[ExI] New wrinkle on power sats and laser propulsion

Keith Henson hkeithhenson at gmail.com
Sat Apr 20 06:22:06 UTC 2013

Skip down to the last two paragraphs if you already know this stuff.

Short economic analysis--power satellites, laser propulsion and new proposal.

I have a paper in for peer review at the Journal of the British
Interplanetary Society (JBIS).  It is a "design to cost" analysis,
based on the assumption that to gain enough market share to be worth
doing, the cost of energy from space had to come in at half the cost
of the least expensive fossil energy (as electricity).  Electricity
from depreciated coal plants is around 4 cents a kWh, so the target is
set at 2 cents per kWh.

Based on levelized cost, 20 years and 6.8% discount, a "no fuel"
energy project can spend ~80,000 times the cost charged per kWh.  For
2 cents per kWh, the owner can spend up to $1600/kw or $1.6 B/GW.
That's about a fifth of the cost of a 1 GW nuclear plant.

It seems likely that the receiving antenna on the surface of the
Earth, can be constructed (in 5 GW blocks) for about $200 M/GW or
$200/kW.  Of that, $50 is the cost of the inverter; the rest is
stamped out antenna, diodes, support poles and labor to build it.
(Not counting land costs, probably leased over farmland.

Of the remaining $1400/kW, the parts and labor should not exceed
$900/kW.  (There is a factor of two because of the transmission loss.)
 The remaining $500/kW is for shipment to GEO.  If the power
satellite's mass is 5 kg/kW (a reasonable number) then the shipment
cost can't exceed $100/kg.

That's roughly 100 times less than the current cost to ship
communication satellites to GEO.  Others (Hiroshi Yoshida and William
Maness) have come to the same cost reduction requirement.

Huge as it is, this reduction seems possible.  The energy cost to GEO
is only a dollar or two per kg.  The business model used for the JBIS
paper has the whole second stage broken up and used for power
satellite parts, making the entire 20-ton dry weight into payload.

The proposed first stage is a second generation Skylon (Reaction
Engines) that carries no oxygen.  Acceleration to low earth orbit (LEO
) above the air-breathing part of the flight is from hydrogen is
heated to ~7500 m/s by a laser in geosynchronous orbit (GEO).  (This
takes about 4000 km of acceleration.)  Part of the substantial cost
reduction comes from the improved payload fraction due to the higher
exhaust velocity.  The rest of the reduction comes from the high
flight rate, 3 per hour at 20 tons delivered to GEO per flight.

Like railroads, the low cost (over wagons pulled by oxen) comes at the
high cost of putting in the rails, or, in this case, multi GW
propulsion lasers.  Incidentally, the calculated time to repay the
energy used to build a power satellites is under two months

A first-pass business analysis puts the peak investment at ~$140 B
over about 7 years.  The entire investment (in the baseline model) is
paid off by year 12 from the start.

This is an impossible amount of money for private enterprise to risk,
though it is within the ability of a number of governments to fund.
Along that line, last November the Chinese surprised an Indian
delegation to Beijing:

"Besides briefing the 82-year-old Kalam about its recent mission to
send three astronauts, including China's first woman to space, CAST
officials have shown "great interest" in partnering the mission with
international collaboration for Space based Solar Power initiative,
said V Ponraj, a scientist who is part of Kalam's delegation.

"Wu Yansheng, President of CAST has said his organisation is very much
interested to collaborate with India and ISRO on the space mission and
would like to establish a formal initiative from both the nations," he
said in a statement."


In a more recent article:

Chinese government promises "whatever it takes" to cap coal use Thu,
07 Feb, 2013 12:36 AM PST

"There is widespread fear that Chinese coal consumption, which nearly
rivals the entire rest of the world combined, will undo efforts to
combat climate change."


The Chinese seem to be serious about power satellites as the best way
to stop burning coal.  They are one of the few energy sources that
scale to the size needed.  Years ago, G. Harry Stein figured there is
room for power satellites in GEO for more than ten times the current
energy consumption.

Three years ago we have this from CAST:

"The CAST SPS research team conceives that there are four imperative
sections for SPS development: launching approach, in-orbit
construction/multi-agents, high efficiency solar conversion and
wireless transmission.  Except for launch, the other aspects do not
seem to be insurmountable issues for China in the upcoming years."


"Except for launch" indicates that three years ago they didn't have a
launch solution where power satellites made economic sense.  Up to
early 2012, there were no proposed solutions to launch cost.

The difficulty with the proposed method is getting the first
propulsion laser to GEO.  That is a major part of the $140 B estimated
price tag.  That is a considerable cost even to cap (or end) coal use.

An April 2, 2013 suggestion from Steve Nixon (patent pending) leads to
the concept of powering the first laser from the ground via a
10-km-sized microwave transmitter and a 1-km high power receiver on
the end in GEO.  Reciprocity
http://en.wikipedia.org/wiki/Antenna_%28radio%29#Reciprocity indicates
this will work.  It would be much smaller and lighter than a power
satellite.  An unclassified NASA study from 1987 indicates a 5-GW
rectenna in space would mass less than 1,000 tons.  Another large
advantage over the JBIS paper baseline design is that it could be
built and tested in LEO (much easier to reach than GEO).

With 5 GW available, Hall thruster engines
http://en.wikipedia.org/wiki/Hall_effect_thruster would push the
propulsion laser to GEO in about ten days, fast enough to avoid much
damage from space debris.  It could cut the cost by as much as half.
More important it could cut years off the time frame to build the
transport infrastructure to build large numbers of power satellites,
massively reduce the amount of coal burned and perhaps hold the CO2
build up to 450 ppm and (assuming CO2 is connected to global warming)
the global temperature rise to 2 deg C.


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