When it comes to storing energy on the grid, giant batteries are the only game in town. Now, a number of companies are building mechanical systems that use air as the storage medium.
U.K.-based Highview Power Storage last week said that it has been awarded an £8 million grant from the U.K. Department of Energy and Climate Change to build a commercial-scale facility that uses liquified air to store energy. Highview is already running a smaller pilot plant, but the full-scale version will be able to store enough energy to deliver five megawatts of power for three hours. That puts it on a scale that would entice utilities to use the technology, says company CEO Gareth Brett.
"We're treating this (demonstration plant) as a shop window on the technology," he says. "Utility companies are pretty conservative and they want to see this bit of kit working at a scale that they can buy one."
Liquid air energy storage is similar to compressed air energy storage in that air is compressed and released to store and then generate power. With Highview’s technology, though, ambient air is compressed, then cooled and liquified. That liquefied air, which is almost -200 °C, is stored in large tanks.
When power is needed, the liquid air is released and pumped to high pressure. That causes the liquid to evaporate, turning it into a high-pressure gas which is then run through a turbine to generate power. The planned demonstration plant will be located at a waste processing center. Heat from the waste plant’s gas turbines, which run on captured landfill methane, will be piped in to improve the efficiency of the evaporation process.
One of the advantages of liquid air storage is that it uses off-the-shelf equipment. The tanks for storing liquid air, for instance, are the same as those used in the industrial gas industry. Highview’s expertise is in engineering the different components into a working system with the highest possible efficiency. “Getting the supply chain right is really what our technology is all about. What we’re trying to do is get a system to work with widely available kit,” Brett says.
This commercial-scale plant also gives an indication of how much liquid-air energy storage costs. For 15 megawatt-hours of storage, it will cost about £533 (about $900) per kilowatt-hour. But Brett projects the economies of scale from a larger plant would allow Brightview to get the cost under $500 per kilowatt-hour. At that price, energy storage on the grid can be cost competitive with power plants for a number of applications, such as storing wind and solar energy for delivery during peak hours, say experts.
Highview’s plant will be used to relieve congestion on the grid. For example, stored energy can supply power to the local distribution grid when substations are maxed out during peak hours.
The U.K. has emerged as a leader in advancing liquid air storage technologies. Last year, the Center for Low Carbon Futures released a report that identified liquefied air and liquid nitrogen as a valuable “vector” of research for storing energy for both the grid and transportation. And last year, the University of Birmingham won a £6 million grant from the U.K. government to establish a Center for Cryogenic Energy Storage.
Although liquid-air energy storage is a relative novelty in the utility industry, storing energy in the form of compressed air has a long track record. Two compressed air plants—one in Huntorf, Germany, and one in Alabama in the United States—have been operating for decades. Though the technology is considered reliable and relatively low-cost, these two plants use underground caverns to store pressurized air.
A few companies are now developing systems to store compressed air in above-ground tanks or pipelines, which makes siting easier. Because liquefied air is stored in tanks, Highview facilities can also be located in a wide range of industrial sites, Brett says. And because liquid air is four times as dense as compressed air, its tanks take up less space than those holding compressed air; liquid-air storage is also cheaper because the medium can be contained in less-expensive, low-pressure tanks, Brett says.
Still, Highview Power faces a number of competitors, including many battery makers that hope to slash the cost of storage. One of the biggest advantages Highview has is its potential to scale up quickly. Because it’s using off-the-shelf equipment, it doesn’t need to build a new plant or develop new manufacturing methods as new battery companies may need to. In theory, that means, if it has the orders, it can build and engineer storage plants without having to invest in its own production facilities.
Its large-scale demonstration site is projected to go online next year and, if utilities like what they see, the technology’s next step is a commercial plant.
Image: Highview Power Storage