How CAES works
Storing energy in compressed air has been around for decades, and has been used world-wide in many systems.
When power is abundant and demand is low, it’s drawn from the grid to compress air into a salt cavern hundreds of metres underground. When the power is needed, the air is let out from the cavern to regenerate the electricity.
Salt caverns have been used for many decades to store natural gas because not only are they naturally hermetic, but also salt undergoes plastic flow under pressure, so it seals any cracks that may occur. And what is safe for gas is safer for air.
There are salt basins globally, in which caverns can be made cheaply and easily.
Existing CAES Plants
Huntorf, Lower Saxony, Germany
Compressed air energy storage (CAES) has been in operation since 1978 in Huntorf in Germany, and since 1992 in McIntosh, Alabama, USA. Both of these plants regenerate the electricity by feeding it into a gas-fired power station, roughly tripling the efficiency of the power station.
When compressing the air, by the laws of thermodynamics it heats up. When expanding it again, that heat needs putting back into the air. Both Huntorf and McIntosh waste that heat, leading to only 42% overall efficiency in Huntorf and 54% in McIntosh (the difference being because McIntosh's power station is Combined Cycle).
McIntosh, Alabama, USA
Huntorf and McIntosh have proved useful additions to their local grids, in operation daily. Storelectric has another technology, more closely resembling these two plants, which can be retro-fitted to CCGT power stations that are near appropriate geologies, which burns gas and has an efficiency expected to be above 60%
There are three adiabatic CAES projects under way in America since 1999: General Compression, Lightsail and SustainX. Despite combined funding of $250m, since then they have only built just over 2MW in combined demonstrator capacity, with significant technical, up-scaling and down-time issues. Within 2-3 years of funding, we will construct a 40MW, 1GWh pilot plant that will by then be tradingprofitably, at a fraction of the cost.
The CARES PCI project will use a development of these technologies, Thermal Energy Storage (TES) CAES, licensed from TES CAES Technology Limited.
In contrast to the existing CAES Plants, TES CAES is adiabatic. This means that it seeks to balance the heat throughout the plant. Thus it uses the heat stored during compression (which the existing plants do not utilise) to heat the air during expansion / generation. Emissions are zero and round trip efficiencies are 60-70% depending on plant size.
TES CAES plants are built entirely of known subsystems, proven at comparable scales and load cases. As with most large rotating equipment, they become more efficient with increasing size, unlike batteries which become less efficient due to the increased requirement for cooling.
A CAES plant can deliver into various revenue streams including:
It can also deliver additional benefits for infrastructure deferrals and replacements and further grid services such as:
A 500+40MW adiabatic Compressed Air Renewable Energy Storage project in Cheshire, UK, providing large scale and long duration (5 hours) energy storage efficiently and without emissions
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