The National Building Research Institute

Professor Dan Constantinescu
www.incerc.ro
hp@cons.incerc.ro

The work of the Department of Building Services and Efficient Use of Energy includes a study on:

‘Inverse modelling method for the identification of the real thermal characteristics of existing buildings’. The identification of the real thermal characteristic of an existing building implies mainly the assessment of the invariants specific to that structure, namely the equivalent thermal conductivity of the materials used for the opaque external envelope of the dwelling space.

The method used is of the inverse modelling type, based on the building global balance and on the thermal response of the heating system to random climatic conditions. The problem implies two phases as follows:

• The preliminary phase, consisting in the integration of the heat balance differential equation of the dwelling spaces;
• The final phase, based on the thermal response characteristic of the heating system in real operation conditions.

The average indoor air temperature ta1 used in the preliminary phase is assessed from the thermal balance equation of the heating system where the m coefficient specific to the heat flow, specific to the heating units, is a variable value. In the end it results a correlation of the value of the equivalent thermal conductivity of the heat insulating material, kE, with the value of ventilation rate na, specific to each experiment. Value kE also includes the dissipative effects of thermal bridges.

The INVAR software assisted simulation of the building dynamic thermal behaviour for a couple {kE, na} (for a certain value m1) and the use of the measured heat flow-rate values QM(t) lead to the value series ta1(t). Based on ta1(t) values, the hourly values of the outlet heat carrier temperature tR(t) is determined. The decision on the accuracy of values kE, m1 and na is given by the analysis of the daily average values tR and tRM as well as by the standard deviation of tR(t) and tRM (t) values.