Chapter 3 Energy Efficiency

Goals:

To define and calculate efficiency of an energy conversion deveice.

To understand and articulate the concept entropy

To understand and explain operating principles of a heat engine.

To calculate overall efficiency from step efficiencies.

3.1 Energy Conversion Devices

3.2 Efficiency of Energy Conversion Devices

3.3 Measuring Thermal Energy

3.4 Kelvin Scale

3.5 Heat Engines

3.6 The Carnot Efficiency

3.7 Entropy and Quality of Energy

3.8 Overall Efficiency

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Electrical Energy

Light

Heat

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3.1 Energy Conversion Devices

In the first lesson, we have seen that energy can be transformed from one form to another

and during this conversion, all the energy that we put into a device comes out. However,

all the energy that we put in may not come out in the desired form.

For example, we put in electrical energy into a bulb and the bulb produces light (which is

the desired form of out put from a bulb) but we also get heat from the bulb (undesired

form of energy from an electric bulb).

Therefore, energy flow into and out of any energy conversion device can be summarized

in the diagram below:

Energy Flow Diagram for an Energy Conversion Device

Energy

Conversion

Device

Energy Input

Useful Energy Output

Energy Dissipated to the Surroundings

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When all forms of energy coming out of an energy conversion device are added up, it

will be equal to the energy that is put into a device. Energy output must be equal to the

input. This means that energy can not be destroyed or created. It can only change its

form.

In the case of an electric bulb the electrical energy is converted to light and heat.

Light is useful form and heat is not desired from an electric bulb. This means that the all

the energy that is put in will come out, but all of it will not be in a useful form.

More Information

Say you go to the mall with $100 and you come back with only $10. You need to

account for the $90 that you spent. After thinking about it, you come up with the follow

list:

Gas $15

Sandwich, fries and Drink $8

Lost $5

New Clothes $62

So you spent $62 dollars on something useful, the clothes, but you spent additional

money for other things that were necessary for your trip.

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Flash activity: Identify the useful energy output and undesirable energy

output in the energy conversion devices below:

First 2 Coulmns given – students must fill out last two columns. Can we do this the same

way as the Lawnmower exercise in Lesson 1??

Input Energy Converter Useful Energy Undesirable EnergyChemical Lawn Mower Mechanical Thermal (heat) and radiation

(sound)Chemical Automobile Mechanical Thermal or heat (tail pipe) and

radiation (sound)

Heat (friction) – moving parts in the engine, tires, etc.)

Electrical (generator, dome lights, flood lights)

Electrical Television Radiation (Sound and Light)

Heat from circuits

Electrical Computer Radiation (Sound and Light)

Heat from circuits (electrons moving through system) and Mechanical (fan to cool)

Add a hint button to the “Undesirable energy column for each:

Lawnmower – Hint: How do you know the neighbor is mowing the lawn?

Automobile – Hint: Think about: Mufflers, tires and generator.

TV – Have you ever felt the back of your TV after it had been on for a few hours?

Computer: What’s in your tower and why?

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3.2 Efficiency of Energy Conversion Devices

Efficiency is the useful output of energy. To calculate efficiency the following formula

can be used:

Want to see another example? (Give one more example like above in pop-up textbox.) I will get this info.

Instead of example, Sarma wants cautionary note – wants it to have the yellow caution tape in it.

Caution! This is a simple example because both variable are measured in Watts. If the two variables were measured differently, you would need to convert them to equivalent forms before performing the calculation.

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Illustration

An electric motor consumes 100 watts (a joule per second (J/s)) of power to obtain 90 watts of mechanical power. Determine its efficiency.

Solution:

Input to the electric motor is in the form of electrical energy and the output is mechanical energy.

Using the efficiency equation:

Or efficiency is 90%.

NEW SCREENVinit’s problems – 3-1

Example for equation 3.1

Illustration:

An electric motor consumes 100 watts (a joule per second (J/s)) of power to obtain 90 watts of mechanical power. Determine its efficiency?Now lets see the solution............

Step 1: Input to the electric motor is in the form of electrical energy and the output is mechanical energy. Using the given formula for efficiency

Efficiency = Useful Energy Output         Total Energy Input  = 90 W     100 W   = 0.9      = 90 %   

Why dont you give it a shot now....................

An electric motor consumes 92 watts (a joule per second (J/s)) of power to obtain 83 watts of mechanical power. Determine its efficiency?

Your Answer :

 

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NEW SCREENIllustration 3-1 is a very simple case because both mechanical and electrical power is given in Watts. Units of both the input and the output have to match.

I need to find an example

Illustration 3-2

The United States Power plants consumed 39.5 quadrillion Btus of energy and produced 3.675 trillion kWh of electricity. What is the average efficiency of the power plants in the U.S.?

Total Energy input = 39.5 x 10 15 Btus and the Useful energy output is 3.675 x 10 12 kWh. Recall that both units have to be the same. So we need to convert kWh into Btus. Given that 1 kWh = 3412 Btus,

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Vinit’s Problem 3-2

Another Example for equation 3.1

Illustration:

The United States power plants consumed 39.5 quadrillion Btus of energy and produced 3.675 trillion kWh of electricity. What is the average efficiency of the power plants in the U.S.?Now lets see the solution............

Step 1: To find the efficiency, both the units of input energy and the output energy have to be same. So we need to convert kWh into Btus.

1 kWh = 3412 Btus        Therefore 3.675 x10^12 kWh = 3.675 x 10^12 kWh x 3412 Btus

               1 kWh      = 12539.1 x 10^12 Btus   

Step 2: Use the formula for efficiency.

Efficiency = Useful Energy Output         Total Energy Input  = 12539 x 10^12 Btus        39.5 x 10^15 Btus   = 0.3174      = 31.74 %   

Why dont you give it a shot now....................

The United States power plants consumed 37 quadrillion Btus of energy and produced 2 trillion kWh of electricity. What is the average efficiency of the power plants in the U.S.?

Your Answer :

 

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Energy efficiencies are not 100% and sometimes they are pretty low. The table belows

shows typical efficiencies of some of the devices that are used in day to day life.

Table 3-1 Typical Efficiencies of Some of the Commonly Used Devices

Device EfficiencyElectric Motor 90%Home Gas furnace 95%Home Oil Furnace 80%Home Coal Stove 75%Steam Boiler in a Power Plant 90%Overall Power Plant 36%Automobile Engine 25%Electric Bulb Incandescent 5% Fluorescent 20%

From our discussion on national and global energy usage patterns in Lesson 2, we have

seen that:

About 40% of the US energy is used in power generation

About 27% of the US energy is used for transportation.

Yet the energy efficiency of a power plant is about 35%, and the efficiency of

automobiles is about 25%. Thus, over 62% of the total primary energy in the U.S. is used

in relatively inefficient conversion processes.

Why power plant and automobile design engineers allowing this? Can they do better?

There are some natural limitations when converting energy from heat to work.

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3.3 Measuring Thermal Energy

Thermal energy is energy associated with random motion of molecules. It is indicated by

temperature which is the measure of the relative warmth or coolness of an object.

A temperature scale is determined by choosing two reference temperatures and dividing

the temperature difference between these two points into a certain number of degrees.

The two reference temperatures used for most common scales are the melting point of ice

and the boiling point of water.

On the Celsius temperature scale, or centigrade scale, the melting point is taken as

0°C and the boiling point as 100°C, and the difference between them is divided into

100 degrees.

On the Fahrenheit temperature scale, the melting point is taken as 32°F and the

boiling point as 212°F, with the difference between them equal to 180 degrees.

It is important to realize, however, that the temperature of a substance is not a measure of

its heat content, but rather, the average kinetic energy of its molecules resulting from

their motions.

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Below is a 6-ounce cup with hot water and 12 ounce cup hot water at the same

temperature.

1. Do they have the same heat content?

2. Do they have the same amount of energy?

Click the play button to obtain a magnified view of what is happening. Draw your

conclusions and then check your answer below.

Ok’d

Flash: Show 6 and a 12 ounce cups (clear would be good) with thermometers in them,

show close up of thermometer and temperature (same temperature). Then show close up

of each, with molecules. The 12 ounce cup should show a lot more molecules than the 6

ounce, though both should be moving around a bit.

Answer: They do not have the same heat content. Because they are at the same

temperature the average kinetic energy of the molecules is the same; however, the water

in 12 ounce cup has more molecules than the 6 ounce cup and thus has greater motions or

heat energy.

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3.4 Kelvin Scale

When water molecules freeze at 0°C, the molecules still have some energy compared to

ice at -50°C. In both cases, the molecules are not moving, so there is no heat energy.

So what is the temperature at which all the molecules absolutely have zero energy? A

temperature scale can be defined theoretically for which zero degree corresponds to zero

average kinetic energy. Such a point is called absolute zero, and such a scale is known as

an absolute temperature scale. At absolute zero, the molecules do not have any energy.

The Kelvin temperature scale is an absolute scale having degrees the same size as those

of the Celsius temperature scale. Therefore, all the temperature measurements related to

energy measurements must be made on Kelvin scale.

Combine thermometers with an animation. Press play and observe what happens:

Based on your observations, answer the following questions:

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http://www.weldbend.com/

Images/Diagrams/Technical

%20Diagrams/Temp.gif

Animation goes here:

Show ice – thermometers show temperature for water freezing.

Place ice into a pan on the stove and it melts – thermometers show temperature for melting water.

Water in ban starts to boil – thermometers show temperature for boiling water.

At what temperature does water freeze?

___ Kelvin ____ Celsius ____ Fahrenheit

At what temperature does ice melt?

___ Kelvin ____ Celsius ____ Fahrenheit

At what temperature does water boil?

___ Kelvin ____ Celsius ____ Fahrenheit

Ok:

Add pop-up “More Information” text box with the above screen:

You can convert a temperature in Celsius (c) to Kelvin (k) with this formula:

k = c + 273.15

You can also change a temperature in Kelvin to Celsius:

c = k - 273.15

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Need to emphasize that the objective is to maximize Work to increase efficiency.

If High T increases OR Low T decreases, the efficiency increases

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3.5 Heat Engines

Energy conversions occurring in an automobile are illustrated below:

Energy Conversions in an Automobile

Any device that converts Thermal energy into mechanical energy - such as automobiles

or power plants - is called a heat engine. In these devices, high temperature heat (thermal

energy) produced by burning a fuel is partly converted to mechanical energy to do work

and the rest is rejected into the atmosphere, typically as a low temperature exhaust.

Animate this – An animated version is on Athena in animations folder – but needs learner

controls

Energy Flow in a Heat Engine

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Chemical Energy

Thermal Energy

Mechanical Energy

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A general expression for the efficiency of a heat engine can be written as

We know that all the energy that is put into the engine has to come out either as work or

waste heat. So work is equal to Heat at High temperature minus Heat rejected at Low

temperature. Therefore, this expression becomes

Where, QHot = Heat input at high temperature and Qcold= Heat rejected at low temperature.

The symbol is often (Greek letter eta) used for efficiency this expression can be rewritten

as

The above equation is multiplied by 100 to express the efficiency as percent.

French Engineer Sadi Carnot showed that the ratio of QHighT to QLowT must be the same as

the ratio of temperatures of high temperature heat and the rejected low temperature heat.

So this equation, also called “Carnot Efficiency,” can be simplified as:

This is only equation in blackboard format

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3.6 The Carnot Efficiency

The Carnot Efficiency is the theoretical maximum efficiency one can get when the heat

engine is operating between two temperatures:

The temperature at which the high temperature reservoir operates (Thot).

The temperature at which the low temperature reservoir operates (Tcold).

In the case of an automobile, the two temperatures are:

The temperature of the combustion gases inside the engine (Thot).

The temperature at which the gases are exhausted from the engine (Tcold).

Can we show a car engine? Maybe animate it? YES Here’s one from

http://auto.howstuffworks.com/engine1.htm Can we develop something similar?

The following may be best explained via audio and narration: When the exhaust is leaves

the automobile at a higher temperature, it carries more energy out so that amount of

energy is not available to be converted to work (moving piston). Therefore, we can

conclude that the higher the Tcold, the lower the efficiency. Similarly, if the Thot is

increased by increasing the temperature of the combustion gases, we can get higher

efficiencies.

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Then, why should we operate the automobiles at low efficiencies?

It is not that we cannot achieve high temperatures, but we do not have the engine

materials that can withstand the high temperature. As a matter of fact, we do not let the

engine gases go the maximum that they can go even now and instead try to keep the

engine cool by circulating the coolant.

So we are taking the heat out of the gases (thus lowering the Thot) and making the engine

operate at cooler temperatures so that the engine is protected - but lowering the efficiency

of an automobile.

More Information: It’s like Taxes. The more money you earn (heat), the more money is

taxed (cold), leaving you with less money to take home (efficiency). However, if you

could earn more money (heat) and find a way to have less taxes taken out (better engine

material), you would have more money to take home (efficiency).

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Below are temperature ranges for the heat (hot) and exhaust (cold) of a car engine.

Using the scales above, enter several combinations of numbers for hot and cold, and

observe the graph: (wants it to be like home simulations)

Hot

Cold

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Efficiency

Function of 2 Temperatures

Based on your observations of the graph:

1. What happens as High T increases? Answer: Efficiency Increases2. What happens as High T decreases? Answer: Efficiency decreases3. What happens as Low T decreases? Answer: Efficiency increases4. What happens as Low T increases? Answer: Efficiency decreases

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I need to find an example

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Illustration 3-3

For a coal-fired utility boiler, the temperature of high pressure steam would be about 540°C and Tcold, the cooling tower water temperature would be about 20°C. Calculate the Carnot efficiency of the power plant?

Solution:Carnot efficiency depends on high temperature and low temperatures between which the heat engine operates. We are given both temperatures. However, the temperatures need to be converted to Kelvin

Thot = 540°C+273 = 813 KTcold = 20°C + 273 = 293 K

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Vinit’s Problem 3-2

Example for equation 3.4

Illustration:

For a coal fired utility boiler, the temperature of high pressure steam would be about 540 degrees C and Tcold, the cooling tower water temperature would be about 20 degrees C. Calculate the Carnot efficiency of the power plant?The solution............

Step 1: We are given the high temperature and the low temperatures between which the heat engine engine operates which is needed to calculate Carnot efficiency. To calculate, Convert the temperatures to Kelvin.

Thot = 540 degrees C + 273  = 813 K    Tcold = 20 degrees C + 273   = 293 K   

Step 2: Use the formula for Carnot efficiency.

Carnot Efficiency =(1 - T hot ) x 100           Tcold

  =(1- 293 K) x 100          813 K   = 64%   

Why dont you give it a shot now....................

For a coal fired utility boiler, the temperature of high pressure st

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3.7 Entropy and Quality of Energy

From the Carnot Efficiency formula, it can be inferred that a maximum of 64% of the

fuel energy can go to generation. To make the Carnot efficiency as high as possible,

either T hot should be increased or T cold (temperature of heat rejection) should be

decreased.

Let’s look at the Energy conversions in a power plant:

Audio with animation: Coal or oil or gas has chemical energy stored in the chemical

bonds of the fuel. When the fuel is burned, the chemical bonds in the fuel are broken and

new bonds are formed releasing thermal energy . This thermal energy is transferred to

water that turns into high pressure, high temperature steam. The high pressure, high

temperature steam turns the turbine and converts the thermal energy into mechanical

energy. The steam after turning the turbine will still have some energy but not enough to

turn the turbine. The low pressure steam is condensed into water and the water is sent

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Schematic of a Coal fired Power Plant

back to the boiler. The turbine is connected to a generator and in the generator the

mechanical energy is converted into electricity by turning a conductor in a magnetic field.

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Video demo goes here – will record on 12/16/04

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The basic energy conversions in the three main components in a power plant are shown

quantitatively in the image below:

(Can we make this a little bigger?)

Audio with animation: Let’s say the boiler takes in 100 Btus of chemical energy and

produces 88 Btus of useful thermal energy. The 88 Btus of thermal energy from the boiler

goes into the turbine and 36 btus equivalent of mechanical energy (movement of turbine

blades) is produced. This 36 Btus of mechanical energy is transferred to the generator

which converts it to 10.26 Wh of electrical energy.

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Boiler

TurbineGenerator

Chemical Energy Input (100 BTU)

Thermal Energy (88 BTU)

Mech. Energy (36 BTU)

Elec. Energy Output (10.26 Wh)Energy Conversions in a Power Plant

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Same exercise as with car engine (hot, cold and graph), only use these ranges

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NEW PAGE

3.8 Overall Efficiency

Using the energy efficiency concept we can calculate the component and overall

efficiency.

Here the electrical energy is given in Wh and Chemical Energy in Btus. So Wh can be

converted to Btus knowing that there are 3.412 Wh in a Btu.

This overall efficiency can also be expressed in steps as follows

Applying this method to the above power plant example,

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It can be seen that the overall efficiency of a system is equal to the product of efficiencies

of the individual subsystems or processes. What is the implication of this?

We have been looking at the efficiencies of an automobile or a power plant individually.

But when the entire chain of energy transformations from the moment the coal is brought

out to the surface to the moment the electricity turns into its final form, true overall

efficiency of the energy utilization will be revealed. The final form at home could be light

from a bulb or sound from a stereo. The series of steps are

Need an image of this from Sarma: Production of coaltransportation to power plant

Electricity Generation Transmission of electricity Conversion of electricity into

light

If efficiency of each step is known, we can calculate the overall efficiency of production

of light from coal in the ground. The table below illustrates the calculation of overall

efficiency of a light bulb.

Overall Efficiency of a Light Bulb

Step Step Efficiency Cumulative Efficiency or

Overall Efficiency

Extraction of coal 96% 96%

Transportation 98% 94% (0.96 X 0.98)*100

Electricity Generation 35% 33% (0.96 X 0.98 X 0.38)

Transmission of Electricity 95% 31%

Lighting

Incandescent bulb 5% 1.5%

Fluorescent 20% 6.2%

AUDIO: It can be seen that to generate 6.2 units of light from a relatively efficient

fluorescent bulb, we used up 100 units of energy from coal from the ground. This also

means that during various conversion steps 93.8 units of energy is dissipated into the

environment.

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A similar analysis on Automobile shown in Figure 3-7 and Table 3-3 shows that only

about 10% of the energy in the crude oil in the ground is in fact turned into mechanical

energy moving people.

Figure – missing from word document but in printed document – need to get from Sarma:

Titled “Overall Automobile Efficiency”- shows sequence of steps in converting chemical

energy in crude oil in the ground to movement of a car:

Production of crude Transportation to refinery Refining Transportation of

gasoline Engine Transmission Movement

FigureOverall Efficiency of an Automobile

Step Step Efficiency Overall or Cumulative

Efficiency

Production of Crude 96% 96%

Refining 87% 84%

Transportation 97% 81%

Engine 25% 20%

Transmission 50% 10%

Perhaps audio to explain the above more fully??

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OMIT ALL OF FOLLOWING - NOT IN ONLINE LESSONBUT There is a crossword puzzle at the very end…

Selected References:Hinrichs, R. A., “Energy,” Saunders College Publishers, Philadelphia, PA, 1992.

Aubrecht, G. L., “Energy,” Prentice Hall, Inc., Englewood Cliffs, NJ, 1995.

Fay, J.A. and Golomb, D. S., “Energy and the Environment,” Oxford University Press,

New York, NY, 2002.

Christensen, J. W., “Global Science: Energy Resources Environment”, 4th edition,

Kendall/Hunt Publishing Company, Dubuque, IA, 1996.

Questions for Review and Discussion1. A heat engine has Carnot efficiency of 30%. Useful output from the engine

is 1000J. How much heat is wasted? 2. How can we improve the Carnot efficiency of a heat engine by changing

the hot and cold reservoir temperatures?3. Most of the energy conversion devices that we use in our day-to-day life

can be classified as Heat Engines. Give two examples4. The following diagram shows the energy flow to and from a furnace.

Energy input is 30,000,000 cal

Calculate is the efficiency of the furnace. You need show your work very clearly to get complete credit

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Coal furnaceHeat energyRejected though the tail pipe = 29,045,000 J

Multiple Choice Questions

1. Approximately what percentage of electricity does an incandescent light bulb convert into visible light?

a) 5b) 20c) 40d) 90

2. The following step efficiencies apply to the use of gasoline in a car: Crude production: 96%, Refining 87%, Transportation 97%, Engine efficiency 25%. What is the total efficiency of the process?

a) 40% b) 30% c) 20% d) 50%

3. The flame temperature in an automobile is 1,000 °C, and the exhaust is emitted at 70 °C. What is the Carnot efficiency?

a) 25%b) 65%c) 73%d) 33%

4. If the energy input of a system is 50 calories and the output is 25 calories, what is the system efficiency?

a) 100% b) 50% c)200% d) 25%

5. Heat engines are inefficient because the energy conversion is

a) From low entropy to high entropy

b) From high entropy to low entropy

c) From low temperature to high temperature

d) From high temperature to low temperature

6. Automobile engine efficiency increases in

a) Summerb) Winter

7. The useful output from a heat engine is 238 cal. The energy that is wasted is 5667 J? What is the Carnot efficiency of the engine?

a) 4%b) 15%c) 17.6%d) none of the above

8. Three energy conversion processes take place in succession. The first has an efficiency of 50%, the second is 40% efficient, and the third 5%. What is the overall efficiency (%) of the entire process? (2 points)

a) 10b) 40c) 50d) 5

9. The turbine is the _______ efficient component in a power plant

a) Least b) Highest

10. The flame temperature in an automobile is 1652 °F, and the exhaust is emitted at 212 °F. What is the Carnot efficiency?

a) 88.8%b) 87.2%c) 68.2%d) 25%

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11. The function of a generator in a power plant is to convert

a) Chemical energy to mechanical energy

b) Mechanical energy to electrical energy

c) Thermal energy to mechanical energy

d) None of the above

12. Which of these is False (2 points)

a)

b)

c)

d)

13. In a power plant_________energy is converted into_______energy.

a) Chemical, Mechanicalb) Chemical, Electricalc) Mechanical, Electricald) Mechanical, Thermal

14. Which of the following devices is least energy efficient?

a) Power plantb) Electric motorc) Light bulb

15. Most energy conversion process produce----------- as by product

a) Lightb) Heat c) Soundd) Motion

1 2

3 4 5 6

7

8

9

10

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1232

Energy Basics Review Puzzle

Across 1. Conversion of chemical energy to this form is notoriously inefficient.

3. This law states that we cannot create or destroy energy

7. Temperature is a measure of this form of energy

9. Rate at which energy is spent

10. Ability to do work

11. Useful energy divided by the total energy is called

12. This source of energy supplies most of the US energy needs

Down 2. This primary source is used for most of the electricity generated in this country

4. Second largest primary source for electricity generation

5. Measure of disorder

6. Most households use this fuel for home heating

8. The form of energy in gasoline

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