Researchers Assess Lifecycle Water Intensity of a Range of Light-Duty Vehicle Fuels
15 October 2008
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| Water consumption (left stacked bars read on left axis) and withdrawal (right stacked bars read on right axis) in gallons of water per mile (gal/mile) for various fuels for light duty vehicles. Water use from mining and farming is designated differently from that used for processing and refining. Click to enlarge. Credit: ACS |
Building on their prior studies (earlier post, earlier post), researchers at the University of Texas at Austin have assessed the lifecycle water intensity in “gallons of water per mile traveled” for light duty vehicle (LDV) travel using selected fuels based upon petroleum, natural gas, unconventional fossil fuels, hydrogen, electricity, and two biofuels (ethanol from corn and biodiesel from soy).
Carey King and Michael Webber analyzed the amount of water withdrawn (used and returned directly to its source) and consumed (not directly returned to its source) during the production and use of different fuels. Their findings suggest that producing alternative fuels could strain already limited water supplies in some regions of the country.
They found that fuels more directly derived from fossil fuels are less water-intensive than those derived either indirectly from fossil fuels (e.g., through electricity generation) or directly from biomass.
The lowest water consumptive (<0.15 gal H2O/mile) and withdrawal (<1 gal H2O/mile) rates are for LDVs using conventional petroleum-based gasoline and diesel; non-irrigated biofuels; hydrogen derived from methane or electrolysis via non-thermal renewable electricity; and electricity derived from non-thermal renewable sources.
LDVs running on electricity and hydrogen derived from the aggregate US grid (heavily based upon fossil fuel and nuclear steam-electric power generation) withdraw 5-20 times and consume nearly 2-5 times more water than by using petroleum gasoline.
The water intensities (gal H2O/mile) of LDVs operating on biofuels derived from crops irrigated in the United States at average rates is 28 and 36 gal H2O/mile for corn ethanol (E85) for consumption and withdrawal, respectively. Ethanol processed from corn grain from non-irrigated fields results in water consumption and withdrawal intensities of 0.15-0.35 gal H2O/mile and 0.33-0.56 gal H2O/mile, respectively.
If ethanol is processed from corn stover in irrigated fields, then water consumption is 2.6-46 gal H2O/mile (average of 19 gal H2O/mile) and withdrawal is 5.6-63 gal H2O/mile (average of 23 gal H2O/mile). Ethanol processed from corn stover from non-irrigated fields results in water consumption and withdrawal intensities comparable to corn grain at 0.25 gal H2O/ mile and 0.41 gal H2O/mile, respectively.
For biodiesel derived from soy in irrigated fields, the average consumption and withdrawal rates are 8 and 10 gal H2O/mile. If the soy fields are not irrigated, the consumption and withdrawal are 2 orders of magnitude less at 0.01-0.02 gal H2O/mile and 0.03-0.12 gal H2O/mile, respectively.
The difference in water intensity between irrigated and non-irrigated biofuel feedstocks (up to 3 orders of magnitude in gallons per mile) shows the tremendous amount of need to properly plan for their incorporation. Due to water resource limitations at aquifers that are already being used intensively for food crop production, using those same aquifers for fuel production may exceed existing limits. The enhanced use of biofuel crops that need less water and the organized planting of crops in water and rain rich areas can lessen the water impact of biofuels.
—King and Webber (2008)
Moving to other fossil resources (coal, shale oil, tar sands), other than natural gas, to make liquid fuels approximately doubles the water consumption intensity compared to petroleum fuels, and the water used will likely be from inland sources where fresh water is already scarce, the researchers note.
Water consumption—without considering proposed technological reductions in water consumption—for converting oil shale to gasoline for use in LDVs is 0.15-0.37 gal H2O/mile. For oil sands the water consumption is calculated a little higher, at 0.20-0.46 gal H2O/mile. Water withdrawal rates are 0.71-0.86 gal H2O/mile for oil shale and 0.76-0.95 gal H2O/mile for tar sands.
The water consumption for converting coal and natural gas to Fischer-Tropsch diesel is 0.19-0.58 gal H2O/mile and 0.12-0.43 gal H2O/mile, respectively. Water withdrawal for coal and natural gas to F-T diesel is essentially the same as consumption because most water is used for processing the syngas, they said.
Making decisions while only considering aggregate water consumed and withdrawn on the basis of a region as large as the United States is too simplified. In practice regional impacts will dictate the successful implementation of any of the discussed fuels for LDV travel. Example regional impacts range from relatively localized around shale oil mining and coal to liquids refining to larger agricultural regions used to grow biofuel crops.
Future work needs to show the viable areas of the US where each fuel can be mined, farmed, refined, and consumed to minimize the regional impacts while maximizing broad economic and policy objectives that include water resource and energy sustainability. Where possible, the use of low-quality water sources, such as saline or reclaimed waters, can minimize the quantity of fresh water impact from most of the fuels included in this study. Policy makers should be aware that, due to the inherent distribution of water (through geology and weather), fossil, and natural resources, each state or region may not be able to contribute to the production of future transportation fuels in the same manner.King and Webber (2008)
Resources
Carey W. King and Michael E. Webber (2008) Water Intensity of Transportation. ASAP Environ. Sci. Technol., doi: 10.1021/es800367m
The water-consumption figures claimed for fuels from corn stover surprised me; the variation is nearly 20:1. I didn't see any support for these figures in the article itself, so I wonder how reliable they are and whether they are allocated to the processing or to the growing of the corn itself.
If the water input is assumed to be due to the growing (from irrigation), there is nothing to be done about it. But if it's due to processing, a shift from ethanol production to gasification could reduce or eliminate the added water use.
Posted by: Engineer-Poet | 15 October 2008 at 08:34 PM
when corn based ethanol was pitched, no one really considered that corn prices would rise significantly since everyone assumed that with the need for more corn, more acreage would be planted.LOL, that didnt happen ( farmers understand supply and demand very well). Any boifuel crop support should require that new acreage be planted. Clean water will always be the limitation, supplies are already stretched. A biofuel that can be grown in contaminated water is the best solution, so it doesnt affect current clean water supplies.
Posted by: fred | 15 October 2008 at 10:52 PM
@Fred
When the water is contaminated with N, P, K, it is called fertilizer. WWTP operators have a mindset of de-nitrifying water so they can put the water in the river or ocean. Public heath issues can be regulated to allow waste water to grow non food crops.
Posted by: Kit P | 16 October 2008 at 06:38 AM
E-P
In plant water consumption to produce 1 gallon of corn based ethanol varies from 3.5 gal. to 6.1 gal. for an average of about 5.0. This is about the same as for oil extraction from tar sands.
Water required to produce enough corn for one gallon of ethanol is 700 gallons. (i.e. 1700 gal of water to produce one bushel of corn)
Total is 700 + 5.0 = about 705 gallons of water to produce one gallon of corn ethanol.
It may be wise not to have too many corn based ethamnol plants in dry places. New Orleans general area may be ideal. However, growing enough corn could become a nightmare. Jatropha and other plants requiring less water may be more sustainable.
Water required to produce cellulosic ethanol has not been clearly established yet.
Posted by: HarveyD | 16 October 2008 at 07:00 AM
For FCV's using hydrogen produced from electrolysis using solar or wind electricity, there will be no net water used, since the water produced by the FC can be collected and recycled (good enough for drinking).
Posted by: Roger Pham | 16 October 2008 at 10:26 AM
@Kit P.
A very good and apt observation.
Here in Phoenix, we use gray water to enhance the environment and support our Tourist Industry, using it to grow grass on golf courses, parkland, and for landscaping uses. Irrigation, from separate "grey" water supplies reduces the energy load on purification plants too.
The ignorant greens would have you believe that water is really "consumed". All that has happened is that it has been utilized and is available for other uses. Some are irrigation uses, as described, but others are simply evaporation, filling the skies with H2O to be deposited elsewhere as gentle rain; or re-filling rivers, reservoirs or seas. And some is just leaking way into the ground, re-filling the aquifers.
Water is never really consumed except in in electrolysis or thermonuclear fusion of its constituent atoms.
Posted by: stas peterson | 16 October 2008 at 10:35 AM
"filling the skies with H2O to be deposited elsewhere as gentle rain"
And while in the sky those clouds of water vapor (ahem, the 75% of GHG) are increasing albedo, reducing the amount of solar heat energy striking the Earth.
Posted by: nrg nut | 16 October 2008 at 04:07 PM
"Water required to produce enough corn for one gallon of ethanol is 700 gallons."
LOL. Rain.
"It may be wise not to have too many corn based ethamnol plants in dry places."
You mean we shouldn't grow corn in the desert? LOL, you doomers never learn.
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Posted by: jsoe | 19 October 2008 at 07:05 AM
John:
USA is by no mean short of fresh water. The total availability is still over 20 times the total usage. However, fresh water supply is very badly distributed. Over 25% of USA is already short of fresh water and that will grow as usage goes up and supplies go down. Using agriculture to produce feedstocks for the production of liquid fuels on a massive scale will accellerate this trend.
Canada, USA and Australia are the industrial world per capita top polluters and top water users. For unknown reasons, there seems to be a very close relationship between per capita water usage and per capita GHG emissions. We are the champion for both.
Recycling industrial and domestic used water may help (if the recycled water is re-used or required locally) but one cannot recycle most of the water used for agriculture.
Fresh water is today in a similar situation as was oil 60-80 years ago. As more and more areas run dry, we may learn to appreciate and preserve the fresh water we have.
Posted by: HarveyD | 19 October 2008 at 11:35 AM