|
Page 1 | 2 | 3 | 4
Project Ideas continued
2. Measure the Effect of Clouds.
Clouds cause major changes in sunlight on the ground. Puffy cumulus clouds block considerable sunlight when they pass between the sun and the ground. Clouds also reflect sunlight back into space and toward the ground away from the cloud's shadow. This means that you will usually measure less solar current on clear days than on days with cumulus clouds. This is clearly shown by comparing the graph on the previous page with the SKY CONDITION column in the table on the previous page. Even better is to compare the table and graph above with fisheye photos of the sky taken when the sunlight measurements were made.
 Click to view enlarged version.
Digital fisheye photographs at Geronimo Creek Observatory by Forrest M. Mims III. Used with permission.
Thanks to these fisheye photos that show the entire sky, it's quite clear that the highest solar current occurred on days with cumulus clouds. Note the lower values when the sky was free of clouds.
Project: Measure the effects of clear skies and various kinds of clouds on sunlight. Make a graph of your results.
3. Measure the Effects of Haze, Smoke and Air Pollution.
Haze, smoke, dust and air pollution absorb sunlight and reflect it back into space. When the measurements in the chart above were made, dust from the Sahara desert arrived over Texas on 26 July. Dust was still present the next day, and there was much less dust on 28-29 July. The fisheye sky images above show that the sky was darker when the dust was present. The chart above shows that the solar current was indeed reduced when the dust was present.
Dust from Asia can arrive in the Western US during spring. Dust from Africa arrives in the southeastern US during summer and fall. Natural haze usually occurs during summer, as does smoke from forest, brush and agricultural fires. Haze caused by sulfur emitted by coal-burning power plant pollution covers much of the Eastern US from spring to fall.
Project: Measure the changes in sunlight caused by haze, smoke, dust or air pollution on clear days. Avoid days with clouds near the sun, for sunlight reflected from the clouds usually increases the solar current that you will measure.
4. Measure Sunlight Under Trees.
Plants that grow under tree canopies receive very little direct sunlight. Some sunlight penetrates the branches and reaches the ground, where it forms small bright spots known as sun flecks. Some plants under the canopy may be briefly illuminated by sun flecks only a few times each day.
Project: Use the solar radiometer to measure the sunlight in a sun fleck and in the shade. Compare these data with the sunlight in an open field. Make a list of plants that live under a tree canopy.
5. Measure Sunlight in the Shade of Buildings.
Cities with tall buildings form "urban canyons" which can be shady much of the time, especially during winter. Even small buildings cause significant shading of the landscape.
Project: On a day with few or no clouds, measure the sunlight away from buildings. Then measure the sunlight scattered from the sky while standing in the shade of a building. What is the difference? If you have a thermometer, measure the air temperature in the open and in the shade of the building. Be sure to keep direct sunlight from striking the thermometer. What is the difference in the sunlight and in the temperature that you measure in the open and in the shade?
6. Measure the Albedo (Reflectance) of Plants, Soil, Rocks and Building Materials.
According to NASA's Earth Observatory, "Albedo is the fraction of incoming sunlight that our planet reflects back to space. If Earth was covered in ice like a giant snowball, its albedo would be about 0.84, meaning it would reflect most (84 percent) of the sunlight that hit it. On the other hand, if Earth was completely covered by a dark green forest canopy, its albedo would be about 0.14, meaning most of the sunlight would get absorbed and our world would be far warmer than it is today. Satellite measurements made since the late 1970s estimate Earth's average albedo to be about 0.30."
Project: On a day with few clouds, stand in an open space and point the solar radiometer directly at the sun. Record the solar current. Then turn around with your back to the sun and point the solar radiometer straight down at the ground and away from your shadow. Record the solar current. Divide the solar current when the radiometer is pointing straight down by the solar current from the direct sun. The result is the albedo.
For example, if the solar current is 7 mA when the solar radiometer is pointed straight down at grass and 21 mA when pointed directly at the sun, the albedo is 7/21 or 0.33 (33 per cent).
Albedo is very important to understanding how much the Earth is warmed by sunlight. Measure the albedo of various materials, including grass, soil, gravel, asphalt, wood and so forth. Be prepared for some surprises!
For best results, shield the solar module from sunlight scattered from the sky by means of a sun screen made from black construction paper that can be slipped over the solar module. Fold the paper to form a rectangular box around 5 cm (2 inches) inches long that is open at each end and can be slipped over the solar module and the top end of the multimeter. Use tape to hold the box together and in place over the end of the multimeter where the solar module is mounted.
7. Measure Sunlight Through Leaves.
The chlorophyll in leaves strongly absorbs the blue and red wavelengths (colors) of sunlight. The energy in the blue and red light permits the leaf to produce starch.
Project: You can use the solar radiometer to measure how much sunlight is absorbed by various leaves. Begin by pointing the solar module directly at the sun. Record the solar current. While keeping the solar module pointing at the sun, place a leaf over the module. (The leaf must cover the entire module). Record the solar current. Divide the solar current when the leaf covers the solar module by the solar current from the direct sun. The result is the transmission of sunlight through the leaf.
For example, if the solar current is 2 mA when the leaf covers the solar module and 20 mA when pointed directly at the sun, the transmission is 2/20 or 0.10 (10 per cent).
Questions:
1. True or false: When light strikes a solar cell or a module of several solar cells, an electrical current is produced.
2. When puffy cumulus clouds are floating in the sky near (but not before) the sun, is the sunlight at the ground (a) unaffected, (b) slightly dimmer or (c) slightly brighter? Why?
3. What is albedo?
4. Do haze, dust, air pollution and smoke cause a slight (a) increase or (b) decrease in sunlight?
5. On a clear day, sunlight is brightest in the (a) morning, (b) noon or (c) afternoon?
Going Further:
The links listed above provide much more information about sunlight.
While the intensity of sunlight peaks in the green (about 500 nanometers), silicon solar cells have a peak response in the near-infrared (around 900 nm). This means that solar cells exaggerate the near-IR wavelengths of sunlight over the visible ones. Nevertheless, solar cells have long been used to make solar radiometers.
References:
Mims, Forrest M., Getting Started in Electronics, Master Publishing, p. 77, 2004.
Ibid., Science and Communication Circuits and Projects, Master Publishing, pp. 82-92, 2004.
|
Project Idea contributed by Forrest M. Mims III, Geronimo Creek Observatory, Texas
|
|
