The electrical system of the International Space Station is a critical resource for the
International Space Station
The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ...
(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 cell
A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. s to directly convert sunlight to
electricity
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
. Large numbers of cells are assembled in arrays to produce high power levels. This method of harnessing
solar power
Solar power is the conversion of energy from sunlight into electricity, either directly using photovoltaics (PV) or indirectly using concentrated solar power. Photovoltaic cells convert light into an electric current using the photovoltaic e ...
is called
photovoltaic
Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially us ...
s.
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
A space station is a spacecraft capable of supporting a human crew in orbit for an extended period of time, and is therefore a type of space habitat. It lacks major propulsion or landing systems. An orbital station or an orbital space station i ...
in orbit. The ISS power system uses
radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics.
A radiator is always a ...
s 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 is the largest ever deployed in space, weighing over 2,400 pounds and using nearly 33,000 solar arrays, each measuring 8-cm square with 4,100 diodes. When fully extended, each is in length and wide. Each SAW is capable of generating nearly 31 Kilowatts (kW) of direct current power. When retracted, each wing folds into a solar array blanket box just high and in length.
Altogether, the eight solar array wings can generate about 240 kilowatts in direct sunlight, or about 84 to 120 kilowatts average power (cycling between sunlight and shade).
The solar arrays normally track the Sun, with the "alpha
gimbal
A gimbal is a pivoted support that permits rotation of an object about an axis. A set of three gimbals, one mounted on the other with orthogonal pivot axes, may be used to allow an object mounted on the innermost gimbal to remain independent of ...
" used as the primary rotation to follow the Sun as the space station moves around the Earth, and the "beta
gimbal
A gimbal is a pivoted support that permits rotation of an object about an axis. A set of three gimbals, one mounted on the other with orthogonal pivot axes, may be used to allow an object mounted on the innermost gimbal to remain independent of ...
" used to adjust for the angle of the space station's orbit to the
ecliptic
The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
. Several different tracking modes are used in operations, ranging from full Sun-tracking, to the drag-reduction mode (''
night glider
''Night Glider'' is an album by the American jazz organist Groove Holmes recorded at New York City's Bell Sound Studios in 1973 and released on the Groove Merchant label. '' and ''Sun slicer'' modes), to a drag-maximization mode used to lower the altitude.
Over time, the photovoltaic cells on the wings have degraded gradually, having been designed for a 15-year service life. This is especially noticeable with the first arrays to launch, with the P6 and P4 Trusses in 2000 (
STS-97
STS-97 was a Space Shuttle mission to the International Space Station (ISS) flown by Space Shuttle '' Endeavour''. The crew installed the first set of solar arrays to the ISS, prepared a docking port for arrival of the Destiny Laboratory Module, ...
) and 2006 (
STS-115
STS-115 was a Space Shuttle mission to the International Space Station (ISS) flown by . It was the first assembly mission to the ISS after the ''Columbia'' disaster, following the two successful ''Return to Flight'' missions, STS-114 and STS-1 ...
).
STS-117
STS-117 ( ISS assembly flight 13A) was a Space Shuttle mission flown by Space Shuttle ''Atlantis'', launched from pad 39A of the Kennedy Space Center on 8 June 2007. ''Atlantis'' lifted off from the launch pad at 19:38 EDT. Damage from a hail ...
delivered the S4 truss and solar arrays in 2007.
STS-119
STS-119 ( ISS assembly flight 15A) was a Space Shuttle mission to the International Space Station (ISS) which was flown by space shuttle Discovery during March 2009. It delivered and assembled the fourth starboard Integrated Truss Segment (S6), ...
(ISS assembly flight 15A) delivered the S6 truss along with the fourth set of solar arrays and batteries to the station during March 2009.
To augment the oldest wings, NASA has launched two pairs and is set to launch one more pair of large-scale versions of the
Roll Out Solar Array
The Roll Out Solar Array (ROSA) and its larger version ISS Roll Out Solar Array (iROSA) are lightweight, flexible power sources for spacecraft designed and developed by Redwire.
This new type of solar array provides much more energy than trad ...
aboard three
SpaceX Dragon 2
Dragon 2 is a class of partially reusable spacecraft developed and manufactured by American aerospace manufacturer SpaceX, primarily for flights to the International Space Station (ISS). SpaceX has also launched Private spaceflight, private m ...
cargo launches from early June 2021 to early June 2023,
SpaceX CRS-22
SpaceX CRS-22, also known as SpX-22, was a Commercial Resupply Services (CRS) mission to the International Space Station (ISS) that launched at 17:29:15 UTC on 3 June 2021. The mission is contracted by NASA and is flown by SpaceX using a Carg ...
,
CRS-26 and
CRS-28.
These arrays are intended to be deployed along the central part of the wings up to two thirds of its length. Work to install iROSA's support brackets on the truss mast cans holding the Solar Array Wings was initiated by the crew members of
Expedition 64
Expedition 64 was the 64th long-duration expedition to the International Space Station (ISS), that began on 21 October 2020 with the undocking and departure of Soyuz MS-16. The Expedition started with the three crew members launched onboard So ...
in late February 2021.
After the first pair of arrays were delivered in early June, a spacewalk on 16 June by
Shane Kimbrough
Robert Shane Kimbrough (born June 4, 1967) is a retired United States Army officer and NASA astronaut. He was part of the first group of candidates selected for NASA astronaut training following the Space Shuttle ''Columbia'' disaster. Kimbrou ...
and
Thomas Pesquet
Thomas Gautier Pesquet (; born 27 February 1978 in Rouen) is a French aerospace engineer, pilot, and European Space Agency astronaut. Pesquet was selected by ESA as a candidate in May 2009, and he successfully completed his basic training in Nove ...
of
Expedition 65
Expedition 65 was the 65th long duration expedition to the International Space Station. The mission began on 17 April 2021, with the departure of Soyuz MS-17, and was initially commanded by NASA astronaut Shannon Walker serving as the third fe ...
to place one iROSA on the 2B power channel and mast can of the P6 truss ended early due to technical difficulties with the array's deployment.
![ISS new iROSA deployed](https://upload.wikimedia.org/wikipedia/commons/9/9c/ISS_new_iROSA_deployed.jpg)
The 20 June spacewalk saw the first iROSA's successful deployment and connection to the station's power system.
The 25 June spacewalk saw the astronauts successfully install and deploy the second iROSA on the 4B mast can opposite the first iROSA.
The next pair of panels were launched on 26 November 2022.
Astronauts
Josh Cassada
Josh Aaron Cassada (born July 18, 1973) is an American physicist, test pilot, and NASA astronaut. Prior to his selection to join NASA in 2013, Cassada served as a test pilot in the US Navy, and has over 3,500 hours in more than 40 aircraft, and 2 ...
and
Frank Rubio of
Expedition 68
Expedition 68 is the 68th long-duration expedition to the International Space Station. The expedition began upon the departure of Soyuz MS-21 on 29 September 2022, with ESA astronaut Samantha Cristoforetti taking over as ISS commander. Initiall ...
installed each one on the S4 and P4 truss segments on 3 and 22 December 2022, respectively.
Batteries
Since the station is often not in direct sunlight, it relies on rechargeable
lithium-ion batteries
A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
(initially
nickel-hydrogen batteries) to provide continuous power during the "eclipse" part of the
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
(35 minutes of every 90 minute orbit).
Each battery assembly, situated on the S4, P4, S6, and P6 Trusses, consists of 24 lightweight lithium-ion battery cells and associated electrical and mechanical equipment. Each battery assembly has a nameplate capacity of 110
Ah (
C) (originally 81 Ah) and .
This power is fed to the ISS via the BCDU and DCSU respectively.
The batteries ensure that the station is never without power to sustain life-support systems and experiments. During the sunlight part of the orbit, the batteries are recharged. The nickel-hydrogen batteries and the battery charge/discharge units were manufactured by
Space Systems/Loral
SSL, formerly Space Systems/Loral, LLC (SS/L), of Palo Alto, California, is a wholly owned manufacturing subsidiary of Maxar Technologies.
SSL designs and builds satellites and space systems for a wide variety of government and commercial cust ...
(SS/L),
under contract to
Boeing
The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and product ...
. Ni-H2 batteries on the P6 truss were replaced in 2009 and 2010 with more Ni-H2 batteries brought by Space Shuttle missions.
The nickel-hydrogen batteries had a design life of 6.5 years and could exceed 38,000 charge/discharge cycles at 35% depth of discharge. They were replaced multiple times during the expected 30-year life of the station.
Each battery measured and weighed .
From 2017 to 2021, the nickel-hydrogen batteries were replaced by
lithium-ion batteries
A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...
.
On January 6, 2017,
Expedition 50
Expedition 50 was the 50th expedition to the International Space Station.
Robert S. Kimbrough, Andrei Borisenko and Sergey Ryzhikov transferred from Expedition 49. Expedition 50 began upon the departure of Soyuz MS-01 on October 28, 2016 and ...
members
Shane Kimbrough
Robert Shane Kimbrough (born June 4, 1967) is a retired United States Army officer and NASA astronaut. He was part of the first group of candidates selected for NASA astronaut training following the Space Shuttle ''Columbia'' disaster. Kimbrou ...
and
Peggy Whitson
Peggy Annette Whitson (born February 9, 1960) is an American biochemistry researcher, retired NASA astronaut, and former NASA Chief Astronaut. Whitson has a total of 665 days in space, more than any other woman or American.
Her first space mi ...
began the process of converting some of the oldest batteries on the ISS to the new lithium-ion batteries.
Expedition 64
Expedition 64 was the 64th long-duration expedition to the International Space Station (ISS), that began on 21 October 2020 with the undocking and departure of Soyuz MS-16. The Expedition started with the three crew members launched onboard So ...
members
Victor J. Glover
Victor Jerome Glover (born April 30, 1976) is a NASA astronaut of the class of 2013 and Pilot on the first operational flight of the SpaceX Crew Dragon to the International Space Station. Glover is a captain in the U.S. Navy where he pilots ...
and
Michael S. Hopkins
Michael Scott Hopkins (born December 28, 1968) is a United States Space Force colonel and NASA astronaut. Hopkins was selected in June 2009 as a member of the NASA Astronaut Group 20. He made his first spaceflight as a Flight Engineer on Soyu ...
concluded the campaign on February 1, 2021.
There are a number of differences between the two battery technologies. One difference is that the lithium-ion batteries can handle twice the charge, so only half as many lithium-ion batteries were needed during replacement.
Also, the lithium-ion batteries are smaller than the older nickel-hydrogen batteries.
Although Li-Ion batteries typically have shorter lifetimes than Ni-H2 batteries as they cannot sustain as many charge/discharge cycles before suffering notable degradation, the ISS Li-Ion batteries have been designed for 60,000 cycles and ten years of lifetime, much longer than the original Ni-H2 batteries' design life span of 6.5 years.
Power management and distribution
![Electrical_Power_Distribution](https://upload.wikimedia.org/wikipedia/commons/a/a2/Electrical_Power_Distribution.png)
The power management and distribution subsystem operates at a primary bus voltage set to ''V''
mp, the
peak power point of the solar arrays. , ''V''
mp was 160 volts DC (
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
). It can change over time as the arrays degrade from ionizing radiation. Microprocessor-controlled switches control the distribution of primary power throughout the station.
The battery charge/discharge units (BCDUs) regulate the amount of charge put into the battery. Each BCDU can regulate discharge current from two battery
ORUs (each with 38 series-connected Ni-H
2 cells), and can provide up to 6.6 kW to the Space Station. During insolation, the BCDU provides charge current to the batteries and controls the amount of battery overcharge. Each day, the BCDU and batteries undergo sixteen charge/discharge cycles. The Space Station has 24 BCDUs, each weighing 100 kg.
The BCDUs are provided by SS/L
Sequential shunt unit (SSU)
Eighty-two separate solar array strings feed a sequential shunt unit (SSU) that provides coarse voltage regulation at the desired ''V''
mp. The SSU applies a "dummy" (resistive) load that increases as the station's load decreases (and vice versa) so the array operates at a constant voltage and load. The SSUs are provided by SS/L.
DC-to-DC conversion
DC-to-DC converter
A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low (small batteries) ...
units supply the secondary power system at a constant 124.5 volts DC, allowing the primary bus voltage to track the peak power point of the solar arrays.
Thermal control
The thermal control system regulates the temperature of the main power distribution electronics and the batteries and associated control electronics. Details on this subsystem can be found in the article
External Active Thermal Control System
The International Space Station (ISS) External Active Thermal Control System (EATCS) maintains an equilibrium when the ISS environment or heat loads exceed the capabilities of the Passive Thermal Control System (PTCS). Note Elements of the PTCS ...
.
Station to shuttle power transfer system
From 2007 the Station-to-Shuttle Power Transfer System (SSPTS; pronounced ''spits'') allowed a docked
Space Shuttle
The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...
to make use of power provided by the
International Space Station's solar arrays
A photovoltaic system, also PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and c ...
. Use of this system reduced usage of a shuttle's on-board power-generating
fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
s, allowing it to stay docked to the space station for an additional four days.
SSPTS was a shuttle upgrade that replaced the Assembly Power Converter Unit (APCU) with a new device called the Power Transfer Unit (PTU). The APCU had the capacity to convert shuttle 28 VDC main bus power to 124 VDC compatible with ISS's 120 VDC power system. This was used in the initial construction of the space station to augment the power available from the Russian ''
Zvezda'' service module. The PTU adds to this the capability to convert the 120 VDC supplied by the ISS to the orbiter's 28 VDC main bus power. It is capable of transferring up to 8 kW of power from the space station to the orbiter. With this upgrade both the shuttle and the ISS were able to use each other's power systems when needed, though the ISS never again required the use of an orbiter's power systems.
In December 2006, during mission
STS-116
STS-116 was a Space Shuttle mission to the International Space Station (ISS) flown by Space Shuttle '' Discovery''. ''Discovery'' lifted off on 9 December 2006 at 20:47:35 EST. A previous launch attempt on 7 December had been canceled due to cl ...
,
PMA-2
The Pressurized Mating Adapter (PMA) is a class of spacecraft adapters that convert the Common Berthing Mechanism (CBM) used on the US Orbital Segment to APAS-95 docking ports. There are three PMAs located on the International Space Station (IS ...
(then at the forward end of the ''Destiny'' module) was rewired to allow for the use of the SSPTS.
The first mission to make actual use of the system was
STS-118
STS-118 was a Space Shuttle mission to the International Space Station (ISS) flown by the orbiter '' Endeavour''. STS-118 lifted off on 8 August 2007 from launch pad 39A at Kennedy Space Center (KSC), Florida and landed at the Shuttle Landing Fac ...
with
Space Shuttle ''Endeavour''.
Only
''Discovery'' and
''Endeavour'' were equipped with the SSPTS.
''Atlantis'' was the only surviving shuttle not equipped with the SSPTS, so could only go on shorter length missions than the rest of the fleet.
References
*
*
External links
NASA Glenn Contributions to the International Space Station (ISS) Electrical Power System
{{ISS modules
Components of the International Space Station
Electrical systems
Solar power and space