Methodology for the modelling of thermally activated building components in low exergy design
Abstract: There is still an obvious and indisputable need for an increase in the efficiency of energy utilisation in buildings. Heating, cooling and lighting appliances in buildings account for more than one third of the world’s primary energy demand and there are great potentials, which can be obtained through better applications of the energy use in buildings.This thesis focuses on the development of methods and models for heat and mass transfer processes in buildings, which have a vital impact on the energy utilisation. These models can be used in optimisation procedures aiming at increasing efficiency in the energy use, i.e. at minimising consumption of the necessarily supplied high quality energy, i.e. exergy, in buildings.Through the use of the method of analysing exergy flows in buildings, similar to the analysis applied on other thermodynamic systems, such as power stations, it is possible to identify the potential of increased efficiency in energy utilisation. It has been shown that calculations based on the energy conservation and primary energy concept alone are inadequate for gaining a full understanding of all important aspects of energy utilisation processes. Thus, a method for exergy analyses, based on a combination of the first and second laws of thermodynamics, is presented and an assessment tool has been developed for a better understanding and design of energy flows in buildings.Ventilation heat losses account for a significant fraction of the overall heating energy use in buildings. The implementation of natural ventilation strategies allows for the possibility of supplying indoor space with the required fresh air volume, without any fan power. Because of the ability to create high air flows, the use of natural ventilation can be beneficial to for night cooling processes. All in all, it is important to estimate the expected air flow rates during the design and planning stage of a building. That is why a model, based on earlier published works on single sided natural ventilation on tilting windows, has been developed for natural cross ventilation conditions with tilting windows.There are also building service system solutions which can help to reduce exergy consumption caused by the heating and cooling of rooms. The thermally activated building components are examples of these systems; they use very low temperature differences between the heat carrier medium and the room to be tempered. Earlier derived models of such systems are not always satisfactory for the design of all system configuration or new regulation strategies. The developed macro element modelling (MEM) approach is based on research conducted on the modelling of dynamic heat flows in solid constructions with discrete resistances and capacitances. In this work, it has been expanded by the simultaneous modelling of heat carrier flows and used on the thermally activated components. A methodology for modelling thermally activated components has been developed and verified. Optimised resistance-capacitance (RC) networks combined to so-called macro elements are used to model the solid parts of the system, the fluid temperatures are calculated under the precondition of a linear variation of mass node temperature between the calculation nodes. It has been demonstrated and verified that the MEM method is generally suitable for modelling the dynamic behaviour of combined systems with a heat carrier flow and solid construction parts with substantial heat storage capacity.
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