Experimental study of phase change material performance as thermal insulation of a double-walled spherical tank under partial vacuum for storing solar energy

In solar systems, conservation of stored energy is even more important than energy storage. In this study, as an innovative idea, the thermal behavior of a phase change material (PCM) is investigated as thermal insulation of an energy storage tank based on the unique feature of phase change at a constant temperature. Condition for creating an ideal insulation is the existence of a heat source at the border. The PCM surrounding the tank stores the solar energy latently in the process of thermal charging, and as a heat source in the process of thermal discharge. By releasing energy at a constant temperature through the PCM, the heat loss factor that is the temperature difference between the hot water and the surface of the tank is controlled. The PCM shows three different behaviors as the thermal insulation. One, as long as the energy stored in the PCM is sensibly released, the heat loss coefficient is almost constant, in which the PCM acts as a super insulator. Two, by releasing the latent energy, the heat loss coefficient tends to zero, and the PCM acts as an ideal insulator. Three, by releasing all the stored energy, the PCM acts as a normal insulator. Creating the relative vacuum between the tank and the glass cover improves the thermal performance, thereby the hot water temperature decreases by 7.4 °C in the best case, and the thermal efficiency reaches 85%.