6. Energy from and to the Earth
This module compares the spatial availability of geothermal resources for heating and cooling and evaluates different ground exchange systems for practical application. The operation of enhanced geothermal systems is described in relation to the operations for extracting fossil fuels via hydraulic fracturing. An in-class exercise is completed to demonstrate ground heat exchange and its broad availability as a renewable energy source.
Students will be able to:
- Examine global tectonic plate maps and identify areas of thermal activity near Earth's surface.
- Compare different rock types for thermal exchange capacity.
- Measure the temperature differential in a ground exchange system.
Context for Use
Description and Teaching Materials
This is an indoor activity that involves the examination and interpretation of different geothermal maps that infer and/or display the variation in energy availability near Earth's surface. The module narrative is supplied on the student information page.
Structuring Your Classroom Time
1. Quiz & Discussion. This module typically will be delivered after the initial module on Electricity, Work, and Power, but can function as a stand-alone module. Begin the class with a quiz from the prior week's module. Have students come to class with quiz questions, and select five students to read their questions aloud. After each student recites her/his question, pause for the appropriate time for their classmates to write answers. After all five questions have been completed, ask five different students how they answered. This format allows focused discussion on the topic. Students have an opportunity to work through what is right and what is wrong with their understanding. The teacher gets feedback on the effectiveness of teaching materials and teacher delivery—what is clear and what is still muddy. Use the instructor-regulated discussion pedagogy as explained in the course overview for developing discussion of the current module.
It is important to help the students use what they learned in the first module to build deeper understanding of this module. As the professor, you can help them apply their general knowledge of energy to a consideration of how to exploit geothermal energy. Energy from and to Earth considers the spatial arrangement of geothermal energy and ground-based energy for the generation of power and for thermal exchange systems. The opening quiz reinforces the scaffolding of learning and forces students to revisit the previous module twice, once during their studies when they review for the quiz and formulate the quiz questions, and again when they participate in class discussions.
The flipped-classroom structure advocated for this course facilitates the development of metacognition by the students, directly involving them in the learning process. The use of student-generated questions for quiz and discussion helps students become aware of how they learn and understand the material. This is a big departure from the simple memorizing of terms and concepts that characterized much of their earlier education. Having students think of questions for quizzes and discussion will inform their approach to learning in general. Another metacognitive strategy used in the course is requiring the students to apply basic science concepts to understand a technology, and then requiring them to think about the application of that technology in the real world. The exercise having students examine tectonic plate distribution provides a global overview of the location of geothermal activity. The exercise of building a ground-based heat exchange loop brings the learning to a local level. The description of enhanced geothermal systems allows students to understand this particular method of energy extraction for electricity production, but also provides parallel understanding of natural gas extraction via hydraulic fracturing.
A major theme of this course is that students see the various technologies in the context of the global system and this requires systems-thinking. This module on Energy from and to Earth provides excellent opportunities for students to exercise systems-thinking. The simple exercise of examining the distribution of tectonic plates allows students to consider global variation in geologic systems—and the spatial and temporal scales at which they occur. The use of geothermal heat production for electricity generation (closed and open loop systems) and the exploitation of shallow-earth environments for ground-based heat exchange (closed loop) bring home the point that human systems are intimately related to Earth environmental systems. The use of energy from and to Earth provides alternates to fossil fuel use, CO2 production and other contributions to climate change.
2. Student Presentation. Each class should have one or two PowerPoint presentations by students, given either during the current module or at the beginning of the next module. This activity involves peer instruction. Example Questions:
- Present a case study examining geothermal use in Iceland.
- Present a case study of geothermal power generation in California.
- Present a world map that shows potential places of geothermal resources, and discuss your findings.
- Present a cost-benefit analysis of using ground exchange for heating and cooling in your home state.
3. Hands-on laboratory work. The heat exchange activity will involve cooperative learning as the students will work in groups to accomplish the task.
Teaching Notes and Tips
Geothermal energy use is variable, and students must understand the opportunities and applications of different strategies to exploit energy from Earth, whether extracting hot water to heat homes, or generating steam to create electricity, or using different heat exchange systems for home heating/cooling. Geysers and seafloor spreading demonstrate locations where the heated interior of Earth comes in close contact with the surface. Ground -exchange systems, however, do not exploit heat from Earth's interior but instead use heat conducted from the surface (heated by the sun) and retained in the ground.
The most geothermal energy capacity in the world to date is generated by the United States, mostly in California. In other western states Earth's crust is thin, and hotter rocks are nearer the surface, making them available for geothermal power generation. The 3000 megawatts of power generated in the United States represents about 1/3 of the total geothermal power generation among twenty-four countries. Other countries that produce significant amounts of their country's energy needs from geothermal include Iceland, El Salvador, and the Philippines.
Think-Pair-Share Exercise: Even far away from hot spots of concentrated heat at the surface, there is heat below ground. If one could drill down many km into the ground, the surrounding rock is warm. Why?
Because geothermal heat pumps move at least three times more heat than they use in electricity to power the circulation, the efficiency of heat pumps is 300% or more.
Set-up for the demonstration of ground exchange is pretty simple: use narrow diameter flexible tubing (aquarium pump tubing works well), and coil it on the bottom of an empty 8- to 10-gallon aquarium. Add sand to cover the tubing, leaving inlet and outlet to tubing hanging outside aquarium. Put the entire aquarium in a drying oven and leave overnight at 40C. For demonstration, remove from oven and place inlet tube in an ice bath. On the outlet end, attach a hand pump to pull the ice water into the tubing underground in the aquarium. Collect the water from the outlet tube and compare temperature between water in ice bath and water having passed through the ground exchange system. What factors would lead to variation in heat exchange? See if students can conduct experiments demonstrating this variation.
Possible Activity: Contact your facilities coordinator on campus and if available, visit a campus building that has ground exchange HVAC.