Underground Heat Exchangers In Ground Source Heat Pump System

In the new period,one of the basic principle in China's energy development is to implement the sustainable development strategy. The renewable energy application in the building is an important part of energy conservation. As a new energy-conserving and environment-protected energy technologies, ground source heat pump(GSHP) may obtain massive regenerative energy by consuming less primary energy.when our country's energy supply is in growing tension, increasing attention has been paid to the GDHP and people begin to promote its use. Much experiment research for underground heat exchangers of GSHP has been done by oversea researchers. Domestic research focured on the test of single U-tube heat exchanger and horizontal buried pipes. Few was done about double U-tube underground heat exchangers.Much researches have been conducted about the heat transfer performance of under ground heat exchanger based on the linear theory foundation or the two- dimensional computation. Due to the dissymmetry of heat transfer, some error exsited in application. Based on three dimensional circular cylindrical coordinate heat transfer principle,calculation model of underground temperature field of double U-tube heat exchanger is established, and mathematic analysis method has been put forward.As a new kind of heat exchanger, few test were done about the heat transfer of double U-tube. Based on the project , this article established experimental platform analyzed tesed data of double U-tube whose deep is 70m on initial operating condition in the give heat flow, following conclusions were obatined: as the flow increaing, q_h(per depth heat) increaed; In sumer, the subsurface temperature trested point assumes but actually trapezoidal distribution when the system running; The running time is longer, the underground ground's recovery period is longer.Double U-tube is appliede more and more in ground source heat pump systems, but the cmparation of heat tranfer permance and economic between double U-tube and single U-tube is in-depth. Through the calculation use the phoenics software, draw the conclusions that as the flow inscreasing, q_h of double U-tube insreased, the effected depth instreased. q_h of double U-tube is 1.5¹.7 times of it of single U-tube in the same velocity of flow. The smaller flow is, the more performance advavtage of double U-tube. The araticle evaluation the economic performance of underground heat exchange by C_1q, which is investment per depth heat,draw the fllowing conclusions: double U-tube is better than single U-tube when the velocity is same. For Chongqing, Clq of double U-tube is 3.14 $/W, C_1q of single U-tube is 4.69$/W. If the flow is the same, in some conditions, double U-tube didn't exsit application advantages.In operation process of ground-source heat pump system, heat transfer perfermance of underground heat exchangers may be copact by each orther. Current researches focus on the haplopore heat exchanger. This article draw the conclusion through the system test that the uderground temperature during a trapezoid distribution in winter. Zoning regulation conducive could improve the utilization of double u-shaped heat exchanger and the performance of the system.The smaller load is , the smaller EER of air conditioner is.Many factors can influence the heat tranfer performance of double U-tube ground source heat pump systems. Based on the measured for single heat exchanger, data, this article drew the following conclusions: I. The best flow axsited, which made the q_h maximum. According to the test conditions, the best flow is 1.1 m3 / h. II. In summer, q_h increaed with the inlet water'temperature increasing. q_h increased 1.5% 4.5% when the inlet water'temperature increased 1℃. In winter, q_h reduced with the inlet water'temperature increasing. Test data shows that when water temperature increase 2℃, q_h reduced by 40%.â…¢. The bigger Ks(thermal conductivity of the buckfilling soil) is, the bigger q_h is. Based on the theoretical calculation, when the backfilling materials thermal conductivity increases 45%, q_h increased by 25%. Moderate increases ojf K_s is in favor of heat transfer capability of double u-shaped buried tube.The main purpose of heat tranfer analysis of double U-tube is for practical applications. This article applied the analsized result, put forward that designer can choose straight hot water supply or storage hot water supply when the ground source heat system undertaked airconditioning load and living hot water load.