In the central receiver concept of solar energy collection, a large number of heliostats (reflectors) provide a concentrated solar flux of q''S = 80,000 W/m2 to the receiver, which is positioned at the top of a tower. The receiver wall is exposed to the solar flux at its outer surface and to atmospheric air for which T?,o = 300 K and ho = 25 W/m2 ? K. The outer surface is opaque and diffuse, with a spectral absorptivity of ?? = 0.9 for ? 3?m. The inner surface is exposed to a working fluid (a pressurized liquid) for which T?,i = 700 K and hi = 1000 W /m2 ? K. The outer surface is also exposed to surroundings for which Tsur = 300 K. If the wall is fabricated from a high-temperature material for which k = 15 W/m' K, what is the minimum thickness L needed to ensure that the outer surface temperature does not exceed Ts,o = 1000 K? What is the collection efficiency associated with this thickness?

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Receiver wall Tower Central receiver T h. Working fluid Atmospheric air I Heliostats