Resource: Solar Car

The Sun provides Earth and its inhabitants with a great deal more than warmth and daylight. In fact, it is safe to say that without the Sun's energy, almost all life on Earth would cease to exist. However, the role that this all-but inexhaustible energy source currently plays on Earth has the potential to expand beyond heating the planet and providing the light that drives photosynthesis. Many experts think that the Sun may one day replace fossil fuels as human civilization's primary source of power, and thus solve some of the most pressing environmental issues of the day.

Solar cells, also known as photovoltaic cells, are devices that convert sunlight into electricity, which can then be stored in batteries for later use. However, this process relies on the intensity of sunlight striking the surface of the solar cell; the less light energy that the cell captures, the less electricity it produces. As a result, the orientation of the cell is important.

The angle at which sunlight strikes a surface dramatically affects the amount of energy that it receives. Energy transfer is at its greatest when the Sun's rays are perpendicular to a surface. At shallower angles, the Sun's energy is spread over a larger surface area and is less intense. For example, the direction of Earth's axis relative to the Sun changes throughout the year. As a result, the intensity of sunlight that strikes a given point on Earth's surface varies. When the north pole of Earth's axis points toward the Sun, the northern hemisphere receives more direct sunlight and it is summer there. Conversely, the southern hemisphere receives less direct sunlight and experiences winter.

In order for a solar cell to be most effective, sunlight needs to strike its surface as directly as possible. Thus, solar cells generate electricity most effectively during the summer. In addition, due to the rotation of Earth, there are daily variations in the angle at which sunlight strikes. For example, solar cells achieve better performance at noon than in the morning or evening because the Sun appears highest in the sky during mid-day hours and the angle of sunlight is at its steepest. To maximize performance throughout the course of a day, the orientation of solar cells should adjust to always face toward the Sun.

The use of solar power is limited by the efficiency of the technology. For example, conventional solar cells convert only about 15 percent of the light that strikes them into electricity. However, higher-efficiency solar cells continue to be developed; for instance, in December 2006, researchers created a solar cell that achieved a conversion efficiency of over 40 percent.

To learn more about using the Sun's energy to meet human energy demands, check out Solar Still Part I: Salt Water, Solar Still Part II: Juice, and Cooking Cookies with Solar Power.

To learn more about energy use and renewable energy sources, check out Snapshot of US Energy Use, Energy Sources, and Global Warming: Beyond Fossil Fuels.

To learn more about seasonal variations in the Sun's angle, check out Global View of the Seasons and Earth in Motion: Seasons.

• How does the angle at which the Sun's rays strike Earth's surface affect the amount of energy received by a given amount of surface area?
• If the car's speed increased as the angle of the Sun increased, do you think the Ferris wheel would go faster at noon than early in the morning? Support your answer by explaining your reasoning.
• How would you go about answering the question above? What measurements would you make?
• In what ways was the investigation you saw in the video a "fair test?" What conditions did they keep the same (constant)? What was the variable?