Improvised fridge

[K'Luuppo]

I have used to put things outside if they had to be kept in cold, but now the outside temperatures are rising and after few weeks i don't know if that will be possible anymore but I have to keep things cold. Our fridge isn't really an option, so it came to my mind to make my own "fridge".

I thought it could be made with a 80W peltier-element, a cooling "hedgehog", a computer chassis fan, styrofoam and a thermometer. It could be an styrofoam box/cylinder with peltier-thing on bottom. "Hedgehog" is then glued on the peltier with some heat glue. Then the fan is bolted onto "hedgehog". Finally, the thermometer is put through lid and all wires are connected to a power supply. A picture about thing is on http://koti.mbnet.fi/verkku/fridge/fridge.bmp

I have few problems still: I have no idea about how temperature could be adjusted, but more importantly, would 80W peltier be enough to cool ~300ml water to near freezing point?

[IDTB]

It could be an styrofoam box/cylinder

I wouldn't recommend styrofoam as instillation. I'm not sure how well it'd work scientifically, but my expierence with it tells it's not a wise choice. A decent commercial 'mini-fridge' costs around $100. I have no complaints with mine. Though, I've never been much of a do it yourself'er. Good luck with your project.

[Boomer]

I once built a peltier fridge to use up the large access of photovoltaic energy (had installed 550Wp), not wanting to buy any more batteries (were already 800Ah).

The styrofoam box was approx. 30 litre net volume, cooled with one 50W peltier. It reached 20C under ambient temp at full current. :)

You need GOOD cooling with a THICK heat sink + BIG fan on the outside, plus some means to distribute the cold inside, i.e. another heat sink + fan. Alternately you can line the bottom of the box with quite thick aluminium (3-5mm) to increase surface. Between the 3-5mm sheet and the peltier should be another block of alu of 5-10mm.

So forget about the PC fan. At a COP of 1 you must dissipate 160W from a 80W TEC module. A good PC fan has a rating of 0.5K/W (Pentium-2 fans were above 1K/W). This gives you a hot side of +111C at room ambient, i.e. your "cold" side is +45C at best (= you have built a heating chamber). :p

With a "real" fan (see www.fischerelektronik.de, look for "Hollow-Fin Cooling Aggregates") you get down to 0.04K/W. This gives you a warm side of 6.4C above ambient, so your cold side can go 30C below this (allowing for some losses and thermal resistance between the parts).

And you need a perfect heat transfer: The best is silver-filled paste between perfectly parallel + smooth surfaces. You can still expect to pay 100 bucks for electricity over the summer! Or rather your mum... ;)

Good luck!

[K'Luuppo]

Seems more complicated than I thought...

How about those commercials mini-fridges? They probably aren't using electricity worth of 100$ so there must be some kind of more efficient method to carry the heat out from the box. Is it same method as in real fridges, gas compressing and decompressing or are they built with some other cooling system? 100$/summer sounds quite much, our whole consuption would be doubled...

[Tuatara]

I'd have thought a used bar fridge could be had dirt cheap. I got a big deep freeze for $NZ150 second-hand.

Peltiers, while they work, are never the cheap option. The COP is poor, the maximum working delta T is poor, they are expensive for the power tranfer rating.

Remember that an 80W peltier can only pump 80W when the delta T across the device is just a couple of degrees. When you get up to 60 degrees delta the pumping capacity drops to almost nothing, despite the 160W of electrical power you are pouring in.

[mongo blongo]

You can make a multi-stage peltier by stacking them on top of each other if you want to increase delta T across the device.

[Fatboy]

Seems more complicated than I thought...

How about those commercials mini-fridges? They probably aren't using electricity worth of 100$ so there must be some kind of more efficient method to carry the heat out from the box. Is it same method as in real fridges, gas compressing and decompressing or are they built with some other cooling system? 100$/summer sounds quite much, our whole consuption would be doubled...

From my experience , mini fridges use a peltier element.

Its probably best to go looking round a scrap yard or asking people for old fridges , as there much bigger. If you had a busted fridge im sure the compressor and refrigerant system could be rigged up to make a much more powerful version of this improvised fridge. Take a look on various phase-change PC cooling websites.

[Marvin]

An 80W peltier will only pump 80W at 0 degrees difference and there is a roughly linear relationship between temperature difference and power pumped.

Stacking peltiers is not usually helpful. Becuase they use rather a lot of power, as you stack them pumping capacity falls expoentially to very small amounts. Worse still becuase of the way silicon peltiers work, as the hot side temperature drops the pumping capacity and the max temperature difference (which depends on the area, the materials and the pumping capacity) drop rapidly as well as the fact that each lower stage has to pump all the power being wasted by all the stages above, regardless of if they are doing real work or not, so each additional stage only increases the max temperature difference by a much smaller amount. One stage can get you a maximum of 70-80C temperature difference (at zero pumping capacity) you need 3 stages to get to around 120C difference and from that point things just get much worse. There is probably a single stage fluorocarbon that can get 150C from room temp in 1 stage. On the bright side peltiers are silent and dont wear out.

Boomer also has a very good point made perhaps not strenously enough. Gluing the heatsink will do very badly indeed. USe thermal paste for a CPU, and boly it on tightly. Bolting it on without alowing the screws to thermally short the peltier will be a chore in itself. Styrofoam should provide decent insulation.