2.2.1. Hydrogen

One of the greatest opportunities from low-cost renewable electricity technologies is to produce renewable hydrogen from water. PEM electrolyzers are now emerging as a preferred technology for this opportunity. The key advantage to PEM is their flexibility—PEM electrolyzers can accept partial, dynamic loads and are available from kW to MW scale. The technology functions by applying a current across a cell with two halves separated by a selective polymer that allows only hydrogen to move between the two, as shown in Figure 3. Water is fed into one side and the electrical energy splits the liquid into gaseous oxygen and hydrogen that can be collected and used in a range of applications.

Hydrogen is a good candidate for long-term energy storage to meet emergency requirements and the seasonal variation in energy demand which occurs in hospitals [29] and energy networks. There are many ways to store hydrogen, the most mature forms being as a compressed gas in high-pressure tanks and as a cryogenic liquid in insulated low-pressure vessels. Either of these storage systems can be situated on-site as stationary installations or mounted on truck trailers or rail cars. Solid-state hydrogen storage is an alternative with the potential for much greater energy density and it is now being demonstrated at scale [30]. Hydrogen can also be blended into the natural gas network, which will have a role to play in areas where it currently exists as complete fuel switching from gas to electricity is likely to cost more than rethinking existing infrastructure [31].
