WO2014089597A1

WO2014089597A1 - Heat transfer system and method

Info

Publication number: WO2014089597A1
Application number: PCT/AU2012/001518
Authority: WO
Inventors: John GAL; Robert KERY
Original assignee: Empire Technology Development Llc
Priority date: 2012-12-13
Filing date: 2012-12-13
Publication date: 2014-06-19
Also published as: US20150083361A1

Abstract

A heat transfer system including a first heat exchange assembly, a second heat exchange assembly, and a heat transfer circuit interconnecting the first and second heat exchange assemblies, the circuit incorporating a working fluid to transfer heat between the first and second heat exchange assemblies. The heat transfer circuit preferably takes the form of a closed loop heat-pipe system. The heat transfer system is operative in a first mode where the working fluid is at or below a threshold pressure and in a second mode where the working fluid is at a higher pressure than the threshold pressure. A pressure regulating device is also provided to increase the pressure of the working fluid above the threshold pressure to effect change of operation of the heat transfer system from the first mode to the second mode. Optionally the pressure regulating device is a pressure vessel having at least one heat transfer surface. An air-conditioning system for conditioning air in a region, including the heat transfer system wherein the first heat exchange assembly is operative to moderate air temperature in the region and wherein the second heat exchange assembly is disposed adjacent a source/sink of substantially constant temperature.

Description

Heat transfer system and method

TECHNICAL FIELD

This disclosure relates generally to heat transfer systems and methods of operating heat transfer systems, especially passive heat transfer systems employing loop heat-pipes. Particularly, although not exclusively, the heat transfer system and method are suited to conditioning air in a region, such as a habitable internal space.

BACKGROUND

Approximately forty percent of world energy usage is associated with domestic and commercial buildings and in developed countries a significant portion of this energy usage is involved in the heating and cooling systems including the energy required by air-conditioning systems. Savings in energy in all areas, including these, is rapidly becoming an important element in the adoption of renewable energy sources to address climate change.

SUMMARY

In one embodiment, there is provided a heat transfer system including a first heat exchange assembly, a second heat exchange assembly, and a heat transfer circuit interconnecting the first and second heat exchange assemblies, the circuit

incorporating a working fluid to transfer heat between the first and second heat exchange assemblies. The heat transfer circuit may take the form of a closed loop heat-pipe system but is not limited to such an arrangement. The heat transfer system is operative in a first mode where the working fluid is at or below a threshold pressure and in a second mode where the working fluid is at a higher pressure than the threshold pressure. A pressure regulating device is also provided to increase the pressure of the working fluid above the threshold pressure to effect change of operation of the heat transfer system from the first mode to the second mode.

Optionally the pressure regulating device is a pressure vessel having at least one heat transfer surface. In another embodiment, there is provided an air-conditioning system for conditioning air in a region, including a heat transfer system having a first heat exchange assembly operative to moderate air temperature in the region and a second heat exchange assembly disposed adjacent a source/sink of substantially constant temperature. A fluid circuit including a working fluid transfers heat between the first and second heat assemblies and a pressure regulating device is also provided to increase the pressure of the working fluid. In another embodiment, a pressure regulating device for a heat transfer system includes a pressure vessel with at least one heat transfer surface. Optionally the pressure vessel is coupled by a valve to a working fluid circuit of a heat transfer system.

In another embodiment, a method of controlling a heat transfer system is provided. The system is operative in a first mode where the working fluid is at or below a threshold pressure and in a second mode where the working fluid is at a higher pressure than the threshold pressure. The method includes regulating working fluid pressure in a heat transfer circuit containing the working fluid by increasing the pressure of the working fluid above the threshold pressure to effect change of the heat transfer system from the first mode to the second mode.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the accompanying drawings and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an illustrative embodiment of a heat transfer system.

FIG. 2A is a schematic representation showing the heat transfer system of

Fig. 1 in a heating mode.

FIG. 2B is a schematic representation showing the heat transfer system of Fig. 1 in a cooling mode.

FIG. 3 is a detailed schematic representation of a heat transfer system of a further embodiment. DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the drawing figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein.

This disclosure is directed generally to heat transfer systems and methods of operating heat transfer systems, especially passive heat transfer systems employing loop heat-pipes. While the disclosure is described generally in the context of systems to condition air in a region, such as a habitable internal space, it is not limited to such installations, and may be used in other applications.

Heat transfer systems for application to habitable internal spaces, such as domestic dwellings and commercial buildings, have employed heat pump systems which consume energy or solar passive arrangements. One prior art example involves controlling the insolation and radiation of passive solar thermal storage columns for heating and cooling of homes and other structures. The example includes rotatable insulating panels which control the exposure of thermal storage columns to daytime sunlight and the nighttime sky.

Whilst there are examples of heat-pipe based heat transfer systems, the typical focus is on cooling applications for electrical equipment and for satellite, space vehicle or similar micro-gravity applications. These heat transfer solutions are not appropriate for application to habitable internal spaces, such as required for domestic dwellings or commercial buildings.

Disclosed in some embodiments is a heat transfer system including a first heat exchange assembly; a second heat exchange assembly; a heat transfer circuit interconnecting the first and second heat exchange assemblies. The circuit includes a working fluid to transfer heat between the first and second heat exchange assemblies. The heat transfer system has a first mode where the working fluid is at or below a threshold pressure and a second mode where the working fluid is at a higher pressure than the threshold pressure. The heat transfer system further includes a pressure regulating device to increase the pressure of the working fluid above the threshold pressure to effect change of the heat transfer system from the first mode to the second mode.

Also disclosed in some embodiments is the heat transfer circuit in the form of a closed loop heat-pipe system. In some forms, the first and second heat transfer assemblies may each include a wicking material. In some embodiments, the first and second heat transfer assemblies may be operative mutually alternatively as evaporators and condensers.

Disclosed in some embodiments, the working fluid is arranged to transfer heat to the first heat exchange assembly from the second heat exchange assembly in the first mode. In some embodiments, the working fluid is arranged to transfer heat to the second heat exchange assembly from the first heat exchange assembly in the second mode.

Disclosed in some embodiments, the pressure regulating device comprises a pressure vessel having at least one heat transfer surface. In some forms, the pressure vessel may be coupled to the heat transfer circuit via a fluid flow control valve.

In some embodiments, the working fluid may be selected from the group including demineralised water, ammonia, acetaldehyde, ether, pentane, ethyl chloride, and refrigerants R-245fa (1,1,1,3,3 Pentafluoropropane) and its substitutes.

Disclosed in some embodiments is an air-conditioning system for

conditioning air in a region including a heat transfer system having a first heat exchange assembly operative to moderate air temperature in the region and a second heat exchange assembly is disposed adjacent a source/sink of substantially constant temperature. The heat transfer system may be in any form as described above.

Disclosed in some embodiments, the pressure regulating device may use a pressure source based on temperature of a separate region. Disclosed in some embodiments is an air-conditioning system for

conditioning air in a habitable internal space including a first heat exchange assembly to moderate the temperature of air in the habitable internal space, a second heat exchange assembly adjacent a source of substantially constant temperature; and a pressure vessel having at least one heat transfer surface exposed to ambient temperature conditions.

Disclosed in some embodiments, the source/sink of substantially constant temperature is a subterranean location below the habitable internal space. The the subterranean location may be in the range of 1 to 5 metres below ground surface, preferably 2 to 3 metres below the surface. In some embodiments, the second heat exchange assembly may be ground-coupled at the subterranean location.

Disclosed in some forms, the first threshold pressure is established so that in the first mode the second heat exchange assembly acts as an evaporator at the substantially constant temperature to allow heat transfer from the second heat exchange assembly to the first heat exchange assembly. In some forms in the second mode, the pressure of the working fluid is at a level above the threshold pressure such that the second heat exchanger acts as a condenser at the substantially constant temperature to allow heat transfer from the first heat exchange assembly to the second heat exchange assembly.

Disclosed in some forms is a pressure regulating device having a pressure vessel with at least one heat transfer surface. Optionally a valve may be included for coupling the pressure vessel to a working fluid circuit of the heat transfer system. Disclosed in some forms is a method of controlling a heat transfer system, the system being operative in a first mode where the working fluid is at or below a threshold pressure and a second mode where the working fluid is at a higher pressure than the threshold pressure, the method including regulating working fluid pressure in a heat transfer circuit containing the working fluid by increasing the pressure of the working fluid above the threshold pressure to effect change of the heat transfer system from the first mode to the second mode.

In some form the method further includes selectively reducing the pressure of the working fluid to at or below the threshold pressure to revert the operation of the heat transfer system from the second mode to the first mode. A pressure regulating device may be provided to regulate working fluid pressure. In some forms, the pressure regulating device may be coupled to a working fluid circuit via a valve to enable the selective pressure regulation of the working fluid.

As illustrated in the Figures, some illustrative embodiments of a heat transfer system is suitable for use in conditioning air in a habitable space using the ground as a heat source or sink and ambient temperature to regulate pressure of a working fluid in system. FIG.l is a schematic representation of an illustrative embodiment of a heat transfer system 100. The design of the heat transfer system 100 as illustrated is based on a reversible looped heat pipe that connects a heat sink or source servicing a region, such as an internal space in a building, in a circuit to a sink or source of constant, near or substantially constant temperature. One such source is a subterranean location some 1 to 5 metres deep, typically located about 2 or 3 metres below ground surface. The system may include a heat transfer circuit suitably in the form of closed loop heat-pipe system where a working fluid, such as demineralised water, evaporates at a hotter end and condenses at a cooler end thus transferring heat from the hot region to the cooler region.

In a typical temperate climatic zone the temperature at a location 2 or 3 metres underground is a near constant 15°C. When the temperature in the building is above this value the heat-pipe system can be configured and operated to cool the building by extracting heat and dissipating it underground. Similarly, during winter when the building internal temperature goes below 15°C the heat-pipe system can be reversed to act to heat the building internal space.

The heat transfer system 100 includes a first heat exchange assembly 101 and a second heat exchange assembly 102, and a heat transfer circuit 103 which interconnects the two heat exchangers. The heat transfer circuit 103 contains a working fluid (not shown), which exists in a vapour phase and/or a in liquid phase, in order to transfer heat between the first heat exchange assembly 101 and second 102 heat exchange assembly. The heat transfer circuit includes two pipeline sections 104, 105 which carry the working fluid and couple respective ends of the first and second heat exchangers. The heat transfer system 100 also includes a pressure regulating device, here in the form of a pressure chamber or vessel 106, which is coupled to the heat transfer circuit 103 by a fluid flow control valve 107.

The embodiment employs a reversible heat-pipe system which depends only on the atmospheric temperature for setting the direction of flow of working fluid in the heat transfer circuit 103. The heat transfer system of the embodiment is applied in an air-conditioning system arranged to condition air in a region, here the air in a habitable internal space 111 enclosed by a building 110. The first heat exchanger 101 is operative to moderate the temperature of the air in the habitable internal space 111, for example to a typical desired temperature of approximately 22°C, within the building 110. The typical temperature range of atmospheric air of the building's immediate external environment, here located in a temperate climatic zone, is from 40°C to 5°C. The second heat exchanger 102 is disposed adjacent a source 112 of substantially constant temperature, here being about 2 to 3 metres below ground level 113. In a typical temperate climatic zone, the temperature 2 or 3 metres underground is a near constant approximately 15°C.

When the internal air temperature within the building is above the

approximate 15°C value, typical of summer, the heat-pipe system 100 of the embodiment can be operated to cool internal air in the building 110 by extracting heat from the internal habitable space 111 and dissipating it underground. Similarly, during winter when the building internal ambient temperature goes below 15°C the heat-pipe system can be reversed to act to heat air in the habitable internal space 111 of the building 110. The first and second heat exchangers in this embodiment can operate as either condensers or evaporators, as required, preferably in a substantially mutually alternative arrangement.

The reversing capability of the heat transfer system 100 of the embodiment is achieved by regulating the pressure of the working fluid within the heat transfer circuit 103, including the piping sections 104, 105. For example, when water is the working fluid at a pressure of about 3.5kPa water boils at 25°C and when the pressure is reduced to about 2.0 kPa water boils at 15°C. A passive pressurising /de- pressurising device is included in the loop heat-pipe arrangement, namely the pressure vessel 106. The vessel 106 is a sealed chamber where one heat-absorbing surface 109 is exposed to the atmosphere. The vessel's chamber is connected to the heat-pipe by the control valve 107. By controlling the working fluid pressure in the heat transfer circuit 103 the heating-cooling operating cycle or mode can be reversed. When the atmospheric temperature rises to a threshold value, the control valve 107 opens on the pressure vessel 106 connected to the heat pipe portion 104. This equalises pressures in each of the pressure vessel 106 and heat transfer circuit 103, thus increasing the heat pipe 104 pressure. This pressure increase acts to reverse the direction of flow of the working fluid and thus the heat transfer direction in the heat pipe from cooling to heating, as will be explained further in relation to FIGs 2A and 2B. The exposed surface 109 of the pressure vessel 106 is arranged to absorb and discharge heat based on the external ambient temperature. The vessel 106 is placed outside the building 110, but is desirably protected from direct sunlight and shielded as much as is practical from wind and other environmental influences so that only the ambient air temperature affects the heat transfer in either direction across surface 109.

FIG. 2A is a schematic representation showing the heat transfer system 100 in a heating mode. With an external ambient temperature lower than the below ground temperature, e.g. in the range of 5°C to 10°C and the control valve 107 closed, the initial working fluid pressure is set to a level where the heat transfer system 100 is in a heating mode, as depicted in FIG. 2A. In the heating mode, the first heat exchange assembly is operating as a condenser 101.1 for heating air in the internal space 111 and the below-ground heat exchanger is operating a evaporator 102.1, at 15°C. In the heating mode the heat transfer fluid in pipe section 104 is substantially in the vapour phase and flows from the second heat exchange assembly/evaporator 102.1 to the first heat exchange assembly/condenser 101.1. The heat transfer fluid in piping section 105 is substantially in the liquid phase and flows from the first heat exchange assembly/condenser 101.1 to the second heat exchange assembly/evaporator 102.1, as depicted by the arrow in FIG. 2A.

When the external ambient temperature rises above the constant below ground 112 temperature, e.g. in the range of 25°C to 40°C, this increases the pressure within the pressure vessel 106, including by heat transfer across the surface 109. When the control valve 107 is opened, pressures equalise across the pressure vessel 106 and the heat transfer circuit 103, including in the heat-pipe section 104. When the working fluid pressure in the heat transfer circuit 103 increases above a pre-determined threshold, the heat transfer system will change operating modes and the flow of working fluid and thus the heating/cooling cycle will be reversed. This changed operating mode resulting from opening of the control valve 107 is depicted in FIG. 2B.

FIG. 2B is a schematic representation showing the heat transfer system 100 in a cooling mode. In the cooling mode, the first heat transfer assembly operates as an evaporator 101.2 to absorb heat and thus moderate the temperature in the internal space 111 of building 110 towards the desired 22°C. Heat exchange fluid in the vapour phase flows in pipe section 104 from the first heat exchanger to the second heat exchange assembly which now operates as a condenser 102.2 at near constant 15°C, as depicted by the arrow in FIG. 2B. The pipe section 105 now carries condensed heat exchange fluid in liquid phase from the second heat exchanger back to the first in pipe section 105, to complete the heat transfer circuit 103.2.

One skilled in the art will appreciate that, for this and other processes and methods disclosed herein, the functions performed in the processes and methods may be implemented in differing order. For example, it will be appreciated that by selectively reducing the pressure of the working fluid in the heat transfer circuit 103 to at or below the threshold pressure allows the heat transfer system 100 to revert from the second mode to the first operating mode. Furthermore, the outlined steps and operations are only provided as examples, and some of the steps and operations may be optional, combined into fewer steps and operations, or expanded into additional steps and operations without detracting from the essence of the disclosed embodiments.

FIG. 3 is a detailed schematic representation of a heat transfer system of a further embodiment. The heat transfer system 200 of the further embodiment includes a reversible heat-pipe arrangement with first and second heat exchange assemblies 201, 202 that can act optimally as both evaporators and as condensers. Also, for reversibility, where each heat exchanger needs to act as an evaporator as well as a condenser in each of the operating modes, the structure for both heat exchangers is preferably similar or even the same.

The heat exchange assemblies 201, 202 are both of heat-pipe design and include a wick 208 arranged adjacent at least a portion of the inner surface of the heat-pipe wall. For a reversible heat-pipe, the wick material should have small enough pore size in order to generate sufficient capillary pressure to allow the liquid- vapour flow of working fluid within the heat-pipe to be maintained in a 2 to 3 metre gravity field. This gravity field results from the vertical separation due to ground- coupling of the second heat exchange assembly 202 at a subterranean location 212 of substantially constant temperature. Wick construction materials including nickel, copper and titanium are likely to satisfy these requirements.

A heat transfer circuit 203 is formed by the first and second heat-pipe exchangers that are coupled together by pipe sections 204, 205 as shown in FIG. 3. The pipe section 204 is further coupled to a pressure vessel 206 by a fluid valve 207, which pressure vessel incorporates at least one heat transfer surface 209. The heat transfer surface 209 is exposed to external ambient temperature conditions in order to effect reversal of operating mode, as described in relation to the embodiment of FIG. 1. In FIG. 3, a first operating mode wherein heat is absorbed by the first heat exchanger 201 is represented by the arrows in solid outline; whilst a second operating mode wherein heat is radiated by the first heat exchanger 201 is represented by the arrows in dashed outline, including for the heat fluid phases in pipe sections 204, 205 linking to the second heat exchanger 202.

The material of the wick 208, its effective pore radius, the permeability and thermal conductivity determine the level of the capillary pressure achieved in the heat-pipe system. The smaller the pore size the greater the capillary pressure. The smaller this pore size is, however, the larger the flow resistance which slows the rate of fluid/vapour flow through the system. The capillary pressure is developed when the evaporator section, by absorbing heat, allows the cold liquid to transfer through the wick to be converted to vapour which flows to the condenser while effectively acting as a "thermal and hydraulic lock" to prevent the heated vapour from mixing with the liquid. For efficient operation of the heat transfer system, the capillary pressure developed in the wick Δρ has to be greater than the sum of the pressure losses due to gravity APg, and due to pressure losses arising in the liquid/vapour lines as a result of flow resistance.

Another factor in the efficient operation of the heat transfer system is the characteristics of the working fluid both in the liquid and vapour phases. For example, the heat flow rate through the system depends of such properties of the working fluid as latent heat of evaporation, as well as on the inner and outer radii of the wick and its thermal conductivity.

The pressurising/depressurising chamber or vessel 206 is linked to the heat transfer circuit 203 and operates according to the external ambient temperature T where the pressure P in the chamber can be modelled according the standard gas law:

P = RT/V where the volume V is fixed and the universal gas constant R is also fixed for a given gas/vapour. Thus, as the ambient temperature T rises the pressure in the chamber also rises and hence, when valve 207 is opened, the pressure in the heat- pipe also rises. The parameters of the chamber, the working fluid and the dimensional and thermodynamic characteristics of the heat-pipe can be designed to allow the system to operate efficiently using the near-constant temperatures existing below ground.

It is believed that heat transport rates of 70W/°C to 290W/°C can be achieved depending on some of the above described parameters. With ammonia as the working fluid, maximum transport capability of about 800W may achieved. The evaporation temperature as well as the temperature differences between evaporation and condensation can also affect these values and generally the larger the

temperatures differences the more efficient is the heat transfer.

Since the direction of heat flow is primarily dependent on the pressure of working fluid in the heat-pipe and the pressure is dependent on the external ambient temperature, the system is passive since it does not require energy input to control the process. Furthermore, the liquid-vapour flow of working fluid is controlled by capillary pressure, which is a function of the physical design of the heat-pipe including the wick material, the vapour and liquid channels and again this pressure rises in a passive manner without using external energy.

A desirable property of any working fluid to achieve reversibility of the system is the capability of its boiling point to be changed by changing the pressure. This property allows the operating mode of the system to be reversed by changing the pressure in the system and thereby changing the boiling point from being at or below the ground temperature (heating cycle) to being at or below the set room temperature (cooling cycle). Suitable working fluids, such as water, ammonia, acetaldehyde, ether, pentane, ethyl chloride, and refrigerants R-245fa (1,1,1,3,3

Pentafluoropropane) propylene, nitrogen, freon and its substitutes, may have boiling points close to the estimated ground temperature at normal atmospheric pressure. This allows relatively small changes in pressure to move the boiling point above or below the ground temperature as required for reversibility.

In embodiments suited to domestic housing applications, the proposed heat- pipe system can be implemented by incorporating the heat exchanger into the floor or walls of the internal space of the house. The underground or ground-coupled heat exchanger may be of a similar design where it is implemented to maximise the efficiency of the dissipation or absorption of heat from the ground. The piping connecting the above surface and below-surface heat exchangers can also take a number of different forms depending on the local situation as well as on design details to achieve the highest possible capillary pressure.

Certain disclosed embodiments take advantage of constant ground

temperatures as a source of renewable energy to drive a reversible heat pipe for assisting both heating in cold ambient temperatures and cooling in hot ambient conditions, passive control of the reversing process which involves consumption of very little or no electrical energy. Some embodiments show potential for

implementation in temperate as well as sub-tropical environments where ground temperatures remain constant at relatively low depths, together with the ability to use low-cost working fluids such as water in some applications. The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is to be understood that this disclosure is not limited to particular methods, reagents, compounds, compositions or biological systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted systems are merely examples, and that in fact many other systems can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected", or "operably coupled", to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being "operably couplable", to each other to achieve the desired functionality. Specific examples of operably couplable include but are not limited to physically mateable and/or physically interacting components.

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g. , bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., " a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., " a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

As will be understood by one skilled in the art, for any and all purposes, such as in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non- limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as "up to," "at least," and the like include the number recited and refer to ranges which can be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a group having 1-3 cells refers to groups having 1 , 2, or 3 cells. Similarly, a group having 1-5 cells refers to groups having 1 , 2, 3, 4, or 5 cells, and so forth.

From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

1. A heat transfer system comprising:

a first heat exchange assembly;

a second heat exchange assembly;

a heat transfer circuit interconnecting the first and second heat exchange assemblies, the circuit incorporating a working fluid to transfer heat between the first and second heat exchange assemblies;

the heat transfer system being operative in a first mode where the working fluid is at or below a threshold pressure and a second mode where the working fluid is at a higher pressure than the threshold pressure; and

a pressure regulating device operative to increase the pressure of the working fluid above the threshold pressure to effect change of the heat transfer system from the first mode to the second mode.

2. The heat transfer system of claim 1 wherein the heat transfer circuit comprises a closed loop heat-pipe system.

3. The heat transfer system of any preceding claim wherein the first and second heat transfer assemblies each include a wicking material.

4. The heat transfer system of any preceding claim wherein the first and second heat transfer assemblies are operative mutually alternatively as evaporators and condensers.

5. The heat transfer system of any preceding claim wherein the working fluid is arranged to transfer heat to the first heat exchange assembly from the second heat exchange assembly in the first mode.

6. The heat transfer system of any preceding claims wherein the working fluid is arranged to transfer heat to the second heat exchange assembly from the first heat exchange assembly in the second mode.

7. The heat transfer system of any preceding claim wherein the pressure regulating device comprises a pressure vessel having at least one heat transfer surface.

8. The heat transfer system of claim 7 wherein the pressure vessel is coupled to the heat transfer circuit via a fluid flow control valve.

9. The heat transfer system of any preceding claim wherein the working fluid is selected from the group including demineralised water, ammonia, acetaldehyde, ether, pentane, ethyl chloride, and refrigerants R-245fa (1,1,1,3,3 Pentafiuoropropane).

10. An air-conditioning system for conditioning air in a region, comprising the heat transfer system of any one of claims 1 to 9 wherein the first heat exchange assembly is operative to moderate air temperature in the region and wherein the second heat exchange assembly is disposed adjacent a source/sink of substantially constant temperature.

11. The air-conditioning system of claim 10 wherein the pressure regulating device uses a pressure source based on temperature of a separate region.

12. An air-conditioning system for conditioning air in a habitable internal space, comprising the heat transfer system of either claim 7 or claim 8 wherein:

the first heat exchange assembly is operative to moderate the temperature of air in the habitable internal space;

the second heat exchange assembly is disposed adjacent a source of substantially constant temperature; and the at least one heat transfer surface of the pressure vessel is exposed to ambient temperature conditions.

13. The air-conditioning system of either claim 10 or claim 11 wherein the source/sink of substantially constant temperature is a subterranean location below the habitable internal space.

14. The air-conditioning system of claim 12 wherein the subterranean location is in the range of 1 to 5 metres below ground surface, preferably 2 to 3 metres below the surface.

15. The air-conditioning system of either claim 12 or claim 13 wherein the second heat exchange assembly is ground-coupled at the subterranean location.

16. The air-conditioning system of any one of claims 12 to 15, wherein the first threshold pressure is established so that in the first mode the second heat exchange assembly is capable of acting as an evaporator at the substantially constant temperature to allow heat transfer from the second heat exchange assembly to the first heat exchange assembly.

17. The air-conditioning system of any one of claims 12 to 15, wherein, when in the second mode, the pressure of the working fluid is at a level above the threshold pressure such that the second heat exchanger is capable of acting as a condenser at the substantially constant temperature to allow heat transfer from the first heat exchange assembly to the second heat exchange assembly.

18. A pressure regulating device for a heat transfer system comprising a pressure vessel with at least one heat transfer surface.

19. The pressure regulating device of claim 18 further comprising a valve for coupling the pressure vessel to a working fluid circuit of the heat transfer system.

20. A method of controlling a heat transfer system, the system being operative in a first mode where the working fluid is at or below a threshold pressure and a second mode where the working fluid is at a higher pressure than the threshold pressure, the method comprising:

regulating working fluid pressure in a heat transfer circuit containing the working fluid by increasing the pressure of the working fluid above the threshold pressure to effect change of the heat transfer system from the first mode to the second mode.

21. The method of claim 20 further comprising selectively reducing the pressure of the working fluid to at or below the threshold pressure to revert the operation of the heat transfer system from the second mode to the first mode.

22. The method of either claim 20 or claim 21 wherein a pressure regulating device is provided to regulate working fluid pressure.

23. The method of claim 22 wherein a pressure regulating device is provided to regulate working fluid pressure and is coupled to a working fluid circuit via a valve to enable the selective pressure regulation of the working fluid.