TRNSYS (acronym for TRaNsient SYstem Simulation Tool) is a transient dynamic simulation software for energy systems, considered since 1975 the world reference tool for detailed modeling of solar thermal systems, HVAC, low energy buildings, thermal storage and complex renewable processes.

Originally developed at the Solar Energy Laboratory at the University of Wisconsin–Madison by the team of John A. Duffie, William A. Beckman and Sanford A. Klein"), it is today maintained by an international consortium that includes TESS (Thermal Energy System Specialists, USA), TRANSSOLAR (Germany), CSTB (France) and Aiguasol (Spain), among others.

• Modular architecture based on “Types” (components pre-programmed in Fortran or C++): solar collectors, pumps, stratified tanks, multi-zone buildings (Type 56), controllers, meteorology, loads, etc.

• More than 500 standard Types in version 18 (2024).

• Hour-by-hour or sub-hour simulation (steps of 1 second or less).

• Entry of meteorological data TMY2, TMY3, EPW, IWEC and own formats.

• Native coupling with EnergyPlus (Type 56), Google SketchUp (TRNSYS3d), Excel, MATLAB, Python, COMIS, CONTAM.

• Automatic generation of reports and graphs.

• Design and sizing of large solar thermal systems (DHW, district heating, solar absorption cooling).

• Detailed simulation of buildings with passive solar gain, natural ventilation, thermal storage (PCM, stratified tanks).

• Research in seasonal storage, 5th generation heat networks, power-to-heat.

• Validation of standards (ISO 9806, ASHRAE 93, EN 12975-2, IRAM 11601/11604).

• Advanced energy certification and Net-Zero / NZEB projects.

That's why physicists, solar engineers, and research centers tend to prefer TRNSYS (“it's the only one that does exactly what I want”), while EnergyPlus dominates in certification and generalist architecture, and DesignBuilder is chosen by those who prioritize speed and visualization.

• 1975 – TRNSYS 1 (Fortran IV, punched cards)

• 1994 – TRNSYS 13 (Windows)

• 2004 – TRNSYS 16 (introduction of TRNSYS3d and SketchUp)

• 2017 – TRNSYS 18 (64-bit support, Python coupling)

• 2024 – TRNSYS 18.05 (new Types for thermal batteries, PCM and heat networks).

Almost 50 years after its creation, TRNSYS remains the most used simulation tool in scientific publications on solar thermal energy and complex systems. It is reference software in solar energy postgraduate courses in Europe, the USA, Japan and Latin America (UNLP, UNSa, UNAM, UChile, USP, etc.).