Numerical investigation on the performance, sustainability, and efficiency of the deep borehole heat exchanger system for building heating

Abstract

IntroductionWith the growing demand on renewable energy all over the world, geothermal heating and cooling is increasingly utilized by applying borehole heat exchanger (BHE) coupled ground source heat pump (GSHP) systems. One of the factors preventing the further application of GSHP system in densely populated urban areas is the limiting land surface for installing the BHEs. Therefore, deeper boreholes with a depth of 2302 m have been AbstractIn 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. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

A Fast Simulation Approach to the Thermal Recovery Characteristics of Deep Borehole Heat Exchanger after Heat Extraction

“…Wang et al [30] carried a numerical investigation on the heat transfer characteristics and optimal design of the heat exchangers of a deep borehole ground source heat pump system, a field test was also performed in their work considering the lack of experiment verification of simulation study of DBHE, however, in their model only continuous condition was studied without intermittent state simulation. Chen and Shao et al [31] implemented a FEM numerical model in the open source software OpenGeoSys for the performance analysis of the DBHE. Kong et al [32] used FEM to demonstrate the feasibility of long-term and short-term operations of DBHE by considering the effect of geothermal gradients.…”

Optimization of the utilization of deep borehole heat exchangers

“…Over the past few years, more and more research work has been carried out to investigate the DBHE system performance. Several studies suggest that the heat transfer performance of the DBHE correlates significantly with the geothermal gradient and thermal conductivity of the surrounding rock (Bu et al 2012;Chen et al 2019;Cheng et al 2013;Kong et al 2017a, b;Le Lous et al 2015;Nalla et al 2005;Noorollahi et al 2015;Templeton et al 2014). Le Lous et al (2015) also conducted the sensitivity analysis of the volumetric heat capacity of the rock.…”

“…Furthermore, Nalla et al (2005) pointed out that a key parameter affecting the performance of the DBHE is the fluid residence time, which represents the flow rate. The other DBHE design parameters have also been investigated, including the borehole diameter, the diameter of the outer pipe, the diameter of the inner pipe, the depth of the borehole, grout materials and pipe materials (Alimonti and Soldo 2016;Beier and Holloway 2015;Chen et al 2019;Lhendup et al 2014;Nalla et al 2005;Wang et al 2017). According to the results of these analyses, borehole diameter, the outer pipe diameter, grout thermal conductivity, outer pipe thermal conductivity, and borehole depth can positively influence the heat transfer performance of DBHE (Alimonti and Soldo 2016;Chen et al 2019;Nalla et al 2005;Wang et al 2017).…”

“…The other DBHE design parameters have also been investigated, including the borehole diameter, the diameter of the outer pipe, the diameter of the inner pipe, the depth of the borehole, grout materials and pipe materials (Alimonti and Soldo 2016;Beier and Holloway 2015;Chen et al 2019;Lhendup et al 2014;Nalla et al 2005;Wang et al 2017). According to the results of these analyses, borehole diameter, the outer pipe diameter, grout thermal conductivity, outer pipe thermal conductivity, and borehole depth can positively influence the heat transfer performance of DBHE (Alimonti and Soldo 2016;Chen et al 2019;Nalla et al 2005;Wang et al 2017). In contrast, the inner pipe diameter and inner pipe wall thermal conductivity could negatively influence the heat transfer performance of DBHE (Alimonti and Soldo 2016;Beier and Holloway 2015;Chen et al 2019;Lhendup et al 2014;Nalla et al 2005).…”

“…According to the results of these analyses, borehole diameter, the outer pipe diameter, grout thermal conductivity, outer pipe thermal conductivity, and borehole depth can positively influence the heat transfer performance of DBHE (Alimonti and Soldo 2016;Chen et al 2019;Nalla et al 2005;Wang et al 2017). In contrast, the inner pipe diameter and inner pipe wall thermal conductivity could negatively influence the heat transfer performance of DBHE (Alimonti and Soldo 2016;Beier and Holloway 2015;Chen et al 2019;Lhendup et al 2014;Nalla et al 2005). Therefore, it can be concluded that all these DBHE design parameters can affect the system performance, and the optimal design of DBHE can be obtained, while it was rarely given in the literature.…”