But sooner or later we’ve just got to get over our dependence on the burning of hydrocarbons in general and foreign oil in particular. Bombers and tanks and the crews to operate them are expensive and messy and they don’t stop global warming anyway. The so-called hydrogen economy is an oft-opined alternative. But how to obtain the hydrogen required is an open question. There’s three main ways to do it: electrolysis, thermolysis, and thermochemical cycles. The efficiency of using electricity to split water into hydrogen and oxygen via electrolysis is typically about 75-80%. Sounds good, until you remember that electric power generation efficiencies (steam from whatever heat source into electrical energy) is typically 30-35%. This makes the true overall conversion efficiency of water to hydrogen via commercial electrolysis only 20-25%. Thermolysis, a process in which steam is heated sufficiently to dissociate it into hydrogen and oxygen without a catalyst, requires very high temperatures that just aren’t economical.
That leaves thermochemical hydrogen production – the use of chemical reactions that free hydrogen at lower temperatures than are required by pure thermolysis. The two most highly developed thermochemical cycles are the sulfur-iodine and the calcium-bromine cycles. Both contain at least one reaction that requires temperatures greater than 750C, higher than is achieved by current electrical powerplants in normal operations. Argonne National Laboratory anounced research results this week on a new copper-chlorine thermochemical cycle in which the chemicals used are not consumed and are recycled. The reaction proceeds at 40% efficiency at only 500 degrees C. Thus a hydrogen production facility could be piggybacked on existing electrical powerplants at a higher overall efficiency than electrolysis. This research – or something it leads to – may well allow gasoline powered cars to be one day be replaced with fuel cell versions.