a better measure than wind generator power curves

home power 132 / august & september 2009 128 A Better Measure than Wind Generator Power Curves Back Page Wind generators produce electricity at varying levels, depending on rotational speed (rpm). Graphing generator output (in watts) against wind speed (mph or m/s) yields a “power curve” for the wind generator (see the “Power Curves” graph). The untrained eye is drawn to the top of the curve—the peak power. For gasoline-powered generators, this is useful information. As long as it’s supplied with gasoline and a load, it continues to produce at or near its rated output. Peak power for a wind generator is very different—at most sites, the wind speed at which a turbine generates its peak power occurs only a very small percentage of the time. A wind “distribution” plots the frequency of each wind speed (see “Distribution” graph). For example, a site may experience 15 mph winds 18% of the time and 40 mph winds less than 2% of the time. If you assume that a wind generator will give you peak power most of the time, you’ll have wildly exaggerated energy output expectations. A crucial fact to understand is that the power available in the wind is related to the cube of the wind speed (V 3 ). For instance, doubling the wind speed gives eight times the power: a 20 mph wind has eight times the energy (20 x 20 x 20 = 8,000) of a 10 mph wind (10 x 10 x 10 = 1,000). If a wind generator produces 3,000 watts at 24 mph, it will produce only about 375 watts in a 12 mph wind. Understanding the V 3 law and a wind distribution curve helps you look at power curves—and wind energy—more appropriately. For most people, wind generator power curves only create confusion about performance. When we buy a car, most of us don’t look at the displacement of the cylinders or the cold cranking amps of the battery. We turn to more important overall measures like fuel economy. So we should leave power curves to the number nerds, and stop distracting ourselves from the prize—energy output. Trying to compare one wind generator to another using the peak from power curves is a common mistake. While there is some useful information in the curves, it’s not a simple comparison. For example, I’ve had two turbines with about the same peak power rating, yet one produced 2.3 times more energy than the other in similar conditions. Instead of using power curves, look on the manufacturers’ Web sites or in their literature for energy curves or graphs (see the example below). With an estimate or measurement of the average wind speed at your site, these curves can help you project the energy yield (kWh) from a particular turbine. Then you can determine how that projection matches up with your energy needs, determine which (and if) wind generator is right for your site, and get on with the job of designing and installing your wind-electric system. —Ian Woofenden Power Curves for Three Turbines 0 5 10 15 20 25 30 35 40 45 Wind Speed (mph) Power (kW) 0 1 2 3 4 5 6 7 8 9 10 11 ARE442 Proven 2.5 Bergey XL.1 Governing Speed Rated Power Rated Power Governing Speed Cut-in Speed Sample Energy Curve Average Wind Speed (mph) *Based on a Rayleigh distribution Energy (kWh) 8 10 12 14 0 400 800 1,200 1,600 2,000 2,400 Wind Speed Distribution 0 2 4 6 8 10 12 14 16 18 20 Frequency of Occurrence (%) 0 10 20 Weibull 30 40 50 60 70 80 90 100 Wind Speed (mph) Rayleigh

Upload: azmat

Post on 16-Dec-2015

217 views

Category:

Documents

2 download

Report

Download

Embed Size (px):

DESCRIPTION

Wind Generator Power Curves

TRANSCRIPT

home power 132 / august & september 2009128

A Better Measure than Wind Generator Power Curves

Back Page

Wind generators produce electricity at varying levels, depending on rotational speed (rpm). Graphing generator output (in watts) against wind speed (mph or m/s) yields a power curve for the wind generator (see the Power Curves graph).

The untrained eye is drawn to the top of the curvethe peak power. For gasoline-powered generators, this is useful information. As long as its supplied with gasoline and a load, it continues to produce at or near its rated output.

Peak power for a wind generator is very differentat most sites, the wind speed at which a turbine generates its peak power occurs only a very small percentage of the time. A wind distribution plots the frequency of each wind speed (see Distribution graph). For example, a site may experience 15 mph winds 18% of the time and 40 mph winds less than 2% of the time. If you assume that a wind generator will give you peak power most of the time, youll have wildly exaggerated energy output expectations.

A crucial fact to understand is that the power available in the wind is related to the cube of the wind speed (V3). For instance, doubling the wind speed gives eight times the power: a 20 mph wind has eight times the energy (20 x 20 x 20 = 8,000) of a 10 mph wind (10 x 10 x 10 = 1,000). If a wind generator produces 3,000 watts at 24 mph, it will produce only about 375 watts in a 12 mph wind. Understanding the V3 law and a wind distribution curve helps you look at power curvesand wind energymore appropriately.

For most people, wind generator power curves only create confusion about performance. When we buy a car, most of us dont look at the displacement of the cylinders or the cold cranking amps of the battery. We turn to more important overall measures like fuel economy. So we should leave power curves to the number nerds, and stop distracting ourselves from the prizeenergy output.

Trying to compare one wind generator to another using the peak from power curves is a common mistake. While there is some useful information in the curves, its not a simple comparison. For example, Ive had two turbines with about the same peak power rating, yet one produced 2.3 times more energy than the other in similar conditions.

Instead of using power curves, look on the manufacturers Web sites or in their literature for energy curves or graphs (see the example below). With an estimate or measurement of the average wind speed at your site, these curves can help you project the energy yield (kWh) from a particular turbine. Then you can determine how that projection matches up with your energy needs, determine which (and if) wind generator is right for your site, and get on with the job of designing and installing your wind-electric system.

Ian Woofenden

Power Curves for Three Turbines

0 5 10 15 20 25 30 35 40 45Wind Speed (mph)

Pow

er (

kW)

0

1

2

3

4

5

6

7

8

9

10

11

ARE442

Proven 2.5

Bergey XL.1

Governing Speed

RatedPower

RatedPower

Governing Speed

Cut-inSpeed

Sample Energy Curve

Average Wind Speed (mph)*Based on a Rayleigh distribution

En

erg

y (k

Wh

)

8 10 12 140

400

800

1,200

1,600

2,000

2,400

Wind Speed Distribution

0

2

4

6

8

10

12

14

16

18

20

Freq

uen

cy o

f O

ccu

rren

ce (

%)

0 10 20

Weibull

30 40 50 60 70 80 90 100

Wind Speed (mph)

Rayleigh