>For operating conditions of the reduction at 1500 °C Unfortunately, this figure is fatal to any solar-thermal project. While the color temperature (hence theo

>For operating conditions of the reduction at 1500 °C Unfortunately, this figure is fatal to any solar-thermal project. While the color temperature (hence theoretical limit) of sunlight is a blistering 5000 K, real concentrators achieve much less. For practical purposes they are divided into two categories: - single-stage concentrators, which employ a single heliostat (array of mirrors) or parabolic trough to focus sunlight onto a collector; - double-stage concentrators, which use a second mirror to achieve very high temperatures. The largest double-stage concentrator ever built is the Odellio solar furnace operating at just one megawatt: https://en.wikipedia.org/wiki/Odeillo_solar_furnace https://en.wikipedia.org/wiki/Odeillo_solar_furnace For practical purposes in the foreseeable future, single-stage concentrators are the technology of interest, currently representing around 7 GWp of installed capacity around the world and growing rapidly. These reach a maximum temperature of about 550 C, which is just barely enough to run the copper-chlorine cycle, but not many of the more fanciful solar fuel cycles. Some single-stage "dish" systems reach 750 C at much smaller sizes (the startup that was selling this pivoted to batteries [1]). Some research has proposed ways to boost power tower temperatures to as high as 800 C [2]. But 1500 C? In a single-stage system? It would require a level of accuracy that just isn't available. Such a system requires a solar concentration ratio of around 2500 suns [3]. This is extraordinarily difficult, because the sun subtends a finite angle for an Earth observer and hence reflects off a mirror with a significant divergence; furthermore an ideal reflector is not planar but slightly curved, and every mirror on a flat field would be curved slightly differently, creating impossibly difficult manufacturing issues. Real-world systems hover around 500 suns, with 1000 suns being a commonly stated practical limit [4]. 1: https://en.wikipedia.org/wiki/TEXEL https://en.wikipedia.org/wiki/TEXEL 2: https://asmedigitalcollection.asme.org/solarenergyengineering/article-abstract/129/2/141/459741/High-Temperature-Liquid-Fluoride-Salt-Closed https://asmedigitalcollection.asme.org/solarenergyengineerin... 3: https://opg.optica.org/oe/fulltext.cfm?uri=oe-24-14-A985&id=340823 https://opg.optica.org/oe/fulltext.cfm?uri=oe-24-14-A985&id=... (See Figure 4) 4: https://www.sciencedirect.com/science/article/pii/S0038092X20305867#b0145 https://www.sciencedirect.com/science/article/pii/S0038092X2... (Some thermal efficiencies in Figure 13; cost figures are, in my view, highly optimistic)