Reuse of abandoned underground structures - the compressed air energy storage test plant in Switzerland

The storage of compressed air as an energy vector is a promising option to balance the unsteady electricity production caused by the growing share of renewable energy sources such as wind and solar photovoltaic. One of the main challenges, however, is the enhancement of the efficiency of the process, by utilizing the heat produced during compression of air. Another important factor is the availability of safe storage volume for air at pressures reaching 70 to 100 bar. In Ticino, a research program exploits a decommissioned tunnel of the AlpTransit project for exploring the possibility of high pressure air storage in metamorphic rock of the Alpine arch (Leventina Gneiss). At the same time, the recuperation of heat generated during the compression process through storage in a packed bed of gravel is evaluated. The ultimate scope of the project is the assessment of the possibility to use large, existing, under-ground facilities for the purpose of energy storage, using technologies well known in under-ground construction, and new developments tested in high temperature concentrated solar energy plants. First results of the transformation of the AlpTransit Gotthard drift and of the CAES test are presented.