Power Hungry Air Conditioning

[ssullivan]

I just wanted to see if anyone felt like poking a few holes in my plan for power generation aboard the new boat.

We are doing charters to pay for her, so we have to run the two 10,000 BTU Cruisair units from time to time during charters. This puts us into the genset group, for sure. I plan to purchase a 5KW (peaks at 6KW) diesel unit.

My plan when NOT chartering is to run the genset 1 hour each day to charge the house batteries through an existing Triad 50 Amp Charger. Not having any experience with this charger, I am not sure how long this will take.

Our power requirements when not chartering will be minimal, but will include a laptop computer and some computer equipment with a draw similar to having your GPS and and instruments on. We also have a 12V Alder Barbour fridge that will be running 24/7.

Without doing the math (just by guesstimate) does this sound like a workable power management plan?

Thanks, guys!

PS: We fly out to take possesion of the boat 1 week from tomorrow. After a 4 or 5 week delivery from WI to NY, I'll be back online. (Not sure if that's a good thing or a bad thing... ha ha)

Sean

[GordMay]

How big is your house battery bank?

FYI: You'll be running a 400 Amp (5kW) generator to operate a 50A charger.

Your A-B fridge will be consuming something on the order of 120 amps/day (24 hrs x 5 A), so 1 hour gen' run time (@ 50A) won't cut it.

Gord

[Rick]

Most 5 kW gensets will not start a 10,000 BTU airconditioner much less run two of them. Here is where the latest technology comes in to play as a solution. The latest generation genset uses an alternator (not the automotive type) to create a DC output around 300V. The prime mover (diesel) speed is regulated to control that voltage. An AC inverter converts the DC to regulated 60 (or 50) Hz and the appropriate voltage. As a result a small AC load will cause the prime mover to merely idle (or so) a large load causes full rpm. An example of this type is the Onan 7kW genset, there are others. With sufficiently high capacitance of the DC "rail" a large load can be started without having to oversize the prime mover, which you would have to do in order to start normal airconditoning compressors.

In addition, newer technology, like Glacier Bay (there are others) have high efficiency, low starting load airconditioning units which further reduce the demand on the size of the genset prime mover.

Utilizing both technoligies reduces the total fuel consumption and overall size of the genset required to do the job.

I am famaliar with Triad Magnetics and power supplies yet not specifically any battery chargers. To properly load a genset without creating an undue lagging power factor you need a multiple-step switch-mode charger. This obviates any triac or scr controlled chargers which DO create a huge lagging power factor and gensets do not do well with them. If the data on your charger indicates that it accepts "universal input voltage" then it is a switch-mode charger. They cost more than conventional chargers yet EXTRACT energy from the genset waveform over the entire cycle instead of merely at the peaks like conventional chargers.

Like Gord points out, you need to know just how much energy that you require to replenish in one hour. A 50 Amp unit will NOT do what you want. I'm guessing that you need a minimum of a 100 Amp unit even without doing the math because an Adler Barber reefer goobles up the energy goop in a very innefficient manner over 24 hours. In warm ambient environments they run continuously.

Rick

[ssullivan]

I'll do the math in the next couple days and post the results up here to further the info on the board. I will apply the math to what Gord and Rick have been so kind to share.

[ssullivan]

I suppose Gord is right in that I can't really come up with even rough calculations without knowing the battery bank's capacity and all information in the electrical system.

Since the boat I'm purchasing is in WI and I'm on the East Coast, I can't find all this information out before I go to fetch the boat and make the delivery back.

So... I will continue this thread later on in August, after I return from the trip (and once I get the internet up in the boat).

Thanks again... TO BE CONTINUED!

[ssullivan]

The Cruisair site informed me that the 10,000 BTU AC's comsume the following:

Starting Amps: 33A@115 V
Running Amps: 11.1A@115V

Assuming I am at 120V (worst case scenario), wouldn'd the starting wattage be 3.96 KW? Also, wouldn't the running wattage be 1.33KW?

If so, why couldn't I start one AC at 3.96KW, settling to 1.33KW, then start the other for a total max peak of 5.292 KW?

The only time it wouldn't work is if you started both ACs simultaneously, right?

[Rick]

Your Cruisair data is missing the information regarding power factor. The 33A starting current is what is called "nominal" and does not tell you the lagging power factor.

Air conditioning compressors will draw a "peak" current exceeding the so-called "starting current". When coupled with the power factor you need a generator which will output a much higher peak starting current than the given data suggests. Conventional generators do not have much of a peak output current exceeding the continuous rating (which assumes resistive loads, NOT what air conditioners are).

IN ADDITION your genset WILL NOT output a true sine wave which further degrades the ability to start a compressor in an air conditioner. If you ever have a chance to put an oscilloscope on your genset you will see what I mean. The waveform that you observe will have a peaked look as compared to a nicely rounded sine-wave like line voltage has (should have). What this means to you is that in order to run a given rated output power that more curent must be delivered at lower voltages for a significant conduction angle of the waveform. The genset just cannot deliver as much current at the peaks as can shorepower.

Large gensets (about 12kW rating and above) tend to not have such a problem. I can just about guarantee that even if your genset can start one load it will not be able to start the other.

Rick

[ssullivan]

Thanks, Rick. Got it. Based on your description of the peaks (non-sine), I assume power is the integration of the area under that curve?

(remnants of a physics degree... ha ha)

Anyway, thank you for helping me to avoid a foolish purchase. So everyone who runs ACs must have 7-12KW gensets? The ones I have seen in this range weigh in at 500-1400lbs. I may be stuck here, since I was planning a "deck mount" with fiberglass enclosure, rather than a true marine genset.

Thanks,

sean

[ssullivan]

I just called a couple of Cruisair shops to just to find out what figure they would give me.

Each told me a 5KW genset would run these two 10,000 BTU Cruisair units simultaneously. They went further to mention that the hot water heater will probably kick on as well, so it would be best to have a 7KW to be safe.

[Rick]

I am truly surprised at the Cruisair recommendation as a blanket statement. In the RV industry 7kW is generally specified to run two 10k BTU airconditioners.

The REAL answer is to find someone with the exact model airconditioners and exact same model genset along with the ambient temperatures and humidity before KNOWING in advance if it will work. In general, 5kW is too marginal to me yet this will change when the newer technology gensets and airconditioners are commonplace.

[Rick]

The problem common to most 5kW gensets running compressor loads like airconditioners and reefer units have been in conjunction with the use with triac controlled chargers like the Heart/Trace/Xantrex/(and others) inverter/chargers even at relatively low charger loads.

Sometimes the inverter/chargers would disconnect with overtemp errors caused by the aggrigate total of power factor. Sometimes the genset would blow the breaker, sometimes other breakers would open.....That is not somethihng that Cruisair might not be taking into account. Adding a heater can actually help the overall power factor as it is a resistive load only.

Again I submit that you approach the spending of your money in this area with great caution so as to not be burdened with any bad surprises later.

Regards,
Rick

[ssullivan]

Rick once whispered in the wind:

Again I submit that you approach the spending of your money in this area with great caution so as to not be burdened with any bad surprises later.

Regards,
Rick

Thanks, Rick. Your help on this subject has been a godsend so far. I am scared to death of making the wrong purchase here, since I have no margin of error from a financial standpoint.

I am applying great caution and trying to explore every single option before making a purchase. As of this moment, I am thinking it might be best to run only 1 Cruisair unit at a time (with the switch for the 2nd off).

Either way, I cannot make a purchase today, since I do not know the capacity of my house battery bank or the exact specifications of the 2 battery chargers aboard the boat already.

Thanks again, Rick. I'll continue this thread again later on after I bring the boat back on its 1000+ nm delivery back from WI.

Sean

[GordMay]

I'm not at all comfortable contradicting Rick, but FWIW:
Adding a Resistance Heater to an Inductive Motor Load will decrease the Power Factor Ratio - but (more importantly) will also increase the total Apparent Power (VA) that the Generator must power.
The conventional means Power Factor Correction strategy, due to Inductive Loads (Lag), is to add Capacitive Reactance (Lead) to the circuit, reducing the Total Reactance - hence Total Apparent Power, and Generator size.

[Alan Wheeler]

The use of an inductive load on small Gensets is more because of poor voltage regulation in those small ones. Untill you step into the big boy's toys, the smaller sets became very hard on sensitive electronics. If for some reason a large load was tripped off circuit, the Genset would suddenly rev as it lost load and induve a far higher voltage and frequency. This used to wreck havock on our sensitive Processer controled sound equipment. Even for small event, I would spec at least a 20KVA genset, to ensure it was top of the line in regulation. Maybe it's changed somewhat now, but you couldn't get good regulation till you stepped over 20KVA unit's.

[Alan Wheeler]

I should also add, for any inductive starting device, (Microwave oven is a classic example) you need as a ruff rule of thumb, 3x the Genset rating to the load rating. If you go under that, the unit you are running may start sometimes and not others. If you get too low it will never start. And what happens, well the Genset doens't stall compleately, it loads down, the engine RPM drops till the current supply equalls the circuit load and as a result, the frequency and voltage drop to a dangerousely low voltage, one that is beloow the ability of the breaker to trip on and The unit that is not starting up, simply overheats and burns out.