Energy from heat

[Whitefire]

What exactly makes it so difficult to obtain energy from what we call "heat"? It seems that by making things colder, we should get the energy back.

 

There's so much talk about global warming, and that it comes from additional energy of the sun, stopped in our atmosphere by CO2 etc.

 

On the other hand, we are always short on energy, always looking for more sources, nuclear and what not.

 

It would be wonderous if we could use the first problem to solve the other, would it not?

[iNow]

Seems simple, doesn't it? Now, if only we could snap our fingers to change our entire societal infrastructure, we'd be all set!

 

Btw... welcome to SFN, whitefire. Enjoy.

[Bignose]

What exactly makes it so difficult to obtain energy from what we call "heat"? It seems that by making things colder, we should get the energy back.

 

But, just how are you going to make things colder? Heat flows from warm to cool, and the many(, many, many, ...) times validated laws of thermodynamics show how no matter how efficiently you tap into that warm going to cool, there you'll never be able to get all the energy back. Some is always lost. Whenever you cool something down artificially, like your refrigerator for example, that takes more energy to so do than we'd get back from tapping the heat flowing back into the fridge.

[swansont]

Entropy is what you should be studying.

[YT2095]

Heat pumps are a viable technology and are indeed commercially available.

 

http://en.wikipedia.org/wiki/Heat_pump

http://en.wikipedia.org/wiki/Geothermal_heat_pump

[CaptainPanic]

Because it goes a bit far to explain all the thermodynamics, I'll just add some more keywords.

 

If I may rephrase your question "why can't we just use all heat?"

into: "why can't steam engines run at 100% efficiency, (even when we assume zero friction and a perfect design)?"

 

the wikipedia website that shows some interesting info about this is:

http://en.wikipedia.org/wiki/Carnot_cycle

 

Mr. Carnot was the first to describe the reason why... and he used "entropy" to make that description.

 

For the rest, YT2095 and swansont give more nice keywords to study.

 

What it basically comes down to: you need a temperature difference between two points, not just heat, to do something with it.

[thedarkshade]

What exactly makes it so difficult to obtain energy from what we call "heat"?

It is a very common thing to hear people say "heat is useless energy", and it actually turns out that heat indeed represent loss of energy, unless you use than energy with heat being the purpose of achieving.

 

There's so much talk about global warming, and that it comes from additional energy of the sun, stopped in our atmosphere by CO2 etc.

Well all that talk is because this is what is happening. the CO2 in air and water is the substance that regulates the temperatures, and makes it possible not to have drastic changes in temperatures between day and night since that would be harmful to life.

 

BTW, do take a look on links provided in this thread. You will find the reason why.

[Whitefire]

Btw... welcome to SFN, whitefire. Enjoy.

Thanks

Whenever you cool something down artificially, like your refrigerator for example, that takes more energy to so do than we'd get back from tapping the heat flowing back into the fridge.

 

he used "entropy" to make that description

 

Right - entropy is exactly the word I was looking for. Thank you!

 

But I don't think this fully answers my question. Don't take me wrong, I know we are unable to make "high temperature --> low temperature = energy" work.

 

I am asking why. Is there any molecular explanation for entropy?

 

Heat pumps are ok, but I think they need some pretty high temperature, don't they? Plus they are just pumps, not engines.

[YT2095]

it`s not a Molecular explanation you need, it`s a Thermodynamic explanation, Molecules are just subject to this.

 

something like "the Universe is organised towards Chaos".

I Think it`s the Second rule of thermodynamics, but could be wrong, Swansont will know though

[Mr Skeptic]

What exactly makes it so difficult to obtain energy from what we call "heat"? It seems that by making things colder, we should get the energy back.

 

Actually, heat is one of the simplest forms of energy to produce other energy from. Heat has been used to create light for thousands of years, and with the invention of the steam engine, into motion or electricity.

 

However, to extract energy from it, you need a temperature difference. There is also a limit to how efficiently you can extract energy from it based on how big the temperature difference is. If there is no temperature difference, then you can't extract energy and you have "waste" heat that can't be used.

 

There's so much talk about global warming, and that it comes from additional energy of the sun, stopped in our atmosphere by CO2 etc.

 

On the other hand, we are always short on energy, always looking for more sources, nuclear and what not.

 

It would be wonderous if we could use the first problem to solve the other, would it not?

 

It would indeed be wonderous, but also against the second law of thermodynamics.

 

I am asking why. Is there any molecular explanation for entropy?

 

Sort of. You can imagine atoms/molecules as perfectly bouncy balls. The hotter a group of these are, the faster they are moving. If the faster ones bump into slower ones, then the slower ones speed up and the faster ones slow down -- the transfer of heat from high temperature to lower temperature. The opposite won't happen. This is just one aspect of entropy, though. I hope it is what you were looking for.

[Whitefire]

The hotter a group of these are, the faster they are moving. If the faster ones bump into slower ones, then the slower ones speed up and the faster ones slow down -- the transfer of heat from high temperature to lower temperature.

 

I know this and it doesn't explain entropy. The opposite is possible - actually it's the same. What you describe is "getting warmer". The opposite would be "getting colder" and it has the same description. It would be better to call it "merging" of temperatures.

 

But is it possible to make hot atoms ("balls") influence larger structures? Or maybe using heat (faster "balls") to create movement of electrones (electricity). It should be doable.

 

Imagine a long rod of, say, copper. We leave it on a desert. In a day it is 1m long, at night 99 cm. So it does some mechanical work all by itself.

 

It's just not very efficient.

 

Btw, does any of you know anything about the relation of pressure and temperature? If I squeezed that copper rod on a desert, would it prevent it from going hotter?

[thedarkshade]

I know this and it doesn't explain entropy.

I wouldn't say that. Molecules move in a chaotic way (Brownian movement), and when heat is increased that means molecules move faster and faster, and since their movements is chaotic and is becoming faster, that means more chaos!

 

But is it possible to make hot atoms ("balls") influence larger structures? Or maybe using heat (faster "balls") to create movement of electrones (electricity). It should be doable.

I've not heard of the term "hot atoms' before, but heat is actually a result of the movements of atoms (molecules)

 

Imagine a long rod of, say, copper. We leave it on a desert. In a day it is 1m long, at night 99 cm. So it does some mechanical work all by itself.

Not entirely itself, that is under the influence of outside factors. Besides, contraction and expansion are characteristics of all objects.

 

 

Btw, does any of you know anything about the relation of pressure and temperature? If I squeezed that copper rod on a desert, would it prevent it from going hotter?

If you increase the temperature the pressure will increase.

 

http://en.wikipedia.org/wiki/Gay-Lussac%27s_law

[swansont]

I've not heard of the term "hot atoms' before, but heat is actually a result of the movements of atoms (molecules)

 

I think you mean thermal energy rather than heat, because a microwave over certainly does not rely on the movement of atoms to raise the temperature of the item placed inside.