Research Of In-situ Measurement Of Ground Thermal Properties

Ground-source heat pump air-conditioning systems use the soil as a source of hot and cold and underground pipe makes heat exchange with the soil through the circulation of the medium .Heat pump implents cooling or heating buildings. Compared with the traditional air-conditioning, ground-source heat pump air-conditioning systems use renewable sources of energy, with the characteristics of energy saving and environmental protection. For the use of U-pipe heat exchanger for ground-source heat pump it makes more and more attention to apply a wide range of its stable performance, high efficiency heat transfer.Designers are facing an important challenge the determination of formation thermal conductivity in the design of ground source heat pump systems. Size and cost of heat exchanger are highly dependent on the thermal properties. In order to be able to predict the physical properties of the thermal parameters ,heat injection and heat extraction device are setted up on-site borehole .This paper describes an in situ test facility to measure ground thermal properties.In contrast to previously described facilities,this one allows both heat injection as well as heat extraction using realistic temperature ranges.The test rig has been used to determine the thermal characteristics of several sites,using both energy injection and extraction,leading to improved and more confident designs.In practice,the test rig provides a reliable and robust method of estimating ground thermal properties.The test apparatus,built into a shipping container,are sufficiently mobile and can be operated without supervision.The power needed to run the test facility can either be obtained from the power grid or from a(low noise emission)generator.To allow remote monitoring,a telemetry system has been developed.The object of this paper is vertical underground single and double U-pipe heat exchanger. It consists of recycled water, U-type pipe, backfill material and soil outside geothermal wells. U-pipe heat exchanger is the high-density polyethylene pipe.Its diameter is 32mm and it has the advantages of higher thermal conductivity of the material, good toughness, anti-extrusion capability. Stratigraphic test vehicle for thermal properties developed by Jilin University and Design Institute of China National Aviation is used to test in situ geothermal well testing. Analysis of test datas can be the entire depth of geothermal wells in the framework of the effective thermal conductivity of soil.Line source model, column source model and numerical model of heat conduction model are setted up to calculate the soil thermal conductivity and parameter estimation method is also used to calculate the soil thermal conductivity and borehole thermal resistance. Line source model and source model column is an analytical model and they are simplified into one-dimensional linear heat transfer problem when we make some calculations. Analytical method development model has been more mature but now people are making more and more study on numerical model. The application of the numerical simulation can compare the real simulation of the ground heat exchanger in the actual heat transfer process .Therefore the results of thesis analytical method will be applied to numerical simulation and make sure if the geothermal wells verify between the heat transfer process and the actual situation .Thesis detailediy analyties model of the underground heat exchanger process and derives formula for calculating the borehole thermal resistance. Field test datas on the 2nd single U-type heat exchanger and the 4th double U-type heat exchanger are used to calculate soil thermal conductivity and borehole thermal resistance and it also analyzes the impact factors(test time, initial soil temperature, soil volume hot, physical parameters for backfill materials et) of measurement and calculation of soil thermal conductivity and the borehole thermal resistance. Measured through the testing and data processing, we can see: Thermal properties in situ formation tester to provide a constant flow of hot are able to meet the design requirements of the precision and this will enlarge the scope of application of extraction and impart heat into geothermal wells.It has more advantage in providing a constant heat flow than electric heating cycle. Because line source model does not consider the shape and structure of the underground heat exchanger, calculation accuracy depends on measurement data, and vulnerable to the impact of external environment.Using simplified column heat source model,it is equivalent U-tube to a cylindrical tube and measurements at longer time it matches well with column source model . However, there is much deviation in the model and the actual structure of the heat exchanger and it can not explain the effects of tube spacing. Application of MATLAB PDETOOL is to simulate the heat transfer process in geothermal wells and to verify if the analytical model described above is correct. Analog computing tests the actual thermal conductivity of the borehole and its results are matched with the line source model and the column source model.