Water is key to the hydrogen economy

accounts for both the direct and indirect uses of water in a hydrogen economy. The direct use is water as a feedstock for hydrogen, in which water undergoes a splitting process which separates hydrogen from oxygen. Production can be accomplished in several ways, such as steam methane reforming, nuclear thermochemical splitting, gasification of coal or biomass, and other ways. Note, though, that one of the major production methods in the transitional stage will likely be electrolysis.

The atomic properties of water tell us that 1 kg of hydrogen gas requires about 2.4 gallons of water as feedstock. In one year, 60 billion kilograms of hydrogen would thus require 143 billion gallons of fresh, distilled water. This amount is similar to the amount of water required for refining an equivalent amount of petroleum (about 1 to 2.5 gallons of water per gallon of gasoline). The biggest increase in water usage would come from indirect water requirements, that is, when water is used as a cooling fluid for the electricity needed to supply the energy that electrolysis requires. Since electrolysis is likely to use existing infrastructure, it would pull from the grid and therefore depend on thermoelectric processes. At 100 percent efficiency, electrolysis would require close to 40 kWh per kilogram of hydrogen — a number Webber derives from the higher heating value of hydrogen, a physical property. Today’s systems, however, have an efficiency of only about 60 percent to 70 percent, with the Department of Energy’s (DOE) pushing to for a target of 75 percent. Now, depending on the fraction of hydrogen produced by electrolysis (Webber offers estimates for values from 35 percent to 85 percent), the amount of electricity required based on electrolysis efficiency of 75 percent would be between 1134 and 2754 billion kWh — with up to 3351 billion kWh for a lower electrolysis efficiency of 60 percent. For comparison, the current annual electricity generation in the United States in 2005 was 4063 billion kWh. In 2000 thermoelectric power generation required an average of 20.6 gallons of water per kWh, leading Webber to estimate that hydrogen production through electrolysis, at 75 percent efficiency would require about 1100 gallons of cooling water per kilogram of hydrogen. This comes to 66 trillion gallons per year just for cooling. The NRC report predicts that by 2050 hydrogen demand may well exceed 100 billion kg, that is, nearly twice the 60 billion kg on which Webber’s estimates are based. By then, PhysOrg.com’s Lisa Zyga writes, scientists — helped by DOE’s funding, which will exceed $900 million in 2008 — may find better ways of producing hydrogen.

“That most of the water use is for cooling leaves hope that we can change the way power plants operate, which would significantly ease up the potential burden on water resources, or that we can find other means of power production at a large scale to satisfy the demands of electrolysis,” said Webber. If electrolysis becomes a main method of hydrogen production, Webber suggests that researchers may want to look for an electricity-generating method other than thermoelectric processes to power electrolysis. With this perspective, he suggests hydrogen pathways such as wind or solar sources, as well as water-free cooling methods such as air cooling.

Which brings us back to water. “Each of the energy choices we can make, in terms of fuels and technologies, has its own tradeoffs associated with it,” Webber say. “Hydrogen, just like ethanol, wind, solar, or other alternative choices, has many merits, but also has some important impacts to keep in mind…. I would encourage the continuation of research into hydrogen production as part of a comprehensive basket of approaches that are considered for managing the transition into the green energy era. But, because of some of the unexpected impacts — for example on water resources — it seems premature to determine that hydrogen is the answer we should pursue at the exclusion of other options.”

No alternative energy option should be excluded, but hydrogen becomes ever more appealing if we do find ways to produce large quantities of water in a manner which will not compete with current usage (we are saying this, not Webber). This is where a compelling business proposition can be made: If innovative researchers come together with savvy investors, solutions will be found to feed and sustain the hydrogen economy.

-read more in Michael E. Webber, “The Water Intensity of the Transitional Hydrogen Economy,” Environmental Research Letters 2 (July-September 2007) (034007 doi:10.1088/1748-9326/2/3/034007) (sub. req.)