Hands-on Activity: Efficiency of a Water Heating System

Contributed by: Office of Educational Partnerships, Clarkson University, Potsdam, NY

Photo shows a clear pot of bubbling water on a gas stove burner.
What's the efficiency of a water heating system?
copyright
Copyright © NASA http://blogs.nasa.gov/cm/blog/ISS%20Science%20Blog/posts/post_1301433765536.html

Summary

Students use a watt meter to measure energy input into a hot plate or hot pot used to heat water. The theoretical amount of energy required to raise the water by the measure temperature change is calculated and compared to the electrical energy input to calculate efficiency.

Engineering Connection

Engineers strive to improve the energy efficiency of processes, since some energy is lost in each of the conversion processes. They have learned that more complicated processes that have many components are typically less efficient than simple systems. Making systems that include energy conversions more efficient can help to reduce our nation's consumption of fossil fuels and production of greenhouse gas emissions.

Learning Objectives

After this activity, students should be able to:

  • Explain where energy is "lost" in conversions and why based on the second law of thermodynamics.
  • Compute the efficiency of an energy conversion given input and output.
  • Identify system by-products and explain how they can be used effectively to increase overall system efficiency.
  • Use collected data to calculate the efficiency of a system.

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  • Plan an investigation to determine the relationships among the energy transferred, the type of matter, the mass, and the change in the average kinetic energy of the particles as measured by the temperature of the sample. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
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  • Use appropriate tools and techniques to gather, analyze, and interpret data. The use of tools and techniques, including mathematics, will be guided by the question asked and the investigations students design. The use of computers for the collection, summary, and display of evidence is part of this standard. Students should be able to access, gather, store, retrieve, and organize data, using hardware and software designed for these purposes. (Grades 5 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
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Materials List

Each group needs:

  • electric water-heating device (such as a hot plate, hot pot, microwave; it is best to have different appliances among the groups)
  • thermometer
  • graduated cylinder (100 or 500 ml)
  • stopwatch
  • watt meter (can be shared among groups to some extent)
  • insulated pot holder
  • Student Worksheet, one per student

Introduction/Motivation

(Start with the combustion activity setup or block diagram on the board as a reference point to what students learned earlier.) Does all of the heat go into heating the water and/or spinning the turbine? Where is energy lost? (Or, at least lost from our ability to do work?) Where does this energy go?

We've already seen the first law of thermodynamics in action – the law of conservation of energy states that energy can neither be created nor destroyed (by ordinary means) - only converted into different forms.

A second law of thermodynamics helps to explain these losses. The heat losses can be called "entropy," which is a measure of how much energy is dispersed to the environment and is no longer usable. The second law of thermodynamics states that the entropy of the universe always increases. That means that things get increasingly disordered and are irreversible (or, there will always be heat losses in any energy conversion process).

Lets look at a simple heating process. (Show an example of a hot pot or other simple water heating device, and draw a process flow diagram.)

Block flow diagram shows cold water and electricity coming into a hot pot and hot water leaving.
An example block flow diagram for hot pot.
copyright
Copyright © 2008 Clarkson University, Potsdam NY

Explain the system that will be studied in this activity.

  1. Energy into the system determined by electricity supply.
  • Energy = power X time (W s = J)
  • We can use a watt meter to measure power and stop watch to measure time.
  1. The theoretical amount of energy needed to heat a substance such as water can be calculated based on the mass, temperature rise and specific heat of the substance.

Q = m*Cp* ∆T

Where:

  • Q is the energy required (joules, J);
  • m is the mass of the substance (g) (calculated from volume (V) and density (p) of water, 1 g/ml);
  • Cp is the specific heat (J/g/°C)
  • ∆T is the change in temperature (°C).
  • The specific heat of water is 4.186 J/g/°C. Q = m*Cp*T
  1. Efficiency is defined as energy in output / energy in input:
  • E = (pVCp ∆T)/(Pt)
  • If power is used in units of watts and time in seconds, then both the denominator and numerator have units of Joules.

Procedure

Before the Activity

  • Set up stations around the room with all equipment.
  • Make copies of the Student Worksheet.

With the Students:

  1. Show how the watt meter is operated.
  2. Proceed with the activity.
  3. Conclude with a class discussion comparing the efficiencies that students found in the various water heating systems. Which system is most efficient? Least? What is it about the design of the system that affects the efficiency?

Attachments

Safety Issues

Provide potholders or insulated gloves for handling the thermometers and hot water containers.

Assessment

Worksheets: Collect completed student worksheet to check to make sure calculations are correct and answers to discussion questions show appropriate insight.

Other Related Information

This activity was originally published by the Clarkson University K-12 Project Based Learning Partnership Program and may be accessed at http://www.clarkson.edu/highschool/k12/project/energysystems.html.

Contributors

Susan Powers; Jan DeWaters; and a number of Clarkson and St. Lawrence University students in the K-12 Project Based Learning Partnership Program

Copyright

© 2013 by Regents of the University of Colorado; original © 2008 Clarkson University

Supporting Program

Office of Educational Partnerships, Clarkson University, Potsdam, NY

Acknowledgements

This activity was developed under National Science Foundation grant nos. DUE 0428127 and DGE 0338216. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: May 10, 2017

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