Prospects of life cycle assessment of renewable energy from solar photovoltaic technologies: A review

Renewable and Sustainable Energy Reviews 96 (2018) 11–28

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Prospects of life cycle assessment of renewable energy from solar photovoltaic technologies: A review

T

⁎

Norasikin Ahmad Ludina, , Nur Ifthitah Mustafaa, Marlia M. Hanafiahb, Mohd Adib Ibrahima, Mohd Asri Mat Teridia, Suhaila Sepeaia, Azami Zaharimc, Kamaruzzaman Sopiana a

Solar Energy Research Institute, The National University of Malaysia, 43600 UKM, Bangi, Selangor, Malaysia School of Environmental and Natural Resource Sciences, Faculty of Science and Technology, The National University of Malaysia, 43600 UKM, Bangi, Selangor, Malaysia c Faculty of Engineering and Built Environment, The National University of Malaysia, 43600 UKM, Bangi, Selangor, Malaysia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Life cycle assessment Solar PV technologies Cumulative energy demand Energy payback time GHG emission rate

Life cycle assessment (LCA) is a comprehensive method used to investigate the environmental impacts and energy use of a product throughout its entire life cycle. For solar photovoltaic (PV) technologies, LCA studies need to be conducted to address environmental and energy issues and foster the development of PV technologies in a sustainable manner. This paper reviews and analyzes LCA studies on solar PV technologies, such as silicon, thin film, dye-sensitized solar cell, perovskite solar cell, and quantum dot-sensitized solar cell. The PV life cycle assumes a cradle-to-grave mechanism, starting from the extraction of raw materials until the disposal or recycling of the solar PV. Three impact assessment methods in LCA were reviewed and summarized, namely, cumulative energy demand (CED), energy payback time (EPBT), and GHG emission rate, based on data and information published in the literature. LCA results show that mono-crystalline silicon PV technology has the highest energy consumption, longest EPBT, and highest greenhouse gas emissions rate compared with other solar PV technologies.

1. Introduction Climate change, impacts of mining fossil fuels, resource depletion, and energy shortage worldwide are the most pressing environmental concerns that need to be addressed in addition to the challenges in finding renewable energy solutions for the future. Among renewable energy resources, solar energy is the most abundant natural resource on earth. Solar energy is generated by converting sunlight into thermal or electrical energy powered by the application of photovoltaic (PV) devices. Solar energy is easily exploitable, clean, discreet, inexhaustible, long lasting, and reliable. These advantages make solar energy a key factor viable for meeting the world's growing electricity energy demand because of the increase in human population and infrastructure expansion. Presently, the global market of solar PV technologies has exhibited

impressive growth rates over the past decades. According to the International Energy Agency (IEA) [1] (change reference), by the end of 2016, at least 303GWs of solar PV have been installed over the world including grid-connected and off-grid installations as reported in Fig. 1. Continuous expansion of global PV market was observed from 2009 to 2016 which reflects the positive outlook of PV trend. Moreover, over 265 GW of cumulative PV have been installed in 25 countries under the IEA Photovoltaic Power Systems Programme (PVPS). China had the largest cumulative installed PV capacity, leads the global PV market with double growth from 43.5 GW in 2015 to 78.1 GW in 2016, followed by Japan at 42.8 GW, and Germany with 41.2 GW in 2016 as reported in Fig. 2 [1]. Other countries, such as the United States of America (USA) and India have installed PV systems more than 50% growth reported from 2015. The following countries for instance Italy, United Kingdom, France, Australia, and Spain, grew significantly and

Abbreviations: a-Si, amorphous silicon; BIPV, building integrated photovoltaic; BOS, balance of system; CdTe, cadmium telluride thin film; CIS, copper indium selenide thin film; DSSC, dye sensitized solar cell; EG-silicon, electronic silicon; EPBT, energy payback time; FTO, fluorine tin oxide; GHG, greenhouse gases; GWP, global warming potential; ITO, indium tin oxide; LCA, life cycle assessment; LCI, life cycle inventory; MG-silicon, metallurgical grade silicon; mono-Si, monocrystalline silicon; multi-Si, multi-crystalline silicon; NER, net energy ratio; PET, polyethylene terephthalate; PR, performance ratio; PSC, perovskite solar cell; PV, photovoltaic; QDSSC, quantum dot sensitized solar cell; SiO2, silicon dioxide; TCO, transparent conducting oxide; ZnO, zinc oxide ⁎ Corresponding author. E-mail address: [email protected] (N.A. Ludin). https://doi.org/10.1016/j.rser.2018.07.048 Received 25 July 2017; Received in revised form 4 April 2018; Accepted 27 July 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. Total PV installations for IEA PVPS countries and non IEA PVPS countries from 2002 until 2016 [1].

Fig. 2. Top 10 countries for total cumulative PV installed capacity as of 2015 and 2016.

the life cycle assessment (LCA) of solar PV becomes crucial in determining their environmental performance over the past decades. Up to present, there are few review papers on LCA of PV have been published [5–11]. Sherwani et al. (2010) [5] analyzed the LCA of silicon-based panels. The studied of Peng et al. (2013) [6] and Baharwani et al. (2014) [7] only considered the energy payback time (EPBT) and greenhouse gas (GHG) emissions of crystalline silicon and thin film PV modules. Darling et al. (2013) [8] and Lizin et al. (2013) [9] focused to review the environmental profile of organic PV. Furthermore, the LCA of silicon-based PV panels and balance of system (BOS) were reviewed by Gerbinet et al. (2014) [10]. Meanwhile, J.H. Wong et al. (2016) [11] only reviewed the embodied energy requirements of mono-crystalline and poly-crystalline silicon PV systems. The comparison of environmental impact of the new emerging technologies are not presented elsewhere. Therefore, this paper highlights to summarize up-to-date review and investigated a comparative LCA of different PV technologies including crystalline silicon, thin film and new emerging solar PVs such as DSSC, perovskite, and quantum dot-sensitized solar cell based on earlier studies. It is necessary to evaluate new emerging solar PVs in this review as large academic and industrial research relevance in these technologies while silicon and thin film modules were considered as there are extensively manufactured in the market. Thus, the rational of this study is to compile and investigate the environmental impacts and problems of different solar PV technologies throughout their life cycle and explore improvement modifications of

have cumulative installed capacity of 19.3, 11.6, 7.1, 5.9, and 5.4 GW, respectively, as of 2016. Despite with this significant growth, solar PV technologies have considerable potential as renewable energy sources to meet the world's energy demand. As of 2016, silicon solar cells have conquered 93% of global PV market [2] with highest conversion efficiency reported by the National Renewable Energy Laboratory (NREL) for mono-crystalline silicon (mono-Si) and multi-crystalline silicon (multi-Si) are 25.8% and 22.3% respectively, according to the solar cell efficiency chart in Fig. 3 [3]. The rapid growth in capacity are accompanied by the development of Chinese manufacturers and declining in capital expenditures [1,4]. The second generation of thin film solar cells, amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium selenide (CIS) reveal lower efficiencies than silicon solar cells with corresponding efficiency of 14.0%, 22.1% and 22.6%, respectively [3]. Impressive rapid growing of new emerging solar PV technologies such as dye sensitized solar cell (DSSC), perovskite solar cell (PSC), and quantum dot sensitized solar cell (QDSSC) in last decades have motivated research interests in these technologies toward simplest fabrication and cost effectiveness. According to NREL, the recent record efficiency of DSSC, perovskite and quantum dot solar cells are 11.9%, 22.1%, 13.4%, respectively. These potential candidates play significant role in global PV market even though the technologies presently in R&D stage. As the PV market grows, their development needs to be understood, and their present and future environmental and energy performances must be assessed. Thus, 12

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Fig. 3. Solar cell efficiencies chart reported by NREL from 1976 to the present [3].

4) Interpretation: Summarize the LCI and LCIA results, identify and analyze critical points, and make recommendations and conclusions for future improvements.

each technology. This analysis examined three indicatives for solar PV performance based on cumulative energy demand (CED), energy payback time (EPBT), and GHG emission rate. Those are the most common metrics used in comparative life cycle evaluations of PV system since 2010 [12]. Summing up, the outline of these three indicators were compared among PV types in the last part of this paper.

An effective LCA will quantify the technological, environmental, economic, and social aspects of a product or process that plays a critical role in reducing the life cycle impacts of the product or process toward more sustainable options [18].

2. Life Cycle Assessment (LCA) Life Cycle Assessment (LCA) is a comprehensive method used to evaluate and analyze the environmental performance and energy consumption of the entire life cycle of a product, process, or system and usually follow the “cradle-to-grave” or “cradle-to-gate” approach, starting from the extraction of raw materials, transportation, processing of materials, manufacturing, distribution, usage, and final disposal [13]. According to the standardized framework series of ISO 14040 [14] and ISO 14044 [15], the methodology consists of four distinct steps as shown in Fig. 4.

2.1. System boundaries of LCA solar PV technology According to the Methodology Guidelines on the LCA of PV Electricity [19], the life cycle of PV starts from the extraction of raw materials (cradle) and ends with the disposal or recycling (grave) of PV components, as illustrated in Fig. 5. During the production stage, inputs of raw materials, energy supply, manufacturing of panels, mounting system, cables, inverters, and all other components needed to produce electricity should be included in the system boundaries. In addition, transportation, construction, and installation, including the foundation and supporting structures, should be included in the construction stage. In the usage stage, certain parts, such as auxiliary electricity demand, cleaning of panels, maintenance, as well as repair and replacement (if any) should be included in the system boundaries. Finally, recycling and reuse, waste processing, and transportation should be included in the end-of-life stage. The following parameters need to be presented, as recommended in the guidelines [19]: 1) the goal of the study and functional unit; 2) assumptions for the production of major input materials; 3) location of installation; 4) annual irradiation; 5) PV technology; 6) type of system (i.e., rooftop, ground-mounted fixed tilt, or tracker); 7) module-rated efficiency and degradation rate; 8) lifetime of PV and balance-of-system (BOS); 9) expected annual electricity production; 10) timeframe of data; 11) the life cycle stages included; 12) LCA approach used (if not based on the process); 13) LCA software tool (e.g., SimaPro, GaBi); 14) LCI databases used (e.g., Ecoinvent, GaBi); 15) place of production modeled; and 16) electricity source (if known). Besides to increase transparency of the analysis, it is noteworthy to report these parameters in the LCA of PV system because it can affect the findings significantly when vary with system's boundary conditions, solar irradiation at different locations and modeling approach [19].

1) Goal and scope definition: Determine the research objective, set the system's boundaries, define the functional unit, and formulate some assumptions. 2) Life Cycle Inventory (LCI): Data collection consists of input (such as raw materials, as well as water and energy consumption), intermediate processes, and output (such as waste treatment, by-product and potential emissions to the air, water, and land of the process at each stage in the life cycle). 3) Life Cycle Impact Assessment (LCIA): Interpret the potential environmental impacts of the system, such as global warming, acidification, eco-toxicity, and ozone depletion [16,17] Goal and scope definition

Life Cycle Inventory (LCI)

Data Interpretation

Life Cycle Impact Assessment (LCIA)

Fig. 4. Framework of life cycle assessment (LCA). 13

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Fig. 5. The system boundary of solar PV life cycle.

3. Current solar PV technologies 3.1. Silicon solar cells Silicon is a metalloid discovered in 1824 [20]. As the most abundant semiconductor in the world, this metalloid is essential in modern technology because it produces most types of glasses, high-technology devices, parts of computer semiconductors, and car parts. Silicon is also useful in manufacturing solar PV technologies, such as mono-crystalline and poly-crystalline silicon PVs. Silicon has been proven to have field stability; hence, crystalline silicon PV technologies have dominated the PV terrestrial market for several decades [18]. Crystalline silicon PV modules are produced through several steps. Silicon dioxide (SiO2) or silica from quartz sand is reduced into metallurgical-grade silicon (MG-Si) in an arc furnace. Furthermore, silicon needs to be purified into solar-grade silicon (> 99.99% purity) typically through various methods, including Czochralski, Siemens, and modified Siemens processes [6]. In this process, a considerable amount of energy is consumed because it operates at a high working temperature (typically between 1100 °C and 1200 °C) in the reaction chamber to produce high-purity silicon [11]. Silicon ingots of mono-crystalline crystal or solar-grade poly-crystalline silicon are then sliced by band or wire saw into mono-crystalline and poly-crystalline wafers into 156 × 156 mm2 size [6]. After wafer sawing, solar cell is produced by etching, doping, screen printing, coating, and checking. In the module stage, cells are connected into a string and then encapsulated by two layers of glass and plastics (ethylene-vinyl acetate) prior to installation into the system. Fig. 6 illustrates the overall process of silicon PV module manufacture and its life cycle [11]. Numerous LCA studies on mono-crystalline and multi-crystalline silicon modules have been presented in the literature. Prior 2000, there are few life cycle studies reported by researchers in both technologies such as Kreith et al. (1990) [21], Phylipsen and Alsema (1995) [22], Wilson and Young (1996) [23], Kato et al. (1998) [24], Dones and Frischknecht (1998) [25] and Frankl et al. (1998) [26]. Each of these studies focuses on LCA under different locations and solar irradiations with different types of PV installation. Kreith et al. [21] compared CO2 emissions for 300-kW ground-mounted mono-crystalline plants in the

Fig. 6. Life cycle stages of silicon-based PV modules (Reproduced with the permission from Elsevier Publishing Group) [11].

United States with CO2 production from coal power plants in previous studies [27,28]. Kato et al. [24] performed LCA on a 3-k Wp residential PV system installed on a rooftop under Japan's solar irradiation of 1427 kWh/m2/year using off-grid mono-crystalline silicon (mono-Si), multi-crystalline silicon (multi-Si), and amorphous silicon (a-Si). The results show that mono-Si modules have an EPBT of 11.8 and a GHG emissions rate of 60.1 g CO2-eq/kWh for 10 MW/year. Multi-Si modules and a-Si modules were expected to have lower EPBT with 2.4 years and 2.1 years respectively, compared to mono-Si modules with similar annual cell production of 10 MW/year. From 2000 until presents, various studies were conducted on silicon PV systems for a rooftop application under Southern Europe irradiation of 1700 kWh/m2/year and system lifetime of 30 years. Alsema [29] 14

15

Europe

2006

2007 2008 2008 2009 2010 2010 2011 2012 2014 2015 2016

Jungbluth et al. Fthenakis et al. Alsema et al. de Wild-Scholten Ito et al. L.Lu and Yang Ito et al. Fthenakis et al. Kim et al. Chen et al. Hou G. et al.

Switzerland Europe South Europe Europe China Hong Kong Japan United States South Korea China Northwest China East China

United States United Kingdom Japan Switzerland Italy Europe United States Switzerland Singapore Southern Europe Europe

1990 1996 1998 1998 1998 2000 2001 2005 2006 2006 2006

Kreith et al. Wilson and Young Kato et al. Dones and Frischknect Alsema et al. Alsema Knapp and Jester Jungbluth et al. Kannan et al. Alsema and Fthenakis V. Alsema and de WildScholten Alsema et al.

Location

Year

Authors

Table 1 LCA results review of mono-Si PV systems.

Rooftop Ground mounted Rooftop Rooftop Ground mounted Rooftop Ground mounted Ground mounted Ground mounted N/A Ground mounted Ground mounted

Rooftop

Ground mounted Rooftop Rooftop Rooftop Ground mounted Rooftop N/A Rooftop Rooftop Rooftop Rooftop

Mounting System

1700 and 1000 (MiddleEurope) 1117 1700 1700 1700 1702 1600 1725 1800 1301.35 1139–2453 1600 1200

N/A 573–1253 1427 1117 1700 1700 1800 1117 1635 1700 1700

Irradiation (kWh/m2/yr)

0.75 0.80 0.75 0.80 0.80 N/A N/A 0.8 0.8 N/A 0.75 0.70

0.75

N/A 0.80 0.81 N/A 0.82 0.80 0.80 N/A N/A 0.75 0.75

PR

14 14 N/A 14 N/A 13.3 14.3 20.1 15.96 15.7 17.0 17.0

14

8.5 12–14 12.2 16.5 12.7 14 N/A 16.5 11.86 N/A 14

Module Efficiency (%)

30 30 N/A 30 30 20–30 30 30 30 25 25 25

30

30 20 20 30 25 30 30 30 25 30 30

System Lifetime (year)

N/A N/A N/A 2860 41,947 GJ/MW 2397 3986 4662 0.56 MJ/kWh 653.22 1186.47 1123.11

3.3 N/A 2.0 1.80 2.5 7.3 3.8 1.4 4.65 0.42–0.91 1.7 2.3

2.1–3.6

N/A 7.4–12.1 11.8 N/A 4–8 2.5–3.0 4.1 3.0–6.0 6.74 2.7 2.1

6300 kWh/m2 4387–4970 kWh/m2 3534 11,060 kWh/m2 6000–13,900 5700 8050 N/A 2.94 MJ/kWh N/A 5200 N/A

EPBT (Year)

CED (MJ/m2)

N/A 36 N/A 29 50 671 193,500 64.2 41.8 5.6 – 12.07 65.2 87.3

35

280 N/A 60.1 114 200 50–60 N/A 79 217 45 35

GHG Emission (gCO2/ kWh)

[39] [42] [43] [32] [36] [44] [37] [45] [46] [47] [48]

[41]

[21] [23] [24] [25] [26] [29] [40] [38] [33] [30] [31]

References

N.A. Ludin et al.

Renewable and Sustainable Energy Reviews 96 (2018) 11–28

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demand for both silicon PV technology gradually declined from more than 8050 MJ/m2 in 2000s to less than 1000 MJ/m2 in 2016 with improvement in cell productions and enhancement in conversion efficiency.

analyzed the EBPT and GHG emissions of mono-silicon PV systems and he estimated range EPBT of 2.5–3.0 years and GHG emissions of 50–60 g CO2-eq/kWh. Furthermore in 2006, Alsema and Fthenakis [30] updated the EPBT and GHG emissions of both silicon PV modules from previous research. Under the same solar irradiation of 1700 kWh/m2/ year, the EPBT for rooftop mono-Si and multi-Si were 2.7 and 1.7 years, respectively, and GHG emissions for mono-Si and multi-Si are 45 and 37 g CO2-eq/kWh, respectively. At the same year, Alsema and de-Wild Scholten (2006) [31] performed an environmental LCI data of mono and multi silicon PV module production from 2004 to 2005 collected from manufactures mainly in Western Europe. The results presented that multi-Si PV have shorter EPBT of 1.9 years and lower GHG emission rate of 32 g CO2-eq/kWh compare to mono-Si with EPBT of 2.1 years and GHG emission rate of 35 g CO2-eq/kWh. Moreover in 2009, de-Wild Scholten [32] conducted follow-up LCA studies using mono and multi-Si. The above studies showed that both silicon PV rooftop decrease in EPBT and GHG emissions over the years under the same solar irradiation. Under Singapore's solar irradiation of 1635 kWh/m2/year, Kannan et al.[33] explored the EPBT and GHG emissions of a 2.7 kWp mono-Si PV system. The LCA showed that the EPBT and GHG emissions were 6.74 years and 217 g CO2-eq/kWh, respectively. Furthermore Wei Luo et al. (2018) [34] examined a comparative LCA of three roof-integrated of multi-crystalline PV system under average Singapore's solar irradiation of 1580 kWh/m2/year which are AL-BSF (aluminum back surface field); PERC (passivated emitter and rear cell); and PERC solar cells with the frameless double-glass module structure. The results presented that their corresponding EPBT were 1.11, 1.08 and 1.01 years, respectively and the corresponding GHG emissions rate of the above PV systems were 30.2, 29.2 and 20.9 g CO2-eq/kWh. It was observed that using frameless double-glass PV module design extensively reduce the EPBT and perform better in GHG emissions. Ito et al. [35–37] performed the LCA of a ground-mounted very large-scale PV system (VLS-PV) in Gobi desert, China, and in the Hokuto mega solar plant, Japan. In 2003, the authors analyzed the system in the Gobi desert, assuming an irradiation of 1702 kWh/m2/year, to determine whether the VLS-PV system was feasible and profitable. In 2010, the authors examined the actual VLS-PV system at a performance ratio (PR) of 0.78 using six types of PV modules, namely, mono-Si, multi-Si, a-Si/mono-Si, thin film Si, copper indium selenium (CIS), and CdTe. The LCA showed that mono-Si has the largest EBPT of 2.5 years compared to others, while multi-Si PV plants generated lowest CO2 emissions rate of 43 g CO2-eq./kWh because of the relatively higher conversion efficiency [36]. Moreover, the research was continued in 2011 to study the LCA of 20 different PV modules installed at the Hokuto mega solar plant under irradiation of 1725 kWh/m2/year [37]. It was found that the energy requirement of the system ranging from 19 to 48 GJ/kW, EPBT between 1.4 and 3.8 years and CO2 emissions were between 31 and 67 g CO2-eq./kWh. It was concluded that the VLS-PV system will sustain to provide clean energy for a long time. Another researcher, Jungbluth et al. [38] conducted LCA for crystalline silicon in Switzerland with solar irradiation of 1117 kWh/m2/yr. The EPBT and GHG emissions were estimated to be 3.0–6.0 years and 79 g CO2-eq./kWh, respectively. Additionally in 2007, sixteen gridconnected PV systems at different locations were performed by Jungbluth et al., [39]. It was observed that solar irradiation at different locations, types of PV modules and different installation methods affect the EPBT results. Tables 1, 2 revealed the LCA of mono-Si and multi-Si PV system from previous studies up to year 2000 and above. For mono-Si, the EPBT and GHG emissions rate were ranged between 1.4 and 7.3 years and 29.0–671.0 g CO2-eq./kWh, respectively. Meanwhile, the EPBT and GHG emissions results for multi-Si were varied from 0.8 to 4.17 years and 12.1–671.0 g CO2-eq./kWh, respectively. These results proved that multi-Si had shorter EPBT and lower environmental impacts compared to mono-Si. Moreover, it was perceived that the cumulative energy

3.2. Thin film solar cells Fthenakis [65] described the stages of the CdTe PV life cycle during thin film manufacturing, such as cadmium telluride (CdTe) process. First, Cd was extracted from zinc ores (~ 80%), and Te was obtained from Cu ores; most of the Te were recovered from slime produced by the electrolytic refining of copper, which contained Cu, Se, and other metals [42,66]. Cd and Te were then processed and purified through electrolytic purification, followed by melting or vacuum distillation to produce > 99.99% purity to synthesize CdTe. A transparent conducting oxide (TCO) layer was deposited onto a glass substrate and a thin CdS layer followed by a CdTe layer was deposited by vapor deposition. CdCl2 was sprayed, and a thermal treatment was applied. Finally, the CdTe solar cell was completed by the deposition of a metal layer using sputtering techniques to create the back contact [67]. For CdTe cell formation, the individual cells were interconnected in series using laser scribe technology, followed by lamination, in which glass plates were placed and thermally sealed with the glass substrate. The final module was encapsulated and sealed between two glass plates to form the final module at a film thickness less than 10 µm [67]. The PV module, inverter, electric components, and support structures were installed before operation and electricity generation. Fig. 7 presents the flow chart of the production of thin film PV modules [6]. Kato et al. [68] performed LCA on CdS/CdTe PV modules of residential rooftops to estimate primary energy requirements, EPBT, and CO2 emissions, in comparison with multi-Si and a-Si at different annual production rates. The study revealed that the EPBT of CdS/CdTe PV modules was 1.7 year at 10 MW/year, 1.4 year at 30 MW/year, and 1.1 year at 100 MW/year, which are lower than the EPBTs of a-Si and multi-Si. The life cycle GHG emissions of CdS/CdTe PV modules ranged from 8.9 to 14.0 g CO2-eq/kWh for annual production from 10 to 100 MW/year. Ito et al. [56] conducted a comparative LCA study for a 100-MW VLS-PV systems in the Gobi desert, using four types of PV modules, namely, multi-Si, a-Si, CdTe, and CIS at different module efficiencies. The authors observed that CIS modules have shorter EPBTs of 1.6 years compared with those of CdTe and a-Si modules, which have 1.9 years and 2.5 years, respectively. The GHG emissions for these modules ranged from 9 to 16 g CO2-eq/kWh. Pacca et al. [52] assessed three metrics of LCA, namely, Net Energy Ratio (NER), EPBT, and GHG emissions, on a 33-kW rooftop installation using KC120 (multi-Si) module and PV136 (a-Si) module in Ann Arbor, United States, under solar irradiations of 1359 kWh/m2/year. The study determined that the EPBT of a-Si (3.2 years) is shorter than that of multi-Si (7.5 years); however, if the conversion efficiency increases in the future, the EPBT for a-Si and multi-Si could drop to 1.6 and 5.7 years, respectively. The GHG emission rate exhibits the same pattern as EPBT, in which the GHG emission rate for the a-Si of 34.3 g CO2-eq/kWh is significantly lower than 72.4 g CO2eq/kWh for multi-Si. Another researcher, Raugei et al. [53] performed LCA to explore the EPBTs of CdTe and CIS modules compared with those of poly-Si module under solar irradiation of 1700 kWh/m2/year. The authors determined that the EPBTs of laminated and BOS included in the CIS module were 1.9 and 2.8 years, respectively, while the EPBTs of laminated and BOS included in CdTe were 0.5 and 1.5 years, respectively. In 2009, Raugei et al. [69] also analyzed the life cycle impact and costs of PV systems, including a-Si, CdTe, CIS, and other silicon PV systems, such as monoSi, multi-Si, and ribbon Si, under three scenarios and projected the value of the GHG emissions of these modules until 2050. They estimated that CdTe PV modules provide the least amount of GHG emissions at approximately 6 g CO2-eq/kWh compared with other PV modules. 16

17

United States South Europe Switzerland South Europe Italy (Rome) Europe Gobi Desert, China Europe China China Japan Italy Nisyros island

2007 2007 2007 2008 2008 2008 2008 2009 2010 2010 2011 2012 2013

2014 2014 2015 2016

2017 2018

Yue et al. Kim et al. Y. Fu et al. Hou G. et al.

Peishu Wu et al. Tan Y. S. et al.

South Europe Korea China Northwest China East China China Singapore

Europe Japan Switzerland Italy West Europe Europe China Italy Japan Europe Europe Europe

1995 1998 1998 1998 2000 2000 2003 2005 2005 2006 2006 2006

Phylipsen and Alsema Kato et al. Dones and Frischnecht Alsema et al. Alsema and Nieuwlaar Alsema Ito et al. Battisti and Corrado Hondo Fthenakis and Alsema Alsema and de Wild-Scholten Alsema, Fthenakis and de WildScholten Pacca et al. Raugei et al. Jungbluth et al. Alsema et al. Stoppato Glockner et al. Ito et al. De Wild-Scholten Nishimura et al. Ito et al. Ito et al. Desideri et al. N. Stylos et al.

Location

Year

Authors

Table 2 LCA results review of multi-Si PV systems.

Rooftop Ground mounted Ground mounted Ground mounted Ground mounted Ground mounted Rooftop

N/A Rooftop Rooftop Rooftop Rooftop Rooftop Ground mounted Rooftop Ground mounted Ground mounted Ground mounted Ground mounted Ground mounted

Rooftop Rooftop Ground mounted Ground mounted Rooftop Ground mounted Ground mounted Rooftop Rooftop Rooftop Rooftop Rooftop

Mounting System

1700 1310 1263.6 1600 1200 2017 1580

1700 1427 1117 1700 1700 1700 1702 1530 1314 1700 1700 1700 and 1000 (MiddleEurope) 1359 1700 1117 1700 1552 1700 2017 1700 1701 1702 1725 1552 1797

Irradiation (kWh/m2/ yr)

0.75 0.8 0.8 0.75 0.70 0.835 0.785

N/A 0.75 0.75 0.75 N/A 0.75 0.78 0.75 0.78 0.78 N/A 0.8 0.67

0.75 0.81 N/A 0.87 0.75 0.75 0.78 0.80 0.77 0.75 0.75 0.75

PR

13.2 14.9 16.0 17.5 17.5 17.5 15.9 – 16.7

12.9 14 13.2 N/A 16 14.3 15.8 13.2 N/A N/A 13.9 14.4 14.0

13 12.8 14 12.1 15 13.2 12.8 10.7 10 N/A 13.2 13.2

Module Efficiency (%)

30 30 25 25 25 30 30

20 20 30 N/A 28 30 30 30 30 30 30 25 30

25 20 30 25 30 30 30 30 30 30 30 30

System Lifetime (year)

1.6 3.68 2.52 – 6.05 1.6 2.1 2.3 1.01 – 1.11

7.5 2.4 2.9 1.70 3.7 0.8 1.9 1.80 1.73 2 N/A 4.17 N/A

2.7 2.4 N/A 3.4 1.7 3.2 1.7 3.3 N/A 1.7 1.9 1.7

1145 kWh/m2 1643 18,770 kWh/m2 4200–11,600 4600 4200 3300 5150 N/A N/A 4000 3200 4322 N/A N/A N/A N/A 306 MJ/Kg 31 333 GJ/MW 2699 2420 33,068 GJ/MW 2737 N/A 9045 GJ (include BOS) 3010 0.44 MJ/kWh 2522 1094.31 1034.41 2927.04 1037.6

EPBT (Year)

CED (MJ/m2)

31.8 31.5 50.9 60.13 81.0 36.75 20.9 – 30.2

72.4 72 N/A N/A 80 22 12.1 29 43 43 135,200 88.7 6.04–6.09

N/A 24.8 189 200 30 60 44 N/A 53.4 37 32 32

GHG Emission (gCO2/kWh)

[63,64] [34]

[60] [61] [62] [48]

[52] [53] [39] [43] [54] [55] [56] [32] [57] [36] [37] [58] [59]

[22] [24] [25] [26] [49] [29] [35] [50] [51] [30] [31] [41]

References

N.A. Ludin et al.

Renewable and Sustainable Energy Reviews 96 (2018) 11–28

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N.A. Ludin et al.

over conventional PV technologies because of their potentially lower economic and environmental impact. In DSSC production, glass substrates are first treated with transparent conducting oxide (TCO) such as indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO) is deposited with a semiconductor material layer, typically nano-porous titanium dioxide (TiO2) which act as dye container [78]. After deposition, the coated photoelectrode is fired in the furnace to sinter the TiO2. Dye sensitizer molecules, such as N719 or natural dye, are then stained on the TiO2 surface to harvest and enhance light absorption [79]. The dye is surrounded by electrolyte solution, containing I−/I3− redox ions [80] which have low-viscosity solvents that affects ion conductivity and consequently the cell performance [78]. Counter-electrode on the other ITO glass sheets is coated with platinum [81] to facilitate electron collection [79] and completed the DSSC cell [82]. DSSC cells are then manufactured into modules before they are installed and transmitted to the grid. Fig. 8 illustrates the system boundaries for DSSC LCA studies. Few LCA studies have been conducted on DSSCs. Greijer et al.[83] conducted LCA on a nano-crystalline DSSC system at the Sahara desert with 2190 kWh/m2/year solar irradiation assuming a 20-year lifetime. The CO2 emissions ranged from 19 to 25 g CO2-eq/kWh for module efficiencies ranging between 7% and 12%. In addition, de Wild-Scholten et al. [84] performed LCA on a DSSC under three solar irradiations, including low irradiation of 1000 kWh/m2/year, medium irradiation of 1700 kWh/m2/year, and high irradiation of 2190 kWh/m2/year, assuming module efficiency of 8% and PR of 0.75. The results revealed that EPBT of high irradiation is the lowest (0.6 years) compared with EPBTs of low and medium irradiations, which are 1.4 and 0.8 years, respectively. Parisi et al. [85] conducted laboratory-scale environmental analysis on DSSC PV manufacturing in Southern Europe. This study assumed efficiency of DSSC module is 8%, the module's lifetime is 20 years, its PR is 0.75, and the module used European Union for Coordination of Transmission Electricity (UCTE) as energy mix. The authors found that the NER of the DSSC module was 12.67, and that the EPBT and CO2 emissions were 1.58 years and 22.38 g CO2-eq/kWh, respectively. The authors then compared the LCA of the DSSC study with other inorganic and organic thin film PV technologies [86]. The findings revealed that the DSSC PV system has lower GHG emission rate and NER compared with other organic and inorganic thin films. In 2013, the authors conducted another study to assess the environmental impact of the DSSC PV modules using Recipe 2008 method [87]. Parisi et al. [88] conducted further LCA on DSSCs from previous studies and compared the DSSC with other thin film technologies (i.e., polymer, a-Si, CdTe, and CIS).

Fig. 7. The flow chart of the production of thin film PV modules (Reproduced with permission from Elsevier Publishing Group)[6].

Fthenakis et al. [70] conducted LCA studies on thin film technologies, especially CdTe PV modules. The authors discussed the emissions of Cd for the entire life cycle of CdTe modules. Zn smelting/refining stages were observed to produce the highest atmospheric Cd emissions with 40 g Cd/ton, which allocate 0.58% of total emissions, assuming a solar irradiation of 1700 kWh/m2/year and 9% electrical conversion efficiency, compared with other stages (e.g., Cd purification, CdTe production, and PV manufacture). The authors also analyzed GHG emissions and heavy metal emissions from four different types of PV life cycles, including mono-Si, multi-Si, ribbon-Si, and thin film CdTe [42]. Moreover, Fthenakis et al. [71] conducted a systematic review and harmonization of the life cycle GHG emissions of thin film PV (i.e., a-Si, CdTe, and CIS). The results revealed the GHG emissions of a-Si, CdTe, and CIS, were 20, 14, and 26 g CO2-eq/kWh, respectively, for harmonized ground-mounted systems under 2400 kWh/m2/year, 30-year lifetime, and PR of 0.8. Meanwhile, for harmonized rooftop installations under the same irradiation with PR of 0.75, CO2 emissions were 21, 14, and 27 g CO2-eq/kWh, for a-Si, CdTe, and CIS, respectively. Tables 3–5 show reviewed results of a-Si, CdTe and CIS of thin film PV systems from previous LCA studies. The EPBT and GHG emissions of thin film technologies were varied from 0.79 to 3.2 years and 8.1–95.0 g CO2-eq/kWh, respectively. It was noticed that difference in EPBT and GHG emissions were resulted from different solar irradiation and module PV type. 3.3. Dye sensitized solar cell (DSSC) Dye sensitized solar cell (DSSC) has some differentiation advantages Table 3 LCA results review of thin film amorphous silicon (a-Si) PV systems. Authors

Year

Location

Mounting System

Irradiation (kWh/m2/yr)

PR

Module Efficiency (%)

System Lifetime (year)

CED (MJ/m2)

EPBT (Year)

GHG Emission (gCO2/kWh)

References

Alsema

2000

South Europe

Rooftop

1700

0.75

7

30

2.7

50

[29]

Kato et al.

2001

Japan

2005 2007 2008 2008

Japan United States Switzerland Rome

1430 1430 1430 1314 1359 1117 1700

0.81 0.81 0.81 N/A 0.95 0.75 0.912

10.3 11.2 12.4 8.6 6.3 6.5 5.5

20 20 20 30 20 30 20

2.1 1.7 1.1 N/A 3.2 3.0 N/A

15.8 12.1 8.1 26 34.3 N/A 31

[68]

Hondo Pacca et al. Jungbluth et al. SENSE Ito et al.

2008

2017

0.771

6.9

30

57

[56]

2009

1700

0.75

6.6

30

40,990 GJ/ MW 989

2.5

De Wild Scholten DominguezRamos et al.

Gobi Desert, China South Europe

Rooftop Rooftop Rooftop Rooftop Rooftop Rooftop Ground mounted Ground mounted Rooftop

940–1480 (frameless) 1731 1681 1236 N/A 862 17.7 GJ/kWp N/A

1.4

24

[32]

2010

Spain

Ground mounted

1825

0.78

7

30

N/A

N/A

27

[74]

18

[51] [52] [72] [73]

2008 2008

2008

2009 2009 2009 2010

2010 2014

Ito et al. [56]

Raugei et al. Fthenakis et al. De Wild Scholten [32] Ito et al.

Dominguez-Ramos et al. Kim H., et al.

2007

Fthenakis and Kim

Jungbluth et al. SENSE

2006 2006 2006

Alsema et al. Fthenakis and Kim Fthenakis and Alsema

2007 2008

2001

Kato et al.

Raugi et al. Fthenakis et al.

Year

Authors

19

Spain Malaysia

South Europe United States South Europe China

Gobi Desert, China

Switzerland Europe

South Europe Europe

Europe

South Europe United States Europe

Japan

Location

Ground mounted Rooftop

Ground mounted Ground mounted Rooftop Ground mounted

Rooftop Rooftop Rooftop Ground mounted Ground mounted Rooftop Ground mounted Rooftop Rooftop Rooftop Ground mounted Rooftop Ground mounted Ground mounted Rooftop Ground mounted Ground mounted Ground mounted Ground mounted

Mounting System

1825 1810.4

1700 1700 1700 1702

1430 1430 1430 1700 1800 1700 1700 1700 1800 2280 2060 1700 1700 1700 1117 1700 1700 1700 2017

Irradiation (kWh/m2/ yr)

Table 4 LCA results review of cadmium telluride thin film (CdTe) PV systems.

0.78 0.80

0.80 0.80 0.75 0.78

0.81 0.81 0.81 0.75 0.80 0.75 0.80 0.75 0.75 0.75 0.80 0.75 0.80 0.80 0.75 0.912 0.912 0.912 0.772

PR

9 11.2

10.9 10.9 10.9 N/A

10.3 11.2 12.4 9 9 8 9 9 9 9 9 9 9 9 7.6 10 10 10 9

Module Efficiency (%)

30 30

30 30 30 30

20 20 20 30 30 30 30 30 30 30 30 20 30 30 30 20 20 20 30

System Lifetime (year) 1803 1514 1272 N/A 1200 N/A N/A N/A N/A N/A N/A 236 kWh/m2 N/A N/A 14.5 GJ/kWp N/A N/A N/A 30,987 GJ/ MW 853 966 811 34,879 GJ/ MW N/A 0.221 MJ/kWh

CED (MJ/m2)

N/A 0.94

0.8 0.79 0.84 2.2

1.7 1.4 1.1 1.1 1.1 1.0 1.1 N/A N/A N/A N/A 1.5 N/A N/A 2.7 N/A N/A N/A 1.9

EPBT (Year)

17 15.1

17 18 16 50

14 11.5 8.9 25 24 21 25 16 22 17 21 48 21 26 N/A 66 46 36 47

GHG Emission (gCO2/ kWh)

[74] [76]

[69] [75] [32] [36]

[56]

[72] [73]

[53] [42]

[70]

[41] [66] [30]

[68]

References

N.A. Ludin et al.

Renewable and Sustainable Energy Reviews 96 (2018) 11–28

Renewable and Sustainable Energy Reviews 96 (2018) 11–28

[74] 33

[32] [77] [36] 21 58.8 44

[53] [72] [73] [56]

N/A 30

30 30 30

1.45 1.7 1.8

2.8 2.8 1.3 1.6

24.3 kWh/m2 18.6 GJ/kWp N/A 26,826 GJ/ MW 1684 1105 29,637 GJ/ MW N/A 20 30 20 30

Module Efficiency (%)

11 10.7 11.5 11

10.5 10.1 N/A

10

0.75 0.75 0.912 0.776

0.75 0.776 0.78

0.78

Based on evaluations from previous researchers, the range CED, EPBT and GHG emissions rate of DSSC were 277–365 MJ/m2, 0.6–1.8 years and 9.8–120.0 g CO2-eq/kWh, respectively. It was notably also the GHG emissions of DSSC strongly related with the operational lifetime of DSSC modules [84]. The best performance of DSSC could be obtained with longer lifetimes and higher conversion efficiencies.

Ground mounted

1825

Perovskite Solar Cell (PSC) is one of the newly emerging PV technologies that have attracted considerable attention over the past few years because of its high efficiency and low cost of cell production. Fig. 9 illustrates the system boundary considering a cradle-to-grave approach, including component production, module manufacturing,

2010 Dominguez-Ramos et al.

Spain

1700 2017 1702 Rooftop Ground mounted Ground mounted 2009 2009 2010 De Wild Scholten [32] Ito M. Ito et al.

South Europe Gobi Desert, China China

2007 2008 2008 2008 Raugi et al. Jungbluth et al. SENSE Ito et al.

South Europe Switzerland Europe Gobi Desert, China

Rooftop Rooftop Ground mounted Ground mounted

1700 1117 1700 2017

3.4. Perovskite solar cell (PSC)

Year

Location

Mounting System

Irradiation (kWh/m2/ yr)

PR

Fig. 8. LCA for DSSC process from cradle-to-gate.

Authors

Table 5 LCA results review of copper indium selenide (CIS) PV systems.

95 N/A 43 38.5

EPBT (Year) CED (MJ/m2) System Lifetime (year)

GHG Emission (gCO2/ kWh)

References

N.A. Ludin et al.

Fig. 9. System boundary of manufacturing a perovskite solar module using TiO2 (Reproduced with permission from Royal Society of Chemistry)[90]. 20

Renewable and Sustainable Energy Reviews 96 (2018) 11–28

N.A. Ludin et al.

module usage, and disposal, in previous LCA studies of PSCs [90]. The first stage starts from the extraction of raw materials to produce components, such as FTO glass, BL-TiO2 ink, Pbl2, gold, and PET production, to be used in the second stage. Manufacturing perovskite PV module involves several steps, starting with patterning of FTO glass substrates, blocking TiO2 layer deposition, then electron transport layer deposition, followed by perovskite layer and hole transport layer deposition on the substrate, before deposition with gold by thermal evaporation [90]. The module was encapsulated by PET substrates. After the perovskite modules were assembled, they were utilized to generate electricity in the usage phase to generate electricity. Finally, the waste modules were landfilled in the disposal stage. Espinosa et al. [91] conducted LCA on polymer PSC by ProcessOne using roll-to-roll method, assuming an active area of 67%, irradiation of 1700 kWh/m2/year, module efficiency of 2%, PR of 0.80, and a 15years lifetime. The EPBT and GHG emissions of this 2% efficiency module were 2.02 years and 56.65 g CO2-eq/kWh, respectively, but can decrease drastically to 1.35 years and 37.77 g CO2-eq/kWh under 3% module efficiency. In addition, Gong et al. [90] conducted LCA of two types of perovskite solar modules (i.e., TiO2 module and ZnO module) by cradleto-grave approach to observe 16 LCIs and two sustainable indicators, namely, EPBT and GHG emission, under San Francisco's irradiation of 1960 kWh/m2/year, assuming a lifetime of two years and PR of 0.80. The authors found that the TiO2 module consumed more primary energy with 446 MJ/m2 and produced higher GHG emissions of 2.17 g CO2-eq/m2 compared with ZnO module, which consumed 392 MJ/m2 primary energy and produced 1.91 g CO2-eq/m2CO2 emissions, thereby resulting in longer EPBT of the TiO2 module at 0.266 years [90]. Celik et al. [92] evaluated the impact assessment of three perovskite solar cell structures, namely, vacuum, solution, and hole transfer layerfree devices, from cradle-to-gate with 15% module efficiency, PR of 0.75, 15-year lifetime, and 65% active area under 1700 kWh/m2/year solar irradiation. As a conclusion, the obtained EPBT and GHG emissions of the perovskite were between 1.05 to 1.54 years and 100 to 150 g CO2-eq/kWh, respectively, which can be considered lower as compared with crystalline and thin film technologies. Summing up, the findings of EPBT and GHG emissions for perovskite solar cells were indicated in Table 7. The EPBT varied from 0.2 to 5.4 years and the GHG emissions rate within 56.65–497.2 g CO2-eq/ kWh. Compared with silicon and thin film technologies, perovskite solar cells perform a much lower energy consumption with competitive environmental benefits and EPBT which pave the way toward future industrial scale.

Fig. 10. System boundary of QDDSC (Reproduced with permission from Elsevier Publishing Group) [99].

considered Southern European solar irradiation, PR of 0.75, 10% conversion efficiency, and 25-year lifetime based on 1 cm2 laboratory-scale productions. The performance of GHG emissions, EPBT, and NER of variant-type hybrid QD were 2.89 g CO2-eq/kWh, 1.51 years, and 16.66, respectively. Hatice et al. investigated four environmental impacts of QDSSC from cradle-to-gate, namely, CED, GHG emission rate, heavy metal emissions, and acidification potential [99]. Across the analysis, QDPV has the shortest EPBT and lowest GHG emission compared to CIS and DSSC PV technologies at 14% module efficiency. The CED of QD modules is 286 kWh/m2 including PV frame and BOS. The resulted LCA of QDSSC from previous researches were shown in Table 8. 4. Discussions 4.1. Cumulative energy demand (CED) Cumulative energy demand (CED) or embodied energy indicator refers to the amount of primary energy used within the entire life cycle of the product from raw materials extraction until end-of life stage including installation, transportation and energy-mix used. According to [44], CED is divided into two categories, a) CED of the system (PV modules); and b) CED of Balance of System (BOS). The CED of the system (PV module) is defined by Equation 1 [101] and CED of BOS system including BOS of electrical and mechanical equipment as shown in Eq. (2) [44]:

3.5. Quantum dot sensitized solar cell (QDSSC)

Esystem (kWh) = Emat + Emanuf + Etrans + Eints + Eeol

Quantum dot sensitized solar cell (QDSSC) was introduced in the past few years based on the DSCC structure to replace the dye because of its high absorption coefficient, photostability, and low cost [95]. Various QD materials, such as CdS, CdSe, CdTe, PbS, ZnSe and InAs, have been used as sensitizer to absorb visible light [96]. The typical structure of QDSSC is similar to DSSC, which consists of photo-anode and counter electrode, but the dye is replaced with QD materials [97]. Many techniques have been used to prepare QD sensitizers coated on mesoporous TiO2 either by in situ or ex situ fabrication [98]. Two well known in situ fabrication methods to prepare QD sensitizers are the chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR). These methods are commonly used because of its simple techniques and applicability in large-scale production [98]. Rollto-roll coating process is used to manufacture QDSSC particularly for large-area fabrication before module encapsulation and framing [99]. Supporting infrastructure and BOS are required for power generation and solar energy use. Fig. 10 presents the manufacturing system boundary of a QDSSC module. Azzopardi et al. [100] studied LCA in detail in terms of NER, EPBT, and GHG emissions on hybrid QD-based solar cells. The analysis

(1)

where: Emat = Primary energy demand to produce constituent materials for the PV system (kWh) Emanuf = Primary energy demand to manufacture PV system (kWh) Etrans = Primary energy demand to transport materials used during the life cycle (kWh) Einst = Primary energy demand to install the system (kWh) Eeol = Primary energy demand for end-of-life management (kWh)

EBOS (kWh) = Eelectrical + Emechanical

(2)

where: Eelectrical = Primary energy demand of the electrical BOS components (kWh) Emechanical = Primary energy demand of the mechanical BOS components (kWh) Various LCA studies on the embodied energy requirement of mono21

Renewable and Sustainable Energy Reviews 96 (2018) 11–28

N.A. Ludin et al.

substrate glass also provided negative effect on the primary energy of thin film [29,42,68]. Replacing the glass using a polymer cover was also suggested, because it could save energy by 150 MJ/m2 [29]. The aluminum frame also contributed about 15–25% of the total energy [6,68,106]. Designing frameless modules [6,42,49,53,106], enhancing BIPV structure [11], and improving the conversion efficiency of thin film [49] were recommended as the effective ways to improve energy consumption and to shorten the EPBT. The main significant energy consumption of DSSC life cycle was observed from TCO glass substrates manufacturing [83,84,86–88]; followed by cell and module production that involved glass lamination [84,86] and TiO2 layer sintering that required a high temperature of –450 °C [107]. Essentially, high temperature is needed for efficient electron transport in the TiO2 nanoparticles [84,85]. Maria et al. found that when aluminum frames are included for grid connected rooftop installations, about 28.1% of the DSSC embodied energy are consumed [85]. However, among these obstacles in manufacturing DSSCs, aluminum frames still had significant benefit over thin film technologies [87,88]. The substitution of glass substrate with other substrates, such as metal or polymer foil [84] and PET [87,88] or recycling the TCO glass [84,89] may reduce the energy requirement. Improvement in solar cell efficiency includes [89] adopting low temperature approaches for module preparation [84], minimizing the amount of material used in production step [89],and optimizing printing solutions [87],which could probably decrease the embodied energy of DSSC module that range between 277 MJ/m2 to 365 MJ/m2, as shown in Table 6. Another emerging PV technology that has attracted attention worldwide is the perovskite solar cell. The primary energy of perovskite was indicated from 379 MJ/m2 to 821 MJ/m2, as shown in Table 7, which provided major contributions to ITO or FTO glass that consumed 80–87% energy consumption [90,91,108]. Finding an alternative ITO or FTO glass to be used as a transparent electrode was supported [91]. In addition, gold cathode in TiO2 module and silver cathode in ZnO module contributed about 22–36% [90,108] and 28% [91] of the total embodied energy of the perovskite solar cell. This condition is due to high cost and extensively high energy utilization of gold [90]. Future studies in recognizing suitable materials should be made to replace gold and silver cathode [90]. Moreover, high-temperature sintering and thermal evaporation technology have been found to influence energy conservation [90]. This technology deposits high quality thin film of the metal, but incurs very high energy cost [90]. Hence, another deposition technique should be discovered in the future to diminish energy consumption [90]. For QDSSC, the estimated primary energy consumption ranged between 370 MJ/m2 to 1030 MJ/m2, as indicated in Table 8, and was slightly lower than CdTe thin film but higher than DSSC technology. For this reason, QD module production (42%), production of QD solar cells (33%), and BOS (21%) had significant contributions to the energy requirement for total energy consumption [99]. Therefore, efforts in reducing solvent use, incurring low energy consumption, and replacing aluminum foil with other flexible metal substrate such as titanium, must be considered to overcome the problems in QDSSC [99].

Fig. 11. Breakdown of energy requirement for multi-Si module production with conversion efficiency of 13%.

crystalline silicon and multi-crystalline silicon modules have been conducted [21,23–25,33,49,102]. The embodied energy of mono-Si and multi-Si ranged from 1123 MJ/m2 to 8050 MJ/m2 and 1034 MJ/m2 to 5150 MJ/m2, respectively, as shown in Tables 1, 2. The crystalline silicon production process involves several steps, including silicon purification, wafer production, cell/module processing, module encapsulation, equipment manufacturing, and framing [49]. Among these processes, silicon purification step consumes the highest energy consumption because of the Siemens process and Czochralski process [11,24,29,45,46], as shown in Fig. 11 [49]. This process involves conditions under high working temperature where trichlorosilane (SiHCl3) and hydrogen (H2) gases are heated at 1100–1200 °C in the reactor chamber to produce high-purity electronic-grade (EG) silicon [103]. Silicon wafer production also allocated about 1000 MJ/m2 or 22% of total energy consumption because of the material lost in the production process [49]. The large amount of aluminum used in the frame significantly increased the energy requirement the energy requirement of silicon PV [33,49]. A considerable number of studies have begun to focus on overcoming the problem of high energy consumption during silicon purification and module production processes. Implementing new crystallization processes was suggested [6], such as introducing solar-grade silicon process [29]; improving casting method [29]; employing the “modified Siemens” process [42,104] where silane (SiH4) and hydrogen gases are heated to −800 °C [103]; application of fluidized bed reactor [41,103,104]; using modern techniques such as Elkem Solar Silicon [55], which may lead to a reduction in the energy utilization; scaling up the PV production plant [49] from 10 MW/year to 100 MW/year [24]; or multiplying the production by a factor of two intensely diminish half of the primary energy [33]. In addition, reducing the silicon wafer thickness [29] from 300 µm to 150 µm [11,54,104] resulted in decreased energy by 30–40% [49]. Furthermore, enhancement in silicon PV module efficiency [6,11,29,33], recycling of silicon wafers [30,37], replacing or reducing aluminum utilization in module production [33], and needs for frameless PV modules [24,49,104,105] caused a significant decrease in energy consumption. Compared with crystalline silicon solar cell, thin film technology requires much lower energy consumption because the production process involves low temperature and few materials [6]. Among the three types of thin film technologies, CIS consumed the highest embodied energy of 1105 MJ/m2 to 1684 MJ/m2 compared with CdTe and a-Si increasing from 811 to 1803 and 862 to 1731 MJ/m2, respectively, as shown in Tables 3–5. In thin film technologies, cell production processes, such as TCO layer deposition, CdTe layer deposition [42], and CdS layer deposition required major intensive energy [68] leading to long period of EPBT. Thin cells [49] and production scale increase [53,68] lowered the energy requirement. Encapsulation materials or

4.2. Energy payback time Energy payback time (EPBT) is defined as the time required for the solar PV system to generate the same amount of energy used in its entire life cycle starting from raw materials extraction up to construction and decommissioning phase [101]. The EPBT is described as in Eq. (3) [101]:

EPBT = where: 22

(Emat + Emanuf + Etrans + Einst + Eeol ) ((Eagen/ nG ) − Eo & m)

(3)

2001

2007

2011 2012 2014

Greijer et al.

M.J. de Wild-Scholten, A. C Veltkamp

Maria et al. Maria et al. Maria et al.

Southern Europe Southern Europe Central Europe

Netherlands

Sahara Desert

Location

Rooftop Rooftop Rooftop

Rooftop

Rooftop

Mounting System

23

Southern Europe

Southern Europe

2015

2015

2016

Zhang et al.

Nieves Espinosa et al. Ilke Celik et al.

USA

South Europe San Francisco, US

2011 2015

Nieves et al. Jian Gong et al.

Location

Year

Authors

N/A

N/A

Rooftop

N/A N/A

Mounting System

Table 7 LCA results review of perovskite solar cell (PSC) PV systems.

Year

Authors

1700

1700

1860

1700 1960

0.75

0.75 and 0.95 0.80

0.80 0.80

0.75 0.75 0.75 0.75 0.75

1700 2190 1700 1700 1117

PR

0.75

N/A

PR

1000

2190

Irradiation (kWh/ m2/yr)

Irradiation (kWh/ m2/yr)

Table 6 LCA results review of dye sensitized solar cells (DSSC) PV systems.

8 8 8 8 8

7 9 12 8 5,10,30 5,10,30 20 20 20

20 20 20 5,10,30

15.4(vapor deposition)11.5 (spin coating) 15

6.5

11.0(ZnO module)

2 9.10 (TiO2 module)

5

1

5

15 2

821 (vacuum) 665 (solution) 504 (HTL-free)

N/A

379.26 446 (TiO2 module) 392 (ZnO module) N/A

CED (MJ/m2)

12,365 MJ/kWp 12,365 MJ/kWp 277.4 MJ/m2 12.67 233 – 365 MJ (DSSC using PET and glass)

350–366 100–150

1.05 –1.54

56.65 21.7 gCO2/m2 (TiO2 module) 19.1 gCO2/m2 (ZnO module) 497.2

GHG Emission (gCO2/kWh)

22.38 22.29 9.8–24 kgCO2 (DSSC using PET and glass)

19 25 22 20–120 (depending on lifetime: 5,10,30)

GHG Emission (gCO2/ kWh)

16.54–27.32

2.02 0.266 (TiO2 module) 0.193 (ZnO module) 5.3–5.4

EPBT (Year)

0.8 0.6 1.58 1.58 1.4–1.8 (DSSC using PET and glass)

N/A N/A N/A 1.4

100–280 kWh/m2

12,365 MJ/kWp

EPBT (Year)

CED (MJ/m2)

System Lifetime (year)

System Lifetime (year)

Module Efficiency (%)

Module Efficiency (%)

[92]

[94]

[93]

[91] [90]

References

[85] [86] [88]

[84]

[89]

References

N.A. Ludin et al.

Renewable and Sustainable Energy Reviews 96 (2018) 11–28

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[99] 5.0 0.9

EPBT is one of the indicators considered in several reviews of PV LCA studies [32,40,44,45,49,52,53]. Generally, key parameters affect the EPBT of PV system, including solar irradiation, solar cell type and module efficiency, system lifetime, performance ratio, and PV installation type [5,18,52]. Based on previous studies on solar PV, different total per annum solar irradiation affected the EPBT of the PV system [6,18,49,54,84]. For instance, Alsema et al., (2006) [41] analyzed the EPBT of multi-crystalline silicon in Southern Europe (1700 kWh/m2/year); Battisti et al., (2005) [50] conducted the LCA of same technology in Italy with solar irradiation of 1530 kWh/m2/year; and Kim et al., (2014) [46] investigated the EPBT of multi-Si system in Korea (1310 kWh/m2/year). The results show that the EPBT is shorter (1.7 years) at Southern Europe compared with the EPBT at Italy and Korea (3.3 years and 3.68 years), respectively. Similar with CdTe technology, the EPBT of solar PV technology decreases as total per annum solar irradiation increases, and vice versa. It was proven by Jungbluth et al. (2008) [72] that reported the EPBT of 2.7 years under Switzerland's irradiation (1117 kW h/m2/yr), the EPBT of 1.5 years installed in Southern Europe with the irradiation of 1700 kWh/m2/year estimated by Raugei et al. [53], while the EPBT of CdTe reduces to 0.94 years at high solar irradiation region as performed by Kim et al., 2014 [76]. In addition, solar irradiation depends significantly on the location and PV panel orientations [23,44,109]. Fig. 12 illustrates the EPBT of the rooftop BIPV system at different orientations as investigated by Lu and Yang [44]. If the orientation of solar PV module is south-facing at 30°, the EPBT is the lowest (7.1 years) as compared to the other solar PV orientations. The plane should be perpendicular to the sun where the panel is directed to the south and inclined at 30° to the plane to obtain the maximum energy production during the midday of summer [110]. In terms of PV module efficiency, the EPBT of the PV system decreases as module efficiency increases [49,52,53]. Considering the case of rooftop mono-crystalline silicon PV in Southern Europe (solar irradiation of 1700 kW h/m2/yr), the EPBT of 2.5–3.0 years was reported by Alsema [29] in 2000, the EPBT of 2.1 y in 2006 was reported by de Wild-Scholten and Alsema [104], and the EPBT decreasing to 1.75

N/A

1700

0.80

14

25–30

1029.6 (including PV frame and BOS)

2.89 1.51

[100]

Emat: Primary energy demand to produce materials comprising PV system Emanuf: Primary energy demand to manufacture PV system Etrans: Primary energy demand to transport materials used during the life cycle Einst: Primary energy demand to install the system Eeol: Primary energy demand for end-of-life management Eagen: Annual electricity generation Eo&m: Annual primary energy demand for operation and maintenance nG:Grid efficiency, the average primary energy to electricity conversion efficiency at the demand side

2010

2011

B. Azzopardi, J. Mutale

Hatice Sengul, Thomas L. Theis

Southern Europe Southern Europe

Hybrid Quantum Dot Quantum Dot

BIPV

1700

0.75

10

25

369.75 MJ/Wp

GHG Emission (gCO2/kWh) Mounting System PV System Location Year Authors

Table 8 LCA results review of quantum dot sensitized solar cell (QDSSC) PV system.

Irradiation (kWh/ m2/yr)

PR

Module Efficiency (%)

System Lifetime (year)

CED (MJ/m2)

EPBT (Year)

References

N.A. Ludin et al.

Fig. 12. EPBT of the rooftop BIPV system and different orientation systems (Reproduced with permission from Elsevier Publishing Group) [44]. 24

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process resulted in greater GHG emissions rate as compared to other solar PV technologies [6]. For thin film technologies, CdTe emitted the least amount of GHG emissions because of the lower energy consumption during the module production, which ranged between 8.9 and 66.0 g CO2-eq/kWh as compared to a-Si and CIS [42]. In contrast, DSSC, perovskite cell, and QDSSC indicated declining trends toward GHG emissions as they incur low energy utilization during cell productions. Most of these new emerging technologies had few environmental impacts because of their glass substrate, solvent, and raw material waste [84,88,92,112]. Significant concerns have been raised by researchers on the influence of efficiency and the lifetime of the solar module to GHG emissions. Module efficiency improvement of solar PV should result in decreased GHG emissions [46,49,52,56,75,86,93], because high efficiency modules can produce much electricity generated from the same amount of solar energy [76] with few structural materials [71]. GHG emissions are inversely proportional to lifetime [86,92,93]. For instance, Parisi et al. [86] investigated the lifetime sensitivity analysis of DSSC and demonstrated that increasing DSSC lifetime from 15 to 30 years contributed to the reduction of GHG emissions from 38.68 g CO2eq/kWh to 20.79 g CO2-eq/kWh at the same module efficiency of 6%. Finally, the selection of energy mix also played an important role in determining the total GHG emissions [49]. The comprehensive LCA of solar PV technologies, including crystalline silicon, thin film, DSSC, perovskite, and QD solar cells, were reviewed in terms of CED or embodied energy, EPBT, and GWP that translated into equivalent GHG emissions based on previous literature. The findings were summarized in Table 9. From these reported values, it was observed that mono-Si had greatest range of CED (1123–8050 MJ/m2), longest EPBT (1.4–7.3 years), highest environmental impacts (29.0–671 g CO2-eq/kWh) compared to other PV technologies due to silicon purification and crystallization processes. It is noteworthy that the variation results of EPBT for each PV technology are found to be influenced by various factors, such as solar irradiation at different locations, module efficiency, type of PV technology, and module lifetime. Differences in GHG emissions rates were caused by energy mix for each country, different solar irradiations, and lifetime of PV technologies. Extending the comparison with previous LCA studies on other renewable energy sources such as wind power and hydropower, their respective CED were ranged between 0.01–1.20 MJ/kWh and 0.01–0.90 MJ/kWh [113]. The EPBT values were in the range 0.2–2.3 years for wind power and 0.24–3.09 years for hydropower [113]. Meanwhile, GHG emissions data for wind power and hydropower were evaluated between 6.2–46.0 g CO2-eq/kWh and 2.2–74.8 g CO2-eq/kWh, respectively [113]. From these results, it was observed that wind power had the lowest energy consumption and contributes lowest GHG emissions compared to solar PV and hydropower. Even though solar PV has larger impact values due to modul manufacturing process, however it still resulting better environmental impacts in comparison to hard coal plant that has an emission range of 750–1050 g CO2-eq/kWh [114].

Fig. 13. EPBT sensitivity analysis with different conversion efficiency of dye sensitized solar cell (DSSC).

years in 2009 as reported by de Wild-Scholten. In this time frame, when the amount of silicon material needed for the production process is reduced and the manufacturing of PV process is improved, then the energy efficiency is increased and EPBT is reduced [18]. In addition, Lewis and Keoleian [106] investigated the efficiency of a-Si module, increasing from 5% to 8%, the EPBT of 2.3–13 years would decline to 1.4–8.1 years [6]. Parisi et al. [86] examined the EPBT sensitivity analysis for DSSC. From this study, high efficiency of the cell resulted in a short lifetime, as shown in Fig. 13 [86]. According to Kannan et al. [33], the conversion efficiency of PV is affected by the following factors: (i) the operating orientation, (ii) an increase in working temperature would decrease the conversion efficiency of PV modules, and (iii) efficiency losses occurring at inverters and electrical transfer wirings. Different solar PV technologies affected the EPBT of the solar module [18]. For silicon technology, high energy consumption in purification and crystallization processes resulted in EPBT increase. Emerging solar PV technologies, such as DSSC, perovskite, and QD have several advantages over conventional solar PV, such as low processing temperature [84], require few raw materials, and simple production technique, thereby reducing energy utilization and EPBT. 4.3. GHG emission rate The GHG emissions rate, expressed as CO2-equivalents, is a useful index for determining how effective a PV system is in terms of global warming. According to Kim et al. [71], the major non-CO2 GHG emissions estimated include nitrous oxide, chlorofluorocarbons, methane, and per fluorocarbons, are converted to CO2 equivalent by using the global warming potential of the 100-year time horizon recommended by the Intergovernmental Panel on Climate Change. The emissions can be calculated by determining the total GHG emissions during a life cycle divided by the total amount of annual power generation over its lifetime, as presented in Eq. (4) [109]:

GHG emissions rate

Table 9 Summary of CED, EPBT and GHG emissions of different solar PV technologies from previous studies starting year of 2000.

Total GHG emission duringlife cycle(gCO2)

( ) Annual power generation ( ) xLifetime gC 02 kWh

kWh year

(year )

(4) Several factors that can affect the total life cycle emissions of GHGs significantly have been identified in previous studies. Significant environmental impact during solar PV system life cycle is caused by energy consumption during manufacturing stages [25,49,111]. Reduced energy requirement in the production line resulted in low GHG emissions [75]. For silicon technologies, the range of GHG emissions rate for mono-Si is 29–671 g CO2-eq/kWh, which is slightly higher than multi-Si that ranged between 12.1 and 569.0 g CO2-eq/kWh, as shown in Tables 2, 3. High energy consumption in silicon purification and crystallization 25

Type of Solar PV technology

Range of CED (MJ/m2)

Range of EPBT (years)

Range of GHG emissions (g CO2eq/ kWh)

Mono-Si Multi-Si a-Si CdTe CIS DSSC Perovskite Quantum dot

1123 – 8050 1034 – 5150 862 – 1731 811 – 1803 1105 – 1684 277 – 365 379 – 821 370 – 1030

1.4 – 7.3 0.8 – 4.17 1.1 – 3.2 0.79 – 2.7 1.3 – 2.8 0.6 – 1.8 0.2 – 5.4 0.9 – 1.51

29.0 – 671.0 12.1 – 569.0 8.1 – 57.0 8.9 – 66.0 33.0 – 95.0 9.8 – 25.0 56.65 – 497.2 2.89 – 5.0

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5. Conclusions

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This study reviewed several previous LCA studies on conventional and new emerging solar PV technologies including silicon, thin film, DSSC, PSC, and QDSSC. Three main indicators of LCA, such as CED, EPBT, and GWP, are considered and summarized. Across the analysis, mono-Si indicated the highest intensive energy, longer EPBT, and excessive GHG emissions contribution among other PV technologies associated with the purification and manufacturing process. Meanwhile, DSSC demonstrated the lowest energy requirement and shorter EPBT among other conventional PV technologies due to its simple structure, low cost, and eco-compatibility. However, DSSC is still in the demonstration stage. Further improvement in energy reduction of solar PV manufacturing process, conversion efficiency enhancement, and long module lifetime have attracted huge attention to reduce the EBPT and GHG emissions. Acknowledgements This study was supported by Research University Grant (GUP-2015037 and DIP-2016-025) under Solar Energy Research Institute (SERI), The National University of Malaysia (UKM). The author Nur Ifthitah Mustafa would like to thank the Ministry of Higher Education (MOHE), Malaysia for the myPHD program. Reproduced figures in this paper are with permissions of Elsevier Publishing Group and Royal Society of Chemistry Publishing Group. References [1] IEA-PVPS Reporting Countries, Becquerel Institute (BE), RTS Corporation (JP). Snapshot of Global Photovoltaic Markets (1992–2016), IEA PVPS Task 1, International Energy Agency Power Systems Programme, Report IEA PVPS T1-312017; 2017 〈http://dx.doi.org/ISBN 978-3-906042-42-48〉. [2] Burger B, Keifer K, Kost C, Nold S, Philips S, Preu R. Photovoltaics report. Freiburg, Germany; 2017. [3] National Renewable Energy Laboratory. Best Research-Cell Efficiencies. 〈https:// www.nrel.gov/pv/national-center-for-photovoltaics.html〉 [Accessed 23 November 2017]; 2017. [4] Płaczek-Popko E. Top PV market solar cells 2016. Opto-Electron Rev 2017;25:55–64. https://doi.org/10.1016/j.opelre.2017.03.002. [5] Sherwani AF, Usmani JA. Varun. Life cycle assessment of solar PV based electricity generation systems: a review. Renew Sustain Energy Rev 2010;14:540–4. https:// doi.org/10.1016/j.rser.2009.08.003. [6] Peng J, Lu L, Yang H. Review on life cycle assessment of energy payback and greenhouse gas emission of solar photovoltaic systems. Renew Sustain Energy Rev 2013;19:255–74. https://doi.org/10.1016/j.rser.2012.11.035. [7] Baharwani V, Meena N, Dubey A, Brighu U, Mathur J. Life cycle analysis of solar PV system: a review. Int J Environ Res Dev 2014;4:183–90. [8] Darling SB, You F. The case for organic photovoltaics. RSC Adv 2013;3:17633–48. https://doi.org/10.1039/c3ra42989j. [9] Lizin S, Van Passel S, De Schepper E, Maes W, Lutsen L, Manca J, et al. Life cycle analyses of organic photovoltaics: a review. Energy Environ Sci 2013;6:3136. https://doi.org/10.1039/c3ee42653j. [10] Gerbinet S, Belboom S, Léonard A. Life Cycle Analysis (LCA) of photovoltaic panels : a review. Renew Sustain Energy Rev 2014;38:747–53. [11] Wong JH, Royapoor M, Chan CW. Review of life cycle analyses and embodied energy requirements of single-crystalline and multi-crystalline silicon photovoltaic systems. Renew Sustain Energy Rev 2016;58:608–18. https://doi.org/10.1016/j. rser.2015.12.241. [12] Fthenakis V. Life cycle assessment of photovoltaics. first editionPhotovoltaic solar energy: from fundamentals to applications, 14044. USA: John; 2017. p. 646–57. [13] Gekas V, Frantzeskaki N, Tsoutsos T. Environmental impact assessment of solar energy systems. In: Proceedings of the international conference on Protection And Restoration Of The Environment VI; 2002, p. 1569–76. [14] The International Standards Organisation. Environmental management — Life cycle assessment — Principles and framework. ISO 14040 2006;2006. p. 1–28 〈http://dx.doi.org/10.1136/bmj.332.7550.1107〉. [15] The International Standards Organisation. Environmental management — Life cycle assessment — Requirements and guidelines. ISO 14044 2006; 2006. p. 652–68 〈http://dx.doi.org/10.1007/s11367-011-0297-3〉. [16] Hanafiah M. Quantifying effects of physical, chemical and biological stressors in life cycle assessment. 1st ed. Bangi: S&T Photocopy Center; 2013. [17] PRé Consultants. SimaPro Database Manual Methods library; 2008. [18] Canada E. Assessment of the environmental performance of solar photovoltaic Technologies; 2010. [19] Frischknecht R, Heath G, Raugei M, Sinha P, de Wild-Scholten M. Methodology guidelines on life cycle assessment of photovoltaic electricity. 3rd edition United

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