Solar A/C Possible?

[pickpaul]

I've been following the solar discussions with a lot of interest but A/C is a sticking point as I assumed it was impossible and I need it. I'm going to be on the East Coast mainly and it gets hot here in the mid Atlantic region and the Carolina's

If I had a full roof of solar minus the space for 2 or 3 rooftops, could I run 1 on a hot sunny day to keep the bedroom cool? 2? 3? Assuming a large enough inverter of course. A pair of 4000w 24v to give me decent generator level outputs? Enough roof space to produce that kind of juice? On a hot summer day? Enough battery capacity feasible to run the bedroom A/C overnight? if it's not possible would moving to mini splits give me enough extra panel space to run one A/C?

I'd love to here people's thoughts.

Cheers, Paul.

[Jeremy]

Sure it would work, if you let it charge the batteries for long enough.

But have you priced up 'a full roof of solar', twin 4000w inverters, the large battery bank you'd need, plus the charge controllers, and everything else? It would probably cost more than the bus.

But perhaps I'm just jealous of your buying power, because it certainly would cost more than my bus.

Jeremy

[scanzel]

Anything is possible but at what expense. Enough panels, batteries and two inverters, solar charge controllers and all the wiring etc, air units etc. Easily 10 to 15 thousand. Batteries would be the killer with their weight.

[bevans6]

Back of the envelope figureing: 

Bed room only, you can use an 8K btu ac unit.  Figure 800 watts to run it, twice that to start it.  One 2,000 watt pure sine inverter
figure a 50% duty cycle on the compressor at 800 watts, 100% on the fan at 200 watts, 10 hours of operation, that is 6,000 watt-hours, which is 275 amp hours at 24V nominal with 10% loss, which means you could do this with a 550 AH battery bank taking it to a 50% SOC.  Figure 10 typical golf cart batteries.  You'd have to put back in around 20% more than you take out in recharging to count for efficiency losses, so you need enough solar to produce call it 8,000 watt-hours of power in the sunny part of the day.  If you say you can get 3 hours worth of optimum sunshine with all the fiddle factors to reduce the efficiency of the panels, you maybe need 3000 watts of solar panels?  I have no idea if you can put that on a bus roof.

My lack of knowledge on solar is immense, I have no idea if this is real or not...    but it's what the back of my envelope now says...  I have no idea why you think you need 8,000 watts of inverter.  That is immense for a bus solar operation.  350 amps at 24v nominal, it would be hard to run even from an engine alternator!

Brian

Brian

[TomC]

On a 40ft x 8ft roof, that's 320sq/ft or 46,080sq/in.  Each 130 watt solar panel is 58x26.2 for 1519sq/in.  Divided by roof that gives 30 panels.  Take 3 panels off for 3 roof airs and you have 27 panels x 130 watts each for a total output of 3510 watts-which you could power two roof airs.  Solar panels cost about $4.00/watt-so you'd be looking at about $14,000.00 worth of panels, then two inverters at about $2,000.00 each for $4,000.00.  Then you have controllers, installation-probably getting real close to $30,000.00 for the system.

If you buy a $10,000.00 Diesel genset, you'll have $20,000.00 left over for Diesel fuel-which will buy 5,000 gal.  That will run the genset for about 7,500 hours (the equivelent of 900 days of running the genset 8hrs a day).  If you think you'll be using the A/C more then 900 days in the bus, then the solar panel system probably will pay for itself.  Good Luck, TomC 

[chev49]

im working on the same thing, but was just considering upping my batteries to 6 cart ones, and buying smaller diesel genset to replace my gas one, and haven't figured out how many panels i can easily get on the bus with all the other stuff already up there...

[Kevin Warnock]

Much more practical would be to make a tiny bedroom compartment, like a bunk bed compartment. Cool that at night with a 5,000 btu window unit that might only need to run 10 minutes per hour, on and off. You can run this with not too many batteries, and you can charge them during the day with the gen set you will need for AC during the day.

If you want to avoid the gen set during the day, and you don't mind cooling only a small part of the bus conversion, then you can use a 5,000 btu window unit to cool a tiny office around where you sit to work, maybe 20 square feet? That could be done with panels that will fit on the roof and a 1,000 watt inverter for the tiny air conditioner.

This is all speculation, but it's what I have planned for my RTS.

Kevin Warnock
http://KevinWarnock.com - my blog

[buswarrior]

Air Conditioning a bedroom for the night off a set of batteries has been doable for as long as there's been a modern efficient inverter available.

I know a fellow up here who did it in an MC9 back at the turn of the millennium.

He was able to do the night with 8 Trojan T105 golf carts, SOC down to 50%, with room in the rack and plans to make it 16 batteries. All on a pull-out tray in the spare tire compartment.

Starting with the coach already cooled down, it was well insulated, and the bedroom wall and door tightly constructed with this design goal in mind from the outset. AC was run on low fan setting and compressor cycled somewhat less than 50% in his experience at that time, but suggested 50% was a good design number.

The motivation? 

Silence.

And perhaps someone told him he couldn't do it?

I wonder where Stan is now?

How to charge it all back up?
I'd have as small a generator as I could load well, with a modest amount of solar panel to finish the charge, and maintain the batteries when the coach is stored between uses. A good SOC meter to keep me and the system honest, and the batteries alive.

happy coaching!
buswarrior

[technomadia]

We're sizing our battery bank and solar panel install to do this as well... for at least limited amounts of time.  

Our first step was playing around with Lithium Ion batteries, which enables us to get the battery capacity we need in a smaller size and weight range.  Plus a bunch of other benefits, one of which being no loss of capacity with a high draw that comes from an A/C unit.  Currently, our 500 aH battery bank can run one of our 13,500 roof airs for about 2 - 3 hrs, which takes our bank down to a very safe (for LiFePO4) 80% DoD.  In the next few months, we will probably double the capacity.  

We are also contemplating switching to mini-splits if we can figure out where to put them - as they are much more energy efficient and quieter.

We have not yet approached the solar install, but will be aiming to get as much as we can on the rooftop.  For our 35' bus, we're not anticipating being able get more than 700-900 watts up there.  Don't figure on being able to just line your roof minus where your A/Cs are - you also have to take into account shadowing from anything else on your roof.  Just a little bit of shadow from a A/C unit, vent fan or antenna can significantly lessen your panel's efficiency.


When it's hot outside and we absolutely need to keep cool with air conditioning, we'll generally seal ourselves in the bedroom with our laptops - which cools down really quick.  We survived this past summer with 120+ degree temps in Arizona with only enough shore power to run one air at a time.


We don't think it will be feasible to be totally solar dependent during an extended warm/hot spell when full time A/C use is needed day after day.  Our goal is to be able to control *when* we need to run the generator or plug in to top off.  If we can cool off a bit on a warm day, or run the A/C overnight without the generator - that's pretty sweet to us.

- Cherie

[TomC]

Sealing the bedroom off from the rest of the bus to just cool the bedroom sounds good on paper.  But-if you have an air cooled compressor type refrigerator, it will not like the warm surroundings to try to keep your food cool.  Good Luck, TomC