Round-trip efficiencies of green ammonia and green hydrogen

The water electrolysis voltage of conventional green ammonia was 1.84 V. The voltage with the same energy consumption of gray ammonia was 1.50 V, which was similar to the thermoneutral voltage (1.48 V). The round-trip efficiencies using overseas transportation 10,000 km of green ammonia increased with decreasing water electrolysis voltage, having high volumetric energy density 3530 Wh/L in the liquid state. The round-trip efficiencies were 23–42% depending on the voltage reduction from 1.84 to 1.48 V and power generation systems. In addition, the round-trip efficiencies using land transportation (pipeline) of green hydrogen also increased with decreasing water electrolysis voltage. The round-trip efficiencies were 28–52%, although the volumetric energy density was below 0.01 of liquid ammonia.