2.3 Wind Energy Utilization

Transcription

1 Module 2.3 Wind Energy 2.3 Wind Energy Utilization Gerhard J. Gerdes Workshop on Renewable Energies March 9, 2005 Majuro, Republic of the Marshall Islands Content Global and local wind systems Influence factors on the wind flow Wind variation in space and time Statistical description 2

2 Global wind resource map annual average wind speed 1993 to 2003, at 50m height 3 Wind sources atmosphere = thermal engine moving air masses thermal energy is transformed into kinetic energy as the earth moves underneath, circulation and wind speed pattern are created = global circulation movement of air masses (wind) by three basic mechanisms: seasonal cycles regional phenomena local winds determined by orographic conditions 4

3 Average radiation balance Northern hemisphere 5 Solar radiation is not evenly distributed on the earth s surface it decreases towards the poles creates excess energy around the equator causes a deficit in the polar regions for compensation, air moves from the equator to Northern part of the hemisphere, leading to the Rossby circulation (Northern, Southern hemisphere), and the Hadley circulation in the equatorial regions 6

4 Atmospheric Circulation Simplified model of atmospheric circulation Model of tricellular general circulation 7 Global circulations 1 movement near the ground movement higher in the atmosphere 8

5 Influence on the Wind Regime Wind from sea thermals sea beach grass trees hills woods buildings cities valleys & mountains upwind Affecting the wind: Temperature Roughness Obstacles Orography 9 Wind Resources - What do we want to know? Energy yield Average long term wind speed Distribution of wind speed Distribution of wind directions A small deviation of the average wind speed from 5 m/s results in a large fluctuation in annual energy production: Deviation of wind speed from an average of 5 m/s Fluctuations in annual energy yield 10% 20% 20% 45% Power generation: Fluctuation of average wind speed in time Availability 10

6 Wind is variable in time and space Wind variation in space: global and local wind systems wind direction changes wind shear profile (roughness) effects of orography and obstacles Wind variation in time: long term (> year) annual seasonal diurnal short time (gusts and turbulence) 11 Local winds differences in the radiation balance can also be caused by different reflection of solar radiation on the earth s surface major local winds: sea-land breezes mountain-top mountain-valley winds influenced by temperature differences on account of changing surfaces of the earth 12

7 Sea land breeze 13 Geostrophic surface winds undisturbed wind = geostrophic winds, mainly in high regions (~ 1000m) boundary layer near the surface of the earth turbulence increases close to the ground energy transfer through eddies from geostrophic winds to layers below 14

8 Logarithmic power law the variation of wind speed with height, relative to the reference height, can be approximated by a simple law: this formula shows the vertical distribution of wind speed only as a function of roughness length zo thus, the formula is very useful for extrapolation from measuring (reference) to hub height of a turbine, but it is only a first estimate 15 Vertical wind speed distribution depending on roughness relative wind speed 10m = 32,81ft 16

9 Wind Profile and stratification 80 instable Height z, m neutral stable Wind speed, m/s Influence of thermal stratification on the vertical wind profile 10m/s = 22,4 mph 17 Measured Wind Profile during the course of a day at different heat flow Wind profile under stable conditions during winter Unstable wind profile caused by thermal convection during summer 18

10 Wind profiles measured during different stability conditions Share of occurence (Wilhelmshaven) Share of occurence (Wilhelmshaven) Height above ground level, m ,01% 0,15% 3,0% 6,0% 38% Mean wind speed [m/s] unstable... neutral Class of potential temperature difference [K/m]: < -0,022-0, ,016-0, ,009-0, ,004-0, ,004 0, ,02 0, ,04 0, ,08 >= 0,08 1m/s = 2,24 mph stable neutral unstable 0,5% 9,4% 10% 38% 33% Mean wind speed [m/s] stable... neutral 19 Roughness length typical roughness lengths m open sea without waves, ice 0.01 m short grass, lawn 0.1 m bush land 1.0 m forest, towns, area with lot of obstacles limitations of the logarithmic power law only a first estimation for increase of wind speed with height a lot of other factors influence wind speed distribution such as stratification of temperature with height pressure differences with height in practice, roughness length for different wind directions are determined 20

11 Obstacles on the ground the surface roughness z o was assumed to be homogenous in its effect on the the wind speed profile however, real surfaces normally don t have a homogenous roughness structure in addition, obviously, the roughness on the windward side of (=before) the turbine is more important than the leeward very often, the effects of changing surface roughness and obstacles have to be gathered empirically some typical effects of obstacles in the following 21 Cluster of trees in front behind the obstacle 22

12 Buildings 23 Steep embankment (cliff) 24

13 Gentle (ideal) hill 25 Wind direction distribution during different months Monthly average in spring Monthly average in autumn 26

14 Annual wind direction distribution Annual average 27 Wind Speed Fluctuations in Time Wind speed [m/s] Wind direction [ ] Measured time series of wind speed and direction over 29 days 28

15 Spectrum of the Wind Peaks at 4 days, 12 hours and 1 minute if averaging time 10 min < tave < 2 hours, distinction can be made between large scale variation and turbulences 29 Seasonal Fluctuations Seasonal variation and annual average wind speed in Hamburg, For time periods of 9, 5 and 2 separate years. 30

16 Daily Fluctuations Average wind speed a certain times o the day (10m above ground level (a.g.l).) Recording period: mainly Long term variation of the annual average wind speed 180% Normalised Average Production vwind_hub-height 160% 10 years sliding average (production) 14 m/s 12 m/s 140% 10 m/s 120% 8 m/s 100% 80% 60% 40% years of data 6 m/s 4 m/s 2 m/s 0 m/s 32