Given the basic component summation equation for solar radiation (Section 1.4), it is possible to measure all three components with a single radiometer or combination of any two radiometers. Thus, a pyrheliometer and pyranometer measuring global hemispherical or diffuse sky irradiance can be used to compute diffuse or global hemispherical radiation, respectively. Without a pyrheliometer, global and diffuse measurements can be used to compute the direct beam irradiance (Figure 2.4).
Given a single pyranometer, by alternately making measurements in a shaded and an unshaded condition, global and diffuse radiations are measured alternately, and the direct beam can be computed from these measurements . These approaches require computation of the solar geometry. Since the geometry is constantly changing and the radiometers have finite response times, careful consideration to the timing of the measurements has to be considered.
Since photoelectric detectors have extremely fast time responses, several commercial varieties of rotating shadowband radiometers (RSRs) have been designed to measure all three solar components with one system (Figure 2.5). Extensive research has been conducted to develop corrections for spectral (especially for the shaded
FIGURE 2.4 (See color insert.) Left to right: Shaded, ventilated thermal pyranometer for diffuse sky irradiance; unshaded thermal pyranometer for total hemispherical irradiance; same model shaded thermal pyranometer and thermal pyrheliometer (below pyranometers) mounted on a solar tracker.
FIGURE 2.5 (See color insert.) Rotating shadow-band radiometer. Rotating band is below silicon photodiode photoelectric pyranometer (just above horizon line).
diffuse measurements), geometrical, and temperature response of the pyranometers. However, no amount of correction schemes can reduce the basic uncertainty in the pyranometer device for routine monitoring applications under all conditions [23,24]. Only if very detailed and expensive spectral and temperature measurements can be made in conjunction with the broadband radiometer data can some characterization of or improvement in the accuracy of the RSR type of data be made.