Where does hydrogen fit in a sustainable energy economy?

Where does hydrogen fit into a global sustainable energy strategy for the 21st century, as we face the enormous challenges of irreversible climate change and uncertain oil supply? This fundamental question is addressed by sketching a sustainable energy strategy that is based predominantly on renewable energy inputs and energy efficiency, with hydrogen playing a crucial and substantial role. But this role is not an exclusive one as in the original concept of the 'hydrogen economy' proposed in the early 1970s. A hierarchy of spatially-distributed hydrogen production, storage and distribution centres relying on local renewable energy sources and feedstocks would be created to avoid the need for an expensive long-distance hydrogen pipeline system. There would thus be complementary use of electricity and hydrogen as energy vectors. Importantly, bulk hydrogen storage would provide the strategic energy reserve to guarantee national and global energy security in a world relying increasingly on renewable energy; and longer-term seasonal storage on electricity grids relying mainly on renewables. In the transport sector, a 'horses for courses' approach is proposed in which hydrogen fuel cell vehicles would be used in road and rail vehicles requiring a range comparable to today's petrol and diesel vehicles, and in coastal and international shipping, while liquid hydrogen would probably have to be used in air transport. Plug-in battery electric vehicles would be reserved for shorter-trips. Energy-economic-environmental modelling is recommended as the next step to quantify the net benefits of the overall strategy outlined.