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The energy-water nexus Part I

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The principal challenge of this century, in my view, will be adapting to a life without abundant, cheap fossil fuels. It has been the lifeblood of our society, and turns out to have some really fantastic qualities. The jury is still out as to whether we will develop suitable, and affordable replacements.

But additional challenges loom in parallel. Water is very likely to be one of them, which is especially pertinent in my region. For true believers in the universality of substitution, let me suggest two things. First, come to terms with the finite compactness of the periodic table. Second, try substituting delicious H2O with H2O2. It has an extra oxygen atom, and we all know that oxygen is a vital requisite for life, so our new product will be super-easy to market. Never-mind the hydrogen peroxide taste, and the death that will surely visit anyone foolish enough to adopt this substitution. Sometimes we’re just stuck without substitutes.

Substitution silliness aside, water and energy are intimately related in what has been termed the Energy-Water Nexus (see for example the article by Michael Webber from this conference compilation; sorry about the paywall). We’ll explore aspects of this connection here (and in Part II to be published tomorrow), touching on pumping water, use of water for the production and extraction of energy, and desalination. As glaciers and snowpack melt and drought becomes more common in the face of climate change, our water practices will need to be modified, hitting energy right in the nexus.

Household Water

Let’s start at the familiar level. A typical San Diego residence uses 14 hundred cubic feet (1 hcf = 748 U.S. gallons = 2831 L) of water each month—working out to 138 gal/day (520 L/day) per person, assuming an average of 2.5 people per residence. Based on backpacking experience, this is more than one hundred times as much water as is necessary to satisfy basic needs.

Note: only 60 percent of Do the Math readers are from the U.S. The volumetric units in this post will likely elicit a groan or two overseas. Some additional conversions: 1 gallon is 3.785 L; one cubic meter is 264 gallons; and 1 hcf is 2.83 m³.

My own household (two people) averages 20 hcf per year. This past year is slightly anomalous in that we planted a large section of our yard with drought-tolerant California natives, and have supplied 4.25 hcf of water (5.5 inches or 14 cm of rain equivalent) to help establish them in their first year — yes, I record sprinkler use. Even so, our water use over the last year comes to 23 hcf — working out to 47 gal/day (or 38 gal/day ignoring temporary irrigation).

Okay, so I’m shocked² by these sets of numbers. First, I am shocked that we use as many as 38 gallons per day within our house. On what?! We are sparing with showers, each requiring only five gallons and on a roughly two-day cadence. Our low-volume toilets (one of which has a dual flush capability) average something like 1.5 gal per flush, and we don’t flush after every use (of certain types, if you know what I mean). Dishes? Sparing, efficient. Laundry? The same. Vegetable garden? Rainwater catchment (600 gal storage). So it’s hard to get it to add up—yet it must, and I accept that.

The second power of shock (thus the square) is the nearly 10× greater usage (344 gal/day) by typical area households. Yet it’s not an unfamiliar mismatch — also appearing in our use of electricity, natural gas, etc. But where is all this water going?

Presumably, much of it goes into creating green lawns in a semi-arid desert: San Diego typically gets about 10.3 inches, or 26 cm, of rain per year. If I guess that a typical house in San Diego has about 2000 ft² (185 m²) of lawn, then tripling the natural rainfall amount will require 34 hcf of water, or about an extra 3 hcf per month. Hmmm. Not as big a chunk of the monthly 14 hcf as I thought it would be.

Maybe I underestimate how much irrigation people are doing. Overwatering, overspray, evaporation, and leaky sprinkler systems may be a big part of the story, but I am still left a bit puzzled. I guess frequent long showers (also energy-intensive), washing clothes maniacally often, flushing every use, hosing off the driveway, washing cars, etc. may all add up.

One final comment on domestic water. Each month, I pay a $19.33 base rate for water service (not including wastewater service, which is a comparable charge). Then it’s $3.61 per hcf of water used. So my monthly water bill comes to about $26. Meanwhile, the average residence pays $70 for their 14 hcf allocation. I’m paying about 2.7 times as much per gallon as the average resident: my reward for conserving, apparently.

A further amusement is that my water service issued a “20 gallon challenge,” which asks residents to shed 20 gallons per day per person. Setting aside the temporary native-plant irrigation, our 38 gal/day household usage would mean that rising to the challenge would result in our complete abstention of utility water usage. In fact, it would require each of us to cough up (not literally) a gallon of water each month to donate to the utility. Good thing we have rain catchment.

National and Statewide Usage

The USGS provides national water use estimates every five years. For 2005, the total 410 billion gallons per day works out to about 1330 gal/day per person. This drops in half if excluding water used for cooling power plants. For California, 81 percent of the withdrawals were for power plants and irrigation, leaving 240 gallons per day per individual in the state.

Energy for Water

A 2006 report pegs the total amount of water-related energy use in California at 52 billion kilowatt-hours annually. This computes to about 6 GW of continuous power expenditure, or 160 W per person. Not a trivial amount. Of the 52 TWh, 32 are performed by the end user for heating, cooling, moving, filtering, or pressurizing the water. The other 20 TWh goes into pumping water across the state, including over mountain ranges. That’s about 8% of California’s electricity demand!

As a lark, if we dumped all 52 TWh of energy into the annual California water supply (40 million acre-feet: just listen to the Europeans howl), how much would the water warm up? Answer: about 1°C. Not all that much. But it illustrates the degree (ha!) to which water heated by 40°C for a shower or by 80°C for cooking pasta is high-value water, relative to the rest.

Combining the energy and volume numbers in another way, each gallon of water in California comes with an average energy price tag of 0.004 kWh, or 0.0015 kWh for the pumped-delivery charge alone. This means that a 1 hcf unit costs just a bit over 1 kWh to deliver.

Water for Energy

But there is another direction to consider as well. Power plants generally use water as a conveyance for waste heat, and withdraw far more water than any other entity in the U.S. (including agriculture). According to the EPA, every kWh of energy delivered demands the participation of 25 gallons (95 L) of water. Most of this is either returned to the source (warmer by 17°C, I calculate) or preserved in a closed system.

But 2 gallons are lost for each kWh of energy delivered (regionally variable: much higher in arid regions). Luckily, the resulting kWh is sufficient to deliver another 750 gal to the door, so there’s no spiraling trap preventing us from consuming water in this way.

Obviously, hydroelectricity is another place where water and energy collide. In drought conditions, municipalities may increasingly face decisions about whether to use the contents of their reservoir for water or electricity. Interestingly, according to Webber, each kWh of hydroelectric energy produced results in the loss of 18 gallons of water due to increased evaporation off of manmade reservoirs — over-and-above what would have happened in the natural run of the river. So hydroelectricity is more consumptive than the thermo-electric plants by a fair margin.

Production and processing of our fossil energy commodities also require the use of water. Gasoline consumes a few times its volume in water for production. But from an energy standpoint, at 36.6 kWh/gal, every kWh of energy available in the gasoline takes a small fraction of a gallon of water to produce. Thus electric cars turn out to be more water-intensive than gasoline cars, for instance, if each kWh of electricity production gulps a few gallons of water.

For tar sands, the ratio is about 5:1 water:oil-product, plus more for refining. A similar number accompanies oil shale extraction, but in the U.S. the oil shale is primarily situated in arid regions where water availability is limited.

Hydraulic Fracturing (“fracking”) for natural gas appears to require 10 L/GJ of water consumption. For the energy equivalent of one gallon of gasoline, this works out to just about one liter—so not as intensive as tar sands or oil shale, or even gasoline. Mostly this is because gas is happy to flow on its own with little help from water: the water only has to create the cracks (fracks?). However, water does enter the equation in other ways for fracking. Contamination of ground water is a principal concern, and some residents have complained of being able to ignite the effluent from their kitchen sink faucets!

But the real water hog for energy purposes is biofuels. Each gallon of fuel produced may consume something on the order of 1,000 gallons of water (regionally variable). That’s about 25 gallons per kWh, to put it in the same units as we discussed for power plants. Naturally, the impact depends on the degree to which irrigation is employed vs. natural rainfall. I would assume that the amount of water used in the refining/processing is similar to that for gasoline and therefore does not contribute much additional burden.

Stay tuned for the second part of this two-part series, and later this week we’ll publish The energy-water nexus Part II!

This post originally appeared on Tom Murphy’s blog, Do the Math: Using physics and estimation to assess energy, growth, options.

Tom Murphy is an associate professor of physics at the University of California, San Diego. An amateur astronomer in high school, physics major at Georgia Tech, and Ph.D. student in physics at Caltech, Murphy has spent decades reveling in the study of astrophysics.

Image courtesy of JoshuaDavisPhotography, laRuth, Photoctor, dr_relling, RichardBarley, RennettStowe.

2 Responses to “The energy-water nexus Part I”

  1. Misleading comparison for Hydro generating plants…” Hydroelectric dams are associated with a significant amount of water consumption for
    power generation primarily because the increased surface area of man-made reservoirs
    beyond the nominal run-of-river accelerates the evaporation rates from river
    basins.[21] Notably, the estimates for this increased evaporation depend significantly
    on regional location. Furthermore, whether all the evaporation should be attributed to
    power generation is not clear, as reservoirs serve multiple purposes, including water
    storage, flood control, and recreation.”… Please compare on the same basis… where does come from the water for the other generating plants… from lakes, rivers, reservoir… then it’s not equal to zero for them compare to hydro generation or it’s zero for both of them!

  2. Kelly S.

    Reblogged this on A Girl in Motion and commented:
    Worth a read – as well as the follow-up article (Part II). We had a discussion at dinner last night on the energy water nexus as it relates to fracking. We are delaying an inevitable move away from exploiting finite resources by putting our water resources at risk, which are likely the next to come under pressure, in order to extract the last of our fossil fuel resources.