Launch HN: Prometheus (YC W19) – Remove CO2 from Air and Turn It into Gasoline

(Not that they need it - gas diffusion is fast - but it's a cute synergy.)

What steps are in place to prevent the process from displacing other renewable use? Take the fact that it is less than 100% efficient to store renewable energy as gasoline. Others might make more efficient use of this renewable energy by immediately using it to displace fossil fuels, with no or lower storage losses. Under this scenario Prometheus is again a net carbon generator. (edit: The same dilemma is faced by any other type of storage.)

It strikes me that Prometheus will come into its own if it can guarantee that it doesn't displace more efficient users. This will happen when fossil fuel usage becomes negligible or we (as a society) come up with a sophisticated energy scheduling algorithm that can take account of efficiency.

It comes back to that fact that whilst it is green to replace fossil with renewable energy, it's even greener if we can increase efficiency and avoid consuming that energy in the first place.

Don't get me wrong, correctly used it's a great thing you are doing.

The reality is that there's almost never a practical direct use of the heat at the facility where it is created. It is too low quality for the onsite processes. Economizers and Regenerators are two common pieces of equipment that suck out what heat can be used - what's left is too low temperature for whatever they're making onsite.

Sometimes there's an opportunity to broker a deal between two adjacent facilities, where one can use the heat that the other considers waste. But those are rare.

Happy to talk about this until you wished you'd never asked. Feel free to email.

Being able to work with existing infrastructure and vehicles and other components designed to work with gas is a huge advantage. And of course the simpler storage of gas vs. hydrogen is nice too.

In terms of efficiency we can do a little back of envelope calcualtions. Prometheus converts electricity to gas at 60 kWh a gallon (33.7 kWh) or about 56% efficient. Hydrogen production via electrolysis comes in at between 44.5 - 55 kWh/kg-H2 (I've seen a wide range of estimates) for a kg of Hydrogen (33 kWh) so about 60-70% efficient.

Of course then there's the reverse process of burning gas or using a fuel cell for electricity production. The Toyota Mirai gets about 68 miles per kg of hydrogen and the roughly equivalent Toyota Camry hybrid gets about 52 mpg.

So boiling this down (if I got this right and my data is good):

Gas method: 60kWh -> 1 gallon of gas -> 52 miles = .86 miles / kWh Hydrogen method: 55 kWh -> 1 kg of h2 -> 68 miles = 1.23 miles / kWh

Of course this ignores all the challenges to building the hydrogen infrastructure, public fear (warranted or not) about hydrogen, capital costs of both systems, etc. It's also likely the hydrogen conversion gets more efficient than the 55kWh to product 1kg but wanted to keep this conservative.