Influence of Varying Focal Length on the Performance of a Solar Cooker

Abstract

Access to clean and sustainable cooking energy remains a major challenge in many developing regions, where reliance on biomass fuels contributes to deforestation and adverse health effects. Parabolic solar cookers offer a promising solution due to their high-temperature potential; however, their performance is often constrained by strong thermal non-uniformity resulting from excessive energy concentration at the geometric focus. This study investigates the influence of focal length variation on the thermal performance of a parabolic solar cooker under moderate solar irradiance conditions. A steady-state numerical analysis was carried out using a coupled Tonatiuh–ANSYS Fluent approach, based on the climatic conditions of Ngaoundéré (DNI = 450 W·m⁻², ambient temperature = 298.15 K, wind speed = 1.5 m·s⁻¹). Three focal configurations, including the geometric focus and two defocused positions, were examined. The results indicate that a slight downward displacement of the absorber improves both thermal performance and temperature uniformity. An optimal focal length of 0.70 m increases the maximum absorber and water temperatures by approximately 2.2 K and reduces the temperature non-uniformity factor by 23%. Controlled defocusing thus emerges as an effective strategy for enhancing the efficiency and practical reliability of parabolic solar cookers.