Mathematical model for optimising the performance of a ground source heat pump.

Energy demand for the twenty first century is expected to increase many fold along with corresponding diversification of energy sources and generation methods. Of the many energy sources available, use of Ground Source Heat Pump (GSHP) system is the focus of the analysis in this research. This work is carried out to identify the key parameters which affect the performance of the GSHP system. A mathematical model has been developed to understand the complex operation of the heat pump under typical working conditions. Individual sub-systems, such as Ground Heat Exchanger (GHE), evaporator, condenser, compressor and radiator are modelled in MathCAD and coupled together and solved simultaneously. The performance of the system is predicted while varying air temperature, power input to the compressor and the ground temperature beneath the earth's surface. In addition a special sub-model was developed for the single vertical U-tube GHE in FLUENT, a Computational Fluid Dynamics (CFD) software, to calculate the overall heat transfer coefficient for varying outer surface temperature of the borehole.The overall system results are validated against the published results with the system operating range of 18°C to 33°C with around 10 percent deviations. It is determined that the COP of the system increases with surface area and overall heat transfer coefficient (OHTC) of the heat exchanger. An increase in up to 500 m2 surface area, steep raise of COP from 10.05 to 10.3 is observed. Similarly increase of 10 W/m2K of OHTC has steep COP rise from 10.05 to 10.28. The temperature gradient across the system also has influence on its operating performance, where a 15°C increase in the ground temperature for cooling mode reduces the COP by around 5%. Finally the degree of refrigerant sub-cooling has a positive effect, for every 5°C temperature drop the COP improves by 0.5 similarly for degree of super-heating, COP improves by 0.25. Scope for performance enhancement for GSHP is investigated by tuning operating conditions. The effect of operating variables to the sensitivity of performance of heat pump is also determined.