Problems understanding the 2nd law of thermodynamics

[Tony Hau]

So my professor says that the implication of 2nd law of thermodynamics is that high quality energy will be degraded into low quality energy. By high quality energy he means something like coal or fuel. By low quality energy he means something like heat entering the cold resevoir in a heat engine. The logic is that u must need to pump some heat into the cold resevoir for a heat engine. But I am thinking why can't we recycle the heat entering the cold resevoir so that eventually all heat input can be used to give work done, for example by connecting a even cooler resevoir to it.

 

[sophiecentaur]

for example by connecting a even cooler resevoir to it.

And. how do you propose to keep that colder cold sink cold? In the context of heat engines your cold sink temperature is the best you can obtain. Cooling it further would involve a heat pump and where woulds that get you?

We don't "recycle" thermal energy; it always goes 'downhill' when left to itself.

 

[Tony Hau]

And. how do you propose to keep that colder cold sink cold? In the context of heat engines your cold sink temperature is the best you can obtain. Cooling it further would involve a heat pump and where woulds that get you?

We don't "recycle" thermal energy; it always goes 'downhill' when left to itself.

You must put in some work to convert it back to higher temperature. I think in this sense, you are burning more and more fuel for this conversion. So high grade energy must go to low grade energy.

 

[Tony Hau]

Even if I supply a work into the heat pump so that the the amount of heat is pumped back into the hot resevoir, fuel is burnt eventually.

 

[Lnewqban]

This is an example of "recovering" some heat for useful purposes:

https://en.wikipedia.org/wiki/Cogeneration

You can convert most of mechanical energy into heat, but the opposite is harder to achieve.
Therefore, achieving a cold source that is not naturally cold or cold enough, requires the consumption of energy and a process that is less that 100% efficient.

 

[gmax137]

for example by connecting a even cooler resevoir to it.

The cold reservoir is generally at environmental conditions. So for a terrestrial heat engine, something above 0C. I worked at a plant in Florida where the cooling water to the condenser would approach 35C in the summertime. When all the customers were running their air conditioners...

Therefore, achieving a cold source that is not naturally cold or cold enough, requires the consumption of energy and a process that is less that 100% efficient.

Are you guys talking about chilling the condenser cooling water? While the chillers would be "less than 100% efficient" -- overall it's a negative, the plant output would go down not up.