Oxygen, water, food, and heat are essential for human survival, but on the International Space Station (ISS), electricity is the most critical resource. Designed by NASA's Glenn Research Center in the early 1990s, the electric power system of the ISS is what allows crews to live comfortably, safely operate the station and perform scientific experiments. Without the electric power system, the world's habitable artificial satellite wouldn't exist.
The ISS electric power system uses solar cells to directly convert sunlight into electricity. Though it works to provide more than enough power on the station, this method, called photovoltaics, builds up excess heat that can damage spacecraft equipment. To eliminate this, the ISS electric power system uses radiators to keep the heat away from the spacecraft. The radiators are aligned toward the cold void of deep space, shaded from the sunlight.
Solar array wings
When most people think of the ISS, the solar array wings are what come to mind. The wings each consist of two retractable blankets, deployed and supported by an extendable mast, and are mounted with 33,000 solar cells. When fully extended, the 2,400-pound wings reach 115 feet in length and are 39 feet wide. When retracted, they fold like an accordion into a solar array blanket box that's just 20 inches high and 15-feet long.
The solar cells, which are made from purified chunks of the element silicon, were installed over several space shuttle missions, the last being in March 2009. Altogether, the four sets of solar arrays can generate 84 to 120 kilowatts of electricity — enough to provide power to more than 40 homes.
Two blankets of solar cells make up a solar array wing. Image via NASA.
Batteries
At times, some or all of the solar arrays are in the shadow of Earth or the shadow of part of the ISS. When the station isn't in direct sunlight, it uses rechargeable nickel-hydrogen batteries to provide continuous power. Thirty-eight individual pressure vessel Ni-H2 battery cells are connected in series and packaged in an orbital replacement unit (ORU), and two ORUs are connected in series to form one battery, using a total of 76 cells. During the sunlit part of the orbit, the batteries are recharged.
Designed with a life expectancy of 6.5 years, the batteries must be replaced multiple times during the expected 20-year lifespan of the ISS.
ISS battery subassembly ORU. Image via NASA.
Power management and distribution
Through a series of switches, the power management and distribution subsystem disburses power at 160 volts of direct current around the station. The switches have built-in microprocessors that are controlled by software and are connected to a computer network that runs throughout the station. The dc/dc converter units condition the voltage from 160 to 120 volts dc to form a secondary power system to service the loads. The converters also isolate the secondary system from the primary system and maintain uniform power quality throughout the station.
Whether it's used to power the life-support system or to manage a computerized data network system, electricity is essential on the ISS. Because of the enormous panels of photovoltaic cells, long-lasting rechargeable batteries, and careful testing and planning, the International Space Station's electrical power system keeps the world's wondrous artificial satellite habitable as it soars 200-plus miles high.
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