PV concentrators are able to capture only direct-beam solar radiation and require tracking mechanisms to keep in focus the solar cells. Therefore, they are best suited to sunny sites with low scattering (or low diffuse radiation) and good solar resources. Figure 22.22 compares the amount of normal direct radiation for a PV concentrator mounted on a two-axis tracker, with the amount of global radiation, collected by a conventional flat-plate PV collector, at a fixed optimal tilt. The data represents the 30 locations in Table 22.4. The ratio between both quantities, 5dy(A)/Gdy(eopt),
СЛ X CO
(column 9 divided by column 8 of Table 22.4) is plotted against the annual clearness index. As a general rule, two-axis tracking concentrators collect more radiation than fixed flat-plate modules in places where KTy> 0.55. More information about PV concentrators is found in Chapter 10.