Egypt’s cities face great challenges such as overcrowding, transportation congestion, greenhouse gas emissions, and the urban heat island effect. The Egyptian government is concerned with road planning and neglecting green spaces, and this leads to ignoring the achievement of the community’s sustainability goals (Kamel et al., 2012). New Cairo is one of the new cities that is suffering from the absence of green areas in terms of quantity and distribution. The actual amount of green areas that are available per resident in New Cairo is approximately equivalent to 0.33 square meters (Attia, S., & Amer, A, 2009), but by comparing this value to large cities around the world it is still less. The World Health Organization (WHO) suggests that every city provide a minimum of 9 square meters of urban green space for each person (Morar et al., 2014); it also suggests that an ideal amount of urban green space can be generously provided as much as 50 square meters per person (Takano et al., 2002). As shown by this viewpoint, every city in the world should try hard to have a sufficient provision of urban green spaces. Residential buildings, in general, are an efficient sector of energy consumers in Egypt, accounting for approximately 40% of total energy consumption for buildings. This paper will investigate if the green roof solution would be effective in Egypt to keep buildings cooler, use less energy and increase the percentage of green areas (Fahmy et al., 2018).
The design of an effective green roof system is a complex procedure that depends on several factors of sustainability, including social, cultural, climatic, and environmental considerations (Williams et al., 2010). Green roofs are one of several passive cooling methods that are known and used in many countries. Green roofs are classified as intensive or extensive based on plant height. The extensive type has a growing medium thickness of less than 20 cm, while the intensive type has a thickness of more than 20 cm (Jones & Alexandri, 2006). Green roofs are being considered by building owners as a means to decrease energy consumption and CO2 emissions. Green roofs are effective at reducing urban air temperatures and have a sufficient effect on air quality by improving the absorption of pollutants (Theodosiou, 2003).
The purpose of this paper is to study the impact of green roofs on the electricity consumption of social housing by analyzing their impact on annual energy usage and air temperature in a typical New Cairo neighborhood by using a Design-builder simulation program and ENVI-met program. This research’s empirical studies are based on computer simulation and the evaluation of software products. The first software used in this project is Design-builder 4.5, which is one of the building performance simulation tools that includes over 389 different tools (Attia et al., 2012). Design-builder can be utilized at any stage of the design process. It also has a visually oriented interface that is used by architects. The case study was created and edited with the Design-builder software to measure the cooling and heating consumption of a traditional roof and a green roof. Finally, the results were entered into Microsoft Excel software and displayed as tables and charts. The second software used in this project is ENVI-met, which is a critical tool for analyzing micro-scale thermal exchanges in urban environments. It is a three-dimensional simulation model for top-layer interactions throughout urban environments (Sharmin et al., 2017). This encourages a detailed evaluation of microclimatic changes, which are especially relevant to urban geometry and significant for comfort considerations and the analysis of small-scale interactions between plants, specific buildings, and surfaces.
Many studies have been conducted on the use of green roofs in different locations (Radwan, 2017), which revealed that the huge rooftop areas of buildings in Egypt were neglected for storing furniture, building materials, and food waste. These areas could be utilized to create green areas or social spaces that can be used by people in a better way. Green roofs are an idea that could be easily applied in Egypt to solve the problem of the absence of green spaces. Ragab, Ayman, and Ahmed Abdelrady discussed the impact of several types of green roofing with varying thermal conductivity on energy consumption for cooling school buildings in Egypt (Ragab & Abdelrady, 2020). The results showed that the proposed green roof types saved between 31.61 and 39.74% of energy. Green roofs were shown to be more efficient at reducing energy consumption as compared with traditional roofs, especially in hot climates. Andrews revealed that California State University San Marcos completed a new university student union in 2014 that included a 223 m2 live roof system on its upper terrace. The green roof met the university’s goals for aesthetic design, energy efficiency, and stormwater management through its selection of materials and local flora. This project demonstrates the successful implementation of a green roof in an arid climate (Andrews III, 2016).
As seen above, green roofs play an invaluable role in the environment. This paper focuses on the use of green roofs in social housing buildings in Egypt, which has not previously been addressed to decrease electricity consumption and improve temperatures.
This study includes two parts: the first presents the theoretical framework for green roofs. It describes the layers that transform a traditional roof into an environmentally green roof. It will also present the definition of green roofs, their benefits, their types, and the goal of their use. The second explores a case study in New Cairo city by examining energy consumption and the indoor comfort temperature of residential units using the Design-Builder simulation program, while the ENVI-met program will investigate outdoor comfort temperatures.
The case study method relies on simulation software from the Design-builder program and ENVI-met to compare two buildings. One of them has a traditional roof, whereas the other has a green roof. The Design-Builder will show how much energy is consumed each year and how comfortable the temperature is inside. The ENVI-MET program will show how comfortable the outdoor temperature is and how fast the wind is.
This case study is located in New Cairo, Cairo Governorate, Sakan Misr Serial 14, at latitude 29.98035 N and longitude 31.44269 E, northeast of Greater Cairo, as shown in Figure 3. A residential building consists of five floors. Each floor has four units with an area of approximately 95 m2. Each unit contains three bedrooms, a kitchen, a bathroom, and a living room, as shown in Figure 4.
The simulation in the Design-builder program includes modeling building performance for cooling and heating consumption, then identifying the green layer, which reduces energy use and could improve the indoor thermal comfort of the building.
An ENVI-met program is a critical tool for analyzing micro-scale thermal exchanges in urban environments. The ENVI-met was selected to evaluate the outdoor thermal comfort simulation for a traditional building without a green roof (case A) and a traditional building with a 50% green roof (case B), which is measured on June 21 at 2 p.m. Air temperature values range between 34.45°C and 37.65°C for case A, and air temperature values range between 32°C and 35.5°C for case B.
For this study, unit number 2 of the residential building was selected, as shown in Figure 4. Simulation results in the Design-Builder program are divided into two sections. The first section discusses the annual energy consumption of roof layers for a traditional building as shown in Figure 5 and green-roof layers for a building as shown in Figure 6. The second section identifies the indoor thermal comfort temperature of a traditional building and a green roof building. The paper will replace 50% of a standard roof with a green roof and compare the rooftop before and after using a green roof. The 50% with a green roof has been chosen to use the other part of the building for some other services, such as using it to put a water tank or satellite dish. The energy consumption per year was analyzed from January to December. The results of the building model were used to compare a traditional building without a green roof (case A) and a traditional building with a 50% green roof (case B).
Table 1 shows the cooling consumption, heating consumption, and total energy consumption of a traditional building without a green roof (case A) and a traditional building with a 50% green roof (case B) to compare the two cases.
|ENERGY CONSUMPTION||CASE A (TRADITIONAL ROOF)||CASE B (50% GREEN ROOF)|
|Cooling Consumption||8367.32 k.w.h||7365.43 k.w.h|
|Heating Consumption||2078.91 k.w.h||1817.51 k.w.h|
|Total energy Consumption||10,446.23 k.w.h||9,182.94 k.w.h|
|Total energy Saving||10,446.23 – 9,182.94 = 1,263.29 k.w.h|
The simulation results as shown in Figure 7 showed that the green roof reduced the annual energy consumption for cooling and heating as compared to the traditional roof, as shown in Table 1. For green roofs, annual energy consumption is reduced by 12%. The average cooling energy savings decreased from 8367.32 k.w.h to 7365.43 k.w.h by 11.9%, whereas the average heating energy savings decreased from 2078.91 k.w.h to 1817.51 k.w.h by 12.5%. So, the green roof method decreases annual energy consumption better than the traditional roof.
Table 2 shows the air temperature, operative temperature, and outside dry bulb temperature of a traditional building without a green roof (case A) as shown in Figure 8, and a traditional building with a 50% green roof (case B) as shown in Figure 9 to compare the two cases. The meaning of operative temperature is the combined effects of the mean radiant temperature and air temperature (Tdb). But the outside dry bulb temperature is the temperature of air that does not take into consideration any moisture content. An ordinary thermometer placed indoors or outdoors will measure the dry bulb temperature. The amount of moisture in the air, called relative humidity, cannot be determined from the dry bulb temperature alone.
|THERMAL TEMPERATURE||CASE A (TRADITIONAL ROOF)||CASE B (50% GREEN ROOF)|
|Out Side Dry Bulb Temperature||30.18°C||27.21°C|
According to the program results, on June 21, the operative temperatures were examined for a traditional roof and a green roof model. Results showed that the operative temperature for case A without a green roof achieved 28.31°C on June 21, as shown in Figure 8, while case B with a green roof achieved 25.87°C on the same day, as shown in Figure 9. The difference between the traditional roof and the green roof model was 2.44°C, as shown in Table 2 and Figure 10. These results reflected the effectiveness of green roofs on hot buildings.
In this part, the simulation software tool of the ENVI-met program examines a cluster of residential buildings in New Cairo City. A cluster consists of five buildings and a general service parking area, as shown in Figure 11. The simulation sequence goes as follows: First, the base case will be modeled in ENVI-met with min. air temperature 28°C, max. air temperature 38°C and windspeed 3.5 m/s as shown in Figure 12. After that, the green roof will be added with selected material in the ENVI-met program as shown in Figure 13 to compare the effects of adding vegetation to 50% of traditional roofs to clarify the impact of green roof buildings on temperature reduction. The simulation was measured at 2 p.m. on June 21.
The simulation results indicated that the green roof effectively reduced the outdoor air temperature, as shown in Table 3 and Figure 16. The air temperature values range between 34.45°C and 37.65°C for case A, as shown in Figure 14, whereas the air temperature values for case B range from 32°C to 35.5°C, as shown in Figure 15.
|OUTDOOR AIR TEMPERATURE||CASE A (TRADITIONAL ROOF)||CASE B (50% GREEN ROOF)|
|Min. Air Temperature||34.45°C||32°C|
|Max. Air Temperature||37.65°C||35.50°C|
Green roofs are an appropriate method of saving energy in hot climates. This study was conducted to improve the energy efficiency of indoor areas, the air quality of indoor buildings, and the air temperature of outdoor buildings. The results of this study show that green roofs can reduce energy consumption in hot climates. The case study showed that the energy consumption for the base case was 10,446.23 K.W.H., while the energy consumption for using 50 percent of the green roof was 9,182.94 K.W.H. The energy savings for the investigated green roof method are estimated to be in the range of 12%. Additionally, green roofs are efficient at reducing and improving air quality and microclimate performance. The average temperature inside buildings was decreased by 2.44°C, whereas the average temperature outside was decreased by 3°C.
The authors have no competing interests to declare.
Attia, S, Hensen, JLM, Beltrán, L and de Herde, A. 2012. Selection criteria for building performance simulation tools: contrasting architects’ and engineers’ needs. Journal of Building Performance Simulation, 5(3): 155–169. DOI: https://doi.org/10.1080/19401493.2010.549573
Fahmy, M, Morsy, M, Abd Elshakour, H and Belal, AM. 2018. Effect of thermal insulation on building thermal comfort and energy consumption in Egypt. Journal of Advanced Research in Applied Mechanics, 43(1): 8–19.
He, H and Jim, CY. 2010. Simulation of thermodynamic transmission in green roof ecosystem. Ecological Modelling, 221(24): 2949–2958. DOI: https://doi.org/10.1016/j.ecolmodel.2010.09.002
Jones, PJ and Alexandri, E. 2006. Developing a one-dimensional heat and mass transfer algorithm for describing the effect of green roofs on the built environment: Comparison of experimental results. Building and Environment, 42(8): 2835–2849. DOI: https://doi.org/10.1016/j.buildenv.2006.07.004
Kamel, B, Wahba, S, Nassar, K and Abdelsalam, A. 2012. Effectiveness of green-roof on reducing energy consumption through simulation program for a residential building: Cairo, Egypt. Construction Research Congress 2012: Construction Challenges in a Flat World, 1740–1749. DOI: https://doi.org/10.1061/9780784412329.175
Karachaliou, P, Santamouris, M and Pangalou, H. 2016. Experimental and numerical analysis of the energy performance of a large scale intensive green roof system installed on an office building in Athens. Energy and Buildings, 114: 256–264. DOI: https://doi.org/10.1016/j.enbuild.2015.04.055
Mohamed, H, El, A and Amr, A. 2019. The impact of Vernacular architecture on the thermal comfort of office building-using court and green roofs techniques. In ENGINEERING RESEARCH JOURNAL (ERJ), 1: 40. www.feng.bu.edu.eg.
Radwan, AH. 2017. Green Roofs-A Sustainable Tool of Healthier Cities, Applications in Egypt. 1st International Conference on Towards a Better Quality of Life. DOI: https://doi.org/10.2139/ssrn.3163416
Ragab, A and Abdelrady, A. 2020. Impact of green roofs on energy demand for cooling in Egyptian buildings. Sustainability, 12(14): 5729. DOI: https://doi.org/10.3390/su12145729
Sharmin, T, Steemers, K and Matzarakis, A. 2017. Microclimatic modeling in assessing the impact of urban geometry on urban thermal environment. Sustainable Cities and Society, 34. DOI: https://doi.org/10.1016/j.scs.2017.07.006
Takano, T, Nakamura, K and Watanabe, M. 2002. Urban residential environments and senior citizens’ longevity in megacity areas: the importance of walkable green spaces. Journal of Epidemiology & Community Health, 56(12): 913–918. DOI: https://doi.org/10.1136/jech.56.12.913
Theodosiou, TG. 2003. Summer period analysis of the performance of a planted roof as a passive cooling technique. Energy and Buildings, 35(9): 909–917. DOI: https://doi.org/10.1016/S0378-7788(03)00023-9
Williams, NSG, Rayner, JP and Raynor, KJ. 2010. Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia. Urban Forestry & Urban Greening, 9(3): 245–251. DOI: https://doi.org/10.1016/j.ufug.2010.01.005