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In this work, melting of a high-temperature inorganic phase change material (PCM) eutectic (with a melting point of 569 °C) within a vertical cylindrical tank has been experimentally investigated. To promote the heat transfer rate, a periodic structure that is constructed by a commercial SS-304 mesh screen has been considered and immersed into the PCM tank. Thermal characteristics of the PCM-periodic structure tank under different initial temperatures (450, 490 and 546 °C) and wall temperatures (620, 640, 660, 680 and 700 °C), are then investigated and reported. The presented experimental data can facilitate practical engineers to find the best operating condition of similar PCM tanks; meanwhile, it can be also employed for the investigation of thermal response of transient heat conduction before melting starts.

•Si3N4and SiC NPs have marginal effort on specific heat capacity of solar salt.•The enhancement is 4.3–9.7% by doping 3 wt.% of SiO2particles.•Specific heat capacity was reduced when adding 5 and 7 wt.% of 30 μm SiO2particles.•Formation of sodium silicate is probably correlated to the reduction.

Latent heat thermal energy storage systems can regulate the intermittency of electricity generation in a grid powered by renewable energy sources. Such systems could be used in conjunction with Brayton power generation cycles operating at high temperatures (520 °C–750 °C) delivering higher efficiency. However, a trade-off of higher thermal efficiency can be higher thermal stresses. The isothermal latent heat thermal energy storage and release can reduce the thermal stresses for a specific design. This study investigates the impact of the orientation on the melting process and the consequent thermal stresses in a lab-scale latent heat shell and tube system using a combination of transient computational fluid dynamics and finite element analyses. With a high temperature phase change material melting at 705.8°C, two initial temperatures of 700°C and 650°C were considered in transient analyses, leading to a 50 K and 100 K initial temperature difference across the tubes, respectively. For the structural analysis, an elastic method was employed for the condition of 50 K temperature difference, and an elastic-plastic analysis for the condition of 100 K to account for plastic deformation. The results of the elastic analysis presented the maximum thermal stresses of 63.5 MPa and 106.7 MPa in tubes in the horizontal and vertical orientations, respectively. The stress in the tube sheet was slightly higher (94.5 MPa) for the horizontal system compared with 90.4 MPa in the vertical position. For the condition of 100 K, the results of the elastic-plastic analysis identified the highest stresses of 172.5 MPa and 147.4 MPa in the tube sheets of the horizontal and vertical systems, respectively. For both initial temperature conditions, the stress level in tubes is lower in the horizontal orientation, which is where most cost occurs in a shell and tube system. Particularly important for the 100 K condition, the results show a lower stress of 100 MPa in tubes for a horizontal system compared with 140 MPa in a vertical orientation. In general, the lowest thermal stress occurs where a melt layer forms around tubes during the early stages of a melting process. A higher stress occurred in parts in contact with argon. The highest level of stress occurred in parts in contact with solid PCM, which has direct implications for larger vessel designs. This study provides insights into hydrothermal and mechanical design of a PCM system to prevent and/or manage localised maximum thermal stresses for a cost-effective thermal energy storage system.

The application of phase change material (PCM) for thermal energy storage (TES) has become one of the viable solutions in energy saving achievable with renewable thermal systems. Packed bed of spheres has been used as TES to enlighten the encapsulated PCM availability in space within the TES system. In order to minimise the high manufacturing cost, the simulation on the TES model is proposed. In accordance to that, a model of TES containing a PCM sphere has been studied in this paper. A commercialised computational fluid dynamics (CFD) software is used to simulate the TES model and the result is then compared experimentally, thus validating the simulation model. The model has been validated for both charging and discharging processes. It is also found that with the use of the CFD model, one could determine the effective thermal conductivity of the PCM at different temperatures.

Mathematical representations of the encapsulated phase change material (PCM) within thermal energy storage (TES) models are investigated. Applying the Effectiveness - Number of Transfer Unit (ɛ-NTU) method, the performances of these TES are presented in terms of the effectiveness considering the impact of different variable parameters. The mathematical formulations summarized can be used for future research work with the suggestion to maximize the heat transfer within the storage. Thus the optimisation on the configuration of the encapsulation can be done through a parametric analysis.

Mathematical representation of the thermal energy storage (TES) containing an encapsulated phase change material (PCM) in Medium Density Polyethylene sphere is investigated. The model applied the Effectiveness - Number of Transfer Unit (ɛ-NTU) method. The performance of a TES is presented in terms of the phase change effectiveness. The mathematical formulation of the model discussed will be used in future research with the suggestion to maximize the heat transfer between the heat transfer fluid and the PCM.

This paper presents the experimental result of a small scale packed bed of random spheres with encapsulated PCM being charged and discharged. A vapor compression refrigerator and heated room with fan heater were used to supply constant heat transfer fluid at a minimum temperature of -28ºC for charging and 16ºC for discharging. Even though the temperature differences were not fixed in the experiments, the performance of the thermal energy storage is depicted in the form of effectiveness values. Different results were obtained for charging and discharging the thermal storage unit. The differences are expected to come from natural convection and super cooling. The super cooling during the charging process was as high as 6oC.

The utilisation of phase change material (PCM) for thermal energy storage (TES) can significantly enhance the energy savings achievable with renewable thermal systems. Sphere based packed bed systems have been used as TES for many years. However, due to the thermal resistance within these systems, the heat transfer is limited and not all the PCM can be used effectively. This study focuses on heat transfer enhancement options for single PCM sphere in a TES system. An experimental investigation has been conducted using water as the PCM. The thermal performance of plain plastic sphere containing PCM has been compared to plastic sphere encapsulated with conducting pins. The heat transfer rate of the sphere with conducting pins was more than 34% that of the sphere without pins.

•A methodology for high temperature pycnometric density measurement was developed.•An in-house apparatus was constructed to measure the density of molten salts.•The apparatus was calibrated using sodium chloride with an error of ±0.6%.•The density of 5 molten salts were measured up to 900 °C.

•Cost of seven sensible and/or latent high temperature thermal storage systems for concentrated solar power technoeconomically evaluated•Costs ranged from $22.11/kWhtto $57.60/kWht•2-tank molten chloride was the lowest cost storage but when integration costs added a 2-PCM hybrid system was more economically feasible•System costs could be reduced by decreasing the turbine inlet temperature or improving the material properties•A full system analysis is required when comparing differing storage technologies if a fair and reliable comparison is to be made

Moving into a carbon constrained world it is imperative that new forms of energy generation and storage are implemented to replace the fossil-fuel driven economy. Additionally, this dependence on fossil-fuels has generated large amounts of waste material (such as fly ash and black slag) which currently occupies large amounts of landfill area. Therefore, this current study investigated the thermal performance and stability of a waste-based alkali-activated material (AAM) for use as a high temperature storage material. Samples contained fly ash from a coal fire power plant, slag from an iron refinery, and commercially available sodium silicate solution. Thermo- and physical properties such as density and heat capacity were measured showing excellent properties as a storage material. Microstructure analysis by scanning electron microscopy, energy dispersive x-ray spectroscopy (SEM/EDS), and x-ray diffraction (XRD) was also conducted. Starting waste material indicated high concentrations of iron oxides, aluminosilicates, calcium hydrates, and minimal alkali activators such as sodium or potassium. A sodium silicate solution provided the necessary alkali activator. The thermal stability of this material was measured using a simultaneous thermal analyser (STA) indicating that the material is stable until 800°C while the formation of aegirine, a mineral with thermal stability up to 990°C, was found in the thermally cycled AAM suggesting this value could be higher. Lastly, the thermo- and physical properties, as well as the microstructure and thermal stability, of the material were reassessed after 50, 100, and 200 thermal cycles (635-800°C) showing minimal weight loss and chemical change after the initial heating cycle. The desirable thermo- and physical properties coupled with excellent thermal stability and sample repeatability, indicate that this material could provide an option for high temperature thermal storage in the future.

•ɛ-NTU an in-house numerical method for energy and exergy assessment.•Exergy delivery of indirect cascade thermal energy storage systems compared with two-tank.•Cascade PCM systems can deliver required exergy in a comparable efficiency to two-tank systems.•Proposed method provides a means for a first level design and selection of cascade thermal storage systems.

•Numerical models to describe heat transfer process in PCM composites were reviewed.•Natural convection in liquid PCM is considered in metal and ceramic composites.•A simplified heat conduction model is usually applied in graphite composites.•Graphite leads to higher heat transfer enhancement than metal with similar porosity.

•A ternary chloride eutectic (Ba-K-Na-Cl) and binary carbonate eutectic (K-Na-CO3) have been studied as high temperature PCMs.•Melting point and latent heat determined as 542 °C and 165 J/g, respectively for Ba-K-Na-Cl eutectic, while it is 707 °C and 133 J/g for the K-Na-CO3eutectic.•Thermal cycling around each PCMs melting point show that there is minimal change in melting point or latent heat due to cycling.•Expected cost of the PCMs is $US 0.28–0.51/kg.