Currently, more than 50% of the world’s population resides in urban environments (United Nations, 2011), which will reach 68% by 2050 (United Nations, 2018). According to the UN (2015), cities are responsible for nearly 70% of energy consumption and more than 70% of greenhouse gas emissions in the world (United Nations, 2015). This statistic is situated in a context where most cities are experiencing rapid growth of population and are adopting dense configurations during their urban transition phase. Although a higher density for urban environments is more economical in terms of land use (Sundborg, 2018), it can reduce solar access due to the shadow cast by the surrounding buildings, resulting in the lack of healthy, energy-efficient living environments (Šprah and Košir, 2020). A contradiction as regards maximizing the use of available space (Dogan and Knutins, 2018) as opposed to performing better with respect to the energy efficiency of the built environment sector surfaces. The building sector accounts for nearly 40% of the total energy demand in Western countries, 24.8% and 11.6% of which is associated with the electricity consumed for lighting purposes in commercial and residential buildings (Energy, 2010). In other words, “buildings are the primary energy-use sector of cities” (Shi et al., 2021), from which an average of 20% is reported to belong to the electricity used for artificial lighting in buildings globally (International Energy Agency, 2016).
Daylighting provides an opportunity for climate change mitigation by improving health levels and reducing CO2 emissions indirectly by cutting the energy demand for lighting (Yu and Su, 2015). Improving daylight availability in indoor spaces brings numerous advantages to its users. Daylighting boosts people's mood (Shishegar and Boubekri, 2016), health, and well-being by balancing circadian rhythms and body (Chen et al., 2019). It can also enhance employees' productivity in office buildings (Bellia et al., 2011). However, very often occupants of buildings located in dense urban environments suffer insufficient daylight access. This is especially true in office interiors (Turan et al., 2020), where people spend most of their time. Currently, 74% office spaces experience insufficient daylight levels in a dense city like Manhattan (Turan et al., 2020). On the other hand, in such dense urban contexts, daylit spaces transact for a higher price (5–6% more) and are considered more economical during their operation period. A substantial financial value is thus associated with daylight and this value should accordingly be considered in project financing and investment phases (Turan et al., 2020). In other words, apartments with sufficient daylight are energy-efficient and thus more valuable to users and investors. These buildings are usually more in demand in markets and have higher lease rates and lower utility costs (Ahmadi, 2019). However, nowadays, even tropical cities that typically enjoyed ample solar radiation are struggling with insufficient solar access due to increasing densification of the urban fabric (Jayaweera et al., 2021).
Several studies have focused on daylight and its associated visual comfort in urban environments employing different methods. A study by Jayaweera et al., in 2021, employed a parametric approach to optimize solar access in terms of both daylight and energy savings in different urban contexts. Their findings indicate that an optimum daylight level (sDA of 75%) can decrease lighting energy demand by up to 12% and 36% for east-west and north-south directions, respectively (Jayaweera et al., 2021). Natanian et al. in 2019 also uses a similar approach (using a Radiance-based tool and daylight autonomy metric) to explore the impact of both building and urban design factors, including typology, window to wall ratio and glazing properties and distance between buildings, floor area ratio, and orientation, on daylight and energy performance of buildings. A considerable performative difference is reported between different designs and densities (Natanian et al., 2019). A comprehensive range of design parameters are also investigated in a recent study by Pan and u (2021), exploring building coverage ratio, floor area ratio (FAR), mean nearest neighbor distance (DOS), mean building height (MBH), vertical uniformity, tree coverage ratio, aspect ratio, sky view factor, total site factor, direct and indirect site factor, canyon axis orientation, and ground surface albedo. Based on the results of this study, an increment of 10% sky view factor (SVF) increases the average daytime horizontal illuminance level up to 71.6%, which accounts for 59.6% when combined with a 1m increase in MBH factor. Nevertheless, DOS has shown a negative effect on daytime site illuminance uniformity (D-SUo). Moreover, sky factor, building height, ground surfaces albedo, and vertical uniformity are found to have the highest impact on outdoor illumination, respectively (Pan and Du, 2021). A multi objective evolutionary algorithm (MOEA) based study of Martins et al. (2014) also considers different design indicators of absolute roughness, porosity, contiguity, FAR, plot ratio, aspect ratio, verticality, number of floors, street width, building setbacks, and their thickness, width, and height to evaluate both irradiance and illuminance levels on the building's facade in a tropical Brazilian climate. Accordingly, design parameters of albedo, aspect ratio, the distance between buildings, building width, and shape factor have the greatest impact on illuminance levels on all building facades facing four main directions (Martins et al., 2014). Generally, it is assumed that randomness; both vertically and horizontally, which refers to the difference in buildings height and the distance between buildings (DBB), has a great influence on the amount of daylight received in urban canyons. Random configurations provide a higher level of useful daylight illuminance (UDI); up to 10.8% compared to uniform ones (Ahmadi, 2019). Nevertheless, the parallel placement of buildings (less horizontal randomness) reduces daylight access and the associated average vertical daylight factor (VDF) compared to shifted patterns. A higher DBB is preferable in daylight-based (Francis and Groleau, 2002), energy-efficient urban design (Chang et al., 2019), and a distance equal to the width of the opposite buildings provides for a sufficient daylight level. This value can decrease for shifted building clusters (Fig. 1, b &c) (De Luca, 2019). Along with the DBB factor, the width of the urban canyon and the street geometry play a crucial role in an urban context. According to Mehjabeen (2020), doubling the streets width improves daylight access by at least 60% in typical high-rise residential dense urban contexts (in Dhaka, Bangladesh) (Mehjabeen, 2020).
In a later study by Natanian and Auer (2020), high-rise buildings are reported to increase Spatial Daylight Autonomy (sDA) as they present lower site coverage and higher DBB (Natanian and Auer, 2020). It is also reported that increasing FAR has the highest impact on VDF, while building typology and their position tend to have a moderate impact on VDF (Šprah and Košir, 2020). A building’s form showed a slight impact of 13.5% on sDA (Mehjabeen, 2020), while shape factor has a direct impact on daylight levels. In other words. a larger building envelope (in relation to their built volume) is more favorable for dense urban environments aiming to improve daylight access (Martins et al., 2014). Building width is also found to have a considerable effect on illuminance level on the west and east fronts of the building, while the average depth of a building is a more important design factor in south and north orientations (Martins et al., 2016). Linear buildings provide a higher level of VDF on their facade compared to point buildings (cubic) in Slovenia (Šprah and Košir, 2020) and Copenhagen, while box-shaped building towers are the most appropriate building typology in Hong Kong. The shape and density of tall buildings have a relatively slight impact on the VDF of their facade. However, when they have the same height as their surrounding buildings, a simpler shape would be more beneficial (higher VSC values) to prevent self-blocking (Li and Tian, 2020). A study by Jung and Yoon suggested that a building’s orientation is an important parameter that has the highest impact on the amount of natural light received in apartment interiors (floor area ratio, building coverage ratio, the distance between buildings, were other design factors studied) (Jung and Yoon, 2018). Increasing the window size of a building’s facade (WWR; from 50–75%), and their material reflectivity (from 30–50%) also increases sDa daylight metric from 51 to 56, and 58 percent, respectively (Ahmadi, 2019). Optimum facade design (WWR, window number, height, and sill height) have also been proven to improve illuminance levels (UDI) and energy use intensity (EUI) by up to 20.56% and 141 kWh/m2/yr in semi-arid areas such as Tabriz, Iran (Shahbazi et al., 2019).
Considering this diversity of design variables at the urban as well as the building scale, this study adopted the use of an evolutionary algorithm-based methodology to investigate and enhance daylight penetration into urban canyons while improving visual comfort in building interiors simultaneously. Optimum configuration showcasing maximum outdoor illuminance (avg. VDI), and indoor useful illuminance (spatial % of UDI 300–3000 lux) are accordingly presented. An additional evaluation of the correlation between VDIoutdoor and HILIndoor is also conducted since this correlation has the potential to bridge a significant gap in existing daylight metrics, namely that existing daylight metrics used in exteriors (open spaces) are unable to estimate indoor lighting conditions (inside the buildings). This research thus provides a novel design framework and valuable insights to integrate outdoor daylight access in urban canyons with indoor illuminance levels and ultimately, enhance environmental quality in contemporary cities.