12v refridgerators compressor or absorption?

[Ursula123]

Hi,
I'm looking to fit a fridge in a yacht I'm refitting, it hasn't got one at present. I've seen absorption fridges advertised, they seem good value and are advertised as suitable for boats but has anyone had any experience of them?
Thanks

 

[prv]

As far as I know, the absorption cycle always involves a heater, to separate two chemicals by boiling one out of the other. This has the benefit of being very simple with no moving parts, and can use all kinds of heat sources such as gas or paraffin - or even an old cyclic type that you recharged by putting one end in a camp fire. But if run electrically, it uses vastly more power than a compressor fridge, because heating up coils is a pretty wasteful use of electricity.

These fridges work quite well as three-way types for caravans, where you run them off 12v from the tow car with its engine running with power to spare, then when you get to site either off 240v mains or off gas (the versatility of only needing heat).

On a boat, the 12v side is too greedy to be practical, and the gas option is nowadays considered dangerous in a closed hull. Most absorption fridges also need to be level to work, because of the way the chemicals have to drip downwards through their pipework (there is no pump). They may cope with constant movement in all directions, as when being towed behind a car or just possibly in a motorboat or catamaran, but they need to average upright which is not usually the case in a monohull under sail.

Get a compressor fridge

Pete

 

[VicS]

Hi,
I'm looking to fit a fridge in a yacht I'm refitting, it hasn't got one at present. I've seen absorption fridges advertised, they seem good value and are advertised as suitable for boats but has anyone had any experience of them?
Thanks

I think you will find that an absorption fridge uses more power than a compressor fridge.

Advantages as I see it are that they are silent and can also be gas powered , but the gas powered option may not be as attractive on a boat as it might be in a caravan.

We used to have an absorption ( domestic) fridge ..... but it was a bit of pain because the heating element used to burn out periodically.

Check the average power consumption of an absorption fridge and compare it with an equivalent compressor fridge.


Oh Pete beat me to it...... at lest we seem to agree!

 

[Arcady]

Precisely as prv states. I'm just changing out an absorption fridge for a compressor fridge for all of these reasons.

Many years ago at anchor on a boat with no split between engine start and domestic supply, I left an absorption fridge plugged in - only for a couple of hours whilst we rowed ashore. By the time we got back I couldn't start the engine. They are VERY greedy!

 

[Mistroma]

Absorption types are fundamentally less efficient than compressor types. Some absorption types will be better than others but always worse than a compressor one in good condition. Peltier type weren't mentioned but these go right to the bottom of the list when looking at efficiency. Not certain if that's a theoretical limitation or just implementation of existing Peltier devices. Of course I shouldn't really be using the word efficiency here but I think it gets the meaning across.

 

[PaulRainbow]

Get a compressor fridge

Pete

Nuff said

 

[William_H]

You are looking at around 10 amps continuous for an absorption type 3 way fridge on 12 DC for mediocre cooling capacity.
Versus about 3 amps for 20% to 40 % on cycle for serious freezing of a compressor type. No question compressor always.
olewill

 

[VicS]

You are looking at around 10 amps continuous for an absorption type 3 way fridge on 12 DC for mediocre cooling capacity.
Versus about 3 amps for 20% to 40 % on cycle for serious freezing of a compressor type. No question compressor always.
olewill

That's even worse than a Peltier effect one!

 

[vyv_cox]

You are looking at around 10 amps continuous for an absorption type 3 way fridge on 12 DC for mediocre cooling capacity.
Versus about 3 amps for 20% to 40 % on cycle for serious freezing of a compressor type. No question compressor always.
olewill

Using 12 volts for a heating element is never going to make much sense. But gas is far more effective, the flame is little bigger than a pilot light, consumption very low. The disadvantages for a boat are that they are very sensitive to level and turn themselves off at more than about 3 degrees from vertical and the chimney draught is critical, the flame is easily blown out in a downdraught and the slightest obstruction, e.g. a spider's web, stops it working. We have had many of them in motorhomes. The current van has a big fridge, 125 litres I think, plus a large freezer that works well even in Greek summer temperatures. I would not even consider one in a boat, too many disadvantages, e.g. we carry 2 x 13 kg refillable gas cylinders in the van but only Camping Gaz bottles on board.

 

[JumbleDuck]

Absorption types are fundamentally less efficient than compressor types. Some absorption types will be better than others but always worse than a compressor one in good condition. Peltier type weren't mentioned but these go right to the bottom of the list when looking at efficiency. Not certain if that's a theoretical limitation or just implementation of existing Peltier devices. Of course I shouldn't really be using the word efficiency here but I think it gets the meaning across.

Technically fridges have "coefficients of performance" (CoPs) rather than efficiencies, because ^{what you get}/_{what you pay for} is often more than one (more heat is transferred than energy is used to transfer it) and people get twitchy about efficiencies over 100%.

I think the rotten performance of Peltier devices (CoP <1) is down partly to some fundamental physics and partly to poorly performing materials - even with better materials I don't think they come near the possibilities of phase-change cooling systems. Not my field, though, and happy to be corrected. I've used a lot of very cold stuff in my time, but I didn't make it cold.

 

[BabaYaga]

I think the rotten performance of Peltier devices (CoP <1) is down partly to some fundamental physics and partly to poorly performing materials - even with better materials I don't think they come near the possibilities of phase-change cooling systems.

I have Peltier based system that comes pretty close to the compressor type system in power efficiency. It was called MarinCool when Origo (then part of Electrolux) marketed it for a while some +15 years ago.
The main difference from a normal Peltier system is the absence of a fan, instead the heat transport takes place by a massive copper braid to a cooling plate on the outside of the hull under water.
My system is 20 yo and still works fine. I don't think there is anything similar on the market today, though.

 

[Daverw]

I suspect that if you compared both directly the compression ref system will always come out more efficient, laws of physics etc, large scale absorption are good and very efficient as they use high grade waste heat but on your boat that would then mean running the engine and then when factor this in costs go up.

The Peltier system is simple though but so is compression refrigeration, the issues that many refrigeration engineers try and keep it a black art and try and charge as it is. Over the years many alternate systems have come and gone, most due to running costs being higher in the real world.

The real test is if alternates are the way forward why haven't the industrial boys moved over the last 70 years, plant today is very similar to that design then.

 

[AntarcticPilot]

There are supposed to be developments of the Einstein refrigeration system in hand for use as a vaccine cooler. There is also research going on into making it much more efficient by judicious changes of the working fluids. To me, this looks very promising for marine use - it is constant pressure so leakage should be less likely, and it only requires a heat source to operate. No moving parts, and no parts under stress looks good to me!

 

[blackbeard]

For the record.
I have an absorption type portable fridge I use when camping. Needs to be set up with a spirit level if the field isn't flat. Uses about one Camping Gaz cylinder per fortnight which doesn't sound bad until you consider the cost of Camping Gaz refills.
The boat has a 25 litre Waeco compressor fridge. The handbook says it uses about 3 amps at 24 volts but I haven't found a figure for 12 volts, which I suspect is higher. Not too worried as duty cycle is, as already said, very low.

 

[JumbleDuck]

There are supposed to be developments of the Einstein refrigeration system in hand for use as a vaccine cooler. There is also research going on into making it much more efficient by judicious changes of the working fluids. To me, this looks very promising for marine use - it is constant pressure so leakage should be less likely, and it only requires a heat source to operate. No moving parts, and no parts under stress looks good to me!

Good heavens. Malcolm McCulloch, referred to in that article as restarting development of the idea, was the internal examiner for my doctoral thesis. What's an expert in superconducting electric motors doing mending fridges?

That aside, don't normal absorption cycle fridge work at constant pressure, using the presence of absence of hydrogen to change the partial pressure of ammonia and so promote evaporation and condensation? I used to know that, once.

 

[VicS]

Good heavens. Malcolm McCulloch, referred to in that article as restarting development of the idea, was the internal examiner for my doctoral thesis. What's an expert in superconducting electric motors doing mending fridges?

That aside, don't normal absorption cycle fridge work at constant pressure, using the presence of absence of hydrogen to change the partial pressure of ammonia and so promote evaporation and condensation? I used to know that, once.

There is a lot of physics in fridges! It's a miracle they work at all.

 

[JumbleDuck]

There is a lot of physics in fridges! It's a miracle they work at all.

Clearly they shouldn't, particularly the absorption sort. Distressingly asymmetrical.

Other things are equally worrying. It's all wrong that an injector can take steam from a boiler and use it to force water into the same boiler without any moving parts.

 

[Dougal]

We're still using an absorption fridge on the boat. It's a 3way mains/12v/gas and we have a 15kg butane bottle, for which we pay about 2/3 the price of a 907!!!
The fridge is a 45ltr model and draws 7.6amps about 80% of the time on 12v. The thermostat does actually switch it off occasionally, unless the ambient temp is very high.
We spend at least 2/3 of our time plugged in to shore power, so i haven't the incentive to splash out on a newer fridge? Maybe I should have that on my headstone when we explode;-)

 

[superheat6k]

I suspect that if you compared both directly the compression ref system will always come out more efficient, laws of physics etc, large scale absorption are good and very efficient as they use high grade waste heat but on your boat that would then mean running the engine and then when factor this in costs go up.

The Peltier system is simple though but so is compression refrigeration, the issues that many refrigeration engineers try and keep it a black art and try and charge as it is. Over the years many alternate systems have come and gone, most due to running costs being higher in the real world.

The real test is if alternates are the way forward why haven't the industrial boys moved over the last 70 years, plant today is very similar to that design then.

Not sure why people still think refrigeration engineering is a black art, plenty of books and other sources of how a vapour compression heat engine works, but the problem with cost is it is disproportionate to equipment size. To send a fridge engineer to look at a boat's fridge costs the same as sending him to Tesco's to fix their fridges. It never ceases to amaze me that fridge engineers are somehow meant to be cheap, after all mechanics, lawyers, doctors, et al do not provide their services for free, and fridge engineers are hardly routinely running superyachts.

The major change since I have been in the fridge industry (26 years) has been the move from piston compressors to screw (for large systems) and scroll (for smaller systems), but the basic fridge system remains pretty much the same as used in the late 19th Century. Modern electronics have improved larger systems, but on smaller units the electronics only really switch the compressor on or off for capacity control.

The BIG issue not yet resolved within the Refrigeration Industry is what to do about the ever changing situation of the available / acceptable refrigeration fluids to use. CFCs (R12 / R502) went in the 95, HCFCs (R22) went in 2010. HFCs that replaced these (R134a / R407C / R404A / R410A) will be massively reduced over the next 13 years. The next group are HFOs, which are less efficient and notably flammable, as are the HC group used for some domestic fridges (AKA Isobutane). The phase out programme has been set down in law, but as yet the detailed rules governing use of flammable refrigerants (EN378) has not yet been re-issued to our industry.

This is directly hampering development of more effective equipment, and the supermarkets have spent much of the past 10 years experimenting with CO2, now there's a dichotomy, and a very very difficult gas to use for refrigeration.