Research On Heat Transfer Calculation Of Vertical Ground Heat Exchangers

Ground source heat pumps (GSHP) have been recognized as a highly efficient, renewable energy technology for space heating and cooling in residential and commercial buildings. Heat transfer analyses of ground heat exchanger is crucial in the application of GSHP systems. This article mainly discusses the heat transfer model of vertical ground heat exchangers and calculation method of the design & simulation for GSHP systems.On the basis of analyzing the available heat transfer models, the heat transfer model in this article will be divided into two parts by borehore wall:heat transfer model outside of the borehore wall and inside borehole wall. Two important new explicit solutions have been obtained for both the heat transfer processes of outside and inside borehole.Firstly, a new analytical solution of g-function for middle and mean temperature response of borehole wall has been derived on the basis of finite line source model, which can significantly improve the calculation speed and can be applied to vertical ground heat exchanger design for ground-source heat pump.Secondly, a new analytical solution of borehole thermal resistance estimated by different average fluid temperature on the basis of quasi-three-dimensional model, logarithmic mean temperature for single borehole and geometric mean temperature for double boreholes, which avoids bias in estimation of borehole thermal resistance by arithmetic mean temperature.Thirdly, the design and simulation for GHEs has been then discussed on the basis of explicit solutions. A new equation for the fluid temperature response has been derived by the superposition of step response of the heat transfer rate, then a fast algorithm for the hourly simulations of GHEs, referred to as "part load algorithm" (PLA), has been established in the article. Results of a comparison between the PLA and no load aggregation algorithm (NLAA) are presented. Several cases are examined with three different borefields. Results obtained for one-year and twenty-year simulations show that the PLA is in very good agreement with the NLAA. In the worst case, the maximum difference in fluid temperature is of the order of 1.8℃and 2.4℃. This level of precision is more than adequate to perform accurate hourly simulations of GHEs.The study in article has developed heat transfer analyses of GHEs, which is based on analytical solutions and the principle of superposition, and established a fast algorithm for the hourly simulations of GHEs. All these findings can be directly applied to design and simulation for GHEs.