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Concentration of CO2 in the Atmosphere

Liquid Air Energy Storage in Vermont

Cryogenic energy storage system. Image: Highview Power.

George Harvey

Two energy companies announced that they will co-develop a highly unusual energy-storage project in Vermont. It will be the first commercial cryogenic energy-storage system in the United States. It will use air that has been cooled to the point of being liquid to store energy.

A cryogenic system is similar to a steam engine that recycles its steam. Both systems require energy to make them work, and both produce energy. In a steam system, energy is used to boil water, and condensation requires only cooling off in a heat exchanger, releasing heat to the atmosphere. In the liquid air system, energy is used to compress the air, which causes it to liquefy, and the boiling is done by drawing heat from the atmosphere in a heat exchanger.

Liquid air is held in highly insulated tanks, at a temperature of -196°C (-320°F). At this temperature, it boils very slowly, and the boiling removes heat. In a very efficient cryogenic system, very little of the liquid is lost to keep the rest cold.

Compressing the air takes a fair amount of energy, but boiling liquid air releases energy in the same sort of way boiling water does. The liquid air expands by a factor of 700, as it becomes a gas, and that can drive a turbine.

Much of the theory behind cryogenics has been known since the 19th century. Of course, turning theory into reality is often not terribly easy. Highview Power, a British company that is partnering with Encore Renewable Energy (ERE) for the Vermont storage project, has been pursuing the business of cryogenic energy-storage since 2005. After laboratory work and patent development, it produced a 350-kilowatt pilot project in the UK. Next, with support from the British government, it built a larger project of 5 megawatts (MW). Last October, it built a 50-MW project that could provide 250 megawatt-hours (MWh) of electricity. The project to be built in northern Vermont is of similar power capacity, 50 MW, but with more energy storage, at 400 MWh.

We should note that these projects are both technologically uncomplicated and rather small compared with lithium-ion batteries of similar power capacity, so they do not need much land. They have the same advantage over pumped storage, with the added benefit that they do not have much potential effect on wildlife. Highview Power says they are not expensive. They are very clean and can be powered without any need for fossil fuels.

The decision to build the plant in Vermont solves a problem that exists in the northern part of the state with an old and limited electric transmission system. As the system goes through regular use, with changes in supply and demand of electric power, local power lines can be asked to deliver more electric power than they were designed to conduct. In such a case, one or more tricks have to be employed to reduce the load. In a worst case, the supply may have to be cut off. The flip side of this is that if too much local power is produced, whether from a wind farm or net-metered solar panels on rooftops, that power has to be curtailed; even if it can be produced, it will not make it to the grid. By putting storage into a place with a limited power grid, it becomes possible to deal with both excess power and excess demand locally. Using the battery means that electricity that would otherwise be lost can be saved for use when it is needed.

Another advantage to local storage is that it provides emergency power support in situations where the grid would otherwise fail altogether. The cryogenic system in northern Vermont could wind up being important, as more extreme weather comes with climate change.

Highview Power’s website is www.highviewpower.com. ERE’s website is encorerenewableenergy.com.

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