The University of Alaska Fairbanks (UAF) Alaska Hydrokinetic Energy Research Center was tasked with developing a real-time data telemetry / remote power generation system to monitor frazil ice conditions in the Kvichak River in support of the U.S. Department of Energy funded “Next Generation MHK River Power System Optimized for Performance, Durability and Survivability” project. A real-time telemetry system was requested because of the short time span between the end of the frazil ice season when the instruments would be recovered, limited vessel availability and the project end-date.
To meet the project objectives, UAF designed and assembled a remote power/real-time data telemetry system that included an auto start propane generator, a small PV array, a small battery bank and line-of-sight radios as well as two sonar systems to monitor river velocity and water column acoustic backscatter strength. Both sonars included internal batteries for powering the instruments in case of failure of the shore based power system. The sonars, deployed in ~5 m of water on the bed of the Kvichak River, adjacent to the Village of Igiugig, Alaska were tethered to shore via a waterproof armored cable that conveyed power to the subsurface instruments and data from the instruments to the shore based telemetry system. The instruments were programmed to record data internally as well as to transmit data serially over the cables to the shore based system.
The system was in-place between November, 2016 and June, 2017. While the real-time data telemetry system was not successful and the remote power generation power system was only partially successful, the system design included sufficient redundant power in the form of internal instrument batteries to enable the collection of nearly three months of overlapping velocity and backscatter data (from November through February) and a record of acoustic backscatter strength spanning the entire ~150 day frazil ice season between November, 2016 and ~April, 2017.
The acoustic Doppler current profiler (ADCP) ceased recording data during a site visit in February during which communication to the ADCP was lost when personnel on-site were midway through re-programming the ADCP after the failure of the shore based remote power system. Based on battery bank voltages and ambient temperatures recorded by an on-site data logger, the remote power system functioned until mid-February just prior to the arrival of UAF personnel on-site, when very cold air temperatures (< -30 °C) caused the battery bank voltage to drop. An accumulation of ice from an icing event earlier in the deployment appeared to interfere with the generators ability to self-start and thus the generator was unable to recharge the battery bank. In addition, solar panels at the site were iced over and solar insolation was insufficient to clear the panels and/or deliver sufficient power to recharge the battery bank. While the generator was able to be restarted, UAF personnel on-site were not equipped to deal with the frozen batteries.
The results of the monitoring are summarized as follows: briefly, the sonars captured multiple time periods when frazil ice was present at the deployment site. Frazil was detected at the site beginning in early December when water temperatures first dipped below -0.1 deg. C. There is a ~2 week period in the record (from ~1/7/2017-1/22/2017) when frazil ice was continuously detected. Outside of this two week period, frazil is intermittently present. Later in the season, in late February, there appears to be enough solar gain during the day to warm water temperatures above the cutoff for frazil (~-0.2 deg. C) and there is a distinct diurnal signal in the backscatter and water temperature records. While the sonars are unable to definitively identify the presence of frazil ice, the increase in acoustic backscatter strength is correlated with periods when super cooled water was present at the site (temperatures below zero degrees Celsius). Both the ADCP and the Shallow Water Ice Profiler (SWIP) record water temperature. Note that video or physical sampling would be required confirm that the increase in acoustic backscatter is indeed frazil as well as to determine the accumulation rates of frazil on any submerged infrastructure to determine the risk frazil poses to hydrokinetic energy converters in this environment.