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Geothermal: Classroom Activities

Exploring the Transfer of Energy between Two Fluids in a Binary Power Plant


Goal

To explore the transfer of heat between two fluids with different boiling points.

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Background Information

In geothermal power plants with binary heating systems, heat energy from hot geothermal fluid is transferred to a secondary fluid with a lower boiling point. This second fluid then boils at a lower temperature and produces steam to drive turbines and generate electricity.

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Activity Overview

Students observe how heat energy from one fluid can be transferred to another fluid. One fluid has a lower boiling point than the other. To do this, heat energy is transferred through a piece of metal by conduction. Students observe changes in temperatures of the water and the fluid with a lower boiling point.

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Materials and Equipment

Styrofoam cups with lids, hot water, alcohol or antifreeze (or other fluid with low boiling point), thermometers, tongs, watch, graduated cylinders, U-shaped piece of metal, or carefully bent nail, to transfer heat between the cups.

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Activity

  1. Describe to the students how the investigation will work. Instruct them to take care with their dependent and independent variables. You might want to demonstrate how to set up the investigation.
  2. Remind students of the safety procedures for working with hot fluids.
  3. It is a good idea to have students perform a dry run of the investigation so that the can be accurate and efficient during the "live" investigation.
  4. Have students add a measured volume of hot water to a Styrofoam cup.
  5. Have students add a measured volume of alcohol or other fluid with a low boiling point to a second Styrofoam cup.
  6. Have students place a lid on each cup and a thermometer in each fluid.
  7. Have students insert the U-shaped piece of metal through the lids and into the fluid in each cup.
  8. Have students record their observations over selected intervals of time (e.g. every 10 seconds).
    Depending on the materials that are used for the heat conductor, the second fluid, and the volumes of fluids, results will vary. Water has specific heat capacity of 4.19 kJ/kg.K, alcohols, depending on the type typically have values from 2.3 - 2.5 kJ/kg.K. Students should observe that the temperature of the water will decrease while the temperature of the second fluid increases.
  9. Have students discuss their results as a class. Ask them:
    1. How do their findings relate to heat exchange in a geothermal plant?
      Heat can be exchanged from one hot fluid to another by: a) convective heat transfer from fluids to pipes and b) by conductive heat transfer through the walls of pipes to other fluids. By using fluids with lower boiling points vaporization can be achieved to drive turbines using heat received from hot geothermal water.

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