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Old 12-09-2009, 01:03 PM   #1
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solar system for running an A/C

ok i'm new. i've been researching all the ins and outs of building your own rv. this seems to be the place to get good answers. my background is mechanical engineering and design. but i could use some guidance with this electricity stuff.

my conversion will have interior space similar to a short bus (8'x8' floor) and 8' ceiling. it's not a bus. it has FRP sides, and i'll probly add a few layers of foam board insulation, with 5-6 inches on the roof. wile the air conditioned space inside is small the total vehicle length should be 30'. i should only use it for short trips, 2-7 days, 10-20 times a year, off grid. the rest of the time it will sit. on the short trips we will never have power hookups. i wanna run a small window a/c without pissing everyone else off by running a genny.

so 1000W A/C, in 100* 70-100% humidity weather. it'll runn a lot. i'm guessing 7 hours a day. it gets all the way down to 90-85 at night. some sites say i'll get 5 hours of peek sun per day. i know the sun is up and hot for 12 hrs+ all summer. not using the A/C is not an option. this project is about making trips to see the country more comfortable. if i wanted to "cut back" we would just drive in my car and camp. and we will probly still do that, just less often.

i'm thinking i'm gunna need 4 230 watt REC panels. that's 920watts of panels. i haven't found a single charge controller that will support that many panels. i will only accept a 3 stage charger.

so can you run multiple charge controllers to a single battery bank? can you run multiple isolated battery banks into the same inverter?
should i give up on the huge solar system idea and just cart 1500lbs of batteries around?
is there a generator that isn't gunna bother the tent campers in our group? it's gotta be VERY quiet and i haven't found one yet.
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Old 12-09-2009, 06:13 PM   #2
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Re: solar system for running an A/C

Im looking at the honda eu generators. I used one a few weeks ago and it was quiet. I mean incredibly quiet. It was under the table we were sitting and playing cards at. Just hummed along. Honestly couldnt believe it. I was so surprised. It was the 3000w unit. Cost on it is around $1800. There is a similar unit from Kipor. Looks almost exactly the same except its yellow. It only costs $999. I have never used the kipors, but have heard good things. Either way, the honda or the kipor, I think it will be way cheaper than trying to build a solar setup to run the a/c all the time. Plus, like smitty said....you may have a really really hot rainy cloudy day, and you would be just fine.

*EDIT* Honestly, since you are running just a window unit in a small space, you could run the smaller honda or kipor units. You can get the 1000w honda versions for under $1000, the 2000w for around $1200. Kipors smaller ones are: 1000w for $400 and the 2000w for $550. All these prices are the norm if you just google them. I put some links to them at the bottom of this post. I know I saw the honda 2000w on sale at camping world a few weeks ago for $999. Also, check ebay or craigslist for them. Yamaha makes a really quiet genny too. Hope this helps.

http://www.go2marine.com/product.do?...F&WT.mc_id=gb1

http://www.islandwaterworld.com/brow...n/4,12010.html


http://www.americanrvcompany.com/Kip...RV_p_1523.html

http://www.americanrvcompany.com/Kip...RV_p_1519.html
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Old 12-11-2009, 12:54 PM   #3
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Re: solar system for running an A/C

i wanna get back to making a solar system big enough to run an a/c. 850 amp hours a day. plus some wiggle room for the fridge, lights, and tv. we wont run a microwave, heater or any other high demand devices. so it looks like 1000 amp hours should hold me. i know this isn't the budget minded idea that skoolie.net is full of. but it IS possible.

ok so lets think money no object theoretical huge cabin sized system. i know you can do this. i've just got to find parts big enough or than you can run in parallel.

the panels i've got all figured out. the batteries i've got all figured out, wire is easy. the inverter is purchased.
panels: 4 220 watt REC's
Batteries: 10 220 amp hour trojan T 105's
wire: 4 gauge from each panel to the controller(s). 2/0 for the controller(s) to the batts and the batts to the inverter. 4 gauge from inverter to breaker box. 12 gauge for house circuits.
Inverter: go power 3000Watt pure sine

pieces that aren't clear:
charge controllers: i haven't found one over 100amps. so i'm gunna need two that can have their outputs wired together. i've heard people complain about two crappy controllers being used in the same system that aren't rated for it. so what about the good ones?
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Old 12-11-2009, 02:59 PM   #4
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Re: solar system for running an A/C

We have a 13K BTU Coleman TSR Rooftop AC unit on our RV. We have an Onan 2.5KW genset. This does handle the surge required to get the compressor motor running. In the RVing world, a 2.5KW genset is basically the minimum to run an AC unit. Our RV is 22 ft and we can freeze ourselves out with the AC unit (I run low fan and bump the thermostat up fairly warm). I have always been told that powering an AC unit with solar was impractical. Supposed to require too many PV panels and batteries. For that reason, we plan on using a couple solar powered deck vents like these: http://www.westmarine.com/webapp/wcs...classNum=12407

At most, we only use AC for a few hours per day (few windows in the RV) and mostly use the fan of the unit. I did run the AC unit while heading out west (temps inside were 90+, outside temps were 100+). I also ran the genset while heading down here to Corpus from NM. The genset powered the 120VAC 3cf fridge and 120VAC 4 cf freezer (all lights are 12DC and powered off the battery... which is charged by the genset or shoreline) along with a couple hours of 120VAC TV/DVD watching. We used about 1/4 gal gas per hours (or a little less).
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Old 12-11-2009, 05:57 PM   #5
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Re: solar system for running an A/C

found it!

morningstar tristar ts-60
http://www.morningstarcorp.com/en/suppo ... .03.EN.pdf
page 8

ruff quote: any number of tristar controllers can be run in parallel in battery charge mode.
ok so i need two tristar 60's.
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Old 12-11-2009, 08:01 PM   #6
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Re: solar system for running an A/C

Try using watt-hours per day instead of amp-hours. Remember, watts equal volts times amps. Figuring losses, it takes 11 amp-hours from a 12-volt battery bank to produce 1 amp-hour at 120 volts.

You say you need 1000 Ah per day. I suspect that's at 120 volts, so that's 120,000 watt-hours per day.

Four 220 watt panels give you 880 watts in standard insolation. We get the equivalent power of 4-5 hours of standard insolation up north in winter. You might get 6+ in the southwest. Since we are talking summer heat, let's make it 10 hours with never a cloudy day just for fun.

Solar panels max output: 8,800 watt-hours per day. A/C draw: 120,000 watt-hours per day.

If you really want solar air conditioning, you can develop my convection idea: Take apart a propane refrigerator, use the icebox coil in an air duct, and have a vertical solar collector on one side to activate the heat tube. Small computer fans run by PV would lift interior air through the coils to outlets near the ceiling.
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Old 12-13-2009, 06:58 PM   #7
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Re: solar system for running an A/C

Quote:
Originally Posted by Redbear
If you really want solar air conditioning, you can develop my convection idea: Take apart a propane refrigerator, use the icebox coil in an air duct, and have a vertical solar collector on one side to activate the heat tube. Small computer fans run by PV would lift interior air through the coils to outlets near the ceiling.
That is a really good idea! One could use lenses to focus the sun on the plate where the propane would usually burn..This is a very workable idea, if you could get the pipes & hoses to where you needed them to be!
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Old 12-13-2009, 07:50 PM   #8
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Re: solar system for running an A/C

nope i converted everything to 12 volts already. 1000Ah at 12 volts for everything. it isn't really that hard to run the A/C. but it ISN"T cheap. i'm looking at
2500$ in panels
400 in charge controllers
1300 in batteries
500 inverter
200 battery monitor

so that's 5 grand. plus wire and other things i forgot
but it's silent.
and i have plenty of roof space for this. i just don't have a rooftop a/c, sky lights, or excessive venting.


edit: i'll look into how the propane fridges work and get back to you. if i went propane on heating and cooling it would save me a huge amount in electricy needs. i originally didn't plan on using any propane since i'm trying to limit the number of systems. but if i used it for a/c and water heating, and fridge... then it might be worth it.

double edit: i'd have to get one hell of a huge propain fridge. 5K btu might do it.
also i need to add 2 more panels to the roof. still plenty of space on a skoolie. but i m might have to rig up a folding awning over the cargo area of my truck to make it fit.
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Old 12-14-2009, 08:19 PM   #9
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Re: solar system for running an A/C

i converted the energy usage (or is that power usage? i need to take physics again) from 120 volts to 12 volts. taking into account losses in the inverter. i got a 120 volt, 10K btu a/c that draws 9.2 amps (at 120 volts) according to the label. so that's 92*1.1=101.2. I rounded to 100 amps @ 12volts dc for my spreadsheet. it could be a lot lower but i havent gotten my kill-a-watt meter yet.
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Old 12-14-2009, 09:46 PM   #10
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Re: solar system for running an A/C

AC Hard Start Capacitor:

http://www.modmyrv.com/2009/05/27/rv...tart-capacitor
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Old 12-21-2009, 06:34 PM   #11
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Re: solar system for running an A/C

did some research on propane powered refrigerators. now i'm no expert but i found information on a few independent sites.

the biggest propane fridge i could find is 1500BTU. that's an 8 cubic foot model. it's said to use 2.5 gallons of propane a week. and it weights 200 lbs. so i would need four of these units to equal the BTU output of the smallest window a/c i can find. to equal my $200 window unit it would cost...$8000. and if the run time stays the same as if it were a fridge then i would use 17 lbs of propane a week.

it looks like solar might be a better option.

also i wanted to mention that my set up is somewhat unique. i have a flat roof. i can layout the vents in any way i want. my a/c doesn't take up valuable roof space. this lets me make the solar panel installation first priority. not an after thought.

crunching more numbers and finishing the planning stages
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Old 01-11-2010, 12:50 PM   #12
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Re: solar system for running an A/C

Okay a plug against refrigeration....

Do you really need AC? I lived for 4 summers in western SC without AC. It's not that bad, really. My bus is painted white, and that helps a whole lot (as I could tell when it got dirty and heated up.) When the bus was clean, if I had the windows open, it never got hotter inside than it did outside. Of course, there were days where it was around 100 in the bus, but it was 100 outside, too. MOST of the time, it was much more comfortable than that, those were just the high temperatures in the heat of summer. I was working primarily outside at the time, so I was accustomed to the heat, which can make a personal difference.

To me, it just seems like a huge labor and capital investment for the marginal benefit of having cool air.

But then, I don't like refrigeration much.
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Old 01-11-2010, 07:30 PM   #13
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Re: solar system for running an A/C

hey differing opinions. cool. i really wanted to get some of the smart guys in here thinking.

i'm considering changing my plans slightly to make the solar system easier. i'm intending to use my skoolie(or whatever truck i start with) as a way to get my offroad toy to faraway destinations and make my g/f more cumfy during the day and both of us more cumfy at night. i was gunna make the living space small and put the offroader on the back of the truck. but if i get a slightly larger (5 extra feet) truck and get a trailer for the offroader, i can double my roof space.

so there will be a lot of time during the day when we will not be in the bus and can let it heat up. also the size of the solar system means it will cover (shade) 80% of the roof. also the thing will be white and i will insulate the heck out of it. the planned system right now is:
10 220 Ah 6V trojan batteries
4 230 watt panels (if i go truck with a trailer i can double this number. 8 panels just equals the cost of a diesel genny that will last 15 years.)
2 60 amp 3 stage charge controllers.
4/0 wiring.
3000 watt pure sine inverter
right now i'm messing with designing my own tilting system that i can set up in less than a minute from the ground.
i'm not so sure that a generator is even cheaper in the long run. my kill-a-watt came today. so by this time next week i should have solid numbers on usage for everythign besides the a/c. but i like leaving margin in my spreadsheet for that one anyway.

i'm not gunna run any DC stuff in my conversion. i just like AC better. don't ask me why. i've already gotten all the electrical components in ac versions.

o yea it's usually between 95 and 105 where i plan to go, when i plan to go. july and august in central texas, utah, arizona, and so cal are not the places you want to go without a/c. people who think they can get by often DIE in the places i'm going. yes you can do it with enough water and shade. but why suffer if you don't have to. what's the worst that could happen with my system? i'd have to set the thermostat at 85 instead of 70? i think i can live with that.
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Old 01-12-2010, 03:31 PM   #14
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Re: solar system for running an A/C

Personally, I looked into this a long time ago...and lost all the data I had collected....and am 100% sure it is possible. However, I am also convinced that it will be an extremely expensive system to set up and is one of the reasons so many here are saying that it is a crazy idea. I remember crunching some numbers on what it would take to build my ideal, not realistic, bus and this concept would have accounted for well over 60% of the total cost of the project. I'm talking a lot of money, but if you've got it to throw around and it's what you want, who's to tell you different?
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Old 01-14-2010, 10:59 AM   #15
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Re: solar system for running an A/C

ok guys. i know both of you are trying to help. all the input is appreciated. but the bickering isn't helpful.

to address handy bob: his blog is basically where i learned what i know about solar. i've since gone deeper and found pretty much everything he's said to be true. there are a few nickpicky things i don't like about his site. he talks in numbers of panels. that is the worst unit of measure i've ever heard of. the panels i want to use are 230 watt. i can get 15 watt panels. he is using a few panels near 135 watt. also i think that solar implementation is way behind solar capability in the rv industry because of the lack of electrical knowledge on the part of RV'rs and retailers alike.

he says you can run a/c on solar. but i'm betting that it's too expensive and wasteful for him. at heart he is into being efficient. and anyone who's crunched the numbers knows that a/c is one of the least efficient things ever created. i'm not interested in efficient. i'm interested in cumfyness.
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Old 03-19-2010, 12:13 AM   #16
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Re: solar system for running an A/C

Quote:
Originally Posted by jam0o0
ok i'm new. i've been researching all the ins and outs of building your own rv. this seems to be the place to get good answers. my background is mechanical engineering and design. but i could use some guidance with this electricity stuff.

so 1000W A/C,... 7 hours a day... i'll get 5 hours of peek sun per day
Here are a few things to think about when designing a Photovoltaic system:

You only charge during daylight hours. You have to get all the power you need in that time. One commonly overlooked factor is how quickly you can push that energy back into the batteries. Here's a good rule of thumb: the fastest you can charge a battery bank is 1/10 the total Amp hour capacity.

You said 1000Watts 7 hours per day. To make that 110V alternating current power to run the A/C you need an inverter. Let's say it's a REALLY nice one and it 90% efficient. So you need to make 7hrs * 1000W * 1.1 = 7.7 Kw-hours of power and get it into your batteries.

7.7kw-hrs = 640 Amp-Hours in a 12 Volt battery. Now keep in mind you can only run the battery down 50% AT MOST (better to stay above 80%) so you should have a battery bank capacity of at least 1300 Amp hours. Let's say you use Trojan T-105 Batteries (a common choice) which have 220 Amp-hours. So you need 6 pairs of batteries. (it takes two batteries in series to make 12V and capacity only adds in parallel).

12 batteries at $140 each. Now how fast can you charge them? Max charge rate is about 1300 * 1/10 or 130 amps. You need to replace 640 amp-hours so you'll need to charge at the max rate for 640/130 = 4.9 hours.

Oh, good we have 5 hours of daylight! Seems doable right? But wait, let's look at battery charging a bit more carefully. When you first start charging a "dead" battery, the battery will take lots of energy (the full 130 Amps) but as it charges it will take less and less. This is because the difference in voltage get smaller and smaller. In reality, it will take MUCH longer to get you batteries back to full charge. So you need to push even MORE current. That means a more batteries and a bigger charger.

So right now we have $1700 worth of batteries, a single 130 amp voltage controller or, yes, multiple 70 amp controllers and finally enough PV panels to push 130 Amps at 15 volts (14.4-14.8 is the charging voltage) That's 15Volts*130Amps = almost 2000 Watts worth of solar panels! We're talking over 15 big panels at $500-600 each. That's at least $7500 worth of panels (way conservative estimate on panel price).

When you add up wire, fuses, bus bars, charge controllers, charge status monitors, bla bla bla We're talking at least $12000 for the whole system. (Don't ask me where you'll put 15 120 Watt PV panels on an RV).

And all that will build you a system that runs it's batteries down too low and can't charge them in the available daylight.

Can you build a PV system to run your AC? Yes. Is it hard? Yes. Is it expensive? Yes. The biggest danger is spending thousands (even 10's of thousands) and still being hot with no AC.

Quote:
Originally Posted by jam0o0
is there a generator that isn't gunna bother the tent campers in our group? it's gotta be VERY quiet and i haven't found one yet.
Honda and Yamaha make generators that are 60dB, I don't think you can get much quieter. But you pay for quiet. Still cheaper than the solar.
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Old 03-19-2010, 08:42 AM   #17
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Re: solar system for running an A/C

I'm going to throw my hat in the ring here. Do you think a 1000 BTU window unit is really going to do what you want it to do? My bus is NOT well insulated, but does have a white roof and off white ("International Harvester White") sides on it. The closed off living room area is about equal in size to a short bus. When running a 1000 BTU window unit hooked to a generator I was not able to make an appreciable temperature difference occur in 85+ degree weather. It was much more "pleasant" to just drop every window and take advantage of what breeze did exist.
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Old 03-21-2010, 01:17 AM   #18
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Re: solar system for running an A/C

I'll add my own thoughts given your usage pattern-

1) Insulate that little area as best as you can. (4" foil-backed foam?)
2) Buy the biggest AC unit that will fit. (it sounds like you're in Africa or something- better over-provision)
3) Buy a Honda/Yamaha generator that can do double that wattage and just run it. (they are seriously quiet, you could sleep next to them)

Running something like an AC unit (1000-1500W, constant) is just really *really* hard on solar/battery stuff.
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Old 04-07-2010, 08:11 AM   #19
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Re: solar system for running an A/C

Quote:
Originally Posted by gwreddragon
Don't forget wind, either (which may help on those cloudy days). I've seen pictures of several buses with wind turbines.
yes don't forget about wind power, especially for at night to.
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