The instantaneous efficiency and the incidence angle modifier can represent the thermal performance of the collector.

4.1 INSTANTANEOUS EFFICIENCY

The instantaneous collector efficiency, q,, is a measure of collector performance that is defined as the ratio of the useful gain over some specified time period to the incident solar energy over the same time period [1]:

By introducing the Equation 3.2.5, the instantaneous efficiency becomes [1]

П; = Qu/AcGT = [Fr (та ) — FrUl (T; — T) Gt] (4.1.2)

Where the absorbed energy Sc based on the gross collector area has been replaced by

Sc = Gt (та) (4.1.3)

(та) is the effective transmittance-absorptance product based on the collector gross area. It is defined as:

(та) = SAp / GtAc = (та )avg Ap/Ac (4.1.4)

In Equation 4.1.2, two important parameters, Fr (та) and FrUl, describe how the collector works. Fr (та) indicates how the collector absorbs energy while FrUl is an indication of how energy is lost from the collector.

4.2 INCIDENCE ANGLE MODIFIER

To express the effects of the angle of incidence of the radiation on thermal performance of the flat-plate solar collector, an incidence angle modifier KTa is employed. This describes the dependence of (та ) on the angle of incidence of radiation on the collector. It is defined as [1]:

KTa = (та) /(та)п (4.2.1)

Where subscript n indicates that the transmittance-absorptance product is for the normal incidence of solar radiation.