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).''

Describe the purpose and focus of the paper:''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 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''

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],''

Which of the following is a renewable energy source commonly used in developing countries?*1 pointNatural GasCoalSolar PowerNuclear Power

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''