The aircraft Environmental Control System (ECS) is the primary consumer of non-propulsive power at cruise conditions, hence, its performance optimization is crucial for the reduction of specific fuel consumption. A novel integrated system design optimization method is presented: thermodynamic cycle, component sizing and working fluid are taken into account simultaneously. This method was applied to the ECS of large rotorcraft based on a Vapour Compression Cycle system electrically driven by a high-speed centrifugal compressor. Steady-state and lumped parameter system component models have been developed using the Modelica acausal modelling language. The optimization design framework consists of an in-house code, featuring a Python-Modelica interface. The study case refers to a critical operating condition: the helicopter is on the ground during a hot and humid day. The working fluid is R-134a. The multi-objective optimization targets the maximization of the system efficiency and the minimization of system weight. The results show that more efficient systems can be designed only with heavier components. The design feasibility of high-speed centrifugal compressors is demonstrated. The advantage of an integrated system design optimization framework for complex energy systems is proved, allowing for the analysis of the impact of both component design and working fluid on system performance.

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The aircraft Environmental Control System (ECS) is the main consumer of non-propulsive energy, accounting for 3% of the total energy consumption among all the aircraft subsystems. The ECS efficiency can be improved by recurring to an electrically-driven Vapour Compression Cycle (VCC) system for cabin cooling. This work documents the detailed design and the commissioning of a novel experimental test rig, called Inverse organic Rankine cycle Integrated System (IRIS). The setup has been conceived for testing the performance of VCC systems and some of their components for aircraft ECS applications in different operating conditions, and for validating the numerical models developed for systems and components simulations. The facility implements a single-stage compression refrigeration cycle with two test sections: a volumetric compressor testing setup and an air-cooled condenser test bed. The evaporator is heated by a glycol-water mixture, warmed up in an independent loop. The design working fluid is R-1233zd(E). The successful commissioning of the facility is documented by discussing the data recorded during steady-state operation at the design operating point, together with the operation of the setup during start-up and shut-down procedures. The system cooling capacity is equal to 17.88 ± 0.8 kW, which is slightly higher than the design value of 15.5 kW. The difference has a positive effect on the system efficiency, which is 4% higher than the one calculated at design.

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The environmental emergency is one of the most critical challenges of modern times. The exponential increase of industrial activities is the primary cause of anthropogenic climate change, with negative consequences for the environment, society and economy. In the transport sector, decarbonization is the most significant challenge. In aviation, the environmental objectives are to halve international CO2 emissions by 2030, and to reach net-zero carbon emissions by 2050. The achievement of these goals implies a step-change in the traditional practice of aircraft design, with many resources invested in research for promoting the use of fossil fuel-free propulsion systems and electrified auxiliary systems.

In this framework, the research presented in this dissertation is on methods for automated design optimization with applications to a novel electrically-driven Environmental Control System (ECS) for aircraft cabin cooling. The ECS is the main consumer of non-propulsive power onboard aircraft. The founding idea of the project, which has been carried out in collaboration with several companies, is to replace the traditional ECS equipping airliners, which is based on Air Cycle Machine (ACM) technology, with Vapour Compression Cycle (VCC) systems powered by high-speed centrifugal compressors. Work performed within this project demonstrated that such a VCC-based ECS can be more efficient and lighter than an ACM-based ECS in the case of mainstream passenger airplanes. The main goal of the research documented in this dissertation is to provide design methods and guidelines for the optimal design of aircraft ECS whose core is a VCC system powered by novel high-speed electrically-driven compressors and using low-Global Warming Potential (GWP) working fluids in place of the conventional R-134a refrigerant. Additionally, the study encompasses the analysis of the impact of the selected working fluid on the optimal design of the main system components, i.e., the heat exchangers and the centrifugal compressor. For this purpose, a novel integrated design optimization framework has been developed: it allows to perform the multi-objective optimization of the aircraft ECS across different points of the aircraft operating envelope. This method enables the concurrent automated optimization of thermodynamic cycle, preliminary component sizing and working fluid selection. Moreover, the successful application of the method to this complex case demonstrates that the approach is generally applicable to any thermal energy conversion system.

The method was applied to the design of the ECS of two different aircraft to demonstrate its capabilities: a large passenger rotorcraft and a single-aisle short-haul aircraft, i.e., the A320. Results show that it is possible to design an efficient VCC system for aircraft ECS that is powered by an electrically-driven centrifugal compressor and uses low-GWP refrigerants as working fluids. In particular, in the case of a small-capacity ECS for large rotorcraft, it was demonstrated that the use of high-molecular complexity refrigerants, such as haloolefins, enables the design of lighter and more efficient VCC systems if compared to the state-of-the-art. The test case of the airliner ECS provided the specifications to further develop and test the methodology: a so-called physics-based equation of state model was adopted for the computation of the thermodynamic properties of the working fluid. Molecular parameters allow to define the fluid, therefore they can be optimized as part of the global optimization of the design of the system. Parameters are constrained so as to define a realistic molecule, though non-existing, called pseudo-fluid. Actual working fluids whose molecular parameters are similar to those of the optimal pseudo-fluid}are selected in a following step of the design procedure. Optimal working fluids are therefore natural refrigerants. The use of these working fluids with null GWP would reduce the environmental footprint of the considered environmental control systems, while enabling an (albeit small, in the considered case) reduction of specific fuel consumption.

Complementary to the numerical investigation, a novel experimental setup called IRIS (Inverse organic Rankine cycle Integrated System) was designed, realized and successfully commissioned at the Propulsion & Power laboratory of Delft University of Technology. The setup was conceived to enable testing and performance analysis of VCC-based aircraft ECS and to validate in-house software for system and components design. The setup hosts two main test sections: one to test compressors and another to test air-cooled condensers. The results of the commissioning show that it is possible to continuously operate the IRIS setup in steady-state conditions at temperature levels which are very close to those at the design point, thus achieving a Coefficient of Performance (COP) equal to 3.76±0.48. @en

The Environmental Control System (ECS) is the main consumer of non-propulsive power onboard aircraft. The use of an electrically-driven Vapor Compression Cycle (VCC) system, in place of the conventional air cycle machine, can lead to a substantial increase of the coefficient of performance. This work documents the development of an integrated design optimization method for VCC-based aircraft ECS, where the sizing of the system is performed along with the conceptual design of the compact heat exchangers and the high-speed centrifugal compressor. A data-driven model of the compressor has been developed to reduce the complexity of the VCC system model and the computational cost of the associated optimization problem. The model is based on artificial neural networks and has been trained on a synthetic dataset of 165k centrifugal compressor designs, generated with an in-house tool. The case study selected to demonstrate the capabilities of the proposed methodology is the multi-objective design optimization of an electrically-driven VCC system for the ECS of a single-aisle, short-haul aircraft, flying at cruise conditions. The results show that the number of function evaluations needed to identify the Pareto front reduces by a factor of three when using the data-driven model, in place of a meanline method. At the same time, the robustness of the numerical solver is improved, leading to the identification of optimal solutions covering a wider design space. Finally, the proposed methodology enables the analysis of the trends established between the system performance metrics and the design of the individual components.@en

This study concerns the assessment of a novel concept for the Environmental Control System (ECS) of large rotorcraft, based on Vapour Compression Cycle (VCC) technology. Its uniqueness stands in the adoption of an electrically driven high-speed 6-8 kW centrifugal compressor, in place of the traditional volumetric machine. A steady-state model of the system has been developed and implemented using the Modelica acausal modelling language. The working fluids selected for this investigation belong to the class of low-Global Warming Potential (GWP) refrigerants. The results show that to obtain feasible compressor designs, and in particular sufficiently large flow passages, a high molecular complexity fluid must be employed as it results in adequate volume flow rate. However, compared to fluids made of simpler molecules, heat exchangers are larger and possibly heavier. The tradeoff between thermodynamic performance, weight, volume, aircraft drag penalty and system integration in general is being investigated.

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