Installation of a thermal energy storage site in an abandoned mine in Picardy (France). Part 1: Selection criteria and equipment of the experimental site

As part of the new French law on energy transition, the Demosthene research project is studying the possibility of reusing old abandoned mines to store thermal energy in the Picardy region. The aim is to store the heat required for a small collective unit, which corresponds to a volume of water of 2000–8000 m³, depending on the temperature (from 15 to 70 °C). An inventory shows around 3700 theoretically available sites in this region. These are mostly shallow dry mines, or mines that are partially flooded with around 1 m of water depth. Based on this water depth and an extraction ratio of 75%, the required mine area is approximately 10,000 m². From the 40 sites that have a sufficient surface area, only 1 is naturally flooded, although statistically many others will exist that are currently not known. For this experimental site to be reproducible, the decision was made to select dry mines but with a sufficient area to achieve an artificial flooding device. Theoretically, this represents more than a thousand sites in Picardy. The most interesting one is the old limestone mine of Saint-Maximin, where a sealed basin can be built. Before installing an experimental underground thermal energy storage basin in this site, the thermomechanical and hydrothermal behaviors were modeled. The aim was to optimize the position of the various sensors that will be used to monitor the basin, and to predict the future deformations induced on the walls by the thermal variations. A 100-m³ basin, sealed with a liner, was built and fitted with 18 sensors to measure temperature, humidity and strain. These sensors allow the stored water, the rock walls and the surrounding atmosphere to be monitored. This device must now operate for 6 months, i.e. a complete heating–cooling cycle, and its results will be analyzed.