Re - New solar sail proposal might win one of NASA's CentennialChallenges near term.

I was reading about methods of concentrating sunlight onto solar
cells using large lenses and mirrors when I came across this method:

Cool Earth Solar: Technology.
http://www.coolearthsolar.com/technology

This provides a lightweight means of increasing the power output of a
solar cell by up to 400 times by focusing the light from a large
collecting area reflecting surface onto the solar cell. Since a space
based solar cell might be able to get 100 watts of power per kilo of
weight, but a thin reflecting surface would only weigh a small
fraction of this, this essentially means the power to weight for the
space solar power system could be increased to about 40,000 watts per
kilo(!)
It seems to me this is what was being described on this page:

Of Lightsails and Solar Arrays.
by Administrator on May 8, 2006
"Landis speculated on a lightweight sail that focuses power on a small
solar array, noting that a basic problem with laser-propelled
lightsails is their low energy efficiency:

'The energy efficiency may be greatly improved, at the cost of a
reduction in specific impulse, by combining the laser sail with a
photovoltaic powered electric (ion) engine. Ion engines in principle
have no physical limits on the specific impulse, although extremely
high specific inpulses require proportionately high energy
consumption. Such a laser-powered rocket would have the ability to
decelerate at the target star (with some loss of efficiency), and
could also greatly decrease the amount of power required from the
laser.'

"Landis then presents a schematic for a rocket like this, with solar
array mounted so that the sail acts as a mirror to focus light on it."
http://www.centauri-dreams.org/?p=652

EXCEPT the Rudolf Meyer proposal is for a simpler system that only
uses solar light, not a laser, to power ion engines within the solar
system.

In any case the key fact is that by just using already existing
lightweight reflecting material, instead of trying to solve the more
difficult problem of creating ultra-lightweight solar cells, we
achieve the same thing discussed by Rudolf Meyer and can create 1-year
to Pluto solar powered ion drives.


Bob Clark


================================================== =
Newsgroups: sci.astro, sci.physics, sci.space.policy, sci.energy
From: "Robert Clark"
Date: 7 May 2006 15:39:00 -0700
Subject: New solar sail proposal might win one of NASA's Centennial
Challenges near term.

NASA unveils its toughest challenges yet.
17:15 09 February 2006.
"The Station-Keeping Solar Sail Challenge A solar sail pushed through
space by the force of the Sun's photons to a target could earn $2.5
million with an equal amount available for keeping a solar sail for 90
days at a fixed point in space."
http://www.newscientistspace.com/cha...ght/dn8701-nas...

Via Solar Array to the Outer Planets.
"New Scientist is covering the work of Rudolph Meyer (UCLA), who
envisions a vehicle that sounds for all the world like a cross between
a solar sail and an ion engine. And in a way, it is: Imagine a
flexible
solar panel a solid 3125 square meters in size, and imagine this
'solar-electric membrane' weighing no more than 16 grams per square
meter, far lighter than today's technology allows."
....
"Update: Geoffrey Landis was kind enough to forward the complete text
of his comments to New Scientist (the magazine quoted only the last
sentence). Landis wrote: "Professor Meyer suggests an interesting
thought-experiment about what may be possible in the future. The solar
array needed for his mission requires reducing the mass of solar
arrays
by several orders of magnitude from existing technology."
http://www.centauri-dreams.org/?p=638

The solar sail is required to only weigh 50 kg for 3125 square
meters.

But a recent advance involving carbon nanotubes can produce thin
sheets
of arbitrary size for which a 1 kilometer square sail, or 1000m x
1000m
= 1,000,000 square meters, would only weigh 30 kg:

Researchers produce strong, transparent carbon nanotube sheets.
"Strength normalized to weight is important for many applications,
especially in space and aerospace, and this property of the nanotube
sheets already exceeds that of the strongest steel sheets and the
Mylar
and Kapton sheets used for ultralight air vehicles and proposed for
solar sails for space applications, according to the researchers. The
nanotube sheets can be made so thin that a square kilometer of solar
sail would weigh only 30 kilograms. While sheets normally have much
lower strength than fibers or yarns, the strength of the nanotube
sheets in the nanotube alignment direction already approaches the
highest reported values for polymer-free nanotube yarns."
http://www.physorg.com/news5890.html

The sheets do not have the strength of individual carbon nanotubes but
are stronger than steel on a per weight basis.

To be made into solar cells will require the sheets to made in both
p-type and n-type semiconductors. The sheets appear to be formable
into
both types:

Strong, Transparent, Multifunctional, Carbon Nanotube Sheets.
SCIENCE,VOL 309, 19 AUGUST 2005, p. 1215-1219.
"However, black sheets of solution spun
MWNTs have been used as nontransmissive
hole-collecting electrodes in solar
cells (10), and transparent p-type SWNT
sheets have been used as hole-injection electrodes
in inorganic LEDs based on gallium
nitride (32)."
http://www.cnrs-imn.fr/GDRE_NanoE/Te...rentsheets.pdf

Transparent, Conductive Carbon Nanotube Films.
Science, 27 August 2004:Vol. 305. no. 5688, pp. 1273 - 1276.
"For the nanotubes, the ease of chemical charge-transfer doping to
obtain such transparency-versus-conductivity optimization (via
exposure
of the nanotubes to vapors of appropriate chemicals) provides an
additional advantage for the t-SWNTs. Moreover, charge transport in
these t-SWNTs is p-type, unlike the far more common transparent
conducting oxides [e.g., indium tin oxide (ITO)], which are n-type.
This should permit new complementary applications and alternative
photonic coupling schemes (3)."
http://www.sciencemag.org/cgi/conten.../305/5688/1273

And there are various ways now available for making general nanotubes
of either type:

CARBON NANOTUBE COMPUTER CIRCUITS
Novel processing and microfabrication lead to first single-molecule
logic gate.
"The principal challenge in constructing NOT gates out of nanotubes,
Avouris explained, is that invariably, without special processing,
transistors fashioned from nanotubes are p-type--that is, they conduct
positive charge carriers (holes). But NOT gates require n-type
transistors, the type that conduct negative charge carriers
(electrons), as well as p-type.
"Some researchers have demonstrated recently that doping nanotubes
with
electropositive elements such as potassium is a viable method for
preparing n-type nanotube transistors. Now the IBM team has discovered
another way to do it. Simply heating (annealing) p-type nanotube
transistors in vacuum converts p-type into n-type, they reported."
http://pubs.acs.org/cen/topstory/7936/7936notw1.html

Nanotubes: Surprising Sensitivity To Oxygen Creates New Possibilities.
March 29, 2000
"We've demonstrated that carbon nanotubes can behave as both n-type
and
p-type semiconductors. Until now, all nanotube measurements had
suggested p-type conducting behavior only."
"In their paper, the Berkeley researchers found that the degree of
oxygen exposure is the determining factor as to whether a carbon
nanotube functions as an n-type or p-type semiconductor. The ability
to
function as either type is critical if nanotubes are to ever replace
conventional silicon devices."
http://www.lbl.gov/Science-Articles/...nanotubes.html

And another research team may have already created thin-flim nanotube
solar cells of comparable lightness to the transparent nanotube films:

Sensational Materials: Cheap solar cells based on nanotubes.
Future Materials News - February, 2006
"Professor Nunzio Motta of QUT's School of Engineering Systems and Dr
Eric Waclawik from the School of Physical and Chemical Sciences are
developing the new renewable energy source using nanotechnology.
Weighing only 10 micrograms per square centimetre, it's possible to
generate additional power by linking up the polymer tiles in a
patchwork that increases the size of the device to suit the user's
needs."
http://www.future.org.au/news_2006/feb/cheap.html


Bob Clark
==================================================

Dear Robert Clark:

"Robert Clark" wrote in message
...
I was reading about methods of concentrating sunlight
onto solar cells using large lenses and mirrors when I
came across this method:

Cool Earth Solar: Technology.
http://www.coolearthsolar.com/technology

This provides a lightweight means of increasing the
power output of a solar cell by up to 400 times

400% is not 400 times. It is either 4 or 5 times, depending on
how you figure the boost.

by focusing the light from a large collecting area
reflecting surface onto the solar cell.

Much of the energy gain, is lost in cooling the solar cell back
down.

David A. Smith

On Jul 23, 12:54*am, "N:dlzc D:aol T:com \(dlzc\)"
wrote:
Dear Robert Clark:

"Robert Clark" wrote in message

...

I was reading about methods of concentrating sunlight
onto solar cells using large lenses and mirrors when I
came across this method:

Cool Earth Solar: Technology.
http://www.coolearthsolar.com/technology

This provides a lightweight means of increasing the
power output of a solar cell by up to 400 times

400% is not 400 times. *It is either 4 or 5 times, depending on
how you figure the boost.

by focusing the light from a large collecting area
reflecting surface onto the solar cell.

Much of the energy gain, is lost in cooling the solar cell back
down.

David A. Smith

No it's 400 *times* more. There are several companies using the same
simple idea:

August 1, 2007 9:15 AM PDT
FAQ: A concentrated power boost for solar energy.
By Martin LaMonica
Staff Writer, CNET News
http://news.cnet.com/FAQ-A-concentra...3-6199933.html

It's simply due to the fact that the power put out by a solar cell
depends on how much light energy falls on it. So if you get several
different mirrors or a large reflector to focus their light on that
solar cell then it will put out more power than it would if it were
just using the normal sunlight that falls on its surface.
Since solar cells are expensive, compared to other forms of
electricity generation, but simple reflecting surfaces are cheap, you
can put out much more power for the same cost.
This is also important of course for space applications since the
weight of solar cells make up a big part of the mass of large
spacecraft such as communications satellites, and also for planetary
missions that were previously thought to require a nuclear powered
drive.
For the extra heat produced, efficient heat exchangers can also turn
this into usable energy.


Bob Clark

Dear Robert Clark:

On Jul 23, 6:50*am, Robert Clark wrote:
....
*For the extra heat produced, efficient heat
exchangers can also turn this into usable energy.

No, it can't. The amount of usuable energy is defined by the highest
and lowest temperatures (Carnot efficiency), and the high temperature
must be less than the diffusion temperature for the solar cell. In
the case of silicon-based cells, this is not very hot.

And in the case of "lightweight reflectors", if your active cooling
system fails (and you'd have to have one) you destroy your solar
panels. So now you are back to the mass of 1x solar panels, by the
time you throw in active cooling for all of them, add a redundant
system, or put in rapidly adjustable mirrors for protection.

Now if your pointing system fails, and your "cold sink" surfaces
become lit, you also risk frying the solar panels form the *back*
side...

On Earth, the limitations to light amplification such as you
descriobe, is getting the current out of each chip... (and cooling of
course).

David A. Smith

On Jul 23, 11:03*am, dlzc wrote:
Dear Robert Clark:

On Jul 23, 6:50*am, Robert Clark wrote:
...

*For the extra heat produced, efficient heat
exchangers can also turn this into usable energy.

No, it can't. *The amount of usuable energy is defined by the highest
and lowest temperatures (Carnot efficiency), and the high temperature
must be less than the diffusion temperature for the solar cell. *In
the case of silicon-based cells, this is not very hot.

And in the case of "lightweight reflectors", if your active cooling
system fails (and you'd have to have one) you destroy your solar
panels. *So now you are back to the mass of 1x solar panels, by the
time you throw in active cooling for all of them, add a redundant
system, or put in rapidly adjustable mirrors for protection.

Now if your pointing system fails, and your "cold sink" surfaces
become lit, you also risk frying the solar panels form the *back*
side...

On Earth, the limitations to light amplification such as you
descriobe, is getting the current out of each chip... (and cooling of
course).

David A. Smith


You don't HAVE to have high efficiency conversion of this heat to
electrical power. When conversion rates for solar cells are given the
fact that some of the impinging light energy is given off as heat is
included in that efficiency rate.
Then we can use the propellant gas for the ion engine such as xenon
to carry off the excess heat as the propellant gas is expelled. If we
convert some of that excess heat to further electrical power then that
is just a bonus.
For the weight of the heat exchanger system, we might look at
examples where saving weight is critical. One such is the proposed
Skylon spaceplane. This uses a combined turbojet/rocket engine. It
required a lightweight means of cooling the incoming air at high Mach
speeds. The Skylon team was able to demonstrate a lightweight heat
exchanger capable of 110,000 watts of cooling per kilo of weight by
using very thin cooling channels:

HEAT EXCHANGER DEVELOPMENT AT REACTION ENGINES LTD.
Richard Varvill
Reaction Engines Ltd, United Kingdom
"This type of precooler construction is extremely
lightweight whilst achieving high
thermodynamic efficiency. For example the
SCIMITAR precooler has a predicted mass of
940 kg. At Mach 5 cruise conditions the
precooler handles 172 kg/s of air at a recovered
temperature of 1250 K that it cools to 665 K
whilst incurring an airside pressure drop of 0.4
bar. This gives an installed power/weight ratio
of about 110 kW/kg."
http://www.reactionengines.co.uk/dow...8%20C4.5.2.pdf

For every propulsion system having the cooling system work properly is
critical to the system. The space shuttle main engines and most liquid
fuel rocket engines operate at *above* the melting point of the
combustion chamber metals. If that cooling system fails, the engines
fail and so does the mission.
And even if the radiator in your car engine fails your engine can be
destroyed.


Bob Clark