Effect of vacuum degradation on heat losses in parabolic trough collector receivers

This study investigates the thermal performance of linear evacuated receivers designed for parabolic trough collector (PTC) systems under varying operating conditions. A parametric thermal analysis was performed using ANSYS Fluent for a stainless steel receiver tube with Therminol® VP-1 as the heat transfer fluid. This fluid can produce low-boiling-point compounds, including hydrogen, which permeate through the absorber wall into the evacuated annulus between the absorber and the glass envelope of the receivers. Two annulus conditions were evaluated: a fully evacuated state (10−2 mbar) and hydrogen-filled environment (1 mbar). Additionally, the impact of wind speeds (5 and 10 m/s) on key performance parameters was assessed under two extreme operating conditions. Key metrics included temperature and velocity profiles, circumferential temperature gradients, convective heat transfer coefficients, and heat losses. Simulations covered mass flow rates of 3–6 kg/s (in 1 kg/s increments), inlet temperatures from 290 ◦C to 390 ◦C (in 20 ◦C steps), and direct normal irradiance levels of 800 and 1000 W∕m2. Results revealed how these annulus conditions and wind effects influence thermal performance. Hydrogen presence increased heat losses by up to 3 times compared to the evacuated case, with glass temperatures peaking at 142.5 ◦C. Increasing the mass flow rate from 3 to 6 kg/s reduced the absorber tube’s maximum circumferential temperature difference from 39.4 ◦C to 23.5 ◦C, with different annulus conditions showing diminished impact on this parameter at higher flow rates. Heat losses increased under windy conditions, whereas the glass temperature decreased significantly, especially for the hydrogen-filled annulus.