Chapter IV, Section A, Item 2:  The Energy Asset of Freshwater

The Earth’s hydrologic system puts an investment of solar energy into delivered freshwater. The Earth’s ecosystems receive freshwater from precipitation ultimately liberated from ocean water, and this freshwater facilitates all of the other nutrient cycles of an ecosystem that make life possible. A growing awareness of the value of that investment is at the heart of the growing environmental awareness.

Freshwater is not a fuel, so how can this energy investment in freshwater be measured in the complete energy economy? Though it may seem like energy thinking in reverse, the second law does provide an exact measure of the energy value of freshwater, expressed as losses as the freshwater returns to the sea. The losses include both the dissipation of gravitational potential energy, including that dissipated through our hydroelectric systems, and the actual mixing of fresh and saltwater. The reverse order of these two events is the forward order of the energy invested in freshwater delivery.

Contaminated water can be treated, but it takes energy and other expenses.  Though some contaminant treatment requires even more expense, the example of salt contamination (salinization) provides a baseline of energy expenditure.  When 35 grams of salt are mixed into a liter of pure freshwater, producing a concentration equal to seawater, energy equal to 20,500 Joules, about the same as a gram of sugar or 5 diet calories, are lost to the universe. The temperature did not change, which is to say no energy was delivered to or absorbed from the surroundings. No reaction took place. What happened?  What’s the problem?  Why can a loss be expressed in terms of energy? The bottom line is that the entropy of the universe changed. The system of salt and water moved from ordered to disordered, from the order of perfectly separate, unique entities of salt and water, with the potential for change, to the disorder of a dispersed, uniform mix of inertness. During the actual mixing, the potential energy did some work within the system, moving molecules apart, even creating pressures on membranes that may have existed between the salt and water, such as the cell wall of some unwitting microbe, subsystems within the salt and water system. But no energy was exchanged with our surrounding system, so how does it affect us? The energy that was lost is noticed by us when we want the salt and water separated again.

A finite amount of energy associated with the entropy change was dissipated to the universe in the mixing of the salt and water. Unfortunately an infinite amount of energy would be required to return a closed saltwater system to the absolutely pure state of the separate water and salt that existed previously, theoretically an infinitely ordered state. An absolutely pure state of water is not needed for drinking, and thus desalination is possible and in some cases, economically feasible. The 35 grams of salt in a liter of seawater needs to be reduced to a half gram to make potable water at a target concentration of 500 ppm. Given a large source of salt water–the ocean–desalination of relatively smaller quantities of potable drinking water is certainly feasible and locally implemented.

Desalination plants are open systems that continuously exchange an input volume of seawater into a small percentage output volume of potable water, but they thus produce a large percentage output volume of brine, a saltwater that is even more concentrated than seawater and must be disposed of environmentally. After all, the salts, referred to as “concentrate,” have the potential to increase entropy by mixing with freshwater, i.e. to cause the loss of available energy in a system by moving it to an unavailable form. Reverse osmosis desalination plants can currently operate at about 4 kW-hr of energy per cubic meter of potable freshwater produced, slightly less than the entropic energy lost in mixing an equivalent volume of seawater. This is the equivalent of 14 Diet calories for every gallon of water. Thermal distillation desalination plants are more advantageous when heat energy is more abundant, e.g. with solar energy, but are more energy intensive, operating at about 17.5 kWatt-hr / m3, roughly 3 times the energy of mixing. In either case, a geometrically increasing amount of energy is needed for increasing water purity.

Liberating the freshwater from seawater is only the beginning of the required energy investment in water: the water needs to be delivered. Californians are notorious for living in places they shouldn’t, and engineering around the admonishments to make that living possible, if only temporarily. The California Aqueduct provides an estimate of the energy value of water conveyance. The largest aqueduct in the world, it moves freshwater from the Sacramento Delta 444 miles and over 3,500 feet of elevation to the dry populace of Southern California. Ironically the delta region source receives the flow of the Sacramento and San Joachim Rivers, water collected largely from the western slope of the Sierra Mountains, and the aqueduct returns the flow up a large portion near the route of the San Joachim River itself. A recent study by the California Department of Water Resources summarized the Califormia Aqueduct’s pumping energy requirements and power generating recoveries along the entire system, with a net energy of 4.4 kW-hr used per cubic meter of water conveyed the entire length of the aqueduct, 444 miles. This is slightly more than the energy requirements of reverse osmosis desalination. Energies now being equal, desalination is becoming a competitive alternative for Southern California coastal communities. However, living in a dry place 444 miles from the coast would require both the combined desalination and conveyance energy inputs.

A gallon of rainwater collected 444 miles from the ocean thus has roughly equal parts desalination and conveyance energy invested in it, each part about 14 Diet calories. Another 14 Diet calories must be added for each 444 miles from the ocean. Thus the rainwater in Chicago, water largely taken for granted, is equal to about 55 Diet calories per gallon. This energy is a minimum. It assumes reverse osmosis desalination, and does NOT include the equivalent energy costs of building the desalination plant and aqueduct infrastructures, NOR include the equivalent energy costs of those infrastructures’ operation and maintenance. An area that cannot take its water for granted, Phoenix, is about 170 miles from the Gulf of California in Mexico, 300 miles from the Pacific Ocean near San Diego, and 1,450 miles from Lake Michigan near Chicago. Assuming water from Lake Michigan is not available, desalination, and access to the ocean through Mexico, Phoenix water is a minimum energy equivalent of 19 Diet calories per gallon.

If we just consider domestic potable water use in gallons, a mere fraction of total water use, the energy value is illuminated by multiplying that volume times the energy per gallon. In a recent New York Times magazine article on the impending effects of climate change on water supply, Jon Gertner quoted several western cities’ domestic water uses, ranging from 125 to 160 gallons of water per person per day. Even using the low-end per capita water consumption, combined with the low-end energy equivalent of the California seawater desalination and aqueduct conveyance example (reverse osmosis desalination, aqueduct-conveyed 444 miles from the sea, not including infrastructure and maintenance costs), the equivalent energy value of water consumed by a US population of 300 million people is over 30 million gallons of gasoline per day. Compare this to the 390 million gallons of gasoline per day consumed in transportation.

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