How do thermal generators work?

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du hast mich
Registered Senior Member
Before you all shout at me. I did google it, and couldn't get a clear answer.

Is it just heat + water creates steam, which turns a turbine and creates electricity? or is there another way that they work?
are you asking about thermal genrators that direcly make (at low efficiency) electricity with no moving parts or the type with turbins driving an elecric generator?

To understand the first, you need to know some solid state physics, especially about the how the Fermi levels differ in different metals, etc. The second type is comparatively simple and much more efficient in the use of the heat, but still limited by the Carnot cycle limits.
Very large turbines and Stirling engines can utilize heat efficiently. Electrical induction has its own aspects for energy conversion too.
Sounds like a thermocouple, they used thermocouple to generate power from the warmth of plutonium decay. Energy is generated (albeit very inefficiently) when heat transfers through the thermocouple layer as the lutonium's warmth discipates into space. It's something like this, called a Radioisotipe Thermal Generator (RTG) that powers the voyager probes, or any probe that goes so far from the sun that solar power becomes insufficient.

I don't know much about how it works, but I do know that thermocouple makes power when heat transfers through it. You wouldn't just stick it in a steam pipe, you would apply steam to one side of it and cool outside air or water on the other side, forcing heat to transfer through the thermocouple.

But I think efficiency is only like 10-15%, hardly something you'd want to create bulk power with.
Billy T: I know how the turbine ones work, I'm more interested in the first one. I know about fermi levels - at a high school level at least.

Thermocouples sound pretty right on track, but do you know if it's possible (ignoring turbine systems) to create energy from a temperature, not from a temperature difference?

Thanks everyone
Thermocouples are subject to the Carnot restrictions, they only work on temperature differences. To extract any electrical power, you must have a circuit. And in any circuit, the characteristics of the individual junctions sum to zero _unless_ at least two of the junctions are at different temps. In a way of speaking, you have a Carnot cycle device which produces electrical energy instead of mechanical work and whose working fluid is the current itself.
IF THERE is no temp difference... then you have equilibrium... and nothing happens.

why dont you study stirling engines... really cool.. complex in operation but uniquely simple.....

they can generate work, and motion, using the heat of your hand, or a cup of warm water.... really neat... and easy to build

Thanks Kevin

Hmm, that's 2 votes for Stirling Engines, I shall look into them
Thanks Kevin

Hmm, that's 2 votes for Stirling Engines, I shall look into them
Stirling engines can be (often are) Carnot cycles - I.e. take all the heat in at one temperature (moving on the higher isotherm of closed cycle) and reject the waste heat all at one lower temperature (moving on the lower isotherm of closed cycle). As these two isotherm legs of the closed cycle are the only part of the cycle with therrmal energy transfer, they are connect by adiabatic curves to complete the Carnot cycle, the most efficient cycle possible. Real Sterling engines often closely approximate this.

However, there are two big reasons* why less efficient internal combustion engine is used: the input and output heat exchangers. If you make them economically small there is a great temperature difference or drop across both, so in practice the Sterling engine is operating on a lower temperature than the heat source and is rejecting the waste heat at a higher temperature than the "cold sink." Both factors make for a reduction in efficiency and even two small heat exchangers cost money and add weight.

The IC engine brings the fuel, which will produce the heat, inside the engine (that is were it is converted to heat and why it is called an "IC" engine) avoiding the cost and weight of the "input heat exchanger."

When in the cycle it is time to dump the waste heat, that is what is literal done. - Namely the hot gas exhaust is dumped out, never needing to transfer any heat thru the second or lower-temperature heat exchanger.

I do not know what application you are thinking of for the Sterling engine, but be prepared to pay at least 2 or 3 times more for one of power equal to an IC engine alternative, and if it is for you car, expect the large heat exchangers to add about 50% to the total weight of the car, if temperature drop across them is relatively small; or if small exchangers are used, be willing to see your gas mileage drop about in half.

Detroit is dumb; they brought you the SUV gas hog, etc, but not dumb enough to try to put a Sterling engine in your car.
*actually there are three - the "recuperator," which is sort of an internal heat exchanger/ heat storage unit, but I will not go into that detail.
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Hey Billy. Thanks for the info, I understood most of it :p

I wasn't thinking of using it in a car, but basically as an airconditioner that produces electricity instead of consuming it.
Hey Billy. Thanks for the info, I understood most of it :p

I wasn't thinking of using it in a car, but basically as an airconditioner that produces electricity instead of consuming it.

Sorry, that won't ever work. Any heat engine (which is really what an air conditioner is, since it moves heat from one place to another) is going to expend energy to do it. You could create a device that had some power source and worked as a heat engine AND an electrical generator, but then really you're just using that energy for two purposes.

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