Date of Award

2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Jeffrey S. Marshall

Second Advisor

Paul D. Hines

Abstract

Growing concerns about the environmental impact of fossil fuel energy and

improvements in both the cost and performance of wind turbine technologies has spurred

a sharp expansion in wind energy generation. However, both the increasing size of wind

farms and the increased contribution of wind energy to the overall electricity generation

market has created new challenges. As wind farms grow in size and power density, the

aerodynamic wake interactions that occur between neighboring turbines become

increasingly important in characterizing the unsteady turbine loads and power output of

the farm. Turbine wake interactions also impact variability of farm power generation,

acting either to increase variability or decrease variability depending on the wind farm

control algorithm. In this dissertation, both the unsteady vortex wake loading and the

effect of wake interaction on farm power variability are investigated in order to better

understand the fundamental physics that govern these processes and to better control

wind farm operations to mitigate negative effects of wake interaction.

The first part of the dissertation examines the effect of wake interactions between

neighboring turbines on the variability in power output of a wind farm, demonstrating

that turbine wake interactions can have a beneficial effect on reducing wind farm

variability if the farm is properly controlled. In order to balance multiple objectives, such

as maximizing farm power generation while reducing power variability, a model

predictive control (MPC) technique with a novel farm power variability minimization

objective function is utilized. The controller operation is influenced by a number of

different time scales, including the MPC time horizon, the delay time between turbines,

and the fluctuation time scales inherent in the incident wind. In the current research, a

non-linear MPC technique is developed and used to investigate the effect of three time

scales on wind farm operation and on variability in farm power output. The goal of the

proposed controller is to explore the behavior of an "ideal" farm-level MPC controller

with different wind, delay and horizon time scales and to examine the reduction of

system power variability that is possible in such a controller by effective use of wake

interactions.

The second part of the dissertation addresses the unsteady vortex loading on a

downstream turbine caused by the interaction of the turbine blades with coherent vortex

structures found within the upstream turbine wake. Periodic, stochastic, and transient

loads all have an impact on the lifetime of the wind turbine blades and drivetrain. Vortex

cutting (or vortex chopping) is a type of stochastic load that is commonly observed when

a propeller or blade passes through a vortex structure and the blade width is of the same

order of magnitude as the vortex core diameter. A series of Navier-Stokes simulations of

vortex cutting with and without axial flow are presented. The goal of this research is to

better understand the challenging physics of vortex cutting by the blade rotor, as well as

to develop a simple, physics-based, validated expression to characterize the unsteady

force induced by vortex

Language

en

Number of Pages

166 p.