Class Notes (839,246)
United States (325,890)
CEE 265 (11)
Lecture

# November19th.CEE265.docx

5 Pages
78 Views

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

This preview shows pages 1 and half of page 2. Sign up to view the full 5 pages of the document.
Description
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
More Less

Only pages 1 and half of page 2 are available for preview. Some parts have been intentionally blurred.

Unlock Document

Unlock to view full version

Unlock Document
Me

OR

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Join to view

OR

By registering, I agree to the Terms and Privacy Policies
Just a few more details

So we can recommend you notes for your school.