Carbon Footprint of Hydrogen Produced with Photovoltaic Electricity: Comparison
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The production of hydrogen as both chemical feed and energy carrier using low-carbon technologies is one of the solutions to reach net-zero emissions. The publications on the life-cycle assessment of photovoltaic (PV)-based hydrogen production focused on the carbon footprint are reviewed. The global warming potential (GWP) values of this H2 production process considering the state-of-the-art PV panels for installation in Italy are updated. 

  • hydrogen
  • photovoltaic
  • carbon footprint
  • electrolysis

1. Introduction

Over the last decades, hydrogen has been produced mainly for industrial applications through conventional production processes based on fossil fuels. Global warming, increasing energy demand, and energy storage challenge made hydrogen a potential solution for future energy transition scenarios. Therefore, on the one hand, a significant amount of hydrogen should be produced to satisfy the growing market demand. On the other hand, these production methods should meet economic and environmental needs. According to hydrogen production data available from International Energy Agency (IEA) [1], in 2021, less than 1% of the produced hydrogen can be considered low-carbon. Low-carbon hydrogen refers to the hydrogen obtained through technologies with a low climate profile, specifically, low CO2 equivalent emissions. Figure 1, based on the IEA predictions, shows the current and future status of hydrogen production from different sources. Around 94 Mt of H2 was produced in 2021, of which more than 80% has been obtained directly from fossil fuels without the application of carbon capture, utilization, and storage (CCUS). Currently, the chemical sector demands the largest share of hydrogen in the market to produce ammonia (36%) and for hydrocracking and desulfurization of fuels in refineries (42%) [2]. Based on a net-zero emissions (NZE) scenario, it is forecasted that in 2030, hydrogen production will reach up to 180 Mt. Production via the application of electricity (34%), and fossil fuels with CCUS (18%) will be responsible for this increase in supply. The corresponding demand for this increase in the production volume comes from new applications (heavy industry, power generation, and synthesis of H2-based fuels) [1].
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Figure 1.
Current and future hydrogen production from different sources (data from [1]).
Hydrogen can be produced using a wide range of processes, including thermochemical, electrochemical, and biological methods. Regarding the diffusion of these processes in new installations, based on the statistical analysis of the available raw data for hydrogen production projects in different stages (operational, under construction, feasibility study, and concept) from IEA [3], more than 80% of new projects are based on electrolysis and less than 10% are powered with natural gas with CCUS. Achieving a low impact from hydrogen production is particularly important in view of its increasing role in the future energy system. For example, the current EU classification for sustainable activities (EU Taxonomy Regulation 2020/852) requires hydrogen production to remain below 3 kgCO2 eq./kg H2 [4] to comply with a criterion of “substantial contribution to climate change mitigation”. Figure 2 shows the number of new hydrogen production projects in Italy and Europe (without Italy). Among the 32 H2 projects in Italy, 28 of them use electrolyzers. The type of electrolysis system is not disclosed for a considerable number of projects. However, the alkaline electrolyzer has a larger share in Italy for those plants with the known electrolyzer type, followed by proton-exchange membrane (PEM) electrolysis systems. In contrast, in Europe, PEM electrolysis systems are used more frequently.
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Figure 2. Number of H2 projects based on different processes for (a) Italy and (b) Europe without Italy, built based on the raw data from IEA [3].
The electricity powering the electrolyzers to produce H2 can be supplied from the grid, nuclear, or renewable sources (e.g., wind, photovoltaic, hydro, biomass), usually labeled in the literature as yellow, pink, and green hydrogen. Table 1 introduces the colorful classification of hydrogen production pathways and briefly describes the related process and energy source for each color. The first two lines represent the production processes with high environmental impacts. In contrast, the second group shows the potential of low-impactful production approaches, including the green pathway considered herein.
Table 1.
Color classification of hydrogen production pathways.
Category Color Process Source
High environmental footprint (high carbon) Brown/Black Gasification Fossil fuels

Coal
Grey Steam reforming Fossil fuels

Natural gas
Low environmental footprint (low carbon) Blue Steam reforming or gasification + CO2 capture and sequestration (CCS) Fossil fuels

Natural gas
Turquoise Pyrolysis Fossil fuels

Natural gas
Yellow Electrolysis Water and electricity from the grid (generic)
Pink/Red Electrolysis Water and electricity from nuclear source
Green Electrolysis (+biomass *) Water and electricity from renewable sources/biomass *
* Classification not consolidated.
Photovoltaic (PV) and wind-based power plants are today the main sources of renewable electricity for new installations. This text focuses on the case of PV electricity and its utilization for green hydrogen production. The environmental impacts of producing H2 via electrolysis systems powered by renewable electricity have been studied using the process-based life-cycle assessment (LCA) method in several publications (attributional modeling). In 2014, Bhandari et al. [5] reviewed some LCA studies on H2 production via electrolysis and reported that the global warming potential (GWP) for PV-based hydrogen could vary from 2 to 7 kg CO2 eq./kg H2. They mention that higher GWP values for solar PV than hydro or wind are caused by the emissions related to PV module manufacturing processes. In 2018, Parkinson et al. [6] re-estimated the life cycle GHG emissions of a solar PV-based electrolysis system around 2.21 kg CO2 eq. per each kg of hydrogen. Their calculations assumed a generalized overall energy requirement of 51.2 kWhel/kg H2 and a 40 g CO2 eq./kg H2 for electrolyzer contribution. The related emission of PV systems was estimated based on the review work of Nugent and Sovacool [7].

2. Hydrogen Production

The water electrolysis process to produce hydrogen and oxygen has been known for two centuries, with the advantage of producing extremely pure hydrogen and a relatively straightforward process based on electricity consumption as energy input. Most applications have been limited to small-scale and unique situations in which access to large-scale fossil fuel-based hydrogen production plants—which dominate today’s production, as reported in Figure 1—was not possible or not justified (e.g., electronic industry, food industries as well as medical applications) [11][8]. Recent developments towards a lower cost of electrolyzers and the perspective availability of low-cost renewable electricity are making their application more attractive. The electrolysis processes can be grouped based on the electrolyte, which may feature different pH (alkaline and acid) or physical state (liquid or solid) [12][9]. In the following, the 4 most common types of electrolyzers proposed for hydrogen production are shortly explained.
  • Alkaline
Among hydrogen production technologies, alkaline water electrolysis is a mature process [13][10]. Troostwijk developed the first design of an alkaline electrolyzer in 1789 [14][11], and the technology has followed a long trajectory of research and development, today showing the lowest plant-specific cost (€/kW) with respect to the other types [15][12]. An aqueous potassium hydroxide (KOH) solution with typical concentrations of 20–40% KOH is used as an electrolyte in alkaline water electrolysis [11][8]. The cathode and anode electrodes usually are based on low-cost materials (iron or nickel). The mature and low-cost alkaline electrolyzers show higher lifetime and production capacity. However, this electrolysis system is mostly designed for a continuous power supply and limited turndown capacity to avoid damage and safe operation [20][13]. Corrosive environment conditions, risks of leakage of liquid electrolytes, gas permeability, and low current density, bringing to relatively heavy and high-footprint installations, are other disadvantages of this system.
  • Proton-exchange membrane
A newer generation of electrolyzers with respect to alkaline electrolysis systems is known as proton-exchange membrane (PEM) electrolyzers. PEM electrolyzers use a thin solid polymer electrolyte (membrane) instead of a liquid electrolyte. Commercial systems use Nafion® as the proton-conducting membrane with a typical thickness of 60–200 µm [21][14]. Nafion is prepared as a copolymer from tetrafluoroethylene (TFE) and fluorinated vinyl ether, e.g., perfluoro (4-methyl-3,6-dioxane-7-octene-1-sulfonyl fluoride) [19][15]. Compact design, high current density, faster response to load variation, wide turndown capacity, and dynamic operation are obtained by the use of a solid membrane [25][16]. PEM electrolyzers’ other advantages are the possibility of producing highly compressed and pure hydrogen and high efficiency [26][17]. On the downside, the technology relies on expensive noble metals, such as platinum and iridium (PGM) [16][18]. The application of such noble metals in addition to titanium and the proton-exchange membranes makes the PEM electrolyzers a tendentially more expensive option than conventional alkaline electrolysis systems.
  • Solid oxide electrolyzer cells
Solid oxide electrolyzer cells (SOEC) operate in temperatures higher than 500 °C, typically in the range of 600–850 °C [22][19]. They can produce hydrogen with a lower electricity input than alkaline and PEM electrolyzers. At the same time, industrial waste heat can be utilized to provide the thermal energy demand of the high-temperature SOEC electrolysis. A SOEC consists of a dense ionic conducting electrolyte and two porous electrodes. Zirconia with different dopants (typically yttria) is the most used material for electrolytes, which allows the conduction of oxygen ions [19][15]. SOEC electrolysis systems offer significantly higher efficiency and may achieve a lower total cost of hydrogen production compared to conventional low-temperature electrolysis, thanks to favorable thermodynamics [29][20]. Additional opportunities are given by the possible simultaneous electrolysis of CO2 and H2O for the production of synthesis gas and the possibility of reversible operation as a fuel cell (particularly interesting for energy storage applications) [28][21].
  • Anion-exchange membrane
An anion-exchange membrane (AEM) is an alkaline solid polymeric membrane, which is the core component of an AEM electrolyzer [16][18]. Applying AEM instead of the conventional diaphragms used in alkaline electrolyzers is the main difference between these technologies [30][22]. A dilute alkaline electrolyte (KOH) is used on the anode side, while no solution is supplied for the cathode side [16][18]. One of the advantages of AEM electrolysis is its overall lower cost due to the application of transition metal catalyst and the quaternary ammonium ion-exchange-group-containing membrane instead of more expensive noble metal (PGM) catalysts and the Nafion-based membranes [30][22]. Also, applying a low-concentration alkaline solution as an electrolyte instead of concentrated KOH (without a corrosive liquid) results in the absence of leaking, volumetric stability, ease of handling, and a reduction in the size and weight of the electrolyzer [30][22]. Catalyst performance deterioration may occur due to the intermittent nature of solar and wind power supplies in the AEM electrolysis. AEMs are susceptible to damage if frequent shutdowns occur on the electrolyzer. Therefore, the durability and conductivity stability of the AEMs should be reinforced considering the large-scale applications and real-world size and conditions within the desired life duration [32][23].

3. Electricity Source—Photovoltaic

Utility-scale photovoltaic (PV) plants include modules, mounting systems, inverters, transformers, cables, electrical protection systems, measurement equipment, and system monitoring [33][24]. The PV modules produce direct current electricity using solar irradiance and then convert it into alternating current in an inverter for further applications. Crystalline silicon PV panels production starts with silica mining, carbothermic/quartz reduction (removing oxygen from silica), metallurgical-grade silicon (MG-Si) purification, solar-grade (SOG) silicon construction, silicon ingot crystallization, wafer slicing, PV module assembly and finishes with module and laminate construction [34][25]. The silicon ore is reduced to metallurgical-grade silicon with silicon purity of around 99% via the use of carbon in a large arc furnace [35][26]. Metallurgical-grade silicon is further refined to 99.999% purity using the modified Siemens process, which is more advanced and less energy-consuming than the original Siemens process [36][27]. To produce high-purity silicon, the modified Siemens process consumes a considerable amount of energy due to the operation of the reaction chamber at a high working temperature (typically between 1100 °C and 1200 °C) [37][28], while over the last decade, the energy and material efficiency of the Siemens process has improved remarkably [38][29]. Single-crystalline and multi-crystalline silicon ingots are obtained by crystallization through the Czochralski process and casting of solar-grade silicon, respectively. These ingots are sliced using a multi-wire saw combined with a slurry of cooling liquid and abrasive particles and then treated by subsequent etching with sodium hydroxide (NaOH) and washing with water and sulfuric acid (H2SO4) [36][27]. PV cells are produced via different steps, surface preparation, dopant diffusion, junction formation, and coating, in which some materials like electric pole printing ribbons, nitrogen, oxygen, and argon are used [35][26]. Finally, solar cells are connected into a string and then encapsulated by two layers of glass and plastics (ethylene-vinyl acetate) to form the PV module [37][28].

4. Life-Cycle Assessment

The process-based life-cycle assessment (LCA) method is considered one of the most analytical methods to evaluate the environmental profile of products (goods and services), such as hydrogen. Since then, numerous directives, communications, and recommendations of the European Commission have been referred to LCA. The LCA is an internationally recognized method according to the principles defined in ISO 14040 and 14044 [39,40][30][31]. Italy is one of the 6 European countries with an annual renewable electricity potential capacity higher than 600 TWh [42][32]. The outcomes were calculated using SimaPro 9.4 software [43][33], the ecoinvent 3.5 library cut-off method as the Life Cycle Inventory database [44][34], and the Environmental Footprint (EF) 3.0 as the characterization method [45][35]. The environmental burdens of co-production and end-of-life treatment processes were assessed using the partitioning method in compliance with the ecoinvent library. For modeling the end-of-life scenarios, waste producers bear the burden of waste treatment based on the “polluter pays” principle; consumers of recycled products receive them without charge. The Life Cycle Inventory data (LCI) or foreground data for the alkaline electrolysis system are based on Sundin [46][36], which relies on Koj et al. [18][37]. The functional unit is defined as the hydrogen weight (1 kg) following the proposed harmonized life-cycle global warming impact of renewable hydrogen [47][38]. The boundary system is cradle-to-gate (CTG), in which only the life cycle of hydrogen is considered up to its production. Figure 3 shows the schematic boundary system.
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Figure 3.
H
2
production schematic boundary system.
Two scenarios have been defined: baseline and updated PV systems.
  • For the baseline scenario, the electrolysis system is powered by Italy’s default ground-mounted PV system in the ecoinvent database. The ground-mounted PV system in ecoinvent is equipped with multi-Si panels. To make a comparison, the panel is substituted with the available single-Si module in ecoinvent. Consequently, the relative factors, like the area of the PV panels and the mounting system, are adjusted. Also, a sensitivity analysis was carried out, evaluating the impact of: (i) the PV system lifetime, (ii) electrolyzer operating hours in its lifecycle, (iii) the specific electricity consumption of the electrolyzer, and (iv) the solar irradiance (insolation) for the PV plant equipped with ecoinvent default multi-Si panels. The baseline scenario assumptions and limits of sensitivity analysis are summarized in Table 2.
  • In the updated scenario, the market supply chain and the main parameters in LCI of PV modules are based on the International Energy Agency special report on solar PV global supply chains and the Task 12 PV sustainability LCI report, respectively [38,48][29][39].
Table 2.
Description of electrolyzer and PV system in the baseline scenario and sensitivity analysis limits.
Parameter Description Sensitivity Analysis Limits
Location (Insolation [kWh/m2 year]) Italy (1250) 1250–2200
PV system lifetime [years] 30 15–30
Annual specific yield [kWh/kWp] 1033  
Module types Single-Si, Multi-Si  
40]. Over the last decade up to 2018, the multi-Si modules had a higher share in the market. In the last few years, single-Si panels become the dominantly produced modules with more than 90% of cumulative global production [38][29].
Both single- and multi-Si panels with higher efficiencies are considered in the updated scenario. The annual specific yield of PV systems, which defines the amount of electricity produced for each kWp of PV module over a year, is updated for Italy according to Frischknecht et a. [48][39]. Also, in order to have a more realistic approach, the annual specific yield is calculated based on the real net PV electricity production data. Values were collected from Terna—the Italian electricity transmission operator [50][41]. The annual specific yield is estimated based on the published data for installed capacity and net production of PV systems over the three-year period of 2019–2021. This value is calculated for Italy and the three regions of Lombardy, Lazio, and Sicily as representative of the country’s north, center, and south, respectively. The related values used in the updated scenario are reported in Table 3. The assumptions corresponding to the electrolyzer remain unchanged.
Table 3.
PV system assumptions for the updated scenario.
Parameter Description
PV system lifetime [years] 30
Annual specific yield—Italy [kWh/kWp] 1376 [48][39]
Annual specific yield—Terna, Italy [kWh/kWp] 1114
Module efficiencies 14%, 13%  
Annual specific yield—Terna, Lombardy [kWh/kWp952
Annual specific yield—Terna, Lazio [kWh/kWp] PV system installation Ground-mounted *  
] 1189 Electrolyzer type Alkaline  
Annual specific yield—Terna, Sicily [kWh/kWp] 1242 Electrolyzer lifetime [hours]
Module types Single-Si, Multi-Si 80,000 30,000–80,000
Module efficiency 20% [51][42], 18% [48][39] Electrolyzer capacity [MW] 6  
Electrolyzer specific consumption [kWhel/kg H2] 50 50–65
* The ground-mounted PV system available in ecoinvent has a capacity equal to 570 kWp. Based on the electricity demand volume, SimaPro software considers a linear relationship and satisfies the demand. In practice, the solar PV plant capacity should be designed to be 3 to 4 times higher than the electrolyzer capacity to satisfy the demand.
The default PV systems considered in the ecoinvent database do not represent current or near-future PV system installations. On the one hand, both the module technology (type and efficiency) and PV panel production processes have evolved. On the other hand, the relative supply chain of PV panels has changed. For example, the single-Si and multi-Si modules in the ecoinvent have efficiencies equal to 14 and 13%, respectively, while the available panels in the current market can offer efficiencies of up to 22.4% [49][40]. The total weighted efficiency of crystalline silicon wafer-based modules regarding the total shipment in 2020 is 20.4% [49][
PV system installation
Ground-mounted

5. Photovoltaic Module Supply Chain

The supply chain of products can play an important role in their LCA analysis. The effect of the supply chain on the final impact becomes more significant for products with high manufacturing energy consumption, such as PV modules. Considering the supply chain for the different stages of solar PV manufacturing, China dominates the market and then the Asia-Pacific region countries (APAC), as is evident in Figure 4, in red and yellow colors, respectively. According to IEA [38][29], China is responsible for almost 80% and 97% of solar-grade polysilicon and wafer production in 2021. Similarly, 85% and 75% of cells and modules were produced in China. APAC countries are the second largest cell and module producers, with around 12% and 15% share in the market. While the ecoinvent database still relies on outdated supply chain data, in which the share of Asia is less than 50% (China is less than 25%) in all stages of PV panel production. Correspondingly, a higher production share is assigned to Europe and North America in ecoinvent. On the demand side, China still is the biggest PV module consumer, while Europe, North America, and APAC countries rank as the second, third, and fourth PV panel end users (almost 18, 17, and 13% of the market, respectively).
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Figure 4. Solar PV production by country and region, 2021; the plot is based on [52].
Solar PV production by country and region, 2021; the plot is based on [43].
Regarding the search words, almost 150 articles are found. The articles that do not provide the possibility to extract the GWP impact of H2 produced via PV-powered electrolysis are excluded. For example, in some articles, a hybrid electricity source is considered; therefore, these publications are out of the scope of this wdiscussiorkn. Comparing the articles found in the search campaign and the review work of Kanz et al. [8][44], in which only 13 works were found for the review, a total number of 22 publications (21 peer-reviewed and 1 book chapter) on the LCA of PV-based H2 are identified. 

Among the 22 studies, 50% used different versions of the ecoinvent as a background database [59,68,70,73,76,78,79,81–84][45][46][47][48][49][50][51][52][53][54][55].  In five articles, the background data are not introduced [55,56,63,74,80] [56][57][58][59][60]. Only one study is based on Gabi [66][61], while another used both Gabi and ecoinvent [85][62]. Similarly, one study used the GEMIS database [53][63], while another one is based on both GEMIS and GREET [64]. Also, only one study used the Japanese IDEA database [71][65]. Lombardi et al. [62][66] reported SimaPro as a database, whereas SimaPro is a software. Figure 5a shows the library distribution used.

The Life Cycle Inventory or foreground data can play an important role in LCA. The majority of the reviewed studies (80%) are based on available data from literature or reports (secondary data) [55,56,59,63,66,68,70,71,73,74,76,78,79,81–85][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][61][62][65]. Only two of the studies reported that partially they had access to some primary data [62,80] [60][66]. The remaining two studies did not reveal their inventory data [53,64][63][64]. The distribution of LCI is indicated in Figure 5b.

Figure 5.

Distribution of (

a

) background databases and (

b

) foreground data.

 

The main component of a solar PV system is the module. Among the reviewed studies, 40% did not mention the solar PV panel type considered in their work [53,59,64,66,68,70,71,83,85][45][46][47][54][61][62][63][64][65]. Multi-Si PV modules are assumed in 9 (40%) of the articles [62[48][49][50][51][52][53][55][60][66],73,76,78–82,84], while only one paper considered single-Si panels [74][59]. Three of the studies are based on thin-film panels [55,56,63][56][57][58]. The module efficiency is an important factor related to the nominal power of the panel and its area. Higher panel efficiency means a lower area is needed to reach a specific power output.

Some studies did not specify the panel efficiency. In 10 studies, the panel efficiency was reported or it could be realized. FigureFigure 6 6 shows the trend of PV modules’ efficiency (left axis) and the corresponding GWP values for each kg of H2 (right axis) with respect to the year of the publication. The panel efficiency has increased over time; however, in the case of using the default version of ecoinvent, the increase in efficiency is not applied. The studies [55,56,63][56][57][58] used thin-film PV panels with an efficiency of around 5% according to the reference they have used, i.e., Meier 2002 [57][67]. The multi-Si panels considered in [62,73][48][66] have an efficiency of 12.7% and 14.2%, respectively. The single-Si modules used by Sadeghi et al. [74][59] are 16% efficient. The solar PV farm by Palmer et al. [78][50] is based on multi-Si panels with 16.8% efficiency. Some studies are based on ecoinvent without any further changes for open-ground multi-Si solar plants [76,79,82][49][51][53]. Therefore, they are based on modules with 13% efficiency. The average efficiency of the panels considered in the related literature is only 12.95%. As shown in Figure 6, the GWP value (orange line) does not necessarily decrease with the increase in module efficiency (blue line). Two points can explain this non-trivial behavior. Firstly, the location of the PV site is an important factor in the GWP value of PV-based H2. For example, the value of 4.6 kg CO2 eq./kg H2 shown for multi-Si in 2022 is the average GWP of 6 values for five different locations (Spain = 3.8 kg CO2 eq./kg H2 and Germany = 5.65 kg CO2 eq./kg H2). Secondly, the quality of the LCA analysis depends on the data quality and transparency, which can significantly affect the climate footprint and lead to inconsistency in the result.

 

Figure 6.

Efficiency and type of the modules and corresponding GWP value of the produced H

2

with respect to the year of publication.

 

The GWP values for producing 1 kg of H2 via electrolysis systems powered by PV, extracted from 21 studies, are divided into two groups: Europe and the rest of the world. Among the 22 studies reviewed, only the work of Suleman et al. [68][46] is excluded since their GWP value (0.37 kg CO2 eq./kg H2) is relatively low and is not comparable with other studies. Their electrolytic cell co-produces chlorine and caustic soda in addition to H2, resulting in very low GWP values due to: (i) a different electrolysis system compared to the more common alkaline and PEM electrolyzers and (ii) the allocation approach used, which the authors do not specify.

The GWP values are plotted in Figure 7, providing statistical information (i.e., mean, standard deviations, 25th and 75th percentiles). The first column represents 16 GWP values for PV-based H2 production in Europe. The values fluctuate from 3 to 6.4 kg CO2 eq./kg H2, where the mean is 4.83 kg CO2 eq./kg H2 with a standard deviation equal to 1.05 kg CO2 eq./kg H2. The second column is based on seven studies located outside of Europe (rest of the world), in which exist values as low as 2.1 kg CO2 eq./kg H2 and as high as 9.37 kg CO2 eq./kg H2. The PV-based H2 GWP values for the rest of the world show a mean equal to 3.82 with a standard deviation of 2.17 kg CO2 eq./kg H2. Comparing the two columns suggests that GWP values for the production of H2 via electrolysis powered by PV systems in Europe might be higher with respect to production sites outside of this continent. According to the variation of GWP results in the literature, it can be noted that comparing or interpreting the carbon footprint or, in general, the environmental profile of the PV-based H2 is not trivial. On the one hand, some works’ unclear assumptions or lack of data make comparing the studies even more difficult. On the other hand, different system boundaries and different electrolysis systems can alter environmental footprints. However, this work tried to present the extracted GWP values in a quasi-harmonized manner (in case of possibility compression or liquefaction steps are excluded).

 

Figure 7.

Distribution of GWP values of solar-PV-powered electrolysis system of each kg of H

2

from the literature.

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