Spatial Exergy Destruction for Heat and Mass Transfer in Building Components. Formulation and Implementation in Standard Finite Element Software.

In recent years there has been a development of exergy analysis as a tool for exploring the efficiency of the components and the whole energy system for a building. An important part of this development has taken place within IEA ECBPS Annex 37 Low Exergy Systems for Heating and Cooling of Buildings that is finished and Annex 49, Low Exergy Systems for High-Performance Built Environments and Communities, that is ongoing. The work has included modeling components and system parts as an input/output operator. In the present work the analysis is broken down into a continuous modeling of the exergy destruction or the entropy production for field problems where mass and heat flow takes place. The paper gives formulation of the exergy destruction and implementation of this formulation into a standard finite element software. The use of this methodology is demonstrated for steady state two dimensional heat flow in thermal bridges and combined heat and mass flow for hybrid constructions for heating and cooling in buildings. A discussion follows on the relevance of the gained information for understanding the nature of the components and to develop more effective solutions. For components with given input and output the exergy destruction can be calculated and compared to the integrated exergy destruction from the field model. The relation between integrated exergy destruction from the finite element calculation and the exergy destruction based on the in and output values can be used as an indicator for the convergence or the quality of the solution. This could prove useful even for calculations where exergy analysis is not the primary target.