The research debris diversion platform (RDDP) has proven a robust platform for protecting surface-mounted river energy converters (RECs) from floating debris. With funding from the Alaska Energy Authority (AEA), grant ADN #R1416 "Debris Characterization and Mitigation," the design of the RDDP, as well as our ability to detect and understand its use with RECs, was significantly improved. In addition, the support of the AEA has enabled the development of new analysis techniques and technologies for describing and quantifying debris and its effect on RECs in Alaska’s environments. Work on this project was divided into tasks as follows: (1) improvements to the RDDP and debris impact characterization, (2) the hydrodynamic impact of the RDDP, (3) video observations of debris, and (4) sonar debris monitoring. Though beyond the scope of the original project, an existing discrete element method (DEM) model, COUPi, was tested for simulating the interaction of debris with hydrokinetic infrastructure. A description of this DEM work is included here as a fifth task.
Task 1: Improvements to the RDDP and debris impact characterization. Tests and analyses of debris impacts on the RDDP indicate that the platform’s ability to divert and clear debris improves significantly when all surfaces that come into contact with debris are covered with low-friction material, such as high-density plastic. The RDDP profile in the water is improved by properly ballasting the platform to counterbalance the downward drag at the front of the RDDP caused by water displaced under the debris sweep. The RDDP and mooring buoy system demonstrated the capability of withstanding significant debris impact during long-term deployments. In August 2013, the RDDP cleared debris after three large-scale impacts of up to 29 kN (6,600 lbf), with some debris taking more than 6 hours to clear. Interpretation of load cell data indicates that the debris impacting the infrastructure in the Tanana River likely consists of a mix of single and multiple debris objects. The RDDP will be redeployed during summer 2015 to protect two RECs: in July, the Oceana turbine operating behind the RDDP, and in August, a 5 kW NewEnergy turbine operated by the University of Alaska Fairbanks as part of the ALFA project funded by the U.S. Department of Energy (U.S. DOE).
Task 2: Hydrodynamic impact of the RDDP. A method for analyzing cross-river acoustic Doppler current profiler (ADCP) transects was developed to (1) maximize information derived from such standard, widely used ADCP measurements, and (2) enable the analysis of large-scale turbulence to determine its effects on REC performance. River velocity measurements performed during summer 2013 at the Tanana River test site (TRTS) were rotated into along- and cross-stream directions and projected onto an ideal straight transect. An optimal interpolation procedure was applied by means of MATLAB routines developed for the project that allowed interpolation of irregularly spaced measurements onto an equally spaced grid. From these gridded transects, averages and deviations from the average were calculated to characterize turbulence velocity fluctuations. The gridding analysis showed very little influence on the mean flow field (or on the vorticity) due to the presence of the RDDP, though questions about smaller-scale turbulence remain and measurement and analysis techniques need improvement. A short series of ADCP and acoustic Doppler velocimeter (ADV) measurements were made on the last day of the 2014 Oceana turbine deployment to determine the influence of the RDDP on REC performance. These measurements demonstrated that power output behind the RDDP is reduced and that the influence of the RDDP decreases with increasing distance from it. As no statistically significant difference was found between the mean velocity at hub height at three distances behind the RDDP, we concluded that the reduction in power output is most likely due to the effect of the RDDP on small-scale turbulence.
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 is then used to determine the size, location, and amount of surface debris in the river and to observe the interaction between debris and the RDDP. The VDOS was first built and tested in a breadboard configuration (i.e., a lab setting), and subsequently, a long-term performance test at the TRTS was completed in conjunction with the Oceana turbine deployment. Presently, the VDOS is capable of long-term autonomous operation and thus is a suitable tool for use in remote locations where hydrokinetic projects are being considered.
Task 4: Sonar debris monitoring. The preliminary investigation of whether a BlueView sonar “camera” is suitable for detecting debris was largely unsuccessful because we were unable to obtain long-term observations with the system. In 2013, BlueView’s operating software continually “hung up” during multiple daylong deployments. Additionally, it was determined that the AHERC BlueView system is unable to detect debris beyond ~15 m due to signal scattering by the sediment carried in the Tanana River. Discussions with Teleydyne BlueView indicate that the company has a lower-frequency (450 kHz) sonar that may perform better in this heavily silted river than higher-frequency sonars, such as the one AHERC currently possesses. Teledyne BlueView indicated an interest in working with AHERC to solve the resolution and software reliability issues. The most recent versions of the Teledyne BlueView operating software appear to address past reliability issues, though these remain to be field tested. A split-beam sonar owned by the University of Alaska Fairbanks, deployed in September 2014for fisheries monitoring in support of the Oceana turbine, repeatedly operated for periods of up to a full day unattended using only a small Honda generator as a power source. With funding from the U.S. DOE, a power and data system has since been developed for long-term operation of the BlueView and split-beam sonar systems.
Task 5: COUPi DEM modeling. In addition to the four tasks that comprise the core of the project, the capability and techniques for simulating debris interactions with hydrokinetic infrastructure such as the RDDP were developed as a complement to the four core tasks. 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. Simulations indicate that the ability of the RDDP to clear debris is influenced by the shape of the debris object and the contact friction between debris and the platform. With funding from the U.S. DOE ALFA project, the COUPi DEM simulations will be improved by adding more realistic buoyancy effects and RDDP features, including a rotating debris sweep and a variable opening angle between the platform’s pontoons.
Overall, the RDDP and buoy system, developed as part of prior work directed by Alaska Power and Telephone and refined under this project, has proven a strong platform for protecting surface-mounted RECs from floating debris. Despite progress, however, many questions about subsurface debris remain, including its potential to disrupt subsurface RECs. The question of how best to bring hydrokinetic power to shore in debris-infested river waters remains largely unexplored. While we have made substantial progress in maximizing the utility of ADCPs for making hydrodynamic measurements relevant to REC installations, shortcomings in the current generation of ADV mounts require a greater investment of time so that smaller-scale turbulence and its effect on REC power output can be better understood. While the RDDP/buoy/pontoon barge protection scheme, developed partly with funding from the AEA, has proven successful, cost-effective installations of RECs in remote communities will require refinements in REC+debris protection systems to achieve affordable and practical implementations. The tools developed during this study are a step forward in our ability to evaluate any future hydrokinetic projects, both within and outside Alaska. As additional steps are taken toward employing hydrokinetic energy systems in Alaska’s waterways, we must consider the effects of any new hydrokinetic infrastructure on habitat, including that of the state’s fisheries.
Finally, given the short open-water season in Alaska, the economics of hydrokinetic energy must be determined, including a realistic projection of the levelized cost of energy for hydrokinetic energy systems. Remaining issues require systematic development of new platforms (e.g., improved ADV mounts or simpler, lighter, and stronger debris-mitigation schemes integral to REC systems) and techniques (e.g., improved understanding of the limits of sonar for fisheries monitoring in turbid rivers that carry large amounts of woody debris). Such efforts require continuing cooperation between state and federal agencies, the university, and the private sector.