Study On The Heat Exchanging Performance Of Vertical U-tube

The ground-coupled heat pump (GCHP) system of an office building is studied in this thesis. DeST is used to calculate the hourly cooling and heating loads of the building for a whole year's time. All the results are dealt with to get two series of important parameters, which are monthly total cooling and heating loads and monthly peak cooling and heating loads. These parameters are exactly necessary to the following simulation. Three key evaluating indexes of soil thermal characteristics, which are thermal capacity, heat conductivity and thermal diffusion rate, and their affect factors are analyzed in detail respectively. A mathematical model is established to describe the fluctuation of the underground soil temperature basing on the unsteady heat transfer theory. After further study of this model, a calculate formula of soil initial temperature is formed. It can be used to calculate the soil temperature hourly throughout a whole year. Two sets of curve fit equations are introduced to describe the performance of heat pumps. By using the performance data of the heat pump from the product brochure, all the coefficients of the equations are decided and four curve fits are produced in graphs. Moreover, the theoretical basis of underground heat exchanger design and the backfill material characteristics are described concretely.GLHEPRO 3.0, which is recommended by ASHRAE as a good designing and simulating software for GCHPs, is chosen to carry on the task of simulation. On the basis of monthly total cooling and heating loads and monthly peak cooling and heating loads, several different work conditions are designed by considering all the parameters referred upon. Under these work conditions, the main impact factors of vertical U-tube heat exchanger size and performance are simulated and analyzed respectively through the GLHESIZE module and GLHESIM module of GLHEPRO 3.0. The purpose of doing this is to give some concrete suggestion and reference for the promotion of GCHPs in our country.Results show that the main influential factors of heat exchanger size are soil characteristic, backfill material conductivity, initial ground temperature, separation distance of the boreholes, separation distance of two legs of the U-tube, and the flow rate of the circular fluid. And the main affect factors of heat exchanger performance are separation distance of two legs of the U-tube, separation distance of the boreholes, initial ground temperature, soil conductivity, backfill material conductivity and borehole configuration. The size and performance of underground heat exchangers have a lot to do with the running performance, energy-saving effect and initial investment of the whole GCHP system. They are two key indexes to evaluate whether GCHPs have advantages in the aspect of technology and economy compared with traditional air conditioning methods. All the designers and researchers should give enough consideration to these factors when designing and studying GCHPs.