Small RV system design (request for comment)

PeterN · February 2011

I'm working on a solar installation for a small RV (a SportsMobile Ford E-350 EB conversion), and I would appreciate your feedback on my design.

I want to operate typical RV systems (lights, pumps, fans, etc.), two computers (8+ hours/day, 5 days/week), and satellite Internet indefinitely without shore/grid power and with minimum generator operation. I estimate about 2.4 kW-hr/day demand for everything (about 1 kW-hr/day without the computer equipment). This seems to be a difficult requirement for such a small vehicle, but here are my ideas:

* Three SolarWorld SW-245 Modules (245 Watts each, total 735 Watts)

* Flush mount on roof with Iron Ridge XRL rail system (see first page of http://www.ironridge.com/assets/files/IRONRIDGE_XRL_2_Rail_System.pdf)

* Morningstar Tristar MPPT 60 charge controller with temperature sensor

* Bogart TriMetric battery monitor

* Morningstar SureSine inverter (but most of the demand will be 12 VDC)

* Three Group 8D AGM batteries mounted under vehicle (245 A-h each, total 735 A-h)

The "roof layout" attachment shows how the panels and mounting rails fit on the roof.

The "power estimate" attachment shows:

1) Estimated daily power requirements for summer: about 2.4 kW-hr/day

2) Estimated solar array requirements: about 3 panels assuming 5 full sun hours per day and 70% efficiency

3) Estimated battery requirements: about 4 batteries for a 3 day reserve with 50% depth of discharge

I can only fit three batteries under the van, which gives me about two days of reserve rather than three. Also, the full sun hour and efficiency estimates will vary a lot based on location and weather. Ideally, I'd add a few extra batteries and panels for a larger margin, but they won't fit.

But, I think, with a bit of care and conservation, the estimate is close enough to satisfy my requirements. This estimate doesn't include charge from the alternator. I expect to run the computers only 5 days per week. And, if the weather is consistently bad, I'll drive somewhere else or add a generator.

Here are my questions:

1) Am I missing anything in my power estimate? Is my estimate consistent with your experiences?

2) Is my design reasonable? Am I missing anything? How can I improve it?

3) Is the rail mounting system, such as one from Unirac or Iron Ridge, likely to work on an RV? The ends of the panels are unsupported with the rails, but the cantilever is within the Iron Ridge guidelines. My roof isn't flat (the ends are rounded), so typical RV flush mounts won't fit.

I'd appreciate your ideas and links to relevant posts that I've overlooked.

Thanks,
-Peter

icarus · February 2011

Re: Small RV system design (request for comment)

The general rule of thumb for off grid PV installation (and it is worse for RVs since they seldom if ever have ideal panel orientation) is take the name plate rating of your pv, divide that number by 2 to represent all cumulative system loses and then multiply that number by 4 to represent the average number of hours of good sun one might expect per day over the calendar year. You can adjust that number somewhat due to local variance, but 4 hours works out pretty well on average.

So your proposed 735 watts of power might look like this:

735/2=377.5*4= 1470 wh/day

I personally would round that DOWN to 1kwh to be safe.

So you're going to be able to power your said loads less than half.

What you are missing in your quick calcs is, while you have derated for the PV, you forget wiring loses, inverter loses, basic battery charging efficiency (it takes ~ 120 ah of charge to replace ~ 100 ah of draw,, 20% loss right out of the box).

So you will either need to reduce your loads significantly, increase your PV significantly, or run the genny more.

Tony

As an example, we live off grid with 400 watts of PV, we consume ~ 5-800 wh/day and most days we stay just about even. On a perfect day, we can produce upwards of ~ 1.5 kwh, but on average we produce ~ 800 wh over the course of the year.

Your 700 ah of batteries will produce ~ 140 ah to draw to 80% DoD, or about 1.6 kwh, so your battery is about the right size. Ideally you need to charge that 700 ah with ~ 35-70 amps. Your 735 watts of panel might put out ~ 40 amps on an ideal day. The other problem with most RV installations is that most people prefer to park in the shade to reduce heat in the vehicle. As a result, the Pv works even worse.

T

SCharles · February 2011

Re: Small RV system design (request for comment)

735/2=377.5*4= 1470 kwh/day

I am in bed with a terrible cold as I type, so my brain is off some. However, I do believe there should be 1470 watt-hr, not kilowatt hours. No doubt a typo.

Or, this cold has fried my brain.

Fixed: Thanks, Icarus

Photowhit · February 2011

Re: Small RV system design (request for comment)

I don't know what your plans and goals are, perhaps that would be a better place to start. Regaurdless Icarus has the starting place with good info.

Might consider higher effiecency panels, or just charging off grid and using your massive battery bank, 4 - 245a 12 volt batteries is a lot of storage. Your solar system will buy a lot of RV camping, every 3rd day you'll likly want to dump your holding tank.

PeterN · February 2011

Re: Small RV system design (request for comment)

Thank you. The rule of thumb for efficiency and full sun hours is helpful. I couldn't find a definitive efficiency estimate for off-grid, so I calculated my own based on the PVWatts estimate. Apparently, I missed something.

I really want to stay off-grid and out of typical RV parks. Separately, I'm looking into composting toilets to give me 1 month or more between emptying, so power is the limiting factor.

Anyway, it sounds like I need to take a hard look at cutting my demand.

Are there any thoughts on my component selection or mounting system?

Thanks,
-Peter

BB. · February 2011

Re: Small RV system design (request for comment)

For PV Watts, use 0.52 (or 0.50) for system derating. And 1 kW as your panel size (does not go any smaller). And your mounting choice (tilted to latitude or flat on roof).

If you do winter camping, tilting the panels can be a big help. Use PV Watts to see what your location and panel mounting options will do for you. Say you are near Tulsa:

"Station Identification"
"City:","Tulsa"
"State:","Oklahoma"
"Lat (deg N):", 36.20
"Long (deg W):", 95.90
"Elev (m): ", 206
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 36.2"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 4.01, 64, 4.93
2, 4.46, 63, 4.85
3, 5.33, 81, 6.24
4, 5.88, 85, 6.54
5, 5.61, 80, 6.16
6, 5.77, 78, 6.01
7, 6.06, 83, 6.39
8, 5.95, 82, 6.31
9, 4.99, 69, 5.31
10, 5.41, 81, 6.24
11, 4.11, 61, 4.70
12, 3.70, 58, 4.47
"Year", 5.11, 884, 68.07

And with the array flat:

"Station Identification"
"City:","Tulsa"
"State:","Oklahoma"
"Lat (deg N):", 36.20
"Long (deg W):", 95.90
"Elev (m): ", 206
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 0.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 2.52, 37, 2.85
2, 3.25, 44, 3.39
3, 4.45, 67, 5.16
4, 5.59, 81, 6.24
5, 5.96, 86, 6.62
6, 6.43, 87, 6.70
7, 6.61, 91, 7.01
8, 5.91, 82, 6.31
9, 4.45, 61, 4.70
10, 3.99, 57, 4.39
11, 2.63, 36, 2.77
12, 2.25, 32, 2.46
"Year", 4.51, 761, 58.60

You will get an hour or more "worth of sun" during the winter months with the array tilted.

But during the summer, you will get a 1/2 hour or a bit more of sun...

So, you can use the above chart to esitmate your output with hours of sun:

735 watts of panels * 4.5 hours of sun * 0.52 sys derate =1,720 Watt*Hours per 4.5 hour of sun day

Regarding battery bank vs panel sizing:

(735 WH * 0.77 derating) / (735 AH * 12 volts) = 0.064 = 6.4% rate of charge

Normally, we recommend around 5% to 13% rate of charge--So you are within that range... It will take a bit longer to recharge you battery bank with 6% rate of charge.

Generator sizing--Again with the 5% to 13% rate of charge, assume 50% genset loading:

735 AH * 12 volts * 1/0.80 charger eff * 1/0.50 loading * 0.05 rate of charge = 1,103 watt genset
735 AH * 0.05 = 37 amp minimum charger
735 AH * 12 volts * 1/0.80 charger eff * 1/0.50 loading * 0.13 rate of charge = 2,867 watt genset
735 AH * 0.13 = 96 amp optimal maximum charger

So--You could possibly get away with a eu1000i (900 watt) generator, but a Honda eu2000i genest (1,600 watt) would be a better fit. And you could go larger if you need the extra AC power.

It is actually pretty difficult to find a good quality AC battery charger that works well with smaller generators... If you are into the technical details, this thread walks through one person's search for the optimum charger for a eu2000i generator:

Question about battery charger selection with EU2000 generator

-Bill

PeterN · February 2011

Re: Small RV system design (request for comment)

Makes sense. Thanks.

Regarding battery bank vs panel sizing:
(735 WH * 0.77 derating) / (735 AH * 12 volts) = 0.064 = 6.4% rate of charge

Where does the 0.77 derating come from? Is the difference between this derating and the earlier 0.52 derating the inverter and AC wiring efficiency?

-Peter

BB. · February 2011

Re: Small RV system design (request for comment)

The 0.77 derating is ~81% PTC vs STC panel rating (real world vs lab/marketing spec. + a bit of dirt) and ~95% charge controller efficiency:

0.81 * 0.95 = 0.77 derating for panels + charge controller

So 0.77 derates from solar panel energy through charge controller into charging the battery bank.

The 0.52 derating is the end to end derating:

0.77 panel+charge controller
0.85 inverter efficiency
0.80 Flooded Cell battery (use 0.90 for AGM)
0.77 * 0.85 * 0.80 = 0.52 derating.

So the 0.52 assumes solar panel->charge controller->battery cycling->AC Inverter->AC load.

-Bill

dwh · February 2011

Re: Small RV system design (request for comment)

I wouldn't want to leave those panels unsupported on a vehicle. There are two issues, one is wind loading - most quality PV modules have a pretty high wind load rating, like over 100mph.

The other issue is flex, and with those large modules supported only down the sides, I would worry about flex. For such large modules I'd want them supported on all four sides - and still I'd worry about flex. It won't take much flex to break the glass and the bigger the unit, the greater the chance of flex.

Your issue of the curve front and rear might be approached by using more but smaller PV modules which might allow you to fit a greater total number of watts of PV on the roof.

mike95490 · February 2011

Re: Small RV system design (request for comment)

PeterN wrote: »

Separately, I'm looking into composting toilets to give me 1 month or more between emptying, so power is the limiting factor.

Look for a commercial design called "Natures Head" developed for boating use, or look into the "5 gal bucket" composting toilet. Designs with a small fan are OK, but forget the ones with heater elements

PeterN · February 2011

Re: Small RV system design (request for comment)

I saw the Nature's Head a few months ago, and it looks pretty nice. The toilet is rated at 12 Watts, but the unit I saw had a 6 Watt (12V @ 0.5A) fan.

The Unirac rails with mounting feet every 2.5 feet are supposed to be acceptable for up to 170 mph winds according to the Unirac installation guide, but I also recognize that an RV roof is much different than a house roof. I've seen examples of mounting panels on roof racks from Thule or Yakima. Although that approach is more expensive, more time consuming to install, and higher profile than the Unirac system, the roof rack approach is proven on RVs -- getting the mounting wrong could be a very expensive mistake.

Thanks,
-Peter

Small RV system design (request for comment)

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