Hydrogen production from renewables

IF WE ARE TO HAVE A SUSTAINABLE TRANSPORTATION INFRASTRUCTURE THAT INCLUDES HYDROGEN FUEL CELL VEHICLES, WE MUST PRODUCE THE ENERGY CARRIER – HYDROGEN – IN LARGE QUANTITIES FROM CLEAN, NON-FOSSIL ENERGY SOURCES, AND THAT MEANS FROM RENEWABLES. SO EXACTLY WHAT OPTIONS DO WE HAVE TO PUT US ONTO THE CLEAN HYDROGEN PATHWAY, AND WHAT CHALLENGES NEED TO BE OVERCOME ALONG THE WAY?