Seems about right. Pumped storage is at about 70% efficiency round-trip, this is 40%. Also, not producing greenhouse gases sounds like a good idea.

Well I think in this case the iron is feeding a furnace so it's not going through a heat engine, it's just giving off heat.

Right, conversion of iron to rust yielding heat is 100% efficient, less only the work needed to move the rust out of the way.

If you wanted to convert that heat back to electricity, there would be losses, but nowhere near as much as some people are saying. 1800 degrees minus ambient is a very big delta T. You are not bound to a closed cycle, so Carnot does not apply. So it is likely 80+% would be achievable, maybe over 65% round trip.

Yes and anyway you could use combined heath and power systems to get higher efficiency.

Not sure I follow.

Cogeneration. It's common in cogeneration for heat to be a secondary output that makes the overall system profitable, but isn't the primary output.

But in this case heat is the primary output.

Gravity storage IS subject to the same law, why do you believe it isn’t?

Burning hydrocarbons is problematic for a host of reasons besides CO2 release: fracking, oil spills, release of other pollutants, political issues with the supplier countries, pipelines, ...

Gravity storage converts work to potential energy, and back, efficiencies can be 90-almost 100%... heat engines at room temperature generally top out in the 35%ish range. Using the "waste" heat improves that, but nowhere close to 90%.

Gravity storage is not a heat engine, ergo Carnot's rule does not apply.

But the second law of thermodynamics still does. Gravity wells are still a mechanical system, just not a heat engine.

>Not even close. Second law of thermodynamics puts a limit on heat engine. Gravity storage is not subject to such limit.

Emphasis mine. Don't be a pedant.

Well, my important point was that hydrocarbons are fucking terrible but keep focusing on the first part.

Laws of thermodynamics apply to the entire universe, ergo i dont think anyone is claiming to break laws of physics

2nd law of t only applies to mass. Gravity works via gravitons which are massless so the 2nd does not apply..

Because the Carnot limit is particularly pessimistic.

70% vs 40% is a big difference, especially when you consider it from the losses column instead of the wins column. If I gave you a system that was losing 30% of a resource to replace a system that lost 60%, you've cut your losses in half.

If I told you a mechanical system could get that to 15%, you'd replace that again. Anyone who tells you they can get a heat engine down to 15% losses is a charlatan and should be reported for fraud.