TULKKI, Jaakko (Vyökatu 9 B 12, Helsinki, FI-00160, FI)
TULKKI, Jaakko (Vyökatu 9 B 12, Helsinki, FI-00160, FI)
| Claims: 1. A method for transferring heat, where heat energy is transferred in the direction opposite to the direction defined by the second law of thermodynamics with the aid of light or other electromagnetic radiation generated in a structure from an element emitting radiation to an element absorbing radiation, characterized in that the electromagnetic radiation mediating the heat energy is generated by electroluminescence and that the radiation absorbed in the absorbing element is absorbed without exploiting the energy of the absorbed light or other electromagnetic radiation in other form than as heat energy. 2. A method as claimed in claim 1 , characterized in that the emitting and the absorbing element have been coupled by an element that is transparent to the electromagnetic radiation. 3. A method as claimed in claims 1 or 2, characterized in that the emitting element is a semiconductor structure and/or includes a light emitting diode. 4. A method as claimed in any of the claims 1-3, characterized in that efficient transport of the light or other electromagnetic radiation between the emitting and the absorbing element is arranged by enclosing the emitting element and the absorbing element in the same optical cavity and/or by connecting the emitting and the absorbing element to one another by (i) a material layer whose refractive index has been substantially matched with the emitting and the absorbing element, or (ii) a material or a vacuum layer whose refractive index substantially differs from the refractive index of the emitting and the absorbing element, but that is so thin that it does not substantially hinder the transport of photons across the layer, or (iii) nanoparticles or nanostructures so that the space between the nanoparticles or nanostructures forms between the combined structures a gap that is so thin that is does not substantially hinder the transport of photons across the gap, or (iv) an substantially lossless wave guide that does not substantially hinder the transport of radiation, or (v) any structure or structures combining and/or repeating the structures in the above items (i)-(iv), where some material layer(s) function as thermally insulating layers. 5. A method as claimed in any of the claims 1-4, characterized in that some of the thermally insulating material layers in between the emitting and the absorbing materials have been implemented by using small particles so that the space in between the particles is a vacuum or consists of another thermally insulating material so that the thermally insulating material layer is so thin that it allows efficient coupling of the light over the thermal insulator, but the small contact area of the particles reduces the heat transfer between the elements. 6. A device configured to transfer heat in the direction opposite to the direction defined by the second law of thermodynamics and that comprises: an element emitting radiation configured to transfer heat by using light or other electromagnetic radiation to an absorbing element; the absorbing element configured to absorb the electromagnetic radiation and the energy transported with it emitted by the emitting element characterized by the device being configured to generate the electromagnetic radiation mediating the heat energy by using electroluminescence and to absorb the electromagnetic radiation mediating the heat energy in the absorbing element without exploiting the energy of the electromagnetic radiation in other form than as heat energy. 7. A device as claimed in claim 6, characterized in that the emitting and the absorbing element have been coupled by an element that is transparent to the electromagnetic radiation. 8. A device as claimed in claim 6 or 7, characterized in that the emitting element includes a semiconductor structure and/or a light emitting diode. 9. A device as claimed in any of the claims 6-8, characterized in that efficient transport of the light or other electromagnetic radiation between the emitting and the absorbing element is arranged by enclosing the emitting element and the absorbing element in the same optical cavity and/or connecting the emitting and the absorbing element to one another by (i) a material layer whose refractive index has been substantially matched with the emitting and the absorbing element, or (ii) a material or a vacuum layer whose refractive index substantially differs from the refractive index of the emitting and the absorbing element, but that is so thin that it does not substantially hinder the transport of photons across the layer, or (iii) nanoparticles or nanostructures so that the space between the nanoparticles or nanostructures forms between the combined structures a gap that is so thin that is does not substantially hinder the transport of photons across the gap, or (iv) a substantially lossless wave guide that does not substantially hinder the transport of radiation, or (v) any structure or structures combining and/or repeating the structures in the above items (i)-(iv), where some material layer(s) function as thermally insulating layers. 10. A device as claimed in any of the claims 6-9, characterized in that the emitting element is a light emitting diode and in between the emitting element and the passive absorbing element there is at least one thermally insulating material layer or a vacuum that is so thin that it allows efficient transport of the electromagnetic radiation across the insulating layer. 11. A device as claimed in any of the claims 6-10, characterized in that some of the thermally insulating material layers in between the emitting and the absorbing materials have been implemented by using small particles so that the space in between the particles is a vacuum or consists of another thermally insulating material so that the thermally insulating material layer is so thin that it allows efficient coupling of the light over the thermal insulator, but the small contact area of the particles reduces the heat transfer between the elements. 12. A device as claimed in any of the claims 6-11 , where injection of charge carriers into the emitting element has been configured to take place through an electrical contact characterized in that the emitting element and metal functioning as the contact have been separated from one another by a material layer or a vacuum in a part of the contact and the current transport in between the emitting element and the metal has been configured to take place across protrusions extending over the gap that enable the electrical contact between the emitting element and the metal. 13. A device as claimed in any of the claims 6-12, characterized in that the device has been configured to use wave guides, optical fibers or nonresiprocal components like optical isolators based on Faraday rotation in the transfer of electromagnetic energy. 14. A device as claimed in any of the claims 6-13, characterized in that the device includes asymmetry so that the absorbing element is different, preferably simpler, than the emitting element. 15. An optical or an electrical device that includes a device of any of the claims 6-14 generally as a part of the optical or the electrical device or in particular integrated on the same substrate with an electrical or an optical integrated circuit. |
TECHNICAL FIELD
The present invention relates in general to energy transfer. The invention relates especially to transferring heat energy with the aid of electromagnetic radiation, such as light.
BACKGROUND ART
Known heat transfer methods conventionally use various refrigerants (for example compressor based solutions in refrigerators) or electric current (Peltier elements). The weaknesses of these solutions are large size, harmful impact on the environment and wearing out of the moving parts for the mechanical heat pumps, and in case of thermoelectric heat pumps the low coefficient of performance and the small achievable temperature difference.
An earlier patent application Fl 20080434 of the inventors of the present application has introduced a thermophotonic heat pump that solves at least a part of the above problems.
SUMMARY
An object of the present invention is to provide a method and device for transferring heat, where a large temperature difference is reached with a relatively simple device structure.
According to a first aspect of the invention there is provided a method as claimed in claim 1. In embodiments of the invention, heat may be transferred in the direction opposite to the direction of the heat flow determined by the second law of thermodynamics.
In embodiments of the invention, light or other electromagnetic radiation may be used to transfer heat in a solid state heat pump. Embodiments of the invention may achieve the benefits of the Peltier element as a compact solid state heat pump, but also reach a higher temperature difference, typically over 50-100 K, between the heating and the cooling surface. In the heat transfer method of certain embodiments of the invention, radiation emitted by an element emitting light or other electromagnetic radiation is coupled to an element absorbing radiation, in which the energy of the radiation is released as heat. In certain embodiments, heat is transferred from an emitting element to an absorbing element with the aid of photons. The radiation emitted by the emitting element may be, for example, light produced by electroluminescence in a semiconductor. The structure may be asymmetric so that the absorbing element is structurally different, preferably simpler, than the emitting element. In certain embodiments the absorbing element is of any material absorbing the radiation emitted by the emitting element.
In certain embodiments the emitting and the absorbing element have been coupled to one another by an element that is transparent to electromagnetic radiation.
In certain embodiments efficient transport of light or other electromagnetic radiation between the emitting element and the absorbing element is arranged by enclosing the emitting element and the absorbing element in the same optical cavity.
In certain embodiments the efficient transport of light or other electromagnetic radiation in between the emitting element and the absorbing element is arranged by connecting the emitting element and the absorbing element to one another by (i) a material layer whose refractive index has been substantially matched with the refractive index of the emitting and the absorbing element. In certain embodiments the efficient transport of light or other electromagnetic radiation in between the emitting element and the absorbing element is arranged by connecting the emitting element and the absorbing element to one another by (ii) a material or a vacuum layer that has a refractive index substantially different from the refractive index of the emitting and the absorbing element and that is so thin that it does not substantially hinder the transport of photons across the layer.
In certain embodiments the efficient transport of light or other electromagnetic radiation in between the emitting element and the absorbing element is arranged by connecting the emitting element and the absorbing element to one another by
(iii) nanoparticles or nanostructures so that the space between the nanoparticles or the nanostructures forms between the connected elements a gap that is so thin that it does not substantially hinder the transport of photons across the gap.
In certain embodiments the efficient transport of light or other electromagnetic radiation in between the emitting element and the absorbing element is arranged by connecting the emitting element and the absorbing element to one another by (iv) a substantially lossless wave guide that does not substantially hinder the transport of radiation.
In certain embodiments the efficient transport of light or other electromagnetic radiation in between the emitting element and the absorbing element is arranged by connecting the emitting element and the absorbing element to one another by (v) any structure or structures combining and/or repeating the structures in the above items (i)-(iv).
In certain embodiments the efficient transport of light or other electromagnetic radiation between the emitting element and the absorbing element is arranged by enclosing the emitting element and the absorbing element in the same optical cavity and by connecting the emitting and the absorbing element to one another by any structure according to the above items (i)-(v) or a combination thereof. In certain embodiments at least some layer(s) in the structures in accordance with the above items (i)-(v) may function as thermally insulating layer(s).
In certain embodiments the emitting element is a semiconductor structure and/or includes a light emitting diode.
In embodiments of the invention, the energy absorbed by the absorbing element is not exploited in another form of energy than heat energy, which decreases the coefficient of performance of the device but allows a large temperature difference between the emitting and the absorbing element. This enables case-specifically a simpler structure. The large temperature difference between the emitting and the absorbing element can be reached for example by placing between them a layer that is thermally insulating and in particular has a low thermal conductance, and that allows an substantially unrestricted transport of the radiation emitted by the emitting element through the layer. In certain embodiments of the invention the thermally insulating layer is implemented by nanostructures. In an embodiment the emitting and the absorbing element have been separated by nanoparticles (sparsely) scattered on their surfaces and by creating a vacuum in the space formed between the nanoparticles. In some other embodiments the nanostructure may be continuous so that there is no empty space between the emitting and the absorbing element.
In certain embodiments the material layer used as a thermal insulator between the emitting and the absorbing element has been implemented by using small particles so that the space in between the particles is a vacuum or consists of another thermally insulating material so that the thermally insulating material layer is so thin that it allows efficient coupling of the light over the thermal insulator, but the small contact area of the particles reduces the heat transfer between the elements.
In certain embodiments implemented with semiconductors injection of charge carriers into the semiconductor takes place through an electrical contact where in a large part of the contact the semiconductor and metal functioning as the contact have been separated from one another by a material layer with a differing refractive index and where the current transport in between the semiconductor and the metal takes place through protrusions extending over the layer. In certain embodiments the protrusions may have been fabricated by growing them on the surface of the semiconductor.
In accordance with a second aspect of the invention there is provided a device as claimed in claim 6.
In certain embodiments the device comprises an element emitting light optically coupled to an element absorbing light, of which the emitting element cools down as it emits light and the absorbing elements heats up as it absorbs light. The emitting element can be thermally coupled to a cold heat reservoir (the cooled object/target) and the absorbing element correspondingly to a hot heat reservoir (the heated object/target), in which case the device transfers heat from the cold heat reservoir to the hot heat reservoir. In some embodiments implemented with semiconductors the energy gap of the active region in the emitting element is smaller than energy gaps of the doped semiconductors on both sides or (or surrounding) the active region.
The said device can be a device using photons to transfer heat, that is, a thermophotonic heat pump. In certain embodiments the thermophotonic heat pump is a solid state heat pump suitable for both cooling and heating applications. Its advantages compared to compressor based heat pumps are small size and the lack of moving parts and refrigerants. In addition it may reach a larger temperature difference between the cooled and heated object than conventional solid state heat pumps.
In certain embodiments the emitting element is a light emitting diode structure and in between the emitting element and the absorbing element there is at least one thermally insulating material layer or vacuum that is so thin that it allows transport of radiation across the thermal insulator. In certain embodiments injection of charge carriers into the emitting element has been configured to take place through an electrical contact. In certain embodiments the emitting element and metal functioning as the contact have been separated from one another in a part of the contact by a material layer or a vacuum of differing refractive index and the current transport between the emitting element and the metal takes place along protrusions which extend over the layer and enable the electrical contact between the emitting element and the metal.
In accordance with yet another aspect of the invention there is provided an optical or an electrical device that includes a device of claim 6 or any of the devices presented in its embodiments generally as a part of the optical or the electrical device or in particular integrated on the same substrate with an electrical or an optical integrated circuit.
The method and device in accordance with the embodiments of the invention can be used for transferring heat in particular in applications requiring a large temperature difference, like cooling of detectors or medical sensors requiring low temperature.
Certain embodiments of the present invention are described in the detailed description and in the dependent claims. The embodiments are described in the context of certain selected aspects of the invention. The person skilled in the art will understand that any embodiment may typically be combined with another embodiment or other embodiments under the same aspect of the invention. Any embodiment may also typically be combined with another aspect or other aspects of the invention by itself or together with any other embodiment of embodiments. BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows an example of the principle of the heat transfer in an embodiment of the invention and Figure 2 an example of a structure or a cross section of a device enabling the disclosed heat transferring method.
DETAILED DESCRIPTION
In the following examples the operation principle and the structure of a heat pump using light are described according to certain embodiments of the invention. It should be noted that instead of light, the heat pump may transfer heat by using other form of electromagnetic radiation.
In Fig. 1 the element 1 emitting radiation emits radiation 3 by using an external energy source 4 and heat energy 5a obtained from the cold heat reservoir 9a. Element 1 can include for example a light emitting diode that emits light by electroluminescence and the external energy source 4 can be a voltage source (or more generally an energy source) Uo, that injects a current Io for the light emitting diode through the electrical circuit of Fig. 1. The emitted radiation 3 is transferred to the element 2 absorbing radiation, where the energy included in the radiation is transformed to heat. The released heat energy 5b is released to the hot heat reservoir 9b. Element 2 can be made of any material absorbing the said radiation. The area 6 surrounding the emitting element 1 can be thermally connected to the cold heat reservoir 9a and can include for example elements belonging structurally to element 1 , such as the substrate and/or electrical contacts. Area 6 is separated from the area 7 surrounding the absorbing element 2 by a thermally insulating area 8 which reduces the conduction of heat between the emitting element 1 and the absorbing element 2, but is transparent to the electromagnetic radiation between the emitting element 1 and the absorbing element 2. Area 7 may be thermally connected to the hot heat reservoir 9b and can include elements belonging structurally to the absorbing element. Areas 8, 6 and 7 form in this embodiment an optical cavity that enables strong optical coupling between elements 1 and 2. The term optical cavity should be interpreted widely in this document: it can mean the optical cavities formed by areas 6 and 7 independently as well as the cavity they form together when they are optically coupled by area 8.
Fig. 2 represents an example of a cross section of a device or a structure that utilizes the disclosed heat transfer method. For the sake of the clarity of the figure, the structure has not been drawn to correct scale, and in reality the width of the structure is much larger than the height. In Figure 2 the emitting element is formed by the part above intersection A and the absorbing element is formed by the part below intersection B. The emitting element can in practice comprise a semiconductor diode structure, metallic contacts and a mirror structure.
In an embodiment the emitting element operates so that photons are generated when charge carriers recombine after they are injected to the active area 12a through metallic contacts 15a,b and 16a and doped semiconductor layers 10a (n-type doping) and 11a (p-type doping). When the materials are of high quality, the energy of the emitted photons is larger than the energy provided by the external power source. The part of the energy of the emitted photons that is not provided by the external energy source is provided by the heat energy of the emitting element. Therefore the emitting element and the cold heat reservoir cools down.
In an embodiment the absorbing element is a structure absorbing light or other electromagnetic radiation. The energy of the absorbed photons is released into the absorbing element and given to the hot heat reservoir as heat, which results in the heating up of the absorbing element and the hot heat reservoir.
In certain embodiments the external voltage source U 0 of Figure 1 feeds energy to the emitting element through contacts 15a,b and 16a and generates photons by electroluminescence or another applicable mechanism. When the device is packaged the structure of Fig. 2 may be connected to the external circuits, encapsulated tightly and evacuated of any gases. The emitting element forms the cooling side of the device and the absorbing element forms the heating side of the device. To make the heat transfer more efficient, heat conducting elements like heat pipes, heat sinks and/or fans can be placed between the cooling side (cold heat reservoir) and the object to be cooled and the heating side and the object to be heated (hot heat reservoir), that help to transfer heat from the cooled object to the heated object through the device.
The operation of the device in Fig. 2 as a heat pump is based, depending on the embodiment, on the very high quantum efficiency of photon emission, strong optical coupling of the emitting and the absorbing element, small heat conduction between the emitting and the absorbing element and small resistive losses. To accomplish these requirements, following factors play a role:
(1) The absorption of the emitted photons outside the active region 12a should be small in the emitting element. This can be accomplished for example by fabricating the conducting semiconductor layers 10a and 11a from indium phosphide and the active region 12a from a GaAsSb or InGaAs -layer whose energy gap is smaller that in the InP layers. The semiconductor layers 10a, 11a and 12a should be lattice matched with the substrate or pseudomorphic i.e. strained structures in which the strain has not relaxed through the formation of dislocations. The thickness of the active region 12a can typically be of the order of the wavelength of light, the thickness of the semiconductor layer 11a can be of the order of the diffusion length of the holes and the thickness of the semiconductor layer 10a can be of the order of the thickness of the substrate and it can be formed of the substrate itself, provided that the optical losses of the substrate material are sufficiently small. Other compound semiconductors that enable light emission based on electroluminescence and absorption, and that can be used to fabricate a structure where the energy band gap of the active region is smaller than the energy gap of the doped semiconductor layers can be used to fabricate the device of Fig. 2 as well. For example using GaAs/AIGaAs material system is possible, but typically requires removing the GaAs substrate from the complete structure in order for the absorption of the substrate not to cause problems.
(2) The optical coupling between the emitting element and the absorbing element should be strong so that the transport of photons between the elements occurs with a high efficiency, but simultaneously the heat conduction between the elements should be small. The strong optical coupling can be obtained for example by adjusting the effective refractive index of the materials used in the device to be substantially the same as the refractive index of the emitting and the absorbing element, so that the reflections taking place at the boundaries of the materials do not substantially affect the transport of radiation in the structure. The small thermal conductivity and strong optical coupling between the structures can be achieved for example by fabricating the structure in Fig. 2 in two parts so that the emitting and the absorbing element are fabricated separately and placed close to one another for example by attaching them together using small particles 13. The particles can be for example commercially available nanoparticles scattered between the emitting and the absorbing element, self organized quantum dots grown in the gap or nanostructures processed in the gap. This way the gap between the elements can be made so thin that it does not substantially change the refractive index experienced by the light and allows efficient coupling of light between the elements. The small contact area of the particles 13 will however strongly reduce the heat conduction by phonons between the elements. When the device is packaged a vacuum can also be formed in area 14, which further significantly reduces the heat conduction between the elements.
(3) The absorption losses at the interface R a of the semiconductor layer 11a and the metal contact 16a should be small. To this end, an air gap 17a that fills most of the area between the semiconductor and the reflector or contact metal can be used to increase the portion of the internal total reflection at the interface of the semiconductor and the air gap without giving rise to excessive resistive losses. In the configuration of Figure 2 the electrical contacts are formed by the electrically conducting extrusions 18a fabricated to the surface of the semiconductor with a suitable fill factor. Also other mirror structures with a high coefficient of reflectivity are suitable for this purpose.
(4) Reaching a high external quantum efficiency typically requires a large internal quantum efficiency. This requirement can be reached by using high quality materials, advanced fabrication technology and optimization of the structure. The proportion of the nonradiative recombination taking place at the surfaces of the structure can be reduced by passivating the interfaces close to the active region 12a, which reduces the amount of the nonradiative surface states and allows reducing the rate of recombination through these states.
(5) The resistive losses of the structure should be small. The electric contacts 15a,b to the structure in region 10a can be made through the side and in area 11 a so that light is efficiently reflected by the interface between the semiconductor 11a and the electrical contact 16a. Since the width of the structure is considerably larger than the thickness, the current transport in the structure is mainly lateral between contacts 15a,b and 16a. The resistive losses in the structure represented in Fig. 2 can be affected by optimizing the width of the structure, the thickness and doping concentration of the semiconductor layers 10a and 11a and the fill factor of the contact extrusions 18a.
The method in accordance with some embodiments of the invention described above can be exploited by various structures of which only an example has been presented above. Other modifications are for example structures made of other materials than inorganic semiconductors and structures in which optical fibers, photonic crystals, other wave guides or non-reciprocal components like optical isolators based on Faraday rotation are used to transport photons between the emitter and the absorber. Furthermore the structure can also be integrated as a part of an electrical or optical integrated circuit which may allow further advantages in fabrication technology.
The foregoing description provides non-limiting examples of certain embodiments of the invention. It is clear to a skilled person that the invention is not restricted to the presented details and that the invention can also be implemented using other equivalent ways. In this document the terms comprise and include are open expressions and they are not meant to be limiting.
Some of the features of the presented embodiments can be utilized without using other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. The scope of the invention is only restricted by the appended patent claims.
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