# pvl_isotropicsky

Determine sky diffuse irradiance on a tilted surface using the isotropic sky model.

## Contents

- Syntax
- Description
- Inputs
- Outputs
- Example
- References
- See Also

## Syntax

`SkyDiffuse = pvl_isotropicsky(SurfTilt, DHI)`

## Description

The isotropic sky model [1], [2] regards the sky as a uniform source of diffuse irradiance. Thus the sky diffuse irradiance on a tilted surface can be found from the diffuse horizontal irradiance and the tilt angle of the surface.

## Inputs

- `SurfTilt` - a scalar or vector of surface tilt angles in decimal degrees. If SurfTilt is a vector it must be of the same size as all other vector inputs. SurfTilt must be >=0 and <=180. The tilt angle is defined as degrees from horizontal (e.g. surface facing up = 0, surface facing horizon = 90)
- `DHI` - a scalar or vector of diffuse horizontal irradiance in W/m^2. If DHI is a vector it must be of the same size as all other vector inputs. DHI must be >=0.

## Outputs

- `SkyDiffuse` - the diffuse component of the solar radiation on an arbitrarily tilted surface defined by the isotropic sky model as given in Loutzenhiser et. al (2007) equation 3. SkyDiffuse is the diffuse component ONLY and does not include the ground reflected irradiance or the irradiance due to the beam. SkyDiffuse is a column vector vector with a number of elements equal to the input vector(s).

## Example

Calculate sky diffuse on a 30 deg tilted array when diffuse horizontal irradiance equals 200 W/m^2.

    SkyDiffuse = pvl_isotropicsky(30, 200)

    SkyDiffuse =

      186.6025

## References

[1] Loutzenhiser P.G. et. al., 2007. Empirical validation of models to compute solar irradiance on inclined surfaces for building energy simulation, Solar Energy vol. 81. pp. 254-267.

[2] Hottel, H.C., Woertz, B.B., 1942. Evaluation of flat-plate solar heat collector. Trans. ASME 64, 91.

## See Also

`pvl_perez`, `pvl_reindl1990`, `pvl_klucher1979`, `pvl_haydavies1980`, `pvl_grounddiffuse`, `pvl_kingdiffuse`

Copyright 2014 Sandia National Laboratories