Comprehensive assessment of double skin façades: A mathematical model for evaluating influence of KL ratio on electrical and thermal performances, and indoor conditions

Abstract

The present study proposes a mathematical model to evaluate the electrical and thermal performance of a novel Double Skin Façade (DSF) system and its impact on indoor conditions. DSF system comprises opaque PV and transparent Glass panels integrated as an exterior skin. Further, the cavity between the DSF’s exterior and interior skins provides airflow that increases PV efficiency and delivers preheated air into the room for space conditioning. The mathematical model is based on energy balance equations written, considering the DSF as an integral part of the room. This system of energy balance equations is then solved analytically under quasi-steady-state conditions for developing a mathematical model in terms of DSF’s design and climatic parameters. The model is employed to investigate multiple design parameters of DSF, including KL ratio (proportion of PV façade height to total façade height), Glass façade transmissivity, PV packing factor, cavity dimensions, and airflow rate. Montreal (Canada) is considered a pilot location for the study. The results indicate that increasing the KL ratio reduces room temperature and increases electrical output. For a fixed KL ratio, the room temperature can also be modified by adjusting the transmissivity of the exterior and interior glass façade without impacting the electrical output. A 20 % increase in the interior façade’s transmissivity results in a 2.3 ˚C rise on a winter design day and a 2.6 ˚C rise on a summer design day. Additionally, the room temperature increases with an increase in the mass airflow rate up to a certain value, after which it decreases. This behavior is attributed to the interplay between the increased convective heat transfer coefficient (which tends to increase room temperature) and the reduced flow rate factor (which tends to decrease room temperature).