Imnavait Creek Power and Communications System

The first of two power and communications systems nears completion at the Imnavait Creek ridge site. Note the webcam on upper left of tower. Overview: The Imnavait Creek drainage of northern Alaska is an area that has undergone significant scientific scrutiny for many years. The reasons are that the hilly terrain supports a wide range of tundra types in a relatively small area. Proximity to the Dalton Highway provides fairly easy access to undisturbed areas of vegetation. Nearby Toolik Field Station provides researchers with a well equipped base to work from. At over 68 degrees north, this area lies within the Arctic Circle. The elevation of the site is approximately 2,300 ft above sea level. Seasonal variation is extreme, with summer temperatures of over 75F possible, and the sun in the sky 24 hours a day

in June. During the long polar winter, temperatures can drop to -50F, with the sun not appearing over the Brooks Range from the middle of November to the end of January. Imnavait’s autonomous power and communications systems were initially installed in 2007 as temporary support infrastructure to support the Shaver/ Brett-Harte AON IPY proposal, which requires a year-round power supply as well as data telemetry. Two carbon flux towers measure gases released from the tundra. The two towers are located in an area of diverse terrain and vegetative composition. There is a third tower from a previous project which is still marginally operational. Taken together, the towers form a transect from dry heath ridge line to wet fen bottom.

The Systems: The Imnavait Creek area was forecast to become an area of intense study, with several other projects slated for deployment. As such, initial design efforts focused on developing a centralized hybrid power/communications system, with an AC grid feeding electricity to the various study sites. A large battery bank was to form the core of the system, with an inverter feeding the local grid. Solar and wind power were to be the primary inputs, with an engine driven generator starting automatically to recharge the battery bank when RE inputs were inadequate and battery voltage dropped below a pre-determined threshold. This system was to incorporate a WLAN (Wireless Local Area Network) that would allow data transmission back to the power station, for transmission to researchers home institutions via broadband satellite. Unfortunately, due to time and monetary constraints, developing such a sophisticated system was not possible. In short, we needed a temporary solution and we needed it now. Once the immediate project requirements were defined, we approached three vendors for quotes on systems capable of meeting the need and surviving in this unforgiving environment. ABS Alaskan was chosen as the best solution at a competitive price. The systems were based on a unit design previously deployed in several Alaska bush locations. They had proven to be reliable and cost effective solutions. We specified a few changes to better fit this application, and ABS was able to comply. A hybrid renewable energy design is not entirely optimal for this type of environment. The solar resource in the summer is outstanding, but is essentially non-existant for several months of the winter. The wind resources are generally fairly low in the summer, but are relatively good in the shoulder seasons. In the depth of the winter, there is often no sun or wind for extended periods of time, so a significant amount of energy storage is essential. Here an on-demand power supply would potentially have been the best solution, but the distance of the sites from the road access would have made refueling very difficult and costly. The timeline and budgetary realities forced a scaling back of the project. Initially, the researchers had wanted instrument de-icing capabilities. Of course, ice forms on the instruments during the winter, when the environmentally derived power sources are at their lowest ebb. The power requirement for this can be quite high. Without an on-demand power source, this requirement simply could not be accommodated.

With the instrument de-icing requirement eliminated, the power requirements became manageable with available renewable energy sources alone. The average system load was estimated to be about 20 Watts continuous. While this sounds like a pretty small load, remember that it is 24 hours a day, 365 days a year. Learning from past projects, we planned for long periods of sub-optimal energy harvest and the inevitable scope creep. The power requirement was doubled to 40 Watts continuous to ensure reliable performance even in the depths of winter. Specifications: The system is designed with efficiency as a key driver. Operating at 48VDC nominal, an inverter was not required to supply power to the instrument tower, which is approximately 100’ distant from the power unit. With an anticipated current of less than one amp, #8 MC cable carries the electrical power to the tower with minimal voltage drop. The separation is to avoid creating turbulence at the instrument tower. At the tower, the DC voltage is stepped down by an efficient DC/DC converter to the 12VDC required by the instrumentation. By eliminating the inverter and running a strictly DC system, overall efficiency was improved by nearly 10%. The ABS system consists of a 5’ x 10’ heavy duty steel sub-frame supporting about 3,500 lbs of equipment, most of the weight accounted for by the battery bank. The battery bank consists of 32 Concorde PVX-2240T, 6-volt AGM batteries wired in series/parallel. Four series strings of 8 batteries each (48 volts) allows for uninterrupted service in the event battery strings need to be maintained or replaced. The battery bank is rated at 43,000 Watt-hours at the 24 hour discharge rate. This is adequate to support the electrical load unassisted for 15 days of autonomy. Although part of the specifications, a low voltage load disconnect was not originally included. The battery bank and all electronics are housed in a robust fiberglass enclosure, 48” x 48” x 56” high. The enclosure is insulated with 3” of 2lb density polyisocyanurate foam for an R value of 19.84. Access is via an insulated and gasketed refrigerator type door. The original system control was designed to dump any excess power from the wind turbine or PV array to internal or external resistors, based on interior temperatures. A Southwest Windpower “Whisper 200” turbine sits atop a single Rohn tower section. The tower is hinged at the bottom for tilt-down functionality. The hub height on the turbine is approximately 15’ above the ground. While this height is absolutely minimal in terms of getting out of turbulent air and into the undisturbed airstream, the fact that the surrounding vegetation is only a few inches tall helps to somewhat reduce the penalty of a short tower. The turbine has a rotor diameter of 7 feet and is rated to produce 900 Watts at a wind speed of 28 mph (12.5 m/s). The tower also provides primary structural support for 5 Kyocera KC-130TM photovoltaic panels, wired in series for 60VDC nominal. The tower is guyed to the steel base, but also to four guy points in the tundra. Special anchors were designed for this terrain. They consist of 4’ lengths of ¾” diameter steel re-bar, sharpened to points. An adjustable collar slides up or down the re-bar, so it can be locked down at ground level. Surprisingly, we were able to drive the anchors in all the way at both sites using an electric jackhammer. Guy cables are 5/16” diameter and can be adjusted via turnbuckles at the lower end.

Inside the box, power from the 60VDC nominal PV array is reduced to the 48VDC nominal required for the battery bank by an Outback MX60 charge controller. The wild 3-phase AC produced by the Whisper 100 turbine is rectified to 48VDC by the controller that comes with the unit. Originally, the MX 60 handled the wind power regulation via the auxiliary relay control in the MX60. When battery voltage exceeds the threshold point of 58VDC, the relay in the MX60 activates, closing the circuit on a solid state relay. A thermostat located in the enclosure then activates a double throw contactor to send current to the resistors, either inside or outside, dependent upon internal temperatures. This allows for excess power to be put to beneficial use by heating the interior of the enclosure in the winter.

Primary power control electronics inside the enclosure. Wall space is at a premium.

The system is earth grounded via two 4’ ground rods, bonded together with #4 copper. The bare copper is also below grade, creating a good ground reference. The 48VDC is transferred to the instrument tower via #8 MC cable. This “mining cable” is very tough stuff, being both armored and water tight. We have utilized it elsewhere, running it

exposed on the top of the tundra. In time, the tundra grows up over it. In some instances we have had “tundra grids” running for years. At the instrument tower, the MC cable terminates at a DC rated breaker inside a weather tight electrical box. Initially we had intended to use a DC/DC converter made by Analytical Systems. Unfortunately, upon testing, this converter was found to have too high of an idle draw, taking nearly 5 Watts just to turn it on. Fortunately we had a super efficient DC/DC converter, built up by our colleagues at SRI International. Although rated for less power, it was adequate for our needs and pulled only half a Watt at idle. Instrumentation circuits are protected by a Blue Sea fuse block.

Weather proof box at instrument tower. The 48VDC is converted to 12VDC for the instrumentation by the

super-efficient DC/DC converter in the upper right corner. The Analytic Systems converter on the left proved to be too power hungry.

Deploying the Systems: Despite thorough planning, the deployment of these systems into the field was fraught with difficulties. Although ABS Alaskan did their best to accommodate our schedule on short notice, it was an incredible rush for them to complete the systems. The manufacturer of the enclosures delayed them by over a week, and of course much of the equipment installation had to wait for a

place to put it. Due to the diligence of the ABS staff working long hours, the systems were essentially ready to ship many days after the intended deployment. That said, the systems were never fully assembled and tested as complete units, as was specified in the contract, and this did lead to some difficulties in the field. Schedule slip in any area tends to have a cascading effect down the line. The delay also precipitated a last minute staffing change on the field crew. While his assistance was invaluable during the preparation stage, Andy Young had to deploy to Palmer Station, Antarctica. Fortunately, Allen Cornelison was able to fill in at the last minute and proved to be extremely capable and hard working.

Allen at the ridge site as we begin assembling the systems.

The units were disassembled as far as necessary to survive truck shipment up the Dalton Highway, a 400 mile trip between Fairbanks and Toolik Field Station, much of it on dirt road. The units arrived intact. While we had intended to unload the truck at the gravel parking area at Imnavait Creek, Toolik staff insisted that the truck must be unloaded at the station for obscure reasons that defied comprehension. This meant that the helicopter had to sling the loads eight miles, rather than the .75 miles it would otherwise have been. Helicopter hours are not cheap, and this at least doubled the amount of time it took to deploy the systems. Nevertheless, the field team persevered and made the best of the situation. The helicopter was relatively small and the pilot inexperienced with handling sling loads. As such, the systems had to be disassembled almost completely for the final journey to the field

locations. This obviously all took a great deal of time and added to the complexity of the task. Suffice it to say that ultimately all system components were delivered to the sites without incident, a major accomplishment in and of itself.

“Some assembly required”. A semi-organized pile of stuff that is about to be put together completely for

the first time on location. Not recommended practice. Fortunately, the weather was stellar.

Once the primary logistical challenges were overcome, the team of two set to work in earnest, assembling the units completely for the first time on site. This is certainly not an ideal scenario, and several glitches were encountered as a result. For instance, the battery systems are typically 24 volt, rather than 48 volt. As a result, the wiring schematics and cabling were not correct. Fortunately, we had brought along a range of cables and terminals, and were able to fabricate high quality conductors on site. We have found that “Arctic Flex” conductors are absolutely top quality and easy to work with in the field. Quality components and a well-stocked field kit can save a project from disaster, and that was certainly the case here. By the end of the deployment, virtually all of the cable and terminals had been used, as well as distribution blocks and mechanical lugs. We were also able to get a resupply in from ABS Alaskan in time to re-do some work that was functional if not quite to code.

The bottom line is that we were able to complete the deployment of these systems despite numerous setbacks and challenges. The system design and components appeared to be quite good. Everything relating to the power systems came on line and operated as anticipated. Data Acquisition and Communications:

Data acquisition and communications hardware panel.

Advances in system telemetry has revolutionized how we approach remote power system design. Previously, power systems were deployed and the designers and researchers had no idea whether things were working properly, or if it had failed after the first day. Much irretrievable data was lost at great expense over the years as a result. Satellite communications has changed all that. In terms of the power systems, we would not even have known that we had problems with the wind power charge control had we not the ability to see the overcharging events in graphic display. The ability to monitor system performance has had direct bearing on the evolution of these systems and those that will follow. The SRI International developed system employed in support of this project uses low power PC/104 single board computers for data acquisition and control of the Iridium transceivers and Webcam. The communications utilize Data Transport Network and Iridium RUDICS protocols. System Operation: With over two years of running time on these systems, we can now relate some of the successes and failures experienced to date. At no time has the power supply been inadequate for the load at either site, even in the depths of winter. Intermittent system telemetry has made it difficult to precisely monitor performance, but it is clear that battery capacity has never dropped below 65%. The power system components at the instrument towers have also performed well, with a steady 12VDC supplying instrumentation without interruption for this period. There was one report that the wind turbine at the fen site had frozen in the furled position. However, during subsequent wind events, power production appeared to be normal. During the 2008 site visit, the turbine was found to be operating normally. Although there were no problems with any of the cable anchors, the platform at the fen site was found to have gotten a bit out of plumb, one corner having sunk into the ooze by several inches. It was re-leveled with plywood shims and cables re-adjusted. The one significant problem we experienced with the power system is a chronic overcharging of the batteries during wind events. We tried several different things to correct the problem, utilizing remote troubleshooting and assistance from individuals not directly associated with the project. Several components were found to be inadequately rated for the system voltage, and these were replaced or wired around, but the problem persisted. Ultimately, it was determined that the underlying problem was the system architecture, as explained in the section below. In February of 2009, the ridge system failed, forcing an emergency field trip to address the problem. While at the site, it was determined that the second system had also failed while the technician was in route to the site. In both cases, the mechanism of failure was the same. The solid state relay had failed in the closed position, allowing for complete discharge of the battery banks to the external dump loads. This resulted in about 2,500 lbs of frozen batteries at each site. The batteries were thawed out using a portable generator powering an electric space heater and series battery charger. At -40 with high winds, even keeping the little generator running and

fueled was problematic, but ultimately stubborn persistence won out over the elements. The batteries were thawed and re-charged. Temporary system architecture changes were made to restore functionality and ensure there would not be a repeat of this type of failure. While overall battery capacity has likely taken a hit, both banks appear to remain viable in terms of energy storage. Shortly after returning from this field trip, another problem developed at the ridge site. Although the battery voltage was fine at >50 volts, the voltage at the tower had dropped to ~36 VDC. Working with the science tech on the project, Glenn Scott, the problem was ultimately found to be low ampacity through the current shunt used to monitor the science load. This was temporarily wired around. Lessons Learned: First and foremost, rushed deployments lead to problems. Inadequate time for system design, component procurement, integration and testing all contributed to issues that proved rather difficult to resolve one the units were deployed into the field. While the 48 volt system allows for greater system efficiency and smaller wire sizes, it created some significant problems in finding appropriately rated components. Relays, contactors and thermostats operating at 12 volts or 24 volts are common. While components rated for 48 volts on the main contacts can be found, the coils that operate the contactors are typically 12 volt. The only 12 volt signal voltage readily available in the system was from the Outback MX60 charge controller, the primary function of which is regulation of the PV input to the batteries. Utilizing the 12 volt auxiliary relay as the primary control for the wind power diversion regulation tied all the charge control from both sources to one component, creating a potential single point of failure that could take the entire system down. It also limited the range of adjustment for the wind power regulation set points. Initially, it seemed that having the ability to dump excess power as heat inside the module was a very desirable feature. However, in order to do this, the system became significantly more complex, and introduced several more potential single points of failure. Reviewing the data, it became fairly clear that there was not a significant benefit to this functionality. Understand that the energy conversion in a battery based system is only about 80% efficient. The remaining 20% of the incoming power ends up being expressed as heat. As such, whenever there is a charging event, a fair bit of heat comes off of the batteries, the rectifier for the wind turbine, and other electronics to a lesser extent. We already saw a rapid temperature rise within the small, well insulated enclosure due to this. Whenever the interior dump load was functioning properly, the temperature rise was quite rapid, and it was not long before the thermostat re-directed the excess power to the external dump loads. Indeed, it seemed that running too high a temperature within the enclosure was the greater risk, as we saw temperatures of 40C and higher, approaching the upper limits of some of the electronics. The premise that it was important to utilize every bit of excess power to heat the interior of the enclosure was flawed. It must be noted that this was a design requirement that originated with

the author, not with ABS Alaska. They went about designing the system architecture in what seemed the best manner possible, but it proved to be highly problematic, and resulted in chronic battery overcharging during every sustained wind event, as well as borderline overheating of the module interior, and ultimately, system failure. During the 2009 emergency field maintenance visit, the interior dump load and all of the components required for that functionality were wired around. Instead, the system now utilizes the charge regulation that came with the Whisper 200 wind turbine and only the exterior dump load. In addition to the greater reliability achieved by simplifying the architecture and eliminating several potential points of failure, the charge control for the PV and wind inputs are now entirely independent, joined only at the battery bank. With this system architecture, the wind power system could fail and still allow the PV system to input energy to the batteries and vice versa. Human beings often try complex methodologies first, before reverting back to what is simple and known to work, as this project well exemplifies. Due to time and environmental constraints, the system repairs were not perfect. The wiring will be re-worked and additional features will be incorporated. This will include a low voltage load disconnect and a redundant diversion charge control. Both functions will be handled by separate Morningstar, Tristar TS45 multi function charge controllers. It is essential to reduce the number of connections to the bare minimum, as every connection is a potential point of failure. This proved to be true in reference to the low tower voltage as a result of corroded connections on the current shunt. Hall affect current transformers allow for fairly accurate current measurements without any additional connections. This is the preferred method for monitoring current flow in any system. Conclusion: Despite a few problems that proved to be rather difficult to resolve, the Imnavait Creek power systems have fulfilled the power requirements of the project with minimal downtime for over two years. The 48 volt nominal system exacerbated some of the problems in finding appropriately rated components, but also allowed for greater system efficiency and lower costs in wiring.