Storing Sunlight for the Evening Meal: A Phase Change Material Approach to Round-the-Clock Solar Cooking, with an Experimental Comparison of Erythritol, Wax, and Glucose

Cooking is one of the largest and least discussed energy demands in an Indian household, and roughly a third of household emissions arise from it. Solar cookers remove the fuel and the emissions, but they stop working when the sun goes down, which is precisely when most Indian families cook. This paper investigates whether a phase change material (PCM) can be used as a thermal battery to carry solar heat into the evening, and which material is best suited to the task. Five candidate materials were screened against seven selection criteria, and four thermal retention trials were carried out in which boiling water was held in contact with a candidate medium and its temperature logged over forty minutes. Plain water fell by 47 °C over the trial; Glucose limited the fall to 28 °C, wax to 35 °C, and Erythritol to only 13 °C, retaining 87 °C at the end of the run. Erythritol also carries the highest latent heat of fusion among the candidates (about 340 J/g) and melts at 118 °C, a temperature that sits usefully close to normal cooking temperatures. A cooker built around a copper inner vessel, an erythritol-filled outer jacket, a metal foam matrix, parabolic reflectors, and a nichrome coil driven by a small solar battery is described, with a bill of materials of about ?1,900. The design pays for itself against a conventional LPG set-up within the first year of use and emits nothing in operation. The study concludes that latent heat storage, not better insulation, is the decisive variable in making solar cooking practical and portable after sunset.