4. Creating Electricity from Light
Students will be able to:
- Recount the fundamental principles of electricity to include the concepts of charge, current, voltage, resistance, and conduction.
- Contrast parallel and series circuits.
- Articulate how the photoelectric effect is harnessed in photovoltaic (PV) cells to create the flow of electrons.
- Distinguish between concentrated and dispersed solar-electric production and note the advantages and disadvantages of each.
- Evaluate the important parameters used for location of solar systems (latitude, slope, aspect, shading, and cloudiness).
- Determine economic cost and environmental impacts of PV technology.
- Differentiate between the different approaches using concentrated solar production to make electricity.
- Distinguish between grid-tied and stand-alone photovoltaic systems and the advantages and disadvantages of each.
- List and explain the limitations of using sunlight to produce electricity.
Students are expected to synthesize information, collect and analyze real-world experimental data, graph and understand the outcomes, and write a comprehensive report. The students also use published geoscience data.
Context for Use
This module is suitable for beginning to advanced college students, depending on the level of expectations set by the professor. It includes extensive reading and a well-developed hands-on laboratory component. 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. It could also be used in an environmental science course, or a class devoted to renewable energy. The module is designed to work best with class sizes up to 20 students. A full 2 hours and 50 minutes will be needed to complete all aspects of the module. Some professors may wish to spread the activity over two or even three class meetings.
Description and Teaching Materials
We have prepared an extensive illustrated reading for your students. We suggest you read it in preparation for class.
Structuring your classroom time
Here are examples of ways to handle poor questions. First for the quiz, Festus asks, "Define solar thermal collector." You might say, "OK Festus, it is important to know what a solar thermal collector is, 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, what are the characteristics of a good solar thermal collector?" Or, "why do most solar hot water heaters have two circuits, a closed one with glycol and an open one for the water?" 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 questions, it does not mean they know correct responses 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. The students should have brought four questions based upon the readings/videos assigned for the new material (that day's module). 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.
It is important to help the students use what they learned in the prior 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. Point out that photovoltaic modules consist of many photo cells connected in series, which results in high voltages, as they learned in Module #1. Note that PV systems for houses are rated in kilowatts, (the amount of electricity they can produce) as learned in Module #1. Remind them that kilowatts and watts are units of work or power (energy per unit time). Note also that the previous module on Thermal Energy from Light informs the new module in various ways, i.e., the importance of proper orientation with respect to the path of the sun through the sky governs both solar thermal and photovoltaic systems. Also help students make the link between the second module,Using Wind to Do Work, and the current module, noting that these are the two dominant green technologies, and they both suffer from intermittent resource availability, and are both ultimately driven by energy from the sun. 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.
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 understanding a technology, and then requiring them to think about the application of that technology in the real world. The portion of the exercise that has them look at light availability in their own state and apply that information to thinking about the efficacy of solar power moves along those lines. One of the hands-on exercises has students use solar panels to create electricity that causes the hydrolysis of water. That the students can see the streams of hydrogen and oxygen bubbles turning on and off with the manipulation of the panels with respect to sunlight provides a visual sense of the consequences of the flow of electrical current.
This module is a good place to teach the concept of flux, and about relationships that are not interactive, but essentially unidirectional. The students will work with photovoltaic cells that use the energy from the sun to produce electricity. Solar energy reaches Earth at nearly a constant rate over time. This flux of photons is intercepted by photovoltaic cells that facilitate the excitation of electrons and the production of electrical current. Ask the students if using solar cells can use up the energy from the sun. How is this different from using deposits of fossil fuels such as coal or petroleum to create electricity? The more fossil fuel that is used, the less there will be available to use in the future. And the reduced availability means it becomes more expensive. That is not the case with solar energy. In the long-run, using fossil fuel to make electricity is subject to negative feedback control. Although this is not the case for solar energy, it is possible to use up all the available space to place solar panels in a particular area. That is an example of a system reaching saturation.
- The US federal government provides a tax incentive for individuals putting PV systems on their homes. Some states provide additional incentives. In addition, owners of such solar systems may earn money under the Solar Renewable Energy Certificates (SREC) system. Explain how the federal incentive works and its monetary value. Find a state that offers additional incentives and explain those. Explain how the SREC system works. If possible, present a case study of a home or business that took advantage of these opportunities. What does it cost these days to install a 5 kw PV system on a house? How long will it take to pay for itself based upon the electricity produced, state and federal incentives, and SREC sales?
- Use the latest information you can find to discuss how extensively solar energy is being used for the production of electricity in the United States and around the world. What portion of the United States' and the world's electricity now comes from solar? What are the shared characteristics of the places with the most use of solar energy? Be sure to consider geoscience, economics, policy and social science as you address this.
- Solar and wind are the two most important sources of renewable energy currently available. Explain in what ways these two renewable resources are similar and in what ways they differ from each other.
- Storing excess energy produced by photovoltaics or wind turbines will become a larger problem as both of these technologies become more prevalent. Discuss various schemes for storing such energy and use case studies if available.
The laboratory work will require at least four small photovoltaic modules. It is convenient to use the solar battery maintainers sold at auto parts and marine stores. They are designed to produce about 18 volts and around 2 amps (example of a solar battery charger for purchase). Be sure to get the clip terminals so it will be easy to arrange the panels in circuits. You will need a volt-ohm meter and an ammeter.
Teaching Notes and Tips
The students should first read the extensive reading created for the student section. That will enable them to better understand the hands-on work. The professor should supply the following items:
- At least 4 small solar panels for making circuits
- A Digital Multi Meter or an ammeter and a volt meter
- A means of shading the panels (piece of cardboard)
- Pencils sharpened at both ends
- Glass beakers or jars
- Tap water
The pencils serve as anode and cathode for the production of hydrogen gas. A complete hand-out for you and the students (Microsoft Word 2007 (.docx) 83kB Jul12 17) is included.
The students must read their assignment before class. They will need help clipping the PV panels together to make circuits, and clipping on the the pencils for the hydrolysis experiment. Point out that they are splitting H2O, so the bubble production will be twice that for H2 as it will be for O2. Avoid the temptation to collect each gas in an inverted test tube and then recombine to make a small explosion. This is rocket fuel!
Supervise the student groups so that one person does not dominate the activity. The hands-on work is best done on a sunny day, but will work less dramatically when overcast.
Summative Learning Assessments for Course and InTeGrate Goals
The assessment methods for this module can be found on the course assessment page.
Pre/Post Test Questions:
References and Resources
- How Photovoltaics Work from US Department of Energy
- annual solar cycle graph from physicalgeography.net
- Good background: Science@NASA story How do Photovoltaics Work
- map showing Solar Potential from the US DOE
- Better, use this Photovoltaic Solar Resource of the US map from NREL to ask students to find annual average energy for where they live.
- Planning a home PV system article from the US DOE
- Experiment to do electrolysis using solar energy from the New Mexico Solar Energy Association (p. 136)