Many of the cheaper car-fridges are already based on these.
Efficiency of a peltier is inherently low, before you even get started on inefficiencies of the power supply. Example characteristic curves and a good FAQ. A typical "freezer" application requires a temperature difference of about 40C to ambient, and many devices will have an efficiency coefficient down near 0.5 for that temperature gradient, i.e. you need to put in 60W of electrical power to suck 30W of thermal power from the cold side, then dissipate 90W from the hot-side. A good compressor-based system will achieve an efficiency coefficient of about 3 though, e.g. put in just 10W to transfer 30W and then dissipate the 40W total from the hot side.
A 12V 4A peltier is about 50W (plus power supply inefficiencies), which is 438kWh/year and it will struggle to keep a few drinks cold. In comparison, my 150L chest freezer uses only 220kWh/year running at -20C.
A bang-bang thermostat running very slowly (large hysteresis) could probably work well for lack of thermal shock. I set my first attempt to have a 5C window and it was sitting on top of a 65W CPU with only about 50g of Cu as thermal mass, so it was cycling every few seconds. The thermal cycling stress cracked one (of the about 400) peltier elements which meant the whole thing went open-circuit and ceased to cool. Bad-times. I put in a PID controller running in a PIC16F84 and PWM'd the current to the peltier at about 30Hz; it was all good after that and held temp within 1C regardless of CPU load.
You really don't want to know the power consumption of that PC - turning it off paid for my next machine The heat exhaust was a 4x6" car-heater radiator core and there was a continuous warm blast issuing from it. Great in winter and with near-unbeatable bragging rights on 15C core temperature (above dewpoint!) under load.
An often-overlooked feature is that they're physically symmetrical, i.e. pushing heat through a Peltier device gives you electrical power! Attach one side to a big heatsink and the other to a few candles and you can get measurable power from it to run LEDs and the like. I believe you can now buy portable wood-burning cooking stoves that produce 5V USB-power for phone-charging while camping. Most phone chargers are only 10W and a small woodfire is a couple kW, so a bit of waste-heat off the side will do that nicely.
Edit: what I *meant* to say before all that bullshit is, "these things are huge fun".
Last edited by polyglot; 08-08-2012 at 02:50 AM. Click to view previous post history.