Pierre Nadeau

Solar irradiance, or the intensity of the light reaching the Earth, is dependent on the sun’s angle of elevation. Solar radiation is always at highest on a plane that is perpendicular to the sun’s ray. As the azimuth and the elevation angle change throughout the day and the year, the incidence angle of the solar radiation varies constantly on given areas. Orienting panels to keep them facing the sun can achieve significant energy gains in comparison of any fixed position. Gains of 50% during summer and 300% during winter have been mentioned for a comparison between tracked and an horizontal planes¹⁾. It is interesting to realise that rarely panels will be installed on horizontal plane. An elevation angle near 35^o is normally used in Kingston area. Despite the fact that percentage gains appear lower during summer than winter, the yield increase is predominant during the summer period of the year (area under the curve on Fig. 1).

Fig. 2: Single Axis trackers

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%. A tracking device is more expensive than a fixed mounting rack. It requires an adjustable upright that can withhold larger wind pressure during storms. It can either be equipped with an electric drive or be moved by thermohydraulics means. Thermohydraulic systems are based on the principle of heating liquids and the resulting difference in pressure.

In general, the higher energy yield of tracked PV systems cannot compensate for the higher investment for a tracking device. However, more and more cheaper one-axis tracking devices are offered whose usage can make sense under certain conditions. In some cases the economic profitability of PV systems can be increased by using tracking devices when a higher price is paid for the energy produced. Outward appearance and public relations effects are, apart from the obvious increased yield, arguments in favour of such a device.²⁾

Polar trackers have one axis aligned to be roughly parallel to the axis of rotation of the earth around the north and south poles. 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. The polar axis should be angled towards due north, and the angle between this axis and the horizontal should be equal to your latitude.

Simple polar trackers with single axis tracking may also have an adjustment along a second axis: the angle of declination. This allows to angle the panel to face the sun when it is higher in the sky during summer, and to face it lower in the sky during winter. It might be set with manual or automated adjustments, depending on the polar-tracking device. If one is not planning on adjusting this angle of declination at all during the year, it is normally set to zero degrees, facing your panel straight out perpendicular to the polar axis, as that is where the mean path of the sun is found. Occasional or continuous adjustments to the declination compensate for the northward and southward shift in the sun's path through the sky as it moves through the seasons over the course of the year. When the manual method is used for adjustment of the declination, it should be done at least twice a year: Once at the autumnal equinox to establish the best position for the winter, and a second adjustment on the vernal equinox, to optimize it for the summer.

In horizontal axis trackers, a long horizontal tube is supported on bearings mounted upon pylons or frames. They may be oriented by either passive or active mechanisms. 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. 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.

Fig. 3: Double axis tracker

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

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.

Fig. 4: 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. More details about the technology behind this tracking system can be found in «How the Track Rack Follows the Sun» section of this actual link.

• The cost of the Tracker.

• The extra energy gained by tracking. It is not enough to say that the panel(s) may put out twice as much power under given circumstances. It is the accumulated amp-hours (amps times hours) over the course of the day that determines your daily gain.

• The gain is not consistent throughout the year. The greatest gain is usually in summer when the hours between sunrise and sunset are the longest and the sun sweeps its greatest arc across the sky. If this potential for an increased gain in summer also coincides with a wet season or consistently overcast weather then the actual gain may be very little or nothing at all.

• The gain is also dependent on latitude. At increased latitudes the sun's arc across the sky in summer is also increased but in winter it is decreased.

• Solar energy usually has its greatest strength in the middle of the day and often the greatest cloud cover is in the mornings and evenings. The energy from the sun has to penetrate through the greatest depth of atmosphere at the horizon.

• Immediate environment and geographic location may play a major role in the hours of direct sunlight (without shading) that the panels may receive. Nearby mountains, hills, trees, tall buildings etc may considerably reduce the number of hours of direct sunshine that your panels receive.

• Shadow throwing objects tend to have their greatest effect on a solar panel site when the sun is lowest in the sky.

• After having determined how much gain you could expect in mid summer and mid winter you may find that you get the most benefit when you least need it. An automatic solar tracking system usually costs more than a 75W solar panel. Unless your loads are predominantly summer loads (eg fridge, freezer, space cooling, pumping) and you already have at least 8 solar panels, you may be better off with another solar panel. An extra solar panel gives greater benefit in mid winter when there is the greatest demand for night time lighting and entertainment. If you need more power in winter for lighting and entertainment because of the shorter daylight hours then an extra solar panel may be money better spent than having a tracking device. ³⁾

• www.treehugger.com
• Array Technologies