Energy efficiency recommendations for the city of Cuiabá – Mato Grosso
Elaise Gabriel1, Érika Fernanda Toledo Borges2Abstract: The significant increase in electric energy consumption in recent years has encouraged research on the application of passive strategies that aim to achieve energy efficiency and thermal comfort of the user. The objective of this study was to compare and analyze the influence of different constructive types on the building energy consumption in order to propose measures that increase the thermal efficiency of the building, spending less electric energy. For this, a reference room with a 1:2 geometry and an ambient index of 0.80 was determined, and the ideal opening areas and different constructional systems for façades and cover were simulated through the DesignBuilder software. The results were analyzed and it was noticed that due to high solar radiation in the city of Cuiabá-MT the ideal opening area resulted in a value close to 10% of the facade area for both materials tested and the four orientations. It was also concluded that materials with lower thermal transmittance have better thermoenergetic performance. Between the systems of hollow brick and stone wool, when comparative simulation was carried out, the energy consumption difference was 83.68%, six times lower with accumulated techniques.
Keywords: Thermal transmittance. Thermal performance. Ideal window area. Energy simulations. 1 Introduction
The growing concern about sustainable construction has worsened since the oil crisis in 1973. Following the Kyoto Protocol treaties in 1997, a major challenge for the 21st century is the guarantee of a sustainable future, as increasing energy consumption has been detrimental to the future preservation of environmental resources.
The search for energy efficiency in buildings is already a reality in several nations and due to this; several researches have been carried out to achieve this objective (TAVARES, 2006; MOLDOVAN et al., 2014; DUARTE, 2014; GIORDANO et al., 2015). According to Mazzaferro (2013), measures such as thermal performance of building elements, user thermal comfort and energy efficiency are strongly interconnected in the development of techniques with higher performance and functionality.
As Yoshino et al. (2017) says, the six main contributors to energy consumption in buildings are: climate, envelopment, building energy services and systems, operation and maintenance, occupants' activities and internal environmental quality. Thus, it is necessary to study strategies that aim at saving energy and increasing thermal comfort, taking into account the bioclimatic zone of the building and passive strategies such as natural lighting and ventilation.
Faced with this Ander (2005) states that a large percentage of energy can be saved by using natural lighting as a source of light for indoor environments. However, natural lighting does not result directly in energy reduction, but rather when the artificial lighting load is reduced through the use of natural lighting. According to EPRI (1993), the use of natural lighting should be taken into account from the early stages of design, another point to consider is the daily and seasonal variation of light to provide appropriate lighting for longer time and with lower thermal load. Although variable throughout the year and even minutes, should be studied in order to develop lighting projects in which natural lighting is used whenever possible and artificial
lighting only when natural lighting does not meet lighting needs (GHISI, 1997).
Still within the scope of energy consumption by illumination, an essential factor to be considered is the ideal window area is the one whose energy consumption integrated between lighting and air conditioning is the smallest possible (GHISI et al., 2005). According to Ghisi (1997), considerable potential for energy savings can be achieved if natural lighting were used as a light source for indoor environments.
Ander (2005) states that the use of natural light increases user satisfaction and visual comfort, thereby increasing performance. The author further states that natural light can reduce between 35% and 50% of the total energy consumption of commercial buildings. In addition, its use can reduce the cooling load from 10% to 20% by lowering the total amount of energy to be used. According to Lam (2000), in addition to the energetic consumption of artificial lighting natural light has a higher luminous efficacy than most of the artificial lighting systems, also generating less thermal load in the environment. However, the difficulty in using natural lighting is found in its design, which should be done in order to avoid excessive glare and unwanted heat gain, some research done in this area is discussed below. As researches to promote greater performance and energy efficiency was carried out, it was concluded that the window area influences much of the energy consumption (RUPP, 2011). Ghisi and Tinker (2001), sized the ideal window areas for seven Brazilian cities in Brazil and for the city of Leeds in the United Kingdom, besides the geometric variation the authors broadly modified the "K" environment index. Other factors such as occupancy density, thermal transmittance and thermal capacity of walls and roofs were also evaluated. By varying the window areas, however changing the types of glass, Leão et al. (2008) evaluated the energy efficiency of windows in 12 Brazilian capitals. Creating relations between window areas and other factors such as floor and facade area, besides showing the ideal glass for certain climates.
Rupp (2011), approached the subject in a different way, besides the integration between natural and artificial lighting, the author used the hybrid ventilation as another factor to be analyzed, for the city of Florianópolis. Finally, Bisinotto (2011) verified the
1Acadêmica de Engenharia Civil, Universidade do Estado de
Mato Grosso, Sinop-MT, Brasil, elaisegabriel@hotmail.com
2Doutora, Professora, Universidade do Estado de Mato
influence of the window area on the consumption of electricity in the city of São Carlos in São Paulo. Through this research, the importance of considering the place of study as a determining factor of the adopted strategies is perceived. With this in mind, the Brazilian Association of Technical Standards launched in 2005 the NBR 15220-3: Brazilian Bioclimatic Zoning, which classifies Brazil in eight different bioclimatic zones according to temperatures and humidity variations. According to bioclimatic zoning, the state of Mato Grosso has five climatic zones, Cuiabá being located in zone 7 treated in this research.
NBR 15575: 2013 sets the minimum criteria of performance of residential buildings, such as thermal performance, acoustic performance, and durability, among others. The regulation establish minimum values of thermal transmittance and thermal capacity for each Brazilian bioclimatic zone.
Thermal transmittance (U) is defined as the transmitted heat flux per unit area and per unit of temperature difference. According to NBR 15575: 2003, for the bioclimatic zone 7, the maximum acceptable value for external walls is variable according to the absorptance (α - depending on the color of the element), for absorptance greater than 0,6 the transmittance required is 2.5 W/m².K, or values smaller than that the transmittance is 3.7 W/m².K. In addition, thermal capacity must be greater than 130 kJ/m².K.
However, the minimum transmittance values for roofs are also dependent on the bioclimatic zone, for zones 7 and 8 the values are as a function of the absorptance (α), if it is less than or equal to 0.4 the transmittance should be less than or equal to 2.3 FT. If it is greater it must be less than or equal to 1.5 FT. The transmittance correction factor (FT) is given as a function of the available opening height between two opposing eaves (NBR 15220: 2003). The standard is silent on the minimum values of thermal capacity for roofing. In order to quantify the energy consumption of the different sites and materials available, the energy simulation method was developed as a way to facilitate this process. According to Haves et al. (2012), the computational simulation is based on the accomplishment of simulations by means of computational software that carry out the energy analysis of a certain building taking into account the thermal loads resulting from the constructive configuration, the air conditioning systems and other equipment used.
According to Mazzaferro (2013), it is possible to create building models for thermo-energy analyzes to be simulated. These simulations range from the characteristics of the building to climatic data, thus providing a forecast of energy consumption of the building for a given climate. After obtaining the results, it is necessary to analyze them in order to have a correct understanding of how each variable affects the thermo-energetic behavior of the simulated building. These simulations help professionals in the Engineering and Architecture area to develop more thermally efficient projects, taking into account the construction techniques and types of conditioning desired, as well as making retrofits possible. Several software were created to carry out these simulations, including Energy Plus and DesignBuilder. The DesignBuilder program is a commercial software
developed by the British company DesignBuilder LTDA and was the first program created with the visual interface. The software has tools that allow efficient maximization of parameters such as natural lighting, natural ventilation and also benefits from the application of different materials for construction.
According to Lopes (2012), DesignBuilder uses the mathematical models coming from Energy-Plus but using a graphical interface that allows modeling the building in 3D, and has already installed a climate database including more than 2,000 cities. Therefore, to carry out the simulations of this work the software chosen was DesignBuilder.
Lamberts et al. (2014) states that energy efficiency can be understood as an intrinsic feature of the building that represents its potential to provide thermal, acoustic and visual comfort to its users with low energy consumption. Thus, one building is more efficient than another when it provides the same environmental conditions with lower energy consumption.
Much has been discussed about saving electrical energy in buildings. Energy-efficient and low-energy devices have been developed in addition to campaigns to reduce the waste of electricity. One example of this is PROCEL (National Program for the Conservation of Electric Energy), which has invested in the awareness of users about this waste.
One of the programs launched by PROCEL was the Energy Efficiency Seal that compares the energy consumption of similar household appliances. In the framework of the buildings, several regulations were launched with the aim of increasing the energy efficiency of buildings.
Among them there is in force Normative Instruction No. 02 of June 4, 2014, which makes it compulsory to obtain the general ENCE project class "A" for public buildings. Among the factors analyzed are the envelope of the building, the air conditioning system and the lighting system, since these are the elements that most interfere in the energy expenditure of the buildings. Within each of these factors are analyzed all the parameters that are in the project, including active and passive techniques of thermal comfort.
There is no prediction yet for mandatory norms for commercial or residential construction, but some companies use the labeling of these building classes as a marketing form for their projects. Therefore, the main purpose of this research is to effectively quantify the impact on energy consumption of different energy efficiency strategies for the city of Cuiabá - Mato Grosso.
2 Method
The methodology of this work was divided into two stages in order to help its execution and understanding. Thus, the first step corresponds to the determination of the necessary parameters for the simulation, as the reference model and the adopted constructive systems and step 2 refers to the simulation itself, as well as its validation; finally, the results were analyzed. Each of the parameters was analyzed independently of the others and at the end of the simulations a study between two cases was analyzed. The first case was an accumulative of all the parameters with passive strategies such as overhang and sidefins window shading types to totally prevent the direct entrance of
the sunrays, was compared to a simple model with common materials and no passive strategies to exemplify the influence of different materials on the building total energy consume.
2.1 Definition of reference model
The model of reference was defined according to the models of Ghisi and Tinker (2005), being constituted of a geometry 1:2 with dimensions of 2.46 m by 4.92 m and room index k equal to 0.80 (non-dimensional).
Figure 1: Reference room. Source: own authorship, 2017.
𝐾 = 𝑊 ∗ 𝐷 (𝑊 + 𝐷)ℎ
Where K is the room index (non-dimensional), W is the overall width of the room (m), D is the overall depth of the room (m) and h is the mounting height between the working surface and the ceiling (m). From the 1:2 geometry the width was defined as 2.46 m and the depth equal to 4.92 m, and the height as 2.80 m and the working surface of 0.75 m, resulting in a coefficient of ambient value equal to 0.80.
2.2 Determination of the constructive standard
Two types of constructional standard for façades were analyzed. Type 1 was simple hollow brick with thermal transmittance of 3.19 W/m²-K and wall thickness of 14 cm.
Figure 2: Hollow brick construction system. Source: own authorship, 2017.
In the second type of constructive system of facades was used stone wool with air layer and plasterboard. This type has a thermal transmittance of 0.224 W/m²-K and a wall thickness of 23 cm.
Figure 3: Constructive system with stone wool. Source: own authorship, 2017.
Two systems were tested for the roof. The first one was the solid slab of concrete with lower air layer and plasterboard, with transmittance of 2.134 W/m²-K and thickness of 15 cm. The second system tested was the precast metal slab with EPS and polyurethane layer, totaling a thermal transmittance of 0.284 W/m²-K and a thickness of 21.2 cm.
Figure 4 - Construction system with plasterboard. Source: own authorship, 2017.
Figure 5 - Cover construction system with EPS and polyurethane.
The purpose of testing these materials was to determine the level of thermal performance according to NBR 15575: 2013, by comparing external and internal temperatures, based on the table below.
Table 1 - Performance levels of bioclimatic zones.
Level Critério
Zones 1 a 7 Zone 8
M Ti,máx ≤ Te,máx Ti,máx ≤ Te,máx I Ti,máx ≤ (Te,máx - 2°C) Ti,máx ≤ (Te,máx – 1°C) S Ti,máx ≤ (Te,máx - 4°C) Ti,máx ≤ (Te,máx - 2°C) Ti,mín ≤ (Te,mín + 1°C) Ti,máx is the maximum internal temperature Te,máx is the maximum external temperature
Ti,mín is the minimum internal temperature Te,mín is the minimum external temperature
Source: NBR 15575:2013. 2.3 Simulation
2.3.1 Ideal window area
The first factor to be analyzed was the ideal window area according to the method of Ghisi and Tinker (2005). In this method the application of different percentages of facade area in each orientation analyzed, according to the representation below.
Figure 6 - Simulated glass area percentages. Source: Ghisi and Tinker (2005).
Only the facade analyzed has normal thermal changes, the other walls of the prototype as well as the cover are configured adiabatically. The purpose was to analyze in isolation the influence of the percentage of ideal glass area for each orientation individually. A single colorless glass of 6 mm was considered. The material applied to the walls was the constructive system 1, with simple hollow brick.
For the ideal window area simulations, a width of 2.46 m and a depth of 4.92 m was used, and a working surface height of 0.75 m, resulting in a room index of 0.80.
2.3.2 Façades
The two facade materials were tested for each main orientation by observing the difference between the external and internal temperature of the building, in order to classify these materials at minimum, intermediate and superior levels.
For the facade, the minimum opening area according to the Cuiabá Building Code was considered, being 1/6 of the floor area of the room (totaling 16.70% of the facade or 2.02 m² of opening). In total, eight simulations were made, four for each material.
2.3.3 Roof
For the coverage, the materials already specified were tested and for the simulations the walls were considered to be adiabatic, in order to only analyze the results of coverage performance. Totalizing two simulations.
2.3.4 Case studies
In order to compare the models cumulatively, two case studies were carried out. The first one to be simulated was the reference room with the simple hollow brick façades and the solid slab cover using the plasterboard. In this case no passive strategy was used and the openings applied were according to the minimum specified by the Building Code of Cuiabá-MT, with its orientation facing east and west.
The second case study was composed of façades with the stone wool system and EPS coating with polyurethane. For the openings, the ideal area of the window, previously simulated, was used for the north and south orientations. And brises were added in the windows.
Figure 7 - Reference room with accumulated passive strategies.
Source: own authorship, 2017.
3 Results
In this research were obtained results of each of the factors derived from the simulations. From the reference case with geometry 1: 2 and ambient index k = 0.80, the indices below were analyzed. For the accomplishment of the simulations, Leão (2011) validated the software DesignBuilder.
3.1 Ideal window area
The ideal window area simulations were performed using the DesignBuilder software, varying the two types of facade materials. After the energy simulations were performed, the ideal window area was around 10% for the city of Cuiabá, in all orientations. This is due to the fact that Cuiabá has high temperatures and strong insolation, causing a large amount of thermal load in the building, so the ideal window area resulted in a lower value than recommended by the building code.
Figure 8 - Energy consumption for hollow brick system. Source: own authorship, 2017.
Even with the variation of materials with distant transmittance the final result of the percentage of opening was the same for the two materials. However, as can be seen from the comparison between the two graphs, the energy consumption related to the constructive system with hollow brick is much higher than with stone wool, this is explained by the greater thermal transmittance of the first material.
Figure 9 - Energy consumption for stone wool system. Source: own authorship, 2017.
A study was also carried out regarding the increase of energy consumption if the ideal window area is not applied, so the Figure 10 below shows the increase in consumption for each applied opening percentage different from the ideal area of 10% and for each orientation taking into account the two materials.
Figure 10 - Percentage of increase in consumption in relation to the systems with hollow brick.
Source: own authorship, 2017.
Figure 11 - Percentage of increase in consumption for the system with stone wool.
Source: own authorship, 2017. 3.2 Envelope
3.2.1 Façades
For the facade simulations, the same materials were considered and their respective consumption compared according to the guidelines.
Figure 12 - Comparative consumption between facade materials.
Source: own authorship, 2017.
In order to determine the performance level of each material, the internal and external temperature differences of the building. For the material to be considered as minimum level the internal temperature of the building must be less than or equal to external temperature. In view of this, the brick with air layer obtained the difference of 0.84°C between internal and external temperatures being classified as minimum. When performing the simulations of the constructive system composed of stone wool, analyzing the internal comfort of the building can determine that this material showed a difference of 2.01°C between the internal and external temperatures of the building, being classified as intermediate. As the transmittance of this material is extremely low, no other tested material has obtained a temperature difference of 4 ° C, and thus the upper level can not be reached.
3.2.2 Roof
When testing the constructive systems for the cover and their respective internal temperature reductions, it was found that none of the systems reached the minimum level, that is, the systems tested did not obtain an internal temperature reduction when compared to the external temperature of the building.
In this way, only the consumption of these two types of coverage was analyzed, as shown in the figure below.
Figure 13 - Consumption of cover materials. Source: own authorship, 2017. 3.3 Case studies
Two case studies were developed in order to compare the performance of materials on a cumulative basis. The first case was considered the same room with simple hollow brick façades and a solid slab cover with plasterboard, but with no passive strategy applied. In the second case study were applied the materials with lower values of thermal transmittance; the façades in constructive system with stone wool and the cover with EPS and polyurethane.
Figure 14 - Comparative consumption of case studies. Source: own authorship, 2017.
The differences between the two studies include besides the variation of materials already mentioned, the application of brise in the windows of the accumulated room and adequate orientation of the openings, facing north and south while those in the reference room face east and west.
The Figure 14 above highlights the excessive difference in energy expenditure, around 83,68% from the base case to the accumulated; about six times lower only with the application of passive measures.
4 Conclusion
From the generated simulations it was possible to identify the performance of the individual and accumulated materials. Materials with low thermal transmittance have superior performance to materials that allow greater heat flow.
The ideal window area resulted in 10% of the facade area for the two materials tested for all four orientations, this area resulted in a low value due to the high thermal load in this region. The city of Cuiabá-MT has high annual temperatures with strong radiations, and two well-defined seasons, dry and humid, both warm. Also, another factor that could be considered a cause for
these results is the Energy Plus Weather Data (.epw) file used to obtain the climate characteristics.
Still in the ideal window area, the increase in energy consumption was determined if different opening percentages were applied to the façades. It was perceived an almost linear increase in relation to the increase of opening, that is, the larger the opening the greater the thermal load infiltrated in the building and consequently the greater the energy consumption to soften this thermal load.
It was also observed that the consumption of western and eastern orientations is superior to the northern and southern orientations, the latter being the one with the best performance. This was due to west orientation receiving the sun during the afternoon, period whose radiation is highest during the day.
The materials tested for façades showed a great difference of thermal transmittance, a factor that strongly influenced the superior performance of the wall with stone wool compared to the conventional brick building system. Another factor that must be considered is the thickness of the wall, the constructive system with stone wool has a thickness almost twice as thick as the system of hollow brick, thus hindering the passage of heat through the material.
Finally, the materials tested were classified as minimum for the case of the hollow brick and intermediate for the stone wool constructive system, since they presented internal temperatures lower than the external temperatures, according to NBR 15575:2013.
For the coverage, the system that presented better performance was also the system with lower thermal transmittance. Using materials such as EPS and polyurethane that have low transmittance generates less heat passing through the cover, which is considered the part of the envelope where heat transmission occurs most. Therefore, it is strongly recommended that materials with better thermal performance be applied at this location.
The materials tested in the roofing did not obtain satisfactory results according to NBR 15575:2013, since they did not reach internal temperatures smaller than the external ones.
As for the case study, the consumptions of the two examples analyzed were compared and it was concluded that the accumulated system with passive strategies has a performance far superior to the reference room. A consumption about six times lower due to the application of sun protection in the openings and proper orientation thereof, taking into account also the application of materials that have much lower thermal transmittance than conventional materials, resulting in the lowest transfer of thermal load possible.
Thanks and greetings
I would like to thank my family first, who gave me support in all the moments I needed despite all the difficulties we had. Especially to my mother Maria Faé Gabriel, there are no words to describe her importance and her greatness as a human being. To my father Ivanir José Gabriel for the support despite the distances. To my sister Emanuelle Gabriel for the words said and never forgotten during the waiting for the results of the entrance exam.
I would also like to thank all my friends, both old and new, who participated in the whole graduation walk and the ones known during the sandwich period in Michigan (Go Green!). I would also like to remind the people that for some reason got distanced during this period but that somehow contributed for me getting here. Lastly, I would like to thank my two advisors, Érika Borges, for her proposal to develop such a fascinating topic, which has an incalculable return not only to the academic community but also to society in general. And my advisor Marlon Leão for the motivating ideas and suggestions that contributed to the accomplishment of this work.
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