Perfomance analysis of hybrid systems based on externally heayed closed-cycle engines

Abstract

This work presents a comparative analysis of hybrid systems that make use of closed-cycle externally heated bottoming systems. Two options are considered: reciprocating (Stirling) engines and supercritical carbon dioxide turbines. These engines share the common feature of working on closed cycles with optimised fluids (H2 and CO2 respectively). However, they differ in their internal structure: Stirling engines make use of volumetric machinery whereas the SCO2 system is composed by turbomachinery. In both cases, the working fluid is subjected to very high pressure and temperature in the range of 50-200 bar and 40-650 ºC. A brief description of both bottoming systems is provided in the article along with the expected performance of each case in on-design and off-design (part load) conditions. The analysis is therefore split into two stages. First, a comparison is shown for on-design operation aiming to evaluate the maximum efficiency attainable by the proposed systems. Second, a preliminary analysis of off-design operation is presented. The paper concludes that hybrid systems based on atmospheric fuel cells and externally heated closed-cycle bottoming engines have the potential to outperform conventional pressurised fuel cells and gas turbines hybrids while preserving the topping system from the demanding operating conditions of the latter configuration