Tracking systems are included in more than one-third of new photovoltaic developments in Europe, but less than 10% in the US. If properly controlled, they can capture more energy from each panel than fixed racks – up to 40% more in most parts of North America – so selecting the best hardware and the right control algorithm is critical to realizing optimum energy production and reliability.
Key requirements for control systems are cost, reliability, and energy productivity. Energy productivity can be defined as the number of kilowatt-hours produced by each panel (or each kilowatt of panels). Photovoltaic panels carry power ratings – typically 200 watts to 400 watts per panel – based on standard testing methods1. Control systems that enable effective tracking of the sun can produce significantly more energy, as shown in Figure 1.
This article considers flat-plate photovoltaic arrays, which should be controlled to within about 1 degree of the optimum orientation east-west and north-south. Concentrated PV technology requires much higher precision tracking.
Hardware for control systems includes printed circuits, connectors, weatherproof enclosures, and cables – all commercial parts or custom components made with common and inexpensive technologies. Thus cost considerations are generally less critical than reliability and energy productivity.
For large systems, hardware costs are typically less than $0.04 per peak watt, compared with $2.00 to $5.00 for all system costs combined. Systems less than 10 kilowatts may have control hardware costs about $0.10 per watt.
Similarly, control systems for photovoltaic tracking systems can be held to high standards of reliability, because they can be protected from extreme weather and generally do not carry high current. Care is taken to use best practices, such as de-rating of components, and sealing of junction boxes against weather. Cables which are exposed to sunlight must be chosen for life under ultraviolet exposure, or shrouded with uv-resistant material.
Types of Mounting Systems
The primary difference among control systems is in energy production, and it depends largely on the type of mechanical motion. The National Renewable Energy Laboratory’s PVWatts database2 defines the following array types:
Horizontal fixed racks: panels mounted on fixed racks and held horizontal at all times. This type does not track the sun and hence requires no tracking control system.
Tilted fixed racks: panels mounted on fixed racks which are tilted toward the south, typically at an angle approximately equal to the latitude of the site. This arrangement is typically about 20% more productive than fixed horizontal racks, and requires no tracking control system.
Horizontal one-axis tracking systems: panels tracking the sun east-west through the day but fixed at a tilt of 0 degrees. This arrangement is typically 40% more productive than fixed horizontal racks, and requires a control system for the east-west motion.
Tilted one-axis tracking systems: panels tilted up toward the south (in the northern hemisphere), typically at an angle approximately equal to the latitude, and tracking the sun east-west through the day. Systems with this configuration typically produce about 60% more energy than horizontal fixed racks, and have control requirements the same as horizontal one-axis tracking systems.
Two-axis tracking systems: panels track the sun east-west through the day and north-south through the seasons, producing typically 70% more energy than horizontal fixed racks (Figure 2). The control system is more complex than one-axis systems but typically not much more expensive.
a. Upright-pole two-axis tracking systems typically pivot in the azimuth direction, requiring a rotational control moving the panels from east to west through the day and a linear or rotational control adjusting the tilt of the panels through the seasons.
b. Rail tracking systems use linear controls to adjust the orientation of the panels in both the east-west direction and the north-south direction.