Problems understanding the 2nd law of thermodynamics

In summary: The chillers would have to be running constantly to keep the water at 35C, and the plant would go out of business.
  • #1
Tony Hau
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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.
 
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  • #2
Tony Hau said:
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? :wink:

We don't "recycle" thermal energy; it always goes 'downhill' when left to itself.
 
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  • #3
sophiecentaur said:
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? :wink:

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.
 
  • #4
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.
 
  • #5
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.
 
  • #6
Tony Hau said:
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...

Lnewqban said:
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.
 

1. What is the second law of thermodynamics?

The second law of thermodynamics states that the total entropy of a closed system (a system that does not exchange matter or energy with its surroundings) will always increase over time. In simpler terms, it means that the natural direction of physical processes is towards disorder and randomness.

2. Why is understanding the second law of thermodynamics important?

Understanding the second law of thermodynamics is important because it helps us make sense of the world around us. It explains why hot coffee cools down, why ice melts, and why energy is not 100% efficient. It also has implications in fields such as engineering, biology, and economics.

3. How does the second law of thermodynamics relate to the concept of entropy?

Entropy is a measure of the disorder or randomness in a system. The second law of thermodynamics states that the total entropy of a closed system will always increase over time. This means that as time goes on, the disorder and randomness in a system will also increase.

4. Can the second law of thermodynamics be violated?

No, the second law of thermodynamics is a fundamental law of nature and cannot be violated. It has been tested and proven to hold true in all physical systems. However, there are some rare cases where it may appear to be violated, but upon further investigation, it can be explained by other factors.

5. How does the second law of thermodynamics apply to living organisms?

The second law of thermodynamics applies to living organisms in that they must constantly take in energy and release waste in order to maintain their complex and ordered structures. This process of energy exchange and waste production increases the overall entropy of the universe, in accordance with the second law.

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