eli5: if you need to put energy to rise the temperature of the substance, why cant you generate energy by cooling something down?
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You can and do, that’s pretty much literally how we generate energy.
The way you “cool” something in a controlled way can use the term “doing work”.
Hot steam “does work” turns to colder steam/water and then generates electricity.
Hot exhaust gases in a car “do work” then turn colder and generate energy for the engine.
etc. etc.
You can. It’s the central concept for renewable energy storage via heating Salt until Molten, and thermoelectric generators. Who told you couldn’t?
Transfer of heat. You are removing energy (cooling) from one source and transferring it to another (heating). You can recover the energy (heat) you are transferring during the cooling process and utilize it.
It’s important to understand what is meant by “generate” when we talk about energy. At the most fundamental level you can’t generate energy- it can’t be created, only moved from one place to another and usually by changing its form (kinetic to electrical, or thermal to kinetic to electrical, for example).
For example, you can’t make energy from a hydroelectric dam unless there’s water above it to fall through the turbines. In order for the water to be up there, it had to be moved via evaporation from lower areas, carried by the wind, then dropped as rain on higher elevations. The energy to do that came from the sun- at no time did the dam actually “generate” energy, except in the local sense- by changing the kinetic energy of the water into a different form. We say “generate” because it’s generating energy in a form we can use directly- electricity- to power stuff, but under the hood it’s changing energy from one form to another.
To be able use energy to do any useful work, it must move from one place to another- the dam must have water flowing through it, for example. Without that motion, there’s no energy.
If you want to create usable energy from moving heat from one place to another, you have to be able to have a difference in temperature and a way to move the energy from one temperature to another. Geothermal energy is one example- using heat from the planet to boil water into superheated steam, it’s carrying energy from the hotter point through turbines or other energy-conversion systems, which generate electricity, and consequently cause the steam to cool down.
But you always have to move energy and that means you’re moving it from where there’s more energy (the reservoir behind the dam, the hot area in the earth’s crust), to where there’s less- the turbines that generate electricity. Along the way, the means of carrying the energy- usually steam- loses that energy and becomes cooler. Some of the energy is converted into electrical or mechanical power, some is lost as waste heat.
So, when you think about it, generating energy *is* “cooling something down” (when you consider “cooling” to be “reducing its available energy”). Using geothermal power takes an infinitesimal fraction of the heat away from the source, and using hydroelectric power takes energy from the system by letting the water flow.
But the energy has to flow from the higher energy state to the lower one- until both states are equal, which is what all energy in the universe seeks in the long term.
Coda: Car brakes do the same thing- by taking the kinetic energy of the vehicle and turning it into heat via friction, it reduces the velocity of the vehicle. Brakes on your car are literally just heat transfer systems.
From what I was told in (southern USA) schools, when things heat up the molecules move faster and are father apart (think of steam, or how metal expands when hot). The molecules cannot move that fast and stay apart forever (that requires energy to maintain), so they will start to slow down and come back together and that’s called “cooling down” or getting “colder”.
So I’m not sure you can harvest energy from something that’s cooling down. Steam engines harvest energy from the pressure buildup of water turning into steam (expanding), and that pressure can generate mechanical energy. In that process the steam dissipates its heat and cools back down into water.
Theoretically you could harvest energy (in my mind and limited knowledge) from something cooling down by heating up metal that expands alot, and you harvest the energy from the moving metal as it cools down and shrinks. The shrinkage/expansion is what could create mechanical movement.
Anyone feel free to correct me if I’m wrong, I’m pretty sure my high-school science wasn’t exactly top tier.
I think that you are confusing WORK and ENERGY–they use the same unit.
Generating work from a difference in heat is what is called a heat engine. This is probably the largest category of engines (outside of electrical engines) with which you are familiar. Internal combustion (automobile), steam turbines, jet engines, rocket engines, etc are all examples of heat engines. They generate work from a difference in temperatures.
You theoretically can, but heat is not useful energy to do work. If you’ve heard of the second law of thermodynamics (over time entropy tends to increase) entropy is a measure of how much energy in a system is no longer useful to do work. If we could capture that energy and use it, that would be great, but we can’t. That’s why when talking about things “losing energy” it’s usually lost as heat.
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