Adiabatic and isothermal CAES are two different processes which sometimes get confused. The way each process treats heat during the compression and expansion processes is very different. Storelectric develops adiabatic CAES as the isothermal process often can depend on on fossil fuels, or other external fuels sources.
What is CAES?
Compressed Air Energy Storage (CAES) uses excess or cheap energy from renewables to compress air to high pressures of around 70 bar. When demand for energy is high, the air is released to power a turbine which re-generates the electricity.
Because compressed air is not very energy-dense, we require large volumes of it and therefore geological storage is used. Existing CAES uses salt caverns, which are man-made within salt basins and can be found world-wide. CAES is a well-established technology currently used for natural gas storage and to store other hydrocarbons, hazardous waste etc.
Salt caverns are safe and can withstand the pressure of the gases they store. Almost 1/3 of Europe’s natural gas stocks are in salt caverns, any none of them have collapsed.
The Issue with Traditional CAES
It is impossible to compress air without increasing its temperature. By putting air into a smaller space, the particles get closer together and collide with each other and therefore become hotter. When air is compressed to 70 bar, it can heat up to levels of approximately 650°C depending on volume.
This causes a problem: air cannot be stored above 42°C in salt caverns, or the cavern will deteriorate. Therefore to keep temperatures low, traditional CAES wastes the excess heat in a cooling tower.
However, the air has to be expanded to a lower pressure to generate electricity. As particles move away from each other in this expansion process, the air cools to about -150°C, which risks freezing and damaging the environment and equipment. Therefore, the heat needs to be put back in and traditional CAES does this by burning gas.
The method existing facilities in Huntorf, Germany and McIntosh, Alabama use is to feed the compressed air into a gas turbine, making the turbine more fuel efficient. But this method burns 50-60% of the gas of an equivalent sized power station. Because the expansion is through specially modified turbines, traditional CAES is only available in fixed sizes.
What is Isothermal CAES?
Companies such as Lightsail, SustainX, and General Compression, realised the most efficient way to compress air is at a constant, low temperature. So, instead of removing all the heat after compression, they invented novel compressors that extract the heat during compression to keep the temperature at a constant 40°C. Therefore, the air is maintained at the same temperature throughout the system. This is why it is called ‘isothermal’; ‘iso’ means ‘same’, and ‘thermal’ means ‘heat’.
However, whilst it is efficient, isothermal CAES has a similar problem to traditional CAES, as the extracted heat is wasted up a chimney, and not re-used within the system. More heat is required during the expansion process.
The companies claim to solve the expansion problem by scavenging heat from external sources and not directly from hydrocarbons. The 40°C temperature is low enough for heat pumps or waste heat that industry could provide it. But this would not cover the sheer quantity of heat needed if this were implemented at scale, other than in special locations, e.g. using waste heat from a smelter.
Additionally, the novel expanders have not been perfected; and the novelty of the compressors does not maximise efficiency, cost-effectiveness or reliability.
What is Adiabatic CAES?
If process is ‘adiabatic’, heat energy does not enter or leave it at any stage—it is a closed system. Adiabatic CAES technologies balance heat over the whole cycle of compression and expansion to ensure no energy is wasted. After Storelectric’s CAES extracts the heat during compression, it is stored separately then re-used during expansion. This means the technology does not require fossil fuels or other external fuel sources. Storelectric’s technology eliminates emissions from the CAES process, making it more sustainable.
Storelectric’s proprietary adiabatic technology is highly efficient (~62% at 40MW, rising to ~67% at 500MW), buildable with existing technologies, cost-effective and validated by numerous multinational engineering companies including Costain, Fortum, Siemens and Mott MacDonald. Because it uses “off-the-shelf” compressors and expanders, it is reliable and can be built with almost any size compressors and expanders available.
The principles of adiabatic CAES are shown in RWE’s defunct Adele proposal which posited to store the heat of compression in ceramic storage. This storage is riddled with capillaries to diffuse the heat through ceramic brick. This technology is effectively two night-storage heaters, each the size of a tower block, but as they expand and contract with the temperature changes they would rub themselves to dust (thereby clogging any channels air can enter) and crushing the capillaries. This would lead to very high maintenance costs and frequent long outages to re-build the storage. Building and insulating such vessels would be prohibitively expensive.
More on the theory behind adiabatic and isothermal CAES can be read here.
