Emission scenarios for a global hydrogen economy and the consequences for global air pollution
Highlights
► This study combines a global energy system model and a global atmospheric model. ► Global emissions of molecular hydrogen might range from 0.3 to 10% of energy use. ► Hydrogen decreases CO, NOX and SO2 emissions, but increases N2O and VOC. ► This decreases CO, NOx and O3 surface concentrations compared to reference scenario. ► H2 leakage leads to increase of methane lifetime and decrease of stratospheric ozone.
Introduction
Hydrogen is named as a possible, attractive, energy carrier for future energy systems (Azar et al., 2003, Barreto et al., 2003, Edmonds et al., 2004, Hedenus et al., 2010, van Ruijven et al., 2007, van Ruijven et al., 2008). Its strengths are that it can be produced from many primary energy sources, can be more easily stored than electricity, and allows for a very low emission of air pollutants during the end-use stage. Moreover, from a climate policy perspective, hydrogen-based energy systems allow for centralised production of hydrogen based on renewables and/or fossil energy combined with carbon capture and storage. As such, hydrogen-based energy systems form a direct alternative to electricity-based systems, and the choice between them depends on (expected) developments in technical improvements of relevant technologies and societal preferences.
The overall impact of a hydrogen-based energy system on atmospheric chemistry is uncertain. On the one hand, the use of hydrogen in clean fuel cells eliminates the end-use phase emission of air pollutants, such as sulphur dioxide (SO2) and nitrogen oxides (NOx). On the other hand, air pollutant emissions could still occur during hydrogen production (see Section 3). More importantly, some studies have claimed that large-scale emission of molecular hydrogen from system leakages could also lead to enhanced climate change (Derwent et al., 2006) or influence the chemical composition of the atmosphere (Price et al., 2007, Sanderson et al., 2003). Such changes may lead, for instance, to a severe reduction in stratospheric ozone concentrations (Tromp et al., 2003). Other studies, however, claim overall positive impacts on tropospheric air quality (Schultz et al., 2003). The uncertainty about the atmospheric consequences of large-scale hydrogen use is obviously important in deciding whether such a system would be attractive or not. In that respect, it should be noted that current studies into the air pollution impacts of large-scale hydrogen use tend to have used very stylised scenarios: they are not based on explicit modelling of the energy system or the emissions associated with it. This is a major limitation as the expected use of hydrogen, the intensity of leakages and emissions form major sources of uncertainty.
For the current paper, we explored the air pollution consequences of large-scale hydrogen use in more detail. The paper contributes to the existing literature in two ways. First, we systematically analysed the impacts of two determining factors on hydrogen emissions: (a) the hydrogen use and (b) the leakage rates, in order to find out which uncertainties could have a crucial influence on the outcomes. Second, we used realistic emission scenarios that fully describe the impact of large-scale hydrogen use in the energy-system, including emissions from hydrogen production and avoided emissions from substituted energy carriers.
For this purpose, we combined two different modelling systems: a global energy model and an atmospheric chemistry model. First, the role of hydrogen in the global energy system and the related emissions have been calculated using the global energy system simulation model TIMER which was used to develop a set of widely diverging scenarios with respect to hydrogen application (van Ruijven et al., 2007, van Ruijven et al., 2008). In the present study, these energy scenarios were combined with different assumptions on hydrogen emission factors. In a second step, the emission data were fed into the Community Atmosphere Model (CAM) (Lamarque et al., 2005, Lamarque et al., 2008) to determine the impacts on atmospheric chemistry.
In Section 2, we first introduce the applied models and scenarios. Next, in Section 3 we discuss the emissions associated with different steps in the hydrogen energy system. In Section 4, we subsequently discuss the impacts of these emissions on atmospheric chemistry, while finally in Section 5 the main conclusions are presented.
Section snippets
Methods
As a basis for the study, we used a set of energy scenarios with and without large-scale hydrogen application recently developed by van Ruijven et al., 2008, van Ruijven et al., 2007. In this section, we first briefly describe the models that have been used in this study, followed by a description of the energy scenarios.
Emission scenarios
In a hydrogen-based energy system using fuel cells, hydrogen emissions may occur during the production, distribution and use of hydrogen. Emission of other air pollutants, however, can only occur in the hydrogen production phase. The emission rates during the various phases, both for hydrogen and other pollutants, are discussed below.
Model calibration of the global atmospheric chemistry model
In the first simulation, we focused on the model performance in representing the present day H2 concentrations on the basis of current emissions. For this purpose, we used global observations of H2 concentrations at surface level, as used by Hauglustaine and Ehhalt (2002), extended to incorporate all observations (through 2006) available from the NOAA Earth System Research Laboratory.5 The results show that, when the Community Atmosphere Model
Discussion and conclusion
The current literature on the impacts of large-scale hydrogen application on air pollution presents very different results: while some studies emphasize an improvement of air quality, other studies emphasize a negative impact on, for instance, the total ozone column. It should, in this context, be noted that most of this literature is based on very stylised assumptions on future hydrogen use and associated emissions. In this article, we have therefore looked in more detail into the impact of
Acknowledgements
The authors thank Annemieke Righart for text editing. We also thank the other members of the IMAGE team for their contributions to this paper.
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