Oil Based Solar Concentrator with Heat Storage
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A test system for storing solar thermal energy in the form of hot oil is constructed and tested. The motivation for the work is the need for a thermal energy storage which provides heat, from solar concentrators, for food preparation after sunset. The work is part of a collaboration project with a group of universities in Ethiopia, Uganda, Tanzania and Mozambique, focusing on renewable energy systems for off-grid communities. Latent heat storage methods can be attractive for cooking purposes, as heat is available at a constant temperature (the phase change temperature). A similar benefit can also be obtained with a sensible heat storage, provided thermal stratification can be established. Natural stratification can be possible in vertical containers, where hot and cold volumes can be naturally separated if the density gradient is stable. However, as natural stratification can easily be disrupted due to the internal flow conditions, the objective of this work was to investigate a forced stratification method. This was done by separating the cold and the hot oil with a piston in a horizontal pipe. The piston moves as hot oil enters the storage inlet, and will be available for usage by exploiting manual valves. To control the temperature from the concentrator into the storage a thermostat is needed. At a given temperature, the thermostat will divert the flow from a circulation loop, and direct it through the concentrator to the storage. Three principles were considered for the thermostat. The first was based on an edge welded bellows containing an evaporating liquid which expands and opens a valve. This showed acceptable opening times but too slow closing times, non-equilibrium effects were therefore prominent. The second was based on expansion of a gas volume connected to a small diameter bellows. This was unsuccessful due to the mechanical resistance of the bellows, in addition to the compressibility of the gas. In the third case, a latching solenoid was used to operate the valve, triggered by a thermostat switch. This solution worked as expected, but is more complex and does require power. A test system with electrical heating of the absorber pipe, instead of solar heating, was constructed and the concept was successfully demonstrated with the solenoid. However, due to the material of the piston used, made of thermoplastic, the temperature in the system had to be adjusted based on its physical properties. Hence, high oil temperatures could not be tested.%Carl: Foreslår ..., which was accomplished/achieved.Nevertheless, focus was on demonstrating the principles of the system setups, in which was obtained. The test system shows the feasibility of a solar heat collection system which can charge and discharge a thermal oil storage unit at a constant temperature. A 1D dynamic network model was programmed. The thermostat was implemented in the network as a switch between the flow rate in the two connected outlet pipes, and the storage was implemented with a dynamic separating section representing the position of the moving piston. The loss coefficients for the different parts of the system were tuned, and acceptable comparisons with experimental measurements were obtained. A test setup with solar heating was tested at Mekelle University in Ethiopia. However, due to the duration of the study, and difficulties with the manufacturing of required components, there were issues with demonstrating the principles of the system.