Gb Gd p

1 + cos(ß)'

1 + F'sin3

Êl

2 _

.2J

where F' is a clearness index given by

The first of the modifying factors in the sky diffuse component takes into account horizon brightening; the second takes into account the effect of circumsolar radiation. Under overcast skies, the clearness index F' becomes 0 and the model reduces to the isotropic model.

Hay-Davies model

In the Hay-Davies model, diffuse radiation from the sky is composed of an isotropic and circumsolar component (Hay and Davies, 1980) and horizon brightening is not taken into account. The anisotropy index, A, defined in Eq. (2.102), represents the transmittance through atmosphere for beam radiation:

The anisotropy index is used to quantify the portion of the diffuse radiation treated as circumsolar, with the remaining portion of diffuse radiation assumed isotropic. The circumsolar component is assumed to be from the sun's position. The total irradiance is then computed by

Reflection from the ground is dealt with as in the isotropic model. Reindl model

In addition to isotropic diffuse and circumsolar radiation, the Reindl model also accounts for horizon brightening (Reindl et al., 1990a,b) and employs the same definition of the anisotropy index, A, as described in Eq. (2.102). The total irradiance on a tilted surface can then be calculated using

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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