Process and apparatus for transferring heat from a first medium to a second medium

The invention relates to a process and an apparatus for transferring heat from a first, relatively cold medium to a second, relatively hot medium.

In current power plants, work is typically generated by means of a Carnot cycle or "steam cycle", employing a high temperature source and a low temperature source (heat sink) . In practice, a high temperature medium, typically superheated steam, is fed to a turbine, which generates work, and is subsequently condensed, (super) heated and once more fed to the turbine. I.e., the difference between the amount of heat contained in the high temperature medium and the amount of heat sunk to the low temperature source is converted into work, in accordance with the first law of thermodynamics. At higher temperature differences between the high and low temperature sources, more heat can be converted into work and the efficiency of the process improves. Typically, the environment (earth) serves as the low temperature source (heat sink) and the high temperature medium is generated by burning fossil fuels or by nuclear fission.

DE 32 38 567 relates to a device for generating temperature differences for heating and cooling. Under the influence of an external force, a temperature difference is established in a gas. By using centrifugal forces and with gases of high molecular weight, this effect is increased to such an extent that it is of interest for technical use.

WO 03/095920 relates to a method for transmitting heat energy, wherein the heat energy is transmitted into an inner chamber (3) of a rotating centrifuge via a first heat exchanger (4, 4a, 4b), in which inner chamber (3) a gaseous energy transfer medium is provided, and wherein the heat is discharged from the centrifuge (2) via a second heat exchanger (5; 5a, 5b) . The amount of energy used can be

reduced substantially by providing the gaseous energy transmission medium inside the rotor (12) in a state of equilibrium and by radially orienting the heat flow in an outward direction. It is essential to the invention underlying WO 03/095920 that convection be prevented (page 2, last sentence).

US 3,902,549 relates to a rotor mounted for highspeed rotation. At its center is located a source of thermal energy whereas at its periphery there is located a heat exchanger. Chambers are provided, accommodating a gaseous material which, depending upon its position in the chambers, can receive heat from the source of thermal energy or yield heat to the heat exchanger.

It is an object of the present invention to provide a process for efficiently generating a high temperature medium.

To this end, the process according to the present invention comprises the steps defined in claim 1.

The hot and cold media thus obtained in turn can be employed e.g. to heat or cool buildings or to generate electricity by means of e.g. a Carnot cycle or "steam cycle" .

The efficiency of the process according to the present invention can be further increased if segments, defined in radial direction, of the fluid are thoroughly mixed to obtain an at least substantially constant entropy in these segments and thus improved heat conduction within the fluid.

Also, heat conduction and hence efficiency increases with the pressure and density of the fluid. Thus, pressure is preferably in excess of 2 bar (at the axis of rotation) , more preferably in excess of 10 bar (at the axis of rotation) . The ratio of pressure at the circumference and pressure at the axis of rotation is preferably in excess of 5, more preferably in excess of 8.

The invention further relates to an apparatus for transferring heat as defined in claim 8.

In a further aspect, at least one of the heat exchangers is coupled to a cycle for generating work. The further cycle can comprise an evaporator or super-heater, which is thermally coupled to the high temperature heat exchanger, a condenser, thermally coupled to the low temperature heat exchanger, and a heat engine, located inside or outside the rotor. In this cycle, the environment will typically serve as a heat sink, but may also serve as a high temperature source, if the operating temperature of the cycle if sufficiently low.

In yet a further aspect, the compressible fluid is a gas and preferably contains or consists essentially of a mono-atomic element having an atomic number (Z) ≥ 18, such as Argon, preferably ≥ 36, such as Krypton and Xenon.

The invention is based on the insight that, although heat always flows from a higher to a lower entropy and usually from a higher to a lower temperature, in a column of an isentropic, compressible fluid positioned in a field of gravity heat can flow from a lower to a higher temperature. In the atmosphere of the earth, this effect reduces the vertical temperature gradient from a calculated 10 °C/km to an actual 6,5 °C/km. A reduced heat resistance further enhances heat flow from a lower to a higher temperature. Instead of or in addition to transferring heat through conductivity, heat can be transferred through heat capacity and mass flow.

In accordance with at least some aspects of the present invention, artificial gravity is employed to reduce the length of the column of the compressible fluid, in comparison with a column subjected merely to the gravity of the earth, and the atmosphere is replaced by a gas allowing a much higher temperature gradient in the fluid. Mixing or

circulation is employed to improve heat conduction within the fluid.

Within the framework of the present invention the term "gradient" is defined as a continuous or stepwise increase or decrease in the magnitude of a property observed in passing from one point to another, e.g. along a radius of a cylinder.

For the sake of completeness, it is noted that US 4,107,944 relates to a method and apparatus for generating heating and cooling by circulating a working fluid within passageways carried by rotors, compressing said working fluid therewithin and removing heat from said working fluid in a heat removal heat exchanger and adding heat into said working fluid in a heat addition heat exchanger, all carried by said rotors. The working fluid is sealed within, and may be a suitable gas, such as nitrogen. A working fluid heat exchanger is also provided to exchange heat within the rotor between two streams of said working fluid.

US 4,005,587 relates to a method and apparatus for transport of heat from a low temperature heat source into a higher temperature heated sink, using a compressible working fluid compressed by centrifugal force within a rotating rotor with an accompanying temperature increase. Heat is transferred from the heated working fluid into the heat sink at higher temperature, and heat is added into the working fluid after expansion and cooling from a colder heat source. Cooling is provided within the rotor to control the working fluid density, to assist working fluid circulation.

Similar methods and apparatuses are known from US 3,828,573, US 3,933,008, US 4,060,989, and US 3,931,713.

WO 2006/119946 relates to device (70) and method for transferring heat from a first zone (71) to a second zone (72) using mobile (often gaseous or vaporous) atoms or molecules (4) in which in one embodiment, the chaotic motion of the atoms/molecules which usually frustrates the transfer

of heat by simple molecular motion is overcome by using preferably elongated nanosized constraints (33) (such as a carbon nanotube) to align the atoms/molecules and then subjecting them to an accelerating force in the direction in which the heat is to be transferred. The accelerating force is preferably centripetal. In an alternative embodiment, molecules (4c) in a nanosized constraint may be arranged to transfer heat by means of an oscillation transverse of the elongation of an elongated constraint (40). JP 61165590 and JP 58035388 relate to rotary-type heat pipes. US 4,285,202 relates to industrial processes for energy conversion involving at least one step which consists in acting on the presence of a working fluid in such a manner as to produce either compression or expansion. The invention will now be explained in more detail with reference to the drawings, which schematically show a presently preferred embodiment.

Figures 1 and 2 are a perspective view and a side view of a first embodiment of the apparatus according to the present invention.

Figure 3 is a cross-section of a rotor used in the embodiment of Figures 1 and 2.

Figure 4 is a cross-section of a second embodiment of the apparatus according to the present invention. Figure 5 is a schematic layout of a power plant comprising the embodiment of Figure 4.

Figures 6A and 6B are cross-sectional side views of a third embodiment of the apparatus according to the present invention. Figure 7 is a cross-section of an exchanger unit for use in the embodiment of Figures 6A and 6B.

Figure 8 is a cross-section of an exchanger tube for use in the unit of Figure 7.

Figures 9A and 9B are a cross-section of a rotor and a perspective view of a plurality of channels,

respectively, of a fourth embodiment of the apparatus according to the present invention.

Figure 10 is a longitudinal section of part of a rotor providing circulation between compartments. Figure 11 is cross-section of part of yet another rotor.

Identical parts and parts performing the same or substantially the same function will be denoted by same numeral . Figure 1 shows an experimental setup of an artificial gravity apparatus 1, in accordance with the present invention. The apparatus 1 comprises a static base frame 2, firmly positioned on a floor, and a rotary table 3, mounted on the base frame 2. Driving means, e.g. an electromotor 4 are mounted in the base frame 2 and are coupled to the rotary table 3. To reduce drag, an annular wall 5 is fastened to the rotary table 3, along its circumference. Further, a cylinder 6 is fastened to the rotary table 3 and extends along a radius thereof. As shown in Figure 3, the cylinder β comprises a centre ring 7, two (Perspex™) outer cylinders 8, two (Perspex™) inner cylinders 9 mounted coaxially inside the outer cylinders 8, two end plates 10, and a plurality of studs 11, with which the end plates 10 are pulled onto the cylinders 8, 9, and the cylinders 8, 9, in turn, onto the centre ring 7. The cylinder 6 has a total length of 1,0 meter. Figure 3 is to scale.

The lumen defined by the centre ring 7, the inner cylinders 9, and the end plates 10, is filled with Xenon, at ambient temperature and a pressure of 1,5 bar, and further contains a plurality of mixers or ventilators 13. Finally, a Peltier element (not shown) is mounted on the inner wall of the ring 7 and temperature sensors and pressure gauges (also not shown) are present in both the ring 7 and the end plates 10.

During an experiment, the rotary table 3 and hence the cylinder 6 was rotated at a speed of approximately 800 RPM. In view of the fact that a process of this type is reversible and in view of the thermal insulation provided by the inner and outer cylinders 8, 9, which insulation enables conducting substantially adiabatic processes, heat transfer within the cylinder 6, from the axis of rotation to the circumference and vice versa, was substantially isentropic. Upon rotation, the temperature of the Xenon at the endplates 10 initially increased and then decreased to ambient temperature as a result of the cooling effect of the endplates 10. The temperature of the Xenon at the ring 7 decreased. When, upon reaching equilibrium, heat was fed at a constant rate to the gas at the ring 7 by means of the Peltier element, temperature at the ring 7 increased but remained lower than ambient temperature, i.e. a constant heat flow from a relatively low temperature (the gas at the ring) to a relatively high temperature (the gas at the end plates) was established. Figure 4 is a cross-section of a second artificial gravity apparatus 1 in accordance with the present invention. The apparatus 1 comprises a static base frame 2, firmly positioned on a floor, and a rotary rotor 6, mounted, rotatable about its longitudinal axis, in the base frame 2, e.g. by means of suitable bearings, such as ball bearings

20. The rotor 6 suitably has a diameter in a range from 2 to 10 meters, in this example 4 meters. The wall of the rotor is thermally insulated in a manner known in itself. The apparatus 1 further comprises a driving means (not shown) to spin the rotor at rates in a range from 50 to 500 RPM.

The rotor 6 contains (at least) two heat exchangers, a first heat exchanger 22 mounted inside the rotor relatively far from the axis of rotation of the rotor 7 and a second heat exchanger 23 positioned at or relatively close to said axis. In this example, both heat exchangers

22, 23 comprise a coiled tube coaxial with the axis of rotation and connected, via a first rotatable fluid coupling

24, to a supply and, via a second rotatable fluid coupling

25, to an outlet. The embodiment shown in Figure 4 further comprises an tube 26, coaxial with the longitudinal axis of the rotor 7 and containing an axial ventilator 27 to forcedly circulate the contents of the rotor. In this example, the rotor is filled with Xenon at a pressure of 5 bar (at ambient temperature), whereas the heat exchangers 22, 23 are filled with water.

Figure 5 is a schematic layout of a power plant comprising the embodiment of Figure 4, coupled to a cycle for generating work, in this example a so-called "steam cycle". The cycle comprises an super-heater 30, coupled to the high temperature heat exchanger 22 of the apparatus 1, a heat engine, known in itself and comprising, in this example, a turbine 31, a condenser 32 coupled to the first heat exchanger 23 of the apparatus 1, a pump 33, and an evaporator 34. The steam cycle is also filled with water. Other suitable media are known in the art.

Rotating the rotor will generate a radial temperature gradient in the Xenon, with a temperature difference (δT) between the heat exchangers in a range from 100°C to 600°C, depending on the angular velocity of the rotor. In this example, the rotor is rotated at 350 RPM resulting in a temperature difference (δT) of approximately 300°C. Water at 20°C is fed to both heat exchangers 22, 23. Heated steam (320°C) from the high temperature heat exchanger 22 is fed to the super-heater 30, whereas cooled water (10 ⁰ C) from the low temperature heat exchanger 23 is fed to the condenser 32. The steam cycle generates work in a manner known in itself.

In another embodiment, the apparatus comprises two or more rotors coupled in series or in parallel. For

instance, in configuration comprising two rotors in series, the heated medium from the first rotor is fed to the low temperature heat exchanger of the second rotor. As a result, heat transfer to the high temperature heat exchanger in the second rotor is increased considerably, when compared to heat transfer in the first rotor. The cooled medium from the first rotor can be used as a coolant, e.g. in a condenser.

In another embodiment, and as an alternative or addition to the aforementioned tube (26) , the apparatus comprises a plurality of at least substantially cylindrical and co-axial walls, dividing the inside of the rotor into a plurality of compartments. The fluid in each of the compartments is thoroughly mixed, e.g. by means of ventilators or static elements, so as to establish a substantially constant entropy within each of the compartments and thus enhance mass transport within each of the compartments. As a result, an entropy gradient, stepwise and negative in outward radial direction, is achieved which enables heat transfer from the axis of rotation of the rotor to the circumference of the rotor.

The walls mutually dividing the compartments may be solid, thus preventing mass transfer from one compartment to the next, or may be open, e.g. gauze- or mesh-like, thus allowing limited mass transfer. The walls may also be provided with protrusions and/or other features that increase surface area and thus heat transfer between compartments .

Instead of transferring heat through conductivity, as in the embodiments above, heat can instead or in addition be transferred through heat capacity and mass flow. An embodiment of an artificial gravity apparatus 1 based on heat transfer through heat capacity and mass flow is shown in Figures 6A to 8. In this embodiment, the first and second heat exchangers comprises a plurality of radially extending exchanger units 22A, 23A evenly distributed, both in axial

and in tangential direction, over the surfaces of the inner walls of the rotor 6.

As shown in more detail in Figure 7, each unit comprises a double-walled exchanger tube 40, shown in cross- section in Figure 8. Each exchanger tube 40 comprises an inlet 41, a central feed tube 42 , an outer return tube 43 concentric with the feed tube, and an outlet 44. The outer tube 43 in turn is provided on its outer surface with means for enhancing heat exchange, e.g. features, such as fins 45, increasing the outer surface of the outer tube 43.

Each unit further comprises a siphon 46 positioned concentrically about the exchanger tube 40 and spanning at least 70%, e.g. about 80% of the radial distance between the walls 6A, 6B of the rotor 6. To equalize the entropy inside and outside the siphon 46 at least the inner surface of the siphon 46 is provided with means for enhancing heat exchange, e.g. features, such as fins 47, increasing the inner surface of the siphon 46.

During operation, a cooling medium, e.g. water, is fed to the exchanger tubes 40 that are positioned relatively- far from the axis of rotation of the rotor 6, thus locally cooling the fluid in the rotor 6 and locally increasing density and decreasing entropy. The dense fluid around the exchangers tubes 22A will be forced outwards by artificial gravity, i.e. co-currently with the water and towards the outer wall 6A of the rotor 6, and caused to spread over its inner surface.

A heating medium, e.g. water, is fed to the exchanger tubes 23A that are positioned relatively close to the axis of rotation of the rotor 6, thus heating the fluid in the rotor 6 and locally decreasing density and increasing entropy. The fluid around the exchangers tubes 40 will be displaced inwards (buoyancy) as a result of artificial gravity, i.e. co-currently with the water and towards the

inner wall 6B of the rotor 6, and caused to spread over its surface .

These two phenomena together generate a circulation between the heat exchangers, which enhances heat transfer outwards from the heat exchanger 23 relatively close to the axis of rotation of the rotor 6 and towards the heat exchanger 22 relatively far from the axis of rotation of the rotor 6.

In this embodiment, the length of the exchanger tubes 22A, 23A, is selected such as to prevent these tubes from reaching the zone of the fluid inside the rotor where the temperature is about equal to the temperature of an associated heat buffer, such as the surroundings of the apparatus . Further embodiments, also based on heat transfer enhanced through heat capacity and mass flow, are shown in Figures 9A to 11. Radial flow of the fluid is limited or prevented altogether, i.e. the fluid remains at substantially the same distance to the axis of rotation. In the embodiments shown, the fluid is divided in segments, such as segments extending in the axial direction of the rotor and parallel to the axis of rotation.

The embodiment shown in Figure 9 comprises a central duct 23 for a source of thermal energy, e.g. water at ambient temperature, and a rotor 6, which in turn comprises a plurality of co-axial, circle-cylindrical walls 51, in this example five in number, dividing the rotor 6 in annular compartments, and a plurality of radial walls 52, in this example sixteen in number, dividing the annular compartments in axial channels 53, more specifically rings or layers of axial channels 53 all located at substantially the same distance from the axis of rotation. A heat exchanger (not shown) is positioned about the rotor 6. To reduce friction and enhance thermal insulation, the assembly

of the rotor 6 and the heat exchanger can be placed inside a vacuum chamber .

Some of the axial channels 53 in the concentric ring nearest the axis of rotation, which will also be referred to as the ^x first ring', are provided, e.g. intermittently provided, with heat exchange elements 54, e.g. sections of a metal such as aluminium, facing said axis. All remaining walls 55 of each of these channels 53 are thermally insulated. Some of the axial channels 53 in the second ring, adjoining the first ring, are provided with heat exchange elements 54 at the interface with the or at least some of the remaining axial channels 53 in the first ring, i.e. intermittently and staggered with respect to the heat exchange elements 54 in the first ring. As shown in Figure 9A, this pattern is repeated up to and including the outermost ring, which is provided with outwardly facing heat exchange elements 54.

During operation and upon reaching equilibrium, a radial temperature gradient is generated in each of the rings and thus is each of the axial channels. The fluid, e.g. Xenon, in the axial channels 53 in the innermost ring is caused to flow turbulently in a first, axial direction (indicated in the usual way by a cross in a circle, away from the viewer) . The fluid is heated by the source of thermal energy, i.e. heat flows from the source, through the heat exchange elements 54, and to the fluid in the corresponding channels 53 in the first ring. At one end of the rotor 6, which will also be referred to as the ^x hot end' , the heated fluid is turned, as shown in Figure 9B, and caused to flow turbulently in the opposite direction

(indicated in the usual way by a dot in a circle, towards the viewer and towards the ^v cold end' of the rotor) through adjoining channels 53, thus heating fluid in the next ring, i.e. heat flows from said adjoining channels 53, through heat exchange elements 54 associated with these channels,

and to the fluid in corresponding channels 53 in the second ring. The heated and heating channels 53 and the heat exchange elements 54 are configured to provide counter- current heat exchange over each of said elements 54 in the rotor, again as shown in Figure 9A.

Means for propelling the fluid, such as a compressor or a fan, can be provided. In an aspect of the present invention, the channels wherein the fluid flows towards the cold end of the rotor, i.e. the heating channels, are positioned closer to the axis of rotation than the channels wherein the fluid flows towards the hot end of the rotor, i.e. the heated channels. As the density of the fluid at the cold end of the rotor is higher than that at the hot end, the pressure difference over the heating and heated channels will be greater at the cold end than at the hot end, thus providing a driving force for axial convection.

Although, in the embodiment shown in Figure 9A, the width and thus the surface area of the cross-section of the channels increases in radial and outward direction, said surface area can instead be kept at least substantially constant, resulting in an increase of the number of channels in each concentric ring or tangential space between at least some of the channels, e.g. when the number of channel in each concentric ring is kept constant.

Figure 10 shows a further embodiment, wherein the surface available for heat exchange between the rings has been increased.

This embodiment comprises a rotor 6 which in turn comprises a central annular duct 23 for a source of thermal energy, e.g. water at ambient temperature, and a plurality of co-axial, cylindrical walls 51, dividing the rotor 6 in annular compartments. To obtain a substantially equal temperature difference over each of the compartments, the height of the annular compartments decreases in radial and

outward direction. An outer heat exchanger 22 surrounds the outermost cylindrical wall 51 and forms an integral part of the rotor 6. As a further option, the rotor can be placed inside a vacuum chamber. The rotor 6 further comprises a plurality of walls

52 dividing the annular compartments into axial channels 53 having a triangular cross-section, i.e. the walls 52 are inclined with respect to the radii of the rotor 6, e.g. zigzag within each ring. To substantially prevent or at least reduce heat transfer between the axial channels 53 within the same ring, said walls are made of a thermally insulating material, e.g. mineral wool sandwiched between metal or ceramic sheets. The remaining wall of each of the channels 53 is defined by one of the co-axial, cylindrical walls 51. In this arrangement, the entire surface area of the co-axial walls is available for heat transfer. Heat transfer between the rings can be further enhanced by increasing the effective surface area of the co-axial walls, e.g. by means of fins, protrusions, undulations, or the like.

Similar to the embodiment shown in Figures 9A and 9B, during operation and upon reaching a equilibrium, the fluid in the axial channels 53 in the innermost ring is caused to flow in a first, axial direction (cross in circle) and is heated by the source of thermal energy. At the hot end of the rotor 6 the heated fluid is turned and caused to flow in opposite direction (dot in circle) through adjoining channels 53 thus heating fluid in the next ring. The heated and heating channels 53 and the cylindrical walls 51 are configured to achieve counter-current heat exchange between the channels 53 and between the outermost channels 53 and the outer heat exchanger 22.

The walls of the axial channels 53 and, respectively, the heat exchange elements or cylindrical walls in the embodiments described above can be made of an

material either impermeable or permeable, e.g. porous, to the fluid. In the latter instance, the pressure differences between the rings and over the heat exchange elements or cylindrical walls, respectively, will be zero or approximately zero.

Figure 10 shows an example of an embodiment wherein the compartments communicate through baffles 55, which enable heat transfer between the rings or layers through heat capacity and mass flow. In yet another embodiment, an additional liquid flows, e.g. inside radially extending tubes, from the centre towards the circumference of the rotor, thus gaining potential energy and pressure. The high pressure liquid drives a generator, e.g. a (hydro) turbine, and is subsequently evaporated by means of the relatively hot compressible fluid (e.g., Xenon) at or near the inner wall of the rotor. Vapour thus obtained is transported back to the centre of the rotor, at least partially by employing its own expansion, and condensed by means of the relatively cold compressible fluid. This embodiment can be used to directly drive a generator.

The invention is not restricted to the above- described embodiments, which can be varied in a number of ways within the scope of the claims. For instance, other media, such as carbon dioxide, hydrogen, and CF ₄ , can be used in the heat exchangers in the rotor. Also, to reduce rotational resistance, the rotor can be operated in a low pressure or vacuum environment.