title: Experimental Characterization and Numerical Modelling of Solar Cooker Prototypes with Thermal Energy Storage
Creators: name: Aquilanti, Alessia
Publisher: name: UNSPECIFIED
Year: 2023

description: Of all the different renewable energy sources, solar thermal energy, is one of the most promising alternatives to the consumption of highly polluting fossil fuels. Among its applications, solar cooking emerges as one of the most attractive ways to harness this type of energy. In the study presented, different types of solar cookers with and without a storage system were designed and tested: a high-efficiency solar box cooker (SBC), a concentrator cooker (CC), two prototype panel cookers (PSCs), and a medium-efficiency solar box cooker (SBC) equipped with thermal storage based on phase change materials (PCMs). Regarding the latter, an extensive experimental campaign was carried out to characterize the selected PCMs both thermophysically and in terms of their thermal stability. From the analysis, sugar alcohols (SAs) were found to be suitable for use as PCMs in solar furnaces for applications in the low to medium temperature ranges (80-250 °C). The high-efficiency SBC had a concentration ratio of 11.57. It consisted of a cooking chamber, a glazed top cover, and a double row of reflective mirrors. The prototype allowed both azimuthal and zenithal manual alignment. Tests without load were conducted to identify the maximum temperature that the cooker could reach, and load tests were conducted using water and peanut oil in various configurations: with one or two aluminum pots, painted black or unpainted. The cooker was able to bring 1 kg of water to the boiling point in about 11 minutes and get the peanut oil to a temperature of 220 °C in about 41 minutes. The CC, called Heliac, had a concentration ratio of 40.97. It consisted mainly of a wooden lattice structure to which two wooden structures of different sizes were attached: one containing the larger Fresnel lens, and a smaller one containing the reflective surface. Tests with load were carried out using water and silicone oil as test fluids. The cooker was able to make 3 kg of water reach 90 °C in about half an hour and bring 3 kg of silicone oil to 170 °C in less than 1 hour. The two PSCs made are the Newton and the Kimono panel solar cookers, respectively. The former consisted mainly of a prism-shaped glazed cooking chamber and two systems of reflective surfaces, a larger primary one and a smaller secondary one. By changing the inclination of the two reflective surfaces, the device, was able to vary its geometry. No-load tests and tests with load were carried out, using water and glycerin as test fluids. During the experimental campaign, two identical prototypes, one shielded from wind and the other not, were tested to assess how much this environmental parameter affects the final performance. Both devices were able to reach a stagnation temperature of 137 °C. The wind-shielded device was able to bring 2 kg of water to a temperature of 90 °C in about 2 hours and 2 kg of glycerin to a temperature of 110 °C in about 3 hours. These times were slightly longer in the case of the device tested without wind shielding. The Kimono panel solar cooker consisted of acrylic panels connected together and covered with a reflective film. The prototype was tested in parallel with three other panel devices during three different times of the year by conducting no-load and water-load tests. The results showed that the Kimono panel solar cooker is among the best performing prototypes in all the three test periods, with a water heating time to boiling point of 1.74 hours. The medium efficiency SBC had a concentration ratio of 4.08. It was mainly composed of a galvanized steel cooking chamber, a glass cover, and a system of 8 mirrors of two different shapes. Tests without load and tests with load were carried out using water and silicone oil as test fluids. In this case, the contribution to the overall system of the inclusion within the cooking chamber of a storage system based on phase change materials would make was also evaluated. The PCMs selected were erythritol and xylitol. The results showed that the cooling times of 1.5 kg of silicone oil in the chosen temperature range 125-100 °C increased by 350% when tested with the erythritol-based TES compared to when tested alone. In contrast, the cooling time of the same mass of silicone oil in the fluid temperature range 110-80 °C increased by 346% when tested with the xylitol-based TES equipped with a hand stirrer to stimulate nucleation of the material compared with when tested alone. Experimental data from the latter device were used to validate the mathematical model developed specifically to simulate its thermodynamic performance. The model results show a very good fit with reality, managing to simulate the temperature of water and silicone oil in the heating phase of the selected tests with an average deviation from experimental data of 3% and 8%, respectively.