SUPERCRITICAL CO2 COMPRESSOR AND EXPANDER DESIGN FOR INDUSTRIAL WASTE-HEAT VALORIZATION

Within the energy transition framework, it is believed that supercritical carbon dioxide (sCO(2)) power cycles can significantly contribute to climate change goals. Compared to other energy conversion technologies, higher efficiency can be achieved thanks to the minimized compression work, driven by the peculiar properties of sCO(2). Footprint reduction coupled with good operational flexibility and reduced water consumption are additional advantages. The present work discusses design challenges and solutions studied for an sCO(2) power system object of the CO2OLHEAT project, "Supercritical CO2 power cycles demonstration in Operational environment Locally valorizing industrial Waste Heat", European Union Horizon 2020 funded program (grant agreement # 101022831). The project aims at demonstrating the first MW-scale waste heat recovery system employing sCO(2) in the EU; the demo cycle will be installed in an existing industrial plant. Object of the authors' work includes the design and testing of a turboexpander driving the cycle compression phase. In this paper, the main challenges faced in the conceptual design phase of turbomachinery will be assessed. The centrifugal compressor, whose inlet conditions are in proximity to CO2 critical point to get the highest cycle efficiency, has been designed using specific equilibrium models and equation of state to both properly consider phase-change regions and strong gradients of thermodynamic properties. Regarding the expander, the paper will describe how the inlet conditions (pressure and temperature) represented specific challenges in particular for mechanical configuration and safe operation of crucial components such as the Dry Gas Seals (DGS), for which dedicated thermal analyses were conducted. The paper concludes with considerations on design choices for turbomachinery made to improve the operability of the complete cycle even in off-design conditions such as possible variations in minimum and maximum CO2 temperatures due to external factors.