Certain features in the Greater Cape Town area invite consideration for ocean-based energy storage and power generation.
Ocean Thermal Energy Conversion (OTEC):
Energy experts in India and Hawaii have been exploring the option of generating electric power from the difference in temperature between surface seawater at 25ºC and the greater depths at 5⁰C.
To the north of Cape Town, Koeberg Nuclear power station releases cooling water at 40ºC into the ocean where water temperature is typically at 15⁰C, with cooler temperatures occurring at greater depths further offshore.
A power generation thermal efficiency of 40% would see the power station delivering 100MW of power from every 250MW of heat energy, rejecting 150MW of power into the ocean.
An OTEC power plant located near Koeberg could theoretically operate at an efficiency of 1% and deliver 15MW (15 000kW) of power for every 1 500MW of low grade heat sent to the ocean.
That level of power could support up to 15 000-homes, given an average household power consumption of 1 000kW. The power output of an OTEC installation next to Koeberg may be insufficient to be of interest to Eskom, but could be of interest to a nearby municipality and private entrepreneurs interested in generating electric power from a reliable source of waste, low grade thermal energy.
Quest for Energy Storage Capacity:
Internationally, many power utilities are seeking to expand grid-scale energy storage capacity to improve the viability of such renewable technologies such as shore and offshore wind energy, ocean wave power conversion and ocean tidal current power generation.
Access to energy storage capacity also improves the reliability of steam-based thermal power stations that can then operate at constant temperature to deliver constant output to reduce maintenance costs.
The continual cyclical heating and cooling of critical components inside a large steam-based power station produces thermal stresses that can in turn result in component break downs and costly repairs.
Underground Pumped Ocean Hydraulic Storage:
During the overnight hours when demand for electric power is low, traditional hydraulic pumped storage involves pumping potable water from a lower dam to an upper dam. When peak hour power is required, water flows downstream from the upper dam, through turbines to the lower dam.
A 400MW pumped hydraulic installation in Ontario, Canada involves pumping water between an abandoned iron mine and a cavern near the surface. South Africa’s ongoing drought situation could require a future pumped hydraulic energy storage installation to operate on seawater and involve a large subterranean cavern excavated far below sea level.
The relatively calm seawater in the southern region of Saldanha Bay may offer a potential site for such an installation, depending on the availability of suitable deep level bedrock located some 600m below sea level.
A German hydraulic turbine builder that builds maritime propellers also offers pumping hydroelectric turbines capable of operating over a height difference of 600m. The Western Cape Economic Development Department is aware of the concept which may be subject to further evaluation. If the geology in the region is suitable, a pumped hydraulic energy storage installation of up to 2 000MW may be possible.
Compressed Air Energy Storage:
Overseas, the natural gas industry seeks deep subterranean caverns of salt surrounded be impervious rock. They partially flush salt from these caverns to create high-pressure storage capacity for natural gas or to store massive amounts of compressed air.
Available off-peak electric power drives massive air pumps that drive massive amounts of air into the storage caverns. When electric power is needed, the compressed air flows toward turbine engine and is heated prior to entering the engines that drive electrical generators.
However, Eskom officials have expressed concern of the amount of energy lost to heat of compression. A variation of compressed air energy storage (CAES) that is possible at coastal locations can actually dispense with the underground cavern.
Giant size inflatable bags are placed on a lake bed or sea floor along with rock ballast to keep them at that location. An extended submerged pipe extends from shore-based air pumps to the giant bags that when electricity is available, are pumped with air and with some of the heat-of-compression sent into a thermal storage chamber.
When peak-hour electric power is required, air flows to the shore and is preheated prior to entering the engines.
Cape Town CAES:
The uniqueness of Cape Town with coastal mountains and deep seawater some 100km offshore makes possible a variation of inflatable bag based compressed-air-energy-storage.
It is based on the air-over-oil suspension on the Citroen car, provided that a cave about 50m below sea-level is available or possible under either Table Mountain or under Kogelberg Mountains.
One group of inflatable bags will be located offshore some 600m below sea level and connected to a second group of identical bags located inside the cave. The bags inside the cave would be pre-pressurized and press against the sealant-coated cave walls.
When electricity is available, hydraulic pumps would push seawater into the cave under high pressure, forcing the air to the inflatable bags located offshore in deep seawater. During peak power-demand hours, seawater from the pressurized cavern would flow through turbines to generate electric power.
The weight of the mountain above the cave would prevent the cave from exploding while the air-over-water energy storage system would greatly minimize heat loss caused by air compression.
Western Cape Department of Economic Development is aware of the concept and is believed to be doing further evaluation for possible future development.
