Model development and performance analysis of the nanofluid-based direct absorption parabolic trough collectors

In this paper, a simple one-dimensional heat transfer model for the nanofluid-based direct absorption parabolic trough collector (NDAPTC) was developed, which was quick to solve, easy to understand, and unnecessary to solve the complicated differential equations. In an actual system, the vacuum condition between the inner tube and glass cover may be changed gradually over time. Therefore, a continuous function for calculating the convective heat transfer in the annulus from near vacuum to atmospheric pressure was established. Combined with it, the derived heat transfer model was suited to evaluating the long-running thermal performance of a NDAPTC quickly. The heat transfer model for a conventional indirect absorption parabolic trough collector (CIAPTC) was also constructed, aiming at comparing the heat transfer processes of this two kinds of collectors. Both the heat transfer models were validated by the test data from the reference. Additionally, the effects of physical parameters, operational and weather conditions on the collector thermal performance were analyzed. It was shown that improvement of the glass transmittance and emittance was crucial to raise the collector efficiency, especially at a higher inlet temperature. The pressure change in the annulus affected the collector thermal performance greatly, thus it tried to avoid the vacuum damage. The collector efficiency was raised with the solar irradiance and ambient temperature increasing and wind velocity reducing, which was more sensible to these weather conditions at a higher inlet temperature and pressure in the annulus. The results in this paper were useful for guiding the collector improvements and engineering designs theoretically, and were beneficial for cleaner production in civil and industrial heat utilization further.