Flying on waves: as green as it gets The use of microwaves for wireless power transmission
A. Vroom Delft, 2012 Delft University of Technology
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. T his 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 conver ted 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 w ould 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 appr oximately 2 megawatt, a Dor nier 228 can fully be powered by microwaves. T here 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. T he price per kilometre is heavily dependent on the amount of aircraft that pass a ground station per year and on the inter val o f the ground stations. If 24 aircraft will pass a ground station per day and the inter val 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 concer ning 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 air borne 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 par ticipate 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. T he 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 mar ket.
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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 pay load? 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 ver y old, Nicola Tesla proposed his theories of wireless power transmission on the late 19 th and early 20th century. Since then, several new ways of sending power wirelessly trough the air have been found and proven to w ork. 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 wor ked 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 Figure 1: Intel demonstrating transmitter, so a laser will not wor k with cloudy weather. (Spencer, Long inductive coupling Distance Wireless Power Transmission, 2010) T he 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 sor t of antenna: a rectifying antenna (rectenna). Microwaves are generated on the ground and send to the aircraft. T he rectenna, which is hanging below the aircraft, as one may see in figure 2, can produce DC power out of micr owaves. A more detailed description on how this rectenna wor ks 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 Figure 2: the Canadian SHARP, a no longer need to take fuel into flight, this 41 percent could also be used microwave powered unmanned aircraft. The disk between the tail and the wings is for payload. T his would mean that the maximum amount of passengers a rectenna 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 air craft, 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 necessar y. 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 pr oduction of CO2 to zero.
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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. T he 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 wor ks.
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
Figure 3: the general concept (Foot, 1993)
Company Goldwind
Diameter [m] 48
Power [kW] 750
Sinovel
70
1.500
Enercon
82
2.000
Suzlon
88
2.100
Gamesa
90
2.000
Vestas
90
3.000
Nor dex
99,8
2.500
GE Energy Siemens
100
2.500
107
3.600
In order to get the generated power to the aircraft, this power Enercon 112 6.000 will need to be converted to a beam. This will be done with a Table 1: wind turbine diameter versus power gyrotron and a propagating antenna. The high-power gyrotron (European Wind Energy Association, 2009) originates from the development of nuclear fusion, but are now commercially available. It is a device that uses a cyclotr on 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 gyrotr on will be a green, 2 megawatt, power source, a 35 Ampere current and a supply voltage of 80 kV. The magnetic field the gyr otron 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/m 2 and a Figure 4: the cross-section of a gyrotron (CRPP) spot diameter of 40 meters may be sent up to an altitude of over 20 kilometres. (Caplan & Friedman, 2005)
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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. T he reasons for this are the tw o 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. Aer osol 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 conver t 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. T he high-efficiency rectenna also wor ks like this, but it uses a “dual polarization design to double the transmitting power and receive the micr owave power with no polarization mismatching loss� (Fujino, Fujita, Kaya, Onda, & Tomita, 1998) The conventional rectenna set-up only pr ovides a flux density 2 kW/m 2, 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 reac hed. 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 Antenna Beam Rectenna
50 97 80 81
1000 ( Kare & Parkin, 2006) 970 (Brown & Eves, 1992) 776 ( Kare & Parkin, 2006) 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 w ould be used, Table 3 would describe the DC to DC efficiency
Concept part Gyrotron Antenna Beam Rectenna
Maximum Efficiency (%) 97 97 80 81
Power left of ~2 MW input (kW) 2000 (Fusion for energy, 2009) 1940 (Brown & Eves, 1992) 1552 ( Kare & Parkin, 2006) 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 Dor nier 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 T PE3315252Ds. 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) Figure 6: a Dornier 228
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 inter val 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 altit ude, 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 Amste rdam – Paris. The second option would take considerably longer, it is therefore more ideal for car go flights than for passenger flights.
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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 ear th 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 NO x, CO, SO x, NMVOC (non-methane volatile organic compounds), particulates and other trace components including hazardous air pollutants”. A few problems that the aviation industr y is having or that will come
Figure 7: Earth’s ozone layer over the arctic in 2011. (The Sciense News, 2011)
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 ver y 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 Figure 8: CO2 emission per person per 1000 kilometres. This table is assuming - As one may see in figure 8 the aviation an average number of passengers of 70% of the maximum capacity (Math! industry produces a lot of CO2, ranging from How much CO2 is released by Aeroplane?, 2007) 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 T yndall-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 Figure 9: carbon dioxide increase between now and 2050. The tickets. green dotted line is today’s carbon emission, the black dotted line is the emission in 2050. (Adams, 2009)
This concept, the use of microwaves, could decrease the pollution of the environment by the aviation industr y 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.
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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. T he maintenance costs are also included in the calculation. In Table 3 one can find the results.
Non-aircraft elements
Invest ment
maintenance
Total
Wind turbine, 2,5 MW 1)
€ 3.312.500 for 20 year lifecycle = € 165.625/year €1.700.000 / megawatt = € 3.400.000 10 year lifecycle = € 340.000/year
4,5% of the investment/year = € 125.875/year
€291.500/year
Estimated:
€510.000/year
Antenna and tracking system
Unknown. Estimated:
Estimated:
€ 100.000, 10 year lifecycle = € 10.000/year
5% of investment = € 5.000/year
Ground and building. Simple configuration.
Unknown/ Estimated:
Estimated:
200.000. 10 year lifecycle = € 20.000/year €7.012.500
5% of investment = € 10.000/year
Gyrotron 2)
Total
5% of investment = €170.000 / year
€310.875
€15.000/year
€30.000/year
€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 unknow n. 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 tw o turboprops, the kerosene tank and the complete fuel system.
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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 ow n 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
Aircraft that pass the ground
Costs per aircraft
Price/ km
Price/ km
station/year
station / day - year
per ground station
(35 km range per
(160 km range per
ground station)*
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 micr owaves 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 industr y 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 concer ning electromagnetic radiation will need to be followed. Passengers, crew and the public need to be protected from the microwaves.
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5. The market Of course, this concept cannot directly be used for all flights. It shall first be used in a smaller mar ket. This market consists of smaller aircraft and unmanned aircraft that need to be airbor ne for as long as possible to observe or for reconnaissance. A few examples are: militar y 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) car go transpor t and the large passenger transpor t. The size of the mar ket is dependent on the amount of power that can be send, because more power means bigger aircraft and a larger mar ket, 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 pr obably, tests will need to be shown to the public, to show that it is completely safe. Once the concept has been accepted, the mar ket 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.
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Spencer, W. (2010). How Wireless Power induction Works. Retrieved april 5, 2012, from Tech-Faq: http://www.tech-faq.com/how-wireless-power-induction-works.html Spencer, W. (2010). Long Distance Wireless Power Transmission. Retrieved april 5, 2012, from TechFaq. Stockholm Environment Institute. (2012, januari 18). Cargo vs. Passengers. Retrieved march 25, 2012, from Core: http://www.co2offsetresearch.org/aviation/Cargo.html The Sciense News. (2011, april 12). Never bigger the ozone hole over the earth. Retrieved march 29, 2012, from The Science News: http://www.thesciencenews.info/2011/04/never-bigger-ozone-holeover-earth.html Transportdirect. (n.d.). CO2 emissions calculator. Retrieved april 3, 2012, from Transport Direct: http://www.transportdirect.info/Web2/JourneyPlanning/JourneyEmissionsCompare.aspx Wilde, D. d. (2010, april 2). windmolen-faq. Retrieved april 6, 2012, from Olino, duurzame energie: http://www.olino.org/articles/2010/01/28/windmolen-faq
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