An experimental study in full spectra of solar-driven magnesium nitrate hexahydrate/graphene composite phase change materials for solar thermal storage applications

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Magnesium nitrate hexahydrate is a preferred material in building heat applications with a moderate temperature (89 °C), suitable latent heat (130~150 kJ·kg−1), and low cost (1000~2000 CNY/Ton). However, as the low heat source temperature in the engineering application (usually less than 150 °C), the temperature difference driving the phase transition of the phase change materials is small, resulting in the slower dynamic process of heat storage. Aiming at this problem, this paper proposes a new method to prepare solar-driven composite phase change materials by combining photo-thermal materials with salt hydrate phase change materials. Solar-driven composite phase change materials can absorb photons and convert them into heat, most of which is stored in the phase change materials as the form of latent heat. The energy driving phase transition of phase change materials came from the solar and the other heat source simultaneously. Herein, a novel solar-driven composite phase change material containing magnesium nitrate hexahydrate, carboxymethyl cellulose, and graphene is prepared successfully and its absorption capacity and photo-thermal storage performance are measured systematically in the full spectrum. Compared with pure magnesium nitrate hexahydrate, the absorption capacity of the composites with graphene can be increased to 79.12% from 46.54%. The photo-thermal conversion and storage efficiency of composite phase change materials as high as 69.73%, and for the pure magnesium nitrate hexahydrate, it is not enough to absorb much heat to make a phase transition due to low absorption capacity. Also, the thermal conductivity of composite phase change materials with 5 wt% of graphene was significantly enhanced by 191.18% (0.99 W∙m−1∙K−1), compared with 0.34 W∙m−1∙K−1 of one for composites without graphene. Although with graphene mass fractions increases, the latent heat slightly decreases, an acceptable latent heat value (122.8 kJ∙kg−1) is obtained. The prepared sample has excellent cycling performance, which is important in applications. Therefore, this novel solar-driven composite phase change material could be potential as a solar heat storage material in engineering applications due to its good photo-thermal conversion and storage performance.