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  • POINT-TO-POINT WIRELESS POWER TRANSPORTATION IN REUNION ISLAND

    48th International Astronautical Congress, Turin, Italy, 6-10 October 1997 – IAF-97-R.4.08

    J. D. Lan Sun Luk, A. Celeste, P. Romanacce, L. Chane Kuang Sang, J. C. Gatina

    University of La Réunion – Faculty of Science and Technology

    15, Av René Cassin – 97715 St Denis Messag Cedex 9

    Email: lanson@univ-reunion.fr

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    Abstract

    This article presents the results of a case study that was carried out in La Reunion on a Wireless Power Transportation system which goal is to deliver 10 kW of electricity power to a small isolated mountain village called Grand-Bassin. The distance between the reception system and the emitter source is 700 m. The use of original wire antennas that integrates well in the natural environment, while exhibiting good performances, is described. Safety issues was accounted for by the use of low power densities and various pilot signals. In order to guaranty the electricity delivery quality to the users, a power conditioning has been studied and is described in this paper.

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    1) Introduction

    Energy control is closely related to Mankind evolution. Among the various forms of energy used, nuclear and fossils take the biggest part of the market. Since the industrial revolution, our fossil energy consumption is growing very fast. In the last few decades, where growing energy demands, increasing environmental problems and declining fossil fuel resources, have raised serious questions on future developments, people have concentrated efforts on finding alternative energy options. Among these options, nuclear plants have been extensively studied, but are not satisfactory in terms of sustainable development because of nuclear waste disposal problems.

    We have focused our attention on the SPS (Solar Power Satellite) concept. After the concept validation period [1], it seems opportune that ground based systems be developed in order to improve the technology and public acceptance necessary before this concept is used on a wide scale in space.

    Following this guideline, we have studied in La Réunion, a point-to-point wireless power transportation system that is used to deliver electricity to a small isolated mountain village.

    2) Presentation of the Grand-Bassin case study

    Grand-Bassin is a small isolated mountain village, in La Reunion island (21° S, 55 °E), a French overseas territory. The beauty of this village has made it a tourist spot, highly visited by walkers. Four lodges have been created for tourist housing, but as the connection to the main electricity network is not easily achievable, there is up to now no refrigerator nor washing machine, and the related tourism activity is thus limited.

    Since few years, inhabitants claim for a connection to the electricity network in order to increase this activity and related benefits that could prevent people from leaving the village.

    Various solutions have been proposed (photovoltaics, embedded lines…) by specialists, but none of them seems to be acceptable in terms of price, running costs and power amount, all taken together. The mayor of the city (Le Tampon) and local administrations have been searching for other solutions.

    So we have proposed to carry a case study on a wireless power transportation for this village.

    After some meetings and visits to people at the Grand-Bassin village, we have defined requirements that the WPT system should meet:

    – the power to be supplied to the village, in a first step, is 10 kW.

    – the distance between the emitter system and the receiving system is 700 meters.

    We have also defined two majors issues to our work:

    • – the safety constraints [2]– and the environmental integration.

    The security for people and natural environment is achieved by using power densities below the level imposed by the international standard for a permanent exposure to the radiation (5 mW/cm2) and a total control of the system.

    Preserving the beauty of the countryside is another issue that was taken into account by a collaborating with a team of architects [3] from the very beginning of the design process.

    In the following of this paper the system that originates from our initial studies is presented. It is divided into three parts:

    – The emitter unit: this unit transforms the AC power taken from the main electricity grid to an RF beam.

    – The receiving unit: this unit collects the energy of the incident RF beam and transforms it to a DC electricity form

    – The power conditioning unit: This unit transform the DC energy into a conventional AC electricity form that is delivered to the end users.

    3) The emitter unit

    Halfway up the south cliff surrounding the village there is a medium voltage transformer connected to the main electricity grid. It is used to furnish electricity to a water pumping station. By installing the emitting unit near the pumping station we can reduce the distance for the power transportation to 700 m and benefit from the electricity and water needed by the system.

    This unit converts the electricity power provided by Electricity of France (EDF) into an electromagnetic wave at a frequency of 2.45 Ghz. Magnetrons are used as conversion elements.

    A beam is focused using an array of projecting antenna and is directed to a receiving antenna situated down in the valley, near the village.

    A new kind of antennas, called MultiFoci Parabolic Reflector antenna (MPR), has been developed for this unit, in order to integrate well in the environment, while achieving high performances.

    The magnetron

    The magnetron is a microwave free running oscillator, that is able to provide a high microwave output power with a good efficiency (up to 80 %) .

    It has been extensively used for years as a microwave source in radar and domestic or industrial microwave ovens. For this reason, the magnetron has become a cheap component compared to other microwave sources.

    It is possible, although not straightforward to control the phase of the output RF signal of a magnetron, by building a phase locked loop in which the reference signal is injected into the cavity of the magnetron via a circulator. The phase error signal is used to change the free running frequency of the magnetron by changing the supplied anode current.

    This work was carried by W. C. Brown in the past decades to build an MDA (Magnetron Directional Amplifier).

    The power supply offers the possibility to control the power at the output by steps [4]. This enables to align the amount of power generated with the one that is consumed by the users at each moment, with the help of a feedback pilot signal mechanism.

    In this unit, the use of several low power magnetron instead of a single high power source was deliberately chosen. This is because low power magnetrons are very cheap and robust, and this configuration facilitates maintenance operations and cost, as well as future power enhancement developments of the system.

    The MPR antenna

    This new kind of antenna uses many parabolic surfaces with different foci (figure 1), but all sharing a common focus. Using this technique the effective area is kept at the same level than conventional parabolic antennas while space occupation is decreased.

    For our study, we have built an MPR reflector prototype by using only plexiglas and wire mesh.

    Simple measurements of gain in the main direction of the hand-made prototype gave a result of 33 dB. In this prototype, the reflector is illuminated by a pyramidal horn placed at the antenna focus.

    mpr-antenna

    Figure 1: MPR antenna

    We believe that the gain can be increased on future more rigorous products using this technique.

    4) The receiving unit

    In the following the average power density of the wave in the normal direction of the receiving is assumed to be equal to 5 mW/cm2.

    This power density level was deliberately kept low compared to previous experiments for safety reasons related to the application and situation of the receiving area.

     

    The H-dipole

    Wire antenna technology was preferred to other solutions for many reasons:

    • • low cost technology• easy environmental integration [3]

      • no concurrence with the solar radiation for the vegetation.

    The H-dipole [5] that we use, was developed by G. Pignolet and J. D. Lan Sun Luk in collaboration with the ISAS team of professor Nagatomo. In comparison with the dipole use by W.C. Brown and R. Dickinson in the Goldstone experiment, it presents a better directivity and is less sensible to a depolarisation of the wave.

    It was demonstrated that the efficiency of the RF to DC conversion increases with the level of the input signal. Because using single H dipoles to feed the rectifying units would lead to a poor conversion efficiency at the power density levels used, it has been necessary to group six H dipoles in series in order to increase the power level at the input of the rectifier unit.

    We use a two-wire to collect the incoming RF signal and drive it to the rectifier circuit. A mesh ground plane, placed at l/4 of antennas, is also used to increase the collected power.

    The figure 2 gives the radiation diagram for this sub-array as obtained by a moment method calculation.

    The theoretical input impedance is 50 W with a very low inductive part.

     

    Figure 2 : Calculated radiation patterns (plane E and H) for the sub-array.

    The rectifier circuit

    In order to minimise power re-radiation that would decrease the efficiency and lead to RF, an input filter is used to match the circuit to the antenna impedance. The input signal passes trough a rectifier circuit constituted of a Schottky diode bridge rectifier that converts the RF signal to a DC current. A Low pass output filter is used to force the RF signal to flow through the bridge rectifier.

    1SS97 schottky diodes from NEC are used. These are low cost diodes with good RF characteristics but with a low standing power rate (150 mW). It is then necessary to make a parallel / series connection of several of these diodes in order to withstand the 2 W incoming RF power.

    This configuration, however increases the series and package resistance responsible for the conversion losses, and further implementations should be using higher power Schottky diodes.

    If the impedance is matched in a wide band, different authors [1,6,7,8] have shown that the RF to DC conversion achieved with this circuits can reach 90%.

    The DC power collection scheme

    The set constituted by a sub-array of 6 H-dipole and the rectifier circuit will be named rectenna in the following.

    Each rectenna is distant to its neighbour to 3l/2. A wire bus collects the DC power (figure 3).

    Figure 3 : The series connection between rectenna

     

    From the theoretical values obtained for a rectenna, we can calculate the dimensions of our receiving system to provide the 10 kW output power.

    5) The conditioning unit

    The DC power collected at the output of the rectenna must be converted to the French electricity distribution standard before utilisation (uniphase network 220VAC /50 Hz/ 10 kVA).

    In this study case we must solve the classical problem : how to deliver a good quality electricity power with an isolated source ? In other terms, all the consumed power (active or reactive) must be provided by the network at every time [9].

     

    First point

    The rectenna delivers a DC power with no reactive part (resistive load). The network needs a static compensator or a free running machine to provide this reactive part.

    Second point

    The load can have some rapid variations, positive or negative. We need an auxiliary source to answer the demand and a short-circuit load to dissipate the excess power.

    Third point

    The DC power provided by the rectennas must be converted in AC with frequency stability and low harmonics generation. This function is realised by a static converter.

    The conditioning unit proposed configuration

    We can see on the figure 4, the proposed solution and the characteristics of our conditioning unit.

    Each static converter also participates to the protection of the system against overload or short-circuits.

    6) Efficiency estimations

    An efficiency estimation is necessary to determine the size of the system. It is based on two strong requirements.First, the power density of the microwave beam should not be over 5 mW/cm2 at the receiving antenna level for safety reasons. Second, the maximum delivered electricity power at the user end must reach 10 kW.

    Let us start from the load. Assuming a 90 % efficiency for the DC to AC converter and a 95 % efficiency for the static DC to AC converter, a dc power of 11.7 kW must be delivered at the output of the rectenna main collecting bus.

    Assuming a 10 dB taper Gaussian distribution of the illumination, and a maximum of 5 mW/cm2 at the centre of the receiving antenna, and taking into account the effective area of the elementary group of H dipoles used as subarray elements as well as the rectifying efficiency against incident power taken from previous experiments [10], we can estimate the size of the receiving antenna. A necessary 17 m aperture radius is obtained.

    Figure 4 : The configuration of the conditioning unit

     

    At a distance of 700 m, the projecting antenna used to illuminate the receiving antenna with the corresponding distribution should be a 3m aperture radius antenna. This is achieved by using 15 MPR antennas. The resulting collection and conversion efficiency is 84 %. Thus the projected microwave power must be about 14 kW. Assuming an 80 % efficiency for the magnetrons, which is readily achievable using switching inverter power supplies and good working points for the magnetrons, the amount of power taken from the main electricity grid should reach 17.5 kW.

    The overall efficiency is then estimated to be 57 %.

    The table 1 resumes the obtained characteristics of the system.

     

    Power delivered to the users

    10 kW

    Power conditioning efficiency

    85.5 %

    Receiving aperture (radius)

    17 m

    Emitting aperture (radius)

    3 m

    number of MPR

    15

    Power taken from the main grid

    17.5 kW

    Overall efficiency

    57 %

    Table 1

    7) Conclusion

    We have presented the characteristics of a point-to-point WPT system for a ground application which is able to deliver 10 kW of electricity to a village situated at 700 m from the nearest electricity network. This study has shown that efficiencies in excess of 50% can be achieved by such a system, which makes it competitive with other solutions. This level of performance can be achieved without denaturing the site beauty nor cause any health and nature damage. These two last requirements have been the guidelines of our study.

    However, the system presented here uses technologies that have still to be reinforced. Indeed, excellent results have been presented on phase locked loop driven magnetrons in the past, but for a single magnetron working alone. More studies are needed to evaluate the possibility to use such microwave sources in a phased antenna array. One more point that still needs to be studied carefully, concerns the rectifying diodes used for converting RF to DC. These diodes should exhibits good RF characteristics, low series resistance and be able to withstand high power levels. The price of such diodes also need to be kept as low as possible considering the large number of pieces that is needed. To our knowledge, such a diode does not seem to be commercially available.

    After this theoretical approach, fruitful contacts have been taken with the city of Le Tampon in order to realise a prototype of the system.

     

    Acknowledgements

    The authors would like to acknowledge the city of Le Tampon for their help and support during this work. We also would like to thank J. McSpadden for allowing us to use the WEFF software for efficiency calculations and G. Pignolet for his hard work to help us to carry out this case study.

    References

    [1] W. C. Brown, “The history of wireless power transmission” Solar Energy, Vol. 56, N° 1, pp. 3-21, 1996.[2] A. J. Berteaud, “Réflexion sur les risques biologiques associés à la mise en oeuvre des centrales solaires spatiales ” Journées d’études sur les Centrales Solaires Spatiales, SEE Club 11, Paris, Ecole Supelec, Juin 1986[3] F. Lefèvre, ” L’intégration du système dans l’environnement” SPS IdR 96, Nov 96, Université de la Réunion
    [4] D Chambers, C Scapellati, “New developments in the design and application of current source power supplies for high power magnetrons” Proceedings of the 30th Microwave Power Symposium, Jul. 95.
    [5] G. Pignolet, J. D. Lan Sun Luk, “Design a low cost rectenna for a low-power SPS-2000/WPT demonstration model” ISAS Research Note, N° 573, July 95
    [6] J. J. Nahas, “Modelling and computer simulation of a microwave-to-DC energy conversion element” IEEE Transactions Microwave Theory and Techniques, Vol MTT-23, N° 12, Dec 1975
    [7] R. J. Gutmann, J. M. Borrego, “Power combining in array of microwave power rectifiers” IEEE Transactions Microwave Theory and Techniques, Vol. MTT-27, N° 12, Dec 1979
    [8] T Razban, M. Bouthinon, A. Coumes, “Microstrip circuit for converting microwave low power to DC energy” IEE Proceedings, Vol 132, Pt. H, N° 2, April 1985.
    [9] P Bastard, M Meunier, “Le réglage fréquence puissance dans un réseau d’énergie” Journée d’études SEE, 01/02/96.
    [10] W. C. Brown, “Electronic and mechanical improvement of the receiving terminal of a free-space microwave power transmission system” NASA contractor Report N° 135194, Raytheon Company.