Input Data

SolarFarmer is designed to work with the following combinations of input irradiance data:

Global Horizontal Irradiance ($G_{\text{h}}$) and diffuse horizontal irradiance ($G_{\text{dif}}$).
Global Horizontal Irradiance ($G_{\text{h}}$) alone;
Global Irradiance in the plane of the array ($G_{\text{poa}}$, typically from measurements made directly at the site with the instruments oriented in the same direction as the modules).

Further, the sun's position at each time step is required to evaluate both the amount of irradiance available to the modules, and shading effects.

The irradiance calculation starts with $G_{\text{h}}$ and $G_{\text{dif}}$. If these quantities are not both available directly in the input data, then they must first be separately calculated from the available input data.

When only $G_{\text{h}}$ is available, the Erbs model is used to derive $G_{\text{dif}}$;
When only $G_{\text{poa}}$ is available, a combined Erbs/transposition model is used based on the Hay model for the diffuse to derive $G_{\text{h}}$ and $G_{\text{dif}}$. This requires the use of the Hay model for subsequent transposition back up to the module planes.

Horizontal Irradiance

Global Horizontal Irradiance radiation has two components, direct horizontal irradiance ($G_{\text{dir}}$) and diffuse horizontal irradiance ($G_{\text{dif}}$) (both on the horizontal plane); it is assumed that there is no reflected component in horizontal input data.

$$G_{h} = G_{\text{dir}} + G_{\text{dif}}$$

$G_{\text{dir}}$ and $G_{\text{dif}}$ form the basis of modelling in SolarFarmer, except when suitable in-plane data are loaded.

If the array were installed with all modules horizontal (i.e. flat on the ground), then the average irradiance on the array at time t (ignoring shading) would be:

$$G_{layout,h}(t) = G_{h}\left( t \right)$$

If $G_{\text{h}}$ is zero for any time interval, then it is to be assumed that the output from the array is zero. In this case, the only calculation necessary is inverter power consumption (and resulting consumption in the AC network and transformer), as described in Night Time Tare Loss.

Irradiance data is loaded into SolarFarmer in several formats depending on the source of the data, and some processing is required to obtain $G_{\text{dir}}$ and $G_{\text{dif}}$.

Global Horizontal Irradiance and Global Diffuse Irradiance

Where the input irradiance data consists of Global Horizontal Irradiance $G_{\text{h}}$ and Global diffuse irradiance $G_{\text{dif}}$, direct horizontal irradiance is calculated simply by:

$$G_{\text{dir}} = G_{h} - G_{\text{dif}}$$

Global Horizontal Irradiance

Where only $G_{\text{h}}$ is available as input data, $G_{\text{dir}}$ and $G_{\text{dif}}$ are obtained through a process called Decomposition. SolarFarmer currently only includes the Erbs decomposition algorithm, described in Erbs, however future versions of SolarFarmer may include DIRINT, described in DIRINT

Ground Reflected Irradiance

Ground reflected irradiance, $G_{\text{r}}$, is the product of the GHI, $G_{\text{h}}$, and the albedo, which the user can supply per month to account for seasonal variations in ground cover, see section Ground (Albedo) Irradiance.

Plane-of-Array (POA) Irradiance

The orientation of modules in an Array is nominally described by the chosen azimuth and tilt angles for the Array. SolarFarmer recognises that whilst an entire PV Plant will often be composed of modules with the same nominal azimuth and tilt angles, the presence of non-flat terrain at ground mounted PV Plants will result in modules across the Plant and within individual Arrays having different azimuth and tilt angles. The term "Plane of the Array" (POA) is used by convention to describe properties pertaining to the faces of modules as opposed to the horizontal plane, but it is noted that the POA may not be constant across individual Arrays (though it is assumed to be constant across individual Racks) and POA should be interpreted as specific to the particular orientation of each rack where it is used in this document.

POA irradiance measurements are more complex to use because:

They include reflected irradiance, which may be specific to the location of the sensor;
If made within an operating Plant Array, they potentially include shading effects from other modules;
They include horizon and terrain shading effects specific to the sensor location;
They typically do not include separate measurements of global and diffuse irradiance, complicating shading analysis and making transposition to other planes difficult where these are necessary.

The approach taken to utilising POA irradiance measurements in SolarFarmer is described in Use of Plane of Array Measurements. In summary, the POA measurements are used to estimate concurrent values of $G_{\text{h}}$ and $G_{\text{dif}}$ which are then used to initiate the model in the same way as if $G_{\text{h}}$ and $G_{\text{dif}}$ had been measured directly.

Sun Position

The sun's position is calculated relative to the site position (meaning a representative set of site coordinates is required) at fixed intervals for the entire period of the solar resource input data. The calculation is described in Sun Position.