# pvl_haydavies1980

Determine diffuse irradiance from the sky on a tilted surface using Hay & Davies' 1980 model.

## Contents

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

## Syntax

`SkyDiffuse = pvl_haydavies1980(SurfTilt, SurfAz, DHI, DNI, HExtra, SunZen, SunAz)`

## Description

Hay and Davies' 1980 model determines the sky diffuse irradiance on a tilted surface using the surface tilt angle, surface azimuth angle, diffuse horizontal irradiance, direct normal irradiance, extraterrestrial irradiance, sun zenith angle, and sun azimuth angle.

## 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)
- `SurfAz` - a scalar or vector of surface azimuth angles in decimal degrees. If `SurfAz` is a vector it must be of the same size as all other vector inputs. `SurfAz` must be >=0 and <=360. The azimuth convention is defined as degrees east of north (e.g. North = 0, East = 90, West = 270).
- `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.
- `DNI` - a scalar or vector of direct normal irradiance in W/m^2. If `DNI` is a vector it must be of the same size as all other vector inputs. `DNI` must be >=0.
- `HExtra` - a scalar or vector of extraterrestrial normal irradiance in W/m^2. If `HExtra` is a vector it must be of the same size as all other vector inputs. `HExtra` must be >=0.
- `SunZen` - a scalar or vector of apparent (refraction-corrected) zenith angles in decimal degrees. If `SunZen` is a vector it must be of the same size as all other vector inputs. `SunZen` must be >=0 and <=180.
- `SunAz` - a scalar or vector of sun azimuth angles in decimal degrees. If `SunAz` is a vector it must be of the same size as all other vector inputs. `SunAz` must be >=0 and <=360. The azimuth convention is defined as degrees east of north (e.g. North = 0, East = 90, West = 270).

## Outputs

- `SkyDiffuse` - the diffuse component of the solar radiation on an arbitrarily tilted surface defined by the Hay & Davies model [2] as summarized in [1] Equation 7. SkyDiffuse is the sky diffuse component ONLY and does not include the ground reflected diffuse irradiance or direct irradiance. SkyDiffuse is a column vector vector with a number of elements equal to the input vector(s).

## Example

    SurfTilt = 30;
    SurfAz = 180;
    DHI = 47;
    DNI = 969;
    GHI = 473;
    HExtra = pvl_extraradiation(60);
    SunZen = 60;
    SunAz = 161;
    pvl_haydavies1980(SurfTilt, SurfAz, DHI, DNI, HExtra, SunZen, SunAz)

    ans =

       68.4462

## 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] Hay, J.E., Davies, J.A., 1980. Calculations of the solar radiation incident on an inclined surface. In: Hay, J.E., Won, T.K. (Eds.), Proc. of First Canadian Solar Radiation Data Workshop, 59. Ministry of Supply and Services, Canada.

## See Also

`pvl_perez`, `pvl_reindl1990`, `pvl_kingdiffuse`, `pvl_klucher1979`, `pvl_grounddiffuse`, `pvl_ephemeris`

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