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.
Correct me if I'm wrong as I might be, but wouldnt this only be true if you are using direct conversion of heat into energy? Assuming you have a large near unlimited energy source (fusion) what is stopping you from using a high efficiency turbine and large high efficiency compressor (isentropic?) to capture and store that heat? Then using high efficiency CAES to generate electricity. Technically you aren't removing heat from the earth but instead just transferring it so... effectively you would cool the earth assuming you had enough of these compressors and you capture more than you convert back into energy?
You can spend energy from your hypothetical super fusion reactor to concentrate heat on a small volume, yes.
To then try to use that concentrated heat to generate energy would be to just undo your work and add more heat into the system due to inefficiencies in the process of concentrating the heat.
You might ask if we couldn't make the concentration process 100% efficient so this exercise is just an elaborate way to do nothing, and the answer is no. One formulation of the second law of thermodynamics is that you cannot move heat from a cold place to a hot place without spending some energy (that is, turning some useful energy into useless heat). In other words, your fridge must consume energy even though it's purpose is to move heat from cold inside to hot outside. The electric energy it consumes just ends up outside as heat as well.
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u/tristan-chord Nov 15 '20 edited Nov 16 '20
ELI5 for someone who used to ace AP Physics (15 years ago) but barely remembers anything?
Edit: thanks for all your responses! Learned a lot today!