Performance of double pass unglazed transpired collectors with energy storage in phase change materials during days with different radiative regimes

The effective operation time of unglazed transpired collectors (UTCs) may be extended by double pass (descending and ascending) air flow and by energy storage in phase change materials (PCMs). The transient operation of a hybrid UTC is described by using an original 3D model. The model has been verified and calibrated against measurements obtained in a temperate-continental region (South-Eastern Europe). The performance of the hybrid UTC does not depend significantly on the diameter of the orifices on the perforated plate and on the vertical distance between orifices. The heat released by the collector increases by increasing the width of the ascending air duct. The transient collector performance is analyzed in days with different radiative regimes. The best performance occurs in days without clouds, as expected. In Summer and Autumn, a large amount of thermal energy is stored in the PCM and it is released after the sunset. The amount of heat released after the sunset increases by increasing the mass of PCM and depends on the PCM type. During Winter, most of the accumulated energy is released before the sunset. Generally, the thermal performance of the UTC depends on the radiative regime in a slightly different way for the two PCM types considered in this paper. The best thermal performance of the UTC is ensured by using the RT27 PCM. This applies for the nearly stable radiative regime, for cloudy sky days, and for the unstable radiative regime. Usage of the RT35 PCM provides a good thermal performance during clear sky days and stable radiative regime. The steady-state approximation is more accurate in days with clear sky and in days with overcast sky than in days with skies partially covered by clouds.