Development and analysis of ventilation models in Building Energy Simulations

The ventilation systems are responsible for the preservation of indoor air quality in buildings. However, their operation involves energy costs that cannot be neglected. This aspect is often overlooked, and this thesis tries to boost the research on the topic. The activities involved in the thesis project adopt a modelling approach as it allows to simulate the system under different scenarios and rapidly evaluate energy efficiency solutions. The reported works focus on the development and analysis of simplified ventilation models to be integrated into building energy simulations. The scale of investigation is progressively enlarged from room to district level. In the first part, three room-scale models are developed to evaluate the steady-state operation of different ventilation systems. Firstly, a zonal model is constructed to detect the airflow patterns induced by a mechanical mixing ventilation system, applying a macro-discretisation of the room volume. The model is applied to three rooms with different end-uses to evaluate the airborne infection risk from COVID-19 and the energy demand for the air handling processes in parallel. Subsequently, a three-node model for predicting thermal stratification in rooms equipped with displacement ventilation systems is developed. It is applied to an office room with different arrangements of the internal heat sources. For each case, a CFD simulation is run to calibrate two parameters of the nodal model, expressing the air entrainment into the inlet jet and the recirculation flows of the convective heat gains within the occupied zone. A preliminary database of optimal model parameters is obtained; the latter can be employed in energy simulations. The last room-scale model calculates the natural ventilation flow rate driven by buoyancy and wind forces through openings on the building envelope. The stack ventilation calculation is verified against CFD analyses concerning a small-sized room with opposite door and windows: the model provides reliable predictions for both summer and winter seasons under different opening configurations (i.e., single-sided, crossflow). In the second part, the operation of natural ventilation was assessed for two building-scale case studies. A monitoring campaign in twelve council houses in the northeast of Italy was carried out comparing different ventilation strategies. The air temperatures measured in summer highlight that natural ventilation systems could not be sufficient to provide adequate passive cooling to indoor spaces. However, the lack of information regarding the tenants’ behaviour and building usage makes it difficult to compare the effectiveness of the different systems. Then, the coupled TRNSYS-CONTAM multi-zone modelling approach is adopted to simulate the operation of hybrid ventilation in two different environments, i.e., a dwelling and a classroom. The switching between mechanical and natural systems is achieved by a control system based on occupancy and indoor-outdoor temperature difference, for which different simulation scenarios are arranged. The results show that a well-controlled hybrid system could maintain the indoor spaces healthy with reduced energy demand for air handling. In the final part of the thesis, the integration of the simplified natural ventilation (NV) model into the urban energy modelling tool EUReCA is proposed. EUReCA applies a lumped-capacitance approach to perform energy simulations at urban scale, modelling each building as an equivalent RC network. The integrated model is applied to a case study of eleven Danish apartments to evaluate the cooling demand in summer, accounting for the benefits of windows’ opening. The results show that the cooling demand is destined to increase due to global warming, but as long as the outdoor air temperatures are favourable, natural ventilation has a significant cooling potential, suggesting its exploitation in combination with an active air-conditioning system.