A grid-connected pilot energy storage system using liquid air can be scaled to GWh volumes, according to its developers Highview Energy Storage. The pilot plant has been in operation on an industrial estate to the West of London, since July 2011.
The capital costs of the Cryogenic Energy System are relatively low, according to Highview, because the system uses standard components that are commercially available – such as low-pressure tanks, liquefiers, compressors, power turbines and generators. Because all components have been proven in industrial use over many years, the costs of a system are predictable, say the developers, who have been working with Leeds University on Cryogenic Energy Storage since 2005. The system can return up to 70% of the electricity it draws from the grid (AC to AC).
How it works
Excess electricity is drawn from the grid and used to freeze air (the pilot plant actually uses nitrogen) until it reaches its liquid state at -196ºC. The liquid is pumped into insulated tanks for storage.
Liquid air and other cryogenic fluids are hundreds of times more energy-dense than water and therefore require far less space than hydro storage. The energy density of fluids in cryogenic storage is comparable to compressed air. But liquid air offers a major advantage. It can be stored at low pressure. A 2,000 tonne tank of liquid air stores roughly the equivalent of 200 MWhs of energy. A 100,000 tonne tank would be enough for 10 GWhs.
When there is demand for electricity, the liquid air is pumped from the tanks to fuel a cryogenic turbine that drives a generator. Like a steam turbine, the four-stage cryogenic turbine is driven by expansion during a liquid-to-gas phase-change. Liquid air expands about 700 times when moving between the two states. The power available can be maximised by increasing the pressure and superheating the liquid air with ambient heat before it enters the turbine. The Slough pilot plant adds waste heat at 115ºC produced by an adjacent biomass plant.
The exhaust from the cryogenic turbine – a stream of very cold air - is fed back to halve the energy needs of the air liquefaction process. If waste heat is added to the cycle, the system returns around 70% of the electricity that it originally drew out.
The price of a plant will depend on configuration because the three main system components – liquefier, storage tank and turbine – can be sized independently to suit the application. Highview’s Chief Operating Officer, Toby Peters, estimated in a recent interview with Plant Engineer magazine that a system configured for daily cycling will cost early adopers around £2,000 (US $3,000) per kW, reducing to £1,000 ($1,500) or less when the plant is mature.
Highview are looking for partners who can help them deploy the technology at utility scale. They are already in talks with companies interested in full-scale plants and they are planning a system with a 3.5MW power generator that is due to go into operation by the end of 2012.
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