| The ground-coupled heat pump has been widely popularized and applied athome and abroad as a new air conditioning technology of energy conservation andenvironmental protection. But because the difference between the building heatingload and cooling load in severe cold area, the soil temperature would descend yearby year due to the long-term operation of the ground-coupled heat pump system. Asa result, the heating performance of the heat pump would drop year by year and theindoor temperature can’t meet heating requirement. Therefore, this paper presentedthe heat pump system with air-source soil heat storage (HPASSHS) based on theidea of the seasonal heat storage using the soil as medium. The heat of the naturalair was injected in the soil in summer and was extracted from the soil by the heatpump for space heating in winter. Compared with the solar energy storage, the air-source heat storage has the advantages of simple device and the less cost ofinvestment and maintenance. If there was no the central heating network and no theenough roof space or other space to install the solar collector, the air-source heatstorage could remove the natural energy from summer to winter too， whichprovides a new way for the application of ground-coupled heat pump in severe coldarea. This study is based on the following aspects:Firstly, the operation modes and principles of HPASSHS system wereintroduced, and the numerical model of each heat transfer part in the system wasbuilt. Considering the computational complexity and accuracy, the quasi three-dimensional transient heat transfer model of the multi-pipe ground heat exchangerwas developed using equivalent single pipe with square cross section instead of U-tube, and the heat transfer boundary condition between the equivalent pipe and thesoil was calculated by shape factor method. Based on the above models, the on/offconditions of the different modes were researched and determined, so that thedynamic operation characteristics of HPASSHS system can be simulated.Secondly, the experimental platform of HPASSHS system was set up in theHarbin University of Technology and about two years experiment was performed.The characteristics of the air-source soil heat storage and heating by the ground-coupled heat pump as well as the temperature variation characteristics of the soilwere analyzed base on the measured data. It can be concluded that the soiltemperature could be increased effectively via the air-source soil heat storage, theair temperature was the main influence factor of the heat storage effect, and the heat exchange efficiency of the ground heat exchanger could be improved by theintermittent operation mode. The reliability and validity of the models establishedabove were verified by comparing the simulation results and the experimentalresults.Thirdly, a HPASSHS system was designed for an energy-saving residencebuilding in Harbin, and the10years operation performance of the system with andwithout the air-source heat storage was simulated, respectively. The annualcharacteristics of the system with the air-source heat storage was studied fromoperation parameters, heating effect, coefficient of performance and soiltemperature field and so on, and comparing the system performance with andwithout the heat storage year by year, it was found out that only with the seasonalheat storage could ground-coupled heat pump system realize the soil temperaturefield balance year after year and be sustainable for heating in severe cold area.Fourthly, the impacts on the system operation characteristics caused by themajor parameters including heat storage time, heat exchange area of ground heatexchanger, heat output of heat pump, heat exchange areas of indoor and outdoorfan-coil units were simulated and analyzed, respectively. On the basis ofparameters analysis, the annual minimum cost was taken as the objective tooptimize the system parameters, and the optimal parameter combination of thesystem was gained under the simulation conditions. The HPASSHS system with theoptimal parameter combination was compared with that of electric boiler heating,electric ground heating and gas heating on the sides of energy-saving and economy.The results show that the application of HPASSHS system has a certain advantagein severe cold area.Finally, the HPASSHS system was applied to an energy-saving build ofBeijing in cold area. The soil cooling directly, the cooling by the ground-coupledheat pump and the air-source heat storage operated alternately in summer, and theheating by the ground-coupled heat pump operated in winter. The annual operationcharacteristics of the HPASSHS system were analysed and compared with those ofthe traditional ground-coupled heat pump system by the simulative calculation. Thefollowing conclusions can be drawn: the HPASSHS system can meet the indoortemperature requirement in summer and winter, the cooling coefficient ofperformance of the system rised significantly by the soil cooling directly. Throughthe air-source heat storage, the heat injected into the soil could be equal with theheat exacted from the soil during the HPASSHS system operation. While the heatinjected into the soil was only65%of the heat exacted from the soil during theoperation of the ground-coupled heat pump system, which was not good for thelong-term heating. The energy-saving and economic performances of the HPASSHS system were compared with those of the two heating and air-conditioning plans, theresults indicated that the application of the HPASSHS system in cold area coulddecrease markly the primary energy consumption, and the total cost was less.The simulative and experimental studies in this paper have proved that the air-source seasonal soil heat storage is feasible in severe cold area or cold area and theHPASSHS system has sustainable and efficient heating performance, which canprovide theoretical basis and technical support for the system application.This research topic is a part of the project ’Research on Key Technologies andEngineering Demonstration for Energy Saving and Consumption Reduction of theHeating System in Severe Cold Area’, which is the country ’11th Five-Year Plan’ tosupport science and technology project (No.2011BAJ05B04). |
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