A novel radiative sky cooling-assisted ground-coupled heat exchanger system to improve thermal and energy efficiency for buildings in hot and humid regions

Abstract

Thermal efficiency degradation of the ground heat exchanger (GHE), caused by the imbalance between heat extraction and rejection, is a severe issue that largely restrains GHE from wider application in hot and humid regions. This study proposed a novel hybrid ground-coupled heat exchanger (HGCHE) system by combining the coaxial ground heat exchangers (GHE) with radiative sky cooling radiators (RSCR) to overcome the heat imbalance so as to improve the thermal efficiency of coaxial GHE. Also, the RSCR, providing sub-ambient cooling under sunlight without extra energy input, was developed as the heat rejector, with consideration of the spectral selectivity of the coated radiative cooling material. Moreover, a case study on a building in Hong Kong was conducted, and the cooling capacity of the RSCR assisted-GCHE system was investigated. The result shows that the RSCR can achieve daytime sky cooling, and the yearly accumulated daytime cooling power can reach up to 127.5 kWh/m2, 31.0% of the total cooling power in the first year. The RSCR has the best cooling performance in January, due to the low solar radiation, ambient air temperature and precipitable water vapor. The thermal efficiency of the RSCR assisted-GCHE system can be improved by 6.6% compared with the GCHE without a heat rejector. The cooling capacity of coaxial GHE is proportional to the mass flow rate and borehole depth, neglecting the ground thermal gradient. The findings of this proposed HGCHE system could provide useful engineering guidance for integrating passive radiative cooling technology into space cooling, serving for low-energy buildings, especially in hot and humid regions.