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ColSim - Collector Simulation Environment

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Current Version is 0.63

ColSim is a modular simulation environment, primaraly designed for development of controllers in thermal systems. High calculation performance with a time horizon of a whole year using timesteps down to some seconds is achieved. it's maintained by Christof Wittwer at Fraunhofer Institute for Solar Energy Systems in Freiburg/Germany. (

ColSim is based on the public domain concept and is delivered with source code in a dual licence concept, which allows commercial use with an exclusive licence. it's written in ANSI-C code, thereby it can be compiled on nearly all platforms. well known PD-tools for visualization are used inside the environment.

Introduction and Features of ColSim The simulation of controller strategies with the simulation code TRNSYS (ColSim is based on a similar structure to the modular simulation code TRNSYS) [1] and other commercially available programs is not possible without modifying numeric algorithms of the programs' solvers in order to achieve convergence with small time steps. Furthermore, the implementation of controller routines in Fortran within the programs is absolutely unsuitable for the code transfer to modern micro-controller hardware. Experiencing these (and other) restrictions of existing simulation programs was the original motivation to create a new simulation environment. ColSim is based on the UNIX multitasking philosophy(ColSim can be compiled as well on 32-bit-MS-windows systems) and the idea of public domain software. The current platform is a LINUX system (default installation SuSE LINUX) with different public domain tools included to setup the simulation environment. Most important is the GCC C-Compiler which represents a solid base for highly reliable software and development environments. ColSim is primarily a command line driven tool, which can be controlled interactively by a simple TCL-menu. The visualization of online results is realized with the interactive function plotting program GNUPLOT [14]. The design of systems to be simulated can be done with the vector graphic tool XFIG [13], which allows to convert a simulation script with all necessary object relations and parameter information.

The initial intention for creating ColSim was not the simulation of a building´s thermal performance, but primarily the simulation of HVAC and solar thermal systems with it's controllers. But the modularity of ColSim with its objects and the graphical representation allowed an extension of a building model which is described below in more detail. The high resolution of time of a simulation in ColSim now enables to investigate particularly non linear physical effects, which often occur in energy saving technologies for buildings (e.g. phase change materials in a wall system, transparent insulation etc.).

Plug Flow Model: The plug flow model allows the tracing of the mass flow through a closed hydraulic network, which guarantees a fast calculation of the object chain. The plug is initiated at a pump object, is shifted through the branched network and is returned to the origin (pump). This mechanism allows simple modeling of flow delay effects and a mass balance at the second call of the pump in one time step. Error detection (e.g. missing connection between two objects) is one of the most important tasks of a simulation code, since the developer is usually spending most of the time for debugging a new system design. Checking the mass flow and system energy balance are the main features of ColSim's error detection mechanism.

screenshot of a colsim session: (click here to zoom up system)