TRNSYS is a simulation

Computer simulation

A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...

 program

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

 primarily used in the fields of renewable energy

Renewable energy

Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...

 engineering and building

Building engineering

Education in the field of Building Engineering, better known as Architectural Engineering in the United States, is the study of the integrated application of engineering principles and technology to building design and architecture...

 simulation for passive

Passive solar building design

In passive solar building design, windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer...

 as well as active solar

Active solar

Active solar technologies are employed to convert solar energy into another more useful form of energy. This would normally be a conversion to heat or electrical energy. Inside a building this energy would be used for heating, cooling, or off-setting other energy use or costs. Active solar uses...

 design. TRNSYS is a commercial software package developed at the University of Wisconsin. One of its original applications was to perform dynamic simulation of the behaviour of a solar hot water

Solar hot water

Solar water heating or solar hot water systems comprise several innovations and many mature renewable energy technologies that have been well established for many years...

 system for a typical meteorological year

Typical meteorological year

A typical meteorological year is a collation of selected weather data for a specific location, generated from a data bank much longer than a year in duration...

so that the long-term cost savings of such a system could be ascertained.

See http://sel.me.wisc.edu/trnsys/