X-Message-Number: 2031 Date: Fri, 26 Mar 93 01:40:04 CST From: Brian Wowk <> Subject: CRYONICS: More Peltier Effect The -130'C Cold Room will require about 50 watts of electric power for fans and the active temperature control system. We should use the Peltier effect to provide this power. A heat sink should be built into the concrete surrounding the vault (and perhaps extending into the earth as well). By attaching thermopiles to this heat sink and the LN2 reservoir, the 210'K temperature difference can be used to passively generate electricity to power everything. Thermodynamics predicts that producing 50 watts in this manner will heat the LN2 at a rate of 20 watts, increasing LN2 boiloff by only 4%. Of course the 50 watts itself is eventually dissipated in the room, also increasing boiloff, but this power would otherwise be piped in from the outside anyway. I dare say this is a great idea. With this system you don't have to worry about external connections to power supply lines, battery backups, or ever switching between the two. Heck, you don't have to worry about anything other than faithfully replenishing your LN2 once a week. Now your cryonics facility can explode, burn, and fall on top of your vault (covered by fire-resistant panels) and your self-contained patient care system will never know the difference. There is another problem the Peltier effect may solve for us. A Cold Room must be "failsafe." I interpret this to mean that the consequences of total mechanical failure, though suboptimal, must be acceptable. The only mechanical parts remaining in the present room design are the fans. We must consider what new equilibrium will establish itself in the room if the fans are stopped. (In fact, during patient loading operations they may be deliberately stopped for extended periods of time.) The design I introduced yesterday would conduct heat toward the insulated LN2 reservoir through the aluminum walls between storage cells. Metal heat conductors attached to the -130'C aluminum walls would pass through the reservior insulation, making direct contact with -196'C interior. This represents a steep temperature gradient, with the icy -196'C temperature constantly "fighting" to get out of the reservoir and into the rest of the room. If the air circulation stopped, I'm afraid it would succeed. I ran some numbers today and concluded that in absence of air circulation, unnacceptably large temperature gradients would arise within the cell walls between the center of the room and the periphery. The inner cells would cool dangerously. The only way to prevent this would be to make the aluminum walls cetimeters thick (ouch!). I suggest we make the walls a more reasonable thickness, and use the savings to go out and buy a couple of dozen state-of-the-art thermoelectric heat exchangers (thermopiles). We insulate the LN2 reservoir heavily and completely, line the inside with thermopiles, and rely on thick electric cables to carry heat from thermopiles at the room periphery to their counterparts in the reservoir. The goal is to move 400 watts of heat across the room without temperature gradients or air flow, and thermoelectricity is the only way to do this. Such a system would be inherently stable and self-regulating, making the fans almost redundant (although we will still have fans anyway). I ran some numbers based on a copper/constantan therocouple and concluded that passively conducting 30 watts between a 60'K temperature difference would require moving thousands of amperes through copper cable thicker than my fist. Since commercial refrigeration systems exist based on the Peltier effect, I infer that they must use something other than copper/constantan (probably semiconductors). When time allows, I will investigate this further and see what hardware is out there available commercially. I am also now thinking that there is really no need for a "utility" cell large enough to lower a man into. Other than the fans, what is there to really service anyway? We simply stick the 2000 litre LN2 reservoir in three central cells, stuffing them full of insulation right up to the very top where the fans are. This now leaves 22 cells for patient storage, increasing total capacity to 132 patients. By the way, my "can within a can" design for the LN2 reservoir addresses the LOX buildup problem nicely. The inner can can be periodically hoisted out of the unit and dumped to get rid of LOX residue at the bottom. This is a far simpler maintenance procedure than purging dewars with patients in them. Chalk up another one for -130'C storage! --- Brian Wowk Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2031