Thermal power sources - future tech alternative to steam

Question

Nuclear power is perhaps the most "sci fi" source of electricity we have in our world today, and yet in the end the process still comes down to using steam power to drive a dynamo. It's kind of a bummer, and in science fiction settings where humanity possesses the ability to warp space itself, a whistling steam turbine feels antiquated. Of course there are many efficient ways of making electricity today and in the future without steam: wind, hydro, solar, and tidal. But in certain contexts thermal energy might still be preferable or necessary, for example in deep space or when geothermal energy is available.

How might humans generate electricity from heat in the future, without the use of steam engines? Please make it something with a futuristic aesthetic - no Stirling engines!

For thermal sources we're talking active lava flows, on earth or elsewhere in the solar system (lava flows on Io can get up to 1,300°C), nuclear power of present or speculative future variety, and fuel sources that are abundant off-earth (methane burns at 1950 °C, hydrogren at 2111 °C). People in the future will be making energy on earth, in the vacuum of space, and on other planetary surfaces, so it is difficult for me to give a single figure for what the ambient temperature is of this power plant. Assume any ambient temperature that you like.

Answer 1

There are lots of possible options. This is not an exhaustive list, but it covers some of the main options:

Solid State Thermoelectric generators

For lower temperatures (i.e. below the melting point of solid materials), a Thermoelectric generator is your friend. The Seebeck effect is when a voltage generates over a length of material that has a temperature gradient (i.e. heat flux) across it. Currently, this has some use in small, low power devices (tens to low Kw). With future handwaved technology, it may be possible to create exotic materials with a much stronger Seebeck coefficient, hence enabling more efficient power conversion.

Brayton Cycle

This is used extensively today, but it fits the bill of not requiring boiling water. Some sort of gas is heated, expands, drives a turbine, cools, and is run through a compressor. This thermodynamic cycle is essentially a closed cycle jet engine. This is used for modern gas turbine power stations (in conjunction with a more conventional steam based cycle), but it is an efficient use of high temperature sources that work at many hundreds of degrees.

MHD Generator

If your thermal source is hot enough to (at least partially) ionise a gas into plasma, then you can use a MHD generator. The hot plasma is ducted down a pipe with a transverse magnetic field. This separates the positive ions and electrons, into two streams, extracting work from the gas/plasma and slowing it down. The separate streams can be passed over high temperature electrodes, which form the terminals of a circuit, creating a voltage difference.

Direct conversion

If your thermal source is extremely high temperature, and directly produces a stream of charged particles (perhaps some exotic fusion reaction) you may be able to directly convert your exhaust. There are a lot of different ways this can be engineered from schemes that look similar to a MHD generator, all the way up to devices that look like a particle accelerator that operates in reverse.

Exotic Photovoltaics

If the high temperature source is extraordinarily hot, it may radiate a significant quantity of blackbody radiation. Enclosing it in some sort of exotic photovoltaics could generate electricity. The "exotic" part comes in because current photovoltaics are only really suitable to harvest sunlight. You may need some exotic (i.e. handwaved) semiconductors to make them work at higher frequencies (perhaps even x-rays or gamma rays), and with efficiencies that make it worthwhile for your alternate heat source.

All of the above

You may notice that there is a bit of a hierarchy to the above systems. As temperature increases, the most efficient engineering solution changes. For even more efficiency, you can chain them together! The most efficient system may be one where, for example, the exhaust of a futuristic aneutronic fusion reactor feeds into a direct conversion generator, which exhausts a lower temperature plasma into a MHD generator, which is cooled by a molten salt loop/heat exchanger into a Brayton cycle gas turbine, which is finally topped off with a "low" temperature coolant loop through a bank of thermoelectric generators. All of this would face numerous engineering constraints such as the cost/reliability/weight/volume limits of the system.