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Thermochemical heat storage is a promising technology for the efficient utilization of renewable energy. Among available thermochemical systems, the CaO/Ca(OH) 2 system is the most popular because of availability and cost. However, poor powder properties and low heat storage rates hinder the successful implementation of this system. This study presented a novel synthetic method of granular composites based on carboxymethyl cellulose sodium (CMC) and vermiculite in order to mitigate the drawbacks of natural materials and stabilize the size of materials for use in moving bed reactors. TGA and DSC experiments and some essential characterizations were done in order to evaluate the improvements on the basis of three objectives: the heat storage rate, heat storage density, and mechanical properties, compared with natural materials. Results showed that the granular composite still had great structural integrity after several dehydration/hydration cycles, whereas compacted natural materials had fragmented. Additionally, composite had a higher heat storage rate than natural materials. The gravimetric storage density of granular composite was slightly reduced while the volumetric storage density was enhanced up to approximately 124% as compared to the powdery Ca(OH) 2 material. It was concluded that present synthetic method is a promising route for the development of Ca-based composite materials. • A novel synthetic method of granular composites based on CMC and vermiculite is developed. • Granular composites still have great structural integrity after several cycles. • The heat storage rate of composite is improved compared with natural materials. • The volumetric storage density of composite materials is slightly enhanced. [ABSTRACT FROM AUTHOR]

Abstract Cascaded Thermal Storage (CTS) technique is an efficient solution for storing solar thermal energy of high quality. In this paper, heat transfer rate and thermodynamic irreversibility are combined for optimizing cascaded PCM thermal storage system based on entransy and entropy. Optimal solutions for temperature of CTS unit are obtained based on entransy and entropy, which cannot only present a benchmark for similar research, but also be used for guiding phase change material (PCM) selection of multi-PCM thermal storage device. Limitations for entransy and entropy optimizations are put forward, and comprehensively analyzed. A parametric study is performed for the optimal thermal performance. The optimal PCM temperature based on entropy is geometric while that based on entransy is linearly distributed. By increasing stage number or heat transfer parameter, optimal performances for optimizations of entransy and entropy are promoted. Results show that thermal efficiency in entransy optimization is greater than that in entropy optimization while the exergy efficiency in entropy optimization is superior to that of entransy optimization. Stage number should be adjusted by balancing earnings and costs for cascaded design of thermal storage system. Heat transfer enhancement is essential for performance promotion of cascaded system and selection of optimization principles. Graphical abstract Image Highlights • CTS system is optimized based on entransy and entropy with optimization process. • Effects of key factors on PCM temperature and thermal performance are examined. • Optimal temperature with entropy is geometric and that with entransy is linear. • The critical stage number is presented for different optimization cases. • Entransy and entropy can be used for optimizing CTS system of different purposes. [ABSTRACT FROM AUTHOR]

The cascaded thermal storage technique has emerged as an important solution for efficient conversion and utilization of thermal energies. In this paper, an exergy optimization was performed for cascaded latent cold/heat storage using multi-stage heat engine model. The optimization solution for both heat storage and cold storage systems was obtained, which was used for guiding the selection of PCMs with two examples presented. Cascaded thermal storage with increased stage number can not only extend temperature band for multi-grade thermal energy, but also reduce the exergy of the outlet HTF. It was found that heat transfer enhancement (improving NTU ) is very necessary for a cascaded thermal storage system. The COP of cold energy may be greater than 1, which is also higher than that of heat for the same temperature difference in a cascaded thermal storage system. The increased environment temperature improves the COP of the cascaded cold storage (from 0.54 to 0.68) but reduces that of the cascaded heat storage (from 0.42 to 0.366). In the practical design of the cascaded thermal storage system, the stage number should be determined by balancing economics and system complexity. [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]

Phase change material (PCM) as highly efficient temperature control material has been used in terrestrial photovoltaic-thermoelectric (PV-TE) coupling system to achieve higher power generation efficiency. However, the mass increase results from addition of PCM limits its application in space system. In present paper, a two-dimensional model for PV-PCM-TE coupling system was developed to investigate the energy transfer and conversion performance under space conditions. Firstly, the suitable PCM was selected, both the pure PCM and its metal foam composite PCM (MFCPCM) are used. Then, performance comparison for PV-TE, PV-PCM-TE and PV-MFCPCM-TE systems were performed. The results show PCM can significantly enhance the total efficiency, but causes power density dramatically decreasing. The average total efficiencies for PV-TE, PV-PCM-TE and PV-MFCPCM-TE are 29.50%, 30.50% and 30.61%, and the corresponding average power density are 30.34 W/kg, 22.39 W/kg and 21.20 W/kg, respectively. After that, an orthogonal experiment was designed to reveal the effects of structure parameters on power generation efficiency and power density of PV-MFCPCM-TE system. The optimum geometric parameters to achieve the best power generation efficiency and power density were derived. The results show that after optimization, the average total efficiency and power density of PV-MFCPCM-TE are 30.83% and 31.35 W/kg, both of them are higher than those of the traditional PV-TE system (29.50% and 30.34 W/kg). The present work is beneficial for the application of PV-PCM-TE system in space conditions. [ABSTRACT FROM AUTHOR]

Thermochemical heat storage is a process of storing and releasing thermal energy with gas-solid reactions, e.g. the interaction of oxides and water vapor. To guarantee a more balanced reaction pressure in the thermochemical heat storage, a multi-layered reactor with multiple reacting zones was designed based on calcium materials. This novel fixed-bed reactor could enable material stacking more rational and eliminate the issue of excessive stacking causing steam obstruction. In comparison to the conventional straight-cylinder reactor, the multi-layered reactor has a more balanced pressure distribution. In this study, a mathematical model for thermochemical heat storage with multiple physical fields coupling is developed based on the multi-layered reactor, and the verification results show that the mathematical model is in good agreement with the experimental data in literature. The thermochemical heat storage process of materials with porosities of 0.5, 0.6, 0.7, 0.8, and 0.9 was numerically simulated by finite element method (FEM) under experimental boundary and initial conditions. Results indicate that for the whole 100–120 min exothermic period, the peak temperature of the process may reach around 510 °C. With different porosities, the total power output can be up to 470 W, 429 W, 330 W, 209 W, and 93 W and the maximum pressures are respectively 2.9 MPa, 1.2 MPa, 0.27 MPa, and 0.21 MPa near the entrance. The mathematic model and numerical simulation for the multi-layered thermochemical reactor with heat extraction process in present work could provide data reference for thermochemical theoretical research, reactor design, and reaction optimization. [ABSTRACT FROM AUTHOR]

Pumped-thermal electricity storage (PTES) is a promising energy storage technology with high-efficiency, energy density, and versatility of installation conditions. In this study, a 20 kW/5 h phase change packed-bed thermal energy storage experimental system is established and employed to validate the accuracy of thermal energy storage (TES) component within the broader PTES system. Moreover, a novel combined system based on latent TES is proposed, incorporating Joule-Brayton and organic Rankine cycle-based subsystems for supplying cold, heat, electricity, and hydrogen. Operational strategies are also explored to optimize the multifunctional capabilities of the system. The thermal energy density of the system, when phase change materials (PCMs) are used instead of sensible heat storage materials, can be improved from 167.85 kWh/m3 to 272.58 kWh/m3. Proton exchange membrane electrolyzer cycle (PEMEC) is coupled into the Carnot battery system, and a comprehensive analysis of the system's performance is conducted. With the charging time of 5 h, the energy performance of the multi-generation system (EPMG) can be reached up to 143 % and 146 % in combined cooling, heating, and power (CCHP) and multi-generation energy system (MGES) modes, respectively. The MGES mode is made 3.44 % more exergy efficient than the CCHP mode as a result of the ingenious coupling of the hydrogen production process with the power generation process and heating/cooling processes. The system in one charging/discharging process has the capability to generate 240.86 kg of hydrogen, 34.82 MWh of heat, 12.97 MWh of cold, and 40.90 MWh of electricity. [ABSTRACT FROM AUTHOR]

Pumped thermal electricity storage is a promising large-scale electricity storage technology that uses thermodynamic cycles and thermal energy storage to achieve electricity storage and release. At present, for pumped thermal electricity storage based on the Joule-Brayton cycle, packed beds filled with sensible heat storage media are widely adopted as the thermal store, while they usually suffer from low energy storage densities and large volumes. In this paper, the thermodynamic feasibility of packed-bed latent heat/cold stores is explored to replace the packed-bed sensible heat/cold stores in pumped thermal electricity storage. A numerical model of a 10.5 MW/5 h storage system based on packed-bed latent heat/cold stores is established. The energy and exergy analysis of components is carried out. The effect of compression ratios, porosities, isentropic efficiencies, and inlet velocities on the systematic thermodynamic performance is investigated and optimal conditions are also investigated. It is concluded that the energy storage density of the system increases from 232.5 kWh/m3 to 245.4 kWh/m3 when packed-bed sensible heat/cold stores are replaced by latent ones. Furthermore, the power density of the system based on packed-bed latent heat/cold stores is 216.5 kW/m3, and the round-trip efficiency reaches 84.7%, which is competitive in large-scale electricity storage technologies. [ABSTRACT FROM AUTHOR]

CaCO 3 /CaO thermochemical energy storage (TCES) system has been widely studied on laboratory scale for its potential in concentrated solar power (CSP) plants. However, there is still need for the large-scale experiments based on which a detailed analysis can be obtained to improve this system for real-time applications. In this study, we developed a packed bed reactor capable of operating at high temperature and pressure providing the carbonation and decarbonation reactions of CaCO 3 /CaO reaction pair. The heat storage process is studied with the aspect of different decarbonation temperatures and different sample sizes (amount of sample in grams) placed in the reactor. Furthermore, we compared the heat storage performance of pure and composite samples. The heat release process is performed by varying the concentration of CO 2 and the sample size of the loaded material. The results showed that the heat storage rate and efficiency improve by increasing the temperature of the decarbonation, implying that this reaction must be carried out above 925 °C for better performance. The heat is released smoothly and at a good rate when the concentration of CO 2 is high in the reactor. Also, carbonation reaction (heat release process) depicts that CaCO 3 /CaO TCES system has a great potential to be used as a heat pump and is capable of upgrading the temperature of the released heat from lower to higher value. [ABSTRACT FROM AUTHOR]