Carbon Commentary…The economics of this technology look interesting. What is even more compelling is that you could bolt together a large plant using conventional components freely available today from a variety of major suppliers. Unlike some of the really wacky suggestions for storing energy, we pretty much know that Highview’s ideas will work. A 350 kW pilot plant alongside the Slough power station has been through extensive testing for the last six months or so.
So how does it operate? You take ambient air and put it through a liquefaction plant using electricity. (Hundreds of these plants around the world today make liquid nitrogen, oxygen or natural gas). Liquefaction works by expanding a gas, which causes its pressure, and thus its temperature to fall. This technology is a hundred years old. The process uses substantial amounts of energy.
Allowing liquid air to expand increases its volume many hundred fold. This will produce high pressure in any sealed container. If the gaseous air is allowed to escape through a turbine, electricity can be generated. This second phase produces about 55% of the input energy, says Highview. This relatively low number can be improved to perhaps 70% by using waste heat from nearby industrial processes, such as the hot water from the cooling processes in a nuclear or fossil fuel power station.
How does efficiency this compare? Here are some very rough figures for other means of storing electricity.
| Pumped hydro | 70% | Water is pumped uphill to a reservoir. When electricity is needed it flows through turbines back into the lower reservoir |
| Lithium ion batteries | 80% | Lithium ion cells are used in electric cars and electronics. They are still expensive and have limited life |
| Compressed air | 60% but perhaps more | Spare electricity drives a compressor. The air is stored at high pressure in deep caves. When released it drives an air turbine. |
| Hydrogen | 40% | Electrolysis uses electricity to make H2 and O2 from water. Hydrogen in a fuel cell generates electricity. |
(A previous article on Carbon Commentary assessed the economics of using stored hydrogen for electricity production).
The huge advantage of Highview’s plant, if it works as planned, is that each of the main alternative storage technologies have intrinsic problems. Hydrogen is inefficient and the equipment is expensive. Compressed air requires large amounts of storage. Lithium batteries are expensive and don’t like being discharged too often. (Other battery systems are less problematic but they have other disadvantages). Pumping water uphill is cheap and well-understood. There just aren’t many places where it can be done economically.
Highview quoted me a figure of £1,000 per kilowatt of output power. Let’s be clear about what this means. The Slough pilot plant can produce 350 kW of electricity. So the cost of a commercial plant would be about £350,000. (The cost of the pilot was much greater, of course). The Slough kit can deliver about 2.5 megawatt hours when fully charged. That is, it can work for seven or eight hours at full power. If it can be achieved, £1,000 per kilowatt of electric power is highly competitive with most other storage technologies, particularly since operating costs are so low. A large pumped hydro plant would be comparable, but hydrogen could be four or five times as expensive.
Build an air liquefaction plant and expansion plant to Highview’s designs and what do you get?
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