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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Water is cheap and abundant, and easy to transport (eg. river). Heat it to make steam and you’ve got a compressed gas.
Just release the steam through a turbine and make electricity. Its actually quite efficient and all you need is a steady supply of fuel, like natural gas or coal.
Steam power has come a long way. The difference between a boiler from 2000 and a boiler from the 1800 is vast
It’s just really, really, really efficient. We haven’t found a better way to turn heat into electricity. Notably, it depends on gravity, and thermoelectric devices which work outside of gravity are much less powerful than simply steam rising through a turbine to spin electromagnets.
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.
You guys are mostly missing the fact that water-to-steam and back is nearly 99% efficiency, it is also linear to a point and predictable so that we have tables to describe it. So, for every BTU of nat gas you burn to make steam, 99% of it is retained. Steam will also carry more BTUs more efficiently than water and is self motive at pressure so that you don’t need to further waste energy to pump it around. There are geothermal processes out there using isopentane instead of water, to basically do the same thing, but it is highly volatile.
water is a VERY EFFICIENT compound for expansion and contraction, and those actions when harnessed for rotational purposes are the best way we know to make alternating current with magnets. This means that even nuclear power is harnessed to heat water past the boiling point, to make a directed force much the same as a steam engine, BUT! the important thing there is that the heat is not produced through combustion, and does not release hydrocarbons. A nuclear plant today, in both the 20th and 21st centuries is a very advanced steam turbine… A nuclear plant in the far future may potentially be concentrated solar panels, directly absorbing the radiated power as electrons into the wires, without the water turning thermal energy into expansive thermal energy as an intermediate step.
It’s hard to turn thermal energy into electricity because heat is kind of the lowest form of energy. It’s hard to contain, even harder to direct/transport, and everything else has a tendency to turn into heat, not the other way around.
You could try to use a FISSION reactor (they haven’t really got fusion to work reliably, yet) to heat up one end of a bunch of thermocouples, but that would be: A) really inefficient, and B) a pain to make AC current, which you want if you are transmitting the electricity any significant distance.
Using heat to boil water, and using the steam to turn a turbine, though? That’s a GREAT way to make electricity. Got some nuclear fuel? Stick it in a tank and heat the water for steam. Live somewhere with a lot of coal? Burn it to make steam! Got some good digging equipment? Dig a hole deep enough so it gets hot, pump some water down in a pipe, and use the resulting steam.
Also, using steam to turn a generator is easier to put in a location of your choice, while other methods of electrical production (wind, solar, hydroelectric) are more area specific (can’t build a hydroelectric dam if you don’t have a river)
The most efficient way to create electrical energy (currently) is via mechanical generation. So how do we get that mechanical energy? We could use flowing water. But what if you don’t have enough flow or the ability to create water storage (i.e. a lake) to allow you to continuously generate electricity? We create mechanical energy and boiling water to create steam is an effective (relatively safe) way to do that. That being said, open system piston based steam engines aren’t really a thing anymore and everything uses much more compact turbines in a closed system which theoretically never needs to have water added as the water you boil is pure and has no impurities to lose and therefor condenses with zero loss.
I believe it has something to do with the phase changes of the working fluid. And water, despite being a common substance, has some unique properties in that regard. So if you use both steam generation and condensation, it offers surprisingly good efficiency as far as heat engines go. An internal combustion engine like that on a car might be something under 25% efficiency at best, but steam turbines with the right setup approach the 70% to 80% range. That’s not bad at all for turning heat from differential temperatures into kinetic energy stored in some large rotating mass. (Which then of course is generally used to drive some generator.)
Maybe there are better explanations than that (somebody could whip out a Mollier diagram and get into enthalpy and all that), but that’s my take on it.
mostly because its really rather effective.
water has a high thermal capacity, nice phase change temperatures well within the ranges of building materials, is not corrosive or harmful if spilled, is in plentiful supply.
we dont do anything different, because its just the best method in most cases
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