STUDY OF BOREHOLE HEAT EXCHANGER HEAT TRANSFER ENHANCEMENT PARAMETERS

Abstract

This paper discusses the study of parameters for improving the heat transfer of a borehole heat exchanger for a ground source heat pump application. The study of efficiency parameters was carried out based on an experimental prototype of a ground source heat pump developed by the authors. A mathematical model has been developed for calculating the efficiency of a ground heat exchanger based on three-dimensional equations of heat and mass transfer in a porous medium. The numerical solution was carried out using the COMSOL Multiphysics software. The numerical calculation algorithm was verified by comparison with experimental data from the created prototype. Calculations were made of the efficiency of a borehole heat exchanger with various geometric configurations of the pipes in the well. With an increase in the tube diameter, the heat transfer increases. With a tube diameter of 40 mm, the thermal efficiency of the heat exchanger was 42.4 W/m in the heat charging mode, which is 24% more with a diameter of 20 mm. With increasing well depth, the heat transfer efficiency increases. The influence of the thermal conductivity coefficients of the pipe material, grout material and various types of ground on the heat transfer efficiency was also studied. It was shown that with an increase in the thermal conductivity coefficients of grout and ground, the heat flux increases, but above 6.0 W/m K, the heat flux practically does not change. When the coefficient of thermal conductivity of the pipe material is higher than 1.0 W/m K, the heat fluxes almost do not change. In general, materials containing plastics are used for piping of ground heat exchangers, the thermal conductivity coefficients of which vary between 0.24-0.42 W/m K.

Keywords

• borehole heat exchanger
• ground source heat pump
• thermal efficiency
• heat and mass transfer in porous media
• thermal conductivity
• heat exchanger geometry
• mathematical model
• numerical solver