The energy transition towards a fossil fuel free society presents ambitious challenges ahead. The highly potential renewable energy sources are becoming competitive and increasingly adopted for power generation, while for sectors such as residential, industrial or transportation still solutions have to be found. Sector coupling and energy storage are presented as alternatives to increase RES integration and reduce emissions. Given the different applications and requirements in each of these sectors the combination of multiple technologies with different characteristics will be needed. Among these technologies, Solid Oxide Cells have shown huge potential and increasing attention based on their unique characteristics. This research advocates for pushing further the integration of renewable energy sources trough Solid Oxide Cell based systems.

At first, a review of the state-of-the-art technologies for sector coupling and energy storage is presented, the limitations and challenges at the sector level are addressed, and the value and research status of Solid Oxide Cell systems is conveyed. A selection of suitable sector coupling applications has been done based on SOC unique characteristics. Their functional requirements have been investigated for the future system design. The selected processes have been modeled in Aspen Plus to further conduct an energy and exergy analysis with the objective to understand the effect of the operating conditions and functional requirements on the thermodynamic performance. To conclude, a techno-economic analysis has been performed to determine the economic competitiveness and feasibility of these systems.

Through the thermodynamic optimization, in electrolysis mode exergy efficiencies of 78.93% and 80.38% are achieved for the methane-based and methanol-based system respectively. When operating thermoneutral, lower temperatures are desirable for both systems, but pressurization only has positive effects in the methane case. The upgrading process has a considerable impact on the process performance highly dependent on the operating conditions. In fuel cell operation, the methane-based system achieves 60.33% in comparison with 52.10% for methanol at high temperatures and environmental pressure. From a techno-economic perspective, stack cost is the main driver of the plant capital cost highly influenced by current density and lifetime, while the operational costs are mainly determined by electricity and carbon price. Onshore wind is presented as the best alternative for renewable energy supply due to higher capacity factors, under realistic considerations a 445 EUR/tn LCOMEOH is accomplished which could become market-competitive for future carbon emission’s prices.

The concept of coproduction has been explored in combined heat and power applications. It is a method of improving the efficiency of the energy generating system by utilising waste heat. In the coming years hydrogen is expected to play an important role in decarbonization as it does not emit greenhouse gas at the point of application. Hydrogen today is primarily generated from fossil fuels and the processes of producing hydrogen are energy intensive, while also emitting large quantities of greenhouse gases into the atmosphere. As the amount of hydrogen generated today is limited, it has restricted the growth of industries such as the automobile industries producing fuel cell vehicles that are to use hydrogen as fuel. In this thesis report a coproduction concept using high temperature molten carbonate fuel cell has been examined. The molten carbonate fuel cells operate at very high temperature, and it is possible to utilise the waste heat for internal reforming reaction of a fuel such as natural gas to liberate hydrogen required by the fuel cell. Excess hydrogen can also be produced from such fuel cell systems when the fuel utilisation in the fuel cell is reduced. This concept has been studied with solid oxide fuel cells and a paper published in 2008 by Hemmes et al. titled "Flexible Coproduction of Hydrogen and Power Using Internal Reforming Solid Oxide Fuel Cells System" has served as the inspiration for this thesis report. Three modes of operations have been simulated in this thesis on the Cycle-Tempo software with varying fuel utilisations, similar to what has been shown with the solid oxide fuel cells in that paper. With molten carbonate fuel cells, overall efficiency of up to 80% was obtained in terms of electricity and hydrogen coproduction. By doing so it is also possible to produce overall power output of nearly three times than what can be achieved by conventional electricity production. The results obtained have also been compared with the solid oxide fuel cells in this report. While high coproduction efficiencies for flexibly coproducing hydrogen and power have been shown to be possible, other factors would also play important roles in the success of this technology. In this report some of those factors such as the status and expected growth of the hydrogen market, molten carbonate fuel cell market, role of actors, role of policy makers have also been examined. As this technology does rely on a fossil fuel that is natural gas, the benefits of using natural gas in hydrogen production has also been highlighted along with the positive effects these systems could have on the society.

As all industries move towards sustainable fuels to reduce emissions, aircraft industry is still at its nascent stages. The adoption of clean energy sources has been slow particularly due to low energy density of commercially available batteries, and high volumetric density of hydrogen as a fuel. Electric aircrafts have been demonstrated but only a few are available commercially. Hydrogen powered flights have also been demonstrated, but only for experimental purposes. As the move towards clean aviation furthers, it is necessary to indicate an approach for designing a mass minimized aircraft powerplant. The aim of this thesis was to indicate exergy-gravimetric approaches towards designing an optimal minimized-mass powerplant for aircraft applications. This thesis work also aimed to use this approach to size a mass-minimized powerplant for the Pipistrel Alpha Electro aircraft. The exergy analysis of fuel cell was carried out on CycleTempo through which sources of exergy destruction were identified and quantized. A mass-minimized model was developed on MATLAB. Using these tools, a number of system configurations for varying battery combinations, endurance requirements, and fuel cell types were analysed. Overall, an exergy-gravimetric approach towards mass minimization was developed in this thesis work. Multiple fuel cell - battery hybrid powerplants were sized for the Pipistrel Alpha Electro. It was demonstrated that at least three system configurations using commercially available batteries and PEM fuel cells exist which would perform superior than the existing battery system on the aircraft, reducing the powerplant weight by as much as 20 kg from the existing battery system, while increasing endurance by about 15 minutes.

Power generation systems based on SOFCs provide a highly efficient alternative to traditional systems. In the present study a sensitivity analysis with cell operating temperature, pressure ratio and fuel utilization as system parameters is performed on a SOFC-GT system fed by Hydrogen and Methane. Exergy losses in different system components and their dependence on system operating parameters and fuel chemistry are investigated in detail. In the considered ranges system efficiency increased with both cell operating temperature and fuel utilization. A flat optimum with pressure ratio was found for Hydrogen (2.5) and Methane (5). The main causes of high losses in the system are found to be very different for Hydrogen and Methane which leads to different optimization strategies.
Following the optimizations comparable system exergy efficiencies were obtained with Hydrogen (76.2%) and Methane (78%). Electrical efficiencies were 74.6% and 80.9% with Hydrogen and Methane respectively. An additional study of a SOFC system without a gas turbine was also undertaken with 4 fuels (Methane, Hydrogen, Methanol and Ammonia). It was observed that it is possible to achieve a high system efficiency by minimizing the excess heat left due to the removal of the gas turbine and utilizing it completely for internal reforming of the fuel. An electrical efficiency of 73.3% was achieved with Methane without the gas turbine.

Unmanned aerial vehicles (UAV) for science missions are becoming increasingly popular in the present context. Science missions require high altitude operation and long endurance (HALE). With rising awareness of climate change phenomenon it becomes imperative to look into more efficient and more sustainable means of powering these missions. A 'well-to-wheel' study is a comprehensive way of comparing different operations for powering a vehicle. For this study , the HALE UAV is powered by a solid oxide fuel cell and gas turbine (SOFC-GT) hybrid. The HALE UAV design is based on concept that has been developed before by NASA. It is also essential that a suitable fuel is selected for powering the hybrid system. Previous studies which were based on liquid hydrogen (LH2 ) concluded that liquefication of hydrogen as an avenue for major exergy losses. For the current study, use of compressed synthetic natural gas (SNG) is analysed. Exergy analysis is carried out for fuel production and the hybrid system operation at cruise altitude. To begin with, syngas is synthesised from woody biomass through gasification. The syngas is then converted into SNG and compressed for storage. This SNG is utilised in the SOFC-GT hybrid. Design calculations are made for the take -off stage. Effect of pressure variation is studied to get the optimum system size. Then the performance of the system is analysed in the cruising stage to estimate the endurance of the flight. Exergy efficiency analysis and mission endurance calculations are made for the system during cruising conditions as a means to evaluate the system performance. The effect of position of air preheater is studied in two different configurations. The exergy efficiency for compressed SNG synthesis is 59.7% and the SOFC-GT operation at cruise is 69.6% effcient. A total 'well-to-wheel ' effciency of 41.6 % is attained. Based on the results of the analysis carried out thus far, the mass for the system components is estimated. Volumetric estimates are also made and the system is fitted in the nacelle volume as defined in NASA concept. The results are eventually compared to the system working on LH2 . The results from compressed SNG study show that specific energy density of the fuel is important. The mass estimates show that the infrastructure required to carry the SNG fuel reduces the specific energy of the fuel. liquid fuels such as methanol should be analysed as another option.