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.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • 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... Give feedback on this alignment...
  • Develop descriptions, explanations, predictions, and models using evidence. Students should base their explanation on what they observed, and as they develop cognitive skills, they should be able to differentiate explanation from description--providing causes for effects and establishing relationships based on evidence and logical argument. This standard requires a subject matter knowledge base so the students can effectively conduct investigations, because developing explanations establishes connections between the content of science and the contexts within which students develop new knowledge. (Grades 5 - 8) Details... View more aligned curriculum... Give feedback on this alignment...
  • Think critically and logically to make the relationships between evidence and explanations. Thinking critically about evidence includes deciding what evidence should be used and accounting for anomalous data. Specifically, students should be able to review data from a simple experiment, summarize the data, and form a logical argument about the cause-and-effect relationships in the experiment. Students should begin to state some explanations in terms of the relationship between two or more variables. (Grades 5 - 8) Details... View more aligned curriculum... Give feedback on this alignment...
  • Use mathematics in all aspects of scientific inquiry. Mathematics is essential to asking and answering questions about the natural world. Mathematics can be used to ask questions; to gather, organize, and present data; and to structure convincing explanations. (Grades 5 - 8) Details... View more aligned curriculum... Give feedback on this alignment...
  • Mathematics is important in all aspects of scientific inquiry. (Grades 5 - 8) Details... View more aligned curriculum... Give feedback on this alignment...
  • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways. (Grades 5 - 8) Details... View more aligned curriculum... Give feedback on this alignment...
  • Heat moves in predictable ways, flowing from warmer objects to cooler ones, until both reach the same temperature. (Grades 5 - 8) Details... View more aligned curriculum... Give feedback on this alignment...
  • Perfectly designed solutions do not exist. All technological solutions have trade-offs, such as safety, cost, efficiency, and appearance. Engineers often build in back-up systems to provide safety. Risk is part of living in a highly technological world. Reducing risk often results in new technology. (Grades 5 - 8) Details... View more aligned curriculum... Give feedback on this alignment...
  • Technological solutions have intended benefits and unintended consequences. Some consequences can be predicted, others cannot. (Grades 5 - 8) Details... View more aligned curriculum... Give feedback on this alignment...
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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.

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.

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