Ground Boundary Impact on Short Tower Turbines: A Conceptual Study

This conceptual study explores the impact of ground boundary on the production of short tower turbines. It investigates whether two turbines with the same rotor diameter but different hub heights can generate the same energy output. The study examines how the ground boundary influences the airflow patterns and energy production of turbines, presenting results from computational fluid dynamics simulations and discussing potential implications for turbine performance in various terrains.

• Turbine Technology
• Wind Energy Production
• Boundary Interaction
• Computational Fluid Dynamics
• Energy Efficiency

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1. Impact of Ground Boundary on Production of Short Tower Turbines - A Conceptual Study

2. Outline of Short Tower Issue Question: For a given input free stream energy (i.e. fixed Veq), can two turbines with the same rotor diameter and power curve, but different hub heights be expected to product the same energy? It is conventional to ignore the interaction of the rotor stream tube and the ground boundary, but is this realistic? Energy Veq = X Energy Veq = X Short/Stubby Tower Tall Tower Two turbines have same diameter, but different hub heights.

3. Axial Flow Theory At what hub height does the influence of the ground boundary being to influence the stream-tube? Far Wake As= Source Area Freestream Vext Vw V Aext Aw p0 p- p+ p0 Ground Boundary How might the ground boundary influence the production of: A single turbine? A wind farm?

4. RES CFD Simulations HH / D = 1.11 HH / D = 1.00 HH / D = 0.89 HH / D = 0.78 HH / D = 0.67 HH / D = 0.56 Draft/Provisional Results 1 Turbine Fixed Diameter, Variable Hub Simulated Power = Velocity at Rotor * Force CFD simulations of wind turbine rotor and ground boundary. Velocity tuned to keep free stream energy at rotor approximately constant. Asymptotic behaviour does not make sense more work needed. Downstream turbines see wake elongation (c.f. ground plane reflection wake model). Increased wake effects. Upstream turbine sees ground blockage effect i.e. reduced rotor speed gives reduced power.

5. SSE CFD Simulations: 4*4 Array Fixed Diameter, Variable Hub Draft/Provisional Results T1 = Upstream Turbine (large dashed lines) T4 = Downstream Turbine (small dashed lines) Upstream of T1 (solid lines) Results indicate reduction in power at both upwind and downwind turbines.

6. Implications for Predicted Pattern of Performance? If the interaction between the turbine and the ground is significant for stumpy towers how might this influence the pattern on production onsite? Are the turbines at the hill top/bottom more/less effect than a turbine in flat terrain? Wind Farm Performance Verification Part 2: RES, BWEA 31 - 22 October 2009

7. Implications for Power Performance Testing? At a given distance upwind of a turbine, could the blockage effect upwind of a stumpy tower be larger than that of a tall tower? Could this be helping mask a performance loss due to stump towers? No Blockage Blockage? Short/Stumpy Tower Tall Tower

8. Questions/Discussion