Hello, Energy 101. We're talking about laws of nature
controlling the energy conversion. And we're deep into the second law of
thermodynamics, or the second law of energy conversion.
We've talked about energy quality, which is really a result of the second law.
But this is probably the most difficult aspect of this whole course energy 101
but it is one that I feel feel strongly about that we have to talk about because
it's it's the one people don't understand when they come to me about new
technologies the processes they feel they have come up with That will solve our
energy crisis because they don't understand it violates the second law of
thermal dynamics most frequently. The first law a lot of time, most of us
have some concept of that and that's fairly easy to grasp.
The second law being a negative statement we have talked about is more difficult.
Stay with me here and we will this is the last one on the laws of energy
conservation, first and second law. [COUGH] But I want to give you a little
bit of feel for those [COUGH] excuse me. I want to give you a little bit of feel
for those, so that you'll know when I say What you can and can't do from the laws
of nature you will understand where it's coming from.
and of course this is the law of controlling the conversion of energy from
input when we convert it to a different form on the at the output.
And you've seen this before. we went through this example about energy
quality, which is inherent in the second law that I'll, we would need to talk
about again today. But just showing that example quickly,
reviewing it. if we have waste heat or any heat that's
available at 240 degrees Fahrenheit. Then
the quality we can calculate of that thermal energy comes out to be between
.29. That means it has a value per unit of
energy of approximately less than 30% less than what work at that unit of work
would have, or unit of electricity, or unit of potential energy, or unit of
kinetic energy. All of what had a quality 1.
So we're not going to pay as much for it. Because it's not only as lower value from
the thermodynamic sense. That means it has lower economic value.
It, it filters through the system. So that's the example that we had.
And the second law, as I briefly put it up last time, tells us that the.
Fraction of the thermal energy Q that can be converted to work has to be less than
the quality of that work. So since the quality of 240 degree
fahrenheit thermal energy is 29 percent that means that the fraction of the
thermal energy produced by whatever is producing the 240 degree fahrenheit
Thermal temperature that can be converted to work is less than 30%.
Now, that's the ceiling. Notice, you can't get 29% out, because
the real world has friction in it. We have temperature drops that,
transferring in through heat exchangers, and.
And and flued flow through pipe friction that we have to pump it that it's not
reverseable. So what the second law does is put a
ceiling on it it doesn't say that we can do that well, it says we can not do
Better than that in the real world we actually cant do that well because in
real world iriversablity we call it. So that's what the second law tells us,
that with thermal energy is not of equal value to work and when will convert
thermal energy to work, we cannot convert 100% of it.
And that's what I'm showing graphically right here.
So, here, we, we've got thermal energy at some temperature coming in.
Where some quality over on the left hand side.
And I show a theoretical process that upgrades that.
Thermal energy, quality is...is the vertical scale here.
The higher up we are, the higher the quality is.
And the second law I've done an X through it, saying you cannot take that 1 unit of
thermal energy And converted to 1 unit of energy with the quality of 1, like
electricity or work. Now, and this is so many times I read it
in...in the Press, or see it on the national news The fact that why are we
wasting all of this energy at a power plant.
Why aren't we using all of this heat, thermal energy that we're paying for, and
coal and natural gas And nuclear thermal energy.
Why are we taking that thermal energy, and only converting a fraction of it into
electricity, and then throwing the west of the, of it away to the atmosphere or
the rivers? Well, it's because the second law tells us we cannot do that.
Why can't we do that? Why does gravity pull down? I don't know.
But it's a hund-, we've known it for 150 years.
When people say that. Well, they found a way around the second
law of thermodynamics. And I tell them they got to go talk to
somebody else. Because I firmly believe in the second
law of thermodynamics. It served me well.
And, and has used it to design many, many energy systems of all types, and new, new
technologies, and it always works. It's worked for 150 years, and we don't
expect to see that happening, like we don't expect to see the laws of gravity
repealed. so that's a statement of the second law.
and so but but so you say how do we get thermal energy to produce, converted to
electricity. Well it what the second law does allow is
upgrade Upgrading some of the thermal or heat energy while downgrading the
remainder, which is exactly what a power plant does.
So, here again, a similar diagram where the higher quality Is on the vertical
scale here. And we have combustion energy or nuclear
reaction from the uranium that produces thermal energy.
And we put it into our power plant. I was taking an example here of 3 units
of energy, or 3 BTUs, 3 kilojoules. Take whatever unit you want.
taking BTUs here, could be a million BTUs.
And we put it through a power system. This is a coal fired power plant.
It's a natural gas fired power plant. It's a nuclear power plant, that, is
taking that thermal energy. We do produce about 1.
Unit of electricity out of three units of thermal energy to electricity what
happens to the rest of it? The rest of it we have to downgrade.
Now we can't get rid of the energy we can't destroy energy so we have to throw
it away and we throw it away to the river generally or to the atmosphere.
More and more we are throwing it away to the air than the river because beating
the rivers up Damages the the biological growth in the river and kills fish etc.
It has a zero quality so people say why don't we utilize this energy the same
reason we don't utilize all the thermal energy in the atmosphere because it has
no quality, it has no value. But this is what the second law does
allow That's the way if we downgrade some we can upgrade others.
And and the limit of what percent we can upgrade is the quality of this energy.
Before if this was 240 degrees fharenheit We could upgrade theoretically as much as
29 percent of that, and throw away the other 71 percent.
But that's the limit that the second law places on us, and the real world Pr-
processes. We can't do that well.
We try to approach it, and we spend more money on, on the power plants.
We get, we can get a little closer to it. But that's what the second law of
thermodynamics does. And it, I, I've talked about power
plants. But it's also true for auto engines, 'cuz
they take in combustion heat. And produce work to drive our rear
wheels. Diesel engines fall into this category,
and power plants. All 3 of those kinds of energy
conversions systems, convert thermal energy to work or electricity.
And the second law limits what fraction of that thermal energy we can convert.
That's just the way life is. And may not be the way we like it.
But that's the way the natural laws have worked out.
And that that's what's been given us. So, thank you.
That's noticed that, talked about the second law of thermodynamics.
And never talked about something called enthalpy.
because enthropy is a very abstract quantity and, and property and you don't
need to know anything about it unless you get deep into the theoretical
calculations of thermodynamics that we don't need to deal with in this course.
Thank you.