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As an alternative energy for fossil fuels, geothermal reserve is abundant. In recent years, scholars have investigated geothermal wells through numerical simulations. To the best of our knowledge, many studies have explored the influence factors of thermal power over time, and few studies have investigated the heat transfer mechanism between wellbore and formation in different circumstances. This paper aims to established a transient numerical heat transfer model of U-shaped closed-loop geothermal system with long horizontal section, which can simulate the temperature distribution and heat transfer mechanism during the whole wellbore formation. In the model, the convection term is discretized by using the second-order upwind scheme, and the finite volume method is used to solve it. The numerical results are proven to be correct by comparing with the field data of a well in China. Subsequently, the influence mechanism of fluid type, insulation casing length, fluid injection temperature, flow rate, and horizontal section length on thermal power is explored. The optimal depth of insulation casing under different injection temperatures, flow rates, and horizontal section lengths is suggested so as to provide crucial guidance for engineering practice. [ABSTRACT FROM AUTHOR]

The hybrid ground-coupled heat pump (HGCHP) systems with supplemental heat rejecter/supplier can effectively solve heat imbalance problems in the subsurface, and consequently improve the operation performance of the geothermal systems. For example, solar energy and/or industrial waste heat may be used as stable heat sources for underground heat storage in northern China with higher heating load, and cooling towers are installed to release heat into the air in southern China, where more cooling demand is needed. This paper reviews and discusses different HGCHP systems, which have been applied in China. And based on the heat transfer model of vertical borehole heat exchangers (BHE) for HGCHP systems, physical and mathematical models of multistage series circuits are developed to illustrate the heat transfer process of the underground thermal storage. A set of parameters, such as borehole spacing, heat recharging rate fractions and thermal properties of soils, which affect the thermal performance of the ground heat exchangers are analyzed, and the optimal solutions are discussed for engineering application. [ABSTRACT FROM AUTHOR]

The energy demand in China is rapidly growing, the development and applications of low energy technologies for green buildings have been paid much concern. The hollow block ventilated wall is proposed in this paper. According to the characteristic of the hollow part from up to down, the supply air passes through the cavity of the block, and then discharges to the outside after the heat exchange with the concrete hollow block, the cooling and heating requirements of envelope caused by outdoor environment can be partially removed; the building cooling/heating load can be greatly reduced or even eliminated. The Frequency-Domain Finite Difference (FDFD) model is established and the thermal performance analysis of hollow wall is carried out by using FDFD method in frequency domain. A CFD model is developed to simulate the thermal characteristics of this structure for reference for validating the FDFD model. The results demonstrate that the predictions of the FDFD and CFD models agree very well, and the FDFD model can provide accurate prediction. When the frequency varies, the frequency response of the heat flux is not obvious in low frequency region and is significant in high frequency. [ABSTRACT FROM AUTHOR]