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Intro: Carnot Efficiency & the Quality of Energy

The

combined with
the Kelvin or Ideal Gas Scale led to:

Efficiency of reversible HE's such as the

In this lesson, we will show that this equation for

Carnot Efficiency

leads to the idea that energy has a quality as well as a quantity.

In other words, some Joules of energy are better or more valuable than others!

The quality or value of a Joule of energy depends on how useful it is.

We will show that work is the most useful or valuable form of energy.

We will also see that heat is more valuable when it is stored or available at a higher temperature.

Lesson 6G: page 1 of 8

In the previous lesson, we saw that the ratio of the amount of heat rejected to the cold reservoir by a heat engine to the amount of heat absorbed from the hot reservoir was equal to the ratio of the cold reservoir temperature to the hot reservoir temperature.
This is a consequence of the 2nd Law and Lord Kelvin’s Temperature scale. This equation also holds for the Rankine temperature scale, but not for the Fahrenheit or Celsius scales.
All we need to know to determine the efficiency of a reversible heat engine is the temperature of the two reservoirs with which the HE interacts.
Cool. So, what is this lesson about ?
In this lesson, we will look at the way that the efficiency of HE changes as the temperatures of the reservoirs change.
This will lead us to the startling conclusion that some Joules are better or more valuable than others.
This is sometimes called the “quality” of the energy.
Joules that are stored at a higher temperature can do more work, so they are more valuable or have a higher quality.
But Joules of work are already pure work, so they have the highest value or quality.
That gives you a road-map of what this lesson is about. So, now, let’s get started.