Peltier Effect Coolers

[AntarcticPilot]

Dear All,

I'm thinking of ways and means of improving the cool box on Capricious, my Moody 31. The cool box is quite small - I'd guess not much more than a cubic foot, two at the most, and is top opening. It is thoroughly built in, and I'm not up for changing it. It is well insulated; once it is cold it will stay cold for up to a day when loaded with cold food (or beer!).

Conventional refrigeration circuits all seem complete overkill, both in cost, capacity, complexity and size.

Can a Peltier effect device like these: http://www.virtualvillage.co.uk/thermoelectric-coolers be used effectively? Of course, it would need hooking up to some sort of thermostat system, but I doubt that would be a problem.

I presume some sort of thermal link between the hot side of the plate and the outside world would be necessary. Any thoughts?

Although the operating current is quite high (10-30 amps depending on the model), I'd tend to guess that the duty cycle would be maybe 10% once the box was down to a reasonable temperature - 4 or 5 degrees C would be ample.

All and any thoughts welcome!

 

[VicS]

Portable cool boxes using Peltier elements must use smaller ones as the current is usually 3 or 4 amps.

Usually have a double fan, one set of blades circulating air inside , the other blowing air over the outside

Mine is finned so assume you'll have to add fins.

Reasonably effective but current consumption is high. Usually run contiuously AFAIK but I guess can be controlled though

 

[tobble]

I think one of the main drawbacks is that when the unit isn't running, you have a large, efficient thermal bridge. I suppose that's a logical reason for running them continuously, as Vic Suggests, albeit under a control system.

Having said that I have no idea what sort of effiecincies you will get, compared to a conventional vapour compression system.

Edit: from Wikipedia:
"Thermoelectric junctions are generally only around 5–10% as efficient as the ideal refrigerator (Carnot cycle), compared with 40–60% achieved by conventional compression cycle systems"

If you're doing shortish trips, I'd think in terms of freezing/refrigerating as much as you can at home, taking lots of ice packs/frozen bottles of water and using the cool box as existing, maybe even upgrade the insulation and seals.

My next blog entry contains some musings on the necessity of fridges, will try to remember to let you know when it's published. Basically, most stuff it's not necessary, except for larger, white wine, and fresh milk for more than 3/4 days. the solution - real ale, red wine, and MArvel, which really isn't that bad!

appologies for the digression

 

[johnalison]

I put a Peltier cooler into my old Sadler 29. It was actually pretty useless, only partly because the fuel tank was the other side of the bulkhead and this got warm. A Moody 31 should be spacious enough for a compressor unit which would be vastly more effective. Of course, you can always reverse the current on a Peltier and warm your food.

 

[AntarcticPilot]

I put a Peltier cooler into my old Sadler 29. It was actually pretty useless, only partly because the fuel tank was the other side of the bulkhead and this got warm. A Moody 31 should be spacious enough for a compressor unit which would be vastly more effective. Of course, you can always reverse the current on a Peltier and warm your food.

Maybe, but a compressor unit will a) be vastly over-specified for the size of the box and b) be MUCH higher priced! I'm not looking for domestic refrigerator efficiencies, or a freezer compartment - just something at refrigerator temperatures of about +4 degrees C. I'm not a liveaboard as yet, so I'm not looking for more than few days use at a time. And I reckon the Peltier effect thingy would only cost about £30 all up. Indeed, it might even be worth cannibalizing a car cold box!

if I were living aboard, then I'd be looking at different kinds of solutions.

One thing I am aware of is that one side of the Peltier effect device has to be cooled so the thing can act as a heat pump. I'm still thinking about that, as the objection above that the heat circuit will spoil the efficiency of the cool box makes a lot of sense.

 

[tobble]

Fair comment on the liveaboard front, but I think you should estimate how much electricity the thing will use during a typical trip, and make sure that you can generate/store sufficient. Another option would be to only use it when the engine is going, and at other times wedge insulation over both sides of the unit to reduce the thermal bridge effect I mentioned. bit of a faf I know but will keep things cooler longer without the power drain.

 

[AntarcticPilot]

Fair comment on the liveaboard front, but I think you should estimate how much electricity the thing will use during a typical trip, and make sure that you can generate/store sufficient. Another option would be to only use it when the engine is going, and at other times wedge insulation over both sides of the unit to reduce the thermal bridge effect I mentioned. bit of a faf I know but will keep things cooler longer without the power drain.

Yes, electricity usage does have me worried - that's why I'm thinking of thermostatic control, if possible. But I'm also considering a wind generator (for other reasons - not to run a refrigerator!).

As commented previously, of course you can do without refrigeration for a few days, so this is very much a "nice to have" not a "must have". What I don't really understand is why, given the price disparity between the two systems, no-one is marketing a "drop-in" unit as they do for compressor systems. If I've got the pricing right, then a unit that would be much easier to fit than a compressor unit should cost less than £50 - allowing for a substantial mark-up! And the peltier effect device is obviously much more robust for a marine application.

 

[johnalison]

I'm a great fan of fridges, having only had one for the last ten years. The main advantage is that you can store fresh produce for a few days as at home, instead of racing to get into harbour before the shops shut or have a meal out of tins.

One option I considered in the Sadler was to have a portable Peltier cool-box which would have sat in the quarter-berth, but realistically, the current drain needed to keep food reliably cool would have been too much. My current fridge stays on throughout our 3 month summer cruises and causes a voltage drop of about 0.1 volts per day (when not plugged in ashore), with around 200 amp/hours available.

 

[DownWest]

Why not get a block of foam and make a new lid, with a cut out to fit one of the lid mounted units sold for cars/caravans (or two) By the time you have bought all the bits and fitted them, you could have have several off the shelf. Also they only reduce the temp by X amount below ambient, so aiming for 4 deg on a warm day might be optimistic.

At least by trying it, the worst you are out is some foam, and being lthe proud owner of a camping unit (or two) )
A

 

[ditchcrawler]

I have fitted a proper fridge compressor unit (under the sink) in my M31.It is the best thing to do.Mine cost £260 two years ago.Peltier type units are heavy on current.If you fit a proper fridge you will not regret it.

 

[bromleybysea]

I've had a couple over the years. I then traded up to a Waeco compressor 'fridge and despite the great difference in cost have never regretted it. IMHO the small, self-contained Waeco units are great and it's one of the best decisions I have made. The Waeco unit really does keep the beer cold! The problem with the peltier units is that they need to run continuously and they will only cool down to a fixed proportion of the ambient temperature. Thus they are least efficient when you need them most.

 

[Bosun Higgs]

A peltier effect cooler ( or heater) is about one fifth the efficiency of a typical compressor fridge ie uses 5 times as much leccy for the same effect.

 

[Pete7]

I have fitted a proper fridge compressor unit (under the sink) in my M31.It is the best thing to do.Mine cost £260 two years ago.Peltier type units are heavy on current.If you fit a proper fridge you will not regret it.

As did we in our M31 last winter, cost £300 for one of these, from Percy M See in Fareham.

http://www.force4.co.uk/736/Isotherm--Classic-GE-80-Fridge.html

We fitted a sink plug to the inside of the fridge to stop cold air sinking down the drain pipe and lined the lower outer sections in extra 5mm radiator insulation. The result is the fridge runs for a couple of minutes every quarter of an hour though it is difficult to hear. Probably a couple of amps an hour in the UK summer. If the thermostat is too high it freezes the tonic Compressor unit fits on the removeable wooden inspection hatch under the sink and infront of the existing cool box so doesn't affect the space under the sink. Big brownie points from SWMBO for fitting it.

Pete

 

[Billjratt]

I agree totally with the previous posters - there's no contest between a peltier device and a compressor unit, however AP has indicated that there is no justification for a compressor.
So, I guess it's a case of how many amp-hours can be afforded, then choose the appropriate wattage. Fit computer processor coolers (blown heatsinks) inside and out - I can pass a couple to Pete if you want to go ahead - and fit the resulting assembly to the lid - or a new specially made lid. When you power it off, you could swap back lids if you were worried about coolth rising. The problem I see with them is the length of time taken to achieve the above-mentiond coolth. I think I would still use pre-frozen meals in the bottom (or ice) and use the peltier to prolong the coolth.

 

[BabaYaga]

I have used one of these for more than ten years now

http://www.ozmarine.se/OZcooler/Eng/IndexOZcoolerEng.html

and it's been working fine.
Perhaps not the low cost solution you are looking for, but might give you some ideas. This is a Peltier based cooler that can be fitted to an existing box (the kit version). No fans involved, completely silent.
I think the big problem with Peltier based cooling is getting rid of the heat produced. In this case the Peltier element is well outside of the box and the heat is transported away through the huge copper braid and dissipated into the sea water.

 

[Rivers & creeks]

We used a £25 peltier cool box for a few years. They don't come with a thermostat so I just turned it on for a few hours a day. It kept stuff nicely chilled and if on for a few hours I was amazed at how chilly the beer got. Downside was the current - 4amps an hour takes some generating - the Rutland 913 only averaged 2 amps so basically I had a bloomin great wind genny to just run a coolbox for half the day

 

[BrendanS]

If you are only talking about several days, then Tobble's idea of pre chilling and freezing can be highly effective. I used to use a few packs of cans of beer (don't try this with soft drinks, the cans will explode!)frozen at home, and transported in a travel cool box. The frozen cans can keep the cool box very cold for 3 or more days, to the extent that if you overdo it, other items in there can freeze, even mid summer. You have to remember to take the cans you are going to drink out before hand, otherwise they will be frozen.
Can be highly effective.

If you have peltier to top up from 3 to 4 days onwards, you will have a pretty workable system for very little cost.

 

[William_H]

Peltier fridge

The Oz cooler heat coupling to the sea should make it more efficient. As said however the device is a bit like a lump of aluminium and with no current would be quite a good conductor of heat into fridge.
The oz cooler controller seems to keep current on at all times just reducing the current. I hope in a high efficiency switch mode controller.
However for all that I think Peltier will only disappoint you would find bottles of ice in your existing cool box will be best. It seems to be most popular around our yacht club. it certainly gets hot here. forecast 35 max yesterday actually hit 39 (nice being a bank holiday) equal hottest summer ever. Only 35 today and relief later in the week. PS forget any need for a thermostat. olewill

 

[Dockhead]

Just using ice (in bags, or frozen in plastic bottles) will be still much cheaper and will not suck down your batteries, compared to the Peltier devices, which really do not belong on a boat as others have shown. Generations of sailors cooled their food with ice; it works.

If you don't want to invest in a real fridge (and that really lovely Isotherm unit above only costs 380 squids at Force 4), and you are only out for short periods, that's the way I would go.

 

[wooslehunter]

Many people have mentioned efficiency. I'm not sure of the actual figures here but can shed some light on why a peltier is not such a good idea if you have the space for a compressor.

Peltier junctions work by creating a temperature gradient across the device. They consume energy when they do this. If you remove energy from a closed space, it gets cold. The energy is dissipated as heat somewhere along with the extra energy it took to remove it. A peltier will do this but consumes vastly more energy than a compressor.

Therefore not only will the peltier consume more power than a compressor, you have to get rid of the extra heat generated somewhere: heat up the boat for instance. For every joule (1 joule = 1 Watt for 1 second) of energy extracted from the fridge, the peltier will consume 10-20 joules. Whereas a compressor will consume around 2 joules. Peltiers are 5-10% efficient whereas a compressor is 40-60% efficient.