Performance of ground heat exchangers: A comprehensive review of recent advances

The ground source heat pumps (GSHPs) have been extensively applied to commercial and residential buildings owing to their high-energy efficiencies and low running costs. The key component of the GSHP is a ground heat exchanger (GHE). The state-of-the-art two-dimensional (2D) and three-dimensional (3D) heat transfer models for borehole heat exchanger (BHE) and energy pile (EP) systems are reviewed in this paper. The physical procedures of heat transfer and the derivation of energy conservation within different channels of BHE (e.g., U-, W-, helical-shaped or coaxial-shaped) are summarized, in addition to the primary merits and demerits of each model. The main influencing factors on 2D and 3D model solutions including axial heat transfer, friction heat, spacing shack, thermal resistance, thermal short-circuiting between the inlet-pipe and outlet-pipe, are analysed and compared. Furthermore, various applications of these 2D and 3D models are elaborated. Finally, the recommendations, standpoints and potential future research on BHE heat transfer model are highlighted. It is believed that the work presented will contribute to the record of information and experiences necessary to develop BHEs for GSHP systems.

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A comprehensive review on 2D and 3D models of vertical ground heat exchangers

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

The present paper investigates numerically the effect of an external magnetic field on heat transfer and entropy generation of Ag–MgO (50:50 vol%)/water hybrid nanofluid flow in a partially heated irregular ventilated cavity. A finite-volume FORTRAN code has been written to solve the governing partial differential equations. New empirical correlations specifically dedicated to predict the dynamic viscosity and the thermal conductivity of the considered hybrid nanofluid were employed. After validation of model, the analysis has been done for a wide range of Reynolds number (10 ≼ Re ≼ 600), Hartmann number (0 ≼ Ha ≼ 80) and total nanoparticle volume fraction (0 ≼ φ ≼ 0.02). The results are presented in terms of streamlines, isotherms and isentropic lines as well as the average Nusselt number (Num), the average entropy generation (Sgen,m) and the Bejan number (Beavg). The criterion ξ = Sgen,m/Num is adopted to discuss the thermal performances of the system. The results reveal that the intensification of the magnetic field tends to attenuate the heat transfer convection and to reduce the thickness of the thermal boundary layer, close to the active walls. Globally, adding nanoparticles to the base fluid improves the heat transfer but increases the total entropy generation.

Second law analysis of MHD mixed convection heat transfer in a vented irregular cavity filled with Ag–MgO/water hybrid nanofluid

Heat transfer simulation, analysis and performance study of single U-tube borehole heat exchanger

An experimental and numerical study has been carried out in order to investigate mixed and natural convection heat transfer in a two-dimensional enclosure. A discrete isothermal heat source is located at one of the vertical walls. Also, two ventilation ports are at the bottom and on top of the opposite wall. A forced flow condition was imposed by providing an inlet of air at the bottom port. A Mach–Zehnder interferometer was used to visualize the temperature field within the enclosure and to determine the local and average heat transfer characteristics of the heat source. Five heater positions on the vertical wall and different Rayleigh numbers (4.5 × 105 to 1.15 × 106) and Reynolds numbers (120 to 1600) were considered in the experiments. A finite volume code has been developed based on the SIMPLE algorithm and hybrid discretization scheme for the numerical study. It is observed that the interaction of natural convection with the forced flow leads to various flow fields depending on the Richardson number...

Experimental and Numerical Study of Mixed and Natural Convection in an Enclosure With a Discrete Heat Source and Ventilation Ports

Thermal resistance capacity model for short-term borehole heat exchanger simulation with non-stiff ordinary differential equations

In the present paper, a simple but accurate transient model to evaluate the thermal performance of earth to air heat exchangers (EAHEs) is developed Governing equations for both pipe flow and soil surrounding the pipe are derived for the transient state and are solved using Laplacian transform To validate the analytical model, the 3D numerical model of the EAHE is also simulated Results of the analytical model and numerical simulations are compared with already reported experimental data In heating period at velocity of 2 m/s, the discrepancy between the analytical model and reported experimental results is 087% and 131% for steel and PVC pipes, respectively For velocity of 5 m/s this was as low as 04% for both pipe materials For the cooling condition, a maximum discrepancy occurs for PVC pipe at 2 m/s equal to 955% Analytical results were compared with numerical simulations for both constant and time-varying inlet temperatures, and great correlation was observed Thermal diffusion into the soil was also investigated after 6 h and 120 h of continuous operation Results revealed that for the fluid velocity of 5 m/s, after 6 and 120 h of continuous operation, heat diffused up to 10 cm and 50 cm distance of the pipe axis, respectively, which showed the thermal saturation of the soil with passing the time in continuous operation mode Results show that thermal saturation of the soil should be taken into the model as ignoring it can cause an overestimation in system performance

Asgar Minaei

Papers

Experimental and Numerical Study of Mixed and Natural Convection in an Enclosure With a Discrete Heat Source and Ventilation Ports

Thermal resistance capacity model for short-term borehole heat exchanger simulation with non-stiff ordinary differential equations

A new transient analytical model for heat transfer of earth-to-air heat exchangers

A new analytical model for short-term borehole heat exchanger based on thermal resistance capacity model

Thermal resistance capacity model for transient simulation of Earth-Air Heat Exchangers