It’s been more than 200 years since the widespread implementation of the steam engine.
Why is this still the most prevalent means of producing electricity? With things like fusion reactors, why is it so hard to convert the thermal energy into electrical energy?
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The following answer assumes Alternating Current (AC) generation; Direct Current (DC) is unusual (other than solar power) and different.
The reason for steam is actually a little more fundamental. Since energy must be passed into a turbine to generate AC and convert heat into electricity, it makes sense to make as much electricity as possible from the same amount of heat by using high energy processes.
The highest known efficiency process to drive a turbine is to use a physical property of matter known as a “supercritical fluid”, whereby a substance has properties of both a liquid AND a gas. In the case of power generation, the liquid state is dense (so carries a lot of energy – think of how much energy it takes to heat a kettle of water to boiling point) and isn’t really compressible. However, it pools at the bottom of systems, and would create compressible gas bubbles while running through a turbine which would reduce efficiency. The gas phase flows easily, fills all available space and will not form any bubbles (since bubbles are also gas). If water is heated to more than 374C and pressurised to at least 218 atmospheres (the “critical” conditions) it becomes supercritical and is dense, incompressible and has energy capacity like liquid, but flows and fills all space like gas.
This is the optimum substance and state to carry large amounts of energy into a turbine, turning it without compression causing efficiency losses, while being easy to handle. Using supercritical steam, generation efficiency greater than any other (readily achievable) method is possible.
As others have noted, regardless of the source of heat (burning coal, fusion reactor, etc.) all while society uses AC electricity, supercritical steam turbines are likely to be used. We simply haven’t found a more efficient way to generate electricity yet…
TLDR: steam in turbines is supercritical, which provides physical properties that, at time of writing, offer the most efficient way to generate electricity.
Source: I’m a doctor of physics, working at a company making power supplies to switch ITER on.
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