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Significant increases in the heat capacity of heat transfer fluids are needed not only to reduce the costs of liquid heating and cooling processes, but also to bring clean energy producing technologies like concentrating solar power (CSP) to price parity with conventional energy generation. It has been postulated that nanofluids could have higher heat capacities than conventional fluids. In this work, nano- and micron-sized particles were added to five base fluids (poly-α olefin, mineral oil, ethylene glycol, a mixture of water and ethylene glycol, and calcium nitrate tetrahydrate), and the resulting heat capacities were measured and compared with those of the neat base fluids and the weighted average of the heat capacities of the components. The particles used were inert metals and metal oxides that did not undergo any phase transitions over the temperature range studied. In the nanofluids studied here, we found no increase in heat capacity upon the addition of the particles larger than the experimental error.

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This paper presents the thermo-physical properties and stability testing results of six high-temperature phase-change candidate materials for potential use as a cascaded storage system for concentrating solar power applications. This type of storage is a promising technology because it offers a higher utilization of the possible phase change and a more uniform heat-transfer fluid outlet temperature, compared with the single phase-change material (PCM) storage system. The tested materials were inorganic eutectic PCMs with reported phase-change temperatures between 300C and 600C. Four PCMs were made from carbonate salts (Na 2 CO 3 , K 2 CO 3 , and Li 2 CO 3 ) and two from chloride salts (NaCl, MgCl 2 , and KCl). The phase-change temperature, phase-change enthalpy, and specific heat of these PCMs were measured using a differential scanning calorimeter. Large material samples were tested in an oven subject to multiple melt-freeze cycles. The results showed that the carbonate PCMs have a very high degree of sub-cooling in the initial cycles, which decreased in subsequent cycles. The chloride PCMs have a negligible degree of sub-cooling. There is some disagreement between the measured and reported thermo-physical property values of the tested materials, which demonstrates the uncertainty associated with published property values. One carbonate PCM and one chloride PCM were recommended as promising latent heat storage materials.

Molten salts are currently the only thermal energy storage media operating with multiple hours of energy capacity in commercial concentrated solar power (CSP) plants. Thermal energy is stored by sensible heat in the liquid phase. A lower melting point in the range of 60–120 °C and a decomposition temperature above 500 °C are desired because such a fluid would enhance the overall efficiency of the plants by utilizing less energy to keep the salt in the liquid state and by producing superheated steam at higher temperatures in the Rankine cycle. One promising candidate is a multicomponent NaNO3—KNO3—Ca(NO3)2 molten salt. Different compositions have been reported in literature as the best formulation for CSP plants based on melting temperature. In this paper, the National Renewable Energy Laboratory (NREL) presents the handling, preparation, thermal properties, and characterization of different compositions for this ternary nitrate salt, and comparisons are drawn accordingly. This system has a high tendency to form supercooled liquids with high viscosity that undergo glass formation during cooling. When the proportion of Ca(NO3)2 decreases, the formulations become more thermally stable, the viscosity goes down, and the system increases its degree of crystalline solidification. Differential scanning calorimetry (DSC) tests showed the presence of a ternary eutectoid solid–solid invariant reaction at around 100 °C. The eutectic invariant reaction was resolved between 120 and 133 °C as reported in the literature. Based on DSC and viscosity results, the best composition would seem to be 36 wt. % Ca(NO3)2—16 wt. % NaNO3—48 wt. % KNO3, which showed a low solidification point.