Tilted irradiance term

Some PV sites, such as rooftops, may involve different slopes and orientations. Installing a large number of irradiance sensors is often not feasible. Instead, you can create a tilted irradiance term to calculate the tilted irradiance on the module plane based on irradiance data from a horizontal pyranometer.

The term can be used for standard charts, KPI calculations, and user-defined alarms
You can compare the tilted and the horizontal irradiance using the Analysis feature in VCOM
A horizontal pyranometer is required

Formula

The tilted irradiance formula is based on the following:

GHI is split into diffuse and direct irradiation using the Perez algorithm[1].
Diffuse and direct irradiation and the current sun position are used to calculate the total inclined radiation using the Klucher algorithm[2].

[1] https://www.sciencedirect.com/science/article/abs/pii/0038092X9090060P
[2] https://www.sciencedirect.com/science/article/abs/pii/0038092X79901105

The formula calculates the tilted irradiance for the given arguments:

Horizontal irradiation
Tilt angle (0 = horizontal, 90 = horizontal)
Orientation angle (0 = north, 90 = east, 180 = south, 270 = west)

Formula	Result
tilted_irradiance(NULL;20;180)	NULL
tilted_irradiance(500;NULL;180)	NULL
tilted_irradiance(500;20;NULL)	NULL
tilted_irradiance(500;20;180)	The result will vary based on the date, time, and location of the system

Example

The example below shows a tilted irradiance term.

In this example, a term is created for irradiance in the module plane as well as for each subsystem with a different tilt.

Example: tilted irradiance terms for subsystems with different orientations

It is then possible to display the tilted and the horizontal irradiance as an analysis:

Analysis comparing irradiance on the module plane and tilted irradiance