Scientists from all across the European Union are working on what could be the next large-scale energy storage option to combine with variable renewable energy technologies to increase their efficiency — air.
According to researchers from SINTEF, Norway’s largest independent research organisation, storing compressed air in sealed tunnels and mines as an alternative to the more cheaper, though regionally restrictive method of pumped hydropower energy storage. Compressed air storage is in no way a “new” concept, but scientists from all over Europe, working under the auspices of the RICAS 2020 research project, are investigating the possibility of storing large amounts of compressed air in disused caverns and tunnels as large-scale storage sites.
The existing premise is already in use in some areas of the world, and essentially uses surplus electric power to compress air, which is stored underground, only to be released through a gas turbine that generates electricity when needed. Such energy storage plants help meet peak electricity demand.
“The more of the heat of compression that the air has retained when it is released from the store, the more work it can perform as it passes through the gas turbine,” said Giovanni Perillo, project manager for SINTEF’s contribution to RICAS 2020. “And we think that we will be able to conserve more of that heat than current storage technology can, thus increasing the net efficiency of the storage facilities.”
Currently, one of the problems with this technology — as experienced at two of the largest compressed air stores in the world, in Germany and the US — is the loss of potential energy through the compression stage, as there is no means to store the heat produced. This contributes to the fact that existing sites lose around 45 to 55% of the produced energy during the compression process. Participants in the RICAS 2020 project have developed a means to minimize heat energy losses in future underground storage caverns — essentially adding an additional station in the final solution:
- On its way down to the underground cavern, the hot compressed air passes through a separate cavern filled with crushed rock.
- The hot air heats up the rock, which retains a large proportion of the heat.
- The cold air is stored in the main cavern
- When the air subsequently returns through the crushed rock on its way to be used to generate electricity, the flow of air is reheated by the stones.
- Hot air is then expanded through the turbine generating electricity
Energy storage technologies such as pumped hydropower and compressed air are significantly cheaper than battery storage, and the new SINTEF-developed solution could raise the efficiency of the system by as much as 70 to 80%.
“The project is based on the belief that our solution will offer better energy storage than batteries can ever provide, thanks to its longer lifetime and lower capital cost per kWh of stored energy,” added Perillo. “We also expect that it can be employed virtually irrespective of the type of geological formation available.”
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