Introduction: Portable Sun Tracking Solar Panel With a Windup Clock Drive
I've just designed and built a homemade solar tracker than can be easily set to accurately align to the altitude of the sun (as it changes daily, with the seasons), and also faithfully track the sun, from sunrise to sunset. The tracking ability of the panel provides about 40% more power than a fixed panel. Another little-known advantage of sun tracking solar panels is that they run cooler, which enhances their output, versus rooftop mounted panels, or panels flat on the ground, which tend to build up a considerable amount of heat.
The lightweight, but sturdy device is a real workhorse, ready to provide power in the field for contractors, third world homesteads, camping trips, vacation cabins, scientifc research field stations, recharging electric powered radio controlled model aircraft, and sound systems for park events.
The heart of the unit is a standard Intermatic wall timer (Model FD12HC), with a 21 tooth spur gear mounted on the 3/16" diameter timer knob shaft, and a 36 tooth spur gear mounted on the 1/4" threaded axis that holds and rotates the solar panel. The electrical contacts of the Intermatic timer were gutted to reduce drag. The massof the lightweight 12 Volt, 12 Watt mono-crystalline solar panel, delicately balanced, slows the speed of the clock mechanism about a third of one percent.
The spur gears were ordered from:
Sterling Instrument / Stock Drive Products
Excellent schematics provided online for all gears.
21 tooth gear:
Part Number: A 1T 2-Y24021
36 tooth gear:
Part Number: A 1T 2-Y24036
Step 1: Articulated Foundation:
The foundation is designed to be lightweight, yet with rigid right angles at the corners, but is very flexible, parallel to the ground. In fact, a leg can be lifted almost 2 inches before another leg lifts. Four levels, one on each side, are required to accurately level the foundation to the ground. The photo shows the foundation knocked down for ease of stowage and transport.
Step 2: Pivot Mount for the Solar Panel:
The solar altitude "U" bracket (with geared clock drive) is hinge-mounted to the foundation, and angled to the proper solar elevation for the specific day, and snugged down with 2 thumb screws and a friction-fit wedge.
Step 3: Geared Clock Drive Details:
A standard off-the-shelf Intermatic Spring Wound Wall Timer (Model FD12HC), with a 21 tooth spur gear mounted on the 3/16" diameter knob shaft, and a 36 tooth spur gear mounted on the 1/4" threaded axis that holds and rotates the solar panel. The electrical contacts of the Intermatic timer were gutted to reduce drag.
It's important to note that these Intermatic™ clock timers are now in short supply, as the Intermatic™ website no longer seems to be running and hardware stores no longer stock these reliable and well-made devices. There are other brand(s) of household windup clock drive wall timers out there, but they are not well made and are unreliable, and are, simply, false economy.
Step 4: Balancing and Mounting the Solar Panel:
A lightweight, 12VDC, 12 Watt solar panel was mounted to a wooden frame (varnished in advance) and carefully balanced, between two push-pins, to find the center of gravity, for the axis. Holes were then drilled and short 1/4" threaded shafts inserted and tightened down with nuts and large washers to provide an absolutely straight axis. The solar panel pivots on roller bearings, with internal greased ball bearings. The mass of the panel, the power-out cord, and the internal resistance of the grease, retards the speed of the geared down clock mechanism only about 1/3rd of one percent. But serendipity played its hand to perfectly match the rotation of the panel to the path of the sun.
A larger solar panel could be mounted on the rig, and solvent could be used to remove the grease from the ball bearings, and light machine oil added instead, to track at the same rate.
Two clock mechanisms, mounted in tandem, should work quite well for even larger solar panels -or a lightweight solar oven, mounted on a horizontal "lazy susan" ball bearing base. But, in the case of a solar oven, it seems that the best way, for a solar tracker, would have the cooking pot resting securely on a firm foundation, with a lightweight clock-driven reflector / glass window housing revolving around the heavy cooking pot. Only upper and lower reflectors would then be needed, to compensate for latitude and solar angle.
Let me know if you try this, as I have to move on to other projects and simply don't have time to develop and refine it. But I love to see how these things progress.
Step 5: Pivot Mechanism:
The clock mechanism is fully wound and the solar panel is then carefully placed into the "U" brackets, and the gears meshed. The fluted wooden inserts are then inserted over the roller bearings and snugged down with rubber bands, to hold fast in high winds.
Step 6: Basic Lead-acid Battery Power Station:
A 12 Volt, 8.5 Amp-Hour sealed lead acid battery pack (a good match for the 12 Volt 12 Watt solar panel) is connected to the solar panel. The portable battery pack has an internal charge controller that prevents the solar panel from overcharging the battery. Lead acid batteries are also far more tolerant of fluctuating solar panel voltages than other types of battery packs. And the new breed of high rate sealed lead acid batteries have twice the capacity of those produced in the 1980's, making them a better choice than lithium-ion and nickel-metal hydride battery packs. The battery pack rests on a special, moveable rack, to provide heft and stability against gusting winds.
Step 7: Sun Tracking Solar Panel - Setup and Ready for Charging:
Portable 12 Volt, 12 Watt solar panel, a real work horse, is set up and now ready to generate power.
To quickly and accurately orient the sun tracker to true north, I prefer to use the "Sun Compass
version 1.8" (freeware), on my Palm TX pda. It's easy to initially set up: Just enter the time and select your city, from a long list on the program. And, when the program is opened, the "sun" will always appear in the exact position.
Holding the pda stylus perpendicular to the screen, to create a shadow across the screen, and
then rotating the pda until the thin shadow crosses both the "sun" and the center of the compass
rose, will provide highly accurate results.
A standard, traditional compass, even when set for magnetic declination, can all to often be
affected by nearby power lines and metallic objects, producing faulty readings. And my digital compass will not give readings if any magnetic interference is detected.
And my trusty vintage handheld GPS device will not funtion as a compass, when standing still.
Setting the tracker to the solar altitude, unlike orienting to North -or pivoting the solar panel to face the sun, is NOT intuitive. A solar altitude formula, or a solar altitude reference chart, is required to quickly and efficiently angle the solar panel to the seasonal angle of the sun. And, as the solar angle will vary wildly with the seasons, the angle of the solar panel should be carefully reset at least every two days.
Basically, the north-south rotating axis of the solar panel needs to always point north and skyward. There seems to be a common misconception that the axis always points directly to the north star, but that is not the case. The elevation of the north-south axis is always at a right angle to the sun's rays.
To orient the solar tracker to the seasonal altitude of the sun, the University of Oregon offers their "Online sun path chart program". It's the simplest, and most intuitive, I've found:
Entering your zip code will generate a chart that shows the angle of the sun, at high noon
(solar time), in your area, for different days of the year. Don't be distracted by the hours on the chart, as high noon is the only position you should be concerned about.
90 degrees, on the chart, is directly overhead. 0 degrees, on the chart, is on the horizon.
Subtracting the angle of the sun, from 90 degrees, will give you the proper angle of the solar
panel (perpendicular to the sun's rays), at high noon. The solar panel will now follow the sun, from sunrise to sunset.
Later on, I found a used Palm m515 pda, at a local flea market, for $ 5.00, and installed "RiseSet" program for the Palm OS, Version 2.1 (freeware). The program allows for setting up to 3 favorite locations with latitude, longitude, and GMT offset, to later provide a quick and easy reference for north-south orientation of the axis of the solar panel, as well as accurately setting the noon solar elevation. For noon solar elevation, just be sure to set the time for "high noon", which can be 11:00am, or 12:00pm, or 1:00pm, depending on daylight savings time. The vintage Palm m515 pda is now conveniently stored with the solar panel.
The tracker is designed to be fully functional, anywhere from the equator to the north pole. A simple modification of the clock drive position will allow it to also function in the southern hemisphere.
Also, being portable, the unit, if necessary, can be easily moved to another nearby location, to take advantage of the afternoon sun, if there is no area that receives full sunlight throughout the day.
As an added bonus, this solar tracking device could also be set up on a picnic table and used, at night, by the casual amateur astronomer. To function as a true equatorial mount, the north-south rotating axis of the solar panel needs to be re-positioned to point directly at the north star. But be advised that windy nights may blow the telescope back and forth, +/- 1 degree on the geared axis. While such tolerances are acceptable for solar panels, it is not suitable for serious astronomy.