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The ground thermal properties are the basic parameters for the design of borehole heat exchanger (BHE) in ground–coupled heat pump system (GCHPs), and their accuracy directly affects the economy and reliability of the heat pump system. In traditional design, ground is usually regarded as an isotropic homogeneous medium, and its thermal properties are obtained by solving the inverse heat transfer problem in BHE through in-situ thermal response test (TRT). In fact, different geological layers can be observed along the depth of BHE, and thermal physical properties of each layer are different based on the ground geological conditions. In order to investigate the effect of geological stratification on estimated accuracy of ground thermal parameters in TRT, a validated numerical layered BHE model (NLBM) is presented to simulated TRT, and the fluid temperature response is used to estimate effective ground thermal conductivity and borehole thermal resistance based on line source model (LSM). Secondly, the distributions of fluid temperature in U-pipe and heat flux of BHE along the depth are compared and analyzed based on the NLBM and the numerical homogeneous BHE model (NHBM). At last, borehole thermal resistance from the NLBM and estimated borehole thermal resistance from LSM are compared. The results show the maximum heat flux in the 3rd layer of the NLBM is 21.1% higher than that of NHBM, and the minimum heat flux in the 1st layer is reduced by 46.9% in the 100 h duration. The estimated λ LSM using the fluid temperature responses in the NLBM is 1.3% higher than the thickness-weighted thermal conductivity in the NLBM. The minimum relative error between R LSM and R b is still up to 10.45% in the duration of 100 h even though extending the duration is helpful to improve the estimated accuracy of R LSM. • A numerical layered BHE model is proposed and validated by the experiment. • The distributions of fluid temperatures are compared from the two models. • The differences of heat flux are illustrated in the two models. • Effect of geological stratification on estimated accuracy is investigated in TRT. [ABSTRACT FROM AUTHOR]

U-bend deep borehole heat exchanger (DBHE) is a new promising kind of ground heat exchanger, but nowadays there are lack of accurate efficient models to simulate U-bend DBHE. This paper has developed a semi-analytical model of U-bend DBHE, which considers the unsteady heat transfer of borehole, heterogenous soil thermal properties and geothermal gradient. A 2D numerical model and an analytical model based on infinite line-source theory are also developed, and the three models are compared with experimental data for different boundary conditions. Based on the soil thermal properties estimated by logging data and rock identification, the three models have large errors when compared with experimental data. Based on a soil thermal conductivity λ s estimated by matching experimental data, the three models match better with experimental data, and it is inferred that λ s should be measured by thermal response test instead of rock identification. The semi-analytical model and 2D numerical model match very well for all the time, but the analytical model has large differences with 2D numerical model, which is because the analytical model ignores borehole heat capacity and time-varying heat flows at the borehole wall. This paper could provide a cost-effective semi-analytical model and some advices for U-bend DBHE. [ABSTRACT FROM AUTHOR]

U-bend deep borehole heat exchanger (DBHE) is a new promising kind of ground heat exchanger, but nowadays there are lack of accurate efficient models to simulate U-bend DBHE. This paper has developed a semi-analytical model of U-bend DBHE, which considers the unsteady heat transfer of borehole, heterogenous soil thermal properties and geothermal gradient. A 2D numerical model and an analytical model based on infinite line-source theory are also developed, and the three models are compared with experimental data for different boundary conditions. Based on the soil thermal properties estimated by logging data and rock identification, the three models have large errors when compared with experimental data. Based on a soil thermal conductivity λ s estimated by matching experimental data, the three models match better with experimental data, and it is inferred that λ s should be measured by thermal response test instead of rock identification. The semi-analytical model and 2D numerical model match very well for all the time, but the analytical model has large differences with 2D numerical model, which is because the analytical model ignores borehole heat capacity and time-varying heat flows at the borehole wall. This paper could provide a cost-effective semi-analytical model and some advices for U-bend DBHE. [ABSTRACT FROM AUTHOR]

This paper proposes an integrated photovoltaic vacuum glazing unit with an intermediate air cavity and a calibrated modelling approach to quantify its thermal properties and evaluate the heat transfer performance. Theoretical analyses of the heat transfer process are conducted with reasonable hypotheses and traceable boundary conditions. Three-dimensional heat transfer models are then established and cross-validated against previous publications. The detailed validation demonstrates the reliability of the developed complex models under different circumstances. Furthermore, four photovoltaic vacuum glazing configurations are compared in terms of the temperature distribution and overall heat transfer coefficient (i.e. U-value). Simulation results show that the photovoltaic vacuum double glazing can achieve the optimum performance among the four configurations based on simultaneous consideration of the PV module temperature and U-value. Sensitivity analyses of glazing design factors are also conducted for the U-value, which is found to be greatly reduced by decreasing the density and diameter of vacuum pillars as well as the glass thermal conductivity. A lowest U-value of 0.23 W/(m2·K) is achieved for the photovoltaic-vacuum double glazing and can be further improved with future design optimizations. This research can provide guidance to design improvement of PV vacuum glazing systems and promote their integration with building modelling tools. • A 3D heat transfer model of PV vacuum glazing is established and cross-validated. • Four compositive PV vacuum glazing configurations are proposed and compared. • The intermediate air cavity can reduce the U-value of PV vacuum glazing by 25%. • The PV vacuum glazing can achieve a U-value of 0.23 W/(m2·K). • Sensitivity analysis can provide a design guideline for PV vacuum glazing. [ABSTRACT FROM AUTHOR]

NaCl–CaCl 2 molten salt is considered as a promising high-temperature heat transfer and storage fluid for advanced nuclear power plants and concentrating solar power plants in the field of renewable energy utilization. However, the comprehensive physical properties and their microscopic mechanisms for the molten NaCl–CaCl 2 are failed to be measured accurately due to the extremely measuring condition. In this work, the ab-initio molecular dynamics simulation is used to investigate its microstructures and thermophysical properties for entire operating temperatures. It reveals that ion clusters are formed in terms of three for face-sharing, two for edge-sharing, and one for corner-sharing Cl− ions between the coordination shells of two neighboring cations. The coordination numbers of Na+-Cl- and Ca2+-Cl- ion pairs decrease from 5.88 to 6.46 at 783 K to 5.33 and 6.02 at 1173 K respectively. Meanwhile, the reliable and meaningful values of densities, ion self-diffusion coefficients, viscosities, and thermal conductivities were evaluated from 783 to 1173 K. It suggests that the distances and interactions between ions pairs determine thermodynamic properties directly. The ab-initio molecular dynamics simulation is proved to be an effective way to obtain the essential data for the designs of heat transfer and thermal energy storage systems in practical applications. • Ab-initio molecular dynamics simulations are applied for NaCl–CaCl 2. • Approach-to-equilibrium molecular dynamics method is used for thermal conductivity. • The correlation between microstructures and thermophysical properties are studied. • The distances and interactions between ion pairs determine its properties. [ABSTRACT FROM AUTHOR]

Effective utilization of solar energy resource enables to improve the thermal performance of solar evacuated tube. In present work, the back side of solar vacuum tube was coupled with mini-compound parabolic concentrator (mini-CPC). The thermal characteristics of the presented mini-CPC solar vacuum tube were analyzed theoretically and experimentally. A mathematical model of heat transfer for the mini-CPC vacuum tube was established according to the energy conservation, and solved by iterative calculation based on home-built C programming language. The obtained numerical solutions fit in well with the experimental results. Furthermore, analysis indicates the vacuum interlayer plays a crucial role in hindering heat transfer. The thermal-convection resistance (R co-air,conv) dominates the heat loss from cover tube to ambient rather than thermal-radiation resistance (R co-sky,rad). The measured final temperature increment of working water for the mini-CPC and ordinary evacuated tube can reach 63.4 K and 49.8 K, respectively. An experimental result also reveals that the thermal efficiency of the mini-CPC vacuum tube is increased by 24.3%–29.2% compared with that of vacuum tube without mini-CPC, considering various weather conditions. Consequently, the designed mini-CPC evacuated tube shows a preferable performance, which may provide a certain reference in technology for engineering applications. • A mini-CPC evacuated tube to utilize effectively solar energy was designed. • Construction and solution of heat transfer model for mini-CPC evacuated tube. • Theoretical solution fits in well with the experimental result. • The temperature increment of working water can reach 63.4 K for the mini-CPC. • The improved ratio of thermal efficiency is 24.3%–29.2%. [ABSTRACT FROM AUTHOR]

Effective utilization of solar energy resource enables to improve the thermal performance of solar evacuated tube. In present work, the back side of solar vacuum tube was coupled with mini-compound parabolic concentrator (mini-CPC). The thermal characteristics of the presented mini-CPC solar vacuum tube were analyzed theoretically and experimentally. A mathematical model of heat transfer for the mini-CPC vacuum tube was established according to the energy conservation, and solved by iterative calculation based on home-built C programming language. The obtained numerical solutions fit in well with the experimental results. Furthermore, analysis indicates the vacuum interlayer plays a crucial role in hindering heat transfer. The thermal-convection resistance (R co-air,conv) dominates the heat loss from cover tube to ambient rather than thermal-radiation resistance (R co-sky,rad). The measured final temperature increment of working water for the mini-CPC and ordinary evacuated tube can reach 63.4 K and 49.8 K, respectively. An experimental result also reveals that the thermal efficiency of the mini-CPC vacuum tube is increased by 24.3%–29.2% compared with that of vacuum tube without mini-CPC, considering various weather conditions. Consequently, the designed mini-CPC evacuated tube shows a preferable performance, which may provide a certain reference in technology for engineering applications. • A mini-CPC evacuated tube to utilize effectively solar energy was designed. • Construction and solution of heat transfer model for mini-CPC evacuated tube. • Theoretical solution fits in well with the experimental result. • The temperature increment of working water can reach 63.4 K for the mini-CPC. • The improved ratio of thermal efficiency is 24.3%–29.2%. [ABSTRACT FROM AUTHOR]

A novel nano phase change material emulsion (NPCE) with low supercooling and high thermal conductivity was prepared by sonication method. N -octadecane was employed as phase change material, multi-walled carbon nanotubes (MWCNTs) were utilised as high thermal conductivity material, and octadecanol was utilised as nucleating agent. The characterization and thermal properties of the nanoemulsions prepared with various concentrations of MWCNTs and octadecanol were measured and analysed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size analyser, differential scanning calorimeter (DSC) and thermal conductivity meter. The results indicated that the nanoemulsions prepared had great stability, low supercooling and enhanced thermal conductivity. The thermal conductivity was enhanced by 4.32% for 10 wt% nanoemulsion with addition of 1 wt% MWCNTs. The supercooling degree of 20 wt% nanoemulsion was decreased by 36.4% from 17.3 °C to 11.0 °C with addition of 1 wt% octadecanol. It can be concluded that the nanoemulsions prepared were able to be utilised as heat transfer and energy storage fluids, with great potential in thermal system applications. • The enhancement ratio of thermal conductivity was 4.32% for 10 wt% nanoemulsion. • The supercooling of 20 wt% nanoemulsion was decreased by 36.4% with addition of 1 wt% octadecanol. • Fabricated nanoemulsions presented good stability and thermo-physical properties. • Fabricated nanoemulsions have great potential application in the thermal systems. [ABSTRACT FROM AUTHOR]

Abstract In the present work, an experimental investigation was conducted to quantify the heat transfer coefficient, thermal resistance and the thermal performance of a thermosyphon heat pipe charged with zirconia-acetone nanofluid. The thermosyphon was designed to operate at high heat fluxes such as 200 kW/m2 (∼200 sun) similar or larger than the evacuated solar tube heat pipes. The zirconia-acetone nanofluid was prepared and stabilized by adding a surfactant, homogenizing with sonication and setting the pH of the nanofluid with a buffer solution. The thermosyphon was fabricated with an oxygen-free copper with the internal and outer diameters of 10.7 mm and 12 mm, respectively and the axial length of 280 mm. The thermal performance of the heat pipe was assessed for various heat fluxes (1 kW/m2-200 kW/m2), filling ratio (20%–75%), tilt angle (5°–70°) and the mass fraction of the nanofluid (0.025%–0.1%). Results showed that the presence of the nanofluid decreases the total thermal resistance of the heat pipe reaching the minimum value at the largest heat flux applied to the evaporator. Also, the heat transfer coefficient of the evaporator was increased. Likewise, the zirconia-acetone nanofluid enhanced the boiling heat transfer mechanism and the geyser boiling, which resulted in the enhancement in the thermal performance of the heat pipe due to the increase in the heat transfer coefficient of the heat pipe by 36.3% at wt. % = 0.1 in comparison with the pure acetone. The optimum tilt angle and filling ratio values were 65° and 60%, respectively, in which the highest heat transfer coefficient was achieved. A correlation was also developed using a dimensionless analysis to predict the Kutateladze number as an index for the thermal performance of the heat pipe. Highlights • Zirconia-acetone nanofluid was assessed in a thermosyphon. • Nucleate boiling and geyser boiling were the dominant heat transfer mechanisms. • A tradeoff between filling ratio, tilt angle and performance was identified. • A new correlation was developed for estimating Kutateladze number. • The boiling HTC was in a good agreement with Stephan-Abdelsalam equation. [ABSTRACT FROM AUTHOR]

Abstract This study proposes a novel ultra-thin aluminum flat heat pipe with thickness of 1.5 mm for the first time in order to satisfy the ever increasing requirements for cost-effective, physically compact and highly efficient thermal management solutions. The extruded aluminum tube contains multiple separate micro cavities that work as independent micro heat pipes and a two-level compressing process is applied to flatten the tube to the ideal thickness. Sintered aluminum fibers and aluminum fiber meshes with surface treatment are inserted into each cavity of the ultra-thin heat pipe as the extra wicks for comparison. The thermal response characteristics of the fabricated aluminum heat pipes, including the temperature uniformity, thermal resistance, and effects of wick structure and inclination angles, are fully investigated. Under large inclination angle (90°), the smooth heat pipe without any extra wick performs lower temperature difference, lower thermal resistance, and higher heat transport capacity compared to that with aluminum fiber wick whereas the heat pipes with wicks present superiority under smaller inclination angles (60° and 30°) due to the liquid backflow enhancement. Besides, comparative study also shows that wick structure optimization holds the potential to improve the thermal performance of this ultra-thin two-phase device. Highlights • Ultra-thin aluminum heat pipe with only 1.5 mm thickness is developed. • It contains multiple separate micro cavities working as independent heat pipes. • The thermal response characteristics are fully investigated. • Wick structure optimization holds potential to improve the thermal performance. [ABSTRACT FROM AUTHOR]

A novel eutectic of Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 improved by LiF, when employed as a heat transfer fluid in concentrating solar power systems, is prepared to eliminate the disadvantage of limited operating temperature range and low specific heat as well as thermal conductivity. Using the static melting method, Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 with 15.0 wt.% LiF which is simply physical mixture determined by X-ray diffraction, is chosen as the optimized candidate. Results indicate that, at 10 °C/min of heating rate, LiF-Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 reached melting point of 368 °C and upper temperature limit of 753 °C, allowing it to be effectively employed in thermal storage and heat transfer in high temperature CSP systems. During temperature range from 400 to 600 °C, the liquid quaternary exhibited notably mean specific heat of 1.917 J/(g·°C), which is larger than that of the original ternary eutectic. Thermal conductivity of solid LiF-Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 reached the stable level of 1.021 W/(m·°C) at 320 °C. Density of improved eutectic tested by the archimedean principle is approximately 1.95 g/cm 3 . In conclusion, LiF-Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 can be employed as the admirable heat transfer fluid for thermal storage and heat transfer in high-temperature CSP plants. This investigation will provide useful information for further improvement of molten salt based heat transfer fluids. [ABSTRACT FROM AUTHOR]