# pvl_ashraeiam

Determine the incidence angle modifier using the ASHRAE transmission model.

## Contents

- Syntax
- Description:
- Inputs:
- Outputs:
- Example
- References:
- See also

## Syntax

- `IAM = pvl_ashraeiam(b, theta)`

## Description:

Calculates the incidence angle modifier as developed in [1], and adopted by ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning Engineers) [2]. The model is used by modeling applications such as PVSyst [3].

Note: For incident angles near 90 degrees, the ASHRAE model has a discontinuity which has been addressed in this function.

## Inputs:

- `b` - A parameter to adjust the modifier as a function of angle of incidence. Typical values are on the order of 0.05 [3].
- `theta` - The angle of incidence between the module normal vector and the sun-beam vector in degrees. Theta must be a numeric scalar or vector. For any values of theta where abs(theta)>90, IAM is set to 0. For any values of theta where -90 < theta < 0, theta is set to abs(theta) and evaluated. A warning will be generated if any(theta<0 or theta>90). For values of theta near 90 degrees, the ASHRAE model may be above 1 or less than 0 due to the discontinuity of secant(theta). IAM values outside of [0,1] are set to 0 and a warning is generated.

## Outputs:

- `IAM` - The incident angle modifier calculated as 1-b*(sec(theta)-1) as described in [2,3]. IAM is a column vector with the same number of elements as the largest input vector.

## Example

This example plots the IAM over a range of incident angles.

    b=0.05; %Typical value.
    theta = 0:89.9;    %angles =>90 generate a warning
    IAM = pvl_ashraeiam(b, theta);
    figure
    plot(theta,IAM)
    xlabel('Incident Angle (deg)')
    ylabel('IAM')
    title('ASHRAE IAM Model Example')

    Warning: It seems that we have encountered a discontinuity in pvl_ashraeiam. Any
    incident angle modifiers calculated to be less than 0 or greather than 1 have
    been set to 0.

Note that the function can result in IAM values <0 and/or >1. When this occurs the out of range results are set to equal zero and a warning is reported.

## References:

- [1] Souka A.F., Safwat H.H., Determindation of the optimum orientations for the double exposure flat-plate collector and its reflections, Solar Energy vol .10, pp 170-174. 1966.

- [2] ASHRAE Standard 93-2010, Methods for Testing to Detemrine the Thermal Performance of Solar Collectors.

- [3] PVsyst Contextual Help. http://files.pvsyst.com/help/index.html?iam_loss.htm retrieved on September 10, 2012.

## See also

`pvl_getaoi` , `pvl_ephemeris` , `pvl_spa` , `pvl_physicaliam`

Copyright 2014 Sandia National Laboratories