# Hvga a bu x WmK

By (3.6), (3.9) and (3.49), and, since eg = eair = 1.0, giving F[2 = 1.0 in (3.44), the radiative loss coefficient for the outer surface is hr,ga = 4a[(Tg + Ta)/2]3 = 6.2Wm-2 K-1

The losses by convection and radiation from the external glass to the environment are in parallel, and since by (3.9) h = 1/r, the combined thermal resistance is

Note how the radiation resistance Rpg dominates, since there is no convection to 'short circuit' it. It does not matter that the mixed convection formula (C.15) applies to a flat surface, since it will underestimate the resistance from a curved surface.

Since each 1 m of tube occupies the same collector area as a flat plate of area 0.05 m2, we could say that the equivalent resistance of unit area of this collector is rpa = 0.40m2 KW-1, although this figure does not have the same significance as for true flat plates.

To calculate the heat balance on a single tube, we note that the heat input is to the projected area of the inner tube, whereas the losses are from the entire outside of the larger outer tube. With no heat removed by a stagnant fluid, input solar energy equals output from losses, so 