How Easy is Wind Enerhy to Use
Wind Energy: Why We Don't Use It
Reed Miller
November 20, 2014
Submitted as coursework for PH240, Stanford University, Fall 2014
Introduction
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| Fig. 1: A typical wind turbine. (Source: Wikimedia Commons) |
One seemingly simple question has always bothered me: if there is such a terrible impending energy crisis, why don't we use more alternate forms of energy? Specifically, why isn't wind being used? After all, isn't wind an unlimited resource? Upon some deeper exploration, I came to understand that while it may always be windy somewhere, that does not mean there is unlimited energy.
How Wind Is Turned Into Energy
With a typical wind turbine, pressure from wind turns the blades or fans of a turbine, which are attached to a shaft that spins, which is in turn connected to a generator. The spinning of the shaft generates energy, stored as electricity, within the generator. As of 2013, "The current operating fleet of wind capacity in the US can power the equivalent of more than 15 million average American homes." [1] The equation for how wind power is captured by a turbine can be expressed as
where 1/2 pv3 is the power density in the wind, A is the swept area and η is the efficiency of the wind turbine. [2] Specifically, 16/27 represents the Betz limit, or the maximum amount of power any turbine can draw from the wind.
The Problem With the Weather
There would not be so many problems with wind energy usage if there was always the same amount of wind in the same places (other problems include maintenance dangers, upkeep, bird deaths, etc.). [3] At a basic level, wind comes from the energy used by the Sun to heat our planet. When certain areas of the world receive more energy from the Sun, their heat rises, creating a region with higher air pressure. Areas receiving less energy of course have lower air pressure. The air flows from areas of high pressure towards ones with lower pressure, creating wind. However, wind's inherent nature of temperature and pressure change creates a problem. With the Earth and its climate constantly changing (whether or not you believe in global warming), wind patterns are always in some form of fluctuation. The main issue is that there needs to be an equal amount of energy output that the wind turbines can supply, but available at any time. Obviously, the cost for maintaining these additional generators or batteries would be extremely high. "Wind power is considered to be an intermittent energy source, that is to say a source for which output is driven by environmental conditions mainly outside the control of the generators or the system operators. The inflexibility, variability, and relative unpredictability of wind power as a means for electricity production, are the most obvious barriers to an easy integration and widespread application of wind power." [4] Thus, the uncertainty of the wind requires a system that is always available to replace all the electrical output created by the wind turbine system. In other words, it is too expensive to have wind turbines lying around that don't do anything.
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| Fig. 2: Wind patterns in the US. (Courtesy of the U.S. Department of Energy) |
Possible Solutions
One way to improve the wind energy industry would be to create some way of storing surplus wind energy for when there was a lack of wind. Xcel Energy Inc. is currently working on a sodium-sulfur battery that could potentially make wind energy more affordable and reliable. [5] The batteries, created by Japan's NGK Insulators, Ltd. "can store roughly seven megawatt-hours of power, meaning the 20 batteries are capable of delivering roughly one megawatt of electricity almost instantaneously, enough to power 500 average American homes for seven hours." [5] This system, or one like it, could be used to address the main problem the wind industry has: the wind doesn't always blow when you want it to. Additionally, such a battery is not cheap. NGK's battery is roughly $3 million per megawatt. With additional testing, that figure could potentially change.
© Reed Miller. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
References
[1] "Global Wind Report - Annual Market Update 2013," Global Wind Energy Council, April 2014.
[2] A. V. da Rosa, Fundamentals of Renewable Energy Processes, Third Ed. (Academic Press, 2012), Ch. 15.
[3] J. Eilperin and S. Mufson, "Renewable Energy's Environmental Paradox," Washington Post, 16 Apr 09.
[4] P. J. Luickx, E. D. Delarue, and W. D. D'haeseleer, "Considerations on the Backup of Wind Power: Operational Backup," Appl. Energy 85, 787 (2008).
[5] D. Biello, "Storing the Breeze: New Battery Might Make Wind Power More Reliable," Scientific American, 22 Dec 08.
Source: http://large.stanford.edu/courses/2014/ph240/miller1/
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