Session 15 wind power

T. Ferguson, University of Minnesota, Duluth. 2008

Wind Energy

GE Wind Turbine, The Netherlands


Wind Energy Basics

• Solar Driven• Immense Resource• High variability, poorly correlated to loads• Non-dispatchable• No economic storage of wind energy• Best resources require significant high

voltage transmission additions• Power proportional to cube of wind speed


Wind Energy Basics

P(v)A

=ρ2

V3

Where P(v) = power, in wattsA = area perpendicular to flow, in m2 ρ = density of fluid, in kg/m3 v = velocity of fluid, in m/s

The ratio of power of a 10 mph wind to a 5 mph wind issubstantial:

(v10/v5)3 = 8

Therefore, site selection is critical.


Wind Energy Basics

• Wind speed increases with height as ~ 1/7 power: v2/v1 = (h2/h1)1/7

• 50 m hub elevation compared to 30 m height: about 7.6% higher speed

• Height enhanced by cube of velocity:

(1.076)3 = 1.245, or 24.5% more power• Above about 1 km above earth’s surface,

wind not appreciably affected by surface


Wind Energy Basics

• ρair = 1.226 kg/m3 at 15 °C (288°K) and 1 atmosphere (29.92 inHg)

• Using PV = nRT (ideal gas law):– ρ variation with Temperature extremes:

• 90°F = 32° = 305 °K: V305/V288 = 305/288 = 1.059

Therefore, ρ305 = 1/1.059 X ρ288 = 0.944 ρ288

Hot air (w.r.t. 15 °C) reduces available power by ~ 6%

• -40°F = 233°K: ρ233 = 1.236 ρ288

Cold air (w.r.t. 15 °C) increases available power by ~ 24%

• Therefore, wind power is relatively sensitive to air temp


Wind Energy Basics

• ρair = 1.226 kg/m3 at 15 °C (288°K) and 1 atmosphere (29.92 inHg, or 760 mmHg)

• Using PV = nRT (ideal gas law):– ρ variation with pressure extremes:

• 30.15 in. = 0.8% increase in ρ (w.r.t. 29.92 in.)• 29.00 in. = 3.1% decrease in ρ

• So, cold air consistent with high pressure systems are advantageous


Wind Energy Systems

Now, place a wind turbine in the air flow:• Maximum attainable wind power coefficient

= Betz ratio

= ηmax = 0.593

• Modern units capable of ~ 50% (40% after gearbox and electrical losses)


Wind Energy Systems

Two or Three Blades: (Lift vs. drag design)1. Electrical generation requires higher rotational speeds

possible with lift design2. Lift design must avoid turbulence; hence, two or three

blades3. Pocket of low pressure on downwind side4. Pocket pulls blade toward it – lift5. Direct force on blade pushes blade – drag6. Force of lift is about 10 times that of drag7. Both forces cause rotor to spin8. Rotor typically at 40-400 rpm9. AC generator requires 1200-1800 rpm10. Hence, gearbox required


Wind Energy Systems

Photo from GE website;2.5 MW Series Turbines

Hub end ofblade

Angle of attack neartip

Angle of attack nearNear hub

Wind

1. Blade more prone to stall near hub2. Twist is self protecting in high winds3. Blades usually glass fiber-reinforced

plastic (steel and Al are heavy andprone to metal fatigue)

4. Yaw control keeps perpendicular towind flow

5. Cable twist counter “yaws” the unitoccasionally to unravel cables

Nacelle


Wind Energy Systems

• Cut-in: Wind speed at which usable power produced

• Rated: Minimum speed to produce rated power• Cut-out: Wind speed at which unit brakes

• If interconnected to grid at speeds below cut-in, unit would run as motor

• Once cut-in reached, generator needs load attached to avoid overspeeding

• Units typically produce 660 v at 60 Hz; transformer at each tower steps up to 10-35 kV


Wind Energy Systems

Source: GE website for 2.5 MW series wind turbines; GE claims to havemanufactured over 5000 megawatt-plus turbines

Note that, even though wind speed(and available power) increases after12 m/s, the turbine output remains flat

4 m/s = 9 mph13 m/s = 29 mph25 m/s = 55 mph


Wind Energy Systems

Source: GE website for 2.5 MW series wind turbine


Wind Energy Systems

• Adjacent turbines interfere to reduce energy• Rules of thumb:

– Space towers at least 3 rotor diameters to get out of windshade

– Usually 3 to 5 diameters perpendicular to prevailing winds

– Usually 5 to 9 diameters in prevailing wind direction– Mitigates the greater turbulence


Wind Energy Systems

A rough approximation of unit performance within a farm:

Prevailing Winds

X

X

X X

X

X

Turbine/towerlocation

x

y

Row1

Row2

Efficiency of Nth row =

F ≈ e-2NR2

Where R = x/DD = rotor diameter

For D = 100 m and x = 1000 m,R = 10.

Efficiency of second row isF ≈ e(-4/100) = 96%

Efficiency of tenth row is F ≈ e(-20/100) = 82%


Wind Energy Systems

“The fewer rows deep, the better”


Minnesota Wind Integration Study

• May, 2005: Legislature requires study of impacts• The objectives of the study are to:

1. Evaluate the impacts on reliability and costs associated with increasing wind capacity to 15%, 20%, and 25% of Minnesota retail electric energy sales by 2020;

2. Identify and develop options to manage the impacts of the wind resources;

3. Build upon prior wind integration studies and related technical work;

4. Coordinate with recent and current regional power system study work;

5. Produce meaningful, broadly supported results through a technically rigorous, inclusive study process.


Minnesota Wind Integration Study

• The work reported here addresses two major questions:1. To what extent would wind generation contribute to

the electric supply

capacity needs for Minnesota electric utility companies?

2. What are the costs associated with scheduling and operating conventional

generating resources to accommodate the variability and uncertainty of wind

generation?


Meteorology

• Affects wind generation output across wide areas

• Also affects electrical demand

• High winds during peak demand would be ideal – integration costs would be low

• However, correlation is far from perfect


Meteorology

• Historical weather data from NWS used

• Wind penetration levels of 15, 20 and 25% of projected 2020 retail electric loads

• Output at 152 grid points, at 40 MW each, calculated every 5 minutes

• A 20% energy penetration in 2020 will require 5000 MW of wind nameplate


Turbine Power Curve

Source: Final Report – 2006 Minnesota Wind IntegrationStudy – Volume 1 – November 30, 2006 (MN PUC)


Operating Reliability Constraints

• Objective: very high reliability; lowest cost

• Continuously match generation to load

• Voltage at all nodes within limits

• Regulate frequency, maintain synchronism

• System capable of withstanding major outages or loss of major elements


Integration StudyConclusions

• Increases costs to load ranging from $2.11 (15% penetration) to $4.41/MWh (25%)

• Combining balancing areas significantly improves financial results

• Wider geographic area = more stable output

• Effective load carrying capability (ELCC) ranged from 5% to 20% of nameplate

• Extensive transmission upgrades needed


Mean Wind Speed@ 80m AGL



Wind Net Capacity Factor



Seasonal Output



Impact on Large Coal Units



Environmental Benefits


Session 15 wind power

Education

Transcript of Session 15 wind power