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.