Collector efficiency is defined as the fraction of solar energy
incident upon the face of the collector that is removed by the fluid circulating through the collector. Several parameters are defined as follows:
Ti = heat transfer fluid inlet temperature
Ta = ambient air temperature
I = solar irradiance on the collector
Ac = solar collector surface area
FR = collector heat removal factor, a dimensionless parameter describing the ratio
of actual energy gained by the collector to that which would be gained, in
the limit, as the absorber plate temperature approaches the fluid inlet
temperature. This value is similar to a conventional heat exchanger's
effectiveness.
UL = overall heat loss coefficient. This factor describes the cumulative heat
transfer between the collector and the ambient surroundings.
t = transmittance of the glazing.
a = absorption coefficient for the absorber plate. Note that this value varies with
wavelength. A selective surface is one that absorbs short wavelength solar
radiation very well while emitting longer wavelength thermal radiation poorly.
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