Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review
Introduction
Power-to-Gas (PtG) and Power-to-Liquids (PtL) are often discussed as important elements in a future renewable energy system (e.g. [1], [2], [3]). The conversion of electricity via water electrolysis and optionally subsequent synthesis together with CO or CO2 into a gaseous or liquid energy carrier enables a coupling of the electricity, chemical, mobility and heating sectors. This opens up enormous storage or absorption capacities for excess energy with high electricity generation from renewable energies in excess of demand. It also supports the integration of fluctuating renewables like wind and solar power in the energy system, including the provision of balancing power. By substituting fossil fuels, this can help to reduce greenhouse gas emissions in the mobility or chemical sectors. The future demand for Power-to-Liquids and Power-to-Gas energy storage represents an emerging market for electrolysis systems. Operating strategies such as the absorption of excess energy at limited operating times per year, providing grid services or arbitrage trading (exploitation of highly fluctuating electricity prices) are possible, which also could be combined [4]. This poses new requirements regarding efficiency, flexibility, part-load and stand-by performance, electrolyser capacity (multi MW to GW plants) and capital costs, depending on the specific application and operating strategies.
There have been several excellent reviews on electrolysis technologies in general [4], [5], [6], [7], [8] as well as on AEL (alkaline electrolysis) [9], [10], PEMEL (proton exchange membrane electrolysis) [11], and SOEL (solid oxide electrolysis) [12], [13], [14]. Moreover, the FCH JU (Fuel Cells and Hydrogen Joint Undertaking) under the EU's funding programme Horizon 2020 has implemented key performance indicators (KPIs) for flexible water electrolysis as a target and for monitoring their multi annual work programme (Table 1) [15].
The scope of this review is on commercial technologies and research related to flexible electrolysis operation and performance relevant for PtG and PtL applications. It provides an overview of the current status of water electrolysis on the way to large-scale flexible energy storage applications. After dealing with the fundamentals of water electrolysis, the major electrolysis technologies (AEL, PEMEL, SOEL) are compared with regard to the available capacity, nominal and part-load performance, flexibility (load range, load gradients, start-up time, stand-by losses) lifetime and investment costs. This comparison is based on the above-mentioned literature reviews, discussions with manufacturers, project reports and an extensive market survey of electrolysis suppliers.
Section snippets
Fundamentals of water electrolysis
The overall reaction of electrochemical splitting of water into hydrogen and oxygen by supplying electrical (and thermal) energy is given by:
The volumetric co-production of oxygen corresponds to half the production of hydrogen. The heat of reaction gives the overall energy demand of reaction ΔH, which can be partly supplied by heat (ΔQ) while another part (change in Gibbs energy ΔG), has to be supplied electrically:
As shown in Fig. 1, the overall energy demand ΔH
Water electrolysis technologies
Water electrolysis technologies can be classified according to the applied electrolyte, which separates the two half reactions at the anode (oxygen evolution reaction) and cathode (hydrogen evolution reaction) of the electrolyser [7]. The main water electrolysis technologies are Alkaline Electrolysis (AEL), Polymer Electrolyte Membrane Electrolysis (PEMEL) and Solid Oxide Electrolysis (SOEL). The principle layout, reactions and relating properties of AEL, PEMEL and SOEL are discussed in the
Market survey of electrolysis technologies
The electrolysis market is very dynamic with several fusions and acquisitions in recent years (e.g. McPhy acquired Piel (IT) and Hytec Enertrag (DE) in 2013, Areva H2Gen, resulting from the fusion of Areva Helion and CETH2 in 2014, Hitachi Zosen Inova (CH) acquired ETOGAS (DE) in 2016, NEL Hydrogen (NO) aquired Proton OnSite (US) in 2017, Accagen (CH) became EnerBlue in 2013). In this context, the companies H2Nitidor [63] (using Voltiana technology from Casale Chemicals) and Avalance [64] in
Nominal and part-load performance
Electrolysers feature an increase in performance in part-load as discussed before. Therefore, the efficiency of an electrolyser including part-load operation is best characterised by its I-U-curve (cell voltage vs. current density). After discussing the rated specific energy consumption of commercial electrolysers, a survey of exemplary I-U-curves for AEL, PEMEL and SOEL from literature is given in the following.
Rated efficiency and specific energy consumption of commercial electrolysis stacks
Conclusions
The main parameters of the water electrolysis technologies investigated (AEL, PEMEL and SOEL) are summarised in Table 3. Alkaline electrolysis (AEL) represents the most mature technology, having been commercial available for over a century. It has the lowest specific investment and maintenance costs. Twenty manufacturers of AEL could be identified that offer single-stack capacities up to 6 MW. Historically, AEL was designed for stationary applications and has to be adapted to the new flexibility
Acknowledgments
This work is related to the HotVeGas III project, which is supported by the BMWi and industrial partners (Air Liquide and RWE) under contract number 0327773I. The authors would like to thank all the manufacturers of electrolysis systems who we contacted for their valuable input and discussions (especially Mr. Hug of Wasserelektrolyse Hydrotechnik and Mr. Barisic of ELB). Thanks also go to the TUM Graduate School and Stephan Herrmann (TUM) for proofreading as well as Maximilian Möckl (ZAE
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