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Some people enjoy baking cookies in the oven, some people enjoy baking themselves in the backyard. Even if oil-soaked sun-worshiping Saturday afternoons are the most direct experience most of us every get with the energy of the sun, we know
instinctively that the sun is essential for life. It turns out it's also essential for just about any type of energy you can think of.
Solar energy is free and inexhaustible, and has been for the 5 billion years or so that the planet has been in existence. In the broadest sense, solar energy supports all life on earth and is the basis for almost every form of energy we use.
- The sun makes plants grow, which are burned as fuel or rot in swamps and are compressed underground for millions of years to become coal and oil.
- Heat from the sun causes temperature differences between areas, causing the wind to blow.
- Water evaporates because of the sun, falls on high elevations, and rushes down to the sea, spinning turbines as it passes.
But the term "solar energy" usually refers to ways the sun's energy can be used as heat, lighting, and electricity.
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One simple, obvious use of sunlight is to light our buildings. The sun can also affect a building's heating and cooling costs: If properly designed, a building can capture the sun's heat in the winter and reject it in the summer, while using daylight year round for lighting. With the exception of that guy down in the bowels of the Grinning Planet accounting department who seems to thrive on flickering fluorescent lighting, most of us prefer natural light.
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Besides using design features to maximize use of the sun, some buildings have active systems to gather and store solar energy. Solar collectors sit on the rooftops of buildings to collect solar energy for space heating, water heating, and space cooling. Most solar collectors are large flat boxes, painted black on the inside, with glass covers. In the most common design, pipes in the box carry liquids that take the heat from the box and bring it into the building. This heated liquid, usually a water-alcohol mixture to prevent winter freezing, is used to heat water in a tank or is put through radiators to heat the air.
Oddly enough, because of the cooling effect moist air has when it evaporates, solar heat can also drive a cooling system. Such systems are currently at work in humid southeastern climates, like that of Florida.
By using mirrors and lenses to concentrate the rays of the sun, solar thermal systems produce high temperatures that can be used to heat water for producing
steam to drive an electric turbine or for industrial applications, like boiling water to sterilize soup cans.
Solar concentrators come in three main designs: parabolic troughs, parabolic dishes and central receivers. The most common is parabolic troughs—long, curved mirrors that concentrate sunlight on a liquid inside a tube that runs parallel to the mirror. Parabolic dish concentrators and central receivers can produce much higher temperatures and produce electricity more efficiently but are more complicated and are not in common use.
In 1839, French scientist Edmund Becquerel discovered that certain materials would give off a spark of electricity when struck with sunlight. Solar cells work because the silicon substrate has a weak grip on its electrons. The cells are made of two layers of silicon, one with too many electrons (the n-layer) and one with too few (the p-layer). When light hits the first layer, electrons are knocked loose. As they flow toward the layer with too few electrons, they pass through an electric circuit, the current from which can be used to power equipment and devices.
In the 1970s, a serious effort began to produce photovoltaic panels that could provide cheaper solar power. Experimenting with new materials and production techniques, solar manufacturers cut costs for solar cells rapidly, as the following graph shows.
SOURCE: UCS/Worldwatch Institute
Many solar panels are used today to power cellular phone transmitters, road signs, and water pumps, as well as millions of solar watches and calculators. But most of the market for solar electric is concentrated in off-grid homes, in the villages of developing countries and the vacation homes of industrial countries. Developing nations see PV as a way to avoid building long and expensive power lines to remote areas.
Recently, even utilities in developed countries have been attaching photovoltaics to their power grids. In some locations, it is less costly and politically difficult to install distributed solar panels than to upgrade the transmission and distribution system needed to meet ever-growing electricity demand.
This distributed-generation approach provides a new model for the utility systems of the future. Small generators, spread out in a city and controlled
by computers, could replace the large coal and nuclear plants that dominate now.
As the cost of photovoltaic systems continues to decline, they will find increasingly larger niches. No other electrical generator is as easy to install or maintain. Even among the various types of renewable energy, photovoltaics have great potential. The cells are made of silicon, one of the most plentiful materials on earth, and they draw power from the everlasting sun, so they will never run into the problem of fuel scarcity. As PV prices continue to fall, solar power will become a significant source of electricity in the 21st century.
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We now pause to remember our dear departed surfing Uncle Sandy, who once started a referendum in Beach City to annex "the warmth of the sun and all of the gnarly waves." We suspect that all those years of sunlight striking his head must have knocked loose a few cranial electrons.
In the next Eco-Logical we'll talk about how solar energy in your home can be a winner for your budget and for the environment.
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This article was adapted, with permission, from the Union of Concerned Scientists' article
How Solar Energy Works, 2/11/2003. UCS is the leading science-based nonprofit working for a healthy environment and a safer world. An alliance of more than 250,000 citizens and scientists, UCS combines independent scientific research and citizen action to develop innovative, practical solutions and to secure responsible changes in government policy, corporate practices, and consumer choices. Visit them at www.ucsusa.org.
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