International Space Station (ISS) power system
This article will outline the ISS power system, starting with the Solar arrays and moving into stability analysis criteria of the rest of the power management system and loads.
Figure 1: Station Solar Arrays and Radiator Panels (Image courtesy of a NASA Expedition 38 crew member on January 2, 2014)
A pinpoint beam of sunlight peeks through a truss-based radiator panel and a primary solar array panel on the ISS in Figure 1. Clouds can be seen over the Earth blanketed by the cold, blackness of space in the background.
Figure 2: ISS Main solar panel view
Figure 3: Solar ‘wings” in space on the ISS
The ISS needs power for life support, lighting, communication, experiments, propulsion and pretty much just about everything up there 220 miles above us on Earth. The system design for reliable power in such a remote region is, to say the least, challenging. If you lose power on Earth, you can call the electric company and wait for their service trucks to arrive. If you lose power on the ISS---all on board can perish.
Sunlight is plentiful up there is space, so the natural candidate for power would be solar energy. The design that NASA and its partners came up with for mounting the solar arrays was a “blanket.” The blanket is capable of folding up like an accordion for the launch into space and once in orbit it is deployed and fully spread out to its maximum size by a command from the ground controllers via radio signal.
The arrays always need to face the sun for maximum power efficiency, so gimbals are used to rotate them so that they face the sun all the time. Each of the eight solar arrays is 112 feet long by 39 feet wide. The entire solar array wingspan (240 feet) is longer than that of a Boeing 777 200/300 model, which is 212 feet. Together the arrays contain a total of 262,400 solar cells and cover an area of about 27,000 square feet (2,500 square meters) – more than half the area of a football field.
The 75 to 90 kilowatts of power needed by the ISS is supplied by this acre of solar panels. Eight miles of wire connects the electrical power system.
Altogether, the four sets of arrays are capable of generating 84 to 120 kilowatts of electricity – enough to provide power more than 40 homes on Earth. To put this in perspective, just think about an active computer and monitor using up to 270 watts or a small refrigerator using about 725 watts.
The solar arrays produce more power than the station needs at one time for the station systems and experiments. When the station is in sunlight, about 60 percent of the electricity that the solar arrays generate is used to charge the station’s batteries. At times, some or all of the solar arrays are in the shadow of Earth or the shadow of part of the station. The on-board batteries power the station during this time.
On the ISS, the electricity does not have to travel as far. The solar arrays convert sunlight to DC power.