Three different types of DC Field tilt technologies are supported by PlantPredict: fixed tilt, seasonal tilt, and horizontal-axis tracking. These three are mutually exclusive and can be configured as part of the DC Field definitions. They impact the available plane-of-array irradiance and row-on-row shading.
DC Field Fixed Tilt
This is the base case. The tilt angle of the DC Field is static and set for all simulation time.
This is a variation of the fixed-tilt case, where the effective tilt angle depends on the 12-month lookup table of desired tilt angles as defined in the DC Field definitions.
Set the instantaneous tilt angle for time t according to the by-month lookup of array tilt angles, where n=1:12:
For horizontal north-south axis tracker, this procedure computes the ideal tracker angle φτ, taking into consideration backtracking shade avoidance strategy and the tracker’s mechanical limits of travel. This procedure is generalized for a tracker whose N-S axis is skewed by a tracker yaw angle γτ (azimuth angle).
The coordinate system used in this suite of algorithms is the “North-East-Down” convention (NED), as follows:
- Solar azimuth angle is measured from North (N=0°, E=90°, S=180°, W=270°)
The other angles are positive when rotating counter-clockwise looking towards the origin.
- Tracker roll (tilt) angle is positive East, negative West, 0° is horizontal (facing up)
- Tracker yaw angles are positive east (e.g. 0° is N-S; 10° has a slight NNW-SSE orientation)
This is illustrated in Figure 5.
Figure 5. Modified Tracker Coordinate System
This procedure returns the tracker angle, as well as the effective tilt and azimuth angles, rotated in the reference
frame of a basic fixed-tilt array. In the case of a horizontal north-south tracker with a yaw angle of 10° degrees
east with the tracker set at an evening angle 45° to the east, the effective tilt β and azimuth γ would
be +45° be and +80°, respectively. The derivation of the rotation angle for optimum tracking of a single-axis
trackers is given in the referenced technical report published by NREL.
PlantPredict only supports a horizontal tracker (where the pitch angle is zero). This section summarizes the referenced equations into a form easily implanted in source code.
Note: Unlike in the NREL technical report, the incidence angle is calculated separately in order to have a generalized solution for both fixed-tilt and tracking array configurations.
Algorithm (True Tracking)
1.) Convert the solar azimuth and yaw angles into the NED reference frame.
If backtracking is activated (as it would typically be for PV technologies using bypass diodes), then compute
the modified tracker angle with shade avoidance:
1.) Find the cutoff tracking angle where the row-to-row shading begins, which depends on the ground coverage
ratio (tracker width / post-to-post spacing)
Algorithm (Tracker Limits)
1.) If the tracker limits are reached, the tracker stops moving.
if the tracker axis is north-south (azimuth is defined relative to the tracking axis, not the collector);
hence in the morning, the tracker has an effective collector azimuth of 90° (facing east), and in the afternoon,
the effective collector azimuth is 270° (facing west).
Marion, William F. and Dobos, Aron P. “Rotation Angle for the Optimum Tracking of One-Axis Trackers.”NREL. July 2013. NREL/TP-6A20-58891, 2003