Among thermo-mechanical storage, LAES is an emerging concept where electricity is stored in the form of liquid air (or nitrogen) at cryogenic temperatures. A schematic of its operating principle is depicted in Figure 1, where three key sub-processes can be highlighted, namely charge, storage and discharge.
During charge, ambient air is first purified, compressed using excess electricity and finally cooled down to reach the liquid phase; liquid air is then stored in near-atmospheric pressure vessels. Despite the cryogenic temperatures (liquefaction temperature for Nitrogen at ambient pressure is -196°C), vacuum or perlite insulation is very effective in limiting boiloff at this stage to only 0.1-0.07% per day.
When discharging, the required electricity is retrieved by pumping, evaporation and expansion of the liquid air stream through a set of turbines, in the power recovery unit (PRU). During the operation of LAES, hot and cold thermal streams are produced, respectively, during air compression (charge) and evaporation (discharge). As illustrated in Fig. 1, and discussed in greater detail later on in the review, such streams can be harnessed and reused within the process itself to improve plant energy efficiency. For this reason, the storage section of LAES typically comprises also thermal energy storage (TES) devices – a hot and a high-grade cold one – in addition to the liquid air tanks.
LAES is a thermo-mechanical storage solution currently near to market and ready to be deployed in real operational environments. LAES exhibits significant advantages with respect to competing solutions: energy density is 1 to 2 orders of magnitude above the alternatives and no site constraints limit its deployment. Because of the cryogenic temperatures of liquid air, the power generation cycle can be driven by largely available heat sources at ambient temperature.
Not only this eliminates the need for combustion and associated carbon emissions, but it also allows the recovery of low-temperature streams such as waste heat within the LAES process. Integration with external sources of heat and/or cold enables energy synergies and symbiosis with other processes, such as industrial sites near the location of LAES process. Underpinned by such compelling features and technical potential, endeavours towards the increase of LAES conversion efficiency – long been identified as a key drawback – and LAES commercialisation have achieved significant milestones in the latest years.
The three components are independently sizeable. LAES cycle produces zero emissions and works with benign materials
Off-peak or excess electricity is used to power an air liquefier to produce liquid air.
The liquid air is stored in a tank(s) at low pressure.
To recover power the liquid air is pumped to high pressure, evaporated and heated. The high pressure gas drives a turbine to generate electricity.
The concept of storing energy by means of liquid air was first proposed in 1977, but experimentally investigated only several years later by Mitsubishi Heavy Industries and Hitachi. A 2.6 MW air-driven Rankine cycle was successfully operated by Kishimoto et al. showing excellent stability characteristics for power generation, while researchers from Hitachi focussed on a layout including a gas combustor and a concrete regenerator to enhance gas liquefaction. Efficiencies as high as 70% were predicted for the system.
Few years later, a joint venture between Highview Power and the University of Leeds, UK, led to the design and construction of the first fully integrated LAES plant in the world. The 350 kW, 2.5 MWh pilot-scale plant was commissioned in 2010 and successfully tested in 2013, when it was relocated to the University of Birmingham for further research and development. This system established a cornerstone for LAES development, stimulating great research interest in the technology.
A further 5 MW, 15 MWh pre-commercial plant by Highview Power was operated in June 2018, leading to the deployment of two LAES 50 MW plants (named CRYOBattery) in the UK and US, recently unveiled from the same company; these will be the first grid-connected LAES plants worldwide. Alongside commercial development, a number of international projects (e.g. the CryoHub project, and the IEA Energy Storage Task 36) have been established to further investigate, characterise and develop LAES technology.
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