A Comparative Analysis of the Two-Bladed and the Three-Bladed Wind Turbine for Offshore Wind Farms

Abstract

Background: As the installation of offshore wind farms continues to increase, the need to optimize on costs involved is a constant factor in the industry. In this regard, research into the two-bladed rotor has garnered some interest in the recent past, to see if the optimum cost can be shifted in the favor of the two-bladed wind turbine. Objective: The objective of this thesis was to determine the costs of energy delivered by a two-bladed wind turbine and a three-bladed wind turbine and to determine which turbine’s cost is lower. Energy cost was treated as the main discerning factor between the turbines. Method: Two concepts were defined; the two-bladed wind turbine and the threebladed turbine. For the three-bladed concept two cases were considered; one with a design tip speed ratio equal to that of the two-bladed concept and another with a design tip speed ratio lower than the first case. Only the rotor and the support structure were designed. For the rotor, Delft University airfoils were used. The Blade Element Momentum theory was used to design the rotors and to obtain the rotor power and thrust. Support structures were designed based on the Rayleigh method for stepped piles. Bladed 3.80 was used to determine fatigue damage of the rotors and support structures. Components in the nacelle were not designed but their masses were estimated using scaling relations. These masses along with rotor and support structure masses were translated into Euro’s using cost factors obtained from literature. An offshore site roughly 70km from shore was selected and hundred of these designed turbines were hypothetically installed at this site. Cost of installation and electrical infrastructure was obtained from data available. From this and the rotor and support structure costs the investment cost was obtained. The Annual Energy Yield of the turbines was also calculated. These, along with operation and maintenance costs were used to calculate the energy cost. Main Results: From the design iterations it was seen that the energy cost of the twobladed turbine was slightly lower than that of the three-bladed concept with a lower design tip speed ratio. Another important result of this report was that a three-bladed turbine with a tip speed ratio equal to that of the two-bladed turbine gave the lowest energy costs. It was also seen that the energy yield of the two-bladed concept is lower than that of the three-bladed concept and support structure costs for the two-bladed concept is higher.