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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Sandia has suggested using a 5th order polynomial function to represent angular optical losses on short circuit current (Isc) [1]. where the coefficient vector, , is determined from fitting experimental data measured outdoors. An example result of this model is shown in Figure 1. Note that the use of a 5th order polynomial causes…
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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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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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The isotropic sky diffuse model is the simplest of the POA sky diffuse models and forms the foundation upon which more accurate models build. The isotropic sky diffuse model assumes that the diffuse radiation from the sky dome is uniform across the sky. The POA sky diffuse irradiance () is calculated as a fraction of…
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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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