Further Study On Heat Transfer Model And Design Of Geothermal Heat Exchangers

Nowadays, energy conservation becomes an important measure to achieve the environment-friendly and resource-saving society. The Ground Source Heat Pump (GSHP) systems, as an energy conservation and environment friendly technology, get more and more attention. Large-scale popularization and application of the GSHP systems in China are restricted mainly by three factors, which are their higher initial cost, land area requirement for installation of the Geothermal Heat Exchanger (GHE) as ell as concerns about annual heating and cooling load imbalance in the GHEs. This article proposes two measures to reduce the high initial investment of the GSHP systems. Firstly, the horizontal slinky ground heat exchanger is considered to replace the vertical borehole heat exchanger in appropriate situations. Secondly, some design procedures used in the "Geostar" software for design and simulation of the GHEs are improved so as to enhance its accuracy.Firstly, this article proposes a heat transfer model suitable for engineering design of the horizontal slinky GHEs. The thermal analyses are based on the one-dimensional transient heat conduction solution in an infinite medium while the effects are considered of the density of pipe disposition and variation in the loads. Key factors which influence the performance of the slinky GHE are discussed such as the thermal properties of the soil and the pipe disposition density. An example of the slinky GHE design is also presented, which is then compared with the vertical borehole GHE.Secondly, in view of the deficiencies in previous method, this article proposes a new method for thermal analysis of the GHEs with multiple boreholes. That is a representative borehole is selected to replace the least-favorable borehole to determine the temperature rise of the GHE. The concept of standard deviation coefficient is proposed to analyze the thermal imbalance among the boreholes. The radio of highest temperature rise and mean temperature rise is calculated, and the result indicates that it is necessary to find location of the representative borehole. In order to achieve this goal, the temperature rises of every borehole in different configurations of borehole matrixes are calculated, and conclusions on the location of the representative borehole are obtained which may be applied in other configurations.Finally, the impact of groundwater flow on performance of geothermal heat exchangers in ground source heat pump systems is analyzed based on an analytical solution of the advection around a line source in porous medium. The temperature at any location in a field with multiple boreholes may be obtained by means of the superimposition principle. The temperature response in an 18-borehole GHE with groundwater advection is computed in detail and shown in an animation. Different parameters which influence the temperature and unbalance of the GHE are discussed such as the groundwater flow direction, velocity, the space among boreholes and boreholes configuration. The temperature rise of the GHE is also computed while variation in the load is considered. The impact of groundwater flow is also studied in comparison between results obtained from modes with or without groundwater advection considered.