Dual Axis

Dual axis tracking devices are similar to telescope mounts in many ways. While there are many styles, they basically fall into two categories: equatorial and altitude-azimuth.
On the previous page for Single-Axis trackers we introduced the azimuth mount which rotates about the vertical axis.

Altitude-Azimuth
Altitude-azimuth trackers, or Alt-Azi for short, typically rotate about a vertical azimuth azis, and tilt up and down on a horizontal altitude axis. This is the most common type of dual axis mount, and is usually the easiest to construct. It's primary drawback is that it requires two degrees of motion in order to track the sun, where an equatorial mostly requires only one (at least on a single day).

Equatorial
Equatorial trackers have one of their rotational axis aligned parallel with the earth's, and naturally use a polar style coordinate system. This means that they point at the north star (celestial north, or south), by simply rotating what is refered to as the Right-Ascension (RA) axis the mount can follow the sun or any other celestial body as it crosses the sky in the course of a day or night. So the RA is like longitude on a map, while the other axis, called the declination, is used to adjust the north/south orientation similar to changing latitudes. Angles A and B in the diagram are fixed by the equatorial mount at the correct position for polar alignment.
The primary difficulty with equatorial mounts is the size/cost of the mount, but this can be overcome with creative installations.

Polar
Polar mounts are a variation of the Equatorial mount which have some imperfections from an astronomical perspective, but are often easier to construct. The significant difference between them is the order of translation. A polar mount uses the elevation motor to change the angle of the primary axis. Equatorial mounts use the primary-axis to rotate the elevation axis. The difference is subtle but results in a different path traced in the sky than when active elevation axis tracking is not employed.

When applied to CSP, dual-axis trackers offer several advantages over single-axis trackers:

  • By pointing directly at the sun they can eliminate off-axis cosine loss of efficiency common with single-axis trackers for highest energy harvesting
  • They are required by prime focus parabolic dish designs which are capable of reaching industrial temperatures.
  • Alt-Azi mounts can also function as heliostats, and can be directed toward different targets