Thermo-economic analysis of geothermal heat pump system integrated with multi-modular water-phase change material tanks for underground space cooling applications

Abstract

The geothermal heat pump system (GHPS) is cascaded with multi-modular water-phase change material (PCM) tanks to cool the underground shelter in the ordinary mode and emergency mode, respectively. Based on the load characteristic of an underground shelter in Beijing, China, the thermo-economic performance of the hybrid system in the two modes is investigated. By utilizing the Taguchi design method, the effect of pipe length, multi-modular water-PCM tank matrix (MMWPM), and cooling water flow ratio of the above-mentioned components on the energy and economic performance are explored and optimized. The result shows that considering 10 years’ operation, 75 % of the base ground heat exchanger (GHE) could guarantee the cooling water temperature within the limited range in the ordinary mode. In the emergency mode, although the buried pipe loop accounts for a small flow rate, e.g., with a value of 10 %, it effectively extends the effective discharging duration (EDD) of the hybrid cooling system by at least 97 %. The average initial cost per unit EDD in the emergency mode is selected as the most important evaluation index in this paper. The order of factors is flow ratio> GHE length> MMWPM. The superior GHE length, MMWPM, and their flow ratio are 2500 m, 3 × 4, and 3:7, respectively. Moreover, the interaction between factors has a descending order of GHE length × Flow ratio> GHE length × MMWPM > MMWPM × flow ratio. With the optimal design of the hybrid system, the initial cost per unit EDD decreased by 24.37 %. This study is expected to serve as a reference for the application of the hybrid system in the underground shelter, meeting the requirement of thermal camouflage and energy-saving requirement.