Thermal performance analysis of coaxial borehole heat exchanger using liquid ammonia

The non-water working fluid is crucial to evaluate and optimize the thermal performance of the coaxial borehole heat exchanger (CBHE). An unsteady heat transfer model is built to evaluate the thermal performance of CBHE using different working fluids. Liquid ammonia, among other eight fluid candidates, is selected as an effective non-water working fluid because of lower injection pressure (5 MPa), a lower pressure loss (2.45 MPa), a better thermal performance (Outlet temperature and thermal power are 0.9 degrees C and 27.17 kW higher than that of water, respectively), a higher coefficient of performance (7.34) and a low cost than other fluid. Moreover, when a double-layered insulation pipe structure insulates 88.9% of the top of the central tube, the thermal power can be 8.41 times that of an uninsulated case. A high flow rate (23 m3/h) can provide a thermal power 15.35 times that of the 1 m3/h flow rate, but with an extra pressure loss of 3.38 MPa. Moreover, the effect of cement sheath, pipe size, injection temperature, injection pressure, and periodic shut-in operation is analyzed and optimized by analyzing the engineering and economic factors. This paper provides a theoretical basis for the future application of CBHE.