I was thinking 60% sodium nitrate and 40% potassium nitrate with a melting temperature nearer to 300 C. Also, as you suggest, it will need concentrated solar.
I think the key question is the maximum temperature of the selective surface in the vacuum tube. Do you know what the over the counter...
SunRain says 300 C. Is this typical? In other words, do solar concentration systems have to have specialized vacuum tubes?
Note: I'm guessing the selective coating is the weak point, but would be interested to know the answer to this too.
As a hobbyist, can any of the current (common) market solar panels withstand/produce in concentrating solar environment - say 30-60X? I'm aware heating issue will arise. I'm just wondering if anything off the shelf could work, and if so, if anyone has a recommendation.