Document Type

Open Access Thesis

Degree Program

Mechanical Engineering

Degree Type

Master of Science in Mechanical Engineering (M.S.M.E.)

Year Degree Awarded

2014

Month Degree Awarded

September

Advisor Name

Matthew

Advisor Middle Initial

A

Advisor Last Name

Lackner

Co-advisor Name

James

Co-advisor Middle Initial

F

Co-advisor Last Name

Manwell

Abstract

The earliest design of a wind power system with multiple rotors on a single support structure dates back to the late 1800s. Such a system called a Multi-Rotor Wind Turbine (MRWT) was proposed by several researchers due to its perceived advantages over a single-rotor wind turbine. As turbine size increases, power produced by a rotor tends to scale up as the square of its diameter, as opposed to rotor weight which varies as its cube. So, several smaller rotors will weigh and cost less than one large rotor producing the same power. MRWTs offer several advantages such as better distribution of loads, better logistics of the components and scope for standardization. The MRWT system can also continue operation even if some of the rotors fail. However, MRWTs require a complex support structure to connect the rotors to the tower and an arrangement to yaw them into the wind. A recent study involving a scaling model for a three-rotor MRWT system estimates a cost saving of 13.1% as compared to the NREL 5 MW single-rotor model. A triangular truss type support structure for the MRWT model is designed and its preliminary static analysis is performed in that study. This thesis is a continuation of that study where the scaling model is extended to include MRWT systems having two to seven rotors. A systematic design method is developed for modeling any MRWT support structure for two to seven rotors for the given 5 MW configuration. The structure consists of frames and cables and the design constraints for the static analysis are stress, deflection and buckling. A dynamic analysis of the MRWT solution is also carried out to verify that the structure can withstand loads induced at varying wind conditions and design load cases – especially steady, turbulent and extreme wind conditions. Some special cases for the three-rotor MRWT system, such as use of two-bladed rotors, direct-drive machines, analysis for zero wind loads, load analysis for each of the assembly stages are also discussed. Finally, as the support structure design for the three and seven-rotor models is the main focus of the thesis, the scaling model is validated by comparing these models with similar turbines having rated power corresponding to the rotors used in the models.

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