PVTCS panels (Credit: Rocketdyne).

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.

Photovoltaic Thermal Control System on the Port 4 Photovoltaic Power Module (Credit: NASA).

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.

Ammonia loading into the TCS (Credit: Rocketdyne).

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.