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electrical system

Electrical system of the International Space Station

The electrical system of the International Space Station is a critical resource for the International Space Station (ISS) because it allows the crew to live comfortably, to safely operate the station, and to perform scientific experiments. The ISS electrical system uses solar cells to directly convert sunlight to electricity. Large numbers of cells are assembled in arrays to produce high power levels. This method of harnessing solar power is called photovoltaics.

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.

Solar array wing

Each ISS solar array wing (often abbreviated "SAW") consists of two retractable "blankets" of solar cells with a mast between them. Each wing uses nearly 33,000 solar cells and when fully extended is 35 m (115 ft) long and 12 m (38 ft) wide. When retracted, each wing folds into a solar array blanket box just 51 cm (20 inches) high and 4.57 m (15 ft) long. The completed ISS will have eight solar array wings.

The solar arrays normally track the Sun, with the alpha gimble used as the primary rotation to follow the Sun as the space station moves around the Earth, and the beta gimble used to adjust for the angle of the space station's orbit to the ecliptic. Several different tracking modes are used in operations, ranging from full Sun-tracking, to the drag-reduction mode ("Night glider" and "Sun slicer" modes), to a drag-maximization mode used to lower the altitude. See more details in the article at Night Glider mode.

Batteries

Since the station is often not in direct sunlight, it relies on nickel-hydrogen rechargeable batteries to provide continuous power during the "eclipse" part of the orbit (35 minutes of every 90 minute 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 batteries have a working life of 6.5 years which means that they must be replaced multiple times during the expected 20-year life of the station. The batteries, and the battery charge/discharge units (BCDUs), are manufactured by Space Systems/Loral (SS/L), under contract to Boeing.

Power management and distribution

The power management and distribution subsystem disburses power, as of December 30, 2005, at 160 volts of direct current (abbreviated as "DC") around the station through a series of switches. This voltage may change as the solar arrays degrade over time and the solar arrays' voltage-max-power (Vmp) point changes. This Vmp is the operating voltage at which the arrays provide the most power. The switches that route power throughout the station have built-in microprocessors that are controlled by software and are connected to a computer network running throughout the station.

SSU

Eighty-two separate strings, or power lines, lead from each solar array to a sequential shunt unit (SSU) that provides coarse electrical power regulation. The job of the SSU is to shunt, or short, the excess current from the solar array to maintain the desired 160 volt bus voltage. The SSUs are provided by SS/L.

DC-to-DC conversion

To meet operational requirements, DC-to-DC converter units step down and condition the voltage from 160 to 124.5 volts DC to form a secondary power system to service the loads. By transmitting power at higher voltages and stepping it down to lower voltages where the power is to be used, much like municipal power systems, the station can use smaller wires to transmit this electrical power and thus reduce launch loads. The converters also isolate the secondary system from the primary system and maintain uniform power quality throughout the station.

Station to shuttle power transfer system

The station-to-shuttle power transfer system (SSPTS) allows visiting shuttles to derive power from the station to extend their missions.. It is on the Pressurized Mating Adapter-2 (PMA-2), at the forward end of the Harmony module where shuttles dock.

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