Marine Air 3500 BTU A/C runs on 12V

[James V]

http://www.marineair.com/pdfs/L-2425C.pdf

About the size of a Battery box and draws 29 amps (12v DC)

Has anybody looked into this?

[Catigale]

doesnt sound right on the thermo end. A 3500 BTU/hr AC converted to watts is about 1000 Watts. 12 VDC * 30 amps is 360 Watts.....?????


Anyway, one group 24 battery would run at 29 amp draw for about 30 minutes to half discharge. My guess it would run less than this at this heavy current draw, so you would need an external generator to provide the juice for this. Once you have a gen set, you might as well run it on AC current of course - higher voltage means lower current which means less loss.

[Moe]

Twenty-nine amps ÷ 3 batteries = roughly a 10 amp draw per battery. Not too bad, but near double the 20 hour discharge rate of the typical Group 27 amp-hour rating. Meaning you don't have the full "rated" amp-hours out of the batteries.

At a 40% duty cycle, you're looking at 0.4 x 29A = 12 amp-hours for every hour of use, or about 13 to replace for each hour's use, or 130 amp-hours for a 10 hour overnight. More with a higher duty-cycle. That's a lot of generator or alternator run time, even if you don't try to charge higher than 80% charge, where charging takes much longer.

[James V]

I also think that this needs a water supply to help transfer the heat out.
So you will need a below waterline though hull and another above water line.

[James V]

One report on this is that it cost $ 3500. A bit to much for me.

[Catigale]

Im still not seeing the math on this one.

A small room AC pulls 10 amps from the house current, which is roughly 1200 Watts (ignoring AC factors etc)

30 amps DC at 12V is only 360 Watts.

You cant match 1200 Watts of power with 360 Watts, simply put.

[tgentry]

Is it just me, or does the Dedicated Power Module (DPM) appear to be a DC-AC inverter?

I agree, the current draw isn't making sense, although 3500 btu isn't much. Also, lower down on the page it lists MINIMUM current draw as 31 amps.

[blueskiesup]

First off it's very cool that someone is working to create small devices for marine applications. This sucker is really tiny!

So the math on this one is related to the size...

3500 BTUs is about 1/3 the size of the smallest window airconditioner you can find at 10,000 BTUs... and they pull about 1000Watts (yeah, I know there are air conditioners around 5000 BTUs also but their not typical and the numbers still add up the same...).

So 10,000 BTUs for 1000 Watts at 115V... that's 10amps.

Then 3500 BTUs at 30 amps at 12V is 360 Watts...

So an A/C unit about 1/3rd the size pulls 1/3rd the energy... The unfortunate part here is there is that I would expect more efficiency from the water cooling capabilities over the condenser which doesn't seem apparent...

This unit will probably supply cooling for a very small area on a boat with little insulation. I would guess you can cool the v-berth or aft-berth but not the whole boat...

Greg

[Moe]

An air-conditioner or heat pump isn't making heat from electricity at the rate of roughly 300 watts per 1,000 btu like resistance heating. It's MOVING heat and a lot of other factors come into play, like the difference between the input and output temperatures.

From what I've seen, a standard efficiency air to air air-conditioner compressor typically draws about 1A at 120V or 120 watts per 1,000 btu. Add to that the fan power. That would put a 3,500 btu unit at 420 watts plus fan power. If it's 120VAC unit powered by an 80% efficient inverter, you're looking at 525 watts plus fan power.

But this isn't an air to air unit, it's a more efficient air to water type. So the math doesn't seem so far out of line to me.

Still, if you use my example above of 130 amp-hours to replace after a 10 hour night at a 40% duty cycle, you're looking at a pair of T-105 size golf cart batteries discharged down to about 20% charge and not charged higher than about 80%, above which it takes much longer charge time. Assuming an average acceptance rate of 45 amps per pair of T-105s, a good 60-75A marine alternator with 3-stage regulator should be able to replenish them in about 3 hours of engine run time. But Mac outboards typically have only 10-20A alternators. Best case, you're looking at 9-10 hours of engine run time.

Now if it's hot an the duty cycle hits 80%, you're looking at two banks of T-105s and double the run time to replace the charge.

There are boats where using this unit as a bedroom air-conditioner might be practical. But they aren't Macs.

--
Moe

[Catigale]

thanks Moe - my brain f***

[Frank C]

. . . Still, if you use my example above of 130 amp-hours to replace after a 10 hour night at a 40% duty cycle, you're looking at a pair of T-105 size golf cart batteries discharged down to about 20% charge and not charged higher than about 80%, above which it takes much longer charge time. . . .

Moe
Nice analysis ... maybe even I understood that one!
Continuing my slow elevation from ElectriDunce ... might a Honda portable generator help?
... and isn't this a really good case for comparing AGM to the GC wet cells?

IIRC, an important AGM advantage is that they can dive deeper and recover faster. Could we:
- ignore any AGM cost penalty;
- commit to a Honda 2kw;
- use the genset to help during daylight hours;
- and use it to drive a 20 amp hi-tech charger in AC mode ... ??
- or ... would we still need a Delco's 65 amps for the AGMs?

[Moe]

Frank, I have a 60 amp 3-stage converter/charger in the Airstream. From posts on the RV forums, an EU1000 won't quite power it at max output, but one of our EU2000s will easily. This feeds a pair of Lifeline 100AH Group 27 12 volt AGM batteries, which at least in theory, could accept 80A or more. And they make an 80A version of my converter/charger, which a single EU2000 should also handle. I didn't get it primarily because of the larger size and limited air flow where it's installed.

Marketing hype aside, any deep-cycle battery can be discharged to 20% regularly, and any battery that is will have a lower cycle life than if it's only discharged to 50%. Yes, it may be that AGMs' and golf cart batteries' cycle lives aren't shortened to the same degree a regular flooded cell battery's is by deep discharge, but there is a cost in life.

At the price of the AGMs, $400 for my two, I don't discharge them deeply unless it's absolutely necessary, and it usually isn't if I run the generators 3-4 hours daily. A pair of T-105 size 6V AGMs in series might be better for deeper discharge, but they're too tall for our battery compartments.

Not getting to 100% charge periodically (say every 3rd-4th charge cycle) also hurts AGM battery life a little more than flooded cells. Besides their many benefits, the main reason I have AGMs in the Airstream is because of corrosive outgassing of flooded cells around the aluminum skin.

There are trade-offs. The space and weight savings of a smaller battery bank discharged down to 20% may make it worth replacing them, say 2-3 times as often. And not charging above 80% saves a ton of wear and tear on the generator, not to mention fuel. Many RVers do this, but typically use low-cost flooded-cell 6V golf-cart batteries. That's why I used them in my example.

[Frank C]

Okay ... sometimes most cost-effective solution is the older, lower-tech solution. I guess the potential of weight savings would be the tipping point, rather than charging efficiency.

SEMI-RELATED TANGENT:
I stumbled across a DIY gadgeteers website this week called Instructables (thumbnail sketch at this Wikipedia page). One very interesting gadget was a "free" air cooler (final link below) posted by a guy whose groundwater stays at ~48F degrees. HMMM ... reminded me of our past discussions regarding air-to-water heat exchangers.

The Wiki page has a link to their home page, from which you can browse thousands of DIY gadgets. I found the following three gadgets particularly interesting, but there's tons more interesting stuff. Enjoy!

Evaporative Terra Cotta Beer Chiller . . . self-explanatory.

The Hilsch Vortex Tube

The hilsch vortex tube, cools and heats air at the SAME time with no moving parts, and NO electricity. cool huh? it's quite simple, and only a matter of getting the dimensions right! Not to mention EXTREME temperatures! ... we're talking -50 degrees cold and 350 degrees hot! that means you could burn and freezer burn your hand at the same time ... but realistically expect below freezing, and just above water boiling. which is still a huge eye opener for friends!

Free Air Conditioning

Technically it should be called a heat exchanger, we lovingly call it the water cooler, but it has been providing us with free cold air for more than 20 years now so it definitely does work.

Shortly after moving here (Montana) I noticed that the water from our well is really cold, under 50 degrees. I built this heat exchanger to take advantage of that cold source for use in the house in the summer and as a byproduct it heats up the water going to the garden a bit before it goes on the plants ...

[Catigale]

Who needs AC in Montana????



The Vortex tube thing sounds like the magnets you put on your fuel line to double your mpg....

[Mac Ziggy]

Vortex tubes are used in the nuclear industry to cool personnel in plastic protective suits during times of high heat. They work (no magnets required) but require a significant air pressure. Of course, you want a significant air pressure to keep the radioactive contamination out anyway, so you might as well be cool about it. They will heat too but we never had that problem.

They are simple tubes and would be neat for a science project if you have sufficient air pressure.

PS Simple does not mean cheap, especially in a nuclear or government environment.