18-02-2011, 02:55 PM
Concepts for wireless energy transmission via laser
1. INTRODUCTION
While the science of light and vision has alwaysfascinated and inuenced humans, the beginning ofmodern optics might be traced back to the famous"Book of Optics" by Ibn al-Haytham_, which for the_rst time describes the theory of vision and light asa ray theory, unifying geometrical optics with philosophicalphysics. His book already described experimentswith lenses, mirrors, refraction, and reection.Modern scienti_c optics, with the invention of thetelescope in the 17th century by Dutch and Italianastronomers and mathematicians revolutionised ourway of viewing the universe and the place of Earthand thus ultimately our own place within it.Optics is already one of the most cross disciplinarydisciplines, spanning from physics, chemistry, mathematics,electrical engineering up to architecture, psychologyand medicine. This paper intends to describethe application of optics and light in an area where itis traditionally only marginally present: energy transmission.
2. WIRELESS ENERGY TRANSMISSION
The _rst attempts to transmit energy wirelesslywith the purpose of doing so are attributed to N.Teslay at his laboratory in Long Island, New Yorkjust 30 years after J. Maxwellz had predicted in 1873the transport of energy trough vacuum via electromagneticwaves, validated in principle 15 years laterby H. Hertzx. [1] Following the invention of the magnetron and theklystron in the 1920 and 1930, the developments duringthe second world war made microwave beamsavailable to a wider scienti_c community. The _rstsuccessful engineering approach to use microwaves fore_ective energy transmission was done by W. Brown{in the 1960s, by powering among other devices a tetheredhelicopter. [2]The _rst power stations in Earth orbit, taking advantageof the absence of day-night cycles to harvestthe energy of the sun were described by the earlyspace pioneers K. Tsiokovski__ and H. Oberthyy. PeterGlaser is recognised as the _rst to combine thevisions of these early space pioneers with the practicaladvances in transmitting energy without wires byW. Brown in his 1968 publication in Science, whichcontained the _rst engineering description of a solarpower satellite (SPS). [3] It established a vision of asustainable, practically non-depletable and abundantsource of energy to meet world energy demands andtriggered the imagination of researchers around theglobe.Since this pioneering article, several small andlarger scale studies and experiments have been performedaround the world in order to mature the conceptof solar power satellites further. For a descriptionof how the concept had evolved since the 1968publication, it is referred to [4]. While these studiesand experiments were generally intensi_ed duringtimes of high carbon fuel prices and received lowerattention during times of low oil and gas prices, theidea was never considered mature enough to be puton a larger industrial scale, but the general conceptof abundant, virtually CO2 emission free power generatedin orbit and transmitted to where needed on Earth has most of the time been considered as tooattractive to not pursue further. The most comprehensivestudy was done during 1979, when the USDoE and NASA made a joint technical analysis (frequentlyquoted as \SPS reference study") on the optionsof SPS. [5]Furthermore, all studied so far essentially concludedthat there were no principal technical \showstoppers"to the concept. On the other hand, witheach redesign cycle based on new technology, the totalmass in orbit, cost and complexity of the entiresystem decreased substantially, indicating a remainingpotential for further improvements. [5] [6] [7] [8]The most daring concepts was probably proposedby Criswell et al. in 1990: The proposed lunarpower stations would be very large installations onthe moon, generating energy from solar irradiationon the moon to transmit it to Earth, passing via relaystations and reectors in Earth orbit. [9] [10]Following the theoretical works in the 1950s andthe _rst presentation of a functioning laser in 1960,the use of lasers as a means to transmit energy becameapparent. [11] [12] At the same time, the _rstphotovoltaic cells were mounted on spacecraft to complementthe energy provided by batteries to extendthe spacecraft operational life.The much higher maturity level of microwave devicesand the resulting order of magnitude higheroverall e_ciency has however prevented the conceptsof wireless energy transmission by laser to enterinto the mainstream SPS concepts for most ofthe last 30 years. Recent exceptions include the papersof Brandhorst, Steinsiek, Cougnet, Fork, andLuce. [13] [14] [15] [16]The present paper argues that advances in lasertechnology and operational as well as engineering advantagesof concepts based on laser power transmissionprovide ground for further interest in this conceptand a stronger involvement of the scienti_c lasercommunity.This paper concentrates on technologies for longdistancewireless power transmission technologies.Short and medium range wireless power transmission(e.g. via induction or evanescent wave coupling) arenot considered. [17]
3. WIRELESS ENERGY TRANSMISSIONTECHNOLOGIES
In general, e_ective wireless energy transmissionconcepts need to comply with a range of fundamentalconstraints:_ possibility to transfer the energy though an atmosphere transparency of the atmosphere tothe used wavelength;_ possibility for directional emission;_ possibility to convert the energy from the formof its source (solar, electric, heat) to a transmittableform (e.g. microwave, laser, accoustic);_ possibility to convert the transmittable energyform back into a useful form of energy (e.g. electricity,hydrogen).While this paper concentrates on laser energytransmission, it is useful to compare its performancesand parameters with microwave energy transmission,the most widely studied wireless energy transmissiontechnology. In principle, laser energy transmissionsystems are very similar to energy transmission viamicrowave technology: the power source (solar, electricity)is converted into an emitter or an emitter arraythat generates the directional electromagnetic radiation,which is subsequently absorbed in a receiver,which transforms the energy back into a more useful,transportable form, e.g. electricity, heat, hydrogen.The key di_erence, the wavelengths used, impliesthe major other di_erences between the laserand microwave-based concepts: While most wirelesspower transmission rely on microwave frequencies ofeither 2.45 or 5.8 GHz (0.12-0.05 m; both in the industrial,scienti_c and medical (ISM) frequency band),laser energy transmission takes advantage of the atmospherictransparency window in the visible or nearinfrared frequency spectrum
download full report
http://esa.int/gsp/ACT/doc/POW/ACT-RPR-N...er-WPT.pdf
