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The design of latent heat thermal energy storage systems is critically dependent on the properties of the chosen Phase Change Material (PCM). Currently published data on the thermodynamic properties of eutectic salts contains contradictory data, impeding the selection of PCMs. In this work, previously elaborated theoretical methods for predicting the composition and enthalpy of fusion for eutectic salt mixtures is experimentally validated for three ternary eutectic mixtures: LiNO3 + NaNO3 + KNO3, Li2CO3 + Na2CO3 + K2CO3, and NaCl + Na2CO3 + Na2SO4. For each combination of salts, the eutectic composition suggested by published sources and the composition predicted by the theory are created. The latent heat of fusion, melting temperature, and congruency of the melting peak of these six mixtures are examined via differential scanning calorimetry. In addition, scanning electron microscopy and energy dispersive spectroscopy are used to verify the expected eutectic structure. Analysis of these six mixtures confirms that the composition can be predicted to within 8%. The enthalpy of fusion can be predicted to within 10% of experimental results. In addition, three binary eutectics (NaCl + Na2SO4, KCl + K2CO3, and NaCl + Na2CO3) are created and the theoretical enthalpy of fusion is experimentally verified.

In the current study pre-formed geopolymer half shells were fabricated from a combination of fly ash and black slag. Five (5) geopolymer shells were filled with a chloride-based phase change material (PCM) with a melting point of 540 °C for use in a high temperature thermal energy storage system (TESS). Additionally, three (3) of these shells contained an inner coating of alumina (various manufacturers) while another geopolymer shell was pre-fired at 610 °C. The half shells were cycled around the melting point of the PCM (500–580 °C) for five (5) cycles, with the mass change of the samples recorded. Of the five (5) samples only the Aremco-coated sample suffered less PCM loss than the control. Furthermore, the melting point and latent heat of PCM samples from the geopolymer half shells were measured by differential scanning calorimetry (DSC) to find if degradation occurs. In all samples the PCM melting point was not affected while the latent heat was significantly affected. Additionally, the Aremco-coated sample resulted in PCM segregation. Refereed/Peer-reviewed

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. Keywords PCM heat exchanger design; Latent heat thermal energy storage system optimization; Phase change material; Thermal energy storage

Highlights - The increase in corrosion due to the presence of nanoparticles is characterized. - Experimental difficulties in previous studies are discussed in relation to the identified corrosion reaction. - The latent heat of fusion is reported for alumina and titania nanofluids. Abstract In this paper, the unsuitability of water-based nanofluids in aluminum environments is demonstrated. In addition, the specific heat capacity (Cp) of water-based nanofluids with alumina or titania nanoparticles is investigated, with the Cp reported for these frozen nanofluids. The results confirm the accuracy of the oft referenced Model II in determining the Cp of these nanofluids. The latent heat of fusion for the nanofluids is also reported. Alodined-aluminum sample pans were used to reduce the rate of the water-aluminum reaction enough such that reliable data was obtained. Initial measurements of nanofluids were performed in aluminum sample pans; the nanoparticles are hypothesized to have catalyzed the aluminum and water reaction which resulted in unrepeatable data. The synthesis of the byproduct of this reaction, the mineral bayerite, is confirmed. Room temperature electrochemical studies confirm that the presence of nanoparticles increases the corrosion rate.

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. Refereed/Peer-reviewed

Degradation of an Inconel 601 superalloy subjected to a eutectic mixture of carbonate salt at 450 °C was studied using advanced microscopy and microanalysis techniques. On the outside of the vessel, long term exposure to elevated temperature led to intergranular stress corrosion cracking via a stress assisted oxidation process. Elemental analysis verified diffusion controlled de-alloying of Ni and Cr from grain boundaries. Transmission electron microscopy showed that a (CrFeNiAl)3O4 oxide phase was formed at grain boundaries as a result of oxygen penetration. Intergranular cracking appeared due to the presence of residual stresses and microstructural heterogeneities facilitated the crack branching.

For solar thermal power to be cost effective as a baseload generation paradigm, the cost and efficiency of every subsystem must be improved. To reduce the cost of thermal storage and increase the working temperature of the subsystems for greater efficiency, appropriate storage materials must be identified for study. Unfortunately, there is an enormous search space of salt mixtures that are suitable candidates for thermal storage and must be analyzed for cost effectiveness to determine which candidates are the best. To lessen the experimental burden, this article combines previously validated theories for estimating the properties of salt mixtures with an analytic design optimization method to better estimate the true cost of each candidate. Five hundred and sixty three binary, ternary, and quaternary mixtures are analyzed. Five mixtures are identified as the prime candidates for a cascaded latent heat thermal energy storage system for a supercritical CO2 Brayton cycle generator. This system is cheaper than the Sunshot Initiative cost targets, bringing the solar thermal paradigm into cost competition with fossil fuels.

A comparative, temperature-dependent (80–500 K at 5 K intervals), micro-Raman spectroscopic study of 300 and 50 nm diameter ceramic BaTiO3 nanoparticles was carried out with the purpose of elucidating the nanoparticle size effect on the temperature dependence of the polar and non-polar phonons. A method for calibrating Raman intensities, along with an iterative spectral fitting algorithm, is proposed for concurrent Raman band position and intensity analysis, increasing the analytical abilities of single temperature point Raman spectroscopy. The 300 nm particles exhibit all three phase transitions, whereas the 50 nm particles do not show evidence of these phase transitions in the same temperature range. The Curie temperature appears to be a phonon converging point, irrespective of the phonon symmetry. An attempt was made to qualitatively relate the temperature-dependent Raman spectra to complimentary non-spectroscopic methods, such as heat capacity and X-ray diffraction studies. The study proves that the temperature-dependent behavior of the polar phonon, 265 cm−1, can be utilized as a sensitive phase transition probe. Copyright © 2014 John Wiley & Sons, Ltd.

In an effort to reduce the cost and increase the material compatibility of encapsulated phase change materials (EPCMs) a new encapsulated system has been proposed. In the current study a molten salt eutectic of barium chloride (53% wt.), potassium chloride (28% wt.) and sodium chloride (19% wt.) has been identified as a promising candidate for low cost EPCM storage systems. The latent heat, melting point and thermal stability of the phase change material (PCM) was determined by DSC and was found to be in good agreement with results published in the literature. To cope with the corrosive nature of the PCM, it was decided that a fly-ash based geopolymer met the thermal and economic constraints for encapsulation. The thermal stability of the geopolymer shell was also tested with several formulations proving to form a stable shell for the chosen PCM at 200°C and/or 600°C. Lastly several capsules of the geopolymer shell with a chloride PCM were fabricated using a variety of methods with several samples remaining stable after exposure to 600°C testing. Refereed/Peer-reviewed