Thermal sustainability of low-temperature geothermal energy systems: System interactions and environmental impacts.

Date
2013-10-01
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Abstract
In storing heat during summer for use in winter, the ground provides a better source/sink of heat than the outside air in regards to heat pump efficiency, being cooler than the outside air in the summer and warmer in the winter. Due to their good efficiency, the use of geothermal energy is often encouraged; however, two issues arise in the long-term use of ground for thermal purposes: the sustainability and impact of these systems on the environment. Studies show the potential of the geothermal heat exchangers for environmental impacts such as undesirable temperature rises from these systems in temperature-sensitive regions. Furthermore, interference between adjoining installations is being reported, raising issues of sustainability in terms of performance and equitable sharing of natural resources. The temperature of the soil surrounding the ground heat exchangers (GHEs) and the heat flows in this region are the key factors in determining environmental impacts and their potential thermal interaction. In this study, analytical and numerical models of vertical heat exchangers are presented. First, the effect of system parameters such as borehole spacing on the transient response of two GHEs is described. Second, a numerical finite volume method in a two-dimensional meshed domain is used to evaluate the temperature rise and the heat flows in the soil surrounding borehole systems over the long term. Finally, to examine the effect of temperature rise in the soil surrounding a vertical GHE on the performance of an associated ground heat pump, a reversible heat pump model is coupled to the heat exchanger analytical model via the heat exchanger running fluid temperature. The heat exchanger running fluid temperature, wall temperature and its heat load profile are the main coupling parameters between the three models. The results of the analytical model are compared with ones of a finite volume numerical model. Below a certain depth, the temperature of the ground remains almost unchanged throughout the year. This phenomenon can be exploited by placing a heat exchanger in the ground and coupling it to a heat pump to store heat in the ground during summer for use in winter. The ground provides a better source/sink of heat than the outside air in regards to heat pump efficiency, being cooler than the outside air in the summer and warmer in the winter. Due to their good efficiency, the use of geothermal energy is often encouraged; however, two issues arise in the long-term use of ground for thermal purposes: the sustainability and impact of these systems on the environment. Studies show the potential of the geothermal heat exchangers for environmental impacts such as migration of thermal plumes away from these systems which may cause undesirable temperature rises in temperature-sensitive regions. Furthermore, interference between adjoining installations is being reported, raising issues of sustainability in terms of performance and equitable sharing of natural resources. With increasing interest in installing such systems in the ground and their potential dense population in coming years, regulations need to be implemented to prevent their thermal interaction and their possible negative effects on the design and performance of nearby systems. The temperature of the soil surrounding the ground heat exchangers (GHEs) and the heat flows in this region are the key factors in determining environmental impacts and their potential thermal interaction. Modeling such systems is important for understanding, designing and optimizing their performances and characteristics. In this study, a number of analytical and numerical models of vertical heat exchangers are presented. Through these models, the temperature of the soil surrounding the GHEs and the heat flows in this region can be determined. Thus, the effect of possible thermal interaction between these systems on their coupling heat pump as well as their environmental impacts can be studied. First, the two-dimensional transient conduction of heat in the soil around single and multiple GHEs is discussed via numerical and analytical methods. The effect of system parameters such as borehole spacing as well as heat store capacity on the transient response of two GHEs is described. The results of the temperature response of the soil around a borehole, calculated with an analytical line source theory, are compared with the soil temperature rise calculated numerically. In addition, a three-dimensional numerical study is performed to examine the axial heat transfer effects in heat conduction in the soil surrounding a borehole and especially near its top and bottom. Second, the long-term performance of multiple vertical GHEs is investigated in order to examine their interaction as well as migration of thermal plumes away from these systems which may cause undesirable temperature rises in temperature-sensitive regions. A numerical finite volume method in a two-dimensional meshed domain is used to evaluate the temperature rise in the soil surrounding multiple borehole systems over the long term, for a period of 5 years. A heat flux from the borehole wall is assumed, reflecting the annual variation of heat storage/removal in the ground. By choosing a heat boundary at the borehole wall, it is assumed that the inlet temperature of the circulating fluid running in the U-tube inside the borehole will be adjusted according to the flow rate. The selection of the sinusoidal function is based on the heat pump power consumption and building heating and cooling needs gained via the bin method for a typical building in Belleville, IL. Next, to account the variation in heating strength along the borehole length due to the temperature variation of the fluid flowing in the U-tube, the finite line source model and the numerical model in a three dimensional domain are both coupled to the model inside the borehole and the results are compared in terms of the soil temperature rise and the borehole wall heat flux. Thus, critical depths at which the maximum heat flow rate occurs, resulting in thermal interaction, can be determined. Finally, with some improvements to the coupling procedure, the coupled model is used to investigate interacting borehole systems with a periodic heat flow rate in the long term system operation (30 years). To examine the effect of temperature rise in the soil surrounding a vertical GHE on the performance of an associated ground heat pump, a reversible heat pump model is coupled to the model inside the borehole via the running fluid temperature in the U-tube inside the borehole. The running fluid temperature, the borehole wall temperature and the heat load profile are the main coupling parameters between the three models. The results of the analytical model are compared with ones of a finite volume numerical model.
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Keywords
Ground heat exchanger, Heat pump, Thermal interaction, Numerical analysis
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