DC Air Conditioning

[gbillman]

Does anyone have any experience with this? I have found several websites that claim all kinds of good things, but still find it difficult to believe that an ac can run on dc (even a small, cabin-only unit). I may be technologically behind the times though.

Specifically, I am looking to put ac into my '06 Leopard 40 (no genset currently). Was wondering if I could get away without a genset if I could find a proven dc air conditioner unit, and put enough solar panels aboard to keep some of the newer batteries powered up.

Thoughts anyone??

Thanks.

[jcapo]

With enough battery capacity and solar capacity to recharge those batteries, yes. And a large boat to carry them.

Got a link to a site that claims DC air cooling is practical?

[valhalla360]

Running air/con on DC is certainly technologically feasible. If you can supply the DC power.


The problem is generating and storing the power to feed a system that can actually cool the boat.


Our 12,000btu air/con which is barely enough on hot days, pulls around 10amps at 120v. Convert that to 12v and you are looking at pulling more like 100amps. Even if it was wildly more efficient, you are still talking about drawing maybe 50amps. Assuming an 80% duty cycle, that's 1000amp-hrs (probably a bit low on hot sticky days when you really want the air/con).
- To provide 1000amp-hrs, you would like to have a battery bank on the order of 3000amp-hrs so you don't destroy the batteries in a couple months.
- To replace that 1000amp-hrs, you need to figure solar panels only put out their rated capacity for maybe 5hrs per day. That translates into 200amps or about 2400watts of panels.


Can it be done? Sure but it gets silly pretty quickly.

[newhaul]

What is meant by efficient? The best I could find draws 40 amps at 12 v nom for 8,000 btu cooling. So it could be run off of a 600 watt solar array.
Hammond Air Condtioning Ltd.

[a64pilot]

Most efficient system I've seen is the 16K I just installed, pulls 11 amps at 120V, so say 6 amps at 8 K, including water pump.
Still 55 amps at 13V. First I bet they are quoting amps at at least 14V, but still it's a whole lotta power.
I'll go out on a limb and state that I bet that 120VAC is more efficient than 12VDC, enough so that you offset the 5% loss in an inverter, 120VAC airconditioners are way more plentiful, therefore less expensive, and are more useable on shorepower and when you give up trying to run the thing on DC. I'm saying buy a small efficient AC one and use an inverter for your experiment.
Heat pumps are actually pretty efficient, just it takes a butt load of power to move heat, sort of like boiling a pot of water, no matter how efficient you get, it takes a tremendous amount of energy to boil a pot of water, regardless if source is electrical or chemical, Solar whatever.


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[StuM]

Originally Posted by newhaul

What is meant by efficient? The best I could find draws 40 amps at 12 v nom for 8,000 btu cooling. So it could be run off of a 600 watt solar array.
Hammond Air Condtioning Ltd.

That link actually says 45 Amps, but anyhoo...

That's OK if you only want to use it for about 5 hours in the middle of the day and don't need additional power for anything else.

And even the bl**dy supplier stuffs up the units (45 amps/hr - No it's just 45 Amps! ) so I wouldn't put too much faith in their claims.

[StuM]

And BTW, 8000 BTU will do very little for a cat like a Leopard 40 in a hot climate.

Our 18,000 BTU has to work hard to get the Belize 43 below about 27°C on a hot day here.

[a64pilot]

It takes a 16K and a 5k to cool my IP. The idea of the 5K was it would cool our Stateroom and a Honda would easily run it. It will cool the Stateroom and a Honda will run it in eco mode. It pulls I think about 5 amps.
Funny thing is my old Cruise Air 16K pulled 16 amps, the new 16K Webasto only pulls 11, but the Webasto is harder to srart.
I think if you go with the idea of a small unit just trying to knock the temp and humidity down a little in your Stateroom for a few hours so you can sleep, then it might work.


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[newhaul]

Originally Posted by StuM

That link actually says 45 Amps, but anyhoo...

That's OK if you only want to use it for about 5 hours in the middle of the day and don't need additional power for anything else.

And even the bl**dy supplier stuffs up the units (45 amps/hr - No it's just 45 Amps! ) so I wouldn't put too much faith in their claims.

My bad on the amp quote that was for the cruise air or a telecom specific unit I found ( don't remember which ones)
Here is the one with the 40 amp draw quote
http://www.cruisencomfortusa.com/marine/

[valhalla360]

Originally Posted by newhaul

What is meant by efficient? The best I could find draws 40 amps at 12 v nom for 8,000 btu cooling. So it could be run off of a 600 watt solar array.
Hammond Air Condtioning Ltd.

On a hot sticky day, it may be running for 20 hours (maybe more).

20hrs * 40amps = 800amp hours

600watts at 12v = roughly 50 amps.
You can count on maybe 5 hrs of output over the course of a day. Reality is morning and late afternoon will be drastically less than 50amps and even during the peak of the day, you are likely only putting out 40-45amps, so for simplicity figure 5hrs at the rated output (assuming it's not a hot sticky...CLOUDY day when you may get no where close to that output).

5hrs at 50amps = 250 amp hours

250amp-hr is a lot less than 800amp-hr

Also for our much smaller Gemini 34' 12k btu is really boarderline. 16k is typically what others suggest. 16k btu would presumably translate in 80amps.

Now you have 250amp-hr of generation and you need 1600amp-hrs.

Wait, you aren't trying to cool a little 34' boat, you are looking at a much larger boat...

And don't forget you need a battery bank and charging system to handle the storage of all those amp hours.

The problem isn't really AC vs DC, it's how to supply the electrons to make either work (Reality is I glossed over some differences in power conversion and efficency but it's so far from realistic that it won't change the final outcome and may make it worse when you actually run the numbers).

If you accept a severly limited system that will just take the edge off in a single small cabin for a few hours at night, it's doable. We did that with out first house way back with a window unit. And what happened...in the dead of summer, as soon as we finished dinner, we retreated to the bedroom. Why buy a beautiful big boat and cheap out so much that you can only use one tiny corner?

[newhaul]

I don't use ac on my boat. A couple little fans is all I need if its to hot I move to cooler weather the op asked about efficient units and the 40 amp draw unit was the lowest draw 12 volt unit I could find. Personally if I was to use ac I would go with a heat pump type unit and a generator.

[HopCar]

I only need 5000 BTU to cool my little cabin. I looked into running it on DC. I bought a generator.

[a64pilot]

Originally Posted by HopCar

I only need 5000 BTU to cool my little cabin. I looked into running it on DC. I bought a generator.

How did your install go? You pleased with the unit?

[HopCar]

Hi Jody, I haven't even ordered it yet. I've still got Bill working on the engine.
I'm not expecting any problems. The Webasto should be almost a drop in replacement for my old one. It should be even easier to run on my little Honda Genny.

[a64pilot]

Yes, the 5 K, the Honda will run in Eco mode, I've done it, the Honda does it easily and stays quiet when it does it. 5K only draws about 600 W, the Honda can make 1600 W continuously I believe, so you can run the AC solid and have plenty left over to do whatever else you want.