At 01:14 AM 9/5/2008, Kevin wrote:

>Keith,
>I think you have misplaced a comma or two. That is 1.357kW/m2 not 1357
>kW. 1,357Watt/m2 / 4.66 kg/m2 = 291 Watt/kilogram

Unless I don't understand what they are talking about, the article
notes "The 40.8 percent efficiency was measured under concentrated
light of 326 suns."

So the light hitting the cells would be 1.357 kW/m2 x 326 or 442
kW/m2. Times efficiency that would put out 180 kW/m2. 180
kW/m2/4.66 kg/m2 is 38.7 kW/kg.

>Our thin film cells at 12.4% efficiency rate at 5,880 Watt/kilogram.
>At 2 microns thickness this increases to 16,800 Watts/kg = 16.8 kW/kg.

The trouble with lower efficiency is that the whole power sat is a
lot larger. This costs you in structure and conductors. I am not
convinced that photovoltaic cells are the best approach. They might
be, but steam turbines are extremely well understood and are at least
40% efficient. If we had a pipeline to lunar or asteroid dirt for
making heat transport fluid, steam would be even more attractive.

>Also there is not much heat sink mass.

True. A concentration of 326 will require big heat sinks. But mild
concentration, say 3 especially with some filtering of the parts of
the spectrum that the cells don't use well probably avoids heat sinks.

This is 1400 words of an 8000 word chapter in a novel I was working
on. The chapter describes the space elevator and building a huge
power sat industry.

*************

The junkyard was attached to the space elevator through the so far
misnamed "driver hubs," a pair of pulleys only a few miles
apart. Uplift intended to install big electric motors, but the cable
wasn't yet strong enough to lift motors that big in one piece
yet. UpLift had attached the junkyard to the driver hubs with
breakaway connections in anticipation of the day natural or
artificial space junk cut the cable. The four quadrants of the
junkyard were minute flower petals on an impossibly long stem.

North and south petals of the junkyard held the local photovoltaic
power sources. They were small-scale versions of power-satellite
wings and rotated to stay pointed at the sun. The openings in the
junkyard plane were much larger than the rotating
"wings." Eventually UpLift would extend them to a GW when they
installed motors on the driver pulleys.

Until recently, when the elevator motors started pushing the power
limits of the Enterprise, there had been tension between building up
the elevator cable and extending the construction facilities.

In the three weeks since Marc had last been up, the fourth
beam-spinner had come on line in the East junkyard. Ton spools of
perforated 5-mil Invar foil had been shipped up, and after passing
through the beam spinners came out as one meter by 1 km long channel
beams for the power satellites and the junkyard frame.

The same beams went into the frame for power-sat
construction--humorously known as "dry dock." It had gantries on
both sides that hinged out of the way to launch the power sats to the
east. The power sats slipped out of dry dock by electrical motors at
6 am local time when the transmitting antenna was in the same plane
as the wings.

The little 60-person inflatable habitat was on the west petal,
pointed north/south with a 45-degree sun-tracking mirror to light
it. The space for the family habitat was just a large hole in the
junkyard plane further west of the inflatable temporary habitat.

Dry dock was on the east side, pointed north/south and rotating on
bearings to stay pointing 90 degrees from the sun except for a
non-rotating section in the middle for the transmission antenna. The
antenna pointed down the elevator toward the earth, parallel to the
plane of the junkyard. The dry dock's rotation kept at 90 degrees
from the sun was so the solar cells could be installed "off." A
power sat lacked an "off" switch other than turning it away from the
sun. Inflated white plastic balls on long skinny arms provided light
for the construction crews without generating lethal voltage on the
solar cell arrays.

Power sats were built in the dry dock. The first, Stubby, was taking
shape as a 1/4 physical scale, 1/5th output power to drive the
elevator in the last stages of the cable buildup. Stubby would
demonstrate the technology at almost full scale. Stubby had two
wings like full-scale power sats, but the wings were only two km by
three km instead of five by five km, giving it an overall size of
five and a half km by three km with a one km round transmitting
antenna in the middle.

The bearings, mercury slip rings and transmitting antenna were
installed first, then the two inner end pieces. The beams were
pulled out of the beam spinners from the outside in, stretched one at
a time, spot-welded to the inner end pieces, then spot welded to the
outer ends. The beams were placed ten meters apart, a hundred beams
to the kilometer so there were three hundred two-km long main beams
in each wing. The ton mass of a 1-km beam amused Marc. In
skyscraper construction, a few meters of beam weighed more than a ton.

The construction process resulted in five flat-bottomed troughs in
each wing connected to a rotating transmitter disk in the middle that
always pointed toward the earth. Each trough was 600 meters wide,
280 meters high, with 45-degree reflector sides and 200-meter-wide
pavements of solar cells in the middle. The cells came in rolls, two
meters wide and 200 meters long. The reflectors raised the light
exposure on the cells to three solars. A square meter of solar cells
generated only ten volts, but 200 of these in series amounted to
2,000 volts across 200 meters, and five such strips in series
generated 10,000 volts at a scary 100,000 amps from each wing.

Twenty thousand 100-kW klystrons made up the one km antenna and each
of them used ten amps of current. They came up in hexagonal bundles
of seven and snapped into place. Two ironworkers could put in a
hundred a day. There would be 19 to a bundle when UpLift upgraded
the elevator to 2,000 tons a day.
The plans had called for two km long, aluminized-Mylar reflecting
film to be unrolled on the reflectors. The intent had been to test
the completed power sat in dry dock at full power, then cut it loose
with a number of ion engines to move it into place.

Vacuum degradation around the junkyard and a major flashover/meltdown
on the junkyard's north power wing had the engineers antsy about
powering up a power sat in the construction frame. To improve the
hardness of the local vacuum, the engineers decided to ship up
uncoated Mylar film and coat the film as it was unrolled with
aluminum. There is nothing like a fresh vaporized aluminum surface
to absorb stray gas molecules.

Dry dock's first long compression members had been a major pain to
bend. The ironworkers pulled the beams between the elevator's
unpowered local drive wheels and stretched the beams a few
percent. That made them straight, but it was hard to get the twist
out. The ironworkers chopped up the first half dozen and used them
to extend the junkyard.

After some fast design work by UpLift's engineers, they put real-time
controls on the rollers in the beam spinners that bent the flat sheet
into channel beams and put a laser target on the end of the beam to
detect twisting. The beam spinners then were able compensate for
twisting. After post-stretching, all of the beams since then had
come out in spec for power sats and most of them were good enough to
carry the dry dock's compressive load where the power sat channel
beams were stretched in place.

Marc's main job this shift was to move the beam spinners along the
start end in the construction frame. The frame was only three fifths
of its final width and half its length. When completed and producing
full-sized power sats, the plans called for a thousand beam spinners.

The target construction rate was a power sat every five days, but
that depended on the elevator reaching full capacity, and that
depended on this first quarter scale sat to power it. Full-scale
power sats would eventually weigh ten thousand tons; Stubby was only
a quarter that mass and bringing up its parts had occupied the
elevator for twenty days. In addition, it took another twenty days
to bring up the parts for the quarter-sized dry dock needed to assemble it.

When the rolls of beam stock first started coming up, Marc had asked
Floyd why they were not using steel or aluminum instead of this
expensive nickel alloy.
"Eclipses." Floyd told him. "For a few weeks around the equinox, an
SPS gets eclipsed by the earth, for up to 70 minutes. The darn
things cool by 200 hundred degrees; steel or aluminum would curl up
like a potato chip. Then when the Sun comes back they flap like
wings. The computer simulations were sad. We could put hinges and
dampers into them, but using Invar we just ignore eclipses."

There were only 60 people at the construction yard, but they were
critical, taking on jobs such as building and unjamming the
automation. The speed-of-light delay made it hard to do most jobs
from the ground.

It was amazing how much you could be done in zero-g riding around on
a magnified version of the ancient shuttle arm. In spite of having
to move the beam spinners around, by the end of Marc's three weeks
they were a quarter done with framing Stubby, having pulled out half
the beams on the north wing. Four more beam spinners had come up, so
the next quarter would take only half as long.

*******

Keith

>Kevin
>
>
>
>--- In solarpowersatelliteplace@yahoogroups.com, hkhenson
><hkhenson@...> wrote:
> >
> > At 07:22 AM 8/30/2008, you wrote:
> > >Thy these numbers from SpectroLab as well (
> > >http://www.spectrolab.com/DataSheets/Panel/panels.pdf ):
> > >370 W/m2, mass add-on coverslide needed on these cells for space
> > >launch front side = 1.76 kg/m2 (5.5 mil thick cell), back side 2.06
>kg/m2
> > >(5.5 mil thick cell ). The cells themselves, no mass add-ons are 840
> > >gram per m2.
> > >
> > > Make these cell 100% efficient and run the numbers again,
> > > theoretical maximum value will be 54% efficient, but for the sake
> > > of argument say 100% efficient.
> > >
> > >0.840 + 1.76 + 2.06 = 4.66 kg/m2
> > >AMO Standard 1,357 m2
> > >
> > >Ouch!
> >
> > I'm sorry, I miss your point here. What's important is kg/kW. 4.66
> > kg/m2/130kW is only 35 grams/kW.
> >
> > 77,000 m2 of them would only mass 359 tonnes. That's 3.6% of a
> > 10,000 ton, 5 GWe (ground) power sat. The reflectors, heat sinks and
> > transmitter are each going to mass way more than these cells.
> >
> > If you are going to this much concentration and big heat sinks I
> > think you might as well just use steam turbines that are already 40%
>efficient.
> >
> > Keith
> >
> >
> >
> > >----- Original Message ----
> > >From: hkhenson <hkhenson@...>
> > >To: solarpowersatelliteplace@yahoogroups.com
> > >Sent: Friday, August 29, 2008 10:00:09 PM
> > >Subject: Re: [Solar Power Satellite Place] NREL Solar Cell Sets
> > >World Efficiency Record at 40.8 Percent
> > >
> > >
> > >At 05:49 PM 8/29/2008, you wrote:
> > > >FYI,
> > > >
> > > >"NREL Solar Cell Sets World Efficiency Record at 40.8 Percent"
> > > >National Renewable Energy Laboratory
> > > >http://www.nrel. gov/news/ press/2008/ 625.html
> > > >
> > > >: Scientists at the U.S. Department of Energy's National Renewable
> > > >: Energy Laboratory (NREL) have set a world record in solar cell
> > > >: efficiency with a photovoltaic device that converts 40.8 percent of
> > > >: the light that hits it into electricity. This is the highest
> > > >: confirmed efficiency of any photovoltaic device to date.
> > > >
> > > >: The inverted metamorphic triple-junction solar cell was designed,
> > > >: fabricated and independently measured at NREL. The 40.8 percent
> > > >: efficiency was measured under concentrated light of 326 suns. One
> > > >: sun is about the amount of light that typically hits Earth on a
> > > >: sunny day.
> > >
> > >This is interesting. Let's take a look at some numbers.
> > >
> > >Consider peak sunlight (on the ground) as a kW/m exp 2. So the
> > >output of a sq meter would be around 130 kW. I am not sure how to
> > >account for the reflected light from the cell surface. Ignoring
> > >that, then 60% of 326 kW/m exp 2 will go into heating the
> > >cell. That's about 195 kW/m exp 2.
> > >
> > >195,000/0.9 = 5.67 x 10 exp -8 T exp 4, T would be almost 1400 deg K
> > >or over 1100 deg C. Solar cells don't operate that hot so it would
> > >have to be cooled.
> > >
> > >Installed in a power sat and kept well below 100 deg C, it would use
> > >almost as much radiator as a steam turbine system.
> > >
> > >Fewer moving parts though.
> > >
> > >10 GW would require about 77,000 square meters of cell and 23 million
> > >square meters of reflectors or a square close to 5 km on an edge.
> > >
> > >15 GW of waste heat at room temperature would need a radiator of
> > >about the same size, a 5 x 6 km radiator.
> > >
> > >Hmm
> > >
> > >Keith Henson
> > >
> > >
> > >
> > >[Non-text portions of this message have been removed]
> > >
> > >
> > >------------------------------------
> > >
> > >Yahoo! Groups Links
> > >
> > >
> > >
> >
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