Patent application title: ELECTRIC WIND ADJUSTABLE FRESNEL REFLECTOR SOLAR CONCENTRATOR
Mario Rabinowitz (Redwood City, CA, US)
IPC8 Class: AF24J240FI
Class name: Stoves and furnaces solar heat collector with means to reposition solar collector for optimum radiation exposure
Publication date: 2009-04-23
Patent application number: 20090101136
Patent application title: ELECTRIC WIND ADJUSTABLE FRESNEL REFLECTOR SOLAR CONCENTRATOR
Origin: REDWOOD CITY, CA US
IPC8 Class: AF24J240FI
This invention deals with novel method and apparatus for positioning and
motion control of the elements (mirrors) of a Fresnel reflector solar
concentrator tracking heliostat array by rapid-response motorless linear
motion, angular deflection, and rotational motion utilizing electric wind
force due to electrons, ions, and/or neutrals. Thus forces and torques
are produced without the use of internal moving parts. Control is
achieved without recourse to magnetic fields, by means of high electric
fields which may be attained at relatively low voltages. At low voltages,
the instant invention exceeds the capability of conventional systems. It
can perform dynamic motion control over a wide range of dimensions and
signal bandwidth with independent amplitude and frequency modulation. It
is ideally suited for maximization of solar energy focused by the array
onto a receiver. Since there are no internal moving parts, the instant
invention is the most adapted for fabrication at the micro and
nanotechnology realms. Furthermore it provides less costly and greater
ease of manufacture from the nano-to the macro-realm.
1. A solar concentrator system comprisinga) an array of adjustable optical
elements;b) said elements coupled to at least a first electric wind
device;c) said electric wind device producing a propelling electric wind
force;d) said electric wind force created by emission from said electric
wind device by at least one of the group consisting of electrons,
negative ions, positive ions, and neutrals; ande) said force producing an
alignment of said optical elements to track and focus sunlight onto a
2. The apparatus of claim 1, wherein at least one detent positioning track holds said elements in fixed aligned position following alignment, permitting said electric wind force to be turned off between alignments.
3. The apparatus of claim 1, wherein said optical elements are in pairs of double back-to-back configuration.
4. The apparatus of claim 1, wherein said optical elements are aligned by a positive feedback loop comprising circuitry to control the operation of said electric wind device to intensify the solar energy density reaching said receiver.
5. The apparatus of claim 1, wherein there is at least a second electric wind device in inverse spatial orientation to said first electric wind device, to provide forward and backward motion.
6. The apparatus of claim 1, wherein at least one optical element is a concave mirror.
7. A method of concentrating light comprising the steps ofa) placing moveable optical elements in the form of a Fresnel reflector in a concentrator array;b) coupling said optical elements to at least a first electric wind device;c) propelling said optical elements of a concentrator array by means of an electric wind force;d) producing said electric wind force by emission from said electric wind device by at least one of the group consisting of electrons, negative ions, positive ions, and neutrals; ande) creating an alignment of said optical elements by means of said electric wind force, to track and focus sunlight onto a receiver
8. The method of claim 7, wherein at least one detent retaining track keeps said elements in fixed aligned position following alignment.
9. The method of claim 7, wherein said optical elements are paired in double back-to-back configuration.
10. The method of claim 7, wherein at least a second electric wind device is placed in inverse spatial orientation to said first electric wind device.
11. The method of claim 7, wherein aligning said optical elements by a positive feedback loop comprising a digital processor and an optimizing sensor, intensifies the solar energy density reaching said receiver.
12. The method of claim 7, wherein storing the location of each said element in computer readable memory facilitates the alignment process.
13. A concentrator apparatus comprising:a) an array of adjustable optical elements;b) said optical elements being adjustable independently of each other;c) said elements coupled to at least a first electric wind device;d) said electric wind device producing a propelling electric wind force;e) said electric wind force created by emission from said electric wind device by at least one of the group consisting of electrons, negative ions, positive ions, and neutrals; andf) said force producing an alignment of said optical elements to track and focus energy flux onto a receiver.
14. The apparatus of claim 13, wherein at least one detent track holds said elements in fixed aligned position following alignments.
15. The apparatus of claim 13, wherein the said optical elements are in pairs of double back-to-back configuration.
16. The apparatus of claim 13, wherein the said optical elements are aligned by a positive feedback loop comprising circuitry to control the operation of said electric wind device.
17. The apparatus of claim 13, in which at least a first optical element is positioned on one side of a receiver, and at least a second optical element is positioned on the opposite side of the receiver.
18. The apparatus of claim 13, wherein the optical elements are at least one of the group of mirrors, reflectors, focusers, and lenses.
19. The apparatus of claim 13, wherein said receiver is one of a group of solar cells and heat engines.
20. The apparatus of claim 13, further comprising an optimizing sensor to enable the maximization of said energy flux reaching said receiver.
FILED OF THE INVENTION
This invention provides a better means to achieve affordable solar energy than by a conventional tracking heliostat array. In the latter, enabling the optical elements to be movable generally requires many large heavy motors that consume a great deal of power. This results in an expensive, bulky, and ponderous heliostat that is unfavorable for rooftop and other applications where much weight cannot be tolerated. The present invention functions in solar concentrators and similar equipment much the same as parabolic dish and parabolic trough concentrators, without their disadvantages of bulk and weight. The angular alignment of the optical elements (mirrors, refractors, lenses, etc.) is accomplished by a rocket-like thrust of an Electric Wind rather than with cumbersome motors.
BACKGROUND OF THE INVENTION
The instant invention related generally to larger scale Solar Energy Concentrators than for example in the U.S. Pat. No. 7,133,183 by M. Rabinowitz. In particular, this invention applies to a Fresnel reflector array that tracks the sum and focuses the reflected light by means of an electric wind.
Description of the Prior Art
No prior art was found related to positioning and motion control by electrons, ions and neutrals in electric fields of adjustable reflecting elements in a Fresnel reflector solar concentrator. The prior art has investigated various forms of rocket propulsion systems and ion engines for high altitude and space applications. The rocket and ion engines of the prior art are for a different purpose and use principles in a different way than the present invention. Such prior art engines generally operate at high temperatures and attempt to either burn or ionize the highest possible percentage of the propellant since the propellant fuel must be carried with the airborne or space borne vehicle and cannot be wasted. They operate at very high levels of power consumption, and utilize exotic types of propellants, producing large amounts of pollution by-products. They are of a considerably larger size and require much higher levels of force than needed to practice the present invention. Except for applications in outer space such devices are for the most part not practical due to their size, weight and power requirements. No prior art was found that utilized field emission for force production in a Fresnel reflector solar concentrator. Furthermore, the instant invention is preferably not operated at high temperatures or high degrees of ionization.
INCORPORATION BY REFERENCE
In a solar energy application (as well as other functions), adjustable reflecting elements are an important feature of a Fresnel reflector solar concentrator which tracks the sun and directs the sunlight to a receiver as described in the following patents and published papers. The following U.S. patents, and Solar Journal publication are fully incorporated herein by reference. 1. U.S. Pat. No. 7,247,790 by Mario Rabinowitz, "Spinning Concentrator Enhanced Solar Energy Alternating Current Production" issued on Jul. 24, 2007. 2. U.S Pat. No. 7,187,490 by Mario Rabinowitz, "Induced Dipole Alignment Of Solar Concentrator Balls" issued on Mar. 6, 2007 3. U.S Pat. No. 7,133,183 by Mario Rabinowitz, "Micro-Optics Solar Energy Concentrator" issued on Nov. 7, 2006. 4. U.S Pat. No. 7,130,102 by Mario Rabinowitz, "Dynamic Reflection, Illumination, and Projection" issued on Oct. 31, 2006. 5. U.S Pat. No. 7,115,881 by Mario Rabinowitz and Mark Davidson, "Positioning and Motion Control by Electrons, Ions, and Neutrals in Electric Fields" issued on Oct. 3, 2006. 6. U.S Pat. No. 7,112,253, by Mario Rabinowitz, "Manufacturing Transparent Mirrored Mini-Balls for Solar Energy Concentration and Analogous Applications" issued on Sep. 26, 2006. 7. U.S Pat. No. 7,077,361, by Mario Rabinowitz, "Micro-Optics Concentrator for Solar Power Satellites" issued on Jul. 18, 2006. 8. U.S Pat. No. 6,988,809 by Mario Rabinowitz, "Advanced Micro-Optics Solar Energy Collection System" issued on Jan. 24, 2006. 9. U.S Pat. No. 6,987,604 by Mario Rabinowitz and David Overhauser, "Manufacture of and Apparatus for Nearly Frictionless Operation of a Rotatable Array of Micro-Mirrors in a Solar Concentrator Sheet" issued on Jan. 17, 2006. 10. U.S Pat. No. 6,975,445 by Mario Rabinowitz, "Dynamic Optical Switching Ensemble" issued on Dec. 13, 2005. 11. U.S Pat. No. 6,964,486 by Mario Rabinowitz, "Alignment of Solar Concentrator Micro-Mirrors" issued on Nov. 15, 2005. 12. U.S Pat. No. 6,957,894 by Mario Rabinowitz and Felipe Garcia, "Group Alignment Of Solar Concentrator Micro-Mirrors" issued on Oct. 25, 2005. 13. U.S Pat. No. 6,843,573 by Mario Rabinowitz and Mark Davidson, "Mini-Optics Solar Energy Concentrator" issued on Jan. 18, 2005. 14. U.S Pat. No. 6,738,176 by Mario Rabinowitz and Mark Davidson, "Dynamic Multi-Wavelength Switching Ensemble" issued on May 18, 2004. 15. U.S Pat. No. 6,698,693 by Mark Davidson and Mario Rabinowitz, "Solar Propulsion Assist" issued on Mar. 2, 2004. 16. U.S Pat. No. 6,612,705 by Mark Davidson and Mario Rabinowitz, "Mini-Optics Solar Energy Concentrator" issued on Sep. 2, 2003. 17. Solar Energy Journal, Vol. 77, Issue #1, 3-13 (2004) "Electronic film with embedded micro-mirrors for solar energy concentrator systems" by Mario Rabinowitz and Mark Davidson.
"Adjustable Fresnel reflector" is a variable focusing planar reflecting surface much like a planar Fresnel lens is a focusing transmitting surface. Heuristically, it can somewhat be thought of as the projection of thin variable-angular segments of small portions of a thick focusing mirror upon a planar surface whose angles can be adjusted with respect to the planar surface.
"Concentrator" as used herein in general is an adjustable array of mirrors for focusing and reflecting light. In a solar energy context, it is that part of a Solar Collector system that directs and concentrates solar radiation onto a solar Receiver.
"Dielectric" refers to an insulating material in which an electric field can be sustained with a minimum power dissipation.
"Double Back-To-Back Mirrors" herein shall mean a pair of flat or slightly concave mirrors (in an array of such mirror pairs) that are joined together about a pivot axis so that when the top mirror becomes occluded, the pair can be rotated 180 degrees thus exposing the clean mirror for further usage.
"Electric field" or "electric stress" refers to a voltage gradient. An electric field can produce a force on charged objects, as well as neutral objects. The force on neutral objects results from an interaction of the electric field on intrinisic or induced electric polar moments in the object.
"Electrical breakdown" occurs when a high enough voltage or electric field is applied to a dielectric (vacuum, gas, liquid, or solid) at which substantial electric charge is caused to move through the dielectric.
"Electric wind" herein shall mean the force due to electrons, ions, and neutrals in electric fields resulting in actuation, motion production, control, and positioning.
"Enhanced or macroscopic electric field" is the electric field enhanced by whiskers very near the electrodes based upon the local (microscopic) geometry on the surface of the electrodes.
"Field emission or cold emission" is the release of electrons from the surface of a cathode (usually into vacuum) under the action of a high electrostatic field ˜107 V/cm and higher. The high electric field sufficiently thins the potential energy barrier so that electrons can quantum mechanically tunnel through the barrier even though they do not have enough energy to go over the barrier. This is why it is also known as "cold emission" as the temperature of the emitter is not elevated.
"Focusing planar mirror" is a thin almost planar mirror constructed with stepped varying angles so as to have the optical properties of a much thicker concave (or convex) mirror. It can heuristically be thought of somewhat as the projection of thin equi-angular segments of small portions of a thick mirror upon a planar surface. It is a focusing planar reflecting surface much like a planar Fresnel lens is a focusing transmitting surface. If a shiny metal coating is placed on a Fresnel lens it can act as a Fresnel reflector.
"Macroscopic electric field" `is the applied electric field on the basis of the imposed voltage and the gross (macroscopic) geometry of the electrodes, and which is relevant as long as one is not too near the electrodes.
"Mean free path" of a particle is the average distance the particle travels between collisions in a medium. It is equal to [number density of the medium×collision cross-section]-1.
"Negative ion" is a neutral atom or molecule which has captured one or more electrons.
"Negative ion emission" as used herein is the induced emission of negative ions near an electron emitting cathode where low energy electrons are captured by electronegative molecules.
"Nanotubes" are graphitic microtubule structures of atomic thickness, of the order of 10 Å inside diameter, which have enormous tensile strength. Nanotubes are named for their cylindrical hollow form with nanometer size diameters. They may have single or multi-walled structure. Nanotubes can function as excellent whiskers.
"Optical elements" are the mirrors, reflectors, focusers, etc. of a concentrator array. As a focuser the optical element may be a lens or include a lens.
"Schottky emission" is the enhancement of thermionic emission from a cathode resulting from the application of a moderate accelerating electric field ˜105 V/cm to ˜106 V/cm. The electric field lowers the barrier height, and hence decreases the effective work function. The electric field is not high enough to sufficiently thin the barrier width, so that field emission is not an appreciable part of the emission at moderate electric fields.
"Thermionic emission" is the liberation of electrons from a heated electrical conductor. The electrons are essentially boiled out of a material when they obtain sufficient thermal energy to go over the potential energy barrier of the conductor. This is somewhat analogous to the removal of vapor from a heated liquid as in the boiling of water.
"Thermo-field assisted emission" involves thermionic emission in the presence of a moderate to high electric field so that it includes the realms of both Schottky emission and field emission. At high electric fields, the emission rate is much higher than just from Schottky emission as the barrier is not only decreased in height, but also in width.
"Torr" is a unit of pressure, where atmospheric pressure of 14.7 lb/in2=760 Torr=760 mm of Hg.
"Receiver" as used herein in general such as a solar cell or heat engine system for receiving reflected light. In a solar energy context, it receives concentrated solar radiation
from the adjustable mirror assembly for the conversion of solar energy into more conveniently usable energy such as electricity.
"Whisker" is the generic term used herein for a micro-protrusion or asperity on the surface of a material with a large aspect ratio of height to tip radius.
"Work function" is the minimum energy needed to remove an electron at 0K from a metal. At higher temperatures, the work function for most electrons does not differ appreciably from this low temperature value. More rigorously, the work function is the difference between the binding energy and the Fermi energy of electrons in a metal.
SUMMARY OF THE INVENTION
In the Electric Wind adjustable Fresnel reflector solar concentrator that has been developed, the Electric Wind aligns the optical elements to concentrate solar energy on a receiver having dimensions small compared to the dimensions of the array. This permits the focal point of the array to remain focused on the receiver over the course of a day and throughout the year.
Receivers, such as photovoltaic cells, convert the solar energy focused and delivered to them by the solar concentrator directly into electrical energy or via heat engines (e.g. Stirling cycle engines) which convert the solar energy into mechanical energy which can be used directly, or indirectly converted to electricity.
There are many aspects and applications of this invention, which provides techniques applicable individually or in combination as an actuator, for motion control, and for positioning of the optical elements of a solar concentrator and similar equipment. The broad general concept of this invention relates to the actuation, motion production and control, and positioning resulting from an "Electric Wind" which herein shall mean the force due to electrons, ions, and neutrals in electric fields. The instant invention can perform dynamic motion control over a wide range of dimensions from nanometers to decimeters, i.e. from the nano-range, through the micro-range, through the mini-range to the macro-range in a broad scope of applications in micro-electro-mechanical systems (MEMS) such as a solar concentrator, and in similar equipment such as optical switching to macro-positioning. Motorless linear motion, angular deflection, and continuous rotation are achieved without recourse to magnetic fields thus eliminating the need for coils. Furthermore, the instant invention permits less costly and greater ease of manufacture while providing well-defined motion and position control.
It is a general aspect of this invention to provide a dynamic system for motion control of an optical system.
Another general aspect of this invention to provide a positioning system of an optical system.
Another aspect of this invention to provide an actuator for an optical system.
Another aspect of this invention is to provide the motive force for an optical system.
Another general aspect of this invention is to produce motion of the elements of an optical system in any direction as well as in the reverse direction.
Another aspect of the instant invention is to produce motorless linear motion of the elements of a solar concentrator.
Another aspect of this invention is to cause motorless angular deflection of the elements of a solar concentrator.
An aspect of the invention is to produce motorless continuous rotation of the elements of a solar concentrator.
An aspect of this invention is to produce motorless rotation with the ability to stop, of the elements of a solar concentrator.
Other aspects and advantages of the invention will be apparent in a description of specific embodiments thereof, given by way of example only, to enable one skilled in the art to readily practice the invention singly or in combination as described hereinafter with reference to the accompanying drawings. In the detailed drawings, like reference numerals indicate like components.
In order to better understand the various means and apparatus for production of an Electric Wind, some descriptions of the basic methodologies are given next.
Force due to Electrons, Ions, and Neutrals in Electric Fields
1. Electric Field Enhancement
Whiskers enhance an electric field near their tips. The electric field enhancement at the tip of the whisker is
to a good approximation, independent of the shape of the whisker (e.g. hemispherically capped whisker, cone, spheroid, etc.) where h is the height of the whisker and r is the tip radius. As long as the whisker height is small compared with the gap d, the electric field enhancement is independent of the size of the whiskers and just depends on the aspect ratio h/r. Thus two whiskers of widely different dimensions may have the same enhancement if h/r=h'/r'.
The enhanced microscopic electric field at the tip of a whisker is
where Emac is the macroscopic electric field that would be present at the tip location if the whisker weren't there. In the case of a nearly uniform macroscopic field,
The microscopic field falls off rapidly, so that at a distance ≧10 r (10 tip radii), the microscopic field differs by less than 1% from the macroscopic field.
For very close whisker separations, the enhancement decreases. A large density (close separation) of sharp whiskers is desirable to increase the total emission current as long as the separation between whiskers
At separations between whiskers greater than 10 tip radii, the enhanced microscopic field of each whisker falls off quickly enough with distance that it hardly affects the microscopic field of an adjoining whisker. Within the approximation δ>10 r, the total current is approximately proportional to the total number of sharp whiskers. One may understand why too high a density of whiskers is disadvantageous by noting that in the limit of contiguous whiskers of the same height, there is negligible enhancement of the electric field.
2. Negative Particle Emission Positioning and Motion Control
Let us first consider the emissive repulsive reaction force, Fe, on a cathode due to the emission of electrons so that we may better understand the various trade-offs that may be undertaken in the choice of parameters for the motive device 1.
F e = p t = t ( N m v ) = m v ( N t ) = m v ( I e ) , ( 2.1 ) ##EQU00001##
where p is the momentum of the emitted electrons, m is the mass of an electron, e is the charge of an electron, N is the number of emitted electrons which have attained velocity v as they emerge beyond the grid, and I is the emission current. The kinetic energy gained by the electrons as they emerge from the grid is equal to the potential energy given up in the electric field:
1 2 m v 2 = e V g v = [ 2 e V g m ] 1 / 2 , ( 2.2 ) ##EQU00002##
where Vg is the grid voltage. Substituting eq. (2.2) into eq. (2.1) we have the emissive force on the cathode:
F e = m [ 2 e V g m ] 1 / 2 ( I e ) = I [ 2 V g ( m e ) ] 1 / 2 . ( 2.3 ) ##EQU00003##
Eq. (2.3) implies that ions would exert a larger force than electrons, due to their larger m/e ratio. Please note that the derivation of eq. (3) is general enough that a current I of negative ions of charge e and mass m from the (negative) electrode 3 could also be carried predominantly by induced negative ion emission (cf. Sec. 3). Similarly, the geometry and largely uniform electric field shown in FIG. 1 is only illustrative. Eq. (2.3) gives the emissive force for any shape electric field, and even in the presence of space charge in the accelerating region. Electron and negative ion emission are presented as preferred embodiments of this invention; as is the grid structure.
Using eq. (2.2), the power expended Pe=IVg to produce the emissive force is
P e = F e v = F e ( 1 2 v ) = 1 2 F e [ 2 e V g m ] 1 / 2 , ( 2.4 ) ##EQU00004##
where ν is the average velocity. Eq. (2.4) says that to reduce the power expenditure in producing a given force, we want the charge carrier to have as large a mass to charge ratio as possible, and to make Vg as small as possible.
Let us look at some sample parameters to get an indication of the magitude of the force that is readily possible and the necessary power. The energy loss can be quite small as the power need be applied for only a short duration. For an electron emission current of 10 Amperes with Vg=40 Volts, Fe =21 dynes which is sufficient for many applications. The mass to charge ratio of electrons is only 5.6×10-12 kg/C. Negative ions can easily have mass to charge ratios that are more than 60,000 times higher than this. Thus negative ions would produce a force of Fe>500 dynes at the same voltage, one tenth the current, and a power requirement of only 40 Watts. As shown in Sec. 7, the corresponding electrostatic force is only about 4.4 dynes for comparable parameters.
3. Negative Ions
Since an important operating mechanism utilizes negative ions, let us briefly gain an insight into them. A negative ion is a neutral atom or molecule which has orbitally captured one or more electrons. The electron affinity of a given kind of atom or molecule is a function of both the species, and the medium it is in. It is equivalent to the ionization potential for removing the electron from the negative ion. The noble gases such as He, Ne, and Ar have a filled outer electron shell, are chemically inert, and do not form negative ions. Most atoms have positive affinities. The halogens such as fluorine, chlorine, bromine, and iodine which require only one additional electron to fill their valence shell, have the largest electron affinities for making negative ions.
Fluorine is very electro-negative, having one of the strongest electron affinities for making negative ions. However because of fluorine's toxicity and corrosiveness, it is generally incorporated as part of less active molecules like SF6 for electron capture. Oxygen also easily makes a negative ion--even double charged. But negative oxygen is not stable in the gas phase, although doubly charged O--is stable in solution illustrating that the ability to form negative ions is also a function of the medium the atom is in.
For production of positive ions, the maximum ionization cross-section is at about 10 times the ionization potential. The ionization cross-section falls steeply below this value, and falls slowly above this value. There is no ionization below the ionization potential, and not much ionization up to 5 times the ionization potential. So for small devices for which the instant invention is exceptionally suited, high electric fields are produced while operating at low voltages. So in this case, there are few if any positive ions, with mainly free electrons and negative ions.
One method of production of negative ions is by the field or thermionic emission of electrons from the cathode at low voltage. Bear in mind that with small spacing d, a large electric field can be created even with small voltage. At these low voltages, and polarization of molecules in the high electric field gradient of the whiskers (refer to Sec. 5), the emitted electrons can attach to electro-negative gas molecules such as SF6. This is referred to herein as induced negative ion emission.
4. Positive Ion Pressure Production
By means of a corona discharge a partially ionized medium may be created with predominantly positive ions, electrons, and few if any negative ions. The medium may be a liquid, or a gas at moderate to above atmospheric gas density. We shall concentrate on the frictional momentum transfer by the positive ions to the ambient gas. Momentum transfer by electrons is relatively low due to their small mass. Momentum transfer by negative ions is relatively small due to their scarcity. In equilibrium, the body force (force/volume) on the ions due to an electric field E is balanced by the gradient in pressure P. In the one-dimensional case:
P x ≈ ρ E , ( 4.1 ) ##EQU00005##
where for small ion currents the viscous losses are negligible, and ρ is the charge density of the ions. This tells us that the pressure is greatest where the electric field is greatest.
The ion current density j is
j = ρ v = ( μ E ) ρ = j μ E , ( 4.2 ) ##EQU00006##
where μ is the mobility. Substituting eq. (4.2) into (4.1):
P x ( j μ E ) E = j μ ∫ P o P P = ∫ 0 x j μ x . ( 4.3 ) ##EQU00007##
We can do a direct integration of eq. (4.3) since j is independent of x, and μ is a slowly varying function of E/P making it to a good approximation independent of x:
∴ P - P o = jx μ . ( 4.4 ) ##EQU00008##
From eq. (4.4) we note that to produce a high pressure differential we want a low mobility μ, and hence heavy ions. We also want a large current density j, and hence we want the ions to be multiply ionized if possible. Thus the total force due to the electric field acting on the positive ions is
F i = P A = ∫ ∇ P Ω ≈ Id μ , ( 4.5 ) ##EQU00009##
where I is the ion current, and dΩ is a volume element.
Even though only a small fraction of the neutral atoms and molecules are ionized, the space charge conditions determine the electric field configuration and the ion flow. For this one-dimensional case, by eq. (4.2) Poisson's equation becomes
E x = ρ = j μ E . ( 4.6 ) ##EQU00010##
Integrating eq.(4.6) and substituting eq. (4.4):
∫ E o E E E = ∫ 0 x j μ x P - P o = jx μ = 1 2 [ E 2 - E 0 2 ] . ( 4.7 ) ##EQU00011##
Since E=-dV/dx, we can integrate eq. (4.7) to get V:
V = ∫ 0 x [ E 0 2 + 2 jx μ ] 1 / 2 x = - μ 3 j [ ( E 0 2 + 2 jx μ ) 3 / 2 - E 0 3 ] . ( 4.8 ) ##EQU00012##
Considering the case where the EO terms are negligible, and substituting eq. (4.4) into (4.8):
V = - μ 3 j ( 2 jx μ ) 3 / 2 = - ( 8 jx 3 9 μ ) 1 / 2 = - [ 8 x 2 9 ( P - P 0 ) ] 1 / 2 . ( 4.9 ) ##EQU00013##
The input power Pi needed for the ions to produce the pressure differential by eq. (4.9) is
P i = IV = - jA ( 8 jx 3 9 μ ) 1 / 2 = ( 8 I 3 x 3 9 A μ ) 1 / 2 = - I [ 8 x 2 9 ( P - P 0 ) ] 1 / 2 . ( 4.10 ) ##EQU00014##
We can eliminate the ionic current I from Pi by using eqs. (4.8) and (4.4)
P i = IV ≈ - μ I 3 j ( 2 jx μ ) 3 / 2 = - μ A 3 ( 2 jx μ ) 3 / 2 = - μ A 3 ( 2 ( P - P 0 ) ) 3 / 2 . ( 4.11 ) ##EQU00015##
In vacuum, only emitted electrons participate in the mechanism for the motive force. As soon as a medium is introduced, a number of mechanisms may be at work simultaneously. For example, the positive ion space charge electric field at the cathode can make a sizable contribution to electron field emission, as well as producing a pressure of its own.
5. Neutral Dielectric Polarization Positioning and Motion Control
For completeness, we should consider yet another mechanism which can be utilized to produce a motive force. On any size scale, as soon as a medium other than vacuum is utilized, new effects in addition to electron flow enter in. Let us next examine another mechanism for positioning and motion control. The force in the emission case results from the flow of charged particles. It is also possible to exert a force when the medium is a neutral dielectric with a permanent dipole moment D, and a polarizability α. The potential energy Vp of a neutral particle in this field is
V p = - DE - 1 2 α E 2 . ( 5.7 ) ##EQU00016##
The polarization force is--the gradient of Vp in eq. (5.7)
F p = - ∇ V p = D ∇ E + 1 2 α E 2 = D ∇ E + α E ∇ E . ( 5.8 ) ##EQU00017##
Eq. (5.8) says that when the medium is a neutral dielectric, it is attracted to a region of increased electric field, provided that its dielectric constant is greater than that of the surrounding area. The force on the dielectric medium is proportional to the gradient of the electric field for the permanent dipole moment term. It is proportional to the gradient of the electric field squared for the induced dipole moment term; or equivalently, proportional to the electric field times the gradient of the electric field The polarization that the field produces on both permanent and induced polar moments is proportional to the electric field strength. The force on the dielectric medium is proportional to the product of the electric field strength and the degree of polarization, resulting in the quadratic dependence. In a uniform field, the field exerts equal and opposite forces on the polarized particle resulting in no net force. With a field gradient, the net force on the particle is always toward the increasing gradient independent of the direction of the field. This is also true for an alternating field, as the polarized particle simply rotates or changes the direction of its induced polarization to accommodate the changing field. So the device can operate on either dc or ac power. Note that the grid structure shown in the various embodiments of the instant invention is also important for the neutral molecules motive force. It was clear that the grid structure is important to the electron and ion motive force because it maintains a given electric field independent of the motion. This is also true of neutral molecule motive force since maintaining a given electric field serves to maintain a given gradient of the electric field. Both for neutrals and positive ion pressure of Section 4, the force and the pressure is greatest where the electric field is greatest.
There are similarities between this mechanism and the outward electrostrictive pressure of a dielectric in a capacitor as both involve polarization of the dielectric. Just as the repulsive electrostrictive force can exceed the attractive capacitive electrostatic force, so can the polarization force given by eq. (5.8) when the field gradient is sufficiently strong. The body or volume force (force/volume) on a dielectric is
F / Ω = ρ E = 1 2 E 2 ∇ κ + 1 2 ∇ [ E 2 μ κ μ ] , ( 5.9 ) ##EQU00018##
where Ω is the volume, ρ is the charge density, κ is the dielctric constant, and μ is the mass density. The third term is the electrostrictive term.
The power disipated is related to the dot product of the force given by eq. (5.8) and the mean flow velocity ν:
6. Temperature Related Pressure Effects
An elevation of temperature and temperature gradients with concomitant pressure elevation and pressure gradients are produced by power dissipation in the many physical motive mechanisms. So it is appropriate to briefly discuss this effect which although it has subtle ramifications simply amounts to the fact that if one side of an electrode is warmer than the other, particles will recoil with greater momentum transfer from the warmer side creating a pressure differential. For low ambient pressure i.e. low gas density n, where the particles have long mean free paths, differences in temperature T and pressure P are easier to maintain, and the net force is proportional to the macroscopic area A of the electrode.
Low Ambient Pressure
For equal number density of molecules on both sides of the electrode, i.e. for n2=n1:
P 2 P 1 = n 2 T 2 n 1 T 1 = T 2 T 1 , ( 6.1 ) ##EQU00019##
The net force in this case is
High Ambient Pressure
However as the ambient pressure is increased, an increase in temperature reduces the gas density over the interior surface of the electrode. This reduces the collision frequency with the electrode and hence the pressure exerted on it making the pressure in this interior region of the electrode roughly equal on both sides. This leaves only a strip of width w and length l around the perimeter of the electrode where eq. (6.1) applies. The net force in this case is
where wl<<A. Near the edges of the electrode the temperature differential decreases since the high gas collision frequency reduces the temperature on the warm side to T'2 and increases it to T'1 on the cool side maintaining the relationship T'2>T'1. Even if T'2/T'1=T2/T1, the purely temperature related pressure effect goes away as w→0.
7. Comparing Emissive Force with Electrostatic Force
An electrostatic force could also be used to align the mirrors. The attractive force, Fc, due to the electrostatic field between conducting plates is
F c = 1 2 E 2 A = 1 2 [ V d ] 2 A , ( 7.1 ) ##EQU00020##
where ε is the permittivity of the space between the plates, E is the macroscopic electric field between them, V is the voltage across them, and d is the gap between the plates. For air, ε˜8.85×10-12 Farad/m, with V=100 Volts, d=10 -1 cm, and A=102 cm2, we find Fc=4.4 dynes. This is a relatively small force, and shows that the electrostatic force is only dominant at high voltages when d is comparable for the different mechanisms.
Contrasting Emissive and Electrostatic Forces
Let us contrast the electrostatic attractive force, Fc in eq. (7.1), with the emissive force Fe as given by eq. (2.3). For comparison with Fe, let the plates have the same gap d as between the grid and the cathode, with the same voltage Vg between them so that E=Vg/d. Thus Fe>Fc when
F e F c = m [ 2 e V g m ] 1 / 2 ( I e ) 1 2 E 2 A = I [ 2 V g ( m e ) ] 1 / 2 1 2 [ V d ] 2 A > 1 ( 7.2 ) ##EQU00021##
Eq. (7.2) implies that the emissive force is greater than the electrostatic force if
V g < [ 2 Id 2 A ] 2 / 3 [ 2 m e ] 1 / 3 = [ 2 jd 2 ] 2 / 3 [ 2 m e ] 1 / 3 , ( 7.3 ) ##EQU00022##
where j is the macroscopic current density in the emissive device. This means that there are a range of parameters for which the emissive device of the instant invention gives greater forces than a corresponding electrostatic device.
Practicality of Emissive Force
Let us illustrate that the field emission electron emissive force dominates for sensible parameters, not just for extremely small nano-parameters and devices. It is thus well-suited for the nanometer to the decimeter range. We note that for d=105 Å=10-5 m, j=103 A/cm2=107 A/m2, with e=1.60×10-19 Coulomb, m=9.11×10-31 kg, and ε=8.85×10-12 Farad/m for vacuum and air eq. (6) says that for Vg≦82 volt, the electron emissive force is dominant. For a voltage, Vg=40 volt, this gives Emac=40 volt/10-5m=4×106 volt/m=4×104 volt/cm. To achieve sufficient field emission we need a microscopic electric field Emic>109 volt/m=107 volt/cm, which requires an easily achieved enhancement factor of β>250. Emac is well below the electrical breakdown field (cf. McGraw-Hill Encyclopedia of Science & Technology, Mario Rabinowitz on Electrical Insulation in any of the editions from 1982-20002). Air at atmospheric pressure has an electrical breakdown field of Ebkn˜105 V/inch=4×104 V/cm for d˜10-1 cm; and for d˜10-3 cm=105 Å=10-5 m, Ebkn is considerably higher. In vacuum (<10-5 Torr=1.3×10-3 Pascal) Ebkn˜2×105 V/inch=8×104 V/cm for d˜10-1 cm; and for d˜10-3 cm=105 Å=10-5 m, Ebkn is considerably higher.
Negative ions readily have mass to charge ratios 60,000 times higher than for electrons. Since by eq. (7.3) Vg is proportional to the cube root of the mass to charge ratio, Vg would be 39 times higher for Fe>Fc for the same parameters.
For small devices, when d is small or comparable to the electron mean free path in the ambient gas, then the motive device 1 operates effectively as if it were in vacuum. In this case operation at atmospheric pressure, is much the same as operation in vacuum. Air at standard temperature and 1 atmosphere pressure has a number density of molecules of n˜3×1019 molecules/cm3. The average spacing between molecules is n-1/3˜3×10-7 cm=30 Å. The mean free path of molecules is ˜10-5 cm=1000 Å. The mean free path of electrons can be much higher than this.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of an electric wind motive device for mirror alignment that produces a force by the emission of electrons.
FIG. 2 is a cross-sectional view of another electric wind motive device for mirror alignment which produces a force by the collection of ions and electrons.
FIG. 3 is a cross-sectional view of an angular deflection device for mirror alignment which moves to a given detent position.
FIG. 4 is an illustration of a solar concentrator array which tracks the sun and focuses sunlight onto a receiver.
FIG. 5 is a block diagram flow chart summarizing a method in which a positive feedback system can optimize the tracking and focusing of the solar concentrator array.
The following is a glossary of components and structural members as referenced and employed in the instant invention with like reference alphanumerics indicating like components: 1--electric wind motive device (electron emission) 2--electric wind motive device (positive ions and electrons) 3--angular deflection device 4--solar concentrator array 1--electron 2--whisker (enhances electric field) 3--electrode d--gap 4--grid 5--field free region 6--ultimate collector electrode 7--dielectric support 8--positive ions 9--wire 10--indented track 11--detent positions 12--operating medium 13--pivot point 14--one of a pair of mirrors 15--the second of a pair of mirrors 14' --optical elements (mirrors) of an array 16--mirror support (dielectric) 17--flexible protrusion (fits into detent positions) 18--inclined support frame 19--receiver (receives or collects light e.g. solar cell, Stirling cycle heat engine) 20--optimizing sensor (e.g. photomultiplier)
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
As is described here in detail, the objectives of the instant invention may be accomplished by any of a number of ways separately or in combination, as taught by the instant invention. A tracking solar concentrator has been developed in which the orientation of individual optical elements (mirrors, reflectors, lenses) is accomplished by creating an electric wind to align them consecutively or concurrently without the need for expensive, bulky, and heavy motors. Thus the improved solar concentrator of the instant invention can be more reliable and lighter than conventional solar arrays.
FIG. 1 shows a cross-sectional view of one embodiment of an electric wind motive device 1, which in this case produces a force by the emission of electrons 1 from whiskers 2 on an electrode 3 that serves as cathode. Upon leaving the whiskers 2, the electrons 1 cross the gap d to pass through a grid 4 at voltage Vg to enter a region 5, and then are collected at the ultimate anode collector plate electrode 6 at voltage V>Vg. When V=Vg, region 5 is field free. A dielectric 7 supports the grid 4 and electrically isolates the grid 4 from the electrode 3. The configuration of the grid 4 at fixed separation d from the electrode 3 serves to maintain an unchanging macroscopic electric field as motion is imparted to electrode 3. Otherwise the cathode-anode gap would change, requiring a modulation of the applied voltage to control the motive force. A repulsive force is produced to the left as shown by the arrow and as explained by the analysis in the SUMMARY OF THE INVENTION section. This section teaches how electric wind motive force can be obtained utilizing electrons, positive ions, negative ions, and/or neutrals. If the device 1 operates in the purely field emission mode, reversing the voltages will stop the emission and there will be no emission force Fi. Thus a spatially reversed similar device 1 is needed for reverse motion to the right, such as illustrated in the embodiment of FIG. 3 for motion to the left or right.
FIG. 2 is a cross-sectional view of another embodiment of an electric wind motive device 2 which produces a force Fi to the left, as shown by the arrow, in this case by the collection of electrons 1 and positive ions 8 produced for example by a corona discharge. As an alternate way of introducing whiskers 2 inside the device 2, wires 9 upon which whiskers are grown (cf. Mario Rabinowitz, Emissive Flat Panel Display with Improved Regenerative Cathode, U.S Pat. Nos. 5,697,827, 5,764,004, and 5,967,873), are attached to electrode 3. A dielectric 7 of length d supports the grid 4 and electrically isolates the grid 4 at voltage Vg from electrode 3. An ultimate collector plate electrode 6 is at voltage |V|≧|Vg| When V=Vg, region 5 is field free. The configuration of the grid 4 at fixed separation d from electrode 3 serves to maintain an unchanging macroscopic electric field as motion is imparted to electrode 3 due to the force of the ions as can be seen from the analysis which follows in Sec. 4. With Vg positive, the force is to the left as shown by the arrow. A spatially reversed motive device 2 can be used for reversed motion to the right, such as illustrated in FIG. 3 for motion to the left or right.
FIG. 3 is a cross-sectional view of an angular deflection device 3 which moves to a given detent position. Shown is an indented track 10 which can be situated below the mirrors 14 and 15, with detent positions 11 operating in a medium 12 so that the device may rotate about the pivot point 13. Motion in other directions is possible with a triangular arrangement of pairs of devices 3 with two devices near the bottom and one near the top of the mirrors 14 and 15. The application of a voltage V1 to conducting mirrow 14 and/or V2 to conducting arm 15 produces a large microscopic electric field with a high field gradient at the whiskers 2 so that the device 3 may operate by any of the physical mechanisms described in the body of the specification. As shown here, the electric wind motive device can operate without the grid 4, dielectric 7, etc. which are shown in FIGS. 1 and 2, but the preferred embodiment includes these items. The main point of FIG. 3 is the illustration of mirror alignment. The mirror surfaces 14 and 15 can be conductors which are separated by a dielectric 16. Alternatively, thin film conductors can be laid down below (or even above) the mirrors to supply the given voltages, with a minimal blockage of sunlight. Although one mirror 14 is sufficient, the advantage of double back-to-back mirrors 14 and 15 is that when the top mirror becomes occluded with grime, the mirrors 14 and 15 can be rotated 180 degrees about the pivot 13 exposing on top the clean mirror that was previously on the bottom. This roughly doubles the time before cleaning of the mirrors is necessary. It also makes possible an uninterrupted duty cycle of the concentrator, as the grimy occluded mirrors facing downward can be cleaned, while the mirrors on top perform their duty. The mirrors can be flat or slightly concave.
There is a flexible protrusion 17 at the radial end of the dielectric 16 which fits into the detent positions 11. The protrusion 17 can be retracted during the alignment process and spring loaded to engage a detent position 11 after each alignment process is complete. The mirror can remain in alignment with the voltages removed, until a new alignment is desired. With three or more such orthogonal detent indented tracks 10, and corresponding protrusions 17, any necessary mirror orientation is possible. Only part of the indented track 10 is shown in FIG. 3. The indented track 10 can be partially or entirely circumferential around the pivot point 13, or be positioned down below the mirrors 14 and 15 so as not to block light. Similarly, the flexible protrusion 17 (that fits into the detent positions) can extend down below the mirrors 2 to engage the indented track 10 below the mirrors 14 and 15. Any part of the indented track 10 above the mirrors 14 and 15 can be narrow to minimize light blockage. Below the mirrors, the indented track 10 may be somewhat hemispherical. In FIG. 4, the indented track 10 is entirely below the mirrors, and is thus not shown. Furthermore, the indented track 10 can act as the ultimate collector that is indicated by the numeral 6 in FIGS. 1 and 2.
FIG. 4 is an illustration of a presently preferred embodiment of a solar concentrator array 4 that tracks the sun and focuses sunlight onto a receiver. The solar concentrator array 4 is comprised of an ensemble of optical elements 14' that are mounted on an inclined support frame 18. The optical elements 14' are the mirrors, reflectors, focusers, etc. of a concentrator array. As a focuser an optical element 14' may be a lens or include a lens. The inclination angle of the support frame 18 can be adjusted to accommodate geographic location such as latitude, and/or inclination of the surface upon which it is mounted such as the ground or a rooftop. The desired inclination is to be oriented toward the sun to minimize the needed angular alignment in terms of attitude and azimuth. Ideally, the array 4 surrounds the receiver 19 with the optical elements (mirrors) 14' being individually adjusted by means of electric wind devices such as 1 and 2 of FIGS. 1 and 2 to maximize the amount of energy flux (solar power) reflected to the receiver 19 on a daily and hence on an annual basis. In FIG. 4, the electric wind devices are below the optical elements, but may also be above with minimal occlusion. In FIG. 4, the somewhat hemispherical indented track 10 of FIG. 3 is below the elements 14', and as such is not shown. FIG. 4 also does not illustrate the mirror pairs described in conjunction with FIG. 3. For rotation of the bottom mirror of FIG. 3 into the upward top position, the swivel or pivot 13 axis is situated adjacent to an element's (mirror's) 14' side edge. Mounted on the receiver 19 is an optimizing sensor 20 that maximizes the reflected light from each element 14' of the array 4 as is described in conjunction with FIG. 5.
FIG. 5 is a block diagram flow chart summarizing a method in which a positive feedback loop can optimize the tracking and focusing of the solar concentrator array 4. As discussed in U.S Pat. No. 6,988,809 (Rabinowitz, Jan. 24, 2006), an approximate alignment (orientation) calibration may be done at the factory using GPS (Global Positioning Satellite) input related to the final placement site and orientation, with in-situ fine tuning at the installation location. Positive feedback optimization can only work when there is an initial signal to guide the feedback process by means of a digital processor.
Preferably with the sun overhead on a clear day, the orientation of the elements (mirrors) 14' can be adjusted to maximize the power output of the receiver 19. For example an optimizing sensor 20 such as a photomultiplier can be attached to the receiver 19 and used to increase sensitivity to the reflected light signal by means of positive feedback. On cloudy days, the optimizing sensor 20 or just the receiver 19 can be used to find that orientation of the elements 14' that maximizes the receiver 19 output. This orientation may be away from the sun, and point toward the most reflective cloud. The elements 14' may be individually oriented, or groups of elements 14' may be collectively oriented to simplify tracking and focusing. When groups are collectively oriented, as a group they may have a projected group concavity to aid in the focusing to the receiver 19.
An optimizing sensor 20 such as shown in FIG. 4, directs the steps shown in the block diagram flow chart of FIG. 5. A given element 14' is actuated by an electric wind device such as 1 and 2 of FIGS. 1 and 2 to adjust the alignment of the selected optical element 14' in a first angular direction. If the signal from the optimizing sensor (photomultiplier) 20 increases in response to the change, the element (mirror) 14' is again adjusted in the same angular direction. This process is continued until a slight decrease is perceived. Then the element 14' is adjusted in the opposite direction, increasing the signal back to its prior value. Now smaller steps are taken back and forth until a maximum signal is obtained. This process is then repeated in a additional angular directions until the alignment of an element 14' is optimized. This process is then continued element by element until all the elements are optimized in their alignment. Computer readable memory stores the location of each element 14'. Alternatively, groups of elements 14' can be similarly optimized in their alignment as a whole.
Now that the instant invention has been described and the reader has a reasonable understanding of it, we can more clearly discuss its advantages with respect to other possible concentrator alignment mechanisms.
1. One of the most important advantages of the invention is related to ease of calibration, and response speed in producing a given desired motion or deflection by means of an electric wind. Electromagnetic systems are inherently slower in responding to an input signal because of self-inductance and mutual-inductance effects. Magnetic systems are inherently slower because of the time the magnetic field takes to diffuse into a conducting medium after it is applied. Furthermore, it would be unwieldy to put motors on each element (mirror) of an array.
To have only one or a few motors accomplish alignment of an array by mechanical coupling means such as long rods and turnbuckles, would be a slow, cumbersome, and painstaking process. In a cloudless sky, speed of alignment may not be critical for a solar concentrator, due to the slowly changing position of the sun relative to the earth. However with fast moving clouds, speed can be a decisive factor. In those cases where speed is not important, the electric wind can produce a low thrust force with negligible losses in a low friction system.
Unlike systems where the motive force varies with the deflection, the fixed position grid in the instant invention allows the electric field and hence the motive force to be constant, independent of deflection, linear motion, or rotation. It is shown by the analysis in the SUMMARY OF THE INVENTION section this can be accomplished for practical operating parameters.
2. The analysis throughout the body of the specification indicates that the power requirements to produce the Electric Wind by the different mechanisms of the instant invention are moderate. The reason the power calculations are presented, is to enable not only a comparison of the different mechanisms with respect to power consumption, but also a comparison with other motive mechanisms, such as magnetic which require more power.
3. Electromagnets in general and electromagnetic motors in particular become quite inefficient as they are scaled down in size. The present invention is more amenable to miniaturization such as required in nanotechnology.
4. One may raise a question regarding whisker lifetime. This is clearly a much less serious problem than for field emission flat panel displays. The instant invention is more robust in this respect. In field emission flat panel displays, even if whisker depletion occurs randomly it affects both pixel intensity and color creation. The instant invention is tolerant of random whisker depletion since the emitted current can be maintained constant by increasing the voltage to get the same force and hence same deflection. Calibration can be done with respect to current rather than voltage. Whisker regeneration in situ is possible as taught in Mario Rabinowitz, Emissive Flat Panel Display with Improved Regenerative Cathode, U.S Pat. Nos. 5,697,827, 5,764,004, and 5,967,873. Thus the effects of whisker tip dulling can be mitigated both by regeneration and by separate control of emission.
5. The combination of a moderate electric field and thermionic emission by heating the emitting whiskers is called Schottky emission. The electric fields in this invention go from moderate to high. In the high field case of thermo-field assisted emission, the emission can greatly surpass Schottky emission. One substantive aspect of thermo-field assisted electron and negative ion induced emission, is that a given current can be sustained at substantially lower temperature than if the process were solely thermionic emission. The enhanced electric field greatly assists thermionic emission. Concomitantly, the thermal aspect of moderately elevated temperature of the cathode assists emission due to the small field lowered barrier (effectively decreased work function), and tunneling through the barrier produced by a high electric field. Synergistically, the two aspects help each other in working together to produce notably higher emission rates than each alone. As taught in the immediately above-mentioned patents, the combination of thermal elevation and field elevation capability in the same cathode permits a novel regeneration of electric field enhancing whiskers on the cathode.
Scope of the Invention
While the instant invention has been described with reference to presently preferred and other embodiments, the descriptions are illustrative of the invention and are not to be construed as limiting the invention. Thus, various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as summarized by the appended claims together with their full range of equivalents. It is to be understood that in said claims, ingredients recited in the singular are intended to include compatible combinations of such ingredients wherever the sense permits. It should be recognized that the methods and apparatus of this invention can be used in other contexts than those explicitly described herein. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall there between.
Patent applications by Mario Rabinowitz, Redwood City, CA US
Patent applications in class With means to reposition solar collector for optimum radiation exposure
Patent applications in all subclasses With means to reposition solar collector for optimum radiation exposure