Recently, the photovoltaic (PV) power generation system has attracted attention as one of clean energies. Especially, residential roofing PV system connected with power grids has been popularized as a result of increasing concerns over global warming and continuing decline in PV manufacturing costs. The power generated by the PV module increases with irradiance, but it decreases as PV module temperature becomes high. The PV temperature depends on ambient temperature, and becomes more than 60°C in summer. Therefore, the power generated does not necessarily increase even if the irradiance increases in summer. However, if the PV modules were cooled under such a high PV temperature condition, more electrical power would be obtained from PV modules. In this study, a PV power generating system equipped with a cooling device has been developed. The major components of the system are an array of PV modules and cooling panels attached to the backside of the PV modules. The respective PV module is cooled with cooling water flowing through a narrow gap in each cooling panel. Hot water discharged from the cooling panel is delivered to a storage tank and can be reused in anywhere. In order to save energy for introducing cooling water into the panel, a siphonage from an upper level of a building to the ground level is utilized. A siphon tube is connected to a discharge port of the cooling panel, thus the pressure at the discharge port becomes negative. Cooling water enters into the bottom end of the cooling panel at atmospheric pressure and goes up to the top, discharge side. By adopting this cooling water system, we could spread the cooling water evenly over the entire backside of the PV module and thus realized an effective cooling device. In addition, we could make the cooling device light and smaller because no auxiliary pumping system is needed for introducing cooling water. To provide field performance data for the present PV power generation system equipped with the special cooling device mentioned above, long-term monitoring tests in a natural environment were conducted in summer for a test facility constructed at the Yatsushiro National College of Technology (YNCT), Japan. As a result, it was confirmed that the cooling of the PV modules increases the electric power and that the reuse of hot water from the cooling panel contributes very much for saving energy consumed for heating water.

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