Talk:Nanofactory engineering/cooling
From Wise Nano
Discuss the support mass needed by a nanofactory. As conclusions are reached, put them in Nanofactory engineering/cooling.
Sorry, I meant discuss cooling, not support mass... by Cphoenix 16:47, 27 Sep 2004 (CDT)
Manufacture of coolant by Brett Bellmore 18:56, 27 Sep 2004 (CDT)
I should like to point out that it's not actually necessary to manufacture coolant for a mechanical refrigeration cycle. While certain compounds do have desirable properties for use as coolants, such as convenient boiling points for refrigeration cycles using phase changes, a refrigeration system based on the stirling cycle can use just about any convenient gas, including reasonably dry air.
Stirling is great! by Cphoenix 19:56, 27 Sep 2004 (CDT)
Very cool! (so to speak...) Thanks, Brett! I found this web page that claims:
- Stirling can achieve Carnot efficiency in theory
- Coefficient of performance of 3.0 between 0 and 30 C has been demonstrated
- A weight 1/3 as much as conventional vapor compressors: 2.25 kg
- 1 gram of helium used by a 9-watt (average) system (a well insulated 200-liter fridge)
- Only two moving components
One of their technical papers claims a 40W cooler weighing 1 kg, but with lower COP. I assume they've improved since that paper was written.
So to cool 150 kW would require 50 kW of energy, within my budget. I'll assume that the flat-out capacity of the 2.25 kg cooler is 100 W. Then the coolers would need 0.5 kg of helium (or other gas, according to Brett). The mass, scaling from a prototype built with today's materials, would be 1,100 kg. But we know we can drop that by a factor of 100, and I think for a chunky design like this (there's a drawing in the technical paper), a factor of 1000 should be safe. So it looks like 1 kg should be enough to cool the nanofactory.
Whoa! by Brett Bellmore 06:05, 28 Sep 2004 (CDT)
Several points you might want to consider:
1. Not all the components of a Stirling cycle are mechanical in nature, subject to being made drastically lighter using stronger materials. The regenerator acts as a thermal bank, is essental to the cycle's efficiency, and the only way a better material would make it lighter, is if you can pull something with a miraculously high thermal capacity out of your pocket. Ok, you can probably, over a narrow range of temperatures, simulate a phenomenal thermal capacity by using phase changes, but you're still not going to make the regenerator 1000 times lighter. Maybe ten times.
2. Performance of a Sterling cycle IS improved by using a gas with a lower molecular weight. Upside: The only reason for using Helium instead of Hydrogen is concerns about the latter being flamible. Downside: Using air, your Sterling cycle will either have a lower COP, or be more massive, than if you used Helium.
3. What's your basis for assuming that a cooler THEY rate at 9 W can actually run at 100 W flat out? That's a ten fold difference! You really think they're lowballing the performance of their own stuff that much?
4. Because COP and thus weight and power consumption, are strongly dependent on the temperatures the refrigeration cycle accepts and rejects heat at, it's REALLY important to nail down exactly how heat sensitive the assemblers are. And the climate where they're being used is going to tremendously impact their replication rate. IOW, in the future, Antarctica rules the world. ;)
OK, no problem... by Chris Phoenix, CRN 10:06, 28 Sep 2004 (CDT)
No problem, take 2... by Chris Phoenix, CRN 10:47, 28 Sep 2004 (CDT)
Oops... If I hit "enter" with the cursor in the Headline box, it submits. I'll have to deal with that. Anyway...
1) I'm concerned about diamondoid mass, not total mass. A mesh of water tanks with diamondoid walls could have very high total mass and thermal mass and very low dry mass. So the regenerator does not need to be massive in the sense I'm worried about; in fact, this improves the mass estimate considerably! Also, I don't know if the "balance mass" shown in the picture (figure 3) is counted in the 1 kg; if so, that could easily be water-filled tanks of negligible dry mass.
2) I'd much rather use hydrogen than helium. Again, this is an improvement. A few kg of hydrogen distributed over hundreds of, basically, low-pressure tanks, is not a big safety concern. (You could include some water in the walls of the structures to eliminate flammability.)
3) I don't think they're lowballing; I think that a freezer that's cooling food has a lot more work to do than a fridge that's maintaining temperature. They did not rate it at 9W; they said it ran at 9W average. That was for a refrigerator. And they said in the technical paper that their 1 kg machine would be running at 40% capacity as a freezer.
"In fact the FPSC will be operating at less than 40% of its maximum rated capacity for the freezer point. From Figure 6, the average input to the cooling unit will be about 11 W when operating as a freezer. The electronics will add another 2 W and the reject fan also another 1.5 to 2 W for a total of about 15 W input. Of course during cool down the FPSC will draw whatever is available up to its maximum capacity. This could be up to 30 W. Power consumption for fresh food conditions would be a lot less. In this case the heat load is 8.1 W at 30°C which suggests an input of about 4 W to the cooler for a total system input of 8 W or less."
This makes it clear that at least half an order of magnitude between average and peak power consumption is reasonable. And that's what I figured: 1 kg for 40 W, 2.25 kg for 100 W. Maybe they discovered their 1 kg design was too light, and so 2.25 kg only goes to 40 W. That would only change my estimate by a factor of 2, which is within the noise level at this stage of the discussion.
4) We can't nail down how heat sensitive assemblers are. If we have to go cryogenic, then cooling efficiency will indeed suffer. Carnot refrigeration effectiveness is low/(high-low). For 0 C to 30 C, that's 273 to 303 K, the max effectiveness is 9. For 77 K (liquid nitrogen) to 303 K, the effectiveness is 0.34. That's 1.4 orders of magnitude difference. Bad, but not outrageously bad--assuming the waste heat from the inefficient refrigeration shows up on the hot side, rather than the cold side! Does anyone know whether Stirling is a good cycle for huge temperature differences?
I think environmental temperature is in the noise level at this point. 0 C to 4 C (deep ocean) would give you an effectiveness of 68; less than an order of magnitude better than 0 C to 30 C. 77 K to 4 C gives an effectiveness of 0.38; definitely a noise-level difference from 0.34. If you go to Antarctica and use water-evaporative cooling, environmental temperatures below 0 C would be an engineering headache. (Hm... they'd also be a headache for water-filled tanks used as structure; I never thought of that before! But you could put salt or antifreeze in the water and store it under pressure and probably drop the freezing point enough that you didn't have to worry in most climates.)
So basically, most of your concerns actually improved the picture!
More on Sterling cycle by Brett Bellmore 11:45, 28 Sep 2004 (CDT)
Good point about the regenerator mass not having to be diamond, but just water filler. And, yes, the ballance mass is just a weight to dynamically ballance the mechanism, it can be dirt, or whatever.
Yes, Stirling refrigeration works well over large temperature differentials; It's used to produce liquid air on a small scale. And enviromental differences will only be significant if the "critical" temperature for assembler operation is near room temperature. If it's cryogenic, refrigeration is relatively inefficient regardless of enviroment, if it's significantly higher than room temperature, "refrigeration" is just a cooling loop. Antarctica only gets an advantage if the critical temperature is above the freezing point of water.
More likely than a critical threshold, is for the efficiency of the assemblers to suffer as temperature rises, due to increases in power needed for logic, more moving mass to increase stiffness, and the like. Which suggests that the alternative to use of a refrigeration cycle might be accepting lower efficiency/more massive assemblers with just the cooling loop.
Another form of "refrigeration", which is very energy efficient, but suffers from other limitations, is the use of thermal diode heat pipes in regions with a cold season, to chill a large thermal mass during the winter, which can be used as a heat sink during the summer.
