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Comment by meta_AU | Hacker News Reader

Heat engines generate work (power) through the movement of heat from hot to cold. Conventional systems (steam turbines, Stirling engines, regular thermoelectric generators) require a temperature difference of much higher than 5C between hot and cold, which limits the situations they can be used in. This apparently can work at much lower temperature differences than other systems while generating 'useful' amount of work.

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I wonder if this could be used to power an electric recovery generator in a Internal Combustion Engine in hybrid applications. If my back of the envelope calculations are correct a typical 2 litre engine exhausts over 1000 litres of exhaust gasses at ~300c per minute. A fair amount of energy is lost to the atmosphere this way.

In combined cycle power plants they use the exhaust gasses of a turbine engine to power a secondary steam power plant. This obviously would work in a car if it weren't for the huge complexity and cost.

Sure,but you are judt bolting one heat engine to another (the motor). I'm assuming the efficiency of the motor is better than the TEG (generally the case), so why not just draw power from the motor instead?

This technology is probably more applicable to situations where you have a source of low quality heat and you want to extract a tiny bit of power.

Yeah, checking wikipedia the efficiency of TEGs are ~5-8% so I guess that is why this idea isn't used. It would be hard to justify the cost / weight at that limit.

Many applications don’t have a weight limit, things like generators powering remote living locations.

There's got to be more to it than that. I would suspect that energy recovered over the service life isn't enough to more than break even.

If up front cost and/or weight/packaging were the issue you'd see them employed at least occasionally in marine/rail/off highway/stationary industrial applications where physical trade-offs aren't as big of an issue and the up front cost is can be more easily amortized over the long service life of the equipment it's tacked on to.

An internal combustion engine relies more on the heat from the chemical reaction than the heat of the engine block. And the hot gasses are just blown out the exhaust pipe. It isn't a closed cycle.

Bolting a heat engine to the internal combustion means that the excess heat from the engine block and the exhaust gases can do some work before convecting, diffusing, or radiating away. The usual problem is not the additional weight of the heat engine part, but the enormous radiator you would need to maintain a proper cold well. This would likely be a ribbed (finned) aluminum plate covering the entire underside of the car, with scoops and fans to ensure sufficient airflow across it.

The combustion engine part could then be redesigned to produce higher temperatures, as the heat engine portion can be actively driven if necessary to cool the engine block--or to warm it, as might be needed for diesel startup.

Combustion engines don't rely on heat so much as pressure (can be viewed as the same thing in some models).

There's no need to have "excess" heat in an engine.

PV = nRT

In a fixed volume (the cylinder, on the time-scale of ignition), pressure is proportional to temperature.

Higher temperature in the cylinder bleeds more heat into the engine block, but also produces more force on the pistons.

Nitrous oxide systems do this, at risk of overheating the engine. If you were to actively drive a Stirling integrated into that engine, it would actively cool the engine, forcing its heat into the cold well. You would overheat your oversized radiator, instead of your engine.

The typical way to extract power from exhaust gas heat is to add a turbo.

And in Formula 1 now this is done "completely" with the waste heat recovery units. Basically, a normal turbo frequently generates more boost pressure than the engine can use, so the recovered power is just wasted again through a wastegate. But in F1, they've managed to build very long turbo shafts, so they can be coupled to an electric motor, and thus remove excess boost by generating electricity.

Formula 1 cars use a much simpler method to recover energy from exhaust gases - a generator attached to the turbocharger.

Aircraft engines had an "simpler solution" to this problem, they geared the output of the turbo charger down to the crankshaft speed and directly recovered the power into mechanical energy.

This was known as a turbo compound engine: https://en.wikipedia.org/wiki/Turbo-compound_engine

These improved fuel efficiency by 15-35%. It's actually the reason my mind jumped to recovering energy from exhaust gasses. Doing this directly from heat would be elegant if it could ever be made cost effective.

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