Single-axis and dual-axes solar trackers move PV panels to follow the sun
Solar PV panels operate at their greatest conversion efficiency when the incoming light rays strike the panel’s surface perpendicular to the plane of the panels. Considering that the sun is a constantly moving light source, this would occur only once per day with a stationary installation! However, a mechanical system, known as a solar tracker, can be used to constantly move the PV panel to directly face the sun. Solar trackers typically increase the power generated by a solar array from 20% to as much as 40%.
There are many variants of solar tracker designs, involving different approaches and technologies for moving the PV panels to follow the sun. Fundamentally, however, solar trackers can be divided into two basic types: single-axis and dual axis.
A few typical single-axis designs include:
Typical dual-axes designs include:
Defining the motion of a tracker to follow the sun can be crudely accomplished using “open loop” controls. These controls calculate the sunrise to sunset movement of the sun based on the time of year and geographic latitude of the installation, and develop a motion program to move the PV array accordingly. However, environmental loads (wind, snow, ice, etc.) and accumulated positioning errors make open-loop systems less desirable (and less accurate) over time. There’s just no guarantee that the tracker is actually pointed where the controls think it should be.
Using position feedback improves the tracking accuracy and helps ensure that the solar array is actually positioned where the controls dictate it should be, based on time of day and time of year, and especially after meteorological events involving strong winds, snow, and ice.
Obviously, the tracker’s design geometry and motion mechanics will help determine the best solution for position feedback. Five different sensing technologies can be employed to provide position feedback on solar trackers. I’ll briefly describe each with its unique advantages.
1. Inclination Sensors
Directly mounted to the PV array, they provide direct feedback on the “tilt” of the array with respect to the horizon. Inclination sensors are great for single-axis trackers like those shown in figures a and b above, or for the “elevation” axis position on trackers shown in figures d, e, and f. Obviously, an inclination sensor would have no value on a vertical-axis tracker like figure c. Absolute position is retentive through power loss --- the inclination sensor will accurately report tilt angle upon restoration of power.
2. Proximity Sensors
These are used to count gear teeth on elevation jackscrews or rotary slew rings. Depending on the specific design of the motion actuators, mounting can range from trivial to problematic. The position data (pulse count) must be maintained in the controller, as the prox sensor itself has no knowledge of angular or rotary position. Thus, the sensor does not provide absolute position --- it only reports incremental motion based on sensing the presence / absence of targets. Despite these shortcomings, proximity sensors are one of the most cost-effective solutions for many tracker applications.
3. Rotary Encoders
These sense and measure rotation of drive motors or motor-driven linear actuators, and usually need to be closely integrated into the design of the actuator itself. (Rotary encoders would not be a good option for a hydraulic cylinder-driven linear actuator, for example.) Absolute, multi-turn rotary encoders can provide retentive, absolute position data even through power loss, and can be applied to either the elevation or rotational axes of any of the tracker types shown above.
4. Inductive Rotary Position Sensors
Position sensors directly mount to a rotating component of the tracker’s elevation axis to sense rotational position. They are ideal for single-axis trackers like those shown in figures a and b above, or for the “elevation” axis on trackers shown in figures d, e, and f. These sensors are retentive through power loss, and usually provide an analog current or voltage value in proportion to the rotational position.
5. Ultrasonic Sensors
Capable of measuring relatively long distances, ultrasonic sensors can be mounted on a tracker frame and provide distance feedback between the sensor and a fixed target mounted on the ground or tracker base. The inclination angle of the solar array can be easily determined using this measured distance and a little high-school geometry. The ultrasonic sensor approach also provides accurate absolute position information upon restoration of power.