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The heat transfer deterioration caused by frosting is the critical factor affecting the liquefied natural gas (LNG) vaporization efficiency of the ambient air vaporizer (AAV). In this paper, an LNG vaporization system using a solar-assisted heat pump (DX-SAHPNV) was proposed for AAV defrosting and LNG auxiliary heating, and a low-temperature LNG bypass flowing through the collector/evaporator was adopted for heat pump performance improvement and LNG preheating. Moreover, a dynamic model of the DX-SAHPNV system was established. Based on the meteorological data in Beijing, the system performance throughout the year and on typical days was analyzed, as well as the main meteorological factors effecting the system performance. Results showed the annual COP and the heat collection efficiency of collector/evporator with LNG bypass were 10.77% and 12.84% higher than that without LNG bypass, respectively. Comparing with the conventional AAV, the two-phase length of AAV in the DX-SAHPNV system was reduced by 12.7% and 20.2% on the typical winter and summer day, while the outlet LNG temperature was improved by 5.33 K and 11.56 K. The proposed system may provide a new perspective for the AAV defrosting and LNG vaporization optimization. [ABSTRACT FROM AUTHOR]

Aimed at applications in ground-coupled heat pump and thermal energy storage systems, this paper presents a heat transfer model of a deep borehole cluster, taking into account the geothermal gradient in the subsurface and the real connection conditions of the two regions inside and outside the borehole. The superposition principle is proven valid in a mathematical sense for such models in certain conditions, and the three-dimensional heat conduction problem for a multi-borehole heat exchanger may be decomposed into a number of two-dimensional single-borehole problems. Then, a new dimension reduction algorithm is proposed for simulation of heat transfer in the deep borehole cluster. Using this algorithm, comparisons with other commercial model and simulations on example cases show that computational efficiency can be improved by orders of magnitude compared with traditional three-dimensional software. • A new heat transfer model of a deep borehole cluster was developed. • The geothermal gradient and connection conditions on boreholes were considered. • The superposition principle was proven valid on the boundary of the borehole walls. • A dimension reduction algorithm was proposed for a deep borehole cluster. • The proposed model has a high computational efficiency. [ABSTRACT FROM AUTHOR]

In order to solve the problem of heat build-up in subway tunnels, a subway heat pump system with capillary network as front-end heat exchanger is proposed. A one-dimensional simplified plate heat transfer calculation method for capillary heat exchanger (CHE) based on surface-heat-source is proposed. Taking into consideration of the changing heat flux during heat transfer, this paper adopts step load, superposition principle, energy conservation and iterative method to establish the heat transfer model of CHE. CHE heat transfer tests were carried out at the Zhiquan Road Station of Qingdao Metro R2 line to prove this model. The results show that the max error of the model is 8.79%. In addition, the impact of such factors as inlet water temperature of CHE, the lining thermal conductivity, the thermal conductivity of surrounding rock and thermal diffusivity of the surrounding rock to the performance of the heat exchanger has been analyzed. • A capillary heat exchanger used in subway source heat pump system is proposed. • A one-dimensional plate heat exchange model for capillary heat exchanger is proposed. • The accuracy of the heat transfer model has been verified by experiment. • The four main influence factor of the capillary heat exchanger capacity are analyzed. [ABSTRACT FROM AUTHOR]

This paper presents the analysis, simulation and performance study of heat transfer in a single U-tube borehole heat exchanger (BHE). Unsteady heat transfer method was used to analyze the heat transfer process of both inside and outside the borehole. Implicit numerical method applied on heat transfer equations obtained from energy balance (accompanied with thermal resistance model) was used to obtain the solution. The variation of mean fluid temperature, average borehole wall, grout and nearby ground temperature as well as borehole loading with borehole depth and time were investigated. Dynamic simulation was also performed to assess the influence of important parameters. The effect of two parameters, fluid mass flowrate and thermal conductivity of grout, on mean fluid temperature, borehole wall, grout and ground temperature as well as on borehole loading and borehole thermal effectiveness were investigated. The outcome of this study can substantially reduce the time devoted for research and to quickly determine the impact of various parameters on performance of vertical single U-tube borehole heat exchanger. Furthermore, the obtained result can be utilized as a reference for the design and optimization of heating and cooling system integrated with ground coupled heat pump system. • Transient heat transfer analysis for both inside and outside the single U-tube BHE was performed. • Modeling equations that govern heat transfer process inside and outside the BHE has been developed. • Variation of borehole loading and effectiveness of BHE with depth and time was examined. • Effect of mass flow rate and grout conductivity on performance of BHE was investigated. • Impact of heat carrier fluids on performance of single U-tube BHE was assessed. [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]

Thermal performance of vertical U-pipe borehole heat exchanger (BHE) is an important research subject for the design and application of ground-coupled heat pump system (GCHPs). This paper presents an improved evaluation method for thermal performance of BHE based on analytical solution model, which is validated by comparing with duct storage system (DST) model and field experiment. Using the evaluation method, impacts of inlet fluid temperature, fluid flow rate and borehole depth on thermal performance of two types of U-pipe BHEs are studied. The study provides a good alternative of the experimental method for thermal performance test (TPT) and a useful thermal performance evaluation tool for BHEs in GCHPs. [ABSTRACT FROM AUTHOR]

This paper presents a numerical heat transfer model for vertical U-tube Ground Heat Exchangers (GHE). This model is uniquely able to take into to account different initial soil temperatures and physical properties at different depths. The model has been validated based on an experimental case study and has been used to simulate the thermal performance of GHEs. The simulation results show that for a 100-m vertical GHE, the first 70 m of the vertically buried GHE has a higher heat transfer capability than its last 30-m section. In addition, the validated model is used to investigate the optimal depth of vertical GHEs in five case studies ranging from 60 to 100 m length. Among them, the simulation results demonstrate that the GHE with buried depth of 70 m is able to provide the highest heat exchange rate per unit depth (54.1 and 47.0 W/m under heat rejection/extraction mode). It consequently results in shortest total GHE length of 11,388 m and the minimum cost of 1.82 million Yuan. However, a larger area is needed for boreholes and GHEs installation. Therefore, the optimal buried depth of the vertical U-tube GHEs for the studied case is 70 m on the condition of allocation of an abundant area to set up the boreholes. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents experimental study of thermal charging the boreholes that are used for inter-seasonal thermal storage of heat and coolth integrated with ground-coupled heat pump and unglazed solar collectors. After 180 days of thermal charging, it was observed that the temperature of the ground at 21 m depth and 1 m distance from the borehole had increased by 2.5 °C. The unglazed collectors are able to collect heat for charging the heat storage borehole at an average of 43.9 MJ day−1. The system is able to charge borehole at an average heat transfer rate of 57 W m−1. A comparison of the experimental results with the simulated results of a TRNSYS model of the system showed a good agreement. The mean efficiency of the unglazed solar collector during 180 days charging operation was found to be 30% and the mean efficiency of the system was found to be 38%. [Copyright &y& Elsevier]

Abstract: In this paper an improvement of the model CaRM (CApacity Resistance Model) is presented to consider the borehole thermal capacitance, both of the filling material of the borehole and of the heat carrier fluid inside the ground heat exchanger. Several models, numerical and analytical, are available in literature for short time step analyses of ground-coupled heat pump systems. According to the modelling for the surrounding ground, the new approach for the inside of the borehole is based on electrical analogy. In this study the double U-tube ground heat exchanger is analyzed. The new model has been validated by means of a commercial software based on the finite elements method as well as measurements of ground response test, using a suitable plant system. In this last comparison, the contribution of the thermal capacitance of the circulating fluid is investigated, since it is frequently neglected in short time step simulations. In both cases, there is agreement between the CaRM results and data from numerical simulations and measurements as well. [Copyright &y& Elsevier]

Abstract: This paper develops previous research on space heating in passive houses (PH). The results reported are based on a complex theoretical model used to simulate the heating system operation in Pirmasens PH (Rhineland Palatinate, Germany). The economic feasibility of different active space heating systems based on ground thermal energy utilization was studied. Both the heating and cooling potentials of the system under real climatic conditions were investigated. On medium- and long-time operation (i.e. longer than 3–10 years), a ground-source heat pump (GSHP) configuration proves to be the best economical solution. The configuration based on a ground heat exchanger is also a better solution than conventional heating in case the operation time is longer than 20–30 years. As compared to the GSHP configuration, it has the advantage of a much lower investment cost. [Copyright &y& Elsevier]

Abstract: This paper describes experiments and analyses on an improved underground heat exchanger by using a no-dig method for the purpose of the cost reduction of a space heating and cooling system using underground thermal energy. First, the improved underground heat exchanger was installed on the campus of Hokkaido University, and it was shown that a ground source heat pump system utilizing the heat exchanger was sufficient for space heating and cooling. Second, evaluation program of the heat exchanger was developed, and the program was verified to give good predictions by comparing with experimental results. As a result of system simulations, an energy reduction for a system installation relative to a conventional vertical earth heat exchanger reached 78%. The primary energy reduction rate including the system installation and operation relative to a typical air source heat pump was 29%. [Copyright &y& Elsevier]

Abstract: Adsorption heat pump and refrigeration systems are noiseless, non-corrosive and environment friendly. For these reasons the research activities in this sector are still increasing to solve the crucial points that make these systems not yet ready to compete with the well-known vapor compression system. There is an increasing interest in the development and use of adsorption chillers due to their various economic and impressive environmental benefits, enabling solar energy or waste heat to be used for applications such as district networks and cogeneration plants. In order to increase the cooling power, much effort has been devoted to enhance materials heat transfer properties. Consolidated adsorbent could be the solution. This paper provides a literature review on current progress of consolidated adsorbent for adsorption systems. A number of consolidated technologies of adsorbent are discussed. The related work at our laboratory is also discussed. [Copyright &y& Elsevier]