12v or 110v/220v generator - HELP!

[Joli]

I gotta agree with Bill. An AC generator seems to be the only viable way to add air conditioning. We have a 5 kw Westerbeke that is pretty small and very quite. It is located in the engine room that is directly under the center cockpit but not in its own sound surround. We do not have air though.

[GordMay]

I’ve successfully used about 15 BTUs of cooling per cubic foot (interior volume), for liveaboard Monohulls in south Florida & Caribbean.
1 watt/hour = 3.415 BTU; but it requires perhaps 20- 25% more than a watt of electrical power to deliver 3.41 BTU’s of cooling (efficiencies).
Rule of thumb: 1 Watt/hour electrical power will deliver about 2.5 BTU of cooling.

Don't Over-Size an Air-Conditioner (nor Heat Pump):
It costs more to buy a larger air conditioner than you need.
The larger-than-necessary air conditioner cycles on and off more frequently, reducing its efficiency.
Frequent cycling makes indoor temperatures fluctuate more, and results in a less comfortable environment.
Frequent cycling also inhibits moisture removal. In humid climates, removing moisture is essential for acceptable comfort.
In addition, this cycling wears out the compressor and electrical parts more rapidly.
A larger air conditioner uses more electricity and creates added demands on electrical generation and delivery systems.

[btrayfors]

Gord,

Not sure about those numbers. The first one seems ballpark (15 BTU per cubic foot), but the others don't track with, e.g., the specs of the 12V A/C mentioned above.

That unit claims a load of just under 30 amps (or 375 watts @ 12.5VDC) in order to deliver about 3,000 BTU. This is about 100 BTU per amp (or 12.5 watts) or 8 BTU per watt. This is over 3 times more efficient than your figure of 1 watt per 2.5 BTU of cooling.

In my own case with 115VAC-powered A/C units, my 18,500 BTU unit draws 13.1 amps (or 1507 watts). This equates to 12.3 BTU per watt (about 54% more efficient than the 12VDC A/C cited above).

Both, however, deliver much more than 2.5 BTU per watt.

Bill

[GordMay]

By definition: 1 Watt/Hour = 3.412141156488 BTU’s
However:
Each air conditioner has an energy efficiency rating that lists how many BTU's per hour are used for each watt of power it draws. For room air conditioners, this rating is the Energy Efficiency Ratio, or EER. For central air conditioners, it is the Seasonal Energy Efficiency Ratio, or SEER. The EER of an air conditioner is its British thermal units (BTU) rating over its wattage. For example, if a 10,000-BTU air conditioner consumes 1,200 watts, its EER is 8.3 (10,000 BTU/1,200 watts). The higher the EER is, the more efficient the air conditioning unit is.

[sailvi767]

I have 22,000 BTU's on a 45 foot cat in 3 units. It is a bit marginal however works fine at night and will keep a main salon with a lot of glass around 80 under a tropical sun. That is about the minimum I would go with on your boat. The 6kw genset on my boat burns between .3 to .5 gallons per hour depending on load. With the genset wired into a battery management system you are also charging your batteries while cooling the boat. Not having to run main engines an hour in the morning and evening offsets some of the genset fuel burn. On a 1 week sailing trip using the genset every night for about 10 hours I average about 40 gallons of total fuel use per week.

[swagman]

Really appreciate all the input especially from those who've done it.
As stated, seems the 110/220AC generator is the most recommneded route, but I'm still drawn towards the 12vDC route but not sure why!
Not worried about the genset kicking in if battery power drops - and most of the new gensets have the automated capability to do this - and equally know we'd not be running the aircon 24/7. It's primary need is to chill off cabin on v hot days and prior to sleep at night.
Has anyone actual experience of the 12vdc aircon units like Breezeair?
And anyone smart enough to work out the BTU needs to chill off a cabin like our saloon (15' x 15' x 6') from say 35 deg celcius to 25 deg?
Again, appreciate all the input.
JOHN

[GordMay]

Originally Posted by swagman

... the BTU needs to chill off a cabin like our saloon (15' x 15' x 6') from say 35 deg celcius to 25 deg?
JOHN

For tropical use (35 deg. C / 95F), I’d recommend* a minimum size of 16,000 BTU/h up to about 20,000 (or up to 24,000 BTU/H depending upon availability).

* Based upon: minimum of 12 BTU/Cubic Ft, up to 15 - 18 BTU per Cu. Ft. (upper size would only be suitable if you had lots of glazing, or other heat gain factors etc)

[swagman]

Thanks Gord.
JOHN

[captainbri]

Hi all I am a constant CF stalker and post once in a while. There has been a lot of good advice on here. The most technical and best reply I have read is from gordy. I am a professional honest residential ac tech so ill throw in my two cents.

Gordy is right about seer and eer ratings the higher the ratings the more efficient the unit is as far as running amp draw. Also a unit that is oversized will cycle on and off more and it actually puts more humidity in to the air making you feel hotter because your sweat is not evaporating. a unit that is to small will just run and run. Most peole are concerned about the start amp draw tripping breakers ect. I can tell you that I can take start amps on a unit from 100 to 200 plus amps to 30 to 40 amps some even less. With about 2 hours of work or less. So here is the golden ticket.. 1. see if there is a start capacitor available for marine units if so buy the best one. A start capacitor is simply a battery that lets its stored energy to be used up in less than a second and then charges when the unit starts pulling power from the capacitor instead of the house batteries. 2. If there is a contactor replace it think of old points on a car dame basic idea. If they are worn or pitted it will arch on start up causing higher amp draw. And to keep the unit efficency clean the condensor and evaporative coils this improves cooling and airflow reducing running amps. If the unit is not disapating the heat it is cycling it back through the system. When you wash the air filter remember it is only stopping about 10% of the dust and dirt so imagine years of dust and sticking to the coils. On the your ac or refrigerator. Sorry im not more detailed but im using my phone here are a couple of photos of a house ac unit just think marin ac. I was the 4th tech at thus guys house and the only one to show him what was wrong. Before and After photos ok im posting from a phone so I MAY have some typos and I tried to look st marine schematics but on the phone its impossible to read it makes legal print look like a large font