CA2205435A1 - Balloon - Google Patents

Abstract

Light disposable balloon borne grounded wires for directing lightning to safe ground areas and prevent forest fires. When the wire is raised to sufficient altitude, a leader emerges from the top end of the wire and propagates toward the cloud, creating a lightning rod reaching nearly to the cloud.
Carbon fiber offers exceptional lightness, high temperature resistance and such fineness so as not present any hazard to aircraft navigation.

Description

~ CA 0220~43S 1997-0~-14 The object of this invention is an apparatus for protecting large ground areas from lightning by directing the lightning strikes to a safe ground spot.
In particular, it is intended to reduce the number of lightning initiated forest fires.

PRIOR ART

Lightning rods used to protect buildings have a protection coverage limited to a radius equivalent to their height. Conductive wires raised by tethered permanent aerostats could provide protection over a large area, but would be uneconomical and would present a hazard to aircra~ft navigation.

A laser produced conductive ionized path is claimed in U.S. Pat. 4,017,767.
U.S. Pat. 3,371,144 related to the protection of transmission lines and the book "LIGHTNING" by M. A. Uman (McGraw-Hill Book Co.) give a broad overview of the subject and the related scientific literature.

CA 0220~43~ 1997-05-14 Rocket launched wires are routinely used in scientific research for artificially triggering strikes in order to study one component of the strike, the return stroke which happen to be similar to the return stroke of a natural strike.
The use of rockets is however impractical for lightning protection since the timing and angle of launch is very critical and the wire does not stay up long enough. In addition, copper wire is used; because of its relatively low melting temperature, the high current pulses frequently cause it to melt prematurely failing to leave behind a conductive ionized path for the lightning strike to occur.

SUMMARY

In natural lightning, a strike is preceded by a downward negatively charged leader that emerges from a negatively charged bottom portion of the cloud progressing in steps toward the positively charged ground. As this leader gets closer to ground, a short positive upward leader emerges from the ground and meets the downward leader establishing a conductive path for the actual strike.

CA 0220~43~ 1997-0~-14 Grounded objects in the presence of the electric field created by a thunderstorm charge emit a glow of positively charged ions. The intensity of this glow increases with height in the field and intensity of the field. From studies of glow from tall structures and rocket launched wires, it is known that once the field across the length of the grounded wire exceeds a critical value, the glow will transform into an arc or a leader that will continue to propagate as long as the field gradient is sufficient The combination of the grounded wire and propagating leader can be viewed as a lightning rod rapidly growing in length toward the charged portion of the cloud As the tip of the leader approaches the cloud charge, the field gradient between the two of them exceeds a critical value and a downward negatively charged leader emerges from the cloud charge to meet the upward leader. The distance between the meeting point and the ground connection is the effective protection range; it is almost the height of the bottom of the cloud, about 4 km for a typical thunderstorm cloud.

Computer simulations of a rocket launched grounded wire in an electric field with a gradient of 40 kV/m at altitude 100 m and 80 kV/m at altitude 1000 m, -- CA 0220~43~ 1997-0~-14 typical when an average thunderstorm is directly overhead, indicate that the progress of the upward leader becomes continuous and self propagating above an altitude of 180 m as the ambient electric field across the length of the wire exceeds about 6.5 MV.

Therefore with a wire raised to a rather modest height, we can obtain the lightning protection range of an equi\lalent lightning rod having 20 times that height. This range can be further extended by having a mid-portion of the wire non conductive and extending the overall wire length. A range of close to 10 km could thus be reached. Thus large ground area could be protected with quite sparsely distributed wires. Such wires could present a hazard to aircraft navigation however, unless they can be made so fine as to be easily and safely ruptured. This requirement is conflicting with the need of being able to sustain large current peaks and not melt prematurely before a conductive ionized path is established. I have determined that high temperature melting wire material and carbon fibers in particular can carry a sufficiently large current and reach a sufficiently high temperature to ionize thermally the surrounding air and reliably create a conductive CA 0220~43~ 1997-0~-14 ionized path for the leader to continue propagating after the wire has disintegrated and still be made so fine as not present a hazard to aircraft navigation. In addition, carbon fibers being very light requires smaller balloon and less lifting gas.

The balloon borne wires can be manually launched from the ground in advance of thunderstorm clouds or at the beginning of the lightning or dry season. The combination of balloon/wire and grounding rod can also be dropped from an aircraft so that the grounding rod implants itself into the ground, for inaccessible locations or in advance of a thunderstorm.

The balloon borne wires can be remotely released upon the approach of a thunderstorm by rangers manning the lookouts. They can be automatically launched by having for instance a capacitor charged by the electric field of the thunderstorm discharge at a preset voltage and trigger a helium filled cannister to inflate one or more balloons, the inflating of the balloon in turn triggering a guillotine to cut off the balloons free. Additional balloons can be automatically launched to replace stricken ones by having lightning ~ CA 02205435 1997-05-14 striking previously kaunehed wires sequentially trigger additional launchings.

The preferred embodiments of this invention are illustrated in the following drawings, of which:
FIG. 1 is a view of a balloon borne continuously conductive wire;
FIG. 2 is a view of a balloon borne wire having a mid portion non conductive for extended range;
FIG. 3 shows the means for launching balloon borne wires either remotely or automatically;
FIG. 4 shows means for sequentially launching addition~l balloon borne wlres;
FIG. 5 shows an aircraft dropped wire a~er implanting itself into the ground.

~n an err bodiment of the invention, FIG. 1 shows a conductive wire 1 of substantial length having a lower end grounded to earth through the grounding means ~. and an upper end raised to altitude by balloon 3 thereby extending the wire up to altitude. A high temperature resistant thermally and elec,trically insulating connector 5 joins the upper end of wire 1 to balloon 3 so as to thermally protect balloon 3.

Balloon 3 is a disposable balloon filled with helium and may be made of metalizecl film or n~bber. The wire length is determined by the expected ambient field encompassing the length of the wire. An encompassing field of about ~.5 MV is required for a self propagating leader to occur. For an average thunderstorm directly overhead the wire, the length may be as short as 180 m; however for a thunderstorm some distance away, the weaker field will ne!cessitate a longer wire. However the longer wire may trigger a strike from a cloud passing overhead that would not normally have produced natural lightning strikes, thereby wasting a wire and balloon unnecess,arily. Therefore a tradeoff has to be made between longer range longer wires widely separated and shorter range shorter wires more closely spaoed. The optimum choice depends on the expected number of strikes per storm ~nd the economics of the gridwork deployment of the wires.
Generally, the length should be sufficient for the wire to an altitude such that when in the electric field of a thunderstorm, it attracts lightning and direct it to the safe ground area.

CA 0220~43~ 1997-0~-14 However, it is preferable to strive for a length such that when in the electric field of a thunderstorm not quite sufficiently strong for a natural lightning strike to occur, the ambient electric field encompassing the wire is sufficiently strong to cause an upward leader to emerge from the top end of the wire and self-propagate indefinitely toward the thunderstorm cloud until a lightning strike is triggered from the cloud to the tip of the leader and along the ionized conductive path left after the wire is thermally vaporized.

For a more extended lightning protection range, wire 1 may have a non conductive mid-portion as shown in FIG. 2 wherein wire 1 has a conductive upper portion 7, an intermediate non-conductive portion 9 of substantial length relative to the upper portion and a bottom conductive portion 11 whose lower end is grounded to earth. Not being grounded, the upper conductive portion 7 needs a stronger field across its length for leader discharges to emerge from its ends. Therefore it has to be both longer and extend closer to the cloud, where the field gradient is stronger.

CA 0220~43~ 1997-0~-14 Such a combination of wire has been rocket launched to simulate and study intra cloud lightning strike between the upper positive charge and lower negative charge of a cloud; here we use the same combination for a different application, namely discharging the negative bottom charge of the cloud safely to the ground. The scientific literature reports that when the upper conductive portion is in a field approximately 10 MV across its length, an upward positive leader emerges from its top end self-progressing toward the negative cloud charge and a few milliseconds later, a downward negative leader emerges from its bottom end and quickly jumping across the non-conductive intermediate portion 9 joinsa short positive leader emerging from the top end of the bottom grounded conductive portion 11;
thus a continuous conductive path is established from the ground to the still progressing top leader. Very quickly though the current increases so much that the conductive wires melt leaving behind an ionized conductive path that keeps the leader progressing for many hundreds of seconds toward the thunderstorm cloud until it gets close enough to trigger a lightning strike along the conductive path.

CA 0220~43~ 1997-0~-14 The wire portions could simply have substantial length, however in order to avoid unnecessary artificial lightning strikes, the length of the upper conductive portion 7 is preferably made and the length of the intermediate portion 9 is such as to raise the upper portion 7 to suffcient altitude so that when in an electric field of a thunderstorm not quite sufficiently strong for a natural cloud to ground lightning strike to occur, the electric field encompassing the upper portion is sufficiently strong to cause an upward leader to emerge from the upper end of the upper portion and self-propagate indefinitely toward the thunderstorm cloud and a downward leader to emerge from the lower end and jump across the intermediate non-conductive portion 9 to the bottom conductive portion 11, so as to bridge conductively the gap between the upper portion 7 and the bottom portion 11 and thereby establish a continuous conductive path from the grounding means to the tip of the self-propagating upward positive leader.

The bottom conductive portion 11 may be rather short, about 50 to 100 m and serves to direct the strike to the designated safe ground spot.
Insulating spacers 13 insulate the lower melting temperature intermediate CA 0220~43~ 1997-0~-14 portion from the hot upper and bottom portions, keeping it from melting prematurely.

WIRE MATERIAL

Very early during the leader self propagation, the current increases so much that the wire quickly melts. If prior to melting the temperature is high enough to thermally ionize the surrounding air, then a conductive ionized path will survive the wire and allow the leader to continue progressing toward the thunderstorm cloud and establish an ionized conductive path reaching close enough to the thunderstorm cloud to trigger an artificial cloud to ground lightning strike along the conductive path.

At 1000~K, air is a good insulator; at 4000~K, it is in an ionized state and is a good conductor. Therefore a high melting temperature material is preferred. As a matter of fact, copper commonly used in rocket launched wire has a rather low melting temperature and has a high rate of failure in triggering artificial lightning.

CA 0220~43~ 1997-0~-14 The highest current carrying capacity of a wire prior to melting is achieved when the resistive power generated within the wire can be radiated at the surface according to the following formula for a unit length round wire:

4 p I2 , or d2 3.74. 10 ( ) d K
p where I = current, A p = resistivity, Qm ~ = emissivity d = diameter, m 6 = 5.67 x 1 o-8 W/m2/~K4 K = absolute temperature (Boltzmann constant) (~C + 273) This equation shows that temperature and diameter, hence density, are the dominant controlling factors for the highest current carrying capacity prior to melting. The highest temperature conducting materials are carbon fibers, tungsten and molybdenum.

~ CA 0220~43~ l997-0~-l4 The following table summarizes the relevant properties near the melting temperature, comparing the required wire diameter and weight of a 100 m long wire capable of carrying 10 A prior to melting:

MELTINGRESISTIVITYDENSITYEMISSIVITYDIAMETERWEIGHT (g) TEMP (oK)(~S2m) (g/cm3) (cm) PER 100 m PAN CARBON FIBER 3925 32 1.8 0.85 0.048 33 TUNGSTEN 3683 1.2 19.3 0.35 0.024 85 MOLYBDENUM 2883 1.1 10.2 0.2 0.038 119 The table shows carbon fibers to be definitely the optimum material. Not only is it the highest temperature resistant, it sublimates instead of melting making it survive much longer and thereby ionize more of the surrounding air. For the same current carrying capacity, it is almost three times lighter than tungsten. In addition, its greater resistivity helps slow the rapid current increase in the electric field.

In the above table, the candidate materials were evaluated on the basis of a single round wire. In fact, a carbon fiber of such diameter cannot be CA 0220~43~ 1997-0~-14 made because of process constraints. Instead they are only produced in extremely small filaments in the range of 7 to 10 microns in diameter. They are commercially available at an acceptable price as a minimum tow of 1000 filaments (1 K).

A single 7-micron polyacrylonitrile (PAN) carbon filament can carry 0.017 A
before starting to sublimate; 600 such filaments could carry 10 A if spread apart and would weigh only4.3 g per 100 m, less than 3% of the weight of tungsten for the same current carrying capacity.

The 1 K tow could be used as is with some reduction in the radiative capacity, or it can be subdivided into smaller tows. Spreading the filaments in a ribbon-like arrangement increases the radiating surface. However as the filaments carry the current in the same direction, the electromagnetic force would tend to bunch them together. To overcome this tendency, the filaments could be glued together in a ribbon-like arrangement or held in this arrangement by crosswise glued filaments. An appropriate adhesive could be phenolic based as at elevated temperature it would also carbonize.

CA 0220~43~ 1997-0~-14 It can be appreciated from the above table that the use of high melting or sublimating temperature material allows extremely fine diameter wire to be used for the application. Such fine wire would therefore have such a low strength as not to present any hazard to a flying aircraft.

Wire as defined here may be construed as being round or flat, being conductive or having conductive and non conductive portions, being made of one or more filaments loosely assembled or twisted or glued in a ribbon-like arrangement. A high temperature resistant wire is preferred for the wire of FIG. 1 and at least the upper conductive portion 7 of FIG. 2; the bottom conductive portion 11 needs not be since it is involved only at a later stage of leader development.

CA 0220~43~ 1997-0~-14 MEANS FOR AUTOMATIC LAUNCHING

The automatic launching a balloon borne wires may be desirable for replacing wires struck by lightning and/or for remote or unmanned stations.
The basic sequence of events may include:

1. Detection of conditions warranting the launch, i.e. the approach of a thunderstorm and in the case of forest protection determining the dryness condition of the forest. Although this could be done by human observation, for instance the rangers manning the lookouts, they could also be done by on site electric field sensors and hygrometers;

2. Triggering the launch sequence. This can be done remotely by human intervention or it could be done automatically by the field sensor;

3. Inflating one or more balloons with lighter than air gas such as helium;

4. Setting the balloons free to rise.

CA 0220~43~ 1997-0~-14 The means for automatically launching one or more balloon raised wires are illustrated in FIG. 3. As a field sensor, capacitor 13 has on the upper side an extension rod 15 extending up in the electric field of a thunderstorm, while the lower side 14 is grounded to earth through contactor 17 being closed by optional hygrometer 19 sensing a dry forest condition. A pointed electrode 21 is placed a calibrated gap 23 away from the upper side of the capacitor and mounted on insulator 18.

The gas filling apparatus is made of a pressurized helium cylinder 25 mounted inside guide 26 and preferably weld sealed; the lower end of the cylinder has a necked extension 27 with an easily piercable diaphragm 28.
The necked extension 27 engages the upper end of flexible sleeve 29. The lower end of sleeve 29 engages one end of elbow 31 having a piercing pin 33 in proximity with diaphragm 28. The other end of elbow 31 is engaging the neck 34 of one or more balloons 3. A conductive weight 35 is held near the top of guide 26 attached by fusible link 37 to insulating cover 39. Electrode 21 is electrically grounded via wire 22, fusible link 37, weight 35 contacting guide 26 which is grounded to earth.

CA 0220~43~ 1997-0~-14 A V-shaped lever 43 has a first leg resting on the balloon and has a weight 45 attached near the end of the second leg and is pivoting around fulcrum 44 at the apex of the V. A blade 49 is held at the top of guillotine 47 by pin 51 linked to the end of the second leg of lever 43 by a loose string 53. Guillotine 47 is mounted over the neck 34 of balloon 3. Wire 1 is coiled and has one end attached to balloon 3 and the other end grounded.

As the thunderstorm approaches, the electric field gradient builds up across the length of extension rod 15. If a dry hazardous condition exists, hygrometer 19 will close contact 17 grounding the lower plate of capacitor 13. As the electric field builds up, capacitor 13 becomes charged.
Gap 23 is adjusted so that as the capacitor becomes charged to a predetermined voltage, arcing will occur across gap 23 and the capacitor will discharge to ground via electrode 21, fusible link 37, weight 35 and guide 26 and melt fusible link 37, releasing weight 35 onto cylinder 25.

~ ' CA 0220~43~ 1997-0~-14 The energy of the falling weight causes the sleeve 29 to collapse allowing pin 33 to puncture diaphragm 28 releasing the pressurized helium and inflating the balloon 3. The inflating balloon tilts lever 43 around fulcrum 44.
As the weight 45 tilts past fulcrum 44, it builds up enough momentum at the end of its travel to pull out pin 51 thereby releasing blade 49. String 53 has enough slack to delay the pulling of the pin, allowing weight 45 time to build up energy and balloon 3 to fill up.

The falling blade 49 slices the neck 34 of balloon 3 releasing it. A check valve 55 in the neck of the balloon retains the helium. The freed balloon raises one end of wire 1 to altitude while the other end of the wire is connected to the earth grounding rod.

The above sequence of events initiated by sensing the presence of a thunderstorm is used to release initially one or more balloons. Means for sequentially launching additional balloons may be provided for replacing wires already struck by lightning using lightning itself as a trigger, as shown schematically in FIG. 4.

CA 0220~43~ 1997-0~-14 Wires 1 and 1a have the bottom end grounded and the upper end raised to altitude by balloons and wires 1b and 1c awaiting to be launched are attached to a common grounding rod 60. A coil 63 surrounds the grounding rod 60 at a safe distance; one end of the coil is grounded and the other end is connected to spring blade 64. Weight 35b is suspended through fusible link from the end of spring blade 64 and deflecting it down away from contactor 68. Electrical continuity from the fusible link to the ground is provided through the grounded guide 26b. As lightning strikes wire 1 a, the current induced in coil 63 melts fusible link 37b dropping weight 35b to launch a replacement balloon raised wire 1 b. Being freed of weight 35b, spring blade 64 springs up, establishing contact with contactor 62 connected to spring blade 70 to which weight 35c is attached through fusible link 37c in a similar arrangement as the previous one, establishing continuity to ground via guide 26c and making the apparatus ready for the next lightning strike to wire 1b to trigger the release of balloon raised wire 1c. When the launching is manually triggered, the capacitor is not used, instead fusible link is directly fused using on electrical impulse from a battery for instance.

The earth grounding means may be the bottom end of the wire being buried into the ground or a rod driven into the ground, or connected to an underground grid; it may be a conductive grounded structure or any other grounding means known to the trade.

Referring to fig. 5, when the terrain is inaccessible or for tar~eting a thunderstorm, instead of being launched from the ground, the balloon, wire and grounding rod could be dropped from an aircraft from a sufficient altitude for the grounding rod to implant itself into the ground and establish sufficientlly effective grounding.

The method comprises coiling a conductive wire 1 in container 12, attaching one end to helium filled balloon 3, attaching the other end of the wlre to the groundin~3 rod 60 and dropping the bal~oon, wire and grounding rod from an aircra~. Optionally a bottom portion 1 d of wire 1 of length extending up past the fine combustibles on the ground or in the tree cover may either have a much larger wire gauge than the rest of the wire so that it does not heat up suffciently to start a firel or i~ can be surrounded with a sheath 62 filled with a light non combust:ible filler 64 such as fiberglass wool so as to keep wire 1d away from the combustible material 66 on the ground or in the tree cover. The drop altitude should be such that the grounding rod reaches the ~round before rising balloon 3 unwinds wire 1 substantially to altitude in order not to break the fine wire off the balloon and not cause a premature artificial lightning s~.rike.

Alternatively sheath 62 and filier 64 can be replaced by a tumescent coating expanding when he!ated by the lightning strike and insulating the rod and/or the boffom wire portion from the fine combustible materiaJ.

Claims (3)

1. Claim 1 A disposable apparatus for directing lightning to a safe ground area comprising:
   a balloon filled with helium;
   earth grounding means located in the safe ground area; and a wire, having an upper end attached to the balloon and a lower end conductively grounded to earth through the grounding means, being light enough to be extended up to altitude by the balloon and weak enough to be safely ruptured by a flying aircraft, being of length sufficient for being extended up to an altitude such that when in the electric field of a thunderstorm it attracts lightning and direct it to the safe ground area, the wire being destroyed in the process.
2. Claim 2 The apparatus of claim 1, wherein the wire is conductive and is of length such that when extended up in the electric field of a thunderstorm, the electric field encompassing the wire is sufficiently strong to cause an upward leader to emerge from the upper end of the wire and self-propagate toward and close enough to the thunderstorm to trigger an artificial lightning strike from the cloud to the ground along the ionized conductive path created by the wire and the leader.
3. Claim 3 The apparatus of claim 1, wherein the wire has:
   an upper conductive portion;
   an intermediate non-conductive portion; and a grounded bottom conductive portion, the upper portion being of length and the intermediate portion being of length allowing the upper portion to be raised to an altitude such that when in the electric field of a thunderstorm not quite sufficiently strong for a natural cloud to ground lightning strike to occur, the ambient electric field encompassing the upper portion is sufficiently strong to cause an upward positive leader to emerge from the upper end of the upper portion and self-propagate toward the thunderstorm cloud and a downward negative leader to emerge from the lower end of the up to altitude, further comprises means for sequentially launching additional balloon raised wires, the launching being triggered by lightning striking the previously balloon raised wire.
   
   Claim 7 A method for the aerial dropping and extending of a wire for safely conducting lightning to the ground comprising:
   coiling a conductive wire;
   attaching one end of the wire to a helium filled balloon;
   attaching the other end of the wire to a grounding rod;
   dropping the wire, the balloon and the grounding rod from an aircraft from a high enough altitude that the grounding rod implants itself into the ground sufficiently deep for effective grounding to earth; and allowing the helium filled balloon to unwind and extend up the wire to altitude.