Graphene-based thermoelectric conversion may make possible fast Mars missions.

[Exoscientist]

 Adding to the many remarkable properties of graphene, they are also highly efficient in converting heat directly to electricity:

SUTD team proposes low-temperature thermionic converter with graphene cathode; about 45% efficiency.
9 March 2015
http://www.greencarcongress.com/2015/03/20150309-sutd.html

 For spacecraft operating too far from the Sun or needing too much power to use solar cells, radioisotope thermoelectric generators (RTG's) have been used, and nuclear space reactors have been proposed in regards to manned missions. RTG's however have very poor efficiencies in the range of 3% to 7%:

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Efficiency

 And the nuclear space reactors that have been tested only have efficiencies in a similar poor range:

http://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx

though a proposed nuclear space reactor could have ca. 25% efficiency.

 The new research on graphene however suggests it could get efficiencies in the range of 45% in converting heat directly to electricity. And using graphene rather than heavy metals for the thermoelectric conversion would also save significantly in weight. 
 
 The greater efficiency at a lighter weight might make possible proposed nuclear electric space propulsion systems such as VASIMR that could cut trips to Mars to weeks travel time instead of months.

 The Mars Society president Robert Zubrin had criticized VASIMR on the grounds that it would require an unreasonablly lightweight nuclear power system for the power it put out:

The VASIMR Hoax By Robert Zubrin | Jul. 13, 2011  http://www.spacenews.com/article/vasimr-hoax

 But taking into account both the higher efficiency and the lighter weight, a graphene based thermoelectric conversion system may allow the required lightweight power system for VASIMR or other nuclear electric propulsion systems.
 

  Bob Clark

Edited June 30, 2016 by Exoscientist
typo

[hms_warrior]

This....sounds pretty cool. Now we just need a reliable, cheap way to produce those large-scale graphene things ^^

[PB666]

16 hours ago, Exoscientist said:

 Adding to the many remarkable properties of graphene, they are also highly efficient in converting hear directly to electricity:

SUTD team proposes low-temperature thermionic converter with graphene cathode; about 45% efficiency.
9 March 2015
http://www.greencarcongress.com/2015/03/20150309-sutd.html

 For spacecraft operating too far from the Sun or needing too much power to use solar cells, radioisotope thermoelectric generators (RTG's) have been used, and nuclear space reactors have been proposed in regards to manned missions. RTG's however have very poor efficiencies in the range of 3% to 7%:

https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator#Efficiency

 And the nuclear space reactors that have been tested only have efficiencies in a similar poor range:

http://www.world-nuclear.org/information-library/non-power-nuclear-applications/transport/nuclear-reactors-for-space.aspx

though a proposed nuclear space reactor could have ca. 25% efficiency.

 The new research on graphene however suggests it could get efficiencies in the range of 45% in converting heat directly to electricity. And using graphene rather than heavy metals for the thermoelectric conversion would also save significantly in weight. 
 
 The greater efficiency at a lighter weight might make possible proposed nuclear electric space propulsion systems such as VASIMR that could cut trips to Mars to weeks travel time instead of months.

 The Mars Society president Robert Zubrin had criticized VASIMR on the grounds that it would require an unreasonablly lightweight nuclear power system for the power it put out:

The VASIMR Hoax By Robert Zubrin | Jul. 13, 2011  http://www.spacenews.com/article/vasimr-hoax

 But taking into account both the higher efficiency and the lighter weight, a graphene based thermoelectric conversion system may allow the required lightweight power system for VASIMR or other nuclear electric propulsion systems.
 

  Bob Clark

Nope, not even close. 

[wumpus]

TL;DR - PB666's response.

Huh?  There are a ton of problems with "fast Mars missions".  The biggest issue is that if VASIMR can get out of the lab, it makes "slow Mars missions" possible nearly the next day.  Then we can worry how to speed up Mars missions.

You need extreme power.  The proposed solution sounds like it would fix a significant number of  the world's problems.  Even on this forum, I'd put Mars missions far down on the list of things I'd do with a high efficiency nuclear engine (no idea if the non-nuclear bits can be produced cheap enough to crank up the efficiency of hybrid cars by 25-50%).

You need heat management.  This doesn't begin to cover the issue that every Watt generated has to leave via black body radiation.  Trying to maintain life support on a high power system cooled by black body radiation makes life interesting (and more power, which requires more cooling, and so on).

And all this ignores the fact that if VASIMR can get itself out of the lab it might make even slow (Hohmann transfer) missions possible.  Right now we have:

Fast: requires magic.  Might as well require warp drive.

Slow: Hohmann transfers.  Takes months, and hundreds (I suspect thousands*) of tons of [chemical] fuel (in orbit for $millions/ton).  Nobody has yet found a source of funding to get this done.  If VASIMR can scale up to provide this level of TWR (basically provide a bit more than 1000 m/s delta-v per Mangalayan maneuver orbit, or at least ~1100m/s on the last one), it will be *huge* in getting us to Mars.  Even discussing "fast travel to Mars" makes no sense if you remove most of the issues with "slow travel to Mars" and replace them with huge new issues just for high speed.  Get to Mars first and then work on going faster.

Mind boggingly slow: ion engines and gravity transfers.  Cheap, but way too slow for human transfer.  If fuel and supplies travel this method, things might happen.

* I think you need a hundred tons of xenon just for the slowest trip possible for 100 tons of life support (which is the current NASA plan).

[magnemoe]

1 hour ago, wumpus said:

TL;DR - PB666's response.

Huh?  There are a ton of problems with "fast Mars missions".  The biggest issue is that if VASIMR can get out of the lab, it makes "slow Mars missions" possible nearly the next day.  Then we can worry how to speed up Mars missions.

You need extreme power.  The proposed solution sounds like it would fix a significant number of  the world's problems.  Even on this forum, I'd put Mars missions far down on the list of things I'd do with a high efficiency nuclear engine (no idea if the non-nuclear bits can be produced cheap enough to crank up the efficiency of hybrid cars by 25-50%).

You need heat management.  This doesn't begin to cover the issue that every Watt generated has to leave via black body radiation.  Trying to maintain life support on a high power system cooled by black body radiation makes life interesting (and more power, which requires more cooling, and so on).

And all this ignores the fact that if VASIMR can get itself out of the lab it might make even slow (Hohmann transfer) missions possible.  Right now we have:

Fast: requires magic.  Might as well require warp drive.

Slow: Hohmann transfers.  Takes months, and hundreds (I suspect thousands*) of tons of [chemical] fuel (in orbit for $millions/ton).  Nobody has yet found a source of funding to get this done.  If VASIMR can scale up to provide this level of TWR (basically provide a bit more than 1000 m/s delta-v per Mangalayan maneuver orbit, or at least ~1100m/s on the last one), it will be *huge* in getting us to Mars.  Even discussing "fast travel to Mars" makes no sense if you remove most of the issues with "slow travel to Mars" and replace them with huge new issues just for high speed.  Get to Mars first and then work on going faster.

Mind boggingly slow: ion engines and gravity transfers.  Cheap, but way too slow for human transfer.  If fuel and supplies travel this method, things might happen.

* I think you need a hundred tons of xenon just for the slowest trip possible for 100 tons of life support (which is the current NASA plan).

Big benefit is an way easier and cheaper way to extract heat energy than steam. 
As you say you would run it on car engines, some companies think of adding an steam turbine to the turbo on trucks to increase efficiency, system is to large to be used on cars but is common on power plants.  

Difference between Hohmann and warp is like walking or private vtol jet, you have other options like cars or trains.
Pretty easy to cut travel time to an faction by using 3-10 times the dV, you save most on long distance travel. 
20 Km/s tend to be my preferred speed in KSP then interplanetary, 10 is low using pulse engine. 


 

Edited June 29, 2016 by magnemoe

[wumpus]

1 hour ago, magnemoe said:

Big benefit is an way easier and cheaper way to extract heat energy than steam. 
As you say you would run it on car engines, some companies think of adding an steam turbine to the turbo on trucks to increase efficiency, system is to large to be used on cars but is common on power plants.  

Difference between Hohmann and warp is like walking or private vtol jet, you have other options like cars or trains.
Pretty easy to cut travel time to an faction by using 3-10 times the dV, you save most on long distance travel. 
20 Km/s tend to be my preferred speed in KSP then interplanetary, 10 is low using pulse engine. 


 

You're claiming that something involving "Advances in nano/micro fabrication" and "carbon nanotubes" is easier and cheaper than steam?  Than 19th century tech (ok, high efficiency, high pressure steam was *expensive* 19th century tech, but still).  Maybe it will happen someday, maybe not.  But large scale steam is pretty efficient, and well known.

Except that nobody is paying for the walking to Mars trip.  And any extra delta-v you spend to get there, you have to spend again to capture (aerocapture with Hohmann speeds is controversial.  Don't even think of pulling it off with "fast Mars" in real life).  And not only that, nobody is planning on using VISIMR for plain old Hohmann (or faster).  But if they had it, you could at least do a Hohmann.  It is more or less exactly what you want to get to Mars.

Ask yourself  why nobody bothered to do a fast trip to the Moon?  Presumably anybody could eat 3-10 times the delta-v just as easily.

[PB666]

3 minutes ago, wumpus said:

You're claiming that something involving "Advances in nano/micro fabrication" and "carbon nanotubes" is easier and cheaper than steam?  Than 19th century tech (ok, high efficiency, high pressure steam was *expensive* 19th century tech, but still).  Maybe it will happen someday, maybe not.  But large scale steam is pretty efficient, and well known.

Except that nobody is paying for the walking to Mars trip.  And any extra delta-v you spend to get there, you have to spend again to capture (aerocapture with Hohmann speeds is controversial.  Don't even think of pulling it off with "fast Mars" in real life).  And not only that, nobody is planning on using VISIMR for plain old Hohmann (or faster).  But if they had it, you could at least do a Hohmann.  It is more or less exactly what you want to get to Mars.

Ask yourself  why nobody bothered to do a fast trip to the Moon?  Presumably anybody could eat 3-10 times the delta-v just as easily.

Russians tried to use steam based fission reactors 30 years ago, it heavy, requires cooling panels and because of wear and tear issues needs to be massively overengineered making the weight prohibative. Solar panels are better in that regard. Given the weight/panel is going down, this will blow anything steam driven out of the water.

[magnemoe]

7 hours ago, wumpus said:

You're claiming that something involving "Advances in nano/micro fabrication" and "carbon nanotubes" is easier and cheaper than steam?  Than 19th century tech (ok, high efficiency, high pressure steam was *expensive* 19th century tech, but still).  Maybe it will happen someday, maybe not.  But large scale steam is pretty efficient, and well known.

Except that nobody is paying for the walking to Mars trip.  And any extra delta-v you spend to get there, you have to spend again to capture (aerocapture with Hohmann speeds is controversial.  Don't even think of pulling it off with "fast Mars" in real life).  And not only that, nobody is planning on using VISIMR for plain old Hohmann (or faster).  But if they had it, you could at least do a Hohmann.  It is more or less exactly what you want to get to Mars.

Ask yourself  why nobody bothered to do a fast trip to the Moon?  Presumably anybody could eat 3-10 times the delta-v just as easily.

Steam is old technology who has reached an limit, new technology can work better especially on smaller scale. And this will have very many uses, more efficient cars  is an killer app as its huge need and loads of money to be made. 

Nobody bother to do an fast trip to moon as its an short trip, going faster and using one day instead of 3 would not save anything. 
Faster than Hohmann give less radiation and less chance of system fails during trip. 

[PB666]

The biggest problem is that the NTGs are heavy per kw and don,t produce much power to efficiently halve the transfer time you need alot of dV and to do that you need mass efficient ion drives, but mass efficient ion drives need MWs if power to carry weights in the 10s of tons, particularly if you are talking about a mars lander abd return vehical. At least to leave Earth orbit efficiently from LEO you need a traditional rocket to take advantage of LEO velocity, NTGs are not it.. You need something like a whole Falcon-9 except with vacuum engines. In terms of power production per kg, the new lightweight panels are better. 

[hms_warrior]

Still a RTG that produces what? 8 times the electrical power for the same amount of termal power, is lighter and has less challenging cooling (since core-temperature needs only to be half as high) is a GOOD thing. Very good. Not everything in spaceexploration can use solar-power. Think about it. What could curiosity do whit 800 Watt instead of that meager 125 ?  What about outher solar system probes? solar-power sucks there.

Heck, if it realy reaches 45% conversion rate than you can replace all those 19th century steam-generators in nuclear/coal powerplants whit it and improve their efficency by 5% while removing a load of moving parts (more robust).

Edited June 30, 2016 by hms_warrior

[wumpus]

2 hours ago, hms_warrior said:

Still a RTG that produces what? 8 times the electrical power for the same amount of termal power, is lighter and has less challenging cooling (since core-temperature needs only to be half as high) is a GOOD thing. Very good. Not everything in spaceexploration can use solar-power. Think about it. What could curiosity do whit 800 Watt instead of that meager 125 ?  What about outher solar system probes? solar-power sucks there.

Heck, if it realy reaches 45% conversion rate than you can replace all those 19th century steam-generators in nuclear/coal powerplants whit it and improve their efficency by 5% while removing a load of moving parts (more robust).

Except that any thermal generator will have the same Carnot limits (because there theoretically exists the inverse heat pump).  Getting those last few percents near Carnot are likely to get *really* hard.  Not that the maintenance crews wouldn't *love* to get rid of high pressure steam.  I'm not holding my breath more a replacement (and the idea of basing a power plant on nanotech seems like distant science fiction.  But it sounds like reverse-osmosis water treatment is being built on similar scales, so who knows)

While I certainly try to pound on cooling issues (cooling is a big thing with electronics on *Earth*.  Now try it in vacuum), mass issues are probably going to keep solar panels being used in favor of RTGs this side of Mars, and don't be surprised if you see large reflector use.  The plutonium supply is also an issue, but I think NASA got the DoD to whip up more plutonium (they were carefully rationing it for awhile).

[PB666]

On June 30, 2016 at 6:41 AM, hms_warrior said:

Still a RTG that produces what? 8 times the electrical power for the same amount of termal power, is lighter and has less challenging cooling (since core-temperature needs only to be half as high) is a GOOD thing. Very good. Not everything in spaceexploration can use solar-power. Think about it. What could curiosity do whit 800 Watt instead of that meager 125 ?  What about outher solar system probes? solar-power sucks there.

Heck, if it realy reaches 45% conversion rate than you can replace all those 19th century steam-generators in nuclear/coal powerplants whit it and improve their efficency by 5% while removing a load of moving parts (more robust).

True you could move safe nuclear into cities because the reactors would be below the critical mass for meltdown, in addition you could use soent rods to generate power. 

But in space, per weight if you have at least 0.5 AU of sun, solar is better.