Studies On The Identification Of Soil And Grout Thermal Properties In Ground Source Heat Pump System

Ground source heat pump(GSHP) is a renewable energy technology with many advantages(e.g. energy saving, environmental protection) which has been widely used in practical engineering. GSHP transfers heat with underground soil through ground heat exchanger therefore it is extremely important to design ground heat exchanger appropriately. Identification of soil and grout thermal properties is an important component in the design of ground heat exchanger(GHE). A study shows that the accuracy of soil thermal properties have a great impact on GHE designing and operating.Many scholars applied parameter estimation method to identify soil and grout thermal properties, but at present parameter estimation method has many problems, such as less estimation parameters and low recognition accuracy. Thus, the identification method of soil and grout thermal properties is investigated from the following two aspects.For the thermal parameter estimation of soil thermal conductivity, soil thermal diffusivity and borehole thermal resistance, the analytical thermal conductive model inside borehole and the numerical thermal conductive model outside borehole are proposed in this paper. Fluid average temperature is calculated by coupling the heat transfer between the inside wall temperature and the outside borehole wall temperature. Each value of decision variables is solved by simplex search algorithm based on the in-situ thermal response test(TRT). The fluid outlet temperature calculated by this method was more close to the measured value compared to 2-D numerical thermal conductive model and p(t)-liner analytical model. Meanwhile, compared with the general 2-D model, the calculation error of thermal resistance of inside borehole was reduced when calculated by multipole model.For the thermal parameter estimation of soil thermal conductivity, soil thermal diffusivity, borehole thermal resistance, grout thermal conductivity and thermal diffusivity, this paper solves the fluid average temperature of U-tube in parallel, on the basis of infinite line source(ILS) model and composite media line source(CMLS) model with wall thermal resistance correction. On this basis, genetic algorithm combined with sandbox TRT is applied to solving optimization of decision variable values. In the solving process of composite media line model, borehole thermal resistance is applied to modify the thermal resistance of pipe wall, and the results of genetic algorithm are compared with simplex method and pattern search method, indicating that the genetic algorithm has high identification accuracy in evaluating thermal properties.This identification model can accurately identify the soil and grout thermal properties in GHE designing. The results have an important reference value for GHE design.