We all depend on a stable electricity supply system, but with growth in clean renewable sources, there is a challenge in storing energy when the wind doesn’t blow, or the sun doesn’t shine.
Compressed air energy storage (CAES) and liquid air energy storage (LAES) are two potential solutions to the problem of long-duration energy storage (see box). Both technologies could prove crucial in accelerating the energy transition to intermittent renewables and maintaining the stability of a future energy system. And, according to their promoters, neither process is in competition with each other.
Mark Howitt, CTO at CAES developer Storelectric, explains: “The market is huge. The first phase of the market is a trillion dollars, which is nearly all the variable energy for the world for a fortnight.”
As its name suggests, CAES is a method of storing energy that has been generated using compressed air. Surplus low-price electricity is used to pressurise air. This is then stored underground; generally in salt caverns (see graphic). When electricity is needed, the air is released, heated and expanded in a turbine to generate power. Heat is always generated when air is compressed; so, if this heat can also be stored and used during expansion, then the system’s efficiency improves significantly.
CAES has been proven to work. The first compressed air storage facility was built in Huntorf, Germany in 1978. The 280MW/560MWh (two hours’ duration) plant originally operated as a short duration peaker plant and, in 2007, it was overhauled and refurbished, expanding to 321MW with the same volume of storage. It has a 42% round-trip efficiency and, according to Howitt, it remains profitable.
In 1991, a second CAES plant went live, this time in McIntosh, Alabama. This 110MW/2,860MWh plant provides around 20 hours’ duration, and currently operates at around 54% efficiency.
The efficiencies of the two plants are comparatively low because neither utilise the heat from the compression process, meaning heat needs putting back into the air during expansion. Howitt says the way the two plants work is analogous to the differences between open-cycle and closed-cycle gas turbines: “An open-cycle gas turbine burns gas in a turbine and exhausts hot fumes; modern ones are 50-54% efficient. A closed-cycle gas turbine runs those fumes through a recuperator (a heat exchanger) to generate more electricity, and therefore its efficiency is around 8% higher.
“Traditional CAES uses modified gas turbines, injecting compressed air into them instead of compressing the air; this does not save all the energy of compression, as the injected air is cold whereas the turbine-compressed air is hot; but it does have the benefit of storing significant amounts of energy.”
Times have moved on, and new ideas are being tested. Storelectric is working towards developing a new CAES plant in the Netherlands. It has built on the success of the existing plants and it can offer diabatic and adiabatic CAES technology.
The diabatic technology can be retrofitted to combined cycle gas turbine power stations based near appropriate geologies, to generate a claimed efficiency above 60%. Perhaps more interesting is its adiabatic design, which stores heat created during compression and uses it in the expansion process, significantly increasing a plant’s efficiency.
Howitt won’t go into the technical details about how it intends on storing this heat, saying it’s confidential. But he says the problem has been “partially overcome”, offering predicted efficiencies of between 62-70%. “The bulk of the losses are in the heat of the air that’s compressed; the second law of thermodynamics,” he says. “The challenge is storing this heat. We are still in concept. But there’s very little technical challenge in it.”
LAES, or cryogenic energy storage, relies on excess energy to cool down the air, or nitrogen, to the point that it liquidises. Electricity is used to cool air from the atmosphere to -195°C, where it liquefies. Liquid air takes up one-thousandth of the volume of the gas and can be kept in large vacuum flasks at atmospheric pressure. When needed, it’s pumped into a heat exchanger, turning it back into a gas. The volume and pressure increase drives a turbine and generates electricity.
Highview Power has been operating the world’s first grid-scale LAES plant in Bury since April 2018, providing short-term operating reserves, and support during winter peaks.
Javier Cavada (inset), CEO of Highview Power, says this Pilsworth plant continuously provides the UK power network with reserve, grid balancing, and regulation services and has a capacity of 5MW/15MWh. It provides power to 1,200 homes for a day, based on an average consumption per home of 4.6MWh a year.
Cavada explains cryogenic energy storage has all the benefits of CAES, but none of the limitations: “CAES systems require one of the following: a geographical feature, such as a disused salt cavern, the combustion of a hydrocarbon, or the installation of vast numbers of high-pressure vessels,” he says. “Liquid air energy storage is an easily locatable, highly scalable technology that produces zero emissions. We use established, clean and safe components that store large amount of energy in a dense fluid at low pressure. That means affordable grid-scale energy storage can be put anywhere it is needed.”
Cavada also says that while existing processes in the market are subject to energy loss, its system is no exception, but he claims “real losses are lower than other technologies.” He adds that its engineering team has achieved a round-trip efficiency of more than 60%, and up to around 70%, by utilising waste heat and cold.
“The growth of renewables and the challenges their intermittency bring to the grid are key drivers of the demand for any long-duration energy storage solution,” he says. “Our cryogenic technology can shift all the energy from solar and wind arrays, enabling 100% renewable societies. As such, it is clear the markets targeted are widening week by week.”