Taking into consideration of all the reviewed sun-tracking methods, sun trackers can be grouped into one-axis and two-axis tracking devices. Fig. 1 illustrates all the available types of sun trackers in the world. For one-axis sun tracker, the tracking system drives the collector about an axis of rotation until the sun central ray and the aperture normal are coplanar. Broadly speaking, there are three types of one-axis sun tracker:
1. Horizontal-Axis Tracker – the tracking axis is to remain parallel to the surface of the earth and it is always oriented along East-West or North-South direction.
2. Tilted-Axis Tracker – the tracking axis is tilted from the horizon by an angle oriented along North-South direction, e. g. Latitude-tilted-axis sun tracker.
3. Vertical-Axis Tracker – the tracking axis is collinear with the zenith axis and it is known as azimuth sun tracker.
Fig. 1. Types of sun trackers
In contrast, the two-axis sun tracker, such as azimuth-elevation and tilt-roll sun trackers, tracks the sun in two axes such that the sun vector is normal to the aperture as to attain 100% energy collection efficiency. Azimuth-elevation and tilt-roll (or polar) sun tracker are the most popular two-axis sun tracker employed in various solar energy applications. In the azimuth-elevation sun-tracking system, the solar collector must be free to rotate about the azimuth and the elevation axes. The primary tracking axis or azimuth axis must parallel to the zenith axis, and elevation axis or secondary tracking axis always orthogonal to the azimuth axis as well as parallel to the earth surface. The tracking angle about the azimuth axis is the solar azimuth angle and the tracking angle about the elevation axis is the solar elevation angle. Alternatively, tilt-roll (or polar) tracking system adopts an idea of driving the collector to follow the sun-rising in the east and sun-setting in the west from morning to evening as well as changing the tilting angle of the collector due to the yearly change of sun path. Hence, for the tilt-roll tracking system, one axis of rotation is aligned parallel with the earth’s polar axis that is aimed towards the star Polaris. This gives it a tilt from the horizon equal to the local latitude angle. The other axis of rotation is perpendicular to this polar axis. The tracking angle about the polar axis is equal to the sun’s hour angle and the tracking angle about the perpendicular axis is dependent on the declination angle. The advantage of tilt-roll tracking is that the tracking velocity is almost constant at 15 degrees per hour and therefore the control system is easy to be designed.