Numerical analyisis of thermal bridges in dynamic conditions

Thermal bridges play a significant role in the heat loss of nearly Zero Energy Buildings (nZEB). In the case of existing buildings, the underestimation of thermal bridges can lead to errors of about 20 % in the assessment of their energy requirements. Nowadays, proper simulation tools for evaluating the building energy performance in dynamic conditions are increasingly needed. Their outputs are important inputs for life cycle costs (LCC) and life cycle assessment (LCA) as well as for energy audits. A weak point is that the tools which are currently well established on the market, do not consider the contribution given by thermal bridges to the overall building energy balance as they rely on a onedimensional approach to recreate heat flows. Several scientific studies deal with different methods that can be applied to evaluate the dynamic behaviour of thermal bridges, but they disregard the wall capacity to accumulate/release heat loads and the role played by internal temperatures. This work analyses some numerical methods proposed by different authors based on the discretization of thermal bridges and their characterization in dynamic regime. A calculation procedure is evaluated to underline its potential as a rapid dynamic calculation algorithm to be integrated in the current software for dynamic analyses. The surface temperatures and the heat fluxes are taken into account. In the present work, the method of the equivalent thermal wall has been implemented to get the input parameters required to dynamically assess the thermal bridges energy contribution. Afterwards a finite volume analysis is developed to compare the outputs coming from different methods in terms of crossing fluxes, surface temperatures and thermal storage capacities. A low percentage error is found between the equivalent thermal wall and the real one in terms of surface temperatures. This achievement allows to carry out proper superficial condensation assessments. Anyway the above-exposed procedure is quite complex and time-consuming. The algorithm is then expected to be refined in the future by simplifying the necessary operations for the evaluation of thermal bridges.