Learning About Solar Energy From the Dept of Energy: It's a bit cloudy
Interest in solar energy as an energy source rises and sets along with the tax credits that fuel its growth. Because I am essentially a kid at heart, I visited the Energy Kid's Page of the Energy Information Administration to learn more.
The Energy Information Administration (EIA), created by Congress in 1977, is a statistical agency of the U.S. Department of Energy. "Our mission is to provide policy-neutral data, forecasts, and analyses to promote sound policy making, efficient markets, and public understanding regarding energy and its interaction with the economy and the environment."
The Energy Kid's Page starts off in a friendly manner in a way that even I could understand:
The sun has produced energy for billions of years. Solar energy is the sun's rays (solar radiation) that reach the earth.
Some of this solar energy is converted to heat, which is used to heat water and building interiors.
So far, so good.
Solar energy can be converted to electricity in two ways:
Photovoltaic (PV devices) or "solar cells" - change sunlight directly into electricity. PV systems are often used in remote locations that are not connected to the electric grid. They are also used to power watches, calculators, and lighted road signs.
Solar Power Plants - indirectly generate electricity when the heat from solar thermal collectors is used to heat a fluid which produces steam that is used to power generator. Out of the 15 known solar electric generating units operating in the United States at the end of 2006, 10 of these are in California, and 5 in Arizona. No statistics are being collected on solar plants that produce less than 1 megawatt of electricity, so there may be smaller solar plants in a number of other states.
With childlike wonder, I pressed further, wanting to learn how energy-efficient, renewable solar energy can help reduce fossil-fuel-generated electricity (half of which comes from burning coal, which in turn creates the 104 tons of mercury annually we have the pleasure of breathing, drinking, and eating).
So I dug into the Photovoltaic Section. Without warning or proper disclosure, The Energy Kid's Page sent me back to college for my PhD in electronics. (Guys: Seriously, do you really think your Kid Page is kid-friendly?!)
With quick googling, I found an online university and enrolled, studied night and day for four entire weeks, got my PhD, and now am delighted to give you this kid-friendly explanation of solar energy.
We'll shine a light on photovoltaic energy and the three types of solar power plants.
Photovoltaic Energy
Photovoltaic energy is the conversion of sunlight into electricity. A photovoltaic cell, commonly called a solar cell or PV, is the technology used to convert solar energy directly into electrical power. The solar cell is a nonmechanical device usually made from silicon alloys.
Sunlight has photons. Photons have energy. The solar cell gets slammed by photons and absorbs some of them. Others bounce right off like basketballs or pass through like ghosts.The absorbed photons knock electrons out of place. Electrons are the stuff that electricity is made of. The electrons bubble up to the surface of the solar cell.This movement to the surface creates an imbalance. When you hook up a wire from the solar cell to an outside battery, you create a path that the electrons will follow.That flow into the battery is electricity.
A single photovoltaic cell will power your calculator and your curiosity, but not much more. At 1 or 2 watts, you need hook up a bunch together to get some real juice flowing. This is called an array, and can include as many cells as you can afford. Definitely a case of the more you spend, the more you save.
Don't Stick It Where The Sun Don't Shine
Sunlight is constant - the clouds and fog are not. Climate conditions significantly effect the amount of solar energy received by a photovoltaic array and, in turn, its performance. Most current technology photovoltaic modules are about 10 percent efficient in converting sunlight. Further research is being conducted to raise this efficiency to 20 percent.
AC/DC: Conversion
The battery collects direct current (DC). You need alternate current (AC) flowing through your light fixtures. Solid state devices called inverters convert DC power to AC. Problem solved.
Photovoltaic Energy Recap: photovoltaic or solar cells absorb the slings and arrows of outrageous photons from the Sun, which shove electrons to the surface of the cells, where you store them by having them flow through a wire into a battery. You break out your inverter to make that battery energy usable. But at 10% efficiency, they suck big time in consistently poorly lit areas, such as Sweden during November to January when they're nearly blacked out and lighting their way with seal blubber candles and Ikea accent lamps.
Next up, kids (and I'll keep it friendly):
Solar Thermal Power Plants
Solar thermal power plants use the sun's rays to heat a fluid, which is put into water and turns it into steam. The steam propels a turbine that powers a conventional generator that creates electricity. Solar thermal power generation works essentially the same as generation from fossil fuels except that instead of using steam produced from the combustion of fossil fuels, the steam is produced by the heat collected from sunlight. Solar thermal technologies use concentrator systems due to the high temperatures needed to heat the fluid.
In short, they replace fossil fuels with solar energy to run turbines which create electricity.
The three main types of solar-thermal power systems are:
parabolic troughs
solar dish, and
solar power tower
Parabolic Troughs
The most common type of solar plant, the parabolic trough is used in the largest solar power facility in the world located in the Mojave Desert at Kramer Junction, California. This facility has operated since the 1980's and accounts for the majority of solar electricity produced by the electric power sector today.
A parabola is U-shaped. The parabolic trough is like a magnifying glass: it takes sunlight and focuses it on a spot to heat it up. That spot is on a tower, surrounded by all the parabolic troughs. It heats the spot up to 400 degrees Celsius. The parabola can follow the Sun's movement during the day in order to get the most sunlight.
The spot that is heated up is filled with fluid. This fluid flows to heat up water and create steam. The steam propels a turbine that powers a conventional generator that creates electricity. The fluid cools down a bit and is returned to the spot on the tower to be reheated to 400 degrees.
The plant is usually designed to operate at full rated power using solar energy alone, given sufficient solar energy. However, all plants are hybrid solar/fossil plants that have a fossil-fired capability that can be used to supplement the solar output during periods of low solar energy. This defeats the purposes of generating renewable energy, but I guess the scientists know what they're doing.
Solar Dishes
A solar dish/engine system utilizes concentrating solar collectors that track the sun on two axes, concentrating the energy at the focal point of the dish (the box-like thing sticking out from the center of it) because it is always pointed at the sun.
The solar dish's concentration ratio is much higher that the solar trough, typically over 2,000, with a working fluid temperature over 750 degrees Celsius. The power-generating equipment used with a solar dish can be mounted at the focal point of the dish, making it well suited for remote operations or, as with the parabolic trough, the energy may be collected from a number of installations and converted to electricity at a central point.
The engine in a solar dish/engine system converts heat to mechanical power by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding the fluid through a turbine or with a piston to produce work. The engine is coupled to an electric generator to convert the mechanical power to electric power.
Solar Power Tower
A solar power tower or central receiver generates electricity from sunlight by focusing concentrated solar energy on a tower-mounted heat exchanger (receiver).
This system uses hundreds to thousands of flat sun-tracking mirrors called heliostats to reflect and concentrate the sun's energy onto a central receiver tower. The energy can be concentrated as much as 1,500 times that of the energy coming in from the sun. Energy losses from thermal-energy transport are minimized as solar energy is being directly transferred by reflection from the heliostats to a single receiver, rather than being moved through a transfer medium to one central location, as with parabolic troughs. Power towers must be large to be economical. This is a promising technology for large-scale grid-connected power plants. Though power towers are in the early stages of development compared with parabolic trough technology, a number of test facilities have been constructed around the world.
What We Learned Today
We learned that solar energy can be used to heat water and interiors.
Next, we learned that solar energy can be converted to electricity using photovoltaic cells and solar thermal power plants. The photovoltaic cells take in photons, which shove aside electrons. The electrons, seeking a new home, journey in mass across a wire to their new land called Battery. Solar thermal power plants work by focusing sunlight to heat a fluid, and that fluid turns water into steam, which runs a turbine, and that turbine is the engine to run an electric generator.
And most important, we learned that, no matter what you label something, such as being designed for kids, that does not make it so. I think we need to call in the Department of Commerce to enforce the Truth-in-Labeling Act with the Department of Energy's Energy Kid's Page, one of the most kid-unfriendly websites known as mankind. And I'm going downtown right now to file a claim for reimbursement of my expensive online tuition for the PhD I had to get just to understand this Energy Kid's Page.
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