Rotation Control

The objective is to keep the panels facing the sun. The pointing accuracy is not critical, as small angular errors do not decrease the apparent panel area by much. For example, a 10 degree error changes the panel apparent area to 98%. (cosine 10 degrees.) For focusing collectors, accuracy is very important.

One source of the data for the sun's position is here. The sun's azmuth and elevation appears in table form, where the date, location, time interval is specified.

One Axis Control = one control motor.

The axis of rotation is either parallel to the earth's axis, or horizontal, but facing N-S.

Pros: Control is easy, as the angle is almost equal to the sun's hour angle. No complicated trigonometry. But the controller can factor in the coordinate transformations found in chapter 4 of Power From the Sun.

Cons: The height of the array can be higher than the 2 axis array, depending on the array size.

The panels start to rotate in the morning, when usable energy can be harvested. The rate of panel motion is constant, at 1 rev per day. The rotation continues, rain or shine, to the end of the day, at which point the panels are motorized back to the start position. Then wake up and start over in the morning.

Control: Microcontroller - see Control Package

Two Axis Control = two control motors.

There are 2 ways to configure the axis of rotation. In one way, there is a vertical axis - the sun's azimuth, and there is a horizontal axis that rides with the vertical axis - the sun's elevation. The equations are found in Power From the Sun, chapter 3. The second way to configure the 2 axis is shown pole tracker picture. Basically the 2 bars are rotated with separate motors. One I call the daily axis (E-W rotation) and the other I call the seasonal axis ( adjusting for the height of the sun at noon, and throughout the day. The equations for this are found in Power From the Sun, chapter 4.

Pros: The height of the array is lower than the single axis array. makes cleaning and assembly somewhat easier.

Cons: The control is more challenging, as you have to compute the azimuth and elevation.

Control: The motors are turned on and off, throughout the day, to emulate 1 rev per day.

For 2 axis trackers, the apparent axis of motion is parallel to the earth's axis. However, the mechanical movement of the panels is a composite of 2 movements - a movement about a vertical axis, and a horizontal axis. The current design has a 20 RPM motor turning a leadscrew, with 8 threads per inch. The motion of the 2 motors is incremental. That is, the motors cause the array to rotate much quicker that the earth's rotation. So they rotate a little bit, and wait for a longer amount of time.

Predicting the sun's altitude and elevation.

The equations for finding the altitude and elevation are generally found in Wikipedia. Power From the Sun. (Also see Astronomical Algorithms by Jean Meeus.) The variables needed are latitude, longitude, day of year, and the time. A microcontroller is used to compute the solar Azimuth and Elevation based on the time and date, and latitude/longitude input, by the user. In order to keep the cost down, the drive motors turn a leadscrew, and through a bar linkage, adjusts the azimuth (or elevation).

A thanks for the people at Matrix Multemedia for developing Flowcode, a GUI for writing C code in the background.

Bottom Line: Accurate tracking is not necessary, because if you are close to alignment, that is good enough. For example, if you are 10 degrees off, the loss is 1 - cosine(10) = 1.5 %. If 20 degrees off, 1-cosine(20) = 6%.