CHAPTER 9 SYSTEMS: ELECTRICAL POWER SYSTEM—THE POWER BEHIND IT ALL 156 International Space Station solar arrays capture sunlight during STS-119/ISS-15A fly around. “Following the light of the sun, we left the Old World.” – Christopher Columbus Electrical power is the lifeblood of the International Space Station (ISS). Literally, it is the energy that keeps the ISS running. The ISS uses electrical power to operate the various systems that ultimately allow crew members residing on the space station and scientists across the globe to perform world-class research. In turn, the Electrical Power System (EPS) relies on those systems to provide command and control, to cool EPS devices, and to pinpoint the location of the sun to harvest its energy. Previous NASA human spacecraft relied on consumables-based energy systems. Fuel cells used hydrogen and oxygen to produce power and, as a side benefit, water. This worked well for short-duration missions. For the long-duration mission of the ISS, resupplying these consumables would be cost prohibitive and would endanger the future of ISS operations, should the resupply be interrupted. Solar arrays and cells, along with batteries, proved to be viable power sources for satellites and were used in the Russian human space program—including the Mir space station. However, the energy demands of the ISS would require the largest solar power system ever (to date) to be designed, built on Earth, assembled in orbit, and operationally maintained for decades. Many design decisions drove the overall architecture and operations of the EPS early in the development of the ISS. Large design differences exist between the Russian Segment (RS) and United States On-orbit Segment (USOS), though they have the capability to transfer power to each other. Even the voltage at which the EPS operates has large impacts on hardware design and actions necessary for crew and ISS safety. Multiple independent power sources provide redundancy for critical systems and allow power to be rerouted in the event of a failure. The USOS EPS can be divided and discussed in many ways, but it may be easiest to compare it to how most homes on Earth receive electrical power. Similar to terrestrial power plants, the Primary Power System, operating at high voltage, is where solar energy is converted to electrical power and stored in batteries for use during eclipse. A combination of