Unless you want to cover really significant amounts of land in solar panels wind farms, you shouldn't use synfuels unless absolutely necessary. Not only do you use half of the energy just making the fuel, all applications where you burn it have terrible efficiency compared to electric alternatives. ICEs have an efficiency of maybe 30%, heat pumps easily produce two or three Joules of heat for every joule of electricity.

We already devote really significant amounts of land to energy in the form of corn for ethanol, ~10% of total US cropland. Corn only captures 1% of incoming light vs ~20% for PV solar, so it would be a huge gain in land efficiency to replace corn ethanol with an energy equivalent amount of PV.

This is a result of corn subsidies, not a naturally economically efficient land use.

Specifically, corn subsidies and the Iowa Caucus.

That's a good point. The amount of resources we waste on bioethanol are huge.

I think creating aviation fuels from renewables is the path towards carbon-neutral aviation as one example where this tech makes sense. Shipping might be another, but I’m less familiar there.

You're completely right.

But it might (big might) be worthwhile for a transition period.

ICEs are my pet peeve, mechanical engineers keep coming up with all those ways of improving efficiency, and to be fair, in the last 30yrs they got it to be "good" but the actual % numbers are awful regardless.

Efficiencies might seem bad, but thermodynamics limits the efficiency of any heat engine (of which an ICE is one) according to the absolute temperature of the hot and cold side of an engine. This is called the "Carnot efficiency" and it's comparable to a no-friction mechanical system, or a no-resistance electrical circuit.

For an ICE, assuming combustion temperatures of 550K and assuming the radiator can cool the system to 50C, the Carnot efficiency is (1 - Tc/Th) = (1 - 333K/550K) = about 40%.

That's a hard theoretical limit that assumes all processes are reversible. Real engines have irreversible thermodynamic processes so the Carnot limit can never really be attained.

In other words, your complaint isn't with the engineers, it's with thermodynamics.

(note: ICE engines are modelled using the Otto cycle rather than the Carnot cycle, but that's a bit more complicated and leads to efficiencies that are even worse. The Carnot efficiency is an upper limit for all heat engines, including steam engines, Stirling engines, turbines, etc.)

550k is a low combustion temperature. “Modern military jet engines, like the Snecma M88, can see turbine temperatures of 2,900 °F (1,590°C).” https://en.m.wikipedia.org/wiki/Turbine_blade

That’s like ~1850k assuming 2900F was rounded.

Absolutely. But I read the comment as complaining about efficiencies of automobile engines, not modern military jet engines. The combustion temperature of gasoline is 553K.

https://hypertextbook.com/facts/2003/ShaniChristopher.shtml

https://www.quora.com/How-much-heat-does-it-take-for-gasolin...

Ignition temperature is just the point when something starts to burn, it has nothing to do with the temperature inside an engine after ignition.

https://www.quora.com/Which-engine-has-higher-temperatures-a...

I am aware of the Carnot efficiency, my issue with the engineers is that they can't make big gains anymore, exactly because of the Carnot limit.

Best way of making a gasoline car (not the engine - the car) efficient is taking the throttle of the engine out and attaching a hybrid/regenerative generator/motor to it.

The best way of making the car efficient is doing something about the fact that we use two tons of steel to move 100kg of human.

Yes, but that's a market problem ;)

Maybe the electric scooters/mopeds are the solution?

Combined cycle thermal efficiency’s are breaking 60%. https://en.m.wikipedia.org/wiki/Combined_cycle_power_plant

But, for very long term storage efficiency is not that import. If you’re buying at an average of say 1c/kWh and selling at 20c/kWh in 6 months it’s mostly a question of what your storage costs per day as well as maximum output is.

I first worked that out in response to a claim that solar thermal plants were better than PV because they can store energy as molten salt and produce power at night. What sinks that claim is solar is cheap enough that you could heat your salt with electricity from solar panels and be ahead.

At the point there is no need to use salt. You can use a referable electrically driven gas turbine and get about 70-80% round trip efficiency.

Sure, but fuels are useful and if the power is effectively free, why not make them?

Also hydrogen is not a viable long term/large scale storage mechanism.

It's complicated and expensive to store (pressure tanks or cooled liquid) and hard to contain (due to the small size it leaks out of everything).

Someone pointed me to a link where round trip efficiency was analyzed, with the claim that ammonia is better than hydrogen or methanol for energy storage. Apparently hydrogen compression takes a big hit, although I'm not clear if that's still the case if the hydrogen is burned in stationary turbines (that compressional energy could be recovered).

Perhaps this..

Ammonia—a renewable fuel made from sun, air, and water—could power the globe without carbon

https://www.sciencemag.org/news/2018/07/ammonia-renewable-fu...

Excerpt:

Ammonia's energy density by volume is nearly double that of liquid hydrogen — its primary competitor as a green alternative fuel and it is easier to ship and distribute. "You can store it, ship it, burn it, and convert it back into hydrogen and nitrogen,".

..he shows off one of the devices, about the size of a hockey puck and clad in stainless steel. Two plastic tubes on its backside feed it nitrogen gas and water, and a power cord supplies electricity. Through a third tube on its front, it silently exhales gaseous ammonia, all without the heat, pressure, and carbon emissions normally needed to make the chemical. "This is breathing nitrogen in and breathing ammonia out,".

Companies around the world already produce $60 billion worth of ammonia every year, primarily as fertilizer, and MacFarlane's gizmo may allow them to make it more efficiently and cleanly. But he has ambitions to do much more than help farmers. By converting renewable electricity into an energy-rich gas that can easily be cooled and squeezed into a liquid fuel, MacFarlane's fuel cell effectively bottles sunshine and wind, turning them into a commodity that can be shipped anywhere in the world and converted back into electricity or hydrogen gas to power fuel cell vehicles..

Hydrogen is a scalable storage technology, unlike batteries, and large projects are underway:

https://amer.mhps.com/world’s-largest-renewable-energy-stora...

You should actually read the linked article.

It's relatively sparse on technical details, but it does mention:

- Renewable hydrogen

- Compressed Air Energy Storage

- Large scale flow batteries

- Solid oxide fuel cells

Seems like hydrogen is not actually used as a storage mechanism. They convert to hydrogen on demand to power turbines.

They will be using sealed salt caverns to storage large quantities of gas cheaply.

Why are batteries not scalable?

Cost increases linearly with capacity, which is undesirable at scale. You want sublinear scaling. For example, to store a year’s worth of US electrical generation using batteries would cost approximately 500 trillion dollars.

1) Huge number, but why would you need an entire year of energy in reserve 2) If there was a desire to build that many batteries, cost would surely decrease exponentially as new mines, factories, etc. open up and competition increases.

By definition, batteries are scalable. Battery is a word that defines a method of scaling.

edit - For example; 'A battery of batteries of batteries of cells.'

Won't we have to do that anyway for air (and space) travel?

(I just made a clarification edit without looking at this reply)

Yes, but the point as mentioned in my edit is for usage of renewable energy without much modifying existing infra.