An exergy metric for building performance evaluation

Building energy performance evaluation is a key action to comply with EU Directive on Energy Efficiency in Buildings as well as the regulations' adoption at National level for Energy Certificates and Label assignments. As a matter of fact, a comprehensive and detailed analysis of heat exchange in existing buildings and new ones is still missing due to the complexity of established procedures and huge amount of buildings to assess. For this purpose, an exergy audit considering the designed indoor temperature, the building envelope behavior and heating production and distribution systems allows computing the building performance with more detail by including the exergy consumption of the whole system. This method could provide more detailed calculations in the summer season even by simplifying the shadows and natural ventilation effects on the cooling demand due to their contribution to the amount of variables and complexity. The method here described starts from the LowEX tool developed in the IEA ECBCS Annex 37 framework to elaborate a final new metric. Accounting for the energy conservation principle, the total amount of exergy exchanged by the building is obtained as the addition of the exergy exchanged in each sub-system, i.e. the building envelope, the energy generation and the distribution system. Finally, the efficiency of the system could be calculated, comparing the amount of energy consumption and exergy exchanges. By using the exergy approach, other indicators could be analyzed, i.e Tonne of Oil Equivalent (TOE), CO2 emission and fossil fuel consumption. Among those ones, the ratio between TOE produced by the heating system and the exergy exchanged by the system represents the new exergy parameter. This latter is tested on an existing building as case study.