[L15] Wind energy

Electricity from wind history

• very old (1890s)

• installed capacity is growing rapidly

Modern wind turbines

• convert kinetic energy of wind

• power proportional to swept area and cube of wind speed

• wind speed is site specific

• air is a low density medium and large swept areas are required for large power conversion

• fraction of wind power that may be converted into mechanical work has a theoretical maximum of 0.593 (Betz limit)

• power coefficient varies with the tip speed ratio (rotor tip speed to free wind speed)

• power coefficient often goes to theoretical maximum, and any increase in power must go with increase in swept area

Installed capacity of grid-connected wind turbines

• exponentially increasing since 2001 (onshore + offshore)

• 1200 today GW

Perceived problems

1. Cost

→ but is estimated to continue decreasing

→ partially thanks to auctions / competition

2. Planning permission: public opinion (noise, visibility, environmental damage)

→ in reality majority is in favor

3. Availability

→ combination of wind & solar capacity gives a stable capacity factor (0.15) all year round

4. Component transport constraint on land (blades)

• true issue, components are huge in size

Wind turbine size

• expansion in number of wind turbines but also in size (larger area = more energy)

• higher turbines → cleaner wind, higher velocities

Wind energy penetration

= Total amount of wind energy produced (GWh) / Total annual electricity demand (GWh)

• enables to see how useful wind power is, useful for investors

Wind power capacity penetration

Installed wind power capacity (GW) / Peak load (GW)

• how much wind power can contribute to producing electricity during peak load

• enables to see how useful wind power is, useful for investors

Capacity credit of wind

= capacity of conventional plants displaced by wind power whilst maintaining the same degree of system reliability

• enables to see how useful wind power is, useful for investors

Installed capacity & capacity factor

• based on ‘nameplate capacity’ or ‘rated capacity’

• ‘rated capacity’ = maximum power that can be continuously delivered

• actual delivered energy < rated capacity

Capacity factor = (actual MWh/a)/(rated capacity*24×365) ~ 20-40%

23.5% for UK farms → 2.5% penetration

29.8% in 2011 → 4.3% penetration

→ progress in UK

Is wind resource a limitation?

8 of 100×100km wind farm = all of EU’s electricity demand

→ resource is not a limitation

UK technical potential ~ 610 GW (wind farm of 20000km² = size of Wales)

Energy content of the wind

• turbines convert the kinetic energy of the wind

→ power flux increases with wind speed

Wind energy & integration

• integration to EU by exporting to countries w/ lower wind speeds

• issues w/ wind farms in North sea (issues w/ over clustering of farms → conflicts)

Wind speed measurements

Anemometer

• gives speed at specific (usually at 10m height)

• must extrapolate w/ theoretical calculations

For accurate measurements:

• number & distribution of anemometer masts depend on size/topography

• earth has boundary layer (wind shear profile)

• boundary layer thickness of several hundred meters

• ideally measurements taken at hub height >100m

• recommended minimum measurement height >=75% hub height

Wind speed statistics

Annual mean wind speed is not usually a good indicator of the likely output of a farm

15m/s mean → wind could blow constantly or at 0m/s for 6 months and 30m/s for other

Need to analyse wind data further

• statistical approach

• speed variation follows Weibull distribution

Data recorded at hourly or 30 min intervals:

• mean wind speed

• max 3 sec gust speed

• standard deviation

• mean wind direction

• mean temperature

• 1 years worth of data minimum

Challenges of wind power

Wind does not always blow

• wind speed varies w/ time of day, weather, time of year

• no phase relationship between variations in available wind power & variations in demand → wind-power is non-dispatchable

• flexible generating capacity is necessary to balance supply variations → pumped storage hydro good balancing solution = large storage capacity

Dispatchable power

• can be turned on and off by grid operators to match demand

Too much turbulence

breakage (50m/s gust on 100m diameter rotor)

Wind turbine types linked to drag & lift

oldest devices use sails spread normal to wind

• utilise drag = force in direction of the relative wind

sail at small oblique angle to relative wind

• more efficient → utilise lift = force normal to the direction of relative wind

Relative wind

• device experiencing wind force F extracts power P=Fv only if device moves with the velocity v in the direction of that force

• wind hits blades, is deflected sideways through lift and moves in the same direction

Lift & drag coefficients

drag D & lift L on a body

C_D=D/(0.5rho*v²A)

C_L=L/(0.5rho*v²A)

C_P=P/(0.5rho*v³A)

v: velocity relative to the body

rho: air density

A: plan-form area

P=F*v

Lift to drag ratio as a function of angle incidence

moderate angle → sharp ratio increase as lift increases quickly

~5-10 degrees = optimate angle → strong lift, relatively low drag

> 15 degrees = stalled region

Velocity triangles

relative wind = incoming wind + own rotational motion

1. free-stream wind velocity

• wind blowing towards the turbine

2. blade tangential velocity

• due to rotation, faster near side, sideways direction

3. relative wind velocity

• vector sum of two above velocities → what blade actually undergoes

Power generated by wind turbine

Power coefficient C_P

Depends on

• ratio of rotor speed to wind speed

= Tip Speed Ratio (TSR)

• Reynolds number Re

→ both functions of wind & velocity

→ power will increase w/ increasing wind speed between square & cube of wind velocity

increase in tip speed ratio = increase in max power coefficient

increase in blades = increase in power coefficient

Betz aerodynamic analysis of wind rotor power

1-D dimensional momentum & energy balance

Actuator disc theory

Maximum power = 8/27*rho*U_w³A_r

Power coefficient = P/P∞

C_Pmax=0.5926 → Betz limit

3 bladed upwind HAWTs

Vertical axis & horizontal axis cross-flow machines

• inherently unsteady = fatigue machines

Horizontal Axis Wind Turbine

• dominate: 95%

• experience steady forces in uniform wind

• slightly more efficient than VAWTs → high TSR which increases efficiency

• also means more noise

1 blade = higher efficiency (less drag from other blades) → but higher TSR to get same power → noise

1-2 blade → mechanical problems w/ movement following wind (changing inertia of rotor) → 3-blade is best compromise

Upwind rotors (blades facing wind in front of tower) avoid tower wake-blade interaction = wind hits blades before the tower (tower causes turbulence)

VAWTs resurgence

• no yaw system needed

• better in turbulent environments

• lower centre of gravity (maintenance at ground level)

• allow closer spacing → could exceed HAWT power densities

Blade element momentum theory

• wind slows & swirls as it passes through the rotor → wake

Offshore wind energy types & locations

• high resources north of Europe

Types:

• fixed into sea bed ~ onshore

• floating platform design (semi submersible)

• spar-buoy

Connected to bed with mooring lines

Floating turbine dynamic motion

• blade deflection from wind + waves = constantly moving, dynamically loaded system

Offshore wind technology challenges

• subsea cables

• grid connection / integration

• offshore access

• offshore logistics installation

• turbine foundations

• need uniform predictions & measurements at every stage w/ data

Problems for wind energy industry

• scaling up: increasing flexibility, gravitational loads, fatigue, control

• wake effects in wind farms

• offshore deployment (maintenance, fixed or floating foundations, wave loading etc)

• public perception: visibility, noise, availability/reliability

• sustainability: what to do with massive blade at the end of life cycle

Wind energy development

Onshore (aerospace & commercial) → offshore (fixed & floating) → multi-rotor…

Increase in size & complexity