Hacker News Reader

Comment by walnutclosefarm | Hacker News Reader

Recovered CO2 if it's recovered from the atmosphere. Essentially all concentrated CO2 today is recovered from fossil fuel consumption, and while turning that into propane and then burning the propane (in a ship or home furnace, e.g.) would get double duty out of the carbon, it would still release it into the atmosphere. So, the real question is the combined efficiency of carbon capture from the atmosphere + catalytics propane production followed by propane combustion. It may pencil out for processes that require combustion heat, or where the portability of propane vs electricity are a huge win. Maybe as a storage medium for electricity production between renewables. All depends on numbers.

View on HN

It’s kind of annoying that this is always brought up as a gotcha. Direct air CO2 capture is not a massive contributor to the total energy usage. It’s still dominated by electrolysis itself. The numbers aren’t that hard to find, either. A kilogram of propane has a specific energy of 50MJ and emits 3kg of CO2. Assuming this electrolysis is 50% efficient, that requires 100MJ to make. CO2 direct air capture is about 4.3MJ/kgCO2, or about 13MJ/kgPropane compared to the 100MJ/kgpropane of electrolysis. So it’s still a pretty small fraction of the total energy costs.

EDIT: note that burning that propane in a cheap generator is only gonna net you 10MJ/kg of electricity, maybe 17MJ/kg in a large expensive generator. So the roundtrip efficiency is just 9-15% efficient. But it potentially saves you a LOT in storage costs if you’re only cycling this storage once or twice a year.

(Note that propane is a great way to store hydrocarbons as the pressure is low but it’s self pressurizing and thus it doesn’t get water ingress or have any of the storage difficulties of gasoline and diesel, which last only 3-6 months or 6-12 months respectively. It’s also very clean burning compared to those two.)

I wasn't looking for a "gotcha." Just saying that you need to consider all the input costs to know how this pencils out.

50% is almost exactly the paper's claimed efficiency for the lab cell, so it's a reasonable number.

Overall, I'm very enthusiastic electrocatalytic methods for producing hydrocarbons as a combustion fuel source for applications where direct electric technologies are not feasible. I'd much rather seen money and energy going into making something like this work, than all the effort on hydrogen. Propane, or any hydrocarbon in the 3C-8C range, is a way better fuel for any fossil fuel replacement energy system than hydrogen.

If you capture the CO2 produced when burning the propane, you can get more than double duty. If you used this process to store energy into propane when electricity is cheap, and then produce electricity by burning propane when electricity is expensive, you could potentially have a system with close to 0 CO2 emission. Compared to batteries, storing propane and CO2 even for months looks very cheap.

Organic reactions have a very low efficiency. Most of them only less than a 50%. Batteries have a better efficiency in the conversion.

Humanity uses around 25 000 TWh of electric energy yearly.

With batteries storing that is technically possible but it would take the whole world decades to build the required infrastructure.

With propane or other similar hydrocarbons it's around 2 million tonnes. 4 million if you account for 50% efficiency of turbines (we have better ones BTW).

4 million tonnes of gas seems like a lot, but currently USA has about 5850 bcf (1.6*10^14 liters) of underground gas storage ready, at 1.8 kg per m3 you could store about 300 million tonnes of propane. Enough to power the electricity grid of the whole world for 75 years.

So it's a choice between spending billions and turning our whole industrial output to it for years - or just using a fraction of what's already there in a slightly different way :)

Another point is - once you have one kind of hydrocarbons - you can burn them in adapted ICEs or transform into other hydrocarbons to be able to use existing cars. Suddenly you can continue to use the whole infrastructure we built in the last 100 years as if nothing happened with net 0 carbon footprint.

It's the only thing that makes sense, really.

I think that you slipped some decimal places.

Propane has a thermal energy content of 13778 watt-hours per kilogram [1]. That's (25000 * 10^12) / 13778 = 1,814,486,863,115 kilograms, or 1.8 billion tons. 3.6 billion tons of propane if you recover electricity at 50% efficiency. That would make the 300 million ton underground storage equivalent to one month of global electricity demand. That's still a lot of storage, of course.

[1] https://en.wikipedia.org/wiki/Energy_density#In_chemical_rea...

Thanks, should have double checked. I always mess up long and short scale when combining from different sources.

Yeah, it's going to be a long time before we can get completely away from hydrocarbon fuel. There's nothing that comes close for energy density for something that is practical to use. (Hydrogen beats it considerably but comes with a million headaches. There's a reason SpaceX doesn't use hydrolox engines despite their considerable performance advantage!)

Thus we should be looking for efficient ways of turning clean energy into hydrocarbon fuel.

At some point, cheap trumps efficient.

Propane is easy to store, easy to transport, propane storage tanks are cheaper than batteries, require no high-tech manufacturing or rare earth elements, and I'd guess the energy storage density is higher.

Many (most) existing cars could be converted to run on propane and the engines will last longer and emissions will be lower as it's a cleaner-burning fuel.

Some newer battery cell chemistries are using very very little rare earth elements

A propane tank is basically just the hollow shell of a battery, made of even cheaper mild steel since weight is far less of a concern. There's no contest in this department.

Not that I'm in favor of combustion engines persisting.

I’m not saying your arguments don’t hold. Just that the calculations may be closer than stated

Sure, and that is an option if you're using propane as storage for renewable electricity, or for large scale industrial uses like cement production. You won't get to full recycle, but you can get close. It's not an option for propane as transportation or heating fuel, however.

Ah, a perpetual motion machine.

This assumes that co2 recapture and propane synthesis require less energy than is produced by burning propane.

This is maybe a workload for excess solar, making renewable propane with electric that would otherwise be wasted

I think the suggestion is that it's a battery with low long term storage costs, not that it's 100% efficient round trip.

Reply on news.ycombinator.com