Peltier cool box trials

[pcatterall]

As I mentioned on a previous thread I was interested to see just how well these cool boxes worked and if they could be improved in terms of battery consumption.
A summarry of the results is below.

I tested a waeco 20 lt box containing 20 lt of water over 4 days.
Initially there was no added insulation, externall temps were around 58 to 65 F no draughts no sun good ventillation.

On the initial cool down it took 16 hours to reduce the water temp from ambient to 15 degrees below ambient where it remained stable.

I added 1" of insulation to the base and sides of the box
After 5 hours the temperature had fallen to 17 degrees below ambient.

I then cycled the power one hour on and one hour off. there was no significant change in the temperature difference.

The unit did give a substantial amount of heat which would have to be disapated if the unit was used in a confined space ( as with any other fridge).
The insulation clearly helped another 1" would improve this.
Once the contents had reached their coolest level cycling the power appeared to maintain that level with only slight changes in temp observed.

Conclusion???
Still a cheapo alternative to a proper cool box but, with extra insulation and cyclying the power, performance/efficiency can be improved.

 

[maby]

Interesting - your tests seem to match our experiences pretty well.

My biggest problem with this type of coolbox is the lack of any kind of thermostatic control on most models. The current consumption is comparable with that of a proper 12v fridge, but it's continuous unless, as you did, you manually cycle it.

 

[pcatterall]

I had just thought I would put a simple time switch on it. With the insulation in place there was less than a degree of 'warming' on the 'off' period and this was always brought back by the on period. So once the optimum temp of the contents is acheived power consumtion is effectivly halved.
Of course opening and closing, removing stuff and putting warm beers back in will affect this!!

 

[RAI]

Do they slowly die?

I have had a cheap one running for a couple of months (aid of shore power). My impression is that its cooling capability is reducing.

It also collects a lot of condensation water inside. A puzzle, as it seems to have no obvious source. Somehow it must get it from outside.

 

[maby]

I can't see any reason why it should slowly die - it's really just a thermocouple running in reverse. I suppose the junction might somehow poison, but I was not aware af any such mechanism...

 

[RAI]

Maybe it uses several junctions and some of them have died...

When it's dead, I'll cut it open.

 

[atol]

i used a small waeco last year across the tropical pacific with good results,though nearly constant 3 amp draw at 35c ambient temps.
it has a built in thermostat so cycles.

moisture build up inside is due to humidity in the air,loss of efficientcy is probably due to lint,fluff n dirt on the fan blades and heat exchanger,clean with soapy water and ear buds!!!

 

[peteK]

As I mentioned on a previous thread I was interested to see just how well these cool boxes worked and if they could be improved in terms of battery consumption.
A summarry of the results is below.

I tested a waeco 20 lt box containing 20 lt of water over 4 days.
Initially there was no added insulation, externall temps were around 58 to 65 F no draughts no sun good ventillation.

On the initial cool down it took 16 hours to reduce the water temp from ambient to 15 degrees below ambient where it remained stable.

I added 1" of insulation to the base and sides of the box
After 5 hours the temperature had fallen to 17 degrees below ambient.

I then cycled the power one hour on and one hour off. there was no significant change in the temperature difference.

The unit did give a substantial amount of heat which would have to be disapated if the unit was used in a confined space ( as with any other fridge).
The insulation clearly helped another 1" would improve this.
Once the contents had reached their coolest level cycling the power appeared to maintain that level with only slight changes in temp observed.

Conclusion???
Still a cheapo alternative to a proper cool box but, with extra insulation and cyclying the power, performance/efficiency can be improved.

I too posted on this subject some time ago and cant understand why the manufacturer doesnt incorporate some sort of timer in the absence of a thermostat with enough insulation it could power up say 1 hour in 3 thus cutting consumtion by 2 thirds.

I researched on the internet for suitable timers and found that possibaly a central heating timer/thermostat that is powered by 2 AA batteries might work.

 

[atol]

I too posted on this subject some time ago and cant understand why the manufacturer doesnt incorporate some sort of timer in the absence of a thermostat with enough insulation it could power up say 1 hour in 3 thus cutting consumtion by 2 thirds.

I researched on the internet for suitable timers and found that possibaly a central heating timer/thermostat that is powered by 2 AA batteries might work.

their aint no free lunch!! a thermostat will cycle 1 minute on,3 minutes off and keep your food at a constant tempreture,

having the thing off for 3 hours and on for 1 will warm and cool your food,causing a perfect environment for food poisening to occur!!!!!

and not save any power!!

 

[pappaecho]

Be a little bit careful over this test. Water has the highest heat capacity of any common substance, and hence it will retain 5 times the amount of heat compared to say a 20 litre slab of concrete.
Hence it would hold rather more coolth, than say a motley collection of stuff in a normal coolbox much of which would be air, so the thermal capacity would be lower.
However the hour on - hour off routine, could of course be modified to off, then no power until box rises to X degrees C, when it then starts again,

 

[Dabhand]

I can't see any reason why it should slowly die - it's really just a thermocouple running in reverse. I suppose the junction might somehow poison, but I was not aware af any such mechanism...

Strictly speaking a Peltier is not a thermocouple as it consists of one basic material. Peltier devices are normally very reliable providing they are treated well, they will certainly die very rapidly if held at temperatures of 80C to 90C, afterall they will heat as well as cool. I believe it is a molecular interaction with the solder and the semiconductor material - normally bismuth telluride, of which the individual junctions are made. Likewise if they are run continuosly at maximum current they will detiorate over time.
A Peltier actually consist of many small elements all connected in a series / parallel arrangement to obtain the required power rating (in Watts) with appropriate current levels.
A Peltier arrangement will achieve temperatures of better than -40C, admittedly only with a small mass to cool rather than a fridge, but does require special techniques to do this. By reversing the current through them they will heat. With suitable temperature monitoring and control very precise temperature stability is achievable.
The most crucial factor for efficient cooling is to keep the fan cooled heatsink as well ventilated with cool air as possible. Ideally pumping sea water through the heatsink would be the best, if impractical method.
If the cooling system should become ineffective then the Peltier can overheat and the solder holding the elements will melt. I know, it has happened to me, although not in a fridge.

 

[SURV69]

Could I be wrong?

I only run my Peltier unit(Coleman), at night, early morning and about 7 in the evening. I run it for between 10-12 hours total per day.

I never run it when the temperature is high and the sun is out.

I have actually frozen 1% milk(solid, like a brick), in August and I'm more than satisfied with the performance . . . although I hate frozen milk.

I plan on making a built-in cooler with a water drip hot side and continue running the units when the ambient temperature tends to be lower.

Am I right assume that if I run the unit on a 75 degree night(possibly maintaining about 40 or so degrees in the cooler, that if I likewise run the unit when the day is hot and the sun is high, that I am actually(in a respective manner) heating the cooler contents.

I plan on making my box accessible to remove food items when I cook(in the evening) and have a much smaller access for removing the smaller things(like a beer), during the day.

It's just that I really like the simplicity of a Peltier unit and replacement parts take no space and cost little money . . . not like a compressor-type cooler.

BTW . . . I'm diabetic and can't eat Ice Cream . . . too often.