Hourly Utilizability

Utilizability can also be defined as the fraction of incident solar radiation that can be converted into useful heat. It is the fraction utilized by a collector having no optical losses and a heat removal factor of unity i.e., Fr(to) = 1, operating at a fixed inlet to ambient temperature difference. It should be noted that the utilizability of this collector is always less than 1, since thermal losses exist in the collector.

The Hottel-Whillier equation (Hottel and Whillier, 1955) relates the rate of useful energy collection by a flat-plate solar collector, Qu, to the design parameters of the collector and meteorological conditions. This is given by Eq. (3.60) in Chapter 3, Section 3.3.4. This equation can be expressed in terms of the hourly radiation incident on the collector plane, It, as

where

Fr =

collector heat removal efficiency factor.

ac =

collector area (m2).

(Ta) =

effective transmittance-absorptance product.

it =

total radiation incident on the collector surface per unit area (kJ/m2).

Ul =

energy loss coefficient (W/m2-K).

t =

inlet collector fluid temperature (°C).

ta =

ambient temperature (°C).

The radiation level must exceed a critical value before useful output is produced. This critical level is found by setting Qu in Eq. (11.35) equal to 0. This is given in Eq. (3.61), but in terms of the hourly radiation incident on the collector plane, it is given by

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