A Design Strategy to Reach nZEB Standards Integrating Energy Efficiency Measures and Passive Energy Use

In this paper a design strategy is presented to integrate energy efficiency measures and passive energy use, in order to meet near Zero Energy Buildings requirements in European temperate climates. In particular, a hybrid system for the integration of natural and mechanical ventilation at different times of the year is proposed. ZEBs and nZEBs usually employ mechanical ventilation systems to provide air changes and energy saving. This common solution is in contrast with the principles of natural ventilation and adaptive comfort. It is well known that natural ventilation can significantly reduce the energy demand during the summer. The case study is represented by a social housing complex located in the periphery of Modena (Italy), dating back to 1980. The project consists in the deep energy renovation of some buildings of this complex to accomplish nZEB standards. The proposal envisages two different modes of operation for the buildings, one for the cold season and one the warm season. For the cold season, a mechanical ventilation system with earth tubes and heat recovery has been designed, together with airtightness, solar greenhouses and high thermal mass and insulation. For the warm season the design allows a free-running use: open trickle ventilators applied to windows which provide background ventilation, mass and insulation mitigate the heat loads, vertical ventilation shafts support natural ventilation and free night cooling. The ventilation shafts are designed with aerodynamic principles to provide each apartment with additional (and maximised) differences of pressure due to the stack effect. The ventilation shafts have an important role for the free cooling. The renovated buildings are also equipped with active systems to compensate the remaining energy demand: a Combined Heating Power System (CHP), PV panels and solar thermal collectors. The indoor comfort conditions in the warm season are evaluated according to the ASHRAE 55 adaptive model for free-running buildings. The internal comfort in the warm season is verified with a multizone dynamic simulation and a CFD analysis. The results of the study confirm that in the warm season acceptable indoor comfort conditions can be achieved in a free running nZEB.