Study On Inner Heat Source Of Underground Heat Exchanger And The Ground Coupled Heat Pump's Performance

The paper introduced theoretical model of inner heat source of underground heat exchanger firstly, and set up the heat and mass transfer model of underground temperature field around the underground heat exchanger. Different with traditional model, a number of factors, such as soil heat and moisture movement and soil property were taken into account in the inner heat source model. The underground heat exchanger was regarded as an equivalent inner heat source term in the model and the numerical simulation study on the underground temperature field around the underground heat exchanger was carried out in the paper. First, the influences of different soil property and different operation mode to the underground temperature field around single borehole heat exchanger was analyzed, then, the paper emphasized the simulation and analysis of a group of U-pipe underground heat exchangers in a real project under two operation modes. One is single-season operation mode, which only extracted heat from soil in a heating season or emited heat to soil in a cooling season in one year. The other was the double-season operation mode, which extracted heat from soil in the heating season and emited heat to soil in the cooling season in the same year. This provided a new method to the simulation and prediction of the heat exchange performance, operation life time for the underground heat exchanger.Different kinds of underground heat exchangers' experimental systems were established, for example, single or double U-shape heat exchanger, sandstone or cement backfills material, and the experimental study was carried out on the performance of the single borehole heat exchanger under different soil property, different backfilled material and different operation mode.A Ground Coupled Heat Pump (GCHP) experimental system with two different backfills underground heat exchangers with sandstone or cement backfills was established based on the single borehole heat exchanger's experimental results. To GCHP system, a Computer Supervisory Control and Data Acquisition (SCADA) system was developed, which could meet the data-collect and auto-acquisition on the GCHP system. On this condition, experiments on operation performance of the GCHP were conducted in winter and summer respectively, emphasized on the GCHP's performance, underground temperature field and underground thermal reservoir. A physical & mathematical dynamic simulation model of the GCHP system with a fuzzy control method was set up. Different from traditional simulation method of skip variety of disturbance or startup mode, this model can be used to simulate the GCHP operation in continuous disturbance variation by surroundings. Finally, a set of dynamic simulation programs was made up based on the Matlab/Simulink. And the simulation results showed good fit with the experimental results. An index of cost of per heat transferred on depth of borehole was brought forward firstly, which can be used to analyze the heat transfer performance and economics of the underground heat exchanger comprehensively. The economics, energy saving and environmental benefit of GSHP compared with the conventional HVAC system was analyzed in the paper.