Memorandum 35
Submission from Bristol Spaceplanes Limited
EXECUTIVE SUMMARY
A very important action the United Kingdom can
take to address both our energy security and climate change problems
is to join our friends in chartering Space Solar Power System
(SSPS) with a view to building an experimental system. An SSPS
would use very large satellites—several kilometres across
to capture the sun's energy in high GeoSynchronous Orbit (GSO)
and cleanly beam it down using wireless power transfer (WPT) to
rectennas on the ground; directly into the electric power grids
of contracting utilities. SSP has the potential to provide virtually
unlimited clean power. The power reaching the Earth from the Sun
is about ten thousand times greater than the power consumed by
the world's population. High space transportion costs have been
a major obstacle, but there are now realistic prospects for large
reductions in the next decade or two. There are still several
issues to be resolved, but a programme of study and experiment
is now well worthwhile.
Solar energy is converted to direct current
by solar, or photovoltaic, cells. That direct current then powers
microwave generators which feed a highly directive satellite-borne
antenna, which beams the energy to the Earth. There a rectifying
antenna (rectenna) converts the energy to direct current. After
processing, this is fed to the power grid.
The first practical WPT demonstration was done
by Bill Brown of Raytheon in 1975. Rectennas would be kilometers
across, however crops or other farming could be done under these,
just as now done under electric power lines. Maxwell's equations
governing power transmission argue strongly for a large scale
solution, which has been impractical to undertake to date. Many
past and current studies and demonstrations of SSP concepts have
been done.
A typical SSP satellite—with a solar panel
area of about 10 km2, a transmitting antenna of about two kilometres
in diameter, and a rectenna about four kilometres in diameter—may
yield an electric power of several Gigawatts. Critical aspects
motivating SSP research are:
(i) low attenuation of the microwave transmission
beam by the Earth's atmosphere;
(ii) 24-hour energy availability, except
around midnight during the equinoxes;
(iii) very carbon dioxide emission per unit
of power generated; similar to dams;
(iv) potential availability of many Terawatts
of clean energy for a billion years into the future; and
(v) zero fuel costs. (Except for station
keeping).
The Pentagon's National Security Space Office
(NSSO) is now objectively exploring SSP for its potential contributions
to tactical, operational, and strategic energy security in addition
to space security. These studies include exploring the political,
scientific, technical, logistical, and commercial feasibility
of SSP.
We recommend UK fund an SSP economic impact
and environmental analysis study.
1. Low cost access to space
The first requirement before an SSPS could be
considered is greatly reduced launch costs. Current commercial
space access prices are far in excess of what known SSPS concepts
could afford. Graphing what they are and what they would be at
higher flight rates, we see the curve below, however. The red
dots below are Elon Musk, SpaceX, $1300/lb and below that, Sandia
National Laboratories projects $20/lb[52]
SpaceX' Demonstration Flight 2 Flight Review Update (PDF version)
has been cleared by DARPA and is approved for public release.
Two flights are scheduled by year end. The key to SSP is being
financially able to charter a company able to financially negotiate
the path to those much higher flight rates—the same market
SSP provides. This is what Sunsat Corp offers.
The key is to move space transportation into
the private sector. Many businesses and settlements will one day
thrive in space; we just have to provide a market that will incentivize
low-cost space transportation. Groups such as the Space Solar
Power Workshop are recommending that Congress charter a space
solar power corporation, to build power satellites, just as they
chartered Comsat in 1962, to build communication satellites. This
is the simplest and fastest way to throw open the doors to space
development, while providing clean baseload power to the planet.
2. Microwave power transfer
An SSP satellite would consist of a solar energy
collector, to convert solar energy into dc electric power; a dc-to-microwave
converter; and a large antenna array to beam the microwave power
to a rectenna (rectifying antenna) on the ground.
For transmitting the power to the ground, frequency
bands around 5.8 or 2.45 GHz have been proposed, which are within
the microwave radio windows of the atmosphere. The antenna array
to transmit the energy to the ground would require a diameter
of about 1—2 km at 2.45 GHz, and its beam direction would
be electronically controlled and locked to an accuracy of significantly
better than 300 m, corresponding to 0.0005 degree (for a
geostationary orbit of the satellite).
In addition to the orbiting SSP satellite, a
ground-based power receiving site, the rectenna,—a device
to receive and rectify the microwave power beam—has to be
constructed to convert the beamed energy back to dc electric power.
The size of the rectenna site on the ground depends on the microwave
frequency used and the transmitting antenna's aperture. A typical
rectenna site would have a diameter of two to three kilometres
for a transmitting antenna of km2. This is frequency dependent,
however.
The rectenna (located on the Earth) receives
the microwave power from the SPS and converts it to dc electricity.
The rectenna is composed of an RF antenna, a low-pass filter,
and a rectifier. It is a purely passive system, apart from a low-power
pilot beam to maintain assured beam lock. A low-pass filter is
necessary to suppress the microwave radiation that is generated
by nonlinearities in the rectifier. Most rectifiers use Schottky
diodes. Various rectenna schemes have been proposed, and the maximum
conversion efficiencies anticipated so far are 91.4% at 2.45 GHz
and 82% at 5.8 GHz. However, the actual rectenna efficiency will
also depend on various other factors, such as the microwave input
power intensity and the load impedance.
The rectenna array, with a typical radius of
approximately 2 km, is an important element of the radio technology
for which high efficiency is essential. The peak microwave power
flux density at the rectenna site would then be 300 W/m2, if a
Gaussian power profile of the transmitted beam was assumed. The
beam intensity pattern would be non-uniform, with a higher intensity
in the centre of the rectenna and a lower intensity at its periphery.
For human safety requirements, the permissible microwave power
level has been set to 10 W/m2 in most countries and the SPS power
flux density would be constructed to satisfy this requirement
at the periphery of the rectenna.
After suitable power conditioning, the electric
output of the rectenna is delivered to the power network.
Besides microwave power transmission very recently
also laser power transmission has been suggested. In such a scenario
highly concentrated solar radiation would be injected into the
laser medium (direct solar pumping) and transmitted to Earth.
On the ground the laser light would be converted to electricity
by photovoltaic cells. Such a system would be fundamentally different
from a "classical" microwave power transmission: In
space there would be the light concentration system and lasers
instead of a photovoltaic cell array and the transmitting antenna,
and on the ground there would be a photovoltaic cell array instead
of a rectenna. Other differences from power density to rectenna/receiver
characteristics would be quite different, if laser were to become
available or preferred by a customer contract.
3. Space photovoltaics
The key elements in the dc power generation
for the SPS system are solar cells. Thin-membrane (amorphous)
silicon solar cells are expected to be the most suitable today,
because of their good performance for a given weight (W/kg), although
their conversion efficiency is lower than the figures for crystalline
cells. But progress beyond 2000 Watts/Kg in several companies
and new technologies continues.
EMCORE Corporation, for example, announced last
month that its PhotoVoltaics Division attained a record solar
conversion efficiency of 31% for an new class of advanced multi-junction
solar cells optimized for space applications. The new solar cell,
the Inverted Metamorphic (IMM) design, is one fifteenth as thick
as conventional multi-junction solar cell.
Developed with the Vehicle Systems Directorate
of US Air Force Research Laboratory, the cell will enable extremely
lightweight, high-efficiency, and flexible solar arrays to power
next generation satellites. EMCORE's investment in technology
innovation will enable the introduction of concentrator solar
cell products with conversion efficiency of 40% as a part of planned
high-volume product roadmap.
David Danzilio, Vice President and General Manager
of EMCORE's PhotoVoltaics Division stated, "The successful
demonstration of this new class IMM cell represents the most significant
improvement in terms of watts/kg and $/watts in the past decade,
which will enable never before envisioned space power applications.
Our industry leading scientists and engineers continue to refine
and optimize our terrestrial concentrator products and production
capabilities to meet our customers' needs and enable CPV systems
to achieve the lowest cost of power."[53]
4. Political/economic planning
There is no question SSP can be built; the question
is how to build it economically—as a private company would.
An engineer has been defined as someone who can build for a dime
what any fool can build for a dollar. When America has faced such
seemingly insurmountable problems as SSP before, often a public/private
corporation has been chartered—a cooperation between government
and individuals. In 1862 the Transcontinental Railroad Act, which
spanned North America with rail, was enacted by Congress.
The process to create a congressionally chartered
corporation, the SunSat Act, is well understood. This was the
same legislative tool used to create Comsat in 1962, 100 years
after the Transcontinental Railroad. An SSP system is no less
a challenge than Comsat or the Transcontinental Railroad were
in their day and would also seem to dictate a public/private corporation
to reduce those risks via compensating appropriate rewards.
The only successful path to build SSP, is a
congressionally chartered corporation, we call it SunSat Corporation.
The purpose in this paper is to explore SunSat Corp's forest as
we look at the trees ahead of us. We want to understand the new
and complex business process which we must cultivate and drive.
Draft Sunsat Legislation has been placed on the web at http://www.sspi.gatech.edu/sunsat-how.pdf
5. Telerobotics
On 16 June 16 2007, Boeing's Orbital Express
system, validated telerobotic and autonomous spacecraft servicing
capabilities, performing a fully-autonomous "fly-around and
capture" of a client spacecraft. During the five-hour test,
the ASTRO (Autonomous Space Transport Robotic Operations) spacecraft
used its onboard cameras and video guidance system to separate
from, circle and re-mate with Ball Aerospace's NextSat spacecraft.
The test primarily used passive sensors with no active exchange
of relative navigation information or involvement by ground controllers.
"Positioned in orbit 60 meters above NextSat,
ASTRO followed an imaginary line called the "Rbar,"
extending from Earth's center to a satellite and beyond, to capture
the spacecraft. Rbar is the approach direction needed to effectively
service a satellite without interfering with its cameras or antennas.
ASTRO and NextSat began Scenario 5-1 in the
Mated Nominal mode. At the predicted time, ASTRO's autonomous
systems separated it from NextSat to a range of up to 120 meters.
ASTRO then circled NextSat using its sensor systems to continuously
track NextSat during the fly-around. If sensor inputs had deviated
outside established limits, an autonomous safing action would
have repositioned the spacecraft to a safe location. It did this
successfully in mid-May when Orbital Express experienced a computer
sensor anomaly. The system's autonomous safing feature maneuvered
the spacecraft to a safe location until the team could re-mate
them. The team has resolved that anomaly.
After completing the fly-around, ASTRO maintained
its relative position with NextSat at 120 meters for 17 minutes
then maneuvered above NextSat to perform a corridor approach to
within centimeters of the client spacecraft. The capture mechanism
grappled NextSat and performed a soft berth, drawing NextSat and
ASTRO together.
During the next major unmated operation (Scenario
7-1), ASTRO will depart NextSat to a range of four kilometers
before approaching the client spacecraft and performing a free-fly
capture using its robotic arm.[54]
Carnegie Mellon's Skyworker, a robot designed
for assembling immense projects in space, in particular SSP satellites,
can be reviewed at http://www.frc.ri.cmu.edu/projects/skyworker/
An award winning film showing Skyworker in action
is also available for viewing.
NASA's Space Telerobotics Office was closed
in 1997, but useful resources remain there: http://ranier.hq.nasa.gov/telerobotics—page/telerobotics.shtm
6. Satellite Control and Programme Interfaces
All modern SPS concepts rely on robotic assembly
and maintenance systems rather than on human astronauts. Suitable
orbit transfer vehicles may need to be developed to transport
very large structures from lower to higher orbits. Solar electric
propulsion orbital transfer vehicles have been suggested for this
purpose. Some corresponding prototype propulsion systems, like
a magneto-plasmadynamic thruster, a Hall thruster, and a microwave
discharge ion engine have been tested ([1], section 2.3.1.2).
Other key issues of SPS technology are subsystem
lifetime, especially photovoltaics, and maintenance. The limited
lifetime of solar cells has already been mentioned, but a long-term
radiation hazard also exists for any solid-state device on the
SPS, such as, for instance, dc-to microwave converters.
Both effects can in principle deform the structure
and change its attitude. In particular, the radiation pressure
exerts a force which is continuously changing in direction with
respect to the line joining the satellite and the rectenna. This
may pose serious problems concerning the control of the orbit
and the orientation of the RF beam. The amplitude of this force
is of the order of 100 N for a solar cell area of 10 km2 (2 *
solar radiation power flux * 10 km2/velocity of light).
Regarding maintenance, the present-day
experiences for low-Earth orbits with the Hubble space telescope
and the International Space Station indicate that maintaining
and servicing a much larger system in a much higher orbit may
be very difficult and much more expensive than for low Earth orbits.
A completely new approach to space maintenance may be required
to maintain large assets at geostationary orbit. Currently, progressive
replacement is the only viable option. An active defense against
"small" incident meteorites could also be valuable.
7. Alternative energy overview
Very active discussions concerning global oil
peak production dates are in progress. We find the most current
and authoritative research as of this date, predicts that global
oil production to peak during the 2008 to 2018 timeframe. While
we will never "run out" of petroleum; it will simply
become too expensive to burn in most cars and trucks. To quote
from that study, "In a worst-case scenario, global oil production
may reach its peak in 2008, before starting to decline. In a best-case
scenario, this peak would not be reached until 2018. These estimates
were made in a Swedish study by Fredrik Robelius, whose doctoral
dissertation estimates future oil production".
http://www.sciencedaily.com/releases/2007/03/070330100802.htm
and http://www.peakoil.net/GiantOil Fields.html
Also very recently, the most current and authoritative
research predicts global coal production to peak around 2025.—"Peak
coal by 2025 say researchers", initiated by a German member
of Parliament. Authors were Dr Werner Zittel and Jrg Schindler.
http://www.energywatchgroup.org/files/Coalreport.pdf
and http://www.energybulletin.net/28287.html
On the Terawatt scale of interest, Biofuels are also
not the answer (from EnergyPulse Weekly):
Peak Soil: Why Cellulosic ethanol and other
Biofuels are Not Sustainable and a Threat to America's National
Security—Part I By Alice Friedemann, Freelance Journalist—two
more parts also linkable from there.
Briefly summarized below, we find no other baseload
energy source as clean, safe or reliable considering the MASSIVE
energy quantities we require.
|
| Clean?
| Safe? | Reliable?
| Baseload? |
|
| Fossil Fuel | No
| Yes | Decades remaining
| Yes |
| Nuclear | No
| Yes | Fuel very limited
| Yes |
| Wind Power | Yes
| Yes | No, intermittent
| No |
| Ground Solar | Yes
| Yes | No, intermittent
| No |
| Hydro | Yes
| Yes | No; drought; complex scheduling
| |
| Bio-fuels | Yes
| Yes | Very limited quantities—competes directly with food production.
| |
| SSP | Yes |
Yes | Yes
| Yes |
|
8. Conclusions
The main conclusion from work done so far on Space Solar
Power is that it has potential for providing virtually unlimited
clean power but that much research work remains to be done to
establish practicability. The emerging space tourism industry
offers the prospect of the economies of scale needed to drive
down the cost of transport to orbit to levels where experimental
SSP satellites can be afforded. The time is therefore right for
HMG to start to fund a programme of research into SSP.
9. Some SSP Links
1. URSI Space Solar Power White Paper, report, and appendices
at http://www.ursi.org/WP/White—papers.htm A major focus
is on Wireless Power Transfer.
2. The Space Solar Power Workshop website at http://www.sspi.gatech.edu
.
3. International Telecommunication Union, Question ITU-R
210-1/1 on "Wireless power transmission", 2006,
http://www.itu.int/itudoc/itu-r/publica/que/rsg1/210-1.html
4. L Summerer, Solar Power from Space—European
Strategy in the Light of Global Sustainable Development, ESA SPS
Programme Plan 2003/2005, GS03.L36, July 2003, http://www.esa.int/gsp/ACT/doc/ESA—SPS—ProgrammePlan2—06.pdf
5. Space Frontier Foundation/National Security Space
Office (NSSO) Public discussion area: http://spacesolarpower.wordpress.com
6. "Pentagon Considering Study on Space-Based
Solar Power" By Jeremy Singer, April 11, 2007, http://www.space.com/businesstechnology/070411—tech—wed.html
July 2007
52
Space Sunshade Might Be Feasible, 3 November 3, 2006,
http://uanews.org/cgi-bin/WebObjects/UANews.woa/wa/MainStoryDetails?ArticleID=13269 Back
53
EMCORE Announces Significant Performance Advancements of Multi-Junction
High-Efficiency Solar Cells for Space and Terrestrial Applications,
http://www.emcore.com/news/release.php?id=158 Back
54
The Boeing Orbital Express, 27 June 2007, http://www.technologynewsdaily.com/node/7266
Other Orbital Express news releases at http://www.boeing.com/ids/advanced-systems/orbital/news Back
| 
