Rotary heat exchanger

Abstract

A rotary heat exchanger 100 having a central axis L and comprising an annular rotor 102 which surrounds and is configured to rotate about the central axis. The heat exchanger also comprises a stator 1

Description

TECHNICAL FIELD The present disclosure relates to a rotary heat exchanger. BACKGROUND Our heavy reliance on fossil fuels in homes, particularly for heating and cooling homes, has significant environmental consequences. Burning fossil fuels contributes to air pollution and releases greenhouse gases, exacerbating climate change. Transitioning to cleaner and more sustainable energy sources, such as renewables and electrification, is imperative if we are to reduce our environmental footprint and combat climate change. Heat pumps offer an eco-friendly alternative by efficiently transferring heat from one source to another, rather than burning fossil fuels. By harnessing the temperature of the environment (such as from outside air), heat pumps can provide heating (and cooling) for homes and buildings. Heat pumps can therefore help to significantly reduce greenhouse gas emissions and help combat climate change, as they have a smaller carbon footprint compared to traditional heaters and air conditioners. In addition to their environmental benefits, heat pumps can also offer cost savings over time, as they are highly efficient, making them a versatile and sustainable choice for indoor climate control. To enhance heat exchange efficiencies, there has been a trend towards increasing the internal volumes of heat pumps, leading to bulkier units that occupy a significant amount of space, making them less ideal for household applications. It is against this background that the present invention was conceived. SUMMARY OF THE INVENTION According to an aspect of the invention there is provided a rotary heat exchanger. The heat exchanger has a central axis. The heat exchanger comprises an annular rotor surrounding, and configured to rotate about, the central axis. The heat exchanger further comprises a stator enclosing the rotor. The stator comprises an upper section, the upper section extending radially away from the central axis to define an annular upper perimeter that surrounds the central axis. The stator further comprises a lower section arranged axially away from the upper section. The lower section extends radially away from the central axis to define an annular lower perimeter that surrounds the central axis. The stator further comprises an annular outer section, the outer section extending axially between the upper and lower perimeters and surrounding the central axis. An annular stator cavity is defined between the upper and iower sections and between the outer section and the central axis and surrounds the central axis. The rotor is arranged within the annular stator cavity. The heat exchanger further comprises an inlet for directing fluid into the stator, the inlet being arranged proximal to the central axis. The heat exchanger also comprises an outlet for directing fluid out of the stator, the outlet being arranged distal to the central axis. In any plane extending radially away from the central axis, the cross-section of the rotor is uniform and the rotor extends both axially between the upper and lower sections of the stator and radially between the central axis and the outer section, such that rotation of the rotor causes fluid to flow both axially and radially along an interior flow surface of the stator between the inlet and the outlet. Due to this configuration, rotation of the rotor causes the fluid to move “helically” or “spirally” (i.e., both axially and radially) as it moves from the centre of the heat exchanger towards its perimeter. As such, the path of the fluid through the rotor between the inlet and outlet is extended, and hence heat transfer between the fluid and the stator is sustained for longer. This is because as the distance travelled by the fluid is increased, the contact time and the contact area between the fluid and the stator is increased, and so heat transfer is maintained for longer. Moreover, due to this configuration, the heat exchanger can deliver fluid through a large area of thermal contact in a relatively small volume. This allows for a more compact and less costly heat exchanger which is particularly suitable for home use. Furthermore, the heat exchanger of the invention does not need to be subject to high entry pressure to draw fluid through the heat exchanger. Instead, rotation of the rotor is sufficient to drive fluid through the heat exchanger due to fluid adhesion to the rotor surface. Because of this, the design of the heat exchanger is simpler and cheaper than those of the prior art that rely on external pumps to pressurise the system. Moreover, because of this arrangement, the width of the fluid channel or flow channel between the stator and the rotor can be minimised and so fhe fluid is provided with sheet-like geometry within the heat exchanger which in turn helps to further improve heat transfer between heat source and heat sink. The relative motion between the rotor and stator (i) constantly shears (reduces) the boundary layer of the fluid and (ii) highly disturbs the fluid, while propelling the fluid through the rotary heat exchanger. It may even form toroidal Taylor vortices and turbulence in the fluid at sufficiently high speeds. As such, heat transfer from the disturbed fluid to the stator is maximised. Furthermore, since the inlet is arranged close to the central axis L and the outlet is arranged away from the central axis L, net fluid flow is consistent with the direction of the centrifugal force acting on the fluid, and so the system has improved efficiencies for this reason too. Finally, the flow of fluid through the heat exchanger is achieved without the need for seals between moving parts. This again means a simpler and more cost effective design. The rotary heat exchanger described above is particularly suitable for heating and cooling homes. The heat exchanger- by way of forced convection between the fluid and the stator- can either heat or cool the fluid flowing therethrough. The fluid may be a liquid or a gas. Additionally or alternatively, for any radial cross-section of the heat exchanger, the rotor is uniform and the rotor extends both axially between the upper and lower sections of the stator and radially between the central axis and the outer section, such that rotation of the rotor causes fluid to flow both axially and radially along an interior flow surface of the stator between the inlet and the outlet. Since the rotor (and optionally the stator) is (or are) uniform in any plane extending radially away from the central axis (and / or in any radial cross-section of the heat exchanger), the rotor (and optionally the stator) may be understood to be symmetrical about the central axis. The term “radial cross-section” used throughout this application relates to a cross-section of the heat exchanger defined by the central axis and the plane that extends radially away from said central axis in any direction. Surrounds means extending all the way around the central axis, i.e., thereby forming a circle or the like around the central axis. The inlet may be arranged in either of the upper and lower sections, while the outlet may be arranged in the outer section. The rotor and the stator cavity are preferably concentrically arranged. Herein, "Out of may refer to extending out from, while "away from" may refer to extending in a direction away from (but not necessarily out from). The rotor may have an upper rotor surface and a lower rotor surface. The stator may have a first inner surface facing the upper surface of the rotor and a second inner surface facing the lower surface of the rotor. The interior flow surface may be provided by the first or second inner surface. Optionally, the first inner surface comprises an inner surface of the upper section of the stator. The second inner surface may comprise an inner surface of the lower section of the stator. The upper rotor surface is preferably the upper surface of the rotor that extends all the way between the inlet and outlet. Likewise, the lower rotor surface is preferably the lower surface of the rotor that extends all the way between the inlet and outlet. The inner surface of the upper section of the stator may be or include the downward-facing surface of upper section, while the inner surface of the lower section of the stator may be or include the upward-facing surface of the lower section. The interior flow surface may be provided by the first inner surface. The distance between the first inner surface and the upper surface of the rotor may be less than 4mm, preferably less than 3mm, more preferably less than 2mm, along the length of the first inner surface. By arranging the rotor and the stator so close together, the heat exchanger provides a large surface area to volume ratio for the fluid. Additionally or alternatively, the distance between the first inner surface and the upper surface of the rotor may be less than 3.5mm, less than 2.5mm, or less than 1,5mm, along the length of the first inner surface. Additionally or alternatively, the distance between the first inner surface and the upper surface of the rotor may be less than 1mm, less than 0.75mm, less than 0.5mm, or less than 0.25mm along the length of the first inner surface. Additionally or alternatively, the distance between the first inner surface and the upper surface of the rotor may instead be less than 0.6mm, less than 0.4mm, or less than 0.1mm, or less than 0.05mm, along the length of the first inner surface. Alternatively (or additionally), the interior flow surface may be provided by the second inner surface. The distance between the second inner surface and the lower surface of the rotor is preferably less than 4mm, preferably less than 3mm, more preferably less than 2mm, along the length of the second inner surface. Additionally or alternatively, the distance between the second inner surface and the lower surface of the rotor may be less than 3.5mm, less than 2.5mm, or less than 1.5mm, along the length of the second inner surface. Additionally or alternatively, the distance between the second inner surface and the lower surface of the rotor may be less than 1mm, less than 0.75mm, less than 0.5mm, or less than 0.25mm along the length of the second inner surface. Additionally or alternatively, the distance between the second inner surface and the lower surface of the rotor may instead be less than 0.6mm, less than 0.4mm, or less than 0.1mm, or less than 0,05mm, along the length of the second inner surface. In one embodiment, for any radial cross-section of the heat exchanger, the lower and upper sections of the stator have substantially the same radial length. The lower section of the stator may be arranged below the upper section such that the stator cavity defined therebetween is substantially rectangular. This configuration of stator can allow for simplified manufacture of the heat exchanger. Rectangular may include square. The stator may further comprise upper and / or lower radial flanges that extend radially outward from either or both of the upper and lower sections respectively. The rotor may have a substantially rectangular radial cross-section. Such a configuration is more easily manufactured. The rotor may comprise a rotor body and a rotor cavity defined within the rotor body. Optionally, the rotor cavity is also annular and surrounds the central axis. This “barrel” shaped configuration of the rotor ensures the rotor (and therefore the entire heat exchanger) is light. It also relies on minimal materials in its manufacturing, and allows for a simple design that mitigates potential for deformation and / or failure of parts. The rotor body and the rotor cavity are preferably concentric. In one embodiment, the stator comprises at ieast one annular stator protrusion that surrounds the central axis. For any radial cross-section of the heat exchanger, the or each stator protrusion may extend axially out of the upper section of the stator, through the stator cavity and towards the lower section of the stator. Alternatively or additionally, any radial cross-section of the heat exchanger, the or each stator protrusion may extend axially out of the lower section of the stator, through the stator cavity and towards the upper section of the stator. This increases the interior flow surface of the stator, thereby increasing heat transfer between the fluid and the stator. The stator protrusions or “fins” act as baffles to the movement of the fluid flowing radially outward through the heat exchanger compelling the fluid to move through a circuitous helical path up and down between the upper and tower sections of the stator before being expelled at the fluid outlet. If there is a plurality of stator protrusions, at least one first stator protrusion may extend out of the upper section and at least one second stator protrusion may extend out of the tower section. In these embodiments, the first and second stator protrusions may be arranged alternatingly, e.g., with one extending out of the upper section then one from the lower section etc.), or they may all extend out of one of the upper and lower sections. If there is a plurality of stator protrusions, each stator protrusion is preferably arranged at a different radial distance from the central axis, such that a concentric arrangement of stator protrusions is achieved. In these ways, the interior flow surface of the stator can be extended and heat transfer improved, while keeping the heat exchanger compact. For any radial cross-section of the heat exchanger, the or each stator protrusion may have a width that is substantially constant along the axial length thereof. The or each stator protrusion may be straight. Between the first and second ends of the or each stator protrusion, the or each stator protrusion may have substantially the same width. Optionally, the rotor comprises an annular radial portion surrounding the central axis and / or an annular axial portion surrounding the central axis. For any radial cross-section of the heat exchanger, the radial portion may extend radially away from the central axis towards the outer section. For any radial cross-section of the heat exchanger, the axial portion may extend axially out of the radial portion and between the upper and lower sections of the stator. This is one way of ensuring that fluid flows both axially and radially along the interior flow surface of the stator between the inlet and outlet. The radial portion may extend radially out of the central axle. For any radial cross-section of the heat exchanger, the axial portion preferably extends out of the radial portion proximal to the outermost end of the radial portion. The rotor may further comprise a plurality of annular axial portions surrounding the central axis. For any radial cross-section of the heat exchanger, each axial portion may extend axially out of the radial portion and between the upper and lower sections of the stator. This allows the fluid to flow close to and along the or each stator protrusion. If there is a plurality of axial portions, each axial portion is preferably arranged at a different radial distance from the central axis. If there are a plurality of axial portions and a plurality of stator protrusions, each axial portion and each stator protrusion is preferably arranged at a different radial distance from the central axis. In one preferred embodiment, the axial portions and stator protrusions are arranged aiternatingly e.g., one stator protrusion, then one axial portion etc. For any radial cross-section of the heat exchanger, the or each axial portion may have a width that is substantially constant along the axial length thereof. The or each axial portions may be straight. Between the first and second ends of the or each axial portion, the or each axial portion may have substantially the same width. For any radial cross-section of the heat exchanger, the or each axial portion of the rotor may be arranged between two neighbouring stator protrusions or between the outer section of the stator and the or a neighbouring stator protrusion. For any radial cross-section of the heat exchanger, the or each stator protrusion may have a width that substantially decreases as the or each stator protrusion extends away from the adjoining upper or lower section of the stator. The or each stator protrusion may have a wedge-shaped radial cross section. The or each stator protrusion may taper towards a point at an end displaced away from the adjoining upper or lower section of the stator. The or each stator protrusion may be hollow such that they have the thicknesses described above. In one embodiment, the stator comprises a plurality of annular stator protrusions. The stator protrusions may comprise at least one first stator protrusion extending axially out of the upper section, through the stator cavity and towards the lower section of the stator. The stator protrusions may comprise at least one second stator protrusion extending axially out of the lower section of the stator, through the stator cavity and towards the upper section of the stator. In this way, the interior flow surface of the stator can be further extended. In this embodiment, the first and second stator protrusions may be arranged aiternatingly, e.g., one extending from the upper section, then one extending from the lower section, one from the upper section etc.). Each stator protrusion is preferably arranged at a different radial distance from the central axis, such that a concentric arrangement of stator protrusions is achieved. Optionally, the rotor comprises a meander portion. For any radial cross-section of the heat exchanger, the meander portion may extend away from the central axis, and meander around the first and second stator protrusions between the first and second portions of the stator, as it extends towards the outer section. This ensures the flow of the fluid along the embodiment of the stator where stator protrusions extend from both above and below. For any radial cross-section of the heat exchanger, the meander portion may have a width that is substantially the same along its length. For any radial cross-section of the heat exchanger, the meander portion may be made up of a plurality of substantially straight line segments each including a first end and a second end. Each line segment may be connected to exactly one other line segment at its first and / or second end. The meander portion may form a zig-zag. For any radial cross-section of the heat exchanger, the angle defined between each neighbouring pair of segments may be less than 90°. Preferably the angle between each neighbouring pair of segments is between 45° and 75°, more preferably 50° and 70°, more preferably 55° and 60°, and most preferably 60°. For any radial cross-section of the heat exchanger, the angle defined between each neighbouring pair of segments may be approximately 90 °. In one embodiment, the heat exchanger comprises a further inlet for directing fluid into the stator. The further inlet may be arranged proximal to the central axis. The heat exchanger may comprise a further outlet for directing fluid out of the stator. The further outlet may be arranged distal to the central axis. The rotor is optionally configured such that rotation thereof causes fluid to flow both axially and radially along a further interior flow surface of the stator between the further inlet and further outlet. The heat exchanger defines two separate fluid paths therethrough in this arrangement. The further inlet, the further outlet and / or the further interior flow surface may be distinct from the inlet, the outlet and / or the interior flow surface. The interior flow surface may be provided by the first inner surface and the further interior flow surface may be provided by the second inner surface. Optionally, the rotor comprises or consists of the annular radial portion and the annular axial portion. For every radial cross-section of the heat exchanger, the axial portion may extend out of the radial portion proximal to an outer end of the radial portion The stator may be shaped such that the stator cavity has an L-shaped radial cross-section, This “bell-shaped” arrangement is light, compact and simple to manufacture. The rotor may comprise or consist of a diagonally-extending portion extending both radially and axially away from the inlet and towards the outlet and at an oblique angle with respect to the central axis. Oblique may mean non-perpendicular here. Preferably, the stator is shaped such that the stator cavity has a rhomboid or rhombic radial cross-section. This “conical” arrangement is light, compact and simple to manufacture. The rotary heat exchanger may comprise two rotors. Optionally, the stator comprises a middle section arranged between the upper and lower sections. A first rotor may be arranged between the upper and middle sections of the stator. A second rotor may be arranged between the middle and lower sections of the stator. The heat exchanger can also perform heat exchange on two fluids concurrently. The heat exchanger can process double the fluid, but with the same heat exchanging efficiency. In one embodiment, the rotary heat exchanger also comprises a cooling channel arranged to convey coolant fluid therethrough. The cooling channel may extend at least partially through the stator. Heat transfer occurs in both the rotor and stator. Such cooling channels) through the stator allow for increased heat transfer, as the coolant fluid is able to absorb heat and be continuously refreshed throughout the system. When the stator comprises stator protrusions, the cooling channel preferably extends through the or each stator protrusion. In one particularly preferred embodiment, the cooling channel extends along the entire length of the or each stator protrusion. The heat exchanger may comprise an axle that may extend along the central axis. The axle may be configured to rotate about the central axis. The rotor may be coupled to the axle such that the rotor and axle rotate together. This position of the axle allows for even distribution of fluid the throughout heat exchanger, and provides a compact arrangement of the heat exchanger. In this embodiment, the annular cavity may be defined between the outer section and the central axle. The stator may substantially be made of copper, aluminium or stainless steel, and the rotor may substantially be made of copper, aluminium or stainless steel. This ensures efficient heat transfer in the heat exchanger. Other materials for the stator and the rotor may be used. For example, the rotor and / or stator may substantially be made of a polymeric material. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of exampie only, with reference to the accompanying drawings, in which: Figure la shows a cross-sectionai side view of a rotary heat exchanger according to a first embodiment of the invention; Figure 1 b shows a perspective view of the rotary heat exchanger of Figure 1a; Figure 1c shows a side view of the rotary heat exchanger of Figure 1a; Figure 1d shows an expioded view of the rotary heat exchanger of Figure 1a; Figure 1e shows a top-down view of a rotary heat exchanger according to Figure 1a with the stator upper section and rotor removed; Figure 1 f shows a schematic view of a portion of the rotary heat exchanger of Figure 1a, showing the direction of flow of fluid therethrough; Figure 2a shows a cross-sectionai side view of a rotary heat exchanger according to a second embodiment of the invention; Figure 2b shows a perspective view of the rotary heat exchanger of Figure 2a; Figure 2c shows a side view of the rotary heat exchanger of Figure 2a; Figure 2d shows an expioded view of the rotary heat exchanger of Figure 2a; Figure 2e shows a top-down view of a rotary heat exchanger according to Figure 2a with the stator upper section and rotor removed; Figure 3 shows a cross-sectional side view of a rotary heat exchanger according to a third embodiment of the invention; Figure 4 shows a cross-sectional side view of a rotary heat exchanger according to a fourth embodiment of the invention; Figure 5 shows a cross-sectional side view of a rotary heat exchanger according to a fifth embodiment of the invention; Figure 6 shows a cross-sectional side view of a rotary heat exchanger according to a sixth embodiment of the invention; Figure 7 shows a cross-sectional side view of a rotary heat exchanger according to a seventh embodiment of the invention; Figure 8 shows a cross-sectional side view of a rotary heat exchanger according to an eighth embodiment of the invention; DETAILED DESCRIPTION Figures 1 to 8 show a rotary heat exchanger 100, 200, etc according to different embodiments of the invention. The rotary heat exchanger 100, 200, etc has a central axis L. In each embodiment, the rotary heat exchanger 100, 200, etc comprises an annular rotor 102, 202, etc and a stator 104, 204, etc. The rotor 102, 202, etc surrounds, and is configured to rotate about, the central axis L. The stator 104, 204, etc functions as a housing for the heat exchanger 100, 200, etc and encloses the annular rotor 102, 202, etc. To this end, the stator 104, 204, etc comprises an upper section 108, 208, etc, a lower section 110,210, etc arranged axially away from the upper section 108, 208, etc, and an annular outer section 112, 212, etc (or outer wall). The upper section 108, 208, etc extends radially away from the central axis L to define an annular upper perimeter 114, 214, etc, which surrounds the central axis L. The lower section 110, 210, etc extends radially away from the central axis L to define an annular lower perimeter 116, 216, etc, which also surrounds the central axis L The annular outer section 112, 212, etc extends axially between the upper and lower perimeters 114, 214, etc and 116, 216, etc, and also surrounds the central axis L. An annular stator cavity 118,218, etc is defined between the upper and lower sections 108, 208, etc and 110, 210, etc, and between the outer section 112, 212, etc and the central axis L The annular stator cavity 118, 218, etc thus surrounds the central axis L. The rotor 102, 202, etc is arranged within the annular stator cavity 118, 218, etc. The rotary heat exchanger 100, 200, etc also comprises an inlet 120, 220, etc and an outlet 122, 222, etc. The inlet 120, 220, etc is arranged proximal to the central axis L, and is used to direct fluid into the stator 104, 204, etc. The outlet 122, 222, etc is arranged distal from the central axis L, and is used to direct fluid out of the stator 104, 204, etc. In any plane extending radially away from the central axis (or for any radial cross-section of the heat exchanger 100, 200, etc), the cross-section of the rotor 102, 202, etc is uniform and the rotor 102, 202, etc extends both 10 axially between the upper and lower sections of the stator 108, 208, etc and 110, 210, etc and radially between the central axis L and the outer section 112, 212, etc, such that rotation of the rotor 102, 202, etc causes fluid to flow both axially and radially along an interior flow surface 124, 224, etc of the stator 104, 204, etc between the inlet 120, 220, etc and the outlet 122, 222, etc. Due to this configuration, rotation of the rotor 102, 202, etc causes the fluid to move “helically’ or “spirally” (i.e., both axially and radially) as it moves from the centre of the heat exchanger 100, 200, etc towards its perimeter. As such, the path of the fluid through the rotor 102, 202, etc between the inlet 120, 220, etc and outlet 122, 222, etc is extended, and hence heat transfer between the fluid and the stator 104, 204, etc is sustained for longer. This is because as the distance travelled by the fluid is increased, the contact time and the contact area between the fluid and the stator 104, 204, etc is increased, and so heat transfer is maintained for longer. Moreover, due to this configuration, the heat exchanger 100, 200, etc can deliver fluid through a large area of thermal contact in a relatively small volume. This allows for a more compact and less costly heat exchanger 100, 200, etc, which is particularly suitable for home use. Furthermore, the heat exchanger 100, 200, etc of the invention does not need to be subject to high entry pressure to draw fluid through the heat exchanger 100, 200, etc. Instead, rotation of the rotor 102, 202, etc is sufficient to drive fluid through the heat exchanger 100, 200, etc due to fluid adhesion to the rotor surface. Because of this, the design of the heat exchanger 100, 200, etc is simpler and cheaper than those of the prior art that rely on external pumps to pressurise the system. Moreover, because of this arrangement, the width of the fluid channel or flow channel 126, 226, etc between the stator 104, 204, etc and the rotor 102, 202, etc can be minimised and so the fluid is provided with sheet-like geometry within the heat exchanger 100, 200, etc, which in turn helps to further improve heat transfer between heat source and heat sink. The relative motion between the rotor 102, 202, etc and stator 104, 204, etc (i) constantly shears (reduces) the boundary layer of the fluid and (ii) highly disturbs the fluid, while propelling the fluid through the rotary heat exchanger 100, 200 etc. It may even form toroidal Taylor vortices and turbulence in the fluid at sufficiently high speeds. As such, heat transfer from the disturbed fluid to the stator is maximised. Furthermore, since the inlet 120, 220, etc is arranged close to the central axis L and the outlet 122, 222, etc is arranged away from the central axis L, net fluid flow is consistent with the direction of the centrifugal force acting on the fluid, and so the system has improved efficiencies for this reason too. Finally, the flow of fluid through the heat exchanger 100, 200, etc is achieved without the need for seals between moving parts. This again means a simpler and more cost effective design. The rotary heat exchanger 100, 200, etc described above is particularly suitable for heating and cooling homes. The heat exchanger 100, 200, etc - by way of forced convection between the fluid and the stator 104, 204, etc - can either heat or cool the fluid flowing therethrough. The fluid may be a liquid or a gas. Now, each of the embodiments of Figures 1 to 8 will be described in turn. Figures 1a to 1fshow a first embodiment according to the invention. As shown in Figure 1a, the heat exchanger extends along central axis L. The upper 108, lower 110 and outer 112 sections of the stator 104 define the outer enclosure of the heat exchanger 100, which in this embodiment is substantially cylindrical (as best seen in Figure 1b). The upper and lower sections 108, 110 of the stator 104 take the form of substantially circular plates with the upper section 108 of the stator 104 being arranged on top of the lower section 110 ofthe stator 104. As such, the upper and lower perimeters 114, 116 take the form of (substantially circular) circumferences. The upper and lower sections 108, 110 ofthe stator 104, and therefore the upper and lower perimeters 114, 116, have substantially the same diameter and are arranged co-axially about the central axis L. The upper and lower sections 108, 110 extend in the plane perpendicular to the central axis L. The outer section 112 ofthe stator 104 is annular and takes the form of a curved surface or wall that extends all the way around the central axis L and axially (i.e., vertically) between the upper and lower perimeters 114, 116. As best seen in Figure 1c, although the outer section 112 extends between the upper and lower perimeters 114, 116, it is not necessary for it to have direct contact with the upper and lower perimeters 114, 116. Instead, the outer section 112 can extend between the upper and lower sections 108, 110 such that the heat exchanger 100 generally takes the shape of a spool, wherein the upper and lower perimeters 114, 116 extend more radially outward than the stator outer section 112, defining upper and lower radial flanges on the upper and lower sections 108, 110. However, in other embodiments the outer section 112 may make direct contact with the upper and lower perimeters 114,116, such that the heat exchanger 100 takes the shape of a right circular cylinder, i.e., without said radial flanges on the upper and lower sections 108, 110. As shown in Figure 1d, the lower section 110 ofthe stator 104 may comprise an annular flange 128 and a circular base plate 130 arranged thereunder. The flange 128 is attached to the base portion 130 by way of an airtight joint. To this end, the flange 128 may be either mechanically connected (e.g. by screws and optionally seals), thermally connected (e.g. by welding or brazing) and / or chemically connected (e.g. by adhesive) to the base portion, 130. In one embodiment, the upper section 108, the outer section 112 and the flange 128 may be secured together as a cover. Hence, to open up the stator 104 and access the stator cavity 118 therein (e.g., for maintenance), a user needs only to unfasten removable fixings such as screws between the flange 128 and the base portion 130 and lift the cover off the base portion 130, Additionally or alternatively, the upper section 108 of the stator 104 may be provided with the same “flanged" configuration, only inverted. In this embodiment, the heat exchanger 100 may be configured such that a user may only need to remove a top plate to access the stator cavity 118. In one embodiment, the stator outer section 112 may be formed integrally with the stator upper section 108 and / or the stator lower section 110. In other embodiments, they may be formed separately and then secured together with e.g., latches or the like. The inlet 120 of the heat exchanger 100 is provided in the stator upper section 108 adjacent to and at least partially surrounding the central axis L (as best seen in Figure 1b). The inlet 120 of the first embodiment is made up of several arc-shaped openings defined in the stator upper section 108. Alternatively, the inlet 120 can be provided by way of one annular opening which surrounds (i.e., completely) the central axis L, or else may be provided by way of an opening arranged radially away from the centre of the stator upper section 108 (i.e., such that the opening is not arranged symmetrically about the central axis L) e.g., in the form of a nozzle extending out of the stator upper section 108. The outlet 122 of the heat exchanger 100 is instead provided in the outer section 112 of the stator 104 at the bottom thereof (as best seen in Figures 1c and Ie). To this end, the outer section 112 may define an outlet flange 132 that extends outwardly from the outer section 112 and curves around the outer section 112 in order to aid in the expulsion of fluid from the heat exchanger 100, The top of the outlet flange 132 is covered so as to provide a channel having a square or circular-shaped cross-section and a gradually increasing width as it curves around the stator outer section 112. In this way, the outlet 122 takes the form of a volute (or a nozzle). Other variations of the outlet 122 are possible. The fluid enters the heat exchanger 100 through the inlet 120 and is moved by the rotor 102 radially outward and towards the outlet 122. In particular, the fluid is compelled, by the relative motion of the rotor 102 and stator 104 and the resulting pressure gradient, to move from the central inlet 120 and to be expelled at the outlet 122 at the perimeter of the heat exchanger 100. The inlet 120 and the outlet 122 may represent the only way in which fluid can enter or exit the heat exchanger 100. No fluid can exit (e.g., leak) from the stator 104 except through the outlet 122. As stated above, the upper 108, lower 110 and outer 112 sections define the annular stator cavity 118 therein. Due to the above-described configuration of the upper and lower sections 108,110 of the stator 104, the stator cavity 118 defined therebetween is substantially rectangular in the radial cross-section. The term “radial cross-section” used throughout this application relates to a cross-section of the heat exchanger 100 defined by the central axis L and the plane that extends radially away from said central axis L in any direction, demonstrated in the figures as Rc. As best seen in Figure la, the stator 104 is additionally provided with at least one annular stator protrusion 134 between the central axis L and the outer section 112. These stator protrusions 134 (also referred to as ‘stator fins’) extend directly downward, i.e. axially, from the upper section 108 of the stator 104 and through the stator cavity 118. The stator fins 134 extend towards (and preferably close to) the lower section 110 of the stator 104. The minimum axial distance the stator fins 134 can extend across the stator cavity 118 may be approximately 60%, 65%, 70%, 75%, 85%, 90% or 95% (preferably 75%) of the axial length of the stator cavity 118. Each stator fin 134 has a radial width or thickness that is substantially constant along the axial length thereof, such that the stator fins 134 extend straight down. All stator fins 134 have substantially the same thickness. As best seen in Figure 1e, each stator fin 134 has a substantially circular cross-section in the plane perpendicular to the central axis L and shares a centre at central axis L. Since each stator fin 134 is provided with a different diameter, the stator fins 134 are arranged concentrically and coaxially about the central axis L. The outer diameter of each stator fin 134 increases from the innermost stator fin 134 (which is proximal to the central axis) to the outermost stator fin 134 (which is proximal to the stator outer section). The stator fins 134 are substantially equally spaced out between the central axis L and the outer section 112. Wille the Figures show this embodiment as having four stator fins 134, in other embodiments the heat exchanger 100 may be provided with more or fewer stator fins 134. As stated above, the annular rotor 102 is also arranged within the annular stator cavity 118 and hence is fully enclosed by the stator 104. To allow the rotor 102 to rotate about the central axis L, the heat exchanger 100 is provided with an axle 136 that extends along the central axis L and through the centre of the heat exchanger 100. As best seen in Figure 1 d, the axle 136 extends through the middle of the upper and lower sections 108, 110 of the stator 104. The rotor 102 is coupled to the axle 136 such that the rotor 102 and axle 136 rotate together about the central axis L. In this way, the stator 104 and rotor 102 are mounted coaxially about the central axis L. This position of the axle 136 allows for even distribution of fluid throughout the heat exchanger 100, and provides a compact arrangement of the heat exchanger 100. Bearings 138 are provided in the upper and lower sections 108, 110 of the stator 104 around the central axis L, which allow the central axis L (and hence the rotor 102) to rotate. The rotor 102 can be rotated clockwise or counter-clockwise within the stator 104, As best seen in Figure la, the rotor 102 is provided with an annular radial portion 140 in the form of a circular plate. The radial portion 140 extends radially away from the bottom of the central axle 136, i.e., in the plane perpendicular to the central axis L. The rotor 102 is further provided with at least one annular axial portion (or protrusion) 142 between the central axis L and the outer section 112. These axial protrusions 142 (also referred to as ‘rotor fins’) extend directly upward, i.e., axially, out of the radial portion 140 and through the stator cavity 118. The rotor fins 142 extend towards (and preferably close to) the upper section 108 of the stator 104. Each rotor fin 142 in the radial crosssection has a width or thickness that is substantially constant along the axial length thereof, such that the rotor fins 142 extend straight up. All rotor fins 142 have substantially the same thickness. As best seen in Figure 1d, each rotor fin 142 has a substantially circular cross-section in the plane perpendicular to the central axis L and shares a centre at central axis L. Since each rotor fin 142 is provided with a different diameter, the rotor fins 142 are arranged concentrically and coaxially about the central axis L. The outer diameter of each rotor fin 142 increases from the innermost stator fin 142 (which is proximal to the central axis L) to the outermost stator fin 142 (which is proximal to the stator outer section 112). The rotor fins 142 are substantially equally spaced out between the central axis L and the outer section 112. It can be seen from Figure 1a that the outermost rotor fin 142 is positioned slightly inwards from the outer perimeter of the annular radial portion 140 of the rotor 102, defining an annular extending portion 144 of the annular radial portion 140, which is housed within a corresponding annular notch 146 in the stator outer section 112 nearthe outlet 122, The extending portion 144 aids the rotor 102 in expelling fluid inside the rotary heat exchanger 100 out of the outlet 122. While the Figures show this embodiment as having four rotor fins 142, in other embodiments the heat exchanger 100 may be provided with more or fewer rotor fins 142. In any case, it is preferable for there to be an equal number of stator fins 134 and rotor fins 142. The rotor 102 is preferably formed integrally and is hence strong and robust. Figure 1 a best shows the arrangement of the rotor fins 142 and the stator fins 134 within the heat exchanger 100 when the heat exchanger 100 is assembled together. Since each rotor fin 142 has a different diameter to each stator fin 134 in the plane perpendicular to the central axis L, the rotor fins 142 (which extend upwards) are arranged to interleave with the stator fins 134 (which extend downwards). In this way, each rotor fin 142 is either arranged between two neighbouring stator fins 134 (or between the stator outer section 112 and the outermost stator fin 134), and each stator fin 134 is arranged between two neighbouring rotor fins 142. The rotor and stator fins 142, 134 are substantially equally spaced out between the central axis L and the outer section 112. The rotor and stator fins 142, 134 act as baffies to the movement of the fluid flowing radially outward through the heat exchanger 100 compelling the fluid to move through a circuitous helical path up and down between the upper and lower sections 108, 110 of the stator 104 before being expelled at the fluid outlet 122. While Figure 1 a shows the thickness of each stator fin 134 being greater than the thickness of each rotor fin 142 in the radial cross-section, each stator fin 134 and each rotor fin 142 may also have the same thickness, or each rotor fin 142 may have a thickness greater than the thickness of each stator fin 134. The axial length of each rotor fin 142 and each stator fin 134 is slightly less than the axial length of the stator cavity 118. This results in an upper gap between the top of each rotor fin 142 and the stator upper section 108, and a lower gap between the bottom of each stator fin 134 and the annular radial portion 140. Moreover, the radial length between two adjacent stator fins 134 (or between the stator outer section 112 and the outermost stator fin 134) is larger than the radial thickness of the rotor fin 142 sandwiched therebetween, and the radial length between two adjacent rotor fins 142 is larger than the thickness of the stator fin 134 sandwiched therebetween, such that there are also lateral gaps between each rotor fin 142 and its neighbouring stator fin(s) 134 (or the stator outer section 112). These lateral gaps, along with the upper and Sower gaps, define a flow channel 126 above the rotor 102 that extends all the way through the heat exchanger 100 between the inlet 120 and the outlet 122, and it is this flow channel 126 through which fluid flows through the heat exchanger 100. As best seen in Figure 1a and 1f, the flow channel 126 follows a meandering path between the inlet 120 and the outlet 122. The interior flow surface 124 of the stator 104, discussed above, defines the upper border of the flow channel 126, while the upper surface of the rotor (facing the interior flow surface 124) defines the lower border of the flow channel 126. As the rotor 102 rotates, the rotor 102 forces the fluid against the interior flow surface 124 of the stator 104, thereby improving heat transfer between the two. More specifically, in this embodiment, the interior flow surface 124 is provided by the exposed surfaces of the stator fins 134, the downwardly-facing surfaces of the stator upper section 108 and the inwardly facing surface of the stator outer section 112, while the upper surface of the rotor 102 is provided by the exposed surfaces of the rotor fins 142 and the upwardly-facing surfaces of the annular radial portion 140. The width of the flow channel 126 is substantially constant along its entire length between the inlet 120 and the outlet 122. In other words, the distance between the interior flow surface 124 and the upper surface of the rotor 102 (i.e., between corresponding / neighbouring points thereon) is substantially constant along the entire length of the flow channel 126. In particular, this width may be less than 4mm, preferably less than 2 mm, more preferably less than 1 mm, more preferably less than 0.75 mm, more preferably less than 0.5 mm, more preferably less than 0,25 mm along the length of the rotor 102. This would provide the smallest possible gap for the fluid to flow through, thereby providing a large surface area to volume for the fluid. In terms of materials, the housing and stator 104 are formed from a thermally conducting material, for example a thermally conducting metal such as copper, aluminium or stainless steel. The stator 104 is preferably highly thermally conducting, and so may be made from copper or aluminium. The rotor 102 does not need to be as thermally conducting and so may be made of stainless steel (or possibly aluminium so as to be weight saving). Other materials for the stator 104 and the rotor 102 are possible. For example, in one embodiment, the rotor 102 may be formed of a polymeric material with heat resistance, low coefficient of thermal expansion and high stiffness to reduce the weight and energy required to rotate and disturb the air throughout the rotary heat exchanger 100. In one preferred embodiment, the rotor 102 and / or the stator 104 are coated with one or more suitable wetting or anti-wetting agent so as to improve the spread of the fluid and improve heat transfer. Additionally or alternatively, the surfaces of the rotor 102 and / or the stator 104 may contain (e.g.,micro-)texturing for example in the form of etching, to increase fluid adhesion / turbulence and hence heat transfer. The second embodiment as shown in Figures 2a to 2e is the same as the embodiment of Figures 1a to 1f except that the stator 204 does not include stator fins and the rotor 202 does not include rotor fins. Instead, the rotor 202 of Figure 2 is “barrel-shaped” in the radial cross-section. In more detail, the rotor 202 takes the form of an annular chamber that extends all the way around the central axis L. The annular chamber defines an annular rotor cavity 248 therein that also surrounds the central axis L. To this end, the annular chamber is made up of a substantially circular top plate 250 and a substantially circular bottom plate 252 (corresponding to the annular radial portion 140 of the first embodiment), with the top plate 250 being spaced axially away from, and arranged on top of, the bottom plate 252. The top plate 250 and the bottom plate 252 have substantially the same diameter and extend in the plane perpendicular to the central axis L. The top plate 250 and the bottom plate 252 are arranged co-axially about the central axis L, and the central axle 236 extends through (and couples to the) rotor at the centre of the annular chamber. The top plate 250 is positioned proximal to the stator upper section 208, while the bottom plate 252 is positioned proximal to the stator lower section 210. The annular chamber also includes an outer curved plate 256 that extends all the way around the central axis L and axially (i.e., vertically) between the upper and lower perimeters of the top and bottom plates 250, 252. The outer curved plate 256 is positioned proximal to the stator outer section 212. Hence, the top and bottom plates 250, 252, the outer curved plate 256 and the central axle 236 define a cavity 248 with a substantially rectangular radial-cross section. In this embodiment, the rotor chamber being hollow (i.e., barrel-shaped) 17 provides a light heat exchanger 200, however in other embodiments the rotor 202 may be solid or filled and not include any rotor cavity 248 at all. The rotor 202 is sized and positioned within the stator cavity 218 such that there is a gap above and outward of the rotor 202 (between the rotor 202 and the stator 204) that defines the flow channel 226. It can be seen from Figure 2 that the length of this How channel 226 is shorter than the length of the flow channel 126 of the first embodiment, and provides a more direct path between the inlet 220 and the outlet 222. The Interior flow surface 224 of the second embodiment is provided by the downward-facing surface of the stator upper section 208 and the Inward-facing surface of the stator outer section 212. The interior flow surface 224 faces the upper surface of the rotor 202, which includes the upward-facing surface of the top plate 250 and the outwardly-facing surface of the outer curved plate 256. The third embodiment shown in Figure 3 is the same as the embodiment of Figures 1a to 1f except that the stator 304 includes both upper and lower stator fins 358, 360, said stator fins 358, 360 are wedge-shaped and the rotor 302 undulates as it extends away from the central axis L around the stator fins 358, 360 such that the radial cross-section of the rotor 302 is zig-zagged. While in the first embodiment, the stator fins 134 only extend downward out of the upper section of the stator 108, in this third embodiment, the stator 304 includes both upper stator fins 358 that extend downward (i.e., axially) from the upper section of the stator 308 as well as lower stator fins 360 that extend upward (i.e., axially) from the lower section 310 of the stator 304. In more detail, each upper stator fin 358 extends from the stator upper section 308, down through the stator cavity 318, and towards the stator lower section 310, while each lower stator fin 360 extends from the stator lower section 310, up through the stator cavity 318, and towards the stator upper section 308. The upper and lower stator fins 358, 360 are wedge-shaped. In other words, in any radial cross-section, the width of each stator fin 358, 360 substantially decreases as it extends axially away from its adjoining section of the stator 304. In other words, each stator fin 358, 360 tapers towards a point in the radial cross-section. Because of this, the stator fins 358, 360 can be understood as a plurality of (annular) ridges. The stator 304 of Figure 3 is provided with two annular upper stator fins 358, two annular lower stator fins 360 and a central lower fin 362 (although other numbers of fins are possible). The two annular upper stater fins 358 and the two annular lower stator fins 360 are arranged concentrically and co-axially about the central lower stator fin 362, which is arranged at the central axis L. The two annular upper stator fins 358 and the two annular lower stator fins 360 have substantially circular cross-sections in the plane perpendicular to the central axis L. Since each of the two annular upper stator fins 18 358 and the two annular lower stator fins 360 has a different diameter (at e.g,, the tapered points thereof), the lower stator fins 360 (which extend upwards) can be arranged to interleave with the upper stator fins 358 (which extend downwards). In this way, each lower stator fin 360 can be arranged between two neighbouring upper stator fins 358 (or between the stator outer section 312 and the outermost upper stator fin 358), and each upper stator fin 358 can be arranged between two neighbouring lower stator fins 360, as shown in Figure 3. The central lower stator fin 362 is instead arranged within the innermost upper stator fin 358. In this way, the upper and lower stators fins 358, 360, 362 can be understood to be alternately disposed. The axial length of each stator fin 358, 360, 362 is less than the axial length of the stator cavity 318. This results in an opening in the stator 304 (i.e., within the stator cavity 318) with a zig-zagged radial cross-section. The outer section 312 of the stator 304 is also wedge-shaped, in that it has a wider radial cross-section where it adjoins the upper section 308 of the stator 304 and a narrower radial cross-section where it adjoins the lower section 310 of the stator 304. The outer section 312 further defines a recess at its bottom 346 to house the annular extending portion 344 of the rotor 302, as will be described further below. Due to the rotation of the rotor 302, fluid is drawn into the inlet 320 and moves in a helical fashion up and down between the stator fins 358, 360, 362, outward from the radial centre to the circumference of the heat exchanger 300, to be expelled off the annular extending portion 344 at the outlet 322. Unlike in the embodiment of Figures la to If, where the central axle 136 extends all the way through the heat exchanger 100 (between the upper and lower sections 108, 110 of the stator 104), in the third embodiment of Figure 3, the central axle 336 extends only part way through the heat exchanger 300, namely down along the central axis L through the stator upper section 308 and midway into the stator cavity 318, where it then couples to the rotor 302. Bearings 338 are provided in the upper section 308 of the stator 304 near the inlet 320, and either side of the central axle 136. The bearings 338 ensure coaxial rotation of the rotor 302 relative to the stator 104. The rotor 302 comprises (or consists of) a meander portion 364 that extends outwardly from the central axis L and towards the outer section 312 and the annular extending portion 344 extending out therefrom. The meander portion 364 meanders around the upper and lower stator fins 358, 360, 362 through the zig-zagged opening defined between the upper and lower stator sections 312, 314. The meander portion 364 therefore mirrors the zig-zagged opening and has a zig-zag-shaped profile in any radial cross-section also. In this way, the rotor 302 can be understood to undulate as it extends away from the central axle 336. To this end, the meander portion 364 is made up of a plurality of substantially flat plate segments. Each flat plate includes an outer perimeter and an inner perimeter (both extending around the central axis L), and each flat plate segment is connected to exactly one other flat plate segment at its outer and / or inner perimeter. In other words, in the radial cross-section, the meander portion 364 is made up of a plurality of substantially straight line segments. Each straight line segment includes a first end and a second end, and each line segment is connected to exactly one other line segment at its first and / or second end. In this embodiment, the angle defined between each neighbouring pair of segments is less than 90°, and is preferably around 60° (as shown in Figure 3). The zig-zagged opening is larger than the zig-zagged rotor 302, such that there is a gap between the rotor 302 and the stator 304 above the rotor 302 that defines the flow channel 326 for the fluid between the inlet 320 and the outlet 322. The interior flow surface 324 of the third embodiment is provided by the exposed surfaces of the upper stator fins 358 and the inwardly-facing surface of the outer section 312. Hence, the interior flow surface 324 faces the upper surface of the rotor 302 in this embodiment. The fourth embodiment as shown in Figure 4 is the same as the first embodiment, with the addition of a system of cooling channels 466, 468 that extend through the stator fins 434 of the stator 404. Coolant fluid such as water is fed through said cooling channels so as to aid the transfer of heat away from the fluid being cooled by the heat exchanger 400. This improves heat transfer at the stator fins 434. To this end, each stator fin 434 is provided with an inner cooling channel 466, and an outer cooling channel 468 coupled together. For example, the innermost stator fin 434 has an inner cooling channel 466 with a semi-circular radial crosssection, with the fiat edge proximal to the side of the stator fin 434 which is nearer the central axis L. The inner cooling channel 466 helically coils downwards through the innermost stator fin 434 to the bottom thereof. At the bottom of the stator fin 434, the inner cooling channel 466 feeds into the outer cooling channel 468 for the same stator fin 434. The outer cooling channel 468 then helically coils upwards through the innermost stator fin 434 to the top thereof. The outer cooling channel 468 also has a semi-circular radial cross-section, but with the fiat edge proximal to the side of the stator fin 434 which is further from the central axis L. In this way, a compact and efficient arrangement of the inner and outer cooling channels 466, 468 is achieved in each stator fin 434, and each part of the stator fin 434 is effectively cooled thereby. The outer cooling channel 468 of the innermost stator fin 434 then extends through the stator upper section 408, where it feeds into the inner cooling channel 466 of the next adjacent stator fin 434 arranged outwardly thereof. To connect the outer cooling channel 468 and inner cooling channel 466 of neighbouring stator fins 434, the system of cooling channels 466, 468 is provided with a transfer tube that extends at least partially through the upper section 408 of the stator 400. The transfer tube may pass outside the stator 400 or else be completely contained within the stator 400. Each stator fin 434 has the same arrangement of the inner and outer cooling channels 466, 468 as the innermost stator fin 434. However, the outer cooling channel 468 of the outermost stator fin 434 feeds into an inner cooling channel 470 arranged in the outer section 412 of the stator 404, as best seen in Figure 4. Once the inner cooling channel 470 of the outer section 412 has helically coiled down through the outer section 412, it then exits the heat exchanger 400 at the bottom thereof. To feed cooling fluid into the stator 404, the heat exchanger 400 is provided with a cooling fluid inlet 472, which passes through the stator upper section 408 and feeds into the inner cooling channel 466 of the innermost stator fin 434. Likewise, the cooling channel 470 of the outer section 412 feeds into a cooling fluid outlet 474 defined in the bottom of the outer section 412 of the stator 404, to allow the cooling fluid to exit the heat exchanger 400. Cooling fluid from the cooling fluid outlet 474 can then e.g., be cooled before being sent back into the cooling fluid inlet 472 for further cooling of the heat exchanger 400. In alternative embodiments, each stator fin 434 may have a cooling fluid inlet and a cooling fluid outlet, providing a separate cooling system for each stator fin 434. Additionally or alternatively, there may be cooling channels provided in the rotor fins 442. Alternatively, each stator fin 434 may instead be hollow to define a single cooling fluid channel in each stator fin 434, through which may continuously pass cooling fluid. The fifth embodiment as shown in Figure 5 is the same as the first embodiment as shown in Figures 1a to 1f, except that the rotor 502 comprises only one radial portion 540 and one rotor fin 542 (and in some embodiments also an annular extending portion 544), and that the stator 504 and the rotor 502 have a substantially L-shaped radial cross-section, as will be explained below. The upper section 508 and the lower section 510 of the stator 504 of this embodiment are the same except that the lower section 510 of the stator 504 has a smaller radial length than the upper section 508, and said lower section 510 is arranged directly under only the outermost portion of the upper section 508, such that the lower section 510 does not extend all the way to the central axis L in this embodiment. As such, the lower section 510 of the stator 504 has the form of a substantially circular plate defining a substantially circular opening therein, i.e., it is substantially annular. The lower section 510 of the stator 504 therefore defines an inner perimeter 576 that surrounds the central axis L, said perimeter 576 taking the form of a substantially circular circumference. Towards the centre of the heat exchanger 500, the stator 504 is provided with an underside section (or radial wall) 578 that takes the form of a substantially circular plate extending in the plane perpendicular to the central axis L. The underside section 578 is also arranged concentrically and co-axially about the central axis L, but since the underside section 578 has a smaller diameter than the upper section 508 of the stator 504, the underside section 578 is arranged directly under only the innermost portion of the upper section 508. The underside section 578 of the stator 504 therefore defines an outer perimeter 580 that surrounds the central axis L in the form of a substantially circular circumference. The underside section 578 and the upper section 21 508 of the stator 504 extend radially away from the central axis L in parallel, and the underside section 578 is arranged proximal to toe upper section 508 such that a thin and substantially straight first stator cavity 582 is defined therebetween. The outer perimeter 580 of the underside section 578 preferably has substantially the same diameter as the inner perimeter 576 of the lower section 510, and hence the outer perimeter 580 of the underside section 578 has a smaller diameter than the upper perimeter 514 of the upper section 508 of the stator 504. The stator 504 can therefore be provided with an annular inner section (or axial wall) 584 that takes the form of a curved surface that extends all the way around the central axis L and axially (i.e., vertically) between the outer perimeter 580 of the underside section 578 and the inner perimeter 576 of the lower section 510. The outer section 512 and the inner section 584 of the stator 504 extend in parallel and axially, and the outer section 512 is arranged proximal to the inner section 584 such that a thin and substantially straight second stator cavity 586 is defined therebetween, said second stator cavity 586 extending perpendicular to the first stator cavity 582. In this way, the upper section 508, the lower section 510, the underside section 578, the outer section 512 and the inner section 584 of the stator 504 define a stator and stator cavity 518 with an L-shaped radial crosssection. The L-shaped stator cavity 518 comprises the first and second stator cavities 582, 586 discussed above. In this way, the stator 504 (and hence heat exchanger 500) can be understood as bell-shaped. This shape ensures a large interior flow surface 524 despite a small volume of the heat exchanger 500. The stator 504 does not include any stator fins in this embodiment such that the design of the stator 504 is simpler. As stated above, the rotor 502 comprises toe annular radial portion 540 and one annular axial portion (i.e., rotor fin) 542 in this embodiment. The annular radial portion 540 is arranged towards the top of the heat exchanger 500, and the annular axial portion 542 extends axially downward out of the outer perimeter 580 of the annular radial portion 540. In other words, for every radial cross-section of the heat exchanger 500, the axial portion 542 extends axially downward out of the radial portion 540 at the outermost end of the radial portion 540. The rotor 502 may be provided with an annular extending portion 544 that extends radially outward from the bottom of the rotor fin 542. The rotor 502 is arranged within the stator cavity 518 such that the annular radial portion 540 is arranged between the upper section 508 and the underside section 578 of the stator 504, and the annular axial portion 542 is arranged between the inner and outer sections 584, 512 of the stator 504. Unlike in the embodiment of Figures la to 1 f, where the central axle 136 extends between the upper and lower sections 108, 110 of the stator 104, in this embodiment, the central axle 536 may only extend between the stator upper section 508 and the stator underside section 578. In other words, the central axle 536 may only extend down through the top part of the heat exchanger 500. This results in a shortened axle 536 as compared to the first embodiment 100. The rotor 502 is sized and positioned within the stator cavity 518 such that there is a gap defined between the upper section 508 of the stator 504 and the radial portion 540 of the rotor 502, and between the outer section 512 of the stator 504 and the rotor fin 542. This gap (i.e., above the rotor 502) defines the flow channel 526 for fluid passing through the heat exchanger 500. The interior flow surface 524 of the fifth embodiment is provided by the downward-facing surface of the stator upper section 508 and the inward-facing surface of the stator outer section 512. The interior flow surface 524 faces the upper surface of the rotor 502, which includes the upward-facing surface of the rotor radial portion 540 and the outwardly-facing surface of the rotor fin 542. The sixth embodiment as shown in Figure 6 is the same as the fifth embodiment as shown in Figure 5, except that the rotor 602 comprises an annular diagonally-extending portion (extending downward along, and outward from, the central axis), and the stator 604 has a substantially rhomboid or rhombic (i.e., rhombus-shaped) radial cross-section, as will be explained below. In this embodiment, both the upper and lower sections 608, 610 of the stator 604 have the form of a substantially circular plate defining a substantially circular opening therein, i.e., they are both substantially annular. Hence the upper and lower sections 608, 610 of the stator 604 both define inner and outer perimeters, said perimeters taking the form of substantially circular circumferences. The outer perimeter 614 of the upper section 608 has a smaller diameter than the outer perimeter 616 of the lower section 610, and the inner perimeter 696 of the upper section 608 has a smaller diameter than the inner perimeter 676 of the lower section 610. The annular outer and inner sections 612, 684 of the stator 604 extend vertically (i.e., axially along the central axis L) at the top of the heat exchanger 600 proximal the upper section of the stator 608, and diagonally (i.e., both axially along the central axis L and radially away from the central axis L) at the bottom of the heat exchanger 600 proximal the lower section of the stator 610. To this end, each of the outer and inner sections 612, 684 of the stator 604 comprises an annular axial wall 688, 690 at the top and an annular diagonally-extending wall 692, 694 at the bottom. The axial wails 688, 690 of the outer and inner sections 612, 684 extend in parallel, and the diagonally-extending walls 692, 694 of the outer and inner sections 612, 684 extend in parallel, i.e., at substantially the same distance apart, between the upper and lower sections 608, 610 of the stator 604. In more detail, the axial wall 688 of the outer section 612 extends axially down from the outer perimeter 614 of the upper section 608 of the stator 604, while the axial wall 690 of the inner section 684 extends axially down from the inner perimeter 696 of the upper section 608 of the stator 604. Thereafter, the diagonally-extending wall 692 of the outer section 612 extends radially outward and axially downward from the lowermost end of the axial wall 688 of the outer section 612 and into the outer perimeter 616 of the lower section 610 of the stator 604, while the diagonally-extending wall 694 of the inner section 684 extends radially outward and axially downward from the lowermost end of the axial wall 690 of the inner section 684 and into the inner perimeter 676 of the lower section 610 of the stator 604. The angle 0 between the diagonally-extending walls 692, 694 and the central axis L is oblique, and is e.g., around 5 to 25 °, or around 10 to 20°, or around 13 to 18°, or around 15° with respect to the central axis L. The axial length of the axial walls 688, 690 is smaller than (e.g., around a third to an eighth in size of) the axial length of the diagonally-extending walls 692, 694. Moreover, the upper and lower sections 608, 610 of the stator 604 have a relatively small radial length compared to the axial length of the inner and outer sections 684, 612 of the stator604. In this way, the upper and lower sections 608, 610 and the inner and outer sections 684, 612 of the stator 604 define a stator 604 and stator cavity 618 with a substantially rhombic or rhomboid radial cross-section. A rhomboid is an oblique-angled parallelogram with only opposite sides having equal length, whereas a rhombus is an oblique-angled parallelogram with all sides having equal length. This results in a substantially conical stator 604, more specifically a substantially frustoconicai stator 604 with a substantially cylindrical neck. In other embodiments, the stator 604 may be substantially cylindrical. Due to the annular configuration of the upper and lower sections 608, 610 of the stator 604(and the inner and outer sections 684, 612 connecting the two together), a substantially conical shaped orifice extends all the way through the centre of the heat exchanger 600, thus defining part of the “outside” of the stator 604. Alternatively, this orifice may be filled and / or may be arranged to allow a coolant to pass therethrough to aid heat transfer of the stator. In this embodiment, the rotor 602 comprises an annular vertical portion 698 extending axially down away from the inlet 620 (arranged in the outer section 612 of the stator 604) and an annular diagonally-extending portion 6100 extending both radially outward and axially down from the lowermost end of the vertical portion 698 and towards the outlet 622. The diagonally-extending portion 6100 of the rotor 602 is preferably arranged at the same oblique angle with respect to the central axis L as the diagonally-extending walls 692, 694 of the stator 604. The rotor 602 may be provided with an annular extending portion 644 that extends radially outward from the lowermost end of the diagonally-extending portion 6100 of the rotor 602. The rotor 602 is arranged within the stator cavity 618 such that the annular vertical portion 698 is arranged between the axial walls 688, 690 of the inner and outer sections 684, 612 of the stator 604, and the diagonally-extending portion 6100 of the rotor 602 is arranged between the diagonally-extending walls 692, 694 of the inner and outer sections 684, 612 of the stator 604. Fluid enters the stator 604 at the fluid inlet 620 at the narrow end of the heat exchanger 600 near the central axis L and exits the heat exchanger 600 at the outer circumference of the wide end of the conical heat exchanger through the fluid outlet 622 (also arranged in the outer section 612 of the stator 604). As such, the fluid moves continually in a helical fashion with an ever increasing radius through the heat exchanger 600. Alternatively, when the stator 604 is cylindrical, the fluid moves continually in a helical fashion at a fixed radius through the heat exchanger 600. In this embodiment, the heat exchanger 600 does not need to include the central axle described above. Instead, the rotor 602 may be rotated on and by an annular bearing 6102 arranged in the stator 604 and surrounding the central axis L, Said bearing 6102 is arranged on the inner surfaces of the axial walls 688, 690 of the inner and outer sections of the stator 684, 612, i.e., near the top of the heat exchanger 600. The annular bearing 6102 may be driven by a motor in a conventional way. The bearing 6102 ensures the rotor 602 is fixed in coaxial alignment with the stator 604. The rotor 602 is sized and positioned within the stator cavity 618 such that there is a gap defined between the rotor 602 and the outer section of the stator 612. This gap (i.e., above the rotor 602) defines the flow channel 626 for fluid passing through the heat exchanger 600. The interior flow surface 624 of the sixth embodiment is provided by the inwardly-facing surface of the outer section of the stator 612. The interior flow surface 624 faces the upper surface of the rotor 602, which includes the outwardly-facing surface of the rotor 602. The seventh embodiment shown in Figure 7 is the same as the embodiment of Figures 1a to If except that the stator 704 has both upper and lower (straight) stator fins 758, 760, the rotor 702 meanders around said stator fins 758, 760 between the inlets 720a and 720b and the outlets 722a and 722b, and the arrangement of the stator 704 and the rotor 702 is such that two separate and distinct flow channels 726a, 726b are defined in the heat exchanger 700. This allows two fluids to be passed separately through the heat exchanger 700 for heat exchange. White in the first embodiment, the stator fins 134 only extend downward out of the upper section of the stator 108, in this seventh embodiment, the stator 704 includes both upper stator fins 758 that extend downward (i.e., axially) from the upper section of the stator 708 as well as lower stator fins 760 that extend upward (i.e., axially) from the lower section of the stator 710. In more detail, each upper stator fin 758 extends from the stator upper section 708, down through the stator cavity 718, and towards the stator lower section 710, while each lower stator fin 760 extends from the stator lower section 710, up through the stator cavity 718, and towards the stator upper section 708. The upper and lower stator fins 758, 760 are straight. In other words, each stator fin 758, 760 has a radial width or thickness that is substantially constant along the axial length thereof, such that the stator protrusions 758, 760 extend straight up ordown. All stator fins 758, 760 have substantially the same thickness. The stator 704 of Figure 7 is provided with two annular upper stator fins 758 and two annular lower stator fins 760, although other numbers of fins are possible. The two annular upper stator fins 758 and the two annular lower stator fins 760 are arranged concentrically and co-axially about the central axis L. The two annular upper stator fins 758 and the two annular lower stator fins 760 have substantially circular cross-sections in the plane perpendicular to the central axis L. Since each of the annular upper stator fins 758 and annular lower stator fins 760 has a different diameter, the lower stator fins 760 (which extend upwards) can be arranged to interleave with the upper stator fins 758 (which extend downwards). In this way, each lower stator fin 760 can be arranged between two neighbouring upper stator fins 758 (or between the central axle 736 and the innermost upper stator fin 758), and each upper stator fin 758 can be arranged between two neighbouring lower stator fins 760 (or between the stator outer section 712 and the outermost lower stator fin 760), as shown in Figure 7. In this way, the upper and lower stators fins 758, 760 can be understood to be alternately disposed. The axial length of each stator fin 758, 760 is less than the axial length of the stator cavity 718. This results in an opening in the stator 704, having a meandering radial cross-section. Unlike in the embodiment of Figures 1a to 1 f, the stator 704 comprises two inlets - an upper inlet 720a and a lower inlet 720b - and two outlets - an upper outlet 722a and a lower outlet 722b. The upper inlet 720a is the same as the inlet 120 of the embodiment of Figures 1a to 1f. The lower inlet 720b may have the same configuration as the inlet 120 of the embodiment of Figures 1a to 1f, except that it is arranged in the stator lower section 710 instead. The lower inlet 720b of the heat exchanger 700 is provided in the stator lower section 710 adjacent to and at least partially surrounding the central axis L. Other variations of the upper and lower inlets 720a, 720b are possible. The tower outlet 722b is the same as the outlet 122 of the embodiment of Figures la to If. The upper outlet 722a may have the same configuration as the outlet 122 of the embodiment of Figures 1 a to 1 f, except that it is provided in the outer section 712 of the stator 704 at the top thereof instead, as shown in Figure 7. To this end, the outer section 712 may define a further outlet flange that extends outwardly from the outer section 712 and curves around the outer section 712 in order to aid in the expulsion of fluid from the heat exchanger 700. The bottom of the further outlet flange is covered so as to provide a channel having a square-shaped crosssection and a gradually increasing width as it curves around the stator outer section 712. Other variations of the upper and lower outlets 722a, 722b are possible. The upper and lower outlets 722a, 722b are preferably arranged on opposite sides of the outer section 712 in the plane perpendicular to the central axle L. In other words, the upper and lower outlets 722a, 722b are preferably located approximately 180 degrees apart in the plane perpendicular to the central axle L. The central axle 736 extends all the way through the heat exchanger 700 (between the upper and lower sections 708, 710 of the stator 704), and couples to the rotor 702 at both ends thereof. The central axle 736 is supported in the stator 704 by axial bearings 738 arranged in the upper and lower sections 708, 710 of the stator 704 and is externally driven, for example by an electric motor (not shown) The annular radial portion 740 of the rotor 702 extends radially out of the central axle 736 at the top of the heat exchanger 700, I.e,, proximal to the upper inlet 720a. Additionally, the rotor 702 is provided with a first annular barrier portion 7104 that also extends radially out of the central axle 736, but at the bottom of the heat exchanger 700, i.e., proximal to the lower inlet 720b, and then extends axially upward until it joins the annular radial portion 740 of the rotor 702. Like other portions of the rotor 702, the annular barrier portion 7104 surrounds the central axis L. The rotor annular radial portion 740 further comprises a meander portion 764 that extends radially out from the annular radial portion 740, through the stator cavity 718 and towards the outer section 712. The meander portion 764 meanders around the upper and tower stator fins 758, 760 through the meandering opening defined between the upper and lower stator portions 708, 710. The meander portion 764 therefore mirrors the meandering opening and has a meandering profile in any radial cross-section also. In this way, the rotor 702 can be understood to undulate as it extends away from the central axle 736. To this end, the meander portion 764 is made up of a plurality of substantially flat plate segments. Each flat plate segment is arranged to extend either in the radial direction or in the axial direction. Each end of each plate segment connects to exactly one other plate segment arranged perpendicular thereto (except at the ends of the meander portion, where the plate segment is adjoined to only one other plate segment arranged perpendicular thereto). In other words, in the radial cross-section, the meander portion 764 is made up of a plurality of substantially straight line segments. Each straight line segment includes a first end and a second end, and each line segment is connected to exactly one other line segment at its first and / or second end. In this embodiment, the angle defined between each neighbouring pair of segments is substantially 90° (as shown in Figure 7). The meandering opening is larger than the rotor 702, such that a first gap is defined above the rotor 702 between the stator 704 and the rotor 702, and a second gap is defined below the rotor 702 between the stator 704 and the rotor 702. The first gap defines a first flow channel 726a for delivering fluid between the upper inlet 720a and the upper outlet 722a, while the second gap defines a second flow channel 726b - which is distinct and separate from the first flow channel 726a - delivering fluid between the lower inlet 720b and the lower outlet 722b. As such, a first fluid may be entered into the heat exchanger 700 through the upper inlet 720a, which is then forced by the rotation of the rotor 702 to move in a spiral motion and outward to the circumference of the heat exchanger 700 where it then exits the upper outlet 722a. Meanwhile, a second fluid may be entered into the heat exchanger 700 through the lower inlet 720b, which is then likewise forced by the rotation of the rotor 702 to move in a spiral motion and outward to the circumference of the heat exchanger 700 where it then exits the lower outlet 722b. The first and secondary fluids do not interact as they move through the heat exchanger 700, but heat is transferred therebetween via rotor 702. In this way, the heat exchanger 700 is configured to exchange heat between the first and second fluids, which may be different fluids or the same. In one particularly preferred embodiment, when the fluids are different, the first fluid may have a viscosity and a relatively low volatility such that it create a seal around the rotor 702 (e.g. the fluid may be an oil), or else the fluids may be gases (e.g. an air-to-air heat exchange) such that any potential limited mixing of the two fluids within the heat exchanger 700 is not problematic. Hence, there are two interior flow surfaces 724a, 724b in the seventh embodiment. The upper interior flow surface 724a is provided by the downward-facing surfaces of the upper section of the stator 708 and the exposed surfaces of the upper stator fins 758, while the lower interior flow surface 724b is provided by the upward-facing surfaces of the lower section of the stator and the exposed surfaces of the lower stator fins 760. Hence, the upper interior flow surface 724a faces the upper surface of the rotor 702, while the lower interior flow surface 724b faces the lower surface of the rotor 702. in one preferred embodiment, the rotor 702 may be provided with annular upper and lower extending portions 744a, 744b that extend radially outward from respective upper and lower portions of the outermost part of the meander portion 764, adjacent to the upper and lower outlets 722a, 722b. Between the annular upper and lower extending portions 744a, 744b, the meander portion 764 may include a second annular barrier portion 28 7105 that extends axially between the upper and lower extending portions 744a, 744b. Due to this arrangement , the first and second flow channels 726a, 726b can be kept separate at both outlets 722a, 722b of the heat exchanger 700 and mixing of the first and second fluids is minimised there. Additionally or alternatively, a high velocity and high-pressure fluid can be blown into the heat exchanger 700 via the upper outlet 722a. This would induce the rotor 702 to turn. In effect, the movement of the first fluid through the heat exchanger 700 would be reversed, moving from radially inward from the circumference of the heat exchanger 700 to the upper inlet 720a where it then exits the heat exchanger 700. The motion in the rotor 702 induced in this way would create a natural contra flow as a secondary fluid would enter the lower inlet 720b at the centre and be drawn in the opposite direction through the heat exchanger 700 (i.e., radially outward) to exit the heat exchanger 700 at the lower outlet 722b as normal. The eighth embodiment shown in Figure 8 is the same as the embodiment of Figures 1a to 1f except that the stator 804 has a middle section 8108 in the form of a membrane arranged between the upper and lower sections of the stator 808, 810 (in place of stator fins), the heat exchanger 800 has two rotors 802a, 802b (an upper rotor and a lower rotor), and the arrangement of the stator 804 and the rotors 802a, 802b is such that two separate and distinct flow channels 826a, 826b are defined in the heat exchanger 800. This allows two fluids to be passed separately through the heat exchanger 800 for heat exchange. As stated, the stator 804 of the embodiment of Figure 8 includes a middle section 8108 that is arranged to extend through the stator cavity 818 between the upper and lower sections of the stator 808, 810. The middle section of the stator 8108 extends (radially) across the entire stator 804, thereby dividing the stator 804 into two separate and distinct chambers: an upper stator chamber 818a and a lower stator chamber 818b, The middle section 8108 is held in position within the heat exchanger 800 by way of its attachment to the outer section of the stator812. To this end, the middle section 8108 is connected to the outer section 812 at its outer perimeter, which preferably takes the form of a substantially circular circumference. The outer perimeter of the middle section 8108 is connected to the outer section 812 towards the top of the heat exchanger 800. in this way, the middle section 8108 acts as a static membrane between the upper stator chamber 818a and the lower stator chamber 818b. The middle section 8108 is made up of a meander portion 8110 that meanders between the upper and lower sections of the stator 808, 810 as the meander portion 8110 extends radially outward from the central axis L of the heat exchanger 800. At the centre of the heat exchanger 800, the meander portion 8110 defines a central portion 8110a that extends radially away from the central axis L at the top of the heat exchanger 800. Thereafter, the meander portion 8110 extends axially downward towards the lower section of the stator 810, before extending radially away and then extending axially upward towards the upper section 808, thereby defining a first lower bend in the meander portion 8110. After extending axially back up to the upper section 808, the meander portion 8110 then extends radially away further before then extending axially back down towards the lower section 810 (to define an upper bend in the meander portion 8110). Thereafter, the meander portion 8110 extends radially away further and before then extending actually back up again (to define a second lower bend in the meander portion 8110). The meander portion 8110 extends axially outward at the top heat exchanger and into the central wail 812. Although the embodiment of Figure 8 includes two lower bends and one upper bend in the meander portion 8110, other numbers of bends / meandering configurations of the middle section are possible. To this end, the meander portion 8110 is made up of a plurality of substantially flat plate segments. Each flat plate segment is arranged to extend either in the radial direction or in the axial direction. Each end of each plate segment connects to exactly one other plate segment arranged perpendicular thereto (except at the ends of the meander portion, where the plate segment is adjoined to only one other plate segment arranged perpendicular thereto and the outer section). In other words, in the radial cross-section, the meander portion 8110 is made up of a plurality of substantially straight line segments. Each straight line segment includes a first end and a second end, and each line segment is connected to exactly one other line segment at its first and / or second end. In this embodiment, the angle defined between each neighbouring pair of segments is substantially 90° (as shown in Figure 8). Unlike in the embodiment of Figures 1 a to If, the stator 804 comprises two inlets - an upper inlet 820a and a lower inlet 820b ~ and two outlets - an upper outlet 822a and a lower outlet 822b. The upper inlet 820a takes the form of a channel that extends axially down from the heat exchanger 800 through the centre of the heat exchanger 800 along the central axis L. The upper inlet 820a directs fluid into the upper stator chamber 818a. In particular, the upper inlet 820a directs fluid towards the centre of the middle section 8108, at which point the middle section 8108 directs the fluid radially outward and through the upper stator chamber 818a. The lower inlet 820b also takes the form of a channel, but instead extends axially upward from the heat exchanger 800 through the centre of the heat exchanger 800 along the central axis L. The lower inlet 820b directs fluid into the lower stator chamber 818b. In particular, the lower inlet 820b directs fluid towards the centre of the middle section 8108, at which point the middle section 8108 directs the fluid radially outward and through the lower stator chamber 818b. Because the middle section 8108 is arranged towards the top of the heat exchanger 800 at the centre of the heat exchanger 800, the lower inlet 820b is axially longer than the upper inlet 820a. Each of the upper and lower inlets 810a, 810b is defined by a curved wall that extends all the way around the central axis L. Said wall preferably has a substantially circular cross-section in the plane perpendicular to the central axis L, such that it takes the form of a substantially circular tube. Both the upper outlet 822a and the lower outlet 822b are arranged in the outer section of the stator 812 towards the top of the heat exchanger 800. The upper outlet 822a is arranged between the upper section of the stator 808 and the middle section of the stator 8108, such that it can direct fluid in the upper stator chamber 818a out of the heat exchanger 800. Meanwhile, the lower outlet 822b is arranged between the middle section of the stator 8108 and the lower section of the stator 810, such that it can direct fluid in the lower stator chamber 818b out of the heat exchanger 800. As stated above, in this embodiment, the heat exchanger 800 includes two rotors 802a, 802b in the form of upper and lower rotors. The upper rotor 802a is configured to direct fluid through the upper stator chamber 818a between the upper inlet 820a and the upper outlet 822a, while the lower rotor 802b is configured to direct fluid through the lower chamber 818b between the lower inlet 820b and the lower outlet 822b. To this end, the upper rotor 802a is arranged within the upper stator chamber 818a while the lower rotor 802b is arranged within the lower stator chamber 818b. The lower rotor 802b of the eighth embodiment has the same configuration as the rotor 102 of the first embodiment, except that it has three straight rotor fins 842b only. The upper rotor 802a has the same configuration as the rotor 102 of the first embodiment, except that (I) it is inverted (such that the radial portion 840a is at the top of the rotor 802a and the rotor fins 842a extend down therefrom) and (ii) that it has two rotor fins 842a only. In other embodiments, the upper and lower rotors 802a, 802b may have any number of rotor fins 842. The innermost rotor fin of the lower rotor 842b extends axially upward and towards the central portion of the meander portion 8110a, i.e., between the lower inlet 820b and the innermost plate of the meander portion 8110. The middle rotor fin of the lower rotor 842b instead extends axially upward and into the upper bend of the meander portion 8110 i.e., between two neighbouring axial plates of the meander portion 8110. Finally, the outer rotor fin of the lower rotor 842b extends axially upward and towards the lower outlet 822b of the heat exchanger 800, i.e., between the outermost plate of the meander portion 8110 and the outer section of the stator 812. The radial portion of the lower rotor 802b extends radially outward from the outer rotor fin to define an annular extending portion 844b, thereby directing fluid out of the lower outlet 822b of the heat exchanger 800. Meanwhile, the inner rotor fin of the upper rotor 842a extends axially downward into the first lower bend of the meander portion 8110 i.e., between two neighbouring axial plates of the meander portion 8110, while the outer rotor fin of the upper rotor 842a extends axially downward into the second lower bend of the meander portion 8110 i.e., between two neighbouring axial plates of the meander portion 8110. The radial portion of the upper rotor 840a extends radially outward of the outer rotor fin 842a to define an annular extending portion 844a, thereby directing fluid out of the upper outlet 822a of the heat exchanger 800. Each of the upper and lower rotor fins 842a, 842b and the axial plates of the middle section 8108 has a substantially circular cross-section in the plane perpendicular to the central axis L and extend in parallel in the radial cross-section. As such, each of the upper and lower rotor fins 842a, 842b and the axial plates of the middle section 8108 has a different diameter, and since they are arranged co-axially and concentrically about the central axis L, they are arranged to interleave each other in the configuration shown in Figure 8. In this way, the upper and lower rotor fins 842a, 842b and the axial plates of the middle section 8108 can be understood to be alternately disposed. Because of the presence of the upper and lower inlets 820a, 820b at the centre of the heat exchanger 800, this embodiment of the heat exchanger 800 does not include a central axle. Instead, upper annular bearings 8102a are provided within the upper section of the stator 808 to which the radial portion 840a is coupled, thereby allowing the upper rotor 802a to rotate, while lower annular bearings 8102b are provided within the lower section of the stator 810 to which the radial portion 840b is coupled, thereby allowing the lower rotor 802b to rotate. Hence, the upper rotor and lower rotors 802a, 802b are therefore fixed in coaxial arrangement within the stator 804 by bearings 8102a, 8102b. Motors for powering each of the upper and lower annular bearings 8102a, 8102b are provided above and below said respective annular bearings 8102a, 8102b. in other words, each rotor 802a, 802b has a separate, independent drive motor, for example an annular electric motor. Said motors are enclosed in the upper and lower sections 808, 810 of the stator 804 such that fluid cannot pass through either motor. Since the middle section of the stator 8108 keeps the upper and lower rotors 802a, 802b completely separate, the only way to achie ve motion of fluid in each of the upper and lower stator chambers 818a, 818b is by driving each rotor 802a, 802b separately. In view of the above-described configuration of this heat exchanger, a first gap is defined between the upper rotor 802a and the stator middle section 8108 (i.e., below the upper rotor 802a), and a second gap is defined between the stator middle section 8108 and lower rotor 802b (i.e., above the lower rotor 802b). The first gap defines a first flow channel 826a for delivering fluid between the upper inlet 820a and the upper outlet 822a, while the second gap defines a second flow channel 826b - which is distinct and separate from the first flow channel 826a - delivering fluid between the lower inlet 820b and the lower outlet 822b. As such, a first fluid may be entered into the heat exchanger 800 through the upper inlet 820a, which is then forced by the rotation of the upper rotor 802a to move in a spiral motion and outward to the circumference of the heat exchanger 800 where it then exits the upper outlet 822a. Meanwhile, a second fluid may be entered into the heat exchanger 800 through the lower inlet 820b, which is then forced by the rotation of the lower rotor 32 802b to move in a spiral motion and outward to the circumference of the heat exchanger 800 where it then exits the lower outlet 822b, The first and secondary fluids do not touch as they move through the heat exchanger 800, but heat is transferred therebetween via the middle section 8108. In this way, the heat exchanger 800 is configured to exchange heat between the first and second fluids, which may be different fluids or the same. In this embodiment, fluids with very different viscosities and flow characteristics can be passed through the same rotary heat exchanger 800 concurrently. The speed and power of each independent motor can be modified to suit the fluid flow characteristics of said fluids. Hence, there are two interior flow surfaces 824a, 824b in the eighth embodiment. The upper interior flow surface 824a is provided by the upper surface of the middle section of the stator 8108, while the lower interior flow surface 824b is provided by the lower surface of the middle section of the stator 8108. Hence, the upper interior flow surface 824a faces the lower surface of the upper rotor 802a, while the lower interior flow surface 824b faces the upper surface of the lower rotor 802b. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. For example, various combinations of the above-stated embodiments are covered by of way the present application. For example, although described with respect to the first embodiment, the cooling systems of the fourth embodiment can be incorporated into the stator and / or rotor of the heat exchanger of any embodiments described above.

Claims

1. A rotary heat exchanger having a central axis comprising:an annular rotor surrounding, and configured to rotate about, the central axis,a stator enclosing the rotor, the stator comprising:an upper section, the upper section extending radially away from the central axis to define an annular upper perimeter that surrounds the central axis,a lower section arranged axially away from the upper section, the lower section extending radially away from the central axis to define an annular lower perimeter that surrounds the central axis,an annular outer section, the outer section extending axially between the upper and lower perimeters and surrounding the central axis,wherein an annular stator cavity is defined between the upper and lower sections and between the outer section and the central axis and surrounds the central axis, andwherein the rotor is arranged within the annular stator cavity,an inlet for directing fluid into the stator, the inlet being arranged proximal to the central axis, and an outlet for directing fluid out of the stator, the outlet being arranged distal to the central axis, wherein, in any plane extending radially away from the central axis, the cross-section of the rotor is uniform and the rotor extends both axially between the upper and lower sections of the stator and radially between the central axis and the outer section, such that rotation of the rotor causes fluid to flow both axially and radially along an interior flow surface of the stator between the inlet and the outlet.

2. The rotary heat exchanger of Claim 1, wherein the rotor has an upper rotor surface and a lower rotor surface and the stator has a first inner surface facing the upper surface of the rotor and a second inner surface facing the lower surface of the rotor, and wherein the interior flow surface is provided by the first or second inner surface, and optionally wherein the first inner surface comprises an inner surface of the upper section of the stator, and wherein the second inner surface comprises an inner surface of the lower section of the stator.

3. The rotary heat exchanger of Claim 2, wherein the interior flow surface is provided by the first inner surface, and wherein the distance between the first inner surface and the upper surface of the rotor is less than 4mm, preferably less than 3mm, more preferably less than 2mm, along the length of the first inner surface.

4. The rotary heat exchanger of any preceding claim, wherein, for any radial cross-section of the heat exchanger, the lower and upper sections of the stator have substantially the same radial length and the lower section of the stator is arranged below the upper section such that the stator cavity defined therebetween is substantially rectangular.

5. The rotary heat exchanger of any preceding claim, wherein the rotor has a substantially rectangular radial cross-section.

6. The rotary heat exchanger of any preceding claim, wherein the rotor comprises a rotor body and a rotor cavity defined within the rotor body, and wherein the rotor cavity is also annular and surrounds the central axis.

7. The rotary heat exchanger of any preceding claim, wherein the stator comprises at least one annular stator protrusion that surrounds the central axis, and wherein, for any radial cross-section of the heat exchanger, the or each stator protrusion extends either:axially out of the upper section of the stator, through the stator cavity and towards the lower section of the stator; oraxially out of the lower section of the stator, through the stator cavity and towards the upper section of the stator.

8. The rotary heat exchanger of Claim 7, wherein, for any radial cross-section of the heat exchanger, the or each stator protrusion has a width that is substantially constant along the axial length thereof.

9. The rotary heat exchanger of any preceding claim, wherein the rotor comprises an annular radial portion surrounding the central axis and an annular axial portion surrounding the central axis, and wherein, for any radial cross-section of the heat exchanger, the radial portion extends radially away from the central axis towards the outer section, and the axial portion extends axially out of the radial portion and between the upper and lower sections of the stator.

10. The rotary heat exchanger of Claim 9, wherein the rotor further comprises a plurality of annular axial portions surrounding the central axis, and wherein, for any radial cross-section of the heat exchanger, each axial portion extends axially out of the radial portion and between the upper and lower sections of the stator.

11. The rotary heat exchanger of Claim 9 or Claim 10, wherein, for any radial cross-section of the heat exchanger, the or each axial portion has a width that is substantially constant along the axial length thereof.

12. The rotary heat exchanger of any of Claims 9 to 11 when dependent on Claim 7 or Claim 8, wherein, for any radial cross-section of the heat exchanger, the or each axial portion of the rotor is arranged between two neighbouring stator protrusions or between the outer section of the stator and the or a neighbouring stator protrusion.

13. The rotary heat exchanger of Claim 7, wherein, for any radial cross-section of the heat exchanger, the or each stator protrusion has a width that substantially decreases as the or each stator protrusion extends away from the adjoining upper or lower section of the stator.

14. The rotary heat exchanger of any preceding claim depending through Claim 7, wherein the stator comprises a plurality of annular stator protrusions, and wherein the stator protrusions comprise at least onefirst stator protrusion extending axially out of the upper section, through the stator cavity and towards the lower section of the stator and at least one second stator protrusion extending axially out of the lower section of the stator, through the stator cavity and towards the upper section of the stator.

15. The rotary heat exchanger of Claim 14, wherein the rotor comprises a meander portion, and wherein, for any radial cross-section of the heat exchanger, the meander portion extends away from the central axis, and meanders around the first and second stator protrusions between the first and second portions of the stator, as it extends towards the outer section.

16. The rotary heat exchanger of Claim 15, wherein for any radial cross-section of the heat exchanger, the meander portion is made up of a plurality of substantially straight line segments each including a first end and a second end, and wherein each line segment is connected to exactly one other line segment at its first and / or second end such that the meander portion forms a zig-zag.

17. The rotary heat exchanger of Claim 16, wherein for any radial cross-section of the heat exchanger, the angle defined between each neighbouring pair of segments is less than 90°.

18. The rotary heat exchanger of Claim 16, wherein for any radial cross-section of the heat exchanger, the angle defined between each neighbouring pair of segments is approximately 90 °.

19. The rotary heat exchanger of any preceding claim, wherein the heat exchanger comprises a further inlet for directing fluid into the stator, the further inlet being arranged proximal to the central axis, and a further outlet for directing fluid out of the stator, the further outlet being arranged distal to the central axis, and wherein the rotor is configured such that rotation thereof causes fluid to flow both axially and radially along a further interior flow surface of the stator between the further inlet and further outlet.

20. The rotary heat exchanger of Claim 9, wherein the rotor comprises or consists of the annular radial portion and the annular axial portion, and wherein, for every radial cross-section of the heat exchanger, the axial portion extends out of the radial portion proximal to an outer end of the radial portion21. The rotary heat exchanger of Claim 20, wherein the stator is shaped such that the stator cavity has an L-shaped radial cross-section.

22. The rotary heat exchanger of Claim 9, wherein the rotor comprises or consists of a diagonally-extending portion extending both radially and axially away from the inlet and towards the outlet and at an oblique angle with respect to the central axis.

23. The rotary heat exchanger of Claim 22, wherein the stator is shaped such that the stator cavity has a rhomboid or rhombic radial cross-section.

24. The rotary heat exchanger of any preceding claim, wherein the rotary heat exchanger comprises two rotors, and wherein the stator comprises a middle section arranged between the upper and lower sections, and wherein a first rotor is arranged between the upper and middle sections of the stator, and a second rotor 5 is arranged between the middle and lower sections of the stator.

25. The rotary heat exchanger of any preceding claim, wherein the rotary heat exchanger also comprises a cooling channel arranged to convey coolant fluid therethrough, and wherein the cooling channel extends at least partially through the stator.

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