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Written by Miranda Huey   
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Thursday, 13 May 2010

Renewable Energy Sources

Renewable energy sources are constantly being analyzed and debated in the news, but do you really know how the actual process works? Where does that energy actually come from? Impress your friends with some scientific knowhow and learn about the science behind six major renewable energy technologies: 1) Wind, 2) Solar, 3) Nuclear, 4) Hydropower, 5) Hydrogen Fuel Cell, and 6) Geothermal.

Wind: The simple wind turbine has become an icon of clean energy.1 Its unique shape and elegant lines are visually appealing to many, but it actually depends on three complex elements, the wind cycle, and the turbine.

Everyone knows that wind is just the movement of air, but what causes the wind to move? Actually, wind is the combination of sunlight and the earth’s rotation. Whenever the sun heats a certain spot of air relative to the surrounding air, that spot of air will become less dense and rise upwards, leaving a relative vacuum underneath. Cold air flows to that relatively empty spot, creating wind on the surface of that spot and in the surrounding area.2 This temperature difference happens very predictably along the coast, since land heats up and cools down faster than water does. During the day, the hot land warms the air, sending it upwards, and creating a breeze from the sea towards the land. At night, the opposite happens.

Another major cause of wind is the rotation of the earth. Air higher up from the spinning earth drags behind the air below, creating a vertical mixing. As air moving from the equator moves towards the poles, it seems to curve relative to the surface, but it’s actually just continuing to move quickly in a slower and slower air movement, creating a horizontal mixing. The two forces combine to make an ever-turbulent atmosphere, perfect for capturing wind energy.3

A wind turbine is able to convert the moving air particles into rotational energy. It does that with blades that curve out on one side and are flat on the other. As wind passes across both sides, the wind on the curved side is compressed against nearby wind, forcing more air towards the flat side of the blade relative to the curved side, pushing the blade on its flat side, spinning the turbine.4

The spinning turbine spins a coil in between the south and north end of two magnets. Electrons always flow forward if the north magnetic pole is to the left and the south magnetic pole is to the right. That’s because all the electrons in the magnet are spinning the same way, and opposite facing spins move in the same direction on opposite sides, repelling the electron forward.5 A spinning turbine alternates the current, creating movement of the electrons back and forth along the metal coil. These electrons can either move across a circuit, otherwise known as an electric current, or can be stored in a battery for future use.6

Solar: Solar cells are another great power source, since the sun gives off abundant amounts of light energy. Solar energy works by exciting electrons to a higher energy level in a conductive material, such as silicon. Silicon atoms next to phosphorus atoms leave an electron easy to break free from the neutron force of the phosphorus atom. On the other side of the wire is silicon mixed with boron, which has a positive charge, and is connected to the negatively charged atoms through a circuit or a battery, creating a charge. When a light wave hits the free electron in the negatively charged material, the electrons flow through the wire towards the negatively charged side, creating a direct electrical current.7

Another form of solar energy is passive solar, which absorbs and traps the sun’s heat energy. A passive solar building maximizes the amount of sunlight that enters the building, such as facing the windows southward. A passive solar heat collector can be used as a space or water heater. A glass-covered box with a black bottom on the roof of the house can trap heat, heating a liquid inside. The liquid can then heat water or air that gets circulated back inside the house for use.8

Nuclear: Nuclear power has always been a contentious topic in the United States ever since Three Mile Island and Chernobyl, as well as the still unsolved problem of nuclear waste disposal. In France, however, nuclear power provides more than 75% of the country’s electricity.9 Whether for good or for bad, it’s odd to think how these massive nuclear power plants are all centered around one simple reaction: nuclear fission, the process of one atom splitting into two.10

The chemical reaction itself is fairly simple. Make 2-3% enriched uranium. Hit a neutron to one of the U-235 nucleuses. Most likely, the nucleus will absorb the neutron, become unstable, and split into two atoms, expelling a couple neutrons and about 200 million electron volts each time. Sounds simple. But if multiple U-235 atoms are nearby, the neutrons expelled from the first reaction could hit them, too, potentially causing a chain reaction and massive nuclear explosion. Engineers want the uranium enriched enough to get a few neutrons to hit only a few neighboring U-235 atoms, maximizing the energy output of that bundle.

Nuclear engineers then lower the bundle into water. If the uranium bundle gets overheated, engineers lower material that absorbs neutrons into the bundle. Otherwise, the uranium bundle heats the water to a boiling temperature, and the steam is then used to heat another container of water, the steam of which is directed through the turbine, generating electricity. The water is then condensed and sent to a water tower outside to cool, or the tapered cylinder tower that has become an icon of nuclear power.11

Hydropower: The global water cycle is powered by the most stable, renewable resources on earth: sunlight and gravity. Because the water cycle is so massive, humans are able to extract energy at many different parts of the cycle. Hydro dams use the flow of rivers, tidal turbines use the rising and falling tide, and wave power uses the energy of waves. To understand the science behind each technology, you need to understand both the natural element and the human ingenuity behind the invention.

Hydroelectric dams are the most commonly used form of hydropower. Water from oceans, rivers, and lakes evaporates into the air and condenses as rain, snow, and ice in a cooler region, often over landmasses. Since land generally slopes downward towards the sea, gravity pulls water back to the sea through rivers.12 Hydropower dams take advantage of this by harnessing the momentum of river water and converting it to electricity. A large hydro dam blocks a large river’s water flow except for a few openings that channel the water to turbines. The flowing water then pushes the blades of the turbine around in a circle, spinning a generator and creating electricity.13

Tides are one of the most regular and predictable movements of water, making tidal power a fairly stable source of energy.14 That’s because they’re caused by the gravitational pull of the moon and the sun on the earth. The moon and the earth are constantly revolving around each other, and whichever part of the earth is closest to the moon is pulled the most by the moon’s gravitational field. This gives the earth a more oval shape, creating a relative bulge at that spot from the gravity and at the spot on the earth facing directly opposite from the moon from the spinning.15 A tidal barrage is one system that harnesses this energy. It basically acts as a temporary dam, filling up with water on an incoming high tide and letting the water flow back to the sea during the low tide through turbines that power a generator. There are also individual tidal turbines, which stand on poles in water and operate just like wind turbines, except that they are underwater.16

Wave power is another high potential energy source that is already being utilized by Japan and New Zealand. Waves are caused by wind blowing over the surface of the water, which, as explained earlier, is caused by sunlight. One method of capturing the energy is by placing a segmented tube on the surface of the water. Waves bend the tube up and down or side to side at its segments, which can be used to power a generator. Another method of capturing the energy is through balloon-like floats tethered by a movable piston to a costal seafloor. As the balloon floats up and down with the wave, it pumps seawater through a turbine, generating electricity.17

Hydrogen Fuel Cells: Hydrogen is often called the fuel of the future,18 so you might never have realized that hydrogen fuel cell technology was actually invented in 1839 by a man called Sir William Robert Grove.19 Also known as PEM fuel cells, or proton exchange membrane fuel cells, hydrogen fuel cells delight environmentalists by producing water as its only byproduct.21 However, hydrogen gas is not naturally found on earth, and requires electricity to make. That’s why scientists have turned to fuel cells as a pollution-free energy carrier,22 ideally replacing the use of gasoline engines and chemically toxic batteries.23,24

How does it work? A fuel cell is essentially composed of three layers: the anode, the cathode, and the proton exchange membrane. Hydrogen gas is pumped to one side of the cell, called the anode. Oxygen gas is pumped to the other side of the cell, called the cathode. The hydrogen gas in the anode is then introduced to a chemical catalyst, which separate the hydrogen molecule, H2, into two protons and two electrons.25 From the anode, a membrane blocks the protons while the electrons flow through the electrical circuit (which powers the car motor or other object like normal electricity) and to the cathode.26

Similarly, the oxygen gets pumped through the cathode and catalyst, where the O2 molecule gets split into two strongly negative oxygen ions. The remaining protons in the anode, attracted by the two strongly negative oxygen ions in the cathode, cross the membrane to the cathode. Combined with the electrons that have passed through the circuit, these all combine together and form a simple water molecule, H20.

Unfortunately, a single fuel cell produces only about 0.7 volts,27 or about half the voltage of an AAA battery.28 To increase the power, a lot of fuel cells are connected together using bipolar plates, which have few to no chemical reactions with the hydrogen or oxygen ions. Hundreds of these fuel cells stacked together can power a whole fuel cell vehicle, which usually carries a 300 V fuel cell battery.29

Geothermal: Surprisingly, geothermal energy is now being used in 24 countries, powering 25% or more of the electricity in the Philippines, Iceland, and El Salvador. If you haven’t heard of geothermal energy, geothermal energy basically exploits the vast stores of heat energy waiting below the earth’s crust: magma. Many regions around the world have “hotspots” of geothermal energy, where the crust is thin enough to feel the heat.

The most common method of using geothermal heat is collecting it from geothermal springs. The springs happen naturally because cool water has somehow sunken deep enough into the soil that it is heated up by the hot rocks or magma. Heated, the water expands and becomes less dense, allowing it to rise up to the surface again in the form of a hot spring. One way to get the steam to power a turbine is to focus the steam directly into a turbine, and then recondense it into water. A second way is depressurizing hot water into pure steam, which is then directed into a turbine to power a generator.

However, there are ways to capture the energy without having to wait for a hot spring to bubble up in your town, if you live in a geothermal “hotspot.” It’s called a ground-source heat pump, and it now powers over 600,000 homes in the U.S. alone. It works by pumping air or antifreeze down in pipes near the magma, which then gets pumped back to the surface as heated air or antifreeze. In the summer, air gets pumped out of the house and through cooler ground before surfacing in the home again.30

For more information on specific alternative energy sources, check out:
http://www.greeniacs.com/GreeniacsArticles/American-Solar-Energy.html
http://www.greeniacs.com/GreeniacsArticles/Nuclear-Power.html
http://www.greeniacs.com/GreeniacsArticles/Wind-Power.html
http://www.greeniacs.com/GreeniacsArticles/Solar-Options-For-Your-Home.html
http://www.greeniacs.com/GreeniacsArticles/Cellulose-Fuel.html
Browse all Greeniacs Articles Browse all Greeniacs Guides        Browse all Greeniacs Articles
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1 http://ewea.org/index.php?id=60&no_cache=1&tx_ttnews[tt_news]=1800&cHash=70611b2e5d
2 http://science.howstuffworks.com/wind-power.htm
3 http://www.bwea.com/edu/wind.html
4 http://science.howstuffworks.com/wind-power3.htm
5 http://science.howstuffworks.com/magnet3.htm
6 http://science.howstuffworks.com/electricity3.htm
7 http://science.howstuffworks.com/solar-cell2.htm
8 http://www.ucsusa.org/clean_energy/technology_and_impacts/
energy_technologies/how-solar-energy-works.html#Solar_Heat_Collectors
9 http://www.usnews.com/news/energy/articles/2009/03/10/
how-france-sees-its-nuclear-powered-future.html
10 http://www.howstuffworks.com/nuclear-power.htm
11 http://science.howstuffworks.com/nuclear-power3.htm
12 http://tonto.eia.doe.gov/kids/energy.cfm?page=hydropower_home-basics-k.cfm
13 http://www.energyquest.ca.gov/story/chapter12.html
14 http://scidiv.bcc.ctc.edu/gj/Ocean101-Ch10.pdf
15 http://www.pol.ac.uk/home/insight/tidefaq.html#2
16 http://tonto.eia.doe.gov/kids/energy.cfm?page=hydropower_home-basics-k.cfm#top-container
17 http://home.clara.net/darvill/altenerg/wave.htm
18 http://environment.about.com/od/fossilfuels/a/fcv.htm
19 http://inventors.about.com/od/fstartinventions/a/Fuel_Cells.htm
20 http://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/fuel-cell1.htm
21 http://tonto.eia.doe.gov/kids/energy.cfm?page=hydrogen_home-basics-k.cfm
22 http://tonto.eia.doe.gov/kids/energy.cfm?page=hydrogen_home-basics-k.cfm
23 http://www.scientificamerican.com/article.cfm?id=can-hydrogen-replace-gas
24 http://www.terradaily.com/reports/Tiny_Fuel_Cell_Might_
Replace_Batteries_In_Laptop_Computers_999.html
25 http://www.hydrogen.energy.gov/pdfs/doe_h2_fuelcell_factsheet.pdf
26 http://www.hydrogen.energy.gov/pdfs/doe_h2_fuelcell_factsheet.pdf
27 http://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/fuel-cell2.htm
28 http://answers.yahoo.com/question/index?qid=20081201200048AAaJkhZ
29 http://www.ehow.com/facts_6027126_determines-voltage-hydrogen-fuel-cells_.html
30 http://www.ucsusa.org/clean_energy/technology_and_impacts/energy_technologies/how-geothermal-energy-works.html




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