岩溶地区红粘土热湿迁移特性及其对地源热泵效率的影响

Southwest China is a typical carbonate karst region，and its overlying red clay layer exists complex heat and moisture migration coupling frequently. At the same time, the karst groundwater being rich in the region and complicated climatic environment has the remarkable influence on the heat transfer between the buried heat tube exchange system and soils. Therefore, in the process of using ground source heat pump technology to exploit shallow ground, there is not a reasonable heat transfer model for the red clay in the karst region, which makes the region facing significant challenges during geothermal energy utilization and promotion. Based on the tests, the project focuses on the core scientific problems of heat and moisture migration of red clay in the karst region, and emphasizes on three key scientific issues. They are the thermo physical characteristics of red clay, heat and moisture migration of soil in the heat transfer process, influence of geological structure and meteorological fluctuation on heat transfer. With laboratory test on thermo physical characteristics, model test of heat and moisture migration and in-situ monitoring test of ground source heat pump, the project will comprehensively reveal the heat and moisture migration mechanism of red clay in karst region and its influence on the heat transfer process between soils and heat pump. On the basis, numerical analysis on heat transfer performance of ground heat exchangers is presented to illustrate the heat transfer mechanism between the heat pump and red clay layers and identify the key influencing factors. At last, the optimization design method of heat transfer system of ground source heat pump is proposed.

我国西南部是碳酸盐岩溶的典型分布地区，其上覆红粘土层中存在着复杂的热-湿迁移耦合作用；同时该地区丰富的岩溶地下水和复杂的气候环境对地埋管与周围土壤之间的热交换过程产生显著影响；因此，在利用地源热泵技术开采浅层地能的过程中，一直未能针对岩溶地区的红粘土建立合理的换热模型，使该地区的地热能利用和推广面临着重大挑战。本项目将在试验的基础上，围绕岩溶地区红粘土中的热湿迁移效应这一核心科学问题，重点研究红粘土的热物理特性、换热过程土体的热湿迁移规律、岩土层地质构造与气象波动对地埋管热交换的影响三个基本问题；通过室内热物性试验、土体热湿传递模型试验和地源热泵现场原位观测，研究岩溶地区红粘土的热湿迁移过程机制及其对土-热泵之间的热交换过程的影响；在此基础上，对制约地埋管换热性能的各影响因素进行模拟分析，阐明红粘土中地源热泵与土层之间的换热机制并识别出关键的影响因素，提出地源热泵热交换系统的优化设计方法。

我国西南部是碳酸盐岩溶的典型分布地区，其上覆红粘土层中存在着复杂的热-湿迁移耦合作用；该地区丰富的岩溶地下水和复杂的气候环境对地埋管与周围土壤之间的热交换过程产生显著影响；因此，在利用地源热泵技术开采浅层地能的过程中，一直未能针对岩溶地区的红粘土建立合理的换热模型，使该地区的地热能利用和推广面临着重大挑战。项目组围绕这一课题，在大量试验的基础上，进行了岩溶地区红粘土的热湿迁移机制及其对土-热泵之间热交换影响的理论分析和数值模拟，取得以下主要成果：1）综合运用X射线衍射、全化学元素分析、Bogue法、K值法等方法确定了所取4种红粘土试样的矿物成分；采用热探针法对红粘土的热特性及变化规律进行研究，发现红粘土的热导率、比热容随体积含水率的增加而线性递增，热导率变化曲线存在临界含水率点；红粘土的热导率、比热容、热扩散率均随干密度的增加而线性递增；采用有效介质理论模型（EMT模型）和并联模型可分段计算广西红粘土的热导率；红粘土的热导率随温度升高、盐浓度增加而增加。2）建立了渗流-传热耦合一维模型试验装置和渗流-热传递-地埋管换热耦合的试验平台，探讨了温度场和渗流场耦合作用下红粘土的传热效能，并进行了数值模拟，发现渗流加快了土壤内热量传递，有利于散热，可有效缓解地埋管换热器周围土壤中热量堆积的现象。3）开展了不同工况的地源热泵运行试验，揭示地源热泵运行过程中土体的热湿迁移效应及其对系统运行特性的影响规律：热泵机组间隙运行有利于土壤温度场的恢复，系统性能系数COP增加；土壤温度场的变化幅度随着与地埋管距离的增加而递减，竖埋管和水平埋管热作用的影响半径分别约为2.0m、1.0m。4）对桂林岩溶地区土层进行了热交换模拟，分析了气象波动、土中渗流对地埋管换热效果的影响，揭示了岩溶地区季节性地下水变动影响下的土壤热交换规律，认为地下水渗流可使竖埋管换热效果提高到2.69~3.69倍，提出了合理的地埋管布置形式，为工程设计提供理论参考。