The thermophysical properties of phase change materials (PCMs) make them attractive for application in solar thermal systems as an energy storage medium. However, several shortcomings of PCMs include low thermal conductivity, diffusivity, and cost effectiveness. Additionally, selection of the material type is highly dependent on geographical location when considering operating temperatures. Therefore current available solar water heating (SWHs) systems may not be as effective in areas with low-moderate operating temperature conditions.  In SWH systems, evacuated tube solar collectors (ETC) have shown to be more efficient than flat plate collectors due to significantly lower heat loss from convection, but their operation is just limited by availability of sunlight, and the heat transfer between the solar collecting surfaces. Common ETCs include a heat pipe with a small droplet of water or glycol acting as liquid-gas PCM to facilitate heat transfer from the collecting surface to the working fluid. Yet, the operating temperature of the system is limited by thermophysical properties of the heat transfer fluid (HTF), therefore it has a critical effect on the overall thermodynamic efficiency. The novelty of this study lies in the unique mixtures of solid-liquid PCM doped with nanoparticles (NPs) as HTFs directly applied in a U-tube inside the ETCs by replacing the effect of the heat pipe. One selected type of HTF in this study is a ternary eutectic molten salt mixture (HITEC; 7% NaNO~3~, 53% KNO~3~, 40% NaNO~2~) due to its optimum melting point of 142°C. To improve upon thermophysical properties of HITEC as the base fluid, varying concentrations of NPs were mixed, separately, into samples. Concentrations of 0.06, 0.1, 0.5, and 1.0 weight percent of TiO~2~, Al~2~O~3~, and multi-walled carbon nanotube (MWCNT) powder were added via liquid dispersion method. A sample of pure HITEC was also prepared for comparison. Another PCM, Erythritol (C~4~H~10~O~4~), was chosen as an alternative base HTF. Erythritol is a widely available, inexpensive, and non-toxic sugar sweetener. Due to its high latent heat storage capabilities, it has also been used as a solid-liquid PCM thermal storage medium. The benefits of Erythritol for use as a HTF include its high specific heat capacity in liquid form relative to that of HITEC molten salt, as well as its unique melting/crystallization temperature behavior. In this study samples of Erythritol were also doped with NPs with the aim of maintaining high specific heat capacity while increasing thermal conductivity and diffusivity. A Thermogravimetric Analyzer (TGA) and Differential Scanning Calorimeter (DSC) were used to test the selected materials. The base HTFs were first tested on the TGA to ensure that the weight percent (wt. %) decomposition of the sample does not exceed 5% of the total weight as this could contaminate the DSC cell. Then, the samples were each tested in a DSC to determine the specific heat capacity, melting and crystallization onset temperature, and peak melting temperature. The DSC tests were ran in heating and cooling cycles as a modulated heating ramp. The figure of merit (FOM) analysis has also been investigated to ensure the adequate flow characteristic within the U-tube circulating in the SWH system in order to reduce the pumping effect. The results from the DSC tests show enhancement of the thermophysical properties of the selected PCMs with the addition of NPs. This study allows for the optimization of the operating temperature range of the collector when it’s integrated with these materials, where direct heat gain can be obtained in the collector.

Keywords: Solar thermal collector, ternary eutectic molten salt, phase change material, nanoparticles.


Thermal Analysis of Phase Change Material Based Heat Transfer Fluid in Solar Thermal Collector

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

HT-ICNMM2020 - Virtual Conference, SHTC - Virtual Conference, FEDSM2020 - Virtual Conference

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