Multidisciplinary design and optimization of innovative electrical power systems for aerospace applications

Enrico Testa, Prof. P. Maggiore
2014
One of the main engineering research areas focuses on the development of new power sources technologies. Proton Exchange Membrane Fuel Cells (PEMFCs) promise low emissions, but also high power densities (W kg -1 ) if compared to conventional electrical batteries, in the meanwhile having the possibility to continuously operate if supplied with hydrogen, instead of needing to be charged as it is for batteries themselves. In the last few years, the electric energy demand has increased dramatically
more » ... worldwide: this is mainly due to the Industrialized Countries tending to absorb a constantly raising amount of energy, but also because of the rapid expansion of the developing countries. So far, about 85% of the energy need is satisfied by fossil fuels combustion releasing dangerous pollutants in the atmosphere, such as NOx, CO, HC. Moreover, the whole international scientific community considers the reduction of the gas emissions as necessary in order to preserve the current earth's climate. A sustainable development based on hydrogen as a clean energetic vector seems to be the best solution to date. All of these aspects make fuel cells attractive also for aerospace applications as candidate substitutes for both primary and secondary power generation systems. The use of fuel cells for aircraft is an ever-growing concept in today's environmentally conscious world. NASA studies have indicated that PEM fuels cells are becoming reasonably practical for propulsion in small aircrafts and Unmanned Aerial Vehicles (UAVs), and could be promising in future large-scale commercial aircraft. Dealing with space applications, the current efforts are devoted instead towards the so-called regenerative fuel cell system, consisting of a fuel cell stack coupled with an electrolyser stack used alternatively to generate and store electric power, in substitution of conventional secondary batteries. Primary batteries could also be easily substituted by fuel cells with the promise of lower weight. For these applications, it is important to understand the fuel cellbased power system from a system integration and power management perspective. Optimizing the integration of each component into the system and understanding the overall power system compatibility is essential for a successful design. Nevertheless, this foreseeing objective cannot be reached without a basic, robust, adaptable and multidisciplinary model of the entire power plant, as well as of the core of the system, the fuel cell stack and the electrolyser stack. The aim of this thesis is the development of a confident and robust fuel cell system lumped model, developed mainly in Matlab and Simulink environments, in order to create a powerful predictive virtual test bench to be used with confidence for robust fuel cell systems design, development, analysis and optimization. The two main cores of the presented model are the fuel cell stack, based on the PEM technology, and an electrolyser stack, based both on alkaline and PEM technologies. All of the possible ancillary equipment is also developed and created in Simulink Summary II environment to create a collection of "model blocks" capable to integrate together to build any possible system configuration. The models merge together all of the main and most relevant aspects involved in fuel cells operations, with a particular attention to the modelling of the transient phenomena. The first part of chapter 1 focuses at getting an introduction to fuel cells and electrolysers technologies, providing an introduction and history of them, and giving the reader a brief description of the potential and current applications available in the engineering world. The second part of chapter 1, instead, shows an identification and classification of the different types of fuel cells and electrolysers, and briefly introduces the basic functioning principles from an electrochemical point of view. Chapter 2 contains the development of the alkaline and PEM electrolysers models. In particular, the alkaline one is considered in depth, since the equations for the PEM one are almost coincident. The electrolyser model was developed as a lumped one. The main element of the electrolyser consisted in the stack itself, the cooling flow circuit and the hydrogen/oxygen storage tanks. The stack model here presented collects most of the already available models in the literature, with a particular attention in modelling most of the phenomena involved. Chapter 3 can be considered the core of the thesis, since it contains the fuel cell stack and system model. All of the physical aspects involved in the fuel cell operation are considered. The first part of Chapter 3 deals with the models already available in literature, considering pros and cons of the most relevant ones. The importance of modelling the transient phenomena involved in this particular kind of technology is underlined and analysed. The second portion of this chapter contains mainly the development of the fuel cell stack. Validation, calibration and simulation of this main component will be considered in Chapter 4. The third portion of Chapter 3 contains the models of all to other elements of the fuel cell system, in particular the model for the air compressor, motor, intercooler, air and water membrane humidifiers and flow mixers. For these secondary elements the calibration and validation of the models with the experimental data are provided. Further simulations will be contained in Chapter 4. Chapter 4 is the main results chapter. The first part of this chapter is dedicated to the stack model calibration and validation process with experimental data. Given the model validation, several system simulation results are presented in the second part of the chapter. The calibration and validation of the electrolyser plant is instead given in the third part of Chapter 4. The final part of this chapter contains the simulation data for the electrolyser plant. Chapter 5 presents an activity done in parallel with the lumped models development. In this chapter, optimization studies for fuel cells is addressed from a CFD model viewpoint. It presents the setup of a complete multidisciplinary design optimization model, automatically performed; sensitivity analyses, multi-objective optimization techniques and robust design optimization techniques are explored. The model used in this chapter is based on a validated proprietary CFD model of a PEM fuel cell described in Chapter 5. Chapter 6 contains the conclusions and possible further improvements of the thesis. V
doi:10.6092/polito/porto/2538902 fatcat:4zhyv7c3avef3itgszsnuoi6ii