Methods and Tools: Explain the methodologies and tools used in the studies outlined in the paper:''2 Increasing Awareness of Power-to-X With the project ‘‘Strategieplattform Power-to-Gas’’ [8], which was initiated in 2011 with the objective to improve and promote Power-to-gas technologies, the terminology Power-to-[K] has been used increasingly in the scientific community during the last years (Fig. 1). The extensive presence of this terminology indicates an increasing interest in related technologies, such that the terminology was also included in the German energy strategy [9], which high- lights its relevance. The more general term Power-to-X was introduced later on to combine all related Power-to-[K] technologies under one common terminology. This termi- nology is often used in the context of sector coupling, where Power-to-X technologies are considered a key element. Interestingly, there is a correlation between the number of publications on Power-to-[...] technologies and the amount of compensation payments for curtailed energy in Germany, which have both increased seemingly exponen- tially in the last decade (Fig. 1). Taking the number of publi- cations as an indicator for public investments into R&D, the correlation may indicate the high effort in avoiding elec- tricity curtailments by investing in Power-to-X technolo- gies. This gets even more plausible by interpreting the much steeper increase of compensation payments for curtailed electricity compared to the linear increase in share of RES in total electricity produced: the potentially usable energy from RES seems to be higher than what the current (grid) technology can handle without curtailment. To identify the key interests in Power-to-X technologies, recent studies are categorized into their main object of investigation and examples for each field are given in the following. Many early publications on Power-to-X evaluate to which extent the integration of a distinct technology into the ener- gy system [13–15] and process industry [1, 16, 17] is benefi- cial, and how they influence one another. Typically, future energy scenarios are developed, and the technology is as- sessed based on performance, economic, and environmental indicators. Independent of the sector the technology is applied in, the authors highlight the need for a consistent and predictable energy policy for a successful integration. Coupled with technological progress, such technologies may even become economically attractive. Concerning the environmental impacts of Power-to-X technologies, Koj et al. [18] give a review of life-cycle assessment (LCA) studies based on their understanding of Power-to-X.The question about the integration of Power-to-X into an existing energy system or industry intrinsically includes the question which Powerto-X technology is suited best for which energy scenario. Most publications consider Power-to-gas [19–23], Power-to-liquid, often also referred to as Power-to-fuel [24–27],''

Methods and Tools: Explain the methodologies and tools used in the studies outlined in the paper.''Most publications consider Power-to-gas [19–23], Power-to-liquid, often also referred to as Power-to-fuel [24–27], 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''

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 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.'' 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].''

introduction:'' On a global scale, the share of renewable energy sources (RES) in electricity production is small but growing contin uously. To prevent economic losses and idle green resources caused by energy curtailments, corrective actions need to be taken. Moreover, RES must become accessible for sectors that still heavily rely on fossil resources, i.e., chemical indus try, heating, and transportation [1], if global climate targets are to be met. In this context, technologies termed Power-to-[K], such as Power-to-gas, Power-to-chemicals, or Power-to-fuels, have attracted increasing interest in recent years (cf. Sect.2). The terminology has been used for an ever-increasing num ber of applications, and the large diversity of applications associated with this terminology has resulted in the term Power-to-X. However, we are not aware of an established definition about what the X may or may not include. Look ing at most instances of Power-to-X concepts, these aim at converting electricity into gases, liquids, heat, fuels, or even back from those into electricity [2–7]. We believe that in addition to the new aspect of bringing renewable electricity into production processes to replace fossil-based products, many recently proposed technologies under the term Power-to-X are closely related to the much older concepts of electricity storage and demand side management (DSM). Therefore, a broad definition of Power-to-X as processes with the goal to exploit the environmental and economic potential of renewable electricity is proposed. This explicitly encompasses electricity storage and DSM as well as the newer aspect that we call e-Production. In the following, first, a brief overview of the literature is given (Sect.2). Then, details on the definition and classification of Power to-X are provided, before key challenges and benefits for Chem. Ing. Tech. 2020, 92, No. 1–2, 1–12 given external conditions are discussed (Sect.3). Illustrative examples that demonstrate how process systems engineer ing (PSE) methods support overcoming these challenges are also given (Sect.4). Finally, the most important findings and still open questions are summarized (Sect.5).''