River Surface Debris: Characterization, Mitigation Strategies, and Impact on River Energy Conversion Devices
This 2015 project improved the design of the research debris diversion platform on the Tanana River at Nenana, AK, and enhanced researchers’ ability to understand its use with river energy converters. The project facilitated the development of new technologies for analyzing debris and its effect on river energy converters in Alaska.
Alaska’s hydrokinetic energy resources represent a promising source of power, but river debris poses a significant challenge to the technology. In 2010, two different Alaskan efforts to demonstrate river energy converters (RECs) deployed from floating platforms were terminated prematurely due to problems with woody debris. These experiences indicate that developing technology to mitigate debris problems will need to be a high priority for practical hydrokinetic power production.
One of the 2010 demonstrations, by Alaska Power and Telephone (AP&T), prompted them to initiate a project with the Alaska Hydrokinetic Energy Research Center (AHERC) to examine ways to reduce the hazard of surface debris for REC devices. During the study, AHERC developed a surface research debris diversion platform (RDDP) to investigate the important factors associated with diverting surface debris around REC devices.
This 2015 project extended the AP&T commission to further characterize river surface debris and examine the performance of the RDDP. Top priorities were assessing the magnitude of debris impact loads on the RDDP and improving the RDDP’s performance, with the following specific tasks:
Task 1: Improvements to the RDDP and debris impact characterization: Tests and analysis of debris impacts on the RDDP indicated that the platform’s ability to divert and clear debris improved significantly when all surfaces that come into contact with debris are covered with low-friction material, such as high-density plastic. The RDDP and mooring buoy system demonstrated the capability of withstanding significant debris impact during long-term deployments.
Task 2: Hydrodynamic impact of the RDDP: A method for analyzing cross-river acoustic Doppler current profiler transects was developed both to maximize information derived from such standard, widely used measurements, and to enable the analysis of large-scale turbulence to determine its effects on REC performance. Results indicated that the RDDP had no significant effect on river flow, with most fluctuations due to natural river variability.
Task 3: Video observations of debris: Significant improvements were made to the video debris observation system (VDOS). The VDOS is now able to record images of the river and floating debris at one frame per second, both from shore and from the RDDP. The imagery can be used to categorize debris by size, location and quantity, and to observe the interaction between debris and the RDDP.
Task 4: Sonar debris monitoring: The preliminary investigation of whether a BlueView sonar “camera” is suitable for detecting debris was largely unsuccessful due to inability to obtain long-term system observations. The most recent versions of the Teledyne BlueView operating software appear to address past reliability issues, though these remain to be field-tested.
Task 5: COUPi Discrete Element Method (DEM) modeling: The COUPi DEM was used for simulating the impact of debris on the RDDP to provide a qualitative way of examining the process of debris interaction with the platform and its debris sweep.
Project Conclusions & Next Steps
Overall, the RDDP and buoy system, developed as part of prior work directed by the Alaska Power and Telephone and refined under this project, proved to be a strong platform for protecting surface-mounted RECs from floating debris. The tools developed during this study will aid evaluation of future hydrokinetic projects, both within and outside Alaska. Despite progress, however, many questions about subsurface debris remain. Future research topics include: the potential of the RDDP to disrupt subsurface RECs, practical installation in remote communities, effect of hydrokinetic infrastructure on aquatic habitat, and how to best bring hydrokinetic power to shore in debris-infested river waters.
For more information, please contact Jeremy L. Kasper, Ph.D., Director, Alaska Hydrokinetic Energy Research Center - firstname.lastname@example.org
Photo 1: ACEP Research Engineer, Jack Schmid and Research Professional Paul Duvoy install monitoring equipment at the Tanana River Test Site. Photo courtesy of T. Paris, UAF.
Photo 2: AHERC researchers study river debris prior to hitting the debris diversion device on the Tanana River. Photo courtesty of T. Paris, UAF.
Photo 3: AHERC researchers inspect the debris diversion device on the Tanana River. Photo courtesty of T. Paris, UAF.