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For design and optimization purposes, understanding the performance characteristics of different configurations of PCM systems during charging and discharging processes is crucial, particularly for thermal storage in CSP. A numerical study using FLUENT has been performed to explore and compare the characteristics and behavior of PCM in different shell and tube configurations; namely counter flow and parallel flow in vertical and horizontal orientations. These designs have been compared with that of an alternative design using plate fins. The main practical advantage of the alternative design is decoupling the PCM location from the path of flow of the heat transfer fluid that reduces the constraints on the materials. Confined heating/cooling pipes in a channel at the bottom of the PCM enclosure makes it more flexible to match the required working pressures, and simplifies the design and material (PCM/plate) selection. This is a significant issue as higher temperatures and pressures are being required for improving the overall solar thermal system performance. Based on the same heat transfer area, amount of PCM and input energy, a performance comparison was determined based on exergy maximization. Using sodium nitrate as the PCM, the results show a higher heat transfer rate for the vertical arrangement of plate fins compared to the counter flow shell and tube configurations, as well as a more uniform heat transfer rate compared to all shell and tube arrangements for both the charging and discharging processes. The proposed design leads to less redundant PCM, as well as a smaller and more cost effective PCM system as a heat storage unit. Overall, the vertical plate fin configuration maximizes the useful heat that can be extracted for a given amount of heat and PCM, which has direct applicability to thermal storage solutions for CSP and other applications. Moreover, its modular design facilitates the selection and optimisation for different scales and applications e.g. as a heat storage for electricity production or as a protection system for a receiver from high thermal stresses in a CSP plant. [ABSTRACT FROM AUTHOR]

According to the SunShot initiative, one sixth of the levelized cost of electricity for Concentrated Solar Thermal Power is thermal energy storage. For this power generation paradigm to be successful, the cost of every sub-system must be dramatically reduced. However, the search space for possible storage mediums is too large for a brute force experimental search to be feasible. Thus, a more refined approach is necessary. In this paper, eutectic salt combinations are considered as storage medium. The state of the selection process for these eutectics is discussed. Various methods to predict the important thermophysical properties are reported and applied to eutectics whose physical properties are known. Based on single salt properties, the density of molten salt eutectics can be predicted, around their melting point, to within 5%. Prediction of the melting point and composition is accurate to within 7%. However, the estimation of latent heat for multi-component eutectics is not always accurate, and requires more work. Finally, the thermal conductivity of multi-component eutectics has not been well studied; further research is required to corroborate the predictions. [ABSTRACT FROM AUTHOR]

One feasible solution to reduce the cost of concentrated solar power is to employ a higher temperature and efficiency supercritical carbon dioxide Brayton power cycle. An alternative heat transfer medium and thermal energy storage (TES) system therefore needs to be explored. This work considered a variety of phase change materials (PCMs) and graphite as the storage media in four indirect shell-and-tube storage configurations, including a 3-PCM and 5-PCM cascade storage, a PCM-graphite-PCM hybrid storage and a single graphite storage. The sizing and design of the TES systems were performed by using a dynamic cycling methodology based on a transient 2D numerical model. The cost of those designs were then determined by using an economic model developed inhouse. This work also investigates the impact of some geometric parameters and cost assumptions on the techno-economic performance of the TES system. The analysis suggests a scenario exists whereby a low thermal efficiency storage system which therefore utilises less tube or storage material could be more cost-effective. Overall, the cost of hybrid TES is the lowest among all studied systems, at $26.96/kWh t and $21.49/kWh t for charging temperature of 720 °C and 750 °C, respectively, followed by the 5-PCM storage at $28.06/kWh t and $21.82/kWh t , respectively. [Display omitted] [ABSTRACT FROM AUTHOR]

A shell-and-tube design with different thermal energy storage (TES) media was investigated as a promising TES system for a next generation concentrated solar power (CSP) plant. Sensible TES using graphite, latent TES using phase change materials (PCMs) and a hybrid of both were investigated. A two-dimensional transient heat transfer model was applied to simulate the thermal performance of multiple shell-and-tube TES modules connected in series. Considering the realistic operation of the CSP plant, an intensive numerical investigation was conducted to design and size the storage system. The TES systems studied were categorized as 3-PCM cascade, 5-PCM cascade, PCM-graphite-PCM hybrid and single graphite. All the PCMs considered in this paper have been experimentally proven as potential candidate storage media. It was found that all the TES systems studied, if well designed, can meet the CSP operational requirements. In comparison, single graphite storage is less problematic, but it has the lowest storage density (47.3 kWhr/tonne). By forming a PCM-graphite-PCM sandwich configuration, the energy density was increased to 60.9 kWhr/tonne and the storage effectiveness of this hybrid system is 70.7%, the highest among all the TES systems studied. • A 2D model was validated and verified using experimental and CFD results. • Six designs of sensible, latent and sensible-latent hybrid storage were investigated. • The overall storage effectiveness of the hybrid design is the highest. • The effectiveness of the graphite system is improved by 21.2% in hybrid design. [ABSTRACT FROM AUTHOR]

The eutectic Na 2 SO 4 -NaCl salt was investigated as a novel high temperature phase change material for solar thermal energy storage due to its low cost and melting point (626.0 °C) interested. The measured melting point and heat of fusion of the eutectic salt are 625.3 °C and 263.3 J/g, which are in agreement with their theoretical values. The eutectic salt shows good thermal stability after 100 melting-freezing thermal cycles in the range of 550–680 °C in air. Three types of the eutectic Na 2 SO 4 -NaCl-ceramic composites have been designed and evaluated to address the salt’ corrosion issues with containment materials. Results show that eutectic Na 2 SO 4 -NaCl/both α-alumina and mullite composites with a mass ratio of salt to ceramics of 1:1 (wt%) have good thermal stability without cracking as well as with weight losses of 0.50% and 0.74% and the decrease of latent heat by 1% and 2% respectively after 20 thermal cycles, where no phase separation and chemical reaction occur. Their measured melting temperature and latent heat are consistent with their theoretical values. Addition of α-alumina and mullite to the eutectic Na 2 SO 4 -NaCl salt can reduce the eutectic Na 2 SO 4 -NaCl salt’ supercooling. In contrast, the sample of the Na 2 SO 4 -NaCl/diatomite composite cracked after the cycles. [ABSTRACT FROM AUTHOR]

The search for optimal phase change materials (PCMs) for latent heat thermal energy storage systems (LHTESS) focuses almost exclusively on the properties of the PCM. This neglects the significant contribution of the cost of the containment vessel on the total cost of the LHTESS. Thus, to accurately assess the thermoeconomic performance of various PCMs, the relationship between the cost of the containment vessel and the properties of the PCM must be understood. This paper presents an analytical method for optimizing the design of tube and shell or tube and fin containment vessels for the least cost, subject to geometric and performance constraints. One of the key performance constraints is the time to charge, whose evaluation typically requires computationally-expensive simulations which are unsuitable for optimization algorithms. To enable efficient optimization, a novel closed-form approximation of the charge time is developed and validated through numerical simulations. Subsequently, the optimization methodology is used to investigate the relationship between the optimal vessel geometry and configuration, fin properties, PCM properties, and cost of two PCMs. For these PCMs, the use of high thermal conductivity fins is shown to dramatically reduce the total cost of the system. The new methodology is an efficient way to compare the thermoeconomic performance of PCMs as latent heat storage solutions, allowing for the accurate assessment of PCMs. [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]

Latent heat thermal energy storage, by using phase change materials (PCMs), is considered as a promising technology that can be integrated into concentrated solar power (CSP) applications to mitigate the load and electricity supply. Multiple PCMs connected in series has attracted considerable research attention as this configuration has the potential to improve the thermal performance compared to single PCM storage. However, this technology has not yet been deployed in commercial CSP plants. One of the major restrictions is the lack of reliable thermophysical property data for high temperature PCMs. This study gives a broad review of the experimentally verified PCMs with melting temperatures from approximately 500 °C–700 °C, applicable to CSP systems. A further twelve PCM candidates, containing low-cost sodium and/or potassium salts, were identified in this study and their thermophysical properties were experimentally evaluated. It was found that nine of the new candidates can be used as PCMs. • Experimentally verified PCMs with melting points of 500 °C–700 °C were reviewed. • 12 potentially low-cost PCM candidates were identified from the literature. • PCMs were experimentally characterised by using the DSC and TGA. • 9 out of 12 candidates are suitable as PCMs. • The measured weight loss by TGA is in proportion to the estimated vapor pressure. [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]