Hydraulic Storage using Coastal Salt Domes
Ocean and wind power technologies can generate power during the off-peak periods and large-scale thermal power stations operate most reliably when boilers, steam lines and turbines remain at constant temperature and constant pressure. The combination of electric railways and thermal power stations serve many cities around the world. The massive and instantaneous fluctuations in power demand for power from such transportation systems create difficulties at thermal power stations. Hydraulic and air-pressure based power stations respond more easily to sudden and extreme fluctuations in demand for power.
Coastal Salt Domes:
There are coastal mountains where evidence exists of sections of emptied salt domes that had been pushed up to elevations above sea level. Some caves that were easily accessible were vertical-circular in shaped while other caves had large domed roofs. Salt domes usually occur in groups and the existence of these caves suggested the possible presence of many other larger phenomena hidden within the coastal mountains or below them along the oceanic coast. Some still intact salt domes may lie hidden below sea level near the coast.
Modern seismic techniques can accurately pinpoint the locations, approximate dimensions and elevations of salt domes or sections of salt domes as well as the presence or absence of rock salt. Salt domes that are located very deep in the earth's bedrock and that measure several miles in vertical height by up to a mile in diameter would be suitable to store pressurized natural gas or compressed air. A dome of perhaps 3000-feet in diameter by 3500-feet in vertical height that lies within less than 2000-feet below maritime sea level would be suitable for underground pumped hydraulic storage.
Seawater under Salt Domes:
Voith-Hydro has developed high-capacity turbine pumps that can pump massive volumes of water uphill over elevations of 2000-feet or 600-metres. The corrosion-resistant version of that technology would be suitable for operation is a seawater environment and could pump water from a subterranean chamber of 3000-feet in diameter. The water volume within the chamber may only need to fluctuate over a height of 100-feet to serve 6-hours of peak hour needs of most coastal cities and their electric rail transportation systems.
The water flow rate would exceed 32,000-ft3/sec or 925m3/sec and deliver over 2400MW of power at 89% overall efficiency over an average vertical height of 1000-feet. The power output could exceed 4500MW over a vertical height of 2000-feet, the apparent design limit of the Voith-Hydro pumping technology. A compound pumping and generation system may be applicable where the vertical-circular section of a low-height salt dome is located at some 3500-feet depth below sea level.
An airshaft may connect the atmosphere to the top of the storage dome to allow air to move in and out of the cavity in response to changing water volume within the cavity. Incoming and outgoing air would pass through a bi-directional impulse-style air turbine that drives electrical generation equipment. The output could sustain a portion of the local overnight off-peak power demand and provide additional power during peak periods.
The Riverbank Power Company of Toronto, Canada is pioneering a technique of excavating cavities into the bedrock next to large rivers at depths of 2000-feet or 600-metres below river surface level. Their technology could also be adapted to excavating such cavities into the interior of coastal mountains at elevations of 2000-feet above sea level. Such excavation is unlikely to attract opposition from environmentalists if there is no seepage of ocean water into the surrounding water that may be at a great distance from the excavation site. The excavated cavity is an option where no suitable remnants of salt domes may exist either within a coastal mountain or below the coastline in the general vicinity of where remnants of salt domes exist. There are plans to introduce excavated subterranean cavities into the State of Maine, USA to convert wind energy and ocean energy to stored pumped hydraulic energy.
Relocating Salt from Salt Domes:
While salt domes in coastal mountains may have long been flushed of rock salt, several deep level coastal salt domes that lie below sea level may still contain rock salt. There would be potential to make productive use of that salt in tropical and sub-tropical locations where seawater desalination may prevail. It may be possible to excavate large circular cavities near the ocean coast into which to place excess salt and brine from thermal desalination plants.
While potable water would reflect the infrared spectrum of solar light, the salt in a brine pond would capture that spectrum. The temperature at the bottom of a brine pond may reach 95-deg C or 200-deg F while ocean temperature may be considerably cooler. Cold northbound ocean currents flow along the western coasts of South America as well as Southern Africa, southbound along the coast of northwestern Africa and eastbound along the southern Australian coast. Water temperature may often be below 25-deg C or 77-deg F and often much cooler.
Energy from the Salt:
The thermal energy in coastal brine ponds in tropical and sub-tropical regions can help desalinate ocean water or it can be used to energize engines that are designed to operate on low-grade thermal energy. In either case, a large sealed and corrosion-resistant spiral pipe may be installed at the bottom of the brine pond to collect and transfer heat to either a desalination plant or low-grade heat engine that may operate at about 9% thermal efficiency. Many commercially available solar photovoltaic panels operate similar efficiency.
A transparent cover that is immune to the UV spectrum would allow the infrared spectrum to enter the brine pond, minimize evaporation and provide insulation during the cooler overnight hours. Ocean water may be used to regulate and optimize the concentration of salinity as well as provide the heat sink for either the desalination plant or the heat engine. In terms of long-term costs and operational longevity, the brine pond may have an advantage over photovoltaic technology.
A company called ElectraTherm offers low-grade heat engines of up to 1MW output each that can generate electric power at up to 10% efficiency from the temperature difference between coastal brine ponds and cold-current ocean water. That efficiency is higher than the 2% efficiency of air-based solar-thermal chimneys. Suitable Pacific Coast locations are available in northern Chile and along the desert region of Peru. Suitable Atlantic Coast locations occur along the desert region of Morocco, northwestern South Africa, Namibia and Angola.
There are many parts of the world where pumped hydraulic storage may involve the use of sections of subterranean salt domes located either below sea level near an ocean coast or within a coastal mountain. An optional technology would involve excavating cavities either inside coastal mountains or into the coastal bedrock located below sea level. Such pumped hydraulic storage operation could enhance the operation of other power generation technologies, improve their cost effectiveness and even extend their longevity. Whatever salt is flushed from coastal salt domes can be transferred to coastal brine ponds to generate electric power or to desalinate seawater.