# Performance of a Supercritical CO2 Bottoming Cycle for Aero Applications
## 1 Introduction  
Faced with a highly competitive marketplace, continually increasing passenger traffic [1; 2] and ever-more demanding environmental constraints, aircraft and engine design and technology will need to make even greater strides toward higher efficiency in the coming decades. In its Strategic Research and Innovation Agenda 2050 (Flightpath 2050) [3], the Advisory Council for Aviation Research and innovation in Europe (ACARE) has set very ambitious goals for aircraft for entry into service (EIS) in 2050. These goals include reductions of 75% in CO2 and 90% in NOx relative to EIS2000 technology. Many studies in recent years, in trying to meet similar targets, have focussed on developing novel aircraft concepts [4] with advanced, though conceptually conventional core engine architectures [5]. An alternative approach is the development of advanced tube and wing aircraft together with ultra-efficient core engine technology. A possible upside of this approach may be higher social acceptance of the more conventional looking new generation of aircraft. The \"Ultra Low Emission Technology Innovations for Mid-century Aircraft Turbine Engines\" project (ULTIMATE) [6] is a collaborative effort of ten leading industrial and research institutions across Europe [7]. ULTIMATE is part of the European Commission's Horizon 2020 programme and is focussed on the development of novel core technology concepts for ultra-efficient engines to meet thegoals of Flightpath 2050. These concepts aim to reduce the extent of \"The Big Three\" loss sources of modern aero engines: the combustor loss, the core exhaust loss and the bypass flow loss [6]. Table 1 lists the overall goals of Flightpath 2050. It also shows the shares of those goals that are to be achieved through improved aircraft and engine designs overall and how much of those benefits are supposed to arise from the novel concepts of ULTIMATE. One such technological concept to mitigate the core exhaust losses is to add a closed-circuit Bottoming Cycle, featuring supercritical carbon dioxide (S-CO\({}_{2}\)) as its working fluid. Bottoming Cycles are already proven to enable a step change in efficiency for gas turbines, when applied to stationary power generation plants. In a combined cycle gas turbine (CCGT) power plant, part of the residual heat in the turbine's exhaust gases is transferred to a secondary power cycle that uses the low grade heat to generate additional shaft power. The vast majority of these Bottoming Cycles use steam as the working fluid. As a result, state-of-the-art CCGTs can achieve a thermal efficiency in excess of 61% [8], an approximate increase of 20%, which would be impossible in a standalone gas turbine cycle. In stationary power applications, combined cycle power plants dramatically outperform recuperated gas turbines. Whether the same will be true for weight and drag constrained aero engine designs, is yet to be determined. Bottoming Cycles for aero application have been a largely overlooked concept to date, as most of the studies predicted the machinery and heat exchangers involved would be too large and heavy. In 1996, [PERSON] et al. [9] proposed an Air Bottoming Cycle (ABC) to enhance the efficiency of an aero-derivative LM2500 PE in off-shore application. While successful in raising the efficiency from 36.1% for the LM2500 PE to 46.6% in the combined cycle, the bottoming cycle had a \"power density\" of only 43 kW/t. This is more than two orders of magnitude below the power density of the main engine. [PERSON] et al. prioritised cycle efficiency over compactness and designed high effectiveness heat exchangers which would be too big for aero applications. Even with smaller heat exchangers, the low power density of an ABC could be a limiting factor at low peak temperatures. [PERSON] et al. [10] recently investigated a turbofan with an integrated open air bottoming cycle, which promised a potential efficiency gain of 5%-8% relative to a projected EIS2025 turbofan.