Tecnologie Chimiche ed Energetiche
ICING ON WIND TURBINES
Dott. Lorenzo BATTISTI
COMMISSIONE
Prof. Jens N. SØRENSEN - Risø DTU (DK) Revisore
Prof. Martin O.L. HANSEN - Risø DTU (DK) Revisore
Prof. Stefano DEL GIUDICE Commissario
Prof. Giorgio PAVESI Commissario
Prof. Fabrizio BEZZO Commissario
Prof. Pietro GIANNATTASIO Supervisore
Author’s e-mail: lorenzo.battisti@ing.unitn.it
Author’s address:
Dipartimento di Ingegneria Elettrica Gestionale e Meccanica
Universit`a degli Studi di Udine Via delle Scienze, 106
33100 Udine – Italia tel. +39 0432 5580xx fax. +39 0432 55xxx
1 Effect of cold climates on wind turbine design and operation 1
1.1 Introduction . . . 1
1.2 Special requirements of wind turbines in cold climates . . . 7
1.2.1 Cold weather packages . . . 7
1.2.2 WT installed in the cold regions . . . 13
1.2.3 WT installed in the Alps . . . 14
1.2.4 General forecasts on potential development . . . 15
1.3 Elevation . . . 16
1.3.1 General effect of high elevation . . . 16
1.3.2 Feature ad effect of the mountain resource . . . 17
1.3.3 Wind resource . . . 18
1.3.4 Variation in density of the air with the elevation of the site . . . . 26
1.3.5 Site-power curve mismatch due to not standard air density . . . . 35
1.3.6 Density reduction mitigation strategy . . . 39
1.4 Offshore icing . . . 42
Bibliography . . . 47
Bibliography 47 Glossary 49 2 Relevance of icing for wind turbines 51 2.1 Effect of ice on wind turbines . . . 51
2.1.1 Icing variables . . . 55
2.1.2 Ice geometry identification . . . 56
2.2 Prerquisites for icing occurrence . . . 57
2.3 Icing parameters . . . 58
2.4 Ice detection . . . 60
2.5 Iced wind sensors . . . 60
2.5.1 Example of data records . . . 62
2.6 Icing event . . . 69
2.6.1 Evaluation of the icing period on a site with few information . . . 71
2.7 Type of ice . . . 81 2.7.1 Glaze . . . 82 2.7.2 Rime . . . 82 2.7.3 Frost . . . 82 2.7.4 Wet snow . . . 82 2.7.5 Mixed ice . . . 83
2.7.6 Physical and mechanical characters of ice . . . 83
2.7.7 Ice growth on wind turbine . . . 84
2.8 Aerodynamic performances of ice contaminated airfoils . . . 87
2.8.1 Generalities on the flow condition past the profile . . . 87
2.8.2 Generality of aerodynamics of wind blade profiles . . . 91
2.9 Generalities on aerodynamics of contaminated profiles . . . 94
2.9.1 Effect of surface roughness . . . 94
2.9.2 Effect of ice contamination on aerodynamics . . . 97
2.9.3 Type of ice induced roughness . . . 101
2.9.4 CFD approach . . . 116
2.9.5 3D effects . . . 118
2.10 Icing effect on power production . . . 119
2.11 Influence of ice on rotor aeroelastic behaviour . . . 127
2.11.1 The aeroelastic model . . . 129
2.11.2 Iced rotor physical model . . . 130
2.11.3 Sensitivity analysis on the physical model . . . 133
2.11.4 20-year fatigue lifetime assessment . . . 134
2.12 Simplified analysis of icing rotor unbalance . . . 140
2.13 Ice throw and icing risk . . . 143
2.14 Economic risks induced by iced sites . . . 157
Bibliography . . . 163
Bibliography 163 Glossary 169 3 Icing process 171 3.1 The physics of the ice formation mechanism . . . 171
3.1.1 Ice accretion modelling path . . . 173
3.2 Body discretisation . . . 173
3.3 External flow and temperature field . . . 175
3.4 Modelling the body wetness . . . 177
3.4.1 Droplet trajectory calculation . . . 178
3.5 Impinging water calculation . . . 181
3.5.1 Translating airfoil . . . 181
3.5.2 Drops interaction with the fixed cylinder . . . 182
3.5.3 The determination of the stagnation collision efficiency . . . 187
3.5.4 Determine the collision efficiency for cylinder . . . 192
3.5.5 The solution for the fixed cylinder . . . 197
3.5.6 Collision efficiency calculation at zero AOA airfoil leading edge . . 202
3.5.7 Collision efficiency calculation at AOA other than zero and scaling effects . . . 205
3.5.8 Rotating airfoil . . . 207
3.5.9 Example . . . 214
3.6.1 Water film continuity and break-up . . . 222
3.7 Thermo-fluid-dynamic processes at the ice surface . . . 224
3.8 The freezing fraction and the Messinger method . . . 225
3.9 Energy conservation equation . . . 226
3.9.1 Analysis of the heat fluxes contributions . . . 227
3.10 The solution of the problem . . . 230
3.10.1 The ice accretion solution Tw< Ts< 0℃ . . . 231
3.10.2 The uniced surface solution Ts> 0℃ . . . 233
Bibliography . . . 235
Bibliography 235 Glossary 237 4 Ice prevention systems (IPS) 241 4.1 Introduction . . . 241
4.2 A procedure of ice prevention system assessments . . . 243
4.3 IPS concepts comparison and discussion . . . 247
4.3.1 IPS classification . . . 247
4.3.2 Wind turbine IPS use . . . 250
4.4 The energetic efficiency of an IPS . . . 261
4.5 Estimating the anti-icing power and energy requirement . . . 262
4.5.1 A simplified approach . . . 265
4.5.2 Assessment of the anti-icing heat requirement of different types of turbines . . . 268
4.5.3 An exercise: the assessment of the anti-icing heat requirement of propeller, wind turbine and aircraft airfoil . . . 272
4.6 Emerging solutions for IPSs . . . 276
4.6.1 Inflatable rubber boots on the LE of the wing and control surfaces (pneumatic de-icing system) . . . 276
4.6.2 Electro impulsive/expulsive devices . . . 277
4.6.3 Microwave . . . 277
4.6.4 Low adhesion coating materials . . . 278
4.6.5 Intermittent (cyclic) hot gas heating . . . 280
4.6.6 The regenerative ice prevention system . . . 288
4.6.7 Effusive heating . . . 291
Bibliography . . . 299
Bibliography 299
5 Thermal anti-icing systems design 305
5.1 Estimation of anti-icing power and energy requirement . . . 305
5.2 The numerical model to evaluate the hot-air thermal anti-icing ice preven-tion systems . . . 305
5.2.1 The geometry module . . . 306
5.2.2 The thermo-fluid-dynamic module . . . 307
5.2.3 The conjugate heat transfer module . . . 308
5.2.4 The rate of intercepted water . . . 312
5.2.5 Results . . . 312 Bibliography . . . 325 Bibliography 325 Glossary 327 Conclusions 329 Other Bibliography 333
1.1 classification of sites. . . 1
1.2 (a) mean minimum temperature in℃ in January (1961-1990), (b) average frost days (source www.klimadiagramme.de). . . 3
1.3 reduction in energy harvest in cold climates compared to conventional installation. . . 4
1.4 difference in yield per month between heated and unheated WEC E-82 2MW at location in Dragaliden (SE) [6]. . . 5
1.5 difference in yield per month between heated and unheated WEC E-82 2MW at location in Krystofovy-Hamry in Czech Republic [6]. . . 5
1.6 effect of cold microclimate on wind turbines. . . 6
1.7 effect heavy rain on WT power curve (Energiewerkstatt, 1995) [7]. . . 6
1.8 definition of minimum and standstill temperatures. . . 8
1.9 special requirements for cold climates. . . 9
1.10 ice effects on Neg/Micon 637 wind turbine [3] (original in poor quality). . 10
1.11 schematic of ice effects on wind turbines. . . 11
1.12 duration of operation disruption and distribution of icing events in Ger-many [8]. . . 12
1.13 wind farms installed in the Alps updated as at 2012: Swiss (red dots), Austrian (orange dots), South Tyrol (yellow dot), France (green dots) and Slovenia (blue dot) sites. . . 14
1.14 capacity in Cold Climates (up to end 2012) and forecasted (2012-2017) in MW. . . 16
1.15 diagram of site-related environmental factors and turbine functional fac-tors and their effect on the principal installation issues. . . 18
1.16 Uref/Vave ratio and maximum Vave for class I turbines as a function of the Weibull shape parameter. . . 21
1.17 relationship between standard deviation and Weibull shape parameter k. . 24
1.18 relationship between equivalent loads with variations of Weibull shape parameter k. . . 25
1.19 trend of the relative density as a function of elevation z for different condi-tions of atmospheric stability (standard density at sea level equal to 1.225 kg/m 3). . . . 28
1.20 dependence of Reynolds number on site temperature. . . 28
1.21 dependence of maximum lift coefficient of NACA profiles on Reynolds number. . . 30
1.22 dependence of maximum lift coefficient of DU profiles on Reynolds number. 30 1.23 effect of density variation on power, power coefficient, thrust and thrust coefficient for the stall regulated (case A) wind turbine. . . 32
1.24 effect of density variation on power, power coefficient, thrust and thrust
coefficient for the pitch regulated (case B) wind turbine. . . 33
1.25 effect of density variation on power, power coefficient, thrust and thrust coefficient for the pitch regulated full variable speed (case C) wind turbine. 34 1.26 idealized power curve. . . 36
1.27 alteration of the optimum power curve due to air density variation. . . 38
1.28 expected percent capacity factor drop compared to standard air density. . 38
1.29 iced sea close to WT [24]. . . 43
1.30 floating sea ice pushed from wind to shore at bay of Bothia. . . 44
2.1 picture collection of iced wind turbines (Maissan, J. F. ”Wind Power De-velopment in Sub-Arctic Conditions with Severe Rime Icing”, Technical Services Yukon Energy Corporation - Circumpolar Climate Change Sum-mit and Exposition 2001., web Foto: Kent Larsson, ABVee). . . 52
2.2 picture collection of typical iced wind turbine sites (Source IEA). . . 53
2.3 comparison of the total expected electrical anti-icing specific power qt [kW/m2] , power Qt[kW] and its magnitude compared to the rated power of the turbine as the size varies [6]. . . 54
2.4 classification of the variables involved. . . 55
2.5 geometrical schematization of ice structure (sample from LEWICE code) [8]. 56 2.6 times required and difficulties of the different approaches to study icing effects. . . 57
2.7 in clouds operation of wind turbine. . . 58
2.8 relation between droplet size, liquid water content and air temperature. Measured data from [11]. . . 59
2.9 severe icing of a control anemometer installed over the wind turbine nacelle. 61 2.10 Monte Agaro (left) and met mast (right) sites. . . . 62
2.11 Monte Agaro site. . . 63
2.12 the Agaro met mast form North. . . 64
2.13 image of the iced met mast. . . 65
2.14 comparison between heated (in red line) and unheated (in blue line) anemome-ter and wind vane during February 2009 (Monte Agaro - Italy). . . 66
2.15 comparison between heated (in red line) and unheated (in blue line) anemome-ter and wind vane during 27-29 October 2007 (Monte Agaro - Italy). . . . 66
2.16 example of ice accretion on cup anemometer in dry-icing conditions in cold climate wind tunnel simulations [13]. . . 67
2.17 theoretical cup anemometer responces at 10 m/s: ice free (green line) versus iced cup anemometer for different ice thicknesses and masses. . . . 68
2.18 scheme of the kinematic model of the anemometer. . . 68
2.19 definition of icing day left. . . 70
2.20 measurements of direct and indirect icing [16]. . . 71
2.21 example of anti-icing thermal power duration curve. Filled area represents the anti-icing energy. . . 74
2.22 example of temperature probability density distribution for a trial site. . . 75
2.24 icing frequency (hours) and ice intensity (kW/m2) charts. . . . 76
2.25 % error in anti-icing energy estimation when mean values are adopted for icing parameters. . . 77
2.26 reduced anti-icing power and energy. . . 77
2.27 % duration of IPS protection (1-t*/tI) and % savings of the required energy (1-Et∗/EI) as a function of the parameter K. . . 79
2.28 wind turbine nacelle remote sensing systems. . . 80
2.29 glaze and rime ice - mechanism of formation [24]. . . 81
2.30 glaze ice. . . 82
2.31 rime ice. . . 82
2.32 example of a glaze ice shape and of mixed ice accretion. . . 83
2.33 example of lift forces caused by ice on the pitch controlled blade in different working conditions. . . 84
2.34 picture of the distribution of a typical ice accretion. . . 85
2.35 mass distribution for three rotors of different size. . . 86
2.36 example of ice accumulation on the area between blades and arms for VAWT (Kindly permission on Tozzi Nord wind turbine Company). . . 86
2.37 general picture of the flow into the boundary layer [29]. . . 87
2.38 variation of the speed within the boundary layer in successive stations along the direction of motion, for a favourable pressure gradient and an unfavourable pressure gradient, respectively [29]. . . 88
2.39 representation of stall onset [29]. . . 89
2.40 possible scenarios of boundary layer evolution in adverse pressure gradi-ents [29]. . . 90
2.41 typical pattern of speed along the profile for the pressure and suction surfaces for four different angles of attack. . . 91
2.42 example of lift CL, drag CD and pitching moment Cm curves [30] for symmetric profiles (NACA0015 airfoil [31]). . . 92
2.43 example of lift CL, drag CD and pitching moment Cm curves [30] for asymmetric profiles (DU 97-W-300 airfoil [32]). . . 92
2.44 example of hysteresis on CL, CD e Cmcurves [33]. . . 93
2.45 coefficients of lift and drag of a NACA 63-425 measured in a clean condition and under conditions of fouling, simulating fouling through the affixing of a tape in a zigzag pattern with thickness 0.35 mm at a distance from the leading edge equal to 5 % of the length of the chord [34]. . . 95
2.46 example of the effect of the degradation of performance for the profile FX-W-270S, through the application of a tripping wire on both surfaces under pressure and depression at a distance from the edge of attack of 3 % [34]. . . 95
2.47 comparison of the NACA 63-425 and profile DU 91-W2-250, developed at the University of Delft [34] for wind turbine blades. . . 96 2.48 comparison between the performance of the NACA 63-425 and profile DU
2.49 example of alteration of the lift, drag and moment coefficient at grow-ing icgrow-ing for NACA 4415, modified from [26]. The different accretions
increment the chord length of 2, 22 and 44 %. . . 100
2.50 definition of roughness height (k) and local velocity (vk). . . 102
2.51 equivalent sandgrain roughness as a function of concentration and shape. 103 2.52 effect of roughness on CL,max and stall reduction after Brumby [47]. . . . 103
2.53 experimental data of CLreduction due to surface roughness from different reference sources. . . 104
2.54 experimental ice shape 290 (up and left), 296 (up and right), 312 (down and left) and 322 (down and right) from [49]. . . 105
2.55 results of lift to drag reduction due to ice induced roughness. Elaboration of data from [49]. . . 106
2.56 geometry of a horn ice shape. . . 107
2.57 effect of horn tip radius on CL at Re = 1.8106 and M = 0.18, for k/c = 0.044 horn at s/c = 1.7% on NLF 0414 airfoil [52]. . . 107
2.58 effect on CL of horn location at Re = 1.8106 and M = 0.18, for k/c = 0.044 horn for NLF 0414 airfoil [52]. . . 108
2.59 Example of streamwise ice. . . 109
2.60 effect of small protuberances on lift loss [56]. . . 110
2.61 effect of large protuberances on lift loss [56]. . . 110
2.62 effect of protuberances shape on lift loss [56]. . . 111
2.63 picture and mould reproducing shapes of horns (left) and streamwise ice (right) [44]. . . 112
2.64 picture and mould reproducing shapes of glaze roughness (left) and span-wise ridge (right) [44]. . . 112
2.65 picture and mould reproducing shapes of low temperature formed rime roughness (left) and fine spanwise ridge (right) [44]. . . 113
2.66 comparison of performance effects of the simulated ice configurations on the NACA 23012 airfoil at Re 15 to 9·106and M = 0.20 (left) and Compar-ison of performance effects of the roughness and streamwise ice simulations on the NACA 23012 airfoil at Re 15 to 9·106 and M = 0.20 (right). . . . 114
2.67 example if IPS induced ice. . . 115
2.68 the accreted ice shape for case A and B. . . 117
2.69 lift and coefficient as function of the angle of attack. S.A. denotes Spalart Allmaras turbulence model. . . 118
2.70 NACA 63-425 blade geometric design. . . 121
2.71 DU 91-W2-250 blade geometric design. . . 122
2.72 Cpcurve, power curve, flapwise bending moment and thrust for the NACA 63-425 equipped rotor in clean conditions. . . 123
2.73 Cp curve, power curve, flapwise bending moment and thrust for the DU 91-W2-250 equipped rotor in clean conditions. . . 124
2.74 Cpcurve, power curve, flapwise bending moment and thrust for the NACA 63-425 equipped rotor in icing conditions. . . 125
2.75 Cp curve, power curve, flapwise bending moment and thrust for the DU 91-W2-250 equipped rotor in icing conditions. . . 126
2.76 icing conditions for the rotor tip sections (radius: r=30.5 m, profile: NACA 4412, liquid water content: LWC=0.8 g/m3, mean droplet diam-eter: MVD=20 μm, absolute temperature T=267.15 K, pressure: P=90 kPa, humidity: Rh = 98%, relative velocity: w=71.92 m/s, chord size:
C=0.924 m, angle of attack: = 5.07◦, accretion time: 180 min). . . 131
2.77 maximum ice thickness distributions. . . 133
2.78 wind turbine motions for the main monitored moments. . . 134
2.79 DLM, damage Level Matrix representation. . . 138
2.80 shaft torque normalized to the clean case as function of azimuthal position for different blade contamination. . . 141
2.81 gravitational unbalance normalized to the clean case as function of az-imuthal position for different blade contamination. . . 142
2.82 picture of ice shedding from a turbine rotor. . . 143
2.83 vectorial representation of the velocity and force fields. . . 144
2.84 wind/turbine relative angles. . . 145
2.85 variables used for the simulation. . . 146
2.86 Monte Carlo procedure for the trajectories computation. . . 147
2.87 icing wind diagram; 105samples. . . 148
2.88 ice pieces mass density function; 105samples. . . 149
2.89 detachment spanwise position; 105samples. . . 150
2.90 detachment azimuthal position; 105 samples. . . . 150
2.91 ice fragments distribution on the ground; standstill condition. . . 153
2.92 ice fragments strikes probability; standstill condition. . . 153
2.93 strike probability; 1 cm thick ice fragments. . . 154
2.94 strike probability; 5 cm thick ice fragments. . . 155
2.95 strike probability comparison; 1 cm thick ice fragments. . . 156
2.96 guiding strategies for analysis of ice throw risk. . . 157
2.97 minimum iced days to achieve IPS economics with electric IPS, wind re-source (expressed as average wind speed) and the annual energy produc-tion during icing period (expressed as percentage of the AEP in absence of ice accretion). . . 160
2.98 minimum iced days to achieve IPS economics with hot air IPS (open chan-nel arrangement), wind resource (expressed as average wind speed) and the annual energy production during icing period (expressed as percentage of the AEP in absence of ice accretion). . . 161
2.99 minimum iced days to achieve IPS economics with hot air IPS (closed channel arrangement), wind resource (expressed as average wind speed) and the annual energy production during icing period (expressed as per-centage of the AEP in absence of ice accretion). . . 161
3.1 basic water contributions involved in icing process. . . 172
3.2 triple deck model (unheated body, left) and double deck model (heated body, right). . . 172
3.3 blade discretization scheme. . . 174
3.5 representation of the droplets motion past a body. . . 178
3.6 2D water trajectories past a body. . . 180
3.7 profile section and impingement quantities. . . 181
3.8 LB model for droplet drag [7] compared to NASA LEWICE drag [10] and Beart and Pruppacher relationships [11]. . . 185
3.9 example on freezing on small size turbine. Presented at HS seminar (Swedish Wind Energy Association), Stockholm, March 26, 2012, repro-duced in the presentation of Gran Ronsten (WindREN) with courtesy of Polar Field Services. . . 187
3.10 comparison of simplified impingement theory of LB and computation from LEWICE 2.0 code. . . 189
3.11 stagnation collection efficiency for NACA 0012 airfoils at 0 deg AOA. Static temperature, -12,2℃; static pressure, 100 kPa; airspeed, 54 to 134 m/s; MVD, 10 to 50 μm; LWC, 1 g/m3. Open symbols, 7-in chord; shaded symbols, 21-in chord; solid symbols, 31.5-in chord. Data represented by symbols are from LEWICE predictions. . . 190
3.12 stagnation collection efficiency β0as function of Stokes number according to LB [7] and Finstad [9] approximations for Φ = 103. . . 191
3.13 overall collision efficiency E as function of Stokes number according to LB [7] for Φ ranging from 0 to 104, and Finstad [9]. . . . 191
3.14 droplet trajectories and impingement points of the inscribed cylinder of section 0 of table 3.3. . . 198
3.15 droplet trajectories and impingement points of the inscribed cylinder of section 8 of table 3.3. . . 199
3.16 droplet trajectories and impingement points of the inscribed cylinder of section 10 of table 3.3. . . 199
3.17 droplet trajectories and impingement points of the inscribed cylinder of section 13 of table 3.3. . . 200
3.18 stagnation points collision efficiency and Finstad correlation. . . 201
3.19 total collision efficiency E and Finstad correlation. . . 201
3.20 convergence and number of particle released. . . 202
3.21 local collection efficiency, β, for drop size from 10 to 1000 μ for NACA63a516 airfoil [12]. . . 203
3.22 maximum collection efficiency as function of the drop size [12]. . . 204
3.23 impingement limits and total impingement as function of the drop size [12].204 3.24 qualitative effect of AOA other than zero of droplet trajectories. . . 205
3.25 comparison of LEWICE determinations of leading-edge collection effi-ciency at angles of attack of 0 and 10°with those from equation (3.5.32). . 206
3.26 scaled and reference collection efficiencies: LEWICE predictions for two NACA 0012 airfoils of different sizes. NACA 0012 Airfoils at 0 AOA. Reference Conditions: cR, 21 in (53.3 cm); VR, 67 m/s, dR, 30.0 μm, Scaled model: cS, 10.5 in (26.7 cm); VS, 117 m/s, dS 15.6 μm. . . . 206
3.27 determinations of leading-edge collection efficiency and impingement limits at angles of attack of 0°, 2°and 4°for the NACA 0012. . . 207
3.29 water trajectories on rotating body [14]. . . 208
3.30 collision efficiency as function of the Stokes number and the blade radius. 210 3.31 water mass impinging on the leading edge, LWC = 0.2 g/m3. . . 211
3.32 3D collision efficiency as function of the blade radius compared to 2D collision efficiency (left) and ratio β3D/β2D(right). . . 212
3.33 3D and 2D stagnation collision efficiency as function of the tip speed ratio for blade mid span section. . . 213
3.34 r/R = 1 (up), 0.98 (down). . . 214
3.35 r/R = 0.95 (up), 0.90 (centre) and 0.80 (down). . . 215
3.36 r/R = 0.70 (up), 0.61 (centre) and 0.51 (down). . . 216
3.37 r/R = 0.41. . . 217
3.38 impinging mass distribution (left) and wet blade coordinate x/c (right) in each blade section. . . 217
3.39 impinging mass distribution per m2 of blade surface (left) and per m of blade length right) versus r/R. . . 218
3.40 mass balance at the control volumes. . . 219
3.41 schematic of the film rupture on the surface. . . 223
3.42 variation of wing-surface-wetness fraction aft of area of water impingement with distance from limit of impingement [15]. . . 223
3.43 possible thermal exchange scenarios deriving from the rupture of water layer over the iced surface. . . 224
3.44 graphical representation of the freezing fraction. . . 226
3.45 schematic of the heat fluxed involved in energy conservation. . . 227
3.46 chord length c, nose diameter D and surface coordinate s of airfoil. . . 228
3.47 schematic procedure of computational solution. . . 232
4.1 basic scheme assessing ice prevention system design. . . 242
4.2 simplified path of the design step (contoured by the dashed line of figure 4.1). . . 243
4.3 ice prevention systems integrated design. . . 244
4.4 ice prevention system executive design, verification and certification. . . . 246
4.5 means to generate the heat source. . . 248
4.6 Vestas V47-660 (left) and Bonus 150 kW (centre) with black-painted flu-orourethane (StaClean) coating. Searsburg turbines (right) with black-painted applied to the blades: note the layer of ice along the blade leading edge. . . 250
4.7 details of the JE-System by Kemijoki Arctic Technology Oy. . . 251
4.8 details of the structural behaviour of the JE-System by Kemijoki Arctic Technology Oy. . . 252
4.9 picture of Kelly aerospace electrothermal de-icer. . . 252
4.10 first description of such an application for wind turbine rotors dates back to 1949 [6]. . . 253
4.11 schematic of the open-circuit and closed circuit systems. . . 254
4.12 arrangement of the open-circuit and closed circuit systems. . . 255
4.14 environmental limits performance of the closed circuit systems. . . 256
4.15 blade surface external temperature of a tip section, Test 1 electrothermal pads, Test 2-3-4 hot air internal flow, increasing air root temperature. . . 257
4.16 details of the induct air circulation of Enercon [12]. . . 257
4.17 picture of a Enercon E82 blade with de-icing on (left) and off (right). . . . 258
4.18 infrared picture of the turbine temperature of the Enercon turbine installed in the Wind Farm Moschkogel located at 1.600 m.a.s.l. in the Austrian Alps [14]. . . 259
4.19 picture of a Enercon showing the piping of the hot air directing the power source towards the outer blade portions. . . 261
4.20 body discretization scheme. . . 263
4.21 sensitivity analysis of relevant heat fluxes at the blade LE. . . 266
4.22 variation of the heat flux terms with respect to the imposed surface tem-perature calculated for the ”baseline condition”, expressed in absolute terms and in percentage of maximum chordwise value of the anti-icing heat flux qtm. . . 267
4.23 variation of the heat flux terms with respect to the free stream air temper-ature (left) and with respect to the wind velocity (right) for the ”baseline condition” (Ts=2℃). . . 268
4.24 distribution of the anti-icing heat flux qtm (W/m2) and the specific heat flux per meter of span qta (W/m) as a function of the dimensionless blade radius. . . 269
4.25 maximum chordwise value of the heat fluxes [W/m2] for the blade 70% section, anti-icing thermal power requirement for the wind rotor [kW] and ratio between rotor anti-icing thermal power and rated turbine’s power (table 4.3). . . 270
4.26 maximum chordwise value of the heat fluxes [W/m2] for the blade 70% section, anti-icing thermal power requirement for the wind rotor [kW] and ratio between anti-icing power requirement and rated turbine’s power (table 4.4). . . 271
4.27 maximum chordwise value of the heat fluxes qt[W/m2] for the blade 70% section, anti-icing thermal power requirement for the wind rotor Qt [kW] and ratio between anti-icing power requirement and rated turbine’s power Qt/PR (table 4.6). . . 271
4.28 results of heat fluxes simulations for the propeller case. . . 275
4.29 results of heat fluxes simulations for the wind turbine case. . . 275
4.30 results of heat fluxes simulations for the aircraft airfoil case. . . 275
4.31 working principle of inflatable rubber boots. . . 276
4.32 pneumatic de-icing system, picture of the experimental rig [16]. . . 277
4.33 blade/ice discretization. . . 281
4.34 temperature distribution for a steady state simulation. . . 283
4.35 outer surface overall heat exchange coefficient distribution. . . 284
4.36 intermittency factor vs. warm-air temperature at T∞,out = -3℃. . . 286
4.37 energy consumption ratio vs. warm-air temperature at T∞,out = -3℃. . . 286
4.39 energy consumption ratio vs. warm-air temperature at T∞,out = -6℃. . . 287
4.40 conceptual scheme of the regenerative ice prevention system. . . 288
4.41 reheat factor and heat flux demand ratio for different air mass flows vs air inlet blade temperature. . . 290
4.42 contribution of the power made available by the electrical generator to the anti-icing power required. . . 290
4.43 possible arrangement to automatically direct the hot air into the rotor. . . 291
4.44 effusive air heating scheme and IPS general concept [27]. . . 292
4.45 heat transfer mechanism for film heating. . . 292
4.46 a comparison of the predicted effectiveness with experimental results from Hartnett (1985) [30]. . . 294
4.47 effect of film heating on convective heat loss. . . 295
4.48 effect of anti-icing reduction as function of the blowing ratio. . . 295
4.49 comparison of the external wall temperatures distribution at the LE region among impermeable wall (current technology) and the permeable wall. . . 297
5.1 blade geometry discretization. . . 306
5.2 control volumes used for the mass and energy conservation analysis. . . . 308
5.3 Tjæreborg wind turbine blade planform. . . 314
5.4 dimension of the hydraulic diameter of the inner channel. . . 315
5.5 layout of the blade geometry. . . 316
5.6 evolution of pressure end temperature in the inner channel. . . 317
5.7 boundary layer edge velocities for sections 6, 7, 8 and 9. . . 317
5.8 impingement mass for sections 8, 10 and 13. . . 317
5.9 temperatures of the inside and outside wall in the impingement area to-gether with the external convective heat transfer coefficient for the sec-tions positioned at r =15.460 m (section 8) r =21.460 m (section 10), and r =28.960 m (section 13). . . 318
5.10 heat fluxes for the tip section as functions of the nondimensional curvilin-ear coordinate s/c. . . 319
5.11 net heat fluxes for sections r =15.460 m (section 8) r=21.460 m (section 10), and =28.960 m (section 13). . . 320
5.12 section averaged anti-icing specific heat flux variation and the total ther-mal power. . . 321
5.13 blade external wall temperature averaged on the 8 panels around the im-pingement region. . . 321
5.14 comparison of the wall external temperatures computed with the present model and with ANYSY for the sections positioned at r =15.460 m (section 8) r =21.460 m (section 10), and r =28.960 m (section 13). . . 323
1.1 regional distribution of lightning damage in Germany, over the period
1992-1994 (DEWI Report). . . 7
1.2 minimum temperatures of utility-scale (1-3 MW) wind turbines, without and with Cold Weather packages (2012). . . 8
1.3 cold weather packages equipped small wind turbine (2004). . . 9
1.4 anti/de-icing systems on commercial wind turbines (2013). . . 11
1.5 strategies suggested for icing operations in cold climates. . . 12
1.6 wind turbine installed capacity in some of the Nordic countries [9] and in cold climates [10]. . . 13
1.7 wind turbine installed capacity in some of the Alps countries [9] and [10]. 15 1.8 assumptions for the estimation of the Fatigue Loads Ratio. . . 26
1.9 effect of the location of the site on the density of the air, the Reynolds number, at work and on the aerodynamic forces (Note: st: value of stan-dard conditions, +: value larger than stanstan-dard conditions, -: less than,≈: value unchanged). . . 27
1.10 data and results of the operation of a 66 m diameter wind turbine in different environments (A - for stall regulated, B - pitch regulated, C - full variable speed. Item 1 - standard density environment, 2 reduced density environment, 3 - reduced density with enlarged rotor size, 4 - increased density environment compared to standard). . . 31
1.11 input data for the calculations. . . 37
1.12 diameter increase to accomplish for the density drop. . . 41
2.1 availability of information of the icing parameters for aeronautic fields and wind energy field [10]. . . 59
2.2 overview of the installed met mast of Agaro (Italy). . . 63
2.3 data for estimating the anti-icing energy. . . 70
2.4 list of main meteorological parameters for input in ice prevention design (a=available, na=not available). . . 72
2.5 main characteristics of wind turbine and anti-ice system. . . 75
2.6 samples of input (distribution parameters) and output (energy EI and maximum thermal power Qmax) for the blade described in table 2.5 and figure 2.23. . . 75
2.7 ambient data leading to selected ice structures, reproduced from [44]. . . . 111
2.8 design condition for the NACA 63-425 profile equipped turbine. . . 120
2.9 design condition for the DU 91-W2-250 profile equipped turbine. . . 120
2.10 main characteristics of the original Tjæreborg turbine. . . 129
2.12 ice accretion along the blade (profile: NACA 4412, LWC = 0.8 g/m3, MVD = 20 μm, T =267.15 K, p=100 kPa, w= 71.92 m/s, C=0.924 m, a
= 5.07◦, accretion time=180 min). . . 130
2.13 main data for the adopted contamination levels. . . 132
2.14 set-up for calculating the 20-year lifetime. . . 135
2.15 20-year fatigue equivalent load ranges (reference frequency 1 Hz). . . 136
2.16 damage level matrixes for the Tjæreborg turbine. Edgewise blade root bending moment, blade 3 (standard load M0 = 1,202 [kNm]). . . 138
2.17 damage level matrixes for the Tjæreborg turbine. Yaw shaft bending mo-ment (standard load M0 = 449 [kNm]). . . 138
2.18 damage level matrixes for the Tjæreborg turbine. Transverse tower root bending moment. (Standard load, transverse M0= 685 [kNm]. In bracket: standard load, longitudinal M0= 2,094 [kNm]). . . 139
2.19 20-year fatigue equivalent load ranges (reference frequency 1 Hz) for CL-O and CL-S.). . . 140
2.20 mass of accreted ice and related degree of irregularity of the various con-tamination levels. . . 142
2.21 turbine geometrical and operating conditions . . . 152
2.22 site weather conditions and IPS data used for breakeven simulation of figures 2.97-2.99. . . 160
3.1 computational steps. . . 173
3.2 general blade data of Tjæreborg wind turbine based cylinder analysis. . . 197
3.3 input data for the simulations. . . 198
3.4 Red, Kst, φ, β0, E, M V Dmin for the simulation involving data of tables 3.2 and 3.3. . . 200
3.5 computational steps. . . 211
4.1 average efficiencies of IPS. . . 262
4.2 range of the variables used for the sensitivity analysis and baselines con-ditions. . . 265
4.3 variables used for a comparison of the thermal anti-icing power between 2-bladed and 3-bladed 1 MW size turbines. . . 269
4.4 data used for a comparison of the thermal anti-icing power between 1-bladed, 2-bladed and 3-bladed turbines of medium size. . . 269
4.5 data used for a comparison of the thermal anti-icing power between 1-bladed, 2-bladed and 3-bladed turbines of medium size. . . 270
4.6 Turbines data used for a comparison of the anti-icing thermal power with respect to rotor’s size. . . 272
4.7 data used for the simulation of the heat transfer fluxes on the propeller blade. . . 273
4.8 geometrical of the propeller. . . 274
4.9 data used for the simulation of the heat transfer fluxes on the WT blade. 274 4.10 geometrical of the propeller. . . 274
4.12 typical adhesion strength of ice to various materials at -10℃. . . 279
4.13 environmental and functional parameters for simulation shown in figure 4.34. . . 284
4.14 multilayer domain features. . . 284
4.15 input in the model of equation (4.6.13). . . 289
5.1 input data for the simulations (ambient and turbine). . . 313
5.2 input data for the simulations (hot air open circuit anti-icing system). . . 313
5.3 general blade data of Tjæreborg wind turbine. . . 313
5.4 input data for the simulations. . . 315
The motivation of this thesis is a comprehensive analysis of the phenomenon of icing on wind turbines. The interest on the icing problem of wind turbines and on methods for its mitigation as well as to predict its effect on load, control system, power production etc., has grown significantly in the very latest years due to the increasing number of installation of WT in icing risk areas. Research and development of ice accretions and anti-icing analysis is a multidisciplinary activity encompassing various research fields and competences as meteorology, aerodynamics, heat transfer and ice physics, together with the knowledge of wind turbine operation, economics, manufacturing details, and not last, regulations. Icing process is one of the more challenging application field of computa-tional thermo-fluid-dynamic. More and more numerical codes have been actually made available in the years to analyse icing process on aircraft where icing is of major concern for more than 80 years. These tools have achieved a very profitable accuracy and com-putational power and are currently used for aerodynamic performance analysis, systems design and certification as well. The mentioned complexity is reflected in the high degree of specialisation of the existing codes. Only a few of them can treat the problem of ice growth as non-stationary phenomenon, and several issues are still open. Although in fact the general feature of the flow field and the conjugate heat transfer are considered solved with minor approximations, the need of a more satisfactory knowledge of water film evolution at the surface and the ice process at crystal level is leading the research to explore micro physics of systems and fully stochastic phenomena analysis. The break-up of the water layer is dramatically affecting the heat losses at the surface, since as rule of thumb, the evaporating heat flux have the same order of magnitude of the convective one. Therefore the prediction of the actual wetted surface fraction is of utmost importance in system design.
Disruption of the ice shapes due to aerodynamic forces, centrifugal actions and vibrations is an other essential feature to assess the actual shape of ice and its actual aerodynamic effect. Micromechanics of fracture is also an other active and promising field of improve-ment. Despite of the enormous work carried in the aeronautical field on the topic (in the general references list at the end of the thesis some relevant contribution used in this work have been listed), very little is appeared on wind turbines in the years. The researcher have to deal with very customized tools developed for aircraft and helicopters application (usually adapted) and a very few infield observations which can possibly help them to frame these models. Wind turbines suffer of being not presided plants and it is very hard to link occasional losses of power or malfunctioning to ice. Decay of power curves are regularly reported. From the aerodynamic point of view, if one refers to pro-file performances with ice contaminated surfaces, there is still a not clear link between ice shape and performance alteration. The vibration level induced on the wind turbine rotors, the unbalance level, etc. are information rarely shared from site owners or
man-therefore the development of general correlation is currently week. There is further a scarce transparency of wind turbines manufactures on the turbine operations under icing and even on the operating principle of their ice prevention equipment (when present). All claim for very satisfactory performances of their systems. Often this strikes against observation and experience gained in flight area. For instance chapter 3 shows that a few mm of ice on the leading edge (2-5 mm) on about 1m chord wing causes a drop of max CL between 20% to 50% and a decay of aerodynamic efficiency that can achieve 80%. A
similar level of contamination in dry environment would cause a very large drop in the power curve due to an abrupt anticipation of the stall, which can in turn trigger an ex-pected sequence for the control systems. What happen further with the mass unbalance caused by ice that unevenly growths on blades? What is the more disturbing problem, the mass unbalance of the rotor, the time fluctuating aerodynamic torque or the overall performance drop? These question got a partial answers in this thesis.
To encompass this huge horizon of aspects, a decision has been made to face the topic from the point of view of a systemic assessment of the matter, providing the necessary engineering tools to approach the problem. The theory presented in the various chapters has been simplified in order to work with easily implementable models, and to give direct access to the order of magnitude of phenomena, rather than pointing on a detailed nu-merical treatment of a single one problem. Nevertheless, same items have been treated, when necessary, with finite elements and finite difference models to serve as a basis for more sophisticated analysis. Despite of these numerical treatments, attention has been paid to get relevant engineering conclusions helpful for the practical comprehension of the matter. The analysis of ice protection system implies a lower level of complexity since, for instance for anti-ice systems, ice formation on the blade surface is not tolerated and there is not need of ice accretion modelling. De-icing systems accept a marginal ice accretion level, so this makes necessary to investigate the effect of light, transitory icing, on aerodynamics and loads.
The thesis work has been built on knowledge matured in more than 10 years of activ-ity on the field, and collects some results presented in conference presentations, articles, and courses. A fundamental effort was made to harmonise the mat and deeping, and completing the knowledge of several topics. In particular it extends the embryonic work developed in the courses held by the author during the period 2004-2009 at the Master Course in Wind energy of the DTU in Lyngby Copenhagen.
Trento, 20 May 2013
Foremost, I would like to express my sincere gratitude to my advisor, colleague and friend Prof. Pietro Giannattasio for his continuous support, encouragement, insightful comments, and hard questions to this my late Ph.D. project.
Besides my advisor, I would like to thank the reviewers of my thesis: Prof. Jens N. SØRENSEN and Prof. Martin O.L. HANSEN of Risø DTU (DK).
Part of this work was developed and discussed during my visiting periods at DTU. My sincere thanks also goes to my former students and now appreciated collaborators Dr. Alessandra Brighenti, Dr. Luca Zanne, Dr. Roberto Fedrizzi, the technicians of the Turbomachinery Lab of the Dept. of Civil, Environment and Mechanics, Dr. Sergio dell’Anna and Filippo De Gasperi for their contributions to my researches.
1
Effect of cold climates on wind
turbine design and operation
1.1
Introduction
Wide areas are recently becoming available in the high latitudes of Eastern Europe and Asia and more generally in Nordic regions for wind energy exploitation. Occasionally some installations have take place in mountainous sites. The increasing number of in-stallations in cold climates (both inshore and offshore) on single wind turbine or parks, failure and maintenance reports have boosted recently the interest in studying the effects of such climates that differ from that conventionally used. Cold climates are part of a wider compass of sites named as not conventional sites opposed to conventional ones. A general classification of the sites is given in figure 1.1.
Conventional sites refer to sites located at open and windy areas, characterised by temperate climate, comprehensive knowledge of actual meteorological data and lack of obstacles in proximity of the turbines. Such sites has primarily been chosen considering
the closeness with the electrical grid and an adequate distance to urban areas.
Not conventional sites refer instead to hostile climate areas, leading the turbine to operate in extreme environmental conditions, and requiring special equipments for safe opera-tions. Among not conventional sites, this thesis focusses on cold climates sites. Such sites exhibit the following characters:
• air temperature Ta < 0℃ for large periods during the year;
• complex terrain;
• site elevation above sea level (more than about 1000 m.a.s.l.); • clouding in proximity of the ground surface;
• water content from atmosphere and/or sea water sprays;
• extreme conditions (high turbulence, extreme gusts, hail,lightening);
Despite these conditions, such sites show some potential for developers due to: • there is a relevant presence of wind, also in macro areas classified with a low wind
energy density, because of local spots (due to peculiar terrains shape as, ridges, etc.);
• this kind of site is the only one available for the land (i.e sub artic regions, China, Russia, Finland, Canada, cold desert regions);
• this kind of site is the only portion of land where the resource is available (i.e. alpine regions);
• low housing density (safety).
Certification Guidelines, i.e. GL Wind, arbitrarily define low temperature as an hourly averaged temperature of less than -20℃ which happens in an average year on the WECS site on more than 9 days per year and/or the yearly average temperature is below 0℃ [1]. The IEA XIX Annex, Wind energy in cold climates, more consistently defines cold climates as: sites that have either icing events or low temperatures outside the operational limit standard wind turbines [2].
The Chinese Wind Energy Association is estimating that about 40% of the 253 GW onshore exploitable power will be located in cold desert areas. Limited efforts have been made to assess the actual potential of wind exploitation in arctic and subarctic areas, but some papers [3] [4] reported potential markets of 20% of the installed capacity by the 2010. This estimate would correspond to some 8000 MW annually if combined with the forecast for 2010 presented in ”Wind Force 12” [5]. These target have been recently achieved. Majority of potential cold climate sites are located in open and forested terrain with average wind speeds higher than 7 m/s. The total potential is estimated to be 10 times more than for easily accessible offshore sites (personal evaluation based on not pub-lished market analysis). There is associated a wide market potential located in Sweden,
Figure 1.2: (a) mean minimum temperature in ℃ in January (1961-1990), (b) average frost days (source www.klimadiagramme.de).
Finland, Norway, Iceland, other European mountainous areas (Pyrenees, France, Aus-tria, Switzerland, Liechtenstein, Italy, Germany. Slovenia, Romania, Slovakia, Ukraine, Hungary, Serbia & Montenegro, Scotland), the North America (Canada, USA), Asia (Hi-malayas in China, India, Nepal, Bhutan), a part of South America and non Himalayan parts of China.
Beside of average air temperatures below zero Celsius degree for large part of the year, highly humid environment lead potentially to ice formation and icing persistence on structures exposed to wind and on the access paths to wind farms. Generally speaking, icing refers to both atmospheric icing, due to precipitation of water, and sea spraying and consists on accretion of iced structure over stationary and moving parts, thus de-termining and alteration of the fluid-dynamic behaviour of aerodynamic profiles and the increase of the masses of the ice-contaminated components.
It has to be recognised that it is still crucial the retrieval of reliable data on characters of cold climate sites. Although actually several maps of average site temperatures and frost are available (in figure 1.2 an example of typical maps is given), there is a substan-tial lack of icing maps that can be used to assess the severity of the phenomenon for preliminary design. It is not enough stressed the fact that the common icing evaluation methodologies for meteorological propose are of limited help in forecasting the severity of icing process on the wind turbine parts. In fact, as it will be explained in more de-tail in the next chapters, it is the dimensions of the hit object (i.e. the blade) and its relative velocity with regard to the water droplet dimension and speed that drives the ice formation on the surface. Therefore, indications of icing maps are indicative of the presence of conditions favourable for icing, but direct measurements of icing parameters to be used in models are necessary for the safe design of the wind farm in cold climates. In general, if special precautions are not taken, operation of WTs n cold climate will lead to a reduction in energy harvest. Figure 1.3 shows a typical drop of energy yield compared to sea level operation for a WT installed in mild climate at 1000 m.a.s.l., and
Figure 1.3: reduction in energy harvest in cold climates compared to conventional instal-lation.
an harsher climate at 1000 m.a.s.l. The combined effect of air density reduction and ice can cause energy reduction as large as 55%.
Direct icing refers to ongoing icing on wind turbines during bad weather conditions during the year, while indirect icing collects the events consequent to indirect icing. These events are the persistence of ice on WT hampering normal operation, causing additional mechanical or electrical sources of malfunctioning, and power adsorption by auxiliary equipments.
These considerations are confirmed by infield observations. Efficiency of the rotor blade de-icing system has been tested for 5 months in winter 2009/2010 in locations at Dragali-den in SweDragali-den and Krystofovy-Hamry in Czech Republic by Enercon technical service. On both locations 2 WECs E-82 2 MW have been compared. Both WECs are located next to each other. On one WT the rotor blade heating was activated while on the other it was deactivated. The percentage of energy yield surplus in relation to WT without de-icing system in the 5 testing months were 54% and 48% for the Czech Republic and Sweden sites respectively have been declared from Enercon technical service [6]. In figure 1.4 the monthly detail on the difference in yield between heated and unheated WT E-82 2MW at location in Dragaliden (SE) is shown while in figure 1.5 the analogous situation for the E-82 2MW at location in Czech Republic is given. Unfortunately no indications were available on the site meteorological conditions, and ice severity.
The phenomena leading to such technical availability reductions are displayed in fig-ure 1.6. Extreme events cause additional loads and fatigue, damages and sudden failfig-ure, energy losses caused by precipitation events or prolonged WT stand still and reduced WT availability. The density alteration (low temperature, high elevation) modifies the energy harvest and has a major impact on the control strategy. Low temperatures affect physical properties of materials and normal operation on electronic devices. Specific cold weather packages are used to let WT survive in cold climates. Finally the combined effect of humidity and icing determine additional loads and fatigue, vibrations, reduced
Figure 1.4: difference in yield per month between heated and unheated WEC E-82 2MW at location in Dragaliden (SE) [6].
Figure 1.5: difference in yield per month between heated and unheated WEC E-82 2MW at location in Krystofovy-Hamry in Czech Republic [6].
availability and energy losses. Furthermore additional ancillary equipments (anti-icing and/or de-icing systems) are required. Slight or moderate rain does not influence the performance of a WT. However during heavy rain massive drop impacts occur at the rotor blades through which the flow around the profile is disturbed considerably. From measurements instantaneous power losses up to 30% could be found according to the intensity of rain.
Figure 1.6: effect of cold microclimate on wind turbines.
Table 1.1: regional distribution of lightning damage in Germany, over the period 1992-1994 (DEWI Report).
Coast North-Germain Plains Sub-Alpine mountains All
WEC numbers 584 455 263 1302 Years of operations 1251 936 465 2652 Observation (all) 85 64 109 258 Direct strike 25 16 22 63 Observation/WEC-year 7% 7% 23% 10% Direct strike/WEC-year 2.0% 1.7% 4.7% 2.4%
Figure 1.7 shows an alteration of the power curve due to rain precipitation. The occurrence of hail represents a rare event and therefore hardly measurable losses in the annual energy yields are observed. Much more important is the damage that can occur by the impact of hailstones on the leading edge at terminal speeds exceeding m/s. Lightning strikes is usually a fatal event for WT. Mountainous regions are particularly affected by lightning. Density of yearly strikes per unit square kilometre as high as 5 to 10 can occur in certain areas. The strike can heavily damage or even destroy the rotor generator and the electrical parts. Two types of lightning impact can be distinguished:
• direct strike: here the WT is directly hit, usually on a rotor blade, and high currents are directed from the discharge point to the rotor hub, the bearing, the tower and foundations into the ground. Serious damages involves the rotor blades and the electric components even if lightening receptors are present into the blade tip; • indirect strike: in this event the lightening falls in the vicinity of the WT. The effects
are felt over the medium voltage grid, and excess voltage waves can propagate along the electrical distribution lines. Damages usually involves components which are insufficiently protected against over-voltage.
Table 1.1 shows the regional distribution of lightning damage in Germany, over the period 1992-1994. One can notice the relevance of the phenomena in sub alpine regions.
1.2
Special requirements of wind turbines in cold
cli-mates
1.2.1
Cold weather packages
Beside general certification stating that the design and construction of a given tur-bine/tower assembly is conform to accepted standards (in terms of design load assump-tions, construction materials and methods, control systems and safety margins), opera-tions in cold climates need further caution toward the applicability of the system design and construction to the site-specific conditions. All wind turbine manufacturers specify temperature operating thresholds for their equipment. Materials and lubricants are de-signed to withstand temperatures within their specified ranges and when temperatures
Figure 1.8: definition of minimum and standstill temperatures.
exceed those ranges, increased maintenance may be required, accelerated decay of equip-ments life will occur and additional damages may result. The most important of these operating limits is a minimum ambient temperature below which operation is stopped, according to the characteristics of the materials and lubricants used in the wind turbine. The standstill temperature refers to the temperature the turbine can withstand while not operating. This temperature reflects the limit on the turbine materials ability to withstand stress without exceeding normal or acceptable wear and tear. According to the specifications of many manufacturers, most turbine models are designed for operation in ambient temperatures down to -20℃, although some companies indicate operational ranges up to -30℃, and structure threshold up to -40℃. Special versions exist for such applications (cold climate version, artic version, cold weather packages, etc.), allowing to widen the operating temperature range of the turbine (figure 1.8).
Table 1.2: minimum temperatures of utility-scale (1-3 MW) wind turbines, without and with Cold Weather packages (2012).
Manufacturer Minimum operating/standstill temperature without CW packages with CW packages
ENERCON -10℃/-20℃ -30℃/-40℃ Gamesa -10℃/-20℃ -30℃/-40℃ GE Wind Turbine -15℃/-20℃ -30℃/-40℃ Nordex -10℃/-20℃ -30℃/-40℃ REpower -10℃/-20℃ -30℃/-40℃ Siemens -10℃/-20℃ -30℃/-40℃ WinWinD -10℃/-20℃ -30℃/-40℃ Vestas -10℃/-20℃ -30℃/-40℃
Table 1.3: cold weather packages equipped small wind turbine (2004).
Manufacturer Size Number Date of Location Minimum In
[kW] installation operating service
temperature [℃]
Bonus 150 1 1993 Yukon -30 yes
Vestas V47 LT II 660 1 1999 Yukon -30 yes
AOC 66 10 n/a Alaska
(Kotzebue)
-40 yes
Northwind 100 1 n/a Alaska
(Kotzebue)
-46 yes
The table 1.2 provides background information regarding cold weather operation of utility-scale (1-3 MW) wind turbines, gathered from manufacturers brochures and data sheets.
Small and micro WT are rarely equipped with cold weather packages. The preferred operating philosophy is the stop of the turbine when extreme conditions occur or even during the whole hostile season period. Some examples of earlier cold weather packages equipping small wind turbine are listed in table 1.3.
The cold weather package is associated always with a cold station service providing the power for preventing damages during the turbine stand still or shut down periods as schematically shown in figure 1.9.
Where icing is expected during operations, anti-icing or de-icing systems, synthetically classified as IPS (Ice Prevention Systems) are necessary. These systems require usually a source of power and energy to be driven, which is taken from the online production of the turbine or from the electrical net. As consequence, operation in cold climates need a cold station service to provide energy for both the cold weather package and the ice
Figure 1.10: ice effects on Neg/Micon 637 wind turbine [3] (original in poor quality).
prevention system and to prevent damages during the turbine stand still or shut down periods.
All such issues need to be examined in the design phase preceding the installation of the turbines in their working environment. Neglecting this features would mean lower energy production to that expected and prolonged periods of inactivity required for safety purpose or because turbine inability to perform satisfactorily.
Equipment resulting from figure 1.9 are usual for large WT, while they are usually not provided to small ones, due to the relevant additional investment costs and energy consumption associated, which make them economically not viable for small turbines. Icing is a major source of additional downtime and loss of availability for WT operating in cold climates. Substantial decay of the power curve is usually reported in case of icing. Figure 1.10 shows the effect of icing on a Neg/Micon 637 [3].
The table 1.4 provides background information about main WT manufactures propos-ing commercial anti-icpropos-ing systems.
Figure 1.11 synthesises the basic effects of ice on the turbine.
Data from 250 MW Wind-Programme, and its accompanying Scientific Measurement and Evaluation Programme (WMEP) analyse approximately 55,350 reports with about 2,000,000 hours of total turbine downtime. In more than 880 cases (1.6%) the reports refer to icing events with a total downtime of approximately 64,200 hours (3.1%).
Table 1.4: anti/de-icing systems on commercial wind turbines (2013).
Manufacturer Anti/De-icing technology
ENERCON Anti-icing hot air system, but a new hybrid hot air-electrical system is under development
Leitwind De-icing heating with electrical system
Nordex Anti-icing heating with electrical system on N100/2500 and N117/2400 models
REpower Passive de-icing with special coating
Siemens De-icing heating with electrical integrated system WinWinD De-icing heating with electrical integrated system
Vestas De-icing heating with electrical system, but a new hot air system is under development
Figure 1.11: schematic of ice effects on wind turbines.
The effects has been classified as: • plant stoppage (89%)
• reduced power output (13%) • noise (2%)
• vibration (5%) • overspeed (1%) • overload (1%)
• causing follow up defects (1%) • other consequences (4%).
Figure 1.12: duration of operation disruption and distribution of icing events in Germany [8].
Table 1.5: strategies suggested for icing operations in cold climates.
CLIMATE CHARACTERISTICS STRATEGIES
Low temperatures (0℃÷-3℃) and light icing None and occasionally stopped of WT
Very low temperatures (less than -3℃) and moderate icing Cold weather packages
Real icing risks Ice prevention systems
In most cases (90%) icing of turbines resulted in plant stoppage. In some cases the turbines remain in operation but effects like noise (2%), reduced power output (13%) and vibration (5%) have been reported to ISET [8].
Figure 1.12 illustrates the results in terms of duration of operation disruption and distribution of icing events over Germany [8].
Turbines operating in cold climates have to be conceived for safe operation: dedicated strategies and special equipments could be considered after the scheme shown in table 1.5 showing the climate characteristics and the suggested strategy, although final decision needs to be accomplished by business plan including real costs of additional devices and their benefits.
Generally, the adoption of icing mitigation strategies (as the IPS, increased preventive maintenance, pre-stocking replacement parts in the site or near each turbine) aims to in-crease the WT availability and performance. As consequence in cold climates, additional costs and the different performance scatter respect to a wind project in conventional site have to be taken into account. The major economic risk is due to:
• increased initial costs (limited installation schedules, higher equipment costs, higher installation costs);
• additional costs for installation of dedicated equipment (as cold weather packages or anti-deicing system) and their operational costs;
• increased periodic (e.g. after snowfalls and icing events) and unscheduled (prema-ture failures due to increased fatigue loading) maintenance costs;
• increased down time or power penalty due to icing events;
• increased down time due to extreme low temperatures (only in very cold weather); • increased down time between repairs due to turbine inaccessibility;
• increased turbine down time for public and labour safety (blades and tower ice throw).
1.2.2
WT installed in the cold regions
Cold climate sites offer large wind energy potential. According to the International Energy Agency (Task 19 - Wind Energy in Cold Climates) the cold climates wind in-stallations located in Northern Europe, Canada, and Asia (North China and Russia) amounted to 3 GW at the end of 2008 and reached 10 GW at the end of 2011 [10], when the total installed worldwide wind capacity has grown to 239 GW in 2011. In table 1.6 the wind power capacity of some countries are given (primarily onshore plants).
Atmospheric icing in Northern Europe is very much a local phenomenon. Icing may occur at all existing wind farm sites in Finland, Sweden and Norway but the icing climate of different regions varies considerably. Despite this, wind power represents one of the fastest growing industry in Sweden and the installed wind power capacity has reached 3,745 MW at the end of 2012. In Norway, wind power installations have growth up to 715 MW (2012) and the major part of these are built in areas where there is a signifi-cant risk of icing and actually several companies have their own programmes for finding solutions to this problem. The installed wind power capacity in Finland was 197 MW at the end of 2011 and 288 MW at the end of 2012. The amount of installed wind power capacity in Finland is rather low compared to that of other European countries, but new projects are expected (some of them planned in the North of the region), thanks to the new long-term subsidy concerning the production of electricity from renewable sources
Table 1.6: wind turbine installed capacity in some of the Nordic countries [9] and in cold climates [10].
Country Production capacity installed [MW]
2010 2011 2012
Finland 194 197 288
(in CC areas) (197) n.a. n.a.
Norway 436 520 715
(in CC areas) (48) n.a. n.a.
Sweden 2,163 2,798 3,745
(in CC areas) (124) n.a. n.a.
Germany 27,191 29,075 31,332
(in CC areas) (1,000) n.a. n.a.
Canada 4,008 5,265 6,200
of energy.
Cold weather occurs in Canada’s vast landscape and the best resources are often located in severe icing areas. Actually, in these regions there are a lot of remote commu-nities (i.e. not connected to the grid) entirely powered by diesel generator. Authorities, governments and companies are currently working for a development of wind power in-tegrated plants in such areas.
In Germany, atmospheric icing has been observed and reported from all site categories, i.e. from coastal sites, the plains of northern Germany and from low mountain regions.
1.2.3
WT installed in the Alps
A map of the wind farms installed in the Alps, updated as at 2012, is shown in figure 1.13, and built up by a personnel site scouting. Among the European countries in the Alpine region, Austria and Switzerland were the first to invest in exploiting wind energy in the mountains. By the end of 2002, the Swiss had an installed wind power amounting to 5 MW.
As part of an action by the Swiss Federal Office of Energy (SFOE) and the Association for promoting wind energy in Switzerland (Suisse Eole), several promising locations have been identified at moderately high elevations and the objective is to reach the target of 80 MW installed power by the year 2010, using modern, medium-to-large scale systems. The highest wind park of Europe is sited in the Swiss Alps (Gtsch), near Andermatt,
Figure 1.13: wind farms installed in the Alps updated as at 2012: Swiss (red dots), Austrian (orange dots), South Tyrol (yellow dot), France (green dots) and Slovenia (blue dot) sites.
Table 1.7: wind turbine installed capacity in some of the Alps countries [9] and [10].
Country Production capacity installed [MW]
2010 2011 2012
Austria 1,014 1,084 1,378
(in cold climates areas) (200) n.a. n.a.
Italy 5,797 6,737 8,124
(in cold climates areas) (3) (3) (3)
Switzerland 42 46 52
(in cold climates areas) (35) n.a. n.a.
at 2,350 m a.s.l., with three Enercon Wind turbines, a 600 kW E40 (erected in 2004) and two 900 kW E44 erected in 2012 and 2012. capable of generating a yearly over 4000 MWh of electricity. All turbines are equipped with a blade anti-icing system, the first erected being one of the first pilot plants in the world. After an temporary dismantling due to structural problems identified in the rotor of the first turbine, the wind plant has been powered with two new turbines. The largest Swiss wind farm is installed at 1100 m a.s.l. on Mont Crosin, in the Bernese Jura alpine region. It comprises 16 Vests turbines (from 600 kW to 2000 kW), which generate a global 45,000 MWh a year. It is also worth adding that this installation has become an attraction, with thousands of visitors walking the nature trail that leads to the farm, which is equipped with stations providing visitors with information on energy-related topics, and on solar and wind en-ergy in particular. It should be emphasized that studies have already been conducted in Switzerland to establish guidelines on the usage of wind energy in the mountains and an ice cover map has been drawn up to identify which of the sites considered suitable in anemological terms would be at greatest risk from this point of view.
Austria began exploiting wind energy already in 1994 and, by the end of 2012, the situa-tion is shown in table 1.7. Although the majority of its windfarms are currently located in low-lying areas or on gentle hillsides in the eastern parts of the country, interest in installing wind turbine systems in alpine sites has grown considerably in recent years, nu-merous feasibility studies have been organized and several windfarms have already been installed. Austria can boast the ”Tauernwindpark Oberzeiring” windfarm, which stands at the highest elevation for a wind farm in Europe (1,835 m a.s.l.) with an installed capacity of more than 20 MW.
1.2.4
General forecasts on potential development
According to BTM World Market Update 2012 [11], who made a forecast in the short term (2012-2017) between 45 and 50 GW of wind energy will be exploited in cold climates by 2017, which would mean an increase of as much as 72 per cent since the end of 2012 and investments amounting to approximately EUR 75 billion. This market has the potential to compete with offshore wind power. The latest published prediction is given by BTM World Market Update 2012, and the complete picture of the actual situation and potential exploitation by interest area is given in figure 1.14.
Moderate to heavy icing Light icing
Low temperatures
Source: BTM World Market Update 2012, Navigant Research, 2013. 7,763 7,426 16,442 8,149 7,570 5,922 2012 (cumulative) Forecast North-America 1,653 2,085 24,393 12,083 3,836 2,053 2012 (cumulative) Forecast Europe 9,529 , 10,514 244 1,852 72 27 2012 (cumulative) Forecast Asia
Figure 1.14: capacity in Cold Climates (up to end 2012) and forecasted (2012-2017) in MW.
1.3
Elevation
1.3.1
General effect of high elevation
The strong drive to exploit wind energy has recently led to new types of location for wind turbine installations being considered, including mountain regions and, to be more specific, areas at elevations ranging between 800 and 2,500 m a.s.l. Authoritative sources, such as the European Wind Energy Association (EWEA), have estimated that 20-25% of the approximately 60,000 MW expected to be installed in Europe between now and 2020 will be situated in cold-climate areas, and a part of them will be on hills and mountains. Clearly, by comparison with the conventional sites, such locations are bound to be more critical, not only in environmental terms and as regards their acceptance by the commu-nity, but also because the more severe microclimate conditions will have adverse effects on the functional capacity of the wind turbines and on the electricity generated. In fact, while numerous studies (see for all [12]) have demonstrated that the wind resource in the mountains is generally greater than on the valley floor or in the neigh boring lowlands, they have also shown that this resource is strongly influenced by the peculiar features of the land, being characterized by a lower air density, complex turbulence mechanisms and - in winter - very cold temperatures and conditions favouring ice accretion. In addition, the particular logistics of certain mountain sites demand a careful assessment of their real accessibility. The installation of wind farms in the mountains consequently demands an in-depth analysis, in the design of such plant, into both the methods for assessing the resource and the more or less direct transfer of procedures and technologies developed for
