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A numerical study has been conducted on a shell and tube latent heat storage system whereby the inlet heat transfer fluid direction is periodically reversed during charging and discharging. The impact of varying the boundary condition at the interface of the tubes carrying the heat transfer fluid and phase change material (PCM) on the evolution of the phase change front, heat transfer area and heat transfer rate have been evaluated during the charging and discharging processes. Results for the charging processes show a higher heat transfer area develops during the early stages and amplification of natural convection after 40% melt fraction, leading to a higher heat transfer rate. In comparison to the fixed flow condition, periodic flow reversal for the discharge cases results in an increased heat transfer area for a longer period of time, leading to a higher heat transfer rate particularly after 75% solidification. This effect is more important for discharging cases in the absence of convection heat transfer. Periodically reversing the direction of heat transfer fluid, which produced a periodic boundary condition at the tube-PCM interface, also resulted in a lower temperature gradient in space and time and consequently higher exergy recovery, and about a 6% increase in the time-average heat transfer rate in the charging and discharging cases. The novel reversal flow method provides a means to implement a periodic boundary condition without changing the heat source/sink, enhancing the thermal performance and cost effectiveness of latent heat storage systems. Phase change storage systems incorporating periodic flow reversal provide higher energy delivery rates, greater power density and more exergy recovery. This method can support fast heat release to respond to a peak load in a CSP plant or fast heat storage to protect a tubular receiver from high thermal stresses. [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]

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]

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]

Highlights • Comparison of overall exergy efficiency of different thermal energy storage systems. • Liquid sodium as HTF reduces the gap in exergy efficiency between PCM and two-tank. • High temperature sCO 2 as HTF in PCM reduces the exergy efficiency gap with two-tank. • Na with a high temperature PCM provides high storage density and exergy recovery. • Optimal temperature of PCM depends on PCM/HTF and inlet hot/cold temperatures. Abstract Different options of sensible and latent heat storage systems comprising different types of heat transfer fluids, heat storage media or phase change materials (PCMs) have been compared considering the upstream and downstream requirements of a concentrated solar power cycle. For an optimal system, analysis of both the energy efficiency and exergy recovery of the storage system and the whole cycle of heat to power is necessary. This study provides this analysis, comparing a two-tank sensible storage system to latent heat shell and tube storage system, using an analytical method, ε-NTU. For the case of molten salt as the PCM and sCO 2 as the heat transfer fluid, results show 4.02% higher exergy recovery for the two-tank system at a working temperature range 450–700 °C compared to 450–660 °C for the latent heat storage system where the upper limit is the PCM melting temperature. Compared to the Carnot cycle efficiency (the highest rate of useful energy (exergy) recovery for an isentropic process) of 70%, the overall efficiency of the two-tank system, the base case of molten salt/sCO 2 , molten salt/Na, and molten metal/sCO 2 are 57.41%, 53.39%, 55.21%, and 53.55%, respectively. The gap in efficiency between the sensible and latent heat storage systems decreases by lowering the thermal resistance of heat transfer fluid side and/or the PCM side. Specifically, the gap in efficiency decreases 45% by using liquid Na instead of sCO 2 and a 4% decrease is observed when replacing molten salt with a molten metal like aluminium as the PCM. Using PCMs with higher melting temperature is preferred with liquids with high thermal conductivity like Sodium. For a heat transfer fluid such as sCO 2 with low thermal conductivity, a higher inlet fluid temperature and/or a low temperature PCM can provide high exergy efficiency. For a specific combination of PCM and heat transfer fluid, an optimal melting temperature exists depending on the inlet temperatures during charging/discharging and the thermophysical properties of the media. The study shows that PCM storage provides opportunities to minimise causes of irreversibilities to achieve simultaneous high volumetric energy density and high exergy recovery for CSP application in comparison with the conventional two-tank sensible storage. [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]

• Industrial refrigeration with solar energy, storage and control system demonstrated. • Role of latent heat thermal energy storage to reduce cost of electricity investigated. • An in-house control code used to manage the storage status and electricity demand. • Optimisation opportunities discovered from the data collected over ten months. • Proposed system can manage different renewable energy resources and variable loads. Thermal energy storage offers a solution to mitigate the intermittency of electricity generation in a grid powered by renewable energy and an opportunity to arbitrage on the temporal variability in the cost of the grid supplied energy. This study investigates the potential of a thermal energy storage system used for thermal load and electricity demand management at the industrial scale. A latent heat thermal energy storage system in conjunction with an advanced control and forecast algorithm have been integrated into an existing industrial food processing plant. This demonstration system also incorporates electricity from onsite PV and the grid to power the refrigeration system. The control and forecast code provide a schedule to charge the thermal energy storage from the grid during low or negative prices; otherwise, it instructs to charge from solar PV or discharge to cover the refrigeration load. The data collected from the demonstration system clearly showed the role of the integrated system in demand side management leading to electricity cost reduction during the ten months of operation. The net estimated electricity cost savings was 10,435 AUD in ten months, while doubling the capacity of thermal energy storage could save about 48,700 AUD per annum. Data analysis of energy demand acquired from all the modules of the plant provided valuable insight into further optimization of the control code for higher realization of its potential in the reduction of electricity demand charges. Moreover, such a control system combined with thermal energy storage can be used for the management of any variable load and renewable energy sources. [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]

The thermal performance of a latent heat energy storage system utilising tightly fitted, thermally bonded and non-bonded metal inserts as a heat transfer enhancement material has been investigated. Two methods were used for this purpose. Firstly, a numerical study using volume averaging method was performed and secondly, an analytical study using scale analysis to obtain the theoretical prediction of the performance of the phase change material. Both models agreed with the experimental data in the literature. A further numeric-parametric study was performed to determine the effect of increasing the thermal resistance gap on the total melting/solidification time. It was observed that the storage performance (complete charge/discharge) asymptotically decreased as the gap grows in size and that convection was negligible when periodic structures are used. Generally, it can be concluded that having tightly inserted metal inserts in the storage improves the performance by about three times compared to a case with a 1 mm thermal resistance gap. Numerical results show that when a thermally adhesive glue is used to make ligaments touch the inner wall (thus eliminating the gap), an improvement of about 10% of heat transfer enhancement in charge/discharge is realised as compared to the case of a 1 mm thermal resistance gap. • Numerical prediction of the effect of thermal contact resistance in the performance of phase change materials. • Analytical model accurately predicting the performance of enhanced phase change materials is developed. • The study can help in preliminary design before experimentation. [ABSTRACT FROM AUTHOR]