Study On Load Imbalance Of Ground Coupled Heat Pump In Northern China

Under the process of modernization,Energy and environmental problems of China have received much attention gradually.Based on advantages of energy conservation,environmental protection and effective application of renewable energy,ground coupled heat pump technology has been widely applied in China.However,due to the large latitude span of China,the total amount of cooling and heating loads of buildings with different functions and located in different areas usually exist big difference.When the ground coupled heat pump systems are utilized in these buildings with unreasonable operation control strategy,which usually occurred during the practical application of the ground coupled heat pump technology,the amount of heat extracted from ground in winter does not match the heat released to the ground in summer for the whole year’s operation.Then the soil load imbalance formed and the soil temperature changed year by year.Afterwards,the ground coupled heat pump systems experience the operation efficiency decrease gradually even failed to run normally,which has largely limited the application and extension of the ground coupled heat pump technology.In order to actively promote the "winter cleaning heating plan in northern China(2017-2021)" promoted by the national energy administration,this paper focuses on the load imbalance problem commonly existed in practical ground coupled heat pump system engineering located in northern China.Research is carried out based on a actual ground coupled heat pump project located in Rizhao,Shandong province.According to the analysis on its long-term operational data and simulation results,the load imbalance problem of the ground coupled heat pump system caused by the improper design and operation strategy is studied,and the corresponding feasible solutions for load imbalance problem are proposed.Firstly,the mathematical heat transfer models of ground heat exchanger utilized in Chinaand abroad are discussed,and the thermal response of borehole wall under varying load conditions is calculated based on the existing models.Secondly,based on the relevant building parameters on a practical campus in Rizhao Shandong province,the design process of conventional ground coupled heat pump system is introduced,the cooling and heating loads unbalance rate of the system is calculated and analyzed,the load imbalance rate is controlled in a reasonable range through the development of operation strategy.Then,the data research is carried out on this practical project.The operation data of this ground coupled heat pump system is collected for processing,the problems of the ground coupled heat pump system appeared in actual operation are analyzed,and the corresponding reasons for these problems are summarized in this proposed research.Once again,in view of the typical load imbalance problem with heating load dominated type appeared in the sample actual project,this study carries out the long term operation simulation of the ground coupled heat pump system using the TRNSYS software based on the load characteristics of the actual project.According to the simulation results,the change of the soil temperature and the system operation data are analyzed,and the operation performance of the heat pump unit is forecasted.Finally,aim at the problem of soil load imbalance,several corresponding solutions are proposed.Then the long term operation data of system running with different solutions are simulated with TRNSYS software,and the change of soil temperature is analyzed emphatically to verify the effectiveness and feasibility of the proposed solutions.The results indicate that,due to the influence of climatic conditions and operation mode,the load imbalance rate of the sample ground coupled heat pump system in actual operation is as high as 77%,far greater than the unbalance rate under the design condition.The soil temperature has been decreased year by year,which seriously affects the operating performance of the system.The specific solutions for load imbalance problem proposed in this paper can effectively alleviate the deterioration of soil temperature field and thermal equilibrium year by year: For the operating condition of a heat pump unit,the solar collector with area of 4000 square meters can supply 1833369kW·h heat to the ground during the two transition seasons in one year’s operation.Then the balance of heat extracted from and rejectedinto the ground can be obtained,and the whole system can maintain the high operation efficiency during long-term operation.The relative stability of the soil temperature field can also be ensured by utilizing the municipal heat sources to share part of the heating loads,which can obtain the high efficiency and stable operation performance of the existing system through low cost renovation.