Research On Experimental Table And Heat-transfer Model Of Concentric-tube Heat Exchanger Of Ground Source Heat Pump

Energy is one of elements human being needed to survive. It is important strategic material for economy development and important material base for social living in modern times. The problem of Chinese energy is faced when bringing continuable development stratagem. As all know, Chinese coal resource accounted for 72.8% in all energy consumption. Chinese energy consumption framework is without reason. China is a country in which the first energy is coal resource. Architecture consumption energy, industry consumption energy and traffic transport consumption energy are the most three aspects on once energy consumption in the world. Architecture consumption energy has a lager proportion in social energy consumption, and is about 30~40% in developed countries. Town and city population in Chinese heating areas is a proportion of 13.6% or so, but heating energy arrives to 9.6% in full energy consumption. Two or more resources should be together utilized in the energy framework of diversification renewable energy, because this way can improve use efficiency of every resource by the way of bringing its advantages into play. Under this condition, ground source heat pump is brought forward as new way or approach of using geothermal. It is necessary and timing. In fact, ground source heat pump technology makes use of solar and geothermal together. In this paper, research achievements at aboard and home has been sum up. An experimental table has been building with the help of scientific research funds from China Geological Survey. The item named "research on a new technique of shallow geothermal drilling and pick-up". Ground heat exchanger is the research emphases of ground source heat pump system. Ground source heat pump system aimed to heating in north areas. And many parameters such as temperature, pressure, velocity of flow, flux and heat flux are measured and controlled. These data are used to evaluate entirely ground source heat pump system. It can support in the theory the technology to be used other engineer projects. The experimental table accounted to 500 m2 and had three heat pumps which heat quantity rating is 65kW. It has two ground source heat pump schemes for heating; one is "Single U, Double U, Concentric-tube heat exchangers system + circulation water pump outdoor +water/water heat pump + circulation water pump indoor + fan coil unit indoor"and "Single U, Double U, Concentric-tube heat exchangers system + circulation water pump outdoor +water/wind heat pump + fan coil unit indoor". There are 4 primary parts in this system: outdoor heat exchanger subsystem; heat pump unit subsystem; indoor circulation subsystem and measure and control subsystem. Outdoor heat exchanger subsystem includes three kinds of heat exchanger: two single-U, two double-U and one concentric tube heat exchanger. In the outdoor heat exchanger three kinds of backfill are be used in order to contrast them and find the best performance of heat transfer. Every heat exchanger built a well for the sank of maintenance and observation. Every one can use independently and combine together in order to contrast each heat exchanger performance. Especially, concentric tube heat exchanger can change its inside tube without changing outer tube. Thus performance of every heat exchanger can be measured and the best one will be introduced to engineer projects in order to reduce the cost. Due to unusual geologic condition influence temperature probe, AD590 temperature sensor can work well in the heat exchanger well. In order to assurethe sensor work order, data line with waterproof and shield is speak for factory. Using AD590 temperature sensor, temperature data got show in table1. Experimental data are processed with the SPSS for windows software. With regression analyze,curve plot is shown by the way of Curve Estimation in fig.1.Results analyze as following: Analysis of Variance: Variables in the Equation: R Square: R2=0.933; Estimation equation: lnT=ln8.001+0.005H H——depth (m) T——soil temperature (℃) Ground heat exchanger has several types such as: single-U, double -U, 1+2 type and tube type and so on.,sketch map shown in fig2. fig2. sketch map of several ground heat exchanger U type ground heat exchanger is usually used at present and including single U type and double U type. Pipe diameter of U type heat exchanger is about less than 50mm. Its drilling hole depth is about 20~200m and heat transferflux is 40~100w/m (hole depth). The best virtue of tube type heat exchanger is augmentation of heat transfer area. It can reduce the number of drilling and the depth of pipe. Concentric tube heat exchanger as a new heat exchanger has been researched and developed, and its structure is shown in fig.3. Outside tube is used by HDPE material. Air interlayer is formed in inside-tube using two layers of PPR pipe material in order to improve the performance of heat insulation. Heat conductivity of inside tube is equal to 0.05w/m.K. The structure is in favor to reduce "heat short circuit". The operating principle of concentric tube heat exchanger is: when heating, work substance enters heat exchanger from the place between outside tube and inside tube. After exchanging heat flux with ground soil, it become warmer anddraws out. On the contrary, work substance loses heat flux to soil and become colder when cooling. Heat transfer model of concentric tube heat exchanger is established through analyzing its heat transfer process. Temperature function formula is arrived by quasi-steady heat transfer process calculation. Work substance exchange heat flux with soil when it flow concentric heat exchanger. Heat is only been thought in this process. Because it happen between liquid and solid, heat transfer is considered as liquid-solid coupled one. Concentric tube heat transfer is researched by numerical modeling with Fluent software. The results show: if inlet velocity of work substance is 0.7m/s with the temperature 274K, and soil temperature is 287K in ground 100m, outlet temperature can arrive to 277K. Principle of energy conservation and performance evaluating indicator are discussed in the last. Energy conservation of ground source heat pump system principally includes pump unit conservation, heat exchanger conservation and indoor system conservation. Quadrature manufacturing cost of it is 400 Yuan/m2 or so according to investing to construct the experimental table of ground source heat pump system. Running cost every year is 9.2～14 Yuan/m2 and cost-recovering is about ten years. Running cost every year will reduce 40.4%～60.9% when heating with ground source heat pump system compared to central heating system. Cost of concentric tube heat exchanger is about 80.78%compared to double U type heat exchanger under the condition of picking up the same amount of heat flux. Performance of heat transfer of concentric tube heat...