Research On The Tiered Approach On Layered Heat Transfer Model Of Vertical Ground Heat Exchangers

By the end of 2015, China's renewable energy capacity has reached 24% of the global total, and therefore China has become the world's renewable energy superpower. However, ground source heat pump development has been obvious constrained and faced great pressure. To improve the quality and efficiency of energy has become "much starker choices-and graver consequences-in" period of focus. Ground heat exchanger is an important part in ground source heat pump. In China, the U-shaped vertical ground heat exchanger by virtue of their small footprint, wide range available, resulting in the promotion, including single and double U type. In this paper, the soil as a homogeneous medium, on this basis, the main heat exchanger for a single U-shaped, the establishment of a hierarchical model of the heat exchanger, so as to provide reference for practical engineering design.First, this paper established the single layered U ground heat exchanger heat trans-fer model, based on existing research, the surrounding soil buried tube heat ex-changer considering the existence of geological layers and groundwater seepage, put forward by temperature sensor and a detailed geotechnical layering position stratifica-tion. Next, Through the analysis of drilling inside and outside heat transfer model, puts forward the calculation sheet around U ground heat exchanger the calculation method of rock and soil thermal physical parameters. Based on the actual engineering, detailed hierarchical model was verified and compared it with the difference between the con-ventional non-layered model and a simplified model, which according to the geological stratifica-tion simple, a new method based on a comprehensive tiered rate of change of the ther-mal conductivity of the soil. Using the actual engineering, comprehensive layered ap-proach was validated, and the use of complex geological test again, thus proving the accuracy and reasonableness of the comprehensive layered approach. Finally, a brief introduction about the design according to the results of a comprehensive layered ap-proach to the process, and by VBA development process, the establishment of software, which used soil thermal parameters and unit length layered composite ob-tained by the method of heat exchange.Measured data and simulation results show that, according to the results of detailed stratification, various geological thermal parameters are not the same, the rate of the different geological changes between 17.56-27.59%, same geological between 0.19- 6.51%. Changes differences are large. Therefore, due to the porosity, groundwater flow and other reasons, consider a combination of the rate of change limit stratified, to avoid errors caused by the sheer volume of work, which as geological stratification and in accordance with the simplified temperature measuring points and detailed geological stratification.Compare the measured data, simulation results show that the relative error of the winter and summer non-layered model for the return water temperature were 30.36%, 30.72%, after simple geological strati-fication based directly on the geological stratification, and the relative errors were 22.74%, 23.84%, detailed hierarchical model were 7.41%, 7.32%. According to three hierarchical simulation results proved that the whole ground heat exchanger uniformly regarded as a unified model has a big error. In simple geological distribution, according to the results of geological stratification after also has a big error. For more stratified according to large workload, and therefore pro-posed a comprehensive tiered approach.The same comparison measured data, simulation results show that the thermal conductivity, respectively, in accordance with the rate of change in soil adjacent to the upper limit of 3%, 5%, 8% were stratified their results as 7,6,4 stratified layers, corresponding for winter and summer return water temperature average relative error is 9.31%, 20.33%, 23.29%. Therefore, the rate of change for the selected upper limit of 3%, a small increase in the workload can achieve high accuracy. Through a complex geology, the upper limit of the rate of change has been verified. The results showed that the average relative error out of the water temperature difference of simulated and measured values is 2.7%, less than 5%, so that the method has a precision and reliability. Using VBA to develop the programming method of calculating program, which facilitate designers.Studies have shown that a comprehensive layered approach combines the variation of geological conditions and geological distribution, avoiding the hierarchical results that geological interval is too large or the inner section of geological soil thermal parameters change is large. The tiered approach scientifically sound, provide a basis for the design process, method is reliable, simple and feasible, and has good prospects.