Chapter 3 ORMAT 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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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
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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.
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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Vinit’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 OutputTotal Energy Input
= 90 W
100 W
= 0.9
= 90 %
Why dont you give it a shot now......
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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 :
Bottom of Form
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Illustration 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
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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.
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.
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......
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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 :
Bottom of Form
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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 31 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.
- Do they have the same heat content?
- 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:
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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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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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.