3. Thermal Energy from Light
In this module, students learn the basic principles and techniques involved with converting solar radiation to useful heat. They explore the concepts of temperature and heat, and the physical basis for solar energy. Students conduct hands-on activities to understand how these solar conversion devices work.
Students will be able to:
- Articulate the relationship between the fundamentals of nuclear fusion and sunlight.
- Use hands-on activities to understand that sunlight is comprised of different wavelengths as represented by colors.
- Recount the historical development of solar heating and solar cooking.
- Create an annotated diagram of a solar-powered hot water system for household use.
- Use data they collect from experimentation to discover the relationship between energy uptake and color for solar collectors.
- Explain the greenhouse effect, including the role of short and long wavelength radiation, and relate this to data they collect from experimentation.
Context for Use
The module should be useful in a variety of courses, from introductory to more advanced settings. It can be used as part of a green energy course, or as a unit in an environmental science class. The module is most appropriate for class sizes of 20 or fewer since students conduct hands-on activities. It requires access to the outdoors and works best on sunny to moderately cloudy days. The activity takes 1 to 3 hours, depending on the depth of exploration. The hands-on activities require the construction of simple solar collectors and the use of a commercial or home-built solar cooker. We also recommend demonstrating a Fresnel lens, as it can set a piece of black paper on fire almost instantly — focusing student attention as well as it focuses light!
Description and Teaching Materials
Be sure that you and your students read the materials in the Thermal Energy from Light student section.
Structuring your classroom time
Here are examples of ways to handle poor questions. Festus asks, "Fill in the blank, a ____ is used to convert energy from wind to electricity." Festus is looking for the answer "wind turbine." You might say, "OK, Festus, it is important to know the names of the technology we are studying, but can you make the question deeper?" If Festus succeeds in improving the question, provide praise and go forward. If Festus cannot find his way to a better question, you could then say, "How about, in comparison to a power plant run on fossil fuel, what are two advantages and two disadvantages of using wind turbines to produce electricity instead?" Or, "Before the invention of the wind turbine, what are two historic wind-technologies that helped shape the human experience over the last 500 years?" Sometimes students will come up with seemingly bizarre questions. Hesitate before rejecting those offerings as it may be interesting to see where the classmates go with their answers. Warning: just because students offer a question, it does not mean they know a correct response themselves! How to deal with student answers? Often the student asked to provide an answer will have it wrong, at least in part. Rather than providing the answer yourself, first ask other students for their responses. If nobody got it right, you might consider asking for a substitute question.
After completing the quiz process, I suggest moving on to the discussion phase. All students should have brought four questions based upon the readings/videos assigned for the new material (that day's module — Thermal Energy from Light). Use the instructor-regulated discussion pedagogy as explained in the course overview for conducting this exercise. The length of this phase will depend on how much time you need for the rest of that day's activities. If possible each student should have had the opportunity to ask or answer a question during this exercise. As with the quizzes, rely first on other students to produce an acceptable answer, rather than simply jumping in yourself.
Scaffolding Learning — It is important to help the students use what they learned in the preceding modules to build deeper understanding of this module. The student readings reference important concepts from the module on Electricity, Work, and Power. As the professor you can help them make the connections. Remind them that Watt demonstrated the equivalence between heat and work. Point out that the energy source for the wind discussed in the previous module is ultimately the sun, which is also the prime source of the thermal energy discussed in this module. Note that solar hot water heaters are rated in kilowatts, different only in orders of magnitude from the megawatts used to rate wind turbines. Remind them that megawatts and watts are units of work or power (energy per unit time). The opening quiz reinforces the scaffolding of learning. It forces the 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 the quiz process in class.
Metacognition — 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 the quiz, discussion, and post-student presentation interactions is an important strategy in teaching the students about 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. You are teaching them a new way to approach their studies that, if mastered, 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 to requiring them to think about the application of that technology in the real world. The portion of the exercise that has them look at wind energy availability in their own state and apply that information to thinking about the efficacy of wind turbines moves along those lines. The thermal energy module also offers students the opportunity to use their sense of temperature-touch to better understand the topic. They can feel the radiant heat emanating from a solar collector.
Systems Thinking — This unit presents a great opportunity to teach the concept of equilibrium. This is when a system is in dynamic stability due to the balance of input and output for a particular entity. In the exercise using different-colored solar collectors, the students will plot the change in temperature over time. As long as the sky has fairly consistent illumination (sunny or same degree of cloud cover), each collector will reach a maximum constant temperature. Ask the students why the collectors do not continue to heat up beyond the equilibrium point. They should be able to identify that the new energy coming in to the system from the sun (shortwave light mostly) is balanced by the heat (long-wave) radiating from the collector.
2. Student Presentation — Remember you must give the students at least one week to prepare their PowerPoints! We suggest following the quiz/discussion with a short (10- minute) student presentation on a topic from the module. It is good to choose questions for the student presentations that go beyond the provided readings. We like to require that all the students in the class write two questions to be asked of the presentation. The next portion of the class should be devoted to one or more student presentations. We suggest that if you have Blackboard or a similar teaching aid, that you have your students post their PowerPoints in a discussion group so they may be reviewed by all the students. This activity involves peer instruction. Potential student presentation questions:
1. You now understand how sunlight is transformed into heat to do some useful tasks. But what if you wanted to store that heat for use at a later time? One way to store large amounts of heat for later use is to warm a substance to cause it to go through a change of state (solid to liquid, or liquid to gas). This requires much latent heat, and when the substance cools, it returns that latent heat. Present a case study where salts are used as the material for heat storage and release. Note, such salts are used both on the industrial scale and for single home-level projects. Explain what kind of salts are used and how the system works.
2. Many people would like to put solar hot water systems in their homes, but are restricted from doing so because they live in a neighborhood that is regulated by a community association or city ordinance that bans placing such devices on the roofs of their house. Discuss three case studies that address this issue.
3. Present a case study of a building modified or designed to use active solar devices to provide hot water and space heating. Be sure to provide details on how each device works, and if available, the cost and savings associated with the installation.
3. Hands-on activity The remaining class time (assuming a three-hour lab setting) should be used for the hands-on laboratory work. There are three types of devices used in this module, different colored solar collectors, a solar cooker, and a Fresnel lens. You will want to download and provide the students the Handout for solar thermal lab (Microsoft Word 2007 (.docx) 32kB Jul1 17).
An alternative to the solar collectors described above is to use mason jars. Place paper or foil of the different colors inside the mason jars. Replace the metal lid with a sheet of plastic wrap (like for covering food containers you put in the refrigerator). You can use the regular screw rings or just rubber bands to secure the plastic. Then insert a thermometer in the plastic. Lay the jars on their side with identical exposure to the sun. For demonstrating the greenhouse effect have one jar with and one jar without the plastic lid in place.
b. Fresnel lens —
d. Prism — Use a prism to demonstrate the spectrum of wavelengths comprising white light by projecting the resulting rainbow on light-colored background, such as concrete or sidewalk.
Teaching Notes and Tips
Students will likely need help graphing the data generated from the different colored solar collectors. Remind them that time is the independent value and goes on the X-axis, while temperature is the dependent parameter and goes on the Y-axis. They should run the experiment for about 20 minutes and plot all the data on the same graph to facilitate comparisons.
The assessment page contains an extensive explanation of different ways to assess each student's work. This includes participation in the quiz and discussion, the student presentation, and scoring of the student write-up of the activity. There are also recommended pre/post test questions on the course assessment page.
Pre/Post test questions:
References and Resources
The Global Alliance for Clean Cookstoves is a United Nations and private partnership to address air pollution from home cookers.
Thermal-based absorptive air-conditioning requires a lot of heat. Another approach is to combine the advantages of absorptive and compression-based systems. Such hybrid systems employ a solar collector to superheat the refrigerant, requiring less work by the compressor. This is described in the article More Efficient Solar Powered Air Conditioner Developed in Australia by Lloyd Alter.