Forty Years of Wind Energy Development - Wind Power for the World: The Rise of Modern Wind Energy - page 27

• NREL, Phase II, III, IV turbines.
• Risø, Tellus turbine.
• TUD Open Air Facility (10 m diameter, 2-bladed, upwind):
the very same fully instrumented blade for pressure distri-
bution measurements could be placed in a wind tunnel for
comparison with well defined flow conditions in a wind
tunnel.
Along with the growth of the capacity of wind turbines, also
the size of wind energy projects (wind farms) grew from farms
with a total capacity of 1 MW to wind farms of several hundreds
of MWs. In order to be able to design wind farms efficiently, know
how was needed about wake interaction and flow phenomena
inside wind farms. Research into those aspects started in the early
1980s with analysis of the physics of wakes and experimental
research in wind tunnels. Since the first investigations in wind
farm flow, this research has become more and more important.
External conditions like wind shear, turbulence intensity and the
stability of the atmosphere play a large role in the propagation of
rotor wakes. As there is a big diference between those conditions
on land and ofshore, economic planning of ofshore wind farms
depends to a large extent on results on wind farm flow research.
Traditionally research was focused on wind turbines and wind
farms. How to keep them operational at low cost during the lifetime
never has been an issue until the first wind turbines installed
ofshore in 1991. At ECN we realised that the single focus on wind
turbine and farm design could make the research community less
relevant to the industry if no more attention were paid to operation
and maintenance (O&M) issues. For the Netherlands this was of
particular importance as the main manufacturers were moving
outside the country and the ofshore contractors and operators
showed interest in developing O&M services. Corrective main-
tenance has to be substituted by preventive maintenance in order to
decrease O&M cost. To this end conditioning monitoring techniques
have to be developed to determine the life time consumption of
critical components for condition based maintenance. Other topics
like a “flight leader system”, access technology and dedicated
logistics all need to be improved or developed in association with
efficient O&M systems.