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Old 16-08-2010, 05:56   #16
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OK, so it sounds like the real disadvantage of using a DC genset for my application is the additional expense of a high power inverter. I currently have a Freedom 2000 which looks like it wouldn't be enough power for the air conditioner startup. I'll concentrate on an AC genset.

I really appreciate all the input, explanations and opinions. What a great resource! Thanks for all your help.

Tom
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Old 16-08-2010, 07:06   #17
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We have a 4.2KW AC generator that is rated for 3.8KW continuous duty. It easily handles the demands of a 16K BTU air conditioner while charging our 450 AH battery bank and running our water heater. We do have a 2KW inverter/charger but have rarely used the inverter for more than powering a microwave oven for a few minutes at a time or the TV/DVD player for an hour or so in the evenings. Frankly, I think the alternative of a DC generator coupled with an inverter capable of powering these appliances adds a level of complexity that would be problematic.

FWIW...
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Old 20-08-2010, 02:23   #18
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G'day, mate. Take it from a licensed professional engineer, if you want to run air conditioners, an AC generator is the only real option. It's plain and simple, you are going to have to replace a huge amount of amps after running the AC unit, no solar or wind units are going to keep up with that demand long term. You're going to need a good size battery charger, at least 100 amps if not 125 or 150.

I have an 8KW genest, NO AC, a 15 amp fridge/freezer, 15 amp hot water cylinder, 30 amp watermaker system, plus the battery chargers. I stagger the load to keep the genset reasonably loaded for about 2.5 hours of run time every other day here in the subtropics. Cheers.
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Old 20-08-2010, 07:11   #19
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I think posters are missing the point, DC v AC generators is an apples and oranges comparison.

If you have big AC loads then simply theres no discussion, you need a big AC system. Secondly because AC gens are common they are cheaper.


However a DC generator has a place in a complete DC boat. Its primary advantage is where large battery banks are common. Trying to recharge 1500 Amp/hours via an AC gen and battery charging is a slow slow process. DC gens can easily generate 400 amps and more for specialised ones.

A DC gen is therefore a battery recharging strategy, whereas AC generator are a power supply option. Each has a place. IN my opinion DC generators will grow in popularity as invertors get cheaper. Also N+1 invertor techniology means that given the source power, large 6-9Kw invertors are quite possible , though expensive today.

WIth increasing effective battery storage techologies, I suspect that battery banks will grow and that brings the DC gen into focus

Dave
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Old 20-08-2010, 07:18   #20
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It might be easier if everyone would simply look at the power that appliances consume. One of my 16k btu AC units pulls ~13 amps @120vac (including raw water pump) during normal running. That's 1560 watts which would require (13) 120w solar panels with full sun (~4-5 hours a day at best). Then I'm not sure a 3000w inverter would be enough to start the units.

So, calculate the power an appliance consumes, volts times amps = watts (don't use the watt rating on the label, calculate it). Do the math and compare against your power source(s). If you can run AC in tropics without an AC genset or shorepower, please let everyone know how because you're doing something magical...
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Old 20-08-2010, 10:24   #21
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I didn't miss the point. I am an electrical engineer. If you wanted to convert the boat to pure DC, ... You could. DC motors are available in all sizes, most appliances have DC versions at triple the cost. Battery power has increased. On my boat I power everything, including the fridge off of an inverter 2kw pure sine wave,(dont even think about a "modified" sine wave; another thread). 200-300 amp hours of house batteries will power the boat all night, ... Except the A/C, Quadrupling the house battery size could give enough raw power to run the A/C, but the surge current requires a hefty inverter. Right now the cost and raw wattage needed to run an A/C of any size limits it to AC power. I have built an electric car, and used DC, but an AC motor is more efficient. I am working on a design for an all electric boat with a DC generator for backup, but again the A/C is a problem. Until someone builds an affordable DC powered A/C in the 10-20000BTU range with a scroll compressor that is where we are at.
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Old 20-08-2010, 11:11   #22
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Quote:
Originally Posted by goboatingnow View Post
However a DC generator has a place in a complete DC boat. Its primary advantage is where large battery banks are common. Trying to recharge 1500 Amp/hours via an AC gen and battery charging is a slow slow process. DC gens can easily generate 400 amps and more for specialised ones.
One of the biggest problems I see to a DC boat is the weight involved with 2000-3000ah worth of batteries. At >1lb per 2ah (12v), that's a lot weight, more than a fair sized AC genset and 500ah worth of house batteries.

There is no rule against having multiple AC battery chargers if you are wanting to charge faster. The limitation is with the battery technology, not the charger technology.
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