RV Solar Part IV – Panel Tilting & Winter Solar Optimization
It’s been almost a year since we made the leap to solar on “the beast”. For those of you who missed the chair-gripping series I’ve got the whole exciting detail of how we decided what to buy and install in Part I (discovery), Part II (equipment) and Part III (installation). Since that time we’ve been geeking out and enjoying our panels through extensive dry-camping travels from FL to CA. I’m happy to say our panels have served us perfectly. On good sunny days we’re usually fully recharged on the batteries by noon plus the panels easily keep-up with our daytime power usage even if we’re on the internet all day. There’s honestly not much we’d change.
But there’s always an opportunity to geek out a little more. As days grow long and the sun sits lower in the sky we’ve started to think about winter solar optimization. Now solar panels actually like cooler temps (their output increases at lower temps), but they have a problem with angle which opens up all kinds of fun thoughts and experiments on tilting. And opportunities to blog, of course. So, here we go:
1/ Why Tilt Your Panels?
If you remember my lessons from last year, getting the best out of your solar system is all about minimizing loss. The same lesson applies to tilting too. The power density of a solar panel is always at its’ maximum when the solar panel is exactly perpendicular (at 90-degrees) to the sun. The further you get away from perpendicular the more power you lose and so the less power output you get. Since sun angle varies by both latitude and time of day that means your power output is varying all the time. So, how do you know what to do?
Well, in summer the sun will get pretty high in the sky and stays there for quite a few hours (as an example, here in Palm Springs it gets to ~70-degrees elevation) so your panel output will be pretty darn good even if they’re flat. However in winter everything changes -> the sun stays closer to the horizon (here it only rises to ~30-degrees) and your power output plummets. Sunearthtools.com has a really geeky cool page that’ll give you the exact angle of the sun any time of year for any direction and spot (just plug in your location):
How much of a deal is this, power-wise? In Palm Springs in summer you really don’t lose anything by keeping the panels flat whereas in winter you’ll lose more than 50% of your power output if you keep them flat. It’s HUGE!! To demonstrate this here’s another cool tool that’ll calculate daily flux (= an approximation of the total amount of energy hitting your panels) based on location, time of year and tilt.
2/ How Do You Tilt?
The fanciest type of tilting systems are “sun trackers” that exactly track the sun all day long, but these are not exactly practical on a free-wheelin’ RV roof. Some RVers keep their panels mobile and just bring ’em out to tilt/track the sun manually whenever they need them. With 6 heavy panels to lug around that wasn’t an option we wanted on our “beast” so it made a lot more sense for us to permanently attach the panels and look for other tilting options.
Our solution was to get the AM Solar mounts (highly recommend them, even if you aren’t going w/ AM Solar for the rest of their gear), and then add-on home-made tilting bars. Some basic 1/4″ aluminum stock from Home Depot cut to whatever length you want with holes drilled in (you can even drill multiple holes to have multiple tilt options). Combine with screw/nuts and you’re good to go!
3/ What’s The Best Tilting Angle?
If you’ve made it this far and manage to remember what we talked about in #1, then you know that what we’re looking for is to get your panels as close as possible to 90-degrees to the sun. The cool sunearthtools.com link will tell you what angle the sun gets to in your area, and some simple geometry will give you the optimal tilt angle:
At our current latitude and time of year we’re talking around 60 degrees tilt at noontime for best results. Now, obviously the sun moves diagonally across the sky during the day and rises/sets somewhat southerly in winter so that number doesn’t stay constant and the real (max. total energy) formula is a rather more complicated (the cool tool shows that). But if you face your RV East-West and tilt panels facing due south more or less at the 90-degree noon-spot you’ll get pretty darn close to getting the best out of the sun.
4/ Beware the Shadow Monster
Tilting is just like everything in solar. You’ve got to make sure you avoid ALL shadows. Together with Marvin we were very particular when we installed our panels last year to make sure they had NO shadows from anything on the roof whether tilted or not, even with long winter shadows. I’ve seen shading models that show just 3% shading of a solar array can lead to a 25% decline in efficiency, with 10% shade producing up to 50% decline! The losses are dramatic and could mean the difference between a system that works and one that doesn’t. Even panels that have special “bypass diodes” (meant to help the shade problem) will suffer voltage loss for each cell shaded. Don’t be caught by the shadow monster!
5/ And Our Results Were….?
We conducted our little tilting experiment out in Owl Canyon BLM. Our tilt-bars only go to ~45-degrees so we knew we wouldn’t get optimal output, but we expected a pretty significant boost. Mid-morning with panels flat our 600W system was putting out ~20Amps. With the tilt we hit ~30 Amps, a 40% improvement. As the day wore on our boost got even better with the MPPT charger kicking up output to a stunning peak of~45 Amps at around noon (MPPT really shines with higher-voltage panels in colder temps). Coooool!
We plan to do even more detailed tracking experiments later this the winter and will undoubtedly share these exciting results with our readers, but for now this is a good start. There is also much more geekiness that can be done with solar so don’t expect to see the last of this sunny series.