- Mankins, John C.
The Case for Space Solar Power.
Houston: Virginia Edition, 2014.
ISBN 978-0-9913370-0-2.
-
As world population continues to grow and people in the developing
world improve their standard of living toward the level of
residents of industrialised nations, demand for energy will
increase enormously. Even taking into account anticipated progress
in energy conservation and forecasts that world population will
reach a mid-century peak and then stabilise, the demand for
electricity alone is forecasted to quadruple in the century
from 2000 to 2100. If electric vehicles shift a substantial
part of the energy consumed for transportation from hydrocarbon
fuels to electricity, the demand for electric power will be
greater still.
Providing this electricity in an affordable, sustainable way is
a tremendous challenge. Most electricity today is produced by
burning fuels such as coal, natural gas, and petroleum; by
nuclear fission reactors; and by hydroelectric power generated
by dams. Quadrupling electric power generation by any of
these means poses serious problems. Fossil fuels may be
subject to depletion, pose environmental consequences both in
extraction and release of combustion products into the atmosphere,
and are distributed unevenly around the world, leading to
geopolitical tensions between have and have-not countries.
Uranium fission is a technology with few environmental
drawbacks, but operating it in a safe manner is very
demanding and requires continuous vigilance over the decades-long
lifespan of a power station. Further, the risk exists that
nuclear material can be diverted for weapons use, especially
if nuclear power stations proliferate into areas which are
politically unstable. Hydroelectric power is clean, generally
reliable (except in the case of extreme droughts), and
inexhaustible, but unfortunately most rivers which are suitable
for its generation have already been dammed, and potential
projects which might be developed are insufficient to meet the
demand.
Well, what about those “sustainable energy” projects
the environmentalists are always babbling about: solar panels,
eagle shredders (wind turbines), and the like? They do
generate energy without fuel, but they are not the solution to
the problem. In order to understand why, we need to look into
the nature of the market for electricity, which is segmented
into two components, even though the current flows through the
same wires. The first is “base load” power. The
demand for electricity varies during the day, from day to day,
and seasonally (for example, electricity for air conditioning
peaks during the mid-day hours of summer). The base load is
the electricity demand which is always present, regardless of
these changes in demand. If you look at a long-term plot of
electricity demand and draw a line through the troughs in the
curve, everything below that line is base load power and
everything above it is “peak” power. Base load
power is typically provided by the sources discussed in the
previous paragraph: hydrocarbon, nuclear, and hydroelectric.
Because there is a continuous demand for the power they
generate, these plants are designed to run non-stop (with
excess capacity to cover stand-downs for maintenance), and
may be complicated to start up or shut down. In Switzerland,
for example, 56% of base load power is produced from
hydroelectric plants and 39% from nuclear fission reactors.
The balance of electrical demand, peak power, is usually generated
by smaller power plants which can be brought on-line and shut down
quickly as demand varies. Peaking plants sell their power onto
the grid at prices substantially higher than base load plants,
which compensates for their less efficient operation and higher
capital costs for intermittent operation. In Switzerland, most
peak energy is generated by thermal plants which can burn either
natural gas or oil.
Now the problem with “alternative energy” sources such
as solar panels and windmills becomes apparent: they produce
neither base load nor peak power. Solar panels produce
electricity only during the day, and when the Sun is not obscured
by clouds. Windmills, obviously, only generate when the wind is
blowing. Since there is no way to efficiently store large
quantities of energy (all existing storage technologies raise
the cost of electricity to uneconomic levels), these technologies
cannot be used for base load power, since they cannot be relied
upon to continuously furnish power to the grid. Neither can they
be used for peak power generation, since the times at which they
are producing power may not coincide with times of peak demand.
That isn't to say these energy sources cannot be useful. For example,
solar panels on the roofs of buildings in the American southwest
make a tremendous amount of sense since they tend to produce power
at precisely the times the demand for air conditioning is greatest. This
can smooth out, but not replace, the need for peak power generation
on the grid.
If we wish to dramatically expand electricity generation without
relying on fossil fuels for base load power, there are remarkably
few potential technologies. Geothermal power is reliable and
inexpensive, but is only available in a limited number of areas
and cannot come close to meeting the demand. Nuclear fission,
especially modern, modular designs is feasible, but faces
formidable opposition from the fear-based community. If nuclear
fusion ever becomes practical, we will have a limitless, mostly
clean energy source, but after sixty years of research we are
still decades away from an operational power plant, and it is
entirely possible the entire effort may fail. The
liquid
fluoride thorium reactor, a technology demonstrated in the
1960s, could provide centuries of energy without the nuclear waste
or weapons diversion risks of uranium-based nuclear power, but even
if it were developed to industrial scale it's still a “nuclear
reactor” and can be expected to stimulate the same hysteria
as existing nuclear technology.
This book explores an entirely different alternative. Think about it:
once you get above the Earth's atmosphere and sufficiently far from the
Earth to avoid its shadow, the Sun provides a steady 1.368 kilowatts
per square metre, and will continue to do so, non-stop, for billions
of years into the future (actually, the Sun is gradually brightening,
so on the scale of hundreds of millions of years this figure will
increase). If this energy could be harvested and delivered efficiently
to Earth, the electricity needs of a global technological civilisation
could be met with a negligible impact on the Earth's environment.
With present-day photovoltaic cells, we can convert 40% of incident
sunlight to electricity, and wireless power transmission in the
microwave band (to which the Earth's atmosphere is transparent,
even in the presence of clouds and precipitation) has been demonstrated
at 40% efficiency, with 60% end-to-end efficiency expected for future
systems.
Thus, no scientific breakthrough of any kind is required to harvest
abundant solar energy which presently streams past the Earth and
deliver it to receiving stations on the ground which feed it
into the power grid. Since the solar power satellites would generate
energy 99.5% of the time (with short outages when passing through
the Earth's shadow near the equinoxes, at which time another satellite
at a different longitude could pick up the load), this would be base
load power, with no fuel source required. It's “just a
matter of engineering” to calculate what would be required to
build the collector satellite, launch it into geostationary orbit (where
it would stay above the same point on Earth), and build the receiver
station on the ground to collect the energy beamed down by the satellite.
Then, given a proposed design, one can calculate the capital cost
to bring such a system into production, its operating cost, the
price of power it would deliver to the grid, and the time to recover
the investment in the system.
Solar power satellites are not a new idea. In 1968, Peter
Glaser published a description of a system with photovoltaic
electricity generation and microwave power transmission to an
antenna on Earth; in 1973 he was granted
U.S. patent 3,781,647
for the system. In the 1970s NASA and the Department of Energy
conducted a detailed study of the concept, publishing a reference
design in 1979 which envisioned a platform in geostationary orbit
with solar arrays measuring 5 by 25 kilometres and requiring a
monstrous space shuttle with payload of 250 metric tons and
space factories to assemble the platforms. Design was entirely
conventional, using much the same technologies as were later used
in the International Space Station (ISS) (but for a structure twenty
times its size). Given that the ISS has a cost estimated at
US$ 150 billion, NASA's 1979 estimate that a complete,
operational solar power satellite system comprising 60 power
generation platforms and Earth-based infrastructure would cost
(in 2014 dollars) between 2.9 and 8.7 trillion might be
considered optimistic. Back then, a trillion dollars was a lot
of money, and this study pretty much put an end to serious
consideration of solar power satellites in the U.S.for almost
two decades.
In the late 1990s, NASA, realising that much progress has been made
in many of the enabling technologies for space solar power,
commissioned a “Fresh Look Study”, which concluded
that the state of the art was still insufficiently advanced to make
power satellites economically feasible.
In this book, the author, after a 25-year career at NASA,
recounts the history of solar power satellites to date and
presents a radically new design, SPS-ALPHA
(Solar Power Satellite by means of Arbitrarily Large
Phased Array), which he argues is congruent with 21st century
manufacturing technology. There are two fundamental reasons
previous cost estimates for solar power satellites have come
up with such forbidding figures. First, space hardware
is hideously expensive to develop and manufacture. Measured
in US$ per kilogram, a laptop computer is around $200/kg,
a Boeing 747 $1400/kg, and a smart phone $1800/kg. By
comparison, the Space Shuttle Orbiter cost $86,000/kg
and the International Space Station around $110,000/kg.
Most of the exorbitant cost of space hardware has little
to do with the space environment, but is due to its being
essentially hand-built in small numbers, and thus never
having the benefit of moving down the learning curve as a
product is put into mass production nor of automation in
manufacturing (which isn't cost-effective when you're only
making a few of a product). Second, once you've paid that
enormous cost per kilogram for the space hardware, you have
launch it from the Earth into space and transport it to
the orbit in which it will operate. For communication satellites
which, like solar power satellites, operate in geostationary
orbit, current launchers cost around US$ 50,000
per kilogram delivered there. New entrants into the
market may substantially reduce this cost, but without a
breakthrough such as full reusability of the launcher, it will
stay at an elevated level.
SPS-ALPHA tackles the high cost of space hardware by adopting
a “hyper modular” design, in which the power satellite
is composed of huge numbers of identical modules of just eight
different types. Each of these modules is on a scale which
permits prototypes to be fabricated in facilities no more
sophisticated than university laboratories and light enough
they fall into the “smallsat” category, permitting
inexpensive tests in the space environment as required. A
production power satellite, designed to deliver 2 gigawatts
of electricity to Earth, will have almost four hundred thousand
of each of three types of these modules, assembled in space by
4,888 robot arm modules, using more than two million interconnect
modules. These are numbers where mass production economies
kick in: once the module design has been tested and certified
you can put it out for bids for serial production. And a
factory which invests in making these modules inexpensively
can be assured of follow-on business if the initial power satellite
is a success, since there will a demand for dozens or hundreds
more once its practicality is demonstrated. None of these
modules is remotely as complicated as an iPhone, and once they
are made in comparable quantities shouldn't cost any more. What
would an iPhone cost if they only made five of them?
Modularity also requires the design to be distributed and redundant.
There is no single-point failure mode in the system. The propulsion
and attitude control module is replicated 200 times in the full design.
As modules fail, for whatever cause, they will have minimal impact
on the performance of the satellite and can be swapped out as part
of routine maintenance. The author estimates than on an ongoing
basis, around 3% of modules will be replaced per year.
The problem of launch cost is addressed indirectly by the modular
design. Since no module masses more than 600 kg (the propulsion
module) and none of the others exceed 100 kg, they do not require
a heavy lift launcher. Modules can simply be apportioned out among
a large number of flights of the most economical launchers
available. Construction of a full scale solar power satellite
will require between 500 and 1000 launches per year of a launcher
with a capacity in the 10 to 20 metric ton range. This dwarfs the
entire global launch industry, and will provide motivation to fund
the development of new, reusable, launcher designs and the volume
of business to push their cost down the learning curve, with a
goal of reducing cost for launch to low Earth orbit to
US$ 300–500 per kilogram. Note that the SpaceX
Falcon Heavy,
under development with a projected first flight in 2015,
already is priced around US$ 1000/kg without reusability of
the three core stages which is expected to be introduced in
the future.
The author lays out five “Design Reference Missions”
which progress from small-scale tests of a few modules in low
Earth orbit to a full production power satellite delivering 2
gigawatts to the electrical grid. He estimates a cost of around
US$ 5 billion to the pilot plant demonstrator and 20 billion
to the first full scale power satellite. This is not a small sum of
money, but is comparable to the approximately US$ 26 billion
cost of the
Three Gorges Dam
in China. Once power satellites start to come on line, each feeding
power into the grid with no cost for fuel and modest maintenance
expenses (comparable to those for a hydroelectric dam), the initial
investment does not take long to be recovered. Further, the
power satellite effort will bootstrap the infrastructure for
routine, inexpensive access to space, and the power satellite
modules can also be used in other space applications
(for example, very high power communication satellites).
The most frequently raised objection when power satellites are
mentioned is fear that they could be used as a “death ray”.
This is, quite simply, nonsense. The microwave power beam arriving
at the Earth's surface will have an intensity between 10–20% of
summer sunlight, so a mirror reflecting the Sun would be a more
effective death ray. Extensive tests were done to determine if
the beam would affect birds, insects, and aircraft flying through
it and all concluded there was no risk. A power satellite which
beamed down its power with a laser could be weaponised, but nobody
is proposing that, since it would have problems with
atmospheric conditions and cost more than microwave transmission.
This book provides a comprehensive examination of the history of the
concept of solar power from space, the various designs proposed over
the years and studies conducted of them, and an in-depth presentation
of the technology and economic rationale for the SPS-ALPHA system.
It presents an energy future which is very different from that which
most people envision, provides a way to bring the benefits of
electrification to developing regions without any environmental
consequences whatever, and ensure a secure supply of electricity
for the foreseeable future.
This is a rewarding, but rather tedious read. Perhaps it's due
to the author's 25 years at NASA, but the text is cluttered
with acronyms—there are fourteen pages of them defined
in a glossary at the end of the book—and busy charts,
some of which are difficult to read as reproduced in the Kindle
edition. Copy editing is so-so: I noted 28
errors, and I wasn't especially looking for them. The
index in the
Kindle edition lists page numbers in
the print edition which are useless because the electronic
edition does not contain page numbers.
June 2014 