A backyard installation of passive single–axis trackers, DC rated at 2340 watts. Seen here in winter midday position, tilted toward the south. The tall poles allow walk-under and use of the ground space underneath the panels for plantings that thrive on protection from the intense midday summer sun at this location

A solar tracker is a generic term used to describe devices that orient various payloads toward the sun. Payloads can be photovoltaic panels, lenses or other optical devices.

In standard photovoltaic (PV) applications trackers are used to minimize the angle of incidence between the incoming light and a photovoltaic panel. This increases the amount of energy produced from a fixed amount of installed power generating capacity. In standard photovoltaic applications, it is estimated that trackers are used in at least 85% of commercial installations greater than 1MW from 2009 to 2012.

In concentrated photovoltaic (CPV) and concentrated solar thermal (CSP) applications trackers are used to enable the optical components in the CPV and CSP systems. The optics in concentrated solar applications accept the direct component of sunlight light and therefore must be oriented appropriately to collect energy. Tracking systems are found in all concentrator applications because systems do not produce energy unless oriented toward the sun.

Photovoltaic Tracker Classification

Photovoltaic trackers can be classified into two types: Standard Photovoltaic (PV) Trackers and Concentrated Photovoltaic (CPV) Trackers. Each of these tracker types can be further categorized by the number and orientation of their axes, their actuation architecture and drive type, their intended applications, their vertical supports and foundation type.

Standard Photovoltaic (PV) Trackers

Photovoltaic panels accept both direct and diffuse light from the sky. The panels on a Standard Photovoltaic Trackers always gather the available direct light. The tracking functionality in Standard Photovoltaic Trackers is used to minimize the angle of incidence between incoming light and the photovoltaic panel. This increases the amount of energy gathered from the direct component of the incoming light.

Accuracy Requirements

In standard photovoltaic systems, the energy contributed by the direct beam drops off with the cosine of the angle between the incoming light and the panel. Thus trackers that have accuracies of ± 5° can deliver greater than 99.6% of the energy delivered by the direct beam and 100% of the diffuse light. As a result, high accuracy tracking is not typically used.

Technologies Supported

The physics behind Standard Photovoltaic (PV) Trackers works with all standard photovoltaic module technologies. These include all types of crystalline silicon panels (monocrystalline, multicrystalline, polycrystalline) and all types of thin film panels (amorphous silicon, CdTe, CIGS, microcrystalline).

Concentrated Photovoltaic (CPV) Module Trackers

The optics in CPV modules accept the direct component of the incoming light and therefore must be oriented appropriately to maximize the energy collected. In low concentration applications a portion of the diffuse light from the sky can also be captured. The tracking functionality in CPV modules is used to orient the optics such that the incoming light is focused to a photovoltaic collector.

CPV modules that concentrate in one dimension must be tracked normal to the sun in one axis. CPV modules that concentrate in two dimensions must be tracked normal to the sun in two axes.

Accuracy Requirements

The physics behind CPV optics requires that tracking accuracy increase as the systems concentration ratio increases. In typical high concentration systems tracking accuracy must be in the ± 0.1° range to deliver approximately 90% of the rated power output. In low concentration systems, tracking accuracy must be in the ± 2.0° range to deliver 90% of the rated power output. As a result, high accuracy tracking systems are typically used.

Technologies Supported

Concentrated Photovoltaic Trackers are used with refractive and reflective based concentrator systems. There are a range of emerging photovoltaic cell technologies used in these systems. These range from crystalline silicon based photovoltaic receivers to germanium based triple junction receivers.

Tracker Types

Photovoltaic trackers can be grouped into classes by the number and orientation of the tracker’s axes. Compared to a fixed mounting, a single axis tracker increases annual output by approximately 30%, and a dual axis tracker an additional 6%.

Single Axis Trackers

Single axis trackers have one degree of freedom that acts as an axis of rotation. The axis of rotation of single axis trackers is typically aligned along a true North meridian. It is possible to align them in any cardinal direction with advanced tracking algorithms.

There are several common implementations of single axis trackers. These include Horizontal Single Axis Trackers, Vertical Single Axis Trackers, and Tilted Single Axis Trackers. The orientation of the module with respect to the tracker axis is important when modeling performance.

The axis of rotation for Horizontal Single Axis Tracker is horizontal with respect to the ground. The posts at either end of the axis of rotation of a Horizontal Single Axis Tracker can be shared between trackers to lower the installation cost.

Field layouts with Horizontal Single Axis Trackers are very flexible. The simple geometry means that keeping all of the axis of rotation parallel to one another is all that is required for appropriately positioning the trackers with respect to one another.

In addition, with backtracking, they can be packed at any density without shading.

Horizontal Trackers typically have the face of the module oriented parallel to the axis of rotation. As a module tracks, it sweeps a cylinder that is rotationally symmetric around the axis of rotation.

Several manufacturers can deliver single axis horizontal trackers. In these, a long horizontal tube is supported on bearings mounted upon pylons or frames. The axis of the tube is on a North-South line. Panels are mounted upon the tube, and the tube will rotate on its axis to track the apparent motion of the sun through the day.

Vertical Single Axis Tracker (VSAT)

The axis of rotation for Vertical Single Axis Trackers is vertical with respect to the ground. These trackers rotate from East to West over the course of the day.

Field layouts must consider shading to avoid unnecessary energy losses and to optimize land utilization. Also optimization for dense packing is limited due to the nature of the shading over the course of a year.

Vertical Single Axis Trackers typically have the face of the module oriented at an angle with respect to the axis of rotation. As a module tracks, it sweeps a cone that is rotationally symmetric around the axis of rotation.

Tilted Single Axis Tracker (TSAT)

All trackers with axes of rotation between horizontal and vertical are considered Tilted Single Axis Trackers. Tracker tilt angles are often limited to reduce the wind profile and decrease the elevated end’s height off the ground.

Field layouts must consider shading to avoid unnecessary losses and to optimize land utilization.

With backtracking, they can be packed without shading perpendicular to their axis of rotation at any density. However, the packing parallel to their axis of rotation is limited by the tilt angle and the latitude.

Tilted Single Axis Trackers typically have the face of the module oriented parallel to the axis of rotation. As a module tracks, it sweeps a cylinder that is rotationally symmetric around the axis of rotation.

Polar Aligned Single Axis Trackers (PASAT)

One scientifically interesting variation of a Tilted Single Axis Tracker is a Polar Aligned Single Axis Trackers (PASAT). In this particular implementation of a Tilted Single Axis Tracker the tilt angle is equal to the latitude of the installation. This aligns the tracker axis of rotation with the earth’s axis of rotation. These are rarely deployed because of their high wind profile.

Dual Axis Trackers

Dual axis trackers have two degrees of freedom that act as axes of rotation. These axes are typically normal to one another. The axis that is fixed with respect to the ground can be considered a primary axis. The axis that is referenced to the primary axis can be considered a secondary axis.

There are several common implementations of dual axis trackers. They are classified by the orientation of their primary axes with respect to the ground. Two common implementations are Tip - Tilt trackers and Azimuth-Altitude trackers.

The orientation of the module with respect to the tracker axis is important when modeling performance. Dual Axis Trackers typically have modules oriented parallel to the secondary axis of rotation.