Modern AC fridges much more energy efficient than modern D/C fridges

[mikedw]

Re: Modern AC fridges much more energy efficient than modern D/C fridg

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The internet shows that a modern DC boat fridge uses an average of 1.2 kwhrs per day at 32 degrees.

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Even the largest 12/24V Waeco compressor Fridge/Freezer (for large yachts and commercial shipping) says 60W and 35% (say 50% at 32 deg) giving only about 0.5-0.7kWh per day.

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My old fridge ... ...runs 80% of the time if on the lowest setting...So for example my fridge's energy consumption in one day is 60Watts*24hours*0.8= 1152 Wh or 1.152kWh

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If the Waeco range run 35W to 60W and run 18-50% of the time (I would believe the Waeco figures as the 18lt model specs comes to 12.6Ah in 24h and AngusMcDoon says he measured 10Ah) maybe you just need a more efficient 24V DC fridge rather than concluding you need to move to AC etc.

 

[mikemonty]

Irrespective of the wrangling over units above, this is never going to be meaningful.
Cut to the chase - you want to compare how much energy each system will use.
That will depend on time of year (hence the kWYears - averaging out over a year - averaged over an hour or day is meaningless you use more power in the summer than the winter and unlikely to settle at the "short term" average)
The only thing that matters in the end is the total amount of power each draws, and that depends on insulation, size, food load, temperature, usage pattern, temperature setting(efficiency for each may change at different temperatures) etc.
The only way you could make a useful comparison would be to sit two side by side, each on a meter, and ensure you used each one exactly the same way and amount.
It's not gonna happen is it?

I suggest that you don't waste your time any further on this. Whichever technology (other than gas) you go for, the thermodynamic efficiencies are not going to be significantly different that you would be able to say "that was the better choice" but I would say that handicapping yourself with conversion inefficiencies is not going to help the AC case!

 

[VicS]

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This brings into question Vic's conclusions

[/ QUOTE ] Quite right. My calculations and conclusions were based on the best interpretation that I felt I could make of the data as they were presented.
The OP has attempted to clarify the figures.

It now seems that the DC fridge typically uses 1200 kWh per day making its average power consumption a very small 50 watts

We now have 0.21kW/hrs quoted for the AC fridge. So that is still open to interpretation as an average power consumption of 210 watts or a typical daily consumption of 0.21kWh. The latter being equivalent to an average power consumption of a minute 8.75 watts.

So I am now comparing 50watts (average) for the DC fridge with, either 210watts or 8.75 watts (average) for an AC fridge and with 126 watts (average) for a domestic fridge.

Hopefully 8.75 watts, and a typical daily consumption of 0.21kWh, the correct figures for the AC fridge.

I realise the blame for the confusion over units probably rests with the manufacturers but unless it is sorted out it is impossible to make proper comparisons.

 

[ytd]

Re: Modern AC fridges much more energy efficient than modern D/C fridg

the efficiency of a fridge - given similar size and insulation depends on motor efficiency and the size of the evaporator and condensor. Generally DC motors are more efficient than AC so in a simple on/off type fridge DC should win. But some super efficient domestic AC fridges have variable capacity compressors so the motor runs continuously but its speed is controlled (usually by an inverter) so that the fridge achieves the desired temperature. These are more efficient because they effectively have bigger evaporator and condensor surface areas than a simple fridge which just runs flat out when its on and nothing when its off.

 

[William_H]

Re: Modern AC fridges much more energy efficient than modern D/C fridg

Hi Vic and all. I reckon my domestic fridge would use at least 1200WH per day ie 50 watts average or 100watts, that it is rated at, at 50% running.

I think we must remember that figures quoted for 250V fridges are there to sell the fridge. Even with no exageration a years operation including UK winter in an unheated house would be much lower than the practical summer use for a fridge in a boat.

So I would tend to be optimistic that the waeco is just as efficient as the 240V Ac fridge.

Actually one of the things that bugs me about my domestic fridge is that the condenser ie the bit that radiates the heat is built into the side walls of the fridge. Not at the back as used to be. This means that the heat source is right against the insulation. It must be (I hope) good insulation as the outside gets quite warm.

Lastly I understand that later Engel 12v fridges are claimed to be only half the current of older Engel and Waeco. Does anyone knom if this is true? olewill