Given the statistical nature of the irradiation profile of a site, annual or monthly average values for the daily irradiation (Hda(0) and Hma(0), respectively) are commonly used when designing PV systems. As commented earlier, these average values are available only for horizontal surfaces in most solar databases. However, for installations located in European sunny climates and with an optimun tilt angle, equation 2.1 is a rule-ofthumb that broadly relates the annual average horizontal irradiation -H(0)- and the annual average irradiation collected on a equator-facing, βopt –tilted surface -H(0,βopt): 2 1 H ( 0, op opt )[ kWh·m · year ] This obviously means that:
1.15 H ( 0 )[ kWh·m 2 · year 1 ]
(2.1)
2 1 H da ( 0, op 1.15 H da ( 0 )[ kWh·m 2 ·day 1 ]·365 (2.2) opt )[ kWh·m ·day ]·365 This is: 2 1 H da ( 0, op 1.15 H da ( 0 )[ kWh·m 2 ·day 1 ] (2.3) opt )[ kWh·m ·day ]
If irradiation collected on surfaces with an arbitrarily azimuth angle α and tilt angle β is to be estimated -H(α,β)- some graphs proposed in literature may be of great help. Thus, figure 2.3 is intended to derive the latter value from H(0) and it can be applied to similar range of latitudes to those of Spain (i.e.: Southern European countries). An example is provided to achieve a better understanding of its use.
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