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Effect of irrigation on the experimental thermal performance of a green roof in a semi-warm climate in Mexico
Energy and Buildings 154 (2017) 232–243
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Effect of irrigation on the experimental thermal performance of a green roof in a semi-warm climate in Mexico M.A. Chagolla-Aranda a , E. Simá a , J. Xamán a,∗ , G. Álvarez a , I. Hernández-Pérez b , E. Téllez-Velázquez a a
Centro Nacional de Investigación y Desarrollo Tecnológico, CENIDET-TecNM-SEP Prol, Av. Palmira S/N. Col. Palmira, Cuernavaca, Morelos CP 62490, Mexico Universidad Juárez Autónoma de Tabasco, División Académica de Ingeniería y Arquitectura, Carretera Cunduacán-Jalpa de Méndez km. 1, Cunduacán, Tabasco, CP 86690, Mexico b
a r t i c l e
i n f o
Article history: Keywords: Irrigation Thermal evaluation Green roofs Energy consumption Semi-warm climate
a b s t r a c t A study of the experimental thermal performance of a green roof in a semi-warm climate of Mexico, which considers the irrigation effect, is presented. The experiment was performed in two stages. The first stage consisted of selecting from among five types of plants, commonly used in green roofs, the one that tolerates the greatest number of days without irrigation. This stage was carried out in the warm season and we selected the Aeonium subplanum plant for the thermal experimental evaluation. In the second stage, we constructed two test cells in which the temperature and the heat flux of a green roof and a concrete roof were measured for a period of 8 days. During the experimental test the green roof was watered in one day of the evaluation period. From the results, it was found that after the irrigation event the maximum temperature of the green roof components is reduced by 6.4, 4.8 and 1.3◦ C for vegetation, substrate and slab, respectively. The experimental results show that the use of a green roof decreased the temperature by 20.5 ◦ C compared to a concrete roof. The test cell with green roof had an accumulated electricity consumption 1.3 kWh lower than the test cell with the conventional roof, representing 10.3% less electricity consumption. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Buildings sector is one of the largest energy consumers in the world and it generates large amounts of waste and pollution. Buildings account for over one-third of the energy consumption and produce the same amount of carbon dioxide [1]. Air conditioning is one of the more important causes of energy use in buildings [2]. In urban areas, the increase of the ambient temperature due to the urban heat island (UHI) effect can increase the energy consumption in buildings and in others public Facilities, as lighting systems [3,4]. The effects of UHI also affect the environment, air pollution concentrations increase in warming cities, high pollution concentrations combined with high temperatures can have a negative impact on people health conditions [5].
∗ Corresponding author. E-mail addresses: [email protected] (M.A. Chagolla-Aranda), [email protected] (E. Simá), [email protected] (J. Xamán), [email protected] (G. Álvarez), [email protected] (I. Hernández-Pérez), ivan [email protected] (E. Téllez-Velázquez). http://dx.doi.org/10.1016/j.enbuild.2017.08.082 0378-7788/© 2017 Elsevier B.V. All rights reserved.
Several building envelope techniques have been developed to improve the thermal comfort, to reduce the energy consumption from air conditioners and to mitigate the UHI effect [6]. The roof is the component of the building envelope with the highest temperature fluctuations and significantly contributes to the building energy load. In this sense, several passive technologies such as evaporative cooling, reflective materials, shading with photovoltaic panels, insulation and vegetation can be used to minimize the energy gain from a roof [7]. Green roofs have been increasingly investigated to determine how they can reduce the energy consumption from buildings and improve the quality of the urban environment. Green roofs are totally or partially covered with a layer of vegetation. The main components of a green roof are: waterproofing, root barrier, substrate and vegetation. Green roofs often include drainage, thermal insulation and irrigation system. There are two types of green roofs: intensive and extensive. The intensive green roof has a thick substrate layer (more than 20 cm) and allows to grow deep root plants such as shrubs and trees. The maintenance is high because required fertilizing, weeding and watering. The extensive green roof has a thinner substrate layer (less than 20 cm) and allows to grow small plants, such as grass or sedum. The installation cost is lower, and
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Nomenclature CR G GR q qCR sen qGR sen RH T TCR TGR Tveg CR Tslab,ext
Concrete roof Solar radiation (W/m2 ) Green roof Heat flux (W/m2 ) Concrete roof heat flux sensor (W/m2 ) Green roof heat flux sensor (W/m2 ) Relative humidity (%) Temperature (◦ C) Concrete roof temperature (◦ C) Green roof temperature (◦ C) Air temperature at the vegetation (◦ C) Concrete roof slab external surface temperature (◦ C)
T CR slab,int
Concrete roof slab internal surface temperature (◦ C)
T GR slab,ext
Green roof slab external surface temperature (◦ C)
Green roof slab internal surface temperature (◦ C) Substrate external surface temperature (◦ C) Substrate internal temperature (◦ C) Temperature difference (◦ C) Temperature difference between concrete roof and green roof (◦ C) Tamb−CR Temperature difference between ambient and concrete roof (◦ C) Tamb−GR Temperature difference between ambient and green roof (◦ C) Time (h) t V Wind velocity (m/s) VWC Volumetric water content (m3 /m3 ) VWCsen Volumetric water content sensor (m3 /m3 ) GR Tslab,int
TSust,ext TSust,int T TCR−GR
Subscript Ambient amb sen Sensor subst Substrate Vegetation veg Superscript CR Concrete roof GR Green roof
it has low weight and required minimal maintenance. The main factors that affect the heat transfer in green roofs are: cooling due to evapotranspiration, increased thermal insulation by adding layers of material and vegetation as a barrier to transmission of solar radiation. The aim of a green roof is to provide aesthetical environmental and economic benefits. Green roofs can reduce polluting air particles and increase the oxygen production which contributes to the improvement of the environment. Green roofs can also reduce the urban storm water runoff problems, absorbing part of the rainfall and distributing the runoff over a long-time; provide food and habitat for native plants and animals; reduce noise pollution; create space where people can interact each other and rest; increases the internal roof layers’ durability. Several of the benefits of green roofs are achievable, but due to lack of research green roofs are generally not designed to meet all those benefits. The benefits of green roofs depend of multiple factors as climate, geometry or materials composition [8]. However, the environment can determine its functionality in terms of the thermal performance. Some experimental studies have been made to understand the green roof thermal performance in different climates, a few of them are presented next. Most of the experimental work found in this research report the
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temperature in different layers of the green roof and some report the heat flow either measured or calculated, the results are often compared with a reference roof in different seasons. In temperate climate, green roofs are an effective solution to achieve thermal comfort and reduce cooling demand in summer. In winter a green roof functions as a thermal insulation, even in conditions of saturation it can reduce the heating energy demand compared with plywood and concrete roofs [9–11]. In tropical climate green roofs can reduce the outer and inner roof temperature in free floating conditions [12–14], saving the 22% of electric power in air conditioning conditions [15]. The previous benefits have been achieved in the summer season. In Mediterranean claimed Bevilacqua et al. [16] reported that during summer a green roof had an exterior surface temperature 12 ◦ C lower than a lightweight concrete roof and it maintained the surface temperature 4 ◦ C higher than the concrete roof in winter. Gagliano et al. [17] reported that during the summer a green roof lowered the maximum outdoor surface temperature by 27.7 ◦ C compared to a concrete roof, the green roof also reduced the temperature range throughout the day. Coma et al. [18] reported for the same climate the electrical energy consumption of a heat pump system in two rooms, one with concrete roof and other with green roof; the results concluded that the green roof can reduce the energy consumption (even 16.6%) during warm season, but present a higher energy consumption (even 11.1%) during cold season. In semi-arid climate, Reyes et al. [19] recommended well irrigated green roofs with substrate depth (more than 10 cm), it can reduce the exterior surface temperature 13 ◦ C below ambient temperature and helps to maintain a stable and suitable substrate water content. In hot climate, Dvorak and Volder [20] reported that even unirrigated green roofs can reduce the exterior roof temperature 37.7 ◦ C in comparison with a concrete roof. In hot climate, La Roche and Berardi [21] reported that highly insulated green roofs can overheat the indoor temperature during summer in comparison with reflective and thermal insulated metal sheet roof. In cold climate, there are net benefit of the use of green roofs compared with concrete roofs, but the benefits are lower in extreme winter conditions when the substrate is frozen, when the roof has a snow layer and also during sunny conditions [22–24]. Zhao et al. [25] reported that a green roof in a cold winter claimed can reduce the building’s heating energy consumption by 23% compared to a reference building with plywood roof. However, this energy saving was reduced to 5% with a snow layer accumulated on the roofs. Due to the importance of climate in green roof thermal performance most of the studies are agree on the need to report green roofs information in as many climates as possible in order to have more knowledge about its benefits. In Mexico, there are few norms [26–29] oriented to the efficient use of energy in buildings and studies related to the application of passive techniques are scarce. Some passive techniques studied in Mexico are natural ventilation [30], earth-to-air heat exchanger [31], room-Trombe wall [32], insulation and reflective materials [33], window shading [34], cool roof [35], among others [36,37]. Below are presented some works that reports the thermal performance of green roofs in Mexico, these studies showed that green roofs reduced roof temperature compared to conventional concrete roofs in the warm season. Ovando-Chacón et al. [38] performed simulations to determine the air temperature distribution inside test cells with a concrete roof and a green roof. The simulation was performed using the climate data of the city of Veracruz, Mexico. The green roof reduced the temperature of the indoor air near the ˜ roof by 10 ◦ C with respect to the concrete roof. Castaneda-Nolasco and Vecchia [39] conducted an experimental study that measured the temperature of a concrete roof and a green roof in a tropical climate in Mexico. The green roof reduced the maximum temperature of the interior surface by 13.7 ◦ C with respect to the concrete roof. ˜ and Pérez-Sánchez [40] carried out an experimenOrdonez-López
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Fig. 1. Plants used in the irrigation experiment:(a) Sedum adolphii, (b) Echeveria prolífica, (c) Aeonium subplanum, (d) Crassula ovata y (e) Sedum Makinoi.
tal study in which the temperature of a reflective roof and a green roof were measured in a building in Yucatán, Mexico. The green roof reduced the air temperature inside the building compared to the reflective roof, achieving values closer to an established comfort range. In the studies reported by Quezada-García et al. [41,42] presented models for prediction of heat transfer of green roofs, however the data used in the models do not reflect the effects of the Mexican climate. In Mexico, green roofs have shown that they are an effective measure to improve comfort conditions, which could reduce cooling loads, which are prevalent in the country because two-thirds of the territory is considered to have warm weather. However, it is necessary to have more information about its benefits for different types of climates and other times of the year. Because of above, in this article we present an experimental thermal evaluation of a green roof and compared its performance with a conventional concrete roof using outdoor test cells. Both test cells are air conditioned and located in a semi-warm climate in Mexico. The parameters measured were temperature at different levels and heat flux for each roof which considered the effect of irrigation. 2. Materials and methods 2.1. Methodology The experiment was carried out in Cuernavaca, Mexico. This city is at the central zone of Mexico and its elevation is 1500 m above sea level. The climate of Cuernavaca is semi-warm sub-
humid with light rains during summer [43]. The experimental study was divided into two stages: • The first stage consisted of selecting a plant, among five different types usually used in green roofs, that tolerates the greater number of days without irrigation conserving a visually healthy state. This stage was conducted from Mar. 3rd 2015 to Apr. 30th 2015. The season in which this experiment was performed is considered the warm season. • The second stage consisted of measuring temperatures and heat fluxes of a green roof and a concrete roof. The experiment was carried out during 8 days, from Nov. 21th to Nov. 28th 2016, corresponding to the cold season. To find out how the irrigation influences the temperatures and the heat transfer of the green roof, the substrate was watered on Nov. 25th 2016. 2.2. Selection of plant type and tolerance to drought The plants of the family Crassulaceae and among them, the genus Sedum is one of the main type of plants used for green roofs due to its small requirement of irrigation and its adaptability to sunny environments [44]. Other types of plants widely used in green roofs are grasses and native plants. The low irrigation requirement is a property of the plants used in this type of roof that becomes even more valuable in places where water availability is limited. Moreover, in semi-arid and tropical climates the water content in a green roof influences the reduction of the roof and ambient temperatures [19,45]. To make a better use of water, it is necessary to know the
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Fig. 2. Exposure of plants to the outdoor environment.
Table 1 Number of days without irrigation for each type of plant. Type of plant
Number of days without irrigation
Sedum Makinoi Sedum Adolphii Echeveria Prolífica Crassula Ovata Aeonium Subplanum
3 4 6 7 9
irrigation requirements of the plants used in green roofs. The objective of the experiment carried out in the first stage was to determine how many days five types of plants can stay visually healthy without irrigation for the climate of Cuernavaca, Mexico. In the first stage the number of days in which the plants should be watered was determined, then the plant that requires irrigation for the longest period was selected to use it for the experiment of thermal performance of the green roof (second stage). The experiment of the first stage consisted of placing five types of plants from the family Crassulaceae in containers 50 cm long by 30 cm wide with substrate of 10 cm depth. Fig. 1 shows the plants used in the study: Sedum Adolphi, Echeveria Prolifica, Aeonium Subplanum, Crassula Ovata and Sedum Makinoi [46]. The experiment was carried out two weeks after the plants were placed in the containers and exposed to the outdoors (Fig. 2). During this period, all plants were watered every third day in the afternoon, and we observed a good adaptation of the plants to the environment. The experiment was conducted from Mar. 3rd 2015 to Apr. 30th 2015, which is considered the warm season. In this period, the ambient temperature reached values up to 35 ◦ C, annual minimum relative humidity and rain events are uncommon (during the experiment period no rain event occurred). The day the experiment was started, the plants were watered until substrate saturation, and the following days the appearance of each plant was observed. The leaf turgor was evaluated visually. Similar qualitative assessments were used by Moterusso et al., Nagase et al., Liu et al. and Farrel et al. [47–50]. The evaluation was done in the afternoon when the sun was hidden, this moment was chosen because the plants had just passed through the period of greater stress of the day. When the loss of leaf turgor was evident, it was considered the need of irrigation. Once the plants were evaluated in irrigation condition, they were watered to saturation and record it as an irrigation event. If the plants conserved the turgor in most of their leaves, the plants remained without irrigation, until they were evaluated in irrigation condition. The number of days in which each plant had an irrigation event was recorded. We observed that this number was constant for each type of plant during the period of the experiment. Table 1 shows the number of days that each type of plant stayed without being watered. The plant that tolerated the largest number of days without being watered was the Aeonium Subplanum with 9 days. This plant was selected to be used in
the green roof experiment of second stage because of its ability to tolerate the longest time without irrigation. 2.3. Test cells and instrumentation 2.3.1. Construction of the test cells We constructed two outdoor test cells to simultaneously evaluate the thermal performance of a green roof and a conventional concrete roof (reference roof). For the construction of each cell, we used 1.8 cm thick OSB boards and joined them to form a cubic cavity. The interior surface of the walls was covered with extruded polystyrene insulation of 2.5 cm thick. The walls of the cavity are 90 × 90 cm2 (Fig. 3a). A metal cube structure of 95 cm side was welded to contain the two wooden cavities and respectively support the weight of the concrete roof and the green roof. (Fig. 3b). The test cells were painted white to reflect solar radiation that could enter the through the walls (Fig. 3c). Two circular holes were made in one of the walls of the cell to connect the ducts of the air conditioning system. The top of the cells was uncovered to place the green roof and the concrete roof (Fig. 3d). We constructed two concrete slabs with a thickness of 10 cm and 120 cm side with reinforcement of metallic mesh in the center. The roof slabs were placed on the test cells with metallic structure (Fig. 4a). In the case of the concrete roof, no other components were added. We installed a 15-cm high sheet guard to contain the substrate and the plants for the green roof. The green roof slab was covered with a waterproofing material that integrates an anti-root layer (Fig. 4b). We used 6 cm of substrate with high mineral stone content, and Aeonium subplanum was added as vegetation for the green roof (Fig. 5(a) and (b)). The test cells were placed on the roof of a four-story building where there is no shading due to neighboring buildings. The test cells are located 20 m away from the weather station from which weather information was obtained. Fig. 6 shows the green roof and concrete roof installed on the test cells, the ducts of the air conditioners in the cells, and the two air conditioners connected to the cells. Each test cell has two ducts, inside one circulates the cold air from the air conditioner, and inside the other circulates the air from the test cell that removed the heat gain. To avoid heat transfer between the cells and the roof of the four-story building the cells were placed at a height of 20 cm. The following table shows the optical and thermo-physical properties proposed for the green roof and concrete components [51,52] (Table 2). 2.3.2. Instrumentation To measure climatic variables, we used a Vaisala Maws100 weather station located at the experiment site (Fig. 7). The weather station has a Vaisala QMS101 pyranometer with uncertainty
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Fig. 3. Construction of the test cells: (a) metallic structure, (b) wood cavity and thermal insulation, (c) Application of paint to the cells and (d) cell holes for connection of air conditioner.
Fig. 4. Construction of the concrete slabs: (a) casting of concrete slabs and (b) waterproofing of the roof slab for the green roof.
Table 2 Optics and thermo-physical properties of the vegetation and concrete roof. Roof type Component
Properties
Thickness (m)
Optics
Green
Concrete
Vegetation Substrate Slab Slab
Thermo-physical
Absorbance
Emissivity
Thermal conductivity (W m−1 K−1 )
Density (kg m−3 )
Specific heat (J kg−1 K−1 )
0.72 0.85 – 0.6
0.90 0.95 – 0.82
– 0.25 1.74 1.74
– 1370 2300 2300
– 800 840 840
0.08 0.06 0.1 0.1
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Fig. 5. (a) Application of the substrate and (b) Vegetation of the green roof.
Fig. 6. Outdoor test cells.
Omega HFS-4 heat flux sensor with an uncertainty of ± 10% on the inner surface of the slabs. For a better contact between the heat flux sensors and the slab, we used a thermal paste and adhesive tape to adhere the flux sensor (Fig. 8b). To measure the air temperature that surrounds the vegetation layer of the green roof, and the temperature of outer surface and the center of the substrate, we used thermocouples type “T”. To measure the volumetric water content (VWC) at the center of the substrate, we installed Sensors of the Decagon Devices EC-5 with an uncertainty of ± 3% (Fig. 8c). All information data from the sensors was stored by using a Keysight 34972A data acquisition system (resolution of 6½ digits). Measurements were taken every 10 min with the Keysight data acquisition 34972. The indoor air temperature of each test cell is maintained at constant temperature of 25 ◦ C by an air conditioning unit controlled with by an electronic thermostat with an uncertainty of ± 1 ◦ C. The air conditioners used have a capacity of 1470 W. The air conditioning systems are connected to the test cells through ducts coated with thermal insulation. Fig. 9 shows a diagram with the location of all sensors used in the outdoor test cells. 3. Results Fig. 7. Weather station.
of ± 1%. The sensors of temperature, humidity, wind speed and precipitation are integrated in a Vaisala WXT536 multi-sensor, the uncertainty of each sensor is: ± 0.3 ◦ C, ± 3%, ± 3% and ± 5% respectively. We placed thermocouples type “T” (with an uncertainty of ± 0.5 ◦ C) on the outer surface, the center and the inner surface of the two concrete slabs (Fig. 8a). In order to avoid that the solar radiation influenced in the measurement of the temperature of the outer surface, the thermocouples were placed inside a pvc guard. To measure the heat flux through the slabs, we placed a thin-film
In the experimental results, we present a comparison between the temperature of the green roof and the concrete roof during a test period of 8 days, from 11/21/2016 to 11/28/2016. 3.1. Climatic conditions The data of the meteorological variables as ambient temperature, relative humidity, solar radiation and wind speed measured during the period of the experiment are shown in Fig. 10. The average ambient temperature was 19.4 ◦ C. The average daily maximum ambient temperature was 25.9 ◦ C. The average daily minimum
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Fig. 8. Sensors used in the outdoor test cells: (a) thermocouple arrangement in the slab, (b) heat flow sensor in the slab and (c) volumetric water content sensor in soil.
Fig. 9. Schematic of the instrumentation.
ambient temperature was 13 ◦ C. The average relative humidity was 47.8%. The average daily maximum relative humidity was 71%. The average daily minimum relative humidity was 29%. The average maximum solar radiation was 752 W/m2 . The average wind speed was 1 m/s. The Fig. 10(c) indicates that the wind speed increases during the day and decreases during the night. No rain event occurred during the experiment period. 3.2. Irrigation event One of the objectives of the experiment was to observe the behavior of the green roof when there is an irrigation event, so the green roof was watered until saturation on November 25th, 2016. Fig. 11 shows the VWC of the substrate during the period of the experiment. At the beginning of the experiment the VWC
was 0.37 m3 /m3 . The next days the VWC decreased with an almost linear behavior, having a greater decrease in the first two days of the experiment (Nov. 21th and 22th, 2016) compared with the two days before irrigation (Nov. 23th and 24th, 2016). The VWC further attenuated on the day of irrigation (Nov. 25th, 2016) until it reached a minimum of 0.28 m3 /m3 . During the day of the irrigation, a sudden increase of the VWC occurred until it reached a maximum value of 0.53 m3 /m3 . After irrigation, the VWC linearly decreased, like the days prior to irrigation. At the end of the experiment the VWC was 0.41 m3 /m3 . The amount of water with which the vegetation was irrigated was 50 l. This amount was used because the substrate was saturated with a CVA of 0.54, the total volume of the substrate being 86.4 l, the corresponding volume of water that can retain the saturated substrate is 46.7 l.
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Fig. 11. VWC of the substrate.
with 34.5 and 27.2 ◦ C, respectively. The next day after irrigation, the peak temperature of vegetation was reduced by 6.4 ◦ C, the substrate peak temperature by 4.8 ◦ C and the slab peak temperature by 1.3 ◦ C. Once the VWC started to decrease again, the temperatures of the three layers of the green roof increased. Between the first day after the substrate was irrigated and the last day of the experiment, the maximum temperature of the vegetation and substrate increased 1.7 ◦ C and that of the slab increased 1.2 ◦ C. 3.3. Comparison between the green roof and conventional roof temperatures
Fig. 10. Meteorological data: (a) temperature and relative humidity, (b) global solar radiation and (c) wind speed.
3.2.1. Green roof temperature profile To understand how irrigation affects the temperature of the roof components, Fig. 12 shows the temperature of vegetation, the temperature of the substrate surface, the temperature of the upper surface of the green roof slab and the VWC during the period of the experiment. The components that are most affected by the irrigation are the vegetation and the substrate. The Fig. 12 indicates that as the VWC decreases the temperature of vegetation and substrate increases, having the peak value the day before irrigation
To quantify the benefits provided by a green roof, Fig. 13 shows the outer surface temperature of the slab for the green roof and the corresponding to a conventional concrete roof. The figure demonstrates that the maximum temperature of the upper surface of the green roof slab is lower than the temperature of the concrete roof, and lower than the ambient air temperature during every day. The green roof reduces the average maximum surface temperature of the slab to 17.1 ◦ C with respect to the concrete roof. On the other hand, the average maximum temperature of the upper surface of the green roof slab is 4.1 ◦ C lower than the ambient temperature. Fig. 13 indicates that after irrigation, the maximum temperature of the green roof was 1.3 ◦ C lower than the maximum temperature it reached before irrigation. At night, the average minimum temperature of the upper surface of the green roof is 6 ◦ C greater than that of the concrete roof, and 3.1 ◦ C greater than the ambient temperature. We found a difference smaller than 0.3 ◦ C for the minimum temperature of the green roof before and after irrigation. Similar results are reported by He et al. (2016) [12], which measured the temperature and heat flow of a green roof and concrete roof of an air-conditioned room in a tropical climate. In their results it is said that the green roof can act as a source of heat releasing heat to the environment during the afternoon and evening. Fig. 14 shows the temperature difference between the exterior surface of the concrete roof slab and the green roof slab (TCR-GR = TCR -TGR ). During the day, the TCR-GR was positive with maximum values ranging between 18.1 and 20.5 ◦ C. During the night, the TCR-GR was negative, with minimums ranging between −7.1 to −5.6 ◦ C. The figure shows that the maximum value of TCR-GR decreased as the irrigation event approaches (when VWC reaches its lowest value), and it increased after irrigation. Furthermore, the figure shows that the daily minimum values of TCR-GR increased after irrigation until the end of the experiment. Fig. 14 demonstrates that most of the day, the temperature of the concrete slab is higher than the corresponding to the green roof. This
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Fig. 12. Temperature profile of the green roof.
Fig. 13. Temperature of the green roof slab and conventional concrete slab.
Fig. 14. Temperature difference between TCR −TGR .
effect is observed with the positive values of TCR-GR which cause the concrete roof to have a greater energy gain than the green roof. Fig. 15 shows the temperature difference between TAmb and TCR (TAmb-CR ), and between TAmb and TGR (TAmb-GR ). The figure indicates that during the day TAmb-CR is negative, having a minimum of −15.3 ◦ C, therefore the direction of the heat flux will be from the concrete roof to the ambient. During the night, the value of TAmb-CR is positive, so the direction of the heat flow is from the ambient to the concrete roof, with TAmb-CR having a maximum of 3.8 ◦ C. This means a drastic decrease in the temperature of the concrete roof, even below the minimum ambient temperature at night. In the case of TAmb-GR , this have a behavior opposite to TAmb-CR , since during day TAmb-GR has positive values and at night negative values. Thus, the green roof absorbs heat from the environment during the day and at night the heat is transferred to the outdoor environment. The maximum and minimum values of TAmb-GR were 6.8 and −5.4 ◦ C respectively. We can also note the reduction of the amplitude of TAmb-GR compared to TAmb-CR . There is a tendency the maximum values of TAmb-GR to be reduced
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Fig. 17. Electricity consumption.
Fig. 15. Temperature difference between TAmb y TCR (TAmb-CR ), and between TAmb and TGR .
for each day. Table 3 presents the daily heat gain of the green and conventional roof. The third column shows the difference between the heat gain of the green roof and the heat gain of the conventional roof. This column indicates that for all days of the experiment, the heat gain of the green roof was up to 55% lower than the one of the concrete roof. 3.5. Electricity consumption
Fig. 16. Heat fluxes from the green roof and from the conventional roof.
It was measured the electricity consumption of the air conditioning equipment that maintained the air temperature of the test cells in a comfort range between 24 and 26 ◦ C. Fig. 17 shows the accumulated electricity consumption during the period of study for each test cell. The test cell with green roof had an accumulated electricity consumption 1.3 kWh lower than the test cell with the conventional roof, representing 10.3% less electricity consumption than the conventional roof. The greatest difference of the daily energy consumption was the day after that the irrigation was performed with a difference of 0.26 kWh, which represents 20% of the difference of the accumulated total.
as the irrigation event approaches and the VWC is reduced. The maximum values of TAmb-GR increase after the irrigation is performed.
4. Conclusions
3.4. Heat fluxes
Based on the experimental results of the thermal performance of a green roof and a concrete roof in the semi-warm climate of Cuernavaca, Mexico, the following is concluded:
Fig. 16 shows the heat flux through the green roof and the concrete roof. We considered that the heat flux is positive when it goes from the outside of the roof to the interior of the cavity and negative in the opposite case. The maximum heat flux of the green roof was 10.7 W/m2 whereas the corresponding to the concrete roof was 64.2 W/m2 , which represents a difference of 83.3%. On the other hand, the minimum heat flux of the green roof and the concrete roof was −21.9 W/m2 and −29 W/m2 , respectively, which indicates that the minimum heat flux of green roof was 24% higher. Fig. 16 indicates that during most of the day, the heat flux of concrete roof is positive, while the one of green roof is negative. When the green roof did not receive solar radiation, most of the time it has a positive heat flux. During the day when the ambient temperature is high, the green roof removes heat from the interior of the test cell and at night when the ambient temperature low, the green roof dissipates heat to the interior of the cell. After the day in which the green roof was watered, we can observe an increase of the negative heat flux of the green roof and a decrease of the positive heat flux. To quantify the daily heat gain of the roofs, we determined the area under the heat flux curve (q) by using the trapezoidal rule
• The plant that tolerated the highest number of days without irrigation was Aeonium Subplanum, with an irrigation requirement every 9 days. The plant that required irrigation more frequently was Sedum Makinoi, with an irrigation requirement every 3 days. • After the irrigation of the green roof, the maximum temperature of vegetation, the substrate, and the slab decreased 6.4, 4.8 and 1.3 ◦ C, respectively. The reduction of the green roof maximum temperature due to the irrigation demonstrates the important role of the VWC as temperature regulator for green roofs. • During the day, the exterior surface temperature of the green roof slab was 20.5 ◦ C lower than the concrete roof. This reduction occurred when the ambient temperature reached its maximum value. The maximum temperature of the green roof was even lower than the ambient temperature by 6.8 ◦ C. On the other hand, the concrete roof had a maximum temperature up to 15.3 ◦ C higher than the ambient temperature. This indicates that the concrete roof dissipates to the environment, whereas the green roof does the opposite, it gains heat from the environment and therefore it helps to reduce the heat island phenomenon.
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Table 3 Daily energy supplied by the green roof and the concrete roof calculated with a numerical integration by the trapezoid rule. Day
21 22 23 24 25 26 27 28 Total
Numerical integration by trapezoidal rule
24 0
q(t)dt
192 0
q(t)dt
Energy gain (Wh/m2 )
Difference between energy gained
Green roof
Concrete roof
(Wh/m2 )
(%)
263 260 253 237 254 275 248 242
507 491 508 449 454 486 494 546
244 231 255 212 200 211 247 304
48.1 47.0 50.2 47.2 44.0 43.4 49.9 55.8
2070
3980
1910
48.4
• At night, the temperature of the exterior surface of the green roof slab was up to 7.1 ◦ C greater than temperature of the conventional concrete roof. During the night, the ambient temperature is up to 3.8 ◦ C higher than the temperature of conventional concrete roof. On the other hand, the ambient temperature is up to 5.4 ◦ C lower than the temperature of green roof. At night, the green roof transfers heat to the indoors, which favors the increase of temperature inside the test cell when the ambient temperature reaches its minimum which on average was 13 ◦ C. • During the day, the green roof acted as a heat sink, the heat flux goes from the interior of the cell to the roof, removing up to 21.9 W/m2 . During the day, the conventional concrete roof acted as a heat source, the heat flux goes from the roof to the interior of the cell, contributing up to 64.2 W/m2 . During the night, the direction of the heat flux is reversed on both roofs. In the green roof the heat flux is up to 10.7 W/m2 , which goes from the roof to the interior of the cell. In the concrete roof the heat flux is up to 29 W/m2 , from the interior of the cell to the roof. • The test cell with green roof had an accumulated electricity consumption 1.3 kWh lower than the test cell with the conventional roof, representing 10.3% less electricity consumption than the conventional roof
Acknowledgement M.A. Chagolla-Aranda wishes to acknowledge the support of the Consejo Nacional de Ciencia y Tecnología (CONACYT), through its graduate scholarship program.
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Effect of irrigation on the experimental thermal performance of a green roof in a semi-warm climate in Mexico M.A. Chagolla-Aranda a , E. Simá a , J. Xamán a,∗ , G. Álvarez a , I. Hernández-Pérez b , E. Téllez-Velázquez a a
Centro Nacional de Investigación y Desarrollo Tecnológico, CENIDET-TecNM-SEP Prol, Av. Palmira S/N. Col. Palmira, Cuernavaca, Morelos CP 62490, Mexico Universidad Juárez Autónoma de Tabasco, División Académica de Ingeniería y Arquitectura, Carretera Cunduacán-Jalpa de Méndez km. 1, Cunduacán, Tabasco, CP 86690, Mexico b
a r t i c l e
i n f o
Article history: Keywords: Irrigation Thermal evaluation Green roofs Energy consumption Semi-warm climate
a b s t r a c t A study of the experimental thermal performance of a green roof in a semi-warm climate of Mexico, which considers the irrigation effect, is presented. The experiment was performed in two stages. The first stage consisted of selecting from among five types of plants, commonly used in green roofs, the one that tolerates the greatest number of days without irrigation. This stage was carried out in the warm season and we selected the Aeonium subplanum plant for the thermal experimental evaluation. In the second stage, we constructed two test cells in which the temperature and the heat flux of a green roof and a concrete roof were measured for a period of 8 days. During the experimental test the green roof was watered in one day of the evaluation period. From the results, it was found that after the irrigation event the maximum temperature of the green roof components is reduced by 6.4, 4.8 and 1.3◦ C for vegetation, substrate and slab, respectively. The experimental results show that the use of a green roof decreased the temperature by 20.5 ◦ C compared to a concrete roof. The test cell with green roof had an accumulated electricity consumption 1.3 kWh lower than the test cell with the conventional roof, representing 10.3% less electricity consumption. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Buildings sector is one of the largest energy consumers in the world and it generates large amounts of waste and pollution. Buildings account for over one-third of the energy consumption and produce the same amount of carbon dioxide [1]. Air conditioning is one of the more important causes of energy use in buildings [2]. In urban areas, the increase of the ambient temperature due to the urban heat island (UHI) effect can increase the energy consumption in buildings and in others public Facilities, as lighting systems [3,4]. The effects of UHI also affect the environment, air pollution concentrations increase in warming cities, high pollution concentrations combined with high temperatures can have a negative impact on people health conditions [5].
∗ Corresponding author. E-mail addresses: [email protected] (M.A. Chagolla-Aranda), [email protected] (E. Simá), [email protected] (J. Xamán), [email protected] (G. Álvarez), [email protected] (I. Hernández-Pérez), ivan [email protected] (E. Téllez-Velázquez). http://dx.doi.org/10.1016/j.enbuild.2017.08.082 0378-7788/© 2017 Elsevier B.V. All rights reserved.
Several building envelope techniques have been developed to improve the thermal comfort, to reduce the energy consumption from air conditioners and to mitigate the UHI effect [6]. The roof is the component of the building envelope with the highest temperature fluctuations and significantly contributes to the building energy load. In this sense, several passive technologies such as evaporative cooling, reflective materials, shading with photovoltaic panels, insulation and vegetation can be used to minimize the energy gain from a roof [7]. Green roofs have been increasingly investigated to determine how they can reduce the energy consumption from buildings and improve the quality of the urban environment. Green roofs are totally or partially covered with a layer of vegetation. The main components of a green roof are: waterproofing, root barrier, substrate and vegetation. Green roofs often include drainage, thermal insulation and irrigation system. There are two types of green roofs: intensive and extensive. The intensive green roof has a thick substrate layer (more than 20 cm) and allows to grow deep root plants such as shrubs and trees. The maintenance is high because required fertilizing, weeding and watering. The extensive green roof has a thinner substrate layer (less than 20 cm) and allows to grow small plants, such as grass or sedum. The installation cost is lower, and
M.A. Chagolla-Aranda et al. / Energy and Buildings 154 (2017) 232–243
Nomenclature CR G GR q qCR sen qGR sen RH T TCR TGR Tveg CR Tslab,ext
Concrete roof Solar radiation (W/m2 ) Green roof Heat flux (W/m2 ) Concrete roof heat flux sensor (W/m2 ) Green roof heat flux sensor (W/m2 ) Relative humidity (%) Temperature (◦ C) Concrete roof temperature (◦ C) Green roof temperature (◦ C) Air temperature at the vegetation (◦ C) Concrete roof slab external surface temperature (◦ C)
T CR slab,int
Concrete roof slab internal surface temperature (◦ C)
T GR slab,ext
Green roof slab external surface temperature (◦ C)
Green roof slab internal surface temperature (◦ C) Substrate external surface temperature (◦ C) Substrate internal temperature (◦ C) Temperature difference (◦ C) Temperature difference between concrete roof and green roof (◦ C) Tamb−CR Temperature difference between ambient and concrete roof (◦ C) Tamb−GR Temperature difference between ambient and green roof (◦ C) Time (h) t V Wind velocity (m/s) VWC Volumetric water content (m3 /m3 ) VWCsen Volumetric water content sensor (m3 /m3 ) GR Tslab,int
TSust,ext TSust,int T TCR−GR
Subscript Ambient amb sen Sensor subst Substrate Vegetation veg Superscript CR Concrete roof GR Green roof
it has low weight and required minimal maintenance. The main factors that affect the heat transfer in green roofs are: cooling due to evapotranspiration, increased thermal insulation by adding layers of material and vegetation as a barrier to transmission of solar radiation. The aim of a green roof is to provide aesthetical environmental and economic benefits. Green roofs can reduce polluting air particles and increase the oxygen production which contributes to the improvement of the environment. Green roofs can also reduce the urban storm water runoff problems, absorbing part of the rainfall and distributing the runoff over a long-time; provide food and habitat for native plants and animals; reduce noise pollution; create space where people can interact each other and rest; increases the internal roof layers’ durability. Several of the benefits of green roofs are achievable, but due to lack of research green roofs are generally not designed to meet all those benefits. The benefits of green roofs depend of multiple factors as climate, geometry or materials composition [8]. However, the environment can determine its functionality in terms of the thermal performance. Some experimental studies have been made to understand the green roof thermal performance in different climates, a few of them are presented next. Most of the experimental work found in this research report the
233
temperature in different layers of the green roof and some report the heat flow either measured or calculated, the results are often compared with a reference roof in different seasons. In temperate climate, green roofs are an effective solution to achieve thermal comfort and reduce cooling demand in summer. In winter a green roof functions as a thermal insulation, even in conditions of saturation it can reduce the heating energy demand compared with plywood and concrete roofs [9–11]. In tropical climate green roofs can reduce the outer and inner roof temperature in free floating conditions [12–14], saving the 22% of electric power in air conditioning conditions [15]. The previous benefits have been achieved in the summer season. In Mediterranean claimed Bevilacqua et al. [16] reported that during summer a green roof had an exterior surface temperature 12 ◦ C lower than a lightweight concrete roof and it maintained the surface temperature 4 ◦ C higher than the concrete roof in winter. Gagliano et al. [17] reported that during the summer a green roof lowered the maximum outdoor surface temperature by 27.7 ◦ C compared to a concrete roof, the green roof also reduced the temperature range throughout the day. Coma et al. [18] reported for the same climate the electrical energy consumption of a heat pump system in two rooms, one with concrete roof and other with green roof; the results concluded that the green roof can reduce the energy consumption (even 16.6%) during warm season, but present a higher energy consumption (even 11.1%) during cold season. In semi-arid climate, Reyes et al. [19] recommended well irrigated green roofs with substrate depth (more than 10 cm), it can reduce the exterior surface temperature 13 ◦ C below ambient temperature and helps to maintain a stable and suitable substrate water content. In hot climate, Dvorak and Volder [20] reported that even unirrigated green roofs can reduce the exterior roof temperature 37.7 ◦ C in comparison with a concrete roof. In hot climate, La Roche and Berardi [21] reported that highly insulated green roofs can overheat the indoor temperature during summer in comparison with reflective and thermal insulated metal sheet roof. In cold climate, there are net benefit of the use of green roofs compared with concrete roofs, but the benefits are lower in extreme winter conditions when the substrate is frozen, when the roof has a snow layer and also during sunny conditions [22–24]. Zhao et al. [25] reported that a green roof in a cold winter claimed can reduce the building’s heating energy consumption by 23% compared to a reference building with plywood roof. However, this energy saving was reduced to 5% with a snow layer accumulated on the roofs. Due to the importance of climate in green roof thermal performance most of the studies are agree on the need to report green roofs information in as many climates as possible in order to have more knowledge about its benefits. In Mexico, there are few norms [26–29] oriented to the efficient use of energy in buildings and studies related to the application of passive techniques are scarce. Some passive techniques studied in Mexico are natural ventilation [30], earth-to-air heat exchanger [31], room-Trombe wall [32], insulation and reflective materials [33], window shading [34], cool roof [35], among others [36,37]. Below are presented some works that reports the thermal performance of green roofs in Mexico, these studies showed that green roofs reduced roof temperature compared to conventional concrete roofs in the warm season. Ovando-Chacón et al. [38] performed simulations to determine the air temperature distribution inside test cells with a concrete roof and a green roof. The simulation was performed using the climate data of the city of Veracruz, Mexico. The green roof reduced the temperature of the indoor air near the ˜ roof by 10 ◦ C with respect to the concrete roof. Castaneda-Nolasco and Vecchia [39] conducted an experimental study that measured the temperature of a concrete roof and a green roof in a tropical climate in Mexico. The green roof reduced the maximum temperature of the interior surface by 13.7 ◦ C with respect to the concrete roof. ˜ and Pérez-Sánchez [40] carried out an experimenOrdonez-López
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Fig. 1. Plants used in the irrigation experiment:(a) Sedum adolphii, (b) Echeveria prolífica, (c) Aeonium subplanum, (d) Crassula ovata y (e) Sedum Makinoi.
tal study in which the temperature of a reflective roof and a green roof were measured in a building in Yucatán, Mexico. The green roof reduced the air temperature inside the building compared to the reflective roof, achieving values closer to an established comfort range. In the studies reported by Quezada-García et al. [41,42] presented models for prediction of heat transfer of green roofs, however the data used in the models do not reflect the effects of the Mexican climate. In Mexico, green roofs have shown that they are an effective measure to improve comfort conditions, which could reduce cooling loads, which are prevalent in the country because two-thirds of the territory is considered to have warm weather. However, it is necessary to have more information about its benefits for different types of climates and other times of the year. Because of above, in this article we present an experimental thermal evaluation of a green roof and compared its performance with a conventional concrete roof using outdoor test cells. Both test cells are air conditioned and located in a semi-warm climate in Mexico. The parameters measured were temperature at different levels and heat flux for each roof which considered the effect of irrigation. 2. Materials and methods 2.1. Methodology The experiment was carried out in Cuernavaca, Mexico. This city is at the central zone of Mexico and its elevation is 1500 m above sea level. The climate of Cuernavaca is semi-warm sub-
humid with light rains during summer [43]. The experimental study was divided into two stages: • The first stage consisted of selecting a plant, among five different types usually used in green roofs, that tolerates the greater number of days without irrigation conserving a visually healthy state. This stage was conducted from Mar. 3rd 2015 to Apr. 30th 2015. The season in which this experiment was performed is considered the warm season. • The second stage consisted of measuring temperatures and heat fluxes of a green roof and a concrete roof. The experiment was carried out during 8 days, from Nov. 21th to Nov. 28th 2016, corresponding to the cold season. To find out how the irrigation influences the temperatures and the heat transfer of the green roof, the substrate was watered on Nov. 25th 2016. 2.2. Selection of plant type and tolerance to drought The plants of the family Crassulaceae and among them, the genus Sedum is one of the main type of plants used for green roofs due to its small requirement of irrigation and its adaptability to sunny environments [44]. Other types of plants widely used in green roofs are grasses and native plants. The low irrigation requirement is a property of the plants used in this type of roof that becomes even more valuable in places where water availability is limited. Moreover, in semi-arid and tropical climates the water content in a green roof influences the reduction of the roof and ambient temperatures [19,45]. To make a better use of water, it is necessary to know the
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Fig. 2. Exposure of plants to the outdoor environment.
Table 1 Number of days without irrigation for each type of plant. Type of plant
Number of days without irrigation
Sedum Makinoi Sedum Adolphii Echeveria Prolífica Crassula Ovata Aeonium Subplanum
3 4 6 7 9
irrigation requirements of the plants used in green roofs. The objective of the experiment carried out in the first stage was to determine how many days five types of plants can stay visually healthy without irrigation for the climate of Cuernavaca, Mexico. In the first stage the number of days in which the plants should be watered was determined, then the plant that requires irrigation for the longest period was selected to use it for the experiment of thermal performance of the green roof (second stage). The experiment of the first stage consisted of placing five types of plants from the family Crassulaceae in containers 50 cm long by 30 cm wide with substrate of 10 cm depth. Fig. 1 shows the plants used in the study: Sedum Adolphi, Echeveria Prolifica, Aeonium Subplanum, Crassula Ovata and Sedum Makinoi [46]. The experiment was carried out two weeks after the plants were placed in the containers and exposed to the outdoors (Fig. 2). During this period, all plants were watered every third day in the afternoon, and we observed a good adaptation of the plants to the environment. The experiment was conducted from Mar. 3rd 2015 to Apr. 30th 2015, which is considered the warm season. In this period, the ambient temperature reached values up to 35 ◦ C, annual minimum relative humidity and rain events are uncommon (during the experiment period no rain event occurred). The day the experiment was started, the plants were watered until substrate saturation, and the following days the appearance of each plant was observed. The leaf turgor was evaluated visually. Similar qualitative assessments were used by Moterusso et al., Nagase et al., Liu et al. and Farrel et al. [47–50]. The evaluation was done in the afternoon when the sun was hidden, this moment was chosen because the plants had just passed through the period of greater stress of the day. When the loss of leaf turgor was evident, it was considered the need of irrigation. Once the plants were evaluated in irrigation condition, they were watered to saturation and record it as an irrigation event. If the plants conserved the turgor in most of their leaves, the plants remained without irrigation, until they were evaluated in irrigation condition. The number of days in which each plant had an irrigation event was recorded. We observed that this number was constant for each type of plant during the period of the experiment. Table 1 shows the number of days that each type of plant stayed without being watered. The plant that tolerated the largest number of days without being watered was the Aeonium Subplanum with 9 days. This plant was selected to be used in
the green roof experiment of second stage because of its ability to tolerate the longest time without irrigation. 2.3. Test cells and instrumentation 2.3.1. Construction of the test cells We constructed two outdoor test cells to simultaneously evaluate the thermal performance of a green roof and a conventional concrete roof (reference roof). For the construction of each cell, we used 1.8 cm thick OSB boards and joined them to form a cubic cavity. The interior surface of the walls was covered with extruded polystyrene insulation of 2.5 cm thick. The walls of the cavity are 90 × 90 cm2 (Fig. 3a). A metal cube structure of 95 cm side was welded to contain the two wooden cavities and respectively support the weight of the concrete roof and the green roof. (Fig. 3b). The test cells were painted white to reflect solar radiation that could enter the through the walls (Fig. 3c). Two circular holes were made in one of the walls of the cell to connect the ducts of the air conditioning system. The top of the cells was uncovered to place the green roof and the concrete roof (Fig. 3d). We constructed two concrete slabs with a thickness of 10 cm and 120 cm side with reinforcement of metallic mesh in the center. The roof slabs were placed on the test cells with metallic structure (Fig. 4a). In the case of the concrete roof, no other components were added. We installed a 15-cm high sheet guard to contain the substrate and the plants for the green roof. The green roof slab was covered with a waterproofing material that integrates an anti-root layer (Fig. 4b). We used 6 cm of substrate with high mineral stone content, and Aeonium subplanum was added as vegetation for the green roof (Fig. 5(a) and (b)). The test cells were placed on the roof of a four-story building where there is no shading due to neighboring buildings. The test cells are located 20 m away from the weather station from which weather information was obtained. Fig. 6 shows the green roof and concrete roof installed on the test cells, the ducts of the air conditioners in the cells, and the two air conditioners connected to the cells. Each test cell has two ducts, inside one circulates the cold air from the air conditioner, and inside the other circulates the air from the test cell that removed the heat gain. To avoid heat transfer between the cells and the roof of the four-story building the cells were placed at a height of 20 cm. The following table shows the optical and thermo-physical properties proposed for the green roof and concrete components [51,52] (Table 2). 2.3.2. Instrumentation To measure climatic variables, we used a Vaisala Maws100 weather station located at the experiment site (Fig. 7). The weather station has a Vaisala QMS101 pyranometer with uncertainty
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Fig. 3. Construction of the test cells: (a) metallic structure, (b) wood cavity and thermal insulation, (c) Application of paint to the cells and (d) cell holes for connection of air conditioner.
Fig. 4. Construction of the concrete slabs: (a) casting of concrete slabs and (b) waterproofing of the roof slab for the green roof.
Table 2 Optics and thermo-physical properties of the vegetation and concrete roof. Roof type Component
Properties
Thickness (m)
Optics
Green
Concrete
Vegetation Substrate Slab Slab
Thermo-physical
Absorbance
Emissivity
Thermal conductivity (W m−1 K−1 )
Density (kg m−3 )
Specific heat (J kg−1 K−1 )
0.72 0.85 – 0.6
0.90 0.95 – 0.82
– 0.25 1.74 1.74
– 1370 2300 2300
– 800 840 840
0.08 0.06 0.1 0.1
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Fig. 5. (a) Application of the substrate and (b) Vegetation of the green roof.
Fig. 6. Outdoor test cells.
Omega HFS-4 heat flux sensor with an uncertainty of ± 10% on the inner surface of the slabs. For a better contact between the heat flux sensors and the slab, we used a thermal paste and adhesive tape to adhere the flux sensor (Fig. 8b). To measure the air temperature that surrounds the vegetation layer of the green roof, and the temperature of outer surface and the center of the substrate, we used thermocouples type “T”. To measure the volumetric water content (VWC) at the center of the substrate, we installed Sensors of the Decagon Devices EC-5 with an uncertainty of ± 3% (Fig. 8c). All information data from the sensors was stored by using a Keysight 34972A data acquisition system (resolution of 6½ digits). Measurements were taken every 10 min with the Keysight data acquisition 34972. The indoor air temperature of each test cell is maintained at constant temperature of 25 ◦ C by an air conditioning unit controlled with by an electronic thermostat with an uncertainty of ± 1 ◦ C. The air conditioners used have a capacity of 1470 W. The air conditioning systems are connected to the test cells through ducts coated with thermal insulation. Fig. 9 shows a diagram with the location of all sensors used in the outdoor test cells. 3. Results Fig. 7. Weather station.
of ± 1%. The sensors of temperature, humidity, wind speed and precipitation are integrated in a Vaisala WXT536 multi-sensor, the uncertainty of each sensor is: ± 0.3 ◦ C, ± 3%, ± 3% and ± 5% respectively. We placed thermocouples type “T” (with an uncertainty of ± 0.5 ◦ C) on the outer surface, the center and the inner surface of the two concrete slabs (Fig. 8a). In order to avoid that the solar radiation influenced in the measurement of the temperature of the outer surface, the thermocouples were placed inside a pvc guard. To measure the heat flux through the slabs, we placed a thin-film
In the experimental results, we present a comparison between the temperature of the green roof and the concrete roof during a test period of 8 days, from 11/21/2016 to 11/28/2016. 3.1. Climatic conditions The data of the meteorological variables as ambient temperature, relative humidity, solar radiation and wind speed measured during the period of the experiment are shown in Fig. 10. The average ambient temperature was 19.4 ◦ C. The average daily maximum ambient temperature was 25.9 ◦ C. The average daily minimum
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Fig. 8. Sensors used in the outdoor test cells: (a) thermocouple arrangement in the slab, (b) heat flow sensor in the slab and (c) volumetric water content sensor in soil.
Fig. 9. Schematic of the instrumentation.
ambient temperature was 13 ◦ C. The average relative humidity was 47.8%. The average daily maximum relative humidity was 71%. The average daily minimum relative humidity was 29%. The average maximum solar radiation was 752 W/m2 . The average wind speed was 1 m/s. The Fig. 10(c) indicates that the wind speed increases during the day and decreases during the night. No rain event occurred during the experiment period. 3.2. Irrigation event One of the objectives of the experiment was to observe the behavior of the green roof when there is an irrigation event, so the green roof was watered until saturation on November 25th, 2016. Fig. 11 shows the VWC of the substrate during the period of the experiment. At the beginning of the experiment the VWC
was 0.37 m3 /m3 . The next days the VWC decreased with an almost linear behavior, having a greater decrease in the first two days of the experiment (Nov. 21th and 22th, 2016) compared with the two days before irrigation (Nov. 23th and 24th, 2016). The VWC further attenuated on the day of irrigation (Nov. 25th, 2016) until it reached a minimum of 0.28 m3 /m3 . During the day of the irrigation, a sudden increase of the VWC occurred until it reached a maximum value of 0.53 m3 /m3 . After irrigation, the VWC linearly decreased, like the days prior to irrigation. At the end of the experiment the VWC was 0.41 m3 /m3 . The amount of water with which the vegetation was irrigated was 50 l. This amount was used because the substrate was saturated with a CVA of 0.54, the total volume of the substrate being 86.4 l, the corresponding volume of water that can retain the saturated substrate is 46.7 l.
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Fig. 11. VWC of the substrate.
with 34.5 and 27.2 ◦ C, respectively. The next day after irrigation, the peak temperature of vegetation was reduced by 6.4 ◦ C, the substrate peak temperature by 4.8 ◦ C and the slab peak temperature by 1.3 ◦ C. Once the VWC started to decrease again, the temperatures of the three layers of the green roof increased. Between the first day after the substrate was irrigated and the last day of the experiment, the maximum temperature of the vegetation and substrate increased 1.7 ◦ C and that of the slab increased 1.2 ◦ C. 3.3. Comparison between the green roof and conventional roof temperatures
Fig. 10. Meteorological data: (a) temperature and relative humidity, (b) global solar radiation and (c) wind speed.
3.2.1. Green roof temperature profile To understand how irrigation affects the temperature of the roof components, Fig. 12 shows the temperature of vegetation, the temperature of the substrate surface, the temperature of the upper surface of the green roof slab and the VWC during the period of the experiment. The components that are most affected by the irrigation are the vegetation and the substrate. The Fig. 12 indicates that as the VWC decreases the temperature of vegetation and substrate increases, having the peak value the day before irrigation
To quantify the benefits provided by a green roof, Fig. 13 shows the outer surface temperature of the slab for the green roof and the corresponding to a conventional concrete roof. The figure demonstrates that the maximum temperature of the upper surface of the green roof slab is lower than the temperature of the concrete roof, and lower than the ambient air temperature during every day. The green roof reduces the average maximum surface temperature of the slab to 17.1 ◦ C with respect to the concrete roof. On the other hand, the average maximum temperature of the upper surface of the green roof slab is 4.1 ◦ C lower than the ambient temperature. Fig. 13 indicates that after irrigation, the maximum temperature of the green roof was 1.3 ◦ C lower than the maximum temperature it reached before irrigation. At night, the average minimum temperature of the upper surface of the green roof is 6 ◦ C greater than that of the concrete roof, and 3.1 ◦ C greater than the ambient temperature. We found a difference smaller than 0.3 ◦ C for the minimum temperature of the green roof before and after irrigation. Similar results are reported by He et al. (2016) [12], which measured the temperature and heat flow of a green roof and concrete roof of an air-conditioned room in a tropical climate. In their results it is said that the green roof can act as a source of heat releasing heat to the environment during the afternoon and evening. Fig. 14 shows the temperature difference between the exterior surface of the concrete roof slab and the green roof slab (TCR-GR = TCR -TGR ). During the day, the TCR-GR was positive with maximum values ranging between 18.1 and 20.5 ◦ C. During the night, the TCR-GR was negative, with minimums ranging between −7.1 to −5.6 ◦ C. The figure shows that the maximum value of TCR-GR decreased as the irrigation event approaches (when VWC reaches its lowest value), and it increased after irrigation. Furthermore, the figure shows that the daily minimum values of TCR-GR increased after irrigation until the end of the experiment. Fig. 14 demonstrates that most of the day, the temperature of the concrete slab is higher than the corresponding to the green roof. This
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Fig. 12. Temperature profile of the green roof.
Fig. 13. Temperature of the green roof slab and conventional concrete slab.
Fig. 14. Temperature difference between TCR −TGR .
effect is observed with the positive values of TCR-GR which cause the concrete roof to have a greater energy gain than the green roof. Fig. 15 shows the temperature difference between TAmb and TCR (TAmb-CR ), and between TAmb and TGR (TAmb-GR ). The figure indicates that during the day TAmb-CR is negative, having a minimum of −15.3 ◦ C, therefore the direction of the heat flux will be from the concrete roof to the ambient. During the night, the value of TAmb-CR is positive, so the direction of the heat flow is from the ambient to the concrete roof, with TAmb-CR having a maximum of 3.8 ◦ C. This means a drastic decrease in the temperature of the concrete roof, even below the minimum ambient temperature at night. In the case of TAmb-GR , this have a behavior opposite to TAmb-CR , since during day TAmb-GR has positive values and at night negative values. Thus, the green roof absorbs heat from the environment during the day and at night the heat is transferred to the outdoor environment. The maximum and minimum values of TAmb-GR were 6.8 and −5.4 ◦ C respectively. We can also note the reduction of the amplitude of TAmb-GR compared to TAmb-CR . There is a tendency the maximum values of TAmb-GR to be reduced
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Fig. 17. Electricity consumption.
Fig. 15. Temperature difference between TAmb y TCR (TAmb-CR ), and between TAmb and TGR .
for each day. Table 3 presents the daily heat gain of the green and conventional roof. The third column shows the difference between the heat gain of the green roof and the heat gain of the conventional roof. This column indicates that for all days of the experiment, the heat gain of the green roof was up to 55% lower than the one of the concrete roof. 3.5. Electricity consumption
Fig. 16. Heat fluxes from the green roof and from the conventional roof.
It was measured the electricity consumption of the air conditioning equipment that maintained the air temperature of the test cells in a comfort range between 24 and 26 ◦ C. Fig. 17 shows the accumulated electricity consumption during the period of study for each test cell. The test cell with green roof had an accumulated electricity consumption 1.3 kWh lower than the test cell with the conventional roof, representing 10.3% less electricity consumption than the conventional roof. The greatest difference of the daily energy consumption was the day after that the irrigation was performed with a difference of 0.26 kWh, which represents 20% of the difference of the accumulated total.
as the irrigation event approaches and the VWC is reduced. The maximum values of TAmb-GR increase after the irrigation is performed.
4. Conclusions
3.4. Heat fluxes
Based on the experimental results of the thermal performance of a green roof and a concrete roof in the semi-warm climate of Cuernavaca, Mexico, the following is concluded:
Fig. 16 shows the heat flux through the green roof and the concrete roof. We considered that the heat flux is positive when it goes from the outside of the roof to the interior of the cavity and negative in the opposite case. The maximum heat flux of the green roof was 10.7 W/m2 whereas the corresponding to the concrete roof was 64.2 W/m2 , which represents a difference of 83.3%. On the other hand, the minimum heat flux of the green roof and the concrete roof was −21.9 W/m2 and −29 W/m2 , respectively, which indicates that the minimum heat flux of green roof was 24% higher. Fig. 16 indicates that during most of the day, the heat flux of concrete roof is positive, while the one of green roof is negative. When the green roof did not receive solar radiation, most of the time it has a positive heat flux. During the day when the ambient temperature is high, the green roof removes heat from the interior of the test cell and at night when the ambient temperature low, the green roof dissipates heat to the interior of the cell. After the day in which the green roof was watered, we can observe an increase of the negative heat flux of the green roof and a decrease of the positive heat flux. To quantify the daily heat gain of the roofs, we determined the area under the heat flux curve (q) by using the trapezoidal rule
• The plant that tolerated the highest number of days without irrigation was Aeonium Subplanum, with an irrigation requirement every 9 days. The plant that required irrigation more frequently was Sedum Makinoi, with an irrigation requirement every 3 days. • After the irrigation of the green roof, the maximum temperature of vegetation, the substrate, and the slab decreased 6.4, 4.8 and 1.3 ◦ C, respectively. The reduction of the green roof maximum temperature due to the irrigation demonstrates the important role of the VWC as temperature regulator for green roofs. • During the day, the exterior surface temperature of the green roof slab was 20.5 ◦ C lower than the concrete roof. This reduction occurred when the ambient temperature reached its maximum value. The maximum temperature of the green roof was even lower than the ambient temperature by 6.8 ◦ C. On the other hand, the concrete roof had a maximum temperature up to 15.3 ◦ C higher than the ambient temperature. This indicates that the concrete roof dissipates to the environment, whereas the green roof does the opposite, it gains heat from the environment and therefore it helps to reduce the heat island phenomenon.
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Table 3 Daily energy supplied by the green roof and the concrete roof calculated with a numerical integration by the trapezoid rule. Day
21 22 23 24 25 26 27 28 Total
Numerical integration by trapezoidal rule
24 0
q(t)dt
192 0
q(t)dt
Energy gain (Wh/m2 )
Difference between energy gained
Green roof
Concrete roof
(Wh/m2 )
(%)
263 260 253 237 254 275 248 242
507 491 508 449 454 486 494 546
244 231 255 212 200 211 247 304
48.1 47.0 50.2 47.2 44.0 43.4 49.9 55.8
2070
3980
1910
48.4
• At night, the temperature of the exterior surface of the green roof slab was up to 7.1 ◦ C greater than temperature of the conventional concrete roof. During the night, the ambient temperature is up to 3.8 ◦ C higher than the temperature of conventional concrete roof. On the other hand, the ambient temperature is up to 5.4 ◦ C lower than the temperature of green roof. At night, the green roof transfers heat to the indoors, which favors the increase of temperature inside the test cell when the ambient temperature reaches its minimum which on average was 13 ◦ C. • During the day, the green roof acted as a heat sink, the heat flux goes from the interior of the cell to the roof, removing up to 21.9 W/m2 . During the day, the conventional concrete roof acted as a heat source, the heat flux goes from the roof to the interior of the cell, contributing up to 64.2 W/m2 . During the night, the direction of the heat flux is reversed on both roofs. In the green roof the heat flux is up to 10.7 W/m2 , which goes from the roof to the interior of the cell. In the concrete roof the heat flux is up to 29 W/m2 , from the interior of the cell to the roof. • The test cell with green roof had an accumulated electricity consumption 1.3 kWh lower than the test cell with the conventional roof, representing 10.3% less electricity consumption than the conventional roof
Acknowledgement M.A. Chagolla-Aranda wishes to acknowledge the support of the Consejo Nacional de Ciencia y Tecnología (CONACYT), through its graduate scholarship program.
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