In densely inhabited urban areas, deep borehole heat exchangers (DBHE) have been proposed to be integrated with the heat pump in order to utilize geothermal energy for building heating purposes. In this work, a comprehensive numerical model was constructed with the OpenGeoSys (OGS) software applying the dual-continuum approach. The model was verified against analytical solution, as well as by comparing with the integrated heat flux distribution. A series of modeling scenarios were designed and simulated in this study to perform the DBHE system analysis and to investigate the influence of pipe materials, grout thermal conductivity, geothermal gradient, soil thermal conductivity, and groundwater flow. It was found that the soil thermal conductivity is the most important parameter for the DBHE system performance. Both thermally enhanced grout and the thermally insulated inner pipe will elevate the outflow temperature of the DBHE. With an elevated geothermal gradient of 0.04 °C m−1, the short-term sustainable specific heat extraction rate imposed on the DBHE can be increased to 150–200 W m−1. The quantification of maximum heat extraction rate was conducted based on the modeling of 30-year-long operation scenarios. With a standard geothermal gradient of 0.03 °C m−1, the extraction rate has to be kept below 125 W m−1 in the long-term operation. To reflect the electricity consumption by circulating pump, the coefficient of system performance (CSP) was proposed in this work to better quantify the system efficiency. With the typical pipe structure and flow rate specified in this study, it is found that the lower limit of the DBHE system is at a CSP value of 3.7. The extended numerical model presented in this study can be applied to the design and optimization of DBHE-coupled ground source heat pump systems.

中文翻译：

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建筑物采暖深孔换热器系统性能，可持续性和效率的数值研究

在人口稠密的城市地区，已提出将深井热交换器（DBHE）与热泵集成在一起，以便将地热能用于建筑物供暖。在这项工作中，使用OpenGeoSys（OGS）软件应用双连续方法构建了一个综合的数值模型。通过分析解决方案以及与整体热通量分布的比较验证了该模型。在本研究中，设计并模拟了一系列建模方案，以进行DBHE系统分析，并研究管道材料，灌浆导热系数，地热梯度，土壤导热系数和地下水流量的影响。发现土壤热导率是DBHE系统性能的最重要参数。热灌浆和隔热的内管都会提高DBHE的流出温度。随着0.04°C m-1的升高地热梯度，施加在DBHE上的短期可持续比热提取率可以提高到150–200 W m-1。基于长达30年的运行方案的模型对最大热量提取率进行了量化。在0.03°C m-1的标准地热梯度下，长期运行时必须将提取速率保持在125 W m-1以下。为了反映循环泵的用电量，在这项工作中提出了系统性能系数（CSP），以更好地量化系统效率。根据本研究中指定的典型管道结构和流量，发现DBHE系统的下限为3.7的CSP值。