Cloudless summer days are when solar systems run at full capacity. Unfortunately, in such weather, most of us are more likely to be found enjoying ourselves at the pool than at home cooking, washing or seeing to other business that requires electricity. As at such times lighting and heating are not an issue either, the question is: what to do with excess solar power? One possibility is to store it in compressed air energy storage (CAES) systems with heat recovery, by which excess electricity from solar energy or other renewable energy sources is used to compress air underground, in places such as disused tunnels. Once power is in higher demand again, the compressed air is released from the storage system to power turbines and produce electricity.
Such CAES plants are already in operation in Germany and the USA. However, compared to pumped storage power stations, which are the most widespread method of storing electricity worldwide, their inefficiency is a major disadvantage. This is due to the fact that air compression generates unused heat, reaching approximately 500 degrees Celsius. During subsequent electricity production, the air expands and cools down, impairing turbine performance and power generation, which is why the temperature of the air must first be increased. This again requires energy, most of which comes from fossil fuels. This procedure is neither energetically efficient nor particularly environmentally friendly.
Researchers working at the Swiss Federal Institute of Technology (ETH) in Zurich, the Ecole Polytechnique Fédérale de Lausanne (EPFL), the Scuola universitaria professionale della Svizzera italiana (SUPSI), and the Paul Scherrer Institute (PSI) have therefore built a pilot compressed air storage energy system in which the heat does not escape unused but is recovered. The scientists were thus able to increase the efficiency of the storage system from between 45 and 50 % to between 65 and 75 %. The pilot plant is located in a disused tunnel in Ticino. Air is pressed into the mountain and sealed in by five-meter-thick concrete walls and a steel door. The researchers of the joint project "Compressed air energy storage with heat recovery" were able to exclude leaks in the system.