ISS Solar Arrays

The International Space Station's (ISS) only readily available source of energy is through sunlight. It has many solar panels, the most recent and largest of which are two U.S.-made photovoltaic solar arrays attached to the P6 truss segment deployed late 2000 during STS-97. Each of the solar arrays are 112 ft long by 39 ft wide. There are three more nearly identical pairs of solar panels to come on future missions, as well as a Russian-made Science Power Platform.

Electrical power is the most critical resource for the International Space Station because it allows the crew to live comfortably, to safely operate the station, and to perform scientific experiments. One method of harnessing solar power, called photovoltaics, uses purified silicon solar cells to directly convert light to electricity. Large numbers of cells are assembled in arrays to produce high power levels.

However, a spacecraft orbiting the Earth is not always in direct sunlight. Therefore, the ISS relies on nickel-hydrogen rechargeable batteries to provide continuous power during the "eclipse" part of the orbit. The batteries ensure that the station is never without power to sustain life-support systems and experiments. During the sunlit part of the orbit, the batteries are recharged. The process of collecting sunlight, converting it to electricity, and managing and distributing this electricity builds up excess heat that can damage spacecraft equipment. This heat must be eliminated for reliable operation of the space station in orbit. The ISS power system uses radiators to dissipate the heat away from the spacecraft. The radiators are shaded from sunlight and aligned toward the cold void of deep space.

The power management and distribution subsystem disburses power at 160 volts of direct current (abbreviated as "dc") around the station through a series of switches. These switches have built-in microprocessors that are controlled by software and are connected to a computer network running throughout the station. To meet operational requirements, dc-to-dc converter units step down and 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.

The solar arrays will require more than 250,000 silicon solar cells. NASA has developed a method of mounting the solar arrays on a "blanket" that can be folded like an accordion for delivery to space. Once in orbit, astronauts deploy the blankets to their full size. Gimbals will be used to rotate the arrays so that they face the Sun to provide maximum power to the Space Station.

The complete power system, consisting of U.S. and Russian hardware, will generate 110 kW (kilowatts) total power, about as much as 55 houses would typically use. Approximately 46 kW will be available for research activities.

Truss and Solar Array Assembly Sequence

Element	Flight	Launch date	Length (m)	Diameter (m)	Mass (kg)
Z1 Truss	3A - STS-92	October 11,2000	4.9	4.2	8,755
P6 Truss - Solar Array	4A - STS-97	November 30,2000	73.2	10.7	15,900
S0 Truss	8A - STS-110	April 8,2002	13.4	4.6	13,970
S1 Truss	9A - STS-112	October 7,2002	13.7	3.9	12,598
P1 Truss	11A - STS-113	November 23,2002	13.7	3.9	12,598
P3/4 Truss - Solar Array	12A - STS-115	March 4,2006	73.2	10.7	15,900
P5 Truss	12A.1 - STS-116	Under review	13.7	3.9	12,598
S3/4 Truss - Solar Array	13A - STS-117	Under review	73.2	10.7	15,900
S5 Truss	13A.1 - STS-118	Under review	13.7	3.9	12,598
S6 Truss - Solar Array	15A - STS-119	Under review	73.2	10.7	15,900

Diagram of ISS Truss elements