Research On Soil Temperature Field Around Ground Heat Exchanger System In Different Boreholes' Arrangements

GSHP (Ground Source Heat Pump) is common used in the hot summer and cold winter region of China. Ground pipe heat exchanger is an important part of GSHP systems, as well as the core of GSHP technical research. This paper derived a variable heat flux model of ground heat exchangers on the basis of finite linear source of heat flux with the method of step load and superposition principle. With the help of math tool MATLAB, precision results of excess temperature could be given in conditions of different soil types, time or location. By using the self-built platform of experiment, some feature points in the ground heat exchangers field are tasted, and verified the model. In the end, utilize this model in several practical engineering cases to simulate the entire temperature field.By experimental study about ground heat exchanger system, it is shown that after the boreholes start running, the soil which is nearby the boreholes has a shorter temperature response time. Around the center of the boreholes-area, the excess temperature of soil is higher. After the boreholes stop running, the soil in the whole region will still be warming for some time, which will be longer for the soil around the center of the boreholes' unit. During this time, the soil around boreholes-area's center has a higher excess temperature. Then the soil will be cooling. The rate of cooling is basically same in all the region, so the excess temperature after natural cooling is determined by the excess temperature's maximum.By using the model to simulate the actual project case, it can sum up some conclusion that under the standard according to GB 50366-2005 that the spacing of boreholes is from 3.0m to 6.0m, If the continuous exothermic time is less than a month, the smaller tube pitch in standard range,3.0m is acceptable, is suggested to be chosen, because a short time of heat release will not cause excessive temperature rises near the drill hole wall, while can make full use of the edaphic heat capacity. If the continuous exothermic time is lasting for more than one month, it is better to choose a longer distance of standard boreholes form (longer than 5.0m), to make the ground near the boreholes'wall more likely to dissipate heatThe soil's excess temperatures while the boreholes are in aligned or staggered form are not very different. To further enhance the efficiency of ground heat exchanger, the boreholes are well in staggered arrangement when they keep heat exchange for less than a month. If the continuous running time is more than one month, it is better to choose aligned arrangement for the boreholes, as the excess temperature in this arrangement is lower, and the heat interfenence will be more moderate.