Modeling and performance analysis of alternative solar field configurations for parabolic trough collectors.

With reference to a solar field of small-scale Parabolic Trough Collectors (PTC) which must supply heat at given load inlet and outlet temperatures, this work addresses the issue of annual output losses caused by the limitations of the admitted flow within the collectors. These limitations make it not possible to maintain the required load temperatures in case of too low or too high solar irradiation. During low irradiation periods, due to the flow rate limits of collectors, a solar field has to work far from the optimal conditions and this can introduce temperature oscillations in the entire system. These oscillations may affect the performance of a power generation systems or prevent the achievement of specific process requirements. To avoid these drawbacks, I have analyzed configurations and control strategies that can keep the desired temperature unchanged even with large variations of available radiation. In particular, I have explored two solar field configurations designed to solve this issue in order to find the one which maximizes the amount of energy provided at a given set of temperatures. These two configurations are: FLEXY: flexible configurations in which the hydraulic loops can be modified during the day to allow 1 or 2 or 4 collectors per loop at different times of the day depending on the available solar radiation; REC: configurations which include recirculation valves to allow for different flows and temperatures in the solar field and in the served load. These two configurations have been compared with a standard static configuration (STD), in which 1, 2 or 4 collectors are arranged in series in a single hydraulic loop. This comparison has been performed for two different locations, such as Bolzano (Italy) and Seville (Spain) over a broad range of operating temperatures. During this work, a fast and lean lumped-parameter model has been developed and validated versus a more detailed TRANSYS-based one. Validation results of TRNSYS-based and lumped-parameter models were in agreement both on daily and annual basis. As final result, REC configurations have been found to provide the best thermal annual performance in the entire considered temperature range. FLEXY configurations have been found to reduce electrical pumping consumption. Through a comparison on harvested primary energy, REC configurations have been found to provide better performance in a wider temperature range. This thesis work has been completed in collaboration with Italian company Soltigua and has contributed to the project Flexynets (This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 649820)