[Enthalpy]

Hello nice people!

Some 40 years ago, newspapers and a book claimed that hydrogen would be the energy vector of our societies, produced by renewables, to replace the consumption of fossil fuels.

This hasn't happened, but as hydrocarbons got more expensive than renewables even before the embargo against Russia, some politicians want to revive to old idea.

I claim that lithium batteries have killed hydrogen storage meanwhile, and that very few uses justify hydrogen. Opinions welcome!

[Enthalpy]

The most visible evolution is the electric car, more generally terrestrial transports.

==========

Around 2000, many car manufacturers had projects with gaseous hydrogen in pressure tanks, possibly aided by adsorption, and fuel cells to feed electric motors.

I know only of two (2) gaseous car models on the market and never saw them on the streets. As opposed, dozens of battery car models exist, and in Germany I see one or more battery car in every small street.

The range of battery small cars suffices for many uses but not all. Customers often have an additional gasoline or hybrid vehicle for longer trips, which still emits CO₂. Agro-fuels for them?

Range improves with better regenerative braking, and from lower drag. The visionary Tesla did it brilliantly, some German companies make efforts, while the resigned French industry doesn't even try.

Range improves also with bigger batteries, which can exaggerate a bit further. Or they need more capacity per mass unit, which still improves. For instance a massive lithium electrode would weigh less than lithium atoms in a carrier electrode, if someone succeeds. Or do zinc+air, more generally air as a depolarizer, or other reactions provide alternatives? Or charging while moving?

The load time is a worry for long trips only. Supermarkets boast already loading stations. Tesla proposed loading stations at highway restaurants. We could have loading stations at highway rest parks, if bringing electricity there. And: there is no hard limit to the load time, which depends on an electrode area-to-volume ratio. The self-discharge worsens, but research can improve that. Or: replace the battery instead of loading it quickly. Postal services did replace the horses at the stations.

==========

Bigger terrestrial vehicles favor batteries even more as they use far less energy per mass unit. Trucks, trains run easily on a battery charge for a working day. Main obstacle: the operators pay little taxes on fossil fuels.

A truck consuming mean 100kW for 8h in a day needs 2.9GJ from 3.2t Li-ion battery, heavier than the Diesel+fuel. This takes an axle more or reduces the payload, but the mass ratio is much easier than for a small car. The truck could recharge at night in the equipped rest park. It's a matter of economy, not feasibility.

A 3MW locomotive consuming mean 1MW for 16h in a day needs 58GJ from a 87t fast-discharge Li-MnO₂ battery, not the most compact. Fine: that's the mass of the 4-axle locomotive, sparing the cast iron added for adherence. A passenger train with distributed traction is even easier. No need for hydrogen-powered trains.

[Enthalpy]

Are electric boats interesting, despite the operators pay little taxes on fossil fuels?

Battery boats operate already for complete days, they cross bays and straights. Frequent accelerations and limited streamlining didn't stop them. Newspapers claim operation is cheaper. I believe easily that passengers favour the cleanliness.

Container ships as opposed couldn't navigate for long on batteries. Full 2×32MW for two weeks use 77TJ from 85 000 t of Li-MnO₂ batteries in 165 000 t deadweight. Plus the unreasonable fast charge. They develop renewable fuels instead: ammonia, methanol, yuk. Methanol converts efficiently to ethylene in labs, I'd go further to tetradecene or trialkylamines with C+N around 14. Already palm oil fuel must be too expensive for boats.

The same 400m×59m×(14+73)m container ship with 32t of 60% efficient fuel cell would consume 1060t liquid H₂ that fit in 15 000 m³, a D=31m sphere, easy. Here hydrogen makes sense.

[billnotgatez]

Water vapor is the result of burning hydrogen.

I found these 2 quotes on the internet

Water vapor is Earth's most abundant greenhouse gas. It's responsible for about half of Earth's greenhouse effect — the process that occurs when gases in Earth's atmosphere trap the Sun's heat. Greenhouse gases keep our planet livable.

At 30 °C (86 °F), for example, a volume of air can contain up to 4 percent water vapour. At -40 °C (-40 °F), however, it can hold no more than 0.2 percent. When a volume of air at a given temperature holds the maximum amount of water vapour, the air is said to be saturated.

As an aside here is another quote from the internet on carbon dioxide.

The global average carbon dioxide set a new record high in 2021: 414.72 parts per million.

[Enthalpy]

Hi billnotgatez, nice to read you!

The fate of the greenhouse gases in the atmosphere must be considered too.

• Additional water vapour rains down.
• Methane is destroyed over decades.
• Carbon dioxide isn't destroyed. Part of it is absorbed over time by vegetables and by the Ocean, but this too has limits and drawbacks.

Also: the proportion of human-produced carbon dioxide in the atmosphere is very significant. How small would be human contribution as compared to water evaporating naturally?

[Enthalpy]

What about electric planes?

========== Motors

Electric motors are very small and light if allowed to run fast, say 100m/s or more. Only the force costs materials and losses, speed is for free. Look
  wikipedia second picture: wikipedia
the HP stage pair is grey, the three parallel LP stage pairs are yellow, the alternator is red. Or check my estimates
  scienceforums and around
a ring electric motor of Cu, Fe and optionally Nd-Fe-B as slow as the fan fits in the same volume as the turbofan and is 1/3 as light. A gear would squeeze the motor further.

Aeroplanes need no superconducting motors, simply because the motors are fast. Superconductors are dangerous: they can quench and explode, and have more failure modes while Cu+Fe motors are extremely reliable. I strongly believe superconducting motors for aircraft are wasted time and moneys.

========== Batteries

They are still heavy and demand special aeroplane design. This fits hobby or schooling use. Examples
  (Fly 60+30mn) Pipistrel - (paperwork) scienceforums
Airliners must carry many passengers and if needed divert to an airport 100nm away then wait 45min in the air. Batteries don't fit airliners presently. Much effort was already invested in batteries, so progress is probably expensive.

========== Hydrogen

Hydrogen provides more energy hence flight time and range than the same kerosene mass.

Light tanks store liquid hydrogen at atmospheric pressure. My paperwork:
  scienceforums and elsewhere
I consider all designs around gaseous hydrogen under pressure inadequate for aircaft. Too heavy, too dangerous. Liquid hydrogen is a smaller explosion hazard
  Part1 – Part2   on Youtube

Fuel cells are 60% efficient, commercial hot SOFC were measured at 62% in 2018, with progress at limited cost
  fuelcellsworks.com
while the best gas turbine, terrestrial hence heavy, 10* as powerful as an airliner engine, converts 43% of the fuel energy into shaft energy
  ge.com
and gas turbines take two decades and huge investment to gain 6 points efficiency.

I claim that consuming 1.5× as much hydrogen in a hydrogen turbine is nonsense. Hydrogen tanks are already bulky enough. Hydrogen has a cost. This development squanders moneys. Aircraft will fly with fuel cells.

This is even more true for cars. One company develops a hydrogen engine, despite fuel cells are fully operational for cars.

========== Uses

Fuel cells for the Toyota Mirai weigh 0.5kg/kW
  wikipedia
This is perfect for rotating-wing aircraft. Design examples with masses, still 1kg/kW there:
  masses - sketch - masses - sketch
Electric motors being cheap and light, four rotors, or six or more for redundancy, are better than one or two with cyclic pitch.

Light hydrogen brings flight time and range. Kerosene lets operational copters fly for around 1h, too short for search, rescue and surveillance at sea and moutain, badly short for oil platform service or road rescue. Copters could be first to adopt hydrogen.

Hydrogen improves also light and business aircraft with present fuel cells
  scienceforums - scienceforums

Regional and cargo aircraft are slower, they need less power and live with present fuel cell performance
 scienceforums (1kg/kW) - scienceforums (0.5kg/kW)

Long-haul airliners are faster, they need more power, but 0.5kg/kW plus the hydrogen, the tanks and the motors weigh a much at lift-off as the kerosene and the engines. Hydrogen fits under the wings
  scienceforums and next

Only supersonic planes fly badly at M1.3 with fuel cells, still 1kg/kW there
  scienceforums

So even at 1000km/h, lighter fuel cells are no prerequisite to fly, they're only a strong wish. The automotive industry improved them enough for cars. Aeronautics can lighten them further. Fuel cells need area while the volume weighs, so improvement must be possible. Planes may have specific demands too. Less effort was invested in light fuel cells than in gas turbines, letting hope progress at limited cost. That is a useful research direction.

Aluminium cables work at high voltage lines. Aeronautics should make them operational in aircraft. Research topic.

And among the transports, aeronautics is the one that needs hydrogen rather than batteries, boats possibly too.

[billnotgatez]

I happened on this YOUTUBE on a facet of this topic

https://www.youtube.com/watch?v=M0fnEsz4Ks0

[Enthalpy]

Seems to be a catchy name for adsorption or for metal hydrides. Both are much heavier than liquid hydrogen if one includes the tanks.

[Enthalpy]

As some people imagine hydrogen as a general vector of energy, they would like solar cells to feed electrolysis. That's nonsense, I claim.

Typical solar cells are only 1/6 efficient. Their collecting area is expensive. Typical electrolysis is 70% efficient. From the costly area to the hydrogen, this would convert only 12% of the energy.

Compare with the Zn/ZnO or the FeO/Fe₃O₄ cycles. They use heat, say from a sunlight concentrator, and make hydrogen. They demonstrated 30% conversion two decades ago, with 60% as an easy target. That means a sunlight collector 5× smaller, made of cheap steel sheet mirrors, not of expensive semiconductors. Obvious choice.

==========

I've even read that such hydrogen imported from Namibia or the Arabic peninsula to northern Europe shall produce iron from ore.

If someone really wants to consume electricity to make iron, he shall electrolyse the ore, not go over hydrogen. OK, that's still research.

Hydrogen storage for dark days is worse than ore and iron storage, I say. Compare the temperatures, the volumes, the risks.

Transporting hydrogen rather than iron is uneconomic too. Several countries that export iron ore have plenty sunlight: Australia, Mauretania and more. Sunlight near the mine can first reduce the ore to FeO just by hea. Then make hydrogen from sunlight over the FeO/Fe₃O₄ cycle and use the gaseous hydrogen immediately and on site to reduce the FeO or the ore to iron. This consumes no water, it emits only oxygen, and lets transport only the iron, not the hydrogen nor the ore. Obvious choice again.

==========

Electricity storage over hydrogen means 70% inefficient electrolysis and 62% inefficient fuel cells, the cycle being 43% inefficient. Present lithium batteries do it with 92% cycle efficiency, at interesting cost. No brainer.

But useful research can be done over other storage methods. Flywheels, underwater air or vacuum, and hopefully more. Little has been done up to now, it needs no prior theoretical advances, so progress can be affordable there.

==========

While I have nothing against hydrogen in some uses, especially air transport, in other uses it's uncompetitive. I don't believe hydrogen will be a ubiquitious energy vector.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Beyond the FeO to Fe reduction suggested above, hydrogen from sunlight over the FeO/Fe₃O₄ cycle might reduce more metals without emitting CO₂. Heats of formation under normal conditions, that's improvable.

=========================================
From     H₂O   FeO   Ni₂O₃   Co₃O₄   CoO
To       g H₂  Fe    2Ni     3CoO    Co
kJ/mol   242   272   3×163   177     238
=========================================

Mn, Mo are more doubtful.

Maybe some cycle among the oxides of Ni and Co produces hydrogen from sunlight too. It isn't required, since Fe isn't lost in the cycle.

==========

Concentrated sunlight making heat reduces ZnO to Zn and hopefully other metals too. Heats of formation under normal conditions, again.

=============================================================================
From     ZnS   ZnO   Cu₂S  2CuO   Cu₂O   CuO   SnO₂   SnO   PbO₂   PbO   Bi₂O₃
To       Zn    Zn    2Cu   Cu₂O   2Cu    Cu    SnO    Sn    PbO    Pb    2Bi
kJ/mol   206   351    80   146    169    157   297    281    58    219   3×191
=============================================================================

ZnO reduction by heat provides very pure Zn through distillation, optionally as powder or grains.

S is volatile enough to leave the heated zone.

The above table is a first approach only. The ore, intermediates or the metal must be purified. Existing processes do that cheaply and massively.

Sunlight abounds at some mines, in the Atacama or southern USA.

==========

Zn can store and distribute sunlight as Zn batteries, notably zinc-air primary batteries.

Heat from concentrated sunlight reduces recycles ZnO to pure Zn available as powder making new batteries.

Zinc-air batteries bring 1700kJ/kg, better than 500kJ/kg for lithium secondary batteries (=accumulators)
  wikipedia
This solves the range limit of electric cars. Replacing primary batteries avoids the charging time of accumulators. Discharge in 5h fits the range. Car batteries must improve the power density over button cells here.

Zinc-air batteries are still heavy for container ships. The same 64MW for 2 weeks take 45 000 t from 165 000 t deadweight.

The transport of the batteries consumes energy. Lorries, trains, boats outperform small cars.

The cycle is decently efficient: 1.35V is 74% of ZnO's ΔHf = -351kJ/kg = -1.83eV×2q while ZnO reduction exploits typically 40% of the sunlight input
  wikipedia
The 30% round-trip is better than solar cells charging accumulators, and the collecting area is cheaper per surface unit. The ZnO reduction can probably improve.

A zinc-air battery comprises other parts which must be checked, refurbished, sometimes replaced and recycled, yes.

Marc Schaefer, aka Enthalpy

[Enthalpy]

A regional airliner can fly with zinc-air batteries, as their energy density exceeds that of lithium accumulators. Evaluated for 90mn flight at 540km/h and 10mn ground movements plus 160km to an alternate landing airport and 45mn wait there.

An existing frame design fails with batteries while it can fly with hydrogen and fuel cells, see there
  scienceforums
The same 17.9GJ need 10500kg of 1700kJ/kg zinc-air batteries but the Dornier 328-100 can only take off with 14000kg.

Zinc-air batteries can improve with the size and the necessity to save mass. Casing, insulator, gaskets... can shrink. Lighter cells were demonstrated. But a used anode weighs 1.24* more than fresh.
  wikipedia

The solution is a adequate airplane design. Existing frames optimize the commercial operation with turboprop and kerosene. The example Dornier 320-100 has a lift-to-drag ratio around 10 while operational gliders have 60, and 25 still look like an optimized commercial aircraft. That would reduce the battery to 4200kg, less if the battery is optimized. Electric motors, especially if geared, save much of the 2*800kg turboprops. The frame would be designed with the battery mass in mind, typically with a wider wing. Materials improved also a lot over 40 years.

Concentrated sunlight can provide the heat that regenerates efficiently the Zn anodes from used ZnO, and the plane flies from solar energy.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Maersk agreed with Spain to produce methanol from renewables to power its ships. From wind in Galicia and sunlight in Andalusia. Nice to see some people tackling the issue seriously!
  Maersk - Reuters - Hydrogentoday

I'm less than enthusiastic about the process as described in the papers: wind or sunlight to electricity, electrolysis, react hydrogen with carbon dioxide. Each step is wasteful and may let abandon the project as soon as fossil fuels get affordable again.

Sunlight to hydrogen shouldn't pass over electricity. The Zn-ZnO and FeO/Fe₃O₄ cycles are better, as suggested here on 06 Nov 2022.

Hydrogen can propel the boats directly, as noted here on 01 Oct 2022. Fuels cells are very efficient, a combined cycle should outperform the very best engines, while the conversion of hydrogen to methanol wastes energy. Hydrogen brings hazards, but methanol too. 2 months autonomy need 4240t liquid hydrogen that fit in D=49m for a ship 59m wide and 14+73m tall.

Or if really producing it, methanol can be transformed over dimethyl ether and ethylene to alkenes or trialkylamines around C+N=12 to supply the existing engines. Sunheat can regenerate the acid or zeolite. Safer fuels, save the mass of wasteful C-O and O-H.

[Enthalpy]

Wiki cites efficiencies of 30% (demonstrator), 40% (typical) or 60% (goal) from sunlight to hydrogen over the Zn-ZnO cycle. This includes the reduction of water (liquid -286kJ/mol) by ZnO production (-351kJ/mol), which is 82% efficient at best. So the production of Zn from sunlight is 37% or 74% efficient, not 30% or 60%.

How subtle are present demonstrators? The minimum radiative loss by a black body at 1800°C is only 1%. Upscaling must reduce the conduction losses. Heat exchanges by flowing the oxygen through fresh ZnO in opposite directions improves this loss. Hot Zn can preheat ZnO, over an intermediate fluid if needed. Possible waste heat can make some electricity.

==========

Can Zn reduce CO₂ to CO and stop there, to avoid the complexity and losses of intermediate H₂?

I couldn't evaluate the reaction with Propep nor Rpa, as they surprisingly ignore ZnO as a product. But this reaction must be well known.

==========

If making hydrogen from sunlight, we could store and transport the zinc and make hydrogen where and when needed. Produce Zn in Andalucia, ship it by train or boat to some foggy airport, produce and liquefy the hydrogen shortly before filling the aircraft.

Solid Zn takes 3× less volume than hydrogen, Zn powder about 1.5×. But it's 32× as heavy as hydrogen alone, around 16× as heavy as hydrogen in its tank. Paraffin that drowns Zn powder makes it some 15% heavier and much safer.

==========

The EU and Egypt announced yesterday an principle agreement to produce hydrogen from sunlight.

Marc Schaefer, aka Enthalpy

[Enthalpy]

To get rid of fossil fuels, some people revive the old idea of hydrogen as a vector to transport and store energy, but metal cycles are better. Zn-ZnO, FeO-Fe₃O₄, there are a dozen cycles. Most are meant to produce hydrogen, which isn't necessary, so more cycles are possible.

The cycles reduce the metal at the energy source and oxidize it at the consumer. The vector

• Is safe (can store in paraffin)
• Is dense
• Converts efficiently to electricity
• Offers an efficient cycle

Here are (upper) heating powers. The bulk density of ZnO powder is estimated. Zn and H₂ convert well to electricity to run a heat pump, which doubles the heat in a house. Same advantage towards mechanical work, as engines convert 1/3 to 1/2 of the heat.

             MJ_th/kg   kg/m³   GJth/m³
======================================
Fuel oil       46       860     40
Coal           34       800     31
Liq H₂        142        71     10
Zn/ZnO          4.4    2800     12
Zeolite         0.33   1400      0.46
Molten salt     0.24   2000      0.48
======================================

Solar thermal power plants that store heat from day to night exist in Spain. Zn-ZnO outperforms 25 to 50× the molten salts in that application, both for the energy density and through a conversion twice as efficient to electricity. The unconverted energy can feed a urban heat network.

Present research wants to heat houses in winter by summer sunheat stored as desiccated zeolite. Zn-ZnO outperforms zeolite 25 to 50× in that application too. Again, easy and efficient conversion to electricity is precious. I see the hot reduction temperature as a modest hurdle.

House delivery of Zn or FeO reduced at a central plant makes sense too. Lorries and storage need 3× to 1.5× as much room as for coal: realistic, better than zeolite. Alternative to the reduction at the house in summer.

Batteries (or fuel cells) exist in labs that consume only Zn and keep the other components. The unconverted energy can heat the house directly or warm the input of the heat pump. Batteries, supercapacitors can store excess electricity when more heat is briefly sought, water does the opposite.

Marc Schaefer, aka Enthalpy

[wildfyr]

Re: production of water: Water is a big greenhouse gas... but our ability to emit water is microscopic compared to the amount already in the air. Also, per kg, water is a poor greenhouse gas compared to CO2.

I like the idea of metal redox... but like hydrogen is comes with some inherent fire risks, which are greater than hydrocarbon fuels (for instance finely divided Zn+H2O is bad news, and surface area is the name of the game for all these metal cycles)