Moving Towards Large r Rotors: Is It a Good Idea?

1. Moving Towards Large(r) Rotors Is that a good idea? P. K. Chaviaropoulos, CRES H.J.M. Beurskens, SET Analysis S.G. Voutsinas, NTUA EWEA 2013 Conference Vienna, 4-7 February

2. V164-8.0 MW Siemens 6 MW Offshore WT Alstom Haliade 150-6MW Sway 10 MW Turbine Upwind/NREL/DOWEC UpScaled to 10 MW SeaTitan 10 MW (HTS generator) Motivation

3. Motivation Large Multi MW Offshore commercial turbines with Higher Tip Speed (~90m/s) Large Diameter Concept 10 MW designs with Even Higher Tip Speed (~100m/s) Even Larger Diameter High Tip Speed -> High TSR-> Lower Solidity -> Thicker airfoils Higher Diameter-> Higher Loads for OPT Cpmax HOW DOES THAT WORK ?

4. Quantification with BEM Theory Ref: Peter Jamieson, Innovation in Wind Turbine Design, A John Wiley & Sons, Ltd., Publication, ISBN 978-0-470-69981-2, 2011 All Functions of a, , x

5. Low Induction Rotors (LIRs) Conventional Design For given R o and Maximize C P ( ,a ) RIGHT: Plots of non-dimensional coefficients, candidates for blade optimization, versus axial induction coefficient a . Application for =3, k =100 and =8.85 (providing the maximum C P value =0.4966 for the selected B and k combination). New , R is a design variable Maximize {[C P ( ,a ) .R 2 ]/[C P0 ( 0 ,a 0 ) .R 0 2 ]} Constrained by [C M(0) ( ,a ) .R 3 ]/[C M0(0) ( 0 ,a 0 ) .R 0 3 ]=1 Or Maximize { C P ( ,a )/ C M(0) ( ,a ) 2/3 }

6. As a result, the optimal rotor will have a larger radius: R/R 0 = 1.136. It will capture more energy: [C P ( ,a ) .R 2 ]/[C P0 ( 0 ,a 0 ) .R 0 2 ] = 1.087 and it will be less loaded than the initial one (design C T and C M(r) will be less), operating at a lower axial induction value a =0.187 instead of a 0 =0.33. In other words, we sacrificed C P to increase energy capture with a larger rotor diameter, maintaining the aerodynamic bending moments at their initial level. The obvious question is Is that cost effective, as long as we have a 13% longer blade now? In a very primitive approach and taking only the aerodynamic moments into account, we can assume that the new blade can maintain the cross-sections of the initial blade as long as they have the structural strength to undertake the reference bending loads which are not altered. This means that the weight and cost of the new blade will increase by a factor of R/R0. This would result in an increase of the levelized cost of the blade component of 4.6% (13% additional cost, 8.7% more power). Nevertheless, and since the levelized cost of the rotor blades is a small fraction of the overall levelized cost of electricity, particularly offshore, the selection of a larger, less loaded rotor for offshore turbines seems cost effective. Low Induction Rotors (LIRs)

7. Low Induction Rotors (LIRs)

8. Characteristic properties of rotors with the same root bending moment designed for different values of the axial induction factor. Plots are presented for the rotor diameter (D), the power production at design wind speed P (Vdes), the levelized rotor cost (LCE) and the annual energy production AnEP. All properties are divided by their corresponding reference values ( a =1/3) Low Induction Rotors (LIRs)

9. LIRs in Large Wind Farms Capacity factor and wake losses per turbine in a 10X10 offshore wind farm for 8D spacing. Red dots address the initial turbines (highly loaded) and blue squares the less loaded turbines. The dashed red and the blue line correspond to the wind farm mean values. The less loaded turbines increase the wind farm capacity factor by nearly 6%. This comes partly (3%) from the increased annual production of the larger diameter turbine and partly (another 3% roughly) from the reduction of the wake losses due to the lower axial induction and, therefore, thrust coefficients of the larger rotors.

10. LIRs in Large Wind Farms Annual energy production of a 10X10 offshore wind farm as a function of turbines spacing (x times Diameter). Red dots address the initial turbines (highly loaded) and blue squares the less loaded turbines. The 6% gain in annual energy production is more or less flat and independent from the turbine spacing. Evidently, this is very much connected to the accuracy of wake calculation in multi-raw offshore wind farms with engineering models, like the one used .

11. Synthesis We investigated in some depth the impact of low induction rotor designs to the cost of offshore wind energy. We first demonstrated that a low induction high swept area rotor can capture more energy than a conventional design that aims at C P,MAX , without a burden on the aerodynamic loading of the blades. Then we estimated the additional cost of the larger rotor and concluded that this cost is smaller than the expected benefit in energy production, especially offshore where the capital cost of the rotor corresponds to a small fraction (around 5%) of the levelised cost of wind electricity. This analysis was first done in stand-alone operation. Moving to the wind farm level we anticipated additional benefits for the low induction rotor designs in terms of energy capture and wake losses (also related to extra fatigue loading). Closing, we want to make clear that this paper does not produce an actual design. It should be conceived as a first attempt for understanding possible non-conventional concepts that might be beneficial for the design of large offshore rotors. The methodology used and the assumptions made regarding the rotor and wind turbine loading are therefore pretty crude. For instance, in our analysis we have had no discussion on extreme loads, fatigue loads, blade- tower clearance issues etc.

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