Solar tracking systems
Concentrated photovoltaic as well as concentrated solar power systems make use of sun tracking technologies. Some of these tracking control systems are based on CAN networks connecting motors and sensors.
Sun tracking is nothing new: Sunflowers do it, moving their heads in the direction of the sun. Engineers have adapted this for solar power systems and photovoltaic panels. Following the sun improves efficiency, transforming sunlight into electricity. You can gain between 20 % and 45 % more output depending on the chosen sun-tracking technique and the geographical location. Nowadays, sun-tracking algorithms are well known and often come with off-the-shelf host controllers. Communication between sensors in the photovoltaic panels or the solar parabolic trough (SPT) and the host controller is typically done via CAN networks. Of course, the same network is used to communicate commands to the motion controllers and receive their status information. Sun tracking is a slow-motion application, although the sun moves very fast. But observed from Earth, it is a slow motion that does not require high-speed communication in the range of multiple Mbit/s.
Unlike traditional photovoltaic panels which are fixed in position and pointed in the general direction of the sun, concentrated solar power (CSP) and concentrated photovoltaic (CPV) system focus a large amount of sunlight onto a much smaller area. In CSP systems, this focusing is used to create steam, which then drives a turbine to generate electricity. In the case of CPV systems, small but highly efficient PV cells are placed at the focal point to generate electricity directly.
At the beginning of this year, the Noor-1 CSP in Morocco started operating. It is one of the largest solar power systems in the world. The follow-up Noor-2 and Noor-3 systems are planned for 2017. In contrast to the other systems, the Noor-4 plant will run on photovoltaic technology. Noor-1 uses parabolic troughs that follow the sun and has a capacity of 160 MW. Most sun-tracking systems are installed in large plants.
AEG offers for example the Project 3 and Project 4 CSP control systems featuring embedded CAN networks. They are available as stand-alone systems or can be integrated into application-specific control systems. Intercontrol, well-know as a mobile machine supplier, offers the MTC-CSP CAN-based controller, which is able to control two parabolic troughs. The controller comes with CANopen software support. The company also provides the RD40CSP drive units. All these products need to work in harsh environments or in other words: outdoor. They require a high IP-rate (IP65 and higher). The products have been in use in South Australia since 2012, for example.
The CANopen interface for sun-tracking units in PV systems has been standardized in the CiA 437 application profile. It specifies all necessary parameters and process data including night azimuth, storm azimuth, and snow drop azimuth plus elevation. Of course, tracking commands and drive status are standardized, too. The general functionality can also be used for SP systems. When sun-tracking units are used as stand-alone CANopen devices, an appropriate device profile needs to be extracted from the CiA 437 specification.
Following the sun can be achieved with a one-axis solution. A dual-axis approach is more efficient, but also more costly. Most suppliers offer both options. Framo Morat has developed two-axis sun-trackers for PV systems. The east-west alignment, following the path of the sun, is described as the azimuth adjustment, and the vertical alignment according to the position of the sun above the horizon is called the elevation adjustment. The azimuth adjustment features a rotational clearance of two angular minutes. This adjustment is achieved by means of self-locking worm gears. The accuracy of the vertical alignment is specified with 0,1 mm/1000 mm. The lifting power is up to 10 t. For PV systems using thin-layer modules, the company also offers single-axis controllers with absolute encoders connected to the embedded CAN network.
Framo Morat also supports SP systems that use parabolic channel connectors, tower plants with heliostats, or Fresnel collectors. Fresnel collectors are similar to parabolic troughs, but do not consist of one unit but of flat reflector panels whose slope is adjusted towards the sun by means of a single-axis tracker. They are simple in design and thus cheaper. However, the tracking software is more complicated. The Fresnel tracking system by Framo Morat comprises a 5-level brushless DC motor with a gear ratio of 38000:1, an integrated motion controller with a 17-bit absolute encoder connectable to CAN, and one drive which drivels a mirror area of 60 m2.
The decision between encoders and inclinometers is not even a question any more. Of course, you can use both kinds of position measurement techniques, but inclinometers make the system designers’ life easier, stated the Pewatron company, which is part of the Angst + Pfister Group. As is the case with almost all complex systems, solar trackers have conflicting requirements. The sensor used in the trackers has to be accurate. It also needs to withstand extreme weather-conditions for twenty years or more with no maintenance required. At the same time, it needs to be low cost, which is in opposition to the previous two requirements. The challenge is to find the sensor that has the best balance of these requirements.
For single-axis trackers and for the elevation axis of dual-axis trackers, a few different options exist with regard to the kind of sensor that can be used. If a motor is used, a rotary encoder is a possible solution. If a linear actuator is used, then a linear encoder can be used. With both solutions, the control software has to have an algorithm to translate the rotation of the motor or the distance extended by the actuator to the tilt angle of the mirror being controlled. Checks have to be in place to compensate for backlash or other irregularities in the system mechanics. The simpler and truer feedback device is an inclinometer.
Inclinometers measure the angle of tilt with respect to gravity. When it is mounted on the side of the mirror, the controller receives the actual position of the mirror. This angle is independent of the kind of actuator used in the system and any irregularities in the mechanics are transparent to the controller. The control software now becomes much simpler. There is also added confidence in the controller because the feedback signal is taken from the output of the control loop as opposed to from within the system.
Pewatron has developed the T7 absolute inclinometer with a CAN interface. With the CAN connectivity, multiple sensors can be daisy-chained to a single host controller, which reduces the amount and complexity of wiring involved. Up to 64 inclinometers can be networked on one cable that can be up to about 200 m in length.
The product features a resolution of 0,01° and is accurate to 0,1° in a temperature range from 0 °C to 70 °C. The product’s accuracy holds true over the full 360° range. Often, an inclinometer’s accuracy is valid for only a limited range of angles. Because the T7 has no such limitation, its orientation when mounted is not critical since the reported angle can be configured via software after it is mounted. There is also no limitation on the range of movement of the tracker when using the T7.
Inclinometers are typically either a mechanical or solid-state device. Mechanical inclinometers have some sort of weighted pendulum to reference gravity. So when the inclinometer is tilted, the pendulum swings down and the amount it swings is the angle output of the sensor. This pendulum would have to be damped and also would be mounted with bearings. As a result of the friction on the bearings, there is a limit on how small a movement the inclinometer can detect. This limits the resolution and accuracy of the inclinometer.
Solid-state inclinometers, like the mentioned T7, have no moving parts. Therefore, they do not have any errors due to the limits described above. This makes them ideal for solar trackers where frequent small changes are made. For 0,1° movements, the differential error (DNL) is under 0,02°. During characterization of the T7 for movements of 0,1°, the DNL was measured. The error was analyzed and a cumulative distribution showed that the DNL error is 0,01° with a 90-percent confidence interval. The maximum error is under 0,02°. While some are better than others, most solid-state inclinometers drift over time. The T7 on the other hand has an exceptionally low drift according to the supplier. Several products have been permanently mounted and show no measurable drift during life testings.
Renewable energy is on the move. There are many innovative and new ideas being developed. Obviously, you can combine a PV system with wind power systems. IMK, an Austrian company, has developed the PV-Wind tracker combining a concentrated PV system and a wind turbine. It can be operated as a hybrid system as a PV system alone or with a vertical wind turbine. The solar panels follow the sun. The 16 adjusting drives communicate via the embedded CAN network. Night and bad weather positions are configurable. The azimuth angle motor is equipped with a gear for mounting on PV modules. The PV system has a total size up to 45 m2 including the wind turbine on it. The quiet (35 dB) vertical wind turbine can generate 3 kW or 5 kW.
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