Flying on waves: as green as it gets

The use of microwaves for wireless power transmission

A. Vroom

Delft, 2012

Delft University of Technology

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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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