Net energy and carbon footprint analysis of solar hydrogen production from the high-temperature electrolysis process

This paper assesses the viability of the solar-based high-temperature steam electrolysis (HTSE) process by estimating the cumulative energy demand (CED) and carbon emission footprint (CEF) of hydrogen. In the analysis, photovoltaic (PV) and concentrated solar power (CSP) plants are considered as the source of electricity. The trend of variation in CED and CEF with the changing temperature and current density of the solid oxide electrolyzer is obtained. The requirements for the feasibility of the HTSE process is established by comparing the results with the solar-based alkaline electrolysis process. Monte Carlo simulation is done to capture the effects of data and performance uncertainties. The carbon footprint of the solar-based high-temperature and alkaline electrolysis processes are also compared with the renewable, nuclear and hydrocarbon-based routes for hydrogen production. The CED of the PV and CSP based HTSE processes are 15.1-28 MJ/kg of hydrogen and 18.8-30.1 MJ/kg, respectively. This implies a net energy gain of 90-104 MJ/kg of hydrogen. The carbon footprint of the PV and CSP driven processes are 1.03-1.87 kg CO2/kg of hydrogen and 1.05-1.67 kg/kg, respectively. The carbon footprint of the steam methane reforming (SMR) process, solar-based alkaline electrolysis, and thermochemical cycles are 10.6 kg/kg, 2-2.3 kg/kg and 4.3-4.5 kg/kg respectively. Thus, the solar-HTSE process is a sustainable alternative not only to the SMR route but also to other solar technologies like alkaline electrolysis and thermochemical cycles. It is recommended that the solar-HTSE route should be further explored by demonstrating the technology on a pilot scale.