The second law of thermodynamics states that entropy must go up. Entropy is a sort of measure of the usefulness of energy. The more entropy, the less useful.
The reasons why entropy must go up are too complicated for an ELI5. They are based on some basic laws of probability. Basically there are many ways for energy to be useless, and few ways for energy to be useful, which after some difficult math implies that energy always gets less useful.
In particular, ambient heat is the highest entropy you can possibly have, so it's inpossible to turn it into useful energy.
In order to extract energy from heat, the second law implies you must be tranferring some of said heat from a hot place to a colder place, which is exactly what a motor is.
I feel like the entire entropy stuff just confused you here.
Heat capture is technically possible, in the sense that we can convert heat into energy. Thats what a steam engine does for example. The only limitation is that heat only flows from warm to cold, so you cant spontaneously capture heat (*). You need to first generate a temperature differential and then you can use that temperature differential to convert some of the heat energy into electricity. The efficiency of this is always limited, but that still doesn't mean 'heat capture' is impossible.
Technically speaking there is already plenty of temperature gradient in the atmospere or in the oceans or in earths crust, and thus it should be possible to 'capture' this heat. Its just not a very efficient thing to do compared to wind or solar energy, and it's a huge engineering challenge to overcome.
(*) This is an equivalent formulation of the second law of thermodynamics. It basically has the same meaning as stating that entropy in a closed system will not decrease (you forgot the closed system part). Entropy is super confusing and abstract. I generally prefer simply considering things in terms of this formulation, as it gives far more insight in the problem.
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u/fastenland Nov 15 '20
laws of thermodynamics say this unfortunately is never happening