Experimental Investigation of a Concentrating Solar Fryer with Heat Storage

Abstract

Today many of the solar cookers available in the market are direct cookers, without storage, and they are used for low to medium temperature cooking purposes. In this dissertation, experiments of heat collection, transportation and storage have been carried out using parabolic dish concentrators, steam as heat carrier and phase change material (PCM) as heat storage respectively. The design of the system has been focused to meet the demand for high temperature heat storage, in an economical, safe, robust and simplified way. The stored heat has mainly been tested for Injera baking purpose, the national food of Ethiopia, which requires intensive energy. Most households eat Injera three to four times per day. Injera needs a heat supply in the range of 180-220°C and more than 85% of Ethiopians use biomass fuel to bake this food. A nitrate salt mixture (solar salt) that has a melting point in this range of temperature was therefore selected as PCM media in this research. The research starts by developing two polar mounted parabolic dish concentrators that are suitable to closed loop self-circulation heat transportation. The first system was placed at NTNU and was coupled to an aluminum block heat storage that has PCM cavities and steam channels. This system was tested for natural and artificial heat source charging. The stored heat was tested for egg frying and water boiling. The second system, at Mekelle University, was coupled to Injera baking clay plate, which has an Imbedded coiled stainless steel steam pipe as a heating element. This system demonstrated an indirect solar Injera baking at about 160°C. However, the heating up time and the baking time interval were very long 3 hours and about 15 minutes respectively. The steam based solar Injera baking result has led to a new research line on Injera baking process and a review of its actual baking temperature. Therefore, Injera baking was tested on three different stove materials regarding its baking time, temperature and Injera quality on different baking surface temperatures. These experiments have identified the possibility of Injera baking as low as 120°C surface temperatures and the ordinary stove design can then be modified to save about 50% of its energy consumption. Another system was tested for alternative way of using solar energy indirectly. In this system, the high intensity solar radiation from the receiver’s of a double reflector parabolic dish concentrator was transported onto an absorber using a light guide. The system was designed for short distance radiation transportation and water was boiled in an experimental case. A third version of a heat storage was designed with conducting fines coupling a coiled top plate with a solar salt bed in a container below. Two units were made and tested at NTNU and Mekelle University. Injera baking tests were carried out on the top plate of the heat storage. Injera baking on a fully charged storage shows shorter baking times compared to conventional electric stoves. The system was demonstrated to the public and the Injeras baked on it and a solar cooked Ethiopian stews were served as a free lunch to the participants at Mekelle university. This was the first complete solar prepared Ethiopian food in the history of solar research in Ethiopia.