As a society and individually, we use energy every moment of our lives to improve our quality of life. Energy 101 will develop the big picture and connect the details of our energy use, technology, infrastructure, impact, and future.

Loading...

來自 Georgia Institute of Technology 的課程

能源 101：概述

388 個評分

As a society and individually, we use energy every moment of our lives to improve our quality of life. Energy 101 will develop the big picture and connect the details of our energy use, technology, infrastructure, impact, and future.

從本節課中

Energy Conversion

This module concentrates on topics in Energy Conversion

- Dr. Sam SheltonProfessor Emeritus

Woodruff School of Mechanical Engineering - Dr. Richard A. SimmonsSenior Research Engineer & Fellow

Strategic Energy Institute

Welcome back to Energy 101.

Today we're continuing to talk about the conversion process from, that we used to

transform energy from one form to another, which is necessary in our energy system.

And the second law of thermodynamics is the most

obtuse and mysterical of the laws that we've talked about and will talk about and

that we know.

And it's not very intuitive to say the least.

But let's delve a little deeper into

our discussion about the second law of thermodynamics.

And as we've already mentioned, we we've got energies of different form over

here on the left, we've got energies of different form over here on the right.

But by using an energy conversion system,

we can convert, for instance, work into electricity.

That's an energy conversion process, and that's

where the laws of nature come into play.

We can also, that, that's called a generator, by the way, that when we

generate electricity from work. And we can also go back the other way.

We can take electricity, and use it to drive an electric motor and generate work.

So there're lots of different mechanisms here.

We can take we can take work and and convert it to heat

by merely friction process, like a brake. If we use an electric,

if we put all the work into a, a disc brake, then the

work that goes in generates thermal energy, which we call heat, et cetera.

So that, that's the energy, some of the

energy conversion processes that we need, that we use.

And we need to understand what the

limitations are, which we have been talking about.

Let's do some quality calculations now about converting the,

some forms of energy to other forms of energy.

And the one that we've had, that we

focus on here is, of course, heat, thermal energy.

Because most of the other forms of energy that

we talk about are, have a quality of 1.

They all have an equal value,

at least theoretically, from a thermodynamic viewpoint.

But thermal energy, we have, we noted, has

a varying quality that depends on its temperature.

Well, we get a little more insight into how this

works and how it varies, by going through a calculation.

And, we, we look at what is known as the Carnot equation, and the Carnot equation

was developed in the mid

19 mid 1800s.

And it says that the, the quality of energy or

the percent that can be converted into work, work having

a quality of 1, is 1 minus the temperature of

the atmosphere divided by the temperature of the heat source.

Now, say the temperature of the atmosphere, at, to represent the ambient

atmosphere or ambient temperature, whether it might be

a river, it might, but it's a place

where we can dispose of energy and at the, at the zero quality state, state point.

So, that's what we typically call the Carnot equation.

And numerically, it gives us the value for the quality of the

thermal energy or heat.

So let's take a specific example get a little more insight in how this works.

I'm, I've chosen one here that, that says if we've got the atmosphere or the

river at 40 degrees Fahrenheit, and I picked numbers here to make the math easy.

We first have to convert that 40 degrees F into degrees Rankine.

This, this formula must

have degrees Rankine, or what we call absolute

temperature, in order to get the right answer.

another, another scale for absolute temperature is Kelvin that we convert

from in the metric system, or Rankine in the English system.

But if we, in English system, if we take

Fahrenheit and add 460 to it, we get absolute temperature.

And at absolute zero,

there is no thermal agitation, all the molecules stop their motion.

But if we, and if we assume that we have thermal energy available at T

H, the higher temperature, that we produce from

burning natural gas or, from burning coal or,

passing a, a passing electricity through a resistor, then we

assume that we have temperature of warm air, or warm fluid

of some kind, at 240 degrees Fahrenheit. That's called our heat source.

And if we add the 460 to it to convert it to Rankine, we get 1 minus 500 Rankine

over 700 Rankine.

Those, that's the absolute temperature converting 40 degrees F and 240 degrees F.

So that number comes up with 1 minus 0.71, which is 500 over 700.

And we come up with a, a fraction of 0.29, and I'll call it 0.30.

So that means that the quality, the heat quality

that this thermal energy has at 240 degrees

is 30%, and you might say, well, so what?

why, why did we go through that calculation?

What, what does it, what difference does it make?

well, it allows us to gain an insight in what we can and can't do.

And by the way, let me just say that it's very easy

to come up with processes and ideas that are worse than what

we do now.

And it's very, because people aren't looking for ways to do things

that are, that are not as cheap, or as simple as, as we have now.

So they look for better ways.

But a lot of times, I'll have people coming to me and they will believe

that they have a revolutionary idea because

they have a different way of doing something.

But different is not necessarily better. So that's just one side

comment I want to make about coming up with new energy

systems and mechanisms for how we can convert energy and use energy.

We have to, just because it's different doesn't mean it's better.

We need to look at the cost and economics, and what the efficiency is of

that process. so, so the so

what we take away from this calculation of learning that the quality of

thermal energy at 240 degrees

Fahrenheit is, is that it has a quality of about 30%.

That means that 30% of this thermal energy at 240

can be converted into work.

And that's theoretical, that's the theoretical maximum.

You notice I have, that there is a less than sign here.

It doesn't say equal.

You won't always get 30% of that thermal energy that's at 240 degrees

converted into work. That's only the limit that we can go to.

If we get rid of all friction, and if we spend a lot

of time and effort, we can approach the 0.29 amount of work

that we get out of one unit of thermal energy of 240.

We could, we could never, theoretically, get there.

We can only approach it.

And, and reality, if we get 70 or 80% of the theoretical maximum

that the second law tells us that we can, we've done pretty well.

So what, that's, that's the calculation that gives us an

example of how we can use the the

Carnot equation to calculate the quality of thermal energy at a particular

state. Now, other than the, than

the Carnot equation that gives us the upper limit for how much

of the thermal energy can be converted into work, we also

have the second law statement that says what you cannot do.

And let me comment about that, the second law is always a negative statement.

And that's why I think we have so much trouble grasping the second law

because all of our other laws of nature are e, it's a, are equalities,

they say the energy in has gotta equal the energy out.

F has gotta equal m a in Newton's Law.

The left hand side has gotta equal the right hand side.

That's generally how we state our natural laws.

But the second law is totally unique in that it's a negative statement.

And that leads to an additional confusion that there, turns

out there's an infinite number of ways to accurately state the

second law, all of them being equivalent.

So, you hear different statements to the second law, and it confuses us because we

just heard a different statement, and we're

not sure which one the second law is.

Well both of them, if they're accurately stated can

be correct, but they are just different statements of it.

So one of the things that it tells us we cannot

do, we cannot do, is the fact that we bring in thermal energy

at a low quality. We've got a quality scale over here on the

right that show high quality up and low quality down, and so we're

putting in thermal energy, or heat, that's at a low quality.

Even 240 degrees is higher temperature necessary than

boil water. And what we cannot do is upgrade that one

unit of energy that's going in into one unit of higher quality energy.

We cannot do it. Why can't we do it?

It's not necessarily intuitive.

And, an intuition, by the way, is not something you're generally born with.

In fact, I'm convinced you're not born with

anything that would be intuitive about the first or the second law of thermodynamics.

Intuition is essentially based on, on experience.

And we have no experience with it when, and so we have

no intuition about what the second law of thermodynamics might be.

So this is one of the negative statements that we can make, that is come

from the second law of thermodynamics.

We cannot upgrade energy with no other net effect.

Notice there're no other energy inputs here.

There's no other external energy inputs there're no other exper, external energy

outputs. It's the only net effect that I'm sure you

cannot do is take low quality one unit of energy and increase its quality

to a higher form a higher number and with one, at one unit.

What the second law does allow, though, it does allow upgrading,

or some of the therm, heat energy, while downgrading the remainder.

So it doesn't say we can never, ever upgrade thermal energy or heat.

We, it, it is allowed, and

here's how we do it. What nature allows.

It tells us that we can bring in thermal energy like

combustion energy of coal, or natural gas, or oil, or the thermal

energy, high temperature thermal energy generated by the nuclear fusion in our

nuclear plant, and we can put it into a power plant system.

A Rankine

cycle is a, a common one, Brayton cycle is also getting more common that will burn

natural gas, and we can upgrade some, some of that energy, notice not all of it.

We got one BTU down up there, and three BTUs coming in.

We can upgrade part of it.

If we'll downgrade the other part, and the Carnot equation tells us what the limit

is, or the fraction of this incoming thermal energy that can be increased in

quality, to a quality for instance of 1. And that it does allow us to do.

So sometimes we say well, I don't believe the

second law, because the second law says I can't

take heat energy at a lower quality like combustion

and make work out of it or electricity, I know

we do that every day.

Yes, you can do that, but you must downgrade part of

it, and it, it quantifies how much of it we throw away.

In this case, we, in a electric power

plant, we normally throw it away at 0 quality.

So, those are some of the takeaways and some of the insights that

hopefully helps you understand more about what the second law of thermodynamics is

all about.

Notice that I have not said anything about entropy.

Entropy is a derivative of the second law of thermodynamics, and I

think too many times, is used to try to state the second law.

And if you don't understand entropy, you can't

possibly understand statements of the second law, including entropy.

So, I hope that gains, gains you some insight into the

second law of thermodynamics, thank you.