Utility Week

UTILITY WEEK | JUNE 2021 | 15 Speical report on storage can be stored safely. This heat must then be returned to the air or replaced before expansion, otherwise the sub- sequent cooling would freeze the turbine. Tallat Azad, managing director of Storelectric, says his company is looking at three different solutions to this issue. The first is to reheat the air using the exhaust from a gas turbine fuelled by either methane, hydrogen or a mix of the two. The air is then used to drive a secondary turbine, which together with the first form a combined- cycle system: "The flue gas that goes to create steam in a conventional combined-cycle gas turbine, we would actually use that to heat the air from the cavern." Azad claims this solution, which Storelectric has branded as "H2 CAES" but is more commonly known as diabatic CAES, is expected to have an efficiency of 56 to 60 per cent, with this figure representing the output of both turbines against the electricity used to power the compressor and the energy content of the gas used as fuel. The second is to store the heat extracted from the air and then return it to the air prior to expansion. Azad says this solution, which he calls "green CAES" but is gener- ally referred to as adiabatic CAES, is expected to have an efficiency of 65 to 70 per cent. The third solution combines the previous two together into one system. When utilising both, Azad says this "hybrid CAES" is expected to have an even higher effi- ciency of 75 to 80 per cent. Two diabatic CAES plants are already operating in Germany and the US, which were commissioned in 1979 and 1991 respectively. Azad says they differ from the solu- tions proposed by Storelectric in that the compressed air is fed into specially modified gas turbines that have had parts such as their compressor blades removed and heated upon combustion of the fuel. Because the tur- bines do not have to compress air themselves prior to combustion, this increases their efficiency, thereby har- nessing the stored energy. Azad says his company's solutions not only offer significantly higher efficiencies but also allow it to use unmodified gas turbines, which they can continue to run when stores of compressed air are depleted: "The con- cept is already proven. All we're doing is making it better – a lot of more flexible – by using standard equipment." Depending on their size, each plant would be con- nected to one or more salt cavern ranging in size from 100,000 to 500,000 cubic metres. In the case of the hydrogen and hybrid solutions, one of these caverns could be used to store hydrogen, possibly supplied by co-located electrolysers. Azad says a cavern of 350,000 cubic metres could store enough compressed air to gen- erate around 250 to 300MWh of electricity and enough hydrogen to generate 30 to 40 times that amount. He says the company is currently looking at four pos- sible locations – Cheshire, Fleetwood, Humberside and Teesside: "The Cheshire one is probably the most likely one for our first plant right now," he adds. "We're in the process of trying to secure a lease agreement with EDF, which has leased the caverns off Cheshire Salt. "We're looking at four caverns right now but there are another four caverns right next door to it and for us to be able to combine those together is quite easily done." Thermal energy storage Liquid air energy storage (LAES) Liquid air energy storage essentially works the same way as adiabatic CAES, except the air is not only compressed but refrigerated until it becomes a liquid. Javier Cavada, chief executive of Highview Power, says liquefied air is 700 times smaller in volume and can be stored at atmospheric pressure in tanks. He says the technology is "piggy backing" on the oil and gas industry, using "engineering processes that have existed for decades" and standard "off-the-shelf " equipment, including the tanks, which are already used to store liq- uefied gas. He says the efficiency is determined by "the losses of heat and cold across the system, the pumping of the fluids and gas and the rotating equipment", and range between 50 and 70 per cent, depending on the exact configuration. Unlike pumped hydro or CAES, liquid air energy stor- age is not dependent on geography and can be built anywhere. Highview opened a 5MW/15MWh demonstration project at Bury in Greater Manchester in 2018. Cavada described it as a "complete success", proving the com- mercial viability of the technology. The company is now developing its first commercial plant – a 50MW/250MWh facility – at Carrington on the other side of the city, which it expects to start operating next summer. Cavada claims LAES is the "best technology at scale" because additional storage can easily be added at a later date by installing more tanks, with a "negligible impact" on overall costs. While doubling the generation capacity of the plant to 100MW would raise the price tag by 40 per cent, doubling the storage capacity to 500MWh would only add 10 to 15 per cent: "The reason is the tanks are the least expensive part of the system." He believes LAES offers a much better way of filling long gaps in renewable generation than hydrogen – the other main option being considered: "Hydrogen is a very clean gas, but it is difficult to produce, it's difficult to manage. It's the opposite to air. Air is easy to find, air is very easy to store, air is totally inert." Importantly, he says it is a technology that is ready today. "For grid stability and grid services we cannot wait those decades that are needed," he says. Sensible heat storage Sensible heat refers to energy that elicits a change in the temperature of a substance and is already commonplace in the form of hot water tanks in homes. David White, business development director at Mixergy, says his compnay is seeking to make this storage more efficient and more useful by making it smarter. White explains that most hot water tanks have a heat exchanger at the bottom, where the coldest water in the tank sits: "But with that comes some penalties for the end user. One of them is if you only need a small amount of hot water, you can't generate it. You have to heat everything. Mixergy's tanks instead have a heat exchanger at the top, allowing just the upper layer of water to be heated. A continued overleaf

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