A single-axis solar tracker rotates the rows of modules from east to west to follow the sun through the day. Its controller calculates the position with an astronomical algorithm, corrects it with an inclinometer, applies backtracking when the sun is low so that one row does not shade the next, and moves the row to a stow position in strong wind or other events. Each row reports its status to the plant SCADA, and the design qualification of the tracker is done according to IEC 62817.
What a solar tracker is
A solar tracker is a structure that moves the modules so that they face the sun at a smaller angle of incidence throughout the day. In solar plants the most common type is the horizontal single-axis tracker: the axis runs north to south and the row rotates from east to west. Dual-axis trackers also follow the elevation of the sun, but in large PV plants their use is much smaller because of cost and maintenance.
Compared with a fixed structure of the same capacity, the tracker captures more energy, especially at sites with a high fraction of direct radiation. In exchange it adds motors, controllers, power supply, communications and an operating logic that the plant must supervise. The comparison with the fixed structure and the spacing between rows are covered in PV field design, within solar plant engineering and design.
Parts of a single-axis tracker
| Part | Function |
|---|---|
| Torque tube and bearings | Carry the modules and rotate on the piles |
| Drive | Motor and gearbox that rotate the row; there can be one drive per row or one that moves several rows through a transmission |
| Row controller | Calculates the position, reads the inclinometer, commands the motor and monitors status |
| Power supply | From the plant auxiliary service network, or self-powered with its own module and a battery or UPS |
| Communications | Wireless or wired link between the row controllers and a network controller |
| Network or block controller | Groups the rows, receives general commands (stow, maintenance) and delivers the data to the SCADA |
| Sensors | Inclinometer on each row; anemometer and, depending on the system, other weather sensors |
1P and 2P. The designation tells how many modules are placed vertically across the width of the row. A 2P carries more modules per axis and per drive; its row is wider and taller, with different wind loads and a different shading geometry.
How it follows the sun
The controller calculates the position of the sun with an astronomical algorithm, from the date, the time and the coordinates, and rotates the row to the corresponding angle. The inclinometer confirms that the row has arrived and corrects drift. That is why the controller needs the exact time: a clock that is off produces a pointing error on every row.
Backtracking. With the sun low, a row pointed at the sun would shade the next one. The controller moves the row back toward horizontal as far as needed so that the shadow does not reach the neighboring row. The angle depends on the row spacing, the row width and the terrain slope; on uneven terrain, some manufacturers offer backtracking that takes the slope of each row into account.
Tracking under cloudy skies. With overcast skies almost all the radiation is diffuse, and pointing at the sun adds little. Some manufacturers offer modes that move the row toward horizontal in those conditions to capture more diffuse light.
Stow position
The tracker is a large sail. When the wind exceeds the threshold the manufacturer defines, all rows move to a position that reduces aerodynamic loads and oscillations. Manufacturers define other stow positions for hail, snow, flood or loss of power. For example, the Nextracker NX Horizon datasheet declares stow functions for wind, hail, hurricane, snow, flood and loss of grid power, and a self-powered controller per row with a built-in inclinometer and UPS.
The wind stow command can come from the tracker system’s anemometer or from the plant weather station, whose variables are also required by the grid operator’s SCADA supervision requirements. This logic is about structural safety: if the anemometer or the communication fails, the system must go to a safe state. How it behaves on each failure is defined by the manufacturer and verified at commissioning.
Operating modes of a row:
- Tracking: astronomical algorithm corrected by the inclinometer.
- Backtracking: moving back at the start and end of the day to avoid shading between rows.
- Stow: safe position in wind, hail, flood or loss of power.
- Maintenance: fixed position commanded from the controller for cleaning or repair.
- Night: rest position defined by the manufacturer.
Communications and SCADA
Each row reports angle, mode, alarms, motor status and, if self-powered, the status of its battery. The row controllers communicate with the network controller over a wireless mesh link or a wired bus. Nextracker, for example, declares an encrypted Zigbee wireless mesh network. The network controller delivers the data to the plant SCADA over the protocol the manufacturer defines, such as Modbus TCP.
For operation this means:
- Signal list. The angle of each row, mode, alarms and the stow command must be in the SCADA signal list, with the aggregation the operator needs.
- Network. Network controllers join the plant network, which also carries the inverters, the PPC and the weather station. A traffic storm or a degraded wireless link leaves rows unsupervised.
- Time synchronization. The astronomical algorithm needs the correct time; it is advisable for the controllers to take the same time reference as the rest of the plant.
- Cybersecurity. A false mass command to the rows is an operating risk. Remote access to the controllers must pass through the same barriers as the rest of the control system.
Qualification standard
IEC 62817:2014, with its 2017 amendment, is the design qualification standard for solar trackers for photovoltaic systems. It defines test procedures for the main components and for the complete system: some measure parameters that are reported in the tracker specification sheet and others give a pass or fail result. Its aim is that the parameters declared by different manufacturers have been measured with comparable procedures. The structural design for the site’s wind and seismic loads is a separate topic, handled by the project’s civil engineering under the local structural codes.
What is checked at commissioning
- That each row reaches the commanded angle and that the inclinometer matches the real position.
- That backtracking works on the real terrain geometry.
- That the stow command is executed from the anemometer and from the SCADA, and that the system goes to a safe state if it loses the sensor or the communication.
- That all signals reach the SCADA with the correct name, unit and time stamp, through point-to-point tests.
We help with your plant SCADA
Our team does the SCADA and gateway integration of solar plants and substations: signal list, protocols, plant network and point-to-point tests of inverters, trackers, weather station and PPC all the way to the control center. The engineering is done remotely, worldwide; field tests are done in Colombia, or elsewhere on request. Tell us about your project.
Related guides
References
- IEC 62817:2014+AMD1:2017, Photovoltaic systems – Design qualification of solar trackers.
- Nextracker, NX Horizon datasheet (2023): https://info.nextracker.com/hubfs/Datasheets%202023/nxt_NX_Horizon_Datasheet_v2_2023.pdf
Frequently asked questions
What is backtracking in a solar tracker?
It is the strategy the tracker uses when the sun is low, at the start and end of the day. Instead of pointing straight at the sun, the row moves back toward horizontal as far as needed so that its shadow does not fall on the neighboring row. Some direct irradiance is lost, but partial shading is avoided, which in modules with series-connected cells costs much more energy.
What is the difference between a 1P and a 2P tracker?
The designation tells how many modules are placed vertically across the width of the row: one (1P) or two (2P). The 2P puts more modules per axis and per motor, with a wider and taller row; the 1P is lower and narrower. The choice changes the wind load, the number of piles and the shading geometry.
What does a tracker do in strong wind?
It moves to a stow position that reduces aerodynamic loads, according to the manufacturer's logic. The command can come from the anemometer of the tracker system itself or from the plant weather station. Manufacturers also define positions for other events, such as hail, flood or loss of power.