5 ms·

This makes no sense to me. Don't you also need cryogenic systems to keep the H2 cold? Wouldn't those use waaaaay more energy than what you save with superconduc

This makes no sense to me. Don't you also need cryogenic systems to keep the H2 cold? Wouldn't those use waaaaay more energy than what you save with superconducting motors? Maybe they are talking about short haul flights where the H2 is ground cooled and then used as a cold sink but then in order to keep your electronics cold you're carrying lots more fuel than you need. TFA was too thin to pass the sniff test.

XorNot 5y ago
Motors are solid 3D structures. The more power you put in not superconducting, the more heat you have to extract. Superconductors mean you only need to keep your refrigerant cool, and that's a product of insulation effectiveness, not power output.
- ethbr0 5y ago
  This is the difference between electric and combustion powertrains, no? In electric, heat is the enemy and inefficient. In combustion, heat is a key component of the thermodynamic efficiency, unavoidable, and effectively only limited by material tolerances. (I didn't major in physics, so someone keep me honest on the ceiling / floor efficiency equations for combustion engines?)
  - XorNot 5y ago
    You still have to cool a combustion engine - hence why car's have radiators. Combustion engine's are also limited by Carnot cycle engine efficiency - about 64% theoretically, no more then 50% in the real world. So you're still going to wind up having to dump progressively more heat from a fixed surface area.
    - ethbr0 5y ago
      (Post coffee) Cars have radiators to cool the operating structure (e.g. engine block), but that cooling's ancillary to power produced, no? Carnot cycle was what I was grasping for. Original point being that in a thermal engine, increasing your temperature gradient by increasing your high-temperature side (while holding exhaust constant) leads to more potential power. Or IOW, a thermal engine that doesn't burn hot somewhere isn't going to produce much power. (Electric being a completely different ball of wax)
ginko 5y ago
I think the idea is to use the H2 both as fuel and refrigerant.
- Valgrim 5y ago
  So you recover the boiloff and use it to power a generator which powers a drivetrain cooled by the liquid hydrogen. This seems like a good idea.
ncmncm 5y ago
The idea is that a large-scale LH2 flow into the system is being consumed, and perforce heated, and may be used as refrigerant on the way there. It makes perfect sense. Rockets do this today, as indeed they have done from the earliest days, sending liquified gases through channels in the engine bell and combustion chamber before burning, keeping them both from vaporizing. Here, the very high efficiency of electric-driven turbines is offset by the lower efficiency of fuel-cell generation, for what is expected to be a substantial net gain over burning H2 in internal combustion turbines. The latter are the obvious first stage of deployment, taking advantage of LH2's extreme energy-to-weight ratio for reducing the takeoff fuel load by as much as 5:1. This form will probably be fielded in existing airframes, displacing some cargo space with in-hull LH2 tanks.
- ethbr0 5y ago
  Aka regenerative cooling: https://en.m.wikipedia.org/wiki/Regenerative_cooling_(rocketry) https://en.m.wikipedia.org/wiki/Regenerative_cooling_(rocket...
Tuna-Fish 5y ago
This is effectively regenerative cooling, like what rockets are commonly using. There are no cryogenic systems onboard, but all the fuel is loaded as very cold, and you can use the fuel as a coolant just before you burn it.
- candiodari 5y ago
  They will, however, be using kerosene, not liquid Hydrogen. It cannot provide cryogenic temperatures.
  - Tuna-Fish 5y ago
    Did you read the article? It was specifically about future liquid hydrogen-powered aircraft.
upofadown 5y ago
The H2 keeps itself cold by boiling away. You just need some insulation to slow down the rate to something manageable. Insulation normally uses a gas such as air to block the heat flow and as a result tends to be very light.
petertodd 5y ago
> Don't you also need cryogenic systems to keep the H2 cold? Wouldn't those use waaaaay more energy than what you save with superconducting motors? Probably not. Cryogenic liquids like liquid H2 are typically stored in dewers, which use a vacuum as the primary insulation, along with silvered surfaces and layers of mylar film to reduce radiation losses. We've been able to make dewers with boil off rates of 1% per day or better for decades. That's more than low enough that you don't need any refrigeration at all for an aircraft application where the flight just lasts a few hours anyway. Most likely they'll actually use less effective insulation to save weight. Fortunately for them, cryogenic liquids are commonly used in rockets, so there's already lots of know-how and technology out there for making lightweight cryogenic insulation.
Gravityloss 5y ago
You use the boiloff hydrogen in the power source, I assume a fuel cell in this case. This is regularly done by LNG tanker ships. Evaporating LNG is simply routed to the engines.

XorNot 5y ago

Motors are solid 3D structures. The more power you put in not superconducting, the more heat you have to extract. Superconductors mean you only need to keep your refrigerant cool, and that's a product of insulation effectiveness, not power output.

ethbr0 5y ago
This is the difference between electric and combustion powertrains, no? In electric, heat is the enemy and inefficient. In combustion, heat is a key component of the thermodynamic efficiency, unavoidable, and effectively only limited by material tolerances. (I didn't major in physics, so someone keep me honest on the ceiling / floor efficiency equations for combustion engines?)
- XorNot 5y ago
  You still have to cool a combustion engine - hence why car's have radiators. Combustion engine's are also limited by Carnot cycle engine efficiency - about 64% theoretically, no more then 50% in the real world. So you're still going to wind up having to dump progressively more heat from a fixed surface area.
  - ethbr0 5y ago
    (Post coffee) Cars have radiators to cool the operating structure (e.g. engine block), but that cooling's ancillary to power produced, no? Carnot cycle was what I was grasping for. Original point being that in a thermal engine, increasing your temperature gradient by increasing your high-temperature side (while holding exhaust constant) leads to more potential power. Or IOW, a thermal engine that doesn't burn hot somewhere isn't going to produce much power. (Electric being a completely different ball of wax)

ginko 5y ago

I think the idea is to use the H2 both as fuel and refrigerant.

Valgrim 5y ago
So you recover the boiloff and use it to power a generator which powers a drivetrain cooled by the liquid hydrogen. This seems like a good idea.

ncmncm 5y ago

The idea is that a large-scale LH2 flow into the system is being consumed, and perforce heated, and may be used as refrigerant on the way there. It makes perfect sense. Rockets do this today, as indeed they have done from the earliest days, sending liquified gases through channels in the engine bell and combustion chamber before burning, keeping them both from vaporizing. Here, the very high efficiency of electric-driven turbines is offset by the lower efficiency of fuel-cell generation, for what is expected to be a substantial net gain over burning H2 in internal combustion turbines. The latter are the obvious first stage of deployment, taking advantage of LH2's extreme energy-to-weight ratio for reducing the takeoff fuel load by as much as 5:1. This form will probably be fielded in existing airframes, displacing some cargo space with in-hull LH2 tanks.

ethbr0 5y ago
Aka regenerative cooling: https://en.m.wikipedia.org/wiki/Regenerative_cooling_(rocketry) https://en.m.wikipedia.org/wiki/Regenerative_cooling_(rocket...

Tuna-Fish 5y ago

This is effectively regenerative cooling, like what rockets are commonly using. There are no cryogenic systems onboard, but all the fuel is loaded as very cold, and you can use the fuel as a coolant just before you burn it.

candiodari 5y ago
They will, however, be using kerosene, not liquid Hydrogen. It cannot provide cryogenic temperatures.
- Tuna-Fish 5y ago
  Did you read the article? It was specifically about future liquid hydrogen-powered aircraft.

upofadown 5y ago

The H2 keeps itself cold by boiling away. You just need some insulation to slow down the rate to something manageable. Insulation normally uses a gas such as air to block the heat flow and as a result tends to be very light.

petertodd 5y ago

> Don't you also need cryogenic systems to keep the H2 cold? Wouldn't those use waaaaay more energy than what you save with superconducting motors? Probably not. Cryogenic liquids like liquid H2 are typically stored in dewers, which use a vacuum as the primary insulation, along with silvered surfaces and layers of mylar film to reduce radiation losses. We've been able to make dewers with boil off rates of 1% per day or better for decades. That's more than low enough that you don't need any refrigeration at all for an aircraft application where the flight just lasts a few hours anyway. Most likely they'll actually use less effective insulation to save weight. Fortunately for them, cryogenic liquids are commonly used in rockets, so there's already lots of know-how and technology out there for making lightweight cryogenic insulation.

Gravityloss 5y ago

You use the boiloff hydrogen in the power source, I assume a fuel cell in this case. This is regularly done by LNG tanker ships. Evaporating LNG is simply routed to the engines.