Liquid air energy storage (LAES), also referred to as cryogenic energy storage (CES), is a long duration, potentially large-scale energy storage technology that can be located at the point of demand. While the technology has been known for more than 100 years, only recently has it been widely seen as a promising energy storage option for the grid and buildings, alongside smart technology developments.
“I get half a dozen people a week trying to persuade me they have a brilliant invention”, said Tim Fox head of energy at the Institution of Mechanical Engineers (IMechE). “It seems this one might actually be such an invention”.
Greener and cheaper electricity is used to cool air from the atmosphere to -195 °C using the Claude Cycle to the point where it liquefies. The liquid air, which takes up one thousandth of the volume of the gas, can be kept for a long time in a large vacuum flask at atmospheric pressure. At times of high demand for electricity, the liquid air is pumped at high pressure into a heat exchanger, which acts as a boiler. Air from the atmosphere at ambient temperature, or hot water from an industrial heat source, is used to heat the liquid and turn it back into a gas. The massive increase in volume and pressure from this is used to drive a turbine to generate electricity.
Since 2010, 300 kW, 2.5MWh storage capacity pilot cryogenic energy system developed by researchers at the University of Leeds and Highview Power Storage, has been operating at a 80MW biomass power station in Slough, UK. The system uses liquid air (with the CO2 and water removed as they would turn solid at the storage temperature) as the energy store, and low-grade waste heat to boost the thermal re-expansion of the air.
"Cryogenic storage systems are well-suited to capturing electricity from renewables as they can be easily scaled according to the electricity coming in by adjusting the size of the liquefaction unit, the total amount of energy that can be captured by having bigger or smaller containers, and the delivered power through the size of turbine installed", said Professor Richard Williams of Birmingham University.
"The system doesn't have geographic or geological constraints, like pumped hydro storage or compressed air energy storage, so can be placed near generation or demand centres".
Through a three and a half year, €7m EU research grant funded project, entitled CryoHub, London South Bank University will lead a pan-European consortium of researchers to examine whether refrigeration warehouses and food processing plants could store and supply electricity.
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The refrigeration and processing plants at the centre of the project are themselves high consumers of electricity. CryoHub will investigate the potential for CES to shift the energy that these units use from periods of highest demand on the grid to times when demand is much lower. The researchers estimate that if the technology were applied to just 10% of the refrigerated warehouses and food factories in Europe it could save nine million tonnes of carbon dioxide equivalent emissions per year.
In the UK, cooling consumes 14% of UK electricity annually. The Indian Government forecasts an investment need of $15bn on its cold chain over the next five years, and in China refrigerated storage capacity is on course to increase 20-fold by 2017.
Increasingly vast amounts of “cold” are also being wasted, such as during the re-gasification of LNG (liquefied natural gas) at import terminals, which could potentially be recycled to reduce the cost and environmental impact of energy and cooling in buildings.
A report by the Carbon Trust, states that the global demand for cooling could grow to three times the current UK electricity capacity by 2030, due to the world's expanding population and the growing middle class demographic in emerging markets. The report, The Emerging Cold Economy, highlights the need for UK energy policy to address the under-valued cooling sector and for the Government to carry out a full assessment of the opportunities a 'cold economy' could bring.
CES is not yet efficient enough to be rolled out on a large scale, as the system currently has relatively low ‘round-trip’ efficiency when the energy going in is compared to the energy coming out. However, CryoHub hopes to improve CES efficiency by aligning it with the cooling and heating facilities found in industrial refrigeration warehouses and food processing plants. It is hoped that clever design and integration of existing cooling and heating equipment will enable sufficient CES efficiency gains to be made to make the technology market-viable in the near future.