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Old 14-03-2017, 12:00   #1
rom
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Refrigerator: AC vs. DC

Hi everyone ! on another thread (Decreasing charge time of FLA house bank) some are advising the PO to switch his fridges from 230VAC to 12DC, but since it is OT I thought I should start a new thread.

I currently have 2 vitrifrigo C130L that I was actually thinking about changing for 230VAC appliances ! And here is why:

1. There are so many choices among 230VAC appliances that I will be able to purchase one that make 150 litres (instead of 130L) and still fit in my boat.
2. I can get among the best 230VAC fridge for about half the price mine (which means selling mine second hand and buying new will not cost much)
3. For that price I can get a Class A+++ fridge which is given for 64kWh/Year, where my expensive Vitrifrigo have no classification (!) and are supposed to do .47kW/day, which makes 171kWh/year, that is roughly 3 times more than a modern AC appliance !
4. I have been using a victron multiplus 12/3000/120 inverter for about a year, and this one is already ON 24/7.

edit: 5. I forgot to mention, my vitrifrigo have a separate compressor which uses space and that I find to be noisy ...

So, what would you do ?
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Old 14-03-2017, 12:54   #2
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Re: Refrigerator: AC vs. DC

Quote:
Originally Posted by rom View Post
Hi everyone ! on another thread (Decreasing charge time of FLA house bank) some are advising the PO to switch his fridges from 230VAC to 12DC, but since it is OT I thought I should start a new thread.

I currently have 2 vitrifrigo C130L that I was actually thinking about changing for 230VAC appliances ! And here is why:

1. There are so many choices among 230VAC appliances that I will be able to purchase one that make 150 litres (instead of 130L) and still fit in my boat.
2. I can get among the best 230VAC fridge for about half the price mine (which means selling mine second hand and buying new will not cost much)
3. For that price I can get a Class A+++ fridge which is given for 64kWh/Year, where my expensive Vitrifrigo have no classification (!) and are supposed to do .47kW/day, which makes 171kWh/year, that is roughly 3 times more than a modern AC appliance !
4. I have been using a victron multiplus 12/3000/120 inverter for about a year, and this one is already ON 24/7.

edit: 5. I forgot to mention, my vitrifrigo have a separate compressor which uses space and that I find to be noisy ...

So, what would you do ?
My built in fridge of 226 liters or 8 cubic feet uses 26.4 watts when it is running , it runs at 50 % duty cycle . So if it ran at 100 % duty cycle it would use 26.4 watts every hour or 24hrs x 26.4 watts = 633.6 watts every day so that would be .6336 kw a day times that by 365 = 231.264 kwh per year. But it only runs 50% of the time so divide that by two which is 115.632 kwh per year .

Your average home fridge 2.5kwh a day or 912.5kwh a year !

Check my math but I think I got it right.

So what I am saying is , a watt is a watt 12v or 240 v , its all the same . Also remember size matters and insulation is a huge factor.

Regards John.
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Old 14-03-2017, 13:19   #3
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Re: Refrigerator: AC vs. DC

I'm going to get some people upset I think, but the only inherent difference between a 12 VDC fridge and a 120 VAC one is the electricity.
Yes there is some conversion losses, but it may not be as bad as you think, especially if you have the inverter on anyway.

However the insulation on a regular house fridge isn't that good, reason is of course people want the "biggest" fridge possible, and the only way to get there within the same size box is to have thinner walls, Is thinner insulation. You as an example, that was your reason to have 150 L as opposed to 130 L
So, the biggest difference in the efficiency of a built in 12 VDC boat fridge and a box you buy at the home improvement store is the insulation. If you could get a boat fridge to have the R value of a thermos bottle it would be incredibly efficient, then it would likely hardly ever run and when it did it would be for very short periods. Exception being of course when you put that hot case of beer into the thing, that heat has to be removed.
Power is relatively cheap in a house, and unlimited, there is just no compelling reason for super insulation, so you don't get it in a house fridge

My thought is that on a Lagoon your size you ought to be able to put enough Solar on the boat that it doesn't matter, and I would assume you will have an inverter on anyway running something?
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Old 14-03-2017, 13:30   #4
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Re: Refrigerator: AC vs. DC

I just upgraded my old inefficient 120V fridge that was dying after 10 years on the boat, to a new far better 120v two door fridge (Edgestar 3.1CF) that uses only 5.7 amps dc when running (72 watts) OK that's more then a top of the line DC fridge but for a total outlay of $400 it' pretty sweet.

It uses around 20 ah less daily (rough estimate based on the batteries being topped off 2 hours eariler), then the old cheap 120V fridge that had a 2.5 ah duty cycle. So pretty pleased with it. For $400 (FRIDGE AND 1000 WATT PSW INVERTER) I have a small fridge with separate 0 degree F freezer compartment that stores well over 2 weeks of food for me plus ice cream.

So it can be done quite easily.

One nice thing about a separate freezer compartment is I only go into that one time a day to move the next days meats to the fridge compartment. Highly recommend two door fridge/freezer as it's a better way to go.
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Old 14-03-2017, 13:53   #5
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Re: Refrigerator: AC vs. DC

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Originally Posted by rom View Post
Hi everyone ! on another thread (Decreasing charge time of FLA house bank) some are advising the PO to switch his fridges from 230VAC to 12DC, but since it is OT I thought I should start a new thread.

FWIW, I have interpreted haveaday's fridges in that other thread to be 110VAC, vice 230V.

Doesn't really make a difference to your question...

Our fridges are dual-voltage "marine" models, and seem to be efficient in each state (usually DC when at anchor or while underway -- and AC when at the dock or when the genset is running.) Both NovaKool, one double-door type, and one smaller. Don't know specs offhand to compare against yours, but that info would be on the NovaKool website.

We have an inverter, but not to run AC/DC fridges...

-Chris
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Old 14-03-2017, 15:18   #6
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Re: Refrigerator: AC vs. DC

Thank you for your answers ! It seems I should have insisted that I am european, and that in europe we have https://en.wikipedia.org/wiki/Europe...n_energy_label, and so we have a indication of the appliance's consumption. And therefore we have access to 230VAC appliances that are extremely efficient (Class A+++)

I gave you the example of a 230VAC fridge I could buy: 64 kWh per year !

So you do not need to enter insulation and size considerations, it's already taken into account in the equation.
Quote:
Originally Posted by a64pilot
Power is relatively cheap in a house, and unlimited, there is just no compelling reason for super insulation, so you don't get it in a house fridge
That's really not the way it works here ! You should think of Europe as big vessel with very little energy
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Old 14-03-2017, 15:32   #7
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Re: Refrigerator: AC vs. DC

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Originally Posted by rom View Post
Thank you for your answers ! It seems I should have insisted that I am european, and that in europe we have https://en.wikipedia.org/wiki/Europe...n_energy_label, and so we have a indication of the appliance's consumption. And therefore we have access to 230VAC appliances that are extremely efficient (Class A+++)

I gave you the example of a 230VAC fridge I could buy: 64 kWh per year !

So you do not need to enter insulation and size considerations, it's already taken into account in the equation.

That's really not the way it works here ! You should think of Europe as big vessel with very little energy
Some of us caught on that you were EU with the 230V and class A+++ ratings (which we totally lack in the USA btw) I wish we could buy 120V fridges in the USA with a 64kwh per year rating. Most small ones are in the 340 kwh per year range and thought I was doing good with the 230 kwh.
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Old 14-03-2017, 15:35   #8
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Re: Refrigerator: AC vs. DC

Electricity is very expensive in Mexico also. Big box stores carry refrigerators that are very proud of their efficiency . I have had one on my boat for more than 5 years and have been very happy with it. I agree with sailorchic34 that the separate freezer door is good and ice cream is great!
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Old 14-03-2017, 17:46   #9
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Refrigerator: AC vs. DC

Wanna be that the dual voltage fridges really are not, they have an inverter or power "brick"
Unless mistaken and I may be, I think the venerable BD compressor is actually three phase AC, the electronic box does the power conversion?
Sort of like the TV on my boat, it is 120 VAC, but has a power brick that converts 120 VAC to 12 VDC. So if I used it like it was intended, I would take 12 VDC, invert that to 120 VAC, that then is turned into 12 VDC by the brick to run the TV. So I cut the cord on the brick and ran it direct off my bank, so I guess it dual voltage too, even though it's not meant to be?

I bet that a 120 VAC fridge with the same R value insulation would be very close in power consumption to a 12 VDC "marine" one.
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Old 14-03-2017, 18:07   #10
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Re: Refrigerator: AC vs. DC

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I bet that a 120 VAC fridge with the same R value insulation would be very close in power consumption to a 12 VDC "marine" one.
^^^^ THIS.

The issue is amount of heat rejected from the box. Assuming the same basic design but one fridge has a 120V (or 220V) compressor and the other a 12v compressor, all have to remove the same amount of heat with more or less the same compressor design. So with a given box design the energy use will be near about the same.

Even my old fridge which was a huge energy pig still only used about 2.5ah or 60 ah per day. My new fridge uses around 40ah, most of that savings is from the separate freezer compartment that is only opened once a day or so. There is some energy savings with the new one fridge, better cop compared to the 10 year old fridge.

Never assume a 12V system uses less power then a 120v or 220V system.

The danfoss 12v compressors do have 3 phase AC motors and the black box is a dc to ac inverter. They use 3 phase ac, I suspect as the overall energy cost to start and run the motor is less then with a straight 12v motor (which could be done).
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Old 15-03-2017, 05:47   #11
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Re: Refrigerator: AC vs. DC

Quote:
Originally Posted by a64pilot View Post
Wanna be that the dual voltage fridges really are not, they have an inverter or power "brick"
Unless mistaken and I may be, I think the venerable BD compressor is actually three phase AC, the electronic box does the power conversion?

I bet that a 120 VAC fridge with the same R value insulation would be very close in power consumption to a 12 VDC "marine" one.
Quote:
Originally Posted by sailorchic34 View Post
^^^^ THIS.

Never assume a 12V system uses less power then a 120v or 220V system.

The danfoss 12v compressors do have 3 phase AC motors and the black box is a dc to ac inverter. They use 3 phase ac, I suspect as the overall energy cost to start and run the motor is less then with a straight 12v motor (which could be done).

I think I understood from sailorchic's earlier thread about her new fridge...

That the Danfoss fridges always run on 3-phase AC.

And that the 3-phase AC comes either from a DC source (black box conversion) or from an optional "household" AC source which is first converted to DC (when the AC option present). And then that "household"-AC-from-DC gets converted to 3-phase AC by the black box. (Actually, I'm not clear whether this is AC-to-DC-to-3-phase, or whether it's AC-to-3-phase directly.)

(Using "household" here to indicate 120V, 240V, 230V AC ... not 3-phase AC.)

Translating that to our NovaKool AC/DC fridges... we mostly run them on 3-phase-AC-from-DC when underway or at anchor, and then on 3-phase-AC-from-DC-from-110V AC when on shorepower or the genset is running.

I think.

Happy to be corrected if I haven't understood.

-Chris
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Old 15-03-2017, 08:25   #12
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Refrigerator: AC vs. DC

Chris, I think you have the gist of it, I think.
However what I do not understand, is what is the advantage of a fridge that runs off of both AC and DC ?
When I am plugged into shorepower, my battery charger is on, and power essentially passes though the bank, the battery bank is not depleted and then recharged, power just passes thru. I see no advantage of the additional complication of a fridge that is voltage flexible, unless it a portable, then of course you could take it into a building and plug in.
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Old 15-03-2017, 09:16   #13
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Re: Refrigerator: AC vs. DC

I wondered this very question when fitting out my boat. The best choice was in domestic fridges that suited my needs but they are 230V (UK). I was talking to some canal folk and their view was go with a 230V domestic fridge, add a door lock and an inverter.

The fridge runs on shore power, or a generator if not available, and as soon as I set sail I switch to an inverter. Even without powering the fridge on 12v I have traveled on a 16hour sail starting with a cold fridge and two large lumps of frozen meat and all is well on arrival. I realise that on really long sails I would be using the inverter and therefore battery but as a motor sailer the engine running is not unusual.
Our wiring system has a mains powered relay so if mains is incoming it pulls in the water heater and charger in as well as the mains sockets and fridge.
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Old 15-03-2017, 09:22   #14
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Re: Refrigerator: AC vs. DC

best way to improve efficiency of a front opening fridge is to put bins on each shelf. The main losses are that when you open the door all the cold air falls out. If you arrange it like a freezer with draws more air stays in so temp is more stable and it uses less power. That is why traditionally boat fridges where top opening. I use a portable dometic but it is only 35L.
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Old 15-03-2017, 10:00   #15
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Re: Refrigerator: AC vs. DC

rom you've got the right idea. I have two suggestions though: hardwire a dedicated tiny pure sine wave inverter to your refrig since inverters work at max efficiency at about max load (your 3kw inverter might be running at 50-60% at low load); and try to find an A+++ digital inverter refrig which avoids the huge "locked rotor" draws.
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