International Space Station

Electric Power System

The overall architecture of the ISS Electric Power System (EPS) was designed by NASA's Glenn Research Center in the early 1990's. Boeing's Rocketdyne Propulsion and Power division (now Pratt & Whitney) built most of the hardware for the electrical power system. Boeing, along with Pratt & Whitney RPP as a subcontractor, continues to provide EPS sustaining engineering to NASA.

The ISS electrical power system consists of power generation, energy storage, power management, and distribution equipment. Electricity is generated in a system of solar arrays, which total 27,000 square feet (about 1/2 acre) in size. Besides the solar arrays on the Russian element, the station currently has two main solar array wings on orbit, with two more scheduled to be delivered.

The EPS provides all user and housekeeping electrical power and is capable of expansion as the station is assembled and grows. The EPS consists of power generation and energy storage subsystems that feed power into the Power Management and Distribution (PMAD) subsystem.

A photovoltaic (PV) power generation subsystem was selected for the Space Station. A PV system has solar arrays for power generation and chemical energy storage (batteries) to store excess solar array energy during periods of sunlight and provide power during periods of shade. The station orbits the earth every 90 minutes and for about 30 minutes the station must run on batteries while the station is in the Earth's shadow (eclipse).

Power for the Space Station is provided by flexible, deployable solar array wings. This configuration minimizes the complexity of the assembly process. Each 39-foot by 112-foot (11.9m by 34.2m) wing consists of two blanket assemblies covered with solar cells. Each pair of blankets is deployed and supported by an extendable mast.

The primary purpose of the Energy Storage Subsystem (ESS) is to provide electrical power during the eclipse portion of each orbit. The ESS stores energy for this purpose during the isolation portion of the orbit and is capable of providing both peaking and contingency power. The ESS consists of three pairs of nickel-hydrogen (Ni-H2) batteries per solar array, each with a dedicated battery charge/discharge unit (BCDU) for each pair.

The Ni/H2 battery design has been chosen for the Space Station because of its high energy density (lightweight) and proven heritage in space applications since the early 1970's. The 160-volt, DC Power Management and Distribution (PMAD) system is designed specifically to meet aerospace system requirements. The system is based upon rapid semiconductor switching DC to DC Converter Units (DDCUs) and electromechanical devices to tailor voltage and energy levels of the system.

The overall distribution equipment include cables, load converters, regulators, switches and other electrical equipment. The overall distribution subsystem will be composed of equipment necessary to process, control and distribute power to other station subsystems, elements and attached payloads.

The U.S. power system is also integrated with Russian power sources, so that power from the U.S. solar arrays can be transferred to the Russian power system and vice versa.

For more information, read the Electric Power System (PDF) overview.

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