Solar in Space: Powering the International Space Station

August 7, 2017 by Rich Dana

About Solar Energy, Solar Education, solar electric vehicles --

Since the earliest days of the space program, solar panels have been powering satellites, spacecraft and space stations. Today, the International Space Station relies on one of the most advanced solar arrays ever built to support life and to power research that will take humans to new heights.

The International Space Station, or ISS, is the largest human-made orbital satellite in history, with components manufactured and maintained by U.S., Russian, Japanese and European space agencies. It is a modular structure with pressurized and unpressurized sections designed for habitat and life support, research and engineering. The first module was launched into orbit in 1998, and new modules continue to be added to the space station. In the nearly 20 years of continuous use, the ISS has relied on state-of-the-art solar equipment for 100% of its energy needs. No application could demand more reliability than survival in outer space, and solar photovoltaics (PV) is the energy generation technology provides that level of reliability.

A Brief History of Solar in Space

In an August 2015 article here at Solar Tribune entitled Solar in Space: Powering Earth and Beyond, I discussed some of the historical examples of solar usage in the early days of space exploration, as well as possible future applications.

Vanguard 1, launching into orbit in 1958, was the first satellite to use solar panels to power its instruments. The tiny, 3.25 lbs research satellite is not much larger than a grapefruit, but it provided ground-breaking geographical for six years before going dark. Both solar and spacecraft technology have come a long way since Vanguard 1, but solar continues to be the go-to source for reliable, renewable power in space.

The Russian Soyuz spacecraft have always used solar panels, since their introduction in the early 1960s. The Chinese Shenzhou transport vehicles also use solar panels. Readers may be surprised to discover that no NASA transport vehicles have ever utilized solar during their flights. Not Mercury, Apollo, or even the Space Shuttle. The Dragon capsules launched by Space-X (Elon Musk’s privately-owned rocket company) are the first American transport vehicles with integrated solar.

Skylab. Photo:History.com

America has used solar in space on many occasions, despite the exclusion from use on transport vehicles. For many older Americans, the first American space station, Skylab, may have been their first exposure to solar photovoltaic (PV). Launched in 1973, Skylab sported 10 kWs of solar generation, along with hydrogen fuel cells. Many of the space probes launched by NASA to explore other parts of our solar system were also powered by solar panels. The Hubble space telescope, the Mars Observer, and the Rosetta probe all used solar. Juno, which flew to Jupiter, utilized 280 sq. ft. of solar panels. This is the farthest away from the sun that solar panels have been used- beyond Jupiter, current PV technology is no longer effective.

The ISS Solar Array: System Profile

Interestingly, all of the sections of the International Space Station do not share a common electrical generating system. The Russian and American segments of the station actually have separate, and quite different electrical equipment. The Russian portion of the station, comprised of the Pirs (Pier) docking module, the Poisk and Rassvet research modules, the Zarya cargo module and the Zvesda service module are all powered by 28V DC, similar to the power systems used in launch, transport and service vehicles like Soyuz, the Space Shuttle and Dragon X. Each of the sections has its own series of solar arrays; four large pairs on the US side and four small sets on the Russian side. The two “neighborhoods” of the station are not isolated from one another, though. They share power through a series of power converters.

Photo: NASA

The Electrical Power System (EPS) consists of several hardware components called Orbital Replacement Units (ORU). The ORUs are designed to be replaced robotically or by spacewalking astronauts working outside the station.

According to NASA’s website, the eight ISS arrays contain a total of 262,400 solar cells and cover an area of about 27,000 square feet — more than half the area of a football field. Each of the US solar array’s have a wingspan of 240 feet, and the space station’s electrical power system is connected by eight miles of wire.

The Panels

photo:NASA

The long series of linked modules that make up the central fuselage of the ISS is the American part of the station. The shorter Russian section tees off from the center of the American portion with another series of modules.The giant double rows of solar panels that make up the largest portion of the ISS generation system are mounted perpendicular to the American portion, while each of the Russian modules is equipped with their own smaller, independent solar arrays. This illustrates the difference in system design philosophies- the Russian modules are designed to run independently, while the American section runs off of a series of large, centralized solar power plants.

The panels used on the station are quite different from the standard PV panels used here on Earth. They are bifacial- that is, they are two-sided, allowing the arrays to collect sunlight from a wide variety of angles as the station orbits the planet every 90 minutes.

Traveling at 17,500 miles per hour as it orbits 220 miles above the globe in the Earth’s thermosphere, the station experiences 35 minutes of darkness each rotation. Not only that, the station itself shades portions of the array as it moves in and out of the sun. For this reason, 60% of the station’s generating capacity is dedicated to charging batteries at any given time. The four sets of arrays generate anywhere from 84 to 120 kilowatts of electricity — enough to provide power to more than 40 homes.

Even in space, heat is an issue for solar panels. The ISS system uses a series of baffles called “radiators” that run along the base of the arrays and dissipate the heat away from the station and out into space.

Photo:NASA

The Racks

Each array is mounted on an accordion-folded racking system designed to be transported into orbit compressed and then unfolded to its full length when deployed. These fold-out racks of panels are referred to as “blankets.” The station began its life with just one set of blankets, and now has four sets, the latest having been installed in 2009.

Because of the constant and rapid changing position of the station in its elliptical orbit, the racks include gimbals that continually rotate the panels to face the sun. Similar to a dual-axis tracker used here on Earth to track the sun at both time of day and time of year, The ISS system uses an “alpha” gimbal to track the position of the sun while the “beta” gimbal adjusts to compensate for the elliptical orbit.

Photo:community.topcoder.com

One fascinating aspect of the ISS rack design is the way it compensates for the drag that the giant solar “wings” cause in the thin, residual atmosphere in the thermosphere. Left unchecked, the drag would cause the station’s orbit to decay. To counteract the effect, when the station goes into darkness, the arrays go into “night glider” mode. The panels rotate into a low angle to reduce drag by 30%. When the station re-enters sunlight, they return to their tracking position.

The Batteries

The heart of the ISS electrical system is its bank of rechargeable nickel-hydrogen batteries. These batteries are vital to life-support and ongoing work at the station during the sixteen half-hour long “nights” that the station experiences during every 24-hour terrestrial day.

Photo: NASA

The batteries have a life expectancy of 6.5 years and are changed intermittently as part of the systems maintenance schedule. Starting in 2017, the older nickel-hydrogen batteries will be replaced with smaller, more efficient lithium-ion batteries. These batteries are expected to last much longer than the older, larger batteries, and most likely will be the last set of batteries the station will need.

Boeing: The Contractor Behind the ISS

Since the beginning, Boeing has been the primary contractor for the construction and maintenance of the International Space Station, including the solar equipment. The ISS batteries and the battery charge/discharge units (BCDUs), are manufactured by Space Systems/Loral (SS/L), under contract to Boeing. Spectrolab, a wholly-owned subsidiary of Boeing, is the world’s leading producer of state-of-the-art space solar cells and panels. Founded in 1956, Spectrolab has developed high-efficiency solar cells for space missions, including the Apollo 11 mission to the moon and the Juno probe.

On 30 September 2015, Boeing’s contract with NASA as prime contractor for the ISS was extended to 30 September 2020. Part of Boeing’s services under the contract will relate to extending the station’s primary structural hardware past 2020 to the end of 2028.