electrical system of the International Space Station

Image:ISS P6 truss solar array - close-up (ISS014-E-10053).jpg

Image:STS120SolarPanel.jpg mission.]]

{{See also| Integrated Truss Structure#Truss subsystems| Roll Out Solar Array}}

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 {{convert|35|m}} in length and {{convert|12|m}} wide. Each SAW is capable of generating nearly 31 Kilowatts (kW) of direct current power.

{{cite web | date=July 26, 2006 | url = http://www.nasa.gov/mission_pages/station/behindscenes/truss_segment.html | title = Spread Your Wings, It's Time to Fly | publisher = NASA}} When retracted, each wing folds into a solar array blanket box just {{convert|51|cm}} high and {{convert|4.57|m}} in length.

{{cite web | date = November 9, 2000 | url = http://www.shuttlepresskit.com/STS-97/payload81.htm | title = STS-97: Photovoltaic array assembly | publisher = NASA | url-status = dead | archive-url = https://web.archive.org/web/20010123234800/http://www.shuttlepresskit.com/STS-97/payload81.htm | archive-date = January 23, 2001 }}

Altogether, the eight solar array wings

{{cite web | url = http://www.boeing.com/defense-space/space/spacestation/systems/solar_arrays.html | title = International Space Station – Solar Power | publisher = Boeing}} can generate about 240 kilowatts in direct sunlight, or about 84 to 120 kilowatts average power (cycling between sunlight and shade).{{Cite web|url=https://www.nasa.gov/mission_pages/station/structure/elements/solar_arrays.html#.VvIBj2Ou9oI|title=Solar Arrays|last=Wright|first=Jerry|website=NASA|access-date=2016-03-23}}

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.{{Citation needed|date=March 2021}}

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) and 2006 (STS-115).{{Cite web |url=https://www.nasa.gov/mission_pages/station/behindscenes/truss_segment.html |title=NASA.gov |access-date=2021-05-26 |archive-date=2018-12-29 |archive-url=https://web.archive.org/web/20181229020726/https://www.nasa.gov/mission_pages/station/behindscenes/truss_segment.html |url-status=dead }}

STS-117 delivered the S4 truss and solar arrays in 2007.

STS-119 (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 launched three pairs of large-scale versions of the ISS Roll Out Solar Array (IROSA) aboard three SpaceX Dragon 2 cargo launches from early June 2021 to early June 2023, SpaceX CRS-22, CRS-26 and CRS-28.{{cite web |last=Clark|first=Stephen| title=SpaceX launches Dragon cargo ship to deliver new solar arrays to space station – Spaceflight Now | website=Spaceflight Now – The leading source for online space news | date=26 November 2022 | url=https://spaceflightnow.com/2022/11/26/spacex-launches-dragon-cargo-ship-to-deliver-new-solar-arrays-to-space-station/ | access-date=28 November 2022}} These arrays were deployed along the central part of the wings up to two thirds of its length.{{cite web|url=https://www.nationalacademies.org/documents/embed/link/LF2255DA3DD1C41C0A42D3BEF0989ACAECE3053A6A9B/file/D0CE42612418D93FC850A8B8383F306148EC43CCAE2D|title=Current and Future Operations and Challenges with International Space Station|work=ISS Program Office|publisher=NASA|date=15 Oct 2020|access-date=2 May 2021|format=PDF}} 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 in late February 2021.{{cite web|url=http://www.spacefacts.de/iss/english/exp_64.htm|title=Expedition 64 Information Page|date=10 May 2021|access-date=17 June 2021|publisher=Spacefacts.de}}{{Cite web|last=Garcia|first=Mark|date=11 January 2021|access-date=26 April 2021|url=https://www.nasa.gov/feature/new-solar-arrays-to-power-nasa-s-international-space-station-research|title=New Solar Arrays to Power NASA's International Space Station Research|publisher=NASA|archive-date=24 May 2023|archive-url=https://web.archive.org/web/20230524212434/https://www.nasa.gov/feature/new-solar-arrays-to-power-nasa-s-international-space-station-research/|url-status=dead}}{{PD-notice}} After the first pair of arrays were delivered in early June, a spacewalk on 16 June by Shane Kimbrough and Thomas Pesquet of Expedition 65 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.{{cite web|last=Garcia|first=Mark|date=16 June 2021|url=https://blogs.nasa.gov/spacestation/2021/06/16/spacewalk-to-install-first-new-solar-array-concluded/|title=Spacewalk to Install First New Solar Array Concluded|access-date=17 June 2021|publisher=NASA}}{{PD-notice}}{{cite web|url=https://aviationweek.com/defense-space/space/hardware-spacesuit-difficulties-stall-ambitious-iss-spacewalk|title=Hardware, Spacesuit Difficulties Stall Ambitious ISS Spacewalk|date=17 June 2021|access-date=17 June 2021|work=Aviation Week|publisher=Informa Markets}}{{cite web|date=17 June 2021|access-date=17 June 2021|url=http://www.spacefacts.de/iss/english/exp_65.htm|title=Expedition 65 Information Page|publisher=Spacefacts.de}}

File:ISS new iROSA deployed.jpg

The 20 June spacewalk saw the first iROSA's successful deployment and connection to the station's power system.{{Cite web|url=https://www.theguardian.com/science/2021/jun/21/international-space-station-astronauts-complete-six-hour-spacewalk-to-install-solar-panels|title=ISS astronauts complete six-hour spacewalk to install solar panels|author=Guardian, AP and AFP staff|date=20 June 2021|website=The Guardian|publisher=Guardian News and Media Ltd|access-date=26 June 2021}}{{Cite web|last=Pearlman|first=Robert Z.|url=https://www.space.com/spacewalking-astronauts-roll-out-new-space-station-solar-arrays|title=Astronauts on spacewalk deploy first roll-out solar array to boost power for station|date=20 June 2021|website=Space.com|publisher=Future US Inc|access-date=26 June 2021}} The 25 June spacewalk saw the astronauts successfully install and deploy the second iROSA on the 4B mast can opposite the first iROSA.{{Cite web|url=https://www.space.com/spacewalking-astronauts-deploy-second-space-station-solar-array|last=Pearlman|first=Robert Z.|title=Spacewalking astronauts deploy second new solar array for space station|date=25 June 2021|website=Space.com|publisher=Future US Inc|access-date=26 June 2021}}

The next pair of panels were launched on 26 November 2022. Astronauts Josh Cassada and Frank Rubio of Expedition 68 installed each one on the 3A power channel and mast can on the S4 segment, and the 4A power channel and mast can on the P4 truss segments, on 3 and 22 December 2022, respectively.{{cite web | last=Pearlman | first=Robert Z. | title=NASA astronauts unfurl 4th roll-out solar array on spacewalk outside space station | website=Space.com | date=22 December 2022 | url=https://www.space.com/astronauts-spacewalk-international-space-station-fourth-solar-array-deployed | access-date=23 December 2022}}

The third pair of panels were launched on 5 June 2023. On 9 June, astronauts Steve Bowen and Warren Hoburg of Expedition 69 installed the fifth iROSA on the 1A power channel and mast can on the S4 truss segment.{{Cite web |last=Garcia |first=Mark |date=2023-06-09 |title=NASA Astronauts Begin Spacewalk to Install Solar Array |url=https://blogs.nasa.gov/spacestation/2023/06/09/nasa-astronauts-begin-spacewalk-to-install-solar-array-2/ |access-date=2023-06-10 |website=blogs.nasa.gov |language=en-US}}{{Cite web |last=Garcia |first= Mark |date=2023-06-09 |title=NASA Spacewalkers Complete Solar Array Installation |url=https://blogs.nasa.gov/spacestation/2023/06/09/nasa-spacewalkers-complete-solar-array-installation/ |access-date=2023-06-10 |website=blogs.nasa.gov |language=en-US}} On 15 June, Bowen and Hoburg installed the sixth iROSA on the 1B power channel and mast can on the S6 truss segment.{{cite web | title=Astronauts install new roll-out solar array outside International Space Station – Spaceflight Now | website=Spaceflight Now |last=Clark|first=Stephen| date=June 9, 2023 | url=https://spaceflightnow.com/2023/06/09/us-eva-87-coverage/ | access-date=June 10, 2023}}

The last pair of iROSAs, the seventh and eighth, are planned to be installed on the 2A and 3B power channels on the P4 and S6 truss segments in 2025.{{cite web | last=Davenport | first=Justin | title=ISS finishes initial iROSA upgrade with two EVAs this month | website=NASASpaceFlight.com | date=June 15, 2023 | url=https://www.nasaspaceflight.com/2023/06/iss_irosa_roundup/ | access-date=June 18, 2023}}

Since the station is often not in direct sunlight, it relies on rechargeable lithium-ion batteries (initially nickel-hydrogen batteries) to provide continuous power during the "eclipse" part of the orbit (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.{{cite web|last=Garcia|first=Mark|url=https://blogs.nasa.gov/spacestation/2017/01/06/astronauts-complete-first-of-two-power-upgrade-spacewalks/|date=6 January 2017|title=Astronauts complete first of two power upgrade spacewalks|publisher=NASA|access-date=28 February 2021}}{{PD-notice}}{{Cite book|last1=Schwanbeck|first1=Eugene|last2=Dalton|first2=Penni|title=2019 European Space Power Conference (ESPC)|chapter-url=https://ieeexplore.ieee.org/document/8932009|chapter=International Space Station Lithium-ion Batteries for Primary Electric Power System|publisher=IEEE|date=16 December 2019|page=1|doi=10.1109/ESPC.2019.8932009|isbn=978-1-7281-2126-0|s2cid=209382968|access-date=5 March 2021}} Each battery assembly has a nameplate capacity of 110 Ah ({{formatnum:{{#expr:110*3600}}}} C) (originally 81 Ah) and {{cvt|4|kWh}}.{{cite web|url=http://www.grc.nasa.gov/WWW/RT/2003/6000/6910dalton.html |archive-url=https://web.archive.org/web/20050307133848/http://www.grc.nasa.gov/WWW/RT/2003/6000/6910dalton.html |url-status=dead |archive-date=March 7, 2005 | title=International Space Station Nickel-Hydrogen Batteries Approached 3-Year On-Orbit Mark|publisher=NASA|access-date=September 14, 2007}}{{Cite web|last1=Dalton|first1=Penni|last2=Bowens|first2=Ebony|last3=North|first3=Tim|last4=Balcer|first4=Sonia|url=https://ntrs.nasa.gov/api/citations/20200000004/downloads/20200000004.pdf|title=International Space Station Lithium-ion Battery Status|publisher= NASA|date=November 19, 2019|access-date=March 5, 2021}}{{cite web |url=https://www.nasaspaceflight.com/2017/01/spacewalkers-upgrading-iss-batteries/ |title=EVA-39: Spacewalkers complete the upgrading of ISS batteries |date=January 13, 2017 |access-date=March 5, 2021}} 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 (SS/L),{{cite web |publisher=Space Systems Loral |url=http://www.ssloral.com/downloads/products/ispacest.pdf |title=International Space Station |date=February 1998 |archive-url=https://web.archive.org/web/20141227213842/http://sslmda.com/downloads/products/ispacest.pdf |archive-date=December 27, 2014 }} under contract to Boeing.{{cite press release|date=July 8, 2003|publisher=Loral|url=http://www.loral.com/inthenews/030708.html|title=Space Systems/Loral awarded $103 million contract to build critical power systems for the International Space Station|url-status=dead|archive-url=https://web.archive.org/web/20070928032040/http://www.loral.com/inthenews/030708.html|archive-date=September 28, 2007}} 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.{{cite web|title=Nickel-Hydrogen Battery Cell Life for International Space Station|url=http://www.grc.nasa.gov/WWW/RT/RT1999/5000/5420miller.html|url-status=dead|archive-url=https://web.archive.org/web/20090825125740/http://www.grc.nasa.gov/WWW/RT/RT1999/5000/5420miller.html|archive-date=2009-08-25|publisher=NASA}} Each battery measured {{convert|40|by|36|by|18|in|cm}} and weighed {{convert|375|lb}}.{{cite web|url=http://www.shuttlepresskit.com/STS-97/payload81.htm|title=STS-97 Payload: Photovoltaic Array Assembly (PVAA)|publisher=NASA|access-date=September 14, 2007|url-status=dead|archive-url=https://web.archive.org/web/20010123234800/http://www.shuttlepresskit.com/STS-97/payload81.htm|archive-date=January 23, 2001}}

From 2017 to 2021, the nickel-hydrogen batteries were replaced by lithium-ion batteries. On January 6, 2017, Expedition 50 members Shane Kimbrough and Peggy Whitson began the process of converting some of the oldest batteries on the ISS to the new lithium-ion batteries. Expedition 64 members Victor J. Glover and Michael S. Hopkins concluded the campaign on February 1, 2021.{{cite web|last=Garcia|first=Mark|url=https://www.nasa.gov/feature/spacewalkers-complete-multi-year-effort-to-upgrade-space-station-batteries/|title=Spacewalkers complete multi-year effort to upgrade space station batteries|date=1 February 2021|publisher=NASA|access-date=5 March 2021}}{{PD-notice}}{{cite web|last=Garcia|first=Mark|url=https://blogs.nasa.gov/spacestation/2021/02/01/spacewalkers-wrap-up-battery-work-and-camera-installations/|title=Spacewalkers wrap up battery work and camera installations|publisher=NASA|date=1 February 2021|access-date=5 March 2021}}{{PD-notice}}{{Cite web |url=https://www.space.com/spacewalking-astronauts-upgrade-space-station-battery-upgrade-expedition-64 |title=Spacewalking astronauts complete a space station battery upgrade years in the making |last=Gohd |first=Chelsea |work=Space.com|date=February 1, 2021 |access-date=March 5, 2021}}{{cite web|last=Garcia|first=Mark|url=https://blogs.nasa.gov/spacestation/2021/01/27/spacewalk-wraps-up-with-upgrades-on-european-lab-module/|title=Spacewalk wraps up with upgrades on European lab module|date=27 January 2021|publisher=NASA|access-date=28 February 2021}}{{PD-notice}} 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.

File: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. {{As of|2005|12|30}}, V_mp was 160 volts DC (direct current). It can change over time as the arrays degrade from ionizing radiation. Microprocessor-controlled switches control the distribution of primary power throughout the station.{{citation needed|reason=needs to be re-evaluated with more recent info. maybe https://www.nasa.gov/centers/glenn/about/fs06grc.html |date=December 2016}}

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-H₂ 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

The design is same as the S3R Solar Array Regulator which was patented by European Space Agency -Inventor Alan H Weinberg and D O'Sullivan in 1974.{{citation needed|date=March 2025}}

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.{{cite web|url=http://www.grc.nasa.gov/WWW/RT/2004/PB/PBP-delleur.html |title=Options Studied for Managing Space Station Solar Array Electrical Hazards for Sequential Shunt Unit Replacement |publisher=NASA |url-status=dead |archive-url=https://web.archive.org/web/20061008144118/http://www.grc.nasa.gov/WWW/RT/2004/PB/PBP-delleur.html |archive-date=2006-10-08 }} The SSUs are provided by SS/L.

DC-to-DC converter 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.

Uses the "Weinberg Converter" Topology invented by Alan H Weinberg which was published in an ESA conference paper in 1974.{{citation needed|date=March 2025}}

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.

From 2007 the Station-to-Shuttle Power Transfer System (SSPTS; pronounced spits) allowed a docked Space Shuttle to make use of power provided by the International Space Station's solar arrays. Use of this system reduced 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.{{cite web|title=STS-118 crew interview, Station to Shuttle Power System|url=http://www.space.com/common/media/video/player.php?videoRef=06_118SSPTS|publisher=space.com}}

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.{{citation needed|date=July 2010|reason=Dead link removed}}

In December 2006, during mission STS-116, PMA-2 (then at the forward end of the Destiny module) was rewired to allow for the use of the SSPTS.{{cite book|url=http://www.nasa.gov/centers/johnson/pdf/163524main_ASC_116_F_B_1.pdf?page=174|page=174|title=Ascent Checklist STS-116|publisher=Mission Operations Directorate Flight Design and Dynamics Division|date=October 19, 2006|chapter=Aft flight deck payloads switch list for handover|format=PDF|access-date=April 2, 2009|archive-date=May 24, 2011|archive-url=https://web.archive.org/web/20110524224120/http://www.nasa.gov/centers/johnson/pdf/163524main_ASC_116_F_B_1.pdf?page=174|url-status=dead}} The first mission to make actual use of the system was STS-118 with Space Shuttle Endeavour.{{cite web | date = August 10, 2007 | url = http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts118/news/STS-118-05.html | title = STS-118 MCC Status Report #05 | publisher = NASA | access-date = April 2, 2009 | archive-date = October 19, 2007 | archive-url = https://web.archive.org/web/20071019033145/http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts118/news/STS-118-05.html | url-status = dead }}

Only Discovery and Endeavour were equipped with the SSPTS. Atlantis was the only surviving shuttle not equipped with the SSPTS, so it could only go on shorter length missions than the rest of the fleet.{{cite web|title=Fuel Cell 2 issue cleared – Atlantis in perfect launch|date=November 16, 2009 |first=Chris |last=Gebhardt|url=http://www.nasaspaceflight.com/2009/11/launch-day-live-atlantis1/|publisher=NASAspaceflight.com}}

{{Reflist|30em}}

• {{cite web|date=November 13, 2001|title=Power to the ISS!|url=https://science.nasa.gov/headlines/y2001/ast13nov_1.htm|publisher=NASA|url-status=dead|archive-url=https://web.archive.org/web/20091229212013/http://science.nasa.gov/headlines/y2001/ast13nov_1.htm|archive-date=December 29, 2009}}
• {{cite web|publisher=NASA|date=November 2000|title=Powering the Future: NASA Glenn Contributions to the International Space Station (ISS) Electrical Power|url=http://www.nasa.gov/centers/glenn/pdf/84793main_fs06grc.pdf|access-date=2006-12-21|archive-date=2011-04-16|archive-url=https://web.archive.org/web/20110416174513/http://www.nasa.gov/centers/glenn/pdf/84793main_fs06grc.pdf|url-status=dead}}

• [http://www.nasa.gov/centers/glenn/about/fs06grc.html NASA Glenn Contributions to the International Space Station (ISS) Electrical Power System]
• https://ntrs.nasa.gov/citations/20110015485

{{ISS modules}}

Category:Components of the International Space Station

Category:Electrical systems

Category:Solar power and space