Primarily released by the conversion of primary fossil energy sources, anthropogenic greenhouse gas emissions influence global warming fundamentally. Since they enable increasing the share of sustainable energy sources in the energy supply and reducing greenhouse gas emissions through targeted integration, power-to-X technologies promise to be an important element of compliance with impending regulations and laws. VDI 4663 guideline for strategically optimizing (technical) processes applies the physical optimum, a promising performance indicator for a unified, time-independent, and structured evaluation of power-to-X technologies that defines an operation under physically optimal conditions as a limit value. This study applies VDI 4663 to a power-to-X system and evaluates different components. It specifically examines current power-to-gas applications, the physical optimum as a limit-oriented indicator and its application to complex processes, the physically optimal operation of electrolysis and methanation, heat transfer as a critical component of methanation, the evaluation of a heat exchanger based on the physical optimum, and targeted process optimization based on VDI 4663. The outcome is an energy index for the evaluation of a heat exchanger, factoring in its structural design. The physical optimum of electrolysis and methanation developed here can also be employed as the basis for targeted optimization. This study serves as a basis for the evaluation of other power-to-X systems and introduces the application of VDI 4663. Additionally, the applicability of the physical optimum to chemistry-based processes is validated.

The challenges posed by climate change have prompted significant growth in efficiency evaluation and optimization research, especially in recent years. This has spawned a variety of heterogeneous methods and approaches to the assessment of technical processes. These methods and approaches are rarely comparable and are usually only applicable to specific sectors. This paper provides an overview of the literature on efficiency assessment methods and KPIs, leading to a more manageable selection of an appropriate method with special regard to energy system integration technologies. In addition to reviewing the literature systematically, this paper examines existing methods and indicators’ applicability to and significance for efficiency optimization. In this context, a holistic approach to process design, evaluation, and improvement is given with particular regard to power-to-X systems. Within the framework of the study, three overarching goals could be defined as levels of efficiency evaluation of power-to-X systems: 1. identification of the process (steps) with the most significant optimization potential, 2. identification of the process phases with the greatest optimization potential (timewise considered), and 3. derivation of specific recommendations for action for the improvement of a process. For each of these levels, the most suitable evaluation methods were identified. While various methods, such as life cycle assessment and physical optimum, are particularly suitable for Level 1 and Level 2, for Level 3, even the best-identified methods have to be extended on a case-by-case basis. To address this challenge, a new approach to a holistic evaluation of power-to-X systems was developed based on the study’s findings.

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