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Review

A Systematic Review on the Existing Research, Practices, and Prospects Regarding Urban Green Infrastructure for Thermal Comfort in a High-Density Urban Context

1
Institute for Sustainable Industries & Liveable Cities, Victoria University, P.O. Box 14428, Melbourne 8001, Australia
2
College of Engineering and Science, Victoria University, P.O. Box 14428, Melbourne 8001, Australia
*
Author to whom correspondence should be addressed.
Water 2022, 14(16), 2496; https://doi.org/10.3390/w14162496
Submission received: 11 July 2022 / Revised: 6 August 2022 / Accepted: 10 August 2022 / Published: 13 August 2022

Abstract

:
Urban green infrastructures (UGI) have been suggested as a natural solution to tackle the problem of human thermal comfort as well as to reduce energy consumption in buildings under the pressures of rapid urbanization and global warming. However, the acceptance of UGI to mitigate the urban heat effect is not yet universal. The development of such an infrastructure is also not consistent across the regions, emphasizing the different objective parameters and methodologies. A systematic review has been conducted to analyze the published research work on UGI, targeting thermal comfort, in the past decade to identify the trends of UGI development around the world. The result shows that most of the studied locations were situated around the Mediterranean Sea region in a temperate climate, and most of the studied cities are within countries with a high gross domestic product, large urban area and urban population, primary energy consumption, and high greenhouse gas and carbon dioxide emissions. Extensive green roofs are the most popular type of UGI and mostly use Sedum plants. In the published studies, experimental setups are the most common methods by which to collect data. EnergyPlus is the most popular software used to conduct energy analysis for buildings, whereas ENVI-met is more commonly used for microclimate analysis. These results indicated that the direction of UGI studies is driven by climate characteristics and the socioeconomic factors of geographical location, which favor low construction cost and maintenance needs, with a minimal irrigation requirement for small-scale UGI projects. Understanding the trend of UGI approaches for thermal comfort allows researchers to standardize practices that help the decision-making process for future researchers while recognizing the limitations and potential of current UGI practices. It is recommended that future studies should include arid and equatorial climate regions, with more focus on large-scale projects including high-rise building environments to comprehensively evaluate the effectiveness of UGIs.

1. Introduction

Driven by population growth, rapid urbanization has become a common phenomenon in cities around the world. According to the United Nations World Population Prospects [1,2], the projected global population will reach above 8.2 billion by 2030, while over 5 billion people will be living in urban areas. The urban sprawl has irreversible effects on the existing environment and society that cause negative impacts on human health and wellbeing [3]. The high-rise, high-density architecture of the inner city often leads to urban heat island (UHI) effects, which eventually decreases human life expectancy by causing heat-related illness and diseases [4].
To counter the UHI scenario or extreme heat events, urban dwellers tend to pursue thermal comfort through the extensive use of heating ventilation and air conditioning (HVAC) systems, which triggers a general increase in the city’s energy consumption. In Australia, the HVAC system accounts for 40% of the total building energy consumption of a typical office building [5]. Worldwide, as estimated by the International Energy Agency (2018), air conditioning accounts for 10% of all global electricity consumption and is expected to be the second-largest source of global electricity demand by 2050 [6]. The use of an HVAC system is also one of the main contributors to greenhouse gas (GHG) emissions, especially hydrofluorocarbons (HFCs), which have depleted the ozone layer [7]. According to Climate Watch data (2021), the “electricity and heat” sector was the largest producer of global GHG emissions, contributing around 32% in 2018 [8].
GHG emissions are the main driver of global warming, escalating the negative impacts of climate change and will present additional challenges to maintaining human wellbeing [9]. The World Meteorological Organization (2021) predicts that there is an increasing likelihood that the annual average global temperature will rise by 1.5 °C in the next 5 years. This prediction is in line with the estimations made by the Bureau of Meteorology (BoM) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) [10], who has forecast that Australia will experience more frequent and more severe weather events with a warmer and drier climate in the future. The expected higher temperatures will lead to more frequent heatwaves and bushfires [11,12]. The situation may worsen far more profoundly than it appears at present. The bushfires drastically raise the levels of pollutants and toxic fumes in the air, which are extremely harmful to both humans and animals [13]. The situation is even worse in urban areas, as high-rise buildings create barriers that constrain the airflow, meaning that air pollutants will be trapped and recirculate in urban areas for a longer duration in comparison with rural areas [14]. Under such circumstances, people are advised to stay indoors to avoid the toxic smoke and to rely on mechanical ventilation to maintain indoor air quality [15].
The combined threats of urbanization and climate change create pressures on maintaining human wellbeing and energy demands. Although the current technology using an HVAC system is capable of providing thermal comfort for an urban population, it is inevitable that users will generate GHG from this approach; this considerable amount of GHG contributes to climate change, which further speeds up global warming and creates the vicious cycle illustrated in Figure 1. An alternative sustainable solution is urgently needed for a safe and resilient living environment.

1.1. The Role of Urban Green Infrastructure (UGI)

The elements of urban green infrastructure (UGI), such as green roofs, green walls, and green facades, represent a natural base solution to break the vicious cycle shown in Figure 1 by providing thermal comfort to urban residents without placing an additional burden in terms of urbanization and global warming. Integrating UGI into urban design can improve the urban microclimate, achieve energy-demand savings, and create temperate outdoor spaces [16,17]. Studies show that UGI is effective for controlling UHI and improving air quality; it leads to positive health effects by reducing asthma, cardiovascular and respiratory disease, obesity, and circulatory disease [18,19,20]. The indirect positive health impacts associated with socio-economic factors include child cognitive development, elderly longevity, and strengthened immunity [21]. The incorporation of UGI in urban design can help cities to tackle the challenges of limited access to resources and a lack of green space due to urban development, but the success of UGI implantation requires input from the city authorities, businesses, and other institutions, working together to investigate different options to adapt a variety of urban spaces [22].
Although some cities have adapted their UGI as part of urban planning to mitigate urban heat effects, progress is variable and depends on region. As mentioned by the European Commission, the core European cities have an average of 40% surface area that is given over to UGI, yielding around 18 m2 of publicly accessible green space per inhabitant, with 44% of the urban population living within 300 m of a public park [23]. However, when considering individual cities, the cities around the Mediterranean Sea offer less than 9 m2 per person of green space on average, while central and northern European cities have more than 20 m2 per person on average [24]. Similarly, in the United States, San Francisco was the first US city that required new residential and commercial buildings to have at least 15% of the roof area covered by green roofs or solar panels [25]. Despite the fact that the city’s urban policy was in favor of UGI development, San Francisco has allocated only around 20 m2 of green space per inhabitant, whereas Atlanta has allocated more than 100 m2 of green space per person, which is seven times more than in New York City, which has just 13 m2 per person [26]. There are several factors that stop people from installing a green roof on an existing building. As the rooftops are usually used for the HVAC system equipment, and the installation of a green roof requires retrofitting to ensure that the roof load-bearing capacity is satisfied, the property owner must also have the option to choose between a green roof, cool roof, or solar panels [27]. Therefore, it is necessary to understand the trends of UGI practice worldwide to mitigate thermal comfort in an urban environment.

1.2. Research Objectives

Although there are examples around the world of successfully integrated UGI schemes for thermal comfort, the acceptance rate, adaptation, and popularity of UGI are still not up to the mark. There are more cities and countries still struggling to establish a clear agenda and consistent policy regarding the implementation of UGI. There is a clear consensus among the researchers that the reasons for the slow progress of UGI development are mainly because of the competition for space in cities, the difficulty of finding finance, uncertainty regarding the economic benefits of GI, the complexity of dealing with a living infrastructure, a lack of policy and standardized practices, slow adoption and a lack of awareness of new ideas [28,29,30,31,32]. It is necessary to further investigate the current practice of UGI for thermal comfort to provide more information related to UGI practices, to achieve a better understanding of UGI designs, the appropriate methods, performance evaluations, and the potential parameters that can be achieved.
This study aims to provide a review of the different types of UGI that are most applicable in an urban context to gain a greater understanding of the factors that might affect UGI effectiveness in regulating human thermal comfort and energy consumption.
The following questions define the scope of our research:
  • What are the main regions and geographical areas where the concept of UGI is more popular?
  • What are the main study parameters being investigated through the research?
  • What are the main approaches adopted for studying UGI around the world?

2. Materials and Methods

A systematic review approach was used for this study and was divided into two stages. The first stage is the initial screening, which aims to filter out those articles that are irrelevant to the UGI frameworks, using specified keywords that are applicable to the selected UGI categories. The second stage is the subsequent screening, which identifies the research focus of the filtered articles from the previous stage to make sure the articles align with the research objectives. Figure 2 illustrates the workflow of this systematic review. The review process was first conducted on 16 January 2022; the SCOPUS and the Web of Science (WoS) search engines were used to maximize the selection of potential articles with open access.

2.1. Types of UGI

Different countries might use different definitions and configurations to categorize UGIs, based on the existing practice and current building standards. For example, in Australia, there are only extensive green roofs and intensive green roofs; semi-intensive green roofs are not considered. The dimensions also vary between the design guidelines [33,34]. As mentioned by Koc et al., there is a lack of studies that concentrate on the classification of UGIs from a climatological perspective [35]. Table 1 provides a brief explanation of the features of each UGI category and subcategory:

2.2. Initial Screening of the Literature

The initial screening process focuses on those articles that are related to the framework of UGI concepts and filters out papers on irrelevant green infrastructure. This study only focuses on the UGI that is most suitable for a high-density urban context and is applicable to high-rise buildings. Only three main categories were considered: the green roof (GR), vertical greenery systems (VGS), and urban agriculture (UA). As there are corresponding subcategories extending from these three main categories, the detail of the specific keywords is provided in Appendix A. We applied a search filter that limited the selection to documents using the English language, and restricted the document types to only articles, conference papers, and proceedings papers. Eventually, there were 401 results based on GR keywords across two platforms, 145 results were based on VGS keywords, and 172 results were based on UA keywords.

2.3. Subsequent Screening of Filtered Literature

The subsequent screening specifies a research focus that is related to the thermal comfort or electricity energy aspects of the research; the typical keywords for each aspect and the exact searching codes used in Scopus and the WoS are provided in Appendix A. This screening stage was begun by narrowing down the three main UGI categories into two specified areas; a total number of 674 papers were extracted from Scopus and the WoS from the initial screening, while 307 duplicated papers across two platforms were removed. As a result, a total of 217 articles were reviewed (see Appendix B); of these, 112 articles focused on the thermal comfort aspect, 86 articles focused on electrical energy, and 19 articles investigated both aspects. Figure 2 illustrates the filtering and screening process of the articles.
Critical evaluation criteria for the screening process are as follows:
  • The study UGI type must correspond to this research scope.
  • A realistic study location can be identified.
  • The methodology has been validated and used practically in the research.
  • The study focuses on either thermal comfort or electricity energy consumption.

2.4. Establishment of a Bibliography Network Based on Keywords

After the screening, a bibliography network was used to demonstrate the relationship between keywords and the link between topics. The bibliography network used the co-word analysis method on the VOS viewer [41]. A bibliography network is a technique that helps to examine the content of publications by organizing the author’s keywords, based on the frequency of the words that appear together, assuming that the words have a thematic relationship with each other [42]. Figure 3a on the left shows the strength of the link between keywords and was illustrated by the thickness. For example, the word “green roof” has the strongest link to “energy saving”, after which “cool roof”, “thermal performance”, and “urban heat island” appear. The phrases “vertical greenery system”, “green façade”, and “green wall” occurred equally often, with a similar strength of link.
Note that the author keywords are chosen by the authors themselves as being most representative of the content of the publication, while indexed keywords are chosen by the publisher (in this case, Scopus and the WoS), which are standardized to vocabularies including synonyms, alternative spellings, and plurals derived from thesauri that are owned or licensed by the corresponding publisher/company [43]. While author keywords provide a clearer and simpler thematic relationship between the various keywords since authors are more likely to choose the words that more directly fit and best describe their research scope, the indexed keywords in the bibliography provide a wider perspective on the potential connections between topics that might involve the keywords.

3. Analysis and Discussions

The systematic review for this research provides an in-depth analysis of UGI for thermal comfort, covering the geographical distributions, design parameters, and the methodologies/approaches used to cover the study objectives.

3.1. Geographical Distribution of Study Sites

Many studies have been conducted in the last decade to investigate the impacts of UGI in terms of thermal comfort and energy consumption. A study in Turin, Italy showed that the use of green roofs has the potential to decrease the land surface temperature by 2.7 °C, with an energy saving of approximately 14 GWh/year [44]. In Toronto, the average reduction in peak temperatures at the pedestrian level ranged between 0.4 °C and 0.7 °C when using green roofs, resulting in an energy saving of 11.53 kWh/year per unit area [45]. The green roof offers distinct improvements in terms of both thermal comfort and energy consumption; however, the performance differences between two cities can be huge. It is important to investigate the characteristic of a geographic location to identify potential factors that might influence the performance of the UGI.

3.1.1. Regional Trends

The map in Figure 4 illustrated the locations and the corresponding climate zones of the studied cities. The results show that up to 89% of the studied sites are in the northern hemisphere, with 8% of the studies being located around the equatorial regions; only 3% of studies have been conducted in the southern hemisphere.
Figure 4 also illustrated the distribution according to climate zones, based on the Köppen–Geiger climate classification system [46]; the definition and color scheme of each climate zone are provided in Appendix C. The majority of the studies were conducted in a temperate climate (Type C), as mentioned in 107 articles, while equatorial (Type E), arid (Type A), and continental (Type D) climates shared a similar number of studies, with 21, 24, and 26 articles, respectively. There is as yet no study conducted for a polar climate, mainly because suitable vegetation will not survive in such conditions. Under each climate type, the subsequent climate zone can be defined by the figures for precipitation and temperature. Figure 5 provides the distribution of the studied sites according to the individual climate zone. The most frequently studied climate zone is Csa (a temperate climate with dry and hot summers), with 74 articles; these studies are concentrated around the Mediterranean Sea region, which has dry and hot summers. Both Cfa (a temperate climate with a fully humid and hot summer) and Cfb (a temperate climate with a fully humid and warm summer) have a similar number of studies, at 46 and 44, respectively. The studies on these climate zones are mainly conducted in Europe, the east coast of Asia, and a scattering around North America. In terms of the Type D climate, the climate zone Dfb (a humid continental climate with warm summers) is studied most frequently, with 23 articles scattered around North America and northeast Europe. In terms of the Type A climate, the climate zone BWh (a deserts climate with dry and hot weather all year) is examined in 21 studies that are mainly located in the Middle East and the west coast of North America.
Different climate zones require specific considerations that will greatly affect the UGI design parameters. For example, UGI presents a good passive cooling strategy in arid climate areas; however, the design requires more irrigation as the annual rainfall is limited. Cities around the equatorial region have less distinct or even have no seasonal changes compared to other climate zones; instead of having four seasons, the climate is usually divided into wet and dry seasons or the monsoon season. Therefore, extra considerations regarding the loading are required in the GR design, due to the excess rainfall. Unlike a GR, a VGS is more affected by the building’s orientation; the vegetation in a different orientation might grow differently because of changes in the daily solar path. A change in location between the northern and southern hemispheres will have an opposite impact on VGS. Since most of the studied sites were in the northern hemisphere, most of the VGS studies were focused on investigating the south- (32%) and west-facing (24%) facades, only 11% of VGS studies have investigated all four orientations (north, east, south, and west), and only 1 paper simulated eight orientations.

3.1.2. Top-Performing Countries and Cities

Each country and city has a different level of progress in UGI development, depending on the corresponding socioeconomic factors and the level of government interest. The top ten countries with the highest numbers of UGI research publications are shown in Figure 6. From the systematic review, the results show that China has published the most studies in the last decade, with 41 articles; most of the studies in China were conducted in Hong Kong, Shanghai, and Guangzhou. Italy is the second most popular country in terms of UGI studies, with 40 articles; Bari, Rome, and Catania are the most popular Italian cities in terms of UGI research. The third country in terms of the most UGI publications is the United States of America (USA) with 26 articles; Chicago, Los Angeles, and Phoenix are the most frequently studied US cities.
In total, 177 separate cities across 50 countries around the world were identified; as there are some cities that have been studied more than once, a total of 310 counts in terms of the studied locations were recorded. The map in Figure 7 illustrates the distribution of the studied cities and the number of publications that are represented. It shows that most of the studies are conducted in European countries and those countries surrounding the Mediterranean Sea, typically, the United Kingdom (UK), France, Spain, Italy, Egypt, and Greece. Away from Europe, the majority of UGI studies are conducted in southeast Asian and northern American countries, such as China, India, Singapore, the US, and Canada. The rest of the world shows relatively fewer publications that are scattered around the globe; only Brazil, Mexico, and Australia exhibit slightly more UGI studies than the other countries.
To understand why these countries are more advanced in terms of UGI research compared to the others, the following factors have been considered:
  • The overall number of publications, by country;
  • The gross domestic product (GDP);
  • Urban population;
  • Urban land area;
  • Primary energy consumption:
  • Greenhouse gas and carbon dioxide emissions [47].
Based on the above factors, a normalized weight for each factor was generated to compare the patterns in the top 20 countries with the highest numbers of research publications. The normalizing factors are shown in Figure 8, with the trendlines identifying similar patterns. China, the US, Italy, India, Canada, and Brazil demonstrate stronger relationships with the above factors. It is worth mentioning that Germany and Japan were not on the top ten list of countries. With further investigation, it was found that although Germany and Japan do have a fair number of UGI studies, the study focus of this research is on the combined parameters of “electrical energy” and “thermal comfort” performance, in which areas Germany ranks below 20 and Japan ranks 14. As Germany and Japan are not among the top ten countries in the screening process of this research, this does not affect the primary conclusions of this study.
In the case of the Russian Federation, UGI is not the focus of research in the country because of various factors; it has a low urban land area percentage and UGIs do not provide many benefits in an area with an extremely cold climate.

3.2. Study Parameters

Different types of UGI require different design parameters to achieve ideal effectiveness; understanding the current practices will help to standardize the design parameters in the future. Figure 9 shows the number of reviewed papers that have been published between 2012 and 2021, according to the individual UGI categories. Figure 9 shows a general increasing trend for all three UGI categories. There are far more publications studying the GR types, compared with the VGS and UA types.

3.2.1. UGI Categories and Types

The Venn diagram in Figure 10a illustrates the proportions of the three different types of UGI from the 217 reviewed papers. The majority of papers focused only on GR, with 139 papers in total. In total, 75 papers focused only on VGS, and only 9 papers focused solely on UA. A small number of papers studied more than one UGI type, with 14 papers studying both GR and VGS and 2 papers studying both GR and UA, while 6 papers studied both VGS and UA; none have studied all three types of UGI.
The subcategory distribution of UGI types is illustrated in Figure 10b. The most popular type of GR to be studied was the EGR (48%), while the SemiGR shared a similar number of studies as the IGR, which contributed 4% and 5%, respectively. BRs only appeared in 1% of overall studies, while the remainder are not specified (NS). Although this study showed that IGR can achieve a better performance in terms of improving thermal comfort and reducing building cooling energy demand, the construction and maintenance costs favor the use of an EGR [48]. Articles studying VGS have a relatively balanced distribution across the different types of VGS; the most popular to the least popular are GF, GW, and DSGF, respectively, while the remainder are not specified. Only a small number of papers investigated the UA type, making it difficult to group them into a particular structure type of UA, as each of these papers used a different typology that is hard to classify (for example, edible green roofs, rooftop farms, etc.), although the RTGH has been used in more than one paper. Overall, 15% of the combined studies are not specified in terms of the subcategories of UGI types, and a few studies on VGS have not used the terminology of GW and GF. This finding further supports the need for a standardized classification scheme, as suggested by Koc et al. [35].

3.2.2. Scale and Height Parameters

Scale is important when planning and designing the UGI. In terms of city planning, the scale would determine the applicable UGI type and the suitable building types, as well as the level of impact on the surroundings [49]. Therefore, this paper investigates the scale and building types to understand the trends. Table 2 provides the range of each scale level of UGI, along with a brief description. Figure 11a shows the scales of UGI that have been studied; almost half of the UGI schemes are on a microscale (48%), while about one-third are at a local scale (35%), with 10% in the neighborhood scale, and 6% on the city scale. The trend shows a constant pattern, where smaller-scale structures were more common than a large-scale study. This result is also similar to that of another review study that focused only on GR [50]. A case study regarding GW in Hong Kong showed that a 100% increase in greenery coverage could potentially achieve an 88% saving on cooling load [51].
Similar to the scale, the height of the UGI or, otherwise, the height of the studied site also impacts the effectiveness of the UGI. As claimed by Dahanayake and Chow [51], GW provides a higher potential for cooling load reduction when the building height increases. For example, if the study is focused on outdoor thermal comfort, it should prioritize human comfort at the pedestrian level, although some studies claim that a GR will not provide a direct cooling effect at the pedestrian level; however, the existing surrounding buildings will still have an effect on factors such as air velocity and solar angle, etc. [52,53]. Alternatively, if the study is focused on investigating indoor thermal comfort, the building height might not be the main concern; however, the number of levels/floors of the building should be taken into account, as the thermal performance might be different on the upper floors compared with the lower floors. Consequently, the owners of a rooftop terrace on a multi-story residential building will often have the most interest in installing a green roof [54]. Figure 11b shows that the majority of the published studies, about 73%, focused on investigating the UGI in low-rise buildings (less than 4 stories or 15 m tall), compared to the UGI in high-rise buildings, which appeared in only 19% of the published studies.
Building types represent the scale and function of a building, the occupancy level, and the activities within it; hence, the energy demand can be estimated. For example, one study simulated the cooling energy demand of a primary school, with estimations of the occupant numbers and the assumption of mechanical ventilation in the computer lab and staff rooms [55]. Figure 12 shows the ratio of building types that have been studied, as well as the distribution of the building types in relation to the study scales. It shows that most micro-scale papers studied a test cell or prototype structure. In an experimental study, test cells were used with a thermal scenario that was established based on an office profile to evaluate the internal heat load [56]. Although the use of test cell and prototype structures is more economically feasible for conducting basic analysis in the early stages of a study, it might not reflect the actual scenarios at full scale on a real building.

3.2.3. Seasons of Study Focus

Depending on the UGI application and design focus, the performance of the UGI might vary according to seasonal changes. Figure 13 shows the seasons that were studied in the reviewed papers. In terms of both thermal and electricity energy performance, the majority of the studies were focused on summer periods, with 62% for thermal performance and 46% for electrical energy performance. Winter is the second most popular study period in terms of thermal performance, at 21%. However, for electricity energy performance, the annual performance was slightly more popular, with 26% of papers in comparison to the winter period alone, at 24%. There are only a small number of studies that have focused on spring, autumn, and monsoon seasons. The results show that most studies only focused on either summer or winter, which might not represent the annual performance of the UGI. For example, one study shows that the annual HVAC energy consumption with a green roof performed better in summer and winter; however, the spring and autumn underperformed, resulting in a 3% worse performance than a conventional roof with the same effective thermal insulation in terms of annual HVAC energy use [57]. Another study also suggested that detailed seasonal and annual analyses can help to determine the best irrigation schedules with the highest reduction in energy demand [58]. Nevertheless, the lack of studies on transitional seasons might lead readers to overestimate the UGI performance.

3.2.4. Plant Characteristics

The plant characteristics of GFs vary depending on the objectives, locations, and climate of the study area. In total, 150 research papers mentioned the plant types that were used for the investigation of UGI; 55 studies out of 150 only provide the genera of the plant types, while an average of 3.4 plant species types are investigated per article. The maximum number of species that were investigated was 32 plant types. There were 83 articles using the leaf area index (LAI) as the main parameter; while 56 articles have the actual measurements of the LAI of specific plant species, the other papers either assumed the LAI values or used a reference value from other papers with the same plant types.
The most common plant types used in a green roof are sedums and grass, lawn, and herbaceous plants. Most studies also investigated at least four different types of plants to compare the results. Most VGS utilized climbing plants and creepers as they naturally grow vertically, needing less substrate or supports along the building façade. There are 57 articles on VGS that investigated the plant types, including the evergreen climbing ivy (Hedera helix) and Boston ivy (Parthenocissus tricuspidata), which are relatively popular, with 7 and 8 articles, respectively. In Italian cities, a climbing shrub named Rhyncospermum jasminoides and a variegated form of a vine called Pandorea jasminoides were studied extensively and appeared in 7 articles.
The selection of plant types is dominated by multiple factors; for example, the climate zone where the UGI is located affects the survivability of the plants and the requirements for maintenance, especially in terms of irrigation water, growing substrate, and supporting structure [59,60,61]. The types of UGI determine the growing environment of the plants [62,63]. The orientation of the building is particularly important because VGS relies on the vegetation providing shade to reduce the thermal heat by controlling the amount of incoming solar radiation [64]. For example, some plants can achieve a better LAI value to reduce the incoming solar radiation, while other plants can achieve a better evapotranspiration rate [56].

3.3. Approaches

In this review, the approaches of the screened articles can be separated into two stages, the data collection stage and the data analysis stage. The typical methods and tools that were most often used for these two stages are illustrated in Figure 14.

3.3.1. Data Collection Setup and Tools

As shown in Figure 14, there are two main data sources—primary and secondary data. Primary data is collected by either remote sensing or an experimental setup. Remote sensing refers to technology using satellites, aerial vehicles, infrared thermal cameras, or light detection and range (LiDAR) equipment to provide topology and geometry information; these tools allow researchers to generate information for a large area simultaneously [44,65,66]. An experimental setup refers to using a test cell/cube or prototype that has been built or set up in a dedicated study area and that is solely used for research objectives [67]. Sensors and equipment setups are usually long-term or even permanent and are secured in position with limited disturbance. Although an experimental setup is more flexible so that the researcher can set up their own parameters, it requires the researcher to have a good understanding of operating the equipment correctly. Secondary data refers to collecting historical data from literature reviews, databases, or a third-party provider. The advantage of using historical data is that of time efficiency; it allows the researcher to have faster access to information, and to conduct the analysis without physical fieldwork. Figure 15 shows a bibliography network based on the sensors that were used in the reviewed articles, to illustrate how different types of sensors are associated with each other. It shows that the most commonly used sensors are thermocouples, heat flux sensors, pyranometers, anemometers, temperature-humidity sensors, and weather stations. These sensors are mainly used for collecting meteorological data, particularly the air temperature, relative humidity, heat flux of materials, wind speed and direction, and solar radiation.

3.3.2. Data Analysis Software and Models

From the reviews, two main types of data analysis methods appear and can be grouped into spatial and numerical analyses. The spatial analysis is associated with geographical information systems (GIS) such as Landsat, the single-layer urban canopy model (SLUCM), and the weather research forecast (WRF) model [68,69], as well as three-dimensional building modeling using computer-aided design software, such as Revit, DesignBuilder, and Solidworks. Numerical analysis is useful for creating building energy simulations (BES), microclimate analysis, and computational fluid dynamics (CFD). A building energy simulation is useful to analyze energy consumption and demand, indoor climate, and the indoor thermal comfort of an individual building [70]. From the reviewed articles, EnergyPlus is the most popular software used for building energy analyses, with 69 studies. Another commonly used software program for building energy analysis is TRNSYS, a CFD software program that is used in 13 studies. A microclimate simulation is commonly used for thermal comfort analysis. ENVI-met is the most popular program, with 22 articles. Figure 16 represents the popularity of the software.
Note that statistical software such as Excel, SPSS, and CoStat are not always considered simulation software, since the majority of studies would simply generate graphs/figures from raw data without using simulation modeling. However, with the appropriate coding, coupling, and plugin mathematical modules, such as the Sailor’s model or the FASST (Fast All-Season Soil Strength) model to program the software, based on equations and formulas, it will be able to run simulations for analysis [58]. Another study has combined the MATLAB, KASPRO, and TRNSYS systems to calculate the energy balance of investigated greenhouse structures [71].
The use of software is related to the scale of the UGI. For example, the WRF model is typical for mesoscale analysis, while ENVI-met is appropriate for microclimate-scale analysis [72]. EnergyPlus and DesignBuilder are more suitable for building and indoor scales [73,74,75]. As there are more small-scale UGI studies, the use of software for the building scales analysis is thus more common. As a result, when the study needs to analyze both indoor and outdoor environments on different scales, it will require more than one software type to complete the analysis. For example, one study used the ENVI-met to obtain the outdoor microclimate data for an urban area as the input parameter in TRNSYS for building a scale energy analysis [52].

4. Conclusions

About 60% of the studied location were in temperate climate zones (Group C), more frequently than all other climate zone groups combined. In particular, the most studied sites were in a hot-summer Mediterranean climate (Csa), a humid subtropical climate (Cfa), or a temperate oceanic climate (Cfb). The most common features of these climate zones are that they are relatively warm with distinct seasonal changes and constant rainfall during the year. Although the Csa region might experience a dry summer, overall, the temperate climate zone provides a better environment for plants to grow, resulting in better performance in terms of thermal comfort and building energy consumption, with minimal maintenance. Most UGI studies were conducted in countries and cities with a high GDP, a high density of urban area, high urban populations, high primary energy consumption, and high GHG and CO2 emissions. Among all the countries included in the studies, China, Italy, and the USA have published the most UGI research in the past few decades; the most studied cities in these countries are Hong Kong, Bari, and Chicago, respectively. The distribution of the studied cities is concentrated in the European cities around the Mediterranean Sea, Southeast China, and the coastal area of North America.
There is an increasing trend in the number of publications that investigate the thermal and electrical performance of UGI. Green roofs are the most popular UGI types that have been studied (65%), followed by a vertical greenery system, which is the next most popular UGI (31%). There were only a few studies that investigated urban agriculture (4%). The extensive green roof was the most frequently studied UGI type in all sub-categories (48%); therefore, the extensive green roof is already the subject of more studies than both the VGS and UA categories combined. However, other studies suggested that the green roof strategy is not always the best solution to achieve thermal performance and energy-saving, as this depends on other design parameters.
Studies tend to focus on small-scale and low-level UGI structures. Nearly half of the studies were on a micro-scale (49%), then the number decreased as the scale increased from the local scale (35%) to the neighborhood scale (10%), then to the city scale (6%). Around 73% of the UGIs are located on buildings with a height below 15 m or a maximum of 4 stories tall. The most frequently studied building types are test cells or prototype structures (33%), followed by residential buildings (18%) and education buildings (15%).
This study has also investigated the sessional changes in relation to thermal comfort and energy consumption. The results show that the majority of the research into thermal comfort only focuses on either summer or winter. Although some studies have covered all the seasons, spring and autumn are largely neglected by most studies.
There is a huge variety in terms of plant types that have been studied in the past. It appears that the plant selection is highly dependent on the UGI type. For a GR, the most popular plant is sedum, while for VGS, ivy is the most common plant choice.
In terms of methodologies, the experimental method is the most common data collection method for collecting climate data, which usually consists of data from a weather station, a heat flux sensor, thermocouples, a pyranometer, and a humidity/temperature sensor. Simulation is the most common data analysis method. EnergyPlus and DesignBuilder were the most frequently used simulation software programs to analyze building energy use. Research papers studying the topic on a city scale do not usually specify the type of UGI and the plant selections. The GIS method is usually limited by the short data duration.
The current trend of UGI studies indicates that the main approaches and practices are driven by the existing physical environment and the associated economic factors. Thermal comfort performance is highly dependent on microclimate characteristics and the urban topography, but economic feasibility is the main concern when approaching large-scale UGI studies. In light of these considerations, most UGI studies are from developed countries in a temperate climate area, with the focus on extensive green roofs that will flourish with sedum plants, yielding a relatively low-cost green roof with minimal irrigation requirements.

5. Recommendations

Based on the analysis, we believe that UGI research is required in arid climates and equatorial climates, especially in the Middle East and Southeast Asia, in cities such as Riyadh in Saudi Arabic, Tehran in Iran, Jakarta in Indonesia, and Kuala Lumpur in Malaysia as these countries also have noticeable socio-economic issues, including dense urban populations, primary energy consumption, high GHG emissions, and high CO2 emissions.
In addition, future studies should have an additional focus on UGI effectiveness in arid climatic regions, while studying the dry season in equatorial regions is strongly recommended. Therefore, the UGI design parameters in the above countries should be further investigated, especially in terms of the most suitable types of UGI and the feasibility of plant-type selection. The majority of the studies included in this review are focused on small-scale UGIs on low-rise buildings, which represent a partial urban environment in mega-cities.
It is necessary to investigate the implementation of UGI on a larger city scale and include high-rise buildings as well. Developing cost-effective technology and robust procedures to conduct large-scale UGI projects is essential to facilitate large-scale studies that provide better accessibility for the planners to assess and evaluate the UGI designs.

Author Contributions

Conceptualization, C.Y.W. and N.M.; data curation, C.Y.W. and M.A.U.R.T.; formal analysis, C.Y.W. and M.A.U.R.T.; investigation, M.A.U.R.T. and C.Y.W.; methodology, C.Y.W. and N.M.; software, C.Y.W. and M.A.U.R.T.; supervision, M.A.U.R.T. and N.M.; validation, N.M.; visualization, C.Y.W. and M.A.U.R.T.; writing—original draft, M.A.U.R.T. and C.Y.W.; writing—review and editing, M.A.U.R.T. and N.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used in this study can be available on demand.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Keywords for UGI Terminology (Stage 1)Keywords for Focus Aspects (Stage 2)
Green roof category
“green roofs” OR “green rooftops” OR “irrigated green roofs” OR “wetland roofs” OR “roof gardens” OR “rooftop gardens”
Vertical greenery systems category
“vertical greenery systems” OR “green walls” OR “living walls” OR “green façades” OR “vegetation screens” OR “green curtains” OR “vegetation curtains “
Urban agriculture category
“urban gardens” OR “community gardens” OR “garden farms” OR “garden beds” OR “planter boxes” OR “urban farms” OR “urban farming” OR “urban agricultures” OR “hydroponics” OR “aquaponics”
Thermal comfort aspects
“thermal comfort” OR “thermal stress” OR “thermal performance” OR “urban heat” OR “heat island” OR “cooling effect” OR “surface temperature” OR “air temperature”
Electrical energy aspects
“energy consumption” OR “energy saving” OR “energy demand” OR “energy balance” OR “energy efficiency” OR “energy usage” OR “electricity” OR “power”
Codes used for the Web of Science search engine
TS = (“green roof*” OR “green rooftop*” OR “wetland roof*” OR “roof garden*” OR “rooftop garden*” OR “vertical greenery systems” OR “green wall*” OR “living wall*” OR “green façade*” OR “vegetation screen*” OR “green curtain*” OR “vegetation curtain*” OR “urban garden*” OR “community garden*” OR “garden farm*” OR “garden bed*” OR “planter box*” OR “urban farm*” OR “urban farming” OR “urban agricultures” OR “hydroponics*” OR “aquaponics*”) AND TS = (“thermal comfort” OR “thermal stress” OR “thermal performance” OR “urban heat” OR “heat island” OR “cooling effect” OR “surface temperature” OR “air temperature”) AND TS = (“energy consumption” OR “energy saving” OR “energy demand” OR “energy balance” OR “energy efficiency” OR “energy usage” OR “electricity” OR “power”)
Codes used for the SCOPUS search engine
TITLE-ABS-KEY(“green roof*” OR “green rooftop*” OR “wetland roof*” OR “roof garden*” OR “rooftop garden*” OR “vertical greenery systems” OR “green wall*” OR “living wall*” OR “green façade*” OR “vegetation screen*” OR “green curtain*” OR “vegetation curtain*” OR “urban garden*” OR “community garden*” OR “garden farm*” OR “garden bed*” OR “planter box*” OR “urban farm*” OR “urban farming” OR “urban agricultures” OR “hydroponics*” OR “aquaponics*”) AND TITLE-ABS-KEY(“thermal comfort” OR “thermal stress” OR “thermal performance” OR “urban heat” OR “heat island” OR “cooling effect” OR “surface temperature” OR “air temperature”) AND TITLE-ABS-KEY (“energy consumption” OR “energy saving” OR “energy demand” OR “energy balance” OR “energy efficiency” OR “energy usage” OR “electricity” OR “power”)

Appendix B

AuthorsYearTitle
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Convertino et al. [62]2020Energy behavior of the green layer in green façades
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Erdemir and Ayata [119]2017Prediction of temperature decreasing on a green roof by using an artificial neural network
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Evangelisti et al. [121]2020On the energy performance of an innovative green roof in the Mediterranean climate
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Goussous, Siam and Alzoubi [131]2015Prospects of green roof technology for energy and thermal benefits in buildings: case of Jordan
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Heusinger and Weber [142]2017Surface energy balance of an extensive green roof as quantified by full year eddy-covariance measurements
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Hirano et al. [144]2019Simulation-based evaluation of the effect of green roofs in office building districts on mitigating the urban heat island effect and reducing CO2 emissions
Hugo, du Plessis and Masenge [71]2021Retrofitting Southern African cities: a call for appropriate rooftop greenhouse designs as climate adaptation strategy
Jadaa, Aburaed and Taleb [145]2019Assessing the thermal effectiveness of implementing green roofs in the urban neighborhood
Jaffal, Ouldboukhitine and Belarbi [146]2012A comprehensive study of the impact of green roofs on building energy performance
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Jim and Peng [149]2012Weather effect on thermal and energy performance of an extensive tropical green roof
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Madi, Bozonnet and Patrick [151]2020Building and urban cooling performance indexes of wetted and green roofs—a case study under current and future climates
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Yin et al. [66]2017Cooling effect of direct green façades during hot summer days: an observational study in Nanjing, China using TIR and 3DPC data
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Zeng et al. [257]2017Optimal parameters of green roofs in representative cities of four climate zones in China: a simulation study
Zhang et al. [60]2019Thermal behavior of a vertical green façade and its impact on the indoor and outdoor thermal environment
Y. Zhang et al. [75]2019Cooling benefits of an extensive green roof and sensitivity analysis of its parameters in subtropical areas
Zhao and Srebric [258]2012Assessment of green roof performance for sustainable buildings under winter weather conditions
Zhao et al. [259]2015Accumulated snow layer influence on the heat transfer process through green roof assemblies
Zhao et al. [260]2014Effects of plant and substrate selection on thermal performance of green roofs during the summer
Zheng and Weng [261]2020Modeling the effect of green roof systems and photovoltaic panels for building energy savings to mitigate climate change
Zheng, Dai and Tang [262]2020An experimental study of vertical greenery systems for window shading for energy saving in summer
Ziogou et al. [263]2018Implementation of green roof technology in the residential buildings and neighborhoods of Cyprus

Appendix C

Water 14 02496 g0a1

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Figure 1. The vicious cycle of climate change.
Figure 1. The vicious cycle of climate change.
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Figure 2. The workflow of the systematic review.
Figure 2. The workflow of the systematic review.
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Figure 3. (a) Bibliography network using the authors’ keywords; (b) bibliography network using index keywords.
Figure 3. (a) Bibliography network using the authors’ keywords; (b) bibliography network using index keywords.
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Figure 4. The world map illustrating the distribution of the studied cities in the reviewed papers, according to the updated Köppen-Geiger climate classification scheme in 2016 (see Appendix C for a detailed explanation of the Köppen-Geiger climate classification scheme) [46]. In this paper, the area above the 20° N latitude line is considered to be the northern hemisphere, while the area below the 20° S latitude line is considered to be the southern hemisphere; between the two is considered the equatorial region. Map edited using Datawrapper website. Available online: https://www.datawrapper.de/ (accessed on 7 May 2022).
Figure 4. The world map illustrating the distribution of the studied cities in the reviewed papers, according to the updated Köppen-Geiger climate classification scheme in 2016 (see Appendix C for a detailed explanation of the Köppen-Geiger climate classification scheme) [46]. In this paper, the area above the 20° N latitude line is considered to be the northern hemisphere, while the area below the 20° S latitude line is considered to be the southern hemisphere; between the two is considered the equatorial region. Map edited using Datawrapper website. Available online: https://www.datawrapper.de/ (accessed on 7 May 2022).
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Figure 5. The distribution of Köppen-Geiger climate classification zones according to the studied locations. (See Appendix C for a detailed explanation of the Köppen-Geiger climate classification scheme).
Figure 5. The distribution of Köppen-Geiger climate classification zones according to the studied locations. (See Appendix C for a detailed explanation of the Köppen-Geiger climate classification scheme).
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Figure 6. The ranking of the top 10 countries with the most UGI research publications, published between 2010 and 2021, with additional information on the top 3 cities in each country.
Figure 6. The ranking of the top 10 countries with the most UGI research publications, published between 2010 and 2021, with additional information on the top 3 cities in each country.
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Figure 7. A world map illustrating the distribution of the studied cities in the reviewed papers. Map edited using Datawrapper website. Available online: https://www.datawrapper.de/ (accessed on 7 May 2022).
Figure 7. A world map illustrating the distribution of the studied cities in the reviewed papers. Map edited using Datawrapper website. Available online: https://www.datawrapper.de/ (accessed on 7 May 2022).
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Figure 8. The normalized weight of the socioeconomic factors.
Figure 8. The normalized weight of the socioeconomic factors.
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Figure 9. Distribution of the reviewed papers published between 2012 to 2021, according to their UGIB categories.
Figure 9. Distribution of the reviewed papers published between 2012 to 2021, according to their UGIB categories.
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Figure 10. (a) Venn diagram of the number of studies, based on the three main UGI categories; (b) the distribution of UGI studies according to the categories and subcategories.
Figure 10. (a) Venn diagram of the number of studies, based on the three main UGI categories; (b) the distribution of UGI studies according to the categories and subcategories.
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Figure 11. (a) The scale of the studied UGI; (b) the height of the UGI (or studied buildings).
Figure 11. (a) The scale of the studied UGI; (b) the height of the UGI (or studied buildings).
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Figure 12. The scale of UGI in relation to the building type.
Figure 12. The scale of UGI in relation to the building type.
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Figure 13. (a) The studied seasons for thermal performance; (b) the studied seasons for electricity energy performance.
Figure 13. (a) The studied seasons for thermal performance; (b) the studied seasons for electricity energy performance.
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Figure 14. The typical methods and tools for data collection and analysis that were used in the reviewed articles.
Figure 14. The typical methods and tools for data collection and analysis that were used in the reviewed articles.
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Figure 15. Bibliography network of the sensors used in the reviewed articles.
Figure 15. Bibliography network of the sensors used in the reviewed articles.
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Figure 16. The connections and relationships among methods and tools. This figure was produced using a VOS viewer based on the occurrence of the tools and methods keywords. The thickness of the line represents stronger or weaker links between the keywords.
Figure 16. The connections and relationships among methods and tools. This figure was produced using a VOS viewer based on the occurrence of the tools and methods keywords. The thickness of the line represents stronger or weaker links between the keywords.
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Table 1. Abbreviation for and description of each UGI type investigated in this review [33,34,35,36,37,38,39,40].
Table 1. Abbreviation for and description of each UGI type investigated in this review [33,34,35,36,37,38,39,40].
AbbreviationUGI TypeDescription
GRGreen RoofArtificial landscape on a roof surface with vegetated layers.
EGRExtensive green roofLightweight structure, with a substrate thickness of less than 200 mm. Limited vegetation with shallow roots, such as sedums, herbs, and grasses.
IGRIntensive green roofHeavyweight structure, substrate thickness from 250 mm to more than 1 m. Suitable to grow lawns, perennials, shrubs, and small trees.
SemiGRSemi-intensive green roofThe weight is between EGR and IGR, with a substrate between 120 mm and 250 mm to support grasses, herbs, and shrubs.
BRBlue roofBlue roofs involve the use of water-saturated slabs on the building rooftop to provide extra storage for rainwater under the rooftop surface or vegetation layer.
VGSVertical Greenery SystemVertical structures that allow vegetation to grow across the building’s façade and walls.
GFGreen façadeThe vegetation cover is formed by climbing plants or hanging plants that grow directly on the façade.
DSGFDouble skin green façadeSimilar to the green façade, the vegetation cover is formed on a particular support system that is attached to the building’s walls, so that the plant is growing indirectly on the façade.
GWGreen wallA green wall is also known as a living wall, with supporting structures attached to the façade. With substrate-based plants growing in planter boxes or in pockets on the panels, the vegetation cover is formed by sedums, herbs, or moss instead of climbing plants.
UAUrban AgricultureThe vegetation is edible, which provides a food source and offers other benefits within the urban environment.
RTGHRooftop greenhouseA passive system designed and integrated on a building rooftop to improve the thermal performance.
Table 2. The scale of the UGI compared with the size of the studied site.
Table 2. The scale of the UGI compared with the size of the studied site.
UGI ScaleFeaturesMin. Range 1 Max. Range 1
MicroA very small structure/model that is inhabitable.0to<100
LocalBased on a single building, usually filled with occupants.≥100to<10,000
NeighborhoodA group of buildings that are situated across a few streets or blocks.≥10,000to<1,000,000
CityClusters of building blocks or multiple precincts.≥1,000,000
Note: 1 The values are measured in m2.
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Wai, C.Y.; Tariq, M.A.U.R.; Muttil, N. A Systematic Review on the Existing Research, Practices, and Prospects Regarding Urban Green Infrastructure for Thermal Comfort in a High-Density Urban Context. Water 2022, 14, 2496. https://doi.org/10.3390/w14162496

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Wai CY, Tariq MAUR, Muttil N. A Systematic Review on the Existing Research, Practices, and Prospects Regarding Urban Green Infrastructure for Thermal Comfort in a High-Density Urban Context. Water. 2022; 14(16):2496. https://doi.org/10.3390/w14162496

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Wai, Cheuk Yin, Muhammad Atiq Ur Rehman Tariq, and Nitin Muttil. 2022. "A Systematic Review on the Existing Research, Practices, and Prospects Regarding Urban Green Infrastructure for Thermal Comfort in a High-Density Urban Context" Water 14, no. 16: 2496. https://doi.org/10.3390/w14162496

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