Thermal system analysis of embedded building integrated heating - Numerical model and validation of hydronic floor heating systems

Licentiate thesis, 2006

A numerical simulation tool for building integrated heating (BIH) systems is developed. BIH is defined as a heat supply system that maintains the indoor thermal comfort within acceptable limits by means of a controlled heat supply to the inner of the building envelope/structure. Hydronic low temperature floor heating is a typical and well-known example of BIH systems. This thesis encloses a presentation of the structure of the BIH tool, descriptions of mathematical and numerical models as well as results of empirical validation tests and an inter-model comparison. The BIH model is developed as a subsystem of the building itself and is aimed to work jointly with several other subsystems. Together these subsystems outline a prototype of the whole building as a dynamic thermal system. Thus, the complex thermal interaction between the BIH system, the building structure, inhabitants, external climate and technical installations can be explored. The BIH tool is developed as a fully compatible component of the International Building Physics Toolbox (IBPT) which is a general simulation platform for system analysis of dynamic coupled heat, air and moisture transfer processes for use in buildings applications. The entire toolbox is developed using the graphical programming language Simulink. The numerical model of the BIH element is developed by using an explicit finite control volume method. A hybrid three-dimensional model is developed where the BIH element is subdivided in a finite number of two-dimensional section planes. A detailed description of the heat pipe pattern is implemented in the BIH element model. The temperature decline along the longitudinal direction of the heat pipe is considered. The surface of each section plane has a defined shape, position and orientation and is called a subsurface. A three-dimensional wire frame model of the entire enclosure is composed by the entire set of subsurfaces and surfaces. Transient long wave radiation exchange within the wire frame model is considered. For this purpose the ‘Net Radiation Exchange’ method for detailed computation of long wave radiation exchange has been implemented in the Zone model of IBPT. A separate pre-processor for computation of view factors has been developed. Empirical validation in the case of a full-scale test chamber equipped with a floor heating system is considered as well as validation by means of an inter-model comparison. Both tests point out that the BIH model accurately can simulate transient heat transfer from the heat pipe through the BIH element to the internal surface under the influence of thermal inertia within the structure.