Tracking the Sun by moving in prescribed ways to minimise the angle of incidence allow for increasing the incident radiation. Tracking mechanisms are often used in photovoltaics, mainly associated with relatively big grid-connected PV plants, where these mechanisms have already demonstrated very high reliability. As a particular example, the 100 kWp tracking system at the Toledo PV plant is in routine operation with 100% of availability from 1994, and about half of the 1 GWp installed in Spain during 2007 are mounted on some kind of tracker. Figure 22.20 shows some real examples. It must be mentioned that increased collection comes with an increased cost, because trackers are more expensive than static support structures. Roughly, it can be said that trackers will keep some economic sense meanwhile PV module cost remain larger than 1€/Wp.
Tracking about two axes (more often, a vertical axis to adjust the azimuth and a horizontal axis to adjust the tilt, but two other axis combinations can also be found, provided both are perpendicular) maintains the receiver surface always perpendicular to the sun (в = 0ZS; a = yS). Hence, it allows collecting the maximum amount of energy possible. Mainly depending on the clearness index, the comparison with an optimally tilted fixed surface leads to the ratio Gdy(2 axesyGd^^t) varying from 1.25 to 1.55 (column 5 divided by column 9 of Table 22.4).
However, it is expensive to implement, because it uses relatively complicated mechanisms and takes up a great deal of space, due to the shadows cast. For these reasons, several types of one-axis trackers are sometime preferred. Single-axis trackers follow the sun from East to West during the day. They differ in their tilt angle.
Azimuthal one-axis trackers rotate around their vertical axis, in such a way that the azimuth of the receiver PV surface is always the same as that of the sun’s azimuth. Meanwhile, the tilt angle keeps constant (fi = econs and a = t^S). The incidence angle is given by the difference between the surface’s tilt angle and the solar zenith angle (0S = 0ZS – вот). Obviously, the amount of collected radiation depends on the inclination of the surface, being the maximum for a value close to the latitude. Again, the sensitivity of the annual capture of energy to this inclination angle is relatively low. A typical value of approximately 0.4% loss from each degree of deviation from the optimum inclination can be assumed. Note that an azimuthal tracker tilted to the latitude collects up to 95% of the yearly irradiation compared to two-axes tracking (column 6 divided by column 5 of Table 22.4).
Trackers turning around a single axis oriented N-S and tilted at an angle eNS to the horizontal are also of great interest due to their mechanical robustness. It can be seen that, in order to minimise the solar incident angle, the rotation angle of the axis, t^NS – 0 at noon – must be
sin ю
cos ю cos в a — [sign(^)] tan S sin в a where вА = eNS – abs(Ф). The corresponding solar incident angle is given by
cos 0S = cos t^NS (cos S cos ю cos вА — [sign^)] sin S sin вА) + sin t^NS cos S sin ю
A common configuration, called polar tracking, is when the axis is inclined just to the latitude. Then, the rotation axis is parallel to the rotation axis of the Earth, and Equation (22.53) becomes reduced to 0S = S. Because of the variation of the declination during the year, the cosines of the solar incident angle range between 0.92 and 1, having an annual mean value of about 0.95. This way, the polar tracker also collects about 95% of the energy corresponding to the two-axes case (column 8 divided by column 5 of Table 22.4). It is interesting to note that a polar tracker turns at just half the angular speed as that of a standard clock.
Another common configuration is when the axis is just horizontal. Horizontal one-axis trackers are of particularly simple construction and do not cast shadows in the N-S direction. This encompasses significant radiation reduction when compared with two-axis tracking (column 7 divided by column 5 of Table 22.4), but still significant radiation increase when compared with optimally tilted fixed surfaces (column 6 divided by column 8 of Table 22.4). Because of this, they are today a common tracking solution in large PV plants: PVUSA [45], Toledo [38]. And the same is true for solar thermal plants. We should remember that the very first solar tracker used in any significant way for power generation was just a N-S-oriented horizontal one-axis tracker associated to a parabolic reflecting trough, constructed in 1912 by Frank Shumann and C. V. Boys to power a 45-kW steam-pumping plant in Meadi, Egypt [46]. The tracking surface covered an area of 1200 m2. The plant was a technical success, that is, reliable trackers already existed at the time, but it was shut down in 1915 due to the onset of World War I and cheaper fuel prices. The world’s largest solar plant, at the well-known Luz solar thermal field, erected in California from 1984 to 1986, also employs this type of tracking and, again, with great technical success [47]. The rotation angle (which, in this particular case, coincides with the surface tilt) and the solar incident angle are given by
and
cos 0S = cos S ■ [sin2 ю + (cos ф cos ю + tan S sin ф)2]1/2 (22.55)
Finally, it is worth commenting that large PV generators have many, even hundreds, of rows of modules mounted above the ground. The distance between rows affects the energy produced by the PV generator. If the separation is increased, fewer shadows are cast by some rows on the others and more energy is produced. But it also affects the cost, as greater separations lead to more land being occupied, longer cables and more expensive civil works. Therefore, there is an optimum separation, giving the best trade-off between greater energy and lower cost. There is a widely held view that tracking generally requires much more land than static arrangements. The interested reader is encouraged to consult reference [41], which deals in detail with tracking and shadowing in large PV arrays. The ratio between the area of the PV array and the area of the land is called ground cover ratio, GCR. A rough idea for middle latitudes is GCR « 0.5 for static surfaces, 0.25 < GCR < 0.5 for horizontal axis and 0.1 < GCR < 0.2 for two axis trackers. As a relevant example, Figure 22.21 shows trackers at a 48MW plant located in Amaraleja, Portugal.
