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Comment by ragebol | Hacker News Reader

When looking at hat kind of graphs, it's good to keep in mind that burning hydrocarbons is relatively inefficient: only ~20-30% of the energy in the fuel turns into useful work, while electricity is much, much more efficient. So, we need much less of it.

Still a long ways to go regardless, but much less bleak. And also, exponentially growing deployments of wind and solar are harder to spot on this 2-year old graph which goes only up to 2024.

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That is very much a it depends. The best combined cycle power plants get around 60% efficient. The gas engine in your car is in the ideal case around 45% efficient - but the way you drive in the real world is only about 20-30% efficient.

Indeed. That exponential has renewables being ~100% of electricity in the early to mid 2030s, and approximately ~100% of all power in the late 2030s to early 2040s even without the efficiency improvement from direct electricity use.

> only ~20-30% of the energy in the fuel turns into useful work, while electricity is much, much more efficient. So, we need much less of it.

Isn't a lot of energy used for heating, not work?

Maybe, but that other 70-80% is waste - we could turn it into useful heat, but instead we vent it to the environment.

I've seen plans to run an engine in your furnace, thus creating both electric (at 20-30%, and capturing the rest as heat to the house. I've never seen this in the real world and experts who have looked as those systems said they won't enter a building that has one using that design. (it is possible to make a design they would trust, but they haven't seen any)

Yes but by a weird coincidence most of that heat can be provided by heat pumps at a similar 3-5x ratio.

(it's not a coincidence, it's the exact opposite formula. You can only get 20% out of a certain heat engine because running it in reverse would be a 500% efficient heat pump, and the closed loop can't be more than 100%. This still applies even if the engine isn't built to be able to act as a heat pump, because some other engine could.)

I don't see why that would be the case. Why is the 20% of eg. an internal combustion engine coupled to the 500% COP of a heat pump? I see a vague connection as an ICE also uses expanding gasses etc, but that's about it. What am I missing?

An ICE is not an ideal heat engine. But if you did have an ideal heat engine, it could either produce work by reducing a temperature difference, or increase a temperature difference when provided with work. No physical device can do better than an ideal heat engine.

Ah, thanks, had to be something like that indeed. Had a hard time imagining putting work into an ICE and getting anything useful out.

True. For residential heating, heat pumps make for a similar ratio as they make 3-5 units of hear for 1 unit of electric energy, depending on conditions.

The challenge in heating is industrial heat, which is much hotter. Can't do that with heat pumps (yet?)

Process heat would be a good use case for SMR nuclear if the economics turn out to be favourable (a big if, admittedly). Also potential for using green hydrogen (though the economics of that are even more iffy).

Even without those, the continued decrease in solar costs means that there's a decent chance that plain old resistive heating will become viable at some point in the 2040s.

They account for this.

> Primary energy is based on the substitution method and measured in terawatt-hours.

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