Unless the PV array is mounted on a two-axis tracker, the incident angle for the direct component of the solar radiation will not be normal except for a few rare instances, depending on the orientation. When the angle of incidence is greater than zero, there are optical losses due to increased reflections from the module…
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Weather and irradiance data are used as input to PV performance models. These data are directly measured, derived from measured data, or simulated using a stochastic model. Irradiance is to power as insolation is to energy. Or in other words: Irradiance is an instantaneous measurement of solar power over some area. The units of irradiance…
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The simplest approach to estimating was developed by Souka and Safat (1966) and later adopted by the American Society of Heating, Refrigeration, and Air Conditioning (ASHRAE), and is known as the “ASHRAE incidence modifier.” This function has the benefit of needing only one parameter, , but suffers from a discontinuity at and low accuracy at high…
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A series of papers by Martin and Ruiz (2001; 2002; 2004) describe the optical reflectance loss for PV modules and model the effect of these losses on annual energy. Martin and Ruiz (2001) establish that the angular losses (AL) of a PV modules are a function of the solar incident angle (). where T(x) is…
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This empirical model for diffuse sky irradiance was developed at Sandia National Laboratories by David King and has been found to work quite well at the Sandia facility (better, in fact, than any of the other models that have been compared). The model determines the diffuse irradiance from the sky () tilted surface using the…
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The Hay and Davies diffuse model divides the sky diffuse irradiance into isotropic and circumsolar components. Horizon brightening is not included. An anisotropy index, , is defined as: , where is the direct normal irradiance and is the extraterrestrial radiation. The Hay and Davies model formulation for sky diffuse radiation is: , where is the…
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The Reindl sky diffuse irradiance model (;; ) represents three components of diffuse radiation on the POA, including isotropic, circumsolar brightening, and horizon brightening. This model extends the Hay and Davies model by adding an additional factor to the “brightening” term to account for horizon brightening. As with the Hay and Davies model, an anisotropy…
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While the sky diffuse model presented up to this point separated the isotropic, circumsolar, and horizon components explicitly, Perez developed a more complex model that relies on a set of empirical coefficients for each term. The basic form of the model is: , where and are complex empirically fitted functions that describe circumsolar and…
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Diffuse radiation from the sky dome is typically divided into several components: the isotropic component, which represents the uniform irradiance from the sky dome; the circumsolar diffuse component, which represents the forward scattering of radiation concentrated in the area immediately surrounding the sun; the horizon brightening component. Published models use different semi-empirical approaches for estimating…
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