General > General Technical Chat
SciFi movies and pathetic misconceptions of tech failing for the story line.
AVGresponding:
--- Quote from: David Hess on May 05, 2023, 11:25:54 am ---
--- Quote from: coppercone2 on May 04, 2023, 10:53:34 pm ---Eventually you will need a better radiator because its going to look ridiculous because scifi designers don't have the time to do thermal analysis for space x, but I see your point that it should be a more prominent feature.
--- End quote ---
Sure they do:
P = A * ε * σ * T4
P = the power of waste heat the radiator can get rid of (watts)
σ = 5.670373×10-8 = Stefan-Boltzmann constant (W m-2K-4)
ε = emissivity of radiator (theoretical maximum is 1.0 for a perfect black body, real world radiator will be less. Should be at least 0.8 or above to be worth-while)
A = area of radiator (m2)
T = temperature of radiator, this assumes temperature of space is zero degrees (degrees K)
x4 = raise x to the fourth power, i.e, x * x * x * x
--- End quote ---
Modern materials reach 99.9%, though I expect you'd want to factor in contamination and damage over time. The (average) temperature of (interstellar) space is 2.7°K. Why not recycle some waste heat into electricity?
David Hess:
--- Quote from: AVGresponding on May 05, 2023, 06:20:12 pm ---Modern materials reach 99.9%, though I expect you'd want to factor in contamination and damage over time.
--- End quote ---
Since practical emissivities only make a proportional difference in size, emissivity can be ignored unless it is terrible. There is just not much to be gained with "perfect" emissivity.
This reminds me of current efforts to make more efficient electric motors and motor controllers for electric vehicles. Both are already pretty efficient, so there is not much to be gained there.
--- Quote ---Why not recycle some waste heat into electricity?
--- End quote ---
Project Rho discusses that. It results in a greater volume of lower temperature heat, so now the radiator is less effective to the 4th power, and must be even larger, to the 4th power.
Niven might have recognized the problem, and the solution, but only briefly refers to it. In his stories, spacecraft have storage systems and radiators for handling heat, but also concentrate it, adding even more heat and reducing efficiency, but this reduces the size of the radiators because they can operate at a higher temperature, taking advantage of that 4th power.
5U4GB:
--- Quote from: David Hess on May 03, 2023, 09:43:22 pm ---Even more important than the power distribution and reactor is how did you cool it?
http://www.projectrho.com/public_html/rocket/heatrad.php
--- End quote ---
That looks like a SF project, but something very similar was going to be built by NASA until they cancelled it like almost everything else they do that's cool (sigh), look up the Jupiter Icy Moons Orbiter. That would have been an amazing craft...
AVGresponding:
--- Quote from: David Hess on May 05, 2023, 09:25:56 pm ---
--- Quote from: AVGresponding on May 05, 2023, 06:20:12 pm ---Modern materials reach 99.9%, though I expect you'd want to factor in contamination and damage over time.
--- End quote ---
Since practical emissivities only make a proportional difference in size, emissivity can be ignored unless it is terrible. There is just not much to be gained with "perfect" emissivity.
This reminds me of current efforts to make more efficient electric motors and motor controllers for electric vehicles. Both are already pretty efficient, so there is not much to be gained there.
--- Quote ---Why not recycle some waste heat into electricity?
--- End quote ---
Project Rho discusses that. It results in a greater volume of lower temperature heat, so now the radiator is less effective to the 4th power, and must be even larger, to the 4th power.
Niven might have recognized the problem, and the solution, but only briefly refers to it. In his stories, spacecraft have storage systems and radiators for handling heat, but also concentrate it, adding even more heat and reducing efficiency, but this reduces the size of the radiators because they can operate at a higher temperature, taking advantage of that 4th power.
--- End quote ---
After a quick skim, the immediate problem I see with the argument is the misunderstanding of how the second law applies, generally, and specifically in relation to thermo-electric modules.
"What's the problem? Well, the general problem is that pesky Second Law of Thermodynamics. In this context, it tells you that it is impossible to destroy heat, the best you can do is move it around. So using a thermocouple to convert heat into electricity is impossible.
The specific problem is that a thermocouple does NOT convert heat into electricity. It converts a heat gradient into electricity. The original heat is still there. In fact, the conversion process adds even more waste heat to the original total.
As an analogy, think about a hydroelectric dam. The water in the reservoir is at a higher gravity gradient than the water downstream. The hydroelectric dam converts the gravity gradient into electricity. But the water is still there after passing through. The dam does not convert water into electricity, if it did the water would disappear. In the same way a thermocouple does not convert heat into electricity, the heat is still there."
Ok, firstly "the conversion process adds even more waste heat to the original total" implies the creation of energy, which as we know, is impossible. Using their analogy, this would equate to the generation of extra water during the operation of the dam, a clear nonsense idea.
The use of a "hydroelectric dam" as an example is telling; the use of the term "gravity gradient" here is misleading, in the context of use. A dam converts potential energy into kinetic energy by flowing H2O molecules with high potential energy down the gravity gradient, through a turbine where this potential energy is converted into kinetic and then electrical energy (with some losses, inevitably ending up as waste heat), and for the purposes of this argument you may consider the molecules of H2O to be the equivalent of the photons that carry waste heat away from a radiator.
The water indeed does not disappear, but it does lose potential energy, just as thermal energy is converted into kinetic energy in a steam turbine, or electrical energy in a thermoelectric module. TEMs are pretty inefficient, 6% or so for commercially available stuff, NASA are achieving on the order of 20% in their most advanced TEGs. The cold side of your TEM is the radiator, the photons of waste heat don't disappear, nor are they destroyed; they radiate out into their environment, with even local space (within the solar system) being cold enough and large enough to be effectively an infinite heatsink.
The heat is not "destroyed"; there is a flow of high energy photons from the hot side to the cold side, and as they drop to a lower energy level, this causes electrical current to flow, as the photons give up their energy. Yes, you need a thermal gradient for this to happen, just as you need the gravity gradient to create a difference in potential energy for a hydroelectric dam to work.
BrianHG:
--- Quote from: AVGresponding on May 06, 2023, 11:10:18 am ---TEMs are pretty inefficient, 6% or so for commercially available stuff, NASA are achieving on the order of 20% in their most advanced TEGs.
--- End quote ---
Do you have your acronyms backwards? Maybe something has changed in the last 5 years or so...
TEGs, like the ones on the Voyager space probes, which operate on the Seebeck effect, can only achieve maximum 6% efficiency in the best of cases.
The latest tech Stirling generators which use a magnet piston floating in helium as a lubricant can achieve 20% efficiency under optimum circumstances. However, this tech is not usually labeled today as TEG. It is possible to get even better efficiency with a normal Stirling engine, however, you will now have moving and rotating components which will wear out needing maintenance every few years.
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