TRNSYS is a transient systems simulation program that has been commercially available since 1975 is currently maintained by an international collaboration from the United States (Thermal Energy System Specialists and the University of Wisconsin-Solar Energy Laboratory), France (Centre Scientifique et Technique du Bâtiment), and Germany (TRANSSOLAR Energietechnik) . 16 versions of the software have been developed, and latest costs US$2,100 (€1,600) for an educational licence and US$4,200 (3,200) for a commercial license and between 2000 and 2008, 1,143 users bought TRNSYS. It takes approximately one day of training to begin using the model.
TRNSYS is an open modular structure with open source code which simulates all sectors of an energy-system except the transport sector. The TRNSYS model simulates the performance of the entire energy-system by breaking it down into individual components, and it is primarily used for analysing single-project, local, community, or island energy-systems. To create a model, the end user is able to create custom components or choose from the TRNSYS standard library of components. Overall TRNSYS can simulate all thermal and renewable generation except nuclear, wave, tidal and hydro power. Also, the only electrical energy-storage considered by the model is BES. Hydrogen systems are simulated in detail by the formally independent model, HYDROGEMS , which focuses on renewable energy and hydrogen systems and is now part of the TRNSYS model. However, no transportation technologies are simulated by the TRNSYS. The model uses a user-defined time-step, which ranges from 0.01 seconds to 1 hour, and it can analyse a time-horizon of multiple years. Also, TRNSYS facilitates the addition of mathematical models, available add-on components, and the ability to interface with other simulation programs if necessary.
TRNSYS has been used extensively to simulate solar energy applications, conventional buildings, and even biological processes. These include analysing the performance of prototype solar-thermal system , optimising the operation of a solar thermal system , analysing the thermal performance of buildings , and modelling a hybrid PV-thermal solar system in Cyprus . Finally, the model has also been used to simulate a renewable-energy penetration of 110% in the electricity sector  and 90% of the heat sector .
- A TRaNsient SYtems Simulation program, University of Wisconsin-Madison, 27th April 2009,http://sel.me.wisc.edu/trnsys/
- HYDROGEMS, Institute for Energy Technology 27th April 2009, http://www.hydrogems.no/
- Souliotis, M., Kalogirou, S. & Tripanagnostopoulos, Y., Modelling of an ICS solar water heater using artificial neural networks and TRNSYS. Renewable Energy, 34(5), pp. 1333-1339, 2009.
- Hobbi, A. & Siddiqui, K., Optimal design of a forced circulation solar water heating system for a residential unit in cold climate using TRNSYS. Solar Energy, 83(5), pp. 700-714, 2009.
- Datta, G., Effect of fixed horizontal louver shading devices on thermal performance of building by TRNSYS simulation. Renewable Energy, 23(3-4), pp. 497-507, 2001.
- Kalogirou, S. A., Use of TRNSYS for modelling and simulation of a hybrid pv-thermal solar system for Cyprus. Renewable Energy, 23(2), pp. 247-260, 2001.
- The Aldo Leopold Legacy Center Baraboo, Wisconsin, Thermal Energy System Specialists, 2004,http://www.trnsys.com/ProjectProfiles/ALFProjectProfile.pdf.
- Drake Landing Solar Community: Okotoks, Alberta CA, Thermal Energy System Specialists, 2006, http://tess-inc.com/projects/project1.