LIFE CYCLE COST OPTIMIZATION OF A SOLAR COMBISYSTEM IN NEPAL
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Abstract
Nepal has a huge potential for solar energy utilization to meet the growing energy demand. Solar thermal systems have been used in Nepal since the 1960s. In the past two decades, much work was done with a focus on improving design, fabrication, and installation practices of using solar thermal collectors for heating domestic hot water. Little work has been found in literature on the design and analysis of solar combisystems in Nepal. The work presented in this thesis includes the design, modeling, and optimization of a solar combisystem for typical single-family houses in Nepal. The proposed solar combisystem consists of solar thermal collectors, one preheating water tank, one domestic hot water tank with auxiliary electric resistance heating, an electric instantaneous water heater and circulation pumps. TRNSYS software is used to model the solar combisystem. For optimization, life cycle cost is the objective function. The number of hours not satisfying the thermostat set points is treated as the constraint of the optimization problem. Major optimization variables include the thermal collector area, the volumes of both tanks, the flow rate of water circulating between the thermal collectors and the preheating tank, and several building envelope variables such as the wall insulation and roof insulation. The particle swarm optimization and the Hooke-Jeeves algorithms are combined to solve the optimization problem. The solar combisystem is modeled and optimized in two different climatic regions in Nepal: the Terai region and the Hilli region. The optimization for both locations starts from an initial design based on guidelines, conventional construction practices, and engineering judgement. Relative to the initial design, the life cycle cost is reduced by 63% and 72% respectively for the Terai and the Hilli region respectively. The optimization results in a lower collector area and a lower storage tank size for the Terai region but a higher collector area for the Hilli region. One of the important aspects of this research was to include building insulation variables during optimization. The drastic reduction in objective function is also due to the effect of insulation in building envelope which allowed the usage of low value of instantaneous water heater. Lastly this study emphasizes the importance of using optimization tool to achieve the best performance even for the initial design.