Heat pump

BACKGROUND OF THE INVENTION

This invention relates to a heat pump device particularly for air conditioning, refrigeration and heat pumping systems. The device relates especially to systems that contain to fluids known to have adverse effects on the stratospheric ozone layer or to have high global warming potentials relative to carbon dioxide. The device can provide direct replacements for all applications that currently employ mechanical vapor recompression and refrigerant/absorption solvent cooling or heat pumping systems.

Chlorofluorocarbons (CFC's eg CFC 11, CFC 12) and hydrochlorofluorocarbons (HCFCs eg HCFC 22, HCFC 123) are stable, of low toxicity and non-flammable providing low hazard working conditions used in refrigeration and air conditioning systems. When released they permeate into the stratosphere and attack the ozone layer which protects the environment from damaging effects of ultraviolet rays. The Montreal Protocol, an international environmental agreement signed by over 160 countries, mandates the phase-out of CFCs and HCFCs according to an agreed timetable.

The CFCs and HCFCs have been superseded in new air conditioning, refrigeration and heat pump equipment by hydrofluorocarbons (HFCs eg HFC 134, HFC 125, HFC 32) either as pure fluids or as blends. To accelerate the phase-out of CFC and HCFC existing units have also been retrofitted with appropriate HFC blends. Although HFCs do not deplete stratospheric ozone they are known to contribute to global warming. By the provisions of the Kyoto Agreement governments have undertaken to limit or cease the manufacture and release of these compounds. Some countries have already decided that phase-out of HFCs should commence sometime during the next decade and are actively promoting the development of non-halogen containing fluids.

The fluids in devices intended to replace HFC-containing units must have very low or preferably zero global warming potential. Preferably they should be compounds that are found naturally and whose properties are well understood so that damage to the environment from anthropogenic releases can be avoided. Furthermore, devices should be at least as energy efficient as the HFC containing units they are replacing to ensure that their contributions to global warming due to fossil fuel power station emissions are no greater. Preferably the devices should have better energy efficiencies.

Various terms that have been used in this specification are defined as follows:

Macroporous Solid: A solid containing pores some or all of which are interconnected and to the outer surfaces of the solid. The pores diameters are in the range 1 mm down to 50 nm.

Mesoporous Solid: A solid containing pores some or all of which are interconnected and to the outer surfaces of the solid. The pores diameters are in the range >2 nm<50 nm.

Microporous solid: The term is also extended to include a solid that has been impregnated with a liquid. Pore diameters are <2 nm.

Porous Solid: A generic term for the above three types of solids. In this specification the types of porous solids will be specified at the beginning of description of a particular embodiment of the invention. Thereafter the term “porous” will be used for simplicity in the rest of that description.

Macroporous Plate: A macroporous solid in the form of a plate having macropores that allow the direct passage of gas from one face of the plate to the other.

Mesoporous Plate: A mesoporous solid in the form of a plate having no macropores that would allow the direct passage of gas from one face of the plate to the other without passing through mesopores.

Microporous Plate: A microporous solid in the form of a plate having no macropores or mesopores that would allow the direct passage of gas from one face of the plate to the other without passing through micropores.

Porous Plate: A generic term for the above three types of plates.

Vapor: A fluid below its critical point and having no meniscus.

Gas: A fluid above its critical point. For convenience in this specification “gas” will be taken to include “vapor”.

Reversible process: A process that can be moved back and forth at will between all the equilibrium or steady state positions within its scope, by varying one or more imposed conditions, for example temperature, pressure or electric field.

Sorption: When a gas dissolves in a liquid the process is called “absorption”. Likewise when a gas is taken up by interacting with the surface of a porous solid the process is called “adsorption”. The gas may interact with the liquid or solid via relatively weak forces, for example Van der Waal forces, or by strong forces, for example by the formation of covalent bonds, either reversibly or irreversibly. In some instances when a gas is taken up by a solid impregnated by a liquid the process might be described by either “absorption” or “adsorption”. To prevent confusion in this specification the term “sorption” will refer simply to the reversible take-up of a gas by solid, or solid impregnated with liquid, without implying any mechanism by which the process is occurring.

SUMMARY OF THE INVENTION

According to the present invention, a heating or cooling device comprises a closed circuit wherein a fluid is continuously or intermittently circulated in one direction by a pump or compressor, an air conditioning, refrigeration and/or heat pumping system including the heat transfer device comprising a body of macroporous, mesoporous or microporous solid through which the fluid can circulate to create a temperature gradient.

In a preferred embodiment, the heat transfer device comprises a body of said porous material having inlet and outlet surfaces, means for supplying fluid to the inlet surface and means for removing fluid from the outlet surface, wherein fluid supplied to the inlet surface passes through the body to the outlet surface.

Preferably one or more of the inlet and outlet surfaces comprises a sorbent material.

In preferred embodiments, the thermal conductivity of the body of porous material in the direction of passage of fluid is less than the thermal conductivity perpendicular to the direction of passage of fluid.

The unit may comprise a laminate of a porous material between two layers of sorbent material.

The sorbent material may comprise elongate pieces, for example flakes, ribbons, wires, mesh or platelets of thermally conductive material. The conductive material may be a metal or graphite, for example metal or graphite flakes.

In preferred embodiments the porous material may comprise a mixture of sorbent material together with elongate pieces of the conductive material.

The porous solid may be selected so that the majority of pores have diameters less than the mean free path of the selected gas. This means that molecular, rather than bulk, gas flow will occur. The gas preferably also interacts so weakly with the porous solid itself that any enthalpy changes from this source can be ignored. The observed changes in enthalpy result from kinetic energy as gas molecules enter pores at the hot face of the solid and leave pores at the cold face of the solid. Any gas can be used provided it does not interact substantially with the porous solid surface. Suitable fluids include gases boiling <120 K and preferably <90 K. Preferred gases are He, H₂, N₂, O₂, Ne and Ar since these have the longest mean free paths and the highest molecular velocities and thus highest diffusion rates. H₂and O₂are less preferred because of their hazards. Where gas loss from the equipment is a concern then N2, air and Ar are especially preferred.

A wide range of porous materials may be employed. Silica, for example fumed silica, granular silica or aerogel silica, including granular, monolith and flexible blanket aerogels may be used. Natural or artificial glasses, ceramics or molecular sieves may be used. Carbons which may be used include activated granules or monoliths, aerogels or membranes. A range of organic material including resorcinol-formaldehyde foams or aerogels, polyurethane, polystyrene or other polymers including foamed or aerogel polymers may be used. A range of composites, including silica-carbon composites may be employed.

Porous materials may be made by blowing polymeric foams, sol-gel processes for manufacture of porous dense ceramics, silica or other mineral aerogels or organic aerogels. Organic materials may be pyrolysed, for example coconut and coal to produce activated carbons, polymer aerogels to produce carbon aerogels, hydrocarbons to produce carbon membranes, molecular sieves and carbon black or by plasma processes such as the APNEP (Atmospheric Pressure Non-Equilibrium Plasma) system developed by EA Technology Ltd.

Inorganic materials may be converted by thermolysis, for example production of fumed silica from silicon tetrachloride using an oxyhydrogen flame or by plasma processes. Organic-inorganic precursors may be processed by thermolysis to produce molecular sieves. Natural mineral hydrates may be thermalised, for example vermiculite and perlite. Sepiolite also known as Meerschaum, may be simply dried. Synthetic hydrates may also be used.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by means of example, but not in any limitative sense with reference to the accompanying drawings, of which:

FIG. 1 illustrates the essential elements of the device;

FIG. 2 is a schematic view of a first device in accordance with the invention;

FIG. 3 is a schematic view of a second device in accordance with the invention;

FIG. 4 is a schematic view of a domestic refrigerator in accordance with the invention;

FIG. 5 is a schematic view of an air conditioning unit in accordance with the invention;

FIG. 6 is a schematic view of a multistage system in accordance with the invention;

FIG. 7 is a schematic view of a cascade refrigeration system in accordance with the invention;

FIG. 8 is a schematic view of a thermal transpiration unit in accordance with the invention;

FIG. 9 is a schematic view from an idealized gas cooling cycle;

FIG. 10 is a schematic view of a cooling cycle through a throttling aperture;

FIG. 11 is a schematic view of a further device in accordance with the invention;

FIG. 12 illustrates a tubular device in accordance with the invention; and

FIG. 13 illustrates a cooling or heating apparatus in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 indicates the essential elements of a preferred device in accordance with the invention. The device comprises a closed circuit in which fluid is circulated by a compressor or pump (1). A porous plate (2) is disposed in a chamber defined by a casing (5) and a thermally insulating gasket (6) so that fluid is pumped into the chamber, through the plate (2) and returns to the pump (1), The chosen gas is isentropically compressed by a compressor (1) so that its temperature is approximately the same as that at which heat is to be rejected to a convenient sink, such as the atmosphere or a water stream. Preferably the temperature of the gas should be the same. The compressed gas is then allowed to expand through the microporous plate (2) having pore diameters less than the mean free-path of the gas molecules. The operating pressure is less than 20 bar, preferably <10 bar and most preferably <2 bar. The passage of the gas generates a temperature gradient across the plate. To maintain a constant temperature heat is transferred to a suitable fluid stream on the hot side (3) and heat is supplied by second fluid stream on the cold side (4). For this device to function efficiently it is important that the amount of heat transferred from the hot to the cold side of the plate by conduction through the solid is low. Consequently the thermal conductivity of the plate should <0.5 wK⁻¹m⁻¹, preferably <0.1 wK⁻¹m⁻¹and most preferably <0.03 wK⁻¹m⁻¹. This device can be described as a “baric transpiration unit”, or “BTU”. Any device in which a BTU is incorporated may be called a “baric transpiration device”, or “BTD”.

To complete the cycle gas is drawn into the suction port of the pump or compressor (1). The pump or compressor can be of any of the types known to those skilled in the art of compressing gases and vapors. Such compressors include reciprocating and rotary types. Such compressors often require liquid lubricants i.e. oils. In conventional Rankine Cycle systems the oil entrained by the compressed gas can readily be driven through the circuit and returned to the compressor. In the present invention entrained oil would be captured by the adsorbent thus blinding the pores and inhibiting the adsorption of the gas. Preferably oil containing compressors should therefore be fitted with highly effective oil filters to prevent oil reaching the adsorbent. More preferable are compressors that produce very low levels of oil entrainment such as centrifugal types. Most preferable are oil-free compressors which eliminate oil and possible contamination of the adsorbent, even after prolonged operation over 5 years or more. Especially preferred are diaphragm compressors which are oil-free and have good energy efficiencies, especially compared to oil-free reciprocating compressors.

According to a preferred aspect of this invention fluid/porous solid combinations are selected where additional enthalpy changes resulting from the interaction between the gas and the solid are comparable to, or substantially greater than, those resulting from the kinetic energy changes occurring when the gas molecules enter and leave the porous solid faces. For domestic, commercial industrial refrigeration and air conditioning applications technically suitable gases include CO₂, SO₂, NH₃, HFCs (e.g. HVC 134a), H₂, CFCs, HCFCs, and hydrocarbons (e.g. methane, ethane, propane, butane, I-butane). Blends of these gases can also be used, provided that components do not react chemically with each other. The gas or mixture is matched with a porous solid such as: carbon (e.g., graphite, activated carbon, charcoal, aerogel), silica (e.g. fumed, aerogel, alkylated aerogel), alumina, alumino-silicates (e.g. molecular sieves), and organic polymers (e.g. polystyrene, polyurethane, polyacrylate, polymethacrylate, polyamines, polyamides, celluloses), metal sponges (e.g. Ni, Ti, Fe), and metals or metal complexes supported on organic polymers or carbon.

Not all these gases and their combinations with the available porous solids may be appropriate. Although the CFCs and the HCFCs continue to be manufactured and used in the developing world their phase-out under the Montreal Protocol is already occurring. In territories when continued use of these chlorinated fluids is still legal then they might be used in combination with activated carbon, silica, or on organic polymer. SO₂and HFCs can be used with carbon, silica, alumina or an organic polymer, especially those with “basic” atoms such as O and N or “acidic” H atoms. In territories where phase-out of HFCs is not currently being considered, then their use in the present invention is acceptable, but not preferred because their global warming potentials are much higher than some of the other gases listed above. SO₂is not preferred because of its toxicity.

Hydrocarbons can be coupled with carbon, alkylated silica or an organic polymer, especially a hydrocarbon polymer such as polystyrene. Although more preferred than the halogenated fluids and SO₂, hydrocarbons are restricted to applications where the appropriate precautions can be taken against their marked flammability hazard, for example in large industrial applications or low-inventory, hermetically-sealed systems such as domestic refrigerators. A further disadvantage is that the enthalpy changes associated with the sorption/desorption of hydrocarbons is less than for more polar gases, notably CO₂, SO₂and NH₃. In some territories hydrocarbons are also disliked because any leak to the atmosphere and exposure to sunlight may generate “photo-chemical smog”.

Hydrogen is readily sorbed and desorbed from various metal alloys, notably those containing nickel. Although the high thermal conductivity of such alloys, even as porous solids, makes them unattractive for use in BTUs, by distributing the alloys as small particles in carbon, silica or an organic polymer a composite material combining a strong affinity for hydrogen with low thermal conductivity can be produced. Hydrogen is preferred to hydrocarbons because it will interact more strongly with metals than hydrocarbons with the sorbents listed above. Like hydrocarbons hydrogen reacts with atmospheric hydroxyl radicals that play a key role in removing naturally-emitted hydrocarbons as well as man-made pollutants such as HFCs. Increased hydrogen emissions can thus indirectly increase globally warming.

Ammonia can be used with carbon, silica or with an organic polymer. It is suitable for applications where its toxicity and flammability can be controlled, for example large commercial and industrial applications or low inventory, hermetically sealed domestic applications.

The most preferred fluid is carbon dioxide. This has low toxicity, is non-flammable and is readily adsorbed by a variety of sorbents including carbon, silica and organic polymers, especially those containing basic atoms such as O and particularly N. Suitable micro/mesoporous solids might be essentially homogenous in composition incorporating the groups capable of interacting with carbon dioxide on their surface. Alternatively such solids might comprise meso- or micro- or macro porous solid impregnated with a material capable of enhancing the adsorption of carbon dioxide. Nitrogen-containing organic compounds are preferred; more preferred are amines whose vapor pressures are less than 0.1 bar at 25° C. and even more preferably less than 0.01 bar at 25° C. Especially preferred are amines whose formula weight per amine N atom is less than 150, preferably less than 100 and most preferably less than 50.

Although carbon dioxide derived from fossil fuel is the single largest contributor to global warming the quantities required for this invention would be very small. By obtaining carbon dioxide from a natural source, such as biomass fermentation, any gas emitted from the device would have zero contribution to global warming.

In another embodiment of this invention a device comprises a working fluid, a positive displacement compressor driven by a source of mechanical power, an a porous adsorbent through which the compressed working fluid expands intermittently, i.e. the fluid flow is pulsed.

One configuration for the device is shown in FIG. 2. The compressor consists of piston 21, moving in cylinder 22, driven by piston rod 23 attached to crank shaft 24, the latter being powered by an electric motor or other motive source, which is not shown. The compressor is fitted with two valves, 25 and 26. The inlet or suction valve 25 opens when the swept volume between the piston and the cylinder head is just below the pressure in heat exchanger 27. This occurs when the piston is moving towards bottom dead center. Conversely the outlet or discharge valve 26 opens when the swept volume between the piston and the cylinder head is just above the pressure in heat exchanger 29. This occurs when the piston is moving towards top dead center. Heat exchanger 28 rejects the heat of compression to atmosphere. Adsorbent porous solid 30 interacts with the working fluid reducing its pressure to a level for which the device is designed and providing a resistance to its flow so that a pressure difference can be maintained between 27 and 29. The device is charged with sufficient working fluid to maximize the heat pumping capacity but without compromising the pressure limitations of the design.

The device operates in a cycle described by the following steps starting from the state where the suction stroke of the reciprocating compressor has just been completed and the compression stroke is just about to start, i.e. bottom dead center.

- (a) The piston is driven into the cylinder simultaneously raising the temperature and pressure of the gaseous working fluid until the discharge valve 26 opens and compressed gas is discharged into heat exchanger 28.
- (b) Heat exchanger 28 rejects the heat of compression to an air stream.
- (c) The cooled compressed gas then enters the high temperature, porous solid-containing heat exchanger 29 where it is adsorbed and the heat of adsorption rejected to an air stream which is thereby heated.
- (d) Working fluid travels though the porous solid towards the low temperature heat exchanger 27 under the influence of the pressure gradient established across the solid by the compressor.
- (e) The direction of the piston is reversed thus reducing the pressure in the cylinder causing the suction valve 25 to open so that the working fluid is desorbed from the porous solid in the low temperature heat exchanger 27. The heat of desorption is supplied by an external air stream which is thereby cooled.
- (f) The direction of the piston is again reversed, and starts to compress the gas causing the suction valve to close thus completing the cycle.

For the device to operate successfully in the mode described it is preferred that the pressure of the gas at any point in the porous solid connecting heat exchangers 27 and 29 oscillates about a mean value so that working fluid travels through the solid via a series of adsorptions and desorptions induced by the compressor. This process will provide the major contributions to the enthalpy changes in the heat exchangers 27 and 29.

The reciprocating compressor shown in FIG. 2 may be replaced by any positive displacement compressor, including a rotary, sliding vane or diaphragm type. Compressors whose sliding surfaces in the displacement volume are lubricated by a liquid lubricant may require oil separators between the compressor and the adsorption bed to prevent oil in fine droplet form fouling the bed. Oil-free compressors are therefore preferred, i.e. compressors whose sliding surfaces are not lubricated in the displacement volume by a liquid lubricant. Especially preferred are diaphragm compressors which operate nearer to isothermal than isentropic conditions because of the effective cooling of the working fluid through the large surface of the diaphragm and the compressor head augmented some units by the cooling of the circulating hydraulic oil that drives the diaphragm. Diaphragm compressor energy efficiencies can be superior to those of reciprocating compressors. Fluid leakage rate is much lower because an excellent seal can be established between the diaphragm and compressor case, while an oil-free reciprocating compressor lacks the excellent sealing properties of the oil film of a conventional reciprocating unit. For low duty applications, such as domestic refrigerators and room air conditioning units, diaphragm compressors have the advantage over conventional oil-filled hermetic reciprocating and rotary units in providing a combination of an excellent gas seal with an external electrical motor. The heat generated by the latter can be dissipated by a simple cooling fan. In conventional hermetic systems motor cooling is partly provided by the oil, which transfers heat to the casing, and the refrigerant, which transfers heat to the condenser. By removing requirement for internal cooling of the electric motor the energy efficiency of the cycle can be improved.

Various compressor designs can be used, provided they are configured to deliver pulses of compressed gas at the hot/high pressure face of the BTU and remove pulses of expanded gas at the cold/low pressure face. FIG. 2 schematically illustrates one method for achieving this.

FIG. 3 schematically shows a further embodiment of this invention. A pressure difference is maintained between two vessels, 11 at low pressure and 12 at high pressure, by any type of compressor 13 capable of achieving the desired pressure ratio. This includes both pulsed positive displacement and continuous delivery turbo/centrifugal types, if necessary multi-staged. Pulses of compressed working fluid gas or vapor are delivered to the hot/high pressure heat exchanger 14 of the adsorbent bed 15 by periodically opening and closing powered valve 16. Pulses of expanded working fluid are removed from the cold/low pressure heat exchanger 17 by periodically opening and closing powered valve 18. The advantages of the design shown in FIG. 2 over that in FIG. 1 include the independent phasing of the compression and suction pulses and the ability to use continuous delivery compressors. Heat exchanger 19 rejects the heat of compression to atmosphere.

To further enhance the efficiency of the pulsed embodiment of the invention the air flow over the heat exchange surfaces can also be beneficially pulsed. When the suction part of the cycle is generating cooling on the cold face air flows over this face, but with no air flow over the hot face.

In another aspect of this invention the BTU can form part or all of the cooled enclosure, of a refrigerator, cold store or other cooling apparatus. FIG. 4 shows a domestic refrigerator incorporating a BTU. The domestic refrigerator comprises a casing 40 and door 41. The compressor illustrated schematically at 42 is connected between inner and outer portions of the cavity between the inner and outer surfaces of the porous solid 43 and 44. The conventional closed-cell foam insulation used in the cavity of a standard refrigerator 45 is replaced completely or partially with a body of a meso or micro-porous solid 45.

Compressed gas circulated by the compressor or pump 42 is channeled to a void or inlet passageway between the outer case and the porous solid 45 through a network of passages. This gas passes through the body of porous solid 45 and is collected by a void or passageway between the inner case and the porous solid for return to the compressor. Thus the casing and/or floor of the refrigerator may comprise inner and outer skins, a porous body located between the skins and inlet and outlet voids or passageways between the porous body and the outer and inner skins respectively.

A device in accordance with this aspect of the invention has the advantage that it enables the insulation and the cooling circuit to be integrated. The “conventional” closed-cell foam insulation of a standard refrigerator is replaced by a meso- or micro-porous solid. Compressed gas is piped to the region between the outer case and the porous solid. This region contains a macro-porous, thermally-conducting solid that simultaneously allows facile access of the gas to the surface and the removal of the heat of adsorption of the gas at the surface of the micro/meso-porous solid. A suitable macro-porous, thermally-conducting solid may be fabricated from metal, preferably with a high thermal conductivity, Aluminum is especially preferred. Alternatively the macro-porous solid may be fabricated from an organic polymer, preferably filled with a thermally conducting material. Especially preferred are iron, aluminum or graphite powder or flakes. The macro-porous solid may be fabricated in the form of a solid of adhering particles, a gauze or a perforated or grooved plate. The low-pressure gas desorbs from the micro/meso-porous solid and enters a void between the porous solid and the inner lining of the refrigerator, in this region a second macroporous solid may be located to provide both good thermal conductivity to allow heat to be conducted from the fridge interior to supply the heat of desorption, and the facile exit of the gas to the compressor suction port. The macro-porous materials in both the high pressure and low pressure regions also provide mechanical support for the micro/meso-porous solid.

The refrigerator may advantageously incorporate further features to enhance its energy efficiency. An air circulating fan may be included within the refrigerator interior to the inner walls to enhance heat transfer to the cold inner walls. In this context the term “wall” is taken to include top and floor. If the refrigerator is being used to store frozen food, a device commonly called a freezer, the fan will assist in keeping the inner walls frost free, thus preventing ice build up from compromising the good energy efficiency of the device. To enhance heat transfer within the fridge part, or all, of the inner wall area may be fabricated with extended heat transfer area, for example in the form of ridges or fins. Similarly part, or all, of the external wall of the fridge may also be fabricated with extended heat transfer area. To maximize the transfer to the heat of absorption from the surface of the micro/meso porous solid, the external case and the thermally conducting macro-porous solid can be fabricated as a single component using materials with good thermal conductivities. Similarly the inner wall and its associated macro-porous solid can also be advantageously fabricated as a single component.

The micro/meso-porous solid may be formed in sheets with dimensions or other laminar sheet members comparable to those of a fridge wall. Alternatively it may be provided in the form of tiles, for example hexagonal or square members, whose dimensions are significantly less than those of the wall. Such tiles can be efficiently produced by extrusion using an industrially available extrusion equipment. Such tiles would typically have sides in the range 5 to 100 mm. These are then assembled into the fridge walls by binding them together as a mosaic with an organic polymer. Preferably the binder will be a foam material with good thermal insulation properties. Closed cell polyurethane foam with average cell diameter less than 1 mm is preferred. Especially preferred is a polyurethane foam with a cell diameter less than 0.3 mm.

FIG. 5 illustrates a further aspect of this invention wherein a coolant stream (e.g. air or water) removing heat from the hot side of the device runs counter-current to the stream supplying heat to the cold side of the BTU. This arrangement may serve as an air conditioning unit.

The arrangement is generally as shown in FIG. 1 with means for conducting flows of heated air 51 and cooled air 52 across heating or cooling fins 53, 54 respectively.

Two or more BTU devices may be combined to produce a multistage basic transpiration device (BTD) that enables lower temperatures to be achieved more efficiently than is possible with a single device. Such an arrangement may replace a multi-stage compression system as used in conventional refrigeration systems to allow more energy efficient generator of low temperatures that can be achieved by a simple unit. FIG. 6 shows the basic circuit of a BTD. A single fluid may be used. However a particular advantage of devices in accordance with this invention is that different sorbents can be used to optimise the high and low temperature sorption/desorption stages.

Two porous bodies 63, 64 are connected sequentially so that fluid 61 from the first unit is circulated through the second unit 62.

A series of basic transpiration devices (BDTs) may be placed in series as shown in FIG. 7, to produce a cascade refrigeration system. A BTD can use different fluids; for example a 3-circuit cascade system may have nitrogen in the lowest temperature circuit 71, carbon dioxide in the intermediate circuit 72 and propane in the highest circuit 73. However an especial advantage of a BTD approach is that the same fluid can be used in more than one circuit by matching it with a different microporous solid in each circuit. For example in a 3-circuit system cited, CO₂can be used in both the intermediate and upper temperature circuits.

The BTU may be driven by a second meso- or microporous solid unit working as a heat engine. All of the devices described above rely upon mechanical compressors to circulate the working fluid. Applying a temperature gradient to microporous solid can be used to generate a gas pressure difference across its faces by a process which may be called “thermal transpiration”. This pressure generating device can be called a “Thermal Transpiration Unit” (TTU). A thermally-driven air conditioning, refrigeration or heat pumping unit may comprise a TTU operatively coupled to a BTU in a circuit.

For many applications mechanical compressors have provided the most convenient option for operating a heat pumping device. These require a power source which consumes energy derived from fossil fuel combustion, which in turn contributes to global warming by releasing carbon dioxide into the atmosphere, or nuclear fission which has problems in the containment and storage of highly radioactive waste. Consequently heat pumping devices are preferred that can be powered by relatively low temperature hat sources including solar thermal energy, geothermal energy and low temperature streams such as those sometimes produced by industrial process plants. Environmentally, the utilization of such energy sources minimises the man-made contributions to global warming. Consequently an especially preferred embodiment of the invention involves a BTU driven by a second meso- or microporous solid unit, which can be considered as essentially a BTU working in reverse as a heat engine. In this latter case applying a temperature gradient to the microporous solid generates a gas pressure difference across its faces by process called “thermal transpiration”. This pressure generating device can be called a “thermal Transpiration Unit” (TTU). A thermally-driven air conditioning, refrigeration of heat pumping unit may comprises a TTU operatively coupled to a BTU in a circuit.

FIG. 8 indicates a basic layout. One face (81) of the TTU (83) is heated, e.g. by a flame or solar energy, while the other face (82) is cooled, e.g. by a flow of ambient air. The higher pressure is generated on the hot side due to the influence of the temperature difference. Gas flows from the cool side of the TTU to the hot side. As can be seen from FIG. 8 the TTU (83) sets up a pressure difference across a BTU (84). Essentially the TTU replaces the mechanical compressor shown in previous embodiments. To maximize the energy of the device high pressure leaving the hot side of the TTU is cooled by passing through the heat exchanger by transferring heat to the cool gas leaving the low temperature side of the BTU. After leaving the heat exchanger the gas enters the BTU where it is adsorbed on the porous solid with the rejection of heat. Under influence of the pressure difference generated by the TTU the gas passes through the porous solid of the BTU to the low pressure side where it is desorbed to give the desired cooling effect. The cold low pressure gas returns to the TTU via the heat exchanger as shown.

This device has certain similarities to an absorption refrigerator, but has important and advantageous differences. Since there is no liquid absorbent there is no mechanical pump. All the pumping is performed by the TTU. The heat capacity of a liquid absorbent has a significant detrimental effect on the energy efficiency and/or cost efficiency of an absorption heat pump since it requires a large, and therefore expensive, heat exchanger to reduce its impact. Indeed it often dominates design economics and the choice of fluid/solvent pair. The present system shown here eliminates this problem. Furthermore, although many refrigerant/solvent pairs appear to be potentially useful very few are commercially viable. In contrast for the adsorption system a great range of porous solids may be employed, especially because toxicity and flammability constraints are reduced or eliminated.

This embodiment is especially advantageous for the air conditioning of buildings. In one embodiment the TTU can be configured to form a roof of the building which points towards south in the northern hemisphere and north in the southern hemisphere to maximize the capture of solar radiation. To make the system truly independent of fossil or nuclear energy solar voltaic panels can be installed alongside the TTU to produce electrical energy to drive fans to circulate air over the heat exchanger surfaces of the BTU and the ITU.

The liquid-free systems described above can be constructed without any limitations on the positioning of components imposed by a need to allow for the gravity-controlled flow of liquids. This contrasts with conventional mechanical vapor recompression systems and adsorption systems whose construction is constrained by the need for liquid refrigerant to flow under gravity, for example in condensers and evaporators.

In an idealized gas cooling cycle as shown in FIG. 9 expansion of gas through an expander helps drive the compressor. This is more energy efficient than simple expansion through a throttling aperture as shown in FIG. 10. However in a real system some of this advantage is lost because the intrinsic inefficiencies in the compressor and expander are multiplied together to give the overall efficiency. In one embodiment of the invention the efficiency of the simple expansion system is enhanced by expanding the gas through a macro- or meso-porous solid plate that also acts as a partial or complete insulating shell for the enclosure that is being cooled. An important requirement is that the expanding gas absorbs heat being conducted from the warm, outer surface of the porous solid to the cool inner surface. Sufficient pores are provided with a large total internal surface area to provide adequate heat transfer from the solid to the gas. The diameters of the pores and the thickness of the porous solid are selected to provide resistance to the gas flow to maintain the necessary pressure and thus temperature difference across the faces of the solid. In this aspect of the invention the typical diameters of the pores do not need to be less than the mean free path of the gas molecules.

To enhance the heat transfer from the solid to the surfaces of the pores metal or graphite sheets or fibers eg 118 are located inside the porous solid with their long dimensions disposed parallel to the faces of the BTU as shown in FIG. 11. The metal plates disposed facilitate heat conduction parallel to the BTU faces, but not contribute substantially to conduction between the faces. Extended heat transfer surfaces 111 perpendicular to the BTU faces facilitate heat transfer to and from the surfaces of the porous body. A further important requirement is that there is no significant gas pressure drop across the metal plates. This means that the plates should themselves be highly porous or be in the form of large numbers of small platelets around which the gas can pass. an additional advantage of incorporating metal plates/platelets/fibers is that they scatter thermal radiation and thus augment to the insulating effect of the porous solid.

Preferably the gas and porous solid are selected so that under the operating conditions of the cycle the gas undergoes capillary condensation on entering the pores of the solid releasing its latent heat of vaporization. The fluid then passes through the porous solid under the influence of the pressure difference and evaporates on leaving the pores thus producing the required cooling.

In mobile air conditioning a condenser is conventionally situated at the front of the vehicle, typically just in front of the engine radiator, and the evaporator is in the scuttle behind the dashboard with the compressor mounted on the engine block and driven directly from the crankshaft. Modern vehicle design has developed so that the air conditioning unit is efficiently accommodated. In an attempt to provide devices with the same geometry as existing HFC units very high pressure (150 bar) CO₂units have been specifically designed to be direct replacements. Unfortunately by imposing this constraint the new units have proved to be heavier and less energy efficient than those they are intended to replace. Although BTU based units do not readily meet the space requirements of current HFC units they enable advantageous innovative mobile air conditioning configurations to be designed for vehicles. Because of their low operating pressures, BTUs have low mass and, in the form of panels, can be incorporated into the doors, roofs or other surfaces within vehicles. If placed in a door or roof the hot side of a BTU can be cooled by being in thermal contact with the outer body of the vehicle and thus cooled by the air stream when moving, augmented by a fan inside the door when the vehicle is stationary.

As computers, especially PCs, became increasingly more powerful, chips require forced cooling to enable them to run continuously at their maximum speeds. A further aspect of this invention provides a computer casing incorporating a BTU device to provide cooling. Air can be circulated internally by a fan without the need to provide external ventilation, thus minimizing the ingress of dust and other contamination.

For some applications, especially those requiring the cooling or heating of liquid or gas streams, BTUs can be produced in the form of tubes. This arrangement is especially useful where cooling is being distributed via a secondary refrigerant such as brine or ethylene glycol. In this case heat will be pumped from the inner surface of the tube to the outer surface.

Tubular BTUs can be used to maintain a fluid stream at a given temperature rather than being the primary source of refrigeration. In other words the BTU provides of “active insulation” where the inward (or outward) flow of heat due to conduction and radiation is just balanced by the heat pumping effect of the BUT. Such BTUs can find application in the process industries, for example on a chemical plant where a near-saturated solution of a chemical that needs to be transferred from one location to another without the solute crystallising and thus causing a blockage. In this case the heat is pumped from the outside of the tube to the inside, i.e. in the opposite direction to that shown in FIG. 12.

FIG. 12 illustrates a pipe through which a heated liquid 124 may pass. The body of the pipe comprises an outer skin 121 a porous cylindrical layer 122 disposed within the skin and an inner tube 123 which carries the liquid 124. Passageways (not shown) for conduction of the refrigerant fluid may be provided in an analogous manner to that described with reference to FIG. 4. In one embodiment heat from the surrounding environment is pumped into the tube to maintain the temperature of the liquid or fluid flow 124. In an alternative arrangement the coolant flow may be reversed so that a refrigerator flow is facilitated.

In an alternative embodiment an object to be cooled or heated is contained within a BTU. For example an electronic microchip or an electrical transformer that needs to be kept at specific temperature is shown in FIG. 13.

A microchip 131 or other electrical component is disposed within an insulated casing 132 having a BTU unit 133 located in the wall in order to remove heat from the component 131 during use.

The invention is further described by means of the following examples but not in any limitative sense.

Examples 1 to 8

The performances of typical existing air conditioning systems based on R22 and R407c were modeled using NIST's Cycle D program. The isentropic efficiency of the compressor was 0.7 and the motor efficiency was 0.85. The input parameters and the performance results are summarized in Table 1 and provide a comparison with the results from the modeling of units that are embodiments of this invention.

TABLE 1

1
2
3
4
5
6
7
8

R22
R22
R22
R22
R407C
R407C
R407C
R407C

Input Parameters

Evap temperature
C.°
7
10
5
5
7
10
5
5

Cond temperature
C.°
45
40
37
35
45
40
37
35

Performance

Suction pressure
bar
17.3
15.34
14.24
13.55
18.78
16.58
15.36
14.58

Discharge pressure
bar
6.21
6.81
5.84
6.81
6.39
7.06
6.02
7.08

Capacity
KJ₃m⁻
3942
4557
3954
4758
4132
4851
4185
5108

COP

3.50
4.71
4.31
5.84
3.46
4.68
4.29
5.83

Duty
kW
10
10
10
10
10
10
10
10

Examples 9 to 15

BTU-containing air conditioning units were modeled with argon as the working fluid. The flow rate of gas in all cases was 1 gram atom per second, the isentropic efficiency of the compressor was 0.7 and the motor efficiency was 0.85. The input parameters and the performance results are summarized in Table 2. The low pressures and low pressure differentials can be achieved by a centrifugal compressor, which has the further advantage of being oil-free.

TABLE 2

9
10
11
12
13
14
15

Input Parameters

Microporous solid thickness
m
0.02
0.02
0.05
0.03
0.05
0.05
0.05

Microporous solid thermal
W m⁻¹K⁻¹
0.01
0.01
0.1
0.915
0.1
0.015
0.015

conductivity

Microporous solid area
m²
5
5
5
3
5
5
2

Cold side temperature
° C.
5
7
7
7
7
10
10

Hot side temperature
° C.
37
45
45
45
45
40
35

Performance Results

Suction pressure
bar
1.5
1
1.25
1
1.5
1.25
1.5

Discharge pressure
bar
1.97
1.37
1.72
1.37
2.06
1.61
1.85

Capacity
kW m⁻³
178
118
132
119
158
151
183

COP

4.09
3.44
3.08
3.48
3.08
4.51
5.49

Duty
kW
2.74
2.74
2.45
2.78
2.45
2.84
2.88

Examples 16 to 17

The performances of typical refrigeration based on R134a were modeled using NIST's Cycle D program. The isentropic efficiency of the compressor was 0.7 and the motor efficiency was 0.85. The input parameters and the performance results are summarized in Table 3 and provide a comparison with the results from the modeling of units that are embodiments of this invention.

TABLE 3

16
17

R134a
R134a

Input Parameters

Evap temperature
C. °
−10
−25

Cond temperature
C. °
45
50

Performance

Suction pressure
bar
11.60
13.18

Discharge pressure
bar
2.01
1.06

Capacity
KJ m⁻³
1305
613

COP

2.10
1.25

Duty
kW
10
10

Examples 18 to 22

BTU-containing refrigeration units were modeled with argon as the working fluid. The flow rate of gas in all cases was 1 gram atom per second, the isentropic efficiency of the oil-free centrifugal compressor was 0.7 and the motor efficiency was 0.85. The input parameters and the performance results are summarized in Table 3. For comparison Table 4 shows the performance results for a conventional mechanical vapor recompression system working between similar temperature limits. The Coefficients of Performance (COP) for the BTU based units are clearly superior to those of the R134a based units. Consequently the BTU systems using argon will have reduced global warming both by their superior energy efficiency and by avoiding the use of global warming gases as working fluids.

TABLE 4

18
19
20
21
22

Input Parameters

Microporous solid thickness
m
0.02
0.05
0.05
0.05
0.05

Microporous solid thermal conductivity
W m⁻¹K⁻¹
0.01
0.01
0.1
0.001
0.005

Microporous solid area
m²
1
5
5
2
2

Cold side temperature
° C.
−10
−10
−25
−25
−25

Hot side temperature
° C.
45
45
50
50
50

Performance Results

Suction pressure
bar
1.5
1
1
1
1

Discharge pressure
bar
2.41
2.41
1.94
1.94
1.94

Capacity
kW m⁻³
180
176
114
117
117

COP

2.27
2.23
1.49
1.54
1.53

Duty
kW
2.62
2.57
2.35
2.42
2.41

Example 23

The performance of a freezer was modeled when just maintaining the required internal temperature. The unit was insulated by an aerogel through which compressed argon was passed and expanded in a molecular flow regime. The heat from the cooling circuit was emitted to the surrounding air at the external surfaces of the unit, which were cooled by natural (unforced) convention. Under these conditions the heat pumped by the cooling circuit just balanced the heat flow from the exterior to the interior of the freezer and the heat dissipated to the air equaled the work done by the compressor to compressor the gas. The input parameters for the model were the following.

Freezer dimensions
1 m × 1 m × 1 m

Ambient temperature
25°
C.

Required internal temperature
−20°
C.

Gas flow rate
0.045
mol s⁻¹

Insulation thermal conductivity
0.01
W m⁻¹K⁻¹

Wall thickness
0.250
m

Top thickness
0.251
m

Bottom thickness
0.265
m

Suction pressure
1
bar

Motor efficiency
0.85

Compressor isentropic efficiency
0.7

The performance results were the following:

COP
2.52

Input power to motor
45.2
W

External surface temperatures

Sides
27.50°
C.

Top
27.65°
C.

Bottom
30.20°
C.

The performance of a typical hermetic R134a-containing, vapor recompression unit to achieve the same duty was calculated using the NIST Cycle D Program.

Input Parameters:

Condenser temperature
45°
C.

Condenser subcooling
5°
C.

Evaporator temperature
−20°
C.

Evaporator superheating
5°
C.

Compressor isentropic efficiency
0.70

Motor efficiency
0.85

Performance Results:

Condenser pressure
11.60
bar

Evaporator pressure
1.33
bar

COP
1.64

Input power to motor
69.6
W

The new device based on the expansion of argon gas through the aerogel insulation clearly is energy efficient that a conventional system based on HFC 134a.

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CPC

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