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• Experimental investigation of high temperature thermal storage concrete materials. • Phase change materials (solar salt) added to concrete to increase energy density. • Thermal and mechanical characterization of concrete and 2 wt% phase change material. • Thermal and mechanical properties increase with no phase change materials leakage. • Better behaviour of phase change material/diatomite even after 250 °C. Thermal energy storage is one of the most appropriate technologies to correct the gap between the energy generation and supply and to address energy challenges. Concrete is generally the preferred "solid" heat storage material because its low cost and good thermal conductivity. The major disadvantage is its low heat stored density involving the use of large amounts of concrete. Latent heat storage materials (or phase change materials), have received more attention due to much higher heat storage density and extremely smaller temperature variation during the thermal energy charge/discharge process. These materials can be incorporated in the concrete by using different methods. When the phase change material is encapsulated or added in a shape-stabilized new material, as diatomite, the phase change material leakage is avoided. Thermal and mechanical characteristics of a new heat storage material, composed by concrete with the addition of 2% Solar Salts by weight (as phase change material) in powder or capsules form, were analyzed. The results showed an increase of the main thermal (volumetric heat capacity and conductivity) and mechanical properties while phase change material leakage was not observed. In particular, the phase change material/diatomite capsules provide a better behavior even after 250 °C. [ABSTRACT FROM AUTHOR]

Molten salts as phase change materials (PCMs) can be used as thermal storage media in concentrated solar power (CSP) plants. The addition of nanoparticles into a base fluid (producing the so called nanofluid) can enhance its thermal properties. The most common technique involves the use of water. We present a new procedure based on high temperature mixing. In particular, different nanofluids were developed by mixing NaNO 3 -KNO 3 (60–40 wt%) solar salt with 1.0 wt% of SiO 2 , Al 2 O 3 and a mix of SiO 2 /Al 2 O 3 nanoparticles at 300 °C using a twin screw micro-compounder. The effect of different screw speeds (100 and 200 rpm) and mixing times (15 and 30 min) were studied. The results showed that the nanoparticles induce an increase of the heat of fusion of 1.5–7.4% while the onset temperatures decrease for all the nanofluids independently from the processing conditions (up to 9.7 °C). Moreover, an increase in the specific heat ( Cp ) is recorded mainly for the nanofluid with SiO 2 /Al 2 O 3 with a maximum of 52.1% in solid phase and 18.6% in liquid phase after 30 min of mixing at 200 rpm. The same nanofluid showed the highest stored heat. Particle aggregation into clusters in solid state was detected by scanning electron microscopy (SEM) but smaller aggregates resulted for higher mixing times and screw speed related to the highest Cp . Moreover, smaller grains in the nanofluids were detected with respect to the base salt morphology. Thus, the nanofluid produced with SiO 2 /Al 2 O 3 nanoparticles at 200 rpm for 30 min gives the best overall performances. This work showed that nanofluids with enhanced thermal properties can be obtained with an innovative mixing process directly at high temperature, eliminating the water evaporation step. [ABSTRACT FROM AUTHOR]