A Novel Idea for Improving the Efficiency of Green Walls in Urban Environment (an Innovative Design and Technique)

The advantages of low-impact development approaches, such as green walls in an urban environment, are numerous. These systems can be applied for managing stormwater, saving energy consumption, decreasing noise pollution, improving runoff quality, improving life quality, and so forth. Besides, atmospheric water harvesting methods are considered a nonconventional water source. There are many studies about the analysis and advantages of green walls and atmospheric water harvesting conducted separately. However, the use of a combined system that uses fog harvesting in the irrigation of green walls has received less attention in previous studies, and therefore in this research, the feasibility of a novel green wall platform was investigated. At first, the potential of using green walls and atmospheric water harvesting in different climates was analyzed. Then a new combined system was proposed and explained. The study results determined that atmospheric water harvesting can be applied as a source of irrigation for green facilities, particularly in the dry season and in periods with lower precipitation. In the Mediterranean climate, summer fog harvesting yields 1.4–4.6 L/m2/day, and the water consumption of green walls is about 4–8 L/day/m2. This can improve one issue of green walls in an urban environment, which is irrigation in summer. Furthermore, the novel system would protect plants from severe conditions, improve buildings’ thermal behavior by decreasing direct sunlight, and increase conventional green walls’ efficiency and advantages.


Introduction
Buildings are responsible for about 40% of the total energy consumption in cities, and the impact of greening systems is evident [1,2]. In addition, greening systems are known as natural cooling systems [3] and have a high potential to reduce energy consumption in buildings [4][5][6]. The advantages of low-impact development (LID) systems, such as green walls, green roofs [7,8], and rainwater harvesting systems, have been determined in many studies [9,10]. In the design of these systems, many elements need to be considered [11], mainly including statistical analysis of hydrological parameters [12], hydrological effectiveness [13], flow measurement devices [14], and thermal impacts [15,16]. The other potentials of green walls are improvement of ecosystem services, reduction of urban noise, and improvement of air quality [17].
Reitano [18] analyzed the water conditions in Mediterranean countries. The analysis showed that due to geographical and topographical positions, there are different climate types in the Mediterranean region, and thus the water conditions and design solutions may be different. Therefore, to meet

Materials and Methods
In this study, the potential of fog water harvesting methods and green walls in different climates would be determined based on different case studies, and a novel system for improving the efficiency of green walls would be presented with an explanation of its application and advantages. An analysis diagram is presented in Figure 1.
As can be seen in the analysis diagram, first, we analyzed green walls and their water consumption in different climates besides their advantages and the issues with conventional green walls. The result of this section could determine the amount of irrigation in summer and the thermal advantages of irrigation and other useful information for the next section. Second, we explored atmospheric water harvesting methods in different climates, and the outcomes could show the water harvesting potential per m 2 in fog days, especially in summer, in which irrigation of green walls is necessary. Therefore, we evaluated the possibility of applying atmospheric water harvesting methods to irrigate green walls in an urban environment. The bases of these sections of the analysis are previous case studies and literature reviews. Finally, based on the three previous sections, we proposed a novel design for green walls with a higher efficiency in an urban environment, as well as the main usage of the new system. Then, we explained the main issues with conventional GWs and how the new system is supposed to solve them. As can be seen in the analysis diagram, first, we analyzed green walls and their water consumption in different climates besides their advantages and the issues with conventional green walls. The result of this section could determine the amount of irrigation in summer and the thermal advantages of irrigation and other useful information for the next section. Second, we explored atmospheric water harvesting methods in different climates, and the outcomes could show the water harvesting potential per m 2 in fog days, especially in summer, in which irrigation of green walls is necessary. Therefore, we evaluated the possibility of applying atmospheric water harvesting methods to irrigate green walls in an urban environment. The bases of these sections of the analysis are previous case studies and literature reviews. Finally, based on the three previous sections, we proposed a novel design for green walls with a higher efficiency in an urban environment, as well as the main usage of the new system. Then, we explained the main issues with conventional GWs and how the new system is supposed to solve them.

Green Walls in Different Climates
Greening systems, such as green walls and green roofs, have several advantages, including environmental [7,27,28], social, and economic benefits [29][30][31][32]. These systems are widely used in different climates, and their benefits can change in different contexts. Many studies have emphasized the environmental benefits of green systems in the Mediterranean climate, which are more evident in the hot season than in the cold season [33][34][35][36][37][38][39][40]. In these regions, energy savings can reach 84% in the hot season. In the tropical climate, the adoption of a greening solution has demonstrated that environmental benefits have less impact compared with that in the Mediterranean climate, but they were also relevant [41][42][43][44][45]. The rate of energy savings in these regions can reach an average of 65%. In the arid climate, the results were not promising, but energy savings can reach the rate of 52% in the cold season, while energy savings in the hot season are very low [46,47]. The same behavior can be assumed for the Continental climate, where no relevant energy savings were recorded in the literature, especially in the hot season [48].

FWH
•Analysis of fog water harvesting (FWH) and its potential in different climates.

New Idea
•Development of a novel green wall platform with an atmospheric harvesting water system. •Explanation of different elements in the new system.

New System
•Advantages and feasibility of the new system in an urban environment and how the new system can improve the issues of conventional GWs.

Green Walls in Different Climates
Greening systems, such as green walls and green roofs, have several advantages, including environmental [7,27,28], social, and economic benefits [29][30][31][32]. These systems are widely used in different climates, and their benefits can change in different contexts. Many studies have emphasized the environmental benefits of green systems in the Mediterranean climate, which are more evident in the hot season than in the cold season [33][34][35][36][37][38][39][40]. In these regions, energy savings can reach 84% in the hot season. In the tropical climate, the adoption of a greening solution has demonstrated that environmental benefits have less impact compared with that in the Mediterranean climate, but they were also relevant [41][42][43][44][45]. The rate of energy savings in these regions can reach an average of 65%. In the arid climate, the results were not promising, but energy savings can reach the rate of 52% in the cold season, while energy savings in the hot season are very low [46,47]. The same behavior can be assumed for the Continental climate, where no relevant energy savings were recorded in the literature, especially in the hot season [48].

Different Types of Green Walls and the Main Design Elements
Green wall (GW) systems are generally divided into two typologies: green facades (GFs) and living walls (LWs), which are different from each other as regards constituent elements and design features. GFs, which are divided into direct and indirect systems, support the natural growth of climbing plants that can achieve up to 25 m of height and cover a whole wall surface in a few years. The main difference between direct and indirect GFs is the use of a structural support for plant growth. In this regard, direct systems are characterized by plants directly developed on the wall, while in indirect systems, a structural support (continuous or modular guides) sustains the plant's growth. Generally, GFs are easy to install and light and present low costs [29,49]. On the other hand, LWs, the most recent innovation in terms of GW, based on specific climate conditions, can support a wide variety of vegetations (succulents, perennial plants, grasses, etc.). Just like the former category, LWs can also be subdivided into continuous and modular LWs, whose main difference is in the substrate soil. In continuous LWs, the plants are put in a pocket obtained by cutting absorbent and lightweight screens (as felt). While modular LWs are generally prevegetated panels with vessels, flexible bags, trays, and planter tiles as supporting elements for the plant's growth, they are characterized by organic and inorganic growing media adequate for retention capacity [29,49].

Water Consumption of Green Walls
The water consumption of green walls is essentially due to irrigation, and it may depend on several factors, such as (a) the climate of the area where they are installed, (b) the type of planted vegetation, (c) the type of installed system, (d) the average annual rainfall, (e) the distribution of precipitations, (f) the trend of daytime and nighttime temperatures, and (g) the relative humidity (RH) in the air [50]. Moreover, one advantage of green walls is their thermal impacts on buildings and total energy consumption, and this, especially in summer, would depend on the irrigation. However, water scarcity and precipitation periods could affect irrigation and efficiency, particularly in arid and semiarid areas [15]. Ottelé et al. [51] quantified a green wall's water consumption in a Continental climate with an average wind speed of 5.8 m/s, which ranged from 1 to 3 L/day per m 2 depending on the system investigated. Pérez-Urrestarazu et al. [52] analyzed the water consumption of an experimental LW system with a size of 5.9 m 2 in Spain, Seville (Mediterranean climate), and the results determined a range from 4.0 to 8.0 L/d/m 2 .

The Relative Humidity in the Air and the Area near Green Walls
Several studies have demonstrated that GW use contributes to the stable relative humidity of a building, improving climate conditions. Chen et al. [53] investigated RH's role in two identical thermal labs built for this purpose. Their study concluded that the use of green walls provided a more stable relative humidity condition in the air layer next to the wall surface without increasing the mean relative humidity of the indoor space. Widiastuti et al. [54] found in their study an increase in RH from 65% to 72%.
RH is an important factor for atmospheric water harvesting as explained in the next section. The results of a study that analyzed two case studies showed that the potential of water harvesting depends on relative humidity, and it would be near zero with RH at less than 69% [55].

Fog Water Harvesting Methods and Their Potential in Different Climates
Sabino [56] analyzed fog harvesting pilot sites in the archipelago of Cape Verde, which has a desert climate and an altitude between 750 and 1400 m. The daily collected water was reported to be between 3 and 75 l/m 2 /day. Cereceda et al. [57] investigated the fog harvesting potential in Chile, which has a desert climate. The analysis determined that fog is frequently observed in the coastal area (0-12 km seashores) with a harvesting capacity of about 7 L/m 2 /day. Estrela et al. [58] analyzed fog harvesting using a passive cylindrical collector in four locations in Spain. Their results indicated the significant potential of the method in the selected regions. The water produced was between 2 and 7 L/m 2 /day. The maximum summer fog was also determined, which was 4.6 L/m 2 /day. Harb et al. [59] explored the possibility of fog harvesting to improve the situation of water shortage in Egypt, and the results showed the effectiveness of the method. Lekouch et al. [60] analyzed fog and dew harvesting performance by using a passive condenser and mesh collector in Morocco, which has an arid climate, and the results showed the effectiveness of the method. Dodsona and Bargach [61] explored the possibility of fog water harvesting in the southern region of Morocco, which has a semiarid climate. Their pilot site's annual yield with a mesh size of 600 m 2 was 2.3 million liters (10.5 L/m 2 /day).
Park et al. [62] studied the role of surface wettability on the efficiency of fog harvesting. Their analysis determined that in a mild fog with a wind speed of 2 m/s, an average radius of droplets of about 3 µm, and a liquid water content of 0.1 g/m 3 , the water harvesting capacity could be about 2 L/m 2 /day if the used mesh is dip-coated. Almasian et al. [63] analyzed the role of modified nanofiber in the capacity of fog harvesting. The analysis showed that by increasing the hydrophobic nature of the material, the harvesting capacity increases. Therefore, they investigated a new type of fluorinated nanofibers with a super-hydrophobic nature, which means lower surface energy, higher water contact angle, and higher surface roughness. The capacity of the treated nanofibers was about 10 times more than that of the untreated nanofibers. In another study by Zhou et al. [64], a new hybrid surface was prepared and fog harvesting ability was investigated. Analysis of the new hybrid hydrophobic-hydrophilic surface integrated with a Janus copper foam system (HB-HL + JCF) showed that the efficiency of fog collection compared with untreated copper foam increased by 209%.
Ghosh et al. [65] explored the fog harvesting from a thermal power plant's cooling tower since it is considered a major industrial water consumer. The pilot analysis results determined that a recovery of about 40% could be possible by using fog harvesting (10.5 m 3 per hour in a 500 MW power plant). In a more recent study, Ghosh et al. [66] investigated the use of a coating mesh to increase the efficiency of a metal mesh for fog harvesting and contaminants. The results showed that coated metal meshes made of TiO 2 /ZnO could be applied for this purpose. Bagheri [67] developed an experimental setup for analyzing water harvesting in commercially atmospheric water harvesting systems. The results showed that the average water harvesting in a cold and humid climate is about 0.05 L/h (with an energy consumption of 6.23 kWh/L), and in warm and humid climates, around 0.65 L/h (with an energy consumption of 1.02 kWh/L).
Standard collectors from polypropylene mesh nets can be classified as follows [68,69]: The analysis showed that the efficiency of fog harvesting depends on the moisture capture by the mesh and the captured water removal from the mesh [70]. In estimating the potential Fog Water Collection (FWCp), two critical factors are visibility and wind speed [71]. SFCs can harvest fog when winds are present; however, the efficiency can be decreased by winds parallel to the panel [58]. Contact angle is another important factor, allowing small captured droplets to easily slide down into the collector [62]. Furthermore, water harvesting capacity would depend on the geometrical shapes of the Raschel mesh [72]. Another factor that needs to be considered is the sustainability of the techniques [73,74] and, in this case, the sustainability of the used mesh and installation structures [75].
The efficiency of fog collection depends on wind velocity, fog liquid water content, droplet size distribution, and mesh characteristics, which can be calculated as follows [76]: • The wind pressure on the mesh can break the supporting structures; • The mesh and other components can be damaged by UV radiation.
The potential of fog harvesting in different case studies and climates is presented in Table 1.

The New Proposed Green Wall System
The new green wall (living wall) platform with an atmospheric water harvesting and collector system is presented in Figures 2 and 3, and the installation of the platforms in buildings in Figure 4.
Water 2020, 12, x FOR PEER REVIEW 7 of 17

The New Proposed Green Wall System
The new green wall (living wall) platform with an atmospheric water harvesting and collector system is presented in Figures 2 and 3, and the installation of the platforms in buildings in Figure 4. Figure 2. The novel living wall platform with a fog harvesting mesh. 1-living wall platform, 2mesh for fog harvesting, 3-water collector pipe, 4-funnel that is connected to the collector pipe to collect water drops, 5-rod for conveying drops to the collector pipe, 6-metal clamp to fix the mesh to the pipe and rod, 7-support to fix the mesh system to the green wall platform, 8-water droplets, 9-plants, 10-soil, 11-frame to fix the living wall to the wall of the building, H-height of the platform, L-length of the platform, T-thickness of the platform.

Figure 2.
The novel living wall platform with a fog harvesting mesh. 1-living wall platform, 2-mesh for fog harvesting, 3-water collector pipe, 4-funnel that is connected to the collector pipe to collect water drops, 5-rod for conveying drops to the collector pipe, 6-metal clamp to fix the mesh to the pipe and rod, 7-support to fix the mesh system to the green wall platform, 8-water droplets, 9-plants, 10-soil, 11-frame to fix the living wall to the wall of the building, H-height of the platform, L-length of the platform, T-thickness of the platform.
Water 2020, 12, x FOR PEER REVIEW 7 of 17

The New Proposed Green Wall System
The new green wall (living wall) platform with an atmospheric water harvesting and collector system is presented in Figures 2 and 3, and the installation of the platforms in buildings in Figure 4. Figure 2. The novel living wall platform with a fog harvesting mesh. 1-living wall platform, 2mesh for fog harvesting, 3-water collector pipe, 4-funnel that is connected to the collector pipe to collect water drops, 5-rod for conveying drops to the collector pipe, 6-metal clamp to fix the mesh to the pipe and rod, 7-support to fix the mesh system to the green wall platform, 8-water droplets, 9-plants, 10-soil, 11-frame to fix the living wall to the wall of the building, H-height of the platform, L-length of the platform, T-thickness of the platform.  As can be seen in the figures, the water harvesting mesh (2) produces droplets (6) that move through the sloped rod (4) and then enter the collector pipes (3), and the collected water drops from each living wall platform (1) and discharges on the next one on the downside.  As can be seen in the figures, the water harvesting mesh (2) produces droplets (6) that move through the sloped rod (4) and then enter the collector pipes (3), and the collected water drops from each living wall platform (1) and discharges on the next one on the downside. Water storage tank with filters can store the collected water for the green wall's irrigation and other nonpotable purposes.

Different Uses of the New Proposed Living Wall System
• For atmospheric water harvesting in an urban environment; • For improving the thermal impact of buildings; • For improving the efficiency of green walls in water consumption and noise and air pollutions reduction; • For improving the sustainability of municipalities, factories, and buildings in different climates.

Discussion
Green walls are widely used in different climates, and their benefits can change in different contexts. The environmental benefits of green systems in different climates are more evident in the hot season than in the cold season as energy savings in summer can reach 84% in the Mediterranean climate, 65% in the tropical climate, and 52% in the arid climate. The water consumption of green walls, which affects thermal impacts, would depend on the irrigation, especially in summer. According to previous studies, the water consumption is 4 to 8 L/day/m 2 in the Mediterranean climate and 1 to 3 L/day/m 2 in the Continental climate. The analysis shows the irrigation demands for green walls, specifically in the dry seasons. However, in dry and semidry climates and even in the Mediterranean climate, whereas the annual rainfall may exceed 1000 mm, the dry period (July to September) could have no, or in some years low, precipitation.
One way to provide the required water for GWs could be to use a fog harvesting mesh. The potential of fog harvesting based on previous studies, which also depends on altitude, is 2 to 7 L/m 2 /day in the Mediterranean climate (maximum summer fog from 1.4 to 4.6 L/m 2 /day), 11.8 L/m 2 /day in the arid tropical climate, 10.5 L/m 2 /day in the semiarid climate, 7 L/m 2 /day in the desert climate, and 8.6 L/m 2 /day in the hot desert climate. Furthermore, there are some techniques for improving the efficiency of a fog harvesting mesh. The efficiency of a fog harvesting mesh depends on hydrophobic-hydrophilic conditions, which means the moisture captured by the mesh and the captured water removal from the mesh. This efficiency can be improved by using coated meshes made of TiO 2 /ZnO.
Comparisons have determined that in the Mediterranean climate, summer fog harvesting, which yields 1.4-4.6 L/m 2 /day, could be an appropriate source of irrigation as the water consumption of green walls is about 4 to 8 L/day/m 2 . In addition, GWs increase the relative humidity in the surrounding area, and the potential of atmospheric water harvesting depends on the RH. While the result of a study showed that the fog harvesting potential is low in RH less than 69%, the analysis of another study showed that the use of green walls can increase RH from 65% to 72%, which could be enough to use atmospheric water harvesting methods.

The Advantages of the New Proposed Living Wall System
The main advantages of the new living wall platforms compared with conventional ones are as follows: • In conventional green wall systems for installation, many in-site construction works were required, but in the new system, the platforms, with a specific design (shape, size, and installation method), make installation easy, and in several stages based on budget.

•
In conventional green wall systems, all systems could be stopped for a repair, but in the new system, the repair can be done just on the damaged platform very fast (by removing and replacing the damaged platform with a new one), which decreases the maintenance cost. In addition, a combination of a sloped rod and a pipeline in the fog water collecting system decreases the total cost. In this system, the water droplets (fog/precipitations) move through the sloped rod through gravity and reach the water collector pipe, and from each platform to the downside platform.

•
In conventional green wall systems, the focuses are landscape, thermal impacts (confronted with issues), and in some cases, runoff management. However, in the newly designed platform, the transparent mesh keeps the landscape of plants in the living wall and improves the thermal impacts and stormwater management and reduces noise and air pollution besides protecting the plants from direct sunlight, high wind speed, and snow.

•
Green walls can reduce the total runoff volume and reduce and delay the peak discharge into the sewer system [7,13,[88][89][90]. The performance would be improved in the new system since part of the precipitation can be collected by the installed mesh.

•
According to literature analysis, green walls can increase the surrounding air's relative humidity through its wet substrate and the evapotranspiration of plants. This can improve atmospheric harvesting systems' efficiency since it directly depends on the RH%, and the relative humidity in the site of green walls is more than that in the adjacent area. Thus, this phenomenon undoubtedly increases the possibility of generating water with the systems mentioned in this study, but also, the excessive increment of this parameter could be critical considering the alteration of the climatic condition, especially in the humid climate. • In the new system, the necessity of irrigating green walls, particularly in dry periods, is decreased by a transparent fog harvesting mesh and, as a result, reduces the pressure on the water resource. • Green walls can reduce energy consumption in buildings [15,39,[91][92][93] and mitigate the urban heat island [94][95][96][97]. In conventional green wall systems, the thermal efficiency of green walls decreases in dry periods due to water issues for irrigation, but in the new proposed system with a transparent mesh in the front, thermal efficiency is increased and water consumption is decreased in several ways. First, the mesh absorbs fog/precipitation and parts of the evapotranspiration of the plants. The thermal performance would increase due to the increase in irrigation in the summer. Second, the installed mesh decreases the plants' direct exposure sunlight and decreases the total water consumption.

•
In conventional green wall systems, the plants could be damaged by high wind speed and snow conditions, but these two problems are solved in the new platform with an installed transparent mesh in front of the plants.

•
Green walls can enhance the water quality [89,98,99], improve the air pollution concentration, and decrease the noise levels in urban environments [100][101][102]. In the new system, the performance improved due to the installed mesh.

•
Green walls can improve the wildlife and biodiversity conditions [103][104][105]. The same performance would be available in the new systems.

•
Green walls can improve the building's aesthetic and property values and quality of life [106][107][108].
The same results would be achieved in the new systems. However, the total cost of water and energy demand in the new systems are less than those of previous systems.

•
A main part of the cost of the fog harvesting mesh belongs to the supporting structures that stop collapse owing to winds, which affects the final price [75]. However, in the new system, the mesh is installed to the living walls' platform and does not need separate structures, which decreases the entire coast.

Recommendation for Future Works
The fog mesh's efficiency would depend on the type and distance from the green wall, and therefore, the place of the fog harvesting mesh is recommended for future analysis. In addition, to be confident about the exact impact of the mesh on total water reduction or thermal impact, an experimental analysis is necessary and thus is recommended for future studies. Finally, the evaluation of the mentioned advantages and the analysis of the performance in each point (cost analysis, water quality, etc.) could be done by experimental analysis.

Conclusions
Different studies have determined the application, properties, and advantages of green walls in an urban environment, which are mainly managing stormwater, saving energy consumption, decreasing noise pollution, improving runoff quality, and improving life quality. Green walls' daily water consumption depends on the climate type and ranges from 1 to 8 L/m 2 /day, and their thermal impacts, especially in summer, depend on the irrigation. However, analysis showed that in dry and semidry climates and even in the Mediterranean climate with annual rainfall of more than 1000 mm, the dry period (July to September) could have no, or in some years low, precipitation.
In addition, analysis of different case studies determined the potential of fog harvesting as a supplementary water supply even in arid regions. The average daily fog-collected water in different climates was between 2 and 15 l/m 2 in fog days, including 2-7 L/m 2 /day in the Mediterranean climate, 3.75-4.5 L/m 2 /day in the tropical climate, 6.7 L/m 2 /day in the subtropical dry arid climate, 10.5 L/m 2 /day in the semiarid climate, 5.5-7.8 L/m 2 /day in the desert climate, and 8.6 L/m 2 /day in the hot desert climate, which also depends on the elevation, wind speed, and mesh characteristics. The analysis determined that one way to provide the required water for GWs could be to use a fog harvesting mesh as in the Mediterranean climate, summer fog harvesting, which yields 1.4-4.6 L/m 2 /day, could be an appropriate source of irrigation as the water consumption of green walls is about 4 to 8 L/day/m 2 .
Therefore, a new platform of living walls that benefits from a fog harvesting mesh is developed and analyzed. The analysis shows that, compared with conventional green wall systems, the new system may require less installation time and maintenance costs and could be more economical. Furthermore, the newly designed platform gains atmospheric water without consuming energy and could increase the efficiency and advantages of green walls, particularly in summer with lower precipitation. The transparent mesh used in the new living wall could keep the landscape of the plants; improve thermal performance, stormwater management, and water quality; reduce noise and air pollution; decrease water consumption; and protect the plants from direct sunlight, high wind speed, and snow. In conclusion, it seems that the new proposed systems for living walls would improve the advantages of green walls and may decrease conventional systems' negative points. However, future experimental analysis is required to determine the exact impact on each sector. This idea, after further experimental investigations, could be added to the new energy policies, such as nearly zero-energy buildings (NZEBs).

Patents
The manuscript's idea is submitted as a patent in Italy's Ministero dello sviluppo economico, with application number 102020000027038.