The Effect of Tilt Angle on Solar Panels

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1 The Effect of Tilt Angle on Solar Panels Objectives Students become familiar with the concept of solar insolation: radiant energy that strikes the planet. They explore the relationship between the angle of solar radiation falling on a solar panel and the production of voltage by the panel. Supporting concepts include the effect of axial tilt and seasonal change on solar insolation. Independent variable: The angle at which the solar panel is tilted relative to the ground (tilt angle) Dependent variable: Voltage output Procedural Overview To understand the effect of shade on voltage output in a solar panel, students: Construct a circuit to include the voltage sensor and the solar panel Use a protractor to set the tilt angle of the solar panel Differentiate between angles of insolation and tilt angles for the solar panel Measure the output voltage at various tilt angles Determine the tilt angle that results in the greatest voltage output Correlation to the National Science Education Standards This lab correlates to the following portions of the National Science Education Standards: Abilities Necessary to Do Scientific Inquiry Understandings about Scientific Inquiry Transfer of Energy Earth in the Solar System Abilities of Technological Design Understandings about Science and Technology Note: See the Introduction section, "Correlations to National Science Standards," for additional correlations to the National Science Teaching and Content Standards. Time Requirement Preparation time Pre-lab discussion and activity Lab activity 10 minutes 15 minutes 20 minutes 1

2 The Effect of Tilt Angle on Solar Panels Materials and Equipment For each student or group: Data collection system Voltage sensor Solar photovoltaic panel Patch cords with 2-mm probes (2), red and black 4-mm plug to alligator clip adapters (2), red and black Lamp with 60-W or brighter light bulb Ring stand Three-finger clamp Protractor Ruler Safety Add these important safety precautions to your normal laboratory procedures: Follow all classroom and laboratory safety procedures. Use safety goggles. Keep the lamp at least 15 cm from the surface of the solar panel so that it does not melt the plastic. The lamps will get very hot; do not touch the bulb area. Background The surfaces of the world receive different amounts of sunlight because the Earth is spherical and because of the tilt of the Earth s axis. Sunlight that strikes the planet is called insolation (incoming solar radiation). The great distance of the sun to the Earth results in rays from the sun that are essentially parallel to one another as they strike the surface of the Earth. But from our perspective, standing on Earth, the sun appears closer to or farther from the horizon, depending on the time of day, one s latitude, and the season. The angle that the insolation makes with the horizon can be called the angle of insolation and is measured from the horizon to the sun. The astrolabe, an old mariner s tool, has been used in the distant past to measure this angle. Along the equator, the sun s path across the sky appears high in the sky most of the year, and directly overhead (90 angle of insolation) at noon on the spring and fall equinoxes. As latitude increases, the sun s path appears closer to the horizon (a much smaller angle of insolation). Throughout the year, the sun s position at noon above the horizon depends primarily on latitude, and migrates to the north or south of the celestial equator. The angle of insolation changes constantly with this variation in the sun s position. Knowing the angle of insolation helps to position the solar panel so it is exposed to the most direct rays of the sun for the longest period of time, thus increasing the amount of voltage output it can produce. 2 PS-2856A

3 Flat panel photovoltaic systems (PV) sit in a fixed position, and are often mounted on a roof. The tilt angle is the angle the solar panel makes with the horizontal, that is, the ground level, or the horizon. To find the best tilt angle to position the solar panel, subtract the latitude from 90. If the roof is pitched, subtract the pitch of the roof from the tilt angle to find the additional tilt at which to pitch the solar panel. Example: If you live at 30 N latitude, the angle of insolation would be = 60. If the roof is pitched at 35, then the solar panel could be pitched an additional 25 on the roof to meet the most advantageous tilt angle. (The following diagram identifies the angles referred to but is not to scale.) Angle of Insolation Tilt Angle Roof pitch Lab Preparation Before the Lab: Lab Sequence: The solar panel labs can be completed in any order. Each lab explores a particular aspect of positioning and using solar panels to generate voltage. The teacher notes for each lab contain distinct information about the structure and function of solar cells, and the student lab provides the student with some of this information. 2. Sampling method: This lab uses manual sampling. Review the use of manual sampling, particularly saving the last data point before stopping data collection. 3. Discussion: a. Some students have used or seen solar cookers. Solar cookers are made of various materials, but some of the simplest are cardboard boxes with a clear glass panel in the lid. Depending on the design, the interior may be a dark color to absorb more light and radiate more heat. A black cook pot is used to absorb more heat. There is usually some sort of reflective material that can be positioned outside the box to capture as much solar radiation as possible and reflect it into the solar cooker, maximizing the amount of radiation the solar cooker can accumulate. If you have a solar cooker, or if students can bring one in, students can see the connection between positioning the box and the reflective screen, and cooking the food faster at a hotter temperature. Another artifact of human experience students may be familiar with are reflective screens that sun bathers use to get radiant energy to places on their bodies not directly exposed to the rays of the sun, such as under the chin or in the small of the back. These examples will remind students that objects can be used to direct the oncoming rays of the sun to another location. The property of reflectivity is not important here, but positioning the reflective material so it gathers the most light is the important piece that relates to positioning the solar panel. 3

4 The Effect of Tilt Angle on Solar Panels b. Discuss the placement of solar panels and find out if any students have solar panels on the roof of their homes. If so, are the panels positioned flat on the roof, or are they lifted up to some degree? If possible, show pictures from the Internet of solar panels set up on roof tops at various angles, and discuss the reason for different positioning. Also, look at pictures of solar panels placed on the ground and note the angles at which they are positioned. 4. Determine tilt angle: Demonstrate the tilt angle of a solar panel. Use a pencil with an eraser. Hold the eraser against the surface of the solar panel, so that the pencil is perpendicular to the surface of the solar panel. Now point the pencil at some object in the room, such as the wall clock, keeping the solar panel perpendicular to the pencil. Discuss the changing position of the solar panel relative to the ground. This is the tilt angle. 5. Determine angle of insolation: The position of the sun in the sky can be measured in degrees from the horizon to the sun with a mariner s astrolabe. Here is a simpler way to quickly approximate the solar angle of insolation at your latitude for the current time of year and hour. a. Have students gather the following materials: protractor, 2 pencils, and a sheet of white paper. Each student will need a partner. b. On a sunny day, take students outside to a large unshaded patio or sidewalk. Have them sit on the ground and face toward the sun. c. Tell students to place the paper in front of them on the ground, and stand the pencil on its point in the middle of the paper. d. Without lifting the pencil from the paper, have one student move the shaft of the pencil around until the shadow of the pencil is reduced to as small as it can possibly be. (Tell them to minimize the shadow of their hand so they can see the shadow of their pencil more clearly.) e. Their partner can now use the protractor to measure the angle from the paper to the pencil shaft, which will approximate the angle of insolation of the sun. Note: Caution students never to look directly at the sun. 6. Using a protractor: If students have not had previous experience using a protractor, point out features such as the reversibility of the degree numbering system, the fact that 180 is the maximum measurement, and 90 makes a right angle. Remind them that on some models of protractors, the bottom edge is not 0. During the Lab 1. Ensure that students begin the experiment with the solar panel perpendicular to the table top or desk surface. This is the 90 position. Their final position will be when the solar panel is parallel to the surface of the table top or desk surface. Note: A piece of tape from the stand to the back of the solar panel will hold it in place securely after it has been clamped. 4 PS-2856A

5 Sample Data The following screenshots display examples of data similar to what students should expect to see. Data table showing the first data run Graph of Voltage versus Tilt Angle for the first data run 5

6 The Effect of Tilt Angle on Solar Panels Data table showing the second data run Graph of Voltage versus Tilt Angle for the first and second data runs 6 PS-2856A

7 Answer Key Driving Question How does the tilt angle of the solar panel affect the voltage output of the solar panel? In the space below, make a prediction about how the tilt angle will affect voltage output. (Students predict an answer to this question, so all answers are acceptable.) The tilt angle must make the solar panel perpendicular to the direct rays of the light in order for the voltage to be at a maximum. (The tilt angle and the angle of insolation are complementary angles. Their sum equals 90.) Self Check Questions Self Check. There are many angles to consider when using solar panels: the angle of insolation, the angle the solar panel makes with the ground, the angle of the solar panel with the roof, and the angle of the sun on the solar panel as it courses through the day from east to west. Which of these angles is the tilt angle? The tilt angle is the angle between the slanted surface of the solar panel and the horizontal (usually the ground). Analyze Data Questions 1 2. Please refer to the Sample Data. Analysis Questions 1. Describe the consistent pattern on the graphs for all the runs of data. The pattern in the data shows that voltage is lowest when the tilt angle is at 90 or 0. The closer the solar panel gets to being perpendicular to the direct rays of the lamp, the higher the voltage is. Synthesis Questions 1. The data you have collected is reliable as long as the insolation is at 45 to the surface. At different latitudes and different times of year, the angle of insolation varies. How would you change the tilt angle for different angles of insolation (greater than and less than 45 ) to generate the maximum voltage? For an angle of insolation greater than 45, I would calculate the tilt angle as follows: tilt angle = 90 the angle of insolation. The tilt angle will be less than the angle of insolation. For an angle of insolation less than 45, I would also make the tilt angle equal to 90 minus the angle of insolation. In this case, the tilt angle will be greater than the angle of insolation. Conclusion How does the tilt angle of the solar panel affect the voltage output? Explain your answer using your data. In order for the voltage output to be at a maximum, the tilt angle must position the solar panel perpendicular to the direct rays of the sun s light.. Extended Inquiry Suggestions Investigate changes in insolation. Place the solar panel at the optimal tilt angle and vary the angle of insolation by moving the lamp higher or lower. 1. What happens to the voltage output? 2. How could you compensate for seasonal changes in insolation? 7