Steves’ solar tracker – o –
matic mark 1.
Howdy Steve,
I thought you might like to see a few pics of my solar tracker unit.
This is my first attempt at solar tracking but I am confident that it will be a winner.
This pic gives some clue of the size of the tracker unit.
It stands 16 feet high and weighs Ľ tonne.
It stands on a concrete base that is 450mm thick and 2500mm square.
I figure that a concrete base that heavy should help to keep the wind from blowing this baby over :-).
In
this pic can be seen the gin pole at the right of the upright pole. This is
used with a 2 tonne hand puller to raise and lower the platform pole.
In the lowered position access can be made to the solar panels, drive motors and control computers.
At the bottom of this pic can be seen the horizontal driver system. This consists of a 12V PM motor driving a 40:1 gearbox which then drives another gearbox of the type used in the very first barrier arm units made by Hardings – last millenium. Rotary drive is then conducted to the U frame by shafts coupled by universal joints. The drive motor needs to complete 3200 revs in order to turn the U frame by 360 degrees. Rotation is in reality limited to 270 degrees in order to avoid cable tangles.
Horizon scanner leds.
9 leds are wired in parallel and loaded by a 22K resistor.
A 470ohm resistor in series with all of this provides protection for the leds in the event of an electrical fault and also allows testing of the leds
by supplying lighting current.
This assembly has changed somewhat since this pic was taken and it now has a slot plate over the top of it that narrows the leds’ line of sight.
I discovered that this was needed in order to enhance pinpoint accuracy.
The pic above shows the assembled horizon scanner complete with an optical sensor that is intended to indicate to the control system that the scanner is at the zero degree position. In practice the zero position puts the scanners’ optics in a position that is perpendicular to the U frame arms.
A similar assembly exists for the vertical axis but, instead of having its’ leds arranged in an arc, has its’ leds set out in a straight line.
A
close up of the horizontal drive mechanics.
The collar that covers the end of the angled portion of the driveshaft serves as a backstop in case of a failure of the bearing that forms the turntable.
It takes 30 seconds for the turntable to complete 1 full turn.
The
horizontal scanner assembly will be mounted on the left arm of the U frame at
about 1 metre above it.
As the U frame is rotated by the horizontal drive motor it carries the scanner assembly with it and as such all movements made by the scanner are relative to the U frames’ current position.
Solar tracking is conducted by following these steps:
1) Scanner is set to zero degrees,
2) Light level is checked,
3) Scanner rotates clockwise by 1.8 degrees and continues thus until either the sun is detected or the scanner has come full turn,
4) The scanner returns to zero position,
5) The U frame rotates in the appropriate direction until the horizon scanner once again detects sunlight.
6) The horizon scanner rotates 180 degrees to protect it from excessive exposure to the sun.
The leds in the horizon scanner are arranged in an arc so as to allow the scanner to detect the sun anywhere from the lowest point on the horizon to a near vertical
position.
This functionality is employed for another purpose – the scanner is polled every 20 minutes to scan the sky for any evidence of sunlight strong enough to justify further action. Since this action takes only 1 second there is only minimal power draw, 80 mA at 12V, during night time or poor weather.
A
small windmill will be placed above the right arm of the U frame. This serves
to allow the system to protect itself in high winds by leveling the solar
platform in the upside down position.
The vertical scanner unit will be mounted on the end of this shaft.
This puts it in line with the platform and causes its’ scanning movements to occur relative to the platform.
The
vertical scanner also has a zero degree point and this occurs at the
perpendicular to the platform – so the zero degree point in the case of this
picture would be straight up.
Here is the vertical scanner.
A small stepper motor rotates the optical array in search of the ever elusive sun.
The blob on the motor is where the zero degree detector is mounted.
The scanning sequence is carried out in two steps. Firstly the horizon scanner conducts a scan to check for presence and intensity of sunlight.
Upon completion of horizon scanning the U frame acquires the suns’ position and the horizontal aspect computer then sends a ‘go’ signal to the vertical aspect computer.
Vertical scanning is conducted in the same way as horizontal scanning but simply scans a narrow band of sky starting at one horizon and ending at the
opposite horizon. It is the narrow band of the vertical scan that makes it necessary for horizontal scanning and acquisition to occur first.
High wind is detected by the power management system and used by it to disable scanning and acquisition operations for a preset period of ˝ Hr per wind gust in order to prevent excessive wear on the bearings and mechanism. The same protective technique will be employed to protect the solar cells from hail by turning the array upside down upon detecting hail impacts.
Incidentally the panels are mounted on a platform that is topped by a sheet of ˝ inch thick plywood which serves as the shield during storms.
An interesting observation I made while testing the scanner optics:
Since the scanning process uses light intensity as its’ mode of operation it follows the sun right ?. Well on certain cloudy days the suns’ light will be seen to bounce off the edges of clouds and I found that the scanner would prefer this to the sun which was partially or fully occluded by cloud cover.
A test on a solar panel indicated that it produced more power when aimed at the illuminated clouds than it did when aimed at the obstructed sun.
I think this will result in quite odd behaviour of the finished machine on partly cloudy days.
Well Steve I hope you like this little project – any ideas
you have on it will be greatly appreciated.
:-).
Hope to chat with you soon.
Steven Tetlow.