http://www.nearingzero.net (nz396.jpe)

Today’s Agenda

Thermodynamics, part 3.

Next Monday’s Agenda

Times Beach video

Fossil fuels—whatever I have time for.

Oral report grading plan.

Was the topic related to the environment? (0-3) _____

Did the talk include evidence of “science?” (0-5) _____

Was there an effort by the presenter to evaluate the evidence/situation? (0-4) _____

Was the talk organized? Did it flow logically? (0-5) _____

Was there evidence of thought on the part of the presenter? (0-5)

_____

Was the presenter organized and “enthusiastic?” (0-3) _____

Total (0-25) _____

Now back to Thermodynamics…

Physicists classify processes as reversible and irreversible.

There aren't a lot of truly reversible processes around. Letting a gas expand v e r y s l o w l y in a chamber, slowly enough that its temperature doesn't change, is an example of a reversible process.

An irreversible process would be burning a piece of paper, blowing up a stick of dynamite, or frying an egg.

Things that make processes irreversible: friction heat transfer mass transfer mixing.

Here’s a “movie” of an (almost) reversible process, and an irreversible process. Here’s a web link to the movie. Can you tell which is “forward” and which is “reverse?”

The second law of thermodynamics says that entropy increases in all irreversible processes, and remains constant in all reversible processes, so that the total entropy of a system never decreases.

That sounds very abstract, but it has a number of enormously significant consequences.

Equivalently, it says that the entropy of the universe is always increasing towards a maximum.

You can’t even get out work equal to the energy you spent. Sorry.

“Wasted” energy must be nonzero.

temperature of heat sinkefficiency = 1- .

temperature of heat source

I'm going to hit you with an equation here, but it won't hurt too bad. Starting from the second law of thermodynamics, you can show that the efficiency of an ideal heat engine is

Notice that you can have an efficiency of 1 (100% efficient) only if the temperature of the heat sink (the river, in the case of the power plant) is at absolute zero, or if the heat source (the steam) is at an infinite temperature. Neither of these ideal cases is even remotely practical.

heat in (TH)

work done

“waste” heat (TC)

Worse yet, any real engine will have other places where energy is lost (e.g. friction) so real engines have efficiencies lower than the theoretical best.

H

C

Te = 1-

T

Let's suppose our power plant uses steam (under pressure) at 700 C, which is equal to 973 K. (The equation I wrote down before requires absolute temperature in degrees Kelvin.) That's pretty hot steam.

heat dump

heat source

300Tefficiency = 1- = 1- = .692973T

Let's suppose the power plant dumps waste heat into the river at a temperature of 27 C (or 300 K). That's actually hotter than a free-flowing natural river, but not by a whole lot. Our equation says

If this power plant burns a ton of coal, about 30% is “lost” due to Physics. Another 15% or so is lost due to “friction.” About 7% is lost transmitting the electricity over power lines.We’re lucky if even half of the coal was put to “good” use.

This inefficiency is not due to poor engineering or lack of cleverness; it is a limitation built into nature which we cannot overcome.

In fact, any power plant uses mechanical parts to turn the energy of the steam into work, and all of these mechanical parts are subject to inefficiencies such as friction.This is where clever engineering can help; it can help us approach the theoretical maximum efficiency.

According to the textbook I looked in, a power plant operating at the above temperatures might have an actual efficiency of about 47%. Over half the energy it uses is wasted before it “leaves” the plant!

Let me close by reminding you of the second law of thermodynamics, which says no process is possible whose sole result is the extraction of heat from a reservoir and the conversion of it into useful work.

A machine purported to do just that is called a perpetual motion machine of the second kind; no such machine can be made. The equation for efficiency tells you how much of the energy must be wasted.

A question for you psych. people in this class: what (big, general) things motivate people?

Maybe not the most effective motivator, but rather important.

Those who claim to have invented perpetual motion machines usually claim a conspiracy of scientists or utility companies prevents them from getting their machine accepted.According to US law, all they have to do is plug their machine into the power grid (at the proper phase) and start billing the utility companies.If billions of dollars were to be made, would you neglect to plug your perpetual motion machine into the power grid?