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Retail Beamed Power for a Micro Renewable
Energy Architecture: Survey
NARAYANAN KOMERATH, Georgia Institute of Technology
ARAVINDA KAR, University of Central Florida
Retail delivery of electric power through millimeter waves is relevant in developing areas where the market
for communication devices outpaces the power grid infrastructure. It is also a critical component of an
evolutionary path towards terrestrial and space-based renewable power generation. Narrow-band power
can be delivered as focused beams to receivers near end-users, from central power plants, rural distribution
points, UAVs, tethered aerostats, stratospheric airship platforms, or space satellites. The article surveys the
available knowledge base on millimeter wave beamed power delivery. It then considers design requirements
for a retail beamed power architecture, in the context of rural India where power delivery is lagging behind
the demand growth for connectivity. A survey of technology developments relevant to millimeter wave
beaming is conducted, and indicates that massive, mass-produced solid-state arrays capable of achieving
good efficiency and cost effectiveness are possible in the near term to enable such retail power beaming
architectures.
Categories and Subject Descriptors: J.2 [Physical Sciences and Engineering]: Aerospace
General Terms: Design, Economics, Performance
Additional Key Words and Phrases: Micro renewable energy systems, space power grid, millimeter wave,
power beaming, rural India power, systems
ACM Reference Format:
Komerath, N. and Kar, A. 2012. Retail beamed power for a micro renewable energy architecture: Survey.
ACM J. Emerg. Technol. Comput. Syst. 8, 3, Article 18 (July 2012), 25 pages.
DOI = 10.1145/2287696.2287701 http://doi.acm.org/10.1145/2287696.2287701
1. INTRODUCTION
Despite present fears of low end-to-end efficiency, millimeter wave technology offers
attractive architectures for rural beamed power delivery. In this article we present reasons why retail Beamed Power Transmission Systems (BPTS) at such frequencies may
be viable in the not too distant future. There are no market-ready devices presented
here, just an architecture in which such technology will fit. The article is intended
to trigger thought among the electronics and telecommunications community on the
possibilities of synergy between the infrastructure for connectivity devices using embedded systems, and the infrastructure needed to enable very large numbers of their
potential customers to access power for those devices. Micro renewable energy devices,
as we define them here, denote devices that generate power from renewable resources,
This work was partially supported under NASA grant NNX09AF67G S01. Anthony Springer is the Technical
Monitor.
Authors’ addresses: N. Komerath (corresponding author), School of Aerospace Engineering, Georgia Institute
of Technology, Atlanta, GA 30332; email: komerath@gatech.edu; A. Kar, CREOL, University of Central
Florida, 4000 Central Florida Boulevard, Orlando, FL 32816.
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DOI 10.1145/2287696.2287701 http://doi.acm.org/10.1145/2287696.2287701
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sized with a capacity ranging from a few watts, up to 3 kW. These are suitable for a
single family or small shop.
Rapid development of an information-based economy creates a widely dispersed
demand for small amounts of power, in developing parts of the world. Rural India is
a prime example. Ready acceptance of cell (mobile) phones shows popular appetite for
useful technology, even at costs that seem very high given the local income levels of
the recent past. The same reasoning that helped popularize cellphones, which is the
inadequacy of the land-line telephone infrastructure, holds for the electric power grid
as well. This poses a window of opportunity for options that are compatible with micro
renewable energy systems at the single family level, and with a space-based global
power exchange. Retail beaming infrastructure bridges these architectures of vastly
differing scales.
Tesla [1893] demonstrated that lamps could be lit at a distance using beamed electric
power. Recently inductive charging of devices using resonant coupling has been shown
[Hadley 2007]. These are discussed later in this article. As Vaitheeswaran [2003] points
out, the wired grid was preferred in industrialized nations where metal and concrete
are abundant, power generation is centralized, and the customer base is dominantly
urban. In the USA, transmission and distribution losses dropped below 6% in 2001,
rising [ABB 2007] to nearly 8% by 2005 due to congestion. In poorer nations, theft of
power and of the metal in the grid, and delays in recovering from natural disasters,
causes large losses beyond those due to aging equipment and low voltage.
Until recently, the electric grid and communications infrastructure went hand in
hand. Radio reception pervaded India early, along with highly efficient and widely
dispersed multilanguage print media catering to local needs. These created substantial
awareness of the outside world, far in advance of physical transport infrastructure
or the ability to communicate back into the outside world. India’s extensive railway
system, which also accommodated the telegraph system along its right of way, was a
leader in adopting wireless microwave communications. Recently, cellular telephony
(the mobile phone) has become the preferred solution for retail connectivity, with
over 885 million users reported at the end of June 2011 [TRAI 2011]. Although many
urban customers have multiple accounts, this number still implies a significant part
of the 1.1 billion population. Comparing this to the 51% who lack access to the electric
grid shows the opportunity for BPTS. A recent news item [Sheerin 2009] is typical of
innovation driven by the worldwide search for energy solutions. Two young men in
southwest Africa are reported to have built a rudimentary wind turbine and generator
out of scrap metal parts. They claimed to be seeking a nonhuman-powered alternative
to their bicycle dynamo to power a home-built radio so that they could dance to music.
The main customers of their wind turbine business came from neighboring villages
to charge cell phones. This illustrates how the worldwide demand for small personal
communication devices is leaping ahead of wired grid reach. In fact, the cost paid by
impoverished customers for the first few watts or watt-hours of electricity is very high,
as illustrated in Figure 1. The issue in our research is how best to use this demand to
advance opportunities for much larger solution architectures.
2. SPACE SOLAR POWER INTERESTS
Shortly we provide the rationale for exploring solutions in the millimeter wave regime
from the perspective of aerospace concept development. Space Solar Power (SSP)
[Glaser 1968; Landis 2004], as considered by NASA [1995, 1984; Stancati 1996; Henley
et al. 2002], the National Research Council [Council 2001], the Japanese Space Agency
(JAXA), and the European Space Agency (ESA) [Geuder et al. 2004; anon 2004], has
generally focused on microwave transmission below 10 GHz. This is driven by the presumed location of the photovoltaic arrays in Geostationary Earth Orbit (GEO) some
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Fig. 1. Present cost of the first few watts to off-grid customers using batteries. From Komerath and Komerath
[2011b].
36,000 km above the equator, or on the Moon [Criswell 2002], some 300,000 km away.
The large distance to GEO implies a receiver diameter of several, even over one hundred, kilometers. This in turn implies massive grid infrastructure with a capacity of
many Gigawatts built out from the receiver, to justify such size. Such stations will
be few in number, and hence offer few alternative transmission paths during cloudy
weather or rain. This forces a choice of frequencies that are relatively unaffected by water vapor, and the low frequency in turn locks down the large system dimensions. The
cost to first power, once estimated at 300 billion US dollars at a time when the Space
Shuttle was expected to deliver payloads to earth orbit for $220 per kg, is so immense as
to be not worth calculating today. Japanese researchers showed high (38%) conversion
efficiency from the solar spectrum to infrared laser beams using a Chromium-doped
Nd-fiber laser [Saiki et al. 2005], and have proposed a scalable architecture for solar
power satellites using laser modules [Sasaki et al. 2005]. Recent US papers also appear to be considering laser transmission [Potter et al. 2008; Penn and Law 2002];
however, laser transmission through the atmosphere is mostly considered only for delivery to ocean-based receivers. Other JAXA concepts for trial SSP systems call for
wide-area beaming from Low Earth Orbit (LEO) photovoltaic satellites. This may be
suitable over remote areas, for explorers and rescue teams to set up communications
and charge other small devices. ESA emphasis appears to have shifted [Trieb 2006;
Summerer and Ongaro 2004; Geuder et al. 2004] towards developing terrestrial DC
grids to connect renewable power generators in the North Sea and North Atlantic, and
photovoltaic arrays in the Sahara Desert.
2.1. Space Power Grid Concept
The Space Power Grid architecture [Genk 2006] is an evolutionary, revenue-generating
path to overcome the cost-to-first-power hurdle of SSP. The first argument in the Space
Power Grid (SPG) concept is that efforts to win public funding of SSP, as a competitor to terrestrial renewable or nuclear power options, are doomed to failure. Instead,
SPG argues for synergy with terrestrial renewable power generation, which also faces
formidable obstacles that space technology can help overcome. In this architecture,
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initial public funding promotes the establishment of renewable power plants in remote
areas, beaming and receiving power through a constellation of satellites. This helps
these plants to reach customers located all over the world, and in particular, to fill
the gaps in their own generation and smoothing the fluctuations that plague most renewable power technologies. Beyond finding high-valued markets for power from peak
generation periods, real-time exchange will help such plants to command the prices
earned by baseload providers by reducing the need for fossil-burning auxiliary generators. It has been shown that with a starting constellation of no more than 20 satellites
in 2000 km-high near-equatorial and sun-synchronous orbits, and about 100 participating power plants, an economically viable power exchange system can be set up,
able to reach customers in most parts of the world with power from distant generating
stations at reasonable cost of power. Once such a dynamic grid and market are in place,
ultralight reflectors placed in high orbits would beam sunlight to collector-concentratorconverters placed on the low-orbit satellites that replace the first SPG constellation.
Thus the addition of space-based generation comes after the infrastructure and market
are functioning, and is done incrementally using mostly commercial funding. Figure 2
from Genk [2006] illustrates the concept of the Space Power Grid. Transmission frequencies below 100 GHz are not viable [Komerath and Boechler 2006] because of the
very large antenna size, and the attendant weight penalty on the satellites. This is
illustrated by Figure 3. There are acceptable transmission windows near 140 and 220
GHz. In rain, neither window offers acceptable transmission. On the other hand, with
a constellation of satellites in various orbits, alternative reception and transmission
paths can usually be found. Several areas favorable for locating solar and wind plants
are located in high deserts where humidity is low and rain is rare. US rain data [Petty
and Mahoney 2007], for instance, shows that there are major areas where rain periods
are less than a few hours per year, making them ideal for generation and reception
[Komerath et al. 2009]. Similar locations are postulated to exist around the world to
make a real-time power exchange viable. The SPG system was projected to break even
[Komerath and Komerath 2011a] in 17 years at a power cost of $0.2 per kWh with
public funding covering the initial development and investment risk. A second generation of spacecraft built with revenue from these operations would then incorporate
high-intensity solar collector-converters, and these would enable power delivery at less
than $0.2 per kWh. To make the optimistic projection of $0.08 per kWh cited in some
space agency reports viable, cost estimation must be done using something other than
the published NASA-USAF space system cost database.
Recently, the feasibility of using millimeter waves for power delivery from space
has been studied for an experiment to be conducted by the Boeing Company [Potter
et al. 2011] using the International Space Station. Two generators are considered for
installation on the ISS, which moves in an orbit roughly 400 km above Earth. One
generates 10 kW of beamed power at 5.8 GHz, and the other generates 10 kW at
94 GHz using a solid-state conversion system. Since the transmitting antennae in
the experiment are fairly small, it is expected that large receivers would be needed
on Earth for full beam capture. Accordingly, the Goldstone Observatory antenna in
Arizona, and a site in Florida, are chosen. During passages of the ISS over these
locations, power received from the ISS is to be measured. The 5.8 GHz, 10 kW beam is
expected to yield less than 100 watts at either location. The 94 GHz beam is expected
to yield 7000 W at Goldstone, and roughly 4000 W at the Florida site where it is much
more humid. Although numbers on generation efficiency are not available at present,
it is a reasonable presumption that generation efficiency is expected to be quite high.
Otherwise, a large radiator would be required to dispose of the several thousand watts
of waste heat. The mass per unit power of the generator is also evidently quite high.
Devices to generate several kilowatts at 94 GHz have been refined over the years, partly
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Fig. 2. The Space Power Grid concept. From Genk [2006]. Reproduced by permission of the authors.
due to military interest in this frequency for crowd-control beam weapons [Schamiloglu
2004].
Formidable issues remain, in implementing millimeter wave, dynamic power exchange between terrestrial sites and spacecraft, and in meeting the specific power (i.e.,
power delivered per unit mass in orbit) requirements of space-based millimeter wave
generation from solar energy. The preceding arguments serve to point out why research
directed at these obstacles is a worthwhile approach, compared to either arguing for
public funding for GEO-based 5.8 GHz SSP demonstrators or waiting for a massive
drop in space launch costs, both of which run counter to presently evident trends. Before going on to survey the prospects for millimeter wave power beaming technology,
we now consider the retail power beaming end of the market requirements, to better
understand the technology needs.
2.2. Relevance to a Micro Renewable Energy Economy
Power beaming at retail level removes the constraint of hard-wired grid connectivity,
and enables connection of devices that are located in remote and mobile sites. The
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Fig. 3. Variation of the logarithm to base 10 of antenna diameter for 84% capture of a beam, as function of
beaming distance for various frequencies.
connection can be both ways. Thus, for instance, one may consider beamed power to
receivers located on automobile roofs, serving to charge electric vehicles as they drive
on the highway or in parking lots. Less exotic applications might include colocating millimeter wave antennae on solar PhotoVoltaic (PV) panels and receiving beamed power
at night while the PV arrays are otherwise not in use. During the day, excess power
may be beamed back. Power may also be exchanged between millimeter wave generators and storage facilities located centrally in a village, and wind turbines or other
generators. Such connectivity would greatly improve the utility of small renewable
power generators, that suffer greatly from the unsteady nature of their sources. Hence,
the availability of a retail power beaming architecture would make a huge difference to
the utility of micro renewable power generators, and bring such distributed generation
into the overall energy economy in a significant way. While such generators may be
inefficient in generating electric power, the economic multiplier of such a capability is
likely to be large, generating a growing appetite both for micro renewable devices, and
for the retail beamed power market. It also stands to reason that millimeter waves will
be the likely choice for such beaming, because the receivers must necessarily be small,
and in line of sight with the transmitters or relays. Figure 4 illustrates some of these
applications.
Chowdhary et al. [2009] considered the policy issues related to beaming of power
at retail level. Where the amount of power needed is small, but its availability in the
short term makes a big difference, beamed power wins over wired grid construction.
Chowdhary et al. also show that over short distances, beamed transfer is more cost
effective to install than wired transmission, even before considering all the benefits
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Fig. 4. Near-term applications of beamed retail power transmission systems. Courtesy Girish Chowdhary.
of wireless transmission. This is based on the $1M or greater marginal cost per kilometer cost of laying long-distance high-tension wired infrastructure. It is evident in
the preceding that the cost of imported power will stay well above the $0.1/ kWh level
idealized in western utility power architecture. In the short term such power enables
connectivity and opportunity in parts of the world where the wired grid does not and
is not likely to reach. In the long term beamed power allows renewable generators to
level the fluctuations in locally generated power. The local beamed power infrastructure enables micro generators to be integrated into the local grid. Trade studies will
determine whether micro renewable generators should beam their power spikes back
into the grid, or whether they should use those to generate stored energy media such
as hydrogen.
2.3. Commonality between Space and Rural Power Beaming Issues
There is surprising similarity between the space to space power market and the rural
Indian market, to justify retail power beaming. In both cases, the wired grid is inaccessible. Independent power sources and storage have to be installed for survival, and for
basic communications. In both cases, there is strong usage of leading edge communications technology including space-based communications. The individual customer’s
power requirements are modest. Note that many spacecraft require less than a kilowatt. The major differences are in the requirement for extremely precise moving-target
beaming and the much larger distances in the space case, and the need to reduce cost
to extremely low levels in the rural case.
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Fig. 5. Why it is better to conduct horizontal propagation at higher altitudes. Modified from Petty and
Mahoney [2007] and Liebe and Hufford [1989].
2.4. Atmospheric Propagation Considerations
The atmosphere attenuates electromagnetic beams. Four types of attenuation are usually considered.
(1) The first is free-space spreading from an isotropic antenna, which is proportional
to the square of distance. This is minimized down to the diffraction limit using
transmitters with very high gain, that is, by focusing as tightly as possible.
(2) The second is spillover loss, or failure to capture all of the radiation reaching the
receiver location. The diffraction-limited beam typically has a main lobe containing
roughly 84% of the power that reaches the receiver location. Textbook designs for
low-frequency microwave receivers typically aim to capture only the main lobe,
because of the size constraint. However, with millimeter waves (and with laser
optics) it should be possible to design the receiver to capture the second lobe as
well, permitting up to 98% capture. The size difference is roughly a factor of 1.55
in antenna diameter.
(3) Attenuation by air is ascribed to absorption by gaseous nitrogen, oxygen, and water
vapor. Both temperature and pressure are important. The first effect is that of
density, so that as altitude increases, attenuation decreases. The second is of the
actual water vapor content in mass of water vapor per unit volume. Thus, for a
given relative humidity, increasing altitude and decreasing temperature imply a
decrease in water mass per unit volume. For a standard atmospheric temperature
lapse rate, rising from sea level to 4,000 meters brings roughly a 27-degree Celsius
drop in temperature, which implies up to a 6 dB drop in attenuation.
(4) The fourth is scattering by solid (dust or ice) or liquid (rain or cloud) particles.
Above cloud level, water and ice droplet concentration is very low, so that the
biggest problem with millimeter waves, that of rain, is also minimal.
Some of these effects can be seen in Figure 5, from work done by the US Federal
laboratories and NATO [Petty and Mahoney 2007; Liebe et al. 1985; Liebe and Layton
1987; Liebe and Hufford 1989] which shows horizontal attenuation in dB/km over
the microwave and millimeter wave regimes. The upper curve is for a case of 75%
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Fig. 6. Need for burn-through is illustrated by the severe attenuation of millimeter waves through humid
air. Data from Liebe et al. [1985].
humidity at sea level, while the lower curve is for nearly dry air at 4 km altitude. These
models show that there are several good frequency windows for propagation throug
the atmosphere, other than the low-frequency one below 10 GHz. The windows near
94 GHz, 140 GHz, and 220 GHz are of particular interest. There are already practical,
portable, high-power converters in the 94 GHz regime, developed for military and
police applications. The 140 GHz regime has been used in concept studies for beamed
propulsion from Earth to space-launch vehicles, presumably corresponding to some
high-power sources that may become available for such applications. The 220 GHz
window has found both military and civilian applications, primarily for imaging and
radar. We find this window to be attractive because of the strong need to minimize
receiver size and mass for space beaming applications.
The issue of vertical transit through the atmosphere has been considered by astronomers. Data on propagation come from the Mauna Kea observatory in Hawaii,
which is located 4205 meters above sea level, in a region which enjoys clean air. These
data, easily found on the Internet, show that transmission is above 99% below 10 GHz,
above 98% for the 94 and 140 GHz windows, and above 97% near 220 GHz. Given the
large advatage in receiver size, these data present a clear case to select the 220 GHz
window, provided the low-altitude humidity and weather issues can be avoided as
indicated in the prior paragraphs. The effect of atmospheric transit at a 45-degree
inclination to the zenith has been presented in Petty and Mahoney [2007], and drives
a similar conclusion. It shows roughly 90% trasmission near 220 GHz.
The effects of relative humidity are considered in Figure 6. The figure is constructed
from the predictions in Liebe et al. [1985], for sea level air at the highest temperature
considered, which is 310 K. This is taken to be most relevant for Indian surface conditions. Again, the net conclusion is that horizontal propagation of millimeter waves at
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Fig. 7. Surface range vs. relay height of various platform options.
sea level is to be avoided as much as possible. These considerations impact the architecture described in this article. More exotic ideas, such as the burn-through effect of
continuous beaming at high power levels, are considered later in the article.
3. FOUR-STAGE MODEL FOR A POWER BEAMING ARCHITECTURE
In the following, a 4-stage model is considered. Some idea of the range of applicability
can be obtained simply from line of sight considerations as shown in Figure 7. The
terrestrial power grid will primarily be supplied from power plants located in the
region. This grid will reach district capitals and major towns, and be available along
the railroad system. From points on this grid, power can be beamed efficiently over short
distances using the infrastructure for mobile telephony, as well as by using antennae
located on hills and ridges. The four stages are as follows: (Figure 3 compares the
antenna diameters required for these options)
(1) beaming across nations between major plants via satellites;
(2) beaming from regional plants, to stratospheric platforms distributing to local
reception points or aerostats;
(3) beaming from local mini-power plants via tethered aerostats;
(4) last-kilometer beaming using mobile phone / railway telecom infrastructure.
3.1. Beaming across Nations between Major Plants via Satellites
At levels of tens of megawatts, beaming through space or between high-altitude platforms, becomes a practical alternative for the future. An India-US power exchange
using 4 to 6 satellites is discussed in Dessanti et al. [2011], using the SPG architecture.
While rain and fog effects are severe, dense cloud cover is usually limited to moving
storm fronts. Hence alternative beaming locations can be found within a few miles.
Where this is not feasible, such as in the Indian monsoon, additional storage appears
to be the only option. The losses can be represented by the excess power multiplier to be
applied at the transmitter to obtain the desired power at the receiver. For the SPG first
phase, where terrestrial power is beamed up to space and back down to the receiver,
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the mulitplier is 1.57; however, when space-based power generation is incorporated,
the factor comes down to 1.21 because only one atmospheric transit is needed.
3.2. Beaming from Regional Plants, to Stratospheric Platforms
Distributing to Local Reception Points
The second alternative is beaming from a stratospheric platform (stratoform), which is
a large, lighter than air inflated structure. Such platforms have been proposed for solar
and wind power generation, and hence have been conceptualized with both the large
area needed for an antenna to receive power from space, and the propulsive ability
to stay stationary in high-altitude wind streams. Both stratoform and space beaming
are best at 220 GHz, also enabling direct transmission to rooftop antennae where the
weather allows.
Where access to the wired grid is several kilometers away from a village, the shortterm feasibility and cost of a tower-based system must be traded off against the higher
beam efficiency with the high-altitude platform, whether it is a stratospheric platform
or an aerostat tethered to stay in the troposphere. Stratoform beaming requires two
passages through the troposphere. The rest of the transit is through high-altitude rarefied air, above the weather and most of the water vapor. Thus with sufficient relay
efficiency, the stratoform becomes the winning choice for most areas lacking primary
terrestrial grids. It is also the better choice in case of natural disasters. During the
most intense storms where the disturbances may reach far into the stratosphere, the
stratoform will have to be grounded, but probably only for a few hours. A terrestrial
tower-based relay, on the other hand, requires an investment in permanent structures
in remote areas, using hilltops wherever possible. These towers are also susceptible
to damage in major storms, and will be downed by earthquakes, landslides, and high
winds. Satellite-based beamed power transmission achieves transmission efficiencies
nearly as high as the stratoform does, but is substantially more expensive and involves longer lead-time for development. The ground receiver antenna diameter may
be 150 meters with 220 GHz beams. A preferred option, when both systems are operational, may be to use stratoforms as relays for space-based beaming to enable pin-point
retail beaming from the 150 m antennae on the stratoforms directly to rooftop antennae.
3.3. Beaming from Local Mini-Power Plants via Tethered Aerostats
For transmission over a distance larger than about 5 km, a practical and effective way
is using tethered aerostats. Tethered aerostats have been described by many authors,
for instance Krausman [2005] and Peterson [2005]. Our recent studies indicate that
the aerostat architecture offers some interesting options to improve the effectiveness
of rural power beaming.
Pant et al. [2011] discuss the use of tethered as well as powered Lighter Than Air
(LTA) platforms such as aerostats in rural power beaming. Tethered aerostats are
feasible, where the aerostat is anchored at altitudes as high as 4000 to 5000 meters.
This fact opens up the possibility of relaying most horizontal propagation between
aerostats at altitudes where the air is both dry and rare enough to eliminate most of
the losses. Komerath et al. [2011] discuss the option of sending millimeter wave power
through waveguides built into the tethers used with aerostats at 4000 m altitude, as a
way to tunnel through the lossy transmission through the troposphere. The dimensions
of the tether waveguide are compatible with the choice of 220 GHz as the beaming
frequency. The added weight of the metal waveguide within the tether appears to
be quite manageable, within the demonstrated envelope volumes of aerostats. The
antenna structure and related paraphernalia can be placed inside the gas envelope of
the aerostat, close to the center of gravity. This option, if successful, can break through
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the primary objection to the use of millimeter waves, which is the vulnerability to
moisture and the loss due to the dense air near the surface.
3.4. Last-Kilometer Beaming Using Mobile Phone/Railway Telecom Infrastructure
As seen before, beamed power delivery becomes cost effective at the final stage of
distribution, where the power grid is unlikely to reach in the near term. To avoid
long-term health uncertainties, we will assume that the device-level delivery, which
includes indoor reception, occurs through wired charging points rather than wide-area
wireless. Thus all BPTS use considered here is as point-to-point beams.
The last delivery step inside a village home is thus still likely to be using AC or
DC power through wires wherever practical, to reduce expense and technology level.
Sea-level beamed transfer may be done using the 94 GHz window, where the excess
power mulitplier is only 1.11 per kilometer, which means that only 1.11 Watts need
be beamed for every watt that is received with the transmitter only 1 km away.
This shows why low-altitude beaming is at best a stop-gap solution, since a beaming
distance of even a few kilometers would bring efficiency down to less than 20% and
take the excess power multiplier well over 5, driving the cost beyond sustainable levels.
For stratoform beaming, the beam must go up to 30 km and back down, while the
transit loss between stratoforms is negligible. Here the excess power multiplier is 1.51.
Local distribution from high-altitude platforms to rooftops within the last-kilometer
is better done at 220 GHz to keep receiver sizes down.
3.5. Usage of Mobile Telephone Infrastructure
It is reasonable to assume synergy with the existing mobile telephony infrastructure,
because of the separation in wavelengths used, and the pin-point nature of power
beaming. The presence of strong electromagnetic fields at the antennae might interfere
with cellphone signals [Drapalik et al. 2010; Costia et al. 2010]. An alternative is to
colocate the beamed power receivers with railway infrastructure where the railway
reaches the villages to be served. These issues remain to be explored. Concerns have
been expressed regarding the effect of millimeter beams on metal towers. The best
answer may be that the receivers must be placed a short distance away from the
towers themselves, but close enough for maintenance access.
3.6. Baseline Economic Model for Beamed Power Transmission to Rural India
With 72% of India’s 1.1 billion population living in rural areas according to the 2001
census, and some 638,000 villages, the average village has a population of roughly
1700 and a land area of 100 hectares or square kilometers. While 80% of villages
are believed to have at least one electric line, 52.5% of villagers (415 million people)
are estimated to have no access to electricity [Shrestha et al. 2004]. This works out
to roughly 100 million households, or roughly 150 homes per village. Modi [2005] in
2005 estimated the number at 579 million. The 1 square kilometer area is a generous
assumption since irrigated farm areas are usually included in this figure, allowing
for a dispersed housing model. Many Indian villages concentrate houses in a small
area for easy access to community resources and social gatherings, with fields or forest
resources surrounding this area. However, some of the most wealthy homes are likely
to be located away from the center, and these are likely to be leaders in technology
adoption. Thus the local distribution range is well within the definition of the lastkilometer that power has to reach. As early as 1997 [Press 1999] cable television had
penetrated a large number of villages, distributed from satellite receivers, with local
fossil-fuelled generators powering the sets. Today, cellphone usage is prevalent, but
owners often have to go far to charge them.
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Since the villagers who lack access to electricity cannot be assumed to be able to pay
the high cost of beamed power, the cost of power must be borne mostly by any community center and industry located in each village. At the first stage, with ground-level
beam delivery over more than a kilometer, the excess power multiplier is high, meaning
that this is suitable only for low power levels. Such levels are suited to communication devices, lighting systems based on Light-Emitting Diodes (LEDs), and emergency
equipment. In the next stage we assume that each family uses 2.4 kWh per 24 hrs,
with the community and industry (C & I) picking up the rest of the 2920 MWh/yr.
With the families paying only $0.1/ kwh, the C&I pay $0.215/ kWh. With SPG-based
power delivery to a more developed community, the usage level goes up by an order of
magnitude, and the cost is levelized at $0.2/ kWh. This level will permit growth of local
renewable sources, with many families generating enough to reduce their net power
cost below the ideal of $0.1/ kWh, as energy independence grows.
For longer distances and larger amounts of power, stratospheric platforms [Krausman 2005; Peterson 2005] offer the best promise. Assuming that each stratoform can
deliver power simultaneously to 1000 receivers, and that each village is served by two
stratforms, a fleet of nearly 1400 stratoforms are needed to reach all villages, which
should take some 14 years totally to produce and deploy. The business case for this
investment must be based on the economic development arising from power access,
education, and advances in standard of living, and sales of communication equipment.
4. TECHNOLOGY ISSUES
The entire spectral range from 100 GHz to 1 THz poses intriguing opportunities. This
regime spans the transition between traditional electronics and photonics, and hence,
legacy methods from both the microwave and laser optics communities come into play,
with interesting possibilities for integrating these. Applications of millimeter waves
arise in many fields, and the opportunities are just beginning to be realized. Uses in
detecting weapons and contraband using various properties of the human skin, tissue,
thermal characteristics of materials, emission spectra, and scattering characteristics of
textiles are summarized in Appleby [2007]. A modern approach [Crowe et al. 2005] is to
develop solid-state circuits that use nonlinear diodes to extend microwave technology
to much higher frequencies. At another end of the technology spectrum, light is used
directly to generate millimeter waves.
The essential subsystems for beaming power are the power transmitter and the
power receiver. The power transmitter embodies the power beaming station where
ElectroMagnetic (EM) energy is harnessed to a suitable wavelength that can be
beamed to the receiver without significant loss in the transmitting medium. The
power receiver is the electric power generation station where the beamed EM energy
is transformed into electricity. Economical utilization of this energy requires very high
energy conversion efficiencies at the beaming and receiving stations.
4.1. Conversion of the Solar Spectrum to mm-Wave Energy
Solar energy abounds in space. Most of the energy is available in the visible range of
the EM spectrum but there is a significant amount of energy in the ultraviolet region
compared to the spectrum at sea level [Hulstrom et al. 1985]. Considering a particular
photon in the green wavelength of 532 nm, the frequency of the corresponding EM
wave is 564 THz with a quantum of energy 2.33 eV. Therefore, a green photon needs to
be converted to 2560 photons of frequency 220 GHz each for 100% conversion efficiency.
This is a daunting task.
Direct wavelength conversion techniques such as optical heterodyning or wavelength
mixing for frequency down-conversion would require nonlinear optical elements and
the conversion efficiency is expected to be extremely low. Booske [2008] reviewed the
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challenges in frequency conversion and summarized data showing that the output
power of solid-state devices is very low (100 mW). Thus very large arrays of such solidstate devices will be required for high-power applications. Vacuum devices can generate
large amounts of power (1 MW) in the 200–250 GHz range. Although vacuum devices
are proven good for producing high-power microwaves, their specific output power
(output power per unit mass of the equipment) is low. On the other hand, solid-state
devices do not offer nonlinear scaling advantages as power level is increased.
Filip et al. [1997] showed theoretically that microwaves (electromagnetic radiation
in the GHz range) can be produced by controlled motion of electrons in a vacuum in the
presence of electric and magnetic fields. Their vacuum microelectronics device consisted
of a capacitor, a cathode, and an anode, and they showed that the electrons follow a
cycloid-like trajectory. High acceleration of the electrons during this curvilinear motion
leads to the generation of microwaves. Typical values of the speed and acceleration of
electrons in their study were on the order of 106 m/s and 1017 m/s2 , respectively.
Energy is radiated when a charged particle moves in a magnetic field, leading to
cyclotron and synchrotron radiations at nonrelativistic and relativistic velocities of
electrons, respectively. Egorov et al. [2006] and Roy et al. [2007] investigated highpower millimeter wave generation using nonrelativistic electrons in a retarding electric
field, that is, using decelerating electrons. The work of Egorov et al. is, particularly
interesting because they formed an electron beam with an electro-optical system and
studied the oscillations of the beam using a diode scheme. These devices, however,
require an external electricity generator to produce electrons and to create the electric
field for accelerating or decelerating the electrons.
Electrons can be produced by six methods.
(1) Auger electron emission, which occurs due to radiationless transitions of electrons
between orbital energy levels. Ionized atoms can lose energy by releasing an Auger
electron instead of emitting a photon, and the Auger electron acquires its kinetic
energy from the radiationless transition.
(2) Field emission, which occurs when a strong electric field is applied to a metal.
(3) Photoemission, which occurs when photons, that is, EM waves excite electrons to
the vacuum energy level as in the photoelectric effect.
(4) Radioactive nuclear decay, which produces negative beta particles.
(5) Secondary electron emission, which occurs when atoms are excited by charged
particles including electrons.
(6) Thermionic emission, which occurs when a metal is heated.
Electromagnetic radiation itself can be produced by three basic methods: (i) charged
particle acceleration or deceleration, (ii) electronic transitions in ionized or excited
atoms, and (iii) nuclear reactions. Photoexcitation is an approach to produce electrons in the conduction band of a semiconductor by exciting electrons from the valence
band. This can be achieved using incident sunlight. The conduction band electrons
can be accelerated or decelerated using the built-in potential of p-n junctions depending on whether the electrons are injected into the junction from the n-side or
p-side, respectively. The radiation emitted by electrons during deceleration is known
as bremsstrahlung (i.e, braking) radiation. The wavelength of the radiation depends
on the magnitude of the acceleration and deceleration, and the emitted radiation is
highly directional at certain angles around the direction of the electron velocity as
shown in Figure 8. Numerous factors affect the efficiency of converting the sunlight
into electromagnetic waves in the GHz range, including the dopant concentration, builtin potential across the p-n junction, geometry of the p-n junction (flat versus pointed
junction), photogenerated electron density, polarization of the incident radiation, types
of substrate (narrow bandgap (e.g., Si of bandgap 1.1 eV) versus wide bandgap (e.g.,
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Fig. 8. Emission of coherent radiation by electron acceleration.
Table I. Comparison of SiC and Si
Property
Bandgap [eV]
Thermal conductivity [W/mK]
Electrical break down strength [MV/cm]
Junction temperature [C]
SiC
3.2
370–490
5
600–650
Si
1.1
100
0.5
100–150
silicon carbide of bandgap 3.2 eV), and connectivity of several p-n junctions in series
using high electrical conductivity material (e. g., graphene).
Crystalline silicon carbide (SiC) has excellent thermophysical properties and mechanical strength. It is chemically inert and resistant to radiation damage. This radhard property makes it suitable for applications in space and other harsh environments.
Some of the properties of SiC and Si are listed in Table I. High thermal conductivity
and high junction temperature indicate that SiC p-n junctions can be used at high
temperatures. Also high bandgap and high electrical breakdown strength indicate that
SiC p-n junctions will have high built-in potential and the electrons can be accelerated more at the SiC p-n junction than at the Si p-n junction. Additionally the higher
bandgap of SiC indicates that the photons in the visible and near-UV range can be
used efficiently to generate photoexcited electrons in SiC semiconductors. The cost of
crystalline SiC, however, is much higher than that of Si. Other materials may be used
or other methods may be investigated to lower the cost of producing crystalline SiC.
Two p-n junction-based solid-state devices for conversion of sunlight to millimeter
waves are conceptually illustrated in Figure 9. Since sunlight is randomly polarized,
a linear polarizer is used to select only a unidirectionally oscillating electromagnetic
field of the light. While some of the light creates photoexcited electrons, lights of other
wavelengths can be used to inject electrons into the p-n junctions. The p-n junctions
are connected in series to accelerate (or decelerate) the electrons successively in the p-n
junctions as the electrons travel back and forth due to the oscillating electromagnetic
field of the incident light. This mechanism is expected to yield more EM radiation from
the same electron. Another design of the device uses azimuthally polarized light. In this
case, the electrons move in a circular track and emit EM radiation as they accelerate
or decelerate at the p-n junctions.
Another aspect of this p-n junction device is that the operating principle of an
antenna can be applied to transform the solar energy into electromagnetic waves in
the GHz range. Since the electrons and holes accumulate at the ends of the depletion
layer across the p-n junction, the depletion layer can be considered to form a dipole
of length equal to the depletion layer width. The oscillating electric field in the
electromagnetic waves of the sunlight can be utilized to oscillate the dipole in order
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Fig. 9. A set of p-n junctions connected in series to cause acceleration and deceleration of electrons, which
oscillate along a line due to the linearly polarized electromagnetic field of the incident light.
to emit electromagnetic waves with frequency in the GHz range. The resistance
and capacitance of the depletion layer can be designed to achieve this type of downconversion with possibly high conversion efficiency. Salama et al. [2003] fabricated an
antenna-coupled Schottky diode for frequency mixing that operated at about 94 GHz.
The conversion of solar quanta in space directly into a suitable quantum which can
be transmitted to the Earth with minimal transmission loss provides one option for
wireless power transmission. Karalis et al. [2008] used two coils of diameter 60 cm each
to produce radiofrequency energy and supplied power to a 60 W light bulb wirelessly
with 40% efficiency from a distance of 2 m in 2007. Intel Corporation [Schneider 2010]
demonstrated 75% efficiency in supplying power to a 60 W light bulb wirelessly from a
short distance. Mohammed et al. [2010] presented a historical perspective of wireless
power transmission and reviewed microwave generators in various frequency ranges
such as 2.45, 5.8, 8.5, 10 and 35 GHz.
4.2. Generation and Conversion of Millimeter Waves
McMillan [2006] surveys the state-of-the-art of millimeter wave radar and terahertz
imaging, at a time when gyrotrons were the best sources. Kartikeyan et al. [2004] and
Nusinovich [2004] give excellent discourses on the technology of high-power millimeter wave generation using gyrotrons. Schamiloglu [2004] has surveyed high-power
microwave generators, specifically about applications to nonlethal directed energy
weapons.
Today solid-state technology appears to be the preferred option for generation.
Phase-locked loops are used to obtain narrow frequency bands. Johansson and Seeds
[2003] discuss generation and transmission of optical signals modulated by millimeter
waves, using an Optical Injection Phase-Lock Loop (OIPLL). Voingescu [2006] and Winkler et al. [2005] describes the solid-state technology above 60 GHz, and over 200 GHz,
as awaiting mass-production markets. Tracking antenna technology appears to have
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advanced enough to enable a large number of self-aligning phase array receivers to
simultaneously track and communicate with beaming transmitters [Miyamoto et al.
2003].
The possibility of generating millimeter waves by resonance of a microspherical
cavity has been considered by Ilchenko et al. [2000]. Microspherical whispering-gallery
modes of a toroidal lithium niobate cavity electro-optic modulator were superposed
with a stripline resonator. Preliminary results showed resonance Quality (Q) factors
exceeding 5 million in a 9 GHz prototype, and a 33 GHz prototype was being tested.
With efficient coupling of light in and out of the resonators, and high-purity silica, Q
factors of over 10 billion are projected. Wake et al. [1995] describe a way to generate
millimeter wave signals using optical input. A dual-mode multisection distributed
feedback semiconductor laser is used to generate high-power signals between 40 and
60 GHz with excellent spectral purity and stability.
Razavi [2008] describes an inductive feedback technique that substantially raises the
maximum speed of resonant circuits, allowing them to operate near the self-resonance
frequency of the inductors. Optical heterodyning techniques using laser beams directed
at ultrafast photodiodes has been used by Wake et al. [1995] to obtain narrow linewidths
and excellent phase uniformity.
Waveguides for millimeter waves are considered by Hirshfeld et al. [2005] who describe a high-power harmonic generator for infrared and millimeter wave regimes.
Theoretical analysis and computations suggest the use of a uniform dielectric-lined
waveguide in an efficient generator at 103 GHz, using a 34.2 GHz drive beam. A
bunched beam injected into such a waveguide generates intense radiation at a harmonic of the injection frequency. The phase velocity of the design mode of the structure
must be synchronous with the beam velocity. Samoska and Leong [2001] describe two
Monolithic Microwave Integrated Circuit (MMIC) power amplifier designs using InP
HEMT technology. One covers the WR10 and WR8 waveguide bands (75–110 GHz and
90–140 GHz). The other is optimized for the WR10 band, providing over 13 dB large
signal gain over the 75–110 GHz and output power of 40–50 mW. Wang et al. [2001]
describe a further set of W-band power amplifier modules using MMICs, for local oscillators of a far-infrared and submillimeter telescope. The highest frequency power
amplifier covers 100–113 GHz and has a peak output power greater than 250mW at
105 GHz. Waveguide fabrication techniques are described in McGrath et al. [1993].
The most convincing demonstration seen to-date on the deployment of Megawattlevel millimeter wave beams and waveguides with good efficiency is the work of Verhoeven et al. [1998]. They reported generation of over 750 kW, continuously tunable
over the range from 130 GHz to 260 GHz, using a MASER (microwave amplification by
stimulated emission of radiation) powered by an electron beam. They cited over 50%
overall efficiency. The apparatus was designed for long (100 millisecond) pulses in the
given frequency range, with pulse power levels exceeding a Megawatt. They declared
the feasibility of going up to 5 MW and continuous-wave emission in future. Their
equipment used physical waveguides to contain and amplify the millimeter waves.
4.3. Nano-Antennae, Large-Array MMICs and Beam Steering
A digital beam-forming antenna for the 25–30 GHz range, for operation on stratospheric
platforms has been described by Oodo et al. [2000]. Goswami [2004] review emerging
developments in solar energy. Among these is direct conversion using nanoscale antennas with efficiencies exceeding 80 and possibly 90%. These exploit the wave nature
of light, and envisage broadband rectifying antennae. These would have nanoscale dimensions to capture wavelengths in the submicron range. In this way, the fundamental
band-gap limitations of photovoltaic semiconductor cells can be avoided, enabling high
efficiency. By the same argument, nanoscale antenna elements could also be developed,
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tuned to given millimeter wave. Wang et al. [2004] discuss the antenna effect in
receiving and transmitting light like radio waves using random arrays of aligned
carbon nanotubes. They show response consistent with the theory of conventional
radio antennae. They postulate that carbon nanotubes interact with light in the same
manner as simple dipole radio antennae. The reflected signal is suppressed when the
electric field of the incoming radiation is polarized. There are interference colors in
the reflected light from an array, because of the length matching antenna effect.
Torkildson et al. [2010] discuss opportunities for outdoor millimeter wave networks.
They point out that for given areas (antenna sizes), high-frequency systems are
substantially better, with a 21 dB advantage for a 60 GHz system over a 5 GHz system.
They point to the possibilities for electronically steerable arrays of compact printed
circuit antennas to realize highly directional links without manual pointing. Laskar
et al. [2007] and Pham et al. [1998] report that the frequency range of CMOS process
technologies has increased to the point where low-cost, highly integrated 60 GHz
millimeter wave radio is possible, and 60 GHz transceivers have been demonstrated as
being suitable for high-volume products. Nicholson et al. [2007] describe a low-voltage,
small form factor chipset for 77 GHz automotive and imaging applications. The
chipset is fabricated in 0.12 micron SiGe HBT technology. A fine lithography SiGe
BiCMOS process is chosen as an attractive technology to meet digital signal processing
requirements in the millimeter wave spectrum, into the 200 GHz range. A fully
integrated 2.5V, 77 GHz transciever with PLL is described. A solid-state 200 GHz
experimental radar is described by Essen et al. [2007].
In the field of radar applications, there has been intense effort on millimeter wave
generation, pointing, reception and interpretation, along with studies of atmospheric
effects. Brookner Brookner [2002] summarizes the state-of-the-art and trends in MMIC
phased arrays. He describes an electronically steerable plasma mirror, where a plasma
sheet is rotated to steer the beam in azimuth, and electronically tilted to steer the
beam in elevation. A 50 cm by 60 cm plasma mirror is described. While several highend options being developed under the US Defense Advanced Research Projects Agency
(DARPA) sponsorship are indicated, Brookner also points to low-cost mass-produced
77 GHz phased arrays for automobiles. Here separated antennas are used for transmission and reception, with their beamformer networks photo-etched on a sheet of
copper-clad dielectric. Each antenna consists of series-fed columns of patch radiators.
The beam-formers are Rotman lenses. Beams are scanned in azimuth by switching
between input ports of the Rotman lens.
A technical development of immense potential interest is the finding that solar photovoltaic panels can serve as effective antennae for other forms of electromagnetic
energy, including the millimeter wave spectrum [Petty and Mahoney 2007; Gliese et al.
1998; Yao 2010]. Initial interest in this area comes from the noise introduced into
the output of the solar panel by electromagnetic signals, and by the power radiated
away by the solar panel [Drapalik et al. 2010]. However, it appears that panels can be
deliberately designed to serve as photovoltaic collectors and beamed power antennae
simultaneously. This option has a high potential payoff in smoothing the power output
of a solar installation and turning it into a baseload supplier.
4.4. Conversion of Millimeter Wave Energy to Electricity
At the power receiving station, the millimeter wave energy must be converted to
electrical energy in either direct current or alternating current mode to suit end-user
devices. One nonelectrical option may be to use the millimeter waves to heat liquids
and generate steam to run electrical generators; however, this is an inefficient use
of beamed energy in most terrestrial applications. It does offer an inexpensive option
for energy storage at the receiver, and hence may be an option in emergency response
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applications. Another approach is the direct conversion of the mm-wave energy to
electricity using a solid-state device, such as a rectenna which is a rectifying antenna.
The construction of a simple rectenna would be a dipole antenna containing a Schottky
diode across the dipole elements. The incident mm-waves induce an Alternating Current (AC) in the antenna and the diode rectifies the AC to direct current. The Defense
Advanced Research Project Agency [Raghunathan et al. 2005] investigated a rectenna
array for harvesting energy from infrared waves. Corkish et al. [2002, 2003] discussed
rectennas for solar energy conversion and their conversion efficiency. De Vos [1992] and
Berland et al. [2003] reported that the limit of the conversion efficiency is 85.4% for a
nonenergy-selective collector. However, De Vos [1992] also pointed out that the conversion efficiency can be increased to 86.8% in thermal converters by absorbing multiple
wavelengths in multiple materials having different bandgaps. Ries [1983] and Wright
[2000] showed theoretically that the ultimate conversion efficiency, that is, the Landsberg limit [Green 2003; Richards and Shalav 2007] of 93.3%, can be achieved using a
rectenna assembly consisting of nonreciprocal absorbers such as optical circulators.
5. DISCUSSION
Several issues arise with millimeter wave beaming, especially for retail beaming. These
are summarized as follows.
(1) Low efficiency of generation and transmission. The answer here is twofold. First,
even if the efficiency is low, the net result should be compared to the value of the
first watt-hour and first watt of capacity. The opportunity cost of waiting for the
wired grid should also be considered. The second part of the answer is that net
end-to-end efficiency using millimeter wave beaming is not low when compared to
other power transmission options, and the efficiency is rising with research and
development due to strong market interest in millimeter waves.
(2) Expense of solid-state arrays versus photovoltaic arrays. The answer is to design
for synergy. Photovoltaic panels can be modified to serve dual-use as millimeter
wave antennae at night, or even to transmit excess power during the daytime.
Integrated PV-millimeter wave chips to receive, convert, and transmit power could
provide a mass-market solution.
(3) Atmospheric propagation losses. This is a strong argument for minimizing the horizontal path at low altitude. Horizontal propagation should be conducted at high
altitude, using aerostats or stratospheric platforms as far as possible. Options such
as burn-through using different frequencies or evaporation ducts should be explored. The possibility of using waveguides through tethers to aerostats floating
above the cloud layer offers an exciting possibility to break through this fundamental problem.
(4) High beam intensity. With small receivers, intensity will be high. It is standard
practice to have a pilot beam position sensor and automatic cutoff that prevents
transmission unless the link is functioning properly. The high intensity obviously
poses issues with horizontal beaming at low altitude.
(5) Health effects of millimeter waves. This is a large and serious issue. So far, there
are no known ill effects. However, not much research has been published, and longterm phenomena remain to be understood. Ryan et al. [2000] have reviewed health
effects of millimeter waves. Millimeter wave energy is deposited within the first
1–2 mm of the human skin. This effect was used to therapeutic advantage in skin
disorders, gastric ulcers, heart disease, and cancer in the erstwhile Soviet Union.
The advent of so-called nonlethal crowd control weapons employing 94 GHz waves
shows clearly that the heating effects of millimeter waves should be examined
carefully.
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(6) Infrastructure. One option to explore is to use the mobile telephone infrastructure
as convenient last-kilometer transmitters or relays. In India the railway infrastructure has been traditionally used as the path for communications infrastructure to
expand throughout the nation. The mobile telephone network is no exception; it has
also spread along the railways first. Hence it is reasonable to expect that the retail
power beaming infrastructure can in turn use the mobile phone infrastructure.
5.1. Atmospheric Propagation and Burn-Through
Du Bosq et al. [2008] have surveyed millimeter wave effects on other substances such
as soils. Sharma [1974] reported on a 3-year study of the effects of refractive index,
rain, humidity, and sea-breeze on the momentary fadeout of microwave signals in
the tropics. Indian Railways has accumulated extensive experience with microwave
line-of-sight transmission along the railroad right of way [Chakraborty 2009; Dalela
et al. 2008]. Solar photovoltaic powered microwave repeater systems are used, where
railway signals are transmitted between wireless towers [Sreekanth 2003]. The solar
power sources with storage augment the wired power grid which is prone to frequent
failures. One observation is that even with 7 GHz microwave communications, fog in
river valley areas in a tropical climate causes signal outages [Reddy and Reddy 1992].
This should give pause to anyone who believes that using microwave frequencies below
10 GHz guarantees high-efficiency, all-weather transmission. Farinholt et al. [2009]
have explored the use of energy transmitted through microwave to power distributed
sensor nodes. They used a “microstrip patch antenna” which operated over the 2.4 GHz
band to power a wireless sensing device used for structural health monitoring. The
concept has been tested on real devices on the field and in the laboratory. They report
excellent results even when the level of transmitted power was limited to low values
(1 W), which was sufficient to provide power for the sensor nodes. Their results break
new ground towards new embedded devices that communicate and receive power over
the same wireless network.
5.2. Phase Nonuniformity and Atmospheric Turbulence Effects
One criticism of millimeter wave and laser beaming coming from the proponents of
low-frequency microwave beaming is that extreme mirror flatness is required. It is
argued that if such perfection were feasible, the state of astonomical imaging would
be greatly advanced from its present state. However, this argument misses several
points. Firstly, low-frequency microwave beaming systems require a very large product
of transmitter and receiver diameters, for any significant distance. Most such systems
are sized for only 84% reception, that is, only the main lobe, because of the cost of
building or installing antennae large enough to capture the first side lobe. This single
reality more than neutralizes the claimed dry-atmosphere transmission advantage of
low microwave frequencies. Most experiments to-date have used transmitter-receiver
combinations that yield single-digit efficiency values. Atmospheric turbulence, refractive index gradients, dust, and interference with secondary reflections from the spilled
radiation all conspire to produce considerable phase noise and other nonuniformities.
Hence the first conclusion is that low microwave frequencies also suffer large amounts
of noise and spatial nonuniformity.
This problem is not without solutions. In the Boost-Phase Intercept (BPI) problem
of strategic missile defense, one pacing issue in the 1980s was the computer speed
required to compute the spatial transfer function between the low-level pilot signal
sent from the weapon and its reflection received from the target. The transmitter array would then be adjusted to compensate for this transfer function before the main
high-power pulses were sent, so that the beam hitting the target would have the desired spatial characteristics and coherence. This option sounds exotic if done with
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servo motors and mirror deflectors. However, consider that solid-state millimeter wave
power beaming and reception will probably be done with large arrays of integrated
converter/transmitter/receiver modules, each of which is a digitally-controlled device.
Thus, tailoring phase and other characteristics is fundamentally possible at a singledevice level. Once developed, this technology brings astronomy-quality electro-optical
correction to mass-produced devices for the consumer market, and hence its development cost will be quickly recovered. Actual implementation of this and the other
advances postulated before is of course a nontrivial endeavor, which is the reason for
this article.
6. CONCLUSIONS
The state of present research related to retail power beaming appears to be as confused
as our initial presentation of the diverse streams of enquiry relevant to this field. Systematic classification is hindered by the fact that ideas used in one paper for a given
application offer excellent potential in other aspects of the power beaming problem.
This is mostly attributable to the relative obscurity of the retail power beaming application. The technology is primarily being driven by the market for communication
devices or military/security/astronomical imaging applications. However, for the same
reason, there are several streams of technology that offer excellent promise when integrated towards power beaming. The reasons why millimeter wave bands must be
explored for power beaming are laid out in the article. The use of millimeter wave
bands, particularly in the 220 GHz atmospheric window, appears to be much more
near term than was previously suspected, since 220 GHz components and fabrication
technology are already being widely used in device development for other millimeter
wave regimes. Direct optical conversion and various forms of opto-electronic technology appear to offer the potential to enable power beaming at high levels, far beyond
what is presently considered for short-range or imaging applications. In particular, concepts such as the micro-resonators and waveguide bundle resonators offer intriguing
possibilities. Some specific conclusions from the article are enumerated next.
(1) Despite low end-to-end efficiency, millimeter wave power beaming is a viable alternative to bring electrification to rural areas, even in economically undeveloped
areas. It can leap-frog the wired grid.
(2) Direct beaming via towers, relay through tethered aerostats, or stratospheric platforms, and a Space Power Grid constellation are considered. The aerostat offers
small receiver size for retail exchange, while the stratospheric platform enables
use of very small retail receivers when used as relays of power beamed from space.
(3) For low-altitude beaming, the 94 GHz band may be best suited, while for the
stratospheric platform and Space Power Grid, the 220 GHz window is most suited.
(4) Synergy with cellphone and railway infrastructure offer immediate paths to establishing retail power beaming.
(5) Large arrays of solid-state MMICs offer the potential to integrate receiver, converter, transmitter, beam steering, frequency selection, and antenna phase correction possibilities. Digital signal processing and phase-locked loop technologies
enable selection of desired narrow bands for atmospheric transmission and conversion to and from the millimeter wave regime. With a few generations of cost
reduction through efficiencies of scale, beamed power systems may be largely automated and achieve competitive cost effectivness.
(6) Retail beaming technology offers a path to integrate direct conversion of solar
power. A growing body of research is advancing the synergy between photovoltaic
receivers and phase array antennae for round-the-clock power conversion using
direct sunlight and beamed millimeter waves.
ACM Journal on Emerging Technologies in Computing Systems, Vol. 8, No. 3, Article 18, Pub. date: July 2012.
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N. Komerath and A. Kar
(7) Indian village electrification offers an opportunity to establish a retail beaming
infrastructure with a growing fleet of stratospheric platforms.
(8) Low efficiency and weather issues are not show-stoppers for retail millimeter
wave power beaming, but new solutions such as waveguide beaming to and from
aerostats, with horizontal wireless beaming between aerostats, may greatly increase end-to-end efficiency.
ACKNOWLEDGMENTS
The authors acknowledge assistance from Dr. Girish Chowdhary in preparing the ISED conference version
of this article.
REFERENCES
ABB. 2007. Energy efficiency in the power grid. Tech. rep., ABB Inc., Norwalk, CT.
ANON. 2004. Solar power from space- European strategy in the light of sustainable development. Tech. rep.,
European Space Agency, Noordwijk.
APPLEBY, R. 2007. Standoff detection of weapons and contraband in the 100 GHz to 1 THz region. IEEE Trans.
Anten. Propag. 55, 11, 2944–2956.
BERLAND, B., SIMPSON, L., NUEBEL, G., COLLINS, T., AND LANNING, B. 2003. Photovoltaic technologies beyond the
horizon: Optical photovoltaic technologies beyond the horizon: Rectenna solar cell. Final rep., Contract
DE-AC36-99-GO10337 NREL SR-520-33263, National Renewable Energy Laboratory, US Department
of Energy, Golden, Colorado.
BOOSKE, J. 2008. Plasma physics and related challenges of millimeter-wave-to-terahertz and high power
microwave generation. Phys. Plasmas 15, 055502.
BROOKNER, E. 2002. Phased array radars-Past, present and future. In Proceedings of the RADAR’02 Conference. IEEE, 104–113.
CHAKRABORTY, A. 2009. Fault tolerant fail safe system for railway signalling. In Proceedings of the World
Congress on Engineering and Computer Science. Vol. II, 1177–1183.
CHOWDHARY, G., GADRE, R., AND KOMERATH, N. 2009. Policy issues for retail beamed power transmission. In
Proceedings of the Atlanta Conference on Science and Technology Innovation Policy.
CORKISH, R., GREEN, M., AND PUZZER, T. 2002. Solar energy collection by antennas. Solar Energy 73, 6, 395–401.
CORKISH, R., GREEN, M., PUZZER, T., AND HUMPHREY, T. 2003. Efficiency of antenna solar collection. In Proceedings
of 3rd World Conference on Photovoltaic Energy Conversion. Vol. 3, IEEE, 2682–2685.
COSTIA, D., POPESCU, O., POPESCU, C., AND CRACIUNESCU, A. 2010. Photovoltaic solar cell like receiver for electromagnetic waves in VHF-UHF bands. In Proceedings of the International Conference on Renewable
Energies and Power Quality.
COUNCIL, N. R. 2001. Laying the Foundation for Space Solar Power. An Assessment of NASA’s Space Solar
Power Investment Strategy. National Academy Press, Washington DC.
CRISWELL, D. 2002. Solar power via the moon. Industr. Phys. 8, 2, 12–15.
CROWE, T., BISHOP, W., PORTERFIELD, D., HESLER, J., WEIKLE, R., ET AL. 2005. Opening the terahertz window with
integrated diode circuits. IEEE J. Solid-State Circ. 40, 10, 2104–2110.
DALELA, P., PRASAD, M., AND MOHAN, A. 2008. A new method of realistic GSM network planning for rural
Indian terrains. Int. J. Comput. Sci. Netw. Secur. 8, 8, 362– 371.
DE VOS, A. 1992. Endoreversible Thermodynamics of Solar Energy Conversion. Oxford University Press,
Oxford, UK.
DESSANTI, B., PICON, N., RIOS, C., SHAH, S., AND KOMERATH, N. 2011. A US-India power exchange towards a
space power grid. In Proceedings of the International System Dynamics Conference (ISDC’11).
DRAPALIK, M., SCHMID, J., KANCSAR, E., SCHLOSSER, V., AND KLINGER, G. 2010. A study of the antenna effect of
photovoltaic modules. In Proceedings of the International Conference on Renewable Energies and Power
Quality.
DU BOSQ, T., PEALE, R., AND BOREMAN, G. 2008. Terahertz/Millimeter wave characterizations of soils for mine
detection: Transmission and scattering. Int. J. Infrared Millim. Waves 29, 769–781.
EGOROV, E., KALININ, Y., KORONOVSKI, A., HRAMOV, A., AND MOROZOV, M. 2006. Microwave generation power in a
nonrelativistic electron beam with virtual cathode in a retarding electric field. Techn. Phys. Lett. 32, 5,
402–405.
ACM Journal on Emerging Technologies in Computing Systems, Vol. 8, No. 3, Article 18, Pub. date: July 2012.
JETC0803-18
ACM-TRANSACTION
July 11, 2012
17:37
Retail Beamed Power for a Micro Renewable Energy Architecture: Survey
18:23
ESSEN, H., WAHLEN, A., SOMMER, R., JOHANNES, W., BRAUNS, R., SCHLECHTWEG, M., AND TESSMANN, A. 2007.
High-Bandwidth 220 GHz experimental radar. Electron. Lett. 43, 20, 1114–1116.
FARINHOLT, K., PARK, G., AND FARRAR, C. 2009. RF energy transmission for a low-power wireless impedance
sensor node. IEEE Sensors J. 9, 7, 793–800.
FILIP, V., NICOLAESCU, D., PLAVITU, C., AND OKUYAMA, F. 1997. Analysis of microwave generation by field emitted
electrons moving in crossed electric and magnetic fields. Appl. Surface Sci. 111, 185–193.
GENK, M. E., Ed. 2006. Evolutionary path towards space solar power. Proceedings of the AIP Conference.
Institute for Space and Nuclear Power Studies, American Institute of Physics, Albuquerque, NM.
GEUDER, N., QUASCHNING, V., VIEBAHN, P., STEINSIEK, F., SPIES, J., AND HENDRIKS, C. 2004. Comparison of solar
terrestrial and space power generation for Europe. In Proceedings of the 4th International Conference
on Solar Power from Space-SPS.
GLASER, P. 1968. Power from the sun: It’s future. Sci. 162, 856–861.
GLIESE, U., NIELSEN, T., NØRSKOV, S., AND STUBKJAER, K. 1998. Multifunctional fiber-optic microwave links based
on remote heterodyne detection. IEEE Trans. Microwave Theory Technol. 46, 5, 458–468.
GOSWAMI, D., VIJAYARAGHAVAN, S., LU, S., AND TAMM, G. 2004. New and emerging developments in solar energy.
Solar Energy 76, 33–43.
GREEN, M. 2003. Third Generation Photovoltaics: Advanced Solar Energy Conversion. Springer.
HADLEY, F. 2007. Goodbye wires! MIT team experimentally demonstrates wireless power transfer, potentially useful for powering laptops, cell phones without cord. http://web.mit.edu/newsoffice/2007/wireless0607.html.
HENLEY, M., POTTER, S., HOWELL, J., AND MANKINS, J. 2002. Wireless power transmission options for space solar
power. Tech. rep. R.4.08, IAC.
HIRSHFIELD, J., WANG, V., AND MARSHALL, T. 2005. High-power millimeter wave harmonic generator. In Proceedings of the Joint International Conference on Infrared and Millimeter Waves and 13th International
Conference on Terahertz Electronics. Vol. 1, IEEE, 89–90.
HULSTROM, R., BIRD, R., AND RIORDAN, C. 1985. Spectral solar irradiance data sets for selected terrestrial
conditions. Solar Cells 15, 4, 365–391.
ILCHENKO, V., MATSKO, A., SAVCHENKOV, A., AND MALEKI, L. 2000. High efficiency microwave
and millimeter wave electro-optical modulation with whispering-gallery resonators. http://trsnew.jpl.nasa.gov/dspace/bitstream/2014/11669/1/02-0231.pdf.
JOHANSSON, L. AND SEEDS, A. 2003. Generation and transmission of millimeter-wave data-modulated optical
signals using an optical injection phase-lock loop. J. Lightwave Technol. 21, 2, 511–520.
KARALIS, A., JOANNOPOULOS, J., AND SOLJACIC, M. 2008. Efficient wireless non-radiative mid-range energy
transfer. Ann. Phys. 323, 1, 34–48.
KARTIKEYAN, M., BORIE, E., AND THUMM, M. 2004. Gyrotrons: High Power Microwave and Millimeter Wave
Technology. Springer.
KOMERATH, N. AND BOECHLER, N. 2006. The space power grid. In Proceedings of IAC Conference.
KOMERATH, N. AND KOMERATH, P. 2011a. Implications of inter-satellite power beaming using a space power
grid. In Proceedings of the IEEE/AIAA Aerospace Conference.
KOMERATH, N. AND KOMERATH, P. 2011b. Terrestrial micro renewable energy applications of space technology.
In Proceedings of the Space Propulsion and Energy Sciences International Forum. Elsevier.
KOMERATH, N., PANT, R., AND KAR, A. 2011. Antenna considerations for retail beamed power delivery in india.
In Proceedings of the International Symposium on Electronic System Design. IEEE Computer Society.
KOMERATH, N., VENKAT, V., AND FERNANDEZ, J. 2009. Near millimeter wave issues for a space power grid.
In Proceedings of the Institute for Advanced Studies in the Space Propulsion and Energy Sciences
(IASSPES’09).
KRAUSMAN, J. 2005. The 38M aerostat: A new system for surveillance. In Proceedings of the AIAA 5th Aviation,
Technology, Integration, and Operations Conference (ATIO).
LANDIS, G. 2004. Reinventing the solar power satellite. Tech. rep. 2004-212743, NASA.
LASKAR, J., PINEL, S., DAWN, D., SARKAR, S., PERUMANA, B., AND SEN, P. 2007. The next wireless wave is a
millimeter wave. Microwave J. 50, 9, 22–23.
LIEBE, H. AND HUFFORD, G. 1989. Modeling millimeter wave propagation effects in the atmosphere. AGARD
rep. 454, North Atlantic Treaty Organization.
LIEBE, H. J., ALLEN, K., HAND, G., ESPELAND, R., AND VIOLETTE, E. 1985. Millimeter-Wave propagation in moist
air: Model versus path data. NTIA rep. 85-171, U.S. Department of Commerce.
LIEBE, H. J. AND LAYTON, D. 1987. Millimeter-Wave properties of the atmosphere: Laboratory studies and
propagation modeling. NTIA rep. 87-224, U.S. Department of Commerce.
ACM Journal on Emerging Technologies in Computing Systems, Vol. 8, No. 3, Article 18, Pub. date: July 2012.
JETC0803-18
18:24
ACM-TRANSACTION
July 11, 2012
17:37
N. Komerath and A. Kar
MCGRATH, W., WALKER, C., YAP, M., AND TAI, Y. 1993. Silicon micromachined waveguides for millimeter-wave
and submillimeter-wave frequencies. IEEE Microwave Guided Wave Lett. 3, 3, 61–63.
MCMILLAN, R. 2006. Terahertz imaging, millimeter-wave radar. In Advances in Sensing With Security Applications, J. Bymes, Ed. NATO, 243–268.
MIYAMOTO, R., LEONG, K., JEON, S., AND WANG, Y. 2003. Digital wireless sensor server using an adaptive
smart-antenna/retrodirective array. EEE Trans. Vehic. Technol. 52, 5, 1181–1188.
MODI, V. 2005. Improving electricity services in rural India. CGSD working paper no. 30.
MOHAMMED, S., RAMASAMY, K., AND SHANMUGANANTHAM, T. 2010. Wireless power transmission-a next generation
power transmission system. Int. J. Comput. Appl. 1, 13, 102–105.
NASA. 1984. The final proceedings of the solar power satellite. Program rev. NASA-TM-84183, NASA.
NASA. 1995. Space solar power, an advanced concepts study project. In Nasa lerc ssp Technical Interchange
Meeting, NASA, Washington DC.
NICOLSON, S., TANG, K., YAU, K., CHEVALIER, P., SAUTREUIL, B., VOINIGESCU, S., AND ROGERS, E. 2007. A lowvoltage 77-GHz automotive radar chipset. In Proceedings of the IEEEMTTS International Microwave
Symposium (07). 487–490.
NUSINOVICH, G. 2004. Introduction to the Physics of Gyrotrons. Johns Hopkins Univ Press, Baltimore, MD.
OODO, M., MIURA, R., AND HASE, Y. 2000. Onboard DBF antenna for stratospheric platform. In Proceedings of
the IEEE International Conference on Phased Array Systems and Technology. IEEE.
PANT, R., KOMERATH, N., AND KAR, A. 2011. Application of lighter-than-air platforms for power beaming, generation and communications. In Proceedings of International Symposium on Electronic System Design.
PENN, J. AND LAW, G. 2002. Operational and demonstrator laser concept model development. Final rep.,
Contract NAS8-99143, NASA.
PETERSON, S. 2005. The small aerostat system: Field tested, highly mobile and adaptable. In Proceedings of
the AIAA 5th Aviation, Technology, Integration, and Operations Conference (ATIO).
PETTY, K. AND MAHONEY, W. 2007. Weather applications and products enabled through vehicle infrastructure
integration (VII). feasibiity and concept development study. Tech. rep., US Department of Transportation
Federal Highway Administration.
PHAM, A., LASKAR, J., KRISHNAMURTHY, V., COLE, H., AND SITNIK-NIETERS, T. 1998. Ultra low loss millimeter wave
multichip module interconnects. IEEE Trans. Compon. Packag. Manufact. Technol. 21, 3, 302–308.
POTTER, S., DAVIS, D., AND MCCORMICK, D. 2011. Near-Term space solar power demonstrations and applications.
We can get there from here. In Proceedings of the International Space Development Conference.
POTTER, S., HENLEY, M., DAVIS, D., BORN, A., BAYER, M., HOWELL, J., AND MANKINS, J. 2008. Wireless power
transmission optionsfor space solar power. In Proceedings of the State of Space Solar Power Technology
Workshop.
PRESS, L. 1999. A client-centered networking project in rural India. OnTheInternet 5, 2, 36–38.
RAGHUNATHAN, V., KANSAL, A., HSU, J., FRIEDMAN, J., AND SRIVASTAVA, M. 2005. Design considerations for solar
energy harvesting wireless embedded systems. In Proceedings of the 4th International Symposium on
Information Processing in Sensor Networks. IEEE, 457–462.
RAZAVI, B. 2008. A millimeter-wave circuit technique. IEEE J. Solid-State Circ. 43, 9, 2090–2098.
REDDY, L. AND REDDY, B. 1992. Some observations on the influence of hydrometeors on line-of-sight microwave
propagation at 7 GHz in tropics. In Proceedings of the International Conference on Intelligent Systems
Applications to Power Systems (ISAP’92). IEEE, Sapporo, Japan, 889–892.
RICHARDS, B. S. AND SHALAV, A. 2007. Photovoltaics devices. In The Handbook of Photonics 2nd Ed., M. Gupta
and J. Ballato, Eds., CRC Press, Boca Raton, FL. 8–19.
RIES, H. 1983. Complete and reversible absorption of radiation. Appl. Phys. 32, 3, 153–156.
ROY, A., MONDAL, J., MENON, R., MITRA, S., KUMAR, D., SHARMA, A., AND MITTAL, K. 2007. High power microwave
generation from coaxial virtual cathode oscillator. In Proceedings of the Asian Particle Accelerator Conference. 523–525.
RYAN, K., D’ANDREA, J., JAUCHEM, J., AND MASON, P. 2000. Radio frequency radiation of millimeter wave length:
Potential occupational safety issues relating to surface heating. Health Phys. 78, 2, 170.
SAIKI, T., UCHIDA, S., MOTOKOSHI, S., IMASAKI, K., NAKATSUKA, M., NAGAYAMA, H., SAITO, Y., NIINO, M., AND
MORI, M. 2005. Development of solar-pumped lasers for space solar power station. In Proceedings of the
International Astronautical Congress.
SALAMA, I., MIDDLETON, C., QUICK, N., BOREMAN, G., AND KAR, A. 2003. Laser-metallized silicon carbide schottky
diodes for millimeter wave detection and frequecny mixing. In Proceedings of the International Symposia
on State-of-the-Art Program on Compound Semiconductors XXXIX and Nitride and Wide Bandgap
Semiconductors for Sensors, Photonics and Electronics IV. Vol. 2003, 270.
ACM Journal on Emerging Technologies in Computing Systems, Vol. 8, No. 3, Article 18, Pub. date: July 2012.
JETC0803-18
ACM-TRANSACTION
July 11, 2012
17:37
Retail Beamed Power for a Micro Renewable Energy Architecture: Survey
18:25
SAMOSKA, L. AND LEONG, Y. 2001. 65-145 GHz InP MMIC HEMT medium power amplifiers. In IEEE Int.
Microwave Sympos. Dig. 3. 1805–1808.
SASAKI, S., TANAKA, K., HIGUCHI, K., OKUIZUMI, N., KAWASAKI, S., SHINOHARA, M., SENDA, K., AND ISHIMURA, K.
2005. Feasibility study of tethered solar power satellite. In Proceedings of the International Astronautical
Congress.
SCHAMILOGLU, E. 2004. High power microwave sources and applications. Microwave Sympos. Dig.. Vol. 2,
1001–1004.
SCHNEIDER, D. 2010. Wireless power at a distance is still far away, electrons unplugged. IEEE Spectrum 47, 5,
34–39.
SHARMA, V. 1974. Effect of certain climatic factors over microwave propagation. J. Instit. Electron. Telecomm.
Engin. 20, 114–118.
SHEERIN, J. 2009. Malawi windmill boy with big fans. http://news.bbc.co.uk/2/hi/africa/8257153.stm.
SHRESTHA, R., KUMAR, S., SHARMA, S., AND TODOC, M. 2004. Institutional reforms and electricity access: Lessons
from Bangladesh and Thailand. Energy Sustain. Devel. 8, 4, 41–53.
SREEKANTH, P. 2003. Digital Microwave Communication Systems: With Selected Topics in Mobile Communications. Sangam Books Ltd, London, UK.
STANCATI, M. 1996. Space solar power, a fresh look feasibility study, phase 1. NASA contract.
SUMMERER, L. AND ONGARO, F. 2004. Solar power from space: Validation of options for Europe. Tech. rep.,
ACT-RPR-NRG-2004-ESA, European Space Agency.
TESLA, N. 1893. Transmission of power: Polyphase system; tesla patents. USPTO.
TORKILDSON, E., ZHANG, H., AND MADHOW, U. 2010. Channel modeling for millimeter wave MIMO. In Proceedings
of the Information Theory and Applications Workshop. 1–8.
TRAI. 2011. Highlights of telecom subscription data as on 30 june, 2011, Telecom Regulatory Authority of
India. Press Release No. 45 /2011, New Delhi.
TRIEB, F. E. A. 2006. Trans-Mediterranean interconnection for concentrating solar power. Tech. rep. TRANSCP, German Aerospace Center.
VAITHEESWARAN, V. 2003. Power to the People. Farrar, Straus and Giroux, New York.
VERHOEVEN, A., BONGERS, W., BRATMAN, V., CAPLAN, M., DENISOV, G., VAN DER GEER, C., MANINTVELD, P., POELMAN,
A., PLUYGERS, J., SHMELYOV, M., SMEETS, P., STERK, A., AND URBANUS, W. 1998. First microwave generation
in the FOM free-electron maser. Plasma Phys. Control. Fusion 40, A139–A156.
VOINGESCU, S. 2006. Mm-wave in CMOS – Opportunities above 50 GHz? In Proceedings of the CMOS Emerging
Technologies Workshop.
WAKE, D., LIMA, C., AND DAVIES, P. 1995. Optical generation of millimeter-wave signals for fiber-radio systems
using a dual-mode DFB semiconductor laser. IEEE Trans. Microwave Theory Techn. 43, 9, 2270–2276.
WANG, H., SAMOSKA, L., GAIER, T., PERALTA, A., LIAO, H., LEONG, Y., WEINREB, S., CHEN, Y., NISHIMOTO, M., AND LAI,
R. 2001. Power-Amplifier modules covering 70-113 GHz using MMICs. IEEE Trans. Microwave Theory
Techn. 49, 1, 9–16.
WANG, Y. E. A. 2004. Receiving and transmitting light-like radio waves: Antenna effects in arrays of aligned
carbon nanotubes. Appl. Phys. Lett. 85, 13, 2607–2609.
WINKLER, W., BORNGRABER, J., HEINEMANN, B., AND HERZEL, F. 2005. A fully integrated BiCMOS PLL for 60 GHz
wireless applications. In Solid-State Circuits Conference Digest of Technical Papers. IEEE, 406–407.
WRIGHT, S., SCOTT, D., HADDOW, J., AND ROSEN, M. 2000. The upper limit to solar energy conversion. In Proceedings of the Energy Conversion Engineering Conference and Exhibit. 384–392.
YAO, J. 2010. Microwave photonics: Photonic generation of microwave and millimeter-wave signals. Int. J.
Microwave Optical Technol. 5, 1, 16–21.
Received May 2011; revised August 2011; accepted August 2011
ACM Journal on Emerging Technologies in Computing Systems, Vol. 8, No. 3, Article 18, Pub. date: July 2012.