# Performance of a Supercritical CO2 Bottoming Cycle for Aero Applications
## 1 Introduction
This translated into a reduction in thrust specific fuel consumption (TSFC) of 3.7% at cruise, but the study did not calculate a weight increase or estimate a fuel burn benefit. [PERSON] et al. did however estimate the weight for a steam bottoming cycle option and found it cancelled out all of the efficiency benefits. [PERSON] et al. [11] showed Organic Rankine Cycles (ORC) can feature good cycle efficiencies with low grade heat sources. However, they note that most organic fluids feature critical points (\"_The critical point is the point at which the saturated liquid and the saturated vapour states are identical.\"_ as per [12]) below 600 K or often even 450 K. Consequently, to avoid exceeding the fluid's critical point when paired with heat sources between 650 and 900 K, the heat exchangers' effectiveness will need to be limited. This means only a fraction of the available heat energy will be recovered even if the mass flow of the bottoming cycle is increased. This, combined with the highly flammable nature of most organic fluids, challenges their feasibility in aero applications, where safety and compactness are vital. Finally, many popular organic fluids have a large global warming potential (GWP). R245 fa for example has approximately 950 times the GWP of CO\({}_{2}\)[13]. Nonetheless, the power density of ORCs is good for low grade heat applications and [PERSON] et al. [14] proposed an ORC bottoming cycle for a turbofan. That particular bottoming cycle did not produce additional shaft power for the main engine, but was \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Category**} & **Overall** & **Aircraft** & **From ULTIMATE** \\  & **Flightpath 2050** & **Specific** & **Technologies Concept Alone** \\ \hline Energy consumption and CO\({}_{2}\) emissions & \(-75\)\% & \(-68\)\% & \(-15\)\% \\ Total Mission and Cruise NO\({}_{\mathrm{x}}\) & \(-90\)\% & \(-87\)\% & \(-20\)\% \\ Noise per operation & \(-65\)\% (15 dB) & \(-15\) dB & \(-3\) dB \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Flightpath 2050 goals and target contributions of ULTIMATE concepts. designed to replace the conventional power generators for the environmental control system (ECS) of the aircraft and provide anti-icing. The study only predicted a 0.9% fuel burn reduction though this might have risen to 2.3%, if the new components could have been designed to be weight neutral to the reference engine, an unlikely scenario. Another study of an ORC added to an aero engine was presented by [PERSON] et al. [PERSON] et al. (2015). Here, the ORC cycle did not recover waste heat from the exhaust, but was conceptualised as an alternative intercooling concept, that also produced shaft power. The study estimated a possible 3% benefit in TSFC over conventional intercooling concepts, though the likely considerable impact of the installation's increase was not determined. Supercritical CO\({}_{2}\) cycles have been investigated since the 1960s [PERSON] et al. (2015), but technical limitations, both computational and physical, prevented major strides on the subject until the turn of the millennium. Since then, numerous studies have investigated and conceptualised various S-CO\({}_{2}\) cycles: as standalone power cycles for nuclear applications [PERSON] et al. (2015), in concentrated solar power plants [PERSON] et al. (2015) or for waste heat recovery [PERSON] et al. (2015). Many of these studies were conducted using established performance simulation tools such as NPSS [PERSON] et al. (2015), which were expanded in their functionality through supercritical CO\({}_{2}\) fluid property functions. These tools often do not include accurate physical representations of the components by default, but rather work under simplified assumptions such as prescribed effectivenesses and pressure losses in the heat exchangers. The creation of more detailed, though still computationally inexpensive, external models is therefore often necessary to improve the accuracy of the results.