What is Reddit's opinion of Solar Engineering of Thermal Processes?

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I redid your math and got a very similar answer. And then I redid it with my preferred constants, and we're missing something. Because with my preferred constants (0.15 emissivity and 0.9 absorptivity, because the real world is way uglier than spec, and ~30 suns concentration or 10 KW/linear meter), we should be able to heat HTF on the order of 5 deg/second at the top end. And that's a pretty conservative set of numbers.

And at 727 deg. C (1000 K ) we'd only emit 5x more radiation and people would be running molten salt and laughing, and the world would be very different.

EDIT: I pulled out Duffie and Beckman, 3rd ed, and the SEGS systems got up to 390 C, with average 92% reflective mirrors, and a 0.94 absorptivity/0.19 emissivity (at 350 C) selective surface. With a concentration ratio of 80.