Methane is much safer, has existing infrastructure and appliances, works in petrol cars with minor change, and seems to require less extreme conditions and complex plant so may be a viable way to take advantage of carbon sources at smaller scales.
Metal hydride seems like the only potential household scale electricity->fuel->electricity option (not that that's a necessity or even overly efficient, but an affordable complete standalone system would close the book on fossil fuels and finally end the nuclear shilling) as fuel cells, solar power, and electrolyzers can already be had for a combined price of well under electricity retail. It is also safer than ammonia (at least the heavily endothermic versions are). What makes you dismiss it entirely as a research avenue (other than being far too expensive at present)?
Batteries typically can do as much wattage in and out as you are likely to want, but 2x storage is 2x cost.
Metal hydrides can store as much hydrogen as you have bought pricy porous metal. In that way it is much like a battery, with the added complication of the fuel cell and electrolyser limiting discharge and charge rate. I.e., want more watts in, buy more electrolyser, more out buy more fuel cell, more storage more pricy metal.
The appeal of liquid fuels of various sorts is that tankage is cheap. Want more storage, buy more tank. Tank empty, call for a truck to fill it. When your tankage is full, you can fill somebody else's, and call them up to haul away when full, and pay you. They won't want to leave expensive metal or batteries with you to be charged up when you feel like it. But leaving empty tanks around is an easy choice.
I don't see this as a slam dunk. If your expensive, porous material is 10x or 100x cheaper than a battery then you fill a new niche: cheap but not negligible cost capacity with the ability to produce at arbitrarily low scale at 50% efficiency. Methane might wind up competing here, but ammonia won't by any process I know of. If hydrogen storage can hit the $1-10/kWh range then people can start thinking about individual energy independence on time scales of decades.
Ammonia is a slam dunk for utility scale if it ever hits $30/MWh, but I don't think it's so clear cut otherwise (methane may even beat it as it's not clear to me the cost of harvesting CO2 is strictly more than the cost of running at the higher tempsand pressures for ammonia). When utilities are charging some 40c/kWh and over $1/day connection fee and feed in is 5c it's almost always the correct choice to max out the panels. There are a lot of people with energy available during summer that have 30c/kWh + $400/yr + a couple hundred of fuck you money they'd be happy to spend to move it to winter and not continue feeding the ghouls who have been bilking them as well as not having their inverter turned off when there's a brownout due to mismanagement. That becomes about $300/MWh or more than an order of magnitude over what the wholesale costs are.
Any option that is safe enough not to have an excuse for the utility lobby to ban it and under 5x the current cost of methane is going to have a lot of people interested.
I am doubting that the metal hydride storage vessels will be cheaper than equivalent-capacity batteries, but I could be mistaken. Usually they are described as offering better mass-energy density than batteries.
Ammonia can be synthesized electrically from water and air by a cheaper and lower-volume process than is used today to make it from air and NG.
Methane has the unfortunate quality that, for practical transport in liquid form, it needs to be at cryogenic temperature. Ammonia stores and transports in liquid form at room temperature and not too high pressure.
But ammonia is not suitable for home use because of its toxicity. Methane is not, either, because it needs reliable cryogenic refrigeration, or extreme pressure. Likewise, probably, hydrogen.
You probably need synthetic propane for domestic storage, a more expensive proposition than others.
> Ammonia can be synthesized electrically from water and air by a cheaper and lower-volume process than is used today to make it from air and NG.
This is news to me. Can it be done with a catalyst as abundant as nickel, or is it only platinum group or similar? Does it happen in conditions as easy to make as a sabatier reactor (or even better, one of the nickel catalysed methane reactions)?
Edit: Just found the magic words Barium Zirconate proton conductor. I think it works out to $130/MWh (including hydrogen?) https://www.energy.gov/sites/default/files/2021-08/12-proton... and seems to apply to fuel cells as well. This basically obviates nuclear, and is only a factor of four (three with unwanted solar power?) off of replacing many fossil fuels if true. Why isn't it getting more attention? This is far more important than fusion.
> But ammonia is not suitable for home use because of its toxicity. Methane is not, either, because it needs reliable cryogenic refrigeration, or extreme pressure. Likewise, probably, hydrogen.
Methane stores okay in stationary pressurized metal vessels at densities on the order of 1MWh/m^3 or about 5x as much as lpg albeit with a more expensive high pressure tank. This is bulky, but viable to store behind the garage or underground or similar. It's harder to make into electricity though without 70% losses. Hydrogen doesn't really pass this test as it's about 5x again (there's probably enough room for many uses even if people need 50m^3, but a 300bar tank that won't embrittle is far too expensive).
Small (<1kg) retail metal hydride containers are currently about the price of retail batteries by the one shop I found ($8-9k for 3000L). Given it's an extremely niche industry without the benefits of mass production, I see this as very weak evidence for rather than against the hypothesis that a 20 price reduction in 10 years if you want 20x the storage is as plausible as the other things we're talking about.
Neither are much better than ammonia toxicity wise, and with methanol vs ammonia, at least ammonia makes you really really want to leave the area before it sends you blind or kills you.
