Exergy Analysis On Ground Source Heat Pump System Operating Under Cooling And Heating Conditions

In recent years, with energy crisis and environmental pollution worsening, groundsource heat pump technology has been paid more and more attention and applied in thefield of air conditioning building for it is environmental friendly and energy saving.Although the development prospects of ground source heat pump technology is verybroad, there are many problems in practical applications so that the expected energysaving effect can not be achieve. These restrict the further promotion and applicationof ground source heat pump technology in China.An office building in shanghai is selected as a representative example to study.Through the hourly air conditioning load simulation of this building and the hourlyShanghai outdoor temperature distribution frequency analysis, it shows that more than90%of the heating and cooling time of this buildings is under part-load conditions..Interms of the building cooling and heat load, a set of system combining a air-coolingwater chiller with a ground source heat pump is designed, referring to the samplesprovided by manufacturers.A set of mathematical model was developed based on exergy analysis method forthe ground source heat pump and the water chiller, which is more reasonable thanbased on energy analysis method. The method of determining the state points ofrefrigeration cycle is sorted out. Combining this method with the exergy analysismodel, exergy analysis on the ground source heat pump system is programmed by thesoftware of MATLAB and REFPROP.Firstly, exergy analysis under the design conditions were carried out on theground source heat pump and the air-cooling chiller, which supply heating and coolingto the office building we selected for study. It is found that the exergy efficiencies of the ground source heat pump are21.87%under winter design condition and11.58%under summer design condition, respectively. And the exergy efficiency of theair-cooling chiller was8.64%. This indicates that ground source heat pump performsbetter under winter design condition than it does under summer design condition. Andthe exergy performance of the ground source heat pump is superior to that of theair-cooling chiller. Then, the inflences of factors, such as the evaporation temperature,condensation temperature, temperature difference between the supply water and thereturn water of the building side air handling unit, temperature difference between thesupply water and the return water of the borehole heat exchanger side and the differentoutdoor temperature on the performance of the ground source heat pump system werealso analyzed. The results shows that a increase in evaporation temperature, a decreasein condensation temperature, a reduction in temperature difference between the roomand the building side, the smaller temperature difference between the bolehole heatexchanger water inlet temperature ground water and soil and different outdoortemperature, the greater difference between the environment state and the purpose stateof the system lead to the increase in the exergy efficiency.And then, the operational states of the refrigeration and heating cycle under thepartial load conditions were simulated for the ground source heat pump and exergyanalysis were also conducted. It is found that the exergy efficiency of most of thecomponents of the system decreases as the part-load ratio decreases, but theirthermodynamics perfect degree keeps rising. This means the space for performancebettering under part load condition become less. It is concluded that energyefficiencyies and exergy efficiencies achieved under full-load operational condition arehigher than those achieved under part-load conditions.