Solar Thermal Systems

Solar thermal collector performance

A solar thermal collector converts incident solar radiation into useful heat carried by a working fluid. Its performance is judged by the useful energy gain, the thermal efficiency, and the temperature at which the heat is delivered. The two main types are the flat plate collector, which uses both beam and diffuse radiation and works at moderate temperatures, and the concentrating collector, which uses optics to focus radiation onto a smaller absorber.

For a fixed thermal output, the major advantage of a concentrating collector over a flat plate collector is operation at higher temperature, because focusing the radiation onto a small absorber raises the absorber temperature. Concentrators depend mainly on beam radiation, since diffuse radiation cannot be focused effectively. They also have a higher capital cost per unit aperture area, because of optics, tracking and structure. A concentrator needs a large aperture to intercept the radiation; its benefit is a small absorber area, not a small aperture area.

The useful heat gain of a flat plate collector is given by the Hottel-Whillier-Bliss equation, Q_u = F_R A_c [S - U_L(T_i - T_a)], where F_R is the collector heat removal factor, A_c the absorber area, S the absorbed solar flux, U_L the overall loss coefficient, T_i the fluid inlet temperature and T_a the ambient temperature. The bracketed term is the net absorbed flux after thermal losses, and the loss term grows as the inlet temperature rises above ambient. The efficiency is \eta = Q_u / (A_c G), where G is the incident irradiance.

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