Small Wind Site Assessment Produced by the Institute for Sustainable Futures; UTS in partnership with the Alternative Technology Association and TAFE NSW - Northern Sydney Institute This project has been assisted by the New South Wales Government through its Energy Efficiency Training Program

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Small wind site assessment course: Review of Day 2 and Objectives for Day 3

Review case studies Slide 4

Small wind site assessment course: Extrapolating wind speeds to hub height

Small Wind Site Assessment Process 1. Understand the customer2. Assess the site: desktop study3. Evaluate the site: site visit Use alpha to calculate hub height wind speed If necessary, apply displacement height to determine effective wind speed 4. Estimate the energy resource5. Investigate other constraints6. Determine system options7. Do site assessment report

Wind speed at hub height Why do we need to know the wind speed at hub height? What are our options for gathering wind speed data? Slide 7 Are these options always going to give us wind speed at hub height?

Wind speed at hub height Okay – I have my wind speed at 80m from the NSW RE Precinct Wind Map. What is the wind speed at the tower height? Slide 8

Wind speed at hub height Step 1. Estimate tower height based on 10m/150m rule and minimum tower height rule What is the equation for minimum tower height? Slide 9

Wind speed at hub height Step 2. Gather wind speed data from most reliable source Slide 10 Grid-tied: Average annual wind speed Off-grid: Monthly average wind speed

Wind speed at hub height Step 3. Determine the site’s surface friction coefficient Surface Description Wind Shear (α) Smooth hard ground, lake, or ocean.20 Level country with 1/2m-high grass; and tree lines > 300m away0.25 Level and uniform open terrain with crops with occasional trees and buildings 150m-300m 0.30 Terrain is open with scattered trees/buildings <150m0.35 Terrain is less open, not densely wooded; Trees/buildings <150m, OR Mixed densely wooded areas & open areas; Tree lines >150m’ away 0.4 Mixed densely wooded areas & farm fields; Trees/buildings <150m0.45 Densely wooded0.50 Urban areas with tall buildings 0.60 assumed, but must monitor

Wind speed at hub height Step 4. Always extrapolate from higher levels down to hub height. Slide 12 Extrapolating up from a lower level up to hub height overestimates wind speed

Wind speed at hub height Step 5. Use the following equation Slide 13 V hub = (H hub /H ref ) α V ref V hub = Velocity at the hub height V ref = Velocity from reference data H hub = Height of the tower H ref = Height of the reference data α = the wind shear coefficient

Wind speed at hub height: Quiz! Slide 14 Height of tower = 36m Height of wind speed from map (H o ) = 70m Wind speed at map (V o ) = 6.7 m/s Site has has tall row crops, hedges and occasional trees What is the wind speed at the tower height? ? V hub = (H hub /H ref ) α V ref

The impact of misjudging alphas Wind speed (m/s) Wind shear Wind speed The effects of wind shear on wind speed (adapted from SGES, 2008) Estimated wind speed at 30m using reference wind speed at 6.7m/s at 50m

Wind speed at hub height: Quiz! Slide 16 Height of tower = 36m Height of wind speed from map (H o ) = 70m Wind speed at map (V o ) = 6.7 m/s Wind shear alpha estimated at 0.2 What is the wind speed at the tower height? ? V hub = (H hub /H ref ) α V ref

What about displacement height? (Chiras, 2009)

What about displacement height? Slide 18 Wind map resolution: Typically 400m x 400m NSW RE Precinct: 90m Assumptions: Predominant vegetative cover and height Missing: Results are pure modeling outputs that have not been tuned to local observations, e.g. ground clutter, small rises and valleys, shelter belts, groves of vegetation

What about displacement height? Slide 19 Missing: e.g. ground clutter, small rises and valleys, shelter belts, groves of vegetation Which means: Selected heights chosen on wind maps may not always be correct. E.g. dense groves displace the wind flow upwards. When vegetation is dense, the “effective ground level” is not the base of the vegetation.

Wind map speedSpeed in reality (Adapted from Chiras, 2009)

Displacement height rules Slide 21 IF: 1.Obstacle is upwind (towards prevailing wind), and 2.Tree grove is >15m in depth, and 3.If tree grove is closer than 10x (average mature) height of the tree THEN: 1.The wind profile is raised by the displacement height, and 2.The displacement height becomes the effective ground level.

Site with displacement height

Displacement height rules Slide 23 Deciduous trees = 2/3 x mature tree height Evergreen trees = ¾ x mature tree height Residential structures = ½ way between eves and peak Flat roof commercial building = Roof height (AWS Truewind, 2003 )

1.Displacement Height (DH) = Mature Obstacle Height x Displacement Factor 2.Effective Wind Map Height (EWMH) = Wind Map Height + Displacement Height 3. Use effective wind map height (EWMP), to extrapolate the wind speed to real tower height EWMH DH 60m 6.4 [60m + DH] Wind data source: Before displacement After displacement

Displacement height calculation Slide 25 Wind Map Height Original + Displacement Height Effective wind map height Quiz: If the deciduous trees are 18m tall, what is the displacement height of the wind mass? Quiz: What is the effective wind map height of if the original wind map height was at 70m?

Wind speed at hub height: Quiz! Slide 26 Previous example: Height of tower = 36m Height of wind speed from map (H o ) = 70m Wind speed at map (V o ) = 6.7 m/s Wind shear alpha = 0.30 What is the wind speed at the tower height with a 12m displacement height? ? V hub = (H hub /H ref ) α V ref

Application Slide 27 Once calculated, effective wind map heights can be used as: The H ref in the [V hub = (H hub /H ref ) α V ref ] The input into wind turbine performance calculators

Displacement height rules Slide 28 If there is more than 1 prevailing wind direction, E.g. o prevailing wind from open direction 60% of time o prevailing wind from obstructed direction 40% of the time Apply corresponding percentages to annual energy output, e.g (.4 x (Output at (Effective wind map height) + (.6 x (Output at original wind map height). = Annual energy output

Small wind site assessment course Determining energy output from wind turbine Exercise (computer lab)

Estimating energy output from wind turbine Slide 30 After this exercise you will understand how to Estimate turbine annual energy output the old-fashioned way using a spreadsheet, inputting raw data Estimate turbine annual energy output using a software tool, inputting estimated variables