Life cycle assessment of hydrogen production via electrolysis – a review

Highlights

• Twenty-one studies on LCA of hydrogen production technologies were reviewed.

• Most studies discuss only the GWP impact category.

• Electrolytic technologies are better only if renewable energy electricity is used.

• ∼96% of GWP is by set up of turbine and H₂ compression/storage in wind electrolysis.

• GWP of electrolyzer unit is minimal, about 43 g CO_{2 eq.}/kgH₂ in wind electrolysis.

Abstract

There are several hydrogen production technologies. They range from the most widely used fossil fuel based systems such as natural gas steam methane reforming to the least used renewable energy based systems such as wind electrolysis. Currently almost all the industrial hydrogen need worldwide is produced using fossil fuels. Electrolytic hydrogen production based on electricity generated from renewable resources and hydrogen's energetic use could contribute to the global need for a sustainable energy supply. However, these technologies are also not free from environmental burdens. A life cycle assessment (LCA) helps to identify such impacts considering the entire life cycle of the process chains.

This paper reviews twenty-one studies that address the LCA of hydrogen production technologies, a majority of them employing electrolytic technologies. It has been observed that global warming potential (GWP) is the impact category analyzed by almost all the authors. Acidification potential (AP) ranks second. Other categories such as toxicity potential are often not analyzed. The main environmental concern associated with electrolytic hydrogen production is electricity supply. The GWP contribution of the electrolyzer unit is relatively small (e.g. only about 4% in wind based electrolysis including hydrogen production and storage systems). From an LCA perspective, it can be concluded that electrolysis using wind or hydropower generated electricity is one of the best hydrogen production technologies, compared to those using conventional grid electricity mix or fossil fuel feedstocks.

Introduction

Global energy consumption is expected to increase continuously in the next decades, driven by rising standards of living and growing population worldwide. The increased need for more energy will require growth in energy generation capacity and more secure and diversified energy sources (USDOE, 2009). The need for a sustainable energy supply is obvious due to declining fossil energy resources, environmental pollution, climate change and increasing dependency on fossil fuels exporting countries. Fossil fuel combustion or reformation causes adverse environmental impacts, which motivate researchers to look for environmentally sustainable alternative fuels. Those alternative fuels are required to fulfill criteria such as no or less release of carbon dioxide (CO₂), suitability for both mobile and stationary applications and affordable price range (Romagnoli et al., 2011). Hydrogen is one of such candidates (Cetinkaya et al., 2012). It has several advantages associated with its use: can be produced using renewable energy resources; has high yields in fuel cells; undergoes clean combustion without emissions of CO₂ and oxides of nitrogen and sulfur (NO_x, SO_x); and makes the indirect storage of the intermittent renewable energy resources feasible (Balat, 2008, Muradov and Veziroglu, 2008).

Like electricity, hydrogen is an energy carrier and not a primary energy source. Hydrogen is so far mainly used for non-energetic purposes in different industrial applications (>95% of global hydrogen production). Ammonia production is the largest consumer sector (about 62.4%). Its energetic use is very small (Spath and Mann, 2001). Some authors state the global hydrogen production amount of about 500 billion Nm³/yr (Saur, 2008).

Although there might be a mass production of hydrogen using renewable energy in the long term, fossil fuels are the major sources for its production today (Dufour et al., 2011). Iea, 2006, globally, the hydrogen production sources were about 48% from natural gas, 30% from fossil oil, 18% from coal and the rest with electricity via water electrolysis (PE International, 2010). This share may not change drastically in a near future; though coal, the most abundant primary energy source in many countries worldwide, might have a higher share than natural gas. Electrolytic hydrogen production could also be in focus, if the hydrogen is produced using renewable energy generated electricity.

Although hydrogen is generally considered as a clean fuel during its use phase (direct combustion or use in fuel cells), its production has negative impacts to the environment. Examining resource consumptions, energy requirements and emissions from a life cycle point of view gives a complete picture of the environmental burdens associated with hydrogen production (Spath and Mann, 2004). In this regard, hydrogen production can be categorized into three phases: plant manufacturing and installation (hardware), plant operation (energy used to operate the plant as well as feedstock for hydrogen production), and the storage and/or delivery of the produced hydrogen (use phase could also be included as the fourth phase). The environmental impacts associated with hydrogen production in almost all technologies, i.e. from steam methane reforming to electrolysis, are mainly in the plant operation phase. In steam methane reforming it is due to the consumption of natural gas as feedstock, and in electrolysis it is due to the use of fossil fuel dominated grid electricity to operate the electrolyzer. In electrolysis process, these impacts can be minimized when hydrogen is produced using the electricity from renewable resources (a schematic process flow diagram for electrolysis based on renewable energy electricity is shown in Fig. 4). During the operation of such systems there are almost none or less direct emissions. However, manufacturing and installation of renewable energy power plants are responsible for environmental burdens. Life cycle assessment (LCA) helps to understand such impacts in different phases of the process chain.

LCA is an established and internationally accepted method that is defined in ISO standards: ISO 14040 (ISO, 2006a) and ISO 14044 (ISO, 2006b). Life cycle refers to the activities during the product's lifetime from its manufacturing, use and maintenance to its final disposal, including the raw material acquisition required to manufacture the product (Curran, 2006, Iso, 2006a). For the evaluation of environmental effects, there are different impact assessment methods in use (Frischknecht and Jungbluth, 2007). Among them CML 2001 (Guinee, 2001) and eco-indicator 95 (Goedkoop, 1997) methods are mostly used in the studies reviewed under this paper. European Union has developed and recommended a guideline to carry out the LCA of hydrogen production technologies (FC HyGuide, 2011). This method also complies with the ISO series: 14040 and 14044. Since this guideline has been developed recently, its use in LCA analysis has not been noticed in the published literatures yet.

The aim of this paper is to analyze the environmental effects of different hydrogen production routes using LCA with the help of published literatures in the field. For this purpose, an intensive literature search was carried out. LCA related papers and reports on hydrogen production technologies were in focus of the review. Altogether, twenty one studies (Table 2) consisting of 17 peer-reviewed papers and 4 reports or pre-prints were selected for a thorough review. Regionally, articles from every region of the world were considered. In terms of publication dates, articles and reports published between 2000 and 2012 were considered. An extensive review provides state of the art knowledge on LCA of current hydrogen production routes. Although the paper's focus is to analyze electrolytic hydrogen production technologies, other non-electrolytic technologies (e.g. conventional production routes using fossil fuels) are also included in the review for comparison of the data/results.

The following Section 2 introduces the different electrolytic hydrogen production technologies. In Section 3, the studies under review are classified based on publication years, region, technical aspects of hydrogen production plants and LCA methodological aspects of these studies. Detailed review results on LCA of hydrogen production technologies are presented in Section 4. Section 5 concludes the main body of this paper.