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Power to the ISS

Capturing solar energy on Earth can be complicated since sunlight can be blocked by clouds and bad weather. You’d think it would be easier it in space. Not necessarily so. 
True, there are no clouds or storms to get in the way. But there’s something much bigger -- Earth itself.

The Space Station, orbiting the Earth at an altitude of about 400 kilometres, completes a circuit every 92 minutes or so, and thus experiences 16 sunrises and sunsets in each 24-hour period.

For the Station to use solar energy effectively, designers had to develop a system that would compensate for the fact that the Earth blocks the Station from direct sunlight for as long as 36 minutes during each orbit. That’s up to 40% of each orbit in the shade!

The solution is a complex system four sets of huge solar panels—the largest solar power system ever built in space—with rechargeable nickel hydrogen batteries. The batteries store electricity generated by the silicon cells during the sunlit portion of each orbit and discharge electrical energy while in the shade.

Having a uninterrupted and reliable energy source is critical on the Station because power outages could be life-threatening. Electricity is literally the lifeblood of the Station, needed not only for daily activities and running scientific experiments, but also to keep the crew alive.

Each set of solar arrays consists of two panels, each 34 metres long and 12 metres wide, that extend from the sides of the Station like wings. Each panel contains 32,400 silicon cells, each about the size of a computer diskette. The purpose of all this silicon real estate is to soak up as much of the Sun’s energy as possible and convert it directly to electricity in a process known as "photovoltaics."

In total, the Station’s solar power system (including several Russian solar panels) will be able to generate 128 kW of power—about the amount of energy consumed by 64 homes. More than 40 percent of this energy will be devoted to scientific research projects conducted on the Station.

Hot and Cold

The radiators, which consist of seven panels, each about 1.8 by 3.6 metres, are designed to prevent the buildup of heat that could damage delicate components and to equalize the temperature of the Station. Designed to keep the Station’s temperature on an even keel, they extend to more than 15 metres when fully deployed.

Any orbiting spacecraft experiences extremes of hot and cold; the Station can be subjected to temperatures ranging from 149°C to -126°C. And electronic equipment on the Station generates a lot of heat. The solar power system itself does so in the process of collecting solar energy, converting it to electricity and distributing around the Station.

The coolant ammonia circulates through the radiators, absorbing heat from the Station’s systems and carrying it to the panels that dissipate it into the deep cold of space. Ammonia was chosen because it offers the best combination of thermal and safety qualities for the job.

The Station’s temperature is also regulated by a thin coating of thermal material on its exterior surfaces. This coating can be threatened by electrical arcing (power surges) that can occur because the Station will be operating at a relatively high voltage—160 volts direct current, compared with 28 volts DC for most spacecraft.

To solve this problem, NASA developed a "plasma contactor unit" to serve as an electrical ground to dissipate the surges and keep the station’s thermal coating from being zapped. This device contains a supply of gas that is converted to electrically charged particles (ions and electrons) which are released into space. This stream of particles carries away excess electrons that would otherwise cause the buildup of a harmful surface charge that could damage the Station.