Life Cycle Assessment studies on Power-to-X: A review of recent findings

The global need for reduction of greenhouse gas emissions forces a growing number of countries to shift from fossil to renewable energy carriers. Due to the volatility of their power generation a subset of renewable energies is referred to as variable renewable energy [1]. The expansion of electricity generation by variable renewable energies, especially wind and photovoltaics, could exceed the demand [2] and absorption capabilities of electricity grids, in future. These situations go along with so-called surplus electricity generation. To avoid the curtailment of this electricity and to ensure grid stability [3] it has to be converted into a storable form of energy. Power-to-X options are considered to be promising energy storage technologies for this purpose and a driving force for coupling of sectors (e.g. power, industry, and transport) [4]. Several Power-to-X studies consider surplus electricity to be the basis of Power-to-X process chains [5-7]. However, there is also a multitude of studies taking into account electricity from conventional power plants or region-specific electricity mix to be initial point of these process chains [8-11]. Consequently, Power-to-X is here regarded as a multistage process chain that can be based on any type of electricity input. Already existing studies about these Power-to-X projects mainly focus on technological or economic assessments [12-14]. Beyond that a limited but continuously growing number of publications address environmental impacts of Power-to-X (e.g. [6-8, 10]).This study presents a systematic and international literature review of 13 relevant scientific publications at the moment conducting Life Cycle Assessment of Power-to-X options. Beside scientific papers reports and conference proceedings are taken into account. Publications are only considered if they contain Power-to-X terms and accompanying content. As Power-to-X options within considered case studies are not directly comparable they are classified according to their nomenclature and level of aggregation. The considered Power-to-X options of assessed studies, their relevant steps and classification possibilities are illustrated in Figure 1.Power-to-Transport (Mobility) represents the highest aggregation level of Power-to-X within considered studies. In contrast, there are also more itemized assessments considering the product level. On the lowest level of classification it can be distinguished between Power-to-Methane, Power-to-Hydrogen, Power-to-Syngas, Power-to-Methanol, and Power-to-Heat. Some studies additionally name and assess Power-to-Gas or Power-to-Fuel on the intermediate classification level. The present study only considers Power-to-X options if they are named accordingly and set into the context of Power-to-X. For example several review studies assessed the energy conversion of electricity into hydrogen via electrolysis with an environmental focus [15, 16] but without considering this to be Power-to-Hydrogen. Consequently, LCA studies of electricity-based hydrogen production without Power-to-X focus are not part of the present study.Aim of this review is to point out lessons learned about the environmental performance of Power-to-X options and their determinants. For this purpose relevant assumptions (e.g. about technical parameters and scope) are identified and their life cycle-based effects on environmental impacts are pointed out. The sensitivity of results is shown for different parameters (e.g. type of electricity input, power rating, CO2 source or type of electrolysis). The results of the literature are discussed and research gaps detected. Initial results revealed Power-to-Methane to be the most frequently assessed Power-to-X option and global warming potential (GWP) to be the most analyzed environmental impact category. Additionally, a dominating share of relevant LCA studies from European countries has been detected.References[1] L. Hirth, I. Ziegenhagen, Balancing power and variable renewables: Three links, Renewable and Sustainable Energy Reviews 50 (2015) 1035-1051.[2] H. Lund, E. Münster, Management of surplus electricity-production from a fluctuating renewable-energy source, Applied Energy 76(1–3) (2003) 65-74.[3] M. Bailera, P. Lisbona, L.M. Romeo, S. Espatolero, Power to Gas projects review: Lab, pilot and demo plants for storing renewable energy and CO2, Renewable and Sustainable Energy Reviews 69 (2017) 292-312.[4] M. Robinius, A. Otto, P. Heuser, L. Welder, K. Syranidis, D.S. Ryberg, T. Grube, P. Markewitz, R. Peters, D. Stolten, Linking the Power and Transport Sectors-Part 1: The Principle of Sector Coupling, Energies 10(7) (2017).[5] P. Collet, E. Flottes, A. Favre, L. Raynal, H. Pierre, S. Capela, C. Peregrina, Techno-economic and Life Cycle Assessment of methane production via biogas upgrading and power to gas technology, Applied Energy 192 (2017) 282-295.[6] A. Sternberg, A. Bardow, Power-to-What?–Environmental assessment of energy storage systems, Energy & Environmental Science 8(2) (2015) 389-400.[7] V. Uusitalo, S. Vaisanen, E. Inkeri, R. Soukka, Potential for greenhouse gas emission reductions using surplus electricity in hydrogen, methane and methanol production via electrolysis, Energy Conversion and Management 134 (2017) 125-134.[8] D. Parra, X.J. Zhang, C. Bauer, M.K. Patel, An integrated techno-economic and life cycle environmental assessment of power-to-gas systems, Applied Energy 193 (2017) 440-454.[9] A. Sternberg, A. Bardow, Life Cycle Assessment of Power-to-Gas: Syngas vs Methane, Acs Sustainable Chemistry & Engineering 4(8) (2016) 4156-4165.[10] S.B. Walker, M. Fowler, L. Ahmadi, Comparative life cycle assessment of power-to-gas generation of hydrogen with a dynamic emissions factor for fuel cell vehicles, Journal of Energy Storage 4 (2015) 62-73.[11] X.J. Zhang, C. Bauer, C.L. Mutel, K. Volkart, Life Cycle Assessment of Power-to-Gas: Approaches, system variations and their environmental implications, Applied Energy 190 (2017) 326-338.[12] M. Kopp, D. Coleman, C. Stiller, K. Scheffer, J. Aichinger, B. Scheppat, Energiepark Mainz: Technical and economic analysis of the worldwide largest Power-to-Gas plant with PEM electrolysis, International Journal of Hydrogen Energy 42(19) (2017) 13311-13320.[13] D. Parra, M.K. Patel, Techno-economic implications of the electrolyser technology and size for power-to-gas systems, International Journal of Hydrogen Energy 41(6) (2016) 3748-3761.[14] S.B. Walker, D. van Lanen, M. Fowler, U. Mukherjee, Economic analysis with respect to Power-to-Gas energy storage with consideration of various market mechanisms, International Journal of Hydrogen Energy 41(19) (2016) 7754-7765.[15] R. Bhandari, C.A. Trudewind, P. Zapp, Life cycle assessment of hydrogen production via electrolysis – a review, Journal of Cleaner Production 85 (2014) 151-163.[16] A. Valente, D. Iribarren, J. Dufour, Life cycle assessment of hydrogen energy systems: a review of methodological choices, The International Journal of Life Cycle Assessment 22(3) (2017) 346-363