International Space Station - Post Flight ULF-3 Configuration, November 2009 (Credit: NASA).

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.

Generic ORU box (Credit: Hamilton Sundstrand Rocketdyne)

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.

DC to DC Converter Unit (external) (BOTH: Hamilton Sundstrand Rocketdyne)		DC to DC Converter Unit (internal)

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).  

Battery ORU (cover removed to show interior) (Credit: Space Systems Loral)

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 10 years in space.  
 
Starting in 2017, the nickel-hydrogen battery ORUs will be replaced by Lithium-ion batteries.  The design and development of the new Li-ion batteries started in 2011.  One of the advantages of the Li-ion over the Ni-H2 is that it has a higher specific energy, thus two nickel-hydrogen ORUs will be replaced by one Lithium-ion battery.  In order to maintain the integrity of the thermal control loop that runs underneath the EPS ORUs, the empty Ni-H2 ORU slots will be covered by an adapter plate.  Each Li-ion battery will weigh about 425 lb, and each adapter plate will weigh 65 pounds, for a weight savings of 299 lb.

Columbus module (Credit: NASA/George Shelton).

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.

Node 2 (Credit: ESA - D. Ducros).

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.

Pressurized Module (Credit: Japan Aerospace Exploration Agency (JAXA)).

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.

Cupola observation module (Credit: ESA - D. Ducros).

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.

H-II Transfer Vehicle (Credit: JAXA).

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.