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Old 09-07-2009, 19:24   #1
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Question Solar-Powered Air Conditioning ?

Hi,
this is D. This is my first time posting. T and I are using the same profile so I'll delineate myself from him to keep my silly questions from being mistaken as his.

My question is this: We are planning to go on a long cruising trip sometime in the future. Is it possible/practical to have a small air conditioner that runs off of a 12V battery that is powered by solar panels? I would only want this for the times when we are anchored(we are going to stay at marinas as little as possible) at night while sleeping..just the Vberth area..even if its just in the beginning of the night for a few hours. I was looking into some different brands that claim they can do this..
http://www.cruisair.com/sheets/L-2425D.pdf

Finding enough DC Power For Your DC Breeze 12v/24v Air Conditioning System

Does anyone have any experience rigging something like this up?

and if not how do the members that have been out cruising for 2 years or so keep cool when its hot outside. If you are anchored for say 2 weeks and its 85-95 degrees outside and you have no way of cooling down, what do you do?

When you cuddle with your loved one in the heat do you sweat and soak the cushions? then what do you do?

your input is greatly appreciated.

T & D
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Old 09-07-2009, 19:29   #2
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Is it possible/practical to have a small air conditioner that runs off of a 12V battery that is powered by solar panels?
You would need a barge to tow the solar array and it woudn't work at night. The good part is the engine required to tow the barge could power the air onditioner off it's generator at night.

Sorry, there are laws of physics that we can not overcome. One of those time and space things.
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Old 09-07-2009, 19:34   #3
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You use lots of fans.
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Old 09-07-2009, 19:36   #4
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Its mainly marinas where you need air conditioning. At anchor you are usually pointed into the wind so the air plunges down the hatches

In marinas the wind doesnt get in so much, nor in the right direction.

Finally, you will find you acclimatise pretty quickly

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Old 09-07-2009, 19:41   #5
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When you sail you are surrounded by the worlds largest and most efficient AC nit, the Ocean. Go for a dip! About getting the sheets all sweaty, I dunno, it's one of the many awful things about being a human being. You just have to embrace it. I have one of those wind scoops that I put over my v berth hatch. It may end up on the "things you've thrown overboard" thread but I haven't tested it yet in a really heavy heat. We do also have dc fans on the boat.
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Old 09-07-2009, 20:12   #6
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I dunno, it's one of the many awful things about being a human being. You just have to embrace it.
I wouldn't want my wife to catch me.

Scoops are good. I just saw one that will work with our butterfly hatch. It's mostly about where you choose to anchor. Most folks screw up and tuck in tight where there isn't any wind. Scoops are like sailing they need wind.
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Old 09-07-2009, 20:23   #7
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Barge cost?

LOL..a barge huh...I wonder if we can fit that into the budget?? let's see, if i only eat once a day..cut out all alcohol...never leave the boat for fun..I think i could pull it off. Eureka!

Thanks for the input-it made for a good laugh.
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Old 09-07-2009, 20:54   #8
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Yes you can

Lets see 2 people at a heat load of 100 watts each (600 BTU/hr), wringing the moisture out of the air, 80% relative humidity at 85F dropping to 75F at 50% humidity with new air being introduced.
We would need 5000 BTU of cooling (600 WattHr) for the first hour tapering off to 3000 btu/hr (350 WattHr).
So for 2 hours we need 950 WattHours of electrical energy, in Cruiser terminology roughly 90 AmpHours.
So if we were to use solar panels we would need 300 watts of solar panels assuming it is sunny all the time. We would also need to increase the battery bank by 500 ampHrs. All that would add 450 lbs to your boat.

Get a wind scoop or a fan.
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Old 10-07-2009, 06:23   #9
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Lets see 2 people at a heat load of 100 175 watts each (600 BTU/hr ✓)...
Heat gain calculations often assume 600 BTU contribution per person occupancy.
Since 1 Watt = 3.412 BTU (600 ÷ 3.412 = 175.8), each person’s heat load would be about 175 Watts (not 100W).
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Old 10-07-2009, 06:47   #10
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Yes, You Can -- But....

If you can believe the specs for the new mini CruiseAir A/C, it puts out 3500BTU of cooling for a sustained DC amperage draw of 13 amps @ 12V.

Thus, figuring an hour or two of cooldown prior to sleeping in a very small cabin (like the V-berths on many boats), and getting a full 8 hours sleep, the total usage would be:

13 amps x 10 hours = 130AH @ 12V.

Now, the relevant question: could you have sufficient battery storage to draw down 130AH overnight without any recharging?

Clearly, the answer is "yes"....most cruising boats have at least that much capacity (say, around 300AH total capacity in the house banks).

However, after a good night's sleep you are now faced with the challenge of putting back those 130AH into the battery bank. Including losses, this would involve a little more than 150AH charging current.

A good generator could do this in about 90-100 minutes run time. So could a typical engine-mounted alternator of, say, 110A capacity.

None of this calculates other loads on the house system...lights, frig, instruments, anchor light, windlass, etc. If your daily energy consumption -- without the A/C -- is on the order of 150-200AH (which is typical of many mid-size cruising boats these days), then you'd be looking at replacing that amount PLUS the 130AH drawn down by the A/C.

Bottom line: it's probably JUST doable with a capable onboard fuel-driven charging system. It's probably NOT practical with solar-power only unless you have a huge solar panel system and lots of sunlight.

It's probably a great solution for smaller boats -- and maybe bigger ones -- for overnight stays on the boat plugged into a shoreside electrical system.

What's a good night's sleep worth, anyway?

BTW, having spent many years in the Eastern Caribbean, I second the thought that at anchor most times you don't really need A/C, but in sheltered marinas where the tradewinds don't blow it would sometimes be a godsend.

Bill
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Old 10-07-2009, 07:35   #11
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Heat gain calculations often assume 600 BTU contribution per person occupancy.
Since 1 Watt = 3.412 BTU (600 ÷ 3.412 = 175.8), each person’s heat load would be about 175 Watts (not 100W).
Sorry Gord I suppose I should have explained the 600 Watt heat load in greater detail
The heat load is 100 Watts per person for casual activity. I multiplied it by 2 people to give 200 Watts and rounded it out to 600 BTU/hr for sleeping. 175 watts a person is strenuous activity.
It was all intended more as a quick exercise to show the energy related problems with 12 volt battery supply as btrayfors pointed out.
The concern I would have has to do with humidity. After cooling a space overnight all the surfaces are now below the dew point. Shutting off the air conditioner would cause water vapour to condense on all surfaces especially the ones that are hidden. It could cause long term mould issues. It would take longer to dry out the hidden area than the next nights cooling cycle.
Air at 90F and a relative humidity of 80 % has a dew point of 83F. Condensation will form on any surface cooled below this. Leaving the cabin door closed during the day would have a minimal effect.
Condensation is one of the major concerns when power is suddenly shut off (hurricane etc.) to air conditioning buildings in high humidity areas. Only a few windows need to be broken or a door left open. The moisture condenses on and in materials such as drywall. Mould begins growing in a few days time requiring the removal of the materials and a chemical wash down of nearby surfaces.
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Old 10-07-2009, 11:01   #12
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There is often a breeze and your anchorerd boat turns to the breeze, your open hatches pull it in. If the breeze dies you use small fans or take a quick dip in the water. You dont need AC...
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Old 10-07-2009, 13:10   #13
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Paul-
"Sorry, there are laws of physics that we can not overcome." Don't be a rube, our grasp of the "laws" of physics is superficial at best. For instance, in a Bose-Einstein Condensate (a fifth state of matter, beyond liquid, gas, solid and plasma) they've done some wierd things and clocked the speed of light as shifting down to literally walking speeds.
Add a layer of BEC over your solar panels, continue to slow down the light, and you might indeed be able to "expose" your panels during the day--but have the light actually reach the reactive layers many hours later, at night.
I know, it sounds totally impossible. Isn't that what folks told the Wright Brothers? And why the scientists on the Manhattan Project violently protested that the atom bomb might cause total combustion of the atmosphere?

The laws of physics appear to be subject to considerable local variation, and terribly poor local knowledge. Meanwhile...there's always the genset.<G>
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Old 10-07-2009, 13:46   #14
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A good generator could do this in about 90-100 minutes run time. So could a typical engine-mounted alternator of, say, 110A capacity.
I'm not sure the batteries can absorb it that fast though......?
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Old 10-07-2009, 13:59   #15
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I'm not sure the batteries can absorb it that fast though......?
No problem at all for AGMs. They'll take just about as much as you can throw at 'em :-)

But even a sizable flooded bank can easily absorb this. My house bank (six T-105's) is 675AH capacity, and will very easily take 150A....if I could generate that much. All I can do with my paltry setup, though, is about 120A (with a Victron MultiPlus inverter/charger) :-)

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