A HEAT TRANSFER APPARATUS

Abstract

In one aspect the invention provides a heat transfer apparatus which includes a transmitter object which defines an external collection surface and an internal transmission surface. Also provided is a receiver object displaced from the transmitter object, the receiver object defining an internal receiving surface and an external heat delivery surface. A thermal conduit is provided which incorporates at least one side wall connected between the transmitter object and receiver object, this at least one side wall spanning the distance between the transmitter object and receiver object and enclosing a volume between the transmitter and receiver objects. This side wall or walls enclose the internal transmission surface of the transmitter object and the internal receiving surface of the receiver object. The transmitter object, receiver object and thermal conduit are configured to promote heat transfer predominantly towards the receiver object.

Claims

1. A heat transfer apparatus which includes a transmitter object which defines an external collection surface and an internal transmission surface, a receiver object displaced from the transmitter object, the receiver object defining an internal receiving surface and an external heat delivery surface, a thermal conduit which incorporates at least one side wall connected between the transmitter object and receiver object, said at least one side wall spanning the distance between the transmitter object and receiver object and enclosing a volume between the transmitter and receiver objects, said at least one side wall enclosing the internal transmission surface of the transmitter object and the internal receiving surface of the receiver object, wherein an interior surface of a thermal conduit side wall includes one or more thermal reflectors arranged to direct thermal radiation preferentially to the receiver object and the transmitter object, receiver object and thermal conduit are configured to promote heat transfer predominantly towards the receiver object.

2. The heat transfer apparatus as claimed in claim 1 wherein the volume enclosed by the side wall or walls of the thermal conduit is a low-air pressure, partial vacuum or vacuum environment.

3. The heat transfer apparatus as claimed in claim 1 wherein the volume enclosed by the side wall or walls of the thermal conduit contains an inert or low thermal conductivity gas.

4. (canceled)

5. The heat transfer apparatus as claimed in claim 1 wherein the at least one thermal reflector or reflectors include a nanostructure metamaterial adapted to preferentially reflect thermal radiation in specific directions.

6. The heat transfer apparatus as claimed in claim 1 wherein the external collection surface of the transmitter object is utilised to collect heat from the immediate environment of the heat transfer apparatus.

7. The heat transfer apparatus as claimed in claim 6 wherein the external collection surface of the transmitter object includes or has applied a nanostructure metamaterial which is tuned to absorb sunlight and impede blackbody radiation from the external collection surface.

8. The heat transfer apparatus as claimed in claim 1 wherein the external heat delivery surface of the receiver object is utilised to deliver heat to the immediate environment of the heat transfer apparatus.

9. The heat transfer apparatus as claimed in claim 8 wherein the external heat delivery surface of the receiver object includes or has applied a nanostructure metamaterial which is tuned to block the absorption of sunlight and promote the emission of blackbody radiation.

10. The heat transfer apparatus as claimed in claim 1 wherein the internal transmission surface of the transmitter object is utilised to emit black body thermal radiation.

11. The heat transfer apparatus as claimed in claim 10 wherein a nanostructure, nanolayer, or metamaterial is applied to the internal transmission surface of the transmitter object to preferentially emit thermal radiation in specific wavelength bands.

12. The heat transfer apparatus as claimed in claim 1 wherein the internal receiving surface of the receiver object is utilised to absorb the thermal radiation emitted from the internal transmission surface of the transmitter object.

13. The heat transfer apparatus as claimed in claim 12 wherein a nanostructure, nanolayer, or metamaterial is applied to the internal receiving surface of the receiver object to preferentially emit thermal radiation in specific wavelength bands.

14. The heat transfer apparatus as claimed in claim 1 wherein a heat transfer apparatus includes a filter within the volume enclosed by the thermal conduit, the filter being adapted to reflect thermal radiation in specific wavelength bands and transmit thermal radiation in other wavelength bands.

15. The heat transfer apparatus as claimed in claim 14 wherein the filter is formed from of a wavelength depended reflector provided by a dichromatic mirror, thin film interference filter, and/or nanostructure metamaterial.

16. The heat transfer apparatus as claimed in claim 1 wherein the area of the internal transmission surface of the transmitter object is larger than the area of the internal receiving surface of the receiver object.

17. The heat transfer apparatus as claimed in claim 1 wherein a thermal buffer material is provided in contact with or as part of any one or more of: the external collection surface of the transmitter object; the external heat delivery surface of the receiver object; the internal transmission surface of the transmitter object; the internal receiving surface of the receiver object; the internal side wall surface of the thermal conduit; the filter.

18. The heat transfer apparatus as claimed in claim 17 wherein the thermal buffer material experiences an abrupt change in any one or more of the following properties when exposed to a specific control stimulus: thermal conductivity; emissivity; reflectivity.

19. The heat transfer apparatus as claimed in claim 18 wherein said specific control stimulus includes one or more of: a thermal stimulus; an electronic stimulus; a mechanical stimulus; a magnetic stimulus; an optical stimulus.

20. The heat transfer apparatus as claimed in claim 1 wherein an interior surface of a thermal conduit side wall includes one or more thermal reflectors formed from nanostructure metamaterials adapted to preferentially reflect thermal radiation in specific directions, and a nanostructure, nanolayer, or metamaterial is applied to the internal transmission surface of the transmitter object, and/or internal receiving surface of the receiver object to preferentially emit thermal radiation in specific wavelength bands, and the area of the internal transmission surface of the transmitter object is larger than the area of the internal receiving surface of the receiver object, and the volume enclosed by the thermal conduit includes a filter adapted to reflect thermal radiation in specific wavelength bands and transmit thermal radiation in other wavelength bands.

21. The heat transfer assembly formed from at least two heat transfer apparatuses as claimed in claim 1.

22. The heat transfer assembly as claimed in claim 21 which includes a receiver object of a first transfer apparatus engaged with, or in thermal communication with, a transmitter object of a second transfer apparatus.

23. The heat transfer assembly as claimed in claim 21 which is formed from at least two heat transfer apparatuses, each transfer apparatus incorporating a common transmitter object which defines separate external collection and internal transmission surfaces for each transfer apparatus, each transfer apparatus incorporating a common receiver object which defines separate internal receiving and external heat delivery surfaces for each transfer apparatus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0103] Additional and further aspects of the present invention will be apparent to the reader from the following description of embodiments, given in by way of example only, with reference to the accompanying drawings in which:

[0104] FIG. 1 is an orthographic projection diagram of a heat transfer apparatus provided in accordance with one embodiment of the invention;

[0105] FIG. 2a, 2b, 2c provides cross-section diagrams of several versions of a heat transfer apparatus provided in accordance with various additional embodiments of the invention;

[0106] FIG. 3a, 3b provides side view diagrams of two versions of a heat transfer assembly provided by stacked heat transfer apparatuses to improve overall performance in accordance with additional aspects and embodiments of the invention;

[0107] FIG. 4a, 4b provide side view diagrams of two heat transfer apparatuses including a thermal buffer in accordance with various additional embodiments of the present invention;

[0108] FIG. 5 is a side view diagram of a heat transfer apparatus including a thermal control unit in accordance with an additional embodiment of the invention;

[0109] FIG. 6a, 6b provides side view diagrams of arrays of heat transfer apparatus forming an assembly in accordance with further embodiments of the invention;

[0110] FIG. 7a, 7b provides orthographic projections of two different types of arrays of heat transfer apparatus forming an assembly in accordance with additional embodiments of the invention;

[0111] FIGS. 8a, 8b provides orthographic projections of stacked arrays of heat transfer apparatus forming an assembly in accordance with additional embodiments of the invention;

[0112] FIG. 9 is a side view of a flexible array of apparatuses providing a flexible heat transfer assembly in accordance with another embodiment of the invention;

[0113] Further aspects of the invention will become apparent from the following description of the invention which is given by way of example only of particular embodiments.

BEST MODES FOR CARRYING OUT THE INVENTION

[0114] Heat is transferred through one of four mechanisms: conduction, convection, advection, and radiation. Considering only heat transfer between stationary objects, thereby precluding advection, placed in a vacuum, thereby precluding convection, and well thermally insulated, thereby with negligible conduction, leaves radiation as the only significant heat transfer mechanism. From herein we will consider this to be the case.

[0115] All objects emit black body thermal radiation. The power (in Watts) of this emission is related to temperature and surface area by the Stefan-Boltzmann law:

[00001]$P=ϵ\sigma {\mathrm{AT}}^4$

[0116] where ε is the surface emissivity, σ is the Stefan-Boltzmann constant (5.670373×10.sup.−8 Wm.sup.−2K.sup.−4), A is the object's surface area, and T is the object's surface temperature in Kelvin.

[0117] As an object emits thermal radiation, it loses internal energy and its temperature reduces. All objects that emit thermal radiation are also capable of absorbing thermal radiation. When an object absorbs thermal radiation its internal energy, and consequently its temperature increases.

[0118] The amount of black body thermal radiation transfer from an emitting surface to a collecting surface is a function of the temperature and area of the emitting surface, the solid angle subtended by the collecting surface, and the emissivities of the surfaces. It is normally assumed that the emissivities are fixed for a given material, and the effective surface areas of objects interacting through thermal radiation are fixed for a given physical arrangement.

[0119] Therefore, to be in thermal equilibrium, the objects must be at the same temperature. This assumption holds when introducing traditional refractive and reflective optics as any magnification of surface area necessitates an inverse magnification of solid angle subtended, resulting in no net change in energy transfer.

[0120] Generally, heat will transfer from hotter objects to colder objects, as the hotter object emits more thermal radiation than it absorbs from the colder object, resulting in a net emission of heat, cooling it down. The cooler object absorbs more thermal radiation from the hotter object than it emits, resulting in a net absorption of thermal energy, and therefore it heats up.

[0121] Objects at a stable temperature, that is, are not heating up or cooling down, are said to be in thermal equilibrium. Thermal equilibrium is defined as the situation where there is no net flow of thermal heat energy between objects, when they are connected by a path permeable to heat. Even in thermal equilibrium, all objects are still emitting black body radiation and absorbing radiation emitted from other objects, but are absorbing and emitting at exactly the same rate, resulting in zero net heat flow.

[0122] Objects in thermal equilibrium are normally assumed to be at the same temperature because it is normal for the effective surface area of interaction for each object to be the same. However, in accordance with the present invention, thermal equilibrium can be also achieved between two objects of different temperatures under the assumption that a larger surface area of the cooler object can be efficiently coupled to a smaller surface area of a hotter object. From herein we refer to such an apparatus that can achieve thermal equilibrium between objects as differing temperatures as a Thermal Syphon.

[0123] FIG. 1 shows a schematic orthographic projection of a preferred embodiment of a heat transfer apparatus, referred to interchangeably throughout the specification as a ‘Thermal Syphon’ apparatus. This apparatus is configured to preferentially allow the flow of thermal radiation from a transmitter object (101) to a receiver object (102) by concentrating the black body radiation from a larger surface area of the transmitter object onto a smaller surface area of the receiver object. The thermal concentration is achieved by a thermal conduit (103). The transmitter and receiver objects are depicted as flat plates, but other shapes may be beneficial in specific applications. The thermal conduit is depicted as a truncated square pyramid, but other shapes, including but not limited to, a truncated cone or extruded trapezoid, may be beneficial.

[0124] FIG. 2a shows a schematic cross-section view of the Thermal Syphon apparatus of FIG. 1 in more detail. The thermal conduit (103) is comprised of reflecting surfaces (204) adapted to reflect energy preferentially towards the receiver object (102). The apparatus may be sealed and evacuated, or near evacuated, or filled with a gas, to limit conduction and convection heat transfer, thereby improving performance.

[0125] Ray traced light rays (205) illustrate an example preferential reflection of thermal radiation, wherein the reflecting surfaces (204) are substantially physically flat but adapted to focus light and act in a way equivalent to one half of an idealised concave mirror. However, reflecting surfaces with other properties will also be effective. Furthermore, for the purposes of this invention, high quality imaging optics are not required, meaning that the apparatus is tolerant to reflecting surfaces with minor errors or distortions, and thereby may be fabricated with cheaper manufacturing processes.

[0126] FIG. 2b shows a schematic cross-section view of another embodiment of a Thermal Syphon apparatus configured to use wavelength selective elements. This system is configured to preferentially allow flow of thermal radiation from the transmitter object (101) to the receiver object (102) by configuring the transmission surface (206) of the transmitter object to emit radiation at a shifted wavelength (207), for example, through the use of metasurface coatings, and selectively passing those wavelengths with a wavelength selective element (208) such as a dichroic mirror or filter. Radiation emitted (209) from the receiver object will be unshifted in wavelength and reflected back to the same object by the wavelength selective element. The wavelength selective element may be placed in different positions between the transmitter and receiver objects, and that it may be beneficial to also use wavelength shifting metasurfaces on the receiver object surface.

[0127] FIG. 2c shows a schematic of a combination of the apparatuses from FIG. 2a and FIG. 2b to improve efficiency or performance of both.

[0128] By definition, thermal equilibrium between the transmitter object and the receiver object is achieved when there is zero net flow of thermal heat energy between objects. That is, the amount of thermal energy emitted by the transmitter object and absorbed by the receiver object is equal to the amount of thermal energy emitted by the receiver object and absorbed by the transmitter object. When the surface area of the transmitter object is larger than the surface area of the receiver object, and thermal radiation is effectively coupled between them, for thermal equilibrium, the transmitter object must be at a lower temperature than the receiver object.

[0129] In the case of a wavelength selective element placed between the objects, the refection or partial reflection of thermal radiation as an equivalent change in surface area of the surfaces. For example, if 80% of the power of the thermal radiation emitted by a surface is reflected back to that surface, the effective surface area can be considered to also be reduced by 80%, and thereby be mathematically treated as 20% of its actual size.

[0130] For radiative thermal equilibrium, power emitted by the transmitter object and absorbed by the receiver, equals power emitted by the receiver object and absorbed by the transmitter:

[00002]$P_1=P_2{ϵ}_1\sigma A_1{T}_1^4={ϵ}_2\sigma A_2{T}_2^4$

[0131] where variables with subscript 1 relate to the transmitter object and variables with subscript 2 relate to the receiver object.

[0132] For simplicity, we shall assume that both objects have substantially the same emissivity, allowing simplification to:

[00003]$$\begin{gathered} A_1{T}_1^4=A_2{T}_2^4 \\ {T}_2^4=\frac{A_1}{A_2}{T}_1^4 \end{gathered}$$

[0133] In practice, the reflecting surfaces of the concentrator will have limitations on their ability of the preferential energy refection towards the receiver object, making the equilibrium power transfer as:

[00004]$$\begin{gathered} P_2+P_1\left(1-{\eta }_c\right)=P_1{\eta }_c \\ {P}_2=P_1\left(2{\eta }_c-1\right) \end{gathered}$$

[0134] where η.sub.c is the fraction of power emitted by the transmitter object that is absorbed by the receiver object. Here we assume the remaining power is directed back to, and absorbed by, the transmitter object. For simplicity, we shall define a “coupling ratio” as:

[00005]$\eta =2{\eta }_c-1$

[0135] The equilibrium temperature ratio can then be calculated as:

[00006]$P_2=P_1\eta T_2^4=\frac{A_1}{A_2}\eta T_1^4\frac{T_2}{T_1}=\sqrt[4]{\frac{A_1}{A_2}\eta }$

[0136] This equilibrium temperature also represents the operations maximum temperature values for which a single Thermal Syphon apparatus will act as a Thermal Syphon.

[0137] The above equations assume all of the thermal energy emitted by the receiver object will be absorbed by the transmitter object. However, this may not be the case in practice, but herein, for clarity and simplicity, it is assumed that this effect is not material. Should it be material in reality, it will only improve the performance of the system. Those skilled in the art will also recognise that further adaptation of the invention to intentionally limit the transfer of thermal energy from the receiver object to the transmitter object will also improve the overall performance of the apparatus.

[0138] The rate of thermal energy transferred (in Watts) is the difference in thermal power transferred between the object taking into consideration the coupling ratio:

[00007]$\Delta P=P_1\eta -P_2={ϵ}_1\sigma A_1{T}_1^4\eta -{ϵ}_2\sigma A_2{T}_2^4$

[0139] Again, assuming the emissivities of both objects are substantially the same, thermal power transferred is:

[00008]$\Delta P=\mathrm{ϵ\sigma }\left(A_1{T}_1^4\eta -A_2{T}_2^4\right)$

[0140] or, in terms of a ratio of object surface areas:

[00009]$\Delta P=\mathrm{ϵ\sigma }{A}_2\left(\frac{A_1}{A_2}\eta T_1^4-T_2^4\right)$

[0141] If the cooler side temperature and desired power transfer are known, the hotter side temperature can be calculated as:

[00010]$T_2=\sqrt[4]{\frac{A_1}{A_2}\eta T_1^4-\frac{\Delta P}{\mathrm{ϵ\sigma }{A}_2}}$

[0142] FIG. 3a, 3b shows schematic side views of multiple thermal transfer apparatuses stacked together to form an assembly. FIG. 3a shows how the hotter receiver object (301) of a first apparatus (302) also forms, or is in thermal contact with, the cooler transmitter object of a second apparatus (304), increasing the maximum operating temperature. The overall relationship between temperatures and power transferred can now be expressed as the combination of two independent units:

[00011]$T_2=\sqrt[4]{\frac{A_1}{A_2}\eta \left(\frac{A_1}{A_2}\eta T_1^4-\frac{\Delta P}{\mathrm{ϵ\sigma }{A}_2}\right)-\frac{\Delta P}{\mathrm{ϵ\sigma }{A}_2}}$

[0143] and simplified to:

[00012]$T_2=\sqrt[4]{{\left(\frac{A_1}{A_2}\eta \right)}^2{T}_1^4-\frac{\Delta P}{\mathrm{ϵ\sigma }{A}_2}\left(\frac{A_1}{A_2}\eta +1\right)}$

[0144] There is no limit, beyond the obvious such as temperature limits of the component materials, on the number of apparatuses that may be stacked to achieve an even greater maximum operating temperature range, either hot or cold, including cryogenic, or greater power transfer at a given temperature difference, as shown in FIG. 3b. The relationship between temperatures and power transferred can be expressed more generally as:

[00013]$T_2=\sqrt[4]{{\left(\frac{A_1}{A_2}\eta \right)}^n{T}_1^4-\frac{\Delta P}{\mathrm{ϵ\sigma }{A}_2}\underset{i=0}{\overset{n-1}{.Math.}}{\left(\frac{A_1}{A_2}\eta \right)}^i}$

[0145] where n is the number of layered apparatuses in the stack. Hence, the heat transfer for a stacked system can be calculated as:

[00014]$\Delta P=\frac{\mathrm{ϵ\sigma }{A}_2}{{.Math.}_{i=0}^{n-1}{\left(\frac{A_1}{A_2}\eta \right)}^i}\left[{\left(\frac{A_1}{A_2}\eta \right)}^n{T}_1^4-T_2^4\right]$

[0146] Those skilled in the art will also recognise that different surface areas or configurations could be used for each layer of apparatus.

[0147] FIG. 4 shows schematic side views illustrating further embodiments of the apparatus adapted to limit the heating or cooling temperature, whereby a thermal buffer material (401) is added to the hotter side of the Thermal Syphon apparatus, as shown by FIG. 4a, or the cooler side of the apparatus, as shown by FIG. 4b.

[0148] The thermal buffer material is adapted to have a transition temperature at a desired temperature limit and undergo a significant change in thermal conductivity around the transition temperature, thereby limiting heating or cooling temperature range of the apparatus. It is well known that some materials exhibit rapid change in thermal conductivity when undergoing a phase change, such as solid to liquid or vice-versa, and therefore such materials would be good candidates for the thermal buffer. It is also well known that some materials exhibit a change in emissivity or reflectivity when undergoing a phase change. Such materials could be used as a thermal buffer material on the internal surfaces of the apparatus. Further example candidates for a thermal buffer material are thermal components such as thermal diodes, regulators and switches, and tuneable or controllable thermal conductors.

[0149] FIG. 5 shows schematic side views illustrating further preferred embodiments of the apparatus adapted to dynamically control the heating or cooling limit of the apparatus by including a temperature sensor (501) and a controller (502) to selectively control the thermal buffer material's thermal conductivity in accordance with a temperature control algorithm. The same temperature control approach could be applied to the cooler side of the apparatus, or the thermal buffer material placed on the internal surfaces of the apparatus.

[0150] FIG. 6a, 6b show schematic side views illustrating examples of multiple apparatuses arranged together as two different assemblies which form a larger or enclosed group which define a strip in the case of FIG. 6a, or in the case of FIG. 6b, a ring. Other similar arrangements include, swapping orientation of the individual Thermal Syphons to reverse the heat transfer direction, and the use of other structure shapes that may be more appropriate for specific applications.

[0151] Multiple apparatuses may be arranged in other geometries or shapes as appropriate for specific applications, for example: [0152] multi-sided regular or irregular polygons, and especially interlocking polygons such as hexagons, allowing multiple groups of apparatuses to be in thermal contact with one another; [0153] curved profiles, wherein multiple apparatuses are arranged to form or approximate circles, ellipses, or other smooth shapes, wherein the surfaces of the individual apparatuses may also be curved; [0154] arbitrary shapes that are adapted to specific applications, including forming a covering, coating, or surface adapted to follow the form of another object from which thermal energy may need to be supplied or removed; and [0155] all of the shapes are profiles described above in three dimensions.

[0156] FIG. 7a, 7b show schematic orthographic projection equivalents of the side views provided by FIG. 3a, 3b, illustrating how individual Thermal Syphon apparatuses can be formed into shapes such as panels, tiles, tubes or pipes. Those skilled in the art will also recognise how these shapes could then be combined to create even more complex shapes, such as 6 panels forming a cuboid, enclosing or semi-enclosing a zone of heating or cooling, the tube enclosed at the ends to form an enclosed or semi-enclosed cylinder for heating or cooling, or individual apparatuses arranged to form an enclosed or semi-enclosed sphere.

[0157] FIG. 8a, 8b show schematic orthographic projections of the multiple arrays of FIG. 7 arranged in layers or stacks, as demonstrated in FIG. 3, to enhance the performance of the Thermal Syphon. Three layers are shown for clarity, but other numbers of layers could also be used.

[0158] FIG. 9 is a schematic side view showing how multiple individual apparatuses could be connected via a hinge or flexible joint mechanism (601) to form a flexible or semi-flexible sheet. Other forms of flexible material could also be constructed, for example, by embedding multiple small-scale apparatuses in fabric.

[0159] Those skilled in the art will also recognise that the various combinations of apparatuses, as shown in layers and stacks of FIG. 3, FIG. 6, FIG. 7, FIG. 8, FIG. 9, and any other combinations, could also include the use of thermal buffer materials and thermal control units as shown in FIG. 4 and FIG. 5.

[0160] The Thermal Syphons and arrays of Thermal Syphons disclosed herein, could be used for applications including, but not limited to: [0161] heating and cooling of buildings by incorporating the tiles into flooring, walls, or ceiling, or by incorporating the pipes into fluid based heating or cooling systems such as radiators and underfloor heating, and including buildings in remote locations, such as Antarctica, the Moon, and Mars; [0162] creating cool boxes, that not only remain cool, but can actively cool items, for applications such as food or medical supply storage and transport; [0163] bottles that actively heat or cool their contents; [0164] water heating systems; [0165] cryogenic coolers and cryogenic carbon capture, recovery and sequestering systems; [0166] thermoelectric electricity generation; [0167] blankets, and emergency blankets, that can provide active heating.

[0168] In the preceding description and the following claims the word “comprise” or equivalent variations thereof is used in an inclusive sense to specify the presence of the stated feature or features. This term does not preclude the presence or addition of further features in various embodiments.

[0169] It is to be understood that the present invention is not limited to the embodiments described herein and further and additional embodiments within the spirit and scope of the invention will be apparent to the skilled reader from the examples illustrated with reference to the drawings. In particular, the invention may reside in any combination of features described herein, or may reside in alternative embodiments or combinations of these features with known equivalents to given features. Modifications and variations of the example embodiments of the invention discussed above will be apparent to those skilled in the art and may be made without departure of the scope of the invention as defined in the appended claims.