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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]

Abstract: This paper presents the results of a thermal performance analysis of a phase change thermal storage unit. The unit consists of several parallel flat slabs of phase change material (PCM) with a liquid heat transfer fluid (HTF) flowing along the passages between the slabs. A validated numerical model developed previously to solve the phase change problem in flat slabs was used. An insight is gained into the melting process by examining the temperatures of the HTF nodes, wall nodes and PCM nodes and the heat transfer rates at four phases during melting. The duration of the melting process is defined based on the level of melting completion. The effects of several parameters on the HTF outlet temperature, heat transfer rate and melting time are evaluated through a parametric study to evaluate the effects of the HTF mass flow rate, HTF inlet temperature, gap between slabs, slab dimensions, PCM initial temperature and thermal conductivity of the container on the thermal performance. The results are used to design a phase change thermal storage unit for a refrigerated truck. [Copyright &y& Elsevier]

This paper presents a new TRNSYS model of a refrigeration system incorporating phase change material (PCM) for mobile transport. The PCTSU is charged by an off-vehicle refrigeration unit and the PCM provides cooling when discharging and the cooling released is utilized to cool down the refrigerated space. The advantage of this refrigeration system compared to a conventional system is that it consumes less energy and produces significantly lower greenhouse gas emissions. A refrigeration system for a typical refrigerated van is modelled and simulations are performed with climatic data from four different locations. The main components of the TRNSYS model are Type 88 (cooling load estimation) and Type 300 (new PCTSU component), accompanied by other additional components. The results show that in order to maintain the temperature of the products at −18 °C for 10 h, a total of 250 kg and 390 kg of PCM are required for no door opening and 20 door openings during the transportation, respectively. In addition, a parametric study is carried out to evaluate the effects of location, size of the refrigerated space, number of door openings and melting temperature of the PCM on the thermal performance. [ABSTRACT FROM AUTHOR]

Abstract: An innovative refrigeration system incorporating phase change material (PCM) is proposed to maintain refrigerated trucks at the desired thermal conditions. The advantage of using PCM to maintain low temperatures is that a conventional refrigeration system does not have to be located on-board the vehicle. In addition, the system consumes less energy and produces much lower local greenhouse gas (GHG) emissions. The phase change thermal storage unit (PCTSU) is charged by a refrigeration unit located off the vehicle when stationary. The PCM is discharged and provides cooling when in service. A new PCM with a lower cost than currently available PCMs was developed, suitable for maintaining the refrigerated truck at a temperature of −18°C. The PCM has a melting temperature of −26.7°C and a latent heat of 154.4kJkg−1. A prototype system was constructed and test results proved that the proposed refrigeration system is feasible for mobile transport. An analysis shows that delivery of refrigerated products can be made with a PCM system having a weight comparable to that of an on board conventional refrigeration system with less than half of the energy cost. [Copyright &y& Elsevier]

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]

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]

Highlights • Na and Li sulfates solid-solid PCM mixtures are proposed for cascade systems. • The thermal stability, specific heat capacity and latent heat are obtained. • NaLiSO 4 is the most stable material with lower cost. • NaSO 4 shows irreversible phase transition. • A selected mixture of LiSO 4 and NaLiSO 4 shows $50.2 kWh th −1. Abstract In this work, we investigate novel solid–solid phase-change cascade systems based on mixtures of lithium and sodium sulfates. Solid–solid phase-change materials (PCMs) can be coupled with concentrated solar power technologies. They present several advantages over solid–liquid PCMs including lower thermal expansion, lower or no corrosiveness, and no need for encapsulation. In solid–solid PCMs, the energy is stored during crystal structure transitions. Specifically, lithium sulfate undergoes a crystal structure transition (monoclinic to cubic) at 576 °C, which is a suitable temperature for concentrated solar thermal technologies. Due to the high cost of lithium sulfate, we evaluated the potential of mixing lithium with sodium sulfate to create solid–solid cascaded PCM systems to provide higher thermal storage densities. We used differential scanning calorimetry, high-temperature in situ X-ray diffraction and thermogravimetric analysis to evaluate the phase-transition temperature, phase-change enthalpy, specific heat capacity, crystalline phase composition and thermal expansion. The obtained values for heat capacity and enthalpies of phase transitions showed good agreement with available thermodynamic databases. Therefore, further calculations of thermodynamic properties of each mixture in the system were performed for designing cascaded latent thermal energy storage system. From the PCM mixtures studied, NaLiSO 4 shows the greatest stability under ambient conditions. A mixture of 59.17% NaLiSO 4 and 40.83% Li 2 SO 4 allows an optimum charge of both PCMs for power cycles such as supercritical CO 2. Economic assessment revealed that this cascade system has an estimated cost of $50.2 kWh th −1. [ABSTRACT FROM AUTHOR]

Considerable effort has been devoted to the characterization of thermal properties of the different types of materials that can be used as thermal energy storage (TES) media, but scarce literature exists concerning the materials to manufacture the tanks that can be used to contain these storage media. One of the main concerns when selecting the most suitable material for these tanks is its resistance to corrosion due to molten salts that constitute the TES system. Dynamic gravimetric analysis is a newly proposed method for the study of corrosion on metals, which optimizes the standard procedure described by ASTM G1–03. The new technique avoids the direct handling of samples, so more accurate values can be obtained. In this work, the resistance to corrosion of AISI 316 stainless steel samples in contact with commercial grade molten salts of the Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 system, at 600 °C for different exposure times, has been determined by using this new methodology. The results show that the initial corrosion rate is lower at higher amounts of lithium carbonate present in the molten salts mixture. [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]

In the present paper, an experimental study is carried out to evaluate the effect of the dynamic melting concept in a cylindrical shell-and-tube heat exchanger using water as the phase change material (PCM) and a potassium formate/water solution as the heat transfer fluid (HTF). The dynamic melting concept is a new heat transfer enhancement technique which consists of recirculating the liquid PCM during the melting process with a pump and thus increasing the overall heat transfer coefficient as a result of the dominance of the forced convection. The HTF flow rate was kept constant at 1 l/min and four different PCM flow rates of 0, 0.5, 1 and 2 l/min were tested. Results from the experimental analysis showed enhancements up to 65.3% on the melting period, up to 56.4% on the effectiveness, and 66% on the heat transfer rates when the PCM flow rate was twice the HTF flow rate. From these experiments, it can be concluded that dynamic melting is an effective technique for enhancing heat transfer during melting of PCM. [ABSTRACT FROM AUTHOR]

Highlights: [•] Liquid sodium has a quite high convection heat transfer coefficient. [•] Liquid sodium delivered the highest electrical energy to grid −99.4% relative to the ideal case. [•] Gaseous HTFs achieved 87.9–91.3% of the ideal delivered electricity. [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]