Introduction
Over the past two decades, wind energy has rapidly evolved to
the renewable energy with the highest market share worldwide [1]
and an installed capacity of nearly 319 GW (2013) [2]. During this
development, technological innovations in production processes
and the use of new materials enabled turbine manufacturer to
gradually enlarge the design properties of horizontal axis wind
turbines (HAWT), namely rotor diameter, rated power and hub
height, that are associated with the extraction of energy from the
wind [3,4]. Actual, the most powerful available onshore turbine
(2014) has a rotor diameter of 127mwith a hub height of 135mand
a rated power of 7.580 kW [5]. For offshore utilization, there exist
prototypes with rated powers up to 8 MW and rotor diameters of
164 and 171 m, respectively [6,7]. However, the UpWind project
even showed that a 20MWturbine with a diameter of about 252m
is technically feasible in theory [8]. Besides upscaling of turbines,
significant improvements in turbine efficiency could be observed
during recent years, especially occurring in turbines designed for
onshore utilization [9]. This development partly is a result of the
long-term energy policy of several countries in Europe. Investors
were offered incentives in form of subsidy programs, such as the
German “Renewable Energy Act (EEG)”; the thereby generated
market growth was accompanied by the technical progress
mentioned above as well as the implementation of measures to
reduce the costs for turbine manufacturing and operation. As a
result, achievable annual energy production (AEP) of single turbines
rose in context with decreasing production costs, enabling costs of
energy production (COE) of large onshore HAWT that are now
competitive with those of fossil fuels [1]. To summarize, by
upscaling of HAWT up to the technical limitation, investors can
* Corresponding author. expect higher remunerations from electricity sale. Since initial and