Abstract
Understanding the sounds generated by marine energy converters is necessary to evaluate their potential acoustic impacts. To date, there have been few acoustic measurements of marine energy converters, and little has been done to relate noise generated during operation to specific components or processes. In this study, drifting passive acoustic measurements were performed around two cross-flow turbines in a stream in Millinocket, ME. Using operational information provided by the turbine manufacturer, we directly attribute multiple sounds produced by turbines to powertrain components.
The two turbines were first-in-class test units developed for the purpose of demonstration. The only difference between them was their powertrain—one generator was direct drive, while the other had a gearbox. During the measurement period, both were operated over a similar range of rotation rates and power levels. Although their contributions to the soundscape were clear and statistically significant, noise radiated by both turbines would exceed ambient noise levels at ranges of no further than 100 m in similar environments. Both turbines produced strong narrowband signals around 8 kHz, corresponding to the switching frequency of the variable frequency drive (VFD) used to regulate turbine rotation (Fig. 1). The turbine equipped with a geared motor produced narrowband, tonal signals below 4 kHz and sidebands around the VFD tone with frequencies that modulated with the turbine rotation rate. The direct drive turbine also produced a narrowband tone with a frequency that modulated with the rotation rate below 500 Hz and an impulse corresponding to the rotation period. Across all rotation rates, the geared motor turbine produced more intense noise than the direct drive turbine. Overall, our results show the importance of collecting acoustic data at a variety of turbine operating conditions and demonstrate how turbine design and operation directly affect radiated noise.