Methods and Tools: Explain the methodologies and tools used in the studies outlined in the paper.''4 The Role of PSE in Power-to-X – Illustrative Examples In the following, examples for DSM, e-Production, and electricity storage as well as examples in between these extremes are discussed. In particular, it is demonstrated how PSE methods can help to address some of the challenges highlighted above. 4.1 Chlor-Alkali Electrolysis – DSM by Oversizing and Operational Mode Switching CA electrolysis is an electricity-intensive technology, e.g., amounting for 4.25 % of the total German industrial electricity consumption in 2017 [62]. Half of the total production costs are due to electricity consumption [63]. Therefore, DSM of CA processes seems promising and desirable. CA electrolyzers are technologically well fit for variable operation, i.e., variation of the production level, because of their fast dynamics [64]. However, industrial CA electrolyzers are mostly operated at more than 95 % capacity utilization [65]. Thus, further investment costs for oversizing the process are incurred for providing operational flexibility. Additionally, Cl2 is utilized downstream, thus, either necessitating to operate also these downstream processes dynamically or to temporarily store Cl2. Increasing the storage capacity is very costly or even prohibited due to the strict regulations for ensuring safety [66].''

Methods and Tools: Explain the methodologies and tools used in the studies outlined in the paper.''3 Beneficial Utilization of Electricity by Power-to-X Here, Power-to-X processes are defined as processes with the goal to exploit the environmental and economic potential of renewable electricity. This comprises the production of gases and liquids from renewable electricity as well as the provision of heat with the intention to replace fossil-based products, which is called e-Production. Additionally, this definition encompasses the older fields of electricity storage and DSM. With this definition, Power-to-X is not restricted to technologies for the conversion of predominantly electric power into products that are currently based on fossil sources. With DSM, it also incorporates approaches that enable the utilization of renewable electricity for industrial processes that have not been able to utilize such in an effective way until now. We believe that such a broad definition is useful to highlight the relationship and overlap of these fields that become particularly apparent in many Power-to-X technologies falling into more than one of these three categories (gray areas in Fig. 3). We explicitly confine our definition to processes that aim at utilizing renewable electricity and exclude, e.g., general energy management technologies from our definition of Power-to-X. However, contrary to their intended purpose, associated technologies could in principle also utilize other types of energy sources, e.g., for producing synthetic fuels from nuclear power in one country and transport it to another one. In the following, these three main approaches for a beneficial utilization of renewable electricity are briefly introduced, their relations are identified, and their opportunities and common challenges are stated''

Methods and Tools: Explain the methodologies and tools used in the studies outlined in the paper'' 5 Summary and Outlook In this review, Power-to-X technologies are defined more generally than in conventional literature as processes with the goal to exploit the environmental and economic potential of renewable electricity. According to this definition, such technologies comprise DSM, e-Production, and electricity storage. These different approaches for the common goal of utilization of electricity with a high share of RES offer different economic and environmental benefits depending on average electricity prices and fluctuations as well as depending on the electricity average carbon footprint and fluctuations, but they share common challenges. DSM is worthwhile especially for electricity-intensive processes: for CA electrolysis, electricity costs can be saved if variable operation is implemented and oversizing considered. However, the dynamic operation also brings challenges, e.g., limitations on Cl2 storage. It is demonstrated how such a limitation can be overcome by a novel modeswitching operation. For e-Production, two main challenges are identified: technology selection and efficiency improvement. Optimization-based methods give insight into conflicting performance indicators of competing products and processes, e.g., production cost and GWP reduction for e-fuel production, and support decision-making systematically. An additional system-level process analysis using detailed models reveals bottlenecks and can finally lead to significant efficiency improvements as well as GWP and cost reductions. For the ammonia-based electricity storage, optimization-based process design enables a round-trip efficiency of the combined synthesis process that can even exceed the one of electricity storage based on H2 directly. Despite many challenges that have been addressed successfully by PSE methods, there are still open questions and challenges left. Particularly relevant for electrolysis, i.e., a key element of many Power-to-X technologies, the risk of deteriorating equipment lifetime due to flexible operation of Power-to-X processes needs to be investigated. Research regarding this via both experiments and simulations is ongoing; however, long-term compatibility of the material under highly dynamic operation has not been demonstrated conclusively yet. In addition, uncertainties due to external parameters influence decision-making regarding Power-to-X technologies significantly and must be addressed in process design and operation. Especially, uncertainties in future electricity price and carbon footprint in both short and long terms need to be accounted for via prediction methods. This will play a key role for a successful implementation of Power-to-X technologies. All these challenges need to be tackled by further research''

Methods and Tools: Explain the methodologies and tools used in the studies outlined in the paper. ''The common understanding of Power-to-X is exclusively the use of renewable electricity to manufacture products currently based on fossil sources. In this paper, it is argued that beyond such e-Production many of these technologies also include aspects related to demand side management and temporal storage of electricity. Therefore, a definition of Powerto-X is suggested that encompasses all three aspects. It is discussed, which of these are relevant under which conditions and illustrative examples are highlighted, which show how process systems engineering can help address common challenges for Power-to-X technologies.''

Methods and Tools: Explain the methodologies and tools used in the studies outlined in the paper:''Powerto-heat [28], and Power-to-chemicals [7] individually, some of them also collectively [6, 29]. The publications typically consider different performance indicators, i.e., economics, sustainability, efficiency, infrastructure, technology readiness level, etc., as well as a variety of boundary conditions to provide a holistic assessment of the technology. The diversity in boundary conditions and assumptions makes the comparison between the publications difficult. Going through the broad range of different literature on Power-to-X technologies, it is noticeable that their applications in most cases aim at the conversion of predominantly electric power into products that are currently based on fossil energy sources. Therein, the scientific community distinguishes between the product’s state, i.e., Power-to-gas or Power-toliquid, and the product’s intended purpose, i.e., Power-tofuel or Power-to-heat. This product-oriented classification has two shortcomings. First, the notations are ambiguous. Either of the terms Power-to-liquid and Power-to-fuel are used for the same notion. Additionally, ‘‘liquid’’ may refer to fuels, a chemical feedstock, or both, and ‘‘fuel’’ may refer to a liquid, a gas, or both. However, the intended purpose of the product or – even more precisely – of the Power-to-X application is very important for its environmental assessment [6, 29]. Second, the broad usage of the terminology Power-to-[K] and the missing definition lead to the question which technologies to include and which not. For instance, there are DSM endeavors in which some electricity-intensive industrial processes are explicitly operated flexibly to make them utilize renewable electricity effectively (cf. Sect. 3.1). Such approaches are commonly not considered Power-to-X technologies, although flexible operation is a key aspect in many Power-to-[K] processes, and they pursue the same goal of improving the utilization of renewable electricity. This indicates a connection to more established process concepts that were excluded in the keyword search illustrated in Fig. 1. To indicate the increasing interest also in those, Fig. 2 exemplarily shows the number of publications related to some of them using a selection of keywords. In addition to the increasing general interest in these established technologies, the high amount of literature on demand side management with regard to renewable electricity utilization highlights its relevance for Power-to-X processes. Therefore, a broader definition of Power-to-X and a classification based on the way Power-to-X pursues the common goal of a beneficial utilization of renewable electricity are proposed. 3 Beneficial Utilization of Electricity by Power-to-X Here, Power-to-X processes are defined as processes with the goal to exploit the environmental and economic potential of renewable electricity''

Methods and Tools: Explain the methodologies and tools used in the studies outlined in the paper.'' 3.6 Challenges for Successful Implementation of Power-to-X Technologies Power-to-X technologies face a number of challenges for successful implementation, some of which differ from those of classical process systems. Here, technical challenges are discussed and political boundary conditions, e.g., carbon tax or climate targets [49], social acceptance, e.g., preferences for potential Power-to-X products [50], and similar issues are excluded. 3.6.1 Process: Efficient Flexible Operation Power-to-X processes are energy-intensive by definition, and therefore, energy efficiency is of prime importance both for economic and environmental reasons [6, 48, 51]. Beyond high efficiency at a single design point, Power-to-X processes aiming at DSM or electricity storage need to retain high efficiency over a wide range of operating conditions to be able to operate according to electricity price or some other signal for availability of electricity [33] instead of operating at steady-state like today’s process systems. Such changes in operating conditions also need to occur sufficiently fast to follow the changes in electricity supply in the desired time scale. Processes with inherently fast dynamic response can thus easily be used for DSM by appropriate scheduling, e.g., seawater reverse osmosis [41]. Other processes exhibiting slower dynamic response, such as ASU [36], demand advanced control strategies to enable flexible operation for DSM. Additional challenges are associated with the implications of flexible operation on the performance and lifetime of plant equipment and, in particular, catalysts that are not fully understood yet [52].''