Nanoparticles, including Al2O3 and SiO2, and ultrasound were adopted to improve the solidification properties of water. The effects of nanoparticle concentration, contact angle, and ultrasonic intensity on the supercooling degree of water were investigated, as well as the dispersion stability of nanoparticles in water during solidification. Experimental results show that the supercooling degree of water is reduced under the combined effect of ultrasound and nanoparticles. Consequently, the reduction of supercooling degree increases with the increase of ultrasonic intensity and nanoparticle concentration and decrease of contact angle of nanoparticles. Moreover, the reduction of supercooling degree caused by ultrasound and nanoparticles together do not exceed the sum of the supercooling degree reductions caused by ultrasound and nanoparticles separately; the reduction is even smaller than that caused by ultrasound individually under certain conditions of controlled nanoparticle concentration and contact angle and ultrasonic intensity. The dispersion stability of nanoparticles during solidification can be maintained only when the nanoparticles and ultrasound together show a superior effect on reducing the supercooling degree of water to the single operation of ultrasound. Otherwise, the aggregation of nanoparticles appears in water solidification, which results in failure. The relationships among the meaningful nanoparticle concentration, contact angle, and ultrasonic intensity, at which the requirements of low supercooling and high stability could be satisfied, were obtained. The control mechanisms for these phenomena were analyzed.

In this study, a new type of functional hybrid suspension for solar energy conversion and thermal energy storage was prepared by adding carbon nanotube (CNT) and microencapsulated phase-change material (MEPCM) into deionized water. MEPCM with octadecane as the core material and titania (TiO₂) as the shell material was synthesized by the sol–gel method. The MEPCMs were spherical particles with diameters of 2–4 μm, and the thickness of the shell was about 100 nm. The MEPCM achieved better thermal stability and thermal conductivity than the pure octadecane due to the TiO₂ shell. The melting and solidification latent heats of the MEPCM were about 154.24 and 154.26 J/g, respectively. The encapsulation efficiency of octadecane was calculated to be 65.84%. Most of all, the novel hybrid CNT and MEPCM suspensions exhibited remarkable dispersion stability owing to the stable reticular structure composed of CNT in the suspension. Compared with pure water, the thermal conductivity, specific heat of the MEPCM/CNT suspension improved by 34.48 and 43.57%, respectively and the photo-thermal conversion efficiency reached a high value of 86.0%. This work provided a new type of hybrid functional suspension towards direct absorption solar collector for solar energy conversion and storage.