Assessment of the Cooling Potential of an Indoor Living Wall using Different Substrates in a Warm Climate - PDF

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Original Paper Indoor and Built Indoor Built Environ 2012;21;5: Accepted: July 20, 2011 Environment Assessment of the Cooling Potential of an Indoor Living Wall using Different Substrates in a Warm
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Original Paper Indoor and Built Indoor Built Environ 2012;21;5: Accepted: July 20, 2011 Environment Assessment of the Cooling Potential of an Indoor Living Wall using Different Substrates in a Warm Climate Rafael Fernández-Cañero a Antonio Franco Salas b Luis Pérez Urrestarazu b a Department of Agroforestry Sciences, School of Agricultural Engineering, University of Seville, Carretera de Utrera, km 1, 41013, Sevilla, Spain b Area of Agro-forestry engineering, School of Agricultural Engineering, University of Seville, Carretera de Utrera, km 1, 41013, Sevilla, Spain Key Words Green walls E indoor environment E vertical greenery systems E vertical garden E substrate E temperature E humidity Abstract The use of vertical greenery systems in buildings is becoming very popular as they provide several benefits. In this work, the influence of an indoor living wall on the temperature and humidity in a hall inside the School of Agricultural Engineering (University of Seville) was studied. Four different substrates, Geotextile, Epiweb, Xaxim and coconut fibre, were used to grow the plants in order to assess their performance. Several parameters such as temperature, humidity, plant growth or water consumption were monitored and analyzed during a 4-month period. The cooling effect of the living wall was proven, with an average reduction of 48C over the room temperature though maximum decrements of 68C have been observed in warmer conditions. Higher air humidity levels were experienced near the living wall, increasing the overall humidity in the room. All the substrates tested were suitable for plant growing and their behaviour was similar. Geotextile showed the best cooling capacity but higher water consumption, coconut fibre presented degradation problems and Epiweb performance was the poorest. Therefore, these systems have been proven to be very useful and interesting for warm indoor environments due to the cooling effect observed in addition to their bio-filtration capacity and the aesthetic component. Introduction Vegetation plays an important role in our cities where the rampant urban development is causing many problems such as pollution, increased air temperature, lack of green ß The Author(s), Reprints and permissions: DOI: / X Accessible online at Figures 1 6 appear in colour online Rafael Ferna ndez-can ero, Department of Agroforestry Sciences, Escuela Te cnica Superior de Ingenierı a Agrono mica. Universidad de Sevilla. Carretera de Utrera, km Sevilla. Spain. Tel. þ , Fax þ , space and excessive energy consumption. Following the concepts of sustainability, urban greening practices are becoming a popular way of reducing the undesired effects of increasing construction and achieving ecological goals. Nowadays, greenery systems offer the potential to incorporate advanced materials and new technologies to promote sustainable building functions [1]. Vertical greening systems also known as green wall technologies, vertical gardens or bio walls are vertical vegetated structures that may or may not be fixed to a building facade or to an interior wall. Based on the different plants and support structures used, these systems can be divided into two major groups: green facades and living walls. In green facades systems, the vegetation covering is formed by climbing plants or cascading groundcover mainly rooted at the base in the ground or in plant boxes. Living walls are generally more complex and involve a supporting structure with different attachment methods and a waterproof backing to isolate the living wall from the building in order to avoid moisture problems. In this case, the plants fix their roots to a substrate attached to the vertical structure. Some studies have been conducted involving these kinds of systems which proved to have many benefits. For instance, the appropriate use of vegetation on the built environment can adequately adjust the urban microclimate and improve the thermal behaviour of building envelopes [2]. Plant-covering of building surfaces can provide a beneficial cooling effect within the building zone as plants absorb a considerable quantity of solar radiation for their growth and biological functions [3]. Therefore, the opportunity to reduce the cooling load in the summer and the potential to decrease the use of air-conditioning is giving a great impetus to the increasing use of plants within the built environment. For example, Hoyano [4] and Ip et al. [5] employed deciduous climbers to offer seasonal regulation of shading. This potential reduction of temperature inside the buildings has another remarkable consequence: energy savings. With appropriate placement of vegetation, an important reduction of cooling energy demands can be achieved [6] as the temperature reduction needed to match the comfort temperature is lower. Then, the shading and cooling effect of vertical greenery systems can be translated into a reduction of the energy used for cooling by approximately 20% [7], though potential cooling energy savings of up to 60% during warm summer days have been described [8]. There are also acoustics benefits of vertical greenery systems in facades due to the sound-absorbing effect of substrates and they may be useful in enhancing speech privacy if they are installed internally [9] or be used as noise barriers [10]. Nevertheless, most of these studies refer to systems attached to the exterior facades of buildings. Therefore, indoor living walls and their influence on interior environments have not been analyzed to a great degree. People in urbanized societies spend over 80% of their time indoors [11]. For that reason, indoor environmental quality is of critical importance to our health and wellbeing [12]. The indoor environment is a dynamic interrelationship between thermal comfort needs, physical factors and chemical and biological factors [13,14]. Some studies assess the effect of vegetation on air quality improvement [15 17]. It is not only restricted to particle adherence, it is also efficient in absorbing air polluting substances [18] due to a process known as bio-filtration [19,20]. According to the Spanish regulations in buildings, for good indoor air quality in an indoor environment, temperature and humidity levels should be maintained within the range of C and 30% 70%, respectively [21]. In warm climates, it is difficult to maintain these levels without using air conditioning systems, which are high-energy consumers. As living walls must be constantly irrigated, indoor air humidity increases providing a cooling effect that reduces the room temperature when needed. Also the plants evapotranspiration process helps to regulate the temperature. Given that indoor air is usually too dry, particularly in situations with internal heating or cooling systems, this humidity increment is also beneficial [22]. Lohr [23] conducted a study demonstrating that plant transpiration may increase the indoor air humidity by 3% 5% creating a humidity level that matches the recommended human comfort range. The main objective of this work was to analyze the effect of an indoor living wall on the environment inside buildings, in particular, involving temperature and humidity as the main variables. The effect of using different substrates was also evaluated. Methods The living wall was constructed for this study in 2008 inside a small hall (4.40 m m, 4 m height) at the School of Agricultural Engineering in Seville (southern Spain). This area is characterized by long warm periods with temperatures over 308C, so the use of air conditioning is frequently necessary. The data acquisition started in 2009, in order to ensure the proper settlement and Cooling Potential of an Indoor Living Wall Indoor Built Environ 2012;21: Fig. 1. Living wall and substrate disposition. development of the vegetation prior to the beginning of the study. The hall is located in the ground floor with South-east orientation and it is connected to the rest of the building by a long corridor. The access to the hall is provided by a double door in order to enhance insulation from the exterior conditions. The hall is not equipped with an air conditioning system. During the study period, the average outdoors temperature was 16.48C and the average maximum outdoors temperature was 23.68C. The living wall covered nearly 8 m 2 of wall (Figure 1) and consisted of a vertical galvanized iron structure attached to the wall. A tank, built of the same material and with a capacity of 500 L of water, was placed at the bottom of the structure. A substrate layer with pockets for the introduction of plants was attached to the vertical structure. The back of the structure was covered by a waterproof layer to prevent moisture problems in the room wall. Four different substrates were used and tested in this study, two of organic origin, coconut fibre and Xaxim (a material composed by fern roots, mainly Dicksonia sp.) [24]) and two synthetic, Epiweb (based on Polyetylentereftalat) [25] and Geotextile (acrylic textile made of different fibres with a polypropylene base) [26]. A preliminary analysis of organic substrates was performed to determine their ph and salinity levels. Two pulverized samples of each organic substrate (5 g dissolved in 25 ml of distilled water) were tested to determine salinity levels (using measures of electrical conductivity for the saturated extract) and ph (several measures obtained from the Table 1. Selected species for the living wall Selected species Adiantum capillus-veneris Anthurium scherzerianum Asparagus densiflorus Asparagus setaceus Asplenium nidus Chamaedorea elegans Chlorophytum comosum Cissus rhombifolia Codiaeum variegatum Dieffenbachia tropic Epipremnum aureum Ficus pumila Hypoestes sanguinolenta Kalanchoe blossfeldiana Nephrolepis exaltata Peperomia variegata Pilea cadierei Plectranthus australis Saxifraga stolonifera Soleirolia soleirolii Solenostemon scutellarioides Spathiphyllum wallisii Tradescantia spathacea Tradescantia zebrina saturated paste using a ph meter). When the system was already operating, a conductivity and ph analysis was conducted on the draining water as the water used for irrigation was recovered in the tanks and reused until 644 Indoor Built Environ 2012;21: Ferna ndez-can ero et al. Fig. 2. Monitoring system s schematic. certain undesirable conductivity or ph levels were reached. The initial objective was not to exceed an electric conductivity of 3,500 ms/cm, a high level for most horticulture crops [27], but ultimately, the limiting factor was the ph level. The ph recommended value of 7.5 was often surpassed due to the characteristics of the water used to refill the tank. Therefore, in order to optimize the maintenance operations and as the ornamental quality was not affected, the maximum ph level was established at 8. Twenty-four different species were used in the living wall (Table 1). These were ornamental species commonly used in indoor environments. The main criterion used for their selection was their potential to adapt to a vertical structure [26,28]. As the plants were chosen for an indoor living wall, it was desirable that they not emit pollen producing allergies. The different species have been arranged following practical criteria (shading between plants, adaptability to substrates and humidity conditions) though the aesthetic component should also present (grouping of plants, playing with colours). In addition, to detect the influence of planting height, same species were planted at different heights. When it was possible, most species were planted on the four substrates to be able to compare their performance in all of them. Three systems were also required for the correct operation of the living walls: irrigation, monitoring and lightening systems. Irrigation was provided by a network of PVC pipes. Horizontal pipes were placed at regular vertical spacing and small holes along their length allowed water flow. The monitoring system integrated a data logger attached to five digital temperature and humidity sensors SHT75 Sensirion (one for each substrate and one for the room temperature used as control) placed at the same height (1.80 m) and separated by 0.3 m from the living wall to avoid interactions with the conditions under the canopy. The control values (ambient temperature and humidity values slightly influenced by the living wall) were collected by a sensor located at the opposite side of the hall. The data logger was connected to a computer (Figure 2) to ease the analysis of the information. Temperature and humidity data was recorded every 15 min. A software program was also developed in Visual Basic 6.0 to operate the data and to control pumps and lights according to a programmed scheme. Lightening system was composed by six fluorescents Grolux Silvania (58 W). In order to carry out the study of the different substrates properly, the living wall was organized in four independent sections, each one with its own autonomous systems (irrigation, pump and tank). Therefore, different sections could be managed individually. The structures supporting the organic and inorganic substrates were also separated. Once the living wall was planted, a maintenance program was necessary to promote plant survival and growth and to prevent problems such as pump and/or irrigation system malfunctioning. The test started once the system was operating correctly and the plants were fully settled. The data collection occurred from March to mid-june period, when the temperature would be increasing progressively. From mid-june, academic activities in the school would be finished so the occupancy pattern of the School changed. Hence, the test was not prorogued to the warmer months due to this change of conditions. Cooling Potential of an Indoor Living Wall Indoor Built Environ 2012;21: Fig. 3. Temperatures during an average day in the study period. Results and Discussion Substrates Analysis The ph values obtained in the preliminary analysis were 5.81 for coconut fibre and 5.38 for Xaxim. These measures revealed that both substrates were in the medium acid interval, characterized for being the most suitable for plant growth, though Xaxim was in the limit of high acid. The salinity level tests showed 4.23 ds/m for coconut fibre and 2.26 ds/m for Xaxim. The second value can be considered within an acceptable salinity interval for most plants [29], though 4.23 ds/m would be slightly high. Those levels are common for coconut fibre substrates and the excess soluble salts could be easily and effectively leached from the material under customary irrigation regimes [30]. The drainage water was tested periodically for each substrate to obtain the ph and conductivity levels required. Average conductivity values were always below the minimum threshold for hydroponic cropping (below 1500 ms/cm is considered very low) [27]: 435, 457, 635 and 793 ms/cm for Geotextile, Epiweb, Xaxim and Coconut fibre, respectively. This was precisely the objective as an adequate growth was compatible with an acceptable ornamental quality and would reduce water consumption. With this method, the water nutritive solution could be reused for a longer-than-usual period (up to 8 weeks in this case). Similar values of low nutritive supply were suggested in Blanc [26]. Slightly basic average levels of ph were observed (Geotextile: 7.95, Epiweb: 7.5, Xaxim: 7.21 and coconut fibre: 7.25). Most vegetal species would prefer a more acid ph though in this case, apparently, this situation did not have much effect on the appearance or development of the vegetation. Water consumption during June (which had the most extreme temperatures during the study period) was 5.01, 3.94, 3.3 and 3.94 (in l m 2 day 1 ) for Geotextile, Epiweb, Xaxim and coconut fibre, respectively. In this case, the substrate had a high impact on the values as a significantly higher consumption was observed for the synthetic ones. The reason might be the higher retention capacity of the organic substrates, which minimized water evaporation from their surfaces. Influence on the Indoor Environment: Temperature and Humidity In order to study the influence of the living wall on indoor temperature and humidity using the different substrates, those variables were measured and recorded during a 15-week period (from March to mid-june) for each substrate. Figure 3 shows the temperature variations during an average day. The performance of the different substrates was quite similar, but there was a divergence with the ambient temperature of close to 48C during the last hours of the day. This difference of performance was less when the temperature dropped between 7 and 10 am. Epiweb presented the greatest difference with the ambient temperature (nearly 18C more than the other substrates in some cases) though Geotextile produced similar values. The average ambient temperature during the studied 646 Indoor Built Environ 2012;21: Ferna ndez-can ero et al. Fig. 4. Daily maximum and minimum temperatures (8C) for the control and living wall in the study period. Fig. 5. Frequency of occurrence of temperatures in June. period was 248C, being 20.88C when synthetic substrates were used and 21.28C when the substrates were organic ones. Maximum and minimum temperatures were 32.58C and 16.18C for ambient values, 28.68C and 13.88C when using synthetic substrate and 29.28C and 15.48C when using organic substrates. Therefore, the difference in the temperature effect between organic and synthetic substrates was small, less than 18C on average, but the minimum temperature was lower when synthetic substrates were used. Maximum and minimum daily temperatures for the control (ambient temperatures) and near the living wall (average values for all the substrates) are shown in Figure 4. An average of 38C difference between the control and the living wall maximum temperatures was observed along the studied period. The temperature Cooling Potential of an Indoor Living Wall Indoor Built Environ 2012;21: Table 2. Correlation between temperature differences and ambient temperature Values for June Ambient Geotextil Epiweb Xaxim Coconut fibre Average temperature c a b b b (8C) a Máximun (8C) Mı nimum (8C) Average difference to c a b b control temperature (8C) a,b Correlation between temperature difference and ambient temperature c Size effect Medium (almost large) Small (almost medium) Medium Medium a When compared with correlated measures ANOVA, values with same letter were not significantly different at level p ¼ 0.05 (Dunnett s C test) (2-tailed). b Positive number indicates a higher control air temperature. c All correlations have significance at level p50.01 (Tau_b de Kendall). Fig. 6. Humidity average values for the different substrates. difference between maximum and minimum values was less when influenced by the living wall due to a buffering effect on the temperature near the plants. As temperatures during June were higher, the cooling effect of the living wall was more obvious. Figure 5 shows the frequency of occurrence of temperatures in June. The most usual temperatures for the control case were in a range from 258C to 288C and those below 248C were hardly observed. Taking into account the influence of the living wall, the most frequent temperatures registered were between 228C and 258C and exceeded the comfort limit only a few times. Within the different substrates, Epiweb showed the higher temperatures. Geotextile had the best performance though it was quite similar to the organic substrates. Looking into the distribution of temperatures in the second week of June, the results above mentioned were confirmed. During only the last 2 days of the week temperatures near the living wall exceeded the comfort limit, while the ambient temperature went over this limit on several occasions. Once again, the different substrates had a similar behaviour with the exception of Epiweb, which showed a slight divergence for lo
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