SCO2 Bottoming Cycle for Off-Shore Applications – An Optimization Study

Abstract

Abstract Closed Joule-Brayton thermodynamic cycles working with CO2 in supercritical conditions (sCO2) are presently receiving great attention, for their multiple attractive aspects: high energy conversion efficiency, compact size, flexibility of operation, integration with energy storage systems. These features make the sCO2 technology interesting for several energy and industrial sectors, including renewable sources and waste heat recovery. A further promising area of application of sCO2 systems is bottoming gas turbines in combined cycles, replacing steam. The use of steam implies large-scale components and demands for large space availability for the plant installation. In some relevant applications, such as in off-shore platforms, the lack of space complicates the implementation of steam combined cycles, unless accepting a significant efficiency penalty, along with operation issues related to the necessary use of saltwater as cooling medium as well as the need of continuous supervision by dedicated personnel. In such a context, the combination of gas turbines with sCO2 cycles could open the way for developing new efficient, compact, and flexible combined cycles, potentially suitable for off-shore systems but also attractive for several other sectors which might take advantage from the footprint savings, the enhanced flexibility, and the fast dynamics of sCO2 systems. In this work we investigate the thermodynamic potential of combining sCO2 cycles with an existing gas turbine for off-shore applications. In particular, we consider a mid-size (25 MW) gas turbine available on the market, and perform a series of thermodynamic optimizations of the sCO2 bottoming cycle in order to maximize the exploitation of the heat discharged by the gas turbine. Systematic optimization strategies are applied to the sCO2 cycle, considering four alternative configurations and including realistic technical constraints, evaluated by leveraging on the most recent technical outcomes from ongoing sCO2 research projects. A comparison is also proposed with a state-of-the-art steam Rankine cycle, in terms of system efficiency and footprint of the main components. The study clarifies the advantages and challenges of applying sCO2 in combination with gas turbines. The comparison with steam technology clearly indicates the relevance of sCO2 systems for off-shore applications and calls for further technical studies in the field.