Green Roofs for Energy Conservation and Sustainable Development: A Review

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 4 (2016) pp 2776-2780 © Research India Publications. http://www...
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 4 (2016) pp 2776-2780 © Research India Publications. http://www.ripublication.com

Green Roofs for Energy Conservation and Sustainable Development: A Review Vinod Kumar V Research Scholar, Laxminarayan Institute of Technology, R.T.M Nagpur University, Nagpur, Maharashtra, India.

Dr. A.M. Mahalle Associate Professor, Laxminarayan Institute of Technology, R.T.M Nagpur University, Nagpur, Maharashtra, India.

Abstract Global warming has become a threat of our time to the existence of habitat on this earth which has made a clear impact on the level of energy and water consumption. No doubt, increase in the ambient temperature increases indoor and outdoor temperature level of the buildings which catalyzes the use of energy and cost intensive mechanical airconditioning systems. Depletion of fossils also points to alternate energy sources which are to be essentially sought for. Hence thirst for alternate energy is a passion of time for the conducive existence of living creatures in this globe. Research in this area has attained a faster pace where much time and money have been spent to ensure the coexistence of man and living things. Green roof tops belong to such an idea for cutting down the energy consumption and enhancing the comfort level of commercial and residential buildings. A number of collaborative research programmes are underway to reduce the heat in leak to the building and to improve the indoor conditions. Basically green roof technology is a passive cooling system that resists the heat penetrated due to solar radiation. In green roofs, solar radiation is balanced by sensible and latent heat fluxes from the plants and the soil surfaces and with conduction through the soil substrate. Efficiency of green roof depends on the climatic conditions particularly temperature. Green roofs also play an important role in reducing CO2 in atmosphere, used a medium for storm water runoff, vegetation and reducing the noise level in the building. This article critically reviews the research undertaken on green roof buildings thermal potential. It is an effort to shed light into the potential application of green roofs with an emphasis on energy conservation. The cooling potential and thermal performance of intensive and extensive green roofs, green roofs with solar shading and thermal insulation for commercial and residential buildings are also analyzed. This article critically reviews the various research works on energy efficiency and benefits of green roofs around the world.

Introduction Green roofs are increasingly recognized as a passive cooling technique to reduce the heat in leak to the building using growing plants. They inhibit the solar energy to penetration into the buildings, reduce the cooling load, and hence reduce the cost of energy driven systems. Several studies have been conducted to measure the potential benefits of green roofs and the findings indicate that they can offer benefits in summer cooling as well as in winter heating. In green roofs, solar radiation is balanced by sensible and latent heat fluxes from the plants and the soil surfaces and with conduction through the soil substrate. Efficiency of green roof depends on the climatic conditions particularly temperature. The green roof offers other benefits in reducing CO2 in atmosphere, used a medium for storm water runoff, vegetation and reducing the noise level in the building. Energy efficient buildings using green roof has been under the research concerns in the recent past. Many research studies are undertaken to explore the feasibility of using this technique to reduce the heat in leak to the buildings and to improve the cooling efficiency.

Principle The green roof is a vegetative layer grown on a rooftop. The green roof consists of six layers from bottom to the top: waterproofing, root barriers, drainage and retention, filtration sheet, growing medium and, finally, plants. Two types of green roofs are generally distinguished: extensive and intensive as shown in Table.1. Table 1: Types of Green Roofs Extensive Intensive Low soil thickness(5-15 High soil thickness (above 15 cm) cm) Difficult to access Easily accessible Light (50-160 kg/m2) Heavy (more than 200 kg/m2) Can be maintained easily Difficult to maintain Less expensive More expensive

Keywords: Building energy, Green roof, Intensive and extensive roofs, Noise level, Storm water run-off, Thermal performance, Thermal transmittance.

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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 4 (2016) pp 2776-2780 © Research India Publications. http://www.ripublication.com winter seasons. In his studies during the warm period, the green roof reduced heat gain by 92% and 97% in comparison to ceramic and metallic roofs, respectively, and increased the heat dissipation to 49% and 20%. During the cold period, the green roof reduced heat gain by 70% and 84%, and reduced the heat loss by 44% and 52% in comparison to ceramic and metallic roofs, respectively. From the studies it was found that green roof contributes to the thermal benefits and energy efficiency of the building in temperate climate conditions. An experimental study on the selection of appropriate plants for the green roof was done by Liu et al. (2011).The experiment was done at the top floor of an eight floor building located in Taiwan [10]. The results indicate that the plants from CAM families, Portulacaceae, Crassulaceae and Euphorbiaceae are more droughts tolerant by humans. The results showed that the temperature reduction decrease with plant height in the following pattern: 35cm>15cm>10cm. The results also indicate that the plants with green colored leaves are more effective in roof top heat insulation. Thermal analysis and performance of an extensive green roof on a railway station in humid-subtropical Hong Kong was done by C.Y. Jim et al.[3], in (2011). He conducted the study for different weather conditions for a continuous period of 2 years and found that green-roof could reduce daily maximum temperature at the bottom tile surface by 5.2 oC, air temperature at 10 cm height by around 0.7 oC, but little effect at 160 cm. The surface temperature of the vegetation was higher than the bare roof by 3.4oC. Solar radiation and relative humidity were the key meteorological factors of the greenroof thermal effect and could reduce the cooling load of the railway station building by 0.9 kWh m−2 on sunny summer day, 0.57 kWh m−2 on cloudy day and added a slight cooling load on rainy day. A study conducted in University of Lleida, by Gabriel Perez et al. (2011) used recycled rubber from tires as a drainage layer in green roofs instead of porous stone materials [11]. He concluded that the extensive green roofs can be a good tool to save energy during summer in Continental Mediterranean climate, and that the use of rubber crumbs instead of Puzolana as drainage layer material in extensive green roofs is possible, and should not arise any problem for its good operation thereby reducing the consumption of natural materials, which require large amounts of energy in its transformation process to obtain their properties. Moreover it would provide a solution to the problem of waste rubber from the tires. Permpituck, et.al. 2012 studied the energy consumption performance of roof lawn gardens in Thailand experimentally [12]. He studied the thermal performance by varying the soil depth. In his findings the roof top with a soil depth of 0.10 m could achieve 46.24% less heat transfer than the exposed roof, and the roof top with a soil depth of 0.20 m could achieve 93.96% less heat transfer than the exposed roof.

Green roof thermal Performance and Energy Saving Experimental Approach: The various experiments done by eminent researchers from different countries in the field of green roof efficiency are discussed in this session. Onmura et al. [1], in 2001, in his studies in a three story building in Japan, investigated the evaporative cooling effect from roof lawn gardens and it was confirmed that the surface temperature of the roof decreased by 60 to 30oC during day time which was estimated to be a reduction of 50% heat flux. A detailed analysis of the thermal properties and energy performance study through mathematical approach was done by Niachou et al. [2], in 2001. The study was conducted in a hotel situated in Athens, Greece. In his findings the greatest energy savings during a whole year period was 37% for non-insulated buildings, 4% for moderate insulated buildings and 2% in well-insulated buildings. Theodore et al.[4], studied a technique for the inclusion of a model in building energy simulation in 2003. The results were validated by the use of real data taken from an existing construction in the city of Thessaloniki, Greece and a parametric study was performed in order to evaluate the main planted roof characteristics that affect the performance of a planted roof as a passive cooling technique. It was shown that relative humidity is the most important climatic factor which affects the cooling potential of the green roof. Wong et al, (2003) conducted a field study in a low rise commercial building in Singapore [5]. In his studies a maximum reduction of surface temperature of 30 oC was obtained and the temperature reduction varied on the type of plants and density LAI (Leaf Area Index) of the plants. Thermal performance of green roof installed by the Vicenza Hospital, Italy was studied and analyzed by Lazzarin et al. [6], in 2005. The role of the latent flux of the evapotranspiration was studied and with the soil in almost dry conditions the green roof allows an attenuation of the thermal gain entering the underneath room of about 60% with respect to a traditional roofing with an insulating layer. Evaluation of the cooling potential of green roof with solar thermal shading in buildings was developed by Kumar, et. al. in 2005 [7]. The model was validated against the experimental data from a similar green roof-top garden in Haryana, India, and in his findings green roof combined with solar thermal shading reduced averaged indoor air temperature by 5.1 oC, from the average indoor air temperature for the bare roof. Castleton et al.(2010) reviewed the current literature and highlights the situations in which the greatest building energy savings can be made [8]. Older buildings with poor existing insulation were benefited from a green roof as current building regulations require such high levels of insulation that green roofs are seen to hardly affect annual building energy consumption. As over half of the existing UK building stock was built before any roof insulation was required, it was older buildings that will benefit most from green roofs. The case for retrofitting existing buildings was reviewed and it was found there is strong potential for green roof retrofit in UK. Green roofs have been investigated as a bioclimatic strategy to improve the energy efficiency of buildings. Parizotto, et al. (2011) studied the green roof thermal performance of an experimental single-family residence in Florianopolis, SC, Brazil [9]. The studies were conducted during the summer and

Laboratory Analysis: A laboratory study was conducted by Sailor et al. (2011) to measure thermal conductivity, volumetric heat capacity, and thermal diffusivity of 12 green roof soil samples of varying composition [13]. The results indicated that thermal properties vary significantly as a function of growing media design. When the moisture content in the sample was increased there

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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 4 (2016) pp 2776-2780 © Research India Publications. http://www.ripublication.com stage the energy savings were examined. In his findings the greatest energy savings during a whole year period was 37% for non-insulated buildings [2]. A theoretical evaluation of the thermal and energy performance of tropical; green roofs was studied by C.Y.Jim et al. in 2011[19]. Through his theoretical analysis he showed that if green roof can be installed on about 1400 public houses in Hong Kong, an estimate of 43.9 TJ of solar energy can be prevented from penetrating into the buildings in summer seasons. The electricity consumption can be dramatically reduced. For assessing the effects of green roofs, a thermodynamic model and the thermo-physical properties of the green roof components were characterized by Salah Eddine et al. (2011). The proposed model was based on energy balance equations expressed for foliage and soil media [20]. In this study, the influence of the mass transfer in the thermal properties and evapotranspiration were taken into account. A parametric study was performed using the proposed model to classify green roofs depending on the considered climate condition. Comparisons were undertaken with a roof slab concrete model, a significant difference (of up to 30oC) in temperature between the outer surfaces of the two roofs was noticed in summer. The model was experimentally validated according to green roof platform, which was elaborated. The mass transfer effect in the substrate was very effective in reducing the model errors. Simulation results showed that the use of vegetation in the roof building improves not only thermal comfort conditions, but the energy performance of a building. A quasi-steady state heat and mass transfer green roof model that can be incorporated in different energy simulation software or calculation procedures which were investigated and developed by Tabares et al. in 2011[21]. The model analyzed the heat and mass transfer processes between the sky, plants, and substrate. This studies discovered new equations to calculate (1) substrate thermal conductivity for green roofs, (2) substrate resistance to calculate green roof soil evaporation, (3) and set of compiled stomatal resistance functions to calculate plants’ transpiration.

was a substantial increase in the thermal conductivity. Also, it was found that compaction typical of green roof systems that have been installed for multiple seasons can increase thermal conductivity of moist soils by 30% to 40% over their uncompressed values. A new experimental laboratory apparatus, ‘‘Cold Plate’’, was designed by Paulo Cesar Velasco et al. in 2011 and built to quantify heat and mass transfer processes for green roof samples inside an environmental chamber [14]. Experimental data collected in this apparatus showed that evapotranspiration controls the intensity of all other heat fluxes, depending on the plant and environmental conditions. Also, under the described laboratory conditions, the uninsulated green roof samples with plants showed an average heat flux reduction of 25% compared to samples without plants. Tahir Ayata, et al. (2011) in his laboratory set-up created different environmental conditions to measure sensible heat fluxes to/from a vegetated roof assembly [15]. This experimental setup has been successfully used for different wind velocities (0-3 m/s) to create free and forced convection conditions around green roof tested samples. Furthermore a basic model for calculations of the convective heat transfer at green roof assemblies was developed, which is a modified version of the Newton’s cooling law, calibrated and then validated with different sets of data. For forced convection flow regimes, the proposed “basic model” resulted in RMSE (Root Mean Square Error) of 11 W/m2 and R2 value of 0.81. The study of evaluation of green roof thermal performance in terms of the thermal transmittance coefficient, in real scale and under dynamic conditions was done by Kostriris et al. [16], in 2012. For the study’s purposes, five semi-intensive green roof systems were constructed on the roof of an outdoor test cell. The relation between the estimated thermal transmittance and the substrate moisture content was investigated and found to be linear. The green roof systems were also simulated for a single-storey residential building in order to quantify their possible energy savings. The results from the simulation showed that shallow substrates conserve building energy mainly during the summer period of the year. Rock wool and deeper substrates showed significant cooling and thermal insulating features.

Green Roofs: Other Benefits: Reduction of Carbon dioxide: Studies shown that green roofs play a role in the reduction of CO2 in the atmosphere. Carbon is a major component of plant structures and is naturally sequestered in plant tissues through photosynthesis and into the soil substrate via plant litter and root exudates. A green roof will serve as a carbon sink. Getter et al. [22], 2009, in his study in Michigan, USA over a period of two years estimated the carbon sequestered by four species of Sedum in a 6.0 cm substrate depth extensive green roof. The ground plant material and root biomass stored an average of 168 g C m-2and 107 g C m-2, respectively, with differences among species. Substrate carbon content averaged 913 g Cm-2. In total, this entire extensive green roof system net carbon sequestration totaled 378 g C m-2. The potential emission savings using Sailor’s, 2008 model, the campus of Michigan State University in East Lansing has 1.1 km2 of flat roof surface. If all of these roofs were greened, they could avoid 3,640,263 kg CO2 emitted per year. Yang et al., 2008 Illinois, USA estimated the level of air pollution

Modeling and Theoretical Prediction: A mathematical model of the dynamic thermal behavior of actual green roofs was studied by Barrio et al. [17], 1998. Several parametric sensitivity analyses were carried out to evaluate the cooling potential of green roofs in summer. He formulated the most important design considerations for green roofs. Among them the most important factors were selection of plants with large foliage distributed mainly in horizontal direction and selection of light soils, that reduces the thermal conductivity as well as weight. The main conclusion of these analyses was that green roofs will act as insulation ones, reducing the heat flux through the roof. A detailed analysis of the thermal properties and energy performance study through mathematical approach was done by Niachou et al. in 2001. The analysis was done in two stages: during the first stage, extended surface and air temperature measurements were taken at the indoor and outdoor where green land was installed and in the second

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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 4 (2016) pp 2776-2780 © Research India Publications. http://www.ripublication.com Most of the researchers focused on the thermal performance and cooling potential of the green roofs whereas only few studies are seen on the mitigation of air pollution including reduction of CO2, NOx uptake etc. It is observed that most of the research works on cooling potential of green roofs were done in Europe, USA and Greece. Hence research in Indian context is more relevant today than ever before since climatic balance in India has been in turmoil in recent years. Therefore it is high time to carry out research on the effectiveness of green roof technology on cooling potential in Indian scenario with emphasis to their effect on attenuation of sound waves and storm water runoff.

removal in Chicago using a dry deposition model. Annual removal of pollutants per hectare of green roof was 85 kg ha-1 yr-1 with the highest and lowest removal during May and February, respectively. Extensive green roof has been identified as one of the most important means to reduce air pollution in Hong Kong city. The article by Xiaoling Zhanga et al. [23], in 2011, examines the major barriers encountered in promoting extensive green roof systems for the existing buildings in Hong Kong. Case study approach is adopted to investigate how and why the barriers can hinder the implementation of extensive green roof features. Reduction of Noise Level: Van Renterghem and Botteldooren, et.al. 2008, analyzed the effects of intensive and extensive green roofs on noise level. There is a linear relationship between the roof space covered with canopy and the reduction in sound [24]. Because green roof growing substrates tend to be coarse, sound waves enter the pore space and are attenuated by the numerous interactions with the substrate particles. Botteldooren, et.al. in 2009, analyzed that a green roof was able to reduce 10 dB over the frequency range from 500 to 1000 Hz compared to a bare roof. Increase in substrate depth up to a depth of 20 cm improved the noise reduction rate.

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Reduction of Storm Water runoff: The reduction in runoff generally ranges from 50% to 100% depending on the type of green roof system, substrate composition and depth, roof slope, plant species, preexisting substrate moisture, and the intensity and duration of the rainfall. A study was conducted by Hilten, et.al. [25], (2011) on the effectiveness of green roofs to mitigate storm water using computer simulation. Simulation results for runoff in terms of peak flow reduction, retention, and detention time were evaluated for the green roof. Storm data collected as part of a green roof study in Athens were used to validate the HYDRUS-simulated runoff. It was shown that the green roof with growth media depth of 10 cm provides complete retention for storms up to 2.0 cm in depth, while providing detention for storms as large as 7.93 cm when assuming initial soil moisture content of 0.1. Detention time for storms between 5cm and 7.93 cm were approximately 12 hrs. Concerning the effectiveness of green roofs to reduce storm water runoff, simulations showed that green roofs are highly effective for small storms. For larger storms (>2.54 mm), green roofs can act to extend runoff duration thereby reducing surge normally evident with impervious surfaces.

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Conclusion Green roofs are considered as a passive cooling technique and as a sustainable technology that offers benefits to the environment and society. The effectiveness of the green roof depends on type of plants, climatic conditions, geographic locations etc. The potential benefits of the green roofs are: reduction of energy consumption for energy driven airconditioning systems, improvement in comfort of residential and commercial buildings, reduction in storm water run-off, reduction in CO2 in atmosphere, reduction in noise level etc.

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