Solar thermal energy and CO₂ storage in saline aquifers for renewable energy space heating

There are a large number of abandoned oil and gas wells and corresponding depleted oil/gas reservoirs (DOGR) throughout the world, which are recognized as one of the best geological formations of saline aquifers for thermal energy storage and CO₂ sequestration and however, the lack of innovative technique limits the efficient use of underground storage space. Here a novel scheme of storing solar thermal energy and concomitantly sequestrating CO₂ in DOGR for renewable energy heating is proposed, which uses CO₂ as thermal energy storage working fluid. The process of thermal energy storage and release as well as CO₂ sequestration in DOGR are simulated, and thermal performance and CO₂ dissolution in DOGR are analyzed. The results for twenty years of operation show that the thermal recovery efficiency reaches up to 82.93 %, which is 76 % higher than the traditional high temperature aquifer thermal energy storage. Energy storage capacity per heating season, energy storage density per unit volume of underground space and energy efficiency are respectively 57,222.27 GJ, 22,943.01 kJ/m³ and 96.19 %. Furthermore, the dissolved CO₂ during thermal energy storage and extraction is 54.76 % larger than that of mere CO₂ sequestration due to the repeated expansion and contraction of gas and water interface caused by the periodically injection and extraction of CO₂, indicating that energy storage accelerates CO₂ sequestration. To sum up, the new scheme is a high value-added technical route for solar thermal energy storage and CO₂ sequestration as well as renewable energy heating.