Urban built context as a passive cooling strategy for buildings in hot climate

Highlights

• That aim is to quantify the impact of urban context on building cooling loads.

• Building energy modeling is used to test 108 urban context configurations.

• The configurations combine different context heights, distances, and orientations.

• Savings of 26% and 24% were observed for total and peak cooling, respectively.

• Implications on cooling system sizing and urban planning practices are discussed.

Abstract

This paper presents a systematic evaluation of the impact of the built urban context on the cooling energy performance of buildings subjected to extreme hot weather conditions. The proposed approach combines building energy modeling with an extensive parametric variation and statistical analysis scheme. It provides a direct quantification of inter-building shading on energy performance, with an emphasis on the cooling demand patterns for buildings of different types and sizes. Results show that the combined effects of urban context height, distance, and orientation can lead to significant reductions in cooling demand, reaching up to 26% for total cooling loads and 24% for peak cooling loads. The observed savings are particularly notable for residential buildings, making them an ideal target for urban development strategies that aim to leverage inter-building shading effects for energy conservation purposes. Moreover, an in-depth analysis of peak loads illustrates how the density and compactness of the urban form can be used as passive design strategies to reduce the load, and hence, the size of air conditioning systems. Finally, the paper explores how to translate the gained knowledge to design guidelines, bridging the gap between theory and practice.

Introduction

The energy demand of the building sector is projected to grow significantly in the coming decades due to a combination of factors, including population growth, economic growth, and higher standards of living in emerging economies [20]. Moreover, global warming and higher indoor comfort expectations are anticipated to amplify the stress on thermally driven end uses, such as air conditioning systems, particularly in cooling-dominated climates [19]. The above stressors have motivated extensive research efforts on strategies to reduce the energy intensity of building systems while providing comfortable and healthy indoor environmental conditions for occupants [20].

A common approach to reducing thermal loads in buildings is through adopting energy-efficient building technologies, such as low-energy Heating, Ventilation, and Air Conditioning (HVAC) systems, coupled with smart control or automation systems [25]. In contrast to such “active” design strategies, “passive” strategies focus on the design of the building to minimize electric and thermal loads [19]. Passive cooling, in particular, has gained significant interest in the literature as a means of reducing the need for mechanical HVAC work, as documented in multiple review articles on the topic. For instance, Samuel et al. [39] reviewed passive alternatives to mechanical air conditioning, including nocturnal radiation, geothermal, and hydrogeothermal cooling, thermal shading and insulation, among other methods. Santamouris and Kolokotsa [40] reviewed three main passive cooling dissipation techniques (i.e., ground, convective, and evaporative cooling), assessing their contribution to improving indoor environmental quality and reducing cooling needs. More recently, Firfiriset al. [15] conducted a similar review of passive design approaches but dedicated to livestock buildings. In summary, passive cooling (and other design strategies) have consistently shown to be effective at reducing thermal loads, improving energy performance and indoor environmental conditions [11].

While the scope of most building energy efficiency research is on individual buildings, any constructed facility is an active part of a broader urban context or ecosystem, interacting with its various components (e.g., neighboring buildings, vegetation, and microclimate) [54]. This has motivated the need for sustainable planning approaches that promote – at the urban scale – good mobility, efficient use of resources, and overall quality of life [21]. Among the different principles of sustainable urban form, “compactness” and “density” are two principles of particular interest to this study as they can directly impact the energy performance of individual buildings, bridging the gap between the urban and building level scales [47]. Previous studies have explored relationships between compactness, density, and building performance, as detailed next.

Starting with the definitions of the two concepts, Jabareen [21] describes “compactness” as promoting the placement of urban development (i.e., buildings) adjacent to existing urban structures. “Density”, on the other hand, aims to increase the ratio of people or dwelling units to land area (e.g., through taller buildings). The levels of compactness and densification of developments are typically determined by land-use control tools, such as zoning and development codes, among others [52], [37]. For instance, traditional zoning commonly designates the land use of a district into types of development (e.g., residential or commercial), acceptable densities for the district, and acceptable variations (e.g., single-family or multi-family residential developments). Building codes determine the allowable extent of development by imposing restrictions, such as setbacks and proximity to other developments. On an urban scale, the above-mentioned land-use control tools impact the local and regional accessibility, livability, and energy consumption patterns of the residents [52], [37].

In terms of research efforts connecting compactness and density to energy performance, Ye et al. [57] evaluated the premise of compactness as a sustainable city form by studying its impact on household energy use in China. Using regression analysis, the authors found that accessibility to green space and water bodies were negatively correlated with household energy use. The findings highlight the importance of preserving greenery and water bodies when adopting compactness strategies. Shashua-Baret al. [42] explored strategies to mitigate the Urban Heat Island (UHI) effects in a high-density urban environment in Hong Kong. The authors found that the building form can have a significant thermal effect on the urban canopy layer. For high-density residential blocks with low green spaces, they also recommend planting trees in the areal wind paths to maximize the greenery cooling benefits and minimize the UHI effect. Similar results were observed by Tan et al. [45], who estimated that vegetation arranged in wind corridors doubles the cooling effect on air temperature and sensible heat, compared to vegetation in leeward areas. Jiang et al. [23] studied the influence of urban built form on mean wind and turbulent characteristics. Using Large Eddy Simulations (LES), the authors show that building layout, spacing, and height have important impacts on wind patterns. These patterns, in turn, can have important effects on building energy performance [26]. Wong et al. [54] evaluated the impact of air temperature variation from buildings, greenery, and pavement, on building energy consumption. Among the different simulated scenarios, the scenario with the “highest surrounding buildings” resulted in the lowest ambient temperature. The authors argue that the increase in surrounding building height reduces the Sky View Factor (SVF) and provides the needed shading to generate a cooling effect in the urban canopy. In a recent study, Trepci et al. [47] used urban building energy modeling to simulate the performance of a Transit Oriented Development (TOD) and test different compactness and densification scenarios. The authors found that compactness and densification can increase or decrease energy intensity, depending on whether the energy intensity is calculated per unit area or per dwelling unit. Despite its interesting findings, the study was conducted at the urban level and did not isolate effects (e.g., shading) that neighboring buildings have on each other.

The following studies provide more direct attempts to study the impact of the urban built context on building energy consumption. Starting with Steemers [43], the authors studied the impact of urban density on the energy performance of office and domestic buildings in the United Kingdom. Results show that urban density reduces heat losses in building envelopes but can increase energy consumption from reduced solar and daylight availability. An important limitation worth noting of this study is its reliance on the LT (lighting and thermal) calculation method, which is considered a rather simplistic computational method that “should not be regarded as a precision model producing an accurate estimate of the performance of an actual building” [6]. Furthermore, it is unclear if similar results would be observed in warmer climates. Vartholomaios [53] conducted a sensitivity analysis of different urban forms to quantify their impact on heating and cooling loads of a residential building in Greece. The parametric variation covered various urban typologies, number of open space width, orientation, block length, among others. The authors found and recommended two energy-efficient design strategies: high compactness of building volumes and southern orientation (to maximize winter solar gains). Salvati et al. [38] studied the influence of different urban compactness levels and local climate on the cooling and heating loads of an apartment building in the city of Rome. The authors found that that the most compact and dense urban textures are more energy-efficient than sparser and less dense urban patterns, which are more common in recent European urban developments. Javanroodi et al. [22] modeled the urban morphology of the city of Teheran and studied the impact of urban density, patterns, and building form on the cooling and ventilation loads of a high-rise commercial building. Using Computational Fluid Dynamics (CFD), the authors conclude that higher urban density increases average wind speeds, resulting in lower monthly cooling loads. Han et al. [17] introduced the concept of the Inter-Building Effect (IBE) to evaluate the mutual impact that spatially proximal buildings have on each other. Using dynamic simulations of cross-regional and real urban cases, the authors found that shading IBE effects have a more significant impact on building energy consumption than reflection IBE effects.

Among their various benefits, compactness and density have the potential of reducing building energy consumption by leveraging the shading effects of neighboring buildings. Such effects can effectively reduce solar heat gains and corresponding air conditioning loads [17], [43]. The previous section highlighted important research efforts on the topic. However, the listed studies have important limitations that motivate the need for the current work, which include:

• Lack of quantification of direct inter-building shading effects (e.g., [57], [42], [45], [23], [54], [47].

• Limited consideration of joint effects of urban context characteristics on building performance, particularly context’s height and distance (e.g., [43], [38], [17].

• Limited coverage of multiple building types and sizes, which can exhibit different interactions with urban contexts (e.g., [43], [53], [38], [22], [17].

• Focus on total energy performance and lack of consideration of peak cooling loads, which are essential data points for the design and sizing of HVAC systems, as well as energy-generation sources (e.g., [38], [22], [17].

This paper aims to fill the mentioned gap by presenting a systematic quantification of the impact of the built urban context on the energy performance of buildings in a hot climate environment, mainly through inter-building shading effects. The study is comprehensive in its coverage of different building types (i.e., commercial and residential), sizes (i.e., low-rise, mid-rise, and high-rise), urban context (i.e., distance/spacing, height, and orientation), and building performance metrics (i.e., consumption per end-use and peak loads). The results and discussions provide unique insights for both the planning and the building design/construction community by connecting macro (urban) level policies and best practices with their resulting impacts on the micro (building) level.

Prior to proceeding with the methodology section, it is essential to acknowledge that the scope of analysis in this study is set to the impact of inter-building shading effects on building performance. The influence of the urban context on building performance through other phenomena, such as the UHI effect [42] or disturbances to wind patterns [41], is not considered in this paper but can be part of future expansions of the work. The proposed modeling and analysis framework was developed in a generic manner to ease the expansion of its capabilities and its applicability to any other individual or group of buildings.