Numerical study on the effects of design parameters on the heat transfer performance of coaxial deep borehole heat exchanger

Abstract

Summary Deep borehole heat exchanger (DBHE) is attracting attention intensively owing to much more geothermal extraction, higher efficiency for heat pumps, and lesser land demand compared with shallow borehole heat exchanger. DBHE is usually dipped into several thousand meters in the subsurface, having a complicated heat transfer with surrounding rock–soil. However, the heat transfer characteristics below surface under different conditions are rarely studied. In this study, a numerical model considering the comprehensive effects of geothermal gradients and heat loss from inner pipe was proposed. The model was validated with experimental data and Beier analytical solution. Based on the model, the effects of primary design parameters on the heat transfer performance below surface along the pipe were investigated. The results indicate that temperature at pipe bottom increases with inlet flow rate decreasing, while the heat load cannot be extracted fully to the surface because of the heat loss of inner pipe. When the inlet flow rates decrease from 41.39 to 4.52 m3/h, the heat loss ratio increases from 25.5% to 63.7%. It is an effective way of insulating inner pipe to reduce heat loss under low inlet flow rates. Increasing the velocity in inner pipe by lessening the inner pipe diameter can also decline the heat loss well. While by this way, the increasing pumping power resulting from the higher velocity in inner pipe has to be considered. This study is significant to effective optimization of DBHE and energy conservation of buildings.

Longevity and power density of intermediate-to-deep geothermal wells in district heating applications

“…Furthermore, in analysing the performance of well lattices, the principle of superposition is often applied, although it is incorrect-the single borehole solution does not satisfy the global boundary condition of the entire lattice installation. It is no surprise then that some recent studies mix analytical theory with numerical methods [15] or rely on numerical solutions alone in addressing intermediate-to-deep wells [16]. In our analysis, only in the case of a constant mass flow we do find a good agreement with a simplified analytical theory that we have developed in Appendix, and are thus able to accurately explain the characteristics of the more nuanced numerical solution.…”

Geothermal energy extraction using abandoned oil and gas wells:Techno‐economicand policy review

“…The principal variable for the EGS or HDR technology is the geothermal gradient which has been illustrated in Figure 1. Deeper well generally has a higher potential for geothermal energy extraction 49 . Therefore, the selection of abandoned oil well for geothermal energy extraction will strongly depend on the geothermal gradient recorded on the intended well.…”

Simulation Analysis on the Heat Performance of Deep Borehole Heat Exchangers in Medium-Depth Geothermal Heat Pump Systems

“…(1) According to previous studies [27][28][29], the fluctuation of ground surface temperature has little influence on the thermal performance of DHBEs. Thus the surface temperature is set as 13.0 °C , which is the annual average ambient temperature of Xi'an.…”

“…However, due to the lack of engineering experience, the physical parameters of DBHE deviated from the actual values, especially about the heat conductivity coefficient of the outer pipe, thus the simulation results failed to match the field test results. Other researchers also focused on the numerical simulation with FVM model to analyze the heat transfer performance of DBHE [27][28][29]. Better than previous studies, field tests were conducted thus the accuracy of simulation could be checked with operational data.…”