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This paper presents a detailed review of shell materials that have the potential to be used for high temperature thermal energy storage (TES) applications, particularly in conjunction with concentrated solar power (CSP) plants. This paper considers shell materials that are thermally stable at more than 300 °C and have successfully been used to encapsulate a phase change material (PCM). The current review does not consider the thermal performance of the shell material and PCM combinations that have been studied. Using these constraints several feasible materials were identified including: steel (carbon and stainless), nickel (and nickel alloy), sodium silicate, silicon dioxide, calcium carbonate and titanium dioxide. These materials have the potential to encapsulate high temperature PCMs and thus provide a suitable method of high temperature TES. [ABSTRACT FROM AUTHOR]

The performance of parallel and counter flow configurations of a shell and tube heat exchanger used as a latent heat thermal storage system was investigated. The comparative analysis of the melting and solidification processes (charging and discharging) in the vertical and horizontal orientations of counter and parallel flow was included in this study to make the results conclusive for the purpose of design of the storage system to be used in a concentrated solar power (CSP) plant. In this analyses, taking into account natural convection, numerical calculations were performed using ANSYS Fluent. The vertical parallel flow configuration shows 12% higher effectiveness compared to the counter flow for the charging and discharging processes. A higher rate of phase change occurs for both the charging and discharging processes in a parallel flow configuration, as a higher fraction of phase change material (PCM) is exposed to the heat transfer fluid inlet temperature compared to the counter flow. However, a lower temperature gradient and nearly constant effectiveness for a longer period of time are observed in the counter flow arrangement due to the higher rate of natural convection. An interesting result is the higher rate of natural convection in the horizontal orientation compared to the vertical one due to the Benard convection phenomenon, despite the fact that Ra is higher in the vertical orientations (10 10 > 10 7 ). The horizontal counter flow and parallel flow configurations show on average 10% higher effectiveness for the charging process which is constant for a longer period during the process compared to the vertical configurations whereas the horizontal arrangement during the discharging process improves the effectiveness by about 2% due to the minor role of natural convection. The horizontal configurations provide a more uniform phase change process with the lowest peak temperatures (in the melting processes) and temperature gradient which are correlated with the highest second law efficiency and exergy recovery. [ABSTRACT FROM AUTHOR]

The eutectic Na 2 CO 3 -Li 2 CO 3 salt is widely used as an electrolyte in the Molten Carbonate Fuel Cell because of its high both ionic conductivity and operating temperature. The major objective of this study was to investigate the thermal stability of the binary salt as a phase change material for thermal energy storage through thermal decomposition analysis in different environments and thermal cycling. The composition of the binary salt was determined using FactSage software 6.4 and its thermophysical properties were experimentally investigated using a Simultaneous Thermal Analyzer and X-Ray Diffraction. The results show that the eutectic Na 2 CO 3 -Li 2 CO 3 salt has a melting/solidification point of 498.3 ± 0.1 °C/483.9 ± 0.1 °C, with a heat of fusion of 330.8 ± 0.6 J/g and heat of solidification of 329.2 ± 0.3 J/g. The thermal decomposition analysis indicates that the eutectic molten salt has good thermal stability in a CO 2 environment without weight loss at temperatures up to 600 °C. However, 0.8% weight loss is observed at 500 °C in a N 2 atmosphere, which is associated with the thermal decomposition of the eutectic salt to produce corresponding metal oxides and CO 2 . The CO 2 environment could inhibit the thermal decomposition, thus resulting in higher thermal stability of the binary salt in CO 2 than when in N 2 . Hence, an operating environment has a large effect on the thermal stability of the eutectic Na 2 CO 3 -Li 2 CO 3 salt. The eutectic salt exhibits high thermal stability in CO 2 after 500 thermal cycles with little change in its thermal properties compared with the initial sample. [ABSTRACT FROM AUTHOR]

A key drawback of using latent heat thermal storage systems for concentrating solar thermal power plants is the low thermal conductivity of the phase change material during the melting and solidification processes. This paper investigates an approach for reducing the thermal resistance by utilising axially finned heat pipes. A numerical model simulating the phase change material melting and solidification processes has been developed. This paper also includes the models of the evaporation and condensation of the heat pipe working fluid. The results show that by adding four axial fins and including the evaporation and condensation, the overall thermal performance of the storage system is enhanced significantly compared to having bare heat pipes. After 3 h a total of 106% increase in energy storage is obtained during the charging process. The results also show that the combined effect of incorporating the evaporation/condensation process and adding the fins leads to a threefold increase in the heat storage during the first 3 h. During the discharge process, there was a 79% increase in energy discharged and also the combined effect of incorporating the evaporation/condensation as well as adding the fins results in an almost four fold increase in the heat extracted within the first 3 h. A parametric analysis has also been carried out to analyse the effect of the finned heat pipe parameters after incorporating evaporation and condensation of the heat pipe working fluid. [ABSTRACT FROM AUTHOR]

Abstract: Designing a cost-effective phase change thermal storage system involves two challenging aspects: one is to select a suitable storage material and the other is to increase the heat transfer between the storage material and the heat transfer fluid as the performance of the system is limited by the poor thermal conductivity of the latent heat storage material. When used for storing energy in concentrated solar thermal power plants, the solar field operation temperature will determine the PCM melting temperature selection. This paper reviews concentrated solar thermal power plants that are currently operating and under construction. It also reviews phase change materials with melting temperatures above 300°C, which potentially can be used as energy storage media in these plants. In addition, various techniques employed to enhance the thermal performance of high temperature phase change thermal storage systems have been reviewed and discussed. This review aims to provide the necessary information for further research in the development of cost-effective high temperature phase change thermal storage systems. [Copyright &y& Elsevier]

Abstract In the current study the feasibility of using solar-based renewables coupled with thermal energy storage (TES) to displace gas for heating was explored. To assess the feasibility, a numerical model of an air-based encapsulated phase change (EPCM) storage system was developed, validated, optimised, and economically costed. The optimised air-based EPCM system utilised a high and low melting temperature phase change material (PCM) with a sensible storage filler. It was found that a capsule radius of 10 mm and PCM volume of 13% resulted in the lowest cost of discharged thermal energy of $25.55/kWh when storage effectiveness and pumping power was considered. This system was then coupled to solar data for Adelaide, South Australia, to simulate the performance of a 1 MWt heat load over a year. By solving an hourly system generation and demand profile, it was found that a concentrated solar thermal (CST) and photovoltaic (PV) system coupled with TES was able to economically reduce gas consumption by 45–65% when the price of gas was $30/GJ. By employing near-term cost estimates for CST and PV systems coupled with TES, it was found that gas consumption could be reduced by similar amounts with a gas price of $20/GJ. Highlights • Solar-based renewables coupled with TES is explored to reduce commercial/industrial reliance on gas as a heat source. • A numerical model of an air-based EPCM system is developed, validated, optimised, and economically costed. • A 2-PCM hybrid system was found to produce the desired performance at the lowest cost ($25.55/kWh t). • Both PV and CST systems are able to currently compete with gas as a heat source when the gas price is more than $25/GJ. • Predicted cost reductions coupled with low cost grid electricity show these systems capable of being economic at $15/GJ. [ABSTRACT FROM AUTHOR]

The intermittency of renewable energy systems remains one of the major hurdles preventing a large scale uptake of these technologies and concentrated solar power (CSP) systems are no different. However, CSP has the benefit of being able to store excess heat using thermal energy storage (TES). For the uptake of CSP with TES it must be demonstrated that the technology is both economically as well as environmentally feasible. This paper aims to investigate the economic and environmental impact of several TES options that are available for CSP systems. The investigated systems include an encapsulated phase change material (PCM) system, a coil-in-tank PCM system and a liquid sodium TES system. The economic impact in the current study refers to the capital cost (CAPEX) of each system including the tank, storage material, encapsulation cost (if applicable) and allowances for construction and engineering. The environmental impact of each system is accounted by calculating the embodied energy of each of the system components. Each storage system will be required to store a comparable amount of energy so that reliable conclusions can be drawn. The results from this analysis conclude that the encapsulated PCM (EPCM) and coil-in-tank system represent an embodied energy of roughly one third of the corresponding state-of-the-art two-tank molten salt system. Furthermore, the EPCM and coil-in-tank systems result in CAPEX reductions of 50% and 25% over the current state-of-the-art two-tank molten salt system. The liquid sodium system was found to result in higher embodied energy and CAPEX than any previously studied TES system. Finally, the advantages and disadvantages of each system was discussed and compared to previous literature. [ABSTRACT FROM AUTHOR]

In this work, the eutectic Na 2 CO 3 –NaCl molten salt was investigated as a new high temperature phase change material for solar thermal energy storage. The composition of the eutectic binary salt was determined with the aid of FactSage software and its thermophysical properties were investigated using a Simultaneous Thermal Analyzer (STA) and X-Ray Diffraction (XRD). Inductively coupled plasma analysis has shown that the composition of the as-prepared sample is consistent with the nominal one. The STA results exhibit that the melting point of the eutectic salt is 637.0 °C and its heat of fusion is 283.3 J/g whereas its specific heat is a function of temperature, which all are in agreement with the theoretical values determined by the FactSage software. The thermal stability analysis indicates that the eutectic molten salt has good thermal stability without weight loss in a CO 2 environment at temperatures below 700 °C, compared with 0.51% weight loss in a N 2 atmosphere. The weight loss observed in the latter, is most likely to be due to the salt׳s decomposition at high temperature. The thermophysical properties of the salt such as melting temperature, latent heat of fusion and solidification, varied marginally after 50 and 100 thermal cycle tests. This demonstrates that the eutectic Na 2 CO 3 –NaCl salt is a promising high temperature phase change material when used in a CO 2 environment or encapsulation. [ABSTRACT FROM AUTHOR]

A concentrating solar power (CSP) system converts sunlight into a heat source which can be used to drive a conventional power plant. Thermal energy storage (TES) improves the dispatchability of a CSP plant. Heat can be stored in either sensible, latent or thermochemical storage. Commercial deployment of CSP systems have been achieved in recent years with the two-tank sensible storage system using molten salt as the storage medium. Considerable research effort has been conducted to improve the efficiency of the CSP system and make the cost of electricity comparable to that of the conventional fossil-fuel power plant. This paper provides a comprehensive summary of CSP plants both in operation and under construction. It covers the available technologies for the receiver, thermal storage, power block and heat transfer fluid. This paper also reviews developments in high temperature TES over the past decade with a focus on sensible and latent heat storage. High temperature corrosion and economic aspects of these systems are also discussed. [ABSTRACT FROM AUTHOR]

Demand for high temperature storage is on a high rise, particularly with the advancement of circular economy as a solution to reduce global warming effects. Thermal energy storage can be used in concentrated solar power plants, waste heat recovery and conventional power plants to improve the thermal efficiency. Latent thermal energy storage systems using phase change materials are highly thought for such applications due to their high energy density as compared to their sensible heat counterparts. This review, therefore, gives a summary of major factors that need to be assessed before an integration of the latent thermal energy system is undertaken. In addition, challenges faced when constructing and experimenting with the storage systems are mentioned. Finally, an insight on the cost analysis and the general performance metrics of the latent thermal energy storage systems is provided before conclusions are drawn. • High temperature phase change materials for CSP thermal energy storage. • Phase change materials in CSP plants. • Challenges in building LTES systems. • TES sizing and effectiveness. [ABSTRACT FROM AUTHOR]

• Inverse method used to improve original T-history method equations to determine PCMs latent heat. • The method shown to be suitable method to calculate latent heat of low thermal conductivity bulk PCMs. • New transient modelling is more suitable for calculating latent heat with cooling rates than using original equations. High temperature phase change materials (PCMs) are being proposed as thermal energy storage media in concentrated solar power plants. The thermophysical properties of PCMs, such as the latent heat of fusion/solidification need to be defined to enable system design and evaluate system performance and cost analysis. Several methods have been used to measure the latent heat of fusion/solidification of PCMs including the T-history method. The original assumption of the T-history method is the Biot number should be less than 0.1 where the lumped capacitance method is applicable. However, when the Biot number is more than 0.1 then the original equations are not applicable to estimate the latent heat of PCM. This paper demonstrates how a one-dimensional heat transfer mathematical model can be used by applying an inverse method to overcome the limitation of the lumped capacitance method in the original T-history method to determine the latent heat of PCMs. The mathematical model demonstrates that variation of the temperature distributions inside the samples for the discharge curves. Furthermore, this model shows the errors in the latent heat values if the T-history method is used. It is evident from the discharge curves of low thermal conductivity PCMs that the original simplified equations used for interpreting the T-history test results are not applicable for calculating the latent heat at different ambient temperature. [ABSTRACT FROM AUTHOR]