Performance of a solar box cooker with positive temperature coefficient heaters and thermal energy storage

Solar box cookers provide an environmentally friendly cooking option that can reduce greenhouse gas emissions associated with conventional fuels. However, their principal limitations are slow cooking rates and dependence on favourable solar conditions. Even when thermal energy storage (TES) is incorporated, prolonged cloud cover can prevent effective cooking. This study experimentally investigates the thermal performance of a small solar box cooker equipped with six TES containers and two direct-current positive-temperature-coefficient (PTC) heaters connected in parallel. Performance with and without auxiliary heating was evaluated under variable weather conditions. The principal contribution is the integration of PTC-assisted heating with TES to improve cooking capability during periods of low solar irradiance. Six storage media were compared simultaneously. Granite pebbles, silica pebbles, powdered iron filings (Fe), sodium thiosulphate pentahydrate (Na 2 S 2 O 3 ·5H 2 O), and mixtures containing 90 % sodium thiosulphate pentahydrate and 10 % Fe or 80 % sodium thiosulphate pentahydrate and 20 % Fe. Without PTC heating, the measured water-heating thermal efficiency ranged from 11.3 % to 17.3 %, compared with 19.0 % to 28.7 % when auxiliary heating was applied. Silica pebbles generally exhibited the best thermal performance, reaching the highest temperatures in five experimental tests. Iron filings performed best under windy and cloudy conditions with relatively high average solar irradiance. Under very low average solar irradiance (below 200 W/m 2 ), the mixture containing 90 % sodium thiosulphate pentahydrate and 10 % Fe performed slightly better than the other storage media. Without auxiliary PTC heating, the cooker could not bring the water to boiling point, even under clear-sky conditions, because part of the absorbed solar energy was stored in the TES materials. Auxiliary heating enabled multiple cooking sessions during the day, including under cloudy conditions, demonstrating its potential to improve the practical usability of solar box cookers.