Storing solar and wind energy with compressed air

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Salt-water aquifers under British waters could hold 77-96 TWh of renewable-energy-powered compressed air for the two-month windows required of seasonal energy storage, according to a study conducted by the University of Edinburgh.

An investment of $16-59 million in 6,300-7,800 such offshore wells could achieve storage efficiency of 54-59%, according to the Edinburgh researchers, with the resulting electricity having a levelized cost of $0.42-4.71/kWh.

The Scottish university group conceded such wide variation in final power cost made it difficult to assess whether offshore aquifer storage – known as porous-material compressed-air energy storage (PM CAES) – was economically viable but described the process as commercially mature in the paper Inter-seasonal compressed-air energy storage using saline aquifers, published in Nature Energy.

Conventional CAES involves surplus power producing compressed air to be stored underground. That air produces heat which generates electricity using a gas turbine. CAES gas turbines typically produce around 228g of CO2 emissions per kilowatt-hour generated, compared to 388g/kWh for the combined cycle equipment typically used in gas power stations.

Underground sites

Edinburgh researcher Julien Mouli Castillo said two such underground salt cavern facilities are in operation: a 290 MW project developed by electric utility E.on in Huntorf, Germany and a system in McIntosh, in the U.S. state of Alabama. “The technology discussed in my study, however, uses a different type of geological store, which has no commercial plants or demonstrators in the world,” he told pv magazine. “Although a field test was conducted in the 70s in the USA.”

Castillo and his colleagues wanted to consider the potential of using offshore aquifers and arrived at their figures using the ‘Monte Carlo’ method of repeated random sampling to generate numerical estimates related to unknown parameters.

The Edinburgh team suggested offshore compressed air storage could be used in countries which have renewables supply more than 80% of their electricity or in nations with limited onshore land and water.

Densely populated regions, with a consequently high demand for electricity could also benefit although the Edinburgh group warned the industry would have to be regulated as incorrect operation of such facilities could see repeated pressure changes on rock lead to mechanical failure and even well collapses. “The integrity of the sealing rocks surrounding the store must also be considered,” stated the paper.

Deformation

The correct operation of PM CAES facilities would result in deformation of the storage sites which would be expected to stabilize over time, according to the researchers.

Further research would be required into the environmental impacts of the technology, the Edinburgh team said, as heat produced by the generation of compressed air could kill organisms in the wells. Salt precipitation around boreholes and the stability of compressed air reservoir seals would also benefit from further study, according to the academics.

“Further research should focus on identifying sites within the aquifers identified here, paying attention to the extent of both inorganic and organically-mediated chemical reactions within the reservoir, and possibly the overlying seal,” the paper concluded.