Chapter III, Section E: An Ultimate Resource Limitation for
The only byproduct of hydrogen fuel cell energy output is pure
water, but with water as a source of the hydrogen, the pure water
leaving a fuel-cell is derived from purified water entering an
electrolyzer, a chemical electrolytic cell used for the production
of hydrogen. Electrolysis can be conducted on either freshwater or
saline water, however freshwater electrolysis is more efficient,
putting more of the input energy (electricity) into hydrogen rather
than byproducts. Freshwater electrolysis requires specific chemical
solutions to be made with deionized water. Thus to avoid byproducts,
the electrolysis of seawater requires initial desalination, which is
also energy intensive. Desalination is increasingly being used as an
option for producing fresh drinking water in arid regions, but the
energy expense is only justified by the high population demand for
drinking water in areas such as southern California. Alternatively
the electrolysis of seawater or brine can be conducted using
chlor-alkali electrolysis, but in addition to hydrogen, much of the
energy is sunk into other compounds, such as chlorine compounds [ClOx and HCl] and sodium
hydroxide [NaOH], requiring either market use or disposal options for these
byproducts. It turns out the sodium hydroxide [NaOH] can be used to sequester carbon.
Always the optimist and a champion for hydrogen, Rifkin extolled a
1995 project that used solar energy to produce hydrogen with an
electrolyzer (Rifkin, 2002, p. 188). The hydrogen was then used to
power a fleet of trucks using hydrogen combustion in vehicles,
similar to vehicles converted to run on propane gas. Though the
location of this project was coastal El Segundo, California, the
water was de-ionized municipal water from the city’s water supply.
With potable water in relatively short supply, especially in
California, should it really be used for the production of hydrogen?
Reacted in a fuel-cell or in combustion, hydrogen produces pure
water and becomes part of the hydrologic cycle. It is thus fitting
to apply traditional concepts of source water consumption. The fuel
economy of hydrogen fuel cell cars is now well known, with
kilometers per kilogram hydrogen replacing the familiar miles per
gallon gasoline. A value of about 110 km/kg is a good estimate for a
small hydrogen fuel cell vehicle. This fuel economy can also be
expressed in water consumed to make the hydrogen, clearly
elucidating the water consumption implications of a hydrogen
economy. The fuel efficiency is equivalent to 12 km / liter of
water, or if you’d rather, 28 mpg water.
According to 1999 data from the U.S. Department of Transportation,
130 million passenger cars traveled an average 51 km (32 miles) per
day. The fuel efficiency of hydrogen fuel cell cars would translate
this driving into a consumption of 160 million gallons per day of
highly-processed de-ionized water, and that’s just for the
passenger-car-fuel fraction of the hydrogen economy. Given water
scarcity, if we truly want a hydrogen economy, recycling the highly
processed fuel-cell water will be essential.
Geologists are considering calling the present the close of the
latest geological age, the Holocene, and the dawn of the next, the
Anthropocene, a new geological age marked by the human footprint.
The Anthropocene would recognize humans as the greatest agents of
geomorphic and biologic change, from human modifications of the
environment and human-induced extinctions reducing biodiversity. If
we are truly overwhelming nature’s cycles, it is the life out of
balance that requires additional energy to sustain the environment,
beyond the natural energy inputs we take for granted.
How much energy is required to sustain the environment? An amount
that dwarfs human consumption, though it goes mostly unnoticed.
Solar energy is ultimately what removes environmental contaminants,
driving the hydrologic cycle and many other biological nutrient
cycles that either remove the contaminants or recycle them into the
truly-renewable resources. What is that energy worth? Enough to
economically consider the renewable resources–clean soil, air, and
water–as equivalent solar energy resources. A complete energy
economic system would assess the energy value of the order inherent
in these resources. From this economic viewpoint then, solar energy
is actually our most utilized energy resource already. How can these
renewable resources be economically assessed in a new energy-based
economy? The second law.
In the next discussion, an environmental balance sheet will be
presented, a broad overview of our environmentally related energy
assets and debits.