GB2467342A

GB2467342A - Fibre suitable for reflecting solar light.

Info

Publication number: GB2467342A
Application number: GB0901518A
Authority: GB
Inventors: Peter Leaback
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-01-30
Filing date: 2009-01-30
Publication date: 2010-08-04
Also published as: GB0901518D0

Abstract

A solar reflective fibre 100 having a longitudinal axis. The fibre comprises a plurality of filaments, where each filament has a core 111 and a shell where the core and the shell have different refractive indices respectively. The diameter of core of a filament may be a function of the distance of the core from the longitudinal axis of the fibre. The diameter of the shell of a filament may be a function of a distance of a core of that filament from the longitudinal axis of the fibre. Clothing may be made from fabric which may comprise the fibres.

Description

SOLAR REFLECTIVE FIBRE

FIELD OF THE INVENTION

The present invention relates to solar reflectors. In particular the invention relates to fibres arranged to reflect solar radiation.

BACKGROUND

It is well known that white clothing can be effective in keeping a wearer of the clothing cooler when exposed to sunlight compared with coloured clothing including black clothing. This is because less light is absorbed by white cloth compared to coloured cloth.

Light absorbed by clothing is typically transformed into heat which in turn heats the body.

The body compensates for the heating effect of the cloth heating by sweating which causes dehydration and discomfort.

White clothing is however a relatively poor reflector. A white T-shirt will typically transmit 60% of the light falling upon it. A body covered in white clothing, which is in bright sunlight, can have a net absorption of 160 Watts per square meter. By comparison, the difference in energy used by the body when sedentary (e.g. sitting down) and that when performing light exercise (e.g. a light jog) is around 100 watts per square meter.

Thus, the heating effect of sitting in direct sunlight can feel similar to that of light exercise in the shade (on a hot day).

If a polymer fibre could be created that reflected 90% of solar radiation, a body covered in this fabric would have a net absorption of roughly 15 watts per square meter. This absorption rate would be similar to that of a body in shade. Apparel exhibiting this performance would be of particular benefit to endurance athletes.

In hot weather a marathon runner can lose water through sweating at a greater rate than the body can absorb water in the stomach. This unsustainable situation causes reduced performance in the latter stages of a race.

Apparel that reduced the heat load on the athlete by 145 watts per square meter could make the difference between a body remaining fully hydrated and a body experiencing progressive dehydration.

Current reflective clothing technology has concentrated on metalized cloth, which is designed to reflect intense heat encountered by fire fighters. Metalized cloth is not ideal for keeping an object cool when exposed to solar radiation due to the reflective properties of metals over a wide range of wavelengths. Thus although heat incident upon the cloth from the sun is generally reflected away from the object, heat radiating from the object is reflected back towards the object by the cloth (the cloth acting as thermal insulation).

Solar radiation that reaches the ground has wavelengths roughly in the range from around 300 to around 2500nm. The human body radiates at roughly 10,000 nm. An ideal 1 5 material for keeping the body cool in sunlight would therefore be highly reflective in the range 300 to 2500nm but not at around 10,000nm.

It is to be understood that if the material were highly reflective a 10,000nm, body heat will be reflected back towards the body, thus providing thermal insulation.

Aluminised cloth has a high reflectivity (65%) at solar wavelengths and at the wavelengths of heat emitted by the human body. Thus it is not ideal for use in clothing intended to assist in wearer in remaining cool. Aluminised plastic sheeting for example is widely used as a thermal blanket for treating people with hypothermia.

A type of reflector that can be designed to have high reflectivity of solar radiation but low reflectivity of body radiation may be made from a non-metallic (dielectric) material such as a dielectric mirror.

An example of a dielectric reflecting material is a solar reflecting paint. Solar reflecting paint consists of microscopic particles of pigment (usually titanium dioxide) suspended in a clear varnish or other medium. As light enters the paint, it is refracted as it transitions between the titanium dioxide and the varnish. Random scattering occurs which results in some of the light being reflected out of the paint and some of the light becoming absorbed by the paint.

A good solar reflective paint will reflect about 50% of solar radiation incident upon it.

The size of the titanium dioxide particles is chosen to create a high degree of scattering at solar radiation wavelengths (described above), but only a small amount of scattering at wavelengths that a hot building will emit. This is a great advantage of titanium dioxide paint over metal paint; a building will readily re-radiate the energy it has absorbed from the sun when covered in titanium dioxide paint compared with metallic paint.

Solar reflective paint that is used on flat roofs has been proven to significantly reduce an amount of energy required to run air conditioning systems and increase the lifespan of the roof.

If a UV resistant polymer fibre could be created with over 90% solar reflection, the air conditioning costs of buildings could be reduced still further by incorporating such a fibre ontheroof.

Another application of a highly reflective polymer fibre would be as a low cost solar concentrator. A solar concentrator directs light to a small area which contains a device that exploits the solar energy.

Possible advantages of a solar concentrator made from a reflective polymer fibre are the reduced weight of such a concentrator compared with glass/metal reflectors, and the possibility of deforming the reflective surface to best direct sunlight as the sun moves during the day.

A further application of a highly reflective polymer fibre would be in the construction of ultra-lightweight clothing. There are limiting factors in respect of how thin a fabric can be manufactured and still be useful for garments. These factors include the tensile strength of thread from which the fabric is woven and the light scattering properties of the fabric.

In thinner fabrics photons have fewer transitions between air and the fibres as the photons pass through the fabric compared with thicker fabrics. This reduces the amount of light that is scattered/reflected and consequently fabrics becomes increasingly translucent as they become thinner.

The majority of garments are designed to be completely opaque, which limits how thin they can be made. It is to be understood that a more highly reflective fibre could be woven to form a much thinner fabric whilst retaining an opacity in excess of that obtainable using known fibres.

US 7311962 describes a reflective fibre created by producing concentric rings of two materials with differing refractive indices. A disadvantage of this technique is that the cost of the two materials is higher than that of polymers normally used in the fabric industry.

US 2008/0152282 describes a scheme for creating photonic crystal fibres which guide light along the length of the fibres.

US 2003/0174986 describes a photonic crystal formed from a hollow core optical fibre that is itself formed from a collection of smaller hollow core optical fibres.

STATEMENT OF THE INVENTION

In a first aspect of the present invention there is provided a solar reflective fibre having a longitudinal axis, the fibre comprising a plurality of filaments, each filament comprising a core and a shell, the core and shell having different respective refractive indices.

This has the advantage that a highly reflecting fibre can be formed from a single hollow fibre without a requirement to form multiple coatings of a core.

Known dielectric mirrors can be fabricated that have extremely high reflectivity (e.g. 99.999%), however careful deposition of multiple layers of material on a planar substrate is required. Furthermore, such structures have this reflectivity only over certain ranges of angle of incidence of light.

It is to be understood that reference to a diameter of a tube includes reference to an average diameter of a non-circular filament such as an oval, oblong, square, rectangular filament or any other suitable shape.

It is also to be understood that a cross-sectional shape of a filament may change during manufacture due to compressive stress. Thus, a circular filament may become a distorted circular filament in cross-section such as an elliptical filament. The filament may become hexagonal, substantially hexagonal or pseudo-heaxagonal in cross-section.

Preferably a diameter of the core of a filament is a function of a distance of the core from the longitudinal axis of the fibre.

Preferably the core diameter of a filament increases with increasing distance of the core from the longitudinal axis.

Alternatively the core diameter of a filament may decrease with increasing distance of the core from the longitudinal axis.

Preferably a diameter of the shell of a filament is a function of a distance of a core of that filament from the longitudinal axis of the fibre.

Preferably the shell diameter of a filament increases with increasing distance of a core of that filament from the longitudinal axis.

Alternatively the shell diameter of a filament may decrease with increasing distance of a core of that filament from the longitudinal axis.

The filaments may be substantially parallel to the longitudinal axis along a length of the fibre.

The filaments may follow a twisted path along the length of the fibre.

At least one of the filaments may be arranged to follow a substantially helical path along the length of the fibre.

All of the filaments may be arranged to follow a substantially helical path along the length of the fibre.

Preferably at least one of the core and the shell comprise a polymer.

Preferably the shell comprises a polymer.

Preferably a fibre is formed from at least one selected from amongst polyester, acrylic, polypropylene, polytetrafluoroethylnese (PTFE), polyethylene, Ultra High Molecular Weight Polyethylene (UHMWPE) and glass.

Preferably the core comprises at least one void, optionally a gas filled void.

Preferably the core is substantially a single void, optionally a gas-filled void.

Preferably the fibre has a cross-section having a shape selected from amongst circular, oval, elliptical, oblong and polygonal having 3, 4, 5, 6, 7, 8 or more sides.

Preferably the fibre comprises a plurality of concentric tube members, each tube member being formed from a plurality of filaments extending along a ength of the tube member, each tube member having a wall thickness of at least a diameter of one filament, optionally two, three, four, five or more filaments.

Preferably the wall of each tube member is formed from concentric layers of fibres, the wall having a thickness of at least 2 filament diameters, optionally two, three, four, five or more filaments.

A fibre may comprise at least ten concentric tube members.

Preferably a radially inner wall of one tube member is substantially in face to face contact with a radially outer wall of an inner adjacent tube member.

Preferably filaments comprising a given tube member are of substantially the same diameter.

Preferably filaments of respective adjacent tube members are of different respective diameters.

Preferably filaments of respective adjacent tube members are of increasing diameter with radial distance from a longitudinal axis of the fibre.

Preferably filaments of respective adjacent layers are of decreasing diameter with radial distance from a longitudinal axis of the fibre.

Preferably corresponding inner and outer diameters of shells of filaments of respective adjacent layers differ by substantially 10%.

Preferably filaments of an innermost tube have a shell having an inside diameter of around 310 nm and an outside diameter of around 540 nm.

Preferably a diameter of respective different filaments of a fibre varies in a substantially random or pseudo-random manner between filaments through a cross-sectional area of a fibre.

In a second aspect of the invention there is provided a fabric comprising a fibre according to the first aspect of the invention.

In a third aspect of the invention there is provided a garment comprising a fabric according to the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying figures in which: FIGURE 1 shows a cross-sectional view of a fibre according to an embodiment of the invention; FIGURE 2 shows a perspective view of a fibre according to a further embodiment of the invention; and FIGURE 3 shows a cross-sectional view of a fibre formed from a plurality of tube members, members each tube member having a wall formed from a plurality of filaments.

DETAILED DESCRIPTION

In one embodiment of the invention a fibre is formed from an assembly of closely packed hollow polymer filaments as shown schematically in FIG. 1.

A dielectric mirror works well when each layer reflects light in such a way that the phase of each reflection of a given wavelength by that layer is substantially the same.

Reflections from successive layers constructively interfere to produce strong reflection.

Control of the reflected phase is achieved through careful choice of thickness of the layers.

By choosing values of the inside diameter and outside diameter of the hollow polymer fibre and closely packing the fibres, a fibre may be fabricated that approximates a dielectric mirror using layers of air and polymer.

The fibre described above will provide good reflective properties for a relatively small frequency range and angle of incidence. For a fibre to be a good solar reflector it needs to reflect light in the range of 300 to 2500nm and it needs to be able to do this for a large range of angle of incidence.

In some embodiments of the invention this is achieved by varying the inside and outside diameter of the hollow polymer filaments as a function of the distance of a filament from the centre of the fibre.

FIG. 2 shows a cross section through a further example of a fibre 20 according to embodiments of the invention.

It can be seen that a diameter of cores and shells of filaments of the fibre 20 increase as a function of distance of the filaments from a longitudinal axis L of the fibre 20.

The tubes 2A of smallest diameter are at or close to the longitudinal axis of the fibre whilst the tubes 2C of largest diameter are located at or proximate an outer surface of the fibre. Tubes of intermediate diameter 2B are positioned between tubes of the smallest and largest diameters 2A, 2C respectively.

A fabric woven from a fibre according to an embodiment of the invention will have much higher reflectivity to sunlight than solar reflective paint. This is due to the fact that the fabric will more closely resemble the structure of an ideal dielectric mirror.

Titanium dioxide particles of solar reflective paint roughly approximate spheres. A sphere has relatively little surface area that is parallel to a desired reflection plane of solar radiation and therefore a high percentage of the radiation will be allowed to be transmitted deeper into a layer of the paint.

This is in contrast to a fabric formed form elongate fibres. In a fabric, a longitudinal axis of a fibre of the fabric remains substantially parallel to the desired reflection plane. Thus the amount of light that is transmitted to deeper layers of fibres is reduced.

Another advantage of fabric according to embodiments of the invention is that a distance between reflective layers of the fibre may be closely controlled. In contrast, in paint containing titanium dioxide particles, the particles are randomly distributed in a binder or host medium. Thus there is a reduced probability of the occurrence of constructive interference of scattered light.

EXAMPLE

It has been found through experimentation that to achieve greater than 90% reflectance over the wavelength range 300 to 2500nm, the following construction of a fibre is useful.

Other constructions are also useful.

With reference to FIG. 3, a fibre 100 was produced from 10 sets of concentric tube members 110 of which four are shown in FIG. 3 labelled 111, 112, 113 and 114 beginning at a core 111 of the fibre 100. Each tube member 110 was formed to have a thickness of four filaments, a tube member 111 at the core of the fibre having around 8 filaments across a diameter of the tube member 111.

Filaments 120 of a given tube member 110 have substantially the same diameter whilst filaments 120 in respective adjacent tube members 110 have different diameters. In some preferred embodiments the diameter of the filaments 120 of a tube member increase successively between one tube member 110 and another along a radially outward direction from a longitudinal axis of the fibre 100.

In some alternative embodiments the diameter of the filaments 120 of a tube member decrease successively between one tube member 110 and another along a radially outward direction from a longitudinal axis of the fibre 100.

Referring again to FIG. 3, the inside diameter of the shell of the inner most filaments 121 (forming layer or core' 111) is around 310 nm. The outside diameter of the shell of these inner most filaments 121 is around 540 nm.

Filaments of a second tube member 112 each have a shell having inside and outside diameters that are 10% larger than the corresponding diameters of the shells of filaments of the first tube member 111. Similarly, each successive tube member 110 has filaments 120 having inner and outer shell diameters that are around 10% larger 1 0 than the corresponding diameters of filaments of shells of the tube member that that tube member immediately surrounds.

In the example of FIG. 3 immediately adjacent successive tubes are in direct contact with one another.

As described above in a variation of this embodiment of the invention the filaments of largest diameter are provided at the core of the fibre and the filaments decrease in diameter with increasing distance from the centre of the fibre, each successive tube of filaments having filaments of substantially the same diameter.

In some embodiments filaments within a given tube member are of different respective diameters. In some embodiments a random distribution of filament diameters through a cross section of the fibre is provided, resulting in a sufficiently highly reflective fibre for a number of practical applications. However it is to be understood that such a fibre will generally not be as highly reflective as a fibre having an ordered distribution as described above.

An advantage of a random distribution of filament shell diameters may be a simplification for manufacture.

For the purpose of durability of the fibres, it is desirable in some embodiments for the filaments of a fibre to be bonded to one another.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

CLAIMS: 1. A solar reflective fibre having a longitudinal axis, the fibre comprising a plurality of filaments, each filament comprising a core and a shell, the core and shell having different respective refractive indices.
2. A fibre as claimed in claim 1 wherein a diameter of the core of a filament is a function of a distance of the core from the longitudinal axis of the fibre.
3. A fibre as claimed in claim 2 wherein the core diameter of a filament increases with increasing distance of the core from the longitudinal axis.
4. A fibre as claimed in claim 2 wherein the core diameter of a filament decreases with increasing distance of the core from the longitudinal axis.
5. A fibre as claimed in any preceding claim wherein a diameter of the shell of a filament is a function of a distance of a core of that filament from the longitudinal axis of the fibre.
6. A fibre as claimed in claim 5 wherein the shell diameter of a filament increases with increasing distance of a core of that filament from the longitudinal axis.
7. A fibre as claimed in claim 5 wherein the shell diameter of a filament decreases with increasing distance of a core of that filament from the longitudinal axis.
8. A fibre as claimed in any preceding claim wherein the filaments are substantially parallel to the longitudinal axis along a length of the fibre.
9. A fibre as claimed in any preceding claim wherein the filaments follow a twisted path along the length of the fibre.
10. A fibre as claimed in claim 9 wherein at least one of the filaments is arranged to follow a substantially helical path along the length of the fibre.
11. A fibre as claimed in claim 10 wherein all of the filaments are arranged to follow a substantially helical path along the length of the fibre.
12. A fibre as claimed in any preceding claim wherein at least one of the core and the shell comprise a polymer.
13. A fibre as claimed in claim 12 wherein the shell comprises a polymer.
14. A fibre as claimed in any preceding claim formed from at least one selected from amongst polyester, acrylic, polypropylene, polytetrafluoroethylene (PTFE), polyethylene,Ultra High Molecular Weight Polyethylene (UHMWPE), and glass.
15. A fibre as claimed in any preceding claim wherein the core comprises at least one void, optionally a gas filled void.
16. A fibre as claimed in any preceding claim wherein the core is substantially a single void, optionally a gas-filled void.
17. A fibre as claimed in any preceding claim wherein the fibre has a cross-section having a shape selected from amongst circular, oval, elliptical, oblong and polygonal having 3, 4, 5, 6, 7, 8 or more sides.
18. A fibre as claimed in any preceding claim comprising a plurality of concentric tube members, each tube member being formed from a plurality of filaments extending along a length of the tube member, each tube member having a wall thickness of at least a diameter of one filament, optionally two, three, four, five or more filaments.
19. A fibre as claimed in claim 18 wherein the wall of each tube member is formed from concentric layers of filaments, the wall having a thickness of at least 2 filament diameters, optionally two, three, four, five or more filaments.
20. A fibre as claimed in claim 18 or 19 comprising at least ten concentric tube members.
21. A fibre as claimed in any one of claims 18 to 20 wherein a radially inner wall of one tube member is substantially in face to face contact with a radially outer wall of an inner adjacent tube member.
22. A fibre as claimed in any one of claims 18 to 21 wherein filaments comprising a given tube member are of substantially the same diameter.
23. A fibre as claimed in any one of claims 18 to 22 wherein filaments of respective adjacent tube members are of different respective diameters.
24. A fibre as claimed in any one of claims 18 to 23 wherein filaments of respective adjacent tube members are of increasing diameter with radial distance from a longitudinal axis of the fibre.
25. A fibre as claimed in any one of claims 18 to 23 wherein filaments of respective adjacent layers are of decreasing diameter with radial distance from a longitudinal axis of the fibre.
26. A fibre as claimed in claim 24 or 25 wherein corresponding inner and outer diameters of shells of filaments of respective adjacent layers differ by substantially 1 0%.
27. A fibre as claimed in any one of claims 18 to 26 wherein filaments of an innermost tube have a shell having an inside diameter of around 310 nm and an outside diameter of around 540 nm.
28. A fibre as claimed in any one of claims 18 to 21 wherein a diameter of respective different filaments of a fibre varies in a substantially random or pseudo-random manner between filaments through a cross-sectional area of a fibre.
29. A fabric comprising a fibre as claimed in any preceding claim.
30. A garment comprising a fabric as claimed in claim 29.
31. A solar reflective roof comprising a fibre as claimed in any one of claims 1 to 28.
32. An ultra light weight article of opaque clothing comprising a fabric as claimed in claim 29.
33. Use of a fibre as claimed in any one of claims 1 to 28 to form a solar reflective roof.
34. Use of a fabric as claimed in claim 29 to form a garment.
35. Use as claimed in claim 34 wherein the garment is an ultra light weight article of opaque clothing.
36. A fibre substantially as hereinbefore described with reference to the accompanying drawings.
37. A fabric substantially as hereinbefore described with reference to the accompanying drawings.
38. A garment substantially as hereinbefore described with reference to the accompanying drawings.
39. Use substantially as hereinbefore described with reference to the accompanying drawings.