Thermo-physical Characterization and Performance Evaluation of Castor Oil as a Sensible Heat Storage Medium in a Box-Type Solar Cooker

Abstract

Box-type solar cookers (BSCs) provide a clean alternative to conventional biomass fuels, but their operation is limited by intermittent solar radiation. This study investigated castor oil as a sensible heat storage (SHS) medium for improving thermal performance and post-solar heat retention. The castor oil had a density of 956 kg/m³, specific heat capacity of 1900 J/kg K, thermal conductivity of 0.168–0.175 W/m K, and kinematic viscosities of 298.94 and 22.42 mm²/s at 40 and 100°C, respectively. Thermo-gravimetric analysis indicated thermal stability up to approximately 250–280°C, followed by major decomposition between 280 and 350°C. The cooker was fabricated and tested at the Federal University of Technology Minna, Nigeria, during February–March 2026 under outdoor solar irradiance of 615–790 W/m². Experiments were conducted in triplicate. During stagnation testing, the absorber plate reached a maximum temperature of 101.6°C, while the castor oil attained 74.8°C. The calculated thermal efficiency was 50.7%, based on 115.98 kJ of useful energy output from 228.76 kJ of solar energy input. During post-solar testing, 1 kg of water was heated from 30.0°C to 75.6°C within 60 minutes without solar input, demonstrating the heat-retention capability of the castor oil. However, the study did not compare the cooker with an equivalent system without castor oil. Therefore, claims of improved performance and extended cooking capability should be interpreted cautiously. Overall, the results demonstrate the potential of castor oil as a locally accessible and biodegradable SHS medium for extending solar cooking beyond sunshine hours, while further comparative and field-scale studies are recommended.