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Integration of green roof and solar photovoltaic systems

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Green roof and solar photovoltaic (PV) systems are two technologies that could contribute to sustainable building development and reduction of greenhouse gas emissions. When they are combined together on the building roof, it can enhance their functions and effectiveness by cooling and shading effects. This paper explains the major findings of a research to study the benefits of integrating green roof and solar PV systems. The important factors affecting the interactions between the two systems are assessed. The thermal and energy effects are analysed by theoretical models, experiments and field studies. A hypothetical case study to retrofit the roof of an existing building in Hong Kong with such integration is carried out to evaluate the practical design issues. The experimental results showed a positive influence for this integration: green roof surface and soil temperatures are reduced from the shading and higher power output of PV panel is achieved from the cooling. The findings of year-round building energy simulation using EnergyPlus for a low rise commercial building indicated that the energy consumption for air conditioning of the integrated system is slightly lower than the stand-alone system and the PV system on integrated approach generates 8.3% more electricity than the stand-alone option. The extent of the benefits depends on the system design and how to determine the optimum arrangement for a particular building site.
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Integration of green roof and solar photovoltaic systems
Dr. Sam C. M. Hui* and Miss S. C. Chan
Department of Mechanical Engineering, The University of Hong Kong
Pokfulam Road, Hong Kong
*E-mail: cmhui@hku.hk, Tel: (852) 2859-2123, Fax: (852) 2858-5415
ABSTRACT
Green roof and solar photovoltaic (PV) systems are two technologies that could contribute to
sustainable building development and reduction of greenhouse gas emissions. When they are
combined together on the building roof, it can enhance their functions and effectiveness by
cooling and shading effects. This paper explains the major findings of a research to study the
benefits of integrating green roof and solar PV systems. The important factors affecting the
interactions between the two systems are assessed. The thermal and energy effects are
analysed by theoretical models, experiments and field studies. A hypothetical case study to
retrofit the roof of an existing building in Hong Kong with such integration is carried out to
evaluate the practical design issues. The experimental results showed a positive influence for
this integration: green roof surface and soil temperatures are reduced from the shading and
higher power output of PV panel is achieved from the cooling. The findings of year-round
building energy simulation using EnergyPlus for a low rise commercial building indicated
that the energy consumption for air conditioning of the integrated system is slightly lower
than the stand-alone system and the PV system on integrated approach generates 8.3% more
electricity than the stand-a-lone option. The extent of the benefits depends on the system
design and how to determine the optimum arrangement for a particular building site.
Keywords: Green roofs, solar photovoltaic, system integration.
1. INTRODUCTION
Green roof and solar photovoltaic (PV) systems are two technologies that could contribute to
sustainable building development and reduction of greenhouse gas emissions. Nowadays,
some people are interested in developing green roofs for energy saving, reducing storm water
runoff and improving building thermal and environmental performance (Castleton, et al.,
2010; Hui, 2010; Hui, 2006). Others are interested in adopting solar PV systems at rooftops
for renewable power generation (Parida, Iniyan and Goicm, 2011).
Green roofs and roof-mounted solar panels may initially appear as competitors for limited
rooftop space (Peck and van der Linde, 2010). But in fact when they are combined together
on the building roof, the integration can enhance their functions and effectiveness by cooling
and shading effects (Köhler, et al., 2002). It is believed that cooling from evapotranspiration
of green roof plants enables a higher efficiency of PV panels, and at the same time, the panels
shade the plants from excessive sun exposure and evaporation thus improving plant growth.
Some research studies have been done in different countries to evaluate the energy and
thermal performance of green roof and PV systems separately. However, there are very few
studies on the integration of both systems for subtropical climates. This paper explains the
major findings of a research to study the benefits of integrating green roof and solar PV
systems. The important factors affecting the interactions between the two systems are
assessed. The thermal and energy effects are analysed by theoretical models, experiments and
field studies. A hypothetical case study to retrofit the roof of an existing building in Hong
Hui, S. C. M. and Chan, S. C., 2011. Integration of green roof and solar photovoltaic systems, In Proceedings of
Joint Symposium 2011: Integrated Building Design in the New Era of Sustainability, 22 November 2011 (Tue),
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Kong with such integration is carried out to evaluate the practical design issues. It is hoped
that a better understanding of the integration can help promote sustainable building design.
2. BASIC PRINCIPLES
The thermal properties of green roofs, the electrical efficiency of PV panels and the typical
arrangement of system integration are described.
2.1 Thermal Properties of Green Roofs
Figure 1 shows the fundamental concepts to explain the thermal properties of green roofs
(Hui, 2009). The external climatic factor (solar radiation, external temperature, relative
humidity and winds) are reduced as they pass through the foliage of green roof. Large
amount of solar energy are absorbed for the growth of plants through their biological
functions, such as photosynthesis, respiration, transpiration and evaporation. The heat
transfer of green roofs is dominated by four mechanisms (Del Barrio, 1998; Niachou, et al.,
2001; Feng, Meng and Zhang, 2010):
(a) Evapo-transpiration
(b) Shading by plants
(c) Thermal insulation
(d) Thermal mass storage
Figure 1. Thermal properties of green roof (Hui, 2009)
The thermal benefits of green roofs can be studied from two aspects (Hui, 2009; Wong, et al.,
2003). Firstly, the direct effect to the building (internal) can control roof surface temperature
and building heat gain so that the building energy use can be reduced (Castleton, et al., 2010).
Secondly, the indirect effect to the surrounding environment (external) can decrease the
urban temperature and help mitigate the adverse impacts of urban heat islands.
2.2 Electrical Efficiency of PV Cells
The electrical efficiency (η) of PV cells is the result of the relationship between the power
delivered by the cell and the amount of solar irradiation (Meral and Dinçer, 2011; Parida,
Iniyan and Goicm 2011). Heat is one of the primary factors that affect the efficiency of roof-
mounted PV panels. High rooftop temperatures increase the conductivity of the crystalline
semiconductor of PV panel, which in turn inhibits charge separation and lowers the voltage
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of the solar cell. The high temperature can decrease PV panel productivity by up to 25% and
a value of -0.45% per degree celsius can be applied for crystalline silicon PV cells (Peck and
van der Linde, 2010; Makrides, et al., 2009). An effective way of improving efficiency and
reducing the rate of thermal degradation of a PV module is by reducing the operating
temperature of its surface (Meral and Dinçer, 2011).
Another factor influencing the efficiency of PV system is the air pollution or dirt/dust level.
Dirt/dust can accumulate on the PV module surface, blocking some of the sunlight and
reducing output (Meral and Dinçer, 2011). Kaldellis and Fragos (2011) reported that almost
1.5% efficiency reduction is obtained on a dust-deposed PV panel for ash accumulation on
the panel surface reaching 0.4 mg/cm2. However, it is difficult to generalise and quantify the
impact of air pollution on PV system performance because the atmospheric and site
conditions may vary significantly.
2.3 System Integration
Initial research by Köhler, et al. (2002) showed promising results for green roof and solar PV
integration; the electricity generation of PV on green roof is 6% higher than on conventional
roofs (Köhler, Wiartalla and Feige, 2007). The green roof cools ambient temperatures around
the solar panels, allowing the solar panels to stay cooler and function better. A recent
research also confirmed the potential benefit of such integration (Sailor, Wamser and
Rosenstiel, 2010). Since green roof can help reduce dust level and improve air quality, the
efficiency of PV system could be enhanced but no solid evidence can be found in the
literature at present.
On a flat roof with solar PV panels, a green roof installation should be restricted to extensive
or low-profile vegetation. The solar panels should be installed above the vegetation level so
that the panels are not shaded. Lightweight frames are often used to raise and tilt panels
towards the predominant direction of the sun; shade-tolerant vegetation is then planted under
the panels. Figure 2 shows an example of green roof and solar PV integration.
Figure 2. An example of green roof and solar PV integration (Peck and van der Linde, 2010)
The solar panels were mounted on framework which is fixed to plastic boards. The profiled
plastic boards are covered with substrate and allow rain water to drain through and
vegetation to grow underneath the solar panels. Frames usually have a base (also lightweight)
to permit better load distribution. The roof is therefore better protected from damage by point
loads. The racking systems for solar panels may be designed so that the green roof layers act
as ballast, thereby saving the need for roof penetrations or concrete pavers (Peck and van der
Linde, 2010).
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3. ENERGY SIMULATION ANALYSIS
To investigate the energy impacts of green roof and solar PV integration, a building energy
simulation software, EnergyPlus Version 6.0 (www.energyplus.gov), was used. This
software EnergyPlus has a green roof model developed by Sailor (2008) and can calculate
the annual energy consumption of the whole building with solar PV systems. Four simulation
models were set up using EnergyPlus for comparing the energy performance of green roof,
PV and the integrated systems under the Hong Kong climate. Figure 3 shows the basic
descriptions of the models. Model number 1 is the base case reference (bare roof) which is a
12-storey office building with a total floor area of 46,320 m2. Model number 2, 3 and 4 are
derived from the base case with green roof and PV systems added.
Figure 3. Four simulation models for the investigation of energy performance
It is assumed that the PV systems of model number 3 and 4 have efficiency of 12% and 13%,
respectively. Each PV system covers a roof area of 2,494 m2. Also, the power generated from
the PV system is used for general lighting (i.e. decreases the actual lighting energy use). The
leaf area index (LAI) of model number 2 is assumed to be 3 for a typical extensive green roof,
whereas the LAI of model number 4 is divided into two parts: LAI = 3 for 30% of roof area
(exposed) and LAI = 3.5 for 70% of roof area (under the PV panels).
3.1 Annual Energy Consumption
As the PV system will only affect the lighting energy and the green roof system will affect
the building thermal load, it is more effective to assess the energy consumption components
of lighting and space conditioning in order to evaluate the energy impacts. Figure 4 indicates
the annual energy consumption of lighting and space conditioning for the four models. From
the lighting energy consumption, it is found that the PV panels of the integrated system
produce 118 GJ more energy than the PV panels on bare roof. As for the space conditioning,
the figures for the 4 models are very close. This implies that the green roof system has
minimal direct impact on the building energy consumption. It is because the amount of green
roof area is small as compared to the total building floor area.
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Figure 4. Annual Energy Consumption of Lighting and Space Conditioning
3.2 Monthly Energy Performance
Figure 5 shows the monthly energy generation of the stand-alone PV (model number 3) and
green roof integrated PV system (model number 4). It can be seen that the PV systems
generate more energy during summer time than winter time due to high value of solar
irradiance in summer. For each calendar month, the integrated system produces 8.3% more
energy than the stand-alone system because of the efficiency improvement (assumed from
12% to 13%). This amount only considers the direct effect to the building itself. If the
indirect effect to the surrounding environment and urban temperature is taken into account,
there could be benefits for other buildings too.
Figure 5. Monthly power generation of stand-alone PV and green roof integrated PV systems
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4. FIELD MEASUREMENTS
In order to study the practical design issues of green roof and PV integration, field
measurements had been carried out on a rooftop garden of HKU Main Library (see Figure 6)
on a sunny day from 11 am to 2 pm. Control experiments were done by using two identical
PV panels which were placed on a bare roof and an intensive green roof. Figure 7 shows the
four scenarios being assessed. The temperature at different positions and the power output of
the PV panels were recorded using temperature sensors, data loggers and multi-meters.
Temperature sensors were placed inside the soil layer (about 1 to 2 cm depth), at the soil
surface and leaf surface, and on the upper and bottom surfaces of the PV panels.
Figure 6. Measurement locations at a rooftop garden of HKU Main Library
Integrated system Green roof Bare roof with PV Bare roof
Figure 7. Four scenarios being assessed for the field measurements
4.1 Assessment of Temperatures
Figure 8 compares the soil surface temperatures of the integrated system (with shading effect)
and the green roof (without shading by PV panels). A temperature difference of about 4 to 5
ºC can be observed. It is believed that this shading effect is important for the healthy growth
and well-being of the vegetation especially during the very hot summer period. Hui (2009)
has observed at a school green roof project in Hong Kong that the sedum plant can grow
better under partial shading near the ventilating fans of the roof. Köhler, Wiartalla and Feige
(2007) also found that in the shade of the PV panels, the biomass and number of plant species
are significantly higher.
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As for the PV panels, the temperature measurements on the upper surfaces indicate that the
integrated system (with greening around) has 5 to 11 ºC lower than the bare roof PV system.
This temperature difference was observed under bright sunshine when the PV panels were
producing high electrical output. In order to assess the total power yield generated by the PV
systems, further studies are needed to evaluate the conditions throughout the year.
Figure 8. Soil surface temperatures of integrated system and green roof
4.2 Assessment of PV Output
Figure 9 shows the power output of PV panels on bare roof and integrated system. The
average power production over the period for integrated system and bare roof PV panel are
32.2W and 33.6W, respectively. In general, the integrated system can give about 4.3% more
electricity than the PV on bare roof during measurement period. The temperature evaluation
of the PV panels also indicated that the cooling effect by vegetation is quite significant.
Figure 9. PV power output on bare roof and integrated system
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Li, Cheung and Lam (2005) found that the operational performance and efficiency of a PV
system in Hong Kong is influenced considerably by the climatic variables, such as solar
irradiance availability and ambient temperature. In fact, the PV output is also affected by air
pollution or dust level (Meral and Dinçer, 2011) and the surrounding rooftop environment.
However, it is not possible to distinguish them from the current measurement results.
5. CASE STUDY
Hong Kong has high-density and high-rise building developments, therefore, the available
roof space is often very limited (Hui and Chan, 2008). If it is feasible to apply both green
roof and PV systems onto the available roof space, this can facilitate a more effective use of
sustainable technologies and help to reduce the total investment costs (Hui, 2010). When this
idea is applied to existing buildings, it can provide many benefits to improve the building
performance and urban environment, as illustrated in the case study.
5.1 EMSD Headquarters
To evaluate the practical design issues, a hypothetical case study was created to retrofit the
roof of an existing building in Hong Kong. This building is the headquarters of the Electrical
and Mechanical Services Department (EMSD) and has a roof area of about 7,900 m2. The
roof is made of aluminum sheet modules (known as Kalzip aluminum standing seam roofing
system) and a 350 kW grid-connected PV system has been installed on the rooftop (Ho, Chan
and Lau, 2007) (see Figure 10). It is proposed that an extensive modular green roof system
(using sedum plant) will be installed on the roof by integrating with the Kalzip roofing
system. By coordinating the green roof with the existing roofing system, the additional costs
can be reduced and the roofing performance ensured.
Figure 10. PV and roof systems at the EMSD Headquarters
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5.2 Integration Performance
To assess the building energy benefits of adding the green roof to the EMSD headquarters,
an indicator for the energy saving per green roof area (due to thermal benefits of green roof)
was developed from the energy simulation results in Section 3. It was determined by dividing
the total annual energy reduction in air conditioning loads due to green roof by the total
green roof area of the model building. This indicator was found to be 37.32 MJ/m2.
(a) Green roof performance on annual basis
Assuming the green roof will be installed on 80% of the roof area, by applying the energy
saving indicator, the annual energy and cost saving is calculated as shown in Table 1.
Table 1. Green roof performance calculation
Green roof area 6300 m2 (about 80% total roof area)
Energy saving per green roof area 37.32MJ/m2 (based on simulation result)
Energy saving due to green roof installation 235,116 MJ
Electricity consumption reduction 65,310 kWh
Annual cost saving HK$63,351
(b) PV system performance on annual basis
Assuming the PV system efficiency improves from 12% to 13% due to cooling effect of the
green roof. Additional electricity generated from the PV system and the cost saving are
calculated as shown in Table 2. The electricity cost is assumed to be HK$0.97 per kWh. The
reference annual final yield is extracted from Ho, Chan and Lau (2007).
Table 2. PV system performance calculation
Parameter With Green Roof
(Efficiency = 13%) Without Green Roof
(Efficiency = 12%)
Annual final yield (kWh/kWp) 1,021.6 943
Electricity generated (kWh) 357,560 330,050
Cost saving (HK$) 346,833 320,149
Annual cost saving due to green roof : HK$26,685
It can be seen from the above estimates that the potential energy and cost savings are quite
good. There are also other benefits on micro climate, dust level and acoustic performance
(during heavy rainfall, the aluminum Kalzip roofing system will generate noise).
6. DISCUSSIONS
It is believed that green roofs and PV system can be used together and there is no clash
between them (Köhler, Wiartalla and Feige, 2007). Combining the two technologies in an
innovative way to form so called “green roof integrated PV” (Witmer and Brownson, 2010)
could bring positive effects to building energy conservation and improving of the
surrounding urban environment. The extent of the benefits depends on the system design and
how to determine the optimum arrangement for a particular building site. In order to
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maximize the overall performance of the integrated green roof and PV system, it is essential
to study and understand the interactions between them.
6.1 Positive Effects of Green Roofs
The installation of PV panels with green roofs will result in a net cooling of the micro climate
that offsets the warming effect of PV panels. In most project situations the additional costs
for the extensive green roof could be recovered by the higher electrical power output of the
PV system over a few years following installation. To maximize the cooling effect, the layout
and spacing of the integrated system must be designed carefully. The size and scale of the
greening must be large enough to contribute cooling to the surrounding.
Usually, the PV panel and vegetation should be placed at a close distance so that the cooling
by evapo-transpiration can directly influence the solar modules. Green roofs with sprinkler
head irrigation could enhance the cooling by increasing the moisture level and producing
water mist in the air. If the irrigation system and natural rainfall are properly arranged, the
water could help to remove the dirt/dust on the solar panels too.
6.2 Advantages of Solar Panels
The solar panels shade parts of the roof and thereby reduce the sun exposure and high
evaporation rates normally experienced on extensive green roofs. The shading will reduce the
drought stress for plants and could enhance green roof ecosystem viability by promoting
more plant species and biodiversity. This might also enable the use of plants that increase
carbon sequestration and carbon gain.
The electricity generated by PV panels can be used to power the water pumps, lighting and
other electrical appliances. By proper designing and matching of the electrical loads, it is
possible to become self-sufficient on electricity for the building rooftop. This will reduce the
costs of electrical wiring and distribution from the supply mains. Together with rainwater
harvesting and other renewable technologies, a sustainable system can be created for roof
gardens and/or rooftop farming (Hui, 2010).
6.3 Other Considerations
Most design guidelines for green roofs will remind people to check the structural loading and
waterproofing membrane before committing to the project (Hui, 2010). The integrated green
roof and PV system will have many components and must be assessed carefully for the
overall weight and water leakage issues. For existing buildings, the site constraints and
limitations might demand designers to select and arrange the system elements (e.g.
lightweight and modular) so that the load carrying capacity is not exceeded and construction
can be carried out effectively.
Another key issue to consider is related to system maintenance. It should be noted that
intensive green roofs or roof gardens have high maintenance requirements, such as watering,
fertilizing, trimming and weeding. For a properly installed extensive green roof, once it is
well established, its maintenance requirements are usually minimal. Plants for green roofs
must be selected with care if the roof is expected to stay more or less maintenance free. For
maintenance of PV panels, pathways are needed and this mean also more trampling
disturbance of the vegetation.
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7. CONCLUSIONS
Integrating green roofs and solar PV systems can enhance their functions and effectiveness
by cooling and shading effects. The results of literature theoretical study, field measurements
and case study in Hong Kong indicated a positive influence for this integration. It is found
from field measurements that green roof surface and soil temperatures are reduced from the
PV panel shading and higher power output of PV panel is achieved from the green roof
cooling effect. The findings of year-round building energy simulation using EnergyPlus
indicated that the energy consumption for air conditioning of the integrated system is slightly
lower than the stand-alone system and the PV system on integrated approach generates 8.3%
more electricity than the stand-a-lone option. The extent of the benefits depends on the
system design and how to determine the optimum arrangement for a particular building site.
In general, this research study provides useful hints to understand green roof and PV
integration for hot and humid climates.
As we move towards more sustainable buildings, our knowledge of how organic (e.g.
greening) and inorganic (e.g. solar PV) technologies can work together will be critical to the
long-term success of the society (Peck and van der Linde, 2010).
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... However, the effect of the GR on PV electricity output increases in hot climates where there is a high level of solar radiation and the air temperature exceeds 25 • C (298.15 • K) (Nagengast et al., 2013). In the hot summer of the Mediterranean climate, PV energy output increased by 1.29% to 3.33% for GRs planted with gazania and sedum plants, respectively, compared to gravel PV , such increase possibly reaching 8.3% depending on climatic conditions and plant type (Hui & Chan, 2011). ...
... Past studies have used surface temperature and PV cell temperature as parameters to measure the positive effects of GRs on PV electricity output efficiency. Because this efficiency depends partly on the PV cell temperature (Alshayeb & Chang, 2018), (Cook & Larsen, 2021), many studies have found through experimental and modelling that GRs serve as a passive cooling mechanism for PV cells that can improve output electricity by 0.5% to 3.33% in the hot arid climates (Cook & Larsen, 2021), (Kaewpraek et al., 2021), possibly reaching 8.3% in warm climates (Hui & Chan, 2011), as shown in Table 1. This improvement in electricity output can save $0.50/panel/year in the hot arid climate of Pittsburgh in the USA, as compared to black roofs under PV panels (Nagengast et al., 2013). ...
... Most studies neglect the effect of these factors in GR research and make it a constant in the simulation process (Andric et al., 2020), (Mahmoud & Ismaeel, 2019). Nevertheless, it plays a vital role especially for PV/GR systems due to the reduction of reflected solar radiation which results in better PV efficiency (Hui & Chan, 2011), (Lamnatou & Chemisana, 2015). ...
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... In addition to the conventionally accepted benefits of green roofs, there is a growing interest in the integration of green roofs with rooftop-mounted photovoltaic panels as a way of improving the performance of both systems. It has been hypothesized that this integration may improve the performance for both the green roofs and the PV Another important study is that of Hui and Chan [50] in which the results for one ...
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Thesis
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Recent studies suggest that integration of photovoltaic panels with green roofs may improve the performance of both. While vegetation may provide a benefit by reducing the net radiation load on the underside of the photovoltaic (PV) panels, it may also affect convective cooling of panels, and consequently, panel efficiency. Both effects likely diminish with the height of the PV panel above the roof, although placing PV panels too close to the vegetation increases the risk of the plants growing over the edges of, and shading the PV panel. There is a gap in the literature with respect to evaluating these competing effects. The present study aims to fill this gap. Experiments were conducted over a two-month period during summer using two identical PV panels within an array of rooftop-mounted panels. These experiments were performed at two heights (18 cm and 24 cm) using three roofing types: white, black and green (vegetated). Results showed that the mean power output of the system in which the PV panel was mounted above a green roof was 1.2% and 0.8% higher than that of the PV-black roof and the PV-white roof at the 18 cm height. At the 24 cm height, the benefit of the green roof was slightly diminished with power output for the PV panel above a green roof being 1.0% and 0.7% higher than the black and white roof experiments, respectively. These power output results were consistent with measured variations in mean panel surface temperatures; the green roof systems were generally cooler by 1.5˚C to 3˚C. The panel surface mean heat transfer coefficients for the PV-green roof were generally 10 to 23% higher than for the white and black roof configurations, suggesting a mixing benefit associated with the roughness of the plant canopy. As expected, the results ii indicate that the best PV panel performance is obtained by locating the PV panel above a green roof. However, the relative benefits of the roof energy balance diminish with distance between the PV panel and the roof. Moreover, the results of this study showed that the mean power output of the PV panel above the white roof was 0.7% and 0.44% higher than that of the PV panel above the black roof at 18 cm and 24 cm heights, respectively. The results of the power output differences in all the experiments were statistically significant at the 95% confidence interval (P < 0.01).
... The number of PV system installations is exponentially increasing due to the growing concern for greenhouse gas emissions, significant price drops, and government incentives for PV installations [10,11]. In urban areas, the combination of green roofs and rooftop PV systems contributes to sustainable building development [12]. The integration of these two green technologies on the building roof helps in enhancing the effectiveness of the PV system [12]. ...
... In urban areas, the combination of green roofs and rooftop PV systems contributes to sustainable building development [12]. The integration of these two green technologies on the building roof helps in enhancing the effectiveness of the PV system [12]. Research proves that the evapotranspiration cooling process from green roofs helps cool the ambient temperature around the modules [12], thus allowing PV modules to operate at optimal temperatures [13]. ...
... The integration of these two green technologies on the building roof helps in enhancing the effectiveness of the PV system [12]. Research proves that the evapotranspiration cooling process from green roofs helps cool the ambient temperature around the modules [12], thus allowing PV modules to operate at optimal temperatures [13]. The research led by Irga shows that integration of PV system on the green roof generates a daily output that is 13% greater than for conventional roofs, and that this also improves the system efficiency by 3.6% [14]. ...
Article
A fault tree analysis of fires related to photovoltaic (PV) systems was made with a focus of understanding the failure rate of the electric components. The failure rate of different components of these systems was calculated from data obtained from reports, research studies, and fire incident statistics of four countries. The results explain the significant causes of fire on the component level and various failure patterns resulting in PV-related fires. The qualitative analysis identified seven major events that led to incidents caused by a PV-related ignition source, with electrical arcing being the main cause of fires. This finding is highly related to the imprudent installation practices due to negligence and low awareness of the fire risk associated with PV systems by installers. The quantitative results show that 33% of the PV fire incidents are due to unknown or unrelated ignition sources, indicating that great focus should be given to mitigate the consequences caused by PV-related fires. The PV module, isolator, inverter, and connector are the major PV system components that are highly responsible for the ignition of PV-related fires, with the connector being the prime contributor in 17% of the PV-related fires. Finally, the quantitative analysis established an annual fire incident frequency of 0.0293 fires per MW. The results enable estimation of the number of fire incidents linked to the installed PV capacity, and the fault tree analyses highlight where improvements are most critical. Based on the results of the analyses, two questions are suggested for implementation in the post-incident reports of the national fire and rescue services.
... However, not only the optimal use of the -limited -rooftop space is convincing but also the positive interactions between both [3]. Several studies have been conducted to find out more about the manifold synergies -by example, Hui and Chan [4] examined the increase of PV output due to the plant and PV interaction, while Lamnatou & Chemisana have done a selection of appropriate plant species for PV-green roofs [5]. Due to the persuasive study results as well as various possibilities for grants, gradually more projects have been put into practice. ...
... 4) The electrical efficiency of solar panels is affected by accumulated dirt and dust on the PV module surface. Due to the fact, that green roofs can help to reduce dust levels and herewith improve air quality, the performance of the PV panels could be enhanced [4]. ...
Conference Paper
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Since some years, there exist technologies, which enable the combination of solar technologies with a vegetated roof. So-called green roof integrated photovoltaics (GRIPV) feature considerable synergy effects, whereas the increased PV output due to PV/plant interaction is probably the most important one. This study focuses on the implementation of GRIPV on a forty-year-old high-rise complex in Hamburg, taking into consideration the complexity and challenges of its context. The results show that the most important benefits of such a system would be: 1) partial reduction of water stress on the area caused by storm-water runoff , 2) significant reduction of discharge in the existing sewerage system, 3) in-situ production of renewable electricity that could cover the demand of about 26% of the households, 4) recovering the costs of both the green roof and PV-system in a reconcilable amount of time. Even if the results are associated with uncertainties, the beneficial impacts as well as the innovation potential are remarkable and have shown, that green roof integrated photovoltaics can be an essential puzzle piece on the path to sustainable urban development.
... In urban areas, solar energy is the largest renewable energy source that can be flexibly exploited by photovoltaic installations. While photovoltaics and building greenery compete for scarce urban surfaces, synergies between the measures may arise, and hybrid systems featuring solar and green components may allow for a bifunctional use of the available surfaces [3][4][5][6]. Moreover, growing cities experience an increasing shortage of living space, which can most sustainably be countered by redensifying existing districts in an urban context [7]. ...
... Consequently, the separate implementation of both technologies is favoured. However, the evaluation does not consider spatial efficiency, and future concepts of hybrid systems might alter the result of the analysis [4,31,32]. ...
Article
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The current need to redevelop post-war residential settlements opens up the opportunity to exploit the potential for densification and for the climatic and energetic activation of building envelopes through greenery and photovoltaics. The question arises as to which design strategies help to identify and balance relevant solar, green, and densification interventions that would lead to new qualities in the built environment. This work relies on a threefold research by design approach to acquire this knowledge base. Within a research-based design studio, four teams of master’s students in architecture faced the design task in a case study of an inner-city perimeter block development in Munich, thus covering the first two phases of the research by design process: Phase 1—pre-design, comprises a shared knowledge literature research, among other things, and concludes with specific research questions for the subsequent phase; Phase 2—design. Here, design concepts answer the research questions and are iteratively adapted and evaluated in an interdisciplinary expert discourse. Phase 3—post-design, synthesises the design proposals into design strategies. By gaining insights into the benefits and disadvantages of solar and green interventions, the research provides designers and urban planners with strategies to design the practical transformation and upgrading of urban residential structures.
... Particularly, most studies on UA focus on cultivating plants at an urban area because it can potentially contribute to the reduction of GHG emission [25]. UHI problem can be mitigated, as green space in the urban areas increases [27]. The most popular way to implement UA in high-density urban cities is utilization of a green roof [28]. ...
... The most popular way to implement UA in high-density urban cities is utilization of a green roof [28]. This is because it can enhance the community functions (e.g., community participation, education, and aesthetic value) in addition to mitigation of the environmental problems [27]. Moreover, installation cost of a green roof system is relatively easy and inexpensive compared to a new building construction project [29]. ...
Article
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This study aims at introducing a modeling and simulation approach for a green roof system which can reduce energy cost of a building exposed to high temperatures throughout the summer season. First, to understand thermal impact of a green roof system on a building surface, a field-based study has been conducted in Commerce, Texas, U.S., where the average maximum temperature in summer is 104 °F (40 °C). Two types of analyses were conducted: (1) comparison of temperature between different plant type via Analysis of variance (ANOVA) and (2) polynomial regression analysis to develop thermal impact estimation model based on air temperature and presence of a green roof. In addition, an agent-based simulation (ABS) model was developed via AnyLogic® University 8.6.0 simulation software, Chicago, IL, U.S., in order to accurately estimate energy cost and benefits of a building with a photovoltaic-green roof system. The proposed approach was applied to estimate energy reduction cost of the Keith D. McFarland Science Building at Texas A&M University, Commerce, Texas (33.2410° N, 95.9104° W). As a result, the proposed approach was able to save $740,325.44 in energy cost of a heating, ventilation, and air conditioning (HAVC) system in the subject building. The proposed approach will contribute to the implementation of a sustainable building and urban agriculture.
... Such hybrid GRs are highly recommended because of the encouraging results achieved by them. For example, Hui and Chan [99] found that the T s of a hybrid photovoltaic GR (PV GR) was 5 • C cooler than that of a traditional GR, due to the shading effect of the PV panels. An increase of 4.3% in power output from the PV panels also resulted from this combined system. ...
Article
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Water-sensitive urban design (WSUD) has been widely used in cities to mitigate the negative consequences of urbanization and climate change. One of the WSUD strategies that is becoming popular is green roofs (GR) which offer a wide range of ecosystem services. Research on this WSUD strategy has been continuously increasing in terms of both quantity and quality. This paper presents a comprehensive review quantifying the benefits of GRs in papers published since 2010. More precisely, this review aims to provide up-to-date information about each GR benefit and how they have improved over the last decade. In agreement with previous reviews, extensive GRs were considerably researched, as compared to very limited studies on intensive and semi-intensive GRs. Each GR ecosystem service was specifically quantified, and an imbalance of GR research focus was identified, wherein urban heat- and runoff-related benefits were outstandingly popular when compared to other benefits. The results also highlight the recent introduction of hybrid GRs, which demonstrated improvements in GR performance. Furthermore, limitations of GRs, obstacles to their uptake, and inconsistent research findings were also identified in this review. Accordingly, opportunities for future research were pointed out in this review. This paper also recommends future studies to improve upon well-known GR benefits by exploring and applying more innovative GR construction techniques and materials. At the same time, further studies need to be undertaken on inadequately studied GR benefits, such as reduced noise and air pollution. In spite of the existence of reliable modelling tools, their application to study the effects of large-scale implementations of GRs has been restricted. Insufficient information from such research is likely to restrict large-scale implementations of GRs. As a result, further studies are required to transform the GR concept into one of the widely accepted and implemented WSUD strategies.
... Studies show that the temperature of the green surface of the roof and ground is reduced due to shading by solar panels, and greater output power of the panel is achieved by lowering the temperature under the solar panels due to the presence of plants. Thus, in [14] it was found that when using a biosolar roof, the photovoltaic system generates 8.3% more electricity than in the absence of a green roof. ...
Conference Paper
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Urban green infrastructures such as green roofs can reduce building energy demand, mitigate rainfall run-off and improve urban air quality. On the other hand, decentralized renewable energy systems such as rooftop photovoltaics (PV), are one of the key actions towards reducing a building’s energy dependence and greenhouse gas emissions. This study assesses the technical and economic benefits of a combination of green roofs and PV systems and thereby considers increased PV yields, decreased building heat demands, and reduced rainwater runoff mitigation, that can stem from this combination. For this, two workflows within an urban simulation environment, SimStadt, were applied and extended for two city quarters in Stuttgart, Germany. The results show that by installing green roofs with PV systems where possible, annual PV yields increase by about 0.3%, annual space heating demands decrease by 0.1 %, and 30 % of rainwater runoff can be avoided in the case study areas. The economic cost-benefit analysis, however, shows that only around 31% of the initial investment can be recurred over the assets’ lifetime.
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Green roof systems are living vegetation installed on the roofs and can provide many environmental and social benefits for achieving low carbon high performance building. This paper describes the major findings of a research to develop technical guidelines for green roof systems in Hong Kong. The current knowledge and latest trends of green roof technology in the world have been studied. Useful information and experience were examined for assessing the potential benefits and key design factors. By investigating the system components and practical considerations of typical green roof projects in Hong Kong and other countries, key information is established for preparing the technical guidelines. It is hoped that the research findings will not only fill in the information gap, but also enable the making of policy and strategy to promote better urban greenery and assess their performance systematically.
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Informed decision making via an integrative design process and iterative energy simulations (early and often) is crucial during the development of a building design. We report the process and results of energy simulations for the 2009 Penn State Solar Decathlon home, Natural Fusion. The team's home entered the 2009 Solar Decathlon competition organized by the U.S. Department of Energy. According to design criteria of the Decathlon and team concept, we show how iterative modeling can inform decisions throughout the design and build process. Unique systems integrated into the home include a green roof integrated photovoltaic system (GRIPV), a pump-less solar hot water system, a large southern façade for maximum solar gain, a phase change material integrated into the walls and ceilings with a water filled floor system for high thermal capacitance throughout the building while maintaining a low transportation weight, and a close evaluation of loads and demand side management integrated with a user feedback system.
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Green roof systems are living vegetation installed on the roofs and could contribute positively to the mitigation of urban heat island and enhancement of building thermal and environmental performance. Research study has been carried out to investigate the green roof technology and research in the world, with the aim to develop practical information for its applications in Hong Kong and other similar urban cities. The important factors for assessing the performance and designing the systems have been evaluated. This research paper highlights the key findings of the evaluation and discusses the benefits and potential applications of the green roof systems.
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