Energy Efficiency

3. Energy Efficiency

Goals:

To understand and articulate the concept entropy

To understand and explain operating principles of a heat engine.

To calculate overall efficiency from step efficiencies.

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.

Therefore, energy flow into and out of any energy conversion device can be summarized in the diagram below:

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.

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.

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
  

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 Energy Chemical 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?

Efficiency is the useful output of energy. To calculate efficiency the following formula can be used:

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%.