Design of a Small-Scale Vertical Axis Wind Turbine for Testing in the Princeton High Reynolds Number Test Facility and Comparison to Full-Scale Turbines

Abstract

Vertical axis wind turbines (VAWTs) are a growing renewable technology that harvests energy from wind and has great potential in both offshore and urban wind farms. Despite extensive field testing by several research groups, including Professor John Dabiri's Field Laboratory for Optimized Wind Energy (FLOWE) at Stanford University, the fluid dynamics of the airflow around a VAWT at high Reynolds numbers are not yet fully understood. This team conducted experiments on full-sized turbines at high-Reynolds number conditions, but due to the unpredictable and variable nature of field testing, comprehensive experimental results at regular, sustained intervals of Reynolds numbers are difficult to obtain. Using Princeton's High Reynolds number Test Facility (HRTF), a scaled model of the commercial VAWT design is tested at high Reynolds numbers. The HRTF is a pressurised wind tunnel that can operate at a uniquely high pressure, which makes it possible to manipulate air density and - by extension - the Reynolds number. Thus, the effect of the VAWT design on the aerodynamics of the fluid flow in a controlled, experimental environment can be studied for the first time. The power output of the turbine at matching Reynolds numbers but at different physical conditions are also studied. Those results show that the physical conditions are irrelevant, which makes the Reynolds number the determinant of the turbine's power output. The coefficient of power is plotted against the tip speed ratio to show that it arcs to a maximum value at an optimal tip speed ratio, and the magnitude of the maximum value increases then plateaus with an increasing Reynolds number. These results match the trends shown by the full-scale turbine, although the scaled VAWT's maximum coefficient of power is significantly larger than that of the full-scale turbine.

This report details the process taken to study the VAWT design, calculate the necessary wind tunnel conditions to achieve dynamic similarity, and scale the turbine down to a size compatible with the HRTF diameter. Then, the report outlines blade momentum theory to calculate worse case forces and moments experienced by the model, and the process of simulating those forces and moments to ensure against mechanical failure. Once this is achieved, the process of selecting a sufficiently strong material, optimising the turbine design, and machining the resulting model is described. After wind tunnel testing on the model using the HRTF was conducted, the report presents the resulting data and discusses its implications. Future research endeavours may include mapping the fluid behavior around and behind the turbine, and studying turbine-turbine interaction.