Comparative life cycle assessment of power-to-gas generation of hydrogen with a dynamic emissions factor for fuel cell vehicles
Introduction
Power-to-gas, an energy storage technology that provides flexibility and integration to the electrical and natural gas grids, can also be used to produce green hydrogen for fuel cell vehicles. Although hydrogen is currently used mainly as an industrial commodity for the production of ammonia and petroleum, it is clearly becoming a viable solution to use hydrogen as an alternative fuel [1]. At this time, the Department of Energy’s Office of Transportation Technology and the automotive manufacturers clearly see fuel cell vehicles as the path to hydrogen vehicle commercialization vehicles likely to be brought online in the 2015-2020 timeframe [2]. As nations and provinces move to monetize their reduction of CO2 emissions, through carbon taxes and cap-and-trade programs, the market for this technology will only increase. An excellent technology for the production of hydrogen is power-to-gas [3], [4], [5], [6]. Power-to-gas is an effective way to generate emissions-free hydrogen fuel while utilizing cleaner power and can also compliment the function of the electrical grid by providing load flexibility and provision of other ancillary services. As the sources of energy into the electrical grid have changed substantially in many jurisdictions from more carbon-based sources to an increasing amount of intermittent, renewable energy sources that compliments significant a significant base load of nuclear, thus there is a definite need to provide this flexibility. Energy storage through hydrogen generation is a clear alternative.
Another important advantage of power-to-gas is its ability to provide energy storage which allows utilities to better manage excess base load power. When the majority of the electricity from the grid is generated by nuclear energy, there is a constant energy base [7]. At peak demand times, other sources of energy are drawn on to support the base load power and increase the energy supply. At off-peak times, however, the supply of energy exceeds the demand and, as nuclear facilities cannot easily reduce their output, the excess energy is exported to neighboring jurisdictions. In Ontario – the focus of this case study – these jurisdictions include Quebec, Manitoba, New York, New Jersey and Pennsylvania. In 2013, for example, approximately 18.3 TWh was exported to these jurisdictions at a significantly undervalued price by Ontario, potentially costing the utilities up to $1 billion in lost profits [8]. Some other energy storage technologies which can be considered to provide this type of support to the grid include flywheels, lead-acid batteries, lithium-ion batteries, compressed air energy storage (CAES) and pumped hydro energy storage [9], [10]. These energy storage technologies are compared in an earlier work by Walker et al. [11].
The main purpose of power-to-gas, however, is the production of hydrogen. In Fig. 1, below, a number of power-to-gas energy utilization pathways are shown. The supply of energy, on the left hand side, comes from the electrical grid and can be from specifically renewable sources, excess base load energy during off-peak periods or the basic electrical grid mix. Next, the electrolyzer takes water and splits it into H2 and O2. Following the H2 can either be injected into the natural gas infrastructure or stored by tank for immediate use. The H2 stored for immediate use can be used for industrial processes or to power hydrogen fuel cell vehicles, or vehicle fleets [12]. The rest of the H2 is added to the natural gas grid creating Hydrogen Enriched Natural Gas (HENG) [13]. In order to avoid the embrittlement of the materials the pipelines are made with, it is necessary for the H2 concentration to remain below 5% by volume at this time [14].
As discussed earlier, there are a number of advantages of using power-to-gas energy storage including a high energy density, the ability to transport the energy efficiently and the ability to store and distribute the energy in existing natural gas infrastructure for long periods of time. Additionally, the hydrogen can be used as a mixed gas with natural gas and shipped to existing natural gas customers.
As shown in Fig. 1, power-to-gas is a multi-faceted system which can have a number of different pathways from energy input to deliverable electricity or gas outputs. In addition to power-to-gas’s ability to provide efficient, high density energy storage, it can also be used to meet the demand for industrial hydrogen. Although the vast majority of hydrogen is currently used as a commodity, which makes up a $60 billion industry, the future of hydrogen production will be to provide a clean fuel for fuel cell vehicles [2], [15]. There have been a number of works that have considered the well-to-wheel (WTW) emissions of electrolytic hydrogen for vehicles [16], [17], [18]. Most notably McCarthy and Yang [19] consider electrolysis in a comprehensive fashion, however they do not consider different control strategies for the electrolysis operation, specifically control strategies based on an electrical grid dynamic emissions factor.
As billions of kilograms of hydrogen will need to be produced daily to meet the needs of hydrogen fuel cell vehicles, the potential for CO2 equivalent reduction through the substitution of petroleum with electrolytic hydrogen is quite large. In the pathway to producing industrial hydrogen, electricity is used to power an electrolyzer and the hydrogen is then sent by a dedicated pipeline to the customer. Due to the high demand that will accommodate the use of hydrogen as a transport fuel, substituting electrolytic hydrogen for hydrogen from the more commonly used steam methane reformatted hydrogen generation represents an excellent opportunity to reduce greenhouse gas emissions. For example, Simons and Bauer [20] find that in a grid mixture dominated by nuclear energy approximately 3–5 kg CO2 equivalent is emitted per kilogram of H2 produced by electrolysis, while other technologies, such as steam methane reformation, emit approximately 13 kg CO2 equivalent per kilogram of H2. Additionally, the electrolysis of hydrogen can be accomplished in significantly fewer energy-intense steps than steam methane reformation [21].
To encourage these types of reductions in CO2 emissions, a number of carbon reduction policy tools have been proposed. Carbon taxes have been proposed whereby organizations are made to a pay a premium for the carbon emissions that they are responsible for [22], [23]. One criticism of the carbon tax, however, is that it generally increases the costs of energy and transportation for consumers without encouraging innovation [24]. Cap-and-trade systems, however, have been successfully implemented in the European Union and North America [19], [25]. Cap-and-trade systems work on the premise that companies who are able to reduce their CO2 emissions to lower levels should be able to reap financial benefits. It does this by setting a ‘hard cap’ of global emissions from a region or nation and for individual businesses [26]. Under this system, companies who reduce their emissions below the hard cap are given permits that they are able to sell to other companies. Companies who are unable to meet the hard cap must purchase the permits from the market. The importance of carbon trading systems cannot be understated, however. By monetizing technologies that may be more expensive, but are capable of providing carbon reductions, cap-and-trade encourages technological improvements. As the province of Ontario transitions into a cap-and-trade system for industry, the analysis of how such a system could be used for hydrogen production by power-to-gas is of current interest [27]. Thus, Ontario is used as a case study in this analysis, although the results could be applied elsewhere. Also, this case study is useful for accessing the use of low emissions energy for utilizing electrolysis to produce H2 for HFCVs. In this work the simulated production of hydrogen via electrolysis occurs under different control strategies, specifically, continuous operation, price threshold operation, or operation based on an emission factor threshold is compared.
Section snippets
Methodology
Life cycle assessments (LCA) are an important tool for determining the environmental impact of a specific product or service over its life cycle by examining the emissions and resource consumption involved in the creation, use and disposal of the product. Typically an LCA will entail an analysis of all the process steps over the life of the product including the extraction of raw materials, processing of raw materials, and the manufacture of the product, the distribution of the product, its
Characterization of HFCV and ICEV
In Fig. 3 above, the two pathways are given for the production and use of both a HFCV and an ICEV. The arrows represent the movement between process steps while the dotted lines represent the system boundaries for the LCA. At the top of both path (a) and path (b) is an arrow that enters the system boundaries and connects to the vehicle production node. This arrow represents the connection of the steps within the boundaries to those steps which are not shown: raw material extraction and
Mobility fuel production
For the analysis of the WTW stage, in Table 1, emissions requirements for every MJ of fuel produced for the vehicle is given. Here the emissions levels to produce 1 kg of gaseous H2 are either taken from GREET and open source LCA tools or using the emissions factor for electricity [39], [40]. As can be seen here, the production of hydrogen produces more CO2, due to its reliance on the electrical grid. As expected, the amount of CO2 emitted from electrolytic hydrogen production is greatest when
Conclusions
Power-to-gas is an excellent energy storage technology which can be used to produce green electrolytic hydrogen for fuel cell vehicles. The life cycle of a fuel cell vehicle, with hydrogen fuel created through Power-to-gas, offers a significant reduction in greenhouse gases and criteria emissions when compared to internal combustion engine vehicles operating on gasoline. In this work the simulated production of hydrogen via electrolysis occurs under different control strategies, specifically,
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