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WIND ENERGY SYSTEMS(Aeolian Energy Systems)

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ENGS-44 Sustainable Design

Benoit Cushman-Roisin23 February 2012

Renewable Energy Possibilities

Solar1.2 x 105 TW on Earth’s surface

36,000 TW on land (world)2,200 TW on land (US)Wind

Biomass5-7 TW gross

Wind2-4 TW extractable

Tide/Ocean Currents 2 TW gross

Hydroelectric4.6 TW gross (world)

1.6 TW technically feasible0.6 TW installed capacity

0.33 gross (US)

Geothermal 9.7 TW gross (world)0.6 TW gross (US)

(Source: George Crabtree, Materials Science Division, Argonne National Laboratory)

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There are different types of windmills / wind turbines

Old technology:

http://en.wikipedia.org/wiki/Windmill

New technology:

National Renewable Energy LaboratoryPonnequin Wind Farm, Northern Colorado

www.hydrogennow.org/Facts/Wind/wind.htm

Advantages of wind energy over solar energy:

The wind does not stop at night.

Wind energy is efficiently converted into electricity and transported away.

Ideal windy sites are plains, hilltops, wind gaps and offshore, not where people live but in close proximity to them.

Disadvantages:

Wind is fickle. It does not always blow when you want to turn the lights on. So, either an alternate source of energy must exist or electricity needs to be stored somehow. Hydrogen and compressed air are storage options.

Some people raise objections: changing mountain landscape, red lights at night, interference with birds, noise, etc.

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Power extracted from wind increases like the cube of the wind speed:P ÷ V 3

Rate of kinetic energy hitting the windmill: 21VmEK Rate of kinetic energy hitting the windmill:

2VmEK

where the mass flux is: VAm

3

2

1VAEK

in which V = wind speed, = air density, and A = area spanned by windmill.

1In practice the extracted

This means that, when the wind speed doubles, the power output of the windmill is multiplied by 8. Peak winds matter more than average winds.

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2

1VACP p

In practice, the extracted power P is given as: where Cp is called the

coefficient of performance

Wind speed varies over time, according to a probability distribution.

max

min

)()(V

VdVVfVPE

where E = annual energy output, P(V) = power extracted at wind speed V, and f(V) dV = fraction of year during which wind speed falls between V and V+dV.

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Old windmills relied in large part on the drag force and had lousy efficiencies.

Today’s windmills use the lift force almost exclusively, and their efficiency begins to approach the theoretical limit of 59%.

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Low-efficiency turbines designed on drag force:

http://www.windpower.org/en/tour/wtrb/persian.htm

Persian windmill Savonius turbine

membres.lycos.fr/tipemaster/TIPE/Eole/Eole.html

Coefficient of Performance typically no more than 10-15%

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Panemone windmill

A drag device with flipping vanes

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An example of Panemone windmill

The idea…

… the outcome

(http://homepages.paradise.net.nz/albie/projects/panemone.htm)

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Poor performance of drag-based devices is caused by this:

Drag force due to wind speed V :With drag coefficient CD = 1.0 to 1.2

2

2

1VACD

But, plate is moving downwind at speed Uand relative speed of vane is (V – U) and force is reduced to:

2)(2

1UVACD

VelocityForceTime

ntDisplacemeForce

Time

WorkPower

V

U

V

UVACUUVAC

TimeTime

DD

232 1

2

1)(

2

1

Maximum efficiency is only 15% !

Using the lift force with a vertical axis

Advantage of a vertical axis:

Energy is captured regardless of wind direction

Ideal when wind direction→ Ideal when wind direction shifts frequently

Advantages of Darrieus blade over H-rotor:

Longer blade span

Centrifugal force resulting in Centrifugal force resulting in stretching force along the blade

Disadvantage of Darrieus blade:

Efficiency tops at 40%.

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Contemporary turbine with horizontal axis

In green: lift force

In blue: drag force

Use of the lift force in wind turbine design:

SIDE VIEW:lift forceon blade

productivecomponentof lift force

wind

blade speed

air speedrelative to blade In framework moving

with the blade

blade speed

If blade spins faster, the air speed seen by the blade increases and so does the lift force…

… but the angle of projection is less favorable.

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Add the drag force:

+ Blade speed

But, a windmill is not 100% efficient, and there exists an upper limit to the fraction of energy it can extract from the wind. This is called the Betz Limit.

Coefficient of performanceCoefficient of performance = efficiency = ratio of power extracted to available kinetic energy in the wind:

%3.5927

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VA

pVA

KE

P

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= tip-speed ratio = ratio of blade speed at tip to wind speed.

333 )(

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Power

21Power

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pp

C

RAK

AVC

V

R

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Windmill scale Rotor diameter Power rating

micro < 3m 50 W to 2 kW

small 3 to 12 m 2 to 40 kW

medium 12 to 45 m 40 to 1000 kW

Windmill classification

large > 45 m > 1 MW

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750 kW NEG Micon Turbines in Moorhead, Minnesota

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Source: http://rredc.nrel.gov/wind/pubs/atlas/maps/chap2/2-01m.html

Potential for wind energy over New Hampshire and Vermont

Categories:

Class 1: to 4.4 m/s (to 9.8 mph)to 100 W/m2 at 10 m

200 W/ 2 0to 200 W/m2 at 50 m Class 2: to 5.1 m/s (to 11.5 mph)

to 150 W/m2 at 10 mto 300 W/m2 at 50 m

Class 3: to 5.6 m/s (to 12.5 mph)to 200 W/m2 at 10 mto 400 W/m2 at 50 m

Class 4: to 6.0 m/s (to 13.4 mph)to 250 W/m2 at 10 mto 500 W/m2 at 50 m

Class 5: to 6.4 m/s (to 14.3 mph)( p )to 3000 W/m2 at 10 mto 600 W/m2 at 50 m

Class 6: to 7.0 m/s (to 15.7 mph)to 400 W/m2 at 10 mto 800 W/m2 at 50 m

(W/m2 is per m2 of intercepted wind, not per m2 of ground surface.)

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At a finer scale, one notes that wind energy potential in New Hampshire is strongly correlated with orography.

Advantages of going offshore

Stronger wind (= more power) Wind closer to surface (= shorter towers) No need to purchase land (no eminent domain)

Butendiek offshore w

ind faarm in G

ermany

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In 2009 in US, wind:34,300 megawatts installed,up by 39% over 2008

In Year 2009, wind turbines in the United States generated over 73.9 billion kWh of electricity, enough to serve about 7 million households, equivalent to power several large cities. But it is only a small fraction of the nation's total electricity production, about 1.8% (2009 number).

N th t k thi b t thi t f blNow that we know something about this type of renewable energy:

How can we design with wind energy?

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Basic arrangement

Island of Utsira in Norway

Small island of 250 inhabitants located 18 km from the mainland

Energy production:

1.2 MW, approximately 5.1 GWh annually

since July 2004

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Equipment:Equipment:2 Enercon 0.6 MW Wind turbinesNHEL Electrolyser Pout 15 bar 10 Nm3/hHydrogen engine: 55 kWFuel cell: 10 kWFlywheel: 5 kWh

Storage:H2 storage: 2400 Nm3 at 200 bar

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Issue with wind = intermittency

→ need for storage

One idea:

wind → mechanical rotation→ compression of air→ compressed air stored→ decompression thru turbine→ electricity on demand

Wind turbines can be integrated in large buildings.

This opens the possibility of funneling the wind with the structure to increase the wind speed by the time it passes through the turbine(s)by the time it passes through the turbine(s). The power harnessed by a turbine is proportional to the cube of the wind speed. So, a doubling in speed implies a multiplication of power by a factor 8

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Proposed "SkyZED" building at Bermondsey Spa in the UK.

On balance, it has the t ti l f ti itpotential of generating its own

energy over the course of the year.

(http://www.zedfactory.com/bermondspa/bermondsey.htmlobsolote link now...)

The Bahrain World Trade Center

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Another idea: Catching the sea breeze by the sea

The sea breeze is unidirectional, blowing very from sea to land during the day and from land to sea during the night (land breeze). Taking advantage of this property, a butterfly building with wide wings, such as the conceptual seashore resort above, can funnel the wind by a very significant factor. Recall that wind power increases with the cube of the wind speed…

To allow the integration of a full-scale wind turbine, the building needs to be of a certain size. A single-family house is too small. Thus, one has to capture the wind in a different way.

One way is to channel the wind into a duct and push it through a small turbine.

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AeroVironment, based in Monrovia, California

AeroVironment’s AVX400 turbine

(http://www.buildinggreen.com/)

AeroVironment turbine design shown with optional canopy.

AeroVironment s AVX400 turbine designed to take advantage of the rush of wind as it crests a building’s façade. The company launched the new turbines in the fall of 2006.

University of Strathclyde, in Glasgow, Scotland

One ducted wind turbine produces 205 kWh per year. A set of 10 was placed on the roof.