[Enthalpy]

Agreed, Wildfyr!

I hope that between "burn in 0.1s" and "use in hours or months" there is some safe and useful Zn grain size. Zn-air batteries have already found a reasonable compromise. If the powder must be finer, storage in paraffin alleviates the risk, as is done with more active metals.

Metal cycles seduce me because every step looks reasonable and feasible. Reduction by mere heat (1800°C does need engineering), decent risk and volume for transport and storage, easy use even to make electricity, correct overall efficiency, fits many contexts.

When comparing with hydrogen economy, with zeolite energy storage, or molten salts heat storage, I feel metal cycles obviously better. Some nasty bits will emerge when exploring or engineering the idea, that belongs to the game.

[Enthalpy]

Bulky hydrogen is uneasy to move to an airport or other customers. Here an example by train.

Liquid at 20K, hydrogen can fill balloons of round section. Multilayer insulation in vacuum and polymer straps holding the balloon make evaporation minimal. Foam slows the evaporation if vacuum is lost, the overpressure vent can burn the hydrogen over a catalyst and cool the vapour. It needs a vacuum vessel.

One 19m wagon with Jakobs bogies carries 5.3t hydrogen. With end bogies, pantographs and transformers to feed the cryocoolers, one set of 9 wagons carries 48t or slightly more. A locomotive and 4 sets form a 740m train carrying only 200t, as much mechanical energy as 970t kerosene that fit in 15 four-axle wagons. Move 3× as many trains.

A wagon of uneasy design with rectangular 2.8m×4.0m section would carry 11.0t hydrogen instead, and a train 400t. I keep 200t in here under.

240 000 takeoffs/year with hydrogen from a medium-big airport need 2 000 000 t/year or 27 trains/day. Feasible with fast unloading.

The light and streamlined wagons could technically move at 200 or 320km/h.

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A pipeline for gaseous hydrogen is intuitively better than trains, except maybe if the train lane exists but not the pipeline.

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Trains move reduced and oxidized zinc more safely, and zinc can make electricity or hydrogen at the destination.

Many wagon design reach the track limit of 8t/m in the EU, so a 740m train moves 5600t. But 5600t ZnO have produced only 140t hydrogen per slow train.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Planes can bring hydrogen to airports. They use existing runways, can be wider and taller than trains, and they move faster.

While other airplanes carry less volume or mass, the AN-225 has adequate shoulders
  BelugaXL - A380 - Dreamlifter - C-5 Galaxy - C-17 Globemaster - AN-225
The AN-225 can carry 200t on its back, like 170t hydrogen in a 30t tank, and some 50t in its cargo hold. I'd prefer an adaptation to >250t on the back, where leaks are less dangerous.

The ellipsoidal external tank has L=48.3m D=10.6m and S=1130m² per Knud Thomsen. Insulating foam for 20K exists, see Ariane V: 33mW/m/K and less at cold, 50kg/m³, so 0.2m weigh 11t and leak 52kW, which evaporates 0.4t/h at constant 20K or heats the hydrogen by 0.15K/h or 0.07bar/h at constant amount. Glass fabric in epoxy, or something more expensive, can make a sandwich, where 5+5mm weigh 20t. 1bar overpressure pulls one 5mm skin by 100MPa, so some overpressure can remain at altitude and stabilize the shape at varied ground altitudes.

If the tank isn't empty nor full, say to serve several airports, it needs partitions to immobilize the hydrogen. Maybe cones of similar sandwich material.

The only flying AN-225 was bombed, but Antonov will finish the other one. Once all flights sip hydrogen, a medium-big airport will need 27 loads a day like from trains, but an AN-225 can make two 1000+1000km rotations a day from a sunny (sol-zinc!) or windy production site. Meanwhile, one tank plane suffice, possibly shared among airports. Leasing an AN-225 at reported 30kusd/h costs 100kusd/200t or 0.5usd/kg, equivalent to 0.1usd/kg for kerosene.

Maybe a flotilla of AN-225 operates later, or bigger planes powered by hydrogen, until all airports have pipelines for gaseous hydrogen.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Here's a plane designed to bring hydrogen to airports. Don't trust every pixel and digit of this draft, like the short horizontal stabilizer.

It consumes hydrogen to emit no carbon dioxide, and is as big as an A380. The 1008m² wing has W/L=5.7 only, I take L/D=15 from the Spitfire. Winglets are desired, or maybe the chord splits near the tips and the elements spread as for eagles.

• At sea level and 820t, flaps achieve αC_L=1.3 to stall at 100m/s=360km/h=195knots. The displayed blown flaps improve.
• At 8000m (0.526kg/m³), 820t and 200m/s=720km/h=389knots, αC_L=0.76.

Three bodies are slimmer than one, they spread the weight over the runway and the wing. The gears retracted in the tanks drag less. A lifting body or a blended wing-body look worse.

The plane shall take-off or land at 820t. The displayed ellipsoidal tanks host 480t hydrogen if the rest is light enough. 0.2m foam as on Ariane V weigh 3×13t and can be the core of a sandwich as in the previous message. The heat leak evaporates half as much hydrogen as the fuel cells consume, so some liquid is removed too. Again, hours of heat leak have little effect on the pressure, or they waste little hydrogen.

The 6 ducted fans blow over D=3.75m, bigger than the Trent.

• At sea level, they accelerate 6×2131kg/s air from 0 to quiet 158m/s with 90% efficiency to push 6×337kN or 820t×0.25g like the A380 and An225.
• The 98% efficient electric motors obtain 6×30MW_e from 3×30t fuel cells. 2kW/kg exist at the Toyota Mirai.
• At 8000m (0.526kg/m³), the fans accelerate 6×1511kg/s air from 200m/s to 260m/s to push 6×91kN for L/D=15 and 820t.
• The motors draw 6×23MW_e at cruise speed, 77% of the peak power.

The fuel cells need over 6×13kg/s air, easy but must be done. The ducts around the fans can host the fuel cells. A bit of slowed air can flow radially over a short distance and much area to feed the cells, then the warmer vapour and depleted air expand for ramjet thrust.

The fans could reside at the bodies' aft to reduce the drag. They won't blow the flaps then, and the fuel cell must remain before the wing for equilibrium.

Powered wheels, easy with electricity, would taxi and accelerate the plane more efficiently than the fans alone do. Supercapacitors are bad but accumulators could store descent and braking energy to supplement the take-off and ascent.

My turbulators are the topic of
  scienceforums
Thoughts about flight motors, hydrogen and more at
  scienceforums
Waveforms to drive fast electric motors there
  scienceforums
Aluminium wires
  scienceforums - scienceforums

Marc Schaefer, aka Enthalpy

[Enthalpy]

The Toyota Mirai's fuel cell uses a polymer membrane electrolyte and operates around +120°C, so cooling uses many times more air than 13kg/s at each fan of the previous H₂ lifter. Very preliminary estimates suggest 1/10^th the main flux, an incentive to put the cells at the fans.

The losses at the fuel cells, like 40% becoming heat, make my previous proposal more interesting: slow down the secondary flux, cool (and feed) the fuel cells, expand the flux to accelerate it. Very preliminary estimates suggest the gain is equivalent to 70% fuel cell efficiency instead of 60%, wow.

[Enthalpy]

More and more people grasp that hydrogen and fuel cells will fly aeroplanes. Many companies develop the technology, some fly already.

I told already that a hydrogen turbine is a dead end, because fuel cells and electric motors are 1.5× as efficient, and the ratio will spread further
  scienceforums - chemicalforums

Zeroavia has flown a Dornier 228 with hydrogen and fuel cells in the cabin
  Cnn
They would fit in the nacelles of a Dornier 328
  scienceforums
but Zeroavia did a fantastic step. Engineering is harder than a naive drawing, and the Dornier 228 is the biggest to fly on hydrogen up to now. Kudos!

Even Airbus seem to slowly grasp that planes need hydrogen, not batteries, and used in fuel cells, not in turbofans
  Cnn
Wow! Still recently, they were betting on the turbofans by Safran and Rolls-Royce. Have Airbus already understood that superconducting motors are superfluous and risky?

[Enthalpy]

[...] Slow down the secondary flux, cool (and feed) the fuel cells, expand the flux to accelerate it [...]

Here's an illustration and figures for a 30MW, 4m fan as the former H₂ lifter uses. The PEFC fuel cells have a polymer electrolyte as for the Toyota Mirai, with exhaust under +140°C: I take +120°C. ΔHf=-286kJ/mol for H₂O liq @298K.

I compute for air taken as an ideal diatomic gas. It mainly cools the cells, only 3% of the exhaust moles are H₂O but these carry much vaporization heat, partly recovered at expansion, so misused Propep would compute better. Staying near the dew point gains efficiency.

On the sketch, the cooling flux has its own straightener and the stator blades are far. The stator blades could reside more forward and some or all cells behind them. The cooling air flows radially through the cells, with only 6m/s over the big area, and at a higher pressure that enable the expansion and bigger exhaust speed. Less area and a longer path are possible.

163kg/s of the 1250kg/s air are heated by the cells' losses and leave with 293m/s rather than 260m/s. 1.079× as much thrust for the same electric power, which saves 9t cells at the former H₂ lifter. The efficiency improves as much, as if the cells improved from 60% to 65%.

The rotor could blow the cooling flux faster with a local blade angle, especially of the fluxes are separated early. This should increase the gain and remains very simple. Maybe future powerplants have some extra compressor and turbine around the cells to help move the main rotor.

If a fan without a duct holds at a pod, the cooling air can be taken at the pod's leading edge behind the tip of the blades. If a fan resides at a nacelle before a wing, the cooling air can be taken at the wing's leading edge behind the tip of the blades.

Marc Schaefer, aka Enthalpy