Comparative Study of the Performance of Black-Coated and Reflecting-Surface Solar Box Cookers and a Parabolic Dish Solar Cooker

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The thermal performance of a two-compartment solar box cooker and a parabolic dish solar cooker were experimented and compared under same environmental conditions at [name of laboratory/workshop], Bali Town, Taraba State, Nigeria. The identical box compartments (aperture 0.334 m² each) shared one glazing and two booster mirrors; one had black-coated inner surfaces (SC.B) and the other reflecting walls with a black floor (SC.RB). The dish (P.SC) had an aperture of 0.950 m². Stagnation, four-day water-boiling (1 kg of water per pot), heating-and-cooling and single-day rice-cooking tests were carried out. The measured four-day mean maximum water temperatures were 95.1 °C (SC.B), 95.0 °C (SC.RB) and 81.0 °C (P.SC), and the cooling time constants were 182–270 min for the box pots against 50–60 min for the dish pot. Both compartments cooked 1 kg of parboiled rice (maxima 98.3 °C and 97.5 °C), whereas the dish reached only 78.6 °C. The derived first figure of merit was 0.117 (SC.B) and 0.119 (SC.RB) K m² W−1, at the grade A/B boundary. SC.RB heated faster during the first 100 min, but the daily maximum temperatures, useful energies and efficiencies of the compartments did not differ significantly. Referred to the glazed aperture and global irradiance, each compartment was 2.3 times as efficient as the dish (9.7% against 4.2%); allowing for the booster-mirror radiation and for the beam-only acceptance of the dish reduces this ratio to about 1.2. These findings apply only to the prototypes, loads and weather conditions tested.