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Solar tracker

A solar tracker is a device for orienting a solar photovoltaic panel or concentrating solar reflector or lens toward the sun. The sun's position in the sky varies both with the seasons (elevation) and time of day as the sun moves across the sky. Solar powered equipment works best when pointed at or near the sun, so a solar tracker can increase the effectiveness of such equipment over any fixed position, at the cost of additional system complexity. There are many types of solar trackers, of varying costs, sophistication, and performance. One well-known type of solar tracker is the heliostat, a movable mirror that reflects the moving sun to a fixed location, but many other approaches are used as well.

The required accuracy of the solar tracker depends on the application. 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 (or near) the focal point of the reflector or lens. Typically concentrator systems will not work at all without tracking, so at least single-axis tracking is mandatory.

Non-concentrating applications require less accuracy, and many work without any tracking at all. However tracking can substantially improve both the amount of total power produced by a system and that produced during critical system demand periods (typically late afternoon in hot climates) The use of trackers in non-concentrating applications is usually an engineering decision based on economics. Compared to photovoltaics, trackers can be relatively inexpensive. This makes them especially effective for photovoltaic systems using high-efficiency (and thus expensive) panels.

For low-temperature solar thermal applications, trackers are not usually used, owing to the relatively high expense of trackers compared to adding more collector area and the more restricted solar angles required for Winter performance, which influence the average year-round system capacity.

Maintenance

Some solar trackers may operate most effectively with seasonal position adjustment and most will need inspection and lubrication on an annual basis. As most trackers are made from mild steel, maintenance of paint is typically required, and may be critical in highly corrosive environments, such as near saltwater or in polluted industrial localities. In regions with extended Summer dry seasons the periodic washing of the panels may significantly increase performance at a critical demand time, particularly for grid-tied systems.

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. Compared to a fixed mount, a single axis tracker increases annual output by approximately 30%, and a dual axis tracker an additional 6%. There are two types of dual axis trackers, polar and altitude-azimuth.

Polar

Polar trackers have one axis aligned to be quasi-parallel to the axis of rotation of the earth, hence the name polar. Strictly speaking however, the only true polar trackers are used on high accuracy astronomical telescope mounts, which rotate on an axis exactly parallel to the earth's axis. For solar trackers, so called "polar" trackers have their axis aligned perpendicular to the "ecliptic" (an imaginary disc containing the apparent path of the sun). Single axis tracking is often used when combined with time-of-use metering, since 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.

Simple polar trackers with single axis tracking may be set initially to the mean ecliptic at the local latitude. Partial or continuous tracking of the second axis (the elevation angle) is possible with manual or automated adjustments respectively, in order to compensate for the shift of the ecliptic. When the manual method is used for axis adjustment, adjustments occur at least twice a year. Since the rate of change of the ecliptic is greatest during the equinoxes, the yearly tracking accuracy is greatest when axis adjustments are made at these two times. One adjustment is made on the autumnal equinox to establish a position for autumn and winter, and a second adjustment on the vernal equinox for spring and summer. Such trackers may also be referred to as a "single-axis tracker", because only one drive mechanism is needed for daily operation. This reduces the system cost and allows the use of simpler tracking methods, including passive and chronological tracking (described below).

Horizontal axle

Several manufacturers can deliver single axis horizontal 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 latitudes. The principal advantage is the inherent robustness of the supporting 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 (active).

Vertical axle

A single axis tracker may be constructed that pivots only about a vertical axle, with the panels either vertical, at a fixed, adjustable, or tracked elevation angle. Such trackers with fixed or (seasonably) adjustable angles are suitable for high latitudes, 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 Austria (latitude above 45 degrees north) that rotates in its entirety to track the sun, with vertical panels mounted on one side of the building

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.

Multi-mirror reflective unit

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 latitudes. While limited commercial availability was expected in 2007 the company has removed the descriptive web page from their site and is now promoting a two-axis clustered fresnel lens device. The units illustrated each produce approximately 200 peak DC watts.

Drive types

Active tracker

Active trackers use motors and gear trains to direct the tracker as commanded by a controller responding to the solar direction.

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.

Light-sensing trackers typically have two photosensors, such as photodiodes, configured differentially so that they output a null when receiving the same light flux. Mechanically, they should be omnidirectional (i.e. flat) and are aimed 90 degrees apart. This will cause the steepest part of their cosine transfer functions to balance at the steepest part, which translates into maximum sensitivity.

Since the motors consume energy, one wants to use them only as necessary. So instead of a continuous motion, the heliostat is moved in discrete steps. Also, if the light is below some threshold there would not be enough power generated to warrant reorientation. This is also true when there is not enough difference in light level from one direction to another, such as when clouds are passing overhead. Consideration must be made to keep the tracker from wasting energy during cloudy periods.

Passive tracker

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.)

The term "passive tracker" is also used for photovoltaic modules that include a hologram behind stripes of photovoltaic cells. That way, sunlight passes through the transparent part of the module and reflects on the hologram. This allows sunlight to hit the cell from behind, thereby increasing the module's efficiency. Also, the module does not have to move since the hologram always reflects sunlight from the right angle towards the cells.

Chronological tracker

A chronological tracker counteracts the earth's rotation by turning at an equal rate as the earth, but in the opposite direction. Actually the rates aren't quite equal, because as the earth goes around the sun, the position of the sun changes with respect to the earth by 360° every year or 365.24 days. A chronological tracker is a very simple yet potentially a very accurate solar tracker specifically for use with a polar mount (see above). The drive method may be as simple as a gear motor that rotates at a very slow average rate of one revolution per day (15 degrees per hour). In theory the tracker may rotate completely, assuming there is enough clearance for a complete rotation, and assuming that twisting wires are not an issue, such as with a solar concentrator, or the tracker may be reset each day to avoid these issues. Alternatively, an electronic controller may be used, with a real time clock that is used to infer the "solar time" (hour angle). Tracking adjustments can be made incrementally or continuously.

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References

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