Solar tracker

A backyard installation of passive single–axis trackers, DC rated at 2340 watts. Seen here in winter position, tilted toward the south. The tall poles allow walk-under and use of the ground space underneath the panels for plantings that thrive on protection from the severe summer midday sun at this location.

A solar tracker is a device for orienting a solar photovoltaic panel or concentrating solar reflector or lens toward the sun. Concentrators, especially in solar cell applications, require a high degree of accuracy to ensure that the concentrated sunlight is directed precisely to the powered device, which is at the focal point of the reflector or lens. Non-concentrating applications require less accuracy, and a tracker is not necessary, but can substantially improve the amount of power produced by a system by enhancing morning and afternoon performance. Strong afternoon performance is particularly desirable for grid-tied photovoltaic systems, as production at this time will match the peak demand time for Summer season air-conditioning. A fixed system oriented to optimize this limited time performance will have a relatively low annual production. For low-temperature solar thermal applications, trackers are not usually used, owing to the relatively high expense of trackers compared to adding more absorber area and the more restricted solar angles required for Winter performance, which will often determine system capacity. Compared to photovoltaics, trackers are relatively inexpensive, thus making them especially effective for photovoltaic systems using high-efficiency panels. Some solar trackers may operate most effectively with seasonal position adjustment and most will need inspection and lubrication on an annual basis.

Tracker mount types

Solar trackers may be active or passive and may be single axis or dual axis. Single axis trackers usually use a polar mount for maximum solar efficiency. Single axis trackers will usually have a manual elevation (axis tilt) adjustment on a second axis which is adjusted on regular intervals throughout the year. There are two types of dual axis trackers, polar and altitude-azimuth.

Polar

Polar trackers have one axis aligned close to the axis of rotation of the earth, hence the name polar. Only high accuracy astronomical telescope mounts actually use this alignment. So–called "polar" trackers are actually aligned to a perpendicular to the ecliptic, an imaginary disc containing the apparent path of the sun. Simple solar trackers are manually adjusted to compensate for the shift of the ecliptic through the seasons, usually at least twice a year at the equinoxes to establish a position for Autumn and Winter, and another for Spring and Summer. Such trackers are also referred to as "single axis" as only one drive mechanism is needed for daily operation, and so reducing cost or allowing the use of passive tracking methods (described below).

Horizontal axle

Several manufactures can deliver single axis horizontal axis trackers which may be oriented by either passive or active mechanisms, depending upon manufacturer. In these, a long horizontal tube is supported on bearings mounted upon pylons or frames. The axis of the tube is on a North-South line. Panels are mounted upon the tube, and the tube will rotate on its axis to track the apparent motion of the sun through the day. Since these do not tilt toward the equator they are not especially effective during winter mid day (unless located near the equator), but add a substantial amount of productivity during the spring and summer seasons when the solar path is high in the sky. These devices are less effective at higher lattitudes. The principal advantage is the inherent robustness of the suporting structure and the simplicity of the mechanism. Since the panels are horizontal, they can be compactly placed on the axle tube without danger of self-shading and are also readily accessible for cleaning. For active mechanisms, a single control and motor may be used to actuate multiple rows of panels. Manufacturers include Array Technologies, Inc. Wattsun Solar Trackers, (gear driven active), Zomeworks (passive) and Powerlight[1] (active).

Point focus parabolic dish with Stirling system. The horizontally rotating azimuth table mounts the vertical frames on each side which hold the elevation trunions for the dish and its integral engine/generator mount.

Vertical axle

A single axis tracker may be constructed that pivots only about a vertical axle, with the panels either vertical or at a fixed elevation angle. Such trackers are suitable for high lattitudes, where the apparent solar path is not especially high, but which leads to long days in Summer, with the sun traveling through a long arc. This method has been used in the construction of a cylindrical house in Germany that rotates in its entirety to track the sun, with vertical panels mounted high on one side of the building[2].

Altitude-azimuth

Two-axis mount

Restricted to active trackers, this mount is also becoming popular as a large telescope mount owing to its structural simplicity and compact dimensions. One axis is a vertical pivot shaft or horizontal ring mount, that allows the device to be swung to a compass point. The second axis is a horizontal elevation pivot mounted upon the azimuth platform. By using combinations of the two axis, any location in the upward hemisphere may be pointed. Such systems may be operated under computer control according to the expected solar orientation, or may use a tracking sensor to control motor drives that orient the panels toward the sun. This type of mount is also used to orient parabolic reflectors that mount a Stirling engine to produce electricity at the device[3].

Multi-mirror reflective unit

Energy Innovations test units

A recent development, this device uses multiple mirrors in a horizontal plane to reflect sunlight upward to a high temperature photovoltaic or other system requiring concentrated solar power. Structural problems and expense are greatly reduced since the mirrors are not significantly exposed to wind loads. Through the employment of a patented mechanism, only two drive systems are required for each device. Because of the configuration of the device it is especially suited for use on flat roofs and at lower lattitudes. Limited commercial availability is expected in 2007 with availability through regular distribution channels in 2008. The units illustrated each produce approximately 200 peak DC watts.

Drive types

Active trackers

Active trackers use motors and gear trains to direct the tracker as commanded by a controller responding to the solar direction. Devices of this type are manufactured by Wattsun and others.

Active two-axis trackers are also used to orient heliostats - movable mirrors that reflect sunlight toward the absorber of a central power station. As each mirror in a large field will have an individual orientation these are controlled programmatically through a central computer system, which also allows the system to be shut down when necessary.

Passive trackers

Zomeworks passive tracker head in Spring/Summer tilt position with panels on light blue rack pivoted to morning position against stop. Dark blue objects are hydraulic dampers. Click image for additional information.

Passive trackers use a low boiling point compressed gas fluid that is driven to one side or the other (by solar heat creating gas pressure) to cause the tracker to move in response to an imbalance. As this is a non-precision orientation it is unsuitable for certain types of concentrating photovoltaic collectors but works fine for common PV panel types. These will have viscous dampers to prevent excessive motion in response to wind gusts. Shader/reflectors are used to reflect early morning sunlight to "wake up" the panel and tilt it toward the sun, which can take nearly an hour. The time to do this can be greatly reduced by adding a self-releasing tiedown that positions the panel slightly past the zenith (so that the fluid does not have to overcome gravity) and using the tiedown in the evening. (A slack-pulling spring will prevent release in windy overnight conditions.) This type of tracker is supplied by Zomeworks.

External links

• Knowing Sun Tracker, on Solar Energy Blog – Discusses the prospects of using Sun tracker for solar panels.
• An index of tracker manufacturers
• A whole house with photovoltaic turning after the sun
• Energy Innovations Sunflower 250 multi-mirror system
• Stirling Energy and Sandia Laboratories joint development press release
• Solar Concentrators - Pointfocus: Cutting Edge Information on Solar Concentrators, Ring Arrays, Slat Arrays, and Power Towers.