Briefing Paper. Wireless Power Transmission. Peter Vaessen KEMA. September 2009 ELECTRICITY

Transcription

1 Briefing Paper Wireless Power Transmission Peter Vaessen KEMA September 2009 ELECTRICITY

2 Electricity 1 Introduction The definition of Wireless Power Transmission (WPT) is: efficient transmission of electric power from one point to another trough vacuum or an atmosphere without the use of wire or any other substance. This can be used for applications where either an instantaneous amount or a continuous delivery of energy is needed, but where conventional wires are unaffordable, inconvenient, expensive, hazardous, unwanted or impossible. The power can be transmitted using microwaves, millimeter waves or lasers. WPT is a technology that can transport power to locations, which are otherwise not possible or impractical to reach. Maxwell's theory of electromagnetism, published in 1865 mentions electromagnetic waves moving at the speed of light, and the conclusion that light itself was just such a wave. In 1886 Hertz performed a successful experiment with pulsed wireless energy transfer. He produced an apparatus that produced and detected microwaves in the UHF region. Also Tesla did experiments in the field of pulsed wireless energy transfer in Tesla's Magnifying Transmitter, an early type of Tesla Coil that measured 16 meters in diameter, could transmit tens of thousands of watts without wires. Tesla supposedly managed to light 200 lamps, without wires, from 40 kilometers away. No documentation from Tesla's own records has been published validating that this actually happened. In 1897, he filed his first patents dealing with Wardenclyffe tower. This aerial tower was ment to be a pilot plant for his World Wireless System to broadcast energy around the globe. [1] The core facility was never fully operational and was not completed due to economic problems [4] 2

3 Wireless Power Transmission Figure 1 tesla's colorado springs lab The Raytheon Company did the first successful WPT experiment in In this experiment energy was transmitted with a DC-to-DC efficiency of 13%. This company also demonstrated a microwave-powered helicopter in 1964 [2]. The Jet propulsion lab of NASA carried out an experiment and demonstrated the transfer of 30 kw over a distance of 1 mile in They used an antenna array erected at the Goldstone facility. This test demonstrated the possibilities of wireless power outside the laboratory. Rockwell International and David Sarnoff Laboratory operated in 1991 a microwave powered rover at 5.86 GHz. Three kilowatts of power was transmitted and 500 watts was received [2]. This paper provides an overview of the technologies, possibilities and uses of wireless power transmission. An overall view of the past present and possible transmission systems is presented. In this paper also the different systems; economical, ecological and social aspects are discussed. The paper focuses on wireless power transmission systems with microwaves in the power range of about 100 W to 100 kw. WPT systems using optical technologies (laser) are not discussed. 3

4 Electricity 2 State of the art of WPT technology In order to transport electricity is has to be transformed into a suitable energy form. For wireless transmission, this has to be a form that can travel trough air. Microwave frequencies hold this ability. The microwave spectrum is defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. A complete microwave transmission system consists of three essential parts: Electrical power to microwave power conversion Absorption antenna that captures the waves (Re)conversion to electrical power Figure 2 Microwave transmitter and rectenna 4

5 Wireless Power Transmission The components include a microwave source, a transmitting antenna and a receiving antenna. The microwave source consists of an electron tubes or solid-state devices with electronics to control power output. The slotted waveguide antenna, parabolic dish and microstrip patch are the most popular types. Due to high efficiency (>95%) and high power handling capacity, the slotted waveguide antenna seems to be the best option for power transmission. The combination of receiving and converting unit is called rectenna. The rectenna is a rectifying antenna that is used to directly convert microwave energy into DC electricity. It is an antenna includes a mesh of dipoles and diodes for absorbing microwave energy from a transmitter and converting it into electric power. Its elements are usually arranged in a multi element phased array with a mesh pattern reflector element to make it directional [6]. One of the disadvantages is that microwaves have long wavelengths that exhibit a moderate amount of diffraction over long distances. The Rayleigh criterion dictates that any beam will spread (microwave or laser), become weaker, and diffuse over distance. The larger the transmitter antenna or laser aperture, the tighter the beam and the less it will spread as a function of distance (and vice versa). Therefore, the system requires large transmitters and receivers. The used power density of the microwave beam is normally in de order of 100 W/m 2. This is relative low compared to the power density of solar radiation on earth (1000 W/m 2 ) and chosen this way for safety reasons. 5

6 Electricity 3 Applications WTP for space solar The largest application for microwave power transmission is space solar power satellites (SPS). In this application, solar power is captured in space and converted into electricity. The electricity is converted into microwaves and transmitted to the earth. The microwave power will be captured with antennas and converted into electricity. NASA is still investigating the possibilities of SPS. One of the problems is the high investment cost due to the space transport. The current rates on the Space Shuttle run between $7,000 and $11,000 per kilogram of transported material. Recently the idea of Space Solar Power caught again the public attention e.g. by the Obama transition team [17] and The Economist [18]. Figure 3 Space Solar Power satellite 6

7 Wireless Power Transmission Power transfer, bridging applications Using a powerful focused beam in the microwave or laser range long distances can be covered. There are two methods of wireless power transmission for bridging application. First is the direct method, from transmitting array to rectenna. A line of sight is needed and is therefore limited to short (< 40 km) distances. Above 40 kilometers, huge structures are needed to compensate for the curvature of the earth. [16]. The second method is via a relay reflector between the transmitter and rectenna. This reflector needs to be at an altitude that is visible for both transmitter and rectenna. This method is not discussed further. Next three bridging applications of WPT are discussed. Alaska '21 WPT can be an option for power supply to rural areas. In 1993 was a project presented about wireless power supply in Alaska. Because of limited infrastructure, hundreds of small rural communities in Alaska must provide their own electricity. These systems can be expensive not standard or just not available. At the moment, the small communities produce their own power with mainly diesel engines. These produce noise and pollution. Also the needed fuel has to be transported over long distances. All this results in an electricity price in excess of $40/kWh [14]. Cable connections trough water is no option because of ice. With the help of WPT, the needed power production of the communities can be combined. This can reduce noise, pollution and transportation of fuel. WPT may be capable of transmitting electrical energy to Alaska s remote villages. To investigate these possibilities, a pilot project was conducted named "Alaska'21. The system used for the pilot project consisted of a 2.45 GHz phased array design. The distances that should be bridged are between 1 and 15 miles. The status of the project is unknown 7

8 Electricity Figure 4 Alaska 21 Grand Bassin - La Reunion The Grand Bassin project [15] lead by the local university will supply electricity to a remote isolated mountain village. The project supported by CNES, the French space research centre, aims to beam electrical power to the tourist village of Grand Bassin on the island of La Reunion. Grand Bassin lies at the bottom of one of the deep canyons on the island. WPT will be used to preserve the beautiful scenery of the valley. The plan was to build a microwave link down the side of the canyon, operating at 2.45 GHz over a distance of 700 meters and delivering 10kW with an overall efficiency of 57%. The system will work in combination with PV panels, charging batteries of the system when the power is not being used directly. A prototype was build and presented at the Wireless Power Transmission conference in 2001, which was held on the island. The cost of the project was estimated to be "1 million dollars for 10 kw and the project stopped. 8

9 Wireless Power Transmission Figure 5 Grand Bassin La Reunion Hawaii demonstration In May 2008, a long-range WPT demonstration was realized on one of the islands of Hawaii. The demonstration was organized by Managed Energy Technologies of the U.S. and involved the wireless transmission of energy over a distance of 148 kilometers. Although the amount of power transmitted, 20 watts, is barely enough to power a small compact fluorescent light bulb, and most of it was lost in transmission, the system was limited by the budget not the physics. If they had been able to afford more solar panels, more phased array transmitters and a better receivers (the one they had could only receive in the horizontal direction), they could do much better - possibly up to 64% efficiency. [10] The costs of the whole project where less then1 million dollar. Figure 6 Hawaii demonstration 9

10 Electricity 4 Safety issues Bio-effects A general public perception that microwaves are harmful has been a major obstacle for the acceptance of power transmission with microwaves. A major concern is that the long-term exposure to low levels of microwaves might be unsafe and even could cause cancer. Since 1950, there have been thousands of papers published about microwave bio-effects. The scientific research indicates that heating of humans exposed to the radiation is the only known effect. There are also many claims of low-level non-thermal effects, but most of these are difficult to replicate or show unsatisfying uncertainties. Large robust effects only occur well above exposure limits existing anywhere in the world [5]. The corresponding exposure limits listed in IEEE standards at 2.45 or 5.8 GHz are 81.6 W/m 2 and 100 W/m 2 averaged over 6 minutes, and 16.3 or 38.7 W/m 2 averaged over 30 minutes [11]. This low compared to average solar radiation of 1000 W/m 2. A clearly relevant bio-effect is the effect of microwave radiation on birds, the so-called "fried bird effect". Research is done on such effect at 2.45 GHz. The outcome showed slight thermal effects that probably are welcome in the winter and to be avoided in the summer [5]. Larger birds tend to experience more heat stress then small birds [11]. The overall conclusion of bioeffects research is that microwave exposures are generally harmless except for the case of penetrating exposure to intense fields far above existing exposure limits [5]. Further discussions 10

11 Wireless Power Transmission about the maximum microwave power density are necessary. A range of environmental issues and safety-related factors should continue to receive consideration because of public concern about radio wave and micro wave exposure. Compatibility with other radio services and applications It is assumed that WPT systems working with microwaves use frequency bands around 2.45 GHz or 5.8 GHz. These bands are already allocated in the ITU-R radio regulation to a number of radio services. They are also designated for industrial science and medical (ISM applications). The ISM band is, as presently defined, for local use only. The 2.45 GHz is further more used for radio LAN and microwave ovens. The 5.8 GHz is also used heavily for various applications like Radiolocation service and DSRC (Dedicated Short-Range Communications). More investigation is needed to get an image of the possible influencing between the systems [11]. Costs For the very short range (1-10 meters), preliminary demonstrations of WTP at low power levels (less then 1 kw) were in general quite costly, however, the cost estimates are coming down for larger power levels and longer ranges. The slightly over 1million dollar Goldstone test in 1975 delivered 34 kw at 1.6 km. The costs of the test in Hawaii in 2008, where just under 1 million dollar and delivered 20 Watts over 148 kilometers. Tens of km WPT systems are in the range of several $1,000,000/ MW-km, whereas similar range wired systems are of order $10,000/MW-km, at least two orders of magnitude less. Involved organizations NASA and the National Space Development Agency of Japan are still 11

12 Electricity working on Space Power Satellites. France is at the forefront of European interest in wireless power transmission. University La Reunion and CNES (French space research centre) worked on the demo project in the village Grand Bassin. The Raytheon company (military and Technology Company) is also working on the development and application of WPT systems. 5 Conclusions It is clear that WPT systems in the range of 100 W to 100 kw to cannot compete with traditional systems just looking at the costs. At places where economic competition is not the prime consideration, it can be an option. Microwave wireless power transmission can supply power to places that are difficult to reach. Especially small communities in rural areas could be supplied with power using WPT. More successful demonstration projects can help the further development and utilization of this technology. Further investigation concerning compatibility and safety is needed to clarify these issues. Space Solar Power has gained public interest during the last year because of global warming and the energy independency goals of US and EU. References [1] Colorado springs notes [2] Status of international experimentation in wireless power transmission, Gregg E. Maryniak Sunset energy counsel, Solar energy Vol [3] International cooperation for the acquisition of space based energy, R. Bryan ERB, Solar Energy Vol

13 Wireless Power Transmission [4] Cheney, Margaret (1999), Tesla Master of Lightning [5] Health and safety issues for microwave power transmission, John M. Osepchuk, Solar energy Vol. 56, 1996 [6] Research Activities and future trends of microwave wireless power transmission, Djuradj Bubimir and Aleksandar Marincic, International symposium Nikola Tesla, 2006 [7] The results of NASA "Fresh look" at the feasibility of Space Solar Power, John C. Mankins, 1997 [8] Solar power? Ok but why capture it in space? R. Bryan Erb, Fourth international symposium on space means for power utilities, 1997 [9] Comparative analysis of wireless systems as alternative to high voltage power lines for global terrestrial power transmission, Andrey P. Smakhtin, Valentin V. Rybakov, IEEE, 1996 [10] [11] White paper on Solar Power Satellite Systems, URSI, September 2006 [12] Advanced receiver/converter experiments for laser wireless power transmission, Joe T. Howell, 5 th Wireless power transmission conference, pp 1-8, Granada Spain 2004 [13] Laser Power Beaming Market Fact Sheet, Tom Nugent, Lasermotive, july 2008 [14] Alaska '21: A terrestrial point-to-point wireless power transport system [15] Journal de l'ile de La Réunion - Article of 09 February 2002 [16] Comparison of intercontinental wireless and wired power transmission, Richard M. Dickinson, Jet Propulsion Laboratory, John C Mankins, NASA, 1999 [17] [18] Space Solar Power - Let the sun shine in, The Economist print edition, December 4 th