The way you put it its a simple energy balance. Troughs have an efficiency of about 70%, the maximum irradiation you can expect is 940 W/m2, but use maybe 700 W/m2 as a design point.
The problem is however much more complex. The reflected light will illuminate the receiver tube unevenly. The energy distribution on the tube depends on the troughs rim angle and the precision (shape, tracking, torsion). This will cause the receiver tube to bend and move out of focus or worse brake the glass tube. The flow of thermal fluid through the receiver must be high enough as to cool adequate the walls and prevent bowing.
In 2014 at the Solarpaces conference in Beijing Prof. M.H.Abedine-Sanigi from the Shiraz University in Iran presented a paper: "Analysis of Thermal Stress of Absorber Tube of a Parabolic Collector ....."
You can contact me at : juhaven@huiyin-group.com My company produces receivers for parabolic troughs
I wish you good luck
