# Heat engines can organize thermal energy

• I
Hello,

I am reviewing some thermodynamics and heat engines. Heat engines are cyclical machines able to convert a portion (just a portion) of the thermal energy extracted from a hot reservoir into mechanical work. The residual thermal energy must dump into a lower temperature reservoir.

Thermal energy is a random type of energy but I heard heat engines defined as machine that can convert the chaotic thermal energy into a more organized form. What does that exactly mean? What type of order are the engines imparting to thermal energy?

Thanks!

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jbriggs444
Homework Helper
2019 Award
Thermal energy is a random type of energy but I heard heat engines defined as machine that can convert the chaotic thermal energy into a more organized form. What does that exactly mean? What type of order are the engines imparting to thermal energy?
If you have a hot reservoir over here and a cold reservoir over there, that is a sort of organization. There is orderliness in keeping your hot molecules hot and your cold molecules cold. The second law of thermodynamics says (in a vague and hand wavy manner of speaking) that orderliness never increases. You can start with a hot reservoir and a cold reservoir and end up with two medium temperature reservoirs.

The second law of thermodynamics forbids the reverse. You cannot start with two medium temperature reservoirs and end with a hot reservoir and a cold reservoir. That would increase order. The only loophole is if you increase disorder somewhere else by at least as much.

A heat engine does not allow you to create order in thermal energy. It allows you to harvest the order that is already there and make use of it. It's part of the "increase disorder somewhere else" part of the operation.

For instance, a heat engine might provide mechanical energy that could lift a book from the floor and place it on a table. Or it could generate the electricity that runs a refrigerator that heats up one reservoir (the coils) and cools down a different reservoir (your ice cream).

russ_watters
Ok, so while the energy in the universe is constant (some systems absorb the energy that some other systems generate), the overall entropy of the universe is increasing instead in the sense that the places where disorder prevail over the places where order is created.

jbriggs444
Homework Helper
2019 Award
Ok, so while the energy in the universe is constant (some systems absorb the energy that some other systems generate), the overall entropy of the universe is increasing instead in the sense that the places where disorder prevail over the places where order is created.
Right. Though it's not quite right to think that order is created anywhere. It can be piped in from somewhere it already exists. The wires providing power to your house pipe in ordered energy. The windows and walls leak out disordered energy.

anorlunda
Ok. Very interesting. But when create an object (its parts are highly organized), aren't we actually creating order? It does not really seem we are transferring order from anywhere...

On heat engines: I read somewhere that steam engines are the most efficient. Is that still true, even compared to nuclear energy or other processes?

DrClaude
Mentor
Ok. Very interesting. But when create an object (its parts are highly organized), aren't we actually creating order? It does not really seem we are transferring order from anywhere...
You are also very far out of equilibrium! Ultimately, all the "ordering" on Earth is vastly compensated by the vast amounts of entropy created by the Sun.

On heat engines: I read somewhere that steam engines are the most efficient. Is that still true, even compared to nuclear energy or other processes?
You'll have to provide a better source than "I read somewhere". What do you mean by steam engine? (Since a nuclear power plant is a steam engine.) And what are you accounting for in the calculation?

russ_watters
Mentor
On heat engines: I read somewhere that steam engines are the most efficient. Is that still true, even compared to nuclear energy or other processes?
This is confusingly put, so if you are referring to the generation of electricity, you may be unaware that nuclear reactors are just fancy ways to boil water. They still use steam engines to spin the generators.

Hello everyone

We can perhaps make a response from Normand .......
Steam engines (turbines) are one of the best recoveries of thermal energy in general. Especially in thermal power plants for the manufacture of electricity.
Traction engines (cars, trucks, boats, planes ...) have other imperatives. Weight and bulk. But have other advantages for the production of electricity. As a backup source for example.
But a Normand must not be exhaustive.

Dylan