Model for economic evaluation of closed-loop geothermal systems based on net present value

As a renewable energy source, geothermal energy has been widely used to provide space heating and cooling for buildings. The thermal performance of ground heat exchanger (GHE) is significant for the operating efficiency of the ground source heat pump (GSHP) systems. This paper presents a comprehensive review of developments and advances of three kinds of GHE, including vertical borehole GHE (VBGHE), Pile GHE (PGHE), and deep borehole GHE (DBGHE) which are currently popular in larger GSHP systems. Firstly, analytical models proposed to analyze heat transfer process of VBGHE with different geological conditions are summarized, such as homogenous or heterogeneous ground, with or without groundwater advection. Numerical and short-time step models and measures to improve GHE thermal performance are also reviewed. Secondly, a summary of research advances in PGHE is provided, which includes the heat transfer models of PGHE, the effects of geometric structure, operation modes, pile spacing, use of phase change material (PCM), thermal properties of PCM, thermo-mechanical behavior and/or thermal performance of PGHE. The effects of groundwater flow direction and velocity on PGHE are also summarized in brief. Lastly, models of three kinds of DBGHEs, i.e., deep coaxial GHE (DCGHE), deep U-bend GHE (DUGHE) and super-long gravity heat pipe (SLGHP), are reviewed. The physical bases of the different analytical models are elaborated and also their advantages and disadvantages are described. Advances in numerical modelling and improving numerical model calculation speed of DCBHE, DCBHE array, and DUBHE are summarized. The review provides a meaningful reference for the further study of GHEs. 

Advances in ground heat exchangers for space heating and cooling: review and perspectives

The heat generation in the photovoltaic (PV) panels leads to the rise in temperature, which subsequently deteriorates the electrical efficiency of PV panels. The application of phase change materials (PCM) can reduce the operating temperature and perform as a cooling mechanism. However, the cooling performance and energy storage capacity of PCM are drastically affected by its poor thermal conductivity. Therefore, in this paper, the impact of the insertion of metal foam on the heat transfer through PCM and the electrical efficiency of PV-PCM systems has been numerically investigate. A comparative study on the impact of the porosity of metal foam ranging from 0.7 to 0.9 has been carried out. Furthermore, different inclination angles, 0° to 90°, have been considered to evaluate the impact of inclination angle on the natural convection enhancement through liquid PCM. Results indicated that increasing the inclination angle results in better natural convection of molten PCM and better heat transfer through PCM. Moreover, results represented that compared with the PV-PCM system, 6.8% and 9.8% enhancement in the average temperature and electrical efficiency, in the case of insertion of metal foam with porosity of 0.9, is achieved. 

Analysis of the effects of porous media parameters and inclination angle on the thermal storage and efficiency improvement of a photovoltaic-phase change material system

Energy analysis and performance assessment of a hybrid deep borehole heat exchanger heating system with direct heating and coupled heat pump approaches

Is natural convection within an aquifer a critical phenomenon in deep borehole heat exchangers' efficiency?

Heat transfer analysis of U-type deep borehole heat exchangers of geothermal energy

The invention relates to a method for predicting the temperature of the circulating liquid of a medium-deep U-shaped heat exchange well. The medium-deep layer U-shaped heat exchange well comprises a descending pipe, a horizontal pipe and an ascending pipe. A plurality of nodes are arranged in the medium-deep layer U-shaped heat exchange well and rock soil on the periphery of the medium-deep layerU-shaped heat exchange well. And temperature information of the nodes at the circulating liquid inlet and the peripheral rock and soil nodes at the circulating liquid inlet at the initial moment is collected, the collected temperature information is substituted into a prediction model formed by node equations of the multiple nodes in the medium-deep layer U-shaped heat exchange well, and temperature information of other position nodes and other moments of the medium-deep layer U-shaped heat exchange well is obtained. The method is low in equipment investment, and the change rule of the temperature of the circulating liquid along with the position and time can be mastered.

Method for predicting temperature of circulating liquid of medium-deep U-shaped heat exchange well

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Heat transfer analysis of U-type deep borehole heat exchangers of geothermal energy

Method for predicting temperature of circulating liquid of medium-deep U-shaped heat exchange well