Theoretical And Experimental Study On Heat And Moisture Transfer In Soil And Ground-Source Heat Pump

Currently, people all over the world are devoting to the research work of low energy consumption, non-polluting, renewable energy technology.It is reported that the total energy consumption in the world is primarily composed of building energy consumption. While in building energy consumption, heating and air conditioning are the two main energy consumptions, so how to reduce these two main energy consumptions is one of the focuses of the research.By definition, a heat pump is an electrical device that extracts heat from one place with lower temperature and then transfers it to another place with a higher temperature. In the winter time, a heat pump extracts heat from natural resources (the outdoor air, water, soil and solar energy) and waste heat resources, and then circulates it through ducts into needed places. During the summer time, a heat pump reverses the process. A heat pump provides people an effective way to use energy reasonably and rationally, to save fossil fuels and reduce pollution. Therefore the heat pump technology is widely used.The heat pump usually consists of three types: (1) air source heat pump (ASHP); (2) water source heat pump (WSHP); (3) ground source heat pump (GSHP).ASHP is widely used in the warm regions of winter in our country, but its heating efficiency is too low in the cold areas of winter, and if the outdoor temperature falls to near or below freezing when the heat pump is operating in the heating mode, moisture in the air passing over the outside coil will condense and freeze on it, which limits the application of ASHP; WSHP's application is limited by the local water resources; G SHP is a high efficiency, energy saving, low noise and clean heat pump system. Compared to ASHP, GSHP has not been used widely, which may be attributed not only to its comparatively higher installation cost, but also to the lack of reliable system design and simulation models.Among various types of GSHP, the vertical GSHP is a quite worthwhile heat pump system to develop. Because it requires less occupation of land and causes no contamination of groundwater as well.In the existing models of heat transfer in soil, one of the primary assumptions made only studied the heat conduction process while neglecting the moisture migration process, which may induce to deflect from actuality in designing heat exchangers in soil, and result in both bigger invested capital and limited applications.Heat and moisture transfer process in soil is very complex, while the traditional line source model is too simple to meet the practical requirements of the engineering design, which may cause the deviations.In this paper, based on the law of mass and energy conservation, a retrofitted heat and moisture transfer line source model for the vertical GSHP system has been developed. According to this retrofitted model, soil parameters (soil thermal conductivity, initial soil temperature and moisture content, etc.) are analyzed theoretically and experimentally to make the model more in line with practical projects.Heat conductivity is an important characteristic of soil, thermal needle probe is made to measure heat conductivities of soil and sand both with different mass water contents, and to obtain the relationship between density, heat conductivity, heat diffusivity and mass water content.For further study on heat and moisture transfer, an experimental apparatus of one dimension heat and moisture transfer is developed. Soil water content and temperature distribution are acquired along soil column through experiments.Experimental results indicate that temperature gradients affect soil-water potential which induces both liquid and vapor movement; soil-water potential gradients move water, which consequently carries heat. The old water flux equation is used to calculate the change of water content in soil column, but it is not applicable to this experiment. Therefore the equation is modified to describe the heat and moisture transfer process better, and to enable it apply well to the practical projects ultimately.An experimental apparatus of the vertical GSHP is built. Monthly averaged temperature at a depth of 6 m underground, temperature field in soil, and water temperatures entering and leaving the outside heat exchanger are tested, and the actual heat transfer coefficient of well-depth is acquired. A program for heat exchanger simulation is developed to derive the capacity for heat exchanger in soil with different water contents and temperature field, water contents around tube underground.Finally, some suggestions are proposed to further study, the application and development prospects of the GSHP are discussed.