WIND POWER

What is it?

How does it work?

Efficiency

WIND POWER- What is it?

All renewable energy (except tidal and geothermal power), ultimately comes from the sun

The earth receives 1.74 x 1017 watts of power (per hour) from the sun

About one or 2 percent of this energy is converted to wind energy (which is about 50-100 times more than the energy converted to biomass by all plants on earth

and atmosphere induces vertical and horizontal

rotation and contours of the land WIND.

~ e.g.: Land Sea Breeze Cycle

Winds are influenced by the ground surface at altitudes up to 100 meters.

Wind is slowed by the surface roughness and obstacles.

When dealing with wind energy, we are concerned with surface winds.

A wind turbine obtains its power input by converting the force of the wind into a torque (turning force) acting on the rotor blades.

The amount of energy which the wind transfers to the rotor depends on the density of the air, the rotor area, and the wind speed.

The kinetic energy of a moving body is proportional to its mass (or weight). The kinetic energy in the wind thus depends on the density of the air, i.e. its mass per unit of volume.In other words, the "heavier" the air, the more energy is received by the turbine.at 15 Celsius air weighs about 1.225 kg per cubic meter, but the density decreases slightly with increasing humidity.

A typical 600 kW wind turbine has a rotor diameter of 43-44 meters, i.e. a rotor area of some 1,500 square meters.

The rotor area determines how much energy a wind turbine is able to harvest from the wind.

Since the rotor area increases with the square of the rotor diameter, a turbine which is twice as large will receive 22 = 2 x 2 = four times as much energy.

To be considered a good location for wind energy, an area needs to have average annual wind speeds of at least 12 miles per hour.

Sizes and Applications

Small ( 10 kW)Homes

FarmsRemote Application

Intermediate

(10-250 kW)

Village Power

Hybrid Systems

Distributed Power

Large (660 kW - 2+MW)

Central Station Wind Farms

Distributed Power

Community Wind

Site Selection

The Wind Project Development Process

Wind Assessment

Environmental Review

Economic Modeling

Interconnection Studies

Permitting

Sales Agreements

Financing

Turbine Procurement

Construction Contracting

Operations & Maintenance

Land Agreements

Wind Turbine Size-Power Comparison

Aerodynamics of HAWT Blade

Boyle, G., Renewable Energy, 2nd ed., Oxford University Press, 2004

r[L(sin D(cos )] = Torque

Torque x rotational speed= Power

Wind Cost of Energy

0

2

4

6

8

10

12

1990

CO

E (

/k

Wh [co

nsta

nt

2000 $

])

Low wind speed sites

1995 2000 2005 2010 2015 2020

High wind

speed sites

Bulk Power Competitive

Price Band

June 19 20, 2007 Wind Energy 10

Large and Small Wind Turbines

Large Turbines (600-2000 kW)

- 100 MW

$1,300/kW

Designed for low cost of energy (COE)

Requires 6 m/s (13 mph) average wind speed

Value of Energy: $0.02 - $0.06 per kWh

Small Turbines (0.3-100 kW)

-grid and off-grid applications

$2,500-$8,000/kW

Designed for reliability / low maintenance

Requires 4 m/s (9 mph) average wind speed

Value of energy: $0.06 - $0.26 per kWh

June 19 20, 2007 Wind Energy 11

Small Wind Turbines

Blades:Fiber-reinforced plastics, fixed pitch, either twisted/tapered, or straight (pultruded)

Generator:Direct-drive permanent magnet alternator, no brushes, 3-phase AC, variable-speed operation

Designed for:Simplicity, reliabilityFew moving partsLittle regular maintenance required

50 kW10 kW

400 W900 W

Components of Wind Power System

Nacelle

World Wind Power Status

Risingenvironmentalconcernand depletingfossilfuel causethe shiftof powergenerationtoward renewableresources.

Amongthe renewableresources,wind power is the fastest growingenergygenerationsystemwith 31%annualgrowth since2001.

Kyoto University

14

Source: World wind energy association

With this trend, wind power capacitydoublesin everythree years,andwould reachabout1900GWby 2020(WWER09).

Power in Wind

Power in Wind

Specific power vs. wind speed.

power in the wind increases as the cube of wind speed. This means doubling the wind speed increases the power by eightfold.Doubling the diameter increases the power available by a factor of four

Power in Wind

Economies of scale that go with larger wind turbines. The cost of a turbine increases roughly in proportion to blade diameter, but power is proportional to diameter squared, so bigger machines have proven to be more cost effective.

Power in Wind

Power in Wind

Power in Wind

Power in Wind

Power in WindAssuming T is a fairly constant quantity

Power in Wind

Power in Wind

IMPACT OF TOWER HEIGHTSince power in the wind is proportional to the cube of the windspeed, the economic impact of even modest increases in windspeedcan be significant. Air flow is affected by the surface through which it is flowing; for rough surface speed is highly reducedGet reduced friction of air flow by mounting the turbine at high altitude

Power in Wind

Power in Wind

Power in Wind

MAXIMUM ROTOR EFFICIENCYnumber of energy technologies have certain fundamental constraints that restrict the maximum possible conversion efficiency from one form of energy to another.

For heat engines, it is the Carnot efficiency that limits the maximum work that can be obtained from an engine working between a hot and a cold reservoir. For photovoltaics, we will see that it is the band gap of the material that limits the conversion efficiency from sunlight into electrical energy. For fuel cells, it is the Gibbs free energy that limits the energy conversion from chemical to electrical forms. What is the constraint that limits the ability of a wind turbine to convert kinetic energy in the wind to mechanical power?

It is derived by the Beltzlimit

Power in WindThe power extracted by the blades Pbis equal to the difference in kinetic energy between the upwind and downwind air flows:

Power in Wind

CP

Power in Wind

To find the maximum value of Cp, get first derivate of Cp w.r.t and set it to zero.

That gives

Beltzefficiency

For a given windspeed, rotor efficiency is a function of the rate at whichthe rotor turns.

If the rotor turns too slowly, the efficiency drops off since the blades are letting too much wind pass by unaffected. If the rotor turns too fast, efficiency is reduced as the turbulence caused by one blade increasingly affectsthe blade that follows.

Theusualwayto illustrate rotor efficiencyis to presentit asa function of its tip-speedratio (TSR). Thetip-speed-ratio is the speedat which the outer tip of thebladeismovingdividedby the windspeed:

Power in Wind

Power in Wind

Power in Wind

Notice that if the rotor itself is about 43% efficient, as Fig. 6.11 suggests, then the efficiency of the gear box times the efficiency of the generator would be about 66% (43% 66% = 28.4%).

Power in Wind

30.5wP v

3 33(0.5 ) 0.5 ( 0.5 ( ))w P avg avg avgP C v v NOT v

3( )avgv

Calculation of Average Power

We have

Then average power will be

Calculation of

Similarly,

Wind Statistics

Wind StatisticsExample 6.9 Average Power in the Wind. Using the data given in Fig. 6.22, find the average windspeed and the average power in the wind (W/m2). Assume the standard air density of 1.225 kg/m3. Compare the result with that which would be obtained if the average power were miscalculated using just the average windspeed.

Wind Statistics

Wind Statistics

NOTE:In the above example, the ratio of the average wind power calculated correctly using (v3)avg to that found when the average velocity is (mis)used is 400/210 =1.9. That is, the correct answer is nearly twice as large as the power found when average windspeedis substituted into the fundamental wind power equation P = 0.5 Av3.

Wind StatisticsWind Power Probability Density Functions

The type of information displayed in the discrete windspeedhistogram in Fig. 6.22 is very often presented as a continuous function, called a probability density function (p.d.f.).

Wind Statistics

Wind Statistics

Wind Statistics

Wind Statistics

Of the three Weibullp.d.f.sin Fig. 6.24, intuition probably would lead us to think that the middle one, for which k = 2, is the most realistic for a likely wind turbine site;

that is, it has winds that are mostly pretty strong, with periods of low wind and some really good high-speed winds as well. In fact, when little detail is known about the wind regime at a site, the usual starting point is to assume k = 2.

When the shape parameter k is equal to 2, the p.d.f. is given its own name, the Rayleigh probability density function: