Off-Grid Clean Cooking via Thermal Energy Storage : Evaluating a Dual-Tank Prototype with Externally Mounted PTC Heaters

Sammanfattning

Access to clean cooking energy remains a global challenge, particularly in rural sub-Saharan Africa where reliance on biomass fuels causes severe health and environmental burdens. While solar energy is a sustainable alternative, its intermittency limits evening utility. To address this, this thesis evaluates the design, numerical modeling, and physical prototyping of a passive, natural circulating Dual Tank Solar Cooking System. The system uses Duratherm 630 thermal oil as a heat storage medium, circulated via buoyancy forces from externally mounted Positive Temperature Coefficient (PTC) heating elements. The primary objective was to validate the system's thermodynamics and evaluate a mechanical sliding valve designed to route oil between a 60-liter storage tank and a direct-cooking bypass loop. COMSOL Multiphysics simulations established theoretical baselines, which were subsequently validated through experimental testing. The prototype successfully achieved passive thermosiphoning, maintained strict thermal stratification, and utilized the PTC elements self-regulating behavior to ensure thermal safety. Crucially, isolating the storage tank via the bypass loop proved highly effective, reducing the time to boil 2 liters of water from 78 to 45 minutes. However, testing exposed two critical hardware bottlenecks. First, the PTC elements external placement created severe conductive thermal resistance through the steel piping, causing premature power throttling and extending cooking times. Second, the sliding valve failed to achieve hydrodynamic isolation, the oil's decreased viscosity at high temperatures, combined with necessary mechanical clearances, resulted in significant leakage into the storage tank. While the fundamental thermodynamic architecture of the Dual Tank System is validated, practical deployment requires hardware alterations. Future iterations must transition to internal immersion heating for maximum thermal efficiency and utilize a displacement float valve to ensure reliable, frictionless flow control.