4.3 Future energy systems
4.3.1 The hydrogen economy
The simplest use of hydrogen as a fuel depends on the reaction:
which takes place when hydrogen is burnt in air. The reaction is highly exothermic, yielding 1.21×108 J kg-1, roughly twice the calorific value of petroleum products (0.5×108 J kg-1 for petrol). Hydrogen is thus a non-polluting fuel with a high 'energy density'. There are just two problems, the density of liquid hydrogen is ten times less than that of petrol, and hydrogen boils at - 253 °C (20 K), so there are major, but not insoluble, difficulties of transport and storage.
Hydrogen could therefore be used as an energy storage medium to smooth out the vagaries of unpredictable fluctuations in energy supply from renewable sources. Figure 19 illustrates the storage concept; in this case wind energy is used to electrolyse water into its component gases — the reverse of Equation 1 and so an endothermic process. The energy used is thus 'stored' in the gases and can be released when they are recombined. Burning the hydrogen is one way of doing this; another way is to feed hydrogen to the anode and oxygen to the cathode of a fuel cell, where they are recombined into water. As a result of ionisation processes ions migrate between the electrodes of the cell and electrons flow in the external circuit, producing a usable direct electric current. The inverter shown in Figure 19 is required to convert this into alternating current for use on an electricity supply grid.
Electrolysis is just one of several methods by which hydrogen for fuel cells could be produced; other processes include the thermochemical splitting of water by high-temperature chemical reactions and the biochemical liberation of hydrogen by some plants and algae (during photosynthesis) or bacteria (by fermentation, for instance of putrescible waste). All three processes could form part of a futuristic integrated system based on hydrogen as the energy carrier (Figure 20) — the hydrogen economy. Rather than being liquefied, gaseous hydrogen could be stored in high-pressure vessels or as metal hydrides, or in the longer term, perhaps, in depleted aquifers or oil wells. The hydrogen economy offers a promising alternative to the present fossil fuel economy, but considerable technical development of fuel cells and both storage and transport of hydrogen is required before such a system is viable. And, of course, there must be a decisive and dramatic shift to increasing the use of renewables as primary energy sources.