Journal of Cleaner Production 295 (2021) 126403

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Journal of Cleaner Production

journal homepage: www.elsevier.com/locate/jclepro

Life cycle assessment of semi-transparent photovoltaic window

applied on building
Zihao Li a, Wei Zhang a, *, Lingzhi Xie b, Wei Wang a, Hao Tian a, Mo Chen a, Jianhui Li a
a
College of Architecture and Environment, Sichuan University, Chengdu, 610065, China
b
Institute of New Energy and Low-carbon Technology, Sichuan University, Chengdu, 610065, China

a r t i c l e i n f o a b s t r a c t

Article history: The Semi-Transparent Photovoltaic (STPV) Window is a new form of building integrated photovoltaics
Received 18 May 2020 (BIPV), which not only can generate electricity, but also improve indoor light and thermal environment.
Received in revised form The STPV window was designed, manufactured and applied on a test building. Energy and environmental
3 February 2021
benefits were studied. The research site is located in an area with solar radiation diversification. The
Accepted 13 February 2021
Available online 18 February 2021
parameters of the life cycle assessment (LCA) model are derived from the database, experimental data
and the electricity simulation model. The two important indicators, energy payback time (EPBT) and
Handling editor: Dr Sandra Caeiro greenhouse gas payback time (GPBT), are selected to analysis this system, and another seven environ-
mental indicators are considered for different aspects. According to the research findings, the EPBT of the
Keywords: STPV window is 13.8 years, and the GPBT 10.4 years, which are both less than the preset service life of 25
Photovoltaic window years. The results show that the STPV window can reduce pollution and the waste of non-renewable
Life cycle assessment resources. The optimization analysis connected with the LCA study was carried out based on the crit-
Environmental impact ical parameters of the STPV window application in buildings. Optimal directions and preferable locations
Payback time
can result in the STPV window being more economical and sustainable. With increase of the window-to-
wall ratio (WWR) and the coverage ratio of PV cells, the EPBT and GPBT gradually become shorter.
Following to the research findings, suggestions are put forward to effectively reduce the life cycle cost of
BIPV systems.
© 2021 Elsevier Ltd. All rights reserved.

1. Introduction effects of the production of PV modules cannot be ignored. It is

necessary to study the energy and environmental impact of PV
Solar energy plays a crucial role in sustainable development and technology. Up to now, scholars have studied the utilization po-
renewable energy. It can be used to generate electricity sustainably tential of PV technology, considering the potential of technical,
and effectively by means of photovoltaic (PV) technology (Peng and economic and environmental indicators such as solar capacity,
Lu, 2013). Compared with traditional primary energy, PV technol- power contribution rate, life cycle cost, investment payback time
ogy has significant advantages in environmental, economic and and energy payback time (EPBT). Huang et al. (2017) assessed the
societal aspects (Solangi et al., 2011), (Hou et al., 2016). In China, environmental impacts of PV systems quantitatively in China,
since the State Council issued several guidelines promoting the including the recycling process. The life cycle assessment (LCA)
healthy development of the PV industry (State Council [2013] model is well established and can be calculated by the LCA software
No.24) in 2013 (The State Council, 2013), the scale of China’s PV GaBi, and the ReCiPe method is appropriate to quantify the envi-
market has expanded rapidly. By the end of 2018, China supplies ronmental impact. Peng et al. (2013) compared a review of LCA
more than 2=3 of the total global output (Jiang, 2019). studies for five common PV systems, with all five PV systems being
Although PV technology can reduce the consumption of energy sustainable and environmentally friendly. Luo et al. (2018) intro-
and other resources and has a significant impact on the environ- duced LCA studies of three p-type PV technologies in Singapore,
ment through power generation, it is evident that the adverse mainly in regard to EPBT and greenhouse gas (GHG) emissions. The
EPBTs for three PV systems are about 1 year, while their GHG
emissions range from 20.9 to 30.2 g CO2-eq/kWh.
* Corresponding author. In these studies, in order to accurately study the environmental
E-mail address: zhangwei821@scu.edu.cn (W. Zhang). performance of the PV system, LCA is usually used to evaluate its

https://doi.org/10.1016/j.jclepro.2021.126403
0959-6526/© 2021 Elsevier Ltd. All rights reserved.
Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Nomenclature NS the number of solar cells in series

dðTC Þ the thermal voltage
EP energy demand of production stage aImp the temperature coefficient of Imp
ET energy demand of transportation stage aIsc the temperature coefficient of xIsc
EC energy demand of construction stage bVmp the temperature coefficient of Vmp
EO energy demand of operation stage bVoc temperature coefficient of Voc
EW energy demand of waste treatment stage n the empirical diode factor
Isc the short-circuit current k the Boltzmann’s constant
Imp the current at the maximum power point q the elementary charge
Voc the open circuit voltage f ðAMa Þ an empirical function of absolute air massðAMa Þ;
Vmp the voltage at the maximum power point correct the impact of solar spectrum on the short-
Pmp the power at the maximum power point circuit current, Isc
Imp0 the current at the maximum power point under the C0 , C1 empirical coefficients related with Imp to the effective
standard test condition irradiance, Ee , C0 þ C1 ¼ 1,(dimensionless)
Isc0 the short-circuit current under the standard test C2 , C3 empirical coefficients related with Vmp to the
condition effective irradiance (C2 is dimensionless, and the unit
Vmp0 the voltage at the maximum power point under the of C3 isV 1 )
standard test condition EINPUT energy demand input in the life cycle
Voc0 the open circuit voltage under the standard test EOUTPUT annual PV power generation
condition GHGINPUT greenhouse gas emissions in the life cycle
Ee the effective irradiance(suns) GHGOUTPUT greenhouse gas reduction equivalent to the annual
TC the operating temperature PV power generation
TO the temperature of the standard test condition

environmental impact. The two most widely used environmental studies on PV systems in buildings. Ng and Mithraratne (2014)
indicators, EPBT and GHG emissions payback time (GPBT), can studied the environmental and economic performance of com-
directly evaluate the PV system’s sustainability and environmental mercial semi-transparent windows in tropical conditions. Wang
performance. et al. (2016) analyzed the economic and environmental states of
As an important part of global energy consumption, research on BAPV and BIPV systems. In addition, EPBT and GPBT were used to
building sustainability has gradually become a hot issue. There assess the environmental impact. The result shows that BIPV is
have been a lot of studies on building sustainability, in regard to more environmentally friendly than BAPV. Menoufi et al. (2013)
green building materials, geographical environment, renewable compared the energy and environmental performance through
energy and other building energy-saving technology. de Azevedo the entire life cycle of a building attached concentrating PV (BACPV)
et al. (2020) and De la Colina Martínez et al. (2019) proved that system and a BIPV system. The results showed that compared with
primary sludge waste from the paper industry and recycled poly- the BIPV system, the BACPV system has better performance.
carbonate from electronic waste can be used in mortar and con- Through the above research, it was found that although the types
crete to improve the sustainability of buildings. Verichev et al. and application methods of various PV systems are different, they
(2019) studied the assessment of current climatic zoning for can ultimately complete the payback of energy and GHG before the
building construction in two regions in the extreme southern part end of their service life. The payback time varies with the form of
of Chile to determine the main parameters that might affect cli- the system and the application environment.
matic zoning for residential buildings. Chen et al. (2020) proposed a For PV modules applied on buildings, most of the current LCA
hybrid district heating system integrated with solar and studies specifically focused on the PV modules themselves or the
geothermal energy, which could save fossil fuels and improve form of PV modules. There have also been some studies on the
sustainability. optimization factors of PV modules on buildings, Lu and Yang
Recently, the combination of PV modules and buildings has also (2010) studied the BIPV system in different directions and found
become energy-saving and environmental protection solution. that the maximum difference of the EPBT was 6.7 years. Li et al.
There are two main ways to combine PV systems and buildings (2018) studied the application of a low-concentration PV module
(Crawford et al., 2006). One is to install the PV module directly on for buildings in five different cities in China and concluded that the
the building’s external surface (BAPV), which is the primary form of EPBT could be reduced by 40.5%. These critical factors of the PV
the combination. The other is to take specialised materials and system applied on buildings have a significant influence on LCA
processes to produce PV modules directly used as building com- study.
ponents that are building-integrated photovoltaics (BIPV). It is not Based on the literature review, most of the previous articles set
only can be needed as building components, but also generate the PV module in a virtual and ideal condition, and the LCA data
electricity sustainably (Wang et al., 2016). The different application only comes from databases. This situation may lead to inaccurate
forms of BIPV are shown in Fig. 1. The transformation of existing LCA evaluation and overestimation of the economic, energy and
buildings is also an important form of BIPV application. Some environmental benefits of BIPV systems. Besides, the majority of
research was carried out on the overall analysis and optimization of the PV-LCA investigations refer to PV modules themselves without
a project through software simulation (Abd and Khamees, 2018). consideration on BIPV for building components. Moreover, most
Other studies analyzed the various stages in the process of building studies focus on the different forms of PV modules while there is a
transformation and carried out optimization analysis step by step lack of precise research about the critical factors of the PV system
(Mercado Burciaga et al., 2019), (Nikeghbali and Damavandi, 2018). applied on buildings.
Due to the advantages of BIPV, there have also been a few LCA Compared with the previous literature, the LCA model in this

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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Fig. 1. Different application forms of BIPV.

paper was combined with the experimental data and the electricity 2.2. PV module
simulation model to carry out a comprehensive analysis, which is
more realistic. Further, the optimizing factors of BIPV systems on Based on our preliminary research on PV windows in Southwest
buildings are combined with LCA in order to conduct detailed China (Tian et al., 2018), the STPV window was designed and
research, aiming to give scientific conclusions on how to use BIPV manufactured by our partner company. The structure of the STPV
with a shorter payback time and higher return rate. The optimizing window and the real window are shown in Fig. 4. The STPV window
factors include direction, geo-climatic conditions, window area and is composed of four parts; 5 mm glass layers on both sides, 0.8 mm
silicon coverage. crystal silicon module and 9 mm air interlayer. The detailed pa-
The semi-transparent photovoltaic (STPV) window was rameters are shown in Table 1.
designed, manufactured and applied to a building, energy and
environmental benefits is studied. The research site is located in an
area with solar radiation diversification. A solar resources map of 2.3. Test equipment and method
China (Solar resource data: Solargis) is shown in Fig. 2. According to
the abundance of solar energy resources, regions can be divided The test was conducted in 2019, and the test platform is shown
into five grades, all of which are found in Southwest China. Due to in Fig. 5 and Fig. 6. The outdoor test equipment includes a weather
the diversity and complexity of the climate and solar energy re- station and solar radiation test equipment. The outdoor wind di-
sources distribution in this area, the results can represent most rection/speed, temperature and humidity are determined by the
areas with similar solar energy distribution. weather station, and the solar radiation test equipment measures
In order to elaborate on the research content, this paper has the the solar radiation in the window direction. PV room monitoring
following parts: system is mainly used to test the electrical performance parameters
The Case Study introduces an experimental platform of an STPV of the PV system, such as power generation, IeV curve, etc. All the
window in Chengdu, China, and the Research method introduces main instruments and functions are shown in Table 2.
the LCA method based on experimental data and simulation model,
and the life cycle inventory introduces the data collection. The
3. Research method
Results and Discussion introduce energy, GHG emissions,
environmental-related research index results of the case study and
3.1. Assessment methods and data sources
optimization analysis for the application in buildings, and the
research findings are summarized in the Conclusions.
LCA is used to evaluate the associated environmental impacts of
the life cycle (Sherwani et al., 2010). EPBT and GPBT are two of the
most widely used environmental indicators which can be used to
2. Case study evaluate the sustainability and environmental performance of PV
systems (Bany Mousa et al., 2019). Energy demand refers to the
A BIPV scheme in a building in Chengdu City, Sichuan Province, energy consumed in the life cycle of the PV system, in kWh unit.
China, was assembled and tested. Carbon footprint is the sum of direct and indirect GHG emissions
generated in the life cycle of the PV system, mainly in CO2-eq units.
In this paper, the energy demand is estimated by the primary en-
ergy demand (PED) index, and the carbon footprint index is esti-
2.1. The building model mated by the Global warming potential (GWP) index.
The LCA model is established by using the LCA software efoot-
In order to test the power generation of PV modules, and ensure print to calculate the impact of the PED and GWP. The steps to
the accuracy of the parameters of the life cycle model, a crystal establish an LCA model in efootprint are shown in Fig. 7, and can be
silicon (c-si) PV window demonstration house was built in mainly divided into the following three steps:
Chengdu, China, as shown in Fig. 3. The south facade of the pre-
fabricated building was installed with the STPV window, and there (1) Identify the process of evaluation and establish a plan for
were no obstacles in front of the building. each LCA stage.
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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Fig. 2. Solar resource zone in China.

(2) Input the energy, material consumption and emission data of The life cycle of an STPV window is divided into five main
each stage into the material list. stages: production stage (silicon ore mining, industrial silicon
(3) Calculate the impact of the input materials in each stage. smelting, solar grade silicon manufacturing, silicon ingot casting,
silicon wafer slicing, battery chip manufacturing, battery module
The data sources are shown in Table 3. The data mainly comes manufacturing, auxiliary parts manufacturing), transportation
from Ecoinvent database and the survey of previous studies stage (transport PV modules from the factory to installation site),
(Vandepaer et al., 2019), (Liu et al., 2010), survey of factories of the construction stage, operation stage and waste treatment stage
Hanergy Company and the average values from references (Hong (decommission, disposal or other end-of-life ways) (Xie et al.,
et al., 2016), (Li and Yu, 2014). 2015). The Schematic diagram of STPV window life cycle system
boundary is shown in Fig. 8.
3.2. System boundary
3.3. Energy demand
Based on the actual situation for PV systems in Southwest China,
the basic assumption is that the STPV window consists of a normal Energy demand represents the energy used in the life cycle from
window with aluminum frame and the optics and cells array are the exploitation of primary resources to waste (Lu and Yang, 2010).
inserted between its two glass panes. Other basic assumptions are In this paper, the energy demand EL is divided into five stages:
that the lifetime of windows is 25 years and manufacturing is in production stage EP , transportation stage ET , construction stage EC ,
Chengdu, China. The system boundary includes the extraction of operation stage EO and waste treatment stage EW , and analyzes the
raw materials, manufacturing, transportation, construction, use, energy demand of each stage and optimizes the energy demand of
maintenance and waste treatment of the window. a certain stage.
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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Table 1
The parameters of STPV window.

Layer Thickness(mm)

a The structure of PV module

Front glazing layer 5
PV cell layer 0.8
Air gap layer 9
Back glazing layer 5
b The layout of PV module
Glazing size (m2) 1.240.64
PV cell size (mm2) 155155
PV cell number 28
PV coverage radio (%) 85
c The Parameters of PV window
PV cells number 140
PV area (m2) 3.3635
Window area (m2) 4.6715
PV coverage radio (%) 72
Number of cells in series, Ns 140
Number of cells in parallel, Np 1
Short circuit current, Isc (A) 6.968
Open circuit voltage, Voc (V) 85.6
Current at the maximum power point, Imp (A) 6.221
Voltage at the maximum power point, Vmp (V) 68.3

Fig. 3. Prefabricated building installed with STPV window.

used in the PV electricity simulation model, which includes the
influence of solar radiation, solar altitude angle, outdoor wind
The calculation equation is as follows: speed, atmospheric transparency and PV panel temperature, and it
can simulate the actual performance of PV panel operation accu-
EL ¼ EP þ ET þ EC þ EO þ EW (1) rately (Peng et al., 2015).
The calculation equation is as follows:


Isc ¼ Isc0 $ f ðAMa Þ $ Ee $ 1 þ aISC $ ðTC  TO Þ (2)
3.4. Electricity simulation model
  h i
According to the test platform, a simplified BIPV system elec- Imp ¼ Impo $ C0 $ Ee þ C1 $ Ee2 $ 1 þ aImp $ ðTC  TO Þ (3)
tricity simulation model is established to predict the energy-saving
and environmental performance. A Sandia PV empirical model is

Fig. 4. Semi-transparent PV module model.

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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Fig. 5. Weather station and Solar radiation test equipment.

Pmp ¼ Imp $Vmp (6)


 
Ee ¼ Isc Isc0 $ 1 þ aISC $ ðTC  TO Þ (7)

dðTC Þ ¼ n $ k$ðTC þ 273:15Þ = q (8)

Based on the weather data and the relevant parameters of STPV
windows obtained from long-term outdoor cross season experi-
ments, the working condition of STPV windows in Chengdu can be
simulated by the Sandia PV empirical model. In addition to the
weather data and the electrical performance parameters of the
STPV window, the power generation of the STPV window is also
tested. Using the measured data, the Sandia PV empirical model is
simulated and verified.
Moreover, the Sandia PV empirical model is a general model that
can simulate the actual operation of PV according to the local
climate data and the PV electrical properties parameters. By
changing the parameters, different working conditions of PV win-
dows can be simulated, such as different directions, different lo-
Fig. 6. PV room Monitoring System. cations or different PV windows. Through the electricity simulation
model, the calculated power generation is a positive benefit of the
PV system.

3.5. Energy payback time (EPBT)

Voc ¼ Voc0 þ NS $dðTC Þ$lnðEe Þ þ bVoc ðEe Þ$ðTC  Te Þ (4)
In order to determine the feasibility and effect of the PV system
in buildings, it is necessary to analyze the proportion of energy
Vmp ¼ Vmp0 þ C2 $ NS $ dðTC Þ $ lnðEe Þ þ C3 $ Ns $ ½dðTC Þ$lnðEe Þ2 demand of the input and output in its life cycle. Compared with
þ bVmp ðEe Þ$ðTC  To Þ coal-fired power generation, which is the most common form of
power generation, PV power generation can generate electricity
(5)
without consuming any energy resources or energy in the opera-
tion stage. Compared with the emerging clean power generation

Table 2
The main instruments and functions.

Equipment Manufacture/version Function

Weather station J.t Weather condition recorder

Solar radiation test equipment AV87110 Solar radiation recorder
Multi-channel data recorder J.t Data collector

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Fig. 7. The life cycle assessment methods.

Table 3
Life cycle assessment data sources.

Stage Data sources

Production Stage Survey of factories of the Hanergy Company and the average of references (Hong et al., 2016), (Li and Yu, 2014)
Transportation stage CLCD database (Liu et al., 2010)
Construction stage CLCD and Ecoinvent database (Vandepaer et al., 2019), (Liu et al., 2010)
Operation stage The cross-seasonal outdoor test and the PV electricity simulation model
Waste treatment stage CLCD and Ecoinvent database (Vandepaer et al., 2019), (Liu et al., 2010)

methods such as hydropower and wind power, its initial invest- modules) by the GHG reduction equivalent to the PV power gen-
ment is relatively small and more stable. EPBT contrasts the energy eration (De Wild-Scholten, 2013).
generated by the PV system and the energy consumed by the sys- The calculation equation is as follows:
tem to determine whether there is a net energy gain for the user in
the life cycle (Fthenakis et al., 2011). The calculation equation is as  
GHGINPUT kg CO2eq
follows: GPBTðyearÞ ¼    (10)
GHGOUTPUT kg CO2eq year
EINPUT ðkWhÞ
EPBTðyearÞ ¼ (9) GPBT of the PV system is possible because the PV system re-
EOUTPUT ðkWh=yearÞ places the power of the local grid. Diverse power systems such as
coal plants, biofuel plants and nuclear plants, supply power to the
local grid generating CO2-eq emissions. Different locations have
their local grids, with different proportions of different power
3.6. Greenhouse gas payback time (GPBT) systems. Therefore, PV power generation in different locations
should also have different GHG emission factors for grid electricity.
GPBT is also used to measure the sustainability of systems or The GHG emission factors for grid electricity of five major cities in
technologies. Nowadays, global warming has become a global Southwest China are shown in Table 4 (Liu et al., 2010).
environmental problem, so the PV system is recommended as one Due to the broad variation range and rare collection of power
of the best solutions because it does not produce GHG during the generation data in cities, the provincial GHG emission factors
operation stage. However, the PV system still produces carbon di- where the city is located are adopted here. From Table 4, it can be
oxide and other GHGs throughout its life cycle, such as in produc- seen that the GHG emission factors in different provinces are quite
tion, transportation, etc. Therefore, it is essential to study the different and is mainly related to the power generation proportion.
payback time based on GHG emissions to determine the sustain- GHG factors in Tibet and Sichuan are the lowest, 0.26 and 0.36 kg
ability and environmental benefits of the PV system. The GPBT is CO2-eq respectively. This is because Tibet and Sichuan are the re-
calculated by dividing the GHG generated by the system (PV gions with the highest proportion of non-fossil energy power (main
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Fig. 8. Schematic diagram of STPV window life cycle system boundary.

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Table 4
GHG emission factors for grid electricity in different location.

Locations (Province-City) GHG emission factors for grid electricity (kg CO2-eq/kWh)

Sichuan-Chengdu 0.36
Chongqing-Chongqing 0.69
Guizhou-Guiyang 0.77
Tibet-Lhasa 0.26
Yunnan-Kunming 0.54

hydropower) in China, while Chongqing, Guiyang and Kunming Table 6

have mainly thermal power generation, so these provinces have Life cycle inventory of the STPV window.
(2) Transportation stage
large GHG emission factors.
Item Item description/function Material used Quantity
3.7. Other environmental impacts STPV window
PV system PV cells Single-crystalline silicon 3.36 m2
To evaluate the environmental impact of the STPV window on Junction box Wire 6.76 kg
Junction box 3.22 kg
the life cycle more comprehensively, besides energy demand and Window Encapsulation Eva (ethyl vinyl acetate) 5.16 kg
GHG emission, we calculate abundant environmental impacts, such TPT Backplane 2.26 kg
as Chinese Abiotic Resource Depletion Potential (CADP), Water Use Cover White glass 4.19 m2
(WU), Acidification Potential (AP), Eutrophication Potential (EP) Frame Aluminum 18.11 kg
and Respiratory Inorganics (RI), Ozone Depletion Potential (ODP),
Photochemical Ozone Formation Potential (POFP), etc.
The indicators and methods used in the impact assessment are manufacturing and finally forming PV module products. The STPV
shown in Table 5. They are built in the efootprint software to window can be decomposed into five parts: PV cells, encapsulation,
develop build LCA models and perform calculations. cover, frame, junction box, then we can decompose these five parts
into raw materials step by step and input them into the life cycle
4. Life cycle inventory inventory. According to the research on enterprises, a summary of
material consumption in each stage of the STPV window is shown
4.1. Research objectives and scope in Table 6.
Most of the construction materials are transported on highways,
The purpose of this study is to assess the energy and environ- and it is assumed that a medium-sized diesel truck is used for
mental impact of STPV windows. The STPV windows were installed transportation. The relevant life cycle background data is from the
on a prefabricated building in Southwest China as building com- CLCD database (Liu et al., 2010).
ponents with a service life of 25 years.
(3) Construction stage
4.2. Life cycle inventory analysis
In the construction stage, the energy demand is divided into two
4.2.1. Energy demand parts: assembling PV cell modules and windows and installing
assembled PV windows on the building. According to the energy
(1) Production Stage demand of different construction methods, the total energy de-
mand in the construction stage can be calculated (Lamnatou et al.,
In the production stage, the production process of PV systems is 2016). The relevant life cycle background data is from CLCD and
involved with various technologies, mainly including silicon ore Ecoinvent database (Vandepaer et al., 2019), (Liu et al., 2010).
mining, industrial silicon smelting, solar grade silicon
manufacturing, silicon ingot casting, silicon wafer slicing, battery (4) Operation stage
chip manufacturing, battery module manufacturing, auxiliary parts

Table 5
LCIA indexes and methods described in detail.

Abbreviation Name Method name Unit Description

CADP Chinese Abiotic resource ISCP 2010 ADP is divided by the resources in China’s self-sufficiency rate, and the characteristic factor of each
kg Coal-
Depletion Potential R-eq resource was obtained, which was used to express the extent of the scarcity of resources in China.
WU Water Use CML 2002 kg WU is the consumption of water resources
AP Acidification Potential CML 2002 kg SO2-eq
AP is the consequence of acids being emitted to the atmosphere and subsequently deposited in surface
soils and waters.
EP Eutrophication Potential CML 2002 kg EP is referred to the pollution state of aquatic ecosystems in which the over-fertilization of water and
PO3
4 eq soil has turned into an increased growth of biomass.
RI Respiratory Inorganics IMPACT2002þ kg RI represents the respiratory health risks of inorganic particles released into the air from each sector
PM2.5 (kg PM2.5 equivalent).
-eq
ODP Ozone Depletion Potential CML 2002 kg CFC- ODP represents the potential of depletion of the ozone layer due to the emissions of
11-eq chlorofluorocarbon compounds and other halogenated hydrocarbons.
POFP Photochemical Ozone CML 2002 kg Photochemical smog is caused by the degradation of VOCs and nitrogen.
Formation Potential NMVOC-
eq

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and processed, can be used as raw material for glass production

with low requirements for raw material quality, chemical compo-
sition and color. In recent years, Xiao et al. (2020a,b) found that
waste glass can be developed as a raw material for geopolymer
cement and confirmed the potential synergy between waste glass
recycling and geopolymer or pavement industry.
In addition, for the recycling of PV modules, part of the silicon
wafers will crumble during the replace stage due to fragility. The
broken silicon wafer can be treated by metal stripping and hydro-
metallurgy to recover aluminum, silver and other metals. The
complete silicon wafer can be dissolved the packaging material EVA
by organic solvent to recycle (Ardente et al., 2019).

4.2.2. Energy output

According to the cross-seasonal outdoor test throughout the
year, the annual power generation of the experimental modules in
Fig. 9. Monthly power generation of the STPV window in 2019. 2019 is shown in Fig. 9. The modules determine the photoelectric
conversion efficiency. Consequently, the efficiency did not change
much in the different seasons. However, due to the difference of
In the operation stage, the regular operation of PV modules
radiation intensity in each season, its power generation was the
generally does not need other operation and maintenance costs
highest in the transitional season with 33.1 kWh in March, and the
except regular water to wash off the dust from the surface of the PV
lowest in summer with 26.6 kWh in August because of the solar
modules. Due to the small scale and lack of corresponding data, the
altitude angle.
energy demand in the operation stage is ignored.
After calculation, the total annual power generation of the STPV
window in the first year was 346.5 kWh, the annual power gen-
(5) Waste treatment stage
eration was 74.2 kWh/m2. According to equations (2)e(8), the
electricity simulation model was established by EnergyPlus to
In the waste treatment stage, the energy demand can be divided
predict the power generation of PV systems for the future 25 years,
into two parts, demolition and waste treatment. In the demolition
and the cross-seasonal outdoor test measured the relevant pa-
stage, a variety of mechanical equipment and labour are needed.
rameters of PV modules.
After demolition of the building, the treatment method is to
The operation life of the system was set as 25 years. Since the
transport the waste directly to the designated waste treatment site.
attenuation rate of the photoelectric conversion efficiency cannot
The project adopts highway transportation.
exceed 20% in 20 years (Baig et al., 2014), the annual attenuation
In addition, the life cycle of the building is significantly longer
rate of components was set as 0.8%, so the total power generation of
than that of the STPV window. It is necessary to replace or remove
the system after 25 years of operation was 7879.7 kWh. This part
the discarded PV modules in the STPV window in the life cycle of
provides a positive benefit for PV systems.
the building. Generally, the service life of civil buildings is at least
50 years, and the energy demand of the PV modules in STPV win-
dows is considered to be replaced once. The relevant life cycle 5. Results and discussions
background data is from CLCD and Ecoinvent database (Vandepaer
et al., 2019), (Liu et al., 2010). 5.1. Energy demand and carbon footprint
The recycling stage of the STPV window can be focused on the
recycling of waste glass, and which cannot be burned, degraded Based on the collected inventory data and the software data-
naturally in a landfill, or decomposed by general physical and base, according to equation (1), the PED and GWP of each stage of
chemical methods. Generally, waste glass, which is sorted, selected the STPV window are shown in Table 7.
Through analysis, the contribution of PED and GWP of each

Table 7
PED and GWP of the STPV window in each stage of the life cycle.

Project PED/MJ GWP/kg CO2-eq

Production stage Silicon ore mining 14.0 1.3

Industrial silicon smelting 466.3 24.6
Solar grade silicon manufacturing 1322.6 95.0
Silicon ingot casting 377.0 26.8
Silicon wafer slicing 1877.6 124.2
Battery chip manufacturing 903.1 65.8
Battery module manufacturing 2496.8 199.9
Auxiliary parts manufacturing 3579.7 283.1
Total 11037.1 820.7
Transportation stage 669.4 39.6
Construction stage Assembly of PV module and window 3270.9 257.2
Install PV window 1401.8 110.2
Total 4672.7 367.4
Waste treatment stage Replace 593.9 45.2
Disposal 296.9 22.6
Total 890.8 67.8
Total 17270.0 1295.5

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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Fig. 10. Proportion of PED of each stage of STPV window life cycle.

stage of the STPV window was obtained, as shown in Fig. 10 and contributing 28%; the GHG emissions of the transportation and
Fig. 11. waste treatment stage were relatively small, only 39.6 kg and
According to the analysis of the total energy demand of the life 67.8 kg, contributing 3% and 5%; the operation stage had an enor-
cycle of PV systems (Fig. 10), the total energy demand was mous environmental benefit due to the PV power generation,
17270.0 MJ, of which the production stage contributed 11037.1 MJ, reducing the GHG emission by 474.4%.
accounting for 64%; the construction stage 4672.7 MJ, accounting In the production stage, it can be seen from Figs. 10 and 11 that
for 27%; the transportation stage 669.4 MJ, accounting for 4%; the the production process of the auxiliary parts contributes the most,
waste treatment stage 890.8 MJ, accounting for 5%. The total energy with energy demand and GHG emission accounting for 21% and
demand of the life cycle of the BIPV system was mainly concen- 22% of the life cycle respectively. This is followed by the production
trated in the production stage, and the power generation in the process of the battery module, with energy demand and GHG
operation stage generated enormous environmental benefits, emission accounting for 14% and 16% respectively. Then silicon
reducing energy demand by about 164%. ingot slicing with energy demand and the carbon footprint
The total GHG emission of PV systems was 1295.5 kg, and the accounted for 11% and 10% respectively.
GHG emission of each stage of the life cycle is shown in Fig. 11. The It follows that the production process of steel, copper,
results showed that the GHG emission of the production stage was aluminum, glass, silicon and other materials contributes greatly to
the most significant, 820.7 kg, contributing 64%; the second stage the life cycle energy demand of the STPV window. Therefore,
was the construction stage of the STPV window, 367.4 kg, reducing the energy demand and material demand in the

Fig. 11. Proportion of GWP of each stage of STPV window life cycle.

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Fig. 12. Contribution of each stage to the environmental impacts of STPV window.

Table 8
Other environmental impacts LCIA results.

Index Unit Value Index Unit Value

CADP kg Coal-R-eq 1.63Eþ06 RI kg PM2.5-eq 2.38Eþ00

WU kg 1.24Eþ04 ODP kg CFC-11-eq 2.58E-05
AP kg SO2 -eq 7.73Eþ00 POFP kg NMVOC-eq 1.81Eþ00
EP kg PO3
4 eq 6.79E-01

manufacturing process can effectively reduce the environmental

impact in the production stage, and then reduce the environmental
impact in the life cycle.

5.2. Energy payback time (EPBT)

According to equation (9), the total energy demand input of the

STPV window was 17270.0 MJ, equivalent to 4797.2 kWh. In Fig. 13. The EPBT and GPBT of the STPV window with different directions.
contrast, the STPV window’s annual power generation was
346.5 kWh, and the EPBT of the STPV window was 13.8 years, the
energy input can be recovered before the end of the operation life. (ISCP 2010, CML 2002, IMPACT2002þ), the seven environmental
Besides, considering that the BIPV system can replace building impact categories of the STPV window are obtained as shown in
components, the actual energy demand of the STPV windows to be Fig. 12. Table 8 shows the other environmental impacts of the LCIA
recovered should be excluded from the energy demand of a stan- results from the STPV window.
dard window. The actual energy demand input of the STPV window In addition to energy demand and GHG emissions, CADP was the
was 13245.1 MJ, and the EPBT can be further shortened to 10.6 most significant impact on the life cycle of STPV windows, in which
years. the production stage contributes the most, about 96.54%. This is
mainly because of the tin paste and silver paste used in the pro-
5.3. Greenhouse gas payback time (GPBT) duction of PV cells, which are very scarce resources in China, so
their consumption should be reduced as much as possible or find
According to equation (10), The total GHG emission of the STPV replacement materials. The other environmental impacts also
window was 1295.5 kg. In contrast, the annual power generation of contributed the most in the production stage, all above 67.87% (AP).
the STPV window was 346.5 kWh, equivalent to a reduction of To reduce the STPV window’s impact on the environment, from the
124.7 kg GHG emission. The GPBT was 10.4 years, so the cost can be perspective of the life cycle, targeted measures should be carried
recovered before the end of the operation life. The total power out from the production stage, such as improve the production
generation in the life cycle was 7879.7 kWh. Through the electricity process level, save energy and material consumption.
conversion standard coal coefficient, the energy demand can be Due to the power generation of STPV windows, it has replaced
converted into saving 11.3 tonnes of standard coal. The GHG the power of the local grid, and obtained certain benefits. Among
emission can be reduced by about 22.1 tonnes in the life cycle. them, the environmental impact caused by Wu, AP, RI and EP has
been fully recovered, with recovery rates of 140.67%, 215.71%,
5.4. Other environmental impacts 205.85% and 160.99%, respectively. However, in terms of CADP, ODP
and POFP, only partial recovery can be achieved, with recovery rates
Using the different environmental impact assessment methods of 32.24%, 55.91% and 71.11% respectively. However, it can be

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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Table 9
The division and basis of solar energy resources region.

Abundant extent of Solar resource Solar resource zone Annual global solar radiation (kWh/m2) Sunshine duration (hour/year)

Abundant I 1856 3200e3300

Comparatively Abundant II 1626e1856 3000e3200
Medium III 1393e1626 2200e3000
Comparatively Scarce Ⅳ 1161e1393 1400e2000
Scarce Ⅴ <1161 1000e1400

Table 10
Distribution of solar energy resources in cities of Southwest China.

Locations Solar Abundant extent of Annual global solar

resource Solar resource radiation (kWh/m2)
zone

Chengdu Ⅴ Scarce 888.2

Chongqing Ⅴ Scarce 837.9
Guiyang Ⅴ Scarce 743.5
Lhasa I Abundant 1872.2
Kunming III Medium 1441.5

Fig. 14. The EPBT and GPBT of the STPV window with different location.

concluded that the STPV window has excellent benefits in all

environmental impact indicators of the life cycle.

5.5. Optimization analysis

According to our preliminary research on PV windows in

Southwest China, we have found optimization parameters to
improve the efficiency of the PV window, such as window direction,
location, window-to-wall ratio and PV coverage radio. In this paper,
these optimization parameters are selected to explore the effect on
LCA indicators.

5.5.1. Different direction

The EPBT and GPBT of STPV windows installed in different di-
rections are shown in Fig. 13. The results show that the EPBT and
GPBT of STPV windows in different directions are different. The
EPBT ranged from 13.8 to 19.4 years, and the GPBT ranged from 10.4
to 14.6 years. This is due to the difference in solar radiation received
by the vertical facade in different directions, resulting in the dif-
ference in power generation. However, a common point is that the
payback time can be shorter than the service life, usually 20e30
years.
In addition, the PV module investigated in this paper cannot
adjust the opening angle. If the opening angle can be adjusted to
adapt to the optimal solar radiation, the EPBT and GPBT can be
further shortened.

5.5.2. Different location

Southwest China is an area with the broadest and most so-
phisticated solar energy resources classification. Tibet is the most
abundant region in solar energy resources, while Chongqing and
Guizhou are the most deficient regions in solar energy resources.
The division and basis of solar energy resources region are shown in
Table 9 (Jinjun et al., 2014).
The EPBT and GPBT value of the STPV window applied in five
selected cities are shown in Fig. 14. The five representative cities
were selected from three different solar resource zones in Fig. 15. The EPBT and GPBT of the STPV window with different WWR.

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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Fig. 16. The EPBT and GPBT of the STPV window with different PV coverage.

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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

group should contain 85% PV cells covering.

Due to the different WWR, the number of PV cells and their
auxiliary parts in PV windows are different, the energy demand
input and power generation are also different consequently. On the
premise that the coverage ratio is 85%, with the increase of WWR,
the power generation and energy demand input increase. The
trends of EPBT and GPBT are increasing at first and then decreasing
until they tend to be stable. When the WWR is 0.8, the EPBT and
GPBT of the STPV windows are the shortest, which are 12.1 years
and 8.5 years respectively.

5.5.4. Different PV coverage with different WWR

The EPBT and GPBT of STPV windows with different PV coverage
are shown in Fig. 16.
With WWR ¼ 0.3, as the coverage increase, the trend of the EPBT
and GPBT is to decrease gradually. With maximum coverage of 0.85,
the two evaluation indexes are the shortest. With the WWR at 0.5
and 0.8, the trends of the EPBT and GPBT are similar to the trends
with WWR ¼ 0.3. However, when the coverage rate is between 0.65
and 0.85, the differences between the two indexes are within 2
years. Therefore, the selection of coverage mainly refers to the
impact on indoor lighting energy consumption, cooling energy
consumption and heating energy consumption after using the STPV
window.

5.5.5. Global analysis of the STPV window

In order to study the global application of BIPV-LCA, we selected
13 representative cities worldwide according to different solar
energy resource zones and geographical locations. The location and
the abundance of solar energy resources in these cities are shown in
Fig. 17(Solar resource data: Solargis). These city choices are all over
the world, distributed in both the northern and southern hemi-
spheres and all six continents (except Antarctica). The abundance of
solar energy resources varies greatly in different regions. The
abundant areas of solar energy resources are Johannesburg and
Cairo, comparatively abundant areas are Bangkok, Buenos Aires,
Mexico and Sydney, medium areas are Sao Paulo, New York, Madrid
Fig. 16. (continued).
and Rome, comparatively scarce areas are Beijing and Tokyo and a
scarce area is Paris.
Southwest China. Specific information is shown in Table 10 (Solar The basic assumption is that the STPV window is applied on
and wind energy resource assessment (SWERA), 2019). buildings in these 13 cities, and all are installed on the optimal
It can be seen that all five cities can recover the energy and GHG facade (the south facade in the northern hemisphere and the
input within the service life, of which Lhasa in solar resource zone I northern facade in the southern hemisphere). The climate and solar
has the shortest EPBT, at 8.0 years. Secondly, Kunming belongs to radiation data are the historical annual average data. Discussion is
the solar resource zone III with comparatively abundant solar en- made on the optimized simulation model, with focus on EPBT. The
ergy, and the EPBT is slightly longer than Lhasa. Besides, Chengdu, results are shown in Fig. 18.
Chongqing and Guiyang are all located in the solar resource zone Ⅴ From Fig. 18, it can be seen that the EPBT in each city is quite
where solar energy resources are scarce, so the use of STPV win- different. All EPBT value are less than the preset service life of 25
dows still has energy and environmental advantages. The results years, the STPV window has excellent energy recovery performance
show that it is necessary to popularize PV technology in Southwest on a global scale. In particular, the EPBT is the longest with 12.4
China, and in areas with more solar energy resources, so the ben- years in Paris, and the shortest with 7.7 years in Johannesburg, with
efits of energy become greater. a maximum difference of 37.9%. Generally, in areas with more
The results of GPBT are complicated because of the different abundant solar energy resources, the PV system has better power
energy type proportions of the local power grid. Tibet has the most generation performance and shorter EPBT.
power generation, but the GPBT of PV power generation is longer In the life cycle of the STPV window, solar radiation is the
than that of Yunnan, Chongqing and Guiyang, because the local dominant factor affecting EPBT, and the area with the more sig-
power grid is dominated by non-fossil energy (hydropower, etc.), nificant radiation has the shorter EPBT. In addition, latitude and
still was good returns. climate also play a part. In the case of a certain level of solar radi-
ation, the STPV window on the optimal facade applied in the area
5.5.3. Different window-to-wall ratio (WWR) with the higher latitude has a shorter EPBT, such as Bangkok and
The EPBT and GPBT of the STPV windows with different WWR is Tokyo. Therefore, although the STPV window applied in most areas
shown in Fig. 15. Because the maximum coverage of STPV modules can have a good energy return, choosing a better application site
is 85%, in considering the optimization of modules, each control can significantly shorten the recovery cycle.
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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

Fig. 17. Solar resource map of the world.

Fig. 18. The EPBT of the STPV window in 13 representative cities in the world.

6. Conclusions (2) The LCA model was combined with the experimental data
and the electricity simulation model in order to carry out a
A complete LCA model of the STPV window is established comprehensive analysis. In the life cycle, the total power
through cross-season long-term testing, relevant literature, and the generation of the PV system was 7879.7 kWh. It took 13.8
latest PV industry data. Through the above analysis, the following years to recover all the energy input (considering an STPV
conclusions are obtained: window replacing a standard window, the EPBT would be
further shortened to 10.6 years.) The power generation of the
(1) The case study aims to apply STPV Window on buildings, PV system can be converted to save 11.3 tonnes of standard
demonstrating more practical significance. The total energy coal, or 22.1 tonnes of carbon dioxide emission, taking 10.4
demand was 4797.2 kWh, mainly coming from the produc- years to recover all the GHG input. It reduces the consump-
tion stage, accounting for 64%. Similar results can be drawn tion of fossil energy (non-renewable energy) and reduces the
from other environmental assessment factors. This is emission of a large number of harmful gasses.
because the production processes of steel, copper, (3) The optimization analysis connected with the LCA study was
aluminium, glass, crystalline silicon contributes significantly carried out based on the consideration of critical parameters
to the energy demand of PV systems. Therefore, reducing the of PV window applications in buildings. It is evident that
energy and material consumption in the production stage choosing optimal directions and preferable locations can
can effectively reduce the environmental impact in the life utilize PV systems economically and sustainably through
cycle. analysis. Besides, with the increase of the WWR and the

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Z. Li, W. Zhang, L. Xie et al. Journal of Cleaner Production 295 (2021) 126403

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Declaration of competing interest building-integrated photovoltaic (BIPV) system in Hong Kong. Appl. Energy
87, 3625e3631. https://doi.org/10.1016/j.apenergy.2010.06.011.
Luo, W., Khoo, Y.S., Kumar, A., Low, J.S.C., Li, Y., Tan, Y.S., Wang, Y., Aberle, A.G.,
The authors declare that they have no known competing Ramakrishna, S., 2018. A comparative life-cycle assessment of photovoltaic
financial interests or personal relationships that could have electricity generation in Singapore by multicrystalline silicon technologies. Sol.
appeared to influence the work reported in this paper. Energy Mater. Sol. Cells 174, 157e162. https://doi.org/10.1016/
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Acknowledgments integrated concentrated photovoltaic scheme. Appl. Energy 111, 505e514.
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Technology Department Foundation of Chengdu City, China (2017- Energy Rev. 31, 736e745. https://doi.org/10.1016/j.rser.2013.12.044.
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