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Zero Energy Buildings require airtightness and mechanical ventilation systems to provide air changes and energy saving. These requirements contrast with the principles of natural ventilation. Through a case study located in Modena, Italy, a design strategy is proposed as a solution to integrate natural and mechanical ventilation systems at different times of the year to reduce the energy consumption in a newly designed high-density ZEB. The internal comfort evaluation for the warm season is then verified with a multizone dynamic simulation and a CFD analysis.
The proposal consists of two different approaches, the cold season and the warm one. 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 have been designed with aerodynamic principles to provide each apartment with additional (and maximised) differences of pressure due to the stack effect. The indoor comfort conditions in the warm season are then evaluated according to the ASHRAE 55 adaptive model for free-running buildings.
The results of the study confirm that in the warm season acceptable indoor comfort conditions can be achieved in a free running building. The ventilation shaft has an important role for the free cooling of a ZEB and can also be adopted in the renovation of existing buildings.