Improving peltier fridge efficiency??

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pcatterall

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We all know that these little fridges have their limitations, only cooling to X degrees below ambient and using a lot of juice.
Regarding the power consumption; ..... Once the cooler has reached the lowest temp it keeps running ( and using power ) what if it were then to be switched off for, say, half an hour? the internal temp would not increase much and would easilly be reduced again with the next half hour of power.
I guess that there is a suitable device to just keep switching it on and off.
Of course you would have to set this timer to work when the desired temp was reached as I can see that introducing a thermostat would add complication.
 
Thats what a thermostat does. The Peltier effect is simply a thermocouple running in reverse. If you leave it on for ever , it will reach equilibrium when the cooling effect of the Peltier module just balances the heat coming into the fridge through the casing and thus the internal temperatrure is constant. In m y experience this wont be very cold at all.

If this internal temperature is lower than you need then you can use a thermostat to switch the Peltier system off and on. Simple unpowered fridge thermostat would do.
 
Peltier type fridge

Mostly when you want a fridge they will not cool enough, before you flatten the battery. As said you need a thermostat.
The best way to improve efficiency is to cool the hot outlet side. In a boat water cooling is best but a fan to move hot air away or a big Aluminium plate attached to the heat sink of the fridge might help.
Most people just throw em a way... olewill sorry a bit negative
 
First hand experience on this here. I built a thermostat control for my camping gaz cooler box. Its built into the lid, and is set at about 5 deg C. I found that during camping it probably extended the battery life about 30% depending on weather.
It was quite interesting since during the day its operating at 100% but by middle/late eve it would be down to around 50% duty or so. Definitely a worth while 'mod' as these things do eat a lot of power.
 
Oh and I improved the airflow across the heat sinks on both the hot and cold sides by removing some of the plastic fins that are there to stop you putting your fingers in!
 
Agree with beyondhelp. I have a little beer cooler type one on the flybridge and I would normally turn it on when I first get up. By the time I need refreshing upstairs during a passage it is actually quite cold in there - even on a hot day. It takes an age for them to bring the temp down so I only use it when I've got the engines running all day, but for my use it works well.
 
Thanks, I take all your comments on board.
My own experience has been that my little cooler keeps running at the same speed even when the contents are at their coldest.
I have tried it in an insulated box and there is no difference in the minimum temp achievable.
There is however a big difference( as you would expect) in how quickly the contents warm up.
I will do some 'scientific' tests and report back!
 
Because Peltier coolers are pretty inefficient, the most important factor is that the space they are cooling down must be very well insulated; the more insulation you have, the greater the temperature differential that the Peltier cooler will be able to maintain.
 
greenalien said:
Because Peltier coolers are pretty inefficient, the most important factor is that the space they are cooling down must be very well insulated; the more insulation you have, the greater the temperature differential that the Peltier cooler will be able to maintain.
Click to expand...

So are we saying that the peltier coolers inability to cool lower than X dgrees below ambient is down to insulation? I thought it was something more akin to those 'desert' coolers which are limited by humidity and temp.
Will get onto the trials now now!!
 
Think of it like this - a peltier element is a heat pump, moving heat from its cold side to its hot side. If the heat flowing into the box is less than the amount of heat the element can remove, the temperature inside the box will fall. As the box gets colder, the differential between inside and outside increases, so it's more likely that heat will try to flow into the box (temperature gradient). The better the insulation, the less is this inward flow, so the greater the differential that the element can maintain. Most peltier-equipped portable cold boxes aren't very well insulated, so will only maintain a differential of 20 degrees or so. However, some scientific equipment with peltier coolers have over 4" of insulation, and are thus able to get a much larger differential.
 
If a peltier is effectively a thermocouple in reverse - that suggests that it must have a direct thermal connection between hot and cold, so as soon as the power stops being applied, energy will quickly flow back from hot to cold.

Surely it is better to get a small compressor fridge in any circumstances when you are worried about power consumption
 
pcatterall said:
We all know that these little fridges have their limitations, only cooling to X degrees below ambient and using a lot of juice.
Regarding the power consumption; ..... Once the cooler has reached the lowest temp it keeps running ( and using power ) what if it were then to be switched off for, say, half an hour? the internal temp would not increase much and would easilly be reduced again with the next half hour of power.
I guess that there is a suitable device to just keep switching it on and off.
Of course you would have to set this timer to work when the desired temp was reached as I can see that introducing a thermostat would add complication.
Click to expand...

1) Fit one of these to switch it off when the temp drops low enough,
2) add a lot more insulation around it.
3) add a small fan to suck the hot air out from aroung the heatsink (also wire via the thermostat)

Maplin's thermostat kit: Contacts open when the temperature had dropped below the set (adjustable) level.
http://www.maplin.co.uk/temperature-activated-switch-kit-220003
 
greenalien;315657 Most peltier-equipped portable cold boxes aren't very well insulated said:
Two questions:

Are they insulated at all other than by the air gap between the inner and outer boxes. If not then it might be worth filling the gap with expanding foam.

Can anybody explain to me why they are so inefficient (about 5% compared with 40% or more for compressor units.). A link will do :)
Click to expand...
 
The consensus seems to be that it is the insulation which limits the cooling capability.
I will extend my test using a 2" polystyrene outer box.
If it works better I may build an insulated chest to slot the cooler into. Yes I would then need to consider the top of the box air in/out/dissipation etc but if I can reduce the power needed by a good percentage then it may be worth considering.
 
I use one & have wired it to a relay which starts it going when the engine is running & when the engine is not running wired in a redundant mechanical timer off an old tumble drier that can be set for up to an hour.
No thermostat but a degree of control ensuring the battery cannot go flat because of forgetting it is switched on.
Relatively simple circuitry using a 12V relay & tumble drier control (not the digital type)

It is is a locker surrounded by bits of chopped up foam and a little computer type fan in circuit to draw off the warm air away from the locker (very low wattage)

John Lilley
 
we have one on our boat - seems to work ok - not as cold as a compressor type (forget ice cream and ice cubes) but cold enought to keep food and drinks ok - we fitted a lenght of hose to the hot side which runs down to the bilge - our theory being that way is should be more effective at getting rid of heat on that side than the high level position of the unit itself (on theory heat rises) no complaints about the unit at all
 
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