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Revista ingeniería de construcción
On-line version ISSN 0718-5073
Rev. ing. constr. vol.33 no.1 Santiago Apr. 2018
http://dx.doi.org/10.4067/S0718-50732018000100015
Articles
Thermal Response to cold in buildings with Green covers for tropical climate. Green facades and green roofs
*Escuela de Ingeniería de São Carlos, Universidad de São Paulo BRASIL
**Departamento de Zootecnia e Ingeniería de Alimentos - Escuela de Ingeniería de São Carlos, Universidad de São Paulo. BRASIL
***Departamento de Hidráulica y Saneamiento, Escuela de Ingeniería de São Carlos, Universidad de São Paulo. BRASIL
****Universidad Dom Pedro II. Rua Badi Bassiti, São José do Rio Preto. BRASIL
Abstract: The main aim of this paper is to demonstrate the benefits that building using natural elements can provide to indoor environments when temperatures are lower. For this purpose, an experimental procedure was used to allow a comparison of the thermal behaviour of four systems: a prototype called Control (no vegetation) and three different combinations of vegetation (roofs and facades) installed in a region of tropical climate. They recorded simultaneously internal surface temperatures, internal air temperature and the external environmental conditions. The results show that the use of plant systems in buildings establishes a passive technique effective in reducing energy consumption, because of its insulation property and it maintains more pleasant thermal internal conditions than internal ones.
Keywords: Green facades; Green roof; Thermal performance; Thermal comfort; Bioclimatic Architecture
Introduction
Due to the population increase and the growth in urban centres, there have been significant environmental impacts as a result of the massive concentration of buildings, causing the sealing of soil, reduction of green spaces and air pollution increment. All this leads to, a thermal behaviour modification microclimate scale and generates what is called Urban Heat Island (Vecchia 2005).
The gradual concern about these issues has led to a greater interest for a more sustainable architecture. Green spaces inside cities can be considered as environmental quality indicators. The use of vegetation on roofs and facades is a technique which has a positive impact on urban climate, due to the characteristics of the plants that create shaded areas, absorb some of the incident radiation, and encourage rainwater infiltration; therefore, they also increase the relative humidity, all the factors that contribute to Heat Urban Island.
According to (Dunnett and Kingsbury 2008), the vegetation is a technique that has been used for centuries throughout the world to improve the thermal comfort in buildings. This is one of the reasons why the technique is being accepted with a great interest nowadays, in addition to the environmental benefits which are offered, it improves the internal thermal comfort and thus the energy efficiency of buildings. The use of Green Facades with climbing plants is able to reduce the internal temperature of buildings due to the shading that occurs there. Also, in winter, this system protects from the cold and wind, by the branched structure forming by the stem of the plant, avoiding losses of internal heat.
This paper proposed the study of Green Facades and Green Roofs in a tropical climate for a critical cold day, showing experimental results of four test cells in which were measured internal surface temperatures and internal air temperatures to make subsequently, a comparative study between those test cells with and without vegetation test cell. With the final results, it has been verified the potential vegetation respecting to internal heat losses, since, such reduction of heat losses is one of the most useful measures in the sense of improving indoor comfort conditions.
Materials and Methods
This Project about plant covers in architecture aims the use of construction methods that cause a less impact than conventional buildings, and improve the comfort of indoor environmental energy efficiency. For estimation, four experimental cells were planted vegetation in facades (N and W), as well as were installed plants on the roof, as shown in Table 1.
Internal surface temperatures data (IST) and Dry Bulb temperature (DBT) were measured by through thermocouples installed in the test cells. The data of solar radiation and other climatic variables were recorded by the automatic weather station of the Centre of Hydric Resources and Environmental Studies (CRHEA) belonging to the Engineering School of São Carlos (EESC-USP).
To obtain the results, this study was based on the analysis of a critical cold day (winter day), it is defined as a day which was registered, as an exceptional climate form, a minimum temperature lower than the absolute maximum temperatures in the historical data series registered in Normal Climatological 1961-1990, both published in 1992 by the Ministry of Agriculture and Agrarian reform, National Department of Meteorology, Brasilia.
The data were measured by thermocouples installed on the facades North, West and roofs of the four test cells for the period from 07 to 12 July 2015.
Localization and characterization of the study area.
This Project was developed in Itirapina- SP in the margins of the Lobo reservoir, at 733m altitude above sea level (Figure 1). It is a complex area to define because of the climatic origin actions, being a region in transition from polar and inter-tropical atmospheric systems. It is located in an area considered tropical altitude. According to the International Classification of Köppen correspond with a climate Cwa, which is characterized as a warm climate with dry winters in which the average temperature of the coldest month is less tan 18°C and the hottest month exceeds 22°C (Ferrari 2012) (Figure 1).
Test cells’ dimensions are 2 x 2.50m, seated on cement mortar and sand built in situ, they are solid brick constructions with dimensions of 10 x 20 x 5 cm, placed with joints of 1.5 cm of thickness. They have a wooden door with eastern orientation whose dimensions are 2.10x0.60m and a wooden window facing North with standard measurements of 1x0.70m (Figure 2). The test cells were designed with the same orientation, so that they receive equal solar radiation, wind and other atmospheric events. This allows the climatic conditions to act simultaneously and at the same intensity in each cell, as shown Figure 2. Also, they do not create shadow zones among them.
Construction system and development of Green facades and Green roofs.
Green facades were installed in the North and West facades because they are those that receive more sunlight hours. They consist, basically, of a metal hexagonal mesh of 2.40 m wide by 3m high, and anchor to the ground and facades by hooks. The solution adopted was one that did not maintain direct contact with the wall, so it was decided to set up it at an angle of 30º to the top of the wall.
After placing the necessary mesh to enable upward growth of the plants, and cover the entire surface of the facades, a Thumbergia Grandiflora, part of the Acanthaceas’s family was sown at the bottom of the mesh, directly on the ground. This plant is a great low-maintenance vine from tropical and subtropical areas of the world. It is herbaceous and includes within Acanthaceae´s family, twining, dark green and simple, opposite and whole green leaves. The Thunbergiagrandiflora, is commonly known as blue Tumbergia (Figure 3A) and (Figure 3B), blue trumpet vine, blue Bignonia (Martínez 2002).
Source: Own composition.
Plant´s annual cycle and their growth are two important components to ensure the role of plant facades. Despite the consequences of working with living beings, plants are still in fairly constant cycles in relation to development. However, these guidelines are species-specific and depend on the climate in which the plant is located (Pérez 2010). (Figure 4)
The Green roofs were concreted in situ by a preformed ceramic slab with preformed concrete beams with an inclination of 23% and parapets of ceramic brick with size of 0.40m, to form a drawer where the substrate is settled. As it is shown in the figure, the set of the Green roofs are composed by a waterproof layer, a drainage blanket, substrate and vegetation, as shown in Figure 5 and Figure 6.
Below it will be described each item that makes up the system:
Geosynthetic and drainage:
The drainage element used was a geosynthetic blanket MacDrain 2L used for drainage of highways and other civil works, it is a mild and flexible feature whose draining core consists of a tridimensional geomat, composed by filaments of polypropylene, with a thickness between 10 and 18 mm, heat sealed among two geotextiles with polyester resin based at all contact points (Vecchia 2005). For drainage two PVC tubes were placed in lower ends.
Vegetation. Grass.
Paspalumnotatum grass was used as vegetation to cover the Green roofs (Figure 6), which is a native grass of the American continent, it is known in Brazil as the Batataisgrama, grass-forquilha, grama-mato-grosso, grama-de-pasto and grass-Common (Lorenzi and Souza 2000) It has leaves concentrated on the basal part of the plant. One of the outstanding properties is the facility to cover easily the ground, forming large mats, and this is the reason why it is used in football fields and green areas, including protection against soil erosion (Kissmann 1997). It is a species able to adapt to poor soils, water deficit conditions, resistant to the action of sunlight and treading. Although it needs to be cut frequently to maintain good quality (Goatley et al. 1998) (Figure 7).
Automatic measurements.
Each of the prototypes is made up of 15 T-type thermocouples 2x24 WG. Thermocouples are characterized as a very precise instrument. The temperature can be measured with an error of ± 0.1-0.2 ° C (Kinzie 1973). The measurements were used to study the thermal behaviour. For this reason, there were located 64 thermocouples, 16 per constructive elements, 15 (for each test cell) measuring surface temperatures and a thermocouple was placed at the geometric centre of each test cell at a height of 1.20 m approximately, to carry out measurements of the Dry Bulb Temperature (DBT), as the ABNT NBR 15575-1 Standard referred to in Annex A.
The data register was set at intervals of 30 seconds and they are totalled measured every hour. The measures are recorded by a programmable data logger CR1000, Campbell Scientific Inc., connected with two 32-channel multiplexers 416AM Campbell Scientific Inc, which are both currently calibrated. The battery is fed by a solar panel which gives autonomy to the equipment.
The following Figure 8 shows the thermocouples distribution.
Climate analysis. Critical experimental cold day, July 10, 2015.
Under the climate perspective used in this work, the experimental critical Day was defined considering dominant atmospheric state in the testing period. It was used as a reference by the Normal Climatological 1961-1990 which was published in 1992 by the Ministry of Agriculture and Agrarian Reform, National Department of Meteorology (historical series of climatic data from Brazil).
According to (Vecchia 1997), to do a more accurate climate estimation, it can make a climate analysis using representative episodes or observation periods of clime types, that is, studying the intensity and duration of each air mass which dominates on a particular place, related to the atmospheric circulation’s phenomena. The consequences of the advance of a cold front over a given locality have peculiar characteristics on each analysed season (Autumn, Winter,...).
From a point of view of the dynamic climatology, Itirapina is characterized as a region of passage of cold fronts pass throughout a year. The domain of the acting masses in the region can be divided into two main stages, Prefrontal and postfrontal, which are at the same time subdivided into two other stages, harbinger and advance, which happen before the penetration of the mass Polar Atlantic (MPA) with war features. Afterwards there will occur domain and transition phases, where the Polar Atlantic mass (MPA) imposes weather conditions with a cold feature and sometimes a wet feature (Monteiro 1967).
This classification which is beneficial because, according to the requirement of analysis climate facing thermal behaviour, can determinate the most appropriate stage to conduct experiments (Vecchia 2015).Data from external climatic variables used in this experiment from the day in question, were collected by the Climate Station of the Centre for Water Resources and Environmental Studies (CRHEA) of the School of Engineering of Sao Carlos (EESC-USP).
In this study case, the episode from 7 to 12 July 2015, was considered for analysis, days when the Northwest region of the state of São Paulo was under the control of a mass of cold air. In the Figure 9 is shown a climatic analysis of the selected day.
Analysing the climate conditions in the month of July, it is observed a day that can be considered a critical experimental cold day (07/10/2015) because on that day was recorded, as a form of climate exceptionality, a value of solar radiation of 614 (W/m2) and a minimum hourly temperature of 8.8ºC, less than the absolute minimum temperature of the Series History (12.1ºC) for that period.
Internal temperatures results in critical experimental cold day: 10/07/2015.
As it was indicated in the previous section, the experimental data of this work, concerning the winter situation, were recorded on the day July 10, 2015.
Internal air temperature or Dry Bulb Temperature (DBT):
In Figure 10, the graph shows the internal air temperature (DBT), the external air temperature and thermal comfort limits. The effect expected of an environment with vegetation would be a drop in temperature, which could extend the discomfort for cold, especially in an episode as it is presented on 10 July 2015. However, despite the shading of the building and the effect of air humidification, as shown in Figure 12, the alterations observed are predominant in the period of maximum temperatures, and always in the sense of minimizing the discomfort.
In addition, it was achieved a quantification of degrees/hours of discomfort as of the adaptive method indicated by (ASHRAE 2013) - Standard 55. This methodology sets the limits of comfort for each day of the year, either for 80% and 90% of people satisfied in naturally ventilated buildings. To quantify the degrees/hours of discomfort it was necessary to compare hours hourly, the temperature limits (upper and lower) and the operating temperature registered in each of the test cells.
Degrees/hours of discomfort are generated when the internal temperature exceeds the limits set by the standard, (Figure 11) being positive numbers for heat and negative numbers for cold, as shown in the following chart.
Comfort limits were obtained from the following equation (ASHRAE, 2013).
Upper Limit 80% satisfied =0.31 ᵗpma(out) +21.3 = 26.14ºC
Lower limit 80% satisfied =0.31 ᵗpma(out) + 14.3= 19.14ºC
Where, ᵗpma(out) corresponds to the central temperature in the range of comfort, calculated as average of the temperatures of the last 15 days, assuming in this case the value of 15.6ºC.
In Table 2, it is showed the calculation result of hours for discomfort. It is not recorded any hour for heat discomfort, however, it is observed several degrees / hour of discomfort by cold, the maximum value (47°C/h) is recorded in the control test cell and the minimum value (41.41ºC/h) in the cell which Green Roof, followed by the test cell with Green Facades and Green Rood (43.69ºC/h).
In this day and as it shown in Figure 10, it is observed that in the morning period, in the early hours of the day, the four test cells follow the same behaviour. At the time when the lower external air temperature (8.78ºC at 5h) is recorded, it can be viewed that the minimum internal temperatures (15°C) are registered in the cell with Green Facade, Control Cell, and the cell with Green Roof.
By contrast, at the same hour, higher air temperature is recorded in the cell with Green Roof (15,4ºC). In addition, it is noted that since the minimum external temperature is registered until the minimum internal air temperature is recorded, there is a thermal delay of 4 hours, as the minimum internal temperatures recorded happened at 9h.
The difference between internal air temperatures in the evening period is not excessive, since, according to the data, we can see that the higher difference between internal air temperatures of the 4 cells (just 2°C) is between the test cell built with conventional materials (22.6ºC at 18pm) and the cell endowed with Green Facades and Green Roof (21.2ºC at 18pm).
The behaviour of the internal air temperature’s curves when the maximum external temperature is registered (25.9ºC at 14h), shows that there is a thermal delay behind internal air temperatures recorded 4h. Next table exposes the external temperature amplitude of 17ºC, while that, in the indoor environments is about 7ºC for all the test cells studied. See Table 3.
Internal Surface Temperature´s North facades.
Internal surface temperatures (IST) of North facades, as shown in Figure 12, in the early morning when the minimum external temperatures are recorded, presents a difference between recorded values in cells tested about 1°C and 6°C regarding external temperature.
When the external temperature starts to rise, the behaviour of the test cells begins to differentiate, recording in the control cell maximum measure of temperature in the internal surface of the North Facades with maximum internal surface temperature of 25ºC at 17h. Moreover, it was registered the maximum internal surface temperature, 24,1ºC at 17h, in the cell constructed with Green roof. That is, both have a thermal delay respect to the external air temperature (25.97ºC at 15h) of 2 hours.
Figure 12 Internal surface temperature´s North facades vs. External air temperature: Graphic. Source: Own author.
At 18h, minimum surface temperatures in the North facades were recorded in two cells which facades were built with vegetation, that is, the cell with the cell Green Facades and the cell with Green facades and Green Roof. The value of the temperature was 22ºC, that is, 2ºC less than temperatures facades devoid of vegetation in North Facades (conventional materials cell and Green Roof cell). Moreover, the difference respect the maximum external temperature is 4°C.
Give that for an external thermal amplitude of 17 ° C, it is observed an internal thermal amplitude of 7°C for cells with vegetation on its northern facade, and a thermal amplitude of 11°C for cells with solid ceramic brick facades, it can be stated that Green facades have a better behaviour with respect to the internal surface temperature, which was built with brick facades (Table 4).
Internal Surface Temperature’s West Facades.
The West facades have the same behaviour as the North facades discussed above. In the early hours of the day, when the lowest daytime temperatures are recorded, the lowest internal surface temperature is registered in the test cell constructed with conventional materials.
The remaining test cells show almost the same temperature, to the moment when the external temperature begins to rise. The highest external temperature (26°C) occurs at 14h, on the other hand, the two test cells with no vegetation on their West facades (Control cell with Green roof), have the highest temperatures (22.8ºC) at 18h, that is, there is a difference about 3°C with respect to the external temperature and has a thermal delay of 4h.
Conversely, the lower temperatures are found in both cells with vegetation on their walls (Green facades cell and Green facades and Green roof cell) with 21.5°C at 18h, that is, with a difference of almost 5ºC with the external temperature and a thermal delay of 4 hours.
Figure 13 shows the graph of the internal surface temperatures in West facades and external air temperature.
Figure 13 Internal surface temperature of West facades vs. External air Temperature Chart. Source: Own author.
In Table 5, the thermal amplitudes recorded in each test cells are described. There, it can be seen that the highest amplitudes are manifested in the Control cell (9°C) (conventional materials) which has Green Roof (8.2ºC). Conversely, the lowest are observed in the cell constructed with Green facades and the Green roof cell (approx. 7°C).
Internal surface temperatures of Roofs.
In Figure 14, the evolution of internal surface temperatures of the four roofs over the analysed period is represented.
During the first hours of the day and when the lowest internal and external temperatures are recorded, it is observed that the two cells with roofs constructed with ceramic tiles have the lowest internal surface temperatures. However, the two cells with vegetation on their roofs have the highest internal surface temperatures, although this difference in temperature does not exceed two degrees. Nevertheless, there is a difference of almost 8°C in the external temperature with respect of test cells equipped with vegetation on their roofs. Besides this, a thermal delay of three hours is also seen.
In the part of the day where the highest temperatures were recorded, corresponding to the period of the afternoon, the highest temperature is observed in the test cell constructed with conventional materials (22.8ºC at 18h) followed by cell equipped with green facades (22.1ºC at 18h). Conversely, the lowest temperature among the maximum is given in the test cell with Green roof and Green facades (20.6ºC at 19h), followed by the cell with Green roof (21.5ºC at 19h).
In the evening period, it is observed a thermal delay between cells built with vegetation on their roof of 4 hours, while cells constructed with ceramic tile roofs recorded a thermal delay of 3 hours.
The lowest thermal amplitude have been recorded in the test cell covered by vegetation (5ºC), both in facades and roof, and the highest thermal amplitude (8°C) was recorded in the Control cell, built with conventional materials. Then, Table 6 summarizes the indicated thermal amplitudes.
Conclusions
Since the thermal behaviour study with use of vegetation in cold periods for tropical climate, it is concluded that vegetation can reduce internal temperatures variations. The internal temperature of the cell where it was set vegetation on their facades and roof, maintains a stable temperature in the indoor environment. In the early hours of the day, where lower external temperatures are recorded, higher internal temperatures are observed.
In periods when the highest external air temperatures are recorded, the test cell equipped with Green facades and roof show the lowest thermal amplitude, i.e., it has less difficulty in transferring heat to outside. The highest internal air temperatures are presented in the cell built to conventions and lower in cell having Green facades and Green roof, with a difference between them of 1°C.
The internal surfaces temperatures of the areas with vegetation, both facades and roofs remain more resistant to daily temperature variations. The highest difference between the maximum internal surfaces temperatures recorded, occurred between the cell control’s roof and the cell with Green facades and Green roof, with a value of 2.2ºC.
Therefore, with these results can be concluded that the use of vegetation in construction works during tropical winter, protects covers (roofs and facades) of possible heat loss, that is, vegetation acts as insulation. The results obtained show that cells provided with plant maintain their internal air temperature almost 2°C above the others cells in the coldest periods.
Finally, it can be noted that the use of vegetation in architecture is a viable technique with several possible uses for the kind of climate studied, that contributes, in addition, to environmental benefits, provides internal thermal benefits that can improve thermal comfort to the occupants
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Received: August 02, 2017; Accepted: December 12, 2017
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