[This is one of many reasons I call hydrogen “not worth the ink”]
Webber, M.E. 2007. The Water Intensity of the Transitional Hydrogen Economy. Environmental Research Letters 2.
Hydrogen production using thermoelectric powered electrolysis is significantly more water intensive than gasoline production. If 60 billion kg of hydrogen are manufactured a year by electrolysis, it will consume approximately 143 billion gallons of water just as the feedstock. Furthermore, because electrolysis is a very energy-intensive process, manufacturing 60 billion kg of hydrogen annually with that method would require vast amounts of electricity. Since thermoelectric power makes up 90% of the fuel mix in the US, it is likely that some portion of that power for electrolysis will consequently require significant amounts of water for cooling.
Using recent data for water withdrawals by the thermoelectric sector and overall energy consumption, it can be deduced that the water withdrawal and consumption increases for a thermoelectrically powered hydrogen economy are significant. The calculated water withdrawals for electrolytic hydrogen production could increase by anywhere from 27 to 97%, depending on electrolyzer efficiencies from 60 to 90% and the fraction that is produced by thermoelectric power (from 35 to 85%), while consumption (including evaporative losses and conversion of feedwater into hydrogen) might increase by 0.5–1.7 trillion gallons per year. On a per unit basis, thermoelectric power generation for electrolysis will on average withdraw approximately 1100 gallons of cooling water and will consume 27 gallons of water as a feedstock and coolant for every kilogram of hydrogen that is produced using an electrolyzer that has an efficiency of 75%.
Given that water withdrawals have remained steady for decades, this increase in water use represents a significant potential impact of the hydrogen economy on a critical resource, and thus presents a serious technical and public policy problem. If minimizing the impact of water resources is a priority and electrolysis becomes a widespread method of hydrogen production, it is likely that the power for electrolytic hydrogen production will have to come from non-thermoelectric, non-hydroelectric and non-irrigated renewable sources. Consequently, almost all the new electricity generating capacity for hydrogen production would need to be from hydrogen production pathways that do not use much water (such as wind or solar), or effective water-free cooling methods (e.g. air cooling) will need to be developed and widely deployed.
The total water withdrawals for thermoelectric cooling would be anywhere from 19 trillion gallons annually for 90% efficient electrolyzers if 35% of the hydrogen is produced by thermoelectrically powered electrolysis, to nearly 69 trillion gallons for electrolyzers with 60% efficiency if 85% of the hydrogen is produced by thermoelectrically powered electrolysis. These withdrawals correspond to an additional 52–189 billion gallons per day on top of the 195 billion gallons of daily withdrawals already in place for thermoelectric power, representing a potential increase of between 27 and 97%. The total water consumption would increase by between 0.5 and 1.7 trillion gallons over the course of a year for the same cases, presumably mixed 70% freshwater and 30% saline according to the existing ratios. Note that freshwater consumption in 1995 for thermoelectric applications was 1.2 trillion gallons .
For comparison, the reader is reminded from before that gasoline production consumes 1–2.5 gallons of water per gallon of gasoline that is produced, and hydrogen produced via SMR consumes approximately 4.6 gallons kg–1 of hydrogen that is produced , both of which are much lower than the consumption of 27 gallons of water per kilogram of hydrogen for electrolyzers with 75% efficiency operated by average thermoelectric power. Switching to hydroelectric power for electrolysis, which consumes 18 gallons kWh–1 due to increased evaporation at man-made reservoirs , would increase the water consumption to approximately 950 gallons of water per kilogram of hydrogen that is produced with electrolyzers operating at 75% efficiency. Note that withdrawals for hydroelectric power are considered to be zero by convention. These values are summarized in table 3.
Table 2 also lists the annual electricity requirements to produce hydrogen in 2037 based on the fraction of the projected 60 billion kg that is produced by electrolysis as opposed to other pathways (values from 35 to 85% are listed) and as a function of electrolyzer efficiency. If highly efficient electrolyzers are used (e.g. 90% efficient) and only 35% of the 60 billion kg of hydrogen is produced by electrolysis, then 827 billion kWh of electricity will be required annually. If inefficient electrolyzers are used (e.g. 60% efficient) and a great preponderance of the 60 billion kg of hydrogen is produced by electrolysis (e.g. 85%), then 3351 billion kWh of electricity will be required annually. Total annual electricity generation in the US in 2005 was 4063 billion kWh . Thus, producing a fraction of hydrogen from electrolysis, even for very efficient systems, requires significant additional amounts of electricity to be generated.
The indirect water use that is necessary for the power plants depends on the type of power source: thermoelectric power uses water as a coolant, while renewable sources such as wind, solar and hydroelectric do not use water as a coolant. Though hydroelectric power does not use cooling water, it has high water consumption through increased evaporation at man-made reservoirs . It is important to note that more than the 90% of the electricity in the US is generated through thermoelectric processes (either fossil-fuel combustion, biomass combustion, or nuclear reactions). Consequently, it can be expected that a significant fraction of power for electrolysis would be derived from thermoelectric sources that require cooling water.
According to the US Geological Survey, in 2000, thermoelectric power was responsible for about 48% of all freshwater and saline-water withdrawals in the US, requiring 195 billion gallons per day in total, and remaining roughly stable since 1985. Of those withdrawals, approximately 70%, or 132 billion gallons per day, was fresh, which is about the same amount required by the agriculture sector (predominantly for irrigation) . Nearly 99% of all thermoelectric withdrawals were from surface water sources , with almost all of the water returned to the source without being consumed (though at a higher temperature and with a different quality) . Approximately 3% (or 3.3 billion gallons per day) of the freshwater withdrawals were consumed by evaporation .
A comparison of the amount of water used by the thermoelectric sector in 2000 with the amount of electricity generated by thermoelectric sources in 2000 yields an average water withdrawal of 20.6 gallons per kilowatt hour for the nation’s entire thermoelectric fuel mix [21, 22]. Notably, the water withdrawals and fuel mix have not changed very much between 2000 and 2005: in 2000, the total electricity generation was 3840 billion kWh, of which 90% was from thermoelectric power; in 2005, even after significant increases in wind and solar power, the total electricity generation was 4063 billion kWh, of which 90% was from thermoelectric power; and the DOE’s projections out to 2030 show aggressive increases in renewable power, but also show that thermoelectric power is expected to remain 90–91% of the fuel mix [18, 22]. Consequently, it is reasonable to expect that some portion of the power for electrolysis will be derived from thermoelectric sources.
Note that this estimate for water withdrawals per kilowatt hour of generation is an average over geographic locations, cooling systems (e.g. once-through versus open-loop, etc), fuel sources, and power plant designs. There is significant variability of water use, however, with some thermoelectric power plants requiring up to 30–50 gallons kWh–1 for once-through cooling . And, as noted above, a portion of the water withdrawals for thermoelectric cooling are consumed by evaporation, typically in the range of between 0.2 and 0.72 gallons kWh–1 . It is also worth noting that nuclear power is often cited as a suitable carbon-free source of electricity for hydrogen production [2, 10–12, 17], but its water consumption for thermoelectric cooling is at the higher end of the typical range, at 0.4–0.72 gallons kWh–1 . Overall, the average US water evaporation at thermoelectric plants is 0.47 gallons kWh–1 .
3.4. Total water use for electrolytic hydrogen production
Using 20.6 gallons kWh–1 of average water withdrawals for thermoelectric cooling, we can estimate the water use for hydrogen production depending on the fraction that is powered by thermoelectric sources and the electrolyzer efficiencies, as shown in figure 1 for trillions of gallons per year. On a per unit basis, thermoelectric power generation will withdraw approximately 1100 gallons of cooling water on average per kilogram of hydrogen that is produced for an electrolyzer with 75% efficiency. Using 0.47 gallons kWh–1 of average water consumption for thermoelectric cooling, plus 2.38 gallons kg–1 of water as a feedstock for hydrogen, we can estimate the total water consumption of hydrogen production at 60 billion kg per year, depending on the fraction that is produced by thermoelectric power and for a range of electrolyzer efficiencies, as shown in figure 2 for billions of gallons per year. On a per unit basis, thermoelectrically powered electrolysis will consume 27 gallons of water as a feedstock and coolant for every kilogram of hydrogen that is produced for an electrolyzer with 75% efficiency. As expected, as more hydrogen is produced with thermoelectric power, the total water intensity (withdrawals and consumption) increases. Furthermore, as electrolyzer efficiencies improve, the total water intensity decreases. For reference, the thermoelectric sector withdrew 72 trillion gallons of water in 2000 .
|Table 3. Comparative values for water consumption and withdrawals during the production of gasoline and hydrogen via different pathways. Units are listed in gallons of water per gallon of gasoline for refining, and gallons of water per kilogram of hydrogen for SMR and electrolysis [2, 20].|
|Fuel||Production pathway||Water consumption (gal kg–1) or (gal/gal)||Water withdrawals (gal/gal) or (gal kg–1)|
|Hydrogen||Steam methane reforming||4.6||4.6|