Passive temperature regulation in a heat storage for cooking with internal natural oil circulation

Ole Nydal^1*Gunn Helen Nylund¹Andreas Bjørshol¹Peter Mashingo²Alexander Peter Olsen¹Jimmy Chaciga³

• ¹Norwegian University of Science and Technology, Trondheim, Norway
• ²Arusha Technical College, Arusha, Tanzania
• ³Makerere University, Kampala, Uganda

Energy storage solutions are required for renewable energy systems in order to provide energy on demand independently of the time fluctuations of the sources. Electrical batteries provide electrical power, and thermal batteries can likewise provide heating and cooling power directly. In a thermal energy storage, the temperature is the measure of the quality of the energy, and the energy should preferably be available at high temperatures for both partially and fully charged storages. This feature can be achieved in liquid-based sensible heat storage systems where the heat transfer is done with the flow of a heat transfer fluid in and out of the storage. If the storage is such that thermal mixing is avoided, either by physical separation of hot and cold fluid in separate containers or by natural thermal stratification in a single container with hot fluid on top of cold fluid, the thermal energy can be extracted at near the same temperature as the charging temperature. Then, a temperature control method is needed to feed the storage with a high temperature stream, independent of the charging power. This study examines a passive thermal regulation approach in a heat storage system designed for cooking applications, utilizing internal natural oil circulation to manage heat distribution and maintain consistent operating temperatures. An expansion-based temperature control methods is presented, where the expansion of the heat transfer fluid itself is used as basis for controlling the charging temperature of an oil-based heat storage unit. A heating element provides the charging power. The oil is heated in a heating chamber that includes a barrier for the circulation of the oil with the storage unit. At a given temperature, the oil expands beyond the barrier and circulation with the storage unit is initiated.The performance of the method is demonstrated by a set of dedicated experiments and the method is furthermore applied to heat storage systems for cooking. A strong motivation for the work is to find robust and simple solutions for clean cooking technology in the African environment.