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Solar energy

Passive solar energy systems absorb heat directly from the sun without the use of mechanical and electric equipment, and energy cannot be collected or stored. Active solar energy systems use solar energy to heat a liquid through mechanical and electric equipment to collect and store the energy captured from the sun. Photovoltaic solar cells capture light energy from the sun and transform it directly into electrical energy. Their use is limited by the availability of sunlight. Created by Khan Academy.

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Video transcript

- [Narrator] The sun is about 93 million miles away, which means it takes about eight minutes for light from the sun to reach Earth, but it's still close enough for us to take advantage of solar energy. And why wouldn't we want to? After all, solar energy is renewable and essentially inexhaustible. It's also a clean energy source. So more and more we're turning to solar energy to directly warm our homes or indirectly generate electricity. The first type of solar heating we'll talking about is called passive. As the name suggests, passive happens without much work on your end. In my mind it's the equivalent of you just lying out in the sun, which is why throughout the world, this method is the most widely accessible and inexpensive. Some examples of how homes can be passively heated are through the actual materials that the home is made out of. So materials like stone, concrete, adobe, they absorb a lot of the sun's heat during the day and then slowly release it at night. If we use this adobe house as an example, the thick walls and small sunken in windows keep the house sheltered from the sun during the day. And then at night, the clay material that it's made out of releases that heat energy that it stored throughout the day. So this particular home is located in Santa Fe, New Mexico and a lot of homes that are in desert areas like this are made out of clay and for good reason. Because during the day, in desert locations, it can get very hot and that clay will absorb the heat, keeping the inside of the house pretty cool. And then at night desert areas can get very, very cold. And so that clay will release the heat and keep the house moderately comfortable. Or in the Northern hemisphere, some homes are built with South facing windows, which increases the number of hours that sunlight comes into the home. So these are ways to passively warm a home, but there may be situations in which you wanna just block the sun's heat and light to cool down the home instead. For example, the color of the roof can make a big difference in how much heat does or doesn't get stored in the home. Painting the roof a light color will reflect the sun's heat energy, much like choosing to wear a white T-shirt instead of a black shirt on a hot summer day. There's also a specialty type of roof called a green roof where plants are grown on top of the home to provide an additional layer of insulation. Now, I haven't personally been on one, but I can only imagine in the spring time, if there are flowers planted in the green roofs, they must smell and look amazing. And, of course, you can also create shade by planting trees around the home or adding awnings above the windows. To use solar energy in a different way, we can move from passive techniques to active ones. For example, some homes have big panels attached to the roofs called solar collectors. These solar collectors work by heating up a fluid inside of them and then use pumps or fans to circulate that fluid and transfer heat into the home to warm up the air or water in the home. So the use of these pumps or fans to move the heat into the home is what makes this active heating rather than just passive. A different alternative if you happen to have a very large area of space is a concentrated solar power system or CSP for short, which is basically a lot of reflectors that concentrate that solar energy. One specific type uses mirrors to reflect that solar energy onto a single tower. That energy can then be used to boil water, create steam, and ultimately generate electricity. The largest CSP in the United States is in the Mojave Desert very near California's border with Nevada. It's called the Ivanpah Solar Power Facility. You'll see here that there are three sets of concentric circles with mirrors that are reflecting heat energy onto individual towers. And there are hundreds of thousands of mirrors. The plant takes up 3,500 acres of land and can power more than 100,000 homes in California. The downsides of these systems though is that they do cost a lot of money and space up front and if the weather's bad they don't work as well. There are also stories a few years back where there are hundreds of birds quite literally being burned up as they flew between the mirrors and the towers. So you can imagine the enormous amount of heat energy that's being reflected off these mirrors. On the upside, these reflectors can be cheaper to maintain than other technologies and can make use of large stretches of desert that are already empty all over the world. And finally, one other way to harness the power of the sun is through photovoltaic cells. If we break down that big word, photo meaning light, and volt meaning a unit of electric force, then photovoltaic becomes using light to produce electric force. This idea dates back to 1905 when Einstein first published a paper about it. He then went on to win the Nobel Prize for this concept of the photoelectric effect that when light shines on a metal, it causes electrons to be ejected and generates an electric current. Photovoltaic cells are used in solar farms, on the roofs of houses, and you might even have your very own solar powered calculator. You'll notice these types of calculators work best outside or in a bright room, which can definitely be a disadvantage in some cases. Another disadvantage is that sometimes you'll have to install large amounts of these to generate enough electricity for say a home, which can get expensive, and you also have to be sure you're not moving anytime soon. On the other hand, installing solar panels on a home reduces electricity costs and your carbon footprint. So there you go, lot's of ways to harness the power of the sun, both its heat and its light.