There are many factors to consider when selecting a linear actuator for solar tracking application. Depending on the application, they not only have to provide strong and reliable operation under exposed conditions, but they have to be accurate as well. When a storm comes your way, the main thing protecting your tracker's PV investment is the actuator arm stabilizing the array against the wind. A broken actuator arm or mount can result in damaged PV panels or loss of the entire array.
With many different options available how do you know what is the right actuator to use? Home CSP carries standard and heavy duty satellite dish actuators (1.5-3m dish size), as well as purpose built solar actuators. Some other sites show automotive and furniture type actuators, which sometimes seem attractive vs more expensive satellite dish or purpose-built actuators. Let's look at some import features to look for and factors to consider in your selection.
- Weather rating - from freezing cold to baking in the sun, actuators have to function in a wide range of temperatures as well as survive wind, rain, and humidity. Most non-purpose built actuators will fall short with this.
- Limit Switches - while almost all actuators feature internal limit switches, not all are adjustable. Unless you are purpose building your mount to the actuator, adjustable limit switches are a requirement.
- Mounting style - many generic actuators mount at the arm end, and a foot at the motor/gearbox end, while most satellite dish style actuators feature a so-called saddle-clamp mount. The saddle clamp can be positioned anywhere along the length of the actuator tube, making them ideal for adapting to most mounts. This also creates a shorter minimum arm length (distance from arm end to mount point when fully retracted) which can greatly simplify mount design.
- Stroke length - increased stroke length means that the control arm radius (distance from the arm end to the axis of rotation) can also be increased. This is good because it increases the leverage and typically increases resolution as well.
Even with a long stroke actuator, the actuator arm is going to be at a serious disadvantage against the wind with most PV arrays. Calculating actual wind load is complicated, as it depends on many factors including how far the panels are arranged from the axis of rotation, but worst case estimates can be made. A 90mph wind exerts about 25lb/sq.ft. of force. A 200 sq.ft. array (14x14) can experience up to 25lb/sqft x 200sqft/2 x 7ft/2 = 8750 ft.lb. of torque in 90 mph winds. With less than a 9" control radius from an 18" actuator, that number becomes nearly 12,000 lb., while an 18" control radius from a 36" stroke actuator takes it down under 6,000 lb dynamic load.
- Load capacity - is typically listed as either dynamic or static load rating. Dynamic load rating refers to the load while the motor is under operation and represents the actuators capacity to push or pull. Static load ratings are usually significantly higher than the dynamic load rating, and represents the maximum load capacity before the actuator arm will be physically damaged. Load capacity is usually shown with an N which denotes Newtons as the unit of force. 1N = 0.22481 lb of force.
Small 1.5m satellite dish actuators (HARL 3618, etc) have a dynamic load capacity of 2450N (550lb) which is more than most generic linear actuators. Their static load rating of 5390N (1200lb) far exceeds generic actuators, but is still only suitable for arrays of up to about 4 panels under heavy storm conditions.
The QARL HD actuators feature a 10,000N rating which is recommended for approximately 100 sq.ft. of array.
- Speed - solar trackers don't need to move fast. The larger the array, the slower you'll want it to move so things don't shake around. Slow also saves energy, which is what we're trying to make, not consume in the process of making. Typical speeds for tracking applications are measured in mm/sec. Full arm extension/retraction should take at least a minute or more
- Position feedback - digital tracking applications require feedback from the actuator to help calculate the array position. Light sensor based trackers generally do not require position feedback and instead rely on limit switches.
The most common feedback mechanism is a reed-switch based pulse signal. As a magnet connected to the gear system rotates, it causes a sensitive magnetic reed switch to open and close, which creates a pulse that can be monitored by a microprocessor. Hall effect sensors are also sometimes used, as well as potentiometers.
- Operating voltage and amperage - most DC actuators operate at either 12, 24 or 36V. Smaller actuators can draw as little as 1 amp; however there is no reason to accept a larger actuator that draws more than 10A, as our largest capacity actuator draws half of that. Beware of "nominal" 12V conditions: battery, charging, and PV side. So called 24V systems usually operate at 25.4 with fully charged batteries, and charging voltage can easily reach 28.2 or more. Unregulated PV output is several volts higher and may exceed motor driver voltage ratings. Select your actuators with a safe power source in mind.