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Flying on waves: as green as it gets
The use of microwaves for wireless power transmission
A. Vroom
Delft, 2012
Delft University of Technology
2
Table of Contents Summary ......................................................................................................................................3
Introduction ..................................................................................................................................3
1. The concept...............................................................................................................................4
2. Scientific groundwork ................................................................................................................5
2.1 The general concept .............................................................................................................5
2.1.1 The green power source .................................................................................................5
2.1.2 The gyrotron and antenna ..............................................................................................5
2.1.3 The beam.......................................................................................................................6
2.1.4 The rectenna..................................................................................................................6
2.2 Transmit power ....................................................................................................................7
2.2.1 In general.......................................................................................................................7
2.2.2 Case study .....................................................................................................................7
2.3 Ground station interval .........................................................................................................7
2.4 Research fields .....................................................................................................................8
3. Sustainability.............................................................................................................................9
4. Costs .......................................................................................................................................10
4.1 Investment and maintenance ..............................................................................................10
4.2 Who shall be paying these costs? ........................................................................................11
4.4 Regulations and laws ..........................................................................................................11
5. The market ..............................................................................................................................12
6. Conclusion...............................................................................................................................12
Bibliography................................................................................................................................13
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Summary Fuel is expensive, it limits the amount of payload, the range and it pollutes the environment. All of
these problems can be solved by using microwaves for wireless power transmission. This can be done
by generating green power on the ground and convert this power to microwaves with a gyrotron.
These microwaves are send to an aircraft by an antenna and can be converted back to power with the
use of a rectifying antenna. The green power supply, the gyrotron and the antenna together are
referred to as a “ground station”. If microwaves would be used for wireless power transmission, an
aircraft would be able to carry 25-45% more payload, the range would depend on the amount of
ground stations and there would be no more emission of polluting gasses. One wind turbine will
provide enough power to generate a microwave beam. Currently, research is done for more power ful
gyrotrons. At this moment, a beam of 2 megawatt can be generated with an efficiency of 97%. The
use of the newest gyrotron leads to an efficiency of 60%, measured from power supply to aircraft.
With this efficiency and a power supply of approximately 2 megawatt, a Dornier 228 can fully be
powered by microwaves. There are two options for the interval at which ground stations could be
placed. For linear flight, this interval is 35 kilometres. It is also possible to give an aircraft the power
to climb to 20 kilometres at each ground station, after which it will glide to the next. In this case, the
interval could be 160 kilometres. The price per kilometre is heavily dependent on the amount of
aircraft that pass a ground station per year and on the interval o f the ground stations. If 24 aircraft
will pass a ground station per day and the interval is 35 kilometres, the price per kilometre will be
€2.80. If the interval is 160 kilometres, the price per kilometre will be €0.60. This last option has the
same price per kilometre as if kerosene would be used (for a Dornier-228). All of the regulations and
laws concerning fuel will no longer be applicable, instead the laws and regulations concerning
electromagnetic radiation will need to be followed. This concept cannot directly be used for all sorts of
flights. It will first be used for smaller aircraft and unmanned aircraft that need to be airborne for as
long as possible to observe or for reconnaissance. The final market that this concept is aiming for is
the cargo- and passenger transport.
The main advantage of this concept is its sustainability. With this concept, aviation become as green
as it can get: no emission of polluting gasses at all.
Introduction As a first year Aerospace engineering student, I got the opportunity to participate in a race for a ticket
to space: “Space for Innovation”. The assignment for this contest was to come up with a concept that
would have changed the aerospace world in 2040.
This report describes my concept for a sustainable and emission free aviation industry in 2040: using
microwaves for wireless power transmission. In chapter one I shall describe the innovative part of my
concept and point out all the differences between current flight and the type of flight that will be
possible with my concept. Chapter two handles the scientific background of this concept. The third
chapter will elaborate on the sustainability, a very important part. In chapter four, the costs of this
concept will be compared to the current costs. Finally, chapter five shall describe the market.
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1. The concept Fuel is contemporary aviation’s biggest opponent. Be it because it limits the amount of
payload and range or because fossil fuels are running out, nowadays’ pollut ion and the
rising fuel prices. Why do we even take fuel with us? Isn’t it true that more fuel means
less payload? There is a solut ion to all of these problems: generating power somewhere
else and send it wirelessly to the aircraft.
The concept of wireless power is very old, Nicola Tesla proposed his
theories of wireless power transmission on the late 19th and early 20th century.
Since then, several new ways of sending power wirelessly trough the air have
been found and proven to work. A few examples are: inductive coupling
(figure 1), the use of microwaves or by the use of a laser. Inductive coupling,
which is currently used in electric toothbrushes, has only worked up to a few
meters distance (Spencer, How Wireless Power induction Works, 2010) and the
use of a laser will require the device being powered to always be in sight of the
transmitter, so a laser will not work with cloudy weather. (Spencer, Long
Distance Wireless Power Transmission, 2010) The most feasible option for
aviation will be the use of microwaves: these can travel long distances
and can even go through clouds.
The concept of using microwaves for wireless power transmission
is based on a specific sort of antenna: a rectifying antenna (rectenna).
Microwaves are generated on the ground and send to the aircraft. The
rectenna, which is hanging below the aircraft, as one may see in figure 2,
can produce DC power out of microwaves. A more detailed description on
how this rectenna works will follow in chapter two. So, what would change
if we would no longer bring fuel with us, but sent it to the aircraft?
Currently, around 25 - 45 percent (Stockholm Environment
Institute, 2012) (Sadraey, 2009) of an aircraft’s take-off weight is fuel. For
example, the Boeing 777’s fuel fraction is 41 percent. If an aircraft would
no longer need to take fuel into flight, this 41 percent could also be used
for payload. This would mean that the maximum amount of passengers
would for a 777 increases by 130 (KLM, 2011): a major advantage for a
company.
An aircraft’s range would also change. Contemporary aircraft are limited by their range, which
is in turn limited by the amount of fuel. With the use of microwaves to power the aircraft, the range
would no longer be dependent on fuel, but on the amount of ground stations. With enough ground
stations, this could lead to the possibility for any aircraft to travel around the world, without needing
to land for refuelling; any aircraft could fly as far as necessary.
Perhaps the most important change would be the amount of CO2 emission: not a single gram
of CO2 will be produced with the use of microwaves. The energy needed to produce microwaves may
be generated on the ground with a green method, chapter three will elaborate on this subject.
So, a possibility to solve fuel based problems in aviation is to use microwave powered airplanes. The
use of microwaves can increase the amount of payload, expand the range of an aircraft and can
reduce the production of CO2 to zero.
Figure 1: Intel demonstrating inductive coupling
Figure 2: the Canadian SHARP, a
microwave powered unmanned aircraft. The disk between the tail and the wings is a rectenna
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2. Scientific groundwork
2.1 The general concept The wireless power transmission starts with generating power with a
green method (a wind turbine, for instance). This power is fed to a
gyrotron. The gyrotron and antenna will, for example, create a 1
megawatt beam of 35 GHz microwaves out of 2 megawatt DC power.
This beam is following the movements of the aircraft. In the rest of
this article I shall assume a beam of 1 megawatt, to more easily
explain my concept.
A ground station consists of a gyrotron, a power supply and a
propagating antenna. The microwave beam generated by the gyrotron
will be “caught” by a rectenna. The rectenna can convert the
microwaves back into DC power. I shall now explain each part of the
system in detail, to fully explain how this works.
2.1.1 The green power source
The power source is the easiest part of this concept: there are
enough green power sources that can produce the required
amount of power. Wind turbines, for instance, can already
generate up to 6 MW. (Green, 2007) Since the gyrotron only
needs a power supply of 2 megawatt, even a wind turbine
with a diameter of 82 meters would be enough. The average
wind turbine in the Netherlands has a diameter of 65 meters
and produces 1 MW. (Klunne, Beurskens, & Westra, 2001)
Increasing the diameter by only a few meters, results in a
much higher power generation, as one may see in table 1.
2.1.2 The gyrotron and antenna
In order to get the generated power to the aircraft, this power
will need to be converted to a beam. This will be done with a
gyrotron and a propagating antenna. The high-power gyrotron
originates from the development of nuclear fusion, but are now
commercially available. It is a device that uses a cyclotron motion
and electrons in a strong magnetic field to create
microwaves, figure 4 shows the cross-section of a
gyrotron.
These microwaves can have a frequency of 30 GHz up to
300 GHz. The input for the gyrotron will be a green, 2
megawatt, power source, a 35 Ampere current and a
supply voltage of 80 kV. The magnetic field the gyrotron
is using, is 14.000 G. With a propagating antenna of 5
meters in diameter, a 1 megawatt microwave beam with
a frequency of 35 GHz, a flux density of 2 kW/m2 and a
spot diameter of 40 meters may be sent up to an
altitude of over 20 kilometres. (Caplan & Friedman, 2005)
Company Diameter
[m]
Power
[kW]
Goldwind 48 750
Sinovel 70 1.500
Enercon 82 2.000
Suzlon 88 2.100
Gamesa 90 2.000
Vestas 90 3.000
Nordex 99,8 2.500
GE
Energy
100 2.500
Siemens 107 3.600
Enercon 112 6.000
Table 1: wind turbine diameter versus power (European Wind Energy Association, 2009)
Figure 3: the general concept (Foot, 1993)
Figure 4: the cross-section
of a gyrotron (CRPP)
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2.1.3 The beam
35 GHz, 94 GHz or 140 GHz microwaves could be used. This concept uses a 35 GHz microwaves,
because a higher frequency comes with a lower efficiency. The reasons for this are the two types of
attenuation microwave beams suffer: molecular and aerosol attenuation. Molecular absorption has
certain minima around 35 GHz, 94 GHz and 140 GHz, with the amount of absorption increasing with
the frequency. Aerosol attenuation is composed of clouds, rain and other particles. The amount of this
attenuation also increases with the beam frequency. Hence the use of 35 GHz microwaves.
The beam will need to follow the aircraft movements for maximum power transmission. This will be
done with a simple transponder and a passive tracker. The tracker may be a radar system with a
detector, to automatically centre the transponder signal. The coordinates of the tracker are then sent
to the antenna, which will automatically point in the correct direction. (Caplan & Friedman, 2005)
2.1.4 The rectenna
In order to convert the microwaves back to DC power, a high-efficiency
rectenna is used. A rectenna, also known as a rectifying antenna,
consists of a pair of dipole antennas connected by a diode. The dipole
antennas are able to produce a current out of the microwaves, but to
utilize this current, it should flow in a single direction and this is done by
a diode. The high-efficiency rectenna also works like this, but it uses a
“dual polarization design to double the transmitting power and receive
the microwave power with no polarization mismatching loss” (Fujino,
Fujita, Kaya, Onda, & Tomita, 1998)
The conventional rectenna set-up only provides a flux density 2 kW/m2,
which is not enough to power an aircraft. M. Caplan and H.W. Friedman
have shown that by adding a reflector above the rectenna, flux densities
of 100 kW/m2 (a factor 50 larger) can be reached. It is therefore
strongly recommended to place such a reflector in an aircraft. (Caplan &
Friedman, 2005)
Usually fuel is kept inside the wings, but since no more fuel is needed,
this would be an excellent place for the reflector and the rectenna.
The reflector will be on the top sheet and the rectenna on the bottom.
In this way, the rectenna will be just like in figure 5.
Figure 5: an airship with a reflector and a rectenna (Caplan & Friedman, 2005)
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2.2 Transmit power
2.2.1 In general
If we assume the 2 megawatt power supply, 1 megawatt beam and that all of the power at an
altitude of 20 kilometres is caught by the rectenna, table 2 describes the DC to DC efficiency. Of
course, if the aircraft is at an lower altitude, the beam efficiency will increase.
Concept part Maximum Efficiency (%) Power left of ~2 MW input (kW)
Gyrotron 50 1000 ( Kare & Parkin, 2006)
Antenna 97 970 (Brown & Eves, 1992)
Beam 80 776 ( Kare & Parkin, 2006)
Rectenna 81 629 (Fujino, Fujita, Kaya, Onda, & Tomita, 1998)
Table 2: DC to DC efficiency (1992-1998)
In 2009, a 2 megawatt gyrotron was completed with an efficiency of 97%. If this gyrotron and a
beam of 2 megawatt would be used, Table 3 would describe the DC to DC efficiency
Concept part Maximum Efficiency (%) Power left of ~2 MW input (kW)
Gyrotron 97 2000 (Fusion for energy, 2009)
Antenna 97 1940 (Brown & Eves, 1992)
Beam 80 1552 ( Kare & Parkin, 2006)
Rectenna 81 1257 (Fujino, Fujita, Kaya, Onda, & Tomita, 1998)
Table 3: DC to DC efficiency (1992-2009)
2.2.2 Case study
For this case study I consider a Dornier 228-212 (figure 6) and I calculate
whether it can fully be powered on microwaves.
The Dornier 228-212 uses two 560 kW Garrett/AlliedSignal TPE3315252Ds.
This means that the Dornier 228-212 requires 1120 kW for propulsion. If the
2 megawatt beam is used, there is 137 kW left for electrical systems.
(Airliners.net)
2.3 Ground station interval Ground stations will need to be set up at a regular interval. This may be done in two ways:
1. Ground stations are placed at such an interval that the beams are overlapping each other a bit.
Once an aircraft is picked up by a beam, the beam will follow the aircraft until the next beam has
picked up the aircraft. If the maximum distance of the beam is 20 kilometres and the aircraft flies at
around 10 kilometres altitude, the ground stations could be placed at an interval of around 34
kilometres. This will result in 14 ground stations on the route Amsterdam – Paris (for example).
2. At each ground station, the aircraft will receive enough power to climb to an altitude of 20
kilometres. Once the aircraft has reached this altitude, it will glide to the next ground station. If the
minimum altitude is 10 kilometres and the aircraft has a glide ratio of around 1:16, the ground station
interval will increase to 160 kilometres. This will result in 4 ground stations on the route Amsterdam –
Paris.
The second option would take considerably longer, it is therefore more ideal for cargo flights than for
passenger flights.
Figure 6: a Dornier 228
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2.4 Research fields There are still a few fields that require research:
1. how can it be kept save? The beam has a power of one megawatt. This means that the beam is
lethal for anyone or anything that is directly in or near the beam. On the ground, this can partly be
solved by putting a Faraday cage around the gyrotron. The aircraft will act as a Faraday cage for the
passengers inside the aircraft. Of course the ground station should not be placed close to residential
areas. There is still research needed on how to protect birds from the microwave beam.
2. how can the beam power be increased? A 2 megawatt beam is enough for a small aircraft, but it is
not enough for large aircraft like Boeing or a transport aircraft. A way to increase the beam power,
might be by setting up multiple beams or by increasing the gyrotron power. Research is needed to
increase the gyrotron power.
3. how can the spot size be decreased? The current spot size is about 40 meters in diameter. The flux
density will increase if this diameter gets smaller and a higher flux density means that the aircraft’s
rectenna can be smaller. A way to accomplish this might be by increasing the antenna diameter.
4. how can the rectenna be kept perpendicular to the microwave beam? This is an important question
that needs to be solved, because the dipole antennas produce maximum power when set
perpendicular to the microwave beam.
5. how can the rectenna be cooled? The reflector will intensify the microwave beam by a factor 50.
This means that the rectenna will get extremely hot and will need to be cooled.
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3. Sustainability Perhaps the most important part of using microwaves for wireless power
transmission is the sustainability of this concept. Sustainability is becoming more
important by the day. If humanity continues polluting the earth at this rate, the earth
will eventually become inhabitable for future generations. (The Sciense News, 2011)
As mentioned in a report of the UNFCCC (2006): “Aircraft engine emissions are
roughly composed of about 70 percent CO2, a little bit less than 30 percent H2O and
less than 1 percent each of NOx, CO, SOx, NMVOC (non-methane volatile organic
compounds), particulates and other trace components including hazardous air
pollutants”. A few problems that the aviation industry is having or that will come
across in the upcoming years:
- The nitrogen oxide emission of aircraft is
causing an increase of ozone in the
troposphere and a decrease of ozone in the
stratosphere. Both of these are very
undesirable. According to the World
Meteorological Organization, the Earth’s ozone
layer over the arctic has suffered a loss of 40
percent between December and March 2011,
which is even more than the 30 percent loss of
2010. Figure 7 shows the Earth’s ozone layer
over the arctic in 2011
- As one may see in figure 8 the aviation
industry produces a lot of CO2, ranging from
68.7 up to 159.7 kg per person per 1000
kilometres for civilian aircraft. A car in with the same
percentage of occupants, produces 64.3 kilograms of
CO2 over the same distance. (Transportdirect).
- The British Tyndall-Centre for Climate Change
Research calculated the increase of carbon dioxide
emission between now and 2050 for several sectors
(Adams, 2009). As one can see in figure 9, carbon
dioxide emission of international aviation will
dramatically increase. With a carbon dioxide emission
of the proportion mentioned in figure 9, there will be
no space for other sectors to produce carbon dioxide.
- Because the carbon dioxide emission is increasing so
much, governments will be forced to raise the taxes
for this emission. This will eventually lead to, either
companies making less profit or more expensive
tickets.
This concept, the use of microwaves, could decrease the
pollution of the environment by the aviation industry to zero: no more kerosene would be needed.
Modern wind turbines can produce up to 6 megawatt of power. (electricityforum.com)
If the 2 megawatt beam and a 2 megawatt gyrotron would be used mentioned in chapter 2, even one
single windmill would provide enough power to generate the microwave beam and to power a Dornier
228.
Figure 7: Earth’s ozone layer over the arctic in 2011. (The
Sciense News, 2011)
Figure 9: carbon dioxide increase between now and 2050. The green dotted line is today’s carbon emission, the black dotted line is the emission in 2050. (Adams, 2009)
Figure 8: CO2 emission per person per 1000 kilometres. This table is assuming an average number of passengers of 70% of the maximum capacity (Math!
How much CO2 is released by Aeroplane?, 2007)
10
4. Costs
4.1 Investment and maintenance
An estimation has been made of the costs per ground station per year. This is done by dividing the
investment costs per element by the lifecycle of that particular element. The maintenance costs are
also included in the calculation. In Table 3 one can find the results.
Non-aircraft elements
Investment maintenance Total
Wind turbine, 2,5 MW
1)
€ 3.312.500 for 20 year
lifecycle =
€ 165.625/year
4,5% of the investment/year =
€ 125.875/year
€291.500/year
Gyrotron 2) €1.700.000 / megawatt =
€ 3.400.000
10 year lifecycle =
€ 340.000/year
Estimated: 5% of investment =
€170.000 / year
€510.000/year
Antenna and tracking
system
Unknown. Estimated: € 100.000, 10 year lifecycle
= € 10.000/year
Estimated: 5% of investment =
€ 5.000/year
€15.000/year
Ground and building.
Simple configuration.
Unknown/ Estimated: 200.000.
10 year lifecycle = €
20.000/year
Estimated: 5% of investment =
€ 10.000/year
€30.000/year
Total €7.012.500 €310.875 €846.500/year Table 3: the estimated costs per year per ground station
1) Wind turbine, € 1325 / kW (Wilde, 2010)
2) ( Kare & Parkin, 2006)
The costs per aircraft are not included, because:
1. The costs of a high-efficiency rectenna and a electrical engine are unknown.
2. In order to make a good comparison, I assumed that the costs of a rectenna and two electrical
engines weigh up to the costs of two turboprops, the kerosene tank and the complete fuel system.
11
4.2 Who shall be paying these costs? I imagine that these ground stations will be exploited by power companies like Exxon and Shell. They
shall build the infrastructure at their own expense. Airlines will pay these companies per used ground
station. If we assume the costs calculated above, table 4 and 5 show the costs per aircraft per
kilometre.
Costs ground
station/year
Aircraft that pass the ground
station / day - year
Costs per aircraft
per ground station
Price/ km
(35 km range per
ground station)*
Price/ km
(160 km range per
ground station)*
€846.500 12 - 4380 €195 €5,60 €1,20
€846.500 24 - 8760 €97 €2,80 €0,60
Table 4: the costs of the use of microwave per kilometre
* As explained in chapter 2.3
Range Dornier 228 Load of kerosene Price kerosene / litre Price/km
2445 km 2400 litre €0,60 €0,60
Table 5: the costs of the use of kerosene per kilometre
If the range between the ground stations is 160 kilometres, the price/km of kerosene is the same as
the price/km if microwaves would be used. The price/km of microwaves will become less than the
price/km of kerosene if the amount of aircraft that pass the ground station increases.
4.3 Other cost considerations A few things that should be kept in mind are:
- Combustion engines need far more maintenance than electrical engines.
- An aircraft that uses microwaves can take a lot more payload, which means a larger income for the
airline company.
- No tax for pollution has to be paid.
4.4 Regulations and laws A lot of laws and regulations in the aviation industry concern fuel. For example: safe storage and use
of hazardous liquids, the design and construction of fuel tanks and the regulations and laws
concerning fire safety.
All these regulations and laws will no longer be applicable, this means that the regulations and laws
will become less complicated. Instead, the regulations and laws concerning electromagnetic radiation
will need to be followed. Passengers, crew and the public need to be protected from the microwaves.
12
5. The market Of course, this concept cannot directly be used for all flights. It shall first be used in a smaller market.
This market consists of smaller aircraft and unmanned aircraft that need to be airborne for as long as
possible to observe or for reconnaissance. A few examples are: military UAV’s, fire-spotting UAV’s in
areas with a lot of drought or UAV’s used by the police to spot crimes.
The final market that this concept is aiming for is the (unmanned) cargo transport and the large
passenger transport. The size of the market is dependent on the amount of power that can be send,
because more power means bigger aircraft and a larger market, but it is mostly dependent on the
public. At first, the public will be afraid of the radiation, even though it is completely safe while one is
not in direct contact with the beam. Most probably, tests will need to be shown to the public, to show
that it is completely safe. Once the concept has been accepted, the market growth shall no longer
mostly depend on the public, but on the amount of power that can be send.
6. Conclusion The conclusion of this report is that it is technically possible to power aircraft of up to 1500 kW, solely
on microwaves. If more research is done on increasing the gyrotron power and the rectenna, more
powerful aircraft can also be powered by microwaves. This concept can lead to an emission free
aviation industry.
13
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