weather variables I and Ta. The type of process load and system configuration determines

the relative circulation fluid temperature, Ti.

(d) Collector Efficiency Plot. Equation F-1 can be rewritten as a

dimensionless "efficiency" equation by dividing both sides by the product of I and Ac:

Collector Efficiency = FRta - FRUL(Ti - Ta) / I

(eq. F-2)

Note that this efficiency equation is dependent on only one variable that is a combination of

I, Ti, and Ta. This allows it to be graphed in a straightforward manner. Figure F-2 is an

example of a typical collector efficiency plot. Optical losses are shown as a constant

decrease in collector performance, while thermal losses increase as (Ti - Ta)/I increases.

The values of FRta and FRUL can be determined from this type of plot. FRta corresponds to

the intercept value where the collector efficiency curve crosses the vertical graph axis. FRta

is a dimensionless variable with a value between 0 and 1. FRUL is calculated by dividing

FRta by the intercept value on the horizontal axis (it is the negative slope of the plotted

line). FRUL has units of Btu per square foot per hour per degree F.

(e) Performance of Various Collector Types. Figure F-3 shows why

collector efficiency is not always a good indicator of overall collector performance. On

any given day, a solar collector can operate over a wide range of efficiencies as the

solar radiation, ambient temperature, and heat transfer fluid temperature change.

When insolation levels are low early in the day, the efficiency of the collector

approaches zero. As solar radiation levels increase, the collection efficiency increases

until it reaches some maximum level. It will then decrease as the solar insolation and

ambient temperature decrease at the end of the day. Because of the variable position

of the sun, collectors must be oriented so that they are exposed to an acceptable

amount of solar radiation throughout the year. Proper collector orientation and tilt

F-6