Table Of ContentIAC-02-R.4.08
WIRELESS POWER TRANSMISSION OPTIONS FOR SPACE SOLAR POWER
Mark Henley*, Seth Potter ,Joseph Howell ,and John Mankms ,
World Space Congress,
Houston; Texas, USA
Oct. 17, 2002
*Program Manager, Space Solar Power, Boeing ,**Senior Specialist, Boeing
***Manager, Advanced Space Flight Projects, NASA Marshall Space Flight Center
***Chief Technologist; Human Exploration and Development of Space, NASA HQ
Abstract:
Space Solar Power (SSP), combined with Wireless Power Transmission (WPT),
offers the far-term potential to solve major energy problems on Earth. In this paper
two basic WPT options, using radio waves and light waves, are considered for both
long-term and near-term SSP applications. In the long-term, we aspire to beam
energy to Earth from geostationary Earth orbit (GEO), or even further distances in
space. Accordingly, radio- and light- wave WPT options are compared through a
wide range of criteria, each showing certain strengths.
in the near-term, we plan to beam power over more moderate distances, but still
stretch the limits of today's technology. For the near-term, a 100 kWe-class "Power
Plug" Satellite and a 10 kWe-class Lunar Polar Solar Power outpost are considered
as the first steps in using these WPT options for SSP. By using SSP and WPT
technology in near-term space science and exploration missions, we gain experience
needed for sound decisions in designing and developing larger systems to send
power from Space to Earth.
Introduction Satellite photographs of the Earth at
night are, effectively, a means of remote
The demand for power on Earth is sensing for power consumption, in the
growing exponentially, and associated form of electric lights. Figure 1 is a
environmental consequences are composite satellite photograph showing
becoming significant. In this new the entire Earth at night, lit up by our
century, Space Solar Power (SSP) may current levels of power consumption (as
provide a clean, safe energy source, of November, 2000). Bright areas define
alleviating some of the problems we power-rich population centers, generally
would otherwise expect from increasing concentrated near coastlines and
nuclear and fossil fuel use. waterways (note the brightness of the
Nile river in Egypt). Some of the dark
regions indicate barren deserts, while
others,like sub-SaharanAfrica,arewell same peak power level on Earth (1.2
populated, but power-starved. Deserted GWe). Laser-Photovoltaic wireless
locations adjacent to populated regions power transmission is currently less
are potential sites for future large WPT efficient (by about 50%), so a larger
receiving stations. solar array area is needed to collect
sunlight, and the satellite becomes much
Comparison of WPT Options for Far- longer (55 km), and about half as wide.
Term SSP Systems WPT is provided by a series of small
laser transmitters running the entire
Wireless Power Transmission uses length of the satellite. Each light beam
electro-magnetic waves, rather than has a relatively small diameter, so each
wires, to send energy from one location diverges relatively rapidly, but their
to another. Two basic alternatives are combined energy fills a large diameter (6
being considered for power beaming: kin), dual-use photo-voltaic receiver on
radio waves (microwaves) and light Earth with roughly 500 W/m2 of
waves (lasers). monochromatic light. This is half the
peak intensity of sunlight on Earth (~1
Radio waves are beamed in a cloud- kW/m2), but, because the WPT
penetrating radio-frequency band wavelength can match the photo-voltaic
reserved for Industrial, Scientific and cell's band-gap, the cell's efficiency can
Medical (ISM) applications). Figure 2 be much higher (e.g., >40% vs 20%
illustrates an example SSP concept using efficiency). Photovoitaic electrical
the 5.8 GigaHertz ISM band; in this power output may actually be greater
case, a 500 meter diameter phased array with WPT than with sunlight, even
transmitting antenna directs radio waves though the sunlight is twice as powerful.
from GEO to a 7.5 kilometer diameter
rectifying antenna on Earth, Laser-photovoltaic WPT systems do not
approximately 40,000 km away from require large transmitter apertures, so the
power receiving stations. Here, a 15 same net effect (1.2 GWe on Earth)
kilometer long, gravity gradient- could be provided by a large number of
stabilized "backbone" carries 2.8 smaller, co-orbiting satellites.
GigaWatts of power from the solar
panels down to the transmitter. Figure 4 summarizes key features of a
Following a wide variety of loss dual-use photo-voltaic receiver. Large
mechanisms (pointing, side-bands, photovoltaic power plants in major
filtering, power conversion, etc.) the deserts areas can receive & convert light
system provides 1.2 GigaWatts of both directly from the Sun, and via WPT
electrical power on Earth. from the SSP satellite(s). Gravity
gradient-stabilized SSP satellites receive
Light waves are beamed in a wavelength the most sunlight, and transmit the most
which can be generated efficiently and power around 6 AM and 6 PM. Around
easily transmitted through the mid-day and midnight, solar panel self-
atmosphere in an optical or infra-red shadowing or cosine losses significantly
"window". Figure 3 illustrates an reduce the power output. The
example SSP concept using lasers, combination of ambient sunlight (which
instead of microwaves, to transmit the peaks around mid-day) plus WPT
illumination combinesat the terrestrial waveleng!his closeto a million times
PV arrayto matchthe daily electricity larger (centimeters vs nanometers).
demandpattern. Light-wave WPT allows small initial
demonstrationsystemsto be madeand
A satellite without gimbaled arrays tested over significant distances,
further improves the match between whereasradio wave WPT requires a
power output and power demand. huge leap of faith (literally and
Gimbaled solar panelsare heavyand figuratively) to producean initial SSP
complex,and would add only a little system.
more power around mid-day and
midnight. Insteadof using gimbaled RadioFrequencyInterferenceis alsoa
arrays,bothsidesofrigid (notgimbaled) major drawbackfor WPT using radio
solararrayscanproducepower,although waves. With so much power being
the"back"sideproduceslesspowerdue produced,evena small fractionoutside
to occlusion by wires, and partial the ISM band-gap can create major
transmissionof sunlight through the problemsfor the usersof neighboring
translucentsubstrate(e.g.,Kapton). By and harmonic radio frequency bands.
placing the "back" side facing the Theadditionof filters is expectedto be
morningsun(East)andthe"front" side necessary,which reduces the WPT
facing the afternoon sun (West), the efficiencyandincreaseslocalheating.
sateilite'spoweroutputis moreclosely
matched to typical terrestrial power Radio waves, at 5.8 GHz, (or, even
marketdemands,which havea higher better,at the longer wavelength2.45
peak in the afternoon than in the GHz ISM Band)canpenetratethrough
morning. clouds and light rain; light waves,in
contrast,areeasily stoppedby weather
This typeof combinedsystemhasother phenomenon,and require a dry site,
far-reachingbenefitswhencomparedto preferablyathigh altitude. Whilethere
a typical terrestrial solar-photovoltaic is somelogic to placing photo-voltaic
(PV) system. Typical terrestrial PV WPT receivers in desert areas(e.g.,
installationswouldrequireamuchlarger inexpensive, otherwise unproductive
areato collectpower,and would need land),not all areasthatneedpowerare
power storage systems, with large neardeserts. Powerdemandsin many
conversion losses, to match the countriestendto peakwhenthe sunis
electricity demand. By combiningthe bright (e.g., for summer air
naturaltime-dependenotutputof direct conditioning), which correspondsto
solar-PV and SSPWPT-PV systems, relativelyclearskies. NonethelessW, PT
bothcanbenefit. using light from an SSPsystemmay
requirealternatereceiversto makegood
Significant attributesof far-term SSP useofthesystemoncloudydays.
systemsfor radio-waveandlight-waves
WPT are comparedin figure 5 and Somelegal issuesare raised by both
discussedbelow: systems. Radio wave WPT implies
some degreeof RF interference,and
WPT aperturesizemustbemuch,much spectrumallocationsand power limits
larger for radio waves,as the WPT may need to be re-negotiated. Light
waveWPTcanbedesignedto limit the attract certain types of wildlife. Radio-
intensitytosafe,uncontroversiallevels, frequency coupling may be more prone
butthewordingoftreatiesrelatingtothe to transfer energy into small animals,
use of beamed energy for defense whose body length is comparable to the
applicationswouldneedto bereviewed wavelength. Infra-red WPT light falling
carefully, to determinerelevanceand
on an area over a prolonged time might
ensurecompliance,whereappropriate. similarly change the localized plant
growing season.
Infrastructure utilization appears better
for light wave WPT, considering the Efficiency is currently higher for radio-
reduced land area and dual application of wave wireless power transmitters and
the photo-voltaic receiver for terrestrial receivers. Light wave transmitter and
solar as well as SSP-WPT applications. receiver efficiency is rapidly improving,
but it is not clear whether it will ever
Other kinds of dual use have been approach the efficiency of radio
hypothesized for both WPT options. RF frequency transmission. The combined
WPT could potentially be used for data system efficiencies expected for radio-
transmission, and the land beneath wave WPT have been matured through
receiving antennae could potentially be related radar and communications
used to grow crops. A light-beaming applications, and are expected to be in
SSP system could, hypothetically, be the range of 60-70%, when filtering
used to send power to orbital transfer losses are included. For Laser-PV WPT,
vehicles, or even to solar sails (in the the best we expect to achieve about 50%,
very far-term). though this might be improved
significantly, as the technology matures
Public perception is an issue for either through SSP-related flight
option. The radio frequencies suggested demonstrations. When power
are within the "microwave" band, and management and distribution losses are
the public tends to perceive microwaves considered, there is another impact to net
as something to cook with. Similarly, efficiency (and mass) for radio-wave
the public may perceive power WPT, as electricity is carried to a large
transmission by light to be dangerous, central transmitter via wires that are
even though the system is designed to many kilometers long.
ensure safety. Both the micro-wave and
light-wave intensity limits suggested are Near Term Space Flight
weaker than sunlight. Demonstrations
To ensure public safety (and safety of Initial applications of WPT technology
the receiver from public interference or in space may use experimental
vandalism), either WPT option would spacecraft. Two early flight
need to be sequestered. It is unclear as demonstrations are being considered as
to what effects might occur to local flora Model System Configurations for Space
and fauna from long term exposure Solar Power. One concept is concept
within the receiver site. One might known as a "Power Plug", or "LAMP"
expect that a slightly warmer spacecaft, a -100 kWe space-based
environment, with local shelter, might power beaming system whose primary
mission is to supply power to it's own both lunar and Mars exploration, this
solar electric propulsion system and to concept (figure 8) has been referred to as
other spacecraft using WPT. A second the Lunar And Mars Power spacecraft,
concept is a Lunar Lander which or LAMP.
descends to a mountaintop at the moon's
North or South pole, and transmits NASA and Boeing are cooperating to
power to a robot operating in assess a mission that will use SSP and
permanently shadowed lunar polar WPT technologies to investigate the
craters. anomalous high concentration of
hydrogen observed around the poles of
A general view of one Power Plug the moon. Figure 9 provides an
concept is illustrated in figure 6. In this overview of this mission: A solar power
case, to achieve the required power level generation outpost is landed and
in the near-term with minimal risk, the deployed on a high peak near the North
configuration improved upon the or South Pole of the moon. The landing
Integrated Energy Assembly (lEA) site is chosen carefully so that outpost
which is now operating on the remains in full sunlit over many months
International Space Station. Figure 7 is (or even years) as the moon rotates once
a photograph of this existing power per month around it's slightly inclined
system, the largest space solar power axis. Landing site selection also
arrays ever built (so far). With minor considers direct, line-of-sight
modifications, and using current state of communications with Earth, and local
the art photo-voltaic cells, the power terrain features that might obstruct
produced by the lEA can be doubled, Wireless Power Transmission. WPT
while keeping many of the existing, technology is used to send a significant
flight-proven structures and part of this power to a telerobotic lunar
mechanisms. roving vehicle, which is then able to
operate in deep dark craters that are
A derivative of this experimental thought to contain ice. For example, with
spacecraft could be launched on a Delta a beam of light, longer distance WPT
IV-class launch vehicle to Earth escape can be achieved from a transmitter on
velocity (with optional lunar gravity the lander, perhaps using small relay
assist swing-by(s)) and proceed into a mirrors to redirect the beam into any
trans-Mars injection trajectory using regions obstructed by local topography.
solar electric propulsion (SEP). It would More detail on this mission can be found
again uses SEP to enter Mars stationary in a companion paper, "Space Solar
orbit, where it collects solar energy and Power Technology Demonstration for
beams it to a Mars surface infrastructure. Lunar Polar Applications" (IAC-02-
Because of the distance involved (17,038 R.4.04).
km altitude), radio-wave WPT would
require huge transmitters and receivers, Summary
so laser wavelengths are anticipated for
WPT, with a beam diameter on the order This discussion of Wireless Power
of meters (rather than kilometers), Transmission Options for Space Solar
delivering an average 10 kWe of power Power has made the following key
to the surface. Because it may support observations:
energy profile, which allows for better
Far-term micro-wave WPT options are use of [and at receiver sites. To mitigate
efficient, and beam power through weather outages, multiple receiver sites,
clouds / light rain, but require large sizes in desert areas, may be necessary.
for long distance WPT and a specialized
receiver, a rectifying antenna or Gravity gradient-stabilized "Sun Tower"
"rectenna". SSP satellites may make more sense for
WPT light-waves than for microwave
Far- (or intermediate-) term Laser- systems, because the receiver also
Pbotovoltaic WPT options are less converts ambient sunlight into
efficient, but have certain system level electricity. This additional sunlight can
benefits. They allow a smooth transition correct for the cosine loss otherwise
from conventional terrestrial solar power observed in power production at mid-
to SSP, in part because synergistic use of day. With this WPT option, rigid SSP
the same photo-voltaic receiver on Earth arrays, with both sides able to convert
can convert both laser-light and sun-light sunlight to electricity, may be preferable
into electrical power. The smaller to rotating (gimbaled) arrays, as the
aperture size also allows smaller (lower resulting power produced on Earth can
cost) initial systems. match the typical daily energy demand.
Laser-Photovoltaic WPT systems open SSP and WPT technology flight
new SSP architecture options. Efficiency demonstrations can enable advanced
of current / near-term Laser-Photovoltaic space science and exploration in the near
WPT technology would seem to indicate term. The "Power Plug" or "LAMP"
higher mass, but specific power may be spacecraft and the Lunar Polar Solar
competitive for laser systems, as their Power outpost are two near-term
smaller apertures minimize subsystem applications that can advance technology
masses for WPT and power management for far-term commercial SSP systems,
and distribution. Narrow light beams while providing significant value for
can also overlap to generate a "top hat" near-term applications.
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