Advanced Wind Turbine Program Next Generation Turbine ... - NREL

2.3.5 Boundary Layer Control
GE commissioned a review of the current state of the art aerodynamic boundary layer control via
blowing and suction and an evaluation of the potential for applying these concepts to a wind turbine
blade. The study identified four motivations for employing boundary layer control in fluid
dynamic systems:
1. Boundary Layer Control (BLC) via blowing or suction to eliminate leading edge laminar
separation
2. Laminar Flow Control (LFC) using suction to delay or avoid transition of the laminar
boundary layer to turbulent
3. BLC to control the development of the turbulent boundary layer
4. BLC to delay or avoid turbulent boundary layer separation using suction or blowing.
Only Items 2 and 4 have potential for wind turbine applications. GE's study includes an exhaustive
review of research on laminar flow control (LFC and boundary layer control (BLC, including
the drag reductions and lift improvements possible, as well as the requirements for magnitudes
of blowing and suction to achieve desired results. The study also examined practical
methods for implementing suction and blowing, derived equations for calculating the power required
for flow control, and estimated the cost of implementing a flow control system into a
wind turbine blade.
These results were used to conduct power performance and cost modeling of four possible configurations
for introducing suction into a wind turbine rotor blade. All of the system modeling
uses the GE37a blade as a baseline. The key conclusions drawn from this study are that the
maximum benefit expected from applying suction for flow control on the GE37a blade is near a
2% reduction in the COE.
2.4 Variable Diameter Rotor
The essence of the variable diameter concept is to have a rotor that may augment its diameter
and increase energy capture in the frequently occurring moderate wind speeds below rated wind
speed where most energy is available. At the same time, the diameter can reduce in higher wind
speed and avoid loads which would otherwise penalize a rotor of increased diameter. The cost
reducing principle applicable to the present concept is to introduce adaptive characteristics responding
to the value (i.e., energy) and avoiding the penalty (i.e., loads).
Two differing implementations of the variable diameter concept were identified. The first concept,
subsequently referred to as S-VADER (where “S” refers to sliding), employs an outer section
of blade that can retract “telescopically” into the main blade section. The second concept,
referred to as C-VADER, changes the swept area of the rotor by coning the blades. GE subsequently
decided that only the S-VADER concept merited further consideration.
The S-VADER concept employs a telescoping rotor blade configuration, in which an outer, extensible
blade section fits inside of a main blade (see Figure 13). This rotor concept could feasibly
be incorporated into the existing upwind, pitch-regulated configuration. The main blade
would bolt via a pitch bearing to the hub, and the pitch of the blade would be controlled in the
same manner as presently. The outer blade would be extended and retracted by a second mechanical
system in response to gross changes in wind speed (see Figure 14), while pitch control
would still be employed to regulate the power.
38