Large scale underground seasonal thermal energy storage in China

Underground seasonal thermal energy storage (USTES) facilitates the efficient utilization of renewable energy sources and energy conservation. USTES can effectively solve the mismatching characteristics of renewable energy heating system in terms of time, space and strength, which can transfer the renewable energy heating from the summer or transition seasons to the winter, and overcome the instability and low efficiency of the short-term thermal storage system. This research describes the technical classification and operating principle of USTES, reviews the state-of-art of USTES worldwide, and also predicts the development trends of the technology. It reveals that USTES has significant economic, social and environmental benefits. However large heat loss and low solar fraction are still the common challenges for large-scale applications. More work should be carried out on fundamental research including overall design and parameter matching optimization, heat loss mechanism, heat and moisture transfer properties. Moreover, the characteristic and performance of unsteady and transient heat transfer in complex underground environment, and their control strategies of the USTES have been also the most pressing problems. In addition, the supporting policy should be strengthened in planning formulation, legislative support, operation supervision, standard formulation and incentive measures to guide an orderly, healthy and sustainable development of USTES.