Evaluating the potential of mechanically ventilated double-skin façades to enhance energy efficiency and thermal comfort of office buildings in different climate zones of India through numerical modelling

Abstract

The air-conditioning of commercial buildings accounts for approximately 50% of the total energy consumed in India’s commercial sector. Reducing this load is a key step towards minimizing India's greenhouse gas emissions. One culprit is the routine use of fully glazed façades which are chosen for aesthetics but are neither energy efficient nor effective at providing adequate thermal comfort. A double skin façade (DSF) can help reduce the heat gain of a building by exhaustion heat through cavity ventilation, while still providing the aesthetics of a fully glazed façade. The higher thermal resistance can also help provide better indoor thermal comfort to occupants. However, due to the high cost of DSFs, their complex thermal behaviour and overheating risk in the cavity an assessment of the effectiveness of the system in warmer climates of India is needed. This study evaluates the energy saving potential and thermal comfort enhancements with mechanically ventilated DSFs in different climate zones of the Indian subcontinent. A numerical model created in the MATLAB/Simulink platform, employing the zonal approach and verified against Design Builder was used for simulating the thermal behaviour. Optimization of the façade design was based on three parameters: 1) cavity width, 2) cavity ventilation rate and 3) glazing systems. The sensitivity of these design parameters to the performance of the DSF was analysed before proposing optimized configurations for different sites based on their local climate. These optimized configurations are then evaluated against a single skin fully glazed façade. The parameters used for evaluating the energy saving potential are annual heat gain and annual heat loss. For thermal comfort and overheating risk, the temperature distributions were analysed. In comparison with a single skin façade, it is found that the façade orientations with the highest incident solar radiation, usually the South, East and West facing façades, provide the most improvement, up to 50% reduction in heat gain annually. Heat loss was reduced by 25-35% at each site, however, this proves to be irrelevant for sites with mild winters. This corresponded to the arid, semi-arid, humid subtropical and montane climate zones found in India. Thermal comfort was maintained by the DSF for both warm and cool walls. This was also found to be the case for the single skin façade, hence, no additional improvement to thermal comfort was achieved with respect to radiant asymmetries. Overheating in the cavity was mitigated through the application of tinted glazing in the external envelope, ventilation rates of up to 80AC/hr and larger cavity widths. Optical properties of the external envelope have the largest impact on the reduction in heat gain regardless of the façade’s spatial orientation and façades with low direct solar radiation were found to be insensitive to changes in cavity width and ventilation rate. The findings of the study suggest that DSFs have an important energy saving potential in warm climates and the design recommendation can be utilized to mitigate overheating and excess heat gain which is seen as an issue even in cooler climate zones.