Performance and cyclic heat behavior of a partially adiabatic Cased-Wellbore Compressed Air Energy Storage system

Although Compressed Air Energy Storage (CAES) is not a new technology, it has not yet been widely adopted due to location restrictions and inefficiencies. Thermal energy storage improves the round-trip efficiency of CAES systems. This study sets out to investigate the cyclic thermal storage behavior of a small-scale, site-flexible, scalable, cased-wellbore compressed air energy storage (CW-CAES) system in which both heat and mechanical energy can be stored in an array of wellbores. The concept of storing high-temperature compressed air (around 200 ° C) inside cased wells is a promising approach to expanding the utility of CAES systems through site flexibility, partial adiabatic efficiency improvements over conventional non-adiabatic CAES, no need for a separate heat storage system as found in Adiabatic CAES (A-CAES) systems, and small (order 1 MW; 10 MWh) to large (order 150 MW; 600 MWh) capacity that can be achieved through the modular nature of multiple wellbores. This paper provides a detailed numerical model coupled with a semi-analytical model to assess the first year operation of PA-CW-CAES. The results reveal that the semi-analytical model yields results in excellent agreement with the numerical model. To improve thermal storage an array of boreholes is considered. Several charge/discharge cycles are used to appraise the system behavior and determine system efficiency. The simulations confirm that the modeled partial adiabatic CW-CAES has a round-trip efficiency of around 40% which is higher than the corresponding diabatic CW-CAES operating at the same conditions, and that this efficiency increases with time the more charge/discharge cycles CW-CAES experiences. • A small-scale partially-adiabatic compressed air energy storage (PA-CAES) system based on a pressurized wellbore is studied. • A cased-wellbore array is utilized as a repository to store both heat and mechanical energy. • The PA-CW-CAES performance is better than that of an equivalent diabatic CW-CAES system. • An algorithm was developed to decrease the computational costs of numerical simulations. • The surrounding rock acts as a passive low-temperature long-term underground TES.