Electrical initiation of solidification from supercooled state and preservation of supercooled state of sodium acetate trihydrate solution, which is considered as a promising thermal energy storage material, are experimentally investigated with varying the configuration of electrodes and confirmed that the initiation of solidification and preservation of supercooled state are both possible by using the electric field. Further, effect of crystal growth direction on crystal growth rate is also investigated by using the newly developed electrical nucleation method. The result shows that the crystal growth rate, which growth direction is bottom to top, is slightly decreased compared with the direction of top to bottom at certain supercooling temperature range.

In this paper the solid/liquid phase change heat transfer in porous materials (metal foams and expanded graphite) at low and high temperatures is experimentally investigated, in an attempt to examine the feasibility of using metal foams to enhance the heat transfer capability of phase change materials for use with both the low and high temperature thermal energy storage systems. In this research, the organic commercial paraffin wax and inorganic hydrate calcium chloride hydrate salts were employed as the low-temperature materials, while the sodium nitrate is used as the high-temperature PCM in the experiment. The heat transfer characteristics of these PCMs embedded with open-cell metal foams were studied experimentally. The composites of paraffin and expanded graphite with different graphite mass ratios, namely, 3%, 6% and 9%, were also made and the heat transfer performances of these composites were tested and compared with metal foams. Overall metal foams can provide better heat transfer performance than expanded graphite due to their continuous inter-connected structures. But the porous materials can suppress the natural convection effect in liquid zone, particularly for the PCMs with low viscosities, thereby leading to the different heat transfer performance at different regimes (solid, solid/liquid and liquid regions). This implies that the porous materials don’t necessarily mean they can always enhance heat transfer in every regime.

This report describes an experimental investigation into the effect of electric current in reducing the supercooling of erythritol. Previous studies have identified erythritol as a prime material candidate for moderate temperature thermal energy storage (TES) systems due to its high latent heat of fusion and melting temperature (118°C), but it has also shown excessive supercooling, sometimes exceeding 65°C [1]. Various methods for controlling or reducing supercooling are reviewed, including work by Shichiri and Hozumi showing that a small electric current passed through supercooled water is highly effective in initiating nucleation [2,3]. In the present study, the authors demonstrate a similar effect with erythritol by subjecting a sample to repeated thermal cycles with and without the application of a direct electric current. The control cases without electric current showed a highly variable recrystallization temperature ranging from 67°C to 109°C (or supercooling magnitudes from 9 to 51°C). Passing a direct current through the sample using silver wire electrodes significantly shifted the material’s nucleation behavior. The local nucleation temperature only varied from 108°C to 112°C (or 6–10°C of supercooling), and nucleation always occurred on the positive electrode surface. Control cases both before and after the electrical trials indicated no noticeable change in sample crystallization behavior.

When collecting the energy of the sun for domestic use, several options exist, one being the use of evacuated tube collectors with internal heat pipes. This study proposes a system integrating these collectors with a storage unit using the phase change of paraffin wax to store energy. The storage unit makes use of a finned heat exchanger, with paraffin wax on the shell side and glycol on the tube side as the heat transfer fluid. The heat exchanger is embedded within the storage paraffin wax with a volume of 2 ft 3 . The heat exchanger also includes a separate loop for water to flow through and receive thermal energy from the melted wax. Although the wax has the benefit of being inexpensive and nontoxic, it has the problem of low thermal conductivity. Therefore, the heat exchanger has large copper fins brazed to it to extend areas of high thermal conductivity into the wax reservoir. The unit used in this study contains 14 fins. The use of fins will help to speed up the melting of the wax while solar energy is collected, since there is more heat transfer area. When most of the wax is melted, heat can be exchanged to water for domestic use. To determine the benefit of the fins, wax and working fluid temperature data will be taken from a constructed thermal energy storage unit, and then it is used to verify a finite-difference analytical model of the thermal operating characteristics. The maximum operating temperature of the glycol/water mix heat transfer fluid was approximately 65° C when the fluid flowed at 1 gallon per minute. The storage unit was able to store melted wax overnight with a 2–3°C temperature drop with the ambient temperature approximately at 30°C. City water at approximately 3 gpm was used to test the freezing side. The one dimensional model proved useful in predicting the heat storage mode of the system but had some error in predicting the heat release mode of the unit. The model also points to the fact that there are several considerations to be taken when simulating phase change energy storage processes.

The aim of this study is to investigate the enhancement of thermal properties of various high temperature nanofluids for solar thermal energy storage application. In concentrating solar power (CSP) systems, the thermo-physical properties of the heat transfer fluids (HTF) and the thermal energy storage (TES) materials are key to enhancing the overall system efficiency. Molten salts, such as alkali nitrates, alkali carbonates, or eutectics are considered as alternatives to conventional HTF to extend the capabilities of CSP. However, there is limited usage of molten salt eutectics as the HTF material, since the heat capacity of the molten salts are lower than that of conventional HTF. Nanofluid is a mixture of a solvent and nanoparticles. Well dispersed nanoparticles can be used to enhance thermo-physical properties of HTF. In this study, silica (SiO 2 ) and alumina (Al 2 O 3 ) nanoparticles as well as carbon nanotubes (CNT) were dispersed into a molten salt and a commercially available HTF. The specific heat capacity of the nanofluids were measured and applicability of such nanofluid materials for solar thermal storage applications were explored. Measurements performed using the carbonate eutectics and commercial HTF that are doped with inorganic and organic nano-particles show specific heat capacity enhancements exceeding 5–20% at concentrations of 0.05% to 2.0% by weight. Dimensional analyses and computer simulations were performed to predict the enhancement of thermal properties of the nanofluids. The computational studies were performed using Molecular Dynamics (MD) simulations.

Complex macroscale and microscale heat and mass transfer phenomena encountered in several thermal energy storage and transport systems are discussed. Thermal storage and transport systems involving ice slurries and nanoemulsions of phase change materials can be used for either cooling or heating applications or both, which can contribute to the reduced usage of electricity during peak hours. But heat and mass transfer and stability issues are encountered in the production, transport and storage of the heat storage media. Both the heat transfer enhancement effect and detrimental effects such as Ostwald ripening and supercooling will be discussed. Another interesting microscale phenomenon recently encountered in energy transport devices such as heat pipes is the enhancement of heat transport with the use of self-rewetting fluids. Critical heat fluxes in boiling can be enhanced by up to 300% and this helps prevent liquid dryout at high heat fluxes in different types of heat pipes. Both the nature of the enhancement effect and possible mechanisms will be discussed.