Hydrogen is an energy vector. Like electricity it is a man-made form of energy that can be transported and converted into other forms of energy. However, unlike electricity, hydrogen’s energy can be stored on a large scale at relatively low cost. On the Orkney island of Eday, a 0.5MW electrolysis plant exports surplus wind power as hydrogen to Kirkwall on Orkney’s mainland where it powers ferries berthed overnight.
However, this is an inefficient process. Typical efficiencies for electrolysis and fuel cells are respectively 68% and 52%. Compressing hydrogen for storage, typically at 350bar, requires 6% of its chemical energy. So the overall cycle efficiency is 33%. Hence hydrogen traction requires 3kW of electricity to deliver 1kW of power to the wheel. An electric train has no on-board energy conversion so only needs 1.2kW.
This low overall cycle efficiency potentially undermines the green credentials of hydrogen trains as they require 2½ times the electrical energy of a comparable electric train, especially if hydrogen is delivered by the much cheaper CO2-producing reforming process. However, if otherwise surplus overnight wind-turbine generating capacity is used to produce and store hydrogen, this low efficiency is not an issue. Used in this way, hydrogen production also addresses intermittency issues associated with electrical generation from renewables.