Information Overview

For solar trackers to be a worthwhile investment, they must demonstrate to be long term reliable and with minimum maintenance.

Early solar trackers used an optical positioning method. Such a sensor is fully exposed to UV radiation which makes any plastic housing brittle with subsequent failure from weather.

Moving a tracker to a desired target can only happen with a reliable position feedback system. For trackers using the calculated targeting method, the feedback system is typically one of the following:

  • Inclinometer (tilt sensor) is used to measure elevation tilt with respect to gravity. Most inclinometers use a MEMS sensor which has been proven to have an extremely high reliability.

  • For an azimuth tracker axis, a combination of a limit switch and motor encoder is used. At power-up, the tracker is moved up against the limit switch. A motor encoder delivers electrical pulses which is counted by the Encoder Counter module.

Sensors like inclinometers, encoders, and limit switches can be mounted such that they are protected from UV radiation, and weather. However, they do require an advanced control system.

Block diagram of major software modules in Lauritzen Solar Tracker Controller A state of the art solar tracker controller incorporates a set of basic software modules for tracker operations. At the executive level, the Operational module considers environmental conditions such as wind, and current solar position to dispatch a tracker target command. The Fault Handler module accepts the target and acts as the supervisor for the servo module which task it is to move the tracker to its target.

Controller Buttons

Here is a quick review of controller buttons when servicing a tracker.

  • Reset (Black): a controller may be Reset. Reset is used to clear a fault, or test tracker startup. The controller's black Reset buttons has three functions:

    • Depressed for 3 seconds (status LED's rapidly blinking), it will force the controller to contact the server.

    • Depressed for 10 seconds (status LED's turn solid), it will clear all faults, and force a controller reset.

    • Depressed for 30 seconds (status LED's turn off), it will clear all controller parameters to factory default.

  • Service (Red): Enter/exit Service Mode (SM).

  • Up/Down (Green): While in SM, move tracker elevation up or down. Note; when using CX3 controllers with single-axis trackers, the Up/Down is used to move the tracker. Up must move the tracker east, and Down must move the tracker west.

  • East/West (Blue): While in SM, move tracker azimuth east or west.

  • Calibration (Yellow): Push 3 seconds to enter Calibration Mode (CM). If CM is mistakenly entered, Reset button may be used to exit CM. IMPORTANT; ONCE A SYSTEM HAS BEEN PROPERLY CALIBRATED, ADDITIONAL CALIBRATION MUST NOT BE DONE UNLESS CHANGES ARE MADE TO LIMIT SWITCHES OR OTHER SENSORS. A tracker which has been operational, and has now stopped, is most likely due to a sensor or motor failure. Attempting to calibrate such a tracker will overwrite the correct calibration data, and make fault finding even more difficult.

  • Learn (White): The Learn button is used on Master controllers to initiate automatic slave discovery and enter them into the master's domain registration. The Learn process is activated after depressing the button for 3 seconds.

Tracker Mode Definitions

Here are a few useful terms used when using Lauritzen Control Systems with a tracker:

  • Clean Mode: is used to force a tracker into a position where it is convenient to clear the panels. Note: only Operational trackers can enter Clean Mode. Clean Mode is asserted by the local Clean switch.

  • Calibration Mode: is an intermediate step while calibrating a tracker. CM is entered via the Calibration button. If CM is mistakenly entered, Reset button may be used to exit CM. IMPORTANT; ONCE A SYSTEM HAS BEEN PROPERLY CALIBRATED, ADDITIONAL CALIBRATION MUST NOT BE DONE UNLESS CHANGES ARE MADE TO LIMIT SWITCHES OR OTHER SENSORS.

  • Master Stop: most controllers are deployed with a local Master Stop switch. When the MS is engaged, the controller is prevented from moving the controller – both automatically and manually. A Stop State is also transmitted to the server. A controller's Stop State is interpreted as “hands off" because local personal doing maintenance work.

  • Operational Mode: a tracker is operational when it is tracking the sun within its normal operational range and without exceptions or faults.

  • Power Saving Mode: a controller will enter Power Saving Mode (PSM) after 30 minutes of operator inactivity. While in PSM, the controller will continue to be in Operational Mode, but the LED's will be turned-off and only the APP LED will blink. Upstream communication, depending on traffic, may also be curtailed. Any push button is toggled to exit PSM.

  • Service Mode: a controller may be placed in SM to manually move the tracker with push buttons. SM indicates that it is OK for both a local and remote operator to move a tracker. SM is entered and exited by a quick push and release of the controller's red Service mode button.

  • Standby Mode: in a field where master/slave controllers are being used, a slave controller may remotely be forced into Standby Mode. When a slave unit is in Standby, operations will cease until Standby exit. To exit Standby, the local Master Stop is toggled. Standby is used to cease operations, and signal that field attention is required to resolve problem.

  • Storm Mode: is used to force a tracker into its storm position. The storm position is usually horizontal, or near horizontal. Note: only Operational trackers can enter Storm Mode. Storm Mode is asserted from one or more conditions:

    • Triggered by high wind speed.

    • Forced by local Storm switch.

    • Forced by remote Storm command.

LED Indicators

Here is a quick review of some controller LED's and their diagnostic meaning.

  • PWR/BAT/FUSE: Indicators for power, battery charging, or blown fuse.

  • Motor+/-: A pair of LEDs indicate motor output power and indicator.

  • AUX: indicates if power is delivered on the auxiliary terminal.

  • AN: anemometer activity indicator.

  • STP: Indicates when Master Stop is active.

  • AZx/ELx: Indicates encoder state and activity.

  • System Status Indicators: when system is operational, these LED's will alternate back and forth. This is also referred to as “the happy lights". A detected failure is indicated by flashing a corresponding error code.

    • SYS: Controller system status.

    • COMM: Communication with server or master unit.

    • APP: Tracker application status.

  • INCL: External inclinometer status.

  • GPS: GPS radio status.

  • Ethernet: Link carrier and traffic indicators.

  • SW2/SW3: Status of digital inputs.

  • STM: System is in Local Storm Mode.

  • CLN: System is in Local Clean Mode.

  • TX/RX: Activity indicators for master/slave communication.

  • LRN: Learn activity when master controller is discovering connected slave units.

Solar Tracker Accuracy

Optical vs Calculated Positioning

There are two methods commonly used to control solar trackers: 1) optical sensing and 2) calculated sun position. Optical sensing mechanisms only work if the skies are clear, and do not perform well if the sensors become dirty. They commonly use analog electronic circuitry; hence they cannot be controlled remotely. Controllers based on a solar ephemeral equation typically calculate the solar position to 0.01 degree. During early morning and late afternoon, some solar beam deviation will exist due to the atmospheric water vapor deflection, amounting to as much as 1-2 degrees.

Feedback Methods

Solar tracker positional feedback is typically done through the use of encoders, limit switches, or inclinometers. Motor mounted encoders are typically most cost effective, but also introduce inaccuracies caused by the mechanical gears. For those systems, the gear slop can easily reach +/- 1 degree. Another variable is the tracker platform itself, such as bearings or the ductility of the construction materials. Long term failure rates for encoder-based systems can also be higher than expected due to fouling.

How Important is Accuracy?

In the case of using PV modules, the modules are not particularly sensitive with respect to the incoming solar beam. For instance, a solar tracker that is consistently off by 2.5 degrees is operating at 99.9% of its optimal output. Similarly, a tracker operating at 10 degrees deviation from the optimal solar beam will operate at 98.5% of the optimal output. Thus, it should be apparent that constructing a PV solar tracker with emphasis on accuracy may yield a poor value proposition. The table below shows this theoretical relationship:

Deviation of orthogonal solar beam (degree) Relative output w.r.t. orthogonal beam (%)
0.0 100.0
0.1 100.0
1.0 99.98
2.5 99.90
5.0 99.62
10.0 98.48
20.0 93.97

Concentrated Solar Trackers

Concentrated PV solar trackers require by nature quite different tracking accuracies. First, the receivers typically have an acceptance angle of much less than +/- 1.0 degree. Even within the acceptance angle, there is usually some degradation as the solar beam deviation approaches the limits of the acceptance angle. As a result, CPV solar trackers typically have extremely high electro-mechanical design standards. Similarly, encoder and/or inclinometer feedback is typically not sufficient to guarantee optimum positioning.

Movement Frequency

During the winter at noon time at middle latitudes the apparent motion of the Sun can be on the order of 2 to 3 degrees every ten minutes, while during the summer, the movement can be as great as 10 degrees. For PV based trackers, it is generally a good idea to minimize motor starts and stops to promote longevity of the electro-mechanical systems. Such algorithms will consider both a movement time interval and minimum angular adjustment.

Production Gain from Trackers

The energy gain of PV systems based on trackers versus stationary is dependent on location and season. Estimates and actual measurements indicate, on an annual basis, single axis trackers gain \~20% additional power compared to a fixed angle system, while dual axis tracker systems can gain \~40%. Single axis trackers typically follow the sun by elevating the modules around a North-South axis, while dual axis trackers typically rotate panels around the horizontal axis and adjust for elevation in the vertical axis.

Case Studies

Installation, upgrade and reference studies are important learning tools.