LAUSR

Abstract The main idea of this investigation is to increase the thermal efficiency of a flat plate closed-loop pulsating heat pipe solar collector by introducing a novel closed-loop pulsating heat pipe (CLPHP) with a dual-diameter structure. The CLPHP with dual-diameter configuration is made of copper with two diameters; larger in the condensation section (inner diameter of 3.5 mm) and smaller in the evaporation and adiabatic sections (inner diameter of 2 mm). The thermal performance of this type of CLPHP was investigated at different filling ratios and inclination angles. Results showed that the thermal performance of the dual-diameter configuration is considerably better than the single-diameter in the vertical and non-vertical orientations. Moreover, in this study, the performance of a flat plate solar collector coupled with the proposed dual-diameter CLPHP (DD-CLPHPSC) is investigated experimentally in outdoor and indoor modes during July and August of 2020, under local weather conditions of Mashhad, Iran. The effects of the filling ratio, inclination angle, and solar intensity were assessed. Under sunny weather conditions, when the direct normal irradiance was 1030 W/m2, the thermal efficiency of the DD-CLPHPSC with a filling ratio of 60% reached 72.4%.