*Article* **Multidisciplinary Assessment of a Novel Carbon Capture and Utilization Concept including Underground Sun Conversion**

**Andreas Zauner <sup>1</sup> , Karin Fazeni-Fraisl <sup>1</sup> , Philipp Wolf-Zoellner <sup>2</sup> , Argjenta Veseli 1,3,\* , Marie-Theres Holzleitner <sup>1</sup> , Markus Lehner <sup>2</sup> , Stephan Bauer <sup>4</sup> and Markus Pichler <sup>4</sup>**


**Abstract:** The current work investigates the feasibility of a novel Carbon Capture and Utilization (CCU) approach—also known as Underground Sun Conversion (USC) or geo-methanation. The overall objective of the current work is a comprehensive assessment on the technical, economic and legal aspects as well as greenhouse gas impacts to be concerned for establishing USC technology concept. This is achieved by applying multidisciplinary research approach combining process simulation, techno-economic and greenhouse gas assessment as well as legal analysis allows answering questions about technical, economic feasibility and greenhouse gas performance as well as on legal constraints related to large scale CCU using geo-methanation in depleted hydrocarbon reservoirs. CO<sup>2</sup> from the industry and renewable H<sup>2</sup> from the electrolyser are converted to geomethane in an underground gas storage and used in industry again to close the carbon cycle. Process simulation results showed the conversion rates vary due to operation mode and gas cleaning is necessary in any case to achieve natural gas grid compliant feed in quality. The geomethane production costs are found to be similar or even lower than the costs for synthetic methane from Above Ground Methanation (AGM). The GHG-assessment shows a significant saving compared to fossil natural gas and conventional power-to-gas applications. From a legal perspective the major challenge arises from a regulative gap of CCU in the ETS regime. Accordingly, a far-reaching exemption from the obligation to surrender certificates would be fraught with many legal and technical problems and uncertainties.

**Keywords:** power-to-gas; geo-methanation; CCU; life cycle assessment; large-scale energy storage
