Letters
Scramjets
Thanks for the article on hypersonic craft
by Dean Andreadis, “Scramjets
integrate air and space” (August/September, pp.
24–27). I had an idea for increasing the efficiency
of aircraft at hypersonic speeds that
your readers might enjoy: use the boundary
layer air around the entire aircraft (thus
reducing the ram drag) by ingesting it in the
inlets. The boundary layer air would be used
to feed a turbojet, and the inlet air would
feed a scramjet, with both modes operating
at the same time. Here
is a Web post to the sci.astro newsgroup that describes the idea. Robert Clark
Widener University
Philadelphia, Pennsylvania
Energy machines
 In “Big green energy machines” by Jesse
Ausubel (October/November, pp. 20–24),
the author laments that while computers
have grown smaller and more powerful over
the last 25+ years, thus enabling the Internet,
power stations are still about the same
size, if not bigger. As a result, power stations
have not achieved much of an increase in
“power density,” and there is the same sort
of centralization of power production that
we had in mainframe computer days. The
reason power stations are big is that the
input fuels supply only a limited amount of
energy per unit weight; and to generate
3,000–5,000 MW per station, you have to
input or process a large amount of “stuff” on
a continuous basis, all of which requires big
plant and machinery to move it around and
large-bore pipes to carry the fuel to the burners.
To get 10,000 MW of power from the
proposed big zero-emission power plant
(ZEPP) at 70% efficiency, you need to push
about 300 kg of methane and a similar
amount of oxygen into the burner every second—
a process that takes a big burner. Pressurizing,
pumping, and monitoring that sort
of mass flow also take a lot of equipment, as
does removing the waste gases.
Moreover, to bring up a counterargument
to the author’s preamble, 50 years ago cars
were about 20 ft long, 8 ft wide, and 5 ft
high, so to keep pace with computers,
shouldn’t they now be 2 ft long, about a foot
wide, and 6 in. high?
The author states that we put methane
and oxygen in and get only CO2 out. Methane
is CH4, so we actually get CO2 and H2O out,
and the output product is fizzy water under
extreme pressure. I’m not sure what this
does to existing materials, but early powerstation
experience usually included nasty
surprises, for example, stress
corrosion cracking that led to a turbine burst at the
Hinkley Point B station in 1969. So the predictions that the ZEPP would
be easy or cheap, or even necessary, seem
overly optimistic.
The diagram of the ZEPP seems to show
that the CO2/H2O is recirculating. Will
methane and oxygen even burn in a CO2/H2O working fluid?
Won’t the cost of separating O2 from the
normal atmosphere make the whole thing
uneconomic? Couldn’t we get similar energy
density and reduced pollution by using pure
O22 in a conventional gas turbine, which is
what the ZEPP basically is? If you push air
into the burner, reaction with nitrogen and
other trace elements would totally negate
the zero-emission part of the plan.
Do power electronics exist that can rectify
a 10-GW power flow from 30 to 60 Hz? The
author proposes a 10,000-MW generator
with a 30,000-rpm shaft speed.
The second proposal, for a combined
hydrogen–electricity pipeline, seems fatally
flawed in today’s uncertain times too, given
that a small explosive device or a carefully
placed series of devices could sever both
forms of power transmission across the
continent in short order. That is, one
bomb, and we’re left with no electricity and
no hydrogen. Roger Cowles
Marblehead, Massachusetts
[Author replies: Roger Cowles asks many
excellent questions. Limited by space, I
will comment on two, ZEPP design and
footprint. The TIP cartoon of the ZEPP was
not an engineering drawing, and several
readers noted its incomplete design. TIP
reader Don Jenkins, former head of propulsion
for General Dynamics, has already
offered excellent suggestions for starting
the concourse of designs for which my
paper called. I look forward to working
with Don and other TIP readers. We can
certainly deal with fizzy water, but other
nasty problems, such as by-products of the
very high temperatures, will arise and keep
industrial physicists employed.
As for power plant mass and equipment,
pipes will come and go, but high pressures
can narrow them, and siting the plants as
adjuncts to existing natural gas pipelines
minimizes the need for more acreage. The
acreage of a coal plant, including its rail yards
and carbon heap, would easily accommodate
a ZEPP producing 5–10 times the kilowatts,
including associated oxygen or other facilities.
As for shrinking cars, one can think of
the Intelligent Transportation Systems and
many other auto developments, even seatbelts,
as a strategy to increase the flux of
vehicles through an infrastructure of the
same size by denser packing at higher speeds.
Magnetically levitated trains could finally
offer us much lighter, more-compact bubbles
in which to move around.]
As a science writer who deals with ways
to present data to the public, I have a suggestion.
Figure 4, showing how the efficiency
of power generation increases, should
include data points on the Space Shuttle
main engine, which operates in similar
temperature and pressure regimes, namely
~490 atm and ~3,000 K at 37,000 rpm.
The high-pressure pumps are smaller than
what is envisioned for a ZEPP, but the data
points would help show people that the
numbers are achievable.
Information
on the Space Shuttle main engine:
Dave Dooling
Sunspot, NM
[Author replies: Mr. Dooling is right to
point to rockets, and I expect many of the
technical ideas for ZEPPs to come from
rocket engines. A cruise missile engine and
the furnace in your basement may compare
in size, but they differ a bit in power. The
challenge is to make an engine that lasts,
say, 300,000 hours.]
Jesse Ausubel’s article was disappointing
to this advocate of novel but scientific
approaches to practical, clean, sustainable
energy. The article presents no thermodynamic-
cycle efficiency analysis. It also displays
poor appreciation of realistic potential
advances in materials science (both ceramics
and metals); no appreciation of recent
progress in advanced biofuels, wind, or
solar; short-sightedness with respect to natural
gas and nuclear resources; and no
appreciation of the many serious challenges
associated with a hydrogen economy.
At the United Nations-sponsored Expert
Workshop on Abrupt Climate Change (Sept. 30–Oct. 1, 2004), it was shown
that recent progress is beginning to allow advanced
biofuels to compete economically while
dramatically reducing greenhouse gas emissions.
A number of recent analyses show
the highly hyped “hydrogen economy” to
have little chance of living up to any of its
promises in the foreseeable future.
Advanced concepts were shown to have
the potential to produce economically competitive
biofuels (even at oil prices 30%
below today’s prices) with high energy balance;
achieve negative carbon emissions (via
sequestration), along with highly effective
soil-fertility enhancement; substantially
enhance quality of life, especially in developing
countries with limited oil resources; provide
a higher degree of energy security in oilimporting
nations than any other known
option; and contribute to improved health
through the diverse benefits that come from
clean, sustainable, economical energy (such
as improved quality of water and air, a reduction
in international conflicts, and increased
overall economic growth).
I suspect that within a decade at the
most (and probably sooner), the “hydrogen
economy” will be seen to fit in the same
categor y as oceanic thermal gradients.
Readers may find my recently updated
paper interesting and informative: “Fuels
for tomorrow’s vehicles”.
Physicists do themselves and their profession
a disservice by presenting starryeyed
visions with poor appreciation of engineering,
economic, and societal realities.
F. David Doty
Doty Scientific, Inc.
Columbia, South Carolina
[Author replies: Having spent much of my
career working on biodiversity and habitat
for nature, I conclude that biofuels are a
catastrophic use of land on scales that matter
for the United States or world energy. A
huge environmental challenge is to reduce
the human footprint in green nature, to
shrink farming and logging. Many papers at
http://phe.rockefeller.edu address sparing
land. For more on the sad facts of wind,
solar, and biomass, see http://phe.rocke
feller.edu/PDF_FILES/NEIrevision11june04.
pdf and http://phe.rockefeller.edu/Dec-Jan0418/11/04/PDF_FILES/neigraphics11junere
vision.ppt.
Chronic fears about exhaustion of
natural resources abound, and so do
the resources. The famous King
Hubbert charts project human fears,
not geology. Dr. Doty’s paper does
offer welcome realism about using
retail hydrogen for transport. Yet,
globally, the hydrogen industry is
growing nicely, although it is not driven
by the start-ups of the 1990s.
Visit the Dow facility in Freeport,
Texas, to see hydrogen operations at
the scale of hundreds of megawatts.
From the Doty Scientific Web site,
it sounds like your company does
terrific things and that you have a strong
appreciation of technical change. Put your
mind back to 1904, and then think forward
to 1950 or 2000 (the time scale for our big
green energy machines) and the changes in
scale and character of the electric power
enterprise. Compare today’s nuclear magnetic
resonance machines with their ancestors
that came out of General Electric’s Milwaukee
plant. I will wager that the thinking
behind Doty microturbines will matter more
for energy systems than algal ponds in the
Sonoran desert. You should take heart from
your own achievements.]
Although I am not qualified to fully
judge the article’s details, your ideas appear
to be reasonable and workable, and I was
able to understand the basics. Your article
enables one to better imagine future possibilities.
I also appreciated the inclusion of
the Bankside power station in the U.K. as a
historical note.
However, at this geopolitical time, centralized
energy generation and distribution
components would be a terrorist’s dream.
To me, it seems better to have an arrangement
that follows the Internet more closely,
that is, one using smaller generating components
interconnected in a survivable grid
arrangement. Our defense industries have
followed the principle of geographic dispersion
to achieve some measure of survivability
in the event of a national catastrophe.
Bob Downs
Development Associates
Tustin, California
[Author replies: The United States has
about 8,000 large generators. Suppose the
fleet dropped in half because of the success of
ZEPPs and other larger machines, even as
demand rose. If the generators were networked
in a smart mesh that moves electric
power around at a continental scale, saboteurs
(or earthquakes) would probably need
to remove 400 or more plants (10% of the
supply) to make a big problem. Not so easy.
Think of the Supergrid concept as an “eBay” for power, in which everyone
can buy and sell, no matter where you are, with transaction
costs very low. Idaho could be the Saudi
Arabia of hydrogen, shipping to all corners of
North America. Creating this mesh with very
low cost for transport, and putting it underground,
should lift security.]
Regarding the big green energy machines, I
have two problems. The first is that the input
is natural gas and oxygen. The oxygen problem,
which is a huge one, is noted, but not
noted is the fact that natural gas is a scarce
raw material, and too valuable (as a chemical
feedstock) to burn. Somehow, nobody in
developing energy scenarios wants to look at
the supply of fuels.
A table from the Department of Energy's Energy Information Administration
of best-guess worldwide fossil- fuel resources shows that coal far
outweighs gas and oil. I say, We had better use coal or we'll be
out in the cold. Coal can substitute for natural gas in the ZEPP,
and the CO2 can be sequestered from the gas cleanup system.
(By the way, natural gas must be “sweetened”—have
the sulfur compounds removed—before it can be used.) No matter
how we wiggle, we aren’t going to be able to avoid reliance
on coal. In addition, the nuclear option must be considered, but
only if it is possible to “breed” (produce more fissionable
material with surplus neutrons). My second problem is that there
is no economic analysis, and when an analysis is complete with capital,
operating, maintenance, and variable costs and realistic values
for the product-plus-sensitivity analysis, I believe the results
will not be “bankable.” The problem with energy systems
is that they permeate the entire economic framework. For example,
$50/bbl of oil does a whole lot more to the economy than raising
the price of gasoline 50 cents.
E. Gerald Meyer
University of Wyoming and Coal
Technology Corp.
Laramie, Wyoming
[Author replies: We can and should avoid
reliance on coal. The question of the
amounts (and origins) of hydrocarbons
remains open. A team led by Henry Scott
reported in a recent paper entitled “Generation
of methane in the Earth’s mantle” (Proc. Natl.
Acad. Sci. U.S.A.,
Sept 20, 2004) that the hydrocarbon budget of the
bulk Earth might be much larger than conventionally
assumed, for better and worse.]
Correction
In the October/November issue, p. 23,
Figs. 4 and 5 incorrectly included a credit
to Ian Worpole. |
