Written by Rishi Das
|Tuesday, 30 July 2013|
Renewable Energy Sources
While physicists may argue that wind and solar energy are not truly unlimited, the sheer availability of this energy coupled with the ability of this energy form to have “renewed” in a cyclic manner through the earth's natural cycles makes this somewhat heavenly and enigmatic energy a great interest to replace fossil fuels. By this definition, even fossil fuels can be seen as a renewable energy. However, human action and the sheer time scale of replenishing highly pressurized and decayed plankton, also known as oil, makes it possible for the human species to consume nearly every drop before natural geochemical cycles could replenish this form of energy. Renewable energy tends to be confused with clean energy, and often the overlap is well justified. Energy sources such as nuclear energy, however, may be renewable but are subject to environmental pollution in the form of nuclear waste. Hence, not all renewable energies are zero emission or carbon neutral , but thankfully most renewable energies adopt these desired qualities. Let’s look at some of the major sources of renewable energy: 1) wind, 2) solar, 3) nuclear, 4) hydroelectric, 5) fuel cells, and 6) geothermal.
Just a brief walk outside on a chilly, fall evening would expose most of us to the powerful push of the winds. Winds are currents of air driven by atmospheric pressure differentials that reach phenomenal speeds.2 In fact, the highest wind speed ever recorded was on the top of Mount Washington, New Hampshire at 231 miles per hour in 1937,3 making wind an appreciable source of energy. But where exactly does wind come from? Wind is actually the manifestation of the combination of heating effects from the sun as well as the earth’s rotation. Whenever the sun heats a certain mass of air relative to the surrounding air, that mass of air becomes less dense and rises upwards, leaving a relative vacuum underneath. As a result of this vacuum, cold air flows to fill the void, creating wind.4 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.
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.6 While the physical basis of wind helps wrap our heads around an omnipresent earthly phenomenon, harnessing this energy becomes a completely different story.
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.7 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.8 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.9 Although the concept of using a turbine to harness the energy of wind is not immediately groundbreaking, several unique and creative designs have enhanced the way that wind energy can be harnessed.10 Designs can be as simple as adding ridges to traditional fan shaped horizontal turbine blades to complex jet engine shaped turbine systems that maximize efficiency. Some of the most intriguing new designs include:
Solar energy has caught headlines as one of the most sustainable and clean energies that is capable of being practically implemented. The most simple and well developed energy capturing mechanism of the solar energy is the photovoltaic (PV) cell. 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 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.14 The photovoltaic approach makes use of light particles exciting electrons within a semiconductor, but we are also aware of the way in which the sun warms the earth's surface. Harnessing this thermal energy15 has led to the creation of a passive system and an active system that makes use of a special industrial set up to convert thermal energy into usable electricity.
Passive solar systems absorb and trap the sun’s heat energy through creative forms of containment. A passive solar building maximizes the amount of sunlight that enters the building through mechanisms as simple as facing the windows southward. A passive solar heat collector can be used as a space or water heater by literally exposing an insulated space with large amounts of sunlight. In the case of solar water heaters and home heating, a glass-covered box with a black bottom on the roof of the house can trap heat, heating liquid water or air inside. The liquid can then heat water or air that gets circulated back inside the house for use.16
While the most recognized solar panel is the traditional photovoltaic cell18 that relies on photonic stimulation of electrons at the panels surface, active thermoelectric power generation is becoming a more practical and alternative option that can better optimize energy gathering from the sun even as night falls. Developed between 1984 and 1991, thermoelectric solar panels are by no means a novel technology and have been sitting around in California's Mojave Desert for eons. Newer engineering developments and greater optimization of the technology have sparked greater interest in an alternative paradigm to solar cells that uses solar heat instead of light intensity to generate power. Also known as concentrating solar cells, solar thermal energy at this stage is best suited for large scale power generation, relying on a complex mechanism of harnessing thermal energy and heating a liquid to drive a turbine. California's Mojave Desert plant makes use of a creative design that arranges reflective mirrors in a parabolic setup to concentrate and focus solar radiation onto a heat exchanger that transmits the sun's heat to a fluid that vaporizes and generates enough pressure to turn a turbine.19 The focal points of these mirrors can reach temperatures as high as 750F, and all together such plants can create a respectable 80 megawatts of electricity. Nowadays, more advanced systems can make use of more clever geometry in the form of towers with multiple parabolic discs, and even design such systems to follow the sun's rays across the day to optimize performance. Altogether, it is hopeful that combined systems will be able to produce sufficient power to handle the American power grid and turn it into a “smart grid .”
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.20 Regardless, today's nuclear power plant relies on a relative simple fission reaction with various control mechanisms in place that are not so simple.21 In simple terms, a fission reaction involves the ordered breakdown of the nuclei of heavy atoms that are so unstable that their splitting can be induced mechanically. Only very heavy and unstable atoms can perform the fission reaction, and once this reaction is induced, a chain reaction can release a tremendous amount of energy to create what we know as a nuclear bomb. In the context of modern power generation, however, the design and workings of the reactor are centered upon controlling what is basically a large nuclear explosion.22
While there are a variety of heavy elements that have viable radio isotopes for nuclear fission, Uranium 235 (U-235) is perhaps most familiar and popularized radioisotope used in nuclear reactions and the atomic reaction itself is fairly simple on paper.23 First one must start with fissile material, and Uranium 235 must be enriched from naturally occurring Uranium ore that is predominantly the 238 isotope. Uranium 238 is unable to sustain a fission reaction, and therefore a considerable amount of energy must be invested in enriching the ore in specialized centrifuges. After making 2%-3% enriched uranium, shoot a neutron into a U-235 nucleus. 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.
As mentioned earlier, nuclear energy is renewable but at a great environmental cost. The radioactive waste generated may take thousands of years to decompose, and therefore there is no where left to dump nuclear waste except for deep underground mines or into the ocean where it would sink to great depts. The limitations in the standard fission reaction have inspired scientists to pursue the idea of nuclear fusion, the converse process that involves the fusion of two nuclei to generate energy as opposed to breaking down a massive nucleus. Unfortunately, fusion technologies are still far from being a reality given the extreme constraints of such reactions. Some well studied theoretical reactions include hydrogen fusion reactions that combine various isotopes of hydrogen to grant theoretical yields of 17.6 MeV of energy, but with the need for a temperature of 40 million Kelvin to completely dissociate nuclear particles from electrostatic columbic interactions.25 While fusion nuclear bombs have been made in the past, controlling this reaction for reactor purposes becomes a completely different story. The most well studied design is based off of the tokomak concept of magnetic confinement that uses a strong magnetic field through superconductors to heat hydrogen isotopes into a plasma state at 150 million Celsius.26 In this case, the magnetic field acts as a barrier of control as opposed to neutron absorbing moderator rods in a traditional nuclear reactor, but this reactor is still in the experimental phase and has been unable to truly achieve nuclear fusion.
The global water cycle is powered by the most stable, renewable resources on earth—sunlight and gravity. Due to the fact that 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, one needs 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.27 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.28
If river waters flowing at high speeds can effectively generate energy, why not also the characteristic wave-like motion of water that is capable of overturning boats and sinking entire towns? The manifestation of waves are multifaceted, with the final appearance of tides being a culmination of the moon’s gravitational pull, solar heating, and therefore, winds that move water masses in the oceans to create waves. Tides are one of the most regular and predictable movements of water, making tidal power a fairly stable source of energy.30 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.31 Some estimates place the total value of wave energy at a whopping 10 trillion watts of energy,32 and therefore the goal to capture even a fraction of this abundant energy is quite worthwhile. Harnessing of wave energy has proved to be quite difficult, especially since the most powerful waves occur in far from accessible and shallow shores. Not only does the device itself have to be located at quite a distance, but this distance also makes energy transferring quite difficult and the energy yields quite low. Simply extending a long cable could prove inefficient and hazardous to marine organisms and boat traffic.
A tidal barrage is one system that harnesses this hard-to-capture wave 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.33 Another example of a simple wave turbine would be the Pelamis Wave Energy Converter that uses a series of hydraulic cylinders to transmit the up and down movements of the device into energy. The 600 foot device is able to produce around 0.75 megawatts of energy, which is dwarfed by some new generation wind turbines and solar cells.34 A more natural and practical design takes the form of a buoy that is able to translate stretching motion on a spar into wave energy. Autonomous PowerBuoy by Ocean Power Technologies uses this basic mechanism powered by advanced computers making minor adjustments every second to convert a navigational guide for ships into a energy generating device. Unfortunately, the PowerBuoy as a single unit is only capable of generating 0.04 megawatts of energy but there is much hope that greater efficiency combined with mass implementation could greatly improve yields from this hydroelectric device.35
Hydrogen Fuel Cells
Hydrogen is often called the fuel of the future,36 so you might never have realized that hydrogen fuel cell technology was actually invented in 1839 by a man named Sir William Robert Grove.37 Also known as PEM fuel cells, or proton exchange membrane fuel cells, hydrogen fuel cells delight environmentalists by producing water as its only byproduct.38 However, hydrogen gas is not naturally found on earth, and requires electricity to create. That’s why scientists have turned to fuel cells as a pollution-free energy carrier,39 ideally replacing the use of gasoline engines and chemically toxic batteries.40;41 One must heed some caution, however, as the mitigation of pollution is only at the end user level, such as an automobile. Producing hydrogen is an energy intensive, electrolytic process that essentially involves running a circuit in reverse to separate hydrogen and oxygen gas in a process known as electrolysis.
It is questionable to include a hydrogen fuel cell as a renewable energy given the need for non-renewable energy to produce its fuel. Until a method is found to use renewable energy to split water, we cannot by definition include fuel cell technology as a category of renewable energy. Nevertheless, the abundance of water as a starting material satisfies at least some categories on its pathway to being an unquestionable form of renewable energy.
Now, how does a hydrogen fuel cell 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, while 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 assists in separating the hydrogen molecule, H2, into two protons and two electrons.42 From the anode, a membrane allows for charge equalization while the electrons flow through the electrical circuit (which powers the car motor or other object like normal electricity) and to the cathode.43 Similarly, the oxygen gets pumped through the cathode and catalyst, where the oxygen molecule, O2, 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,44 or about half the voltage of an AAA battery.45 To increase the power, a great number 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 volt fuel cell battery.46
Solid Oxide Fuel Cells
Since we are on the topic of fuel cells, it is important to mention a particular kind of fuel cell that is currently in the process of being implemented on a more industrial scale. There are in fact many different kinds of fuel cells following the same basic principle as the hydrogen fuel cell but with different electrolytes that create even higher voltages across the anode and cathode.47 Solid oxide fuel cells have shown remarkably promising potential in dramatically increasing fuel cell efficiencies while eliminating the need for precious metals as catalysts in many conventional fuel cells using the same fuel as hydrogen fuel cells—hydrogen. High operating temperatures couples to advanced ceramics design to produce efficiencies of 60 percent.48 Therefore, as a technology, fuel cells offer a great opportunity to reduce end user pollution but continue to face the drawbacks of hydrogen gas generation for use.
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 arise 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.
There has been an evolution in the ways in which geothermal energy is harnessed from deep underground to produce an estimated 3000 megawatts of power in the United States49. Methods described above fall under a construct known as the direct steam plant that acts to focus vaporized steam from underground into a power generating turbine. While this method is still very effective, it does pose the problem of corroding underground pipes. Water, as a fluid, also has an unusually high boiling point for a molecule so small. Therefore, heat exchangers allow for fluids with higher vapor pressure to drive the turbine instead of vaporized water. Designers did not stop here in the utilization of geothermal energy. At the great depths into which geothermal fluids are pumped, there is not only a temperature differential but a tremendous pressure differential as well.
Flash steam plants make use of a clever mechanism of bringing highly pressurized fluid to a subterranean holding tank at lower pressure in a way that induces the vaporization of the fluid and powering of the steam turbine. As the most commonly implemented geothermal plant design, the method allows for pressurized liquids to be flashed multiple times and maximize steam outputs. Newer binary designs add to the complexity with heat exchange points through a working fluid that further improves efficiencies. However, a more passive and domestic way to capture the energy without having to wait for a hot spring to bubble up in your town makes use of temperature gradients to heat air and water for home use. This is called a ground-source heat pump, which 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.51 As a renewable energy, geothermal power holds the greatest prospect with tremendous yields, low costs, and environmental safety due to the lack of harmful gas emissions.
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|Last Updated ( Tuesday, 30 July 2013 )|