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Evaluation of photovoltaic potential by urban block typology: A case study of Wuhan, China
Renewable Energy Focus Volume 29 June 2019
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Evaluation of photovoltaic potential by urban block typology: A case study of Wuhan, China Shen Xua,b,c, Zhaojian Huanga, Jianghua Wanga, Thushini Mendisa,d and Jing Huange,* a
School of Architecture and Urban Planning, Huazhong University of Science and Technology, China State Key Laboratory of Subtropical Building Science, South China University of Technology, China c Hubei New Technology Research Centre for Urbanization, China d General Sir John Kotelawala Defence University, Sri Lanka e School of Art and Design, Wuchang Shouyi University, China b
Due to the ever increasing energy demand, PV utilisation in the urban scale has become an attractive solution. The purpose of this paper is to demonstrate the effect of urban block type on solar potential. There exists a requirement to analyse solar potential in the real urban context of Wuhan based on block function and type. Therefore, this paper studies a real case in Wuhan’s urban context by selecting 15 cases of real urban industrial, commercial and residential blocks. The PV output was calculated for roofs and fac¸ades respectively. The results obtained showed obvious differences between the three type of blocks, where commercial blocks received the most total solar irradiation, followed by residential blocks, and then industrial blocks. Similarly, the distribution of solar irradiation on roofs and fac¸ades for each block was vividly different based on the block type, owing to differences in building forms based on block function.
Introduction The study of solar potential in the urban environment is becoming a topic of great importance. It is now a widely accepted fact that there exists an environmental and energy crisis. In order to control this growing trend, on-site energy production and use has become a commonly utilised strategy to minimise energy losses due to transformation and transmission [1]. In this case, solar energy is considered to be a renewable energy resource of great potential which is available in abundance and can be conveniently utilised in urban spaces through building integrated photovoltaics (BIPV). This makes the study and examination of solar potential in the urban environment a crucial matter. Many studies have been carried out relating to urban space and solar potential. A hierarchical approach was established by Izquierdo et al. [2] in studying the utilisation of renewable energy sources, which applies gradual restrictions in order to identify various levels of potential: physical potential, geographical potential, technical potential, and *Corresponding author Huang, J. ([email protected])
economic and social potential. Attention has been veering towards attempting to quantify and estimate the amount of global solar irradiation incident on building envelopes, allowing the assessment of active and passive solar heating technologies and methods [3–5]. The potential of implementation of solar technologies on stand-alone buildings has therefore been extensively researched [6–8]. A study conducted in Netherlands was based on attempting to evaluate the technical potential of solar energy in the city in terms of social criterion, which comprise of the most important factors that play a part in the adaptation of solar PV [9]. Current studies that have been conducted involve the investigation of the effects of different horizontal and vertical layouts of built forms on solar potential and daylight availability [10], and the effects of various parameters indicating urban form and density, including, but not limited to, site coverage, plot ratio, and building density [11]. Accordingly, several studies centred on analysing the solar potential in existing urban layouts [12], but were based on characteristic building forms with little or no variation. Some other studies only considered residential buildings [13,6], whilst several
1755-0084/ã 2019 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.ref.2019.03.002
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studies were carried out in order to provide design guidelines for urban spaces by suggesting optimised building shapes and urban layouts for the utilisation of solar irradiation [11]. However, many of these studies have primarily been centred on using characteristic and generic urban layouts in order to explore the effects of urban form on solar potential [14], and have not been applied to real case studies. In addition, although stand-alone buildings have been analysed in detail, due to the undesirable effects of neighbouring buildings and mutual shading, urban blocks are not as capable in capturing as much solar irradiation as stand-alone buildings are. Therefore, the study of the effects of urban form on solar potential is an area of increasing interest [1]. Furthermore, whilst parametric analyses have been conducted in order to determine the effects of varying parameters of urban density on solar potential, and attempting to quantify the amount of solar irradiation that can be obtained in the urban scale, no such studies have been carried out in order to investigate how the building function and form affects the achievable solar irradiation. Finally, even though these parametric analysis studies have been conducted in order to evaluate the effects of these parameters on solar potential, the application potential of these studies in the real urban context in Wuhan is extremely limited. Based on the Chinese urban environment, strict urban planning laws have led to definitive urban blocks in central regions, i.e. urban blocks are segregated based on building function and typology. Furthermore, due to the issue of a shortage of land in the country and high urban density, parameters such as plot ratio, and building density need to lie within a predefined range in order to maximize the land use potential. Due to these restrictions, these parameters cannot be ideally changed in the real urban context as they could be for parametric analyses studies in generic urban models, as they affect each other in the real urban context. Therefore, there exists a requirement to assess the solar potential in real case studies. This study aims to explore the solar potential and obtain the PV potential in the urban scale and its distribution on different block types in central China. Based on the Chinese urban contexts, three types of urban blocks were identified and taken into consideration: industrial, commercial and residential. Solar irradiation incident upon roofs and fac¸ades of these building types were analysed in order to determine which building typology is most suited for PV installation. The solar irradiation available within the urban blocks cannot be directly compared, as reasons for variation in results may owe to the difference in block sizes. Therefore, this research further aims to explore the land-use and PV potential of the individual blocks by estimating the solar irradiation available per square metre of site area. Furthermore, this paper can help urban planners to optimise PV installation methods for the different building types that could aid in future PV deployment projects.
In the actual urban environment, solar potential can be affected by parameters of urban density. However, since there are many different parameters, such as site coverage, plot ratio, building density, etc., these parameters also present mutual constraints on each other. According to Chinese urban development laws, street width is represented by a fixed value, where the adaptability and influence of urban density parameters in the real urban context will be restricted, and therefore, different. This paper classifies buildings based on real blocks, and divides them into industrial, commercial, and residential blocks. After classification based on urban block type, the parameters of the different blocks are accounted for in terms of the site area, building footprint area, fac¸ade area, ratio of fac¸ade to roof area, average heights of the buildings within the block, and the building density of the block. This information is presented in Table 4, and can be significantly distinguished in Figure 1. The urban context of Wuhan is extremely organised, in the sense that urban blocks have been clearly divided with individual blocks being dedicated to one building function, i.e. residential, commercial, and industrial buildings have been clearly segregated into their respective blocks. These block types can therefore be easily identified. Industrial blocks are denser and consist of low-level buildings, whilst commercial blocks consist of more high-rise buildings with more spacing between buildings as can be seen in Tables 1–3. This method of classification was chosen, since classification based on height of buildings may not be entirely relevant as residential blocks tend to be a mix of both high-rise and low-rise buildings. The purpose of this study is also to determine how the building and urban form affects the solar potential in the urban canyon. The Chinese urban context usually results in blocks with individual urban functions, and determining how the urban function affects the solar potential can aid planners in establishing thumbrules for PV integration based on block type. In this sense, it helps to establish which urban function correlates to which method of PV installation, i.e. whether roofs or facades or both. The idea behind this classification of blocks is further enhanced by the graph shown in Figure 1, where it can be clearly seen that when the blocks are considered on the basis of building density, average building height, and the ratio of fac¸ade area to roof area, industrial, commercial and residential blocks lie on different regions of the graph. This signifies a distinct difference in the three types of blocks.
Materials and methods Urban block classification by types Generally, assessing the PV potential of an urban area can be done in three main steps: first, high resolution LiDAR data and aerial digital photography can be used to build a Digital Terrain Model (DTM) and Digital Surface Model (DSM); second, the population distribution needs to be estimated; and third, the Solar Analyst extension for ArcGIS can be used to model the solar radiation [15]. 142
FIGURE 1
Distribution of blocks.
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TABLE 3
Sample information of industrial blocks.
Sample information of residential blocks.
Name
Satellite imagery
Model
Name
Wuhan Heavy Machinery Factory
Vanke Chenghua Jingyuan
Foxconn Factory
Zhonggu Yuan
Satellite imagery
Model
ORIGINAL RESEARCH ARTICLE
TABLE 1
Guanggu Meihuawu
ZMS Cable Factory
Hongxia Mingyuan
Industrial Park Area II
Shahu Gangwan AVIC Cable Factory
TABLE 2
Sample information of commercial blocks. Name
Satellite imagery Model
Oceanwide SOHO
Qunguang Square
Wanda Chuhe Han Street
Optical Valley International Headquarters
Wuhan Times Square
This paper surveys several urban blocks in Wuhan and classifies them according to industrial blocks, commercial blocks, and residential blocks. Five samples were randomly selected of each of these three types of blocks, and the characteristics of solar energy utilization potential were simulated using the selected software.
PV potential estimation In order to assess the solar irradiation incident on the roofs and fac¸ades of the buildings, it was first necessary to establish a method by which to run irradiation simulations on the constructed models. Estimating solar potential in urban landscapes becomes complex, since calculating solar radiation is dependent on time, location and conditions. So the shadow effects also need to be
considered, which include the shadows cast on useable surfaces by buildings, vegetation, or structural elements. For the purposes of this research, it was necessary to determine a suitable tool that could be used for solar irradiation estimation. From the literature review, countless existing models were examined that allowed for the assessment of solar irradiation in urban contexts, some of which include Radiance, Daysim, and ArcGIS Solar Analyst [15,16]. The latter, ArcGIS Solar Analyst was deemed unfeasible to use due to the requirement for GIS data of the city, which would incur high costs. However, Radiance is a highly accurate ray-tracing software system, which applies the Perez diffuse radiation model [17,18], and considers both specular and diffuse reflections from urban obstructions. Based on the way in which light physically behaves in a volumetric 3D model, it uses a refined light-backwards ray-tracing algorithm which can even be used in complicated curved geometries [19]. This software has been authenticated many times, and successfully utilised in many applications regarding the assessment of solar potential on building roofs and fac¸ades for daylighting and electricity generation. Therefore, Radiance was chosen as the method to estimate solar irradiation, and was integrated as plug-in Rhinoceros 5, which is a 3D modelling software. This can be used via the Grasshopper interface, which is a visual programming language and environment, and the open-source Ladybug and Honeybee tools integrated within. These tools are open-source environmental plug-ins for Grasshopper, which help to investigate and assess the environmental performance, where Ladybug imports standard EnergyPlus weather files into Grasshopper. EnergyPlus is a whole building energy simulation programme that allows practitioners to both building energy consumption and water use. The Ladybug tool then supports the preliminary stages of design and the decision-making process by supplying a range of 3D interactive graphics. There are four validated simulation engines which evaluate building energy consumption, thermal comfort, and daylighting, namely, EnergyPlus, Radiance, Daysim, and OpenStudio [20]. Honeybee connects the visual programming environment of Grasshopper to these four simulation engines. In this way, the 143
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TABLE 4
Site information.
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Block
Site area (m2)
Building footprint area (m2)
Façade area (m2)
Ratio of façade area to roof area (m2/m2)
Average height (m)
Building density (m2/m2)
Industrial
395127.0 326305.9 432446.4 124022.1 66420.5
160555.1 135673.6 167487.8 51389.6 25010.5
107975.5 113084.7 152385.8 56325.8 37527.7
0.67 0.83 0.91 1.10 1.50
13.68 15.24 18.52 12.35 11.28
0.41 0.42 0.39 0.41 0.38
Commercial
68940.3 49094.5 32472.2 89502.9 44563.0
10373.0 7021.3 12284.5 14410.3 7136.8
115878.3 85508.4 81314.3 265265.5 54422.4
11.17 12.18 6.62 18.41 7.63
42.45 55.83 44.75 85.00 40.00
0.15 0.14 0.38 0.16 0.16
Residential
72046.9 47478.1 66105.8 73075.1 32582.6
25975.9 16329.3 23571.4 29793.2 12284.5
139529.3 89089.4 156972.6 147386.4 81314.3
5.37 5.46 6.66 4.95 6.62
57.86 51.67 66.67 52.65 44.75
0.36 0.34 0.36 0.41 0.38
validated environmental datasets and simulation engines are dynamically coupled with the adaptable, component-based visual programming interface by these plug-ins. In addition, making use of the urban-scale 3D models coupled with the Ladybug and Honeybee tools is relatively low-cost compared to using GIS data. The suggested method for this research, therefore, was to make use of Rhinoceros and the Grasshopper interface, coupled with the Ladybug and Honeybee tools, which would act as a hub in order to utilise Radiance and OpenStudio to run radiation simulations. By utilising Rhinoceros 5 and the Ladybug and Honeybee tools, it is possible to simulate the radiation incident upon individual surfaces in the urban block being assessed. The simulation engine, RADIANCE, takes into account the building dimensions, orientations and details such as reflections off the ground and neighbouring buildings in order to allow the user to determine the solar radiation available on roofs and facades separately in each urban block. Today the typical performance ratio (PR) is between 80%–90%, and therefore the PR was set as 85% for this research [21], and the Polycrystalline silicon PV efficiency as 15% [22]. In order to obtain the PV potential available in each block, first the solar irradiation incident upon the roofs and/or fac¸ades was calculated, multiplied by the performance ratio of the system and the efficiency of the panel, and then divided by the site area of the block as shown in the equation below, where EPV denotes the PV output in kWh/m2/ year, TSR denotes the total incident solar irradiation in kWh, ɳpoly-Si denotes panel efficiency, PR denotes the performance ratio, and SA denotes the site area in m2: EPV ¼
T SR hpolySi PR SA
u2 ¼
P ðX mÞ2 N
ð2Þ
Results and discussion Overview of simulation results In order to determine the suitability of building types for the installation of solar technologies, the three different urban blocks were compared side-by-side in order to determine the total PV
ð1Þ
In order to consider how the PV potential across different urban blocks of the same urban function varied or spread out, the statistical measure known as standard deviation was used. The standard deviation represents how spread out the values for PV potential are within each block type. The standard deviation, u, in 144
each block type is calculated as shown in Eq. (2) below, where X represents the PV potential being considered in each block, m denotes the mean PV potential over all five blocks under each block type, and N is the number of blocks of each block type (Figure 2).
FIGURE 2
Sample of irradiation simulation.
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output by each block type based on rooftop, fac¸ade, and total PV output. The results indicate that whilst industrial blocks have the lowest PV potential in total, they also have a high rooftop PV potential in comparison to facades. These results suggest that installation of PV on rooftops of industrial buildings would be optimal in order to maximise solar resources. Residential blocks have the second highest potential for PV. However, residential fac¸ades have a higher proportion of PV output than residential roofs, which indicates that residential fac¸ades are most suitable for PV installation when compared with residential roofs. Finally, commercial blocks exhibit the highest PV potential on both rooftops and fac¸ades, with roughly equivalent proportions for each. Therefore, it can be suggested that commercial buildings can be suited for PV implementation on both rooftops and fac¸ades. From the results obtained from the simulations, it was evident that different building block types exhibited respectively varying proportions of PV output on rooftops and facades with little variance between the urban blocks. As can be seen in Figures 3–5, it was noted that each industrial block exhibited similar amounts of rooftop PV output with a variance of 4.97. Commercial buildings displayed roughly equivalent proportions of PV output on both rooftops and facades, although facades showed a slightly higher proportion.
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FIGURE 4
PV output contributions in commercial blocks.
Comparison of PV output Finally, the total PV output including both rooftops and fac¸ades was obtained for the industrial, commercial and residential blocks. These results are shown in Figure 6. Commercial blocks obtained the most PV output with median is 146.2 kW h/m2/year incident on both fac¸ades and roofs. The second highest amount of PV output was obtained by residential blocks with 111.1 kW h/m2/ year which is a 31.6% decrease compared to the commercial blocks. Industrial blocks had the lowest total PV output on both roofs and fac¸ades with only 77.0 kW h/m2/year, which corresponds to a 89.8% decrease compared with the PV output obtained by the commercial blocks.
FIGURE 5
PV output contributions in residential blocks.
FIGURE 3
PV output contributions in industrial blocks.
As shown in Figure 6, industrial blocks exhibit the best performance in regards to PV output with a median value of 55.5 kW h/ m2/year. This could be explained due to the fact that industrial blocks provide larger roof areas when compared to other blocks, since the building form tends to be short and flat, allowing for more rooftop area. This was closely followed by commercial blocks which received a median PV output value of 51.7 kW h/m2/year which shows a difference of 6.9% when compared with the industrial block. Residential blocks achieved the lowest PV output values in terms of rooftop PV with a median value of 27.2 kW h/m2/year, which is approximately half of that achieved by the former blocks. 145
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FIGURE 6
FIGURE 8
Box plot of PV output.
Self sufficiency ratio (PV output/energy demand).
The results indicate that industrial roofs receive a high proportion of rooftop PV output, which can be explained due to the fact that industrial buildings are mainly low-rise factories with low buildings heights, fewer storeys, and little spacing between buildings. Due to the close-spaced nature of the building blocks, fac¸ades tend to receive lower levels of solar irradiation. Likewise, a simulation was carried out in order to determine the PV output of the fac¸ades of industrial, commercial and residential blocks in Wuhan. The results obtained are likewise shown in Figures 6 and 7, which indicate that commercial blocks receive the most PV output on fac¸ades with a median value of 94.6 kW h/m2/year. This can be explained by the building form exhibited by most commercial buildings, where they tend to be high-rise buildings with higher urban density, contributing towards more available fac¸ade area. The second highest level of PV output was obtained by the residential blocks, with a median value of 83.9 kW h/m2/year, which is a 12.7% decrease when compared with the commercial blocks. Finally, industrial blocks received the lowest amount PV output on fac¸ades with only 21.5 kW h/m2/year, which is roughly less than a quarter of the previous blocks. This could be explained due to the building form
of industrial buildings, where most have low levels of elevation and are spaced out with less fac¸ade area and more roof area. Commercial fac¸ades received the highest amounts of PV output in comparison to industrial and residential facades. The distribution of this PV generation can be explained due to commercial block building forms, where commercial blocks are mainly highrise towers with multiple storeys and, therefore, larger fac¸ade areas.
Self-sufficiency ratio The self-sufficiency ratios for the types of blocks were calculated in order to determine how the PV generation of the block type could account for the building energy consumption within the block and can be seen Figure 8. Due to an unavailability of data regarding the energy consumption in industrial blocks, the self-sufficiency ratio was only calculated for commercial and residential blocks by obtaining data available on the China Statistical Yearbook 2018. The ratios obtained indicate that commercial blocks are the most efficient in terms of being energy selfsufficient, since over 50% of its energy demand could be met by photovoltaic-generated electricity, whereas residential blocks have a much lower ratio at less than 25%, indicating a lower level of self sufficiency. However, it should be noted that achieving 25% of energy consumption via renewable energy technologies is still of large significance.
Conclusions
FIGURE 7
Total PV output contribution. 146
This research aimed to evaluate the PV potential in different urban blocks in the real urban context. The results displayed strong differences in PV potential between blocks based on building typology and the reasons for these variations are discussed below. Firstly, industrial blocks exhibited a large proportion of rooftop PV output in comparison to fac¸ades. These results suggest that industrial rooftops are more suitable for PV installation. Secondly, commercial blocks received roughly the same proportion of rooftop to fac¸ade PV potential, with a slightly higher proportion of output incident upon the latter. For PV installation, these results suggest that in high-rise building blocks with higher density, the fac¸ades are more suitable, due to the higher availability of commercial fac¸ade area in the block.
Renewable Energy Focus Volume 29 June 2019
[6] [7] [8] [9]
[10]
Funding This research was funded by the National Natural Science Foundation (No. 51678261), National Key Laboratory of Subtropical Building Science Project (No. 2017ZB08), and Wuhan City, Urban and Rural Construction Committee of Science and Technology Project Funding (No. 201726).
[11] [12] [13] [14] [15] [16]
Conflict of interest
[17]
The authors hereby declare that there is no any conflict of interest regarding the publication of this manuscript.
[18] [19]
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Finally, residential blocks exhibited a higher proportion of incident fac¸ade PV potential compared to rooftops, suggesting that the fac¸ades are more suitable for PV implementation. In general, the results initially displayed that commercial blocks had the highest potential for PV installation, followed by residential blocks, and then industrial blocks.
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Evaluation of photovoltaic potential by urban block typology: A case study of Wuhan, China Shen Xua,b,c, Zhaojian Huanga, Jianghua Wanga, Thushini Mendisa,d and Jing Huange,* a
School of Architecture and Urban Planning, Huazhong University of Science and Technology, China State Key Laboratory of Subtropical Building Science, South China University of Technology, China c Hubei New Technology Research Centre for Urbanization, China d General Sir John Kotelawala Defence University, Sri Lanka e School of Art and Design, Wuchang Shouyi University, China b
Due to the ever increasing energy demand, PV utilisation in the urban scale has become an attractive solution. The purpose of this paper is to demonstrate the effect of urban block type on solar potential. There exists a requirement to analyse solar potential in the real urban context of Wuhan based on block function and type. Therefore, this paper studies a real case in Wuhan’s urban context by selecting 15 cases of real urban industrial, commercial and residential blocks. The PV output was calculated for roofs and fac¸ades respectively. The results obtained showed obvious differences between the three type of blocks, where commercial blocks received the most total solar irradiation, followed by residential blocks, and then industrial blocks. Similarly, the distribution of solar irradiation on roofs and fac¸ades for each block was vividly different based on the block type, owing to differences in building forms based on block function.
Introduction The study of solar potential in the urban environment is becoming a topic of great importance. It is now a widely accepted fact that there exists an environmental and energy crisis. In order to control this growing trend, on-site energy production and use has become a commonly utilised strategy to minimise energy losses due to transformation and transmission [1]. In this case, solar energy is considered to be a renewable energy resource of great potential which is available in abundance and can be conveniently utilised in urban spaces through building integrated photovoltaics (BIPV). This makes the study and examination of solar potential in the urban environment a crucial matter. Many studies have been carried out relating to urban space and solar potential. A hierarchical approach was established by Izquierdo et al. [2] in studying the utilisation of renewable energy sources, which applies gradual restrictions in order to identify various levels of potential: physical potential, geographical potential, technical potential, and *Corresponding author Huang, J. ([email protected])
economic and social potential. Attention has been veering towards attempting to quantify and estimate the amount of global solar irradiation incident on building envelopes, allowing the assessment of active and passive solar heating technologies and methods [3–5]. The potential of implementation of solar technologies on stand-alone buildings has therefore been extensively researched [6–8]. A study conducted in Netherlands was based on attempting to evaluate the technical potential of solar energy in the city in terms of social criterion, which comprise of the most important factors that play a part in the adaptation of solar PV [9]. Current studies that have been conducted involve the investigation of the effects of different horizontal and vertical layouts of built forms on solar potential and daylight availability [10], and the effects of various parameters indicating urban form and density, including, but not limited to, site coverage, plot ratio, and building density [11]. Accordingly, several studies centred on analysing the solar potential in existing urban layouts [12], but were based on characteristic building forms with little or no variation. Some other studies only considered residential buildings [13,6], whilst several
1755-0084/ã 2019 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.ref.2019.03.002
141
Renewable Energy Focus Volume 29 June 2019
ORIGINAL RESEARCH ARTICLE
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studies were carried out in order to provide design guidelines for urban spaces by suggesting optimised building shapes and urban layouts for the utilisation of solar irradiation [11]. However, many of these studies have primarily been centred on using characteristic and generic urban layouts in order to explore the effects of urban form on solar potential [14], and have not been applied to real case studies. In addition, although stand-alone buildings have been analysed in detail, due to the undesirable effects of neighbouring buildings and mutual shading, urban blocks are not as capable in capturing as much solar irradiation as stand-alone buildings are. Therefore, the study of the effects of urban form on solar potential is an area of increasing interest [1]. Furthermore, whilst parametric analyses have been conducted in order to determine the effects of varying parameters of urban density on solar potential, and attempting to quantify the amount of solar irradiation that can be obtained in the urban scale, no such studies have been carried out in order to investigate how the building function and form affects the achievable solar irradiation. Finally, even though these parametric analysis studies have been conducted in order to evaluate the effects of these parameters on solar potential, the application potential of these studies in the real urban context in Wuhan is extremely limited. Based on the Chinese urban environment, strict urban planning laws have led to definitive urban blocks in central regions, i.e. urban blocks are segregated based on building function and typology. Furthermore, due to the issue of a shortage of land in the country and high urban density, parameters such as plot ratio, and building density need to lie within a predefined range in order to maximize the land use potential. Due to these restrictions, these parameters cannot be ideally changed in the real urban context as they could be for parametric analyses studies in generic urban models, as they affect each other in the real urban context. Therefore, there exists a requirement to assess the solar potential in real case studies. This study aims to explore the solar potential and obtain the PV potential in the urban scale and its distribution on different block types in central China. Based on the Chinese urban contexts, three types of urban blocks were identified and taken into consideration: industrial, commercial and residential. Solar irradiation incident upon roofs and fac¸ades of these building types were analysed in order to determine which building typology is most suited for PV installation. The solar irradiation available within the urban blocks cannot be directly compared, as reasons for variation in results may owe to the difference in block sizes. Therefore, this research further aims to explore the land-use and PV potential of the individual blocks by estimating the solar irradiation available per square metre of site area. Furthermore, this paper can help urban planners to optimise PV installation methods for the different building types that could aid in future PV deployment projects.
In the actual urban environment, solar potential can be affected by parameters of urban density. However, since there are many different parameters, such as site coverage, plot ratio, building density, etc., these parameters also present mutual constraints on each other. According to Chinese urban development laws, street width is represented by a fixed value, where the adaptability and influence of urban density parameters in the real urban context will be restricted, and therefore, different. This paper classifies buildings based on real blocks, and divides them into industrial, commercial, and residential blocks. After classification based on urban block type, the parameters of the different blocks are accounted for in terms of the site area, building footprint area, fac¸ade area, ratio of fac¸ade to roof area, average heights of the buildings within the block, and the building density of the block. This information is presented in Table 4, and can be significantly distinguished in Figure 1. The urban context of Wuhan is extremely organised, in the sense that urban blocks have been clearly divided with individual blocks being dedicated to one building function, i.e. residential, commercial, and industrial buildings have been clearly segregated into their respective blocks. These block types can therefore be easily identified. Industrial blocks are denser and consist of low-level buildings, whilst commercial blocks consist of more high-rise buildings with more spacing between buildings as can be seen in Tables 1–3. This method of classification was chosen, since classification based on height of buildings may not be entirely relevant as residential blocks tend to be a mix of both high-rise and low-rise buildings. The purpose of this study is also to determine how the building and urban form affects the solar potential in the urban canyon. The Chinese urban context usually results in blocks with individual urban functions, and determining how the urban function affects the solar potential can aid planners in establishing thumbrules for PV integration based on block type. In this sense, it helps to establish which urban function correlates to which method of PV installation, i.e. whether roofs or facades or both. The idea behind this classification of blocks is further enhanced by the graph shown in Figure 1, where it can be clearly seen that when the blocks are considered on the basis of building density, average building height, and the ratio of fac¸ade area to roof area, industrial, commercial and residential blocks lie on different regions of the graph. This signifies a distinct difference in the three types of blocks.
Materials and methods Urban block classification by types Generally, assessing the PV potential of an urban area can be done in three main steps: first, high resolution LiDAR data and aerial digital photography can be used to build a Digital Terrain Model (DTM) and Digital Surface Model (DSM); second, the population distribution needs to be estimated; and third, the Solar Analyst extension for ArcGIS can be used to model the solar radiation [15]. 142
FIGURE 1
Distribution of blocks.
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TABLE 3
Sample information of industrial blocks.
Sample information of residential blocks.
Name
Satellite imagery
Model
Name
Wuhan Heavy Machinery Factory
Vanke Chenghua Jingyuan
Foxconn Factory
Zhonggu Yuan
Satellite imagery
Model
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TABLE 1
Guanggu Meihuawu
ZMS Cable Factory
Hongxia Mingyuan
Industrial Park Area II
Shahu Gangwan AVIC Cable Factory
TABLE 2
Sample information of commercial blocks. Name
Satellite imagery Model
Oceanwide SOHO
Qunguang Square
Wanda Chuhe Han Street
Optical Valley International Headquarters
Wuhan Times Square
This paper surveys several urban blocks in Wuhan and classifies them according to industrial blocks, commercial blocks, and residential blocks. Five samples were randomly selected of each of these three types of blocks, and the characteristics of solar energy utilization potential were simulated using the selected software.
PV potential estimation In order to assess the solar irradiation incident on the roofs and fac¸ades of the buildings, it was first necessary to establish a method by which to run irradiation simulations on the constructed models. Estimating solar potential in urban landscapes becomes complex, since calculating solar radiation is dependent on time, location and conditions. So the shadow effects also need to be
considered, which include the shadows cast on useable surfaces by buildings, vegetation, or structural elements. For the purposes of this research, it was necessary to determine a suitable tool that could be used for solar irradiation estimation. From the literature review, countless existing models were examined that allowed for the assessment of solar irradiation in urban contexts, some of which include Radiance, Daysim, and ArcGIS Solar Analyst [15,16]. The latter, ArcGIS Solar Analyst was deemed unfeasible to use due to the requirement for GIS data of the city, which would incur high costs. However, Radiance is a highly accurate ray-tracing software system, which applies the Perez diffuse radiation model [17,18], and considers both specular and diffuse reflections from urban obstructions. Based on the way in which light physically behaves in a volumetric 3D model, it uses a refined light-backwards ray-tracing algorithm which can even be used in complicated curved geometries [19]. This software has been authenticated many times, and successfully utilised in many applications regarding the assessment of solar potential on building roofs and fac¸ades for daylighting and electricity generation. Therefore, Radiance was chosen as the method to estimate solar irradiation, and was integrated as plug-in Rhinoceros 5, which is a 3D modelling software. This can be used via the Grasshopper interface, which is a visual programming language and environment, and the open-source Ladybug and Honeybee tools integrated within. These tools are open-source environmental plug-ins for Grasshopper, which help to investigate and assess the environmental performance, where Ladybug imports standard EnergyPlus weather files into Grasshopper. EnergyPlus is a whole building energy simulation programme that allows practitioners to both building energy consumption and water use. The Ladybug tool then supports the preliminary stages of design and the decision-making process by supplying a range of 3D interactive graphics. There are four validated simulation engines which evaluate building energy consumption, thermal comfort, and daylighting, namely, EnergyPlus, Radiance, Daysim, and OpenStudio [20]. Honeybee connects the visual programming environment of Grasshopper to these four simulation engines. In this way, the 143
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TABLE 4
Site information.
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Block
Site area (m2)
Building footprint area (m2)
Façade area (m2)
Ratio of façade area to roof area (m2/m2)
Average height (m)
Building density (m2/m2)
Industrial
395127.0 326305.9 432446.4 124022.1 66420.5
160555.1 135673.6 167487.8 51389.6 25010.5
107975.5 113084.7 152385.8 56325.8 37527.7
0.67 0.83 0.91 1.10 1.50
13.68 15.24 18.52 12.35 11.28
0.41 0.42 0.39 0.41 0.38
Commercial
68940.3 49094.5 32472.2 89502.9 44563.0
10373.0 7021.3 12284.5 14410.3 7136.8
115878.3 85508.4 81314.3 265265.5 54422.4
11.17 12.18 6.62 18.41 7.63
42.45 55.83 44.75 85.00 40.00
0.15 0.14 0.38 0.16 0.16
Residential
72046.9 47478.1 66105.8 73075.1 32582.6
25975.9 16329.3 23571.4 29793.2 12284.5
139529.3 89089.4 156972.6 147386.4 81314.3
5.37 5.46 6.66 4.95 6.62
57.86 51.67 66.67 52.65 44.75
0.36 0.34 0.36 0.41 0.38
validated environmental datasets and simulation engines are dynamically coupled with the adaptable, component-based visual programming interface by these plug-ins. In addition, making use of the urban-scale 3D models coupled with the Ladybug and Honeybee tools is relatively low-cost compared to using GIS data. The suggested method for this research, therefore, was to make use of Rhinoceros and the Grasshopper interface, coupled with the Ladybug and Honeybee tools, which would act as a hub in order to utilise Radiance and OpenStudio to run radiation simulations. By utilising Rhinoceros 5 and the Ladybug and Honeybee tools, it is possible to simulate the radiation incident upon individual surfaces in the urban block being assessed. The simulation engine, RADIANCE, takes into account the building dimensions, orientations and details such as reflections off the ground and neighbouring buildings in order to allow the user to determine the solar radiation available on roofs and facades separately in each urban block. Today the typical performance ratio (PR) is between 80%–90%, and therefore the PR was set as 85% for this research [21], and the Polycrystalline silicon PV efficiency as 15% [22]. In order to obtain the PV potential available in each block, first the solar irradiation incident upon the roofs and/or fac¸ades was calculated, multiplied by the performance ratio of the system and the efficiency of the panel, and then divided by the site area of the block as shown in the equation below, where EPV denotes the PV output in kWh/m2/ year, TSR denotes the total incident solar irradiation in kWh, ɳpoly-Si denotes panel efficiency, PR denotes the performance ratio, and SA denotes the site area in m2: EPV ¼
T SR hpolySi PR SA
u2 ¼
P ðX mÞ2 N
ð2Þ
Results and discussion Overview of simulation results In order to determine the suitability of building types for the installation of solar technologies, the three different urban blocks were compared side-by-side in order to determine the total PV
ð1Þ
In order to consider how the PV potential across different urban blocks of the same urban function varied or spread out, the statistical measure known as standard deviation was used. The standard deviation represents how spread out the values for PV potential are within each block type. The standard deviation, u, in 144
each block type is calculated as shown in Eq. (2) below, where X represents the PV potential being considered in each block, m denotes the mean PV potential over all five blocks under each block type, and N is the number of blocks of each block type (Figure 2).
FIGURE 2
Sample of irradiation simulation.
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output by each block type based on rooftop, fac¸ade, and total PV output. The results indicate that whilst industrial blocks have the lowest PV potential in total, they also have a high rooftop PV potential in comparison to facades. These results suggest that installation of PV on rooftops of industrial buildings would be optimal in order to maximise solar resources. Residential blocks have the second highest potential for PV. However, residential fac¸ades have a higher proportion of PV output than residential roofs, which indicates that residential fac¸ades are most suitable for PV installation when compared with residential roofs. Finally, commercial blocks exhibit the highest PV potential on both rooftops and fac¸ades, with roughly equivalent proportions for each. Therefore, it can be suggested that commercial buildings can be suited for PV implementation on both rooftops and fac¸ades. From the results obtained from the simulations, it was evident that different building block types exhibited respectively varying proportions of PV output on rooftops and facades with little variance between the urban blocks. As can be seen in Figures 3–5, it was noted that each industrial block exhibited similar amounts of rooftop PV output with a variance of 4.97. Commercial buildings displayed roughly equivalent proportions of PV output on both rooftops and facades, although facades showed a slightly higher proportion.
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FIGURE 4
PV output contributions in commercial blocks.
Comparison of PV output Finally, the total PV output including both rooftops and fac¸ades was obtained for the industrial, commercial and residential blocks. These results are shown in Figure 6. Commercial blocks obtained the most PV output with median is 146.2 kW h/m2/year incident on both fac¸ades and roofs. The second highest amount of PV output was obtained by residential blocks with 111.1 kW h/m2/ year which is a 31.6% decrease compared to the commercial blocks. Industrial blocks had the lowest total PV output on both roofs and fac¸ades with only 77.0 kW h/m2/year, which corresponds to a 89.8% decrease compared with the PV output obtained by the commercial blocks.
FIGURE 5
PV output contributions in residential blocks.
FIGURE 3
PV output contributions in industrial blocks.
As shown in Figure 6, industrial blocks exhibit the best performance in regards to PV output with a median value of 55.5 kW h/ m2/year. This could be explained due to the fact that industrial blocks provide larger roof areas when compared to other blocks, since the building form tends to be short and flat, allowing for more rooftop area. This was closely followed by commercial blocks which received a median PV output value of 51.7 kW h/m2/year which shows a difference of 6.9% when compared with the industrial block. Residential blocks achieved the lowest PV output values in terms of rooftop PV with a median value of 27.2 kW h/m2/year, which is approximately half of that achieved by the former blocks. 145
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FIGURE 6
FIGURE 8
Box plot of PV output.
Self sufficiency ratio (PV output/energy demand).
The results indicate that industrial roofs receive a high proportion of rooftop PV output, which can be explained due to the fact that industrial buildings are mainly low-rise factories with low buildings heights, fewer storeys, and little spacing between buildings. Due to the close-spaced nature of the building blocks, fac¸ades tend to receive lower levels of solar irradiation. Likewise, a simulation was carried out in order to determine the PV output of the fac¸ades of industrial, commercial and residential blocks in Wuhan. The results obtained are likewise shown in Figures 6 and 7, which indicate that commercial blocks receive the most PV output on fac¸ades with a median value of 94.6 kW h/m2/year. This can be explained by the building form exhibited by most commercial buildings, where they tend to be high-rise buildings with higher urban density, contributing towards more available fac¸ade area. The second highest level of PV output was obtained by the residential blocks, with a median value of 83.9 kW h/m2/year, which is a 12.7% decrease when compared with the commercial blocks. Finally, industrial blocks received the lowest amount PV output on fac¸ades with only 21.5 kW h/m2/year, which is roughly less than a quarter of the previous blocks. This could be explained due to the building form
of industrial buildings, where most have low levels of elevation and are spaced out with less fac¸ade area and more roof area. Commercial fac¸ades received the highest amounts of PV output in comparison to industrial and residential facades. The distribution of this PV generation can be explained due to commercial block building forms, where commercial blocks are mainly highrise towers with multiple storeys and, therefore, larger fac¸ade areas.
Self-sufficiency ratio The self-sufficiency ratios for the types of blocks were calculated in order to determine how the PV generation of the block type could account for the building energy consumption within the block and can be seen Figure 8. Due to an unavailability of data regarding the energy consumption in industrial blocks, the self-sufficiency ratio was only calculated for commercial and residential blocks by obtaining data available on the China Statistical Yearbook 2018. The ratios obtained indicate that commercial blocks are the most efficient in terms of being energy selfsufficient, since over 50% of its energy demand could be met by photovoltaic-generated electricity, whereas residential blocks have a much lower ratio at less than 25%, indicating a lower level of self sufficiency. However, it should be noted that achieving 25% of energy consumption via renewable energy technologies is still of large significance.
Conclusions
FIGURE 7
Total PV output contribution. 146
This research aimed to evaluate the PV potential in different urban blocks in the real urban context. The results displayed strong differences in PV potential between blocks based on building typology and the reasons for these variations are discussed below. Firstly, industrial blocks exhibited a large proportion of rooftop PV output in comparison to fac¸ades. These results suggest that industrial rooftops are more suitable for PV installation. Secondly, commercial blocks received roughly the same proportion of rooftop to fac¸ade PV potential, with a slightly higher proportion of output incident upon the latter. For PV installation, these results suggest that in high-rise building blocks with higher density, the fac¸ades are more suitable, due to the higher availability of commercial fac¸ade area in the block.
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[6] [7] [8] [9]
[10]
Funding This research was funded by the National Natural Science Foundation (No. 51678261), National Key Laboratory of Subtropical Building Science Project (No. 2017ZB08), and Wuhan City, Urban and Rural Construction Committee of Science and Technology Project Funding (No. 201726).
[11] [12] [13] [14] [15] [16]
Conflict of interest
[17]
The authors hereby declare that there is no any conflict of interest regarding the publication of this manuscript.
[18] [19]
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Finally, residential blocks exhibited a higher proportion of incident fac¸ade PV potential compared to rooftops, suggesting that the fac¸ades are more suitable for PV implementation. In general, the results initially displayed that commercial blocks had the highest potential for PV installation, followed by residential blocks, and then industrial blocks.
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