Heat Storage for oil based solar concentrators
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As the world’s energy demand increases, more and more focus is directed towards different solar energy solutions. In many African countries, a great share of the population cook their food with firewood as the energy source. Since the use of firewood leads to deforestation and bad indoor climate, it is of interest to implement solar cookers in countries where the sun radiation is sufficiently strong. Most commercialized solar cookers are direct systems, meaning the cooking has to take place when the sun is present. To make the cooking more flexible in relation to time, solar cookers connected to a heat storage is of interest to develop. During this work a high-temperature thermal heat storage has been designed, constructed and tested. The heat storage is a part of an oil circulation system where the oil transports heat from an absorber connected to a solar capturing reflective trough. Eight aluminum cylinders, which are immersed in the circulating oil, are connected to the aluminum top plate of the heat storage. Each cylinder contains a solar salt phase change material. The solar salt is the material that stores most of the heat which can be used for cooking purposes. Since the cylinders are connected to the top plate, cooking pots placed on top of the heat storage will be heated by conduction from the solar salt. After the dimensioning of the heat storage was done, a computer-aided design (CAD) program called Autodesk Inventor was used to design drawings which the workshop would use during the construction of the heat storage. The heat storage has been analyzed through simulations done in the finite element method program COMSOL Multiphysics and through experiments in the laboratory. In addition to cooking tests, these analyses have had the main goal of examining the charging and discharging time of the heat storage. The results showed that the charging time decreases if the heat storage is being used on a daily basis. That is because the phase change material does not reach the temperature of the surroundings overnight, causing the next charging to start at a higher temperature than the first. The aim of the cooking tests was to boil one liter of water, which turned out to take at least 40 minutes. If modifications are to be done on the heat storage, one should have the charging time in mind. As the sun shines during a limited time period each day, the charging time, which was found to be between six and nine hours, cannot be allowed to become longer than these experiments indicated.