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07/10/2008
Elusive Hot Energy
With gas prices soaring, virtually everyone is paying attention to
all the talk about alternate energy sources. The debacle resulting
from the conversion of significant amounts of corn to ethanol
and the accompanying increases in food prices illustrates how
complicated it is to wean ourselves away from depending on oil.
Even such a seemingly wonderful and green source of energy,
wind power, garners opposition from those worried about the
killing of birds to those who object to wind turbines spoiling
their views from seaside residences. With the presidential
campaign in full swing, the politicians scramble to come up with
energy programs or ideas that appeal, or some might say, pander
to voters.
The thought occurred to me that this year we’re beginning to see
what the future will look like several generations from now, or
maybe even sooner. Already, airlines are cutting the number of
flights. Undoubtedly, sometime in the future when oil really
becomes scarce, there will be little or no oil for jet fuel and the
age of extensive air travel will be over! It seems to me that must
be the case unless some huge invention or discovery of a new
energy-dense fuel is in the cards. Will sea travel come back to
take air travel’s place? I don’t know. I talked recently with
Alfred, a fellow mall walker who owns a small boat. Alfred says
it takes a gallon of diesel fuel a mile to run his boat. A 20-mile
jaunt ends up costing him almost a hundred dollars. He’s not
taking any long boat trips anymore and, like many others, is
trying to sell the craft. There are no buyers. Alfred did point out
that, if overseas air travel becomes economically untenable,
steamship travel using coal might make a comeback.
Perhaps I shouldn’t be so pessimistic about air travel. In a short
news article by Robert Service in the June 20 issue of Science,
Service mentions that in 1964 an engineer at Raytheon, William
Brown, came up with a flying helicopter borne aloft by a small
engine powered by microwave energy. Brown used what he
called a “rectenna”, an antenna that picked up microwave power
together with a “rectifier” that converted the microwave energy
to electricity. The article doesn’t say whether anyone actually
flew in this craft so I decided to search the Web for Mr. Brown.
On an IEEE Web site I found the answer. William Brown was
quite an inventive guy and was up in the high levels of
management at the Raytheon company. He demonstrated his
helicopter on Walter Cronkite’s CBS Evening News program.
The helicopter was definitely not a manned craft. It was a
tethered model that, from the pictures, seemed to be essentially a
structure of rods and/or wires in a rectangular boxy arrangement
that looked to be about a yard or two square and a foot or so high
with a helicopterish propeller on top. There was a picture
showing the contraption in the air so the feasibility of flight
under microwave power was demonstrated.
Over the past decades I’ve seen many proposals (or is it many
articles about the same proposal?) to put solar collectors in space
that would beam microwave energy down to Earth. I know of no
serious effort or plans to invest the huge amount of money such a
program would require. One wonders, of course, what happens
if one flies through one of those beams of microwave energy?
Would one get cooked, as in your microwave oven? If the
answer is yes, those air traffic controllers would have one more
thing to worry about!
OK, I’ve managed to fill a page or so with speculations about
energy matters. Let’s finish with another speculative item that
was the real reason for this column, a novel type of solar cell that
captures heat instead of light. In your normal silicon solar cell,
photons of visible light (and some ultraviolet light also, I’m
assuming) hit and are absorbed by the silicon, where the photons
supply enough energy to break the bonds between silicon atoms,
kicking off electrons. In a solar cell, which contains a p-n
junction, the charges at the junction attract the electrons,
which end up as an electrical current. It’s a bit more
complicated but that’s good enough for our purposes here.
The Science article by Robert Service reports on the work of
Steven Novack and his colleagues at the Idaho National
Laboratory in Idaho Falls. They have been working on capturing
infrared photons, commonly known as heat. Instead of using a
conventional solar cell, they harken back to old Bill Brown, who
died in 1999, and his use of antennas to pick up microwave
radiation. For antennas to pick up radiation such as microwaves
or radio waves, it’s desirable that the size of the antenna be at
least roughly comparable to the wavelength of the radiation
being detected and picked up. Infrared radiation lies in the
wavelength range of about a millimeter to 750 nanometers
(billionths of a meter).
This means that you can have millions of tiny antennae on a
piece of silicon or a piece of plastic and pick up infrared photons
from the Sun. Novack and his team have harvested as much as
80 percent of the infrared photons hitting their arrays of tiny
antennae, made by patterning spirals of gold on silicon or on
cheap plastic. With the plastic they still harvest 40 to 50 percent
of the infrared photons. All this is great. These infrared plastic
cells could be made very cheaply compared to silicon
conventional silicon solar cells. The Sun gives off lots of heat
and converting that to electricity should be a real advance.
All this sounds fantastic but wouldn’t you know there’s a catch.
When these infrared photons are picked up by the gold antennae,
they prompt the gold’s electrons to oscillate back and forth at
about 30 trillion times a second. This is a slight problem – the
electricity we use in this country generally oscillates at a mere 60
times a second! To use these electrons they have to be slowed
down somehow to our lower frequency. Right now, there
doesn’t seem to be a way to do that. Another solution would be
to convert the oscillating (AC) current to DC, direct current,
which doesn’t oscillate. Brown’s “rectenna” apparently did that
with microwaves and other types of rectifiers do the job at lower
frequencies, but not at such high ones.
So, here we are with another potential alternate energy source but
a critical stumbling block stands in the way. Energy – it ain’t
easy! (If this column is even more disjointed than usual, I blame
it on an unusually heavy schedule of medically related and other
commitments.)
Allen F. Bortrum
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