In the case of a single-row solar generator that is remote from elements such as a building facade, global radiation is somewhat reduced in the plane that is inclined by a1, relative to horizontal global radiation, whereas the solar generator is exposed to the inclined plane at angle (a1 + b). In the case of relatively low a1 values, it is safe to assume that the global radiation arriving at the plane with an angle of inclination a1 relative to the horizontal plane is roughly the same as the global radiation on the horizontal plane. This allows for calculation of a corrected effective portion of reflected radiation:
Inasmuch as the surfaces that form an elevated horizon are not as evenly illuminated as presupposed above (particularly when it comes to the elements near the horizon that have the greatest impact on reflected radiation), irradiance on inclined surfaces is usually overestimated. Hence it is more in keeping with real-world conditions, and also simpler, to calculate the effective portion of reflected radiation RR using Equation 2.21, despite the horizon elevation a1.
In single-row solar generators near a building fac ade (see Figure 2.38), often a2 — 90° — b, i. e. in addition to ground reflection radiation GR1 and HR1, fac ade reflection radiation GR2 and HR2 also need to be taken into account (see Figure 2.40). But in order to compute GR2 and HR2 accurately, the global radiation Gf and Hf incident on the fac ade must first be determined. If a2 is relatively low, this latter calculation is hardly worth the effort. Owing to the relatively low value for the effective portion of reflected radiation RR2, in most instances calculation of the global radiation incident on the fac ade can be
dispensed with, except for very bright facades such as metal ones with a relatively high a2. Forgoing this calculation to some extent compensates for the slight reduction in ground reflection radiation provoked by the reduced global radiation G and Hon the plane in front of the solar generator. For as Figure 2.38 shows, diffuse radiation is reduced by the facade on the surfaces that are immediately in front of the solar generator, as in Equation 2.27. Hence in most such cases it suffices to take account of ground reflection radiation as in Equations 2.19 and 2.20.
Ground reflection radiation for stacked rows of solar generators (see Figure 2.37) is lower than for a single-row generator, particularly in winter. This can be approximately taken into account by using a reduced year-round reflection factor. In stacked solar generator fields (see Figure 2.39), facade radiation from the upper arrays is usually reduced owing to the virtual absence of facade reflection radiation.