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Thermochemical sorption heat storage based on salt hydrates offers the potential advantages such as high energy density, negligible heat loss, small volume change and flexible working modes. Magnesium sulphate (MgSO 4) is a high energy density hydrated salt available for the thermochemical sorption heat storage. In this work, different types of zeolites (including 3A, 5A and 13X zeolites) have been selected as mass-transfer-enhancing and structural stabilizing materials, and a series of zeolite/MgSO 4 composites have been fabricated. To investigate the thermal energy storage performance of zeolite/MgSO 4 composites, the properties of the composites, such as micro-morphology, pore structure, chemical composition and sorption-desorption behavior, have been characterized and analyzed. Results indicated that the mass transfer performance of the composite materials is significantly better than that of the pure zeolites. As the mass fraction of MgSO 4 in the composite sorbents increases, the specific surface area and porosity of the composite sorbent decreases, and the sorption performance has been enhanced. Compared with 3A zeolite-based and 5A zeolite-based composites, 13X zeolite/MgSO 4 composite materials show the best heat storage performance. At the condition of 25 °C and 60% RH, the 13XM20 has the highest sorption capacity of 0.21 g/g, which is 24% higher than that of pure zeolite 13X. Accordingly, the maximum heat storage density is 438.4 kJ/kg, which exhibits the superior heat storage performance among of the hydrated salt sorbents. The potential of 13X zeolite/MgSO 4 as sorption heat storage material in efficient utilization of solar heat and low-grade industrial waste heat has been revealed. [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]