Solar Collectors, Non-concentrating

Glossary

Absorptivity or absorptance (α):

The fraction of the incoming radiation which is absorbed at a specified wavelength (absorptivity) or summed over a specified spectrum range (absorptance). Absorptivity and absorptance are dimensionless quantities.

Collector aperture/absorber/gross area:

Net area (of a flat surface) through which the collector receives solar radiation. Any of these areas can be used for referencing the incoming radiation. For nonflat geometries: a projected flat area is used for the aperture and gross areas, but for absorber areas the full curved surface area is taken into account.

Collector mean temperature (T_m):

Defined, in operating terms, as the averaged value of the inlet and outlet temperatures of the collector heat transfer medium (usually that of water, water/glycol, or air):

${T}_m=\frac{T_{\mathrm{outlet}}+{T}_{\mathrm{inlet}}}{2}.$

Emissivity (ε or e):

The ability of a surface to emit energy by radiation relative to that of a “black body.” For a...