Technical and economic analysis of state-of-the-art electrolytic systems for hydrogen production

At a time when decarbonization constitutes an issue of global relevance, hydrogen is seen as one of the most promising solutions. To date, around 96% of hydrogen is produced from fossil fuels, in processes like steam methane reforming (SMR) or coal gasification, while only the remaining 4% is produced in cleaner ways like thermochemical cycles and, especially, water electrolysis. This last method is based on water splitting into hydrogen and oxygen through chemical reactions driven by electrical energy or solar energy in the case of photoelectrolysis. Nowadays, the main types of water electrolysers, distinguished on the base of the electrolyte used, are essentially three: alkaline water electrolyser (AWE), proton exchange membrane electrolyser (PEM) and solid oxide electrolysis cell (SOEC). While the first two technologies are classified as low temperature electrolysers, operating between 60 °C and 80 °C, SOECs perform a high temperature electrolysis ranging between 800 °C and 1000 °C. Alkaline electrolysers constitute the oldest and more mature technologies and, together with PEM devices, are the most diffused, while SOECs are still in a research and development phase. The thesis mainly focuses on the description of water electrolysers, concentrating in particular on a technical and economic analysis of alkaline and PEM technologies. From the comparison of various electrolysers produced by different suppliers, it emerged that both alkaline and PEM electrolysers can satisfy a wide range of hydrogen demands, varying from values lower than 1 Nm3/h of hydrogen up to typically 4000 Nm3/h, with sizes, in terms of electrical input power, going from 1 kW (or even less) up to around 20 MW. With the same hydrogen flow rate produced, PEM electrolysers usually present higher values of electrical power requirements, which translate into greater specific energy consumptions, expressed as the electrical power required per unit of hydrogen flow rate. AWEs are generally characterized by specific energies comprised between 3.8 kWh/Nm3 and 6.0 kWh/Nm3, while the typical range of PEM technologies is 4.5 - 6.0 kWh/Nm3. Even if the upper extreme is the same in both cases, it is exceeded more times by PEM electrolysers rather than alkaline ones. Anyway, PEM electrolysers are able to tolerate higher pressures (up to 350 bar) compared to alkaline technologies, leading to the possibility of avoiding the expense for a hydrogen compressor. Under the economic point of view, the state of greater development of alkaline electrolysers, together with the possibility of using cheaper materials for the catalysts, results in typically lower investment costs. Indeed, actual values ordinarily reported in several literature articles correspond to 700 - 1700 €/kW for alkaline systems and 1000 - 1500 €/kW for PEM systems. Then, since the operational lifetime of the plants is about 20 years, while the stacks maximum operational lifetime is generally 80000 hours, they need to be replaced, at a typical cost of 340 €/kW for alkaline electrolysers and 420 €/kW for PEM devices.