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Large steam engines seem to get pretty good efficiency, but there are other options such as combustion turbines.

Ignoring all other considerations, what methodology can extract the most energy out of a given amount of fuel?

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    $\begingroup$ Is there an underlying problem that you are trying to solve here? $\endgroup$ – EnergyNumbers Nov 11 '16 at 9:15
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    $\begingroup$ Depends a lot on your definition of "fuel." Mass - to - energy via annihilation will give you a buttload of energy to start even if you settle for the low -efficiency of the Carnot cycle after that :-) $\endgroup$ – Carl Witthoft Nov 11 '16 at 13:58
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    $\begingroup$ @CarlWitthoft that also depends a lot on your definition of "efficiency". Such processes are typically about 2% efficient in terms of potential energy -> electricity. $\endgroup$ – EnergyNumbers Nov 13 '16 at 10:20
  • $\begingroup$ > Is there an underlying problem that you are trying to solve here? – EnergyNumbers 2 days ago Not as such. I admit it is a little open-ended. Mostly I'm looking for general information about what types of technology perform well in this class. $\endgroup$ – Alex Davies Nov 14 '16 at 2:39
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In theory, direct chemical to electrical energy offers the highest efficiency: it can deliver arbitrarily close to 100% efficiency. However, to date, fuel cells haven't delivered on anything close to that promise.

In theory, the worst way to do it, in efficiency terms, is to go via heat - e.g. by burning the fuel. That throws away a lot of the exergy (the ability of the fuel to do work) straight away. The amount that's irretrievably thrown away is given by Carnot's Law, and is determined by the absolute temperatures of the combusting fuel, and of the sink where the cooling happens.

The hotter the combustion, and the colder the sink, the more efficient the process can be.

The best-in-class combined-cycle gas turbines (CCGT) deliver around 60% efficiency, by combining a steam turbine with a gas turbine. And to date, that's about as good as it gets for fuel consumption.

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We need to go directly from chemical energy to electrical energy. Electrical energy can be attained by accelerating electrically charged particles through a coil or magnetic field, which causes electromotive force (voltage) and electrical current through a load to do work to neutralize the voltage. The chemical reaction of fuel with oxygen is known as oxidation reduction and is exothermic. That means that the end products of the reaction are more stable in terms of the resulting molecule's outermost electron sub-orbitals and pairing of electron spin defined by Pauli's exclusion principle and quantum mechanics.

Since the end product is more stable, energy is released (exothermic). This energy is manifested as photon emission as the electrons settle into the lower energy level orbitals, and as increase of the average kinetic energy of the molecules (heat) in this exhaust.

According to Boyle's law, the increased temperature of the exhaust gas causes expansion of said gas to fill a greater volume (less dense). This thermal expansion of the exhaust gas is what drives the turbines in a gas turbine electric power generation setup, jet engine, rocket engine, and what pushes the pistons inside the cylinders in an internal combustion engine.

Now, the trick to converting directly to electrical energy is to ionize the molecules of the exhaust gas at the point of combustion and keep these molecules ionized as they accelerate through a magnetic field producing electrical energy.

The efficiency of this setup would be determined by how much of the exhaust gas can be ionized and how much of the thermal energy of this ionized exhaust could be extracted. As the charged gas goes through the magnetic field it would slow down (cool) and/or possibly lose charge. The greater the temperature difference between the point of combustion and the end of the magnetic field pipe, the greater the efficiency.

We are converting the thermal kinetic energy of the ions directly to electrical energy.

Or, we can extend (stretch) the entire flame, or ionized process of combustion through the entire insulated tube which would be surrounded by magnetic field.

A straight linear tube would avoid x-ray photon emission caused by rotational acceleration if the tube were curved, but this x-ray emission could be useful in keeping the gas ionized. So a circular or spiraling setup may also work.

Also, a pulsed gas setup may work better than a continual stream. The frequency of pulsation could be fine tuned to the physics of the system for optimal efficiency.

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Scale matters as does the type of fuel. Steam power, in general, has the advantage that the boiler is physically separate from the working fluid and from the working parts of the engine itself and you can design the combustion and heat exchange process independently from the engine. So steam power lends itself well to using solid fuels which burn relatively slowly and leave a solid ash residue such as wood, coal and waste biomass and of course nuclear reactors.

Also steam power, using water as a working fluid with phase change means that there are lots of opportunities to recover waste heat as the working fluid is recirculated rather than being dumped to the atmosphere.

Gas turbines are typically a bit less efficient but more flexible as they can achieve peak efficiency at smaller scales so in a grid system steam turbines may be used for the ongoing base load and gas turbines to pick up peak loads.

Obviously gas turbines also require specific types of fuel.

Of course in the real world pure thermal efficiency matters less than economical considerations of fuel cost and availability, capital cost of plant and legislative constraints such as subsidies and penalties.

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