From: John De Armond Newsgroups: rec.outdoors.rv-travel,alt.rv Subject: Re: 12V thermoelectric cooler/warmer Date: Wed, 28 Jun 2000 14:03:18 -0400 Larry wrote: > I have to differ with your comment. We have a Coleman 12V cooler and have > used it in conjunction with lights and a furnace (needed heat a few times a > night) over a four day period with a pop-up and did not drain the 12V > battery. No we didn't have shore power hook-up, did have a large deep cycle > battery. Peltier junction heat pumps have a Coefficient of Performance (COP) of 1 or less, usually less. That is, it pumps 1 watt or less for every watt consumed. Most of these coolers are either around 50 watts (~4 amps) or 100 watts (~8 amps). One can easily determine how long a battery will last from that. Though not exhaustive, I have either owned or closely examined many of the units available on the market. Every one I've looked at either has no thermostat and runs continuously or has a simple ON-OFF thermostat that interrupts the main power. The former will obviously use its rated power continuously. The BIG problem with all these units is that the Peltier junction heat pump is also an excellent Seebeck junction*. The problem comes when the thermostat opens (or the power switch is thrown.) The junctions start generating voltage, in most cases, enough to continue running the cooling fan for awhile. The problem is, the junctions are pumping heat back into the cold space in the process, and about as fast as it removed it when powered up. So the box either must be powered all the time or it will rapidly get hot when disconnected. I've been through several of these things in a stubborn belief that the next one would work better than the last :-( I invariably return to the good old ice box. The one place where I found one of these to be useful was in a motel room on an extended business trip. It was powered from a 12 volt power supply, remained on all the the time and kept cokes and cold cuts nicely cool. In a 70 deg room, of course. * I once bought a large number of Peltier junctions surplus and used some of them to make a propane-powered battery charger to use as the power source for a ham radio Field Day station. Had to be the most unusual power source for that contest. They were arraigned so that a small burner would heat a slab of aluminum attached to one side while the other side got a large heat sink. Because of the soft solder used to fasten the individual junctions to the Peltier pile, I limited the temperature on the hot side to about 300 deg. John From: John De Armond Newsgroups: rec.outdoors.rv-travel,alt.rv Subject: Re: 12V thermoelectric cooler/warmer Date: Thu, 29 Jun 2000 19:48:01 -0400 JOE NEAR wrote: > John: > > Thanks. Knew about the COP but not about the Seebeck junction. Ever try > to run two in series (cold side of one to hot side of another)? Can you > get the cold end of the series 80deg below ambient? Yes! As a matter of fact, that is exactly what is used to cool infrared detectors in precision guided munitions (replaced liquid nitrogen) and closer to my professional home, high performance nuclear radiation detectors. Six of them stacked in series could achieve the temperature of liquid nitrogen IFF the heat load was teeny, tiny. The HPGe (High purity germanium) detectors used in high performance gamma spectroscopy are normally inside vacuum dewars anyway so the heat load matches the devices. They've kinda fallen out of favor, however, except for portable units because of that nasty Seebeck effect. Allowing an HPGe detector crystal to get warm won't instantly destroy it like it would the GeLi crystals they replaced but it does them no good. Lose power and the crystal gets warm REAL quickly. Little micro-sized helium refrigerators have replaced them for that application. If you want to gang 'em, Each pack going toward the cold end has to be 1/2 the capacity of the one above it toward the hot side. This is because the one on the hot side must pump not only the heat that the smaller unit pumps but all the heat it dissipates. A less satisfactory alternative if you happen to find a pile of these things cheap like I did is to run the units toward the cold end at progressively lower current. This is not as good a using smaller piles because the large pile running at low current still has the same heat leakage as it would at full power. But they still will get amazingly cold. I put a stack of five in a small bell jar, evacuated it for insulation and achieved about -120 deg F. I could have done better with optical shielding and such but I was just playing around. John |