This study experimentally investigated the effect of the working-fluid filling ratio (FR) and the cooling-water flow rate (CWFR) on the top heat loss and the performance of a solar collector equipped with a closed-loop oscillating heat pipe (CLOHP). The CLOHP was composed of a heating section, a cooling section, and an adiabatic section; it had a 0.002-m internal diameter and eight turns. The heating section was attached to a copper plate coated with black chrome, which absorbed energy from a solar simulator that had 12 halogen lamps and was controlled by a voltage regulator. The cooling section was inserted into the collector's cooling box, which was made of a transparent acrylic plate. The FR of the working fluid ranged from 30% to 80% with a 10% interval, and the CWFRs were 0.15 l/min, 0.30 l/min, and 0.45 l/min. The experimental results show that the solar collector equipped with the CLOHP has good performance at working-fluid FRs of 60% and 70% with low flow rates of 0.15 l/min and 0.30 l/min.

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