Designing Green Walls: An Early-Design Framework to Estimate the Cooling Impact of Indirect Green Walls on Buildings in Six Different Climates
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AbstractGreen walls are a component of urban green infrastructure and if designed properly, require only moderate human intervention and maintenance during their lifespans (Cameron, Taylor, & Emmett, 2014). The benefits of green walls are numerous for both building and urban scales.
Green walls reduce building heat gain by providing shade (Ip, Lam, & Miller, 2010) and increasing surface albedo (Holm, 1989). They offer thermal insulation for buildings by acting as wind screens and cavity walls (Susorova, Azimi, & Stephens, 2014) and improve indoor air quality by trapping airborne pollutants (Ottelé, van Bohemen, & Fraaij, 2010). Furthermore, they negate the urban heat island effect through solar radiation interception and transpiration (McPherson, Nowak, & Rowntree, 1994). Green walls are also an effective solution for storm water management, provide ecosystem services, and improve the quality of human life (Meier, 1990).
A literature review from this field highlighted three major problems: (a) lack of an effective method for making research findings useful to practitioners, (b) limited understanding of the morphological and biophysical characteristics of vines, and (c) lack of a standardized research methodology across the field.
The objective of this research is to provide practitioners with a series of matrices to easily and quickly evaluate the cooling efficacy of indirect green walls during the early phase of a project. These matrices account for the biophysical traits of the plants used, canopy geometry, and environmental variables for six climatic scenarios.
The climate scenarios are based on summer conditions (e.g., ambient temperatures, precipitation, and relative humidity) and are broken down into the following climates: cool and humid, cool and dry, warm and humid, warm and dry, hot and humid, and hot and dry.
Special attention is given to plant biophysics and performance evaluation methods used in fields such as biophysical ecology and agronomy. The cooling effect of green walls is broken down into two major components: transpiration (W/m^2) and solar radiation interception, or shading (W/m^2).
A modified version of the FAO 56 method was used to evaluate the cooling power produced. FAO 56 is a method developed by the Food and Agriculture Organization of the United Nations to estimate the transpiration rate of crop fields. Additionally, a library of stomatal resistances for 97 vine species expanding through 13 countries was created from measurements from existing field studies.
Variables impacting the transpiration rate were reviewed and possible green wall designs corresponding with the maximum cooling power for each climate condition were investigated. The results show that canopies with a resistance of lower than 100 s/m, a height of 3 to 6 m, and Leaf Area Index (LAI) of 3 or larger produce the maximum possible cooling power. Taking into account the aforementioned design considerations (LAI and height), and average stomatal resistance, it is estimated that the largest cooling power (300 W/m^2) corresponds with the hot and dry climate. The smallest cooling power (50 W/m^2) corresponds with the cool and dry climate.
Similarly, variables impacting the shading effect of indirect green walls are reviewed. A technique for estimating the extinction coefficient of a canopy by combing empirical and statistical methods is introduced. The predicted solar interception calculations show good agreement with field study measurements, with only ±10% margins of error.
Three sets of matrices for designers are introduced. The first two sets provide designers with the cooling power values (W/m^2) of various green wall designs in six climatic scenarios through transpiration and solar radiation reduction. These values show good agreement with the findings of other studies. The results show an average summer cooling power of 42 W/m^2 for the cool and dry climate and 176 W/m^2 for the hot and dry climate. These two sets of matrices are intended to provide designers with back-of-the-envelope estimations of the cooling power of green walls during the early design phase of a project.
The last set of matrices provides designers with the cooling effects of green walls through transpiration and solar radiation interception as a percentage of total incident solar radiation received by the canopy in each climatic scenario.
The cooling power from transpiration accounts for 20% to 30% of the cooling effect of green walls for cool and dry, cool and humid, and warm and humid climates. The contribution from transpiration increases to 48% for warm and dry climates, and 52% for hot and humid climates. The largest contribution from transpiration occurs in hot and dry climates (99%) due to the oasis effect.
For almost all climates, the cooling effect of solar radiation interception was approximately 70%, 80%, and 90% for Leaf Area Indices of 3, 4, and 5, respectively.
The results show that the total cooling power of green walls exceeds 100% for all climates. The canopy not only provides shading, but also acts as a heat sink by storing solar radiation energy in water and releasing it to the environment as vapor via latent heat transfer.
The largest cooling power values correspond with the hot climate, followed by the warm and cool climates. The percentage of cooling power from transpiration in dry climates is larger than that in humid climates. The exception is the cool and dry climate. This exception is due to the high canopy resistance associated with the cool and dry climate.
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