3 ms·

Perhaps you'd like to explain some of those considerations rather then just waving at them. Modern jet turbines are running within a few % of maximum thermodyn

by XorNot 4y ago

Perhaps you'd like to explain some of those considerations rather then just waving at them. Modern jet turbines are running within a few % of maximum thermodynamic efficiency. So what consequences other then CO2 emissions are worse then anything else? And for that matter, how would biofuels possibly be worse fossil fuels, given that the whole point is to not emit net CO2 into the atmosphere. Obviously it's "efficient" because all the energy we put in has to come from the sun, rather then obtained for free but that's the point.

jabl 4y ago
> Modern jet turbines are running within a few % of maximum thermodynamic efficiency. Indeed they are marvels of engineering. But that doesn't make them "sustainable". It just takes a lot of energy to push something through the air at high speed, no matter how efficient the engines are. Apart from the CO2 emissions, there's also the issue of the GHG forcing effect of high altitude water vapor emissions. Current estimates are that flight at typical airliner altitudes of ~10 km results in a GHG forcing (over some decades) of roughly 3 times more than burning the same fuel on the ground. > And for that matter, how would biofuels possibly be worse fossil fuels, given that the whole point is to not emit net CO2 into the atmosphere. Obviously it's "efficient" because all the energy we put in has to come from the sun, rather then obtained for free but that's the point. The major issue is due to land use changes. Biodiversity is already in a sharp decline (arguably this is an as big, if not bigger issue than climate change), largely driven by converting nature into cropland. Secondly, converting nature into crops, whether for fuel or food, tends to result in large CO2 emissions due to the carbon that has been stored into the soils being released. And finally, biofuels just don't scale. There isn't enough arable land on the planet to replace the amount of fossil fuels we use today.
HPsquared 4y ago
Farming causes environmental damage besides greenhouse emissions. Even discounting all the other forms of environmental damage, a lot of biofuel schemes (e.g. corn ethanol) emit more greenhouse gases than they save.
klodolph 4y ago
Thermodynamic efficiency is the wrong way to measure efficiency here. It makes no sense to measure efficiency that way, none at all. I’m talking about biofuels as they exist today, not as they hypothetically exist in the future. Currently, you burn biofuels, you end up releasing more CO2 into the atmosphere, because the production of biofuels produces CO2. It’s not negligible, either. All of the analyses I’ve seen of biofuel paint an incredibly dismal picture. As far as I can tell, the main strategy to reduce CO2 emissions is to stop burning fuels altogether, not switch to something like biofuel. Airplanes need power density & energy density in order to work which makes technologies like batteries unsuitable. Way easier to power something on the ground, where you’re not spending tons of energy just to keep it from crashing. Or better yet, on an electrified rail.

by XorNot 4y ago

jabl 4y ago

> Modern jet turbines are running within a few % of maximum thermodynamic efficiency. Indeed they are marvels of engineering. But that doesn't make them "sustainable". It just takes a lot of energy to push something through the air at high speed, no matter how efficient the engines are. Apart from the CO2 emissions, there's also the issue of the GHG forcing effect of high altitude water vapor emissions. Current estimates are that flight at typical airliner altitudes of ~10 km results in a GHG forcing (over some decades) of roughly 3 times more than burning the same fuel on the ground. > And for that matter, how would biofuels possibly be worse fossil fuels, given that the whole point is to not emit net CO2 into the atmosphere. Obviously it's "efficient" because all the energy we put in has to come from the sun, rather then obtained for free but that's the point. The major issue is due to land use changes. Biodiversity is already in a sharp decline (arguably this is an as big, if not bigger issue than climate change), largely driven by converting nature into cropland. Secondly, converting nature into crops, whether for fuel or food, tends to result in large CO2 emissions due to the carbon that has been stored into the soils being released. And finally, biofuels just don't scale. There isn't enough arable land on the planet to replace the amount of fossil fuels we use today.

HPsquared 4y ago

Farming causes environmental damage besides greenhouse emissions. Even discounting all the other forms of environmental damage, a lot of biofuel schemes (e.g. corn ethanol) emit more greenhouse gases than they save.

klodolph 4y ago

Thermodynamic efficiency is the wrong way to measure efficiency here. It makes no sense to measure efficiency that way, none at all. I’m talking about biofuels as they exist today, not as they hypothetically exist in the future. Currently, you burn biofuels, you end up releasing more CO2 into the atmosphere, because the production of biofuels produces CO2. It’s not negligible, either. All of the analyses I’ve seen of biofuel paint an incredibly dismal picture. As far as I can tell, the main strategy to reduce CO2 emissions is to stop burning fuels altogether, not switch to something like biofuel. Airplanes need power density & energy density in order to work which makes technologies like batteries unsuitable. Way easier to power something on the ground, where you’re not spending tons of energy just to keep it from crashing. Or better yet, on an electrified rail.