Review-Engineering Challenges in Green Hydrogen Production Systems

Today, hydrogen (H-2) is overwhelmingly produced through steam methane reforming (SMR) of natural gas, which emits about 12 kg of carbon dioxide (CO2) for 1 kg of H-2 (similar to 12 kg-CO2/kg-H-2). Water electrolysis offers an alternative for H-2 production, but today's electrolyzers consume over 55 kWh of electricity for 1 kg of H-2 (>55 kWh/kg-H-2). Electric grid-powered water electrolysis would emit less CO2 than the SMR process when the carbon intensity for grid power falls below 0.22 kg-CO2/kWh. Solar- and wind-powered electrolytic H-2 production promises over 80% CO2 reduction 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 H-2 output by solar-powered electrolysis.