[adrianN]

What's missing is a careful differentiation between doing what you said, storing excess energy, and approaches that are popularized, e.g. by carmakers unwilling to invest in R&D, where we keep on relying on hydrocarbons for things like transportation and heating and magically replace them with carbon neutral alternatives.

The first approach of using hydrocarbons as an energy buffer makes perfect ecological and economical sense and is most likely strictly necessary for a carbon neutral energy grid (barring battery breakthroughs), once we reach 60-70% renewable energy in our grids.

The latter approach however makes no sense at all, because we simply don't have enough space for wind and solar to feed our thirst for hydrocarbons with renewable power. The energy losses, both in the step creating the fuel, and in the step turning fuel to useful work, are just too great to fuel every car or heat every home in the world like that.

[leoedin]

> we simply don't have enough space for wind and solar to feed our thirst for hydrocarbons with renewable power.

I'm not sure if this is the case or not. To give some numbers:

Current global oil consumption is 54225 TWh, or 6.2TW [1] Current global solar panel generation capacity is 628 GW (2019) [2]. Median capacity factor maybe 25% [3] Current global wind power capacity is 650 GW [4]. Median capacity factor maybe ~37% [5]

Other renewables are unlikely to achieve the same growth as wind and solar, so can be ignored.

So we need to install 15x current renewables, times whatever the hydrocarbon creation process efficiency is, to produce all our annual oil demand.

Assuming a 50% process efficiency, we'd need to install 30x what we have now. That's a big installation, but it's not outside the realms of possibility. There's a lot of untouched desert in the world.

[1]: https://ourworldindata.org/grapher/oil-consumption-by-region... [2]: https://en.wikipedia.org/wiki/Solar_power_by_country [3]: https://emp.lbl.gov/pv-capacity-factors [4]: https://wwindea.org/blog/category/statistics/ [5]: https://energynumbers.info/uk-offshore-wind-capacity-factors

[adrianN]

Thanks for doing the math. On a global scale it does look better than the math I did for Germany.

However, I think 50% process efficiency is too high. High temperature electrolysis is about 60% efficient, but then you only have hot uncompressed hydrogen. There are a number more steps involved to get to a liquid fuel, each of them with considerable losses. I can't find the sources right now but I believe I remember something like 15% end-to-end efficiency for getting to something you can fuel your car with.

[leoedin]

Yeah, it probably is. There's not really a nice process for making liquid fuel - hydrogen isn't a great fuel really. My understanding is that the biggest problem with making a practical liquid fuel is ironically getting the carbon. Hydrogen is in water, but carbon is only really in carbon dioxide - which is at very low atmospheric concentrations and so takes an incredible amount of energy to capture.

Ammonia would be easier (ubiquitous nitrogen everywhere), but isn't a particularly nice fuel either. At least it's easier to store than hydrogen.

[jfkebwjsbx]

How big are the losses to make such a big impact as to say there is not enough space for enough solar/sun farms?

99% or what?

[adrianN]

I think you're overestimating the amount of space we have realistically available for renewable energy. At some point I did the math for Germany.

The average German needs around 144sqm of solar panels (located in Germany) to meet their primary energy demand of around 48MWh per year (taking into account average production of solar panels in Germany). Germany has a population density of around 4300sqm / person. So in the ideal case with no storage losses you need to cover more than 3% of Germany with solar panels if you want to meet primary energy demand with solar power. (Wind energy is slightly more dense in Germany, but I haven't done the math there).

Realistically you might get something like 1% of the land area without huge resistance of the local population. Probably less, which is why offshore wind is popular. 100% renewable generation is only realistic because you can safely assume that large parts of the primary energy consumption are wasted. For example internal combustion engines are at best 40% efficient. Burning things for heating also only gets you 1J of heating per Joule expended, whereas heat pumps get you 2-3J. You just can't get away with losing another factor two to generate syngas, or even more if you want liquid fuels.

Of course you can justify some inefficiencies if you're willing to transport energy from far away (say solar power from the Sahara), but that is ridiculously expensive compared to more local generation.