_ \ SN--

normal incidence


,d - scattered (s = 0)

i i i\

Fraction of time in which the energy flux exceeds

Fraction of time in which the energy flux exceeds

Figure 3.22c. One-year power duration curves of normal incidence radiation alone, and of scattered radiation alone, based on the Danish reference year (0 = 56°N). The normal incidence curve would correspond to the radiation received by a fully tracking instrument. The curve for scattered radiation is for a horizontal surface.

duration curve may be used directly to predict the performance of a solar energy device, if this is known to be sensitive only to fluxes above a certain minimum value, say, 300 W m-2. The power duration curves for direct radiation alone are shown on the right. They are relevant for devices sensitive only to direct radiation, such as most focusing collectors.

Figure 3.22c gives the power duration curves for normal incidence radiation, SN, and for scattered radiation on a horizontal plane, d. The normal incidence curve is of interest for fully tracking devices, i.e. devices which are being continuously moved in order to face the direction of the Sun. By comparing Fig. 3.22c with Fig. 3.22a,b, it can be seen that the normal incidence curve lies substantially above the direct radiation curve for an optimum but fixed inclination (s = 45°). The power duration curve for normal incidence radiation is not above the curve for total radiation at s = 45°, but it would be if the scattered radiation received by the tracking plane normal to the direction of the Sun were added to the normal incidence (direct by definition) radiation. The power duration curve for scattered radiation alone is also shown in Fig. 3.22c for a horizontal plane. The maximum scattered flux is about 350 W m-2, much higher than the fluxes received during the winter days shown in Fig. 3.18.

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