Balancing demand for energy with timely production is a juggling act that is particularly relevant to renewable sources such as wind and solar. Because the wind isn't always blowing and the sun isn't always shining, the energy produced by these systems needs to be stored efficiently so it can be used when it's needed. While some scientists are looking into storing such energy by converting it to natural gas, Britain's Highview Power Storage has its own approach, which is already in use in a pilot project. In a nutshell, the company is storing excess energy as liquid air.

In Highview's CryoEnergy System (CES), excess energy is used to run refrigeration units which cool air down to a temperature of -196C (-320.8F), at which point it liquifies. The liquid air, also known as cryogen, can be stored in an insulated tank, at an ambient pressure of about 1 bar.

At higher-demand periods, when the direct output of existing energy sources can't meet the needs of the municipal power grid, the liquid air is released into a confined space. The liquid boils as soon as it is heated above -196C, so even room temperatures will superheat it, causing it to regasify and expand in volume by approximately 700 percent. From there, a steam engine effect comes into play, with the high-pressure gas spinning a turbine which in turn powers a generator.

When exposed to ambient air temperatures, the liquid air returns about 50 percent of the energy that went into creating it. If exposed to heated air, however, the phase change from liquid to gas is more intense, resulting in an efficiency of up to 70 percent. If a CES were to be installed at an existing facility where waste heat were already present, the system could use that heat to boost its own efficiency. Conversely, because the only by-product of the system is cold air, it could also be used to provide air conditioning, refrigeration, or even to create more liquid air.

Currently, one of the most common forms of large-scale energy storage is the pumped hydro method. In this system, excess energy is used to pump water from one body of water up to a reservoir at a higher elevation. When power is required, water is released from that reservoir through a dam, spinning turbines as it cascades back down. While that system is more energy-efficient than CES, it is also reportedly more expensive to build and operate, requires a mountainous topography, offers a much lower energy density, and the energy it stores isn't portable. Tanks of liquid air, on the other hand, can be loaded onto a truck and transported to where power is needed – assuming that the energy required to run the truck is less than the amount that is being delivered.

According to a study conducted by consulting firm Frost and Sullivan, the use of batteries for energy storage is inferior to CES, when all criteria are combined. While some types of batteries offer more energy efficiency, they cost about US$4,000 per kilowatt of generating capacity, while CES only costs a quarter of that amount.

A 300-kilowatt pilot demonstrator of the CryoEnergy System has been in use at Scotland's Slough Heat & Power plant for the past nine months, where it has been utilizing the plant's waste heat, and regularly exporting electricity to the national grid. So far, the air has been liquified off-site, but the next phase of the project will integrate a liquifier into the system. The company plans to have a 3.5-megawatt commercial-scale plant operational by late next year, with plans to increase its capacity to 8 to 10 megawatts by early 2014.

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