Small system for an RV

cptdondo · September 2010

I'm trying to figure out if I can fit a small PV system on my little RV, and if it's worth it.

I have enough rooftop for a total of 270W of panels. I can fit an MPPT controller inside. So far so good.

My usage is governed by my DC fridge, which on a moderate temp day uses 17 AH. On a hot day with no breeze and in direct sunlight, it uses about 50 AH.

Will the panels generate enough AH to make up for what the fridge uses? I have batteries and such, so I can store excess power, but my concern is that I really want the panels to cover the use of the fridge.

I live in the Pacific North West, so in the winter (in the moderate temp days) we're typically under cloud cover. Anyone have enough real-world experience to guess if those panels will generate 20AH on an overcast day?

I know I'm asking for guesses; before I drop $1K into solar panels, I'm looking for some assurance that this has a reasonable chance of doing what I need it to do......

BB. · September 2010

Re: Small system for an RV

Rough numbers--4 hours of full noontime equivalent sun per day, 270 watt panel, and 0.52 Off Grid System Efficiency:

4 hours of sun * 270 watts * 0.52 = 562 WH per day
562 WH / 12 volt system = 46.8 AH per day

The above is just a SWAG for planning and for reasonably sunny areas may be true for 9 months of the year.

If you have a marine layer, or really cloudy weather, you may get 1/2 or even less of the above... Solar PV Arrays just do not develop much energy if there is no direct sunlight.

We can be more scientific and use the PV Watts program. Use Eugene Or as your location... Use 0.52 derating and 2.7 kW of panels--Then move the decimal point one place to the left in the kW and $ columns (only does 1kW minimum for panels):

"Station Identification"
"City:","Eugene"
"State:","Oregon"
"Lat (deg N):", 44.12
"Long (deg W):", 123.22
"Elev (m): ", 109
"PV System Specifications"
"DC Rating:"," 2.7 kW" [fix answer below to 0.27 kW by 1/10th]
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.14 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 44.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 7.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 1.89, 7.3, .526 [moved decimal points to left one place]
2, 2.58, 9.3, .670
3, 3.99, 15.9, 1.145
4, 4.48, 17.5, 1.260
5, 5.31, 21.0, 1.512
6, 5.50, 20.4, 1.469
7, 6.29, 24.0, 1.728
8, 6.25, 24.0, 1.728
9, 5.56, 20.8, 1.498
10, 4.03, 15.6, 1.123
11, 1.92, 7.1, .511
12, 1.66, 6.2, .446
"Year", 4.13, 189.2, 13.622

So, you get 6.2 to 21 kWH per month for a fixed array tilted at latitude:

6.2 kWH / 30 days = 0.207 kWH per day (207 Watt*Hours per day)
21 kWH / 30 days = 0.7 kWH per day (700 WH per day)

And you want 17 to 50 AH per day:

17 AH * 12 volts = 204 WH per day
50 AH * 12 volts = 600 WH per day

So--There are some numbers to play with... It looks like 270 Watts of panels is not bad... You still may need a backup generator a couple times of week--and with cloudy days, you will need to make up with a generator daily or every other day (depending on size of your battery bank).

-Bill

vcallaway · September 2010

Re: Small system for an RV

Where in the NW are you?

cptdondo · September 2010

Re: Small system for an RV

WOW! Thanks! That calculator is great.

I have a couple of questions:

the derate factor - how did you get to 0.52? The system uses PV -> MPPT -> battery -> fridge. The fridge runs off of DC.

I can assume that the battery charging process is something like 80% efficient, but I don't see how you got 0.52.

In the model output the lowest figure is 7.3 kwh for the month of January; you used 6.2 I'm not sure if this is a typo, or if this is some other derating thing.

Lastly, my numbers will be a lot lower as the panels will be flat on the roof so I can expect to lose another 25% or so there. No way to tilt the panels without significant further complexity, cost, and weight.

cptdondo · September 2010

Re: Small system for an RV

Eugene, OR - Bill nailed it in one try.

BB. · September 2010

Re: Small system for an RV

cptdondo,

You are correct... I messed up a bit on the calculations. I did AC assuming 85% efficient Inverter--Here is how the deratings stackup:

0.77 (solar panels + charge controller or GT inverter) * 0.80 Flooded Cell Battery Efficiency = 0.62 derating for DC power
0.77 (solar panels + charge controller or GT inverter) * 0.80 Flooded Cell Battery Efficiency * 0.85 AC Inverter Eff = 0.52 derating for DC power

In the end--if your guess are within 10-20% of the real numbers--Count yourself doing really well.

For an average PV system--You normally do not want to count on using 100% of your power every day... Some days you will have more, some day you will have less--and every so often, you will get a week of socked in weather.

I try to be on the conservative side (even with my mistakes :roll:). And always count on having a backup plan (turning off optional power, having a backup genset, etc.).

Regarding mounting--There are RV tilt mounts too. And I think one or two guys here have made their own scissors jack tilt mounts too. If you spend days/weeks at one location, a tilt mount is probably pretty nice... If you move every couple days, it may be more pain than it is worth.

Regarding 6.2kWH per month, that was for December (with the 0.52 mistake).

-Bill

dwh · September 2010

Re: Small system for an RV

cptdondo,

I'd like to ask which DC refrigerator do you have? Also, how do you get your 17ah and 50ah usage figures?

Around here, there is always discussion about DC refrigerators and it would be nice to have some verified real world numbers for the archives.

techntrek · September 2010

Re: Small system for an RV

I'm a long-time camper, popups and travel trailer. Not sure why you are running your fridge on DC. DC is only supposed to be used while on the road - smaller camper fridges use about 10 amps to run the heating element (which drives the ammonia adsorption process) which will kill any RV battery bank very quickly. In propane mode they will last for weeks on one 20 pound tank which is what you should use in-camp. Unless you have "shore power" and then you can use 120 V mode.

Modern RV fridges use some DC power in propane mode to run their control board. One PV panel would be enough to replenish that small load but its really not necessary. A decent battery bank will handle it for quite a while.

LenSatic · September 2010

Re: Small system for an RV

cptdondo,

Can you be more specific on "small RV"?

We have a small RV (17' Casita), and the fridge is the least of our worries. 45w amorphous panels keep us going. The Honda EU2000i handles the A/C and micro/convect oven.

Also, if you mount them on your roof, you have to park in the sun.

Happy trails!

LS

Kamala · September 2010

Re: Small system for an RV

techntrek,

His fridge might not be 3way. It might not be an absorption fridge at all. Might be like one of those 12V cooler type of things.

I haven't run my 3way off of 12V. I'm mostly parked. If and when I do tow, I think that the voltage drop from my tow vehicle's alternator to the fridge will be great enough to degrade performance. My guess is that the wiring is 14 (maybe 12) gauge and about 40 ft round trip. At 10A that's going to be a significant drop.

To the OP, if your load is as small as you say, trust BB.'s comments.

K

BB. · September 2010

Re: Small system for an RV

Except for those comments of mine that contain mistakes.

:roll::p

There are 12 volt mechanical compressor RV fridges (we have discussed them here before)--And they can be pretty power efficient. Certainly more so than absorption fridges on 12/24 VDC/120 VAC or Petlier Cooler types.

-Bill

Kamala · September 2010

Re: Small system for an RV

BB. wrote: »

they can be pretty power efficient. Certainly more so than absorption fridges on 12/24 VDC/120 VAC or Petlier Cooler types.
-Bill

Would this be true for volumes

cubic feet?

BB. · September 2010

Re: Small system for an RV

We had this thread about Engel small DC refrigerators with some power measurements... Even then, there was a huge difference between running in hot weather vs a 70F basement.

Problem is that small refrigerator/freezers are limited as to how much insulation they can have and still have a reasonable footprint--and that their contents goes with the cube of the size, and the surface area goes with the square--So we end up with lots of surface area to chilled volume as there sizes are reduced.

-Bill

cptdondo · September 2010

Re: Small system for an RV

Hehe... Didn't mean to ignite a firestorm in a teapot. My "small RV" is a Roadtrek 200, a 20' long van on a Chevy chassis. The fridge is an Isotherm Cruise 130, a marine fridge with a Danfoss BD-35 compressor and the ASU controller. That controller does some fancy things, mostly varying the compressor speed based on availability of charge voltage.

I got the AH numbers by hooking up a hall-effect ammeter to the fridge, and collecting data on an embedded computer over several trips. They're about as real as I can get.

I also found that the Danfoss compressor is incredibly efficient, and also incredibly sensitive to ventilation and air flow patterns. It's not enough to have air flow; you have to have airflow in the right direction over the compressor itself, and not just the coils.

Basically, when the fridge controller senses charge voltage (above 13.5 volts) it kicks the compressor into high mode, and freezes a cold plate to about 15 deg. F. Once the voltage falls below the charge threshhold, the fridge goes in to econo-mode, running the compressor at reduced RPM and only long enough to maintain temps.

It's pricy, but very neat.

I think the Engels use the same compressor (most boat fridges do) but with better ventilation and without the fancy controller.

cptdondo · September 2010

Re: Small system for an RV

techntrek wrote: »

I'm a long-time camper, popups and travel trailer. Not sure why you are running your fridge on DC. DC is only supposed to be used while on the road - smaller camper fridges use about 10 amps to run the heating element (which drives the ammonia adsorption process) which will kill any RV battery bank very quickly. In propane mode they will last for weeks on one 20 pound tank which is what you should use in-camp. Unless you have "shore power" and then you can use 120 V mode..

Not to get into the RV stuff too much, but a propane fridge just doesn't work for us. We hike and camp in the boondocks; it's often hard for us to find a level spot. Basically, if things don't slide off the counters we call it good. Also, we move around a lot, and constantly switching a fridge from DC to LP was a huge PITA. So we yanked the fridge, and got a Danfoss compressor fridge. Never looked back; once you get the ventilation dialed in it's turn on and forget.

LenSatic · September 2010

Re: Small system for an RV

cptdondo wrote: »

Not to get into the RV stuff too much, but a propane fridge just doesn't work for us. We hike and camp in the boondocks; it's often hard for us to find a level spot. Basically, if things don't slide off the counters we call it good. Also, we move around a lot, and constantly switching a fridge from DC to LP was a huge PITA. So we yanked the fridge, and got a Danfoss compressor fridge. Never looked back; once you get the ventilation dialed in it's turn on and forget.

We boondock exclusively. We carry several 2x6 boards for getting un-stuck or crossing washed out areas on roads. They also help us get level when parked. It's cheaper than relying on DC power for the fridge.

techntrek · September 2010

Re: Small system for an RV

Kamala, you are right that the voltage drop can be an issue. But as long as you use 6 or 8 gauge +12 and ground lines from under the hood to the Bargman connector you are OK. Unfortunately most hitch installation places use the much-cheaper 12 gauge stuff and so after a 4-hour drive with the running lights on and fridge on DC, the camper battery is dead.

techntrek · September 2010

Re: Small system for an RV

Modern absorption fridges are much more tolerant of non-level conditions. Since the first step for boondocking (or designing a PV system) is reducing loads, DC mode on a fridge and the standard furnace (with amp-sucking blower) are always the first 2 things to go. Propane mode for the fridge and a catalytic heater like the Olympian Wave are usually the path chosen. As long as you can always camp with good sun exposure then a DC fridge will work for you, but that's a big IF. 50 amps per day is the total capacity of a group 27 battery @ 50% DOD, so if you get two days with clouds you are sunk.

dwh · September 2010

Re: Small system for an RV

My 1976 camper van has a 30 year old Norcold 323 (smallest 3-way). I ran the numbers on replacing it with a compressor unit, but this thing runs off the built-in 5g propane tank for 4 weeks on high and 6 weeks on low. I run it on low because on high it tends to freeze things.

Running the fridge on high and using the stove (but not that horrid 12k btu furnace), I still got 3 full weeks of camping out of the 5g tank of propane.

So I stopped thinking about replacing the fridge with a DC unit and started measuring to see if I can replace the 5g propane tank with something bigger. I can fit an 11g Manchester horizontal in the same place the 5g is now (under the sliding side door), but jeez...that tank is expensive so I'm just keeping my eyes open for a used tank with the right dimensions.

Small system for an RV

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