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Natural ester is regarded as a potential alternative to mineral insulating oil and has a wide range of applications in power system, but its electrical properties and thermal conductivity still need to be improved. Nano modification technology can improve the performance of natural ester to ensure the stability and reliability of natural ester in power equipment. In this study, nano modified natural ester is prepared with APTES surface modified h-BN, and the mechanism of enhancing the properties of natural ester is explored from the microscopic perspective by molecular simulation. The experimental results showed that nano h-BN had good dispersion stability in natural ester after surface modification with APTES. The electrical properties and thermal conductivity of the nano modified natural ester have been greatly improved, and the relative optimal doping concentration of nanoparticles is 0.03 g/L. The simulation results show that the interfacial thickness between nanoparticles and natural ester is larger and more tightly bound after surface modification, which is beneficial to reduce thermal resistance and improve thermal conductivity of natural ester insulating oil. The traps at the interface between nanoparticles and natural ester can capture free electrons in the oil, thus improving the breakdown voltage of natural ester insulating oil. [ABSTRACT FROM AUTHOR]

Carbonate rocks are one of major geothermal reservoir types around the world. Understanding the thermal properties of carbonate rocks is of significance to the assessment and utilization of deep geothermal reservoirs. Different from conventional test methods, this paper introduced a new approach, molecular dynamics calculation (MDC), to studying the thermal properties of carbonate rocks. By means of MDC, the thermal properties including specific heat, thermal conductivity and thermal diffusivity of four types of carbonate rocks including calcite, dolomite, magnesite, and smithsonite were investigated. The intrinsic relationship among the Debye temperature, elastic and thermal properties of carbonate rocks was revealed. The variations of the thermal properties with the average atom weight and temperature (300–800K) were obtained and regressed as functional formulas for applications. Further, taking carbonate rock samples from North China as an example, the specific heat and thermal conductivity were tested and compared with the MDC results based on actual lattice parameters by using X-ray diffraction method (XRD). The results showed that there was a good agreement between the test and calculation results. Finally, the thermal properties of ferroan dolomite samples were predicted. The present method can provide a promising extension tool for analyzing other crystalline rocks for geothermal applications. [ABSTRACT FROM AUTHOR]

Concentrating solar power is becoming one of options for producing energy to replace conventional polluting energy sources. However, improving the efficiency and reducing the cost of technologies based on this type of energy to make it more competitive is still a work in progress. This study proposes replacing the thermal oil used as the heat transfer fluid in the absorber tubes of parabolic trough solar collectors (PTCs) with nanofluids based on spherical molybdenum disulphide nanoparticles with the aim of improving the thermal efficiency of concentrating solar power plants. The colloidal stability of the nanofluids was verified by Ultraviolet–Visible spectroscopy, Zeta potential and Dynamic Light Scattering monitoring. The presence of spherical MoS 2 nanoparticles resulted in an increase of up to 13% in specific isobaric heat and 6% in thermal conductivity compared to thermal oil. Finally, the efficiency of parabolic trough solar collectors was estimated to increase by 5%, which also favours the decrease of pumping power and the elimination of selective coatings on the absorber tube. To our knowledge, this is the first time that MoS 2 -based nanofluids are tested as heat transfer fluids in PTCs analysing its implementation in the solar energy application. [Display omitted] [ABSTRACT FROM AUTHOR]

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]

This study presents a novel thermal response testing (TRT) approach to minimize test duration, labor, and associated costs. The proposed method, Oscillatory Thermal Response Test (OTRT), employs an oscillating heat flux using a sinusoidal wave with specific frequency and amplitude characteristics. A custom-designed TRT apparatus was developed to regulate the heating injection pattern for precise control. In Sapporo City, Hokkaido Prefecture, and Kai City, Yamanashi Prefecture, Japan, two in-situ OTRTs were conducted. The temperature profiles of the fluid and undisturbed soil were measured through optical fiber cables integrated within the U-tube legs. A novel analytical method was devised to filter, smooth, and fit the recorded data, enabling the determination of the response temperature amplitudes at the borehole inlet and outlet. Also, the temperature time derivative method calculates the effective thermal conductivity in the early 3 h. Comparative analysis with Normal Thermal Response Tests (NTRTs) demonstrated the accuracy and validity of the OTRT approach. Significant conclusions from this study include a remarkable reduction in TRT duration by over 95 %, with a high accuracy of 92 % achieved using only the first 3 h of testing. The proposed method exhibited a Root Mean Square Error (RMSE) of 0.1 W/(m·K) compared to NTRTs. To establish a strong correlation between the Amplitude Ratio and Effective Thermal Conductivity, further experimental studies are essential to address the limitations of the current study. These studies should explore a variety of geological and hydrological conditions, borehole diameters, type of heat exchanger, diameter of U-tubem grouting material, combined with numerical analysis, to enhance our understanding of this relationship. • Successful development and testing of a new Oscillatory Thermal Response Test apparatus in Sapporo and Kai cities, Japan. • Advanced analysis techniques ensure the precise determination of response temperature amplitudes and thermal conductivity. • Innovative Oscillatory Thermal Response Test reduces test time and costs by over 95 %. • OTRT achieves over 92 % accuracy in early thermal conductivity measurement compared with NTRT. • The temperature derivative is used to calculate the thermal conductivity. [ABSTRACT FROM AUTHOR]

Rock salt has high thermal conductivity (>6 W m−1 K−1 at 20 °C) compared to most overlying and adjacent sedimentary rocks, resulting in a thermal blanket effect and a favorable play for low enthalpy geothermal resources. However, the effect of temperature on rock salt thermal conductivity is greater than most other rocks. The result can be a reduction in this contrast between rock types, which has implications for geothermal exploration. The temperature dependence of rock salt thermal conductivity was shown to be a critical factor in our geothermal potential assessment of the Magdalen Islands (Canada). We evaluated the subsurface temperature of this archipelago using numerical simulations of conductive heat transfer, specifying a bulk thermal conductivity of rocks that varies with temperature and porosity, and the latter varying with pressure. A steep geothermal gradient, locally exceeding 40 °C km−1, was found in layers overlying shallow salt domes at depths of ∼1 km but not over a deeper dome due to low thermal conductivity contrasts. This finding highlights the effect of temperature on rock salt thermal conductivity, which should be considered when assessing the geothermal potential of salt domes, intrusions or diapirs, to avoid overestimating the subsurface temperature. • Assessment of the geothermal potential of salt domes in the Magdalen Islands. • Laboratory evaluation of temperature-dependent rock thermal conductivity. • Laboratory evaluation of pressure-dependent rock porosity. • Numerical heat conduction simulations under varying thermal properties conditions. • Temperature dependency of rock salt thermal conductivity shown to be critical factor. [ABSTRACT FROM AUTHOR]

This research aims to explore a novel PCM that can satisfy the needs of vaccine cold chain logistics temperature zone (2–8 °C), with decyl alcohol (DA) and lauric acid (LA) chose to be eutectic. Considering the thermal conductivity and leakage of materials, DA-LA/EG composite phase change material (CPCM) with a mass ratio of 12:1 is prepared by the vacuum adsorption method. The thermal properties of DA-LA/EG CPCM are analyzed by DSC, cooling curve, Hot Disk, SEM etc. From the DSC graphs of DA-LA eutectic phase change material (EPCM), it is found that there is difference between the melting temperature and solidification temperature, which is defined as phase change hysteresis. The results indicate that the phase transition temperature of DA-LA/EG CPCM is 2.08 °C, with high latent heat and thermal conductivity of 188.71 J g−1 and 1.7527 W m−1 K−1, respectively. Due to the unique porous structure of EG, the thermal conductivity is 13.75 times higher than that without EG, which leads to the phase transition time shortened by 79.47%. Furthermore, DA-LA/EG CPCM shows excellent cyclic stability and chemical stability after 500 high-low temperature alternating tests. From TGA, the decomposition temperature of DA-LA/EG CPCM is 186.55 °C, higher than the working temperature range. The applicability is demonstrated by using the CPCM into an incubator. Overall, DA-LA/EG CPCM has good application prospects in vaccine cold chain logistics. [ABSTRACT FROM AUTHOR]

Phase change micro/nanocapsules with high thermal conductivity shells have extensive application prospects in solar energy storage systems and electronics cooling. To improve the heat transfer of the capsules, a method of preparing Ag-shell coated phase change micro/nanocapsules was developed, where the AgBr ultrafine solid particles are introduced to enhance the rigidity of emulsion droplets in the liquid-solid phase transition process, and by employing hydroquinone as the reductant to repeatedly reduce the AgBr that had been adhered to the surface of the emulsion droplets, an Ag shell is formed on the surface of the cores. In this work, palmitic acid (PA) is used as the core material, and the prepared PA/Ag nanocapsules (100–600 nm) exhibit high thermal storage capability and good thermal reliability. The bulk thermal conductivity of the nanocapsules can be as high as 4.072 W/(m·K), indicating that the Ag shell can effectively improve the thermal conductivity of the nanocapsules. Besides, the thermal response of the nanocapsules is estimated by simulating a single-particle heat transfer process considering nanoscale effects, and the results demonstrate that the thermal response time of PA/Ag nanocapsule (350 nm) in working fluid (water, 80 °C) could be as short as 150ns. [ABSTRACT FROM AUTHOR]

The technology of latent heat storage with phase change materials (PCMs) is one of the promising means to improve the utilization of renewable energy. Nevertheless, its broad application will be limited due to the low thermal conductivity of PCMs. In this work, paraffin/expanded graphite (EG) composite PCMs by paraffin having a melting point of 62 °C and expanded graphite with three kinds of particle sizes were prepared and characterized. Environmental scanning electron microscope (ESEM) micrographs showed that EG was dispersed homogeneously in composite PCMs. Meanwhile, the distribution uniformity of expanded graphite in composite PCMs was evaluated quantitatively. The thermal conductivities of composite PCMs increase as the mass fraction of EG increases, and the thermal conductivity of paraffin/EG80 (5 wt.%) was 1.492 W/(m·K) in comparison with the value of pure paraffin was 0.355 W/(m·K). X-ray diffraction (XRD) analysis indicated that the fabrication process of composite PCMs was a purely physical process without a new substance produced. Differential scanning calorimeter (DSC) demonstrated that the latent heat of composite PCM was slightly less than the theoretical value. The addition of EG could lead to the improvement of the melting temperature of composite PCMs, while the decrease of the freezing temperature. [ABSTRACT FROM AUTHOR]

Molten salts/expanded graphite (EG) composite phase change materials (PCMs) could have an anisotropic structure due to the intercalation structure of EG, which may lead to anisotropic thermal properties of the composite. In this study, the anisotropic thermal properties of the cuboid-like Ca(NO 3) 2 -NaNO 3 /EG composite were investigated. The effects of bulk density and temperature on the anisotropic thermal conductivities were studied, and suitable models for prediction of the anisotropic thermal conductivities were analyzed and developed. The results showed that a layer-by-layer structure assembled by EG microsheets with the deposition of salt particles was formed. When the compressing pressure was 4 MPa, the composite showed an isotropic thermal conductivity, while the composites became more and more anisotropic with the increase of compressing pressure. At the room temperature, the axial thermal conductivity increased from 4.016 to 7.694 W/(m K), and the radial thermal conductivity increased from 4.040 to 5.645 W/(m K), when the compressing pressure was increased from 4 to 20 MPa. Contrarily, the working temperature showed little effect on the thermal conductivities. Finally, it was shown that the Maxwell-Eucken model and the modified Parallel model proposed in this work could be used to predict the axial and radial thermal conductivities of the Ca(NO 3) 2 -NaNO 3 /EG composite, respectively. • EG enabled formation of hierarchically structured composite PCM materials. • Thermal conductivity varied linearly with the increase in bulk density. • Bulk density had a greater influence on the thermal conductivity than temperature. • The material with less porous structure provided a higher specific heat capacity. • A modified Parallel model with a synergy factor was proposed. [ABSTRACT FROM AUTHOR]

Phase Change Materials (PCM) possess high heat storage density, but commonly have low thermal conductivity that results in poor heat transfer. Another common problem is the shape stabilization of the storage medium. These problems could be solved by loading the medium with thermally conductive fillers, such as graphite, and encapsulating it in a thermally conductive polymer matrix shell, hence enhancing the thermal properties of both. We suggest a ternary system in which a graphite-epoxy composite provides an encapsulating scaffold to the paraffin, which is also loaded with graphite-based filler. Various graphite-based fillers differing in sizes and geometry were explored, aiming at optimizing their intrinsic properties, such as defect density, and consequently enhancing the thermal properties of the PCM as a whole. It was demonstrated that by judicious choice of the filler, enhancement is achieved for the thermal conductivity of: (1) the shell (epoxy-graphite flakes composite) by 4000% compared to the neat epoxy; (2) the medium (paraffin-graphite flakes composite) by > 2000% compared to the neat paraffin; and (3) the integrated PCM system (paraffin-epoxy-graphite flakes composite) by 1000% compared to that of neat paraffin. The PCM composite is completely preserved during phase change cycling. • Various graphite-based fillers differing in size and geometry were explored for PCM. • Graphite flakes (GF) were found to be the best filler for the overall properties. • Ternary paraffin-GF-epoxy thermally conductive shape-stabilized system was achieved. • Properties of the ternary system were completely preserved during thermal cycling. [ABSTRACT FROM AUTHOR]

Expanded graphite (EG) is often used to encase phase change materials (PCMs) and improve their thermal performance. However, EG's effectiveness when used with disodium hydrogen phosphate dodecahydrate (DHPD) as a hydrated salt is diminished, due to its hydrophobic property. This study proposes modifying the hydrophobic surface of EG with highly hydrophilic nanoparticles, such as SiO 2 and TiO 2 , to address this shortcoming and boost its hydrophilicity. Herein, the nanoparticles were synthesized by the sol-gel technique and mixed with EG to construct a modified EG (MEG). The microstructure, morphology, and adsorption ability of MEG were analyzed. Additionally, the thermal properties, as well as the thermal stability of form-stable composite PCMs, were investigated. The results revealed that by varying the mass ratio of SiO 2 @TiO 2 in MEG, the supercooling degree of the form-stable composite PCMs decreased by 77.36%–78.49%. With 9% SiO 2 @TiO 2 in MEG, the water contact angle of MEG was optimal, and the form-stable composite PCMs prepared (DSMEG3) exhibited a phase change temperature and latent heat of 32.88 °C and 142.13 J/g, respectively. This experimental latent heat was slightly lower than the calculated latent heat by 0.12 J/g. Meanwhile, the supercooling degree decreased to 3.01 °C, and the thermal conductivity increased to 1.23 W/(m ∙ K), which was 1.73 times than that of DHPD. Moreover, DSMEG3 also showed good shape stability, preventing leakage. Therefore, the form-stable composite PCMs, as prepared, are a great potential candidate for the energy storage applications. [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]

For the first time, a complete set of the volume averaged governing equations based on a three-energy equation model has been proposed to attack the transient thermal response of a bi-disperse packed bed thermocline storage system, in which its void is filled by smaller particles (i.e. solid filler). Effective permeability, stagnant thermal conductivity and effective thermal conductivities for three individual phases, namely, the fluid phase, large particle phase and small particle phase of the bi-disperse packed bed storage system, have been evaluated taking full account of tortuosity and appealing to a volume averaging theory. Subsequently, the three individual energy equations in the three-energy model were coupled and analytically solved to reveal a transient behavior of the temperature fields of the individual phases. The effective discharging efficiency of the bi-disperse packing is found to decrease substantially as compared with that of the mono-disperse packing under equal pumping power. Though one can save a substantial initial cost by filling the void by solid fillers, one must pay a penalty of a higher cost in running the bi-disperse packed bed thermocline storage system. The analysis presented in this study serves to estimate both initial and running costs of a bi-disperse packed bed thermocline storage system. [ABSTRACT FROM AUTHOR]

Expanded graphite (EG) is often used to encase phase change materials (PCMs) and improve their thermal performance. However, EG's effectiveness when used with disodium hydrogen phosphate dodecahydrate (DHPD) as a hydrated salt is diminished, due to its hydrophobic property. This study proposes modifying the hydrophobic surface of EG with highly hydrophilic nanoparticles, such as SiO 2 and TiO 2 , to address this shortcoming and boost its hydrophilicity. Herein, the nanoparticles were synthesized by the sol-gel technique and mixed with EG to construct a modified EG (MEG). The microstructure, morphology, and adsorption ability of MEG were analyzed. Additionally, the thermal properties, as well as the thermal stability of form-stable composite PCMs, were investigated. The results revealed that by varying the mass ratio of SiO 2 @TiO 2 in MEG, the supercooling degree of the form-stable composite PCMs decreased by 77.36%–78.49%. With 9% SiO 2 @TiO 2 in MEG, the water contact angle of MEG was optimal, and the form-stable composite PCMs prepared (DSMEG3) exhibited a phase change temperature and latent heat of 32.88 °C and 142.13 J/g, respectively. This experimental latent heat was slightly lower than the calculated latent heat by 0.12 J/g. Meanwhile, the supercooling degree decreased to 3.01 °C, and the thermal conductivity increased to 1.23 W/(m ∙ K), which was 1.73 times than that of DHPD. Moreover, DSMEG3 also showed good shape stability, preventing leakage. Therefore, the form-stable composite PCMs, as prepared, are a great potential candidate for the energy storage applications. [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]

For the first time, a complete set of the volume averaged governing equations based on a three-energy equation model has been proposed to attack the transient thermal response of a bi-disperse packed bed thermocline storage system, in which its void is filled by smaller particles (i.e. solid filler). Effective permeability, stagnant thermal conductivity and effective thermal conductivities for three individual phases, namely, the fluid phase, large particle phase and small particle phase of the bi-disperse packed bed storage system, have been evaluated taking full account of tortuosity and appealing to a volume averaging theory. Subsequently, the three individual energy equations in the three-energy model were coupled and analytically solved to reveal a transient behavior of the temperature fields of the individual phases. The effective discharging efficiency of the bi-disperse packing is found to decrease substantially as compared with that of the mono-disperse packing under equal pumping power. Though one can save a substantial initial cost by filling the void by solid fillers, one must pay a penalty of a higher cost in running the bi-disperse packed bed thermocline storage system. The analysis presented in this study serves to estimate both initial and running costs of a bi-disperse packed bed thermocline storage system. [ABSTRACT FROM AUTHOR]

Polyurethane (PU) possesses excellent thermal properties, making it an ideal material for thermal insulation. Incorporating Phase Change Materials (PCMs) capsules into Polyurethane has proven to be an effective strategy for enhancing building envelopes. This innovative design substantially enhances indoor thermal stability and minimizes fluctuations in indoor air temperature. To investigate the thermal conductivity of the Polyurethane-Phase Change Materials foam composite, we propose a hierarchical multi-scale model utilizing Physics-Informed Neural Networks (PINNs). This model allows accurate prediction and analysis of the material's thermal conductivity at both the meso-scale and macro-scale. By leveraging the integration of physics-based knowledge and data-driven learning offered by Physics-Informed Neural Networks, we effectively tackle inverse problems and address complex multi-scale phenomena. Furthermore, the obtained thermal conductivity data facilitates the optimization of material design. To fully consider the occupants' thermal comfort within a building envelope, we conduct a case study evaluating the performance of this optimized material in a detached house. Simultaneously, we predict the energy consumption associated with this scenario. All outcomes demonstrate the promising nature of this design, enabling passive building energy design and significantly improving occupants' comfort. The successful development of this Physics-Informed Neural Networks-based multi-scale model holds immense potential for advancing our understanding of Polyurethane-Phase Change Material's thermal properties. It can contribute to the design and optimization of materials for various practical applications, including thermal energy storage systems and insulation design in advanced building envelopes. [ABSTRACT FROM AUTHOR]

Accurately estimating ground thermal properties from thermal response tests (TRTs) is critical to design ground source heat pump system (GSHPS). The traditional method may lead to large errors due to the difference between the heat transfer process described by identification models and actual situation. To avoid it, this paper proposes a high-precision identification method based on artificial neural network (ANN), which can directly establish the nonlinear mapping relationship between thermal response parameters (TRPs) and ground thermal properties. Through the inversed orthogonal method, the training and validation samples are obtained from a large number of TRTs on a full-scale simulation platform that is verified by experiments. The estimation accuracy of traditional method and ANN under different ground thermal properties is studied. The results indicate that the estimation accuracy of traditional method varies greatly under different ground thermal properties, and the relative errors of identifying thermal conductivity and volumetric heat capacity vary from −3.61% to 60.14% and −52.06%–110.20% respectively. The estimation accuracy of ANN is almost not affected by the ground thermal properties, and the corresponding errors range from −7.78% to 0.28% and −1.75%–15.6% respectively. This paper provides a new perspective to reduce error caused by identification model. Image 1 • A high-precision identification method for the ground parameters identification is proposed. • The new concept enables deriving ground thermal properties without using identification model. • A three-layer neural network based on back propagation algorithm is constructed. • Ground thermal conductivity and volumetric specific heat are evaluated by conventional method and neural network. • The estimation accuracy of the constructed neural network is almost not affected by the true thermal properties of ground. [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]

The ground thermal properties and borehole thermal resistance are the essential parameters for the design of borehole heat exchanger (BHE) field, and they are usually estimated using the experimental inlet/outlet fluid temperatures of BHE in thermal response test (TRT). This paper proposes the coupled two-step parameter estimation procedure (TSPEP) for estimating ground thermal conductivity and borehole thermal resistance of BHE by evaluating the actual averaged–over-the –depth mean fluid temperature (MFT) using the quasi-three-dimensional model inside the borehole. The simulated annealing algorithm (SAA) is used to iteratively find the minimum values of the two objective functions to obtain the optimal estimated results. In TSPEP, the estimated ground volumetric heat capacity and weighted factor f in the 1st step are transferred to calculate MFT using the experimental data in the 2nd step, which guarantees the direct approach based on the infinite line source model (ILSM)applied to improve the accuracy of the estimated borehole thermal resistance. For 50 m depth BHE, the estimated borehole thermal resistance is increased by 12.1% using TSPEP than the effective borehole thermal resistance evaluated by the arithmetic average fluid temperature (AFT). The estimated ground thermal conductivity in TSPEP is almost same with that from the direct approach based on ILSM, and the maximum relative error between them is only 0.91% even though borehole depth of BHE changes from 50 m to 200 m. • Averaged-over-the-depth mean fluid temperature calculates borehole thermal resistance R b. • A two-step parameter estimation procedure (TSPEP) is proposed to estimate R b in TRT. • The feasibility and accuracy of TSPEP are validated and discussed. [ABSTRACT FROM AUTHOR]

Ground thermal properties (thermal conductivity and heat capacity) are the key parameters for design of borehole heat exchanger (BHE) in ground source heat pump (GSHP). This study presented a novel sequential estimation for ground thermal properties of coaxial BHE by using distributed thermal response test (DTRT) data. Firstly, a comprehensive heat transfer model for coaxial BHE was built to obtain the vertical temperature files. Then a new sensitivity analysis using Spearman correlation method was performed to obtain the correlation between the thermal properties and fluid vertical temperature, determining the estimation sequence. Lastly, according to the sequence, Monte Carlo stochastic method was used to estimate the thermal properties by applying the DTRT data. In addition, the effect of borehole depth and random samples on estimations was investigated. The simulations revealed that the heat input into BHE was mainly used to heat the circulating fluid during the earlier time. It was found that the estimation steps following the thermal conductivity first, then heat capacity got higher precision than other estimation steps with 0.6% and 4% respectively for the two parameters. In addition, the effect of borehole depth on estimation error could be eliminated by using the DTRT data. • DTRT data was interpreted for ground thermal properties estimation. • Sequential parameters estimation was presented for DTRT analysis. • The estimation precision for ground heat capacity was improved to 4%. • Effect of borehole depth on estimate error was eliminated by using DTRT data. [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]

This paper proposes an innovative algorithm using the idea of step-wise to estimate of multiple parameters of the shallow ground and U-shaped geothermal heat exchangers. This procedure uses an infinite composite-medium line-source solution. The short-term solution not only enables testing data collected during short times (< 10 h) to be used in parameter estimation but also provides a feasible approach to estimating geometric parameters and thermal properties inside a borehole. The step-wise estimation consists of four stages: 1) determine soil thermal conductivity k s , 2) estimate the thermal conductivity of grout k b , 3) estimate the distance between centers of U-tube pipe D , the thermal diffusivities of soil and grout, a s and a b , and 4) an iteration process if necessary. The outstanding feature of this step-wise algorithm is that the TRT data used in each step can be controlled separately, which can increase the accuracy of estimation because it alleviates the linear dependence among the sensitivities of parameters. It is shown that the accuracy of the estimates is ranked in descending order as follows: k b (± 1.0%), k s (± 2.0%), D (± 16.0%), a s (± 20.0%) and a b (± 70.0%). • New step-wise parameter estimation method is created and validated by TRTs data. • The new algorithm uses a compositemedium line-source solution. • Multi-parameter of the ground and GHE are determined in a set of procedures. • Early-time (<10 h) data is effectively and sufficiently used for estimations. [ABSTRACT FROM AUTHOR]

Hydrated salt of aluminium potassium sulfate dodecahydrate (APSD)/expanded graphite (EG) form-stable composite phase change material (PCM) has been prepared. Hydrated salt of aluminium potassium sulfate dodecahydrate used as the PCM was firstly modified by using nucleator and thickener with the aiming at solving the supercooling and phase separation. Sulfurized EG is selected as supporting matrix to overcome leakage and low thermal conductivity of MAPSD. Furthermore, hydrophilic and hydrophobic groups on the surface of the expanded graphite is oriented by carrying out titanate coupling agent, which could restrict the volatilization of crystal water and solve the poor cyclic stability problems of APSD. The thermal performance of modified APSD (MAPSD) and modified EG (MEG)/MAPSD composite PCMs are tested and analyzed by using Differential Scanning Calorimeter (DSC), Hot Disk Thermal Constant Analyzer and a small-scale experimental device. The DSC analysis showed that the melting enthalpy of form-stable composite PCM containing 80 wt% APSD and MAPSD are 473.52 and 641.75 J/g, and their corresponding melting points are 84.39 and 84.34 °C, respectively. The 100 times melt-freeze cyclic tests revealed that the mass of MAPSD decreases by 45.57%. However, the mass of MEG/MAPSD composite PCMs decreases by only 7.91% when the sample density (ρ) of 900 kg/m3 and mass fraction of MEG with 20%, which could still maintain its original shape perfectly without any change. What's more, the effective thermal conductivity of MEG/MAPSD composite PCM (6.157 W/(m·K)) is about 11 times higher than that of pure APSD (0.55 W/(m·K)) and Agari-Uno model is used to predict the thermal conductivity of composite PCMs with different MEG contents. The average deviation is only 3.91% between experimental and predicted value. All the results indicate that the form-stable MAPSD/MEG composite PCMs can be considered as high-performance latent heat storage materials and the great potential for practical application in solar thermal utilization. • A novel MEG/MAPSD form-stable composite PCM is prepared. • The composite PCM has good thermal storage capacity and cyclic stability. • Thermal conductivity reaches as high as 6.157 W/(m·K). • The average deviation is only 3.91% between experimental and predicted values. [ABSTRACT FROM AUTHOR]

A geological stratum is a rock or soil layer that has internally consistent characteristics and is distinguishable from adjacent layers. A stratigraphic section contains reference information for the strata of an associated sedimentary area. In this paper, the thermal properties of 36 rock formations in a succession of the Yangtze plate in China are studied. Thermal properties are analyzed with an emphasis on rock formations, rock lithological types and dry/saturated conditions. The results show that thermal conductivity varies dramatically with different rock lithological types, which range from 0.72 W m−1 K−1 to 4.49 W m−1 K−1. The analysis of biochemical sedimentary rocks shows that silicate has the highest thermal conductivity, followed by dolomite and limestone; this variation is due to the differences in rock-forming minerals. The thermal conductivity of the clastic rocks is mainly determined by the type of minerals, the porosity and the water content. Compared to dry samples, the thermal conductivity and volumetric heat capacity of saturated samples are higher. Moreover, the thermal conductivity data of nine other locations in the Yangtze plate are compared with the stratigraphic data. This analysis shows that thermal conductivity matches very well for most of the geological formations, with the exception of sandstone, which exhibits a significant deviation across formations. Finally, the thermal properties for the Yangtze plate are mapped for insight on their spatial distribution. • Thermal properties of 36 rock formations in a sedimentary succession are measured. • Thermal conductivity varies dramatically with lithological types of rocks. • Two categories rocks are studied with emphasis on sedimentogenesis. • Thermal properties of Yangtze plate are mapped for insights on spatial distribution. [ABSTRACT FROM AUTHOR]

Thermal properties are very important for the design of ground heat exchangers (GHE). The line source model is typically used to identify these parameters, based on the inlet and outlet water temperature of an in-situ thermal response test (TRT). Since the model is based on thermal conduction, convection of groundwater can affect identification results. In this paper, a three-dimensional numerical simulation model was established to simulate the dynamic heat exchange for GHEs. A series of TRTs were conducted to analyse the effect of groundwater seepage on identification of thermal properties of soils. The equivalent thermal conductivity (ETC) was found to be increased gradually, while the equivalent volume heat capacity (EVHC) and the change rates of both parameters decreased with increases in groundwater velocity. The seepage direction was vital for identification, especially when the velocity was fast. The ETC was larger and the EVHC was smaller when the seepage direction was perpendicular as opposed to parallel to the buried pipe plane. The height of the seepage layer significantly affected identification results, and the greater the seepage velocity was, the more significant the effect was. In addition, the higher height led to the larger ETC and the smaller EVHC. The location of the seepage layer had little influence on results, but the ETC was larger and the EVHC was smaller if the location was closer to the ground surface. • An accurate 3-D numerical simulation model was established and verified. • The effect of seepage velocity and direction on the identification results of TRT was analysed. • The effect of height and location of the aquifer was also studied. [ABSTRACT FROM AUTHOR]

Paraffin has been one of the most potential phase change materials (PCMs) in low-temperature latent heat storage. However, the low thermal conductivity of paraffin restricts its thermal performance and limits its large-scale applications. Adding a suitable proportion of expanded graphite (EG) into the paraffin can enhance the heat transfer rate, thus improve the thermal efficiency of the whole heat storage system. In this paper, five paraffin/EG composites with 2 wt%, 5 wt%, 10 wt%, 15 wt% and 20 wt% of EG were prepared to study their long-term characteristics. Effects of thermal cycling (up to 100 thermal cycles) on the thermophysical properties were studied, such as, phase transition temperature, latent heat, chemical compatibility, thermal stability and thermal conductivity. The paraffin/EG composite has the potential for converting intermittent electricity directly into heat, therefore, the effect of thermal cycling on electrical conductivity was also studied. The results showed that both phase transition temperature and latent heat decreased slightly after 100 thermal cycles, within 1% and 3% respectively. Thermal cycling changed the distribution and interaction between the paraffin and EG resulting in a lower temperature thermal decomposition process. Long-term thermal cycling also had a more negative effect on the increase rate in thermal and electrical conductivity achieved for higher EG content. • EG addition can enhance PCM's thermal and electrical conductivity. • The properties of PCM composites changes after thermal cycles. • The PCM composite has good chemical stability after thermal cycles. • Thermal conductivity of the PCM composites was reduced after thermal cycles. • Electrical conductivity of the PCM composites was reduced after thermal cycles. [ABSTRACT FROM AUTHOR]

Herein a numerical study was conducted on a ventilation system integrated with the inorganic PCM panels having enhanced thermal conductivity. The overall thermal performance of the ventilation system was improved by adjusting the inlet condition and the thermal properties of the PCM. With the temperature of the inlet air fluctuating from 17.4 to 33.1 °C, the effects of the inlet air flow rate and the thickness and thermal conductivity of the panels were investigated, aiming at making the outlet air temperature closer to the thermal comfort range. It was showed that the outlet temperature fluctuation reduced as the inlet air flow rate decreased or the thickness of the panels increased. Moreover, a PCM utilization rate was introduced to evaluate the PCM performance during the melting-solidification process. It was found that narrowing the phase change temperature range of the PCM in the outlet channel could further reduce the maximum outlet temperature and improve the PCM utilization rate. In addition, when the inlet air flow rate was 11.47 kg/h and the thickness of the PCM panels was 12 mm, the ventilation system with the panels having a thermal conductivity of 13.0 W/(m·K) exhibited the smallest outlet temperature fluctuation of 22.5–27.9 °C. • A numerical model of the PCM-containing ventilation system is constructed. • The effects of air velocity, PCM amount and thermal conductivity of PCM are explored. • Properly increasing the PCM amount can reduce air temperature fluctuation. • Reducing the phase change temperature range can improve the utilization of PCM. • The thermal conductivity of the PCM has an optimum value. [ABSTRACT FROM AUTHOR]

Myristic acid (MA) with a melting temperature around 54 °C has huge prospects for solar passive thermal energy storage applications. However, like all other fatty acids (FA), it is also suffers from issues like leakage and low thermal conductivity (TC), when considered for latent heat thermal energy storage (LHTES) applications. Here, to overcome the seepage problem, MA was incorporated within the silica fume (SF) using simple direct impregnation method. To deal with the TC issue, prepared SF/MA pre-composite was doped with carbon nanotubes (CNTs) at three different weight percentage (0.3, 0.5 and 1.0 wt%) and eventually enhanced its TC by about 9.4%, 21.9% and 43.8%, respectively. The chemical and morphological structures of the SF/MA/CNTs developed as novel shape-stabilized composite PCMs (SS-CPCMs) were evaluated by FTIR, XRD, SEM and DSC analysis. The heat storage capacities of the doped and undoped SS-CPCMs were found in the range of about 87–91 J/g with good thermal stability up to 190 °C. Long standing-thermal cycling study was performed with the selected SS-CPCM doped with CNTs (1.0 wt%). No visible degradation or chemical changes were observed even after 1000 thermal cycles. CNTs doping significantly improved the heating and cooling performance of SF/MA composite. Image 10457 • To overcome the seepage problem of MA, it was impregnated inside SF. • TC of SF/MA pre-composite was enhanced considerably by doping 0.3–1.0 wt% CNTs. • LHTES capacities of SS-CPCMs with doped/undoped CNTs were measured as about 87–91 J/g. • SS-CPCMs showed good long standing-LHTES reliability, chemical stability and high thermal durability. • CNTs doping appreciably improved heating and cooling performance of SF/MA composite. [ABSTRACT FROM AUTHOR]

Abstract The development of high efficient materials for storage and utilization of sustainable thermal energy is very meaningful. Herein, in this research, an oxalic acid dihydrate/glycolic acid (OG) binary eutectic mixtures with the ratio of 80 wt%/20 wt% was prepared as phase change material (PCM), hydrothermal carbon (HTC) and poly (acrylamide-co-acrylic acid) copolymer (PAAAM) were separately and together used as support materials to solve liquid-state leakage problem and improve thermal conductivity. Three form-stable PCM composites (OG/PAAAM, OG/HTC/PAAAM and OG/HTC) were synthesized. The prepared composites were studied by XRD and FT-IR spectra, their thermophysical properties and PCM storage properties were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The OG/PAAAM, OG/HTC/PAAAM and OG/HTC showed the high loading rate to OG content, which are 1718 wt%, 1329 wt% and 1150 wt%, and the latent heat are 289.6, 313.2 and 318.8 J/g, respectively. In addition, the composites possess excellent recyclability after 101 times melting-freezing cycles. Compared to pure OG, OG/PAAAM and OG/HTC/PAAAM (0.2766, 0.3125 and 0.8538 W/m K), the OG/HTC exhibit a higher thermal conductivity of 1.3867 W/m K. Highlights • Three novel form-stable PCMs (OG/PAAAM, OG/HTC/PAAAM and OG/HTC) were prepared. • Three composites have high loading capacity on PCM and show excellent stability. • Composites doped with the HTC show higher heat storage efficiency. • Thermal conductivity has been significantly improved with the doping of HTC. [ABSTRACT FROM AUTHOR]

Stearic acid (SA) and modified expanded vermiculite (EV) shape-stabilized composite phase change materials (ss-CPCMs) with enhanced thermal conductivity were prepared. EV was impregnated with a starch solution, and then a composite of EV and carbon (EVC) was obtained by carbonizing starch in-situ in the EV layers. 63.12 wt % of SA was retained in the SA/EVC ss-CPCMs without leakage. Scanning electron microscopy (SEM) images showed that the EVC with highly porous micro-pores acted as a good support matrix for absorbing molten SA. The thermal conductivity of the SA/EVC ss-CPCMs was 0.52 W/(m K), and this was an increase of 52.9% compared with that of the SA/EV ss-CPCMs. The results from Fourier transform infrared spectroscopy (FT-IR), Differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), and thermal cycling tests indicated that the prepared SA/EVC ss-CPCMs are a promising material for energy efficient buildings because of their optimum phase-change temperature, a high enthalpy of phase change, ideal thermal conductivity, and good chemical and thermal stability. [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]

Abstract: In this paper, a water based suspension containing microencapsulated phase change material (MEPCM) and multi-walled carbon nanotubes (MWCNTs) was prepared as a novel heat transfer fluid. Thermal and rheological properties of the sample were obtained experimentally from differential scanning calorimeter (DSC), thermal gravimetric analysis (TGA), thermal conductivity meter and rheometer. It is found that the addition of MWCNTs into microencapsulated phase change material slurry can effectively improve the thermal conductivity of the suspensions and it is also found that a blend of 10 wt. % MEPCMs and 1 wt. % MWCNTs suspension can achieve an optimal thermal performance and stability. [Copyright &y& Elsevier]

Abstract: The use of microencapsulated phase change materials (MPCMs) is one of the most efficient ways of storing thermal energy. When the microencapsulated phase change material (MPCM) is dispersed into the carrier fluid, microencapsulated phase change slurry (MPCS) is prepared. Due to the relatively large surface area to volume MPCM and its large apparent specific heat during the phase change period, better heat transfer performance can be achieved. Therefore, MPCS can be used as both the energy storage and heat transfer media. This paper studies the thermal and rheological properties of a series of prepared MPCS. In the experiment: MPCS fabricated by dispersing MPCM into water with an appropriate amount of surfactant. The mass ratio of MPCM to water and surfactant was 10:90:1, 25:75:1, 35:65:1 in prepared MPCS samples, respectively. Then the thermal conductivity and specific heat of MPCS were measured by the Hot Disk. The melting/crystallizing temperature and fusion heat/crystallization heat of the phase change materials were obtained from a DSC (differential scanning calorimetry) during the heating/cooling process. Physical properties, such as viscosity, diameter and its size distribution of MPCS were investigated by a rheometer and a particle characterization system. Meanwhile, the chemical structure of the sample was analyzed using Fourier Transformed Infrared spectroscopy (FTIR). The results showed that the thermal conductivity and the specific heat of MPCS decreased with particle concentration for the temperatures below the melting point. Overall, the MPCS can be considered as Newtonian fluid within the test region (shear rate >200 s−1 and mass fraction <0.35). The viscosity is higher for bigger particle slurries. The findings of the work lead to the conclusion that the present work suggested that MPCMs can be used in “passive” applications or in combination with active cooling systems; and it also provided a new understanding for fabricating microencapsulated phase change slurry, it is for sure that to have a better potential for energy storage. Accordingly, it has demonstrated that the MPCS fabricated in the current research are suitable for potential application as heat transfer media in the thermal energy storage. [Copyright &y& Elsevier]

Abstract: Thermal response test (TRT) is crucial for the determination of the ground thermal conductivity and the evaluation of the thermal performance of borehole heat exchangers (BHEs) of ground source heat pump (GSHP) system. This paper presented a novel constant heating-temperature method (CHTM) for TRT. Further, a type of improved TRT equipment was developed and the mathematical model to deal with test data was presented. Based on the measurement of the natural ground temperature distribution, an in situ TRT case was carried out. The experimental results showed that, compared with the conventional TRT with constant heating-flux method (CHFM), CHTM has an obvious advantage of reducing the time period reaching a steady heat-transfer state between the BHE and its surrounding soils. This improved TRT equipment can effectively operate under both heat-injection and heat-extraction modes, and its test data can accurately reflect the thermal properties of the soils as well as the thermal performance of the BHE under different operation conditions. Finally, the advantages and disadvantages between CHTM and CHFM were compared, which can provide a useful reference for the design of GSHP system. [Copyright &y& Elsevier]