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 (CO 2 ) for 1 kg of H 2 (∼12 kg-CO 2 /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 CO 2 than the SMR process when the carbon intensity for grid power falls below 0.22 kg-CO 2 /kWh. Solar- and wind-powered electrolytic H 2 production promises over 80% CO 2 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.