Study on Sustainable Heating Performance and Optimization Design of Medium-deep Coaxial Heat Exchanger
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摘要: 中深层套管式换热器的热提取具有“取热不取水”的特点, 是我国北方地区稳妥推进中深层地热能供暖的重要技术之一。针对当前中深层套管式换热器在长期热提取期间供热性能不明的问题, 本文基于可持续供热的运行特点, 建立数值换热模型分析换热器水温、性能系数随运行年份的变化规律, 并针对性地开展设计优化研究。结果表明: 在可持续供热过程中, 换热器水温在前 5年的下降程度较为明显, 运行至20年以后, 基本不发生变化。换热器季节性能系数的下降程度受岩土体导热系数的影响较大, 在1.5 W/(m·K)条件下的SPF1、SPF2分别下降11.50%和 10.56%, 在 3.0 W/(m·K)条件下的SPF1、SPF2则分别下降4.73%和4.23%。岩土体导热系数越小, 换热器供热性能的下降程度越明显。基于可持续供热性能的变化规律, 提出以“准稳态特征年”的供热性能为基准进行设计, 并综合考虑运行要求与节能性要求优化换热器的运行条件, 明确了在不同地热特征参数、埋管深度下的最优运行流速与最佳供热负荷的分布情况, 对中深层套管式换热器的高效可持续供热设计具有实际指导意义。Abstract: Medium-deep coaxial heat exchanger (MDBHE) has the characteristic of “geothermal utilization without water extraction”, which is one of the important techniques to safely promote the medium-deep geothermal heating in northern China. As the heating performance of a MDBHE is unknown during long-term heat extraction, a numerical heat transfer model is proposed in this study to analyze the evolution of the fluid temperature and performance coefficient with the operating year based on the sustainable heating operation characteristics. Subsequently, a design optimization research was pertinently carried out. The results show that during the sustainable heating operation period, the fluid temperature of a MDBHE declines in the first five years and it barely changes after 20 years. The rock-soil thermal conductivity largely impacts the decline of the MDBHE seasonal performance factor. Under the condition of 1.5 W/(m·K), the heat source seasonal performance factor (SPF1) and heat pump unit seasonal performance factor (SPF2) reduce by 11.50% and 10.56%, respectively.In contrast, under the condition of 3.0 W/(m·K), the SPF1 and SPF2 only reduce by 4.73% and 4.23%, respectively. The decline is more pronounced under the condition of low rock-soil thermal conductivity.According to the sustainable heating performance evolution, the MDBHE design using heating performance in the “quasi-steady characteristics year” as a benchmark is proposed. Subsequently, the operating condition could be optimized by comprehensively considering the requirements of both operation and energy conservation.Consequently, the optimal operating velocities and heating loads of the MDBHE under the conditions of various geological parameters and pipe lengths were clarified, which have practical guiding significance for high-efficiency and sustainable heating design of MDBHE.
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Key words:
- geothermal energy /
- medium-deep /
- heat exchanger /
- sustainability /
- heating
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