Tom & Shelly's build

[noseoil]

Tom, looks really good now. I might turn that plug cover 180 degrees, as those plastic flappy face covers will tend to be in the down position when rolling as it is now. Back there, you will have mud, rain & snow sticking to the open plug if the cover flaps open during travels, just my $.02 worth...

[Tom&Shelly]

Good point Tim! Thanks!

I'd actually forgotten about the flap at first, and had to disassemble a few things to put it on. If I'd noticed how it works beforehand I would have drilled the three screw holes for the plug differently and installed it 180 degrees from what I have.

Rather than re-drill the holes, I'll probably try it with the flap rotated 120 degrees and see if it stays closed while traveling.

Tom

[Tom&Shelly]

I was all ready to begin designing the electrical box in our Galley for the PD4045, battery meter, etc.



when I realized that I should really start thinking about solar now, and figure out where the charge controller will go. Think we'll go with a Renogy Adventurer PWM controller, mainly because I can mount it to the electrical box made from 1/8" or 1/4" Baltic birch, and the wires will be hidden behind the panel. But that's not what this post is about.

The teardrop will be permanently (though we hope relatively infrequently) garaged in our metal garage, 1/8 mile from the nearest AC power.



we'll need to charge and maintain the teardrop battery, and we'd also like a solar power system for the garage. After some practical consideration, it seems the best way is to have two completely separate solar systems. Someday in the future, I'll shop for another charge controller (possibly MPPT) for the garage along with interior 12 volt lights, flood lights for the front, a medium sized inverter to run small power tools, and separate solar panels.

Since the teardrop already will have a charge controller, my immediate need is simply for a separate solar panel mounted to the garage to power that. Later, we'll also buy a separate panel to take camping with us. But that's also not what this post is about.

This post is about mounting the panel, for the teardrop, on the garage. The roof of this garage was not designed to be walked on (we think). But, the wall with the doors faces 202 degrees true, about SSW. Also, as it happens, there is a hill immediately--immediately--to the South, and the Sandia Mountains extending above the Western horizon.

We could mount the panels vertically on the wall, or extended out at some angle. I did some calculations (as a side gig, I occasionally do astronomical calculations. Technically, the Sun is a star) and found these results



The black curve shows the average penalty I'll pay, relative to having the panel always pointing at the Sun, in the Winter. (The Fall curve is identical.) The red curve shows the same for Summer (and Spring). And the blue curve shows the annual average. I'm assuming the panel doesn't really start working until the Sun is 30 degrees above the horizon at sunrise, to account for the hill, and stops when it is 10 degrees above the horizon at sunset.

I'm guessing we'll probably sometimes want to charge the battery in the Summer and usually just maintain it in the Winter, so it looks like a 70-80 degree elevation angle is what we'll go for. If the calculations are correct, in the Summer we'll get about 75% of the power we would have gotten if the panel were always pointed at the Sun. In the Winter, it will drop to about 65%. (I think that's actually current [amperage] out of the panel. Along with a small voltage drop, the actual power out will be slightly less. I think. )

I found a 100 W, 2 ft by 3 ft panel on Amazon for $70, so now I'm thinking about how to mount it. It will probably be above the big middle door. May start with two aluminum L angles up against the painted sheet steel panels (anyone know if that's okay?) screwed threw the sheet into the steel beams on either side of the big door. Then I can use more aluminum to attach the panel to those angles at the correct elevation angle. If I have the 2 ft end stick out, there will be less stress, and there will still be room for two more panels above the door.

Anyone have thoughts or suggestions? Thanks!

Tom

[MickinOz]

My 2 bob's worth:
Presumably, after a trip you'll have at 5 days between weekends to recharge.
So even if your 100W panel only puts out 1/2 power for say 4 hours a day, you'll get 50w, say 4 amps, for 20 hours between weekends. 80 amp hours should be adequate?

Given the longer full sun hours you can expect to get in summer, I think its customary to angle the panel for best winter performance. However, if its only a maintenance charge required in winter you'll probably be fine even if you angle the panel for best summer performance.
P.S. you write about installing a battery meter. Won't the renogy charge controller have all that? My Rover MPPT controller does.

[noseoil]

45-50 degrees faced south & send it. I use a 1 watt per amp hour figure for my solar on the trailer, completely a number I plucked out of the air, but it works well enough in the real world.

I look at solar energy as being similar to passive solar heating. 10% floor area of glazing = heat gain on a sunny day, facing south, on an efficient building (at our latitude). The rest of the time you need a back-up system or something with a plug.

(Please take this with a very large pinch of salt, or for that matter a metric tonne of BS if you prefer)

[tony.latham]

so it looks like a 70-80 degree elevation angle is what we'll go for.

Our 7400-watt grid-tie system is about ready to be turned on. (Idaho's permitting process is a bit like a boar grizzly on a kill.)

The panels are mounted on two pole-mounts that are easily adjusted for tilt with hand cranks. I've got a phone app that breaks the optimal angle if fixed or by month if adjustable. We're at 45º north, and it says they should be at 37.5º if they were fixed –-or 60º for optimal this month.

The app is free and called Optimal Tilt and accesses the GPS in your phone. You might do a quick download.

But you and I may be overthinking this.

Tony

[Tom&Shelly]

My 2 bob's worth:
Presumably, after a trip you'll have at 5 days between weekends to recharge.

Actually, we're retired, so the timeline isn't quite as clean as that, but you're right, it will generally be a few days between trips. So charging efficiency may be compromised a little with no real concern.

So even if your 100W panel only puts out 1/2 power for say 4 hours a day, you'll get 50w, say 4 amps, for 20 hours between weekends. 80 amp hours should be adequate?

Given the longer full sun hours you can expect to get in summer, I think its customary to angle the panel for best winter performance. However, if its only a maintenance charge required in winter you'll probably be fine even if you angle the panel for best summer performance.

You are right Mick. Thank you! I started generating those plots to see what the power penalty is from mis-pointing, but to use them to consider the relative pointing for different seasons, I need to include the longer charging times available in the summer vs winter.

P.S. you write about installing a battery meter. Won't the renogy charge controller have all that? My Rover MPPT controller does.

Actually, the meter keeps track of voltage and current into and out of the battery and estimates capacity, so I'll know when we're around 50% and in trouble.

https://www.amazon.com/AiLi-Voltmeter-A ... 518&sr=8-5

The Adventurer PWM controller will only measure battery voltage. Also, we'll be able to charge the battery from solar, shore power, or towing, so the controller won't see all of the sources of charge. Now that raises another question: Will the voltage from the shore power charger or the tow vehicle damage the controller? I don't think so, but if it does I'll warn the forum. We generally won't be charging from several sources at once.

Thank you for the comments!

Tom

[Tom&Shelly]

45-50 degrees faced south & send it.

Oh sure Tim, that works, but it was snowing earlier in the week, and then in the single digits, and writing the computer code was more fun then shoveling!

Tom

[Tom&Shelly]

so it looks like a 70-80 degree elevation angle is what we'll go for.

Our 7400-watt grid-tie system is about ready to be turned on. (Idaho's permitting process is a bit like a boar grizzly on a kill.)

The Idaho bureaucrats' cousins must live in Bernalillio County, New Mexico! (I neglected to tell them about our plans for stand alone solar power, btw.)

Took us about six months to get the permitting done. When our garage was finally finished, I called to get the final inspection, but the lady on the phone said I needed to call another department to have the (non-existent) plumbing and power inspected first. Since we had the completed garage at that point, rather than make trouble I dropped it. A few months later, after the permit expired, the field inspector came by, passed the building, and told me how I can get a veteran's discount on property tax. He is an all-round great guy.

The panels are mounted on two pole-mounts that are easily adjusted for tilt with hand cranks. I've got a phone app that breaks the optimal angle if fixed or by month if adjustable. We're at 45º north, and it says they should be at 37.5º if they were fixed –-or 60º for optimal this month.

The app is free and called Optimal Tilt and accesses the GPS in your phone. You might do a quick download.

But that would mean I'd have to learn how to download an app to my phone!

Actually, I looked at a few web sites (on my old fashioned computer) and couldn't find one that let me fix the (sub-optimum) azimuth angle. To make the mounting simpler and sturdier, I want it attached so the panel is parallel with the wall. The only variable is the elevation angle. As I was thinking about that, I realized I already had most of the code on the Sun's position that would be necessary to write my own program. Then, as I mentioned to Tim, it was snowing and cold outside...

But you and I may be overthinking this.

Tony

I think I certainly am! Any angle should be good enough, likely even a vertical panel on the side of the wall. But the thread about voltage loss and PWM controllers got me thinking about the details. I'm now convinced that issue won't be a problem. On the other hand, I'm also figuring out what will be best for the system for the garage itself.

Thank you everyone for your comments. I want to get back to building, but right now, the controller, solar panel, and also the teardrop's battery (100 Ah Group 27 AGM) are held up in UPS, which I guess took a beating this week with the weather.

Tom

[bdosborn]

NREL has a free website that will let you estimate your solar production and its super easy to use:

https://pvwatts.nrel.gov/

You can input array size and tilt and it will estimate your harvest based on you location. It also factors in weather data and accounts for if you live in a cloudy area

Bruce

[tony.latham]

NREL has a free website that will let you estimate your solar production and its super easy to use:

https://pvwatts.nrel.gov/

You can input array size and tilt and it will estimate your harvest based on you location. It also factors in weather data and accounts for if you live in a cloudy area

Bruce

That's a fun site. It confirms my 170º azimuth.

Thanks,

Tony

[Tom&Shelly]

NREL has a free website that will let you estimate your solar production and its super easy to use:

https://pvwatts.nrel.gov/

You can input array size and tilt and it will estimate your harvest based on you location. It also factors in weather data and accounts for if you live in a cloudy area

Bruce

Ah! Thank you Bruce. Very good!

Here are updated results of my "role-your-own" code:



Now it's showing Amp-hours per day as a function of day of the year, assuming a 100 W solar panel neglecting all other losses. Of course, we can't neglect all other losses, but it hopefully yields good relative results for day of year, for different elevation angles. I took the results from the earlier curves (plotting cosine of the angle between the sun and panel) and now include the hours that the system collects sunlight--so it accounts for the longer summer days, as Mick suggested.

Relative results seem to be similar, but not exactly the same as the DOE model. I'll attribute that to two things: 1. They have some sort of cloud cover model, where I ignore that effect, and, 2. They most likely assume I live on a plane (as in great, not aero) where I assumed the sun harvesting can't begin until it's 30 degrees above the horizon, and ends when it's at 10 degrees above the sunset horizon to account for the local hillage. Number 1 seems to dominate the differences.

I really wish they had more detail on their cloud cover model, as I could use that for several things. You may notice my curves are essentially symmetric around the solstices, where the DOE results are not (quite). That's how I know a major part of the difference is something besides the geometrical effects, like the cloud cover.

Assuming my model agrees well enough to the DOE model, and is good enough for my purposes, from the plot, it now looks like a 30 - 45 degree elevation angle (complement of the DOE model's "tilt" btw) is the best for my solar panel. That gives me a low of about 35 Ah in the middle of winter, and a low in the summer of 40-50 Ah, with high peaks in spring and fall.

If we came back with our 100 Ah battery half drained in the summer, the plot says it would replenish in about a sunny day, but let's assume about 50% efficiency, and call it two; then perhaps three days in winter. I'm satisfied with that. (And, since we can also charge from the tow vehicle, hopefully we won't be half drained too often.)

Tom

[bdosborn]

PV Watts is always a good reality check whenever I start scheming about PV. I've been looking at a grid tie inverter for a new build and wondered what the payback with a 700 watt array would be. At $90 a year, it wouldn't ever pay back. I think the average lifespan of an inverter is 10 years so I'd need a new inverter before the utility saving would pay for the first one.

Bruce

I think NREL works with NCAR for weather data. My firm is doing the design for the latest supercomputer install at the NCAR Wyoming Supercomputer center. That place is awesome!

[noseoil]

Based on the latest curves you show, which angle (fixed angle) yields the best return each year in power generation?

[tony.latham]

My firm is doing the design....

Holy-CRAP!

T