A comprehensive review on pit thermal energy storage: Technical elements, numerical approaches and recent applications

Pit thermal energy storage (PTES) is one of the most promising and affordable thermal storage, which is considered essential for large-scale applications of renewable energies. However, as PTES volume increases to satisfy the seasonal storage objectives, PTES design and application are challenged. These difficulties trigged an interest in PTES investigations. This paper aims to identify the success factors and research gaps of PTES by an up-to-date evaluation of 160 recent publications. Existing technical elements that affect PTES thermal properties inclusive geometry design, inlet/outlet design, cover design, and materials, are outlined in depth. Numerical studies are categorized in terms of their mathematical theory and research purposes for a systematic discussion. The current application status of PTES systems worldwide is summarized from four aspects: storage material, geological design, operation strategy, and storage duration. For projects in operation, special attention is given to gathering and comparing operational data on solar fraction, storage efficiency, storage cycle, and PTES tem-perature. This review outlines the progress and potential directions for PTES design and numerical studies by identifying the research gaps that require further effort.