X-Message-Number: 2091 Date: Sat, 10 Apr 93 03:50:26 CDT From: Brian Wowk <> Subject: CRYONICS Hybrid System I have been thinking about Steve Harris' idea for a hybrid thermoelectric/mechanical refrigeration system. Basically the idea is that a mechanical system cools a heat sink to -79'C and a thermoelectric system then operates between this temperature and -130'C. If the mechanical system ever needs maintenance then ordinary dry ice can be put in the -79'C heat sink with no disturbance in system operation. While the idea of a simple backup is excellent, I think the hybrid system will be too inefficient. Thermoelectrics have characteristically lower efficiencies than mechanical systems. As Marvin Minsky has intimated, 5% is a good coefficient of performance for thermolectric modules operating between even modest temperature differences. (We would certainly not be able to do better operating between -130'C and -79'C.) This means that moving 200 watts out of the Cold Room thermoelectrically would dump 4000 watts into the dry ice, with most of this being waste electric heat. I think a better solution is to refrigerate mechanically all the way down to -130'C and use LN2 as backup as Steve originally suggested. Only 200 liters of LN2 per day would be required to keep things going, compared to *several tons* of dry ice per day in the hybrid system. Temperature control and stability with an LN2 backup system can be just as good as the dry ice system. In the refrigeration system I currently envision, all heat would be pumped out of the room through a single heat exchanger located near the room fans. (This assumes a system operating below the thermal ballast melting point). The central portion of the heat exchanger could be an LN2 reservoir (normally empty) with insulation designed to pass 500 watts between -130'C outside and the LN2 inside. Refill the reservoir once a day or so, and the heat exchanger will absorb heat exactly as if your refrigerator was still running. Your room would hardly know the difference. --- Brian Wowk Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2091