3.5. Engineered devices for solar tracking
The main elements of a tracking system include [Rockwell Automation, 2011]:
- Sun tracking algorithm: This algorithm calculates the solar azimuth and zenith angles of the sun. These angles are then used to position the solar panel or reflector to point toward the sun. Some algorithms are purely mathematical based on astronomical references, while others utilize real-time light-intensity readings.
- Control unit: The control unit executes the sun tracking algorithm and coordinates the movement of the positioning system.
- Positioning system: The positioning system moves the panel or reflector to face the sun at the optimum angles. Some positioning systems are electrical and some are hydraulic. Electrical systems utilize encoders and variable frequency drives or linear actuators to monitor the current position of the panel and move to desired positions.
- Drive mechanism/transmission: The drive mechanisms include linear actuators, linear drives, hydraulic cylinders, swivel drives, worm gears, planetary gears, and threaded spindles.
- Sensing devices: For trackers that use light intensity in the tracking algorithm, pyranometers are needed to read the light intensity. Ambient condition monitoring for pressure, temperature, and humidity may also be needed to optimize efficiency and power output.
- Limit switches are used to control speed and prevent over travels. The mechanical over travel limits are used to prevent tracker damage.
- Elevation feedback is accomplished by either 1) a combination of limit switches and motor encoder counts, or 2) an inclinometer (a sensor that provides the tilt angle).
- An anemometer is used to measure wind speed. If the wind conditions are too strong, the panels are usually driven to a safe horizontal position and remain in the safety position until the wind speed falls below the set point.
Three classes of tracker drive types to operate the moving receiver:
- Passive trackers use the sun's heat to expand the compressed gas, which is used to move the panel. Selective heating of some cylinders versus others create more expansion on one side of the panel and make it tilt. These systems are relatively simple and low-cost, although may lack due precision necessary for the solar conversion systems using concentrated sunlight.
- Active trackers use hydraulic or electric and an actuator to move the panel based on sensor response. Light sensors are positioned on the tracker at different locations for higher precision. These systems work best with the direct sunlight and are less efficient with cloudy skies.
- Open-loop trackers use pre-recorded data on the sun position for a particular site. Simple timed trackers move the panel at discrete intervals to follow the sun position, but do not take into account the seasonal variations of the sun's altitude. The altitude/azimuth trackers employ astronomical data to determine the position of the sun for any given time and location.
Linear actuators are common technical tools that proved to be effective solution for moving the solar receivers. An electric linear actuator is a device that converts the rotational motion of an electric motor into linear motion. With linear actuators you can lift, slide, adjust, tilt, push or pull objects of various mass, and they are easy to implement in many different applications. Mechanically, linear actuators are quite simple devices that have been extensively deployed in 2-axis and 1-axis trackers due to their precision and service reliabilty.
The following video provides a rather detailed overview of the design, principle of operation, and specifications of electric linear actuators:
The technical details of all the components of tracking systems would be beyond the scope of this course. It is important to understand though that additional components and more complexity, while improving efficiency of the solar panels and reflectors, add to the cost of the whole system and consume additional energy.
This following video (4:25) demonstrates some technical features of a single-axis tracking system:
THOMAS JENKINS: Here we have an example of something that might be at more of a commercial application of photovoltaic systems. We have several photovoltaic panels. We have two sets, we have one type on this structure, we have another type on the structure behind it. Both are mounted on what's called one-axis trackers, in that there are electric motors which turn these panels such that they track the sun as it goes from the east to the west.
With this type of tracker, it's fairly sophisticated in that it's a computer-controlled tracker. There is a little computer in here that runs some very sort of mid-complexity algorithm that knows the latitude of your location, where you are-- Las Cruces, New Mexico, 32 degrees latitude-- and it knows the day of the year-- for example, January 28, day 28-- and it knows the time of the day-- 2:00.
With that information, it can predict exactly the angle of the sun, relative to east and west, and it knows to turn the tracker exactly that many degrees every day to point directly to the sun. This increases the efficiency or the amount of electricity that comes from the solar panels, but you have some additional complexity in your system. These are what's called active trackers, in that it requires electrical motors, it requires some mechanical components, some electrical components, and it tracks the sun, but you get more electricity from this type of system.
This system is a German design, and it's being tested here at SWTDI. Right next to it, we have a good bit of data collection that is brand new, very sophisticated, and it's connected via cell phone and land lines such that all the data-- which is being collected real time-- can be accessed through a cell phone from anywhere in the world. For example, at the headquarters of the German company, who's looking at the system design and the system components and seeing how they're interacting.
You can see on this structure here that we have a couple of instruments that are being used to characterize the sun's energy. These are called pyranometers. They are reading the amount of sunlight so we know how much energy is striking the surface. And, from that, we can see how much energy the panels are delivering to us and determine the efficiency of the panels. So a lot of instrumentation is going in this because this is an evaluation system, but this might be a system that might go into, for example, a desert environment over several acres that might be a large scale electrical production.
Tracking the sun, in some cases, is very important, especially on systems that use a new type of system with lenses, called Fresnel lenses that are used to concentrate the sun onto a smaller section of photovoltaic, so the total amount of photovoltaics that you need is smaller, but they produce the same amount of energy because you're concentrating the sunlight onto the cell. So we're looking at two different types of modules on the same structures, under the same ambient conditions, the same location, the same amount of sunlight, and we're comparing those, seeing how efficient they are relative to one another and relative to traditional solar photovoltaic systems.
PRESENTER: The preceding was a production of New Mexico State University. The views and opinions in this program are those of the author and do not necessarily represent the views and opinions of the NMSU Board of Regents.
Journal paper: Mousazadeh, H. et al., A review of principle and sun-tracking methods for maximizing solar systems output, Renewable and Sustainable Energy Reviews 13 (2009) 1800–1818.