November19th.CEE265.docx

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Department
Civil And Environmental Engineering
Course Code
CEE 265
Professor
Phillip Savage

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CEE 265 November 19, 2013 WIND POWER Renewable electricity  Wind, solar, and other forms share some qualities: o Available throughout most of the world o Low energy fluxes o Intermittent availability (due to periods of no sun or no wind) o High capital cost per unit power output  Relative intensity of energy being received (flux) is a lot lower for renewable o Conventional energy forms are higher o Wind and solar are fairly expensive compared to fossil generation  Electricity generation figures o Effectiveness (EFF): the fraction of energy in a resource that can be collected by the system o Capacity factor (CF): a long-term average power output relative to the maximum  Relates money spent on system to how much we use on an hourly basis  Ideally we want this to be high o But renewable resources are intermittent so the CF is usually low Wind Power  It is not a new resource o They were used for milling in ancient societies o Used to sail ships for many years now  But we are rediscovering it for power generation o Pumping water in the Netherlands o Weathervane on top of farmhouses  Modern wind turbines have a different function then they used to o Now they generate electricity so their structure are very different in design o We want faster rotation and less turbulence o So we made them much taller, with fewer blades, and much larger  The main components to a wind turbine: o Two main portions: the rotor and the generator  The rotor are the blades  The generator converts the mechanical energy extracted from the wind’s energy into electricity o The gear box is needed to match low speed of rotational speed of the rotor blades to the higher speed of the generator  Maximize the amount of kinetic energy that is captured o Can be oriented horizontally (like over a highway)  Or vertically (like a ground based turbine)  Most used for electricity will be horizontal axis on a tower  Gets it above obstructions lower to ground like trees  And horizontal axis we can face downwind or upwind  Most efficient to face them into the wind  Power content of the wind o P =w½ mv 2  We usually don’t talk about wind in mass though  We have to use the volumetric flow rate to convert 2  Pw= ½ (Av)v  Pw= ½ Av 3  So power content of the wind is proportional to the cub of the wind speed o Plot power vs. wind speed  Curve that goes as a cubic function  Makes sense now that we want to put turbines in area where it’s windy o For a HAWT (horizontal axis wind turbine) swept area = A = ¼ D 2  That’s why the blades are getting bigger and bigger  We now have jet sized wind turbines for that reason Tower Height  The higher it’s mounted, more power content in the wind can be harnessed o So how much does the wind speed increase as we go up the tower?  That’s given by a power law relationship too  Depends on a friction coefficient o Based on our judgment of terrain characteristics, we can find the optimum height  Wind flow over hills o The flow stream gets compressed o Increases the mass flow rate in a smaller volume  higher wind speed o So for hillier terrain the friction coefficient will be larger o Changing the tower height will have a bigger impact on hills than on flat terrain Maximum Rotor Efficiency  Remember the Carnot limit o There is only some fraction of the energy that can be converted into useful work o Wind power is not a heat engine, but there is still a maximum amount  Wind turbine aerodynamics o Wind passes through the area swept by the wind turbine o Sheds some kinetic energy to keep the rotational speed of the turbine going o So the power transferred from wind into rotational energy is the difference of the kinetic energy upwind and the kinetic energy downwind  The mass flow rate is the same upstream and downstream (at steady state) o Have to figure out what is happened at wind turbine itself o Speed of wind passing turbine is average of upwind and downwind speeds  Rotor efficiency tells us how much energy is taking from the wind to spin the turbine o So this has to be less than one o But downwind velocity must be greater than zero or else the wind wouldn’t exist o So ratio has to be 0 < x <1  Highest rotor efficiency will give us highest wind content o To locate this maximum, take derivative of v wv and set equal to zero o If you do that you get the answer is 1/3 o If speed downwind is 1/3 of speed upwind you get highest rotor efficiency  So this mean in the best case scenario we can only convert 60% of the wind into power o Actual wind turbine don’t actually get to 60% of power efficiency o They usually get to about 45 -50 % of wind content converted into electricity  Rotor efficiency as a function of tip speed ratio o How fast tip is moving divided by wind speed approaching turbine o Describes how fast the turbine is spinning relative to the wind o Ratio of 5 to 7 is optimal  Get to slow, there is too much drag imposed  Get to fast, there’s not sufficient lift to convert energy o So that’s the range we typically operate within  Wind turbines are designed for rated wind speed (V R o Generators designed to deliver maximum rated power (P ) R o When the wind speed is less than the rated wind speed, the generator puts out electricity at its rated output o Energy produced will be proportional to the cube of the wind speed if we are below the rated wind speed o If the air is nearly idle, the generator won’t run  “cutting” wind speed o If we go above the rated wind speed, the generate can only operate at capacity  So we rea
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