It is straightforward to convert beam irradiance measured on one plane (plane 1) to that on another plane (plane 2). This is particularly important for transforming data commonly available for the horizontal plane. Equation (4.8) gives the angle of incidence of the beam to each plane. Then, for the beam component,
The calculation of the diffuse irradiance on another plane, however, cannot be so precise. Consequently the total insolation H on other than the measured surface remains somewhat uncertain.
Duffie and Beckman discuss many refinements for estimating H. Although the uncertainty is more than 10%, the results are still instructive. For example, Figure 4.16 shows the variation in estimated daily radiation on various slopes as a function of time of year, at a latitude of 45°N, and with clearness index KT = 0.5. Note that at this latitude, the average insolation on a vertical sun-facing surface varies remarkably little with season, and in winter exceeds 10MJm—2 day—1. This is double the insolation on a horizontal surface in winter, and is certainly large enough to provide a
Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Deo.
Figure 4.16 Variation in estimated average daily insolation H on a surface at various slopes, p, as a function of time of year. For latitude 45°N, with KT = 0.50, y = 0° and ground reflectance 0.20. From Duffie and Beckman (by permission of John Wiley & Sons Inc.).
useful input to passive solar buildings, e.g. through insulating windows, atria and conservatories, and active pre-heating systems (Section 6.3).
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