Scrutable

I suspect the immediate solution for aviation will be biofuels, coupled with reductions and modal shifts to reduce flight numbers.

Bird on a Fire wrote: ↑

Fri Oct 02, 2020 10:26 am

I suspect the immediate solution for aviation will be biofuels, coupled with reductions and modal shifts to reduce flight numbers.

Ideally synthetic hydrocarbons, with the carbon pulled from the air or cheap bio carbon that doesn't compete with food, however that is currently very expensive and is doing an R&D.

Food is already in competition for biofuels. Currently 1/3 of corn grown in the USA is for ethanol production, that is something like 120,000 km^2. It nearly all goes into cars and trucks and is a horrendously inefficient process for turning sunlight into vehicle motion. PV at 20% efficiency would yield around 24,000GW in an equivalent area. Note that the US currently has something like 1,100GW generation in total.

Biofuels is a better use than feeding livestock, IMHO. We might have to make that choice.

Just about the only positive about hydrogen powered aircraft is that the fuel tanks don't need to be at the centre of gravity (since they aren't much heavier full than they are empty). This means that the wings can be made with skinny low-drag profiles. Of course, the wing has work to do, but it's also a very significant source of drag.

bjn wrote: ↑

Fri Oct 02, 2020 11:18 am

Food is already in competition for biofuels. Currently 1/3 of corn grown in the USA is for ethanol production, that is something like 120,000 km^2. It nearly all goes into cars and trucks and is a horrendously inefficient process for turning sunlight into vehicle motion. PV at 20% efficiency would yield around 24,000GW in an equivalent area. Note that the US currently has something like 1,100GW generation in total.

Not really in competition. There’s overproduction of corn and other crops in the US. Farmers are facing bankruptcy due to low prices (which are about half what they were during 2011-2014). If US farmers stopped producing for bio fuels they couldn’t switch to supplying it all to the consumer market.

The situation is still mad as the land used for growing biofuels could be reforested instead and become a carbon sink. All that would require would be a tweak to the subsidy regime and farmers would start planting vast areas of trees.

Woodchopper wrote: ↑

Fri Oct 02, 2020 4:01 pm

bjn wrote: ↑

Fri Oct 02, 2020 11:18 am

Food is already in competition for biofuels. Currently 1/3 of corn grown in the USA is for ethanol production, that is something like 120,000 km^2. It nearly all goes into cars and trucks and is a horrendously inefficient process for turning sunlight into vehicle motion. PV at 20% efficiency would yield around 24,000GW in an equivalent area. Note that the US currently has something like 1,100GW generation in total.

Not really in competition. There’s overproduction of corn and other crops in the US. Farmers are facing bankruptcy due to low prices (which are about half what they were during 2011-2014). If US farmers stopped producing for bio fuels they couldn’t switch to supplying it all to the consumer market.

The situation is still mad as the land used for growing biofuels could be reforested instead and become a carbon sink. All that would require would be a tweak to the subsidy regime and farmers would start planting vast areas of trees.

Or even vast areas of wind turbines and PV. Possibly interleaved with trees.

That said, I don't know if the Great Plains were that forested when Europeans arrived, so I'm not clear if reforestation would work that well.

Woodchopper wrote: ↑

Fri Oct 02, 2020 4:01 pm

bjn wrote: ↑

Fri Oct 02, 2020 11:18 am

Food is already in competition for biofuels. Currently 1/3 of corn grown in the USA is for ethanol production, that is something like 120,000 km^2. It nearly all goes into cars and trucks and is a horrendously inefficient process for turning sunlight into vehicle motion. PV at 20% efficiency would yield around 24,000GW in an equivalent area. Note that the US currently has something like 1,100GW generation in total.

Not really in competition. There’s overproduction of corn and other crops in the US. Farmers are facing bankruptcy due to low prices (which are about half what they were during 2011-2014). If US farmers stopped producing for bio fuels they couldn’t switch to supplying it all to the consumer market.

The situation is still mad as the land used for growing biofuels could be reforested instead and become a carbon sink. All that would require would be a tweak to the subsidy regime and farmers would start planting vast areas of trees.

I'm going to be mean and say they shouldn't be growing anything there for which there isn't a demand. Stop farming it, let the buffalo roam again.

Or, y know, put up wind turbines and PV on the land.

dyqik wrote: ↑

Fri Oct 02, 2020 5:30 pm

Or, y know, put up wind turbines and PV on the land.

But only a tiny fraction of it.

bjn wrote: ↑

Fri Oct 02, 2020 5:35 pm

dyqik wrote: ↑

Fri Oct 02, 2020 5:30 pm

Or, y know, put up wind turbines and PV on the land.

But only a tiny fraction of it.

Much of that land is a long way from major population centres, meaning lots and lots of infrastructure needed to wire it all up. So you could focus the power generation in the bits nearer where people live, and the rewilding in the more remote parts.

dyqik wrote: ↑

Fri Oct 02, 2020 5:15 pm

Woodchopper wrote: ↑

Fri Oct 02, 2020 4:01 pm

bjn wrote: ↑

Fri Oct 02, 2020 11:18 am

Food is already in competition for biofuels. Currently 1/3 of corn grown in the USA is for ethanol production, that is something like 120,000 km^2. It nearly all goes into cars and trucks and is a horrendously inefficient process for turning sunlight into vehicle motion. PV at 20% efficiency would yield around 24,000GW in an equivalent area. Note that the US currently has something like 1,100GW generation in total.

Not really in competition. There’s overproduction of corn and other crops in the US. Farmers are facing bankruptcy due to low prices (which are about half what they were during 2011-2014). If US farmers stopped producing for bio fuels they couldn’t switch to supplying it all to the consumer market.

The situation is still mad as the land used for growing biofuels could be reforested instead and become a carbon sink. All that would require would be a tweak to the subsidy regime and farmers would start planting vast areas of trees.

Or even vast areas of wind turbines and PV. Possibly interleaved with trees.

That said, I don't know if the Great Plains were that forested when Europeans arrived, so I'm not clear if reforestation would work that well.

I did a google and it looks like they planted hundreds of millions of trees during the 30s to try to stop the dust bowl. So it would be feasible. https://www.pri.org/stories/2018-02-03/ ... der-threat

shpalman wrote: ↑

Fri Oct 02, 2020 3:12 pm

Just about the only positive about hydrogen powered aircraft is that the fuel tanks don't need to be at the centre of gravity (since they aren't much heavier full than they are empty). This means that the wings can be made with skinny low-drag profiles. Of course, the wing has work to do, but it's also a very significant source of drag.

This does depend on the pressure the hydrogen is kept at. At the pressure of some of the hydrogen tanks being discussed there would be quite significant differences in weight between full (at hundreds of atmospheres) and empty (potentially lighter than air).

Trying to find papers on LH cryogenic tank weights, the only one I could find was this one from 2007. If I read it right (they hid the details for the diameter of the tank), for a 102” diameter tank, they reckon on 150lb for a dual skinned vacuum tank suitable for use in aviation. This is only the two shells, none of the valves, mounts, pumps, inner support rod, etc... Assuming my sums are right, this tank would hold a maximum of 1,100 kg of LH, weigh of the shells alone is around 68kg, and stand 270cm tall.

At work we use liquid nitrogen a fair bit. In those tanks (with no active refrigeration) there is always some boil off leading to pressurisation of the ullage space. Every now and then the pressure builds to a point where the tank vents a bit of gas. With hydrogen you will need to be careful where the vent goes, especially if you have electronics and engines around. DSEAR regulations come into play, making stuff intrinsically safe is very expensive.

bjn wrote: ↑

Sat Oct 03, 2020 8:00 am

Trying to find papers on LH cryogenic tank weights, the only one I could find was this one from 2007. If I read it right (they hid the details for the diameter of the tank), for a 102” diameter tank, they reckon on 150lb for a dual skinned vacuum tank suitable for use in aviation. This is only the two shells, none of the valves, mounts, pumps, inner support rod, etc... Assuming my sums are right, this tank would hold a maximum of 1,100 kg of LH, weigh of the shells alone is around 68kg, and stand 270cm tall.

That’s a good deal lighter than pressure tanks for sure. Not much more than twice the weight of a single skin tank.

I know that any idiot can make a 3-d render and it in no way implies that a real aircraft would be viable but notice the lack of windows in the rear part of the fuselage in the clips in Mentour's video. (For the two models which are at least similar to current designs; not sure how the blended wing will work at all in terms of where the passengers sit and how they'll get in and out (or indeed see in and out).)

Linking to that because it's shorter than Airbus's original video.

There also exists solar-powered creation of liquid hydrocarbons from atmospheric CO2 and water for example.

Grumble wrote: ↑

Sat Oct 03, 2020 9:02 am

At work we use liquid nitrogen a fair bit. In those tanks (with no active refrigeration) there is always some boil off leading to pressurisation of the ullage space. Every now and then the pressure builds to a point where the tank vents a bit of gas. With hydrogen you will need to be careful where the vent goes, especially if you have electronics and engines around. DSEAR regulations come into play, making stuff intrinsically safe is very expensive.

The cryotanks on the space shuttle lost something like 1% per hour, which needed venting. Probably better than that now. This shouldn’t be a problem while flying, just vent straight into the engines to burn it. Being stuck on a runway for too long might end up being a problem though. As Matatouille pointed out, these have to go into the fuselage, increasing the size of it and/or cutting down on useable space for passengers or cargo.

I posted this elsewhere at the time, but network rail published its interim traction decarbonisation strategy a few weeks ago. Worth a look.

https://www.networkrail.co.uk/wp-conten ... s-Case.pdf

AIUI aircraft makers would dearly love to do away with windows in ‘normal’ planes anyway, and just have screens showing the outside view instead, which would be lighter.

nekomatic wrote: ↑

Sat Oct 03, 2020 6:01 pm

AIUI aircraft makers would dearly love to do away with windows in ‘normal’ planes anyway, and just have screens showing the outside view instead, which would be lighter.

Is there anything stopping them?

Grumble wrote: ↑

Sat Oct 03, 2020 6:40 pm

nekomatic wrote: ↑

Sat Oct 03, 2020 6:01 pm

AIUI aircraft makers would dearly love to do away with windows in ‘normal’ planes anyway, and just have screens showing the outside view instead, which would be lighter.

Is there anything stopping them?

Passengers? Imagine being the first airline to buy such planes, marketing the lack of windows will be a very hard sell.

Grumble wrote: ↑

Sat Oct 03, 2020 6:40 pm

nekomatic wrote: ↑

Sat Oct 03, 2020 6:01 pm

AIUI aircraft makers would dearly love to do away with windows in ‘normal’ planes anyway, and just have screens showing the outside view instead, which would be lighter.

Is there anything stopping them?

Probably some safety rules would need updating, as things like emergency safety instructions tell you not to open the emergency exit if you see flames outside.

dyqik wrote: ↑

Sat Oct 03, 2020 9:14 pm

Probably some safety rules would need updating, as things like emergency safety instructions tell you not to open the emergency exit if you see flames outside.

My knowledge of this specifically may be out of date, but windows in the walls (i.e. not in doors) are a safety thing too. Blinds have to be open on takeoff/landing so passengers' eyes can be accustomed to external light conditions in case there is a need for speedy evacuation. In eliminating windows the onus would be on the manufacturer to demonstrate no loss of safety or new introduced hazards, there are precidents for regulations evolving with such changes.

Also passengers like windows. Manufacturers compete with eachother with window size, particularly on long-haul aircraft. A significant chunk of long-haul passengers factor into their airline choice the likely aircraft type and it's comforts. In recent years the trend has been towards bigger windows, window for every row etc. Short haul is different, cost rules the roost there and not many people in that market choose airlines by the aircraft they fly. As H2 is more proposed for this segment of aircraft, windows are quite likely on the chopping block if it reduces cost.

IMO there is a good reason why the Delta design is so much smaller than the other two. Nothing to do with windows, everything to do with meeting the evacuation time and distance to emergency exits regulations. I played with such designs in an airliner design project at uni and this was among the hardest problems to get right in unconventional configurations. We didn't consider pressurised H2 tanks though, putting them in the difficult to evacuate regions of such an unconventional fuselage shape might be a stroke of genius, so long as the passengers don't know how close it is...

... opening the blinds at takeoff and landing is also in case the emergency services need to see in?

shpalman wrote: ↑

Sat Oct 03, 2020 9:44 pm

... opening the blinds at takeoff and landing is also in case the emergency services need to see in?

I've not heard of that, but am likely wrong. I can't think of any situations where they couldn't use the doors & only want to peek inside rather than reach for the saws.