowland coastal areas with the tops of their ceilings being below sea level. Sufficient thickness of rock between them and the ocean would minimize seepage of ocean water into them after they have been flushed of salt. Power would be generated when ocean water is flowed into these subterranean cavities via tunnels and turbines. Turbines, pumps and generating equipment would be located inland and at a maximum depth of 300-metres below sea level so as to ensure safety.
"Recharging" these storage batteries would involve pumping water out of deep level storage and back to the ocean. Salt jugs at vastly different elevations could occur in close proximity to each other. Some of them could occur well above sea level in a coastal mountain and their counterparts could occur well below sea level in nearby lowland area. Such a combination would greatly increase the vertical height over which water would be transferred during recharging and power generation cycles. The power storage capability of such a combination would be extremely high.
Mid-scale Subterranean Hydraulic Storage
The subterranean storage system used by the natural gas industry includes salt domes and salt jugs that have been flushed of rock salt. Salt domes have a domed ceiling and may measure up to one-mile in diameter by up to six-miles in height. Salt jugs are small-scale salt domes that have been found at depths of below 150-metres below the earth's surface. It is theoretically possible for salt jugs to exist in coastal mountains and at elevations that are above sea level. Seismic testing could pinpoint the locations of salt jugs that have floors that are above sea level and that still lie undiscovered in coastal mountains. They could be flushed of salt using ocean water and be used for hydraulic storage of energy also using ocean water after the installation of suitable piping systems and hydraulic equipment (pumps, turbines).
Circular caverns with domed ceilings have been found to exist in mountains in various locations around the world. They may have been salt jugs at an earlier time and may have been naturally flushed of salt (by rainwater) over a period of millions of years. Those that are located above nearby large bodies of fresh water and could be modified into fresh water storage reservoirs and used for inland hydraulic storage of energy. Those that occur along an oceanic coast could become high-altitude reservoirs of ocean water that are used for hydraulic storage of energy.
Mini-scale Subterranean Hydraulic Storage
Caves and caverns of various sizes may often be found at the higher altitudes of various mountains around the world. Some of these caves and caverns may occur in mountains that are in close proximity to a body of water such as a river, a lake or ocean. Many of these caves and caverns could be enlarged and modified into high-altitude water reservoirs that would likely have a minimal impact on the surrounding ecosystems. Evaporation of water from such reservoirs would be minimal. Piping systems and related pumping equipment may be installed to pump water from lower elevations into these reservoirs. Small-site hydroelectric equipment would be installed at lower elevations near the source of the water where power would be generated.
Seismic testing techniques can be used to pinpoint the locations of caves and caverns without entrances that may exist at higher elevations. After their locations are known tunnels may be bored into the surrounding rock or blasted from it to gain access and converted the caverns into water storage reservoirs. Piping and pumping systems may subsequently be installed. Hydroelectrical generation equipment would be installed at the elevation of the source of the water that would be pumped into these caverns. In some cases that source of the water would be a sea or an ocean.
Air-turbines and Hydraulic Storage
A small air-turbine could be installed in an airshaft that connects to the hydraulic storage reservoir. It would generate a small amount of electric power from air that will be pulled into the storage reservoir while water is being released at the lower elevation (to generate power). Air would also pass through the air-turbine when the hydraulic storage chamber is being replenished with ocean water. The turbine could produce a small amount of electric power that could be used in nearby villages. A salt dome could have a diameter of up to one-mile (1600-metres) and could process a very large volume of air when it is being refilled or when it is being emptied. The air that flows through the air pipe could reach sonic speeds (in the turbine) as the ocean water is released through the turbines and flows back to the ocean.
Technology originally developed in Ireland to produces power from the rise and fall of ocean waves to pump air through a large pipe connected to a bi-directional air-turbine. That same air-turbine technology could be adapted to operate along with a hydraulically based energy storage system. Hydraulic storage technology has higher efficiency that pumping compressed air into subterranean caverns. Using emptied salt jugs and salt domes that are located in coastal mountains for hydraulic storage can minimize environmental damage to ecosystems in nearby valleys. The technology would also work well with sources of renewable energy that would supply power at times when there is no demand (wind power, ocean wave power, ocean tidal power).
Conclusions
The hydraulic storage of energy using either ocean water or fresh water can be made environmentally friendly. Valleys do not have to be flooded. Ecosystems and wildlife habitat could remain intact as hydraulic storage technology is developed around the world. Subterranean hydraulic energy storage that uses ocean water as well as fresh water (in different locations) could come to play a significant role in the future of the energy industry.
