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Comment by flakinur | Hacker News Reader

Doing quick napkin math it looks like to process a ton of CO2 it will require 1.6kW of energy, burning that amount of propane generates at 50% power plan efficiency you get ~3kW of energy generation...

8CO2+24H2O+24e→3C3H8+16O2.

Moles of CO2=1000×1000/44.01≈22726moles.

Total moles of electrons= 24/8×22726≈68178moles.

Total charge=68178×96485≈6.58×10^9C.

E=Q×V=6.58×10^9×0.8≈5.27×10^9J.

E_actual=5.27×10^9/0.91≈5.79×10^9J.

E_actual=5.79×10^9/3.6×10^6≈1608kWh.

----------------------------------

Moles of propane= 3/8×22726≈8522moles.

Mass of propane=8522×44.1≈375,670g≈376kg.

Total energy content=376×50.35≈18,932MJ.

Usable energy= 10,791/3.6≈2,997kWh.

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My napkin says you have 8C on the left and 9C on the right, so your stoichiometry is off. I haven’t tried to check any of the rest.

I’m guessing the major error is that the -0.8V is “versus reversible hydrogen electrode ”, so it’s a half-reaction and you need to fill in the correct other half reaction, and there is probably H2 involved. Then you would dig out some free energy values to see how efficient it is.

(I only read the abstract.)

Thx for the catch!

Fixed the math using chat gpt :P.

Reaction: 3CO2 + 12H2 -> C3H8 + 6O2

Number of moles of electrons (n): n = 3 * 4 = 12

Total charge (Q) in Coulombs: Q = (-395 * 1 * 100 * 3600 * 0.91 / 1000) * 12 * 96485

Voltage (V): V = -0.8

Total energy for given current density: E = Q * V

Moles of CO2 in 1 ton: n_CO2 = (1000 * 1000) / 44.01

Total charge for 1 ton of CO2: Q_total = (12 / 3) * n_CO2 * 96485

Adjusted charge considering Faradaic efficiency: Q_adjusted = Q_total * 0.91

Total energy for 1 ton of CO2: E_total = Q_adjusted * V

Convert to kWh: E_kWh = E_total * 2.778e-7 E_kWh ≈ 17707.4 kWh

https://www.engineeringtoolbox.com/co2-emission-fuels-d_1085...

Gives a specific CO2 emission for propane of 13.8 kg carbon / kWh fuel, so your numbers are at least vaguely credible. But your reaction is devoid of CO2, so talking about tons of CO2 is still odd.

> 3CO2 + 12H2 -> C3H8 + 6O2

6 O on the left, 12 on the right.

24 H on the left, 8 on the right.

That is when using propane for electricity.

I think there is a lot more value in burning propane for heating houses. Especially in places (like Europe where I live) that are already build to use gas for heating.

Especially if you consider the massive gas-storage facilities that are common here. We have 12 billion cubic meters of gas-storage here in the Netherlands, on a yearly consumption of 40 billion. That means we could use 'green propane' to load-shift about 30% of our heating from winter to summer. That would be amazing.

And if you sneeze at propane it becomes liquid (-42C). Not sure how natural gas is stored though.

Heating use is indeed very obvious, and so is transportation use. Many countries have an elaborate distribution network for LPG and fitting a car to run on LPG is also very easy.

BTW, hot air balloons also run on propane.

> Not sure how natural gas is stored though.

Mostly stored underground, but it's also shipped and stored in a liquefied state at -162C. Propane is one of the components of natural gas, so I would think any facility that is able to store natural gas should be able to store propane easily.

Burning the propane takes you back to CO2, which you then turn back into propane again and repeat? If your napkin maths is correct, and the process is net energy positive, doesn't this violate the principle of conservation of energy?

It must take at least as much energy to turn carbon dioxide into propane as you generate when you burn that propane to generate carbon dioxide.

Isn't the input electrical energy and the output of burning the propane heat? If you convert the heat back to electrical energy, you get much less than you input into the reaction.

> If your napkin maths is correct, and the process is net energy positive, doesn't this violate the principle of conservation of energy?

In theory, yes, but you need to take into account all the energy that goes into the process, which includes the catalyst, and also the actual capture of the CO2.

The way i read it, this is "just" a way of turning already captured CO2 into fuel again in an efficient way.

Still, it does seem a little bit too good that you can obtain 3 kWh of energy by spending 1.6 kWh, but i guess time will tell.

This is just an educated guess, but . . .

Burning propane produces CO2 AND water. I think the missing bit is the energy required to split the water into hydrogen and oxygen before the hydrogen is used as an input to produce the propane.

Heat pumps can create 3 - 5kWh of heat from 1kWh of electrical energy.

Correction: heat pumps pump heat from 1 place to another. As in: place where it gets pumped out of, gets colder.

Fundamentally different process from exothermal chemical reaction, or electric heating using resistive wire.

Heat pumps do not create heat, they simply move heat from one place to another, which is an entirely different beast.

Using a traditional resistive heater, you're roughly creating 1 kWh of heat by spending 1 kWh of electricity (there is some loss), but with a heat pump you're moving heat, which is also why heat pumps become much less efficient the colder the temperature as there is less heat to move, and many residential heat pumps include a regular resistive heating element as a backup.

There's a great explanatory video on how heat pumps work here : https://www.youtube.com/watch?v=7J52mDjZzto&pp=ygUgdGVjaG5vb...

Good point, thanks for the correction.

Please don't refer to power as energy or use units of power when you really mean energy, it is bad enough when this happens in more traditional media.

According to this article: https://www.rechargenews.com/energy-transition/the-amount-of...

The cost of carbon capture straight from the air is about 10GJ per Tonne (they quote 8.8 and 14 from competing sources). Which is about 2800 kWh That would put a serious damper on the efficiency, but might make it viable even when 'easy' sources of CO2 (like the exhaust of fossil power plants) are no longer available.

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