Fridge power consumption

[Beneteau381]

Absorption fridges are very inefficient, and will only offer a holding capability on 12v, and on LPG (butane or propane gas) are positively lethal on a boat because the risk of exhaust fumes re-entering the boat, and possibility of ignition of gas or flammable vapour accumulations from the bilges due to the continuous flame present when operating on LPG.

For any fridge best efficiency is achieved by keeping the discharge pressure down, which is why water cooled condensers in warmer climates are far more effective. Cooling the compressor is less of an issue becuase most of the cooling is provided by the cold suction gas entering.

A well ventilated condenser low down in the boat where the air will naturally be cooler in the Summer will help with an air cooled condenser.

For a cabinet style fridge then a small computer fan along with suitable vents to allow decent air circulation behind the fridge will make a tremendous difference to its efficiency. Installing a fridge into a sealed or poorly ventilated space is a recipe for poor performance and a short life.

As an aside to this thread a mention of HFC phase down, for fridges notably R134a, is something that will affect newer equipment coming to the market over the next couple of years.

With the phase down of R134a it is likely use of propane R290 or Iso butane R600a for boat fridges will increase, and the small volume of refrigerant should not create any undue additional fire hazard for cabinet fridges, but not so sure about split system fridges with locally plumbed systems. An engineer installing such systems must be Gas Safe registered (F Gas registration alone is not valid for pure hydrocarbon refrigerants) plus be suitably experienced with marine fridges. Not too many of those chaps available.

As an alternative to R134a then R1234YF would provide a low flammability solution, but any locally installed system may require a Fire Safety risk assessment (happy to check the specific rules for this as it might be volume related). R513A would provide a non flammable solution.

For those interested in refrigerant developments those with AC systems have a bigger problem. R407C has no replacement in the pipeline at all, as it was a technical replacement for R22, which has now been largely superseded in large systems by R134a equipment. The other common gas used in AC is R410A, which for small systems in being replaced with one of its base components R32, which itself also has a low flammability, and as a result a room size limit. But R410A provides a better heat volume per equipment volume than R134a, which is why R134a is noot much used for AC on boats - the compressors will typically be over twice the swept volume capacity for the same net cooling effect.

Availability of HFC based gases depends upon their Global Warming Potential and the governing factors behind the phase down and its affect upon market price and availability are very complex. This year will see a marked reduction in HFC availability and procurement of new equipment on an HFC refrigerants should be questioned.

Made me laugh when we had mountains of R12 filled fridges because the country didnt have the capacity to de gas them! Then when they finally started on them we found out that the evacuated gas was going to India and China for re use!

 

[Zing]

Really?

"Each time you turn on you have to pump a vacuum, which produces zero cooling" really? A fundemental lack of understanding of the refrigeration cycle!

No it is not. You don’t get any refrigeration unless you evaporate the gas and for that you need to pump to reduce pressure.

 

[Zing]

Interested to know how you work out that domestic fridges are more efficient?
This link would suggest the DC motor driven compressors we are using are up there in terms of efficiency. I dont see where you work out domestic fridges are inherently more energy efficient.
Direct current compressors

They advertise that their units have good cop figures, but they are not good and most marine fridges with their tiny evaporators don’t achieve great results i.e. within the poorer limits of the range of their performance figures. Good domestic units have massive evaporators and massive heat exchangers in comparison.

 

[Neeves]

Neither did he use one of the more modern 12v fridges with a DC compressor.

Charlie also suggests that buying a domestic fridge is cheaper. Yes, a cheap and nasty, free standing fridge is maybe £150. The Combi he uses in the demo costs over £700, so you spend nearly £900 and have a cheap and nasty fridge. Personally, i wouldn't entertain a free standing fridge onboard. A reasonable quality built in domestic fridge is around £400, so now we're spending £1100 plus. We can go for a cheaper inverter, but then the idle overhead almost doubles and we're still going to have to pay about £400 for a decent inverter, so it's £550 for a small inverter and a cheap and nasty fridge (about the same as a decent 12v fridge) or about £800 if we buy a decent fridge.

As for efficiency, he's having a laugh. He, and you, also fail to take into account the inverter inefficiency and the fact that all of the time the fridge is not running, the inverter is wasting power, it has a standby overhead.

A 4 amp fridge, running for 1/3rd of the time, will use 32ah in a 24 hour period. Run a 4 amp fridge through an inverter and you can add 10% to the consumption for all of the hours it's running, as a minimum, so that's an additional 3.2ah. You can then add another 8ah if the Sterling figure of a 0.5a standby current can be believed, or almost 16ah if we're using the cheaper inverter, for when the fridge isn't running.

12v fridge, running 1/3rd of the time @ 4a = 32ah in 24 hrs
240v fridge, running 1/3rd of the time @ 4a = 51.2ah in 24 hrs using the Sterling Combi
240v fridge, running 1/3rd of the time @ 4a = 43.2ah in 24 hrs using the cheaper inverter

If it was such a great saving, don't you think the production boat builders would be fitting them ?

The suggestion of using the 3 way absorption fridge on a boat is utter nonsense.

Paul,

Its not quite so black and white.

I agree with what you are saying but surely few would only use the inverter for the fridge and would also use it for a variety of other tasks. So the 0.5 amp over 24 hours is 'supported' by other applications (though I suspect it makes only a minimal dent in your thesis). Our, inverter, is also a battery charger - so the capital cost might also be reduced (though as we visit a power point for charging so infrequently we maybe wasted our money (it seemed an excellent idea at the time!)

You mentioned, but did not really emphasise, but most free standing domestic fridges, if not all, have a little notice somewhere advising, or demanding, that an air gap be provided for cooling.

Jonathan

 

[PaulRainbow]

Paul,

Its not quite so black and white.

I agree with what you are saying but surely few would only use the inverter for the fridge and would also use it for a variety of other tasks. So the 0.5 amp over 24 hours is 'supported' by other applications (though I suspect it makes only a minimal dent in your thesis). Our, inverter, is also a battery charger - so the capital cost might also be reduced (though as we visit a power point for charging so infrequently we maybe wasted our money (it seemed an excellent idea at the time!)

You mentioned, but did not really emphasise, but most free standing domestic fridges, if not all, have a little notice somewhere advising, or demanding, that an air gap be provided for cooling.

Jonathan

I accept it might not be entirely black and white, so many different factors that are possible.

You mention reducing the capital cost by only having an inverter, which i think i covered with the cheaper option.

Yes, you could use the inverter for other things. But if it wasn't needed for the fridge, would you not turn it on and off as required to reduce the wasted standby current.

 

[PaulRainbow]

They advertise that their units have good cop figures, but they are not good and most marine fridges with their tiny evaporators don’t achieve great results i.e. within the poorer limits of the range of their performance figures. Good domestic units have massive evaporators and massive heat exchangers in comparison.

Still failed to address the unarguable inefficiencies of leaving an inverter on 24/7, even Sterling didn't factor those in (deliberately ?) He only made his comparisions with the fridges running, no consideration for the wasted over the rest of the time.

 

[Zing]

Still failed to address the unarguable inefficiencies of leaving an inverter on 24/7, even Sterling didn't factor those in (deliberately ?) He only made his comparisions with the fridges running, no consideration for the wasted over the rest of the time.

It’s in 24/7 in a modern boat, so no marginal losses.

 

[geem]

It’s in 24/7 in a modern boat, so no marginal losses.

Whats the COP of a good domestic fridge compared to a good marine fridge? Some test data from a reliable source would be good. I am not sure the Sterling data is accepted as reliable.
Try this Choosing 12v or 240v Fridges & Freezers on Narrowboats & Canal Boats

 

[PaulRainbow]

It’s in 24/7 in a modern boat, so no marginal losses.

Who leaves inverter on 24/7, with standby currents of close to 1 amp ?

Even if someone does, it doesn't mean it's an efficient thing to do. |The fridge, running from an inverter, will still be wasteful, less efficient than running a 12v fridge.

Do you know what efficient means ?

 

[Zing]

Do you know what efficient means ?

If you need it on for other reasons then the marginal losses from running a fridge are zero. Do you know what effect zero has on efficiency calculations Mr Sarcastic?

 

[vas]

Genuine question, could someone explain what would be the other reasons that an inverter would be on 24/7 on a boat?
I've setup my domotics on the boat to turn on/off the Multiplus when I enter/leave (with a delay) the boat just to avoid the extra standby losses (I think around 40W on my setup) That's done based on activity triggered motion sensors.

I'm asking as really most of the devices on board are internally running at some DC voltage. I've bought a dozen or two buck converters (down or up) and some nice small plastic boxes, so when I need something running, I'll tune the 25turn resistor on the converter to the right voltage, solder the cables and fit it in the box with the right plug on the end, job done. Did that last time for the raspberry I have for my victron stuff as well as the accesspoint having the boat connected to the internet (via a local wifi) OK granted these are 1-2Amp territory, don't need more...

cheers

V.

 

[geem]

If you need it on for other reasons then the marginal losses from running a fridge are zero. Do you know what effect zero has on efficiency calculations Mr Sarcastic?

We normally spend 9 months aboard, run lots of electronic gizmos but we dont leave an inverter on full time. We have three separate inverters for different jobs. They are all turned off at night. I dont think we are doing anything exceptional. The norm is not to run inverters 24/7

 

[Zing]

We normally spend 9 months aboard, run lots of electronic gizmos but we dont leave an inverter on full time. We have three separate inverters for different jobs. They are all turned off at night. I dont think we are doing anything exceptional. The norm is not to run inverters 24/7

Agreed. It is not the norm yet, but the bigger the boat, the more it will be on. Also if you have got rid of gas and gone more electric it will be needed more. At night I keep it on for the mosquito killer, but during the day we need it on for easy access to the PC, the kettle, the induction cooker and the hairdryer etc without the guests having their convenience interfered with.

 

[PaulRainbow]

Agreed. It is not the norm yet, but the bigger the boat, the more it will be on. Also if you have got rid of gas and gone more electric it will be needed more. At night I keep it on for the mosquito killer, but during the day we need it on for easy access to the PC, the kettle, the induction cooker and the hairdryer etc without the guests having their convenience interfered with.

OK, so we've established you're usage is not normal. That notwithstanding, your fridge still uses 10% more power running from the inverter than a DC fridge would.

For everyone else, who doesn't like to leave the inverter on 24/7, added to that 10% waste is the fact that all the time the inverter is on and doing nothing it's wasting between 0.5 and 1 amp. That isn't efficient by any standards.

Powering any DC device, such as laptops, phones, tablets, etc is also inefficient, converting from DC to AC (with losses) converting back to DC (more losses) is wasteful.

It may well be that you generate enough power to be able to suffer those losses, but that does not change the fact that they are not as efficient as they could be.

 

[geem]

Agreed. It is not the norm yet, but the bigger the boat, the more it will be on. Also if you have got rid of gas and gone more electric it will be needed more. At night I keep it on for the mosquito killer, but during the day we need it on for easy access to the PC, the kettle, the induction cooker and the hairdryer etc without the guests having their convenience interfered with.

In my experience bigger boats with guests simply have a permanently running genset.

 

[Zing]

In my experience bigger boats with guests simply have a permanently running genset.

Those bigger boats only run the genset permanently if they want aircon. They will have the inverter on 24/7 otherwise.

 

[geem]

Those bigger boats only run the genset permanently if they want aircon. They will have the inverter on 24/7 otherwise.

No. Twin generators. Large for A/C load. Small gen does lighting and small loads. A small gen is cheap compared to large lithium was the answer I got when I asked the question.

 

[Zing]

No. Twin generators. Large for A/C load. Small gen does lighting and small loads. A small gen is cheap compared to large lithium was the answer I got when I asked the question.

That is misleading. This is a function of size. A tiny boat will usually have no AC. The ships you refer to often have no DC. It is obviously in between I am talking about. Medium sized yachts to small superyachts do not run the genset when there is no AC needed. Nearly all gensets cannot be efficiently run at say a 500w load, which is typical of this range.

 

[geem]

That is misleading. This is a function of size. A tiny boat will usually have no AC. The ships you refer to often have no DC. It is obviously in between I am talking about. Medium sized yachts to small superyachts do not run the genset when there is no AC needed. Nearly all gensets cannot be efficiently run at say a 500w load, which is typical of this range.

What size yacht are you sailing on?