Controller Evaluation for Solar-Latent Thermal Energy Applications
Authors:   National and Kapodistrian University of Athens, Department of Agricultural Development, Agrofood and Management of Natural Resources, Energy and Environmental Research Laboratory, Psachna Campus, 34400 Evia, Greece, Konstantaras John , Pagkalos Christos ,   National and Kapodistrian University of Athens, Department of Agricultural Development, Agrofood and Management of Natural Resources, Energy and Environmental Research Laboratory, Psachna Campus, 34400 Evia, Greece, Koukou Maria K. ,   National and Kapodistrian University of Athens, Department of Agricultural Development, Agrofood and Management of Natural Resources, Energy and Environmental Research Laboratory, Psachna Campus, 34400 Evia, Greece, Lymperis Kostas ,   National and Kapodistrian University of Athens, Department of Agricultural Development, Agrofood and Management of Natural Resources, Energy and Environmental Research Laboratory, Psachna Campus, 34400 Evia, Greece, Caouris Yannis ,   University of Patras, Department of Mechanical Engineering & Aeronautics, 26504 Rio Achaia, Greece, Vrachopoulos Michail Gr. ,   National and Kapodistrian University of Athens, Department of Agricultural Development, Agrofood and Management of Natural Resources, Energy and Environmental Research Laboratory, Psachna Campus, 34400 Evia, Greece
Published: June 2022

In this study, the performance of a self-sufficient controller used for a solar-latent heat domestic hot water (DHW) production unit under real-world operating conditions was analyzed. The unit consists of a flat-plate solar collector and a latent heat storage tank. The controller is powered by a small solar panel and governs the charging and discharging of the system, ensuring maximum solar energy absorption, desired hot water temperature, and constant monitoring of the heat-storage tank’s capacity. The system is compact and can be installed on flat and curved roofs as a direct replacement of conventional solar collectors with heat-energy storage tanks. During testing, all internal and external parameters were monitored using a monitoring system that was also used for emulating user profiles. The controller uses self-learning techniques to adjust its parameters and improves its performance by fine-tuning the control equations to the peculiarities of the specific system and installation location. The system was installed and operated for an extended period to allow for the learning equations to train the system. The results for the first, fifth, and twentieth days of operation are presented in this paper. On the 20th operating day, the controller effectively regulated the heat transfer fluid temperature difference in the charging circuit within the efficient band of 2°C–5°C following the irradiance conditions at the testing area. During discharge, the DHW temperature was regulated between 37°C–40°C, with the user’s set temperature as 38°C. The regulation hysteresis time for the DHW temperature regulation was approximately 5 min. The tests were conducted under real-world operating conditions for the charging of the system, while for the discharging, the user profile was emulated using a test rig.