Comparative analysis of the thermal performances of two parabolic trough solar air collectors used for greenhouse heating: An experimental study

In order to improve the solar energy utilization rate and address the issues of high outlet temperature and large temperature difference between the inlet and outlet of a parabolic trough solar air collector with double-receiver tubes (PTSAC-DRTs) used for solar greenhouse heating, a novel parabolic trough solar air collector with triple-receiver tubes (PTSAC-TRTs) was designed in this study. The design was based on increasing the air mass flow rate in the collector to prevent excessively high temperatures at the collector outlet. Experiments were designed and carried out to investigate the influence of four parameters, including the air velocity, solar irradiance, air temperature at the collector inlet, and presence of an external insulation board, on the heat collection performance of the PTSAC-TRTs. The performances of the PTSAC-DRTs and PTSAC-TRTs were compared in terms of the air temperature at the collector outlet, collector efficiency, and normalized temperature difference–efficiency curve. The optimum air velocity was 5.4 m/s for the PTSAC-TRTs, which had an instantaneous collection efficiency (ICE) of up to 76.0 %. The collector did not require any additional insulation for practical application and is thus both efficient and practical. Using the optimum air velocity, the air temperature at the PTSAC-TRTs outlet was consistently 1–4 °C lower than that at the PTSAC-DRTs outlet. Compared with the PTSAC-DRTs, the average heat collection per unit area and average ICE of the PTSAC-TRTs were 10.7 % and 10.4 % higher, respectively. The PTSAC-TRTs was used as a thermal collection device to efficiently supply energy to a solar greenhouse, ensuring a higher indoor temperature (3.6 °C higher on average) than that of a reference greenhouse.