WO2013007798A1

WO2013007798A1 - Electrical light source with thermoelectric energy recovery

Info

Publication number: WO2013007798A1
Application number: PCT/EP2012/063726
Authority: WO
Inventors: Byrapatna Venkatesha SATHISH
Original assignee: GEORGE, John T.; BINU, George; DINU, Sam David; DENUELLE, Pierre
Priority date: 2011-07-14
Filing date: 2012-07-12
Publication date: 2013-01-17

Abstract

A device for the production of light, the device comprising an electrical light source (1), a thermoelectric module converting waste heat generated by said electrical light source into electric energy, comprising at least one layer of a thermoelectric material, wherein said at least one layer of a thermoelectric material is deposited on the outer and/or inner surface of said electrical light source.

Description

ELECTRICAL LIGHT SOURCE WITH THERMOELECTRIC ENERGY RECOVERY

Field of the invention

The present invention relates to the field of electrical light sources. It aims at rendering electrical light sources more energy efficient. More precisely, it relates to a novel electrical light source that uses the heat produced by light generation for generating electricity by a built-in thermoelectric device.

Background of the invention Interior and exterior lighting is done mainly with electrically powered light sources that generate also heat. Incandescent bulbs, halogen lamps, fluorescent lamps, arc discharge lamps, and more recently light emitting diodes (LEDs) are the most commonly used lighting devices. Incandescent bulbs have a high power consumption and short lifetime; they are being phased out in Europe but are still widely used in other parts of the world. Traditional incandescent bulbs or arc lamps convert more than 90% of the input electrical energy into heat and less than 10% into light. Fluorescent tubes convert less than 20% of the input energy into light. Even LEDs generate significant amounts of heat. Improving the energy efficiency of lighting devices would reduce the global demand for electricity from power plants. It is therefore desirable to increase a light source's light output and to lower its heat output.

It has been proposed to convert the heat produced by the light source into electricity. US 2008/0304272 uses a thermionic converter; this requires quite high temperatures. US 201 1/0155200 discloses a light bulb including a thermoelectric device mounted in the bulb's socket; a suitable thermal conductor and/or heat pipe functioning as a thermosiphon is used in order to transport the heat to the thermoelectric device.

Thermoelectric devices have been described for use in other types of products. As an example, US 2003/0041892, US 5,517,468, JP 6109868 and GB 2 395 027 describe the use of a thermoelectric cell in a wristwatch, the heat being transferred from the user's skin into the watch. US 2001/023591 proposes to mount a thermoelectric conversion module on the heat sink of a computer.

The present invention discloses an automated electrical energy harvesting device within or on an electric light source (such as a light bulb) by converting at least part of the wasted heat which is generated within the electric light source into electricity, in order to substantially minimize the usage of electricity for lighting purpose.

Objects of the invention

The principal object of the invention is to provide an automated system with an electric light source (such as an electric bulb) to self-generate part of the required energy to light said electric light source by converting at least part of the waste heat which is generated within said electric light source into electrical energy.

It is another object of the invention to provide an electricity harvesting method for use in an electric light source (such as an electric bulb) to significantly reduce the usage of electrical energy for lighting purpose from the main source.

It is another object of the invention to provide an electricity harvesting method for an electric light source (such as an electric bulb) by providing a thin transparent layer of a thermoelectric material (such as a thermoelectric silicon material) on the outer surface of said electric light source's material (typically glass, hardened glass, quartz, acrylic or any other transparent material capable of forming the outer skin of a lighting device). This thin layer of a thermoelectric material plays a major role in capturing and converting the heat within the light source to electricity.

Yet another object of the invention is to provide an electricity harvesting method for an electric light source (such as a light bulb) to increase its efficiency significantly, to decrease the amount of heat released into the environment, and to save cost for electricity used for lighting purpose.

It is yet another object of the present invention to provide an electricity harvesting method for a light source (such as an electric bulb) which is capable to powering said electric light source when the primary source of electric supply is interrupted or unavailable.

These and other goals are achieved by a device for the production of light, the device comprising an electrical light source, a thermoelectric module converting waste heat generated by said electrical light source into electric energy, said module comprising at least one layer of a thermoelectric material, and a control unit to timely control the flow of electricity from the main supply to said electrical light source, said device being characterized in that at least one layer of a thermoelectric material is deposited on the outer and/or inner surface of said electrical light source. In one embodiment, said thermoelectric module comprises a first transparent, electrically conductive layer, a second transparent, electrically conductive layer and said at least one layer of a thermoelectric material there-between.

Said thermoelectric material can be a nanostructured semiconductor, which can be at least partially p-doped (preferably by boron), and which can preferably be nanostructured silicon, and advantageously comprises at least one rough surface. Said nanostructured silicon can comprise silicon nanowires. Their diameter can be comprised between 1 nm and 500 nm, preferably between 10 nm and 200 nm, and more preferentially between 50 nm and 100 nm; their length can be comprised between 100 nm to 20 000 nm, preferably between 200 nm and 10 000 nm, and more preferentially between 300 nm and 8 000 nm. Said nanowires can be at least partially embedded in a suitable resin or polymer.

The inventive device can further comprise a storage unit to receive and store electricity generated by said thermoelectric module, said storage unit being in electrical communication with said control unit.

The invention can be applied to any type of heat-generating electrical light source, such as an electrical bulb or tube presenting an external glass, hardened glass, acrylic or quartz surface; said electrical light source can be selected from the group consisting of: an incandescent light bulb or tube, a halogen burner, an arc discharge bulb or tube, a fluorescent bulb or tube, a light-emitting diode or diode array.

In another embodiment that can be combined with any of the other embodiments, said device further comprises a separate external switch to stop the supply of power from said storage unit to power said electrical light source. Said storage unit and said control unit can be housed in a housing space. In a specific embodiment, said electrical light source comprises a glass or quartz or acrylic bulb or tube or hemisphere and a socket, and wherein said housing space is either located (i) between said glass or quartz or acrylic bulb or tube or hemisphere and said socket, and preferably has an annular shape, or (ii) in said socket.

In one other embodiment that can be combined with any of the other embodiments, the device comprises an electric bulb or tube comprising (i) at least one thin transparent layer of at least one thermoelectric material (such as a thermoelectric silicon material) on the outer surface of said electric light source's material (typically glass or quartz or acrylic material) to capture and convert the waste heat energy to electrical energy, (ii) at least one storage chamber or storage device (such as a capacitance or battery) to receive and store the electricity generated from said layer of said thermo-electric material, said storage chamber or storage device being in electrical communication with said thermo-electric layer and a control unit, (iii) a control unit to timely control the flow of electricity from the main source to the electric light source, said control unit being in electrical communication with said storage chamber or storage device and the main supply.

The thin layer of a thermoelectric material plays a major role in capturing and concerting the heat within the light source to electricity.

In said device, said electrical light source can comprise a light-emitting diode, and said surface on which said thermoelectric material is deposited can be made from a heat- conductive transparent polymer or resin having a thermal conductivity of at least 10 W/m- K, and preferably at least 20 W/m-K. Said heat-conductive transparent polymer can comprise carbon nanotubes; the weight proportion of carbon nanotubes in said polymer can advantageously be chosen between 0.5 and 10%, and more advantageously between 2 and 10%. Another object of the present invention is a method for manufacturing a device as described according to the invention, in which a first transparent, electrically conductive layer is deposited on a surface of said electric light source to form a bottom contact; a layer comprising at least one thermoelectric material is deposited on said first transparent conductive layer; and a second transparent, electrically conductive layer is deposited on the layer of thermoelectric material to form a top contact. Said thermoelectric material can comprise at least one nanostructured semiconductor, such as silicon nanowires.

Brief description of the drawings

The nature and scope of the present invention will be better understood from the accompanying drawings, which illustrate embodiments of the invention, but do not limit the scope of the invention. Figure 1 schematically shows the different layers forming the thermoelectric module on the surface of an electric bulb; said thermoelectric module comprises nanowires grown onto a transparent, electrically conductive layer.

Figure 2 schematically shows a variant of the embodiment shown in figure 2, in which the space between the nanowires is at least partially filled by a transparent resin. Figure 3 schematically shows an electric light source according to the invention. Detailed description The light source according to the present invention includes a thermoelectric module converting waste heat to electricity. Said thermoelectric module includes at least one thermoelectric material. The converting efficiency of the thermoelectric module depends on the thermoelectric figure of merit ZT of the thermoelectric material, with is defined by the formula: ZT = T

k

where S, o, k and T are, respectively, the Seebeck coefficient of the thermoelectric material, its electrical conductivity, its thermal conductivity, and the absolute temperature. To maximize ZT, the Seebeck coefficient must be large, the electrical conductivity must be large in order to minimize Joule heating losses, the thermal conductivity (being the sum of the contribution of electrons and phonons) must be small to reduce heat leakage and maintain a high temperature difference across the material.

Most thermoelectric materials are semiconductors, such as binary or ternary semiconductors (BiTe, PbSeTe, SiGe), see "Thermoelectric Materials, Phenomena, and Applications: A Bird's Eye View" by T.M. Tritt and M.A. Subramanian, MRS Bulletin 31 , p. 188-194 (2006). In recent years, nanostructured materials have received much attention, such as porous silicon (see "Thermoelectric properties of porous silicon" by J. de Boor et al, Applied Physics A, published online on March 23, 2012 (DOI 10.1007/s00339-012- 6879-5)), thin porous silicon membranes (see "Holey Silicon as an Efficient Thermoelectric Material" by J. Tang et al., Nano Letters 2010 (vol. 10), p. 4279-4283), one-dimensional structures (see "Properties of Nanostructured One-Dimensional and Composite Thermoelectric Materials" by A.M.Rao et al., MRS Bulletin 31 , p. 218-223 (2006)) and in particular rough silicon nanowires (see "Enhanced thermoelectric performance of rough silicon nanowires" by A.I. Hochbaum et al., Nature, vol. 451 , p.163- 167 (2008)), and superlattice structures (see "Aspects of Thin-Film Superlattice Thermoelectric Materials, Devices and Applications" by H. Bottner et al., MRS Bulletin 31 , p. 21 1 -217 (2006)).

Advantageously, the thermoelectric material comprises at least one nanostructure comprising a rough surface, wherein said at least one nanostructure comprises a doped or undoped semiconductor, and wherein said nanostructure contacts a first electrode and a second electrode. Said nanostructure can be one-dimensional (1 D), especially in the form of a plurality of thin wires 14, or two-dimensional (2D), especially in the form of planar structures, such as a stack of thin layers, or zero-dimensional (0D), especially in the form of quantum dots. The thickness of individual 1 D-structrues can be comprised between 1 nm and 1 ,000 nm (where the expression "1 nm" symbolises here the thickness of one atom), and the same applies at least one dimension of individual 2D-structures (such as the thickness or width of planar structures). These dimensions need not be constant over the total length or width of said individual 1 D- or 2D structure.

In certain embodiments of the invention, the nanostructure comprises a rough surface and is one-dimensional (1 -D), or two-dimensional (2-D). Rough surfaces have a reduced thermal conductivity / at low temperatures compared to smooth surfaces. The surface of a nanostructure is called "rough" when the ratio (called here the "r ratio") of the actual (developed) surface area of the surface compared to the theoretical surface area of the surface if the surface was atomically smooth is more than 1. The r ratio can be 2 or more, 3 or more, 4 or more, 5 or more, 10 or more, 20 or more or even 50 or more. Various methods are known in the art to prepare nanostructures with rough surfaces, or to render smooth surfaces rough (in particular by using etching techniques). The inventors have found that rough nanowires of boron-doped silicon with a r-ratio above 5 give very good results.

In some embodiments of the invention, each individual nanostructure (such as each individual nanowire) is uniform in its composition, for example, any dopant is essentially uniformly distributed throughout said nanostructure, and/or the nanostructure is not uniform in composition, and, for example, comprises a p-type dopant in one end and an n- type dopant in the other end.

Any suitable thermoelectric material, and in particular any suitable thermoelectric semiconductor material can be used in the framework of the present invention. Generally, thermoelectric materials with a ZT value of at least 0.5 are preferred, especially for incandescent lamps, halogen burners and arc discharge lamps; ZT values of at least 1 .0 are preferred. For LED lamps and fluorescent tubes, the ZT value should be somewhat higher, preferably at least 1 .5 . In one embodiment, silicon, possibly doped with a valence-three element (for p-type doping), preferably boron, is used as a thermoelectric material; said thermoelectric material can be crystalline. Thus, in some embodiments of the invention, the nanostructure comprises a crystalline 1 -D nanostructure of silicon, such as a nanowire 14, said nanowire comprising an elongated shape with a first end and a second end, and exhibits a rough surface, wherein said silicon nanowire is optionally doped with boron. Rough silicon nanowires are known as such, and describes in particular in patent application WO 2009/026466, the contents of which are incorporated herein by reference in their entireties.

We describe here several methods that can be used to make a thermoelectric module suitable for use in the device according to the present invention.

Nanostructures comprising a plurality of 1 -D silicon nanowires 14 (such as in an array) can be synthesized by using any suitable method. Such methods include an aqueous electroless etching (EE) method, as described for instance in the following publications the contents of which are hereby incorporated by reference in their entireties: K. Peng et al., "Synthesis of large-area silicon nanowire arrays via self-assembling nanochemistry", Adv. Mater. 14, 1 164-1 167 (2002); K. Peng et al., 'Vend rite-assisted growth of silicon nanowires in electroless metal deposition", Adv. Funct. Mater. 13, 127-132 (2003); K. Peng et al., "Uniform, axial-orientation alignment of one-dimensional single-crystal silicon nanostructure arrays", Angew. Chem. Int. Edit. 44, 2737 (2005), and K. Peng et al., "Aligned single-crystalline Si nanowire arrays for photovoltaic applications", Small, vol 1 (1 1 ), p.1062-1067 (2005).

Another suitable method is a technique known as Chemical Vapor Deposition - Vapor Liquid Solid (CVD-VLS), allowing to grow silicon nanowires by decomposition of silane on a substrate (such as a transparent conducting oxide 12) containing small gold particles 16 as a catalyst. Typically, silicon wires 14 obtained by this method had a diameter between 20 and 300 nm and a length between 1 and 10 μηη. In one example, a diameter range between 20 and 30 nm was obtained. In another example, an average length of 4 μηη was obtained, while the diameter was comprised between 40 nm and 250 nm. The substrate 12 was ITO deposited onto a quartz surface.

Nanostructures comprising one or more 2-D nanostructures can be synthesized by any suitable method. Such methods include using the Langmuir-Blodgett (LB) process, described for example in "Langmuir-Blodgett silver nanowire monolayers for molecular sensing with high sensitivity and specificity" by A. Tao et al., Nano. Lett. 3, 1229, 2003 (the content of which is hereby incorporated in its entirety by reference). For example, the LB process can readily produce a monolayer or multi-layer of monodispersed nanocrystals. Such monolayers and multilayers can then be fused together to generate rough 2-D nanostructures. Silicon naocrystals can be used.

Another suitable process of synthesizing 2-D nanostructures suitable for use in the present invention comprises the following steps: (a) providing a physical or chemical vapour deposition (such as, atomic layer deposition or molecular beam epitaxy) to make a thin film with smooth surface, (b) dispersing one or more nanocrystals on the surface of the thin film, and (c) fusing the one or more nanocrystals to the thin films. Again, silicon nanocrystals can be used.

According to the invention, said thermoelectric material is deposited on at least part of a surface of the light source 1. As an example, if the light source 1 is a light bulb, the thermoelectric material can be deposited onto the outer surface 2 of the light bulb or onto the inner surface. Said surface is typically a glass or quartz or acrylic surface.

We describe here the deposition of the different layers required to form the thermoelectric module on the surface of an electric bulb 1 (including incandescence bulbs with a filament heated by Joule effect, halogen bulbs 1 comprising a halogen burner 5 and an envelope 2, arc discharge lamps, fluorescent tubes, hemispheric LEDs or other electrical light sources). Since the surface 2 of the electrical light source 1 (usually made of glass or quartz or acrylic) acts as the base for the growth of nanostructures of the thermoelectric material, such as silicon nanowires 14, proper cleaning and preparation of said substrate is critical to prevent adhesion loss and to increase the durability and performance level of the device. First, the part of said surface 11 on which the thermoelectric material 14 will be deposited has to be treated with a suitable cleaning agent.

Then the different layers forming the thermoelectric device are deposited onto the clean surface 11 :

a first transparent, electrically conductive layer 12 ("bottom layer") of a suitable material (such as a thin, transparent metal layer or, preferably, a transparent conductive oxide (TOO) layer) of the desired thickness is deposited on the substrate (usually glass or quartz or acrylic) to form a bottom contact ;

a layer comprising the at least one thermoelectric material (such as silicon nanowires 14) is deposited as decried above on the bottom transparent conductive layer 12 ;

optionally, a suitable heat-resistant resin 15 (such as epoxy resin or polymer) with good heat resistance and electrical and thermal insulation properties is added to fill holes and isolate the nanowire layer ;

a second transparent, electrically conductive layer 13 (such as a thin, transparent metal layer or, preferably, a transparent conductive oxide (TOO) layer) of the desired thickness is deposited on the layer of thermoelectric material (such as silicon nanowires 14) to form a top contact.

If said resin 15 is deposited onto the nanowire layer 14, said resin 15 should be thin enough in order not to completely cover the nanowires 14, in order to ensure that said second transparent, electrically conductive layer 13 is in good electrical contact with said layer comprising nanowires 14.

The first 12 and second 13 transparent, electrically conductive layers should have a maximum optical transmission in order to avoid loss of light intensity in the desired spectral range.

When the device 1 is in operation, the first transparent, electrically conductive layer 12 and the second transparent, electrically conductive layer 13 are in electrical communication (on one side through the thermo-electric nanolayer 14 between them, and on the other side through the electronic circuit capable of harvesting, stocking and recycling electricity into the lighting device 1 ).

In one embodiment of the present invention, the device 1 comprises:

an electric light source, such as a bulb or tube or hemisphere, which comprises on its outer or inner surface a thermoelectric element comprising a first transparent conductive oxide layer 12, a second transparent conductive oxide layer 13 and at least one layer of silicon nanostructures, such as nanowires 14, there-between, to capture and convert the waste heat energy to electrical energy ; a storage unit to receive and store electricity generated by said thermoelectric element, connected electrically to layer of silicon nanostructure 14 on the outer or inner surface 2 of said electric light source 1 ; said storage unit can comprise an electrical capacitor ;

- a control unit connected electrically to the storage unit and the main supply of said lighting device, to timely control the flow of electricity from the main source to the electric light source.

The storage unit and/or the control unit is advantageously placed in the socket 3 or near to the socket 3 of said electric light source 1 , preferably in a housing space 4 ; the latter may have an annular shape. Said electric light source 1 can be a bulb or a tube or a hemisphere, made of glass or others suitable material.

In one embodiment of the invention, the electric device comprises also a separate external switch.

If the electric light source 1 is turned on, it produces both heat and light. Thus, when said heat falls on the envelope of the lighting device, due to the thermal gradient across the thermoelectric material 14 (e.g. silicon material) coated on the glass, electricity is generated within the layer made of thermoelectric material. Electricity provided by the thermoelectric material (e.g. silicon material) is then transferred to a storage unit placed in the lighting system.

In an advantageous embodiment of the invention, once the electrical energy stored in the storage unit is sufficient to power the electric light source 1 , the external electric supply from the main source is stopped. The supply resumes when the energy level in the storage unit is insufficient. Hence, a control unit is used to timely control the supply from the main source. The control unit is in electrical connection with the storage unit and the main supply.

When the user switches the light off, then recycling heat continues so that the local power storage unit is charged at its maximum in order to feed the lighting device when the user turns it on again. Generally speaking, said storage unit acts as a secondary source of the electric power within the electric light source system. A separate external switch can be provided. The main source of power can be a primary source or a secondary source or back up power source. Hence, the usage of electricity for the lighting purpose is greatly reduced. That is why this invention provides a greater value to the electric light source by reducing the waste heat energy into useful electrical energy which turns out to increase the efficiency of the electric light source significantly.

In a specific embodiment of the device according to the present invention, the electrical light source is a LED or an array of LEDs, and said surface of said electrical light source is made from a material (such as a polymer or resin) presenting a high thermal conductivity. Thermal management of high power LEDs is a concern to designers of electrical light sources, and even so more with the miniaturization of components and the increase of the power to surface ratio, because LED devices are highly temperature-sensitive. According to the state of the art, in a LED nearly all the heat produced is conducted through the backside of the chip, and is conducted through a thermal interface material to a heat sink. On the top transparent side LEDs are usually encapsulated in a transparent resin that exhibits poor thermal conductivity. As a consequence, only half of the space around the LED is heat conductive, whereas the other half is only poorly conducting heat.

According to the inventive embodiment, the top transparent side, too, is heat-conductive, so as to allow a good heat transmission to the thermoelectric module deposited on the outer and or inner surface of said a suitable transparent polymer or resin exhibiting a good thermal conductivity. As an example, said top transparent side can be made of polymer composites filled with high thermal conductivity carbon nanotubes; as an example, the load of carbon nanotubes can be comprised between 0.5 and 10 weight-%, and more preferably between 2 and 10%. Heat conductivity of said heat-conductive polymer is advantageously at least 10 W/m-K, and preferably at least 20 W/m-K.

Claims

A device (1 ) for the production of light, the device comprising :

an electrical light source ;

an thermoelectric module converting waste heat generated by said electrical light source into electric energy, comprising at least one layer of a thermoelectric material;

wherein said at least one layer of a thermoelectric material (14) is deposited on the outer and/or inner surface of said electrical light source.

A device (1 ) according to claim 1 , further comprising a control unit to timely control the flow of electricity from the main supply to said electrical light source ;

A device (1 ) according to claims 1 or 2, wherein said electric light source comprises on its inner and/or outer surface (1 1 ) said thermoelectric module, comprising a first transparent, electrically conductive layer (12), a second transparent, electrically conductive layer (13) and said at least one layer of a thermoelectric material (14) there-between.

A device (1 ) according to any of claims 1 to 3, wherein said thermoelectric material (1 ') is a nanostructured semiconductor, preferably nanostructured silicon.

A device (1 ) according to claim 4, wherein said nanostructured semiconductor (14) comprises at least one rough surface.

A device (1 ) according to claim 4 or 5, wherein said nanostructured semiconductor comprises nanowires (14).

A device (1 ) according to claim 6, wherein the diameter of said nanowires (14) is comprised between 1 nm and 500 nm, preferably between 10 nm and 200 nm, and more preferentially between 50 nm and 100 nm.

8. A device (1 ) according to claims 6 or 7, wherein the length of said nanowires (14) is comprised between 100 nm to 20 000 nm, preferably between 200 nm and 10 000 nm, and more preferentially between 300 nm and 8 000 nm.

9. A device (1 ) according to any of claims 6 to 8, wherein said nanowires (14) are at least partially embedded in a resin or polymer (15).

10. A device (1 ) according to any of claims 4 to 9, wherein said nanostructured semiconductor or nanowire (14) is at least partially p-doped, preferably by boron.

1 1 . A device (1 ) according to any of claims 4 to 10, wherein said nanowires (14) have an r-ratio above 5.

12. A device (1 ) according to any of claims 6 to 1 1 , wherein said nanowires are silicon nanowires (14).

13. A device (1 ) according to any of claims 1 to 12, further comprising a storage unit to receive and store electricity generated by said thermoelectric module, said storage unit being in electrical communication with said control unit.

14. A device (1 ) according to any of claims 1 to 13, wherein said electrical light source is an electrical bulb or tube or hemisphere presenting an external glass or hardened glass or quartz or acrylic or similar transparent surface (2).

15. A device (1 ) according to any of claims 1 to 14, wherein said electrical light source is selected from the group consisting of an incandescent light bulb or tube, a halogen burner, an arc discharge bulb or tube, a fluorescent bulb or tube, a light- emitting diode or diode array.

16. A device (1 ) according to claim 15 referring back to claims 2 to 15, further comprising a housing space (4) for said storage unit and said control unit.

17. A device (1 ) according to claim 16, wherein said electrical light source comprises a glass or quartz bulb or tube or hemisphere and a socket (3), and wherein said housing space (4) is either located (i) between said glass or quartz or acrylic bulb or tube or hemisphere and said socket, and preferably has an annular shape, or (ii) in said socket (3).

18. A device (1 ) according to any of claims 13 to 17, further comprising a separate external switch to stop the supply of power from said storage unit to power said electrical light source.

19. A device (1 ) according to any of claims 1 to 18, wherein said electrical light source comprises a light-emitting diode, and said surface on which said thermoelectric material is deposited is made from a heat-conductive transparent polymer or resin having a thermal conductivity of at least 10 W/m-K, and preferably at least 20 W/m-K.

20. A device (1 ) according to claim 19, wherein said heat-conductive transparent polymer or resin comprises carbon nanotubes.

21 . Method for manufacturing a device (1 ) according to any of claims 1 to 20, in which

(a) a first transparent, electrically conductive layer (12) is deposited on a surface (2) of said electric light source to form a bottom contact;

(b) a layer comprising at least one thermoelectric material is deposited on said first transparent conductive layer (12) ;

(c) a second transparent, electrically conductive layer (13) is deposited on the layer of thermoelectric material to form a top contact.

22. Method according to claim 21 , in which said thermoelectric material comprises at least one nanostructured semiconductor.