Advantages
Flat-plate collectors will absorb energy coming from all directions above the absorber (both beam and diffuse solar irradiance). Because of this characteristic, flat-plate collectors do not need to track the sun. They receive more solar energy than a similarly oriented concentrating collector, but when not tracked, have greater cosine losses.

A. Non-tracking Option
Since tracking is not required, flat-plate collectors may be firmly fixed to a mounting structure, and rigid plumbing may be used to connect the collectors to the remainder of the system. Moving structure, motors, and tracking control systems are eliminated, thereby reducing the complexity of the system. As discussed in Chapter 5, however, because of the cosine effect, less total energy falls on a fixed surface over the period of a day than on a surface that tracks the sun about one or two axes.
In order to increase their output, flat-plate collectors may be repositioned at intervals or placed on a single- or two-axis tracking mechanism. Either of these options increases the output of the collector but eliminates the advantage of fixed piping and mounting structure.

B. Diffuse Solar Radiation Utilization
A flat-plate collector absorbs both the direct and the diffuse components of solar radiation. This partially compensates for the fact that fixed surfaces receive less energy because of the cosine effect. Although the diffuse solar irradiance is only about 10 percent of the direct normal solar irradiance on a clear day, on a cloudy day almost all of the available solar irradiance is diffuse.
A comparison between the energy falling on a fixed and a fully tracking flat-plate collector and on a fully tracking concentrating collector is shown in Table 6.1. The data are yearly average values taken from SERI (1981b). The comparison is location dependent because of the latitude effects on the incidence angle and the difference in cloud cover.
For both locations, the two-axis tracking flat-plate collector receives more energy. However, in the dry, high desert climate of Albuquerque, a fixed.