Optimal design of a hybrid Electrolyser/fuel cell system to produce power and freshwater from seawater

Abstract

Fossil fuels have earned a reputation as unsustainable sources of energy, due to the release of carbon emissions that are attributable to global warming. To overcome the extensive release of carbon emissions into the environment, different approaches are being explored to produce energy, by replacing non-renewable fuels with renewable energy. Additionally, many countries across the world are emerging as water-scarce countries due to the vulnerability of freshwater supply. This work, therefore, focuses on the design and synthesis of a hybrid electrolyser-fuel cell system to generate hydrogen and freshwater from seawater. The proposed system is designed to be integrated with a background process, such as a utility system, that requires both power and water. It has the potential to reduce the burden on freshwater sources and power requirements of background processes, with a significant reduction on carbon footprint. A one-dimensional, mathematical model is developed for a continuous hybrid seawater electrolyser-fuel cell system operated at steady state. The model determines the optimal operating conditions in terms of temperature, current density, electrode thickness and humidity, as well as the performance of the system through the activation overpotential, diffusion overpotential, ohmic overpotential and the open-circuit voltage. GAMS/BARON solver is used to optimise the hybrid system. Furthermore, a techno-economic evaluation is conducted to determine the viability of the system. Results indicate that an overall power conversion efficiency of 41.2 % and a freshwater recovery rate of 48.2 % is achieved. Freshwater produced is expected to have a purity of >99.9% based on the PEME which is assumed to produce pure hydrogen and oxygen. This indicates that the system shows promise of decentralising the production of electricity from fossil fuels, addressing the energy trilemma without straining freshwater bodies, and providing clean supplementary power to background processes. Furthermore, in addition to its environmental benefits, the HEFC system is economically viable presenting a positive net present value, 3.94-year payback period over a 20-year plant life and a 16.3% internal rate of return.