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A numerical heat transfer analysis of the phase change material (PCM) in a high temperature vertical triplex tube thermal energy storage system (TTTESS) was performed, using ANSYS FLUENT 15.0. The model has been validated against experimental data using a low temperature PCM in a TTTESS. The numerical results agree well with experiment. The validated model was used to analyse a high temperature PCM in a TTTESS. Supercritical CO2 was assumed for the heat transfer fluid (HTF) which flows in the inside and outside tubes, whereas the lithium and potassium carbonate (Li2CO3-K2CO3) (35%-65%) PCM with a melting temperature of 505°C was enclosed between them. This study provides a comprehensive parametric analysis to demonstrate the effect of adding more fins relative to the case of no fins on the melting and solidification processes to find the optimum design of the TTTESS. The TTTESS with eight fins is the design that has substantial performance and is cost competitive compared with two-tank system [ABSTRACT FROM AUTHOR]

In the following study cost estimates have been undertaken for an encapsulated phase change material (EPCM) packed bed, a packed bed thermocline and a traditional two-tank molten salt system. The effect of various heat transfer fluids (air and molten salt), system configuration (cascade vs one PCM, and direct vs indirect) and temperature difference (ΔT = 100-500 °C) on the cost estimate of the system was also investigated. Lastly, the storage system boundary was expanded to include heat exchangers, pumps and fans, and heat tracing so that a thorough cost comparison could be undertaken. The results presented in this paper provide a methodology to quickly compare various systems and configurations while providing design limits for the studied technologies. [ABSTRACT FROM AUTHOR]

Latent heat storage using phase change materials (PCMs) can be used to store large amounts of energy in a narrow temperature difference during phase transition. The thermophysical properties of PCMs such as latent heat, specific heat and melting and solidification temperature need to be defined at high precision for the design and estimating the cost of latent heat storage systems. The existing laboratory standard methods, such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC), use a small sample size (1-10 mg) to measure thermophysical properties, which makes these methods suitable for homogeneous elements. In addition, this small amount of sample has different thermophysical properties when compared with the bulk sample and may have limitations for evaluating the properties of mixtures. To avoid the drawbacks in existing methods, the temperature – history (T-history) method can be used with bulk quantities of PCM salt mixtures to characterize PCMs. This paper presents a modified T-history setup, which was designed and built at the University of South Australia to measure the melting point, heat of fusion, specific heat, degree of supercooling and phase separation of salt mixtures for a temperature range between 200 °C and 400 °C. Sodium Nitrate (NaNO3) was used to verify the accuracy of the new setup. [ABSTRACT FROM AUTHOR]

Lithium sulphate (Li2SO4) was evaluated as a solid-solid PCM material to be coupled with concentrated solar power (CSP) technologies. The energy is stored in a cubic crystalline phase that is formed at temperatures above 576°C and can potentially be discharged at temperatures as low as 150°C, providing both sensible and latent thermal energy storage in a hybrid sensible-latent system. These operational conditions are appropriate for current CSP technologies based on subcritical steam Rankine power cycles. Results from thermal cycling experiments in air showed no change in energy storage capacity after 15 cycles. There was up to a 5% reduction in latent thermal capacity and 0.95% in total thermal capacity after 150 cycles in air. In our paper, we evaluate a hybrid sensible-latent thermal energy storage system based on lithium sulphate from an economic and technical performance point of view, demonstrating its potential as a high temperature thermal energy storage material. [ABSTRACT FROM AUTHOR]

Numerical study for phase change material (PCM) in high temperature vertical triplex tube thermal energy storage system (TTTESS) were performed, using ANSYS FLUENT 15. For validation purposes, numerical modelling of a low temperature PCM was initially conducted and the predicted results were compared with the numerical and experimental data from the literature. The average temperature for freezing and melting agree well with the results from the literature. The validated model for the low temperature PCM was extended to high temperature TTTESS; the supercritical CO2 as the heat transfer fluid (HTF) flows in the inside and outside tubes during the charging and discharging processes, whereas the Lithium and Potassium carbonate (Li2CO3-K2CO3) (35%-65%) as the PCM is enclosed between them. To enhance the heat transfer inside the PCM, eight fins have been incorporated between the internal and external tubes. This study also provides results demonstrating the effect of adding more fins relative to the case of no fins on the freezing and melting fraction of the PCM. Compared to 2 tank system, the TTTESS with eight fins can provide significant performance with less size. [ABSTRACT FROM AUTHOR]

In an effort to reduce the cost of thermal energy storage for concentrated solar power plants, a thermocline storage concept was investigated. Two systems were investigated being a sensible-only and an encapsulated phase change system. Both systems have the potential to reduce the storage tank volume and/or reduce the cost of the filler material, thereby reducing the cost of the system when compared to current two-tank molten salt systems. The objective of the current paper is to create a numerical model capable of designing and simulating the aforementioned thermocline storage concepts in the open source programming language known as Python. The results of the current study are compared to previous numerical results and are found to be in good agreement. [ABSTRACT FROM AUTHOR]

A ternary molten chloride has been suggested as a high temperature (550-720 °C) sensible heat storage medium for a two-tank system. An effective thermal insulation is proposed to provide an opportunity for the fabrication of both the hot and cold storage tanks from lower cost metals, e.g. A240-347H, by maintaining the tank wall below 500 °C. To achieve this, the hot storage tank mainly comprises of inner shell (11.4 cm of a refractory brick as liner (KX-99®), 30 cm of insulation brick) and outer metal shell, where 5 cm of frozen layer of PCM800 (NaCl), and 5 cm of ceramic wool fill the annular space between the shells. Another layer of ceramic wool was introduced at the outer side of the metal shell to keep the temperature lower than 60 °C. ANSYS Fluent was used for the numerical study of the cool down behavior of the system during idle period when tank is quarter-filled. The heat transfer modes were considered as radiation and natural convection heat transfer from the hot molten salt at 720 °C, conduction through the solid layers and convection heat transfer from the outer shell to the environment at 15 °C. The results showed that the proposed insulation system including a frozen layer of PCM800 maintains the temperature at the metal wall lower than 500 °C, and lower than 60°C at the outer side of the tank. Apart from the insulation property of the frozen layer of PCM800, it absorbs any leak of hot molten salt, acting as a barrier between salt and the metal shell. The thermal performance analysis provide an insight into the benefits of the multilayer insulation. [ABSTRACT FROM AUTHOR]

The next generation of concentrated solar power plants will need to operate at higher temperatures (> 700°C) so that better efficiencies and lower costs can be achieved. One such design that may be able to achieve this is with sodium as the heat transfer fluid (HTF) and phase change materials (PCMs) as the storage medium. To ensure safe design it is imperative that the consequences of failure are known. Therefore, in the current study, the failure of such a system is investigated by mixing molten PCM into liquid sodium using a custom-built rig. Using this rig, it was possible to introduce molten PCM (Na2CO3−K2CO3) into sodium at 750°C and view the interaction. When combined, it was found that no violent reaction was observed, indicating that a failure in the storage system would not cause detrimental failure if the PCM was to mix with the sodium. Furthermore, the decomposition of the PCM releasing CO2 (potentially reactive with sodium) was studied. [ABSTRACT FROM AUTHOR]

Latent heat thermal energy storage systems working at higher temperatures around 800 °C are considered as a promising alternative in conjunction with the sCO2 Brayton cycle for the next generation of concentrating solar power plants. At higher temperatures, a detailed thermo-mechanical analysis of the system is crucial, particularly during a melting/charging process with a high temperature difference (50-100 K) between the solid phase change material (PCM) and heat transfer fluid. Here, the melt path formation of a high temperature PCM in a horizontal shell and tube system is investigated. Three different levels of modelling were conducted. The results of the study showed that the melted PCM around the tubes connect and form a path after about 50 seconds, which provides a way for the high temperature PCM to flow through. This imposes a temperature gradient along the tubes preventing a localised hot spot and high thermal stresses. [ABSTRACT FROM AUTHOR]

This study established a methodology for designing a cascade latent heat thermal energy storage system. The latent heat thermal energy storage has been investigated by numerous researchers as an energy-dense and lower cost alternative to the two-tank molten salt sensible heat thermal energy storage for concentrated solar power plants. However, in order to achieve a high storage efficiency without decreasing the efficiency of the receiver and power block, many design restrictions should be applied in the design, including the total storage capacity, charging/discharging rate, charging/discharging period and the temperature difference of heat transfer fluid between the inlet and outlet of the storage system for both the charging and discharging process. There is no methodology or guideline currently available in the literature to design such a storage system. Therefore, a new methodology has been proposed, and it was successfully demonstrated in a case study. To achieve the final design, three major steps are involved: preliminary design, coarse optimization and fine optimization. Though intensive simulation is needed, the proposed methodology is appropriate to design the cascade latent heat thermal energy storage system. [ABSTRACT FROM AUTHOR]

In the current study a novel geopolymer created from waste materials (namely fly ash and black slag) has been characterised as a potential high temperature thermal energy storage material option. Several geopolymer samples have been fabricated and characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Additionally, the density and heat capacity have been determined. Lastly, the cost of the material and packed bed system was estimated showing that the geopolymer-based systems are potentially 35% cheaper than the traditional 2-tank molten salt systems. [ABSTRACT FROM AUTHOR]

The thermal behaviour of PCM in different configuration of heat exchangers is analysed for the purpose of design and optimisation of a latent heat thermal energy storage unit. Numerical modelling in conjunction with scale analyses provides a cost effective means to examine the performance of different configurations of latent heat storage systems. The main objective of this work is to scale down an experimental set up to a numerical model which can represent the thermal behaviour of the system with reasonable computational time. A U-tube element from a shell and tube thermal storage experimental set up was used as the model for a numerical study, using FLUENT. The predicted results of the temperature profiles in the PCM domain are in agreement with the measured data. Moreover, the heat transfer fluid outlet temperature and duration of the phase change processes are consistent with experimental results. Using the experimental initial temperature at each point in comparison to using an average initial temperature from all points can improve the predicted temperature profiles. Furthermore, three different mushy zone constants; namely 105, 107, 108 were used to examine the impact on the rate of melting. It was found that the constant 107 provides a closer solution to the experimental results. Results of this study show that the small scale model can represent the lab scale set up, providing more detail about the thermal behaviour of the PCM which is difficult to capture by measurement. The model can also be used for further examination of a high temperature PCM within the same set up for a CSP application. [ABSTRACT FROM AUTHOR]

In the current study the feasibility of using concentrated solar power (CSP) 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 storage system was developed, validated, optimised, and economically costed. The optimised air-based EPCM system utilised a high and low melting temperature PCM with a sensible filler. It was found that a capsule radius of 10 mm and PCM volume of 13 % resulted in the lowest cost of discharged energy of $25.55/kWht. This system was then coupled to typical meteorological year (TMY) solar data for Adelaide, South Australia, to simulate performance of a 1 MWt heat load over a year. By solving an hourly system generation and demand profile, it was found that the system could provide an economical alternative to gas when the gas price was $20/GJ for a variety of demand profiles. [ABSTRACT FROM AUTHOR]

This paper presents the stability and corrosion testing results of a candidate high temperature phase change material (PCM) for potential use in concentrating solar power applications. The investigated PCM is a eutectic mixture of NaCl and Na2CO3 and both are low cost materials. This PCM has a melting temperature of 635℃ and a relatively high latent heat of fusion of 308.1 J/g. The testing was performed by means of an electric furnace subjected to 150 melt-freeze cycles between 600℃ and 650℃. The results showed that this PCM candidate has no obvious decomposition up to 650℃ after 150 cycles and stainless steel 316 potentially can be used as the containment material under the minimized oxygen atmosphere. [ABSTRACT FROM AUTHOR]

In an effort to reduce the cost and increase the material compatibility of encapsulated phase change materials (EPCMs) a new encapsulated system has been proposed. In the current study a molten salt eutectic of barium chloride (53% wt.), potassium chloride (28% wt.) and sodium chloride (19% wt.) has been identified as a promising candidate for low cost EPCM storage systems. The latent heat, melting point and thermal stability of the phase change material (PCM) was determined by DSC and was found to be in good agreement with results published in the literature. To cope with the corrosive nature of the PCM, it was decided that a fly-ash based geopolymer met the thermal and economic constraints for encapsulation. The thermal stability of the geopolymer shell was also tested with several formulations proving to form a stable shell for the chosen PCM at 200℃ and/or 600℃. Lastly several capsules of the geopolymer shell with a chloride PCM were fabricated using a variety of methods with several samples remaining stable after exposure to 600℃ testing. [ABSTRACT FROM AUTHOR]