Solar installation on my 84 Excella Motorhome

[WayneG]

I finished my web article on the installation of a 400 watt solar panel system on my 84 Excella 270 motorhome. It is to large to post here so I have it on my website with my other Airstream articles. You can find the Solar page here: Going Solar
I still have some finishing off to do, but I think it is 98% complete.
I still need to get some data for battery recharge times and follow up on some defective panels.

[WayneG]

I forgot to add some details here so searches will find the gear I used.
My panels are by Renogy RNG-100D.
My panel rocker mounts are from AM Solar
The solar controller SC-2030 and battery monitor TM-2030 are by Bogart Engineering.

Here is a front view of my panels

My roof cad drawing

My overall schematic

[Trailbob]

Thanks for taking the time to write this up. Very informative.

[Kilroy]

Thanks for the write up. Very similar to what I'm planning to do this fall. Afraid your background for the writing makes it difficult for my old eyes to read. Will have to finish reading tomorrow.

[lewster]

Nice job Wayne.

Hope you eventually sealed the base pads of your rocker feet. I noticed that you used screws too. They were not really necessary, as the VHB tape bonds the panels to the roof quite well. I would definitely use a quality polyurethane sealant around the screw heads and the base pad perimeter to assure that you won't be getting any leakage.

[cwbiii]

I did something very similar... except I used 2 MPPT controllers. The solar panels are arranged into 2 banks, each bank has the panels at the same angle...(ie left side, right side) . This can make a significant difference since I monitor each separately and 1 bank can be putting out quite a bit less based upon the angle difference between the 2 banks... especially in the early and late parts of the day in full sun. Typically on a clear full sun day I will put between 3x and 4x the total panel rating into the batteries with this arrangement... 400w * 3.5 = 1600 wh. I also have (4) 75 ah agm batteries to store the energy into. I have the measurement systems permanently in place to accurately quantify those numbers. I'm situated in New England which should similar to your situation in New York.

Chuck

[WayneG]

On the page background, sorry about that, I lightened it up a bit.

On the rocker mounts, I did not seal them yet. I drilled a undersized starter hole and the VHB tape does seal around the screws somewhat, but I do need to seal it when I am satisfied with the installation. I have some other sealing work to do this fall when it is cooler anyway.

I am still thinking of pulling up the passenger side panel mounts to mill out the mounting holes for more flexibility like the drivers side.
That tape is a pain to remove, I have a mini steam cleaner to warm the metal to help pull it looses with a sharpened paint scraper.

Although I could trust the tape alone, the temperatures on that roof in the summer are very close to the tapes limits for maintaining a bond, add a 65 MPH wind to that, and I would be worried about people behind me on the road. The screws add a bit of peace of mind.

I was considering a MPPT controller, but I think the cost .vs. the benefit is marginal. If Bogart made one that went with their battery monitor, I may have gone that way. There are a lot of discussions out there on the topic and I figure I am loosing no more than 10% with the PWM controller.

I wish I had more room for storage capacity or take the chance and replace the chassis battery with a deep cycle and add another 100 AH but I will see how the current setup will work when I am out in the wild.

Someday I bet the Tesla people will make a battery pack for mobile solar. They have something now for home solar, but it works on high voltage, not 12 volts.

My biggest challenge for testing my system while I am home, is that my neighbors all have tall trees so my back yard parking spot is only getting full sun from 10 to 2. As the sun gets further south, I am loosing a lot of solar time. I am almost tempted to ask them if I can lop off the tops of a few of their trees.

I only have 2 more runs in the wild scheduled for this year, neither of them are for very long, but they will be in full sun.

I do love cranking up my Mr. Coffee in the morning without waking others with my generator.

[greghoro]

Originally Posted by Kilroy

Afraid your background for the writing makes it difficult for my old eyes to read. Will have to finish reading tomorrow.

When I find busy backgrounds on a web page with text, I select all text on the page using the Ctrl-A (Select All shortcut). It usually makes the text more readable. Works really well on a page with black background and colored text.


Greg

[cwbiii]

Yes their battery solution is intriguing but for now and the near future it is too expensive. I'm not sure they would be interested in developing a 12V solution.
More likely it would be 24v or higher. This would not be very useful to these old trailers but could be interesting for new ones. The lack of area for solar cells could be a buzz-kill as well. Maybe they will figure a way to install them on the awnings ;-). Roll them out when we are situated.

Chuck

[Kilroy]

Thanks for suggestion and sorry about cluttering up discussion.

Originally Posted by greghoro

When I find busy backgrounds on a web page with text, I select all text on the page using the Ctrl-A (Select All shortcut). It usually makes the text more readable. Works really well on a page with black background and colored text.


Greg

[Alumineer]

Originally Posted by cwbiii

I'm not sure they would be interested in developing a 12V solution.
More likely it would be 24v or higher. This would not be very useful to these old trailers but could be interesting for new ones.

There are MPPT or PWM solar controllers out there that can efficiently convert higher-voltage panel output and charge 12V systems. The benefit that you get is two-fold. First, the current from a higher voltage panel is less (at a given power output) so you have less resistive power loss in your cabling and connectors; and second, your panels will be above the nominal charging voltage requirement at a much lower light level.
The downside is that if one part of one of your panels is shaded (by a post or tree) then it reduces the current available from the entire series string. So there is still benefit from having multiple parallel strings (optimally each tied to its own MPPT controller). All it takes is $$$.

[WayneG]

Here is the article that lead me to go with PWM instead of MPPT controller, it is written by the company that sells the PWM controller I am using, but it seems he took into consideration all the variables.

In this link it is the FAQ question C1 Frequently Answered Questions | Bogart Engineering

C. SC-2030 and charging Frequently Answered Questions

C1. The debate rages: which controller is best PWM ("Pulse Width Modulation") or MPPT ("Maximum Power Point Tracking"). Why did you go the PWM route?

A very good question! They BOTH have good and bad. Plenty of hype has been written already. Here's my (Ralph's) view:
As you know, the Bogart Engineering SC-2030 Solar charger uses PWM technology, not MPPT.

The "good" for PWM:
It is simpler and lower cost technology.
Under some circumstances--it can actually deliver more amps to the battery. That would be when:
(1)days are moderate or warm, with few clouds.

(2) batteries are charging at over 13 volts, (in a 12 battery system) which they almost always are when actually CHARGING. Many places that analyze MPPT charging assume batteries are charging at 12 or even 11 volts, which is unrealistic. Lead acid batteries are typically below 13 volts only when discharging, or perhaps charging with very little charging current--meaning the actual potential gain in amps is not great.

(3) Panel voltage is properly matched to the battery voltage, for example "12V" panels are being used with a 12V system. Some places that analyze MPPT assume that panels with 30V open circuit voltage are being used. Any good MPPT system will easily provide better performance in that case.

PWM is actually more "power efficient" than MPPT--which means less total power loss in the controller itself. So heat sinks in the design can be smaller (and less expensive). Missing in most analysis of MPPT is that there is always a conversion loss with MPPT, which tends to be higher the greater the voltage difference between battery and panels. That's why PWM can actually beat MPPT under circumstances described above.

The "good" for MPPT: If you use panels not voltage matched for the battery, MPPT will utilize more of the potential energy of the panels. For example, if you use 24 volt panels to charge a 12 volt battery system you must use MPPT, otherwise you would be using your panels very inefficiently. If you are trying to use PWM in that case, you are misusing the PWM technology.

Also, with MPPT if wiring is far from batteries to panels, smaller wire can be utilized by running panels at higher voltage to the batteries. Running at twice the voltage reduces wire size to 1/4, which for a long run can be a significant saving in copper wire.

If temperatures are low enough, the slightly less power efficiency of MPPT will be compensated by the higher panel voltages, which will result in a little more battery current. But in my measurements using a commonly sold MPPT solar controller, this would occur at temperatures less than 55 F degrees (in full sun, when charging at more than 13 volts), where there is a slight advantage to MPPT in my location (Boulder Creek, near the California coast). As temperature drops below that (in full sun) MPPT will get some advantage, such as could occur at high elevations in Colorado in the winter. Potentially this would be maximum about a 2.5% improvement in amps output for every 10 degrees F lower in temperature (or 4.6% per 10 degrees C colder. I'm using data from Kyocera KD-140 panels.)

There can be absolutely optimal situations (that I don't personally experience where I live) where MPPT could give some advantage: that is when solar current is present, but the batteries are quite low in charge--but because loads are high and even greater than the solar current the batteries are still discharging despite the solar current. Under these conditions the voltage COULD be at 12.0 volts, or even lower. Again, using data from Kyocera panels, ("Normal Operating Conditions") there is a theoretical maximum gain over PWM of 20% current assuming NO MPPT conversion loss and no voltage drop in the wires to the panels, at 20C (68F). With PWM, the voltage drop in the wires in this case would not affect the charging current. Now if in addition you lower the temperature to below freezing at 28 degrees F (while sun is shining) you might actually get up to a THEORETICAL nearly 30% gain while the batteries are discharging.

The only REALLY BAD part of MPPT, is all the hype surrounding it--for example one manufacturer advertises "UP TO 30% OR MORE" power harvested from you panels. If you are using solar panels properly matched to the batteries, 30% ain't gonna happen unless it's EXTREMELY cold. And your batteries have to be abnormally low in charging voltage--which tends not to happen when it's cold (unless you assume the battery is still discharging while solar is happening). Virtually all the analyses I've seen touting MPPT on the Internet ignore the conversion loss, assume really cold temperatures, assume unreasonably low charging voltages, assume no voltage drop in the wires from panels to batteries, and in some cases assume panels not voltage matched to the batteries, and use STC conditions for the panels (that the marketing types prefer) rather than more realistic NOCT conditions.

The other thing that is misleading about MPPT, is that some manufacturers make meters that show both the solar current and the battery current. In almost all cases the battery current will be greater. The engineers making these know better, but it is implied (by marketing types?) that if you were NOT using MPPT you would be charging your batteries with only the SOLAR current that you read on their meters. That's not true, because the PWM BATTERY current should always be higher than the MPPT SOLAR current. It is the nature of the MPPT that maximum power occurs when the current is lower than the maximum, so they must operate there to get the maximum power. So to properly compare the two you need to compare MPPT with an actual PWM controller in the same circumstances.

Finally, the reason we went to PWM is that I was anticipating that panel prices were going to drop (which they certainly have over the last 5-10 years!) and that the small advantage of MPPT (under conditions where the correct panels are used for the batteries) would not justify their additional cost and complexity. So my thinking, for more total benefit per $, put your money in an extra panel rather than a more expensive and complex technology.

[cwbiii]

The main reason for my recommendation to at least look at MPPT is efficiency. Efficiency counts when collector space is at a premium.
You will put more energy into your batteries with them than standard or PWM.
Pulse Width Modulation (PWM) is one form of DC-DC conversion, basically chopping a DC voltage into a AC signal and then rectifying it back to a DC voltage again... Using the ratio of signal=1 to signal=0 creates a different lower DC voltage than the one supplying. This means that the PWM can change the voltage and recover some of the mis-match energy. How efficiently it does that DC-DC conversion is a primary concern... as is the cost to do it.
Buck and Boost converters have the ability to step-up and step-down the voltage through the use of capacitive and inductive elements. They increase efficiency over simple PWM. These things I know because they are what I've been doing for a significant portion of my engineering life... for your cell phones, laptops, tablets and other personal devices trying to get as much life out of those batteries as possible. Maybe I'm too concerned with efficiency, but as has been said before, waste not want not.

Every manufacturer touts their own system as superior... salesmanship 101.

I'm peaking at 94% efficiency against rated performance (100w panel) ... I checked it myself. The specs say I could do slightly better.

If I were doing a stationary application (house) I would probably go with the the microinverter solution by ENPHASE. Not necessarily the least expensive, at least initially but it is incredibly flexible, if a part fails it only takes out the panel its attaches to, and it communicates its failure to the system in most cases. Shade only affects the panels that are shaded. Damage only affects the panels that are damaged,etc. Long term it depends upon how bulletproof a system is and cobbled systems don't tend to be as reliable as well engineered ones. Things are still changing quite quickly in the solar power arena and will for a while to come. Whats great today will be only good tomorrow.. That crack I made about a flexible panel on your awning was kind of tongue in cheek because it probably will happen sometime in the future. Hopefully sooner rather than later.

We all can do what we like, and use the metrics we prefer to make our choices. "Better" is mostly a matter of opinion and we are all sharing ours.
Hopefully to the benefit of anyone that reads them.

Chuck

[Xicaque]

Thank you for sharing!!!
Great info.

[WayneG]

There will be "more" flexible solar panels in the near future. Our lab does some work for SUNY Poly and they are doing a lot of R&D in the solar field.
SUNY Poly CNSE's Solar Energy Development Center
Your awning, and even the entire upper surfaces of large vehicles can be a collector. It is not far off, but when it will be debugged and affordable, that is what I am waiting for.