TRNSYS simulation program
TRNSYS is an acronym for a ‘transient simulation’ which is a quasi-steady simulation model. This program  was developed by the University of Wisconsin by the members of the Solar Energy Laboratory. The program consists of many subroutines that model subsystem components. The mathematical models for the subsystem components are given in terms of their ordinary differential
or algebraic equations. With a program such as TRNSYS which has the capability of interconnecting system components in any desired manner, solving differential equations and facilitating information output, the entire problem of system simulation reduces to a problem of identifying all the components that comprise the particular system and formulating a general mathematical description of each.
Once all the components of the system have been identified and a mathematical description of each component is available, it is necessary to construct an information flow diagram for the system. The purpose of the information flow diagram is to facilitate identification of the components and the flow of information between them. Each component is represented as a box, which requires a number of constant PARAMETERS and time dependent INPUTS and produces time dependent OUTPUTS. An information flow diagram shows the manner in which all system components are interconnected. A given OUTPUT may be used as an INPUT to any number of other components. From the flow diagram a deck file has to be constructed containing information on all the components of the system, weather data file, and the output format.
Subsystem components in the TRNSYS include solar collectors, differential controllers, pumps, auxiliary heaters, heating and cooling loads, thermostats, pebble-bed storage, relief valves, hot water cylinders, heat pumps and many more. There are also subroutines for processing radiation data, performing integrations, and handling input and output. Time steps down to 1/1000 h (3.6 s) can be used for reading weather data which makes the program very flexible with respect to using measured data in simulations. Simulation time steps at a fraction of an hour is also possible.
Model validation studies have been conducted in order to determine the degree to which the TRNSYS program serves as a valid simulation program for a physical system. It has been shown by analysing the results of these validation studies that the TRNSYS program provides results with a mean error between the simulation results and the measured results on actual operating systems under 10% . The use of TRNSYS for the modelling of a thermosyphon SWH was also validated by the author and found to be accurate within 4.7% . TRNSYS is not a user-friendly program, although some graphical interfacing has been developed recently, like IISiBat, but is the most versatile with respect to the detail that systems are modelled.
More details about TRNSYS program can be found in the program manual  and in Ref. . There are numerous applications of the program in literature. Some typical examples are for the modelling of a thermosyphon system , modelling and performance evaluation of solar domestic hot water systems , investigation of the effect of load profile , modelling of industrial process heat applications [20,116,117] and modelling and simulation of a lithium bromide absorption system .