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International Space Station All Solar Powered

Credits: ©2009 NASA

The most powerful solar arrays ever to orbit Earth capture the sun's energy and begin the process of converting it into power for the International Space Station (ISS). Eight solar panels will supply more than 100 kilowatts of electric power to the station. The panels are being mounted on a metal framework 360 feet (109 meters) long. The International Space Station is a large, inhabited Earth satellite that more than 15 nations are building in space. The first part of the station was launched in 1998, and the first full-time crew -- one American astronaut and two Russian cosmonauts -- occupied the station in 2000. The International Space Station orbits Earth at an altitude of about 250 miles (400 kilometers). The orbit extends from 52 degrees north latitude to 52 degrees south latitude. The station will include about eight large cylindrical sections called modules. Each module is being launched from Earth separately, and astronauts and cosmonauts are connecting the sections in space.


International Space Station above Earth.

International Space Station - Post Flight 10A Configuration. ©2007 NASA

The eight solar array wings will supply an unprecedented 124 volts dc to crew and equipment in the U.S. segment of the ISS. The Space Shuttle and most other spacecraft use 28 volts dc, as will the Russian ISS segment. The higher voltage will meet the higher overall ISS power requirements while permitting use of smaller, lighter-weight power lines. Each 108.6-ft. long solar array wing is connected to the ISS's 310-ft. long truss and extend outward at right angles to it. Altogether, they cover an area of 27,000-sq. ft. When fully extended, a pair of wings and their associated equipment span about 240 feet, the largest deployable space structures ever built.

An array consists of two solar cell "blankets," one on either side of a telescoping mast that extends and retracts to form or fold the solar array wing. The mast turns on a gimbal to keep the arrays facing the sun. The gimbal base is integrated with the ISS truss assembly. Additionally, the outboard portions of the truss assembly of the truss assembly also rotate to keep the arrays facing the sun. A pair of wings and their associated power regulation and power storage hardware is termed a "photovoltaic module." There are four such modules on the U.S. segment of the ISS at assembly complete. The solar arrays normally track the Sun, with the alpha gimbal used as the primary rotation to follow the Sun as the space station moves around the Earth, and the beta gimbal 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.

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.

Since the station is often not in direct sunlight, it relies on rechargeable nickel-hydrogen 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.

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.[6] 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

Damage to the 4B wing of the P6 solar array wing found when it was redeployed after being moved to its final position on the STS-120 mission.

The Station-to-Shuttle Power Transfer System (SSPTS; pronounced spits) allows a docked Space Shuttle to make use of power provided by the International Space Station's solar arrays. Using this system reduces usage of a shuttle's on-board power-generating fuel cells, allowing it to stay docked to the space station for an additional four days.

Sustaining Engineering of Power System Hardware
The complex Electric Power System (EPS) onboard the International Space Station (ISS) provides all the power vital for the continuous, reliable operation of the spacecraft. NASA Glenn Research Center’s Space Operations Division is leading the sustaining engineering and subsystem integration of EPS hardware. Glenn also manages the integration of the EPS with ISS International Partners’ elements.

Once the EPS hardware is built, sustaining engineering is necessary to evaluate, troubleshoot, and repair the hardware in case of failure. This evaluation and maintenance process is performed before and after the hardware is operating on orbit. In this effort, Glenn has partnered with Johnson Space Center, Marshall Space Flight Center, Boeing, and Pratt & Whitney-Rocketdyne.

The EPS consists of several hardware components called Orbital Replacement Units (ORU). Each ORU is considered a subsystem of the entire EPS and can be replaced upon failure either robotically or by Extra-Vehicular Activity (EVA). These components work together to provide power generation, power distribution and energy storage for the ISS.

Energy from the sun (solar power) is collected by the solar arrays, coarsely conditioned by the Sequential Shunt Unit (SSU), tightly regulated by the Direct Current (DC) to DC Converter Unit (DDCU), and stored in the batteries for future use.

The ISS operates in Low Earth Orbit, approximately 250 miles above Earth. Consequently, it is in the sun (insolation) gathering and storing energy for approximately 55 minutes of every 90-minute orbit. During the other 35 minutes of each orbit, the ISS is in Earth’s shadow (eclipse).

The batteries are one of the most important ORUs in the EPS. Efficient energy storage is vital since the ISS must use stored solar energy to power the spacecraft during its eclipse mode. The Battery Charge Discharge Unit (BCDU) will charge the batteries using the power collected by the solar arrays during insolation and must draw energy from the batteries during eclipse to provide power to the ISS. Due to the ISS orbit, this results in a total of 16 battery charge/discharge cycles per day.

The batteries are composed of nickel-hydrogen cells and utilize the same electrochemical method of energy storage as typical satellites, including the Hubble Space Telescope. Each battery consists of two 365 lb ORUs. The battery ORUs should last approximately 7-9 years in space.

Several ORUs provide the EPS with fault protection for added safety and reliability. The DC Switching Unit (DCSU) monitors its output and senses if the circuits are carrying too much current as the power is directed to the BCDU. Similar to the DCSU, the Main Bus Switching Unit (MBSU) provides additional fault protection. It distributes power and enables different power channels to cross-connect if a power channel fails. At the lowest level of power distribution, the Remote Power Controller Module (RPCM) enables power flow control and fault protection with multi-channel, high power circuit breakers.

All of the system hardware components work together as one of the core systems of the ISS to provide safe, reliable power for numerous onboard equipment and experiments. Additionally, most ORUs will have spares onboard the ISS in the event that failures do occur. These units are being produced and tested under the guidance of Glenn’s ISS Subsystem Managers. EPS technologies developed for the ISS may be applied to future lunar and Mars exploration missions.

In addition to the sustaining engineering work, Glenn is also acting as the agent for EPS integration of international elements. Working with international space agency partners, Glenn is ensuring that the Columbus Module, Japanese Experiment Module (JEM), Italian-made Node 2 and Node 3/Cupola, and Japan’s H-II Transfer Vehicle (HTV) can connect to the ISS power system and function properly.

In February 2008, the Columbus module was launched on Space Shuttle Atlantis for the STS-122 mission. Atlantis delivered the 23 by 15 foot research laboratory to the ISS where it can be shared by the U.S. and the European Space Agency.

Node 2, Harmony, is a pressurized module used to link the European Columbus laboratory, the US laboratory Destiny, and the Japanese Experiment Module, Kibo. It was launched in October 2007 on shuttle flight STS-120. Node 3 is also a connecting module that will be used to house life support equipment and will accommodate the European Space Agency’s Cupola observation port, which allows crew members to view the Earth and other objects in space.

JEM is Japan’s first manned facility, which can hold four astronauts performing experiments. JEM consists of the experiment facilities (Pressurized Module and Exposed Facility), the logistics modules attached to each facility, and a Remote Manipulator System for handling experiments. The Pressurized Module is the central part of JEM and is the size of a large school bus. It contains 10 experiment racks primarily used to study microgravity.

Japan’s HTV is a space vehicle that is used to transport up to six tons of food, clothing and equipment to the ISS. After a delivery of supplies, the HTV will return to Earth carrying waste materials like used clothing that are burned up in the atmosphere upon re-entry. The HTV will be launched by the H-IIB launch vehicle, which is still under development.

EPS Cooling System Maintains ISS Safety
The Electric Power System (EPS) components onboard the International Space Station (ISS) must be cooled to sustain the space research experiments and prevent system failures due to overheating throughout the spacecraft. The Photovoltaic Thermal Control System’s (PVTCS) radiator rejects heat into space to keep the power system cool. There will be four PVTCS systems in operation once the ISS assembly is complete.

NASA Glenn Research Center’s Systems Verification Branch provides subsystem management, technical oversight of Boeing’s performance as an ISS contractor, and sustaining engineering and operations in support of the PVTCS hardware. Sustaining engineering is necessary in the event of a failure or a malfunction to troubleshoot, evaluate, repair, remove, or upgrade the flight hardware to maintain proper functionality.

In conjunction with Johnson Space Center, Glenn also monitors the on-orbit performance of the PVTCS, verifies and validates thermal models, identifies problems and resolutions, and handles integration with other subsystems.

As a mechanically pumped, single-phase system, the PVTCS is part of the Thermal Control System (TCS). It can be controlled manually by the astronauts or remotely from the ground via the Photovoltaic Control Unit (PVCU). Using ammonia coolant, the PVTCS keeps the primary EPS components within their proper temperature range by transporting excess heat from the electrical equipment assemblies, batteries and radiators into space.

The PVTCS consists of three main parts: the Integrated Equipment Assembly (IEA) structural framework, the Pump Flow Control Subassembly (PFCS), and the Photovoltaic Radiator (PVR). The cooling system plugs into the IEA framework. The PFCS controls the flow of ammonia coolant to the TCS while the PVR rejects the heat from the photovoltaic electronics into deep space. The PVTCS components work together to help maintain the functionality of the EPS and its related systems while ensuring the safety of the astronauts.

NASA Space Flight Systems Website

NASA International Space Station Website