Solar Tracker Experiment

Several months ago, when I first posted about my manual two-axis solar tracker, a couple of readers asked whether a tracker really made that big of a difference. I had a theoretical answer based on simple trigonometry and the amount of light that falls on a surface relative to its orientation to the sun. Specifically, the amount of direct sunlight that falls on a square surface should be proportional to cosine(θ) x cosine(γ) where θ and γ are the angles between where the panel is pointed and where the sun is, in the x and y axes. By this theory, a panel that is perfectly aligned in one axis, but off by 30 degrees in the other axis would only get about 86.6% as much direct light. (This formula ignores ambient light, which in reality would clearly be present in addition to direct sunlight.) But I had no empirical data to back up my theory… until today.

On a sunny day like today, I’d re-orient my 100W solar panel 2 to 3 times in the course of the day. I usually point the panel to the east before going to bed so that it’ll catch the morning rays, and I’ll move it to point due-south later in the morning. In the afternoon, I might move it one or two times as well. The general idea is to keep the panel pointed to within about 20 degrees of the sun, since that should give me over 94% of available light at all times.

Today, when I went to re-orient the panel a little after 3pm, I decided to get a couple of actual readings. I first checked the voltage of the whole system (charge controllers hooked up to battery array), and got 13.3 Volts. I then measured the current between the charge controller and battery array, with the solar panel in the noon position, and also in its optimal position at the time, which is about 45 degrees from the noon position. With the panel in the “noon position”, I got 3.85Amps, or 51.2 Watts. I then moved it to the 3pm position, and got 5.55Amps, or 73.82 Watts.

The verdict, I might say, is that yes, the tracker makes a significant difference. If the panel had been fixed pointing due south, by 3pm I would only be getting less than 70% of the power that I could be getting, and that number would rapidly diminish as the sun continued moving away. This would also be the case in the morning, when I would get significantly less power than is available for the first few hours of sunlight. And, as it turns out, the numbers fit my theoretical model fairly closely, since according to my theory, my panel should be outputting 70.07% of its maximum when pointed 45 degrees away, while the actual numbers I got today were 69.37% (also, the angular difference was approximate, though, in theory, the sun should move by 45 degrees between noon and 3pm).

One thing to note, however, is that these results were obtained with my monocrystalline panel, which work best in direct sunlight. Thin-film panels, including amorhpous silicone panels, supposedly get more power from ambient light, so they may be less sensitive to orientation, though this is another hypothesis I’ll need to test with my 45Watt amorphous panels sometime.

Another question I got about the tracker was the effectiveness of the “manual” nature of the tracker. Wouldn’t an automatic tracker that constantly aligned the panel with the sun be more effective? Well, yes. But, to get 90% of available energy, the panel can be off by as much as 25 degrees in one axis (arccosine(0.9)). Or, at any given time, if I point the tracker 25 degrees ahead of where the sun actually is, the sun could move through a 50 degree arc and I would still be getting over 90% the whole time. Since it takes the sun over 3 hours to arc through 50 degrees, even manually moving a tracker every 3 hours will ensure that my panel gets 90-100% of available light at all times. So, an automatic tracker with all its complexity only gets maybe 5% more power than a manual tracker that’s re-oriented every 3 hours.

9 thoughts on “Solar Tracker Experiment

  1. Last year I was looking at one of NASA’s web sites where they provide the position of the sun relative to any place on earth.
    If you were to download this information you could accurately position your panel every time, in each plane, to get 100%.

  2. Bravo on your experiments with solar! Some points I would make (as a solar professional).

    First and foremost, it’s not that Amorphous panels get “more” power from ambient light, rather that they are affected “less” by shading, whereas the single crystal silicon modules are greatly affected by even a small amount of shading. Also the results you are seeing in your panels output could be improved (much easier to you) by going from a “on/off” type charge controller to a “Maximum Power Point Tracking” (MPPT) charge controller. You see, the controller you have now, and I’m assuming based on your output readings, obliges the PV panel to operate at or near the momentary voltage point of the battery for charging which, unless your panel is a 30 cell module (or some older 32 cell modules), is causing the module to work less efficiently than it otherwise can (at probably 12-14% efficiency, every little bit counts). With an MPPT charge controller you’d get, on average, about 14-30% more power out of your panels, even if they were in a fixed position at latitude. And you’d get even more power on a tracker, because they take in the PV voltage and current, at their maximum available levels for any given light and temperature condition and send out the voltage at near battery voltage, but at a higher current. In essence they track the maximum power potential on and I, V curve and make sure you get all of the available power by upping the current going into the battery bank. Also, they usually have a 3 stage charging algorithm built in to them which, at the tail end of the charge, does what’s called a “trickle” charge to keep your batteries at the top of their charge, rather than simply turning off. These charge controller also allow you to “equilize” charge your batteries from the PV, which will greatly increase the life of your batteries because it knocks the build up of sulfation off of the plates (but this works best in batteries designed for Off grid power applications because they have more space beneath the cells for build up to accumulate without shorting out the plates, but I digress).

    As for trackers, yes indeed they give you more power but (in the case of large store bought versions) at a significantly greater cost. What you have is perhaps the most elegant version I have ever seen of a home built two axis tracker, and that’s great. However, as you know, it’s only going to be as effective as you are at moving it on time and, with time, you may tire of that. If that happens, I’d suggest getting s simple, mechanical, single axis tracker from a company called “Zomeworks” out of New Mexico. Their trackers are robust and work great. They use canisters of freon, on either side of the tracker with a small tube connecting the two and a series of “shadow plates” so that when the sun hits one tube, the freon in it boils into a gas and travels up the tube to the other canister, thus weighting it, whence it moves into the path of the sun. Once it is directly in line with the sun, a state of equilibrium occurs in the canisters and the tracker stops, until the sun moves.

    Trackers are good, but they are ineffective in cloudy weather so make sure you live in an area where you get enough sunlight to justify the expense if you ever buy one, and you want open space, pretty much from horizon to horizon where your tracker is located or it’s usefulness is significantly limited.

    Anyway, I’d love to talk with you more about this and any solar projects you have. I’ve been installing solar power as my primary job for about 8 years now so I can give you lots of tips on what works and what doesn’t and which are the most reputable brands of products to buy (from hard learned experience).

    Keep up the good work 🙂

    Glenn from VT

  3. There is a free tool called the Solar Advisor Model from Sandia labs that will do relatively detailed simulations for you.

    I wrote about this on my blog, including a reflector and tracking mechanism that looks like it could cut the cost of the photovoltaic collector by about half on a per watt basis.

  4. Hey Glenn, what do you think of the much cheaper photovoltaic collector idea on my blog?

    Be interested in getting your thoughts on it.

    The only concern I have at present is the possibility of reduced panel life due to higher temperatures, but have not been able to quantify it at all.

  5. My brain is not up to high tech, but I have always wondered if a “disco ball” solar collector” would solve this problem.

  6. You may want to consider reflectors placed at 45* on the east-west axis and 11.5* on the north-south axis to help reduce the shading effect. They don’t need to massive things just enough to optimize solar exposure. Seeing how you’re about 30% below maximum efficiency, I’d start with reflectors that are about 30% of the area of your solar panels.

    Here’s something that’s on the horizon:

  7. In my experience it’s almost always cheaper to get x % more panels than get x % more gain with a tracker. Plus trackers are less reliable than solar panels. Although being off grid there is the added factor of getting that sun when it’s out until with grid-tied systems.

    I’ve found with myself and others.. I’m willing to fiddle with this kind of stuff initially, but after a year or two if you’re like me and most others I’ve met you’ll have moved on to other projects at your place and you’ll just want something that works reliably day in and day out. Anything that you hastily set up thinking you’ll be happy to keep tinkering with to keep working, you’ll later most certainly be swearing at for constantly taking your attention from what you’re currently trying to work on.

  8. so basically all you need is like half of a clock face with stop peg holes at the number areas, and each of the ‘hours’ are 30 degrees more or less, and you go out and move it a peg ‘westward’ every three hours? and maybe a slightly finer control half disk for ‘azimuth’?

    Then set it all the way back to the beginning at sunset.

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