A Draft Proposal to
DARPA by Eric & Marty Hoffert, Versatility Energy, South Orange,
NJ 07079

ABSTRACT:
Space solar power (SSP) offers the opportunity of breakthroughs in large-scale
power generation and highly flexible power distribution. To validate
the vision of SSP, a research project is proposed here to demonstrate
basic capabilities of space solar power, including a demonstration of
long-range wireless power transmission from geosynchronous orbit (GEO)
to the surface of the earth using high-energy lasers.
JAXA,
the Japanese space agency, last year announced
plans to launch by 2010 a satellite
that will unfurl a large solar array and beam 100 kilowatts of microwave
or laser power to a receiving station on Earth (Gibbs, 2006). However,
because diffractive beam spreading requires large antennas at microwave
frequencies, it would be virtually impossible to launch a microwave
beamer large enough for efficient space-to-Earth power transfer without
expensive multiple launches and in-space assembly
.
Limitations due to beam divergence from microwave transmission with
its companion large-scale transmitters and receivers can be overcome
using the laser-based system outlined here. Advantages of a laser experiment
include demonstration of continuous (24/7) electric power transfer from
orbit and reception on Earth orders of magnitude greater than anything
that has been done historically. With laser beaming, “on the shelf”
ultralight solar panels, and other early launch opportunities (such
as exploitation of NASA Geo QuickRide or suitable alternatives), this
milestone appears achievable in a three to five year time frame; perhaps
even before the announced JAXA SSP tests. This proposal is focused on
the identification of key technology issues in the real world, setting
the stage for commercial and military space solar power to provide electricity
on-demand where and when needed — a high priority for national security
in a world in which currently inexpensive liquid and gaseous hydrocarbons
will be increasingly scarce and more costly, and where powerful new
methods are needed for flexible power delivery and distribution. Furthermore,
expensive and cumbersome methods of transporting large amounts of fuel
could be replaced with lighter weight and simpler power receiving equipment,
with the potential to revolutionize localized power generation and distribution
capabilities.

GOAL &
MOTIVATION
: Research is proposed to laser beam

solar power collected by space-based ultralight photovoltaics (PV) to
surface arrays tuned to laser bandgaps (Landis, 1994). A specific goal
is GEO orbit-insertion of a to-be-developed experimental payload in
a single launch in five years or less. Strategic applications of space
solar power include global-scale electric baseload supply and electricity-on-demand
(Lior, 2001; Hyde et al., 2006). Satellites in GEO (geostationary orbit
36,000 km above the equator) are widely employed to beam information
worldwide because they appear stationary relative to Earth’s surface.
GEO powersats exploit (almost) continuously available solar flux So
» 1.37 kW/m2 from sun-tracking collectors for order-of-magnitude
more DC power per unit area than the long-term average of highly-variable
& low areal power density solar and wind at the surface (Hoffert,
2006a). Continuous power from SSP is critical because energy storage
is the cost-pacer limiting market penetration of terrestrial renewables
(Love et al., 2003). [That Denmark has the world’s peak renewable
electricity market penetration (20%) is possible only because it’s
windpower is backed-up by pumped storage from Norway’s 100% hydroelectric
system.] The advantage of higher solar flux in space is partly offset
by launch costs and costs of wireless power transmission (WPT). But
even with presently high PV costs (> $5000/peak kWe) electricity
from orbit can have comparable costs to terrestrial-solar for launch
costs close to the present 15,000 $/kg for Russian & Chinese launches
with geosynchronous transfer capacity (Hoffert, 2006a; Futron, 2002).

WHY LASERS?
SPS needs to project through the atmosphere, which is transparent in
visible and microwave bandwidths, but mostly opaque elsewhere. Space-to-Earth
WPT can be as simple as space mirrors bouncing sunlight to surface receivers
(Ehricke, 1979). But geometric optics implies that the subtended sun
angle projects an over 400 km diameter spot size from a GEO onto Earth’s
surface. Too big. Better, but still too big for the near-term, are PV-driven
cloud-penetrating microwave beamers, as in the NASA-DOE reference design
of the ‘70s (Koomanoff and Bloomquist, 1998) and the NASA “Fresh
Look Study”(Mankins, 1997). Microwave beaming is limited by diffractive
spreading proportional to (wavelength)/(transmitter aperture) driving
SSP toward big space antennas (~ 1 km) and bigger surface rectennas
(~10 km). Microwave WPT could work for global-scale electric base load
but require high initial capital costs for “first power.” And small scales won’t work for microwaves.
One solution is laser beams with 100,000 times smaller than microwave wavelengths
and negligible spreading (Hoffert et al., 2004). Lasers are employed,
for example, for lunar ranging by bouncing beams back from corner reflectors
400,000 km distant left by Apollo astronauts on the Moon (Dickey et
al., 1994). Diode lasers are commercially available with 50% efficiencies
at low power (Dickinson, 2002). Comparably efficient lasers with continuous
power of hundreds of kW are under development for DARPA by our partner,
General Atomics
. Siting receivers in deserts can address potentially
intervening cloud scattering, with subsequent routing though terrestrial
power grids, for electric utility applications, and multi-receiver “hopping”
for power “on demand” (Hyde et al., 2003). Supplying the electricity
demand curve of Europe by the laser SSP system depicted in the inset
(PV arrays in GEO, lasers beaming to PV receivers in the Sahara &
electricity transmitting to Europe via HVDC lines) was assessed by Geuder
et al. (2004), who also compared it to purely terrestrial PV with pumped
hydro storage. With economies of scale they find laser SSP cost-effective
at terawatt power levels (1TW = 1012 W). Carbon-emission-free
primary power of 10-30 TW could be needed by midcentury to stabilize
climate against catastrophic global warming with continued economic
growth (Hoffert, 2006b). Today, global electricity generation is 1.9
TWe requiring 5.7 TW primary power to generate, projected
to triple by 2050. Our proposed experiment can test laser SSP technology
near-term, fostering early implementation of this revolutionary global-scale
emission-free electricity source.

POWER, MASS
& COST
: To capture the most solar energy at lowest cost we want
the largest solar collector feasible in a single launch and reliable
space-based components with highest specific power (power per
unit mass, P/M). The largest single-vehicle-launched structure so far
in space may be Signals Intelligence (SIGINT) "Trumpet," with
a reported 100-meter dish (Couvault, 1997). NRO doesn’t release such
information, but reportedly in the unclassified literature Titan 4s
launched three Trumpets since 1994 in Molniya-type elliptic orbits.
It seems safe therefore to specify a thin-film PV collector diameter
D » 100 m. Also assumed are efficiencies in the chain from sunlight
in GEO to DC on Earth: hPV, space º (PV DC power output)/(solar
photon power incident) » 10%, hlaser
º (laser coherent photon power out)/(space PV power in) » 10%, htrans

º (laser photon power captured at surface)/(laser coherent photon power
transmitted) » 80%, hPV, surface
º (DC power to grid or direct consumers)/(laser photon power captured
at surface) » 80% (assumes PV arrays are tuned to the laser bandgap
energy). Some intermediate efficiencies are absorbed in these numbers.
Note our near-term technology conservative assumptions: 10% efficient
amorphous silicon (a-Si) films (not the 20% or more attainable down
the road) and 10% efficient high power diode laser (not the 50% under
development). Electric power available at the surface is then

Psurface
= hPV, space hlaser htrans hPV,
surface
(p/4) D2 So » 70 kWe

(direct current)

The example
illustrates that a single-vehicle launch should be able to orbit a laser
SSP payload package capable of illuminating 70 kilowatts continuously
e.g., electrifying a remote outpost or small village where and when
needed. Moreover, recent breakthroughs indicate high specific power
in the range of 1 to 5 kWe/kg including support structures
may be available for deployable thin-film PV arrays. The inset shows
an ESA-DLR ultra-light 20-meter space-qualified thin film PV array with
supporting booms, with 400 m2 of CP1/a-Si:H thin film cells
of mass 32 kg providing 50 kW of solar power deployable from the small
“suitcase” at center (P/M ~ 1.6 kWe/kg, including support
structure). The record for amorphous silicon on 6 mm CP1 films is ~
4 kWe/kg (Wyrsch et al., 2006). With inflatable-rigidizable
structures gross power densities in the range of 1 to 5 kWe/kg
appears feasible. A 10% efficient array 100 m in diameter in GEO outputting
PPV ~ 1000 kWe (1 MWe) with (P/M)

PV ~ 2 kWe/kg including support structure would mass
out to 500 kg. Adding 200 kg for the laser and 300 kg for “balance-of-system”
(power conditioning, attitude controls, telemetry, etc.) give ~ 1000
kg (one metric tonne) payload. At $15,000/kg launch costs to GEO, the
price to insert this package in orbit would be $15 million. Even if
launch costs were $30 million, perhaps 1/3 of the program costs, a budget
of order $100 million is reasonable for the five-year project.

The funding
outlined here is a very modest level of investment when compared with
competing large-scale power generation alternatives such as nuclear
fusion. In addition, the payback for the approach proposed here could
be very substantial, as foundation technology to enable Megawatt and
Gigawatt class power generation and distribution would be proven and
prepared for scalable deployment.

APPLICATIONS:
The research proposed here is targeted to produce a successful system
for power generation and transmission systems using high powered lasers.
Once such a milestone can be achieved in the development of the core
enabling technologies for WPT and SSP, there are a large number of unique
and important applications which can be supported, such as flexible
power distribution delivered to: (a) isolated or advanced positions;
(b) airplanes, or high altitude airships; (c) large-scale ships at sea;
(d) satellites requiring new power sources (i.e., to address battery
depletion or eclipse constraints); or (e) offshore bodies such as islands
and man-made platforms. Additional capabilities may include support
for on-demand high-scale power generation in remote areas; movement
of objects from LEO to GEO; high-energy electrolysis to produce hydrogen
from water for use in fuel cells in vehicles, aircraft, etc.; and support
of broader goals for energy security and energy independence. Certain
applications such as power delivery to high-altitude airships or ships
at sea requires complementary tracking to ensure power is delivered
accurately to these moving objects.

RESEARCH
PLAN:
We envision this project as a typical $300-500 k/year DARPA
start-up expanding over five years to a full-scale multi-phase space
experiment as critical milestones are accomplished. Early on in a Phase
I program, we plan a high technical level multidisciplinary Workshop
organized by the PIs in which technical issues are identified and analyzed.
For example, the ultralight, or “gossamer,” orbiting structures
envisaged here are subject to solar radiation pressure affecting attitude
control and station keeping; the implications of these effects need
to be better understood. Likewise, the appropriate scale at which meaningful
power beaming tests become feasible must be determined; and the impact
of weather on beam propagation etc must be reviewed. There also needs
to be an evaluation of the deployment of high powered lasers in space
where issues such as cooling are very different and more challenging
as there is only radiation to dissipate easily. Ancillary topics such
as eye safety and other environmental issues need to be covered as well.
We need to understand these examples of issues and many others as we
venture into uncharted territory, despite the availability of selected
parts of the system “on the shelf.”

Phase 1 will
also include design of a comprehensive and detailed research program.
This work will be followed in Phase II by a preliminary design of a
series of experiments, including lab tests, and possible mountain-to-mountain
tests of laser beaming efficiency. In parallel, we will be building
a team of consultants, industrial collaborators and mission analysts,
focused quantitatively on a successful laser SSP experiment as outlined
above. Phase III would be focused on the successful demonstration of
laser power beaming from space to earth. Versatility’s role will be
systems integration and multi-disciplinary science and engineering leadership
in cooperation with the DARPA Program manager. Technical and administrative
details of this project, while eminently doable, are challenging, and
will require a more extensive exposition than space permits here. However,
our basic idea, methods and cost estimates are laid out here to stimulate
discussion and set the stage for the next, we hope more serious, stage.
Upon an indication of interest with respect to this executive summary,
a more detailed project plan and formal proposal will be submitted with
respect to DARPA’s established “Seedling” program for funding
innovative new projects.

REFERENCES & BIBLIOGRAPHY

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, Granada,
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Mankins, J.C.
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Wyrsch, N.
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Waikoloa, Hawaii.

 

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