Review - Engineering Challenges in Green Hydrogen Production Systems

Today, hydrogen (H2) is overwhelmingly produced through steam methane reforming (SMR) of natural gas, which emits about 12 kg of carbon dioxide (CO2) for 1 kg of H2 (∼12 kg-CO₂/kg-H₂). Water electrolysis offers an alternative for H2 production, but today's electrolyzers consume over 55 kWh of electricity for 1 kg of H2 (>55 kWh/kg-H²). Electric grid-powered water electrolysis would emit less CO₂than the SMR process when the carbon intensity for grid power falls below 0.22 kg-CO₂/kWh. Solar- and wind-powered electrolytic H2 production promises over 80% CO₂reduction over the SMR process, but large-scale (megawatt to gigawatt) direct solar- or wind-powered water electrolysis has yet to be demonstrated. In this paper, several approaches for solar-powered electrolysis are analyzed: (1) coupling a photovoltaic (PV) array with an electrolyzer through alternating current; (2) direct-current (DC) to DC coupling; and (3) direct DC-DC coupling without a power converter. Co-locating a solar or wind farm with an electrolyzer provides a lower power loss and a lower upfront system cost than long-distance power transmission. A load-matching PV system for water electrolysis enables a 10%-50% lower levelized cost of electricity than the other systems and excellent scalability from a few kilowatts to a gigawatt. The concept of maximum current point tracking is introduced in place of maximum power point tracking to maximize the H₂output by solarpowered electrolysis.