X-Message-Number: 32702 From: Roko Mijic <> Date: Mon, 12 Jul 2010 19:38:48 +0100 Subject: A proposal for a cryonic grave Dear Cryonauts, This is my first post to the cryonet list, and my subject is a proposal for a cryonic grave to improve the probability of revival of cryopatients. I'd welcome any comments or criticisms, especially from those on this list who are involved running CI or Alcor. Does it sound like something we as a community ought to pursue further? A proposal for a cryonic grave The probability that cryonics organizations fail for some reason in the next 100 years is non-trivial, even with the very best management and organization. There are various modes of failure: - The organizations are closed down by hostile legal action - The organizations run out of money, for example due to a severe recession - Society as a whole breaks down or is severely retarded by a disaster of some kind, but eventually rebounds If this happened, cryopreserved patients may be left to die at room temperature. Furthermore, it seems that with the development of cryoprotectant perfusion and the successful preservation and reanimation of animal organs, the weakest link in the cryonics chain is now the possibility of re-warming in between preservation and "the future". The ideal solution to this problem is a way of keeping bodies cold (colder than -170C, probably) in a grave. Our society already has strong inhibitions against disturbing the dead, which means that a cryonic grave that required no human intervention would be much less vulnerable than a cryonics company. Furthermore, such graves could be put in unmarked locations in northern Canada, Scandinavia, Siberia and even Antarctica, where it is highly unlikely people will go, thereby providing further protection. If modern society collapses reversibly, then the cryopatient will stand a chance of revival if (and only if) the grave can sustain them for longer than it takes for society to bounce back. An engineering case is put forward claiming that such a system could be built for less than $1,000,000, with some uncertainty as to whether a more efficient system with a higher vacuum and smaller size/cost would work. Split between 10 or 20 patients, this cost level is highly feasible, even if these estimates are optimistic by a factor of 2 or even 4. Most importantly, the "peace of mind value" of a 90% chance of reanimation is much more than twice as great as the "peace of mind value" of a 45% chance of reanimation. A cryonic grave would provide a second layer of defence beyond the cryonics organization responsible for the patient: both would have to fail for the patient to be re-warmed. An insulated cryogen grave: preliminary engineering case One way to make a cryonic grave of sufficient endurance would just be a tank of LN2 (or some other cryogen) of sufficient volume and insulation. A preliminary engineering case is presented as follows: Consider a spherical tank of radius r with insulation of thermal conductivity k and thickness r (so a total radius for insulation and tank of 2r) and a temperature difference of deltaT, the power getting from the outside to the inside is approximately 25 * k * r * deltaT If the insulation is made much thicker, sharply diminishing returns are encountered (asymptotically, only another factor of 2 is achievable). The volume of cryogen that can be stored is approximately 4.2 * r^3, and the total amount of heat required to evaporate and heat all of that cryogen is 4.2 * r^3 * (volumetric heat of vaporization + gas enthalpy) The quantity is brackets for Nitrogen and a deltaT of 220C is approximately 346,000,000 J m^-3. Dividing energy by power gives a boil-off time of 1/12,000 * r2 * k^-1 centuries Setting this equal to 1 century, we get: r^2/k = 12,000. (1) Can this constraint be satisfied without an exorbitant price tag? A primitive way of estimating the cost of a large cryo-tank is to scale up the costs of existing dewars and liquid nitrogen tanks, which indicates that Cost in dollars ~ (Volume in litres) * 20 + 5000 so that a volume of 50 cubic meters, i.e. a radius of 2.3 meters would cost on the order of $1,000,000. To get a system that cost less than $1M, one would therefore want a thermal conductivity of or better than 0.0004 W/m-K To attain thermal conductivities in the desired range, there seem to be two routes: Multilayer Insulation (MLI) at high vacuum, or aerogel granules at medium vacuum. It should be noted that this level of thermal conductivity is not "breaking new engineering ground", low temperature systems and satellites routinely achieve values in this range. 5 cubic meter system, high-vacuum multilayer insulation MLI is a sandwich of ultra-thin layers of aluminized mylar interspersed with a low conductivity spacer grid. Its performance relies on a relatively high vacuum in order to cut out gaseous thermal conduction. Suppose that MLI is used at a higher vacuum of 10-5 atmospheres. We might expect a k value of 0.00005 W/m-K, almost an order of magnitude better than what is required, meaning an r of approximately 1 meter could be used. At r=1.06m, the cryogen volume would be 5 m^3, leading to a tank-cost estimate of $90,000. Other costs would dominate in this case (e.g. cost of engineering a reliable vacuum, cost of insulation materials, etc.) meaning that the total system cost would be at least n*$100,000 for some small n. However, it is not clear whether such a vacuum could feasibly and reliably be maintained for 100 years. Further investigation is required. 50 cubic meter system, low-vacuum aerogel insulation The other option is aerogel granules, where a less extreme vacuum is required. Aerogel granules at a rough vacuum yield a good degree of insulation for the following reason: When the mean free path of a gas increases significantly beyond the characteristic dimension of the space that encloses it, the thermal conductivity drops linearly with pressure. If the space is just an evacuated gap of size ~10cm, the required pressure to begin seeing decreased thermal conductivity is one millionth of an atmosphere, 10-6 atm. However, for tiny voids between grains of aerogel of size 0.01mm, the required pressure is a mere 0.01 atmospheres, a very rough vacuum. Figures of 0.0007 W/m-K for perlite powder in vacuum have been quoted, though this is at high vacuum. Fine granules of aerogel would probably outperform this in terms of the vacuum required to get down to < 0.001 W/m-K. On the other hand, aerogel at 0.1 atmospheres can have a thermal conductivity of just 0.004 W/m-K, with approximately linear improvements in k with decreases in pressure, until radiative thermal transport dominates at very low k. A system at 0.01 atmospheres might therefore have a k of 0.0004. The remaining question is whether a sufficiently good vacuum can be maintained for the required period of 100 years. Note that cryogenic dewars maintain a high vacuum for periods of a year at low cost. It seems that maintaining a much rougher vacuum for 100 times longer is very feasible, especially given that the vacuum space and space available for getters and sorbs (which scour incoming gas particles and counteract leaks) scales with r^3, whereas leak rates scale at lower powers of r (linearly for welds, ports, etc.). Further improvements are possible, such as - the use of dry ice to maintain a shield at dry ice sublimation temperature (-78C), as dry ice has 4 times the volumetric heat capacity of liquid nitrogen, and most of the radiative heat transfer will occur at higher temperatures due to the T^4 scaling of blackbody radiation. In fact, this may yield a factor of nearly 4 increased performance, since almost all of the heat loss would be to the dry ice. - installation in cold permafrost at -20C, or installation in Antarctica where winter temperatures reach -80C (though only a relatively light system could get there economically). This would combine well with a heat pipe/thermal reservoir to maintain the system at winter temperature. - the use of liquid oxygen as the main cryogen (1.4 times higher specific heat capacity and latent heat of vaporization than nitrogen) Development costs would have to be amortized, and as with any new piece of equipment these are uncertain. Costs would almost certainly drop with increased demand for such graves, perhaps to the point of one patient per grave being feasible. Roko Mijic 2010 Roko Mijic 07958582685 www.rokomijic.com Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=32702