Feasibility study of a high-temperature thermal energy storage system using CO₂ as working fluid in horizontal aquifers

A novel approach to high-temperature aquifer thermal energy storage (ATES) is proposed, wherein CO₂ replaces water as the working fluid to mitigate scaling and plugging risks. The feasibility of this approach is investigated through numerical simulations. A non-isothermal two-phase flow model, integrating the wellbore and reservoir, is developed to simulate the entire process of CO₂ aquifer thermal energy storage (CATES) including CO₂ storage and subsequent heat storage operations. The study assesses the performance of CATES and identifies factors influencing bottom water coning. Results affirm the viability of CATES in horizontal aquifers, demonstrating its safe operation over 20 years under appropriate conditions. Under the baseline scenario, CATES in horizontal aquifers achieves a heat extraction power of 4723.45 kW at a heat extraction flow rate of 30 kg/s, while maintaining an energy efficiency of approximately 80 %. This capacity can provide heating for an estimated area of 183,154 m² of buildings, assuming a heating load of 35 W/m² and a heat pump COP of 3.8. Increasing the extraction rate to 50 kg/s increases the heat transfer capacity to 8274.36 kW but with an increased risk of water production. Notably, bottom water coning poses a significant challenge to the safe operation of CATES, with aquifers characterized by low vertical permeability, high irreducible CO₂ gas saturation, and high porosity exhibiting lower degrees of water coning.