Chill until needed (Image: Jez Coulson/Highview)

Editorial: “A tankful of sun is what the renewables mix needs“

STANDING in a container full of pipes and valves, wearing a hard hat and sturdy boots, we gaze at a dull grey panel with green and red on and off switches.

“This panel means we’re connected to the grid,” says my companion Rob Morgan, grinning proudly. “To an engineer, this is really exciting.”


We are on an industrial estate in Slough in the UK, on the grounds of a 100-megawatt biomass plant owned by energy firm Scottish and Southern. But what we’ve come to see is the small cluster of containers and a gleaming white liquid nitrogen tank tucked away in one corner of the site. Here Morgan, chief engineer at Highview Power Storage, London, and his colleagues have been running a pilot plant designed to store potential energy in the form of liquid air.

Until recently, the only way grid operators could store energy was in huge hydropower reservoirs. In future, renewable intermittent generators of electricity will form a larger part of our energy mix. So we are going to need ways of storing the power they produce for use in the hours or even days when the sun isn’t shining or the wind isn’t blowing. Since we cannot build a huge new reservoir near every large town and city, more compact storage systems are key to the future of green power.

This is why Highview has been testing its 300-kilowatt pilot plant for the past nine months, supplying electricity to the UK’s National Grid. The process stores excess energy at times of low demand by using it to cool air to around -190 °C. Excess electricity powers refrigerators that chill the air, and the resulting liquid air, or cryogen, is then stored in a tank at ambient pressure (1 bar). When electricity is needed, the cryogen is subjected to a pressure of 70 bars and warmed in a heat exchanger. This produces a high-pressure gas that drives a turbine to generate electricity. The cold air emerging from the turbine is captured and reused to make more cryogen. Using ambient heat to warm it, the process recovers around 50 per cent of the electricity that is fed in, says Highview’s chief executive Gareth Brett. The efficiency rises to around 70 per cent if you harness waste heat from a nearby industrial or power plant to heat the cryogen to a higher than ambient temperature, which increases the turbine’s force, he says.

Unlike pumped-storage hydropower, which requires large reservoirs, the cryogen plants can be located anywhere, says Brett. Batteries under development in Japan have efficiencies of around 80 to 90 per cent, but cost around $4000 per kilowatt of generating capacity. Cryogenic storage would cost just $1000 per kilowatt because it requires fewer expensive materials, claims Brett.

“Lower costs are always better for energy storage, even if it comes at the price of slightly reduced efficiency,” says Aidan Rhodes at the UK Energy Research Centre in London. Highview has so far been receiving cryogen from an external source and using it to store and produce electricity. But the firm has recently added an on-site liquefaction plant, and will begin producing its own cryogen from late March. It plans to build a 3.5-megawatt, commercial-scale system by late 2012, which will be increased to an 8 to 10 megawatt plant by early 2014.

The sponge that soaks up air Another way of storing excess electrical energy is as compressed air. Any unwanted energy is used to compress air, which is stored in tanks. When demand for electricity rises, the compressed air is released and used to drive a gas turbine. However, it is prohibitively expensive to keep the tanks of pressurised air safely above ground so the technology can only be used underground. Timothy Havel at Energy Compression, in Boston, Massachusetts, is using compounds called zeolites to store compressed air at a lower pressure. Zeolites have a honeycomb structure with micropores that trap air molecules when the material is cooled, and release it again when heated. “The material is like a sponge for air,” says Havel.