Abstract
Geothermal heat exchangers (GHEs) represent a buried pipe system, which can be utilised to harness renewable thermal energy stored in the ground to improve the efficiency of heating and cooling systems. Two basic arrangements of GHEs have been widely used: vertical and horizontal. Vertical GHEs generally have a better performance in comparison with the horizontal arrangement, and these systems are particularly suitable for confined spaces. Nevertheless, the main technical challenge associated with GHEs, for either the vertical or the horizontal arrangement, is the performance deterioration associated with an increase in the operation times during summer or winter seasons. In this paper, a combined horizontal-vertical GHE arrangement is proposed to address the current challenges. The combined GHE arrangement can be operated in five different modes, corresponding to different thermal loading conditions. These five operation modes of the combined GHE are analysed based on the transient finite difference models previously developed for the horizontal and vertical arrangements. The simulation results reveal that for the single operation mode (horizontal or vertical only), the vertical GHE performs better than the horizontal GHE due to relatively stable ground temperature deep down. While, for the combined operation mode, the series operations (horizontal to vertical or vertical to horizontal) of the GHE are superior to the split mode. It is found that the effect of the fluid mass flow rate ratio is trivial on the heat dissipation of the split mode GHE. The highest heat transfer rate in the split flow operational mode is rendered by the ratio of the mass flow rate of 40% horizontal and 60% vertical. In addition, the climate condition has more effect on GHE’s performance and the increase of the fluid flow rate it can enhance the amount of energy released by the GHE.
Highlights
Increasing energy demands and crude oil depletion have forced the world to explore new energy resources, those that are sustainable and have a minor environmental footprint.Shallow geothermal energy is one of the sustainable energy resources that could be applied to improve the efficiency of current and future heating and cooling systems
It was shallow region and lower a deeper zone.toAt depth(this of 12corresponded m below the ground surface, thehigher valueinofa the internal source termin was assumed bea zero the ground surface, the value of the internal source term was assumed to be zero
With the same length of the pipe system, the vertical Geothermal heat exchangers (GHEs) could release more energy than the horizontal GHE, as the initial soil temperature at a deeper layer was lower than that at a shallow region; When the GHE operated in combined modes, the amount of energy released by the GHE was increased, as the contact area, where heat was exchanged with the surrounding soil, was increased; The series operations of the GHE could release more energy than could be done in the split mode
Summary
Increasing energy demands and crude oil depletion have forced the world to explore new energy resources, those that are sustainable and have a minor environmental footprint. To address the degradation performance problem caused by an imbalance in the heating and cooling loads, a number of researchers have proposed hybrid GHE systems and different operation strategies [3]. A detailed numerical study was conducted by Zhu et al [15] to investigate the performance and economic characteristics of a combined vertical ground source heat pump and phase change material cooling storage system. Some other studies proposed a combined system of a ground source heat pump with different additional heat rejecter/absorber systems including a boiler, chiller, fuel cell, photovoltaic, and phase change material cooling storage systems. A ground source heat pump system with two different regions of BHEs including warm and cold was recently suggested and analysed It seems a comprehensive study of a combined GHE arrangement with different operation modes has yet to be presented. Based on the analysis of the outcomes of numerical simulations, the recommendations for the optimum operation of combined GHEs are summarised in the conclusion
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