In spring 2010, Alaska Power and Telephone (AP&T) deployed a 25 kW New Energy Corporation EnCurrent hydrokinetic turbine in the Yukon River at Eagle, Alaska, to determine the feasibility of using river in-stream energy conversion (RISEC) devices to supply power to remote communities. The turbine was deployed on a floating platform and operated successfully until problems with surface and submerged debris caused AP&T to end operations. The company found that extensive debris problems on the Yukon at Eagle posed a severe challenge to operating the turbine and created significant safety hazards for their personnel. As a result, plans for deploying the turbine in 2011 were cancelled, and AP&T initiated a project with the Alaska Hydrokinetic Research Center (AHERC) to examine ways to reduce the hazard of surface debris for RISEC devices deployed from floating platforms.
The focus of AHERC’s study was on the characteristics of river debris and strategies for reducing the impact of debris on RISEC infrastructure. This information was used to develop statistics on the occurrence of debris at AHERC’s Tanana River Test Site located in Nenana, Alaska, and to design a research debris diversion platform (RDDP). The RDDP consists of two steel pontoons joined in a wedge with its apex facing upstream (Figure E1a). A verticalaxis freely rotating cylinder (1.1 m diameter) was placed at the leading edge of the wedge. The rotating cylinder initially employed an array of hinged vanes that would exploit the river current to promote rotation, but later was covered with plastic to reduce surface friction. The angle between the two pontoons of the RDDP is adjustable, from 25 to 77 degrees, and the rotating cylinder at the leading edge may be replaced with a fixed angled (62°) wedge (Figure E1b).
The RDDP was moored to a buoy that was connected by a chain to an embedment anchor. The buoy anchor chain was the only tether that traversed the full depth of the river. The tether from the RDDP to the buoy ran parallel to the river surface. This mooring arrangement reduced the probability that debris would catch on mooring tether lines. Any debris that caught on the anchor chain would have a limited effect on RISEC device performance. Tests of the RDDP’s ability to divert debris around a protected zone and the RDDP’s effect on river turbulence were conducted at AHERC’s Tanana River Test Site.
The RDDP system (i.e., anchor, buoy, tether line, RDDP) using the cylindrical debris sweep performed well at diverting river surface debris around the RDDP. Debris impact forces on the RDDP ranged from 3 kN to over 6 kN during the study period. The sharp-angled front end (Figure E1b) did divert debris, but was much less effective than the debris sweep. The mooring buoy’s constant movement caused by river turbulence created an unstable site for debris to accumulate, and the buoy’s large buoyancy made it difficult for debris to override the buoy. The mooring buoy provided a strong first defense against surface debris by reorienting the debris lengthwise, parallel to the current flow direction, making it easier for the RDDP to divert debris.
Any effect of the RDDP on river turbulence was masked by the considerable, natural variability of the river flow. Quasi-stationary acoustic Doppler current profiler (ADCP) measurements detected a decrease in the northward directed near-surface river velocities as well as an increase in the westward directed near-surface velocities in the RDDP’s wake. These changes were attributed to the presence of the RDDP. Changes in river flow direction were negligible several meters downstream from the RDDP.
The opening angle between the RDDP pontoons has a significant effect on the ability of the pontoons to divert debris around the RDDP. The water current force that acts to clear debris from the pontoon surfaces decreases with pontoon opening angle, while the water force acting to pin debris to the pontoon increases with pontoon opening angle. Debris can become pinned against the RDDP when the opening angle of the pontoons is greater than about 58 degrees. Larger opening angles can result in debris counter-rotating (rotating counterclockwise with respect to the river’s right bank) under the RDDP.
High-momentum debris impacts against the RDDP can cause it to rotate about its mooring anchor point, allowing debris to move into the “protected” river current flow path behind the RDDP. To reduce this effect the RDDP should be connected to the downstream RISEC floating platform, such that the combined inertia of the RDDP and the RISEC platform acts to resist the debris impact momentum.
The performance of the RDDP can be improved by covering its pontoon surfaces with a hard plastic sheet to reduce friction and thus provide a smooth contact surface for debris. Moving the safety railing from the outside to the inside of the pontoons will reduce the probability of a debris object branch catching on a railing stanchion. Reducing the inertia of the RDDP debris sweep and operating the RDDP with a pontoon opening angle of less than 40 degrees will improve the RDDP’s ability to clear debris from the front of the RDDP and divert the debris. Increasing the RDDP pontoon draft will reduce the probability of debris counterrotating under the pontoon.
The RDDP system can provide effective protection from river surface debris for RISEC devices deployed from a floating platform. The RDDP is not designed to divert subsurface debris that is moving in the river. Further work is needed in understanding the prevalence of subsurface debris to determine the probability of subsurface debris impact on a RISEC device and to guide concepts for protecting RISIC devices from subsurface debris.