PV at the Polar Ice Cap
The Tower of Power’s triangular design means strength to withstand the arctic blasts and maximum solar harvest without mechanical tracking.
By TRACY DAHL
A few years ago in these pages, I detailed the annual Clean Snowmobile Challenge, in which college engineering students compete to redesign snowmobiles for efficiency, low noise levels and low emissions (November/December 2010). Top finishers in the zero-emissions category may send their machines to Summit Station on the Greenland ice cap, an international research platform for year-round atmospheric science and other research. These snowmobiles offset the use of vehicles whose exhaust can skew data collection. But a diesel-powered generator remains the primary source of electricity at Summit Station. When a wind turbine pilot project there ended, I saw the opportunity for a unique photovoltaic (PV) project.
With an innovative approach, we implemented a purpose-built project, cost-effectively, at one of the most remote and extreme environments on the planet. The tower helps stabilize long-term energy costs and reduces the station’s carbon footprint, while protecting the integrity of research at Summit Station. On a symbolic level, the Tower of Power creates an iconic image of polar solar power at the top of the world.
Understanding the Environment
What drives a solar power design? Two primary criteria: cost and available resources. For this project funded by the U.S. National Science Foundation (NSF), located on Greenland’s ice cap, many other design considerations joined those two main drivers. The project’s impetus was the ending of a wind power pilot project that would leave a specialized foundation available. We had put significant effort into designing and installing the deep-snow foundation for the wind tower, and I wanted to reuse that resource.
The environment of this installation is quite different from most places on the planet: Summit Station is a high-latitude (72.4 deg N), high-elevation (10,600-foot, or 3.23-kilometer) site, located near the highest point of the Greenland ice cap. Although the mass loss of the ice cap was recently established, much of the melting occurs closer to the periphery of the ice cap.
Temperatures at Summit rarely rise above the freezing point, and thus the snow here accumulates permanently, at around 25 inches per year in the general area. Wind-driven snow is the more difficult problem. During winter windstorms, structures directly on the snow surface can be buried to the roof from a single wind event, often lasting days. All of the buildings on station have roof hatches to allow occupants to exit after the winds have subsided. Everything exposed to the elements at Summit, including the people who work there, must be extremely robust.
The solar resource at high latitudes (particularly on an ice cap) can be quite good, but the physics is somewhat complicated. It has taken me nearly 20 years of working in polar environments to learn how to turn conditions to our advantage. While the sun’s rays travel through a greater length of atmosphere due to the angle, relatively low water vapor offsets this disadvantage. When combined with a permanent, highly reflective snow surface, the summertime global irradiance is quite high.
The extreme cold of high polar environments results in significantly higher voltages being produced by PV. Recent technological advances in maximum power point tracking allow us to take advantage of the higher voltages, resulting in greater net power production per unit of PV collector surface. During summer in the high latitudes, the sun stays above the horizon for months — but it remains below the horizon for long periods in the winter.
For most solar collectors, the greatest annual production usually results by setting PV panels at an angle near the latitude of the site and facing the sun. But Summit’s environmental factors argue in favor of orienting the panels vertically. While the angle of incidence affects optimal harvest of direct solar gain, vertical orientation allows for capture and conversion of some light energy reflected from the snow surface. Vertical orientation also tends to reduce snow accumulation on the panel faces. Since snow-covered PV panels make little to no power, keeping them snow-free helps maximize production.
Between the time it came on line on July 21 and mid-November, when the sun dipped below the horizon for the winter, this solar array generated more than 1 megawatt-hour of energy.
Designing for Strength, Light Weight
I had been thinking about how to reuse the wind turbine tower foundation, but answers eluded me — until the basic idea for the solar tower came to me in a dream. Built in a triangular shape, the tower could be made very strong, capable of shedding the force of the wind far better than a conventional, single-plane array. Orienting solar facets in three different directions would maximize the summer solar harvest over the course of each day, while avoiding mechanical tracking devices. The vertical faces would absorb reflected light in the high summer, and provide a good angle of incidence in the shoulder seasons.
But how would we actually build the thing? Summit Station has no crane and limited equipment resources. Any design would have to be built in the field — on the ice — by a couple of guys with hand tools.
Minimizing the overall weight of this structure was an important design criterion for several reasons. First, we had to build it on site without a lot of heavy equipment support. Secondly, Summit Station is at the end of a long logistics chain. Shipping a pound of anything there is expensive, and cost-effectiveness was one of my primary goals.
Initially I thought we’d use the leftover wind turbine monopole tower as a structural element. But the more paper I used drawing complicated frameworks to support the panels, the less enthusiastic I became about the monopole approach. After a couple of epiphanies, I came up with the structural design.
The first breakthrough: using three relatively lightweight vertical poles outside the panels to support the array, rather than just one in the middle. Structurally this results in much greater strength at a reduced weight. The second break-through idea was to use a simple bracket bolted to the ends of two PV panels, pinching down on the vertical tubes with a setscrew accessible from the outside. With this approach, each facet becomes a solid, planar structure. Each connects to the adjacent plane, creating a strong triangular structure; yet the overall structure retains some flexibility due to the manner in which it is attached to the substructure of vertical pipes.
A hinged base allows the tower to be built on the snow and then tilted up into place. With the exception of the hinged base and the top fixture, which captures the tops of the pipes, everything was small and lightweight. The frames of the PV panels themselves are structural members. Diagonal bracing prevents excessive stresses on the panels and creates a triangulated “space frame,” lightweight yet exceptionally strong. The tower base is supported by a central pipe, reinforced with plates set radially. These reinforcements create a sturdy supporting structure by increasing the rigidity and acting as snow flukes, preventing any potential lateral movement. Buried about 15 feet below surface, the foundation base is steadfast.
Meeting the Electrical Design Challenges
I had to address a few unique electrical aspects in the design. First, calculations had to account for a very large temperature differential coefficient to accommodate the coldest anticipated temperature in sunlight, typically in February or March. I used -60°C (-76°F) as the minimum. The equation is the usual one, but the voltage differential for the strings ends up being quite large. For the Solectria inverters I had chosen, I could use from seven to nine panels in the string to achieve an acceptable voltage range. I decided to go with eight panels, more for structural reasons than electrical.
The NSF had purchased a number of Evergreen 205 PV panels at a good price. Having used Evergreen panels previously, I appreciated their quality and the innovative “string ribbon” technology they employed. The fact that the company went bankrupt just as they were commercializing their product was a bad break for the solar industry, but it did allow us to obtain these quality panels at a very low cost.
With three facets facing different directions, I had to keep the DC conductors discrete to the inverters. Theoretically, one of the facets would always be shaded as the sun circled in the sky, and if it was connected to the other two, it would tend to pull them down, reducing overall power output. Because the turbine previously located on this foundation had a three-phase output, we already had a four-conductor plus ground-armored, liquid-tight cable buried under the snow. The AWG #8 wire was adequately sized to keep within a 2 percent voltage drop on the 200-foot (61-meter) run, but we were short a few conductors. Fortunately, station inventory included a roll of used 8-2 plus ground, so we avoided costs again.
At Summit Station, bedrock is under about 2 miles of ice. As a result, the place lacks an earth ground, another electrical design consideration. We addressed this challenge with a robust equipment-grounding system that ties together potential current-carrying metal bits (PV frames, tower components, electrical enclosures, etc.). We then referenced the system back to the single-largest piece of iron on station — the diesel generator — creating a system where all of the electrical-protection devices will work as intended. Because the snow surface is non- conductive, this may actually be safer than most terrestrial systems.
Finally, I needed to tie the solar power into the electrical grid for a local diesel generator operating at 480 volts, three-phase. I tied three Solectria PVI 1800 inverters operating at 208 volts into a three-phase renewable energy subpanel. The panel back-feeds through a 208/480 transformer, then through a breaker into the main distribution panel.
Constructing the Tower
I developed the basic structural design, but I was not working in a vacuum. Fabricator Tony Paradisa of Topar Welding in Trinidad, Colo., offered good ideas and fabrication details in the early stages. Structural engineer Katrina Pearson, from our partner company, CH2MHILL, reviewed the design and made a few changes to increase the structure’s torsional strength. Topar Welding did an excellent job of fabricating and crating it all up for the long trip north.
The downside of using lightweight parts is that it takes a lot of little bolts to hold a structure together — nearly 400 bolts were used in this structure. In particular I had some nightmares about the hundreds of quarter-20 bolts used to attach the PV panels and diagonal supports. Those are some small nuts and bolts for heavily gloved hands operating in a cold, windy place. I purchased abundant extras, expecting that we would lose a few to the snow, but in the end my concern was unfounded.
The small community of Kangerlussuaq serves as the NSF’s logistics hub in Greenland. I passed through “Kanger” en route to Summit with my partner on this project, Nick Salava, during an incredible warm, windy spell in July. Ice cap thawing caused massive volumes of water to race through river channels around Kanger, and the water rose so high, it washed out a section of the main bridge connecting parts of the town.
At Summit, we experienced several days in a row of above-freezing temperatures — very unusual. We found ourselves comfortably working in T-shirts. I’ve been to Summit Station many times and had never witnessed these conditions before. We later learned that NASA satellites observed surface melting across more than 90 percent of the Greenland ice cap.
The warming made this job a lot easier. We wore light gloves, which made all of those little fasteners less of a nightmare than I had feared. The conductors were far more flexible and easier to handle than they would have been at “normal” ice cap temperatures. Finally, it was simply easier to pull long hours outside. We were on a pretty tight schedule with only nine days on snow to “get ’er done.”
We accomplished the goal, due in part to the stellar weather, but also because Nick Salava worked efficiently and produced professional results. Other than connecting the leads on the panels together, I hardly touched a wire, leaving the smart work to Nick while I shoveled snow and generally did the grunt work.
This was the first time I had worked closely with Nick. I later discovered that, though he is a licensed electrician and renewable energy professional by trade, his college degree is in the fine arts: metal sculpture, to be exact. To me, the Tower of Power is a “functional sculpture,” so Nick turned out to be the perfect guy for the job.
So how does the tower work? Even better than expected.
The PV deployed on the tower totals 4,920 watts. Because of the three-facet design, one would never expect to see more than about 2,500 watts at any one time, yet the Tower of Power has produced more than 4 kilowatts (kW) at peak and regularly exceeded 3.5 kW. Further, during summer’s peak, it makes power 24 hours a day. The tower came on line on July 21, a month past the summer solstice, and produced until mid-November, when the sun finally dipped below the horizon for the winter. In that brief period this solar array generated more than 1 megawatt-hour of energy. By the time you read this, the sun will have returned above the horizon in mid-February and the system ”awakened” to start feeding power into the grid again automatically.
Summit has seen several major storms since the tower went up, with wind speeds exceeding 50 mph. Yet the Tower of Power has weathered these with no damage or significant movement.
We kept project costs low by purchasing panels at low prices, reusing the existing foundation and coming up with a design that minimizes materials and allows for a small construction crew to implement on site. All told, the cost per installed watt was about $6.40. Assuming stable fuel costs and even without accounting for emissions reductions resulting from burning less fuel in the diesel generators, the system’s value is considerable. Given the very high cost of electrical energy at Summit, the expected return on investment is around 7.5 years.
The NSF funds Summit Station, Greenland, in cooperation with the government of Greenland. CH2M HILL operates the station under contract to the NSF.
Coming to a Site Near You? Though designed specifically for a high polar environment, solar engineers should consider whether “going vertical” makes sense for applications in more temperate climes. The structure packs a large amount of generating capacity into a small footprint, and the tilt-down design allows for fabrication without a crane. The same basic design could use one live PV facet facing south, substituting inexpensive structural panels on the other two sides. I imagine solar towers in front of public and professional buildings — basically anywhere someone wants to reduce energy costs and carbon footprint while making a bold statement.
Author Tracy Dahl provides sustainable technology solutions to researchers working in the remote Arctic with support from the U.S. National Science Foundation (NSF). For more than 20 years, Dahl has developed remote power systems allowing scientists to run instruments and communications equipment far from established infrastructure. With CH2M HILL Polar Services, the primary research support and logistics provider for the NSF’s arctic program, Dahl has learned, through trial and error, how to work with the polar environment to create reliable and rugged systems.
Dahl is a NABCEP-certified PV installer. He lives and works off the grid in Southern Colorado, where he and wife Amy are often in the midst of developing sustainable technology solutions for professional and personal applications.