COURSE DESCRIPTION This course provides an introduction to the most powerful engineering principles you will ever learn - Thermodynamics: the science of transferring energy from one place or form to another place or form. We will introduce the tools you need to analyze energy systems from solar panels, to engines, to insulated coffee mugs. More specifically, we will cover the topics of mass and energy conservation principles; first law analysis of control mass and control volume systems; properties and behavior of pure substances; and applications to thermodynamic systems operating at steady state conditions. COURSE FORMAT The class consists of lecture videos, which average 8 to 12 minutes in length. The videos include integrated In-Video Quiz questions. There are also quizzes at the end of each section, which include problems to practice your analytical skills that are not part of video lectures. There are no exams. GRADING POLICY Each question is worth 1 point. A correct answer is worth +1 point. An incorrect answer is worth 0 points. There is no partial credit. You can attempt each quiz up to three times every 8 hours, with an unlimited number of total attempts. The number of questions that need to be answered correctly to pass are displayed at the beginning of each quiz. Following the Mastery Learning model, students must pass all 8 practice quizzes with a score of 80% or higher in order to complete the course. ESTIMATED WORKLOAD If you follow the suggested deadlines, lectures and quizzes will each take approximately ~3 hours per week each, for a total of ~6 hours per week. TARGET AUDIENCE Basic undergraduate engineering or science student. FREQUENTLY ASKED QUESTIONS - What are the prerequisites for taking this course? An introductory background (high school or first year college level) in chemistry, physics, and calculus will help you be successful in this class. -What will this class prepare me for in the academic world? Thermodynamics is a prerequisite for many follow-on courses, like heat transfer, internal combustion engines, propulsion, and gas dynamics, to name a few. -What will this class prepare me for in the real world? Energy is one of the top challenges we face as a global society. Energy demands are deeply tied to the other major challenges of clean water, health, food resources, and poverty. Understanding how energy systems work is key to understanding how to meet all these needs around the world. Because energy demands are only increasing, this course also provides the foundation for many rewarding professional careers.
08.02 - More Waste Heat Recovery - Combined Cycles
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En provenance du cours de University of Michigan
Introduction to Thermodynamics: Transferring Energy from Here to There
503 notes
University of Michigan
503 notes
À partir de la leçon
Week 8
In this module, we have a brief discussion of energy carriers – including fossil fuels and battery materials. These lectures highlight the thermodynamic properties of these energy carriers and storage materials that make these systems so attractive and at the same time, so difficult to replace. As this is our last module of the course, I hope you have enjoyed this Introduction to Thermodynamics and that you have learned some new skills. Good luck on all your adventures in energy systems!,
Rencontrer les enseignants
Margaret Wooldridge, Ph.D.Arthur F. Thurnau Professor
Mechanical Engineering, Aerospace Engineering
Okay, welcome back. So we were considering what was the best
operating condition for our fictional manufacturing facility.
So we had our electrical energy target, and we had our heat energy target.
If we work in quadrant A we make excess heat so we don't want that condition
because excess heat simply gets rejected to the environment so that's a complete
loss in the system. So that really eliminates both A and B
from consideration, because both of those are again on the high side of the heat
energy target. Up in quadrant C we regenerate more
electrical energy than what we need. Now you'll often hear people talk about
oh, well I'll install solar panels or wind turbine on my property and if I
generate more energy than I need I'll sell it back to the grid.
Well at this point in time I've yet to hear that that has ever been successful.
So while in principle that can be a feasible operating mode to generate
enough of electrical energy for yourself and more so you could put that energy
back into the grid, it doesn't tend to be that successful.
And that's primarily because we're still trying to figure out how to have these
components interact with the grid. So often what I hear is instead people
generate excess energy and they just put it back into the grid and they're not
paid for that. And again that's just because we're still
trying to figure out how to how the power companies can actually monitor that, how
can they use that energy because it's not dependable it's very intermittent.
So it makes it very challenging, very transient system.
So, while in principle, quadrant C could be a good place to operate.
In practice, it's not. It's a hard place to be.
So that really says is that you want to undershoot both your heat and electrical
energy needs. So what you have to identify is,
essentially, what's the steady baseline electrical energy needs for your company
or your home, and what are the steady baseline heat energy needs.
And then, what you would do is supplement.
So if this is your target, you would supplement this energy difference from
the grid, and you would supplement this heat difference from some sort of
supplementing hidden heating. So if it's a boiler or a furnace or
something like that, but hopefully this got you thinking about how this is a
pretty hard system to design for because we not only have these kind of, these are
average operating conditions. But now imagine that you would have
di-annual variation and seasonal variations that you would have to
consider too. So again hopefully it got you thinking
about all these issues. Okay, so let's step back into this waste
heat recovery business. So we talked about waste heat recovery
for combined heat and power. And we saw our little example of that.
In principle again that's a great thing to do.
In practice we have to be pretty thoughtful about how to do that
effectively. Combine cycles are another form of waste
heat recovery. And in this case what we're going to do
is realize that my air standard braking cycle or if you prefer, my gas turbine
power system generates a lot of waste heat.
So let's go ahead and sketch that up here, and I'll show you how we're
going to take that excess energy and we're going to use it effectively, so
again we have our compressor. We compress air from the ambient.
So this is typically this state, I label this state one.
This is going to be typically just drawing in air from outside.
So 298 Kelvin and pressure it state one. It's typically going to be atmospheric,
right? So we draw this from the ambient.
We compress that air to state two, we add heat by burning natural gas, propane,
whatever our combustion sources is from state two to three and then again as
we've done before, as we harness that energy to generate work out through the
turbine. So this is my work in for the compressor.
And what we recognize again is this is a high energy, high enthalpy, high
temperature fluid at state four. So what we're going to do is take those
exhaust gases, and here's where we're going to be pretty clever, and we are
going to couple the exhaust energy of my braking cycle with a ranking cycle.
So let's show you how we're going to do that.
So remember the basics of our ranking cycles say we have to have work in, that
as pump. And then, remember, we had our steam
generator here. And then, we would expand that high
temperature steam through a steam turbine.
And then we would condense the water back to liquid phase through my condenser.
And what we're going to do in this system is we're going to take this high-energy
fluid from the exhaust of my gas turbine and what we're going to do is pass that
through a heat exchanger to heat the steam.
So this is also referred to as a heat recovery steam generator, so this is heat
recovery, and then we'll exhaust those gasses out the stack, dump that heat to
the environment, now, so this is going to be exhaust, to environment.
Okay, this heat transfer is going to occur without mixing the fluid, I don't
want to mix the air with the water, so this is a two fluid heat exchanger where
there's no mixing between fluids. So this is air, and this is water.
And in this heat recovery steam generator, there is no fluid mixing.
Okay. Now, the pressure of these two systems
are different right, because we're going to exhaust the gas turbine here nominally
to some pressure that's above the ambient.
So this will be greater than the atmospheric condition.
At the exit here this will be, let's go ahead and label these as a, b, c d, and
this is going to be exit state five. So the exit state is going to have a
pressure of about one atmosphere. So when we look at the work out of the
cycle, what we have here is the network for the cycle.
We have work in through the compressor. So this is work in for the compressor.
We have work out from the gas turbine. We have work in from the pump, to the
pump I should say. And then we have work out of the steam
turbine. Okay.
So we know that the work ins are less than zero.
I'll, sorry, I'll put that as work in comma.
This pump is work in. So this is less than zero, this is
greater than zero, this is greater than zero but you can see.
That we have four work transfers in the system.
So now if we consider the net heat transfer for the system, and remember
we're considering the cycle which includes essentially if we were to draw a
dash line around this entire system. So we have key transfer in from the
combuster of the braking cycle, right, so we know that's going to be greater than
zero. I'll draw a line so we understand that
these inequalities refer to the work transfer and these are the inequalities
for the heat transfer, and then we come over here and let's go ahead and.
Label that we know we have Q out, or heat transfer out of the condenser.
So we have plus Q, condenser, out, and we know again that's going to be less than
zero. And here's where we want to be
thoughtful. The heat transfer here that occurs
between the air, the hot air, which is the exhaust of the Brayton cycle, and the
steam in the Rankine cycle is entirely within the overall cycle.
So that key transfer doesn't appear in the accounting for the overall combined
cycle. Okay.
So these are again, these are combined cycle values, okay, for both of these.
So now typically that's not going to be enough heat transfer for us to get a
whole lot of energy that we want for the entrance to the steam turbine so we might
have to supplement that with another form of heat transfer that is external.
Right, so if we have to add coal or whatever.
But the heat transfer that occurs between the braking cycle, between the air and
the steam is entirely within the cycle, so there's no, that doesn't appear within
the cyclic accounting, okay. And then of course if we were to close
the loop here, this is my fictional rejection of the heat transfer to the
ambient. Then we conclude that in my system two,
so we would have plus Q dot ambient. In reality that has to be so, just again
I apologize to have these lines crossing like this, but that's how you would close
the loop, so again this heat transfer that's rejected to the ambient is
different. Than if we had directly connected from
four to one, okay? We're actually connecting from five to
state one, okay? So Q ambient just as a reminder, I will
add that this is five comma one, okay? And this value, again and since it's heat
out of the system is going to be less than zero.
So if we don't have any make up heat, if we don't provide any supplemental heating
within the steam generator, that's it. We only have these three heat transfer
terms. If I do require, let's like, oh we'll be
even more colorful here, we'll add another color to our cycle.
If I do need to add more supplemental heat transfer here.
So this is going to be let's call it Qn supplemental and again this would be
typically coal or some other form of you could do coal.
You could do nuclear or something like that.
Maybe it's a solar power tower who knows. But there's additional heat transfer in
here then we would have to include that heat transfer here.
As a supplemental of the transfer and that would be in, okay?
So, let's back up for a moment and consider that we've learned.
We have high temperature fluid coming out of the gas turbine when we use a Brayton
power cycle. Rather than just send that up the
exhaust, we might as well couple that to a heat exchanger.
And we can couple that to the heat exchanger within another power cycle,
like a Rankine cycle and that's referred to as a combined cycle system.
And that is going to give me a much higher efficiency than if I just had a
simple cycle. Okay.
So, more than 20% of the US in the United States over 20% of the total energy circa
1997 was wasted in just thermal losses, so that's essentially, all of that energy
was just rejected to the environment. That energy if we could find a way to
harness it appropriately could power almost the entire transportation sector.
There is a lot of waste heat generated, not just in the United States, but
everywhere, so combined cycles and combined heat and power co-generation
plants. Are no brainers.
So let's line them up next to each other. So here we have kind of my basic coal
fire power plant, that's shown in the top here.
So we have just for comparison purposes, we'll use a benchmark of 100% fuel comes
into my coal fired steam powered plant. That's my Rankin cycle.
60% of that energy is rejected as waste heat.
40% is generated as electricity. That's the typical Rankine power
efficiency, a cycle efficiency of 40%. A combined cycle takes my natural gas
turbine, the Brayton cycle, which generates about 70% waste heat, so your
Brayton cycle's about 30% efficient. So I have 30% electricity generated form
my 100% fuel bench mark. 70% is generated as waste heat.
I can take that 70% of the waste heat, put it into a Rankine power plant and
again with my 40% efficiency and I can get 28% electricity out of that system.
And 42% is then rejected as waste heat. That means your combined cycle has an
efficiency for just electricity out of 55 to 60%.
So that's a remarkable, that's a huge improvement in the electrical efficiency
out of my combined cycle. Now if we compare combined cycles to
co-gen, that's what we've got in this slide here.
So remember, when we generate power we always reject heat, because these are
heat engines. That's what these are based on.
There are a few exceptions that we're not going to have time to get into in this
class, and that's like, fuel cells for example can generate power.
And they aren't heat engines. So they follow a different sets of
limiting behavior. But for heat engines which are
essentially power cycles that generate power because of the temperature
difference, they always create heat at the same time we create power.
So in combined heat and power cogen, remember we have heat transfer as the
desired output. So I get to include that in my definition
for thermal efficiency. We can use CHP systems at any scale.
You can do this for your home, all the way up to industrial power plants.
And most industrial facilities that use very high power output, like, anything
that's glass processing, cement processing, metal processing.
They've already invoked heating power, and have been doing for many, many years.
Cause they've already realized that's a whole lot of energy that could be wasted
otherwise. So these are very common to see combined
heating power systems around the world. But your heat transfer has to be
co-located. If heat transfer application has to be
co-located. So like in my example of the University
of Michigan power plant. We have a hospital right next door so we
can generate that heat and use it literally right across the street.
So we can't transfer that heat large distances, otherwise CHP is the right
answer, because if we look at my little sketch that I've drawn here the combined
heat and power plant, again if I take 100% fuel in, if I generate electricity
and heat as the desired output. I get 90% of that fuel energy is
effectively used to create electricity and heat.
That's awesome. I only have stack losses of 10%.
That's the right answer. Okay?
But if I don't have a co-located application for heat transfer or if I
can't quite hit the targets. If they're very different right.
If the electrical energy target is very different than the heat transfer energy
target, these systems can be very cumbersome and they aren't as efficient
under those conditions. So instead, the combined cycle power
plant is again. A sort of no brainer, that's where we got
this 55 to 60% useful energy created. And I say it's sort of a no brainer
because well you're adding an entire set of power plant equipment, right?
You have heat exchangers, you have the piping, you have another turbine.
A steam turbine you want to add to that system.
So there's a lot of capital equipment involved.
With creating a combined cycle power. But, if we compare that to our baseline,
where we're only generating 30 to 40% of useful electrical energy, it seems like
we really need to focus on making sure that we're smart about how we make our
power plant, where we locate them, and how we design them to hit our energy and
heat transfer target. Okay, that's going to be the end of our
discussion of the technical details of how to analyze stationary power systems.
Next we're going to move on to talking about the energy carriers and we're
going to start with a discussion of the fossil fuel reserves.
So I want you to think about the answer to this question.
Are fossil fuel reserves scarce? And we'll start with that next time.
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