WO2008155543A2 - Heat pump - Google Patents

Abstract

A heat pump including first and second adsorption beds (1.2, 1.3), each bed including a porous solid, the first bed having a first heat exchanger and the second bed having a second heat exchanger; a circuit for working fluid communicating between the beds; valve means (11.1, 11.2) for switching flows of heat transfer fluid through the first and second heat exchangers (11.3, 11.4); a compressor (1.1., 12.1) connected between the valve means adapted to cause working fluid to flow within the circuit between the beds,- the working fluid comprising a reactive gas capable of adsorption and desorption by the porous solid; the first heat exchanger including a first conduit for a heat transfer fluid; the second heat exchanger including a second conduit for a heat transfer fluid; and a bypass valve (1.7) connected between the valve means in parallel to the compressor (1.1, 12.1) and adapted when opened to allow flow between the beds bypassing the compressor.

Description

HEAT PUMP

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 no fluids known to have adverse effects on the stratospheric ozone layer or to have high global warming potentials relative to carbon dioxide. The device may provide a direct replacement for any application that currently employs a mechanical vapour recompression or fluid/solvent pair absorption cooling or heat pumping system.

In this specification the term 'heat pump' describes any powered device which moves heat from a source to a sink against a thermal gradient. A refrigerator is a particular type of heat pump where the lower temperature is required for the intended application. The term 'heat pump' is also used in a more limited sense than in this specification to describe a powered device which moves heat from a source to a sink against a thermal gradient where the higher temperature is required. The distinction between a refrigerator and a narrowly-defined heat pump is merely one of intended purpose, not operating principle. Indeed, many air conditioning systems are designed to supply either heating or cooling depending upon the user's need at a specific time.

In this specification the term "gas" is taken to include the term "vapour".

Chlorofluorocarbons (CFCs e.g. 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 when used in refrigeration and air conditioning systems. If released they permeate into the stratosphere and attack the ozone layer which protects the environment from the 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.

WO-2006/111773 disclosed a heat pump in which a temperature difference is established between two heat exchangers by inducing cyclical expansion and compression pulses in a coupling fluid which passes through an adsorbent porous solid located between the heat exchangers.

According to a first aspect of the present invention there is provided a heat pump including first and second adsorption beds, each bed including a porous solid, the first bed having a first heat exchanger and the second bed having a second heat exchanger; a circuit for working fluid communicating between the beds; valve means for switching flows of heat transfer fluid through the first and second heat exchangers; a compressor connected between the valve means adapted to cause working fluid to flow within the circuit between the beds; the working fluid comprising a reactive gas capable of adsorption and desorption by the porous solid; the first heat exchanger including a first conduit for a heat transfer fluid; the second heat exchanger including a second conduit for a heat transfer fluid; and a bypass valve connected between the valve means in parallel to the compressor and adapted when opened to allow flow between the beds bypassing the compressor.

According to a second aspect of the invention there is provided a heat pump including first and second adsorption beds, each bed including a porous solid, the first bed having a first heat exchanger and the second bed having a second heat exchanger; a circuit for working fluid communicating between the beds; a compressor adapted to cause working fluid to flow within the circuit between the beds; the working fluid comprising a reactive gas capable of adsorption and desorption by the porous solid; the first heat exchanger including a first conduit for a heat transfer fluid; the second heat exchanger including a second conduit for a heat transfer fluid; valve means for switching flows of heat transfer fluid through the first and second heat exchangers; and a number of adsorption beds, the number being 2N wherein N is a positive integer and wherein each bed has an inlet and an outlet; the heat exchangers being arranged in a plurality of arrays, each array comprising an input for heat from a flow of the heat transfer fluid or an output for heat to the heat transfer fluid; wherein the heat exchangers of each array provide a temperature glide to the respective flow of heat transfer fluid.

The term "reactive gas" used in this specification is intended to refer to a gas which may be reversibly adsorbed onto the porous solid, i.e. by physisorption.

Optionally, but preferably, the device comprises a further heat exchanger located between the compressor and the compressor beds, preferably immediately after the compressor between the valve means and adapted to remove heat of compression from the working fluid gas before the latter comes into contact with an adsorption bed.

In the first aspect of this invention, the bypass valve allows pressure equalisation without a need to stop the compressor. Stopping and restarting a compressor is energy inefficient and will accelerate compressor wear. Furthermore, it may enable a pressure or temperature gradient to be established along the beds. This gradient may advantageously increase the temperature difference between the heat transfer fluid inlet and outlets, maximising the utilisation of the compressor in contrast to the disclosure of WO-2006/111773.

The working fluid may comprise a gas or mixture of gases, including the reactive gas in combination with a porous solid that is capable of adsorbing the gas. Especially preferred is carbon dioxide and a micro-porous/nano-porous solid such as activated carbon. The operation of the device provided below uses CO₂ and coconut carbon as an example but is not intended to be limitative.

The reactive gas may be selected from the group consisting of: hydrocarbons, ammonia, hydrogen, HFCs, fluoro-iodides, unsaturated fluorinated compounds containing 2 to 6 carbon atoms, fluoro-olefins containing trifluorovinyl groups, CO₂, nitrogen and mixtures thereof.

The porous solid may be selected from the group consisting of:

silica, glasses, ceramics, molecular sieves, activated carbons, microporous organic polymers, organometallic polymers, polymers of intrinsic porosity and mixtures thereof.

Polymers of intrinsic porosity are polymers which derive their porosity by having much larger free volumes than conventional polymers. These polymers may be generated during polymerisation by using monomers with rigid 3-dimensional structures which cannot pack closely together, as reported in Angewcmdte Chemie International Edition (Vol. 45, Issue 11, DOI: 10.1002/anie.200504241; p. 1804), McKeown et al. In previous microporous solids, solids pores have been generated in the processing of polymers subsequent to their syntheses.

In the following description only carbon is referred to for simplicity, although other of the aforementioned adsorbent solids may be used.

The working fluid may comprise a reactive gas and an additional gas which may have a ^• higher thermal conductivity than the reactive gas. The additional gas may be selected from the He₅Ne₅H₂ D₂ DH and mixtures thereof. Use of He is preferred when the reactive gas is CO₂.

In use of the device a temperature difference is established between the two beds by desorbing the reactive gas from one bed, which cools down, and adsorbing the reactive gas on a second bed, which is caused to heat up. The adsorbent beds are constructed to facilitate heat exchange with their surroundings. The device thus also comprises a first gas or liquid stream as a heat transfer fluid, which removes heat from the hot heat exchanger and rejects it into a suitable heat sink, and a second stream which supplies heat from a heat source to the cold adsorption bed.

To maximise the area available for heat exchange the adsorption bed heat exchangers may be designed to have an elongate or labrynthine flow path, for example with at least one dimension which is significantly longer than at least one other dimension. The working fluid enters an adsorbent bed heat exchanger at one end of the long dimension and leaves at either the same end or the opposite end.

The quantity of working fluid that a bed can adsorb may be limited by the mass of the adsorbent, its adsorbent properties, the applied pressure and temperature. When this limit has been reached appropriate valves in the circuit are operated so the roles of beds are reversed in order to ensure that heat pumping continues. This is commonly called a "swing" process.

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

Figures 1 and 2 diagrammatically illustrate a heat pump;

Figure 3 illustrates a regenerative heat pump;

Figure 4 illustrates a regenerative heat pump wherein two beds are connected;

Figure 5 illustrates regenerating heat exchange with heat pumping;

Figure 6 is a diagram showing a regenerative heat pumping cycle;

Figures 7 and 10 illustrate a first heat pump in accordance with this invention.

Figures 11 and 12 illustrate further embodiments of this invention;

Figure 13 illustrates an air conditioner in accordance with this invention;

Figure 14 illustrates a 4 bed embodiment of this invention;

Figure 15 illustrates a 6 bed embodiment of this invention;

Figures 16 to 18 illustrate the construction and performance of a heat pump in accordance with this invention;

Figures 19(a) to (c) illustrate an alternative bed configuration;

Figure 20 illustrates a 4-bed, 2 array heat pump using the bed shown in Figure 19; or

Figure 21 (a) and (b) illustrates operation of the heat pump shown in Figure 20.

Figure 1 illustrates a CO₂ circuit which comprises: a compressor 1.1; adsorption beds 1.2 and 1.3; a heat exchanger 1.4 to remove the heat of compression; three- way, powered valves 1.5 and 1.6 which allow the beds 1.2 and 1.3 to be placed alternatively under compression or suction; and a compressor powered by-pass valve 1.7. Figure 2 shows a heat pump arrangement and indicates the air flow configured to provide cold air to a room with the simultaneous removal of stale air. The circuit comprises; adsorption beds 1.1 and 1.2; a powered air valve or damper 2.3; a pusher fan 2.4 and a suction fan 2.5; a negative heat (commonly referred to as "coolth") sink is shown at 2.6. The arrows represent the following flows: 2.7 external fresh entering device; 2.8 room stale air entering device; 2.9 stale being exhausted from device after heating to a temperature higher than the external temperature by bed 1.3; 2.10 fresh air entering after being cooled to the desired low temperature by bed 1.1.

Figures 1 and 2 show the system at one stage of its swing cycle. When the various valves are actuated the cooling and heating functions of beds 1.2 and bed 1.3 are interchanged. The system also comprises pressure and temperature sensors, pneumatic or electric valve actuators and control circuitry which enables the flows of CO₂ and air to be swung between the beds.

A feature of the device claimed in this invention is the integration of three fundamental heat transfer processes in a single unit: a compressor driven adsorption cooling device, a regenerative/recuperative heat exchanger (RHX) and passive (i.e. not driven) adsorption/desorption.

In previous air conditioning practice recuperative heat exchange has taken the form of a heat wheel. In the present invention the two or more adsorption beds perform this duty. The simple regenerative or heat exchange aspect of the present device is explained in relation to Figure 3 which represents fresh air flowing into a room 3.1 from outside a building 3.4 without heat pumping across two air streams 3.5 and 3.6, and without the beds being connected so that no refrigerant gas can pass between them. Bed 3.2 heats exhaust air while bed 3.3 cools incoming fresh air. This process will continue until bed 3.2 is substantially cooled along the whole of its length while bed 3.3 is correspondingly heated to the temperature of the external air. At this point the control circuitry causes the air flows to be reversed so that bed 3.2 cools incoming air and bed 3.3 heats exhaust air. In this mode the device merely operates like a heat wheel or other recuperative heat exchanger relying upon the thermal capacities of the beds to mediate the heat transfer process. Although a simple recuperator provides room ventilation, replacing stale air with fresh air and recovering some coolth, the incoming air is necessarily at a higher temperature than the outgoing air because of the need for finite temperature differences between the air steams and the beds. Secondly Figure 4 shows a situation where the beds are connected by a pipe so that refrigerant gas can pass freely between them. As bed 4.2 heats up refrigerant gas is desorbed taking in heat, while the cooling of bed 4.3 by the outgoing air causes gas to be adsorbed giving out heat. A pressure difference is established between the beds resulting in a refrigerant gas flow from bed 4.2 to bed 4.3. The result is a transfer of heat from bed 4.2 to bed 4.3 mediated by the gas flow, which is added to the heat transfer associated with the heat capacities of the beds described in the previous paragraph. This effect is a passive, recuperative, heat transfer.

Thirdly, Figure 5 shows a system in which there is pumping of CO₂, represented by arrow 5.7, from bed 5.3 to bed 5.2 using compressor 5.8. For clarity only the beds and the compressor are shown. The other components shown in Figure 1 are omitted. The temperature of bed 5.3 is lowered by CO₂ desorption while that of bed 5.2 is simultaneously raised by CO₂ adsorption. As a result the exhaust air 5.5 from room 5.1 is heated to a temperature above the external air 5.4 and the in-coming air 5.6 is cooled below the room exhaust air temperature. This arrangement augments the two recuperative heat transfer effects described in the previous paragraphs. In other words the system is operating both as a recuperator and a heat pump.

An important advantage of this invention is the provision of circuit designs that allow this combined heat recuperation/heat pumping cycle to be realised in practice.

The sequence of the distinct stages of the thermodynamic cycle on which the device operates are summarised in Figure 6. They are: CO₂ pressure equalisation; CO₂ pumping from one bed to the other using the compressor; and recuperative heat transfer.

In one embodiment of this invention the device operates in a cycle where the each stage is distinct and separate from each other, with one stage essentially complete before the next stage commences.

In a further embodiment of this invention the stages overlap so that one stage commences before the previous stage has been completed.

One method of operating the device shown in Figures 1 and 2 is explained in more detail by considering a cycle in Figure 7, where the adsorption beds correspond to the similarly numbered beds in Figures 1 and 2. A convenient starting point in the cycle is shown in Figures 7a and 7c with bed 1.2 at the temperature of the external air (35 ⁰C) and at low internal CO₂ pressure , while bed 1.3 is at the temperature of the exhaust air (20 ⁰C) and high internal pressure. The pressure and temperature values shown in Figures 7 to 10 are taken from a cycle calculation.

Stage 1 (Figure 7a): Valve 1.7 is opened to allow CO₂ to flow through directly from bed 1.2 to bed 1.3. As the pressure difference between the two beds decreases so the temperature of bed 1.2 decreases due to gas desorption. The temperature ultimately drops to the temperature required for entry of air into the room. The temperature of bed 1.3 rises because of gas adsorption to a value greater than the temperature of external air. During the pressure equalisation the air valve 2.3 is switched to its alternative position as indicated in Figure 7d and CO₂ circuit valves are switched to their alternative positions shown in Figure 7b. These actions switch the roles of the two beds. During the switch over period cool air flow is maintained by the coolth sink 2.6, which has been cooled during the previous part of the cycle. When the pressure across the beds has essentially equalised valve 1.7 is closed as shown in Figure 7d.

Stage 2 (Figure 8): External fresh air 2.7 is sucked by fan 2.5 via valve 2.3 over bed 1.2 which is thus progressively heated along its length as indicated by Figures 8a and 8b, while the air stream is cooled, entering the room at 13.5 ⁰C . During this part of the cycle the low temperature of the bed 1.2 is maintained by CO₂ desorption under the suction of the compressor 1.1. The cooling of the air stream can be attributed to a combination of this desorption and the coolth associated with the heat capacity of the bed derived from previously being cooled by outgoing stale air. This is a combination of heat pumping and regenerative heat transfer, in other words the two processes overlap. Stale cool air from the room 2.8 is forced by fan 2.4 over bed 1.3 exiting the bed at 41.5 ⁰C. The bed is progressively cooled as indicated in Figures 8a and 8b. During this part of the cycle the high temperature of bed 1.3 is maintained by CO₂ adsorption driven by the compressor. Pumping CO₂ from bed 1.2 to 1.3 with heat transfer to and from the air streams described will ultimately result in bed 1.2 being at a low pressure and at the temperature essentially that of the external air, with bed 1.3 being at high pressure and at essentially the temperature of the exhaust stale air exiting the room. This position is shown in Figure 9a.

Stage 3 (Figure 9): At this point in the cycle further heat transfer between the beds and the air streams is no longer possible because the maximum acceptable pressure difference has been established between the beds and temperature gradient along the beds has is no longer adequate to provide the required cold and hot air streams. The roles of the beds are reversed by equalising the pressure between the two beds by opening the compressor bypass valve 1.7. Bed 1.3 becomes cold as CO₂ desorbs from the porous solid and bed 1.2 becomes hot as CO₂ adsorbs. The pressure and temperature conditions of the two beds when the pressure equalisation is essentially complete are shown in Figure 9b. Valve 1.7 is closed; the air valve 2.3 set to the position shown in Figure 9d and valves 1.5 and 1.6 to the positions shown in Figure 9b. It will be recognised that Figures 9b and 9d are similar to Figures 7a and 7b but with the roles of beds 1.2 and 1.3 reversed.

Stage 4 (Figure 10): External fresh air 2.7 is sucked by fan 2.5 via valve 2.3 over bed

1.3 which is thus progressively heated along its length as indicated by Figures 10a and 10b, while the air stream is cooled, entering the room at 13.5 ⁰C . During this part of the cycle the low temperature of the bed 1.3 is maintained by CO₂ desorption under the suction of the compressor 1.1. The cooling of the air stream can be attributed to a combination of this desorption and the coolth associated with the heat capacity of the bed derived from previously being cooled by outgoing stale air. This is a combination of heat pumping and recuperative heat transfer, in other words the two processes overlap. Stale cool air from the room 2.8 is forced by fan 2.4 over bed 1.2 exiting the bed at 41.5 ⁰C. The bed is progressively cooled as indicated in Figures 9a and 9b. During this part of the cycle the high temperature of bed 1.2 is maintained by CO₂ adsorption driven by the compressor. Pumping CO₂ from bed 1.3 to 1.2 with heat transfer to and from the air streams described will ultimately result in bed 1.3 being at a low pressure and at the temperature essentially that of the external air, with bed 1.2 being at high pressure and at essentially the temperature of the exhaust stale air exiting the room. This position is shown in Figure 7a. The device has completed a full cycle and returned to the original starting point.

The energy efficiency of a system can be represented by its Coefficient of Performance

(COP). In this specification, the COP for an air conditioner or refrigerator is the ratio of the cooling duty provided by the evaporator to the power supplied to electric motor to achieve that duty. Similarly, the COP of a system operating in heat pumping mode to supply heat is heat provided by the condenser divided by the electric power input to the motor.

An air conditioning unit operating under typical conditions was modelled and compared with an equivalent Rankine Cycle system using refrigerant R22 modelled with NIST' s Cycle D program.

Input parameters

Cooling temperature 13°C

Heat Rejection temperature 41°C External air temperature 35⁰C

Room air exit temperature 2O⁰C

The COP calculated for the adsorption-based unit, which is the subject of this invention, was 6.6 compared to 4.0 for the conventional unit, i.e. the TESL unit was 65% more efficient.

An important feature of the device is that combining heat regeneration with heat pumping allows the device to operate over a required temperature range with a pressure induced temperature swing that is smaller than this range. This point can be appreciated from the system shown in Figures 7 to 10. When the pressure is equalised the temperature, i.e. the stage in the cycle where the beds are swung the hot bed temperature is only changed by +6.5 ⁰C and the temperature of the cold bed by -6.5 ⁰C, compared to the temperature range of 13.5 to 41.5 ⁰C range over which the heat is actually being pumped. This achieved by exploiting regenerative heat exchange within the system.

For a given enthalpy change produced by CO₂ desorption the temperature swing obtained is determined by the combined heat capacity of the carbon or other adsorbent and the metal. Good heat transfer from the bed to the air stream requires the incorporation of high thermal conductivity material into the carbon granules; but this reduces the temperature swing. The approach of this invention can provide a workable compromise between having both an adequate temperature swing and adequate heat transfer.

The hot and cold temperatures generated may depend upon the specific applications for which embodiments of this invention are employed, in particular whether refrigeration or air conditioning is required. In this context air conditioning is taken to include both room cooling and heating. Devices that can provide both heating and cooling depending upon the requirements of the user are sometimes called reversible air conditioners.

For single room air conditioning the heat transfer fluid will be generally be air.

In cooling mode the cold temperature with generally be in the range 5 to 15 ⁰C while hot temperature will generally be in the range 35 to 60 ⁰C. Cooling powers will typically range from 1 kW to 30 kW. In heating mode the output temperature to the room will be typically be 15 to 35⁰C, preferably 20 to 30 ⁰C and input temperatures from the outside air typically in the range from about 2 to 15 ⁰C. Heating powers will typically be in the range from about 2 to about 50 IcW. Alternative devices are adapted simply to provide heating and may generally be described as heat pumps. This is more a restricted meaning of the term than its scope in this specification.

For the air conditioning of large buildings with multiple rooms such as hotels and office blocks the transfer fluid can be water which will be piped through each room where air will be blown over the cold water pipe to provide the required cooling. This system is analogous to conventional chiller installations. The temperature of the water fed into the system is typically in the range from about 5 to about 10⁰C and the water returning to the device is typically in the range from about 10 to about 15 ⁰C. In heating mode the temperature of the water leaving the device is in the range from about 25 to about 40 ⁰C, while the return water is typically in the range from about 15 to about 30 ⁰C.

In one embodiment of the invention the device is used to provide refrigeration typically at temperatures down to -30 ⁰C. In a preferred embodiment of this invention equipment performance is optimised two or more stages. This approach is especially advantageous for temperatures below -20⁰C. Although carbon dioxide is a good refrigerant for Rankine cycle heat pumps with condenser temperatures lower than O⁰C, the critical temperature of 31 ⁰C and high critical pressure of 72 bar means that it is unattractive for heat pumps where the output temperatures above O⁰C. An especially preferred design comprises a device using a conventional heat Rankine cycle stage using carbon dioxide as the working fluid to pump heat from a temperature in the range generally -55 to -10⁰C and reject at a temperature generally in the range -20 to O⁰C to the low temperature side of a second stage device in accordance with the present invention. This second stage may reject heat at temperatures of 35 to 70 ⁰C while operating at maximum pressures in the range 2 to 30 bar, typical of CFC, HCFC or HFC-based refrigeration equipment.

For very low temperature refrigeration involving temperatures below -55⁰C carbon dioxide, is not practical because its triple is -56.7⁰C. For sub -55⁰C temperatures N₂ is preferred as a working fluid with a carbon adsorbent. The preferred heat transfer fluid is atmosphere within refrigerated enclosure, which in many cases will be air. This design will provide cooling in the range -130° to -40⁰C and will reject heat in the range -55 to-25 ⁰C to higher temperature stage.

Preferably the temperature difference between each heat exchanger and an external, single phase heat transfer liquid is near-constant throughout the heat transfer process. Preferably the difference does not change by more than 12°C, more preferably not more than 7°C and most preferably not more than 5⁰C.

Any fluid which is chemically stable in the gaseous state and which can be reversibly adsorbed onto and desorbed from a suitable porous solid is technically acceptable for use in devices of this invention. Preferred fluids include those that are already used in equipment based on other cycles, such as Rankine cycle and gas cycle heat pumps. The working fluid may be a fiuorocarbon or a mixture of fluorocarbons boiling between -14O⁰C and 4O⁰C, preferably between -90⁰C and 0⁰C, more preferably between -9O⁰C and -20⁰C. CFCs, HCFCs and HFCs are acceptable in those territories where their use is permitted but these are not preferred because of their adverse environmental effects. Preferred fluids are those that occur naturally. Hydrocarbons and hydrogen can be used in applications where flammability is not an issue. Ammonia may be used for applications where exposure to humans and animals can be prevented. For applications where a fluorinated fluid is preferred then HFCs, perfluoro-iodides and unsaturated fluorinated compounds containing 2 to 6 carbon atoms can be used. These preferably have global warming potentials relative to CO_2, preferably less than 150, more preferably less than 100, and most preferably less than 10. Particularly preferred compounds are fluorinated olefins. More preferred are fluoroolefins containing a trifluorovinyl group. Even more preferred are fiuoro-olefms containing at least one hydrogen atom. Especially preferred are fluoro-propenes and their blends.

Where fluorinated compounds are not acceptable, especially preferred working fluids comprise CO₂, noble gases and N₂ and mixtures thereof. These combine low environmental impact with low toxicity and non-flammability.

To further promote heat transfer within the adsorption bed to the carbon dioxide can be mixed with an additional gas having a substantially higher thermal conductivity than CO₂. Preferably the ratio of the thermal conductivity of the gas to that of CO₂ is greater than about 1.5, more preferably greater than about 5 and most preferably greater than about 8. Preferred additional gases include N₂, Ne, D₂, and DH. Dihydrogen (H₂) and helium (He) are especially preferred.

The term 'porous solid' may be used for materials with a wide range of properties. Many solids have a very limited porosity including the protective oxide layers found on metals.

In this specification the term, 'porous solid' is used to describe a material with a particular combination of properties. Preferably, the internal surface area of the porous solid is greater than about 10 m² g^"1, more preferably greater than about 100 m² g^"1, most preferably greater than about 1000 m² g^"1. The void space in the porous solid may be distributed between a combination of macro-, meso- and micro- pores. The porous solid may have at least 10% of its void volume in the form of micropores with diameters less than 2 nm. Preferably the porous solid has at least 5% of its void volume in the form of mesopores with diameters less than 50 nm.

The porous solid is selected to be capable of reducing the pressure of the working fluid vapour or gas in contact with it, i.e. it adsorbs the working fluid.

The adsorption process must be reversible, e.g. it must be possible to desorb the working fluid by reducing its pressure or by raising the temperature of the porous solid.

The porous solid must be capable of adsorbing the working fluid gas above its critical temperature.

A wide range of porous materials may be employed. Silica may be used, for example fumed silica, granular silica or aerogel silica, including granular, monolith and flexible blanket aerogels. Natural or artificial glasses, ceramics or molecular sieves may be used. Carbons which may be used include activated granules or monoliths, aerogels or membranes. Examples of porous carbons suitable for this invention are described in PCT/GB 01/04222. A range of organic materials including resorcinol-formaldehyde foams or aerogels, polyurethane, polystyrene or other polymers including foamed or aerogel polymers may be used. Polymers of intrinsic porosity in which the tailored pore sizes are created by the three-dimensional linking of appropriate pre-cursor molecules with constrained geometries are also suitable for this invention. A range of composites, including silica-carbon composites may be employed. Mixtures of porous materials may be employed.

Porous materials which can be used can be manufactured by a variety of known processes. These include, but are not limited to, polymeric foam blowing, sol-gel polymerisation, pyrolysis, thermolysis and direct chemical synthesis. Porous adsorbents which may be used include silica gel, molecular sieves, and aerogels, both organic and inorganic. Organic materials may be pyrolysed in the presence of controlled amounts of oxygen to generate micro-porous adsorbents. Coconut and coal, for example, may be pyrolysed to produce activated carbons. Polymer aerogels may be pyrolysed to produce carbon aerogels. Hydrocarbons may be pyrolysed to produce carbon membranes. Carbon black may be pyrolysed by plasma processes. Carbon based materials, such as activated carbons derived from biomass precursors, e.g. coconut shell or cellulose, are especially preferred since they are obtained from sustainable resources, require minimal energy input in their manufacture and effectively sequester atmospheric carbon dioxide as carbon within heat pump devices. At the end of the working life of a device such carbon adsorbents can be removed recovered and burnt recovering their energy content, originally captured when the biomass was formed, returning the carbon dioxide to the atmosphere. Since the gas originated from this source the combustion is CO₂-neutral. Preferably the carbon adsorbent would be buried in landfill, in the subduction zones at boundaries of tectonic plates, or recycled to new equipment thus ensuring that the carbon is permanently removed from the atmosphere.

Inorganic microporous solids which may be used can be generated by thermolysis. For example fumed silica can be produced from silicon tetrachloride using an oxy-hydrogen flame or by a plasma process. Organic-inorganic precursors may be processed by thermolysis to produce molecular sieves. Natural mineral hydrates may be thermalised, for example vermiculite and perlite.

In preferred embodiments, a particular gas or mixture is used with a selected porous solid such as: carbon (e.g. graphite, activated carbon, charcoal, aerogel), silica (fumed, aerogel, alkylated aerogel), alumina, alumino-silicates (molecular sieves), and organic polymers (e.g. polystyrene, polyurethane, polyacrylate, polymethacrylate, polyamines, polyamides, polyimides, and celluloses).

Although CFCs and the HCFCs continue to be manufactured and used in the developing world their phase-out under the Montreal Protocol is already taking place. In territories when continued use of these chlorinated fluids is permitted then they might be used in combination with activated carbon, silica, or an organic polymer. SO₂ and 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 has not occurred, their use in the present invention is possible, but is not preferred because the global warming potentials are much higher than some of the other gases listed above. SO₂ may not be preferred because of its toxicity.

Hydrocarbons can be coupled with carbon, alkylated silica or an organic polymer, especially a hydrocarbon polymer such as polystyrene. Hydrocarbons are more preferred than halogenated fluids and SO₂. However, hydrocarbons are restricted to applications where the appropriate precautions can be taken against the 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, including CO₂, SO₂ and NH₃. In some territories hydrocarbons are not preferred because any leak to the atmosphere and exposure to sunlight may generate photo-chemical smog.

Hydrogen is readily adsorbed on and desorbed from various metal alloys, notably those containing nickel. Hydrogen is preferred to hydrocarbons because it will interact more strongly with metals than hydrocarbons with the sorbents listed above. Hydrogen can also be used with micro-porous organic materials such as carbon nanotubes, designed to store the gas for power generating applications such as fuel cells. Like hydrocarbons hydrogen reacts with atmospheric hydroxy 1 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 global warming.

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

The most preferred fluid is carbon dioxide. Although carbon dioxide derived from fossil fuel is the single largest contributor to global warming the quantities required for this invention are small. By obtaining carbon dioxide from a natural source, such as biomass fermentation, any gas emitted from the device has a zero contribution to global warming. Carbon dioxide has low toxicity, is non-flammable and is readily adsorbed by a variety of porous solids including carbon, silica and organic polymers, especially those containing basic atoms such as O and particularly N. The ability of porous solids to adsorb CO₂ can be enhanced by impregnating the solids with compounds containing groups capable interacting with the fluid. Nitrogen and oxygen containing substances can be employed. Amines, amides alcohols, esters and ketones are preferred. More preferred are amines, amides, and urethanes with high boiling points, preferably above 100 ⁰C. Especially preferred are substances where the molecular mass per N atom is less than 200, preferably less than 100 and most preferably less than 60. A particularly preferred substance is poly-ethyleneimine.

Preferably the temperature changes generated when the fluid reversibly adsorbs and desorbs should be greater than 5 ⁰C and more preferably greater than 10⁰C. Seven important parameters contribute to the temperature change: (a) the integrated heat of adsorption (IHA) measured between the lowest and highest pressure between which the adsorbent operates in a heat pump device; (b) the heat capacity of the adsorbent (HCA); (c) the density of the adsorbent (DA), (d) the internal surface area of the adsorbent (SAA), (e) maximum operating pressure (MP), (f) the rates of adsorption/desorption and (g) the thermal conductivity of the adsorbent.

The integrated heat of adsorption (IHA) is a function the interaction of the fluid with the porous solid and is defined as the total heat generated when the fluid adsorbs onto the solid as its pressure is raised from a lower pressure to a higher pressure. The higher the IHA the stronger is the interaction of the fluid with solid. Preferably IHA should be at least 50 kJ/kg and more preferably greater than 100 kJ/kg. Most preferably the IHA should be comparable with the latent heats of condensation of existing refrigerants.

The higher the IHA the lower will be the pressure of the fluid above the adsorbent. A maximum working pressure below approximately 2 bar range at the heat rejection temperature is preferred. This keeps the pressure at any point in the device below the pressure at which pressure regulations apply. This allows the device to be manufactured more cheaply. Use of a high volume throughput compressor such as a centrifugal compressor is preferred. Such a compressor is especially suitable for large water chillers for example as employed for air conditioning public buildings. An IHA which reduces the fluid pressure over the adsorbent significantly below 2 bar is not preferred since it increases the size of the components, notably the compressor, without any economic advantage.

In devices working with maximum operating pressures above approximately 2 bar the IHA is preferably chosen so that the pressure of the adsorbent at the lowest working pressure of the device is not less than approximately 1 bar. This serves to prevent the ingress of atmospheric gases which are not significantly adsorbed by the porous solid. In applications where water ingress is not a concern or equipment is designed to prevent it and where the IHA increases disproportionately as the gas pressure falls then pressures below 1 bar can be advantageously used.

A special advantage of the present invention is that it allows even relatively small temperature changes below 5 ⁰C induced by fluid adsorption and desorption to generate the required substantial temperature differences between the ends of the adsorbent tubes, for example typically 10 ⁰C to 45⁰C in air conditioning applications. Despite this advantage larger temperature changes facilitate heat exchange between the bed and the external heat transfer fluid.

Preferably changes on adsorption and desorption are greater than 5 ⁰C and more preferably greater than 10 ⁰C.

The higher the IHA the larger the temperature change obtained. Lower adsorbent heat capacities (HCA) also provide higher temperature changes. Preferably HCA is less than 2.00 kJ/kgK and more preferably less than 1 kJ/kgK and most preferably less than 0.8 lcJ/kgK.

Especially preferred are porous carbon materials and metal adsorbents for hydrogen with HCAs less than 0.75 kJ/kgK.

Although a group of adsorbents may have similar IHA their adsorption capacities (CA) for a working fluid will depend upon the numbers of active sites available per unit mass. The number of active sites tends to be related to the internal surface area (ISA) of the porous solid accessible to the fluid molecules, thus the higher ISA the greater the capacity of the solid per unit mass to adsorb the fluid at a given pressure. ISAs of at least 1000 m³/g are most preferred.

Provided the IHAs, SHAs and ISAs for a series of adsorbents with a specified fluid are similar the temperature changes will be essentially independent of their densities (DA). The temperature changes also depend upon the heat capacities of the materials from which the adsorbent tube is manufactured. The quantities of these materials can be minimised by selecting materials with high densities provided this does not affect the other physical properties indicated.

Furthermore the quantity of the heat exchange fluid which removes heat from and adds heat to the adsorbent tube can also determine the temperature changes. Low inventories and high flow rates of the heat exchange fluid are preferred.

High maximum adsorption pressures maximise the capacity of the adsorbent for the working fluid. However as the pressure increases the incremental capacity of the adsorbent diminishes while the gauge of the pipe required to withstand the pressure increases, with a consequent increase in mass and hence thermal capacity of the heat exchanger. The latter reduces the magnitude of the temperature changes obtained on adsorption and desorption. An optimum maximum pressure which will depend upon the pressure /adsorption properties of the porous solid can be determined by routine experimentation.

An important influence on the design of the adsorbent heat exchangers is the thermal conductivity of the adsorbent. Porous solids, especially in particulate or granular form, have low thermal conductivities. Consequently heat transfer during adsorption and desorption limits the cycling of the beds. The device can be constructed to overcome this constraint. Preferably the device includes adsorbent tubes long in comparison to their width or diameter and progressively removing/adding heat starting at one end. Preferably the ratio of tube length to diameter is greater than about 4:1, more preferably greater than about 10:1 and most preferably greater than about 20:1.

To improve thermal conductivity, bodies of the adsorbents can advantageously be constructed incorporating heat conducting materials to provide adsorbent composites. These materials may include graphite, preferably as flakes, fibres or foams; metal mesh, powder, wire or fibres, preferably comprising a high thermal conductivity metal such as copper and aluminium; organic polymers with high thermal conductivities, such as polyaniline and poly- pyrrolidine. Such polymers generally have good electrical conductivities. In one preferred embodiment of this invention the thermally conducting polymers containing basic nitrogen atoms and comprising at least a proportion of the porous solid also contribute to the adsorption of carbon dioxide. Compressing the porous solids into monoliths also improves thermal conductivity.

Table 1 lists examples of various adsorbents and their thermal conductivities which demonstrates that the addition of a heat conducting additive substantially improves the thermal conductivity of an adsorbent.

Table 1

Adsorbent/Adsorbent Composite Thermal Conductivity

W/(m.K)

Consolidated zeolite 13X 0.58

Consolidated zeolite + expanded graphite 5 - 15

Fused silica 1.3

Silica gel + 20-30% expanded graphite 10 - 20

Monolithic carbon 0.27 - 0.60

Granular carbon 0.1

Monolithic carbon + aluminium laminate 20 Preferred adsorbents or adsorbent composites have thermal conductivities greater than about 0.5 W/(m.K), more preferably greater than about 5 W/(m.K) and most preferably greater than about 50 W/(m.K).

The configuration of the adsorbent heat exchanger is selected to determine the ease with which heat is transferred to and from the porous solid. An important requirement is to maximise heat exchange without unacceptably increasing the thermal capacity of the heat exchanger to such an extent that temperature changes fall below the preferred value of 5 ⁰C.

The adsorption heat exchanger bed may have a cylindrical or tubular configuration and is arranged with a long axis either parallel to or perpendicular to the flow of the external heat transfer fluid.

In one embodiment of this invention the bed includes one or more ducts and wherein the heat transfer fluid flows through the ducts.

In a second embodiment, the bed has an outside wall and heat transfer fluid passes over the outside wall of the bed.

In a third embodiment, the bed has both internal ducts and outside walls and the heat transfer fluid passes both through the internal ducts and over the outside walls.

To enhance the heat transfer area the adsorbent may be contained in a multiplicity of absorber tubes within a single duct, the duct having a circular, square, hexagonal or any other cross-section that is appropriate for a specific application.

In another preferred arrangement a flat plate adsorbent bed heat exchanger is used, the adsorbent material being held between two flat metal plates. Heat is removed from or supplied to the adsorbent by circulating heat transfer fluid on the external surfaces of the plates.

In preferred embodiments, the heat exchangers can be constructed from multiple parallel or series adsorbent beds.

In a preferred embodiment a bed may comprise a conduit containing an array of tubes wherein the heat transfer fluid passes in contact with the tubes heat being transferred between the working fluid, adsorbent and heat transfer fluid through the outer surface of the tubes. The tubes may be arranged in parallel spaced relation. The tubes are preferably connected in parallel to the working fluid circuit. The pressure at each point in a single tube is preferably substantially the same at each instant of the cycle. Small pressure differences are required to produce a flow of CO₂ but these differences are small in comparison to the actual pressure of CO₂ over the adsorbent.

Preferably the pressure drop along a single bed is low. This may be achieved by use of a granular adsorbent or more preferably a monolithic adsorbent with one or more channels therein, the absorbent contacting the inner wall of the tube to ensure good heat transfer to and from the wall. A thermally conducting medium, eg a thermally conducting paste may be used between the wall and adsorbent monolith. For ease of assembly a tube may contain several monoliths.

The multi tube array usually has a temperature gradient from the transfer medium inlet to outlet. The multi tube adsorbent bed has a permanent temperature profile in use.

The external surfaces of the adsorbent heat exchangers which are in contact with the heat transfer fluid in use are preferably fitted with fins to enhance heat transfer. The fins can be held mechanically in contact with the heat exchanger surface. Preferably the fins are soldered, glued, braised or welded to the surface of the heat exchanger to maximise heat transfer. The fin design adopted may depend upon the direction of the flow of the heat transfer fluid relative to the long axis, or axes, of the adsorbent bed. Where this flow is parallel to a long axis longitudinal fins are preferred either in the form of simple strips or as spirals. Where the flow is perpendicular to a long axis then the transverse fins are preferred for example in the form of discs, strips or plates. An especially preferred form of fin is a wire loop, which can be used with both longitudinal and transverse heat transfer fluid flows.

Heat transfer from the adsorbent to the walls of the internal wall of the adsorbent containing tube may include perforated metal plates or discs of metal mesh or fibre preferably arranged perpendicular to the tube axis. For optimum heat transfer these may be in tight contact with the walls of the containing tube. In a preferred embodiment the adsorption bed heat exchanger contains an internal folded wire heat exchanger. This serves to enhance heat conduction from the adsorbent bed to walls of the bed. Preferably the wire is located in contact with the walls of the bed at a large number of points. More preferably the wire is soldered, brazed or welded to the walls at the contact points. The wire is preferably a high conductivity metal such as copper or aluminium.

In another embodiment of this invention, one or more tubes are provided with the adsorbent tube, arranged so that the heat transfer liquid flows through the one or more tubes within the absorption tube. Heat transfer between the adsorbent and the tube containing the heat transfer fluid may be enhanced by fins which are perpendicular to the axis of the tube. Preferably the fins fit tightly or are attached to the outer surface of the heat transfer fluid tube but are not in contact with the inner surface of the adsorbent tube to prevent adventitious heat loss or gain by the adsorbent.

Preferably the adsorbent tube is also internally or externally thermally insulated with a layer of insulating material, for example by a polymer foam. A preferred form of insulation is a closed-cell foam disposed around the adsorption tube.

For minimum heat loss or gain a vacuum jacket may be provided in which molecular mean free path is less than the distance across the jacket. Aerogel containing insulation is especially preferred.

In a further embodiment, the inner wall of the adsorbent tube or other container may be provided with a low thermal conductivity liner or sleeve or coating which inhibits the flow of heat from the adsorbent to the wall of the adsorbent tube or container. This arrangement has the advantage that during the thermal cycling of the adsorbent the thermal capacity of the containing tube does not significantly reduce the temperature changes. The low conductivity liner can also serve as container for the adsorbent and any heat transfer liquid tubing allowing them to be assembled prior to insertion in the adsorbent tube. A further advantage of this design is that adsorbent tube, which is not required for heat transfer, can be fabricated from materials such as mild or stainless steels which are inherently stronger than copper or aluminium. Also, apart from cost and weight, there is no constraint on the tube wall thickness selected which can thus be chosen to resist high pressure. This is especially advantageous when a multiplicity of heat transfer liquid pipes is employed. The liner is preferably fabricated from a low thermal conductivity organic polymer, e.g. polyolefin. More preferred is an open cell organic foam, such as polyurethane foam. Especially preferred is an organic foam reinforced externally with a solid polymer tube to provide mechanical strength. The tube carrying the heat transfer liquid is preferably fabricated from a metal to facilitate heat transfer. Preferably the tube is made from a high thermal conductivity metal such as copper of aluminium. A further advantage is that the tube is exposed to an external pressure of gas rather than an internal pressure. In this mode copper and aluminium, which are less mechanically strong than steel, are acceptable.

In another embodiment of this invention the adsorption tube is fabricated in the form of a spiral and contained within the annulus of two concentric tubes that form the liquid duct. This configuration has the special advantage than it allows a long effective length of adsorption bed to be contained within much shorter actual length. In one embodiment the duct walls fit closely to the adsorption tube so that heat transfer liquid is constrained to flow through the spiral passageways formed between the outer and inner duct walls. This configuration minimises the quantity of heat transfer liquid in the heat exchanger and thus assists in keeping the temperature changes as large as possible. To reduce heat gain or loss to the immediate environment of the device the water duct can be advantageously insulated, for example with a layer of polystyrene, polyurethane foam or glass fibre.

In a further embodiment of this invention the heat transfer fluid is a gas, especially air. Such devices are especially advantageous for heat pumping in microgravity locations because they do not contain any liquid, such as the refrigerant and oil found in conventional units, which requires gravity to ensure it flows properly between components.

In an alternative embodiment of this invention the heat transfer fluid is a liquid. A suitable heat transfer liquid requires a combination of properties which are determined by its intended application. Liquids should have low viscosities so that pumping energy is minimised. Preferably liquids should have dynamic viscosities less than 0.025 Pa-s, preferably less than 0.01 Pa-s, and most preferably less than 0.001 Pa-s. Provided the liquid circulation system is suitably pressurized liquids with a range of boiling points can be used. Preferably for operating convenience the liquid should have a normal boiling point greater than highest temperature reached by the adsorbent. The liquid must not freeze below the lowest temperature generated within the device. Preferably the liquid has a flash point greater than 100 ⁰C, more preferably greater than 130 ⁰C and even more preferably greater than 200 ⁰C. Most preferably the liquid should be non-flammable. Preferred liquids include those already known to the industry as secondary refrigerants. These materials include water, brine, glycols, alcohols, hydrocarbon oils, silicone oils, and halogenated compounds including partially fluorinated ethers, perfluorinated ethers and chlorinated liquids. Where they are mutually compatible these liquids may also be used in mixtures.

For low refrigeration temperatures down to -50 ⁰C compositions with a wide liquid range are required, while retaining the desirable properties of flash points greater than 100 ⁰C and normal boiling points greater than the highest operating temperature. Preferred substances include esters and ethers containing 3 or more carbon atoms. These can be acyclic or cyclic. Preferred substances include, but are not limited to, glycol- or polyol- cyclic carbonates and cyclic ethers. Especially preferred are propylene carbonate, ethylene carbonate and dimethylisosorbide. Blends comprising esters, ethers, glycols with each other and with water can also be used. The liquids may optionally contain additives which enhance one or more desirable composition properties such lower freezing points, higher boiling point, lower viscosity or higher flash point. Such additives preferably constitute less than 50% of composition by mass.

To avoid adverse environmental impacts any compositions containing fluorine or chlorinated substances these compounds should preferably have very low vapour pressures or incorporate reactive groups such as double or triple bonds that facilitate their rapid destruction by reactive species in troposphere.

In another embodiment of this invention the heat is removed from the adsorption bed heat exchanger by a heat transfer fluid which undergoes a change of state from liquid to vapour.

In a further embodiment of this invention heat is supplied to the adsorption bed by a heat transfer fluid which undergoes condensation.

In a further embodiment of this invention heat is transferred to or from the adsorption bed heat exchanger by a combination of a gaseous heat transfer fluid and a liquid heat transfer fluid.

Good heat transfer between the adsorbent and the heat transfer fluid is clearly desirable for good performance. Metal adsorbents which can be used with hydrogen are especially advantageous because they have much higher thermal conductivities than non-metallic materials such as carbon, zeolites and silica gel.

The choice of a fluid/adsorbent combination depends upon a number of factors whose values may be selected to provide an optimum performance for a given application and the design of the adsorbent heat exchangers.

An important aspect of this invention is that the adsorption/desorption process is driven by a compressor. Any compressor may be used including a reciprocating piston, rotary, sliding vane, diaphragm type, screw, scroll, Periflow (trade mark) or centrifugal. However many compressor designs have sliding surfaces in the displacement volume are lubricated by a liquid lubricant and generate oil droplets which are discharged with the compressed gas. Such compressors whose will 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 area of the diaphragm and the compressor head augmented in 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. Refrigerant 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 the 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.

Also especially suitable for this invention are linear compressors such as those developed by Sunpower and LG Industries in which a piston containing a permanent magnet is driven fixed electromagnet. Suitable LG models are the DFL and FA linear compressor ranges.

Adsorption heat pumping requires the refrigerant and heat transfer fluid flows to be periodically swung between beds. The switching of the flows can be actuated by a timer, pressure sensors which generate a signal when the pressure difference between the beds has reached a selected value, or by temperature sensors located either in the beds or in contact with the heat transfer fluid streams exiting the adsorption bed heat exchangers. The switching can be initiated by a combination of these inputs.

Cooling is produced by desorption of working fluid from the adsorbent by reducing the pressure of the gas in contact with the adsorbent. Pressure induced adsorption of the working fluid produces heating. Preferred working fluid/adsorbent solid combinations are selected such that the heats of adsorption and desorption are substantially greater than the heat of compression.

More preferably the heats of adsorption and desorption are similar or equal to the latent heats of vaporisation of CFC, HCFC, HFC, hydrocarbon and ammonia working fluids presently used for any conventional Rankine Cycle based devices which they may be intended to replace. Filters can advantageously be positioned in the pipes carrying gas to and from the beds to prevent adsorbent powder fouling other components in the system.

Suitable valves for this invention include solenoid activated valves which are well- known to air conditioning and refrigeration engineers. Rotary valves, driven for example by electric motors, can also be used. Pneumatically actuated valves can be used. These can be operated by a compressed air supply. Preferred are pneumatic valves that are actuated by the pressure difference of the refrigerant gas generated by the compressor. In a typical system solenoid valves, responding to pressure or temperature sensors or to a timer, send pulses, of gas to the pneumatic actuators thus causing them to change the positions of valves.

An especially preferred embodiment of this invention the device operates as an air-to- air air conditioning unit in which any water condensate formed by the air passing through the cold adsorption bed heat exchanger being cooled below its dew point is retained within the heat exchanger. When the cycle swings so that this bed becomes hot the condensate is evaporated and leaves the heat exchanger as water vapour in the air stream. Current air-to-air a/c units where the hot and cold heat exchangers do not interchange their functions during the cycle condense water on the outer surface the cold evaporator. This has significant disadvantages. Since water has a very high latent heat of evaporation water condensation takes up a significant fraction of the cooling power of a conventional a/c unit since the condensate rejected to a suitable drain. In contrast in the present invention condensate water generated the cooling is re-evaporated when the bed is hot. This allows the bed to operate at a lower temperature during the evaporation period and improves energy efficiency. In a conventional system the need to drain the condensate places limitations on the citing of the evaporator and may also require the installation of a condensate collection tank and pump.

When a conventional a/c unit or refrigerator operating at conditions where the evaporator drops below freezing point then ice will form which reduces the area available for heat transfer and thus reduces energy efficiency. In some instances special provision is made to de-ice evaporators by applying electric trace heating to the external surface of the evaporator, or blowing hot refrigerant gas through its interior. These de-icing provisions require extra equipment to be installed and both consume extra energy. In contrast devices designed in accordance with the current invention are intrinsically de-icing.

In another embodiment of the application of this invention the adsorption heat pump comprises the upper stage of a two stage refrigeration unit where the lower temperature stage may be a conventional Rankine cycle unit using any condensable refrigerant. This can be a fluorine containing fluid such as a hydrofluorocarbon, unsaturated fluorocarbon or a hydrocarbon. Preferably the refrigerant is CO₂ which has low toxicity, low flammability and low environmental impact in this application. In one version of this device the adsorption beds in directly cool the condenser of the lower stage. In a preferred embodiment the adsorption beds cool a heat transfer liquid, for example water, brine or glycol, which in turn cools the condenser.

In a further embodiment of this invention a device comprises two adsorption beds which allow carbon dioxide to be liquified and subsequently evaporated in a separate heat exchanger at low temperature to produce refrigeration.

The refrigerant gas can enter and exit the adsorption bed at same end depending upon whether the adsorption or desorption is occurring. In a preferred embodiment of this invention the working fluid enters at one end and exits at the other. This ensures that the adsorbent is always pushed by the gas pressure in the same direction and this helps to minimise attrition of the adsorbent granules generating fine powder which could escape from the bed and damage other components.

In another embodiment of this invention the adsorption beds of the device shown in

Figure 1 are heated and cooled by water flowing in a closed circuit. One configuration is shown in Figure 11 comprising: two 4-way, powered valves 11.1 and 11.2; two heat exchangers 11.3 and 11.4; a pump 11.5; optionally two water storage buffer tanks 11.6 and 11.7; two fans to drive, or suck, air over 11.3 and 11.4; and two adsorption beds 1.2 and 1.3 corresponding to the beds in Figure 1. Each valve may be switched between two positions so that the water flows through 11.3 and 11.4 are always in the same direction, but water flows through the adsorption beds 1 and 2 reverse direction depending whether they are hot or cold, as determined by the CO₂ circuit. The water and CO₂ circuits are essentially operated in phase with each other. The water flow is driven by a pump 11.5. The pump can be positioned at a convenient location in the circuit where the water flow is uni-directional. Preferably the pump is placed just before the hot heat exchanger 11.4 or just after the cold heat exchanger 11.3 (as shown) to minimize the effect of any heat from the pump motor adversely affecting the energy efficiency and the capacity of the device. Most preferably the pump is placed just before the hot heat exchanger 11.4.

Either or both of the buffer tanks for cold and hot water 11.6 and 11.7 may be placed in the circuit immediately before the heat exchangers as shown. If the device is used to provide cool air to a room the buffer tank 11.6 smoothes any temperature fluctuations during the CO₂ pressure equalization. Tank 11.7 plays an analogous role if the device is being used to heat a room. In one embodiment of this invention the buffer tank 11.6 is sufficiently large in capacity to provide cooling during daytime for at least one hour or longer without the device operating. This is advantageous because it allows the device to cool the water in 11.6 overnight, rejecting heat to the environment when the external temperature is lower, thus providing better energy efficiency. This embodiment also allows air conditioning during peak demand without consuming power apart from the modest amount required for the pump and the fans. This embodiment therefore reduces power during peak air conditioning times, such as summer afternoons when the electricity supply grid can be overload causing "brown-outs".

The operation of the water circuit with respect to the CO₂ circuit is shown in Figures 7, 8, 9 and 10 where the alternative air and water systems options are shown alongside the common

CO₂ circuit. During the pressure equalization stage shown in Figure 7 valves 11.1 and 11.2 are operated simultaneously by actuators so that their positions change from those shown in Figure

7e to those shown in Figure 7f. These changes swap the water flows through 1.2 and 1.3 so that

1.2 is being heated by the return flow from the high temperature heat exchanger 11.9 and 1.3 is being cooled by the return water from the low temperature heat exchanger 11.8. Figures 8e and

8f indicate the positions of during the stage of the cycle where CO₂ is being pumped from 1.2 to

1.3. Figures 9e and 9f indicate the changes in the valve settings associated with the second pressure equalization in the cycle which again swaps the direction of the water flows through the adsorption beds. Figures 1Oe and 1Of indicate the positions of during the stage of the cycle where CO₂ is being pumped from 1.3 to 1.2.

Another embodiment is shown in Figure 12. This comprises a compressor 12.1, two adsorption bed heat exchangers, 12.2 and 12.3, a heat exchanger 12.4 to remove the heat of compression from the refrigerant, and six 2-way valves, 12.5 to 12.10. This system operates on a similar cycle to that described for the system shown in Figure 1 but employs a different valve system, and can be used with an air system such as that shown in Figure 2 or a water circuit such as that shown in Figure 11.

The adsorption beds 12.2 and 12.3 are charged with carbon adsorbent and the unit is charged with the maximum amount of CO₂ without exceeding the maximum pressure rating of the equipment. To enhance the heat transfer between the carbon in the beds and the air passing through the ducts the copper wire fins (not shown) are soldered to both outside and inside walls of the ducts. Figure 12 shows the valve settings during the various stages of the cycle. A black valve indicates that it is closed while a white valve indicates that it is open. Figure 12a shows the valve settings for equalisation the pressure between the two beds and is the same whichever bed is at a high pressure with the other at a low pressure and with CO₂ gas passing either way through valve 12.9. During pressure equalisation the compressor is in bypass mode with compressed gas being allowed to pass immediately back to suction. Figure 12b shows gas being sucked from bed 12.3, which is cold due to adsorption, and compressed onto bed 12.2, which is hot due to adsorption. Figure 12c is analogous to Figure 12b showing gas being sucked from bed 12.3 and compressed onto bed 12.3. For good energy efficiency the compressor should be operated in bypass mode for as short a time as possible when it is not contributing to the cycle. To achieve this valve 12.9 is preferably of greater bore than others in the circuit to ensure that it does not inhibit the gas flow. After the initial pressure drop, gas flow between the beds is determined by the rate of flow between the granules and by the rate of desorption/adsorption from/to the granules. To ensure a continued rapid transfer of gas the compressor can be switched back into the circuit significantly before the equilibrium concentrations of CO₂ on the two beds is achieved. In this way the compressor is usefully contributing to cycle. In terms of the cycle represented in Figure 6 the pressure equalisation stage overlaps with the gas pumping stage.

A further advantage of re-starting gas pumping before equilibrium has been reached is that the cooling can be essentially continuous. However some temperature variations may be observed in the heat transfer fluid, air or liquid. In a further embodiment of this invention these fluctuations may be smoothed fitting a "coolth" sink. An example is shown in Figure 2, situated where coolth sink 2.6 is situated in the cold air flow 2.10. Examples of suitable heat sinks are metal grids and metal tubes containing water. The heat sink is preferably with fitted with fins to enhance heat transfer. In another example the coolth sink in a unit with a water circuit may be a cold water tank such as 11.7 as shown in Figure 11.

In another embodiment of this invention a second gas, which is essentially not adsorbed, is mixed the CO₂ which assists in sweeping the CO₂ from one bed to the other. The gas is selected so that amount adsorbed onto the beds is much less than the CO₂. Gases with boiling points significantly lower than CO₂ are preferred, for example N₂, H₂ and the noble gases, such as He, Ne, and Ar. The device, shown in Figure 13, contains He which both helps to sweep the CO₂ from one bed to the other and because of its high thermal conductivity improves the heat transfer to and from the beds. The application of this embodiment to a room air conditioner shown in Figure 13 comprises: a compressor 13.1; adsorbent beds 13.2 and 13.3 containing activated carbon adsorbent and fitted with internal wire fins to enhance heat transfer; heat exchanger 13.4 to remove heat of compression; 4-way valves 13.5 and 13.6 to enable CO₂ to be alternately sucked from one bed and compressed onto the other; expansion valve or capillary 13.7; valve 13.8 to allow the pressure over the beds to be essentially equalised at the appropriate part of the cycle; expansion valve 6 which allows mainly He gas to pass either way between the beds; and an air or liquid transfer system to add or remove heat from the adsorbent beds, for example as shown in Figures 2 and 11.

The operation of the device is based on the method described in relation to Figure 6 and is similar to other adsorption devices described previously in this specification but with the additional feature that the inert gas He gas moves the CO₂ along the beds. The device operates according the following sequence of operations starting at the point in the thermodynamic cycle where bed 13.3 is essentially at its temperature of the external air (35 ⁰C) and bed 13.2 is essentially at the temperature of the stale air being vented from the air conditioned room (22 ⁰C). The major part of the CO₂ charge in the unit is adsorbed on 13.2 and the remainder on 13.2.

First Pressure equalisation: With the valves set in the positions shown in Figure 13a gas flows from 13.2 to 13.3, bypassing the compressor 13.1. CO₂ desorbing from 13.2 cools the bed to 10⁰C, while 13.3 is heated by CO₂ adsorption to 40 ⁰C.

First Gas Pumping: When temperature difference has been achieved between 13.2 and 13.3 has dropped to an appropriate level valve 13.8 is closed as shown in Figure 13b so the compressor starts pumping a mixture of He and CO₂ from 13.2 to 13.3. In this mode the CO₂ is preferentially stripped by adsorption from the mixture as it enters 13.3. He gas, depleted of CO₂ flows through bed 13.3 and passes via valve 13.6 to the expansion device 13.7 where its pressure drops to that of bed 13.2. The low pressure He sweeps through bed 13.3 where it mixes with desorbing CO₂ which it carries, via valve 13.6, to the compressor. By heating bed 13.2 with a fresh external air flow, 13.9 at 35 ⁰C the air flow is simultaneously cooled to essentially 10 ⁰C along the bed leaving as the cold air stream 13.10 which enters the room. Bed 13.3 is cooled by the stale air 13.11 leaving the room, the air flow simultaneously being heated by the bed and ejected to the external environment at 40 ⁰C (13.12). The use of He to sweep CO₂ through the beds produces sharper defined adsorption and desorption zones providing near-constant hot and cold air stream temperatures. The cyclical compression and expansion of the He gas also provides some additional heat pumping. Second Pressure Equalisation: Once the temperature of the air leaving bed 13.2 starts rising and the temperature of the air leaving bed 13.3 starts falling the pressure difference between the bed is equalised by opening valve 13.8 as shown in Figure 13c. With the valves set in the positions shown in Figure 13a gas flows from 13.3 to 13.2, bypassing the compressor 13.1. CO₂ desorbing from 13.3 cools the bed to 10 ⁰C, while 13.2 is heated by CO₂ adsorption to 40 ⁰C.

Second Gas Pumping: When temperature difference has been achieved between 13.3 and 13.2 has dropped to an appropriate level valve 13.8 is closed as shown in Figure 13d so the compressor starts pumping a mixture of He and CO₂ from 13.3 to 13.2. In this mode the CO₂ is preferentially stripped by adsorption from the mixture as it enters 13.2. He gas, depleted of CO₂ flows through bed 13.2 and passes via valve 13.6 to the expansion device 13.7 where its pressure drops to that of bed 13.3. The low pressure He sweeps through bed 13.2 where it mixes with desorbing CO₂ which it carries, via valve 13.6, to the compressor. By heating bed 13.3 with a fresh external air flow, 13.9 at 35 ⁰C the air flow is simultaneously cooled to approximately 10 ⁰C along the bed leaving as the cold air stream 13.10 which enters the room. Bed 13.2 is cooled by the stale air 13.9 leaving the room. The air flow is simultaneously heated by the bed and ejected to the external environment at 40 ⁰C (13.10). This process returns the unit to its original state and thus completes the cycle.

Although two bed designs have the advantage of simplicity in some instances more beds can be used with advantage. Without being limitative, Figure 14 shows an example of the gas circuit for a four-bed design using a CO₂ and activated carbon gas/adsorbent combination. In addition to the four adsorption beds (14.2 to 14.5) the unit also comprises a compressor (14.1), heat exchanger (14.10) to remove the heat of compression, four three-way valves (14.6 to 14.9) and connecting pipe 14.11. In Figure 14a the four three-way valves are connected such that the compressor 14.1 sucks gas from Bed 14.2, and compresses it onto Bed 14.4. Simultaneously, gas flows from Bed 14.3, which is initially at high pressure, to Bed 14.5, which is initially at low pressure, via connecting pipe 14.11.

Beds 14.2 to 14.5 are alternately heated and cooled by water flowing in the circuit shown in Figure 15. The circuit comprises a pump 14.16; two heat exchangers 14.12 and 14.13; and two valves for periodically reversing the water flow in phase with the switching of CO₂ flow. The valves 14.6 to 14.9 and 14.14 to 14.15 can be solenoid-, motor- or pneumatically- driven and are controlled by a micro-processor or electro-mechanical system with similar functionality. Other methods of reversing the water flow may be employed in place of 14.14 and 14.15; for example a pump capable of pumping in either direction. Alternatively they might be replaced by a single 4-way valve.

The operating cycle can be described starting from the initial condition summarised in the Table 2. This represents the unit condition just after the valves have switched at the end of the previous stage. The temperatures and the relative pressures are indicative and may vary according to the conditions and the design details of the equipment. The valves are set to the positions as shown in Figure 14a and 14b.

Table 2

Bed Pressure Temperature (⁰C)

14.2 Intermediate 15

Stage 1 14.3 High 30

14.4 Intermediate 35

14.5 Low 20

1. The compressor sucks CO₂ from bed 14.2 which cools as a result of gas desorption cooling the water to 5 ⁰C. 2. The cold water enters heat exchanger 14.12 where it cools room and its temperature rises to 15 ⁰C. The block arrow associated with 14.12 indicates the flow of heat from the room air into the heat exchanger. 3. The gas is compressed it onto the bed 14.4 where it is adsorbed raising the temperature of the water to 45 ⁰C. 4. The water enters heat exchanger 14. 13 where it rejects heat to the atmosphere and is cooled to 35 ⁰C.

5. Simultaneously with the compressor-driven transfer of gas from 14.2 to 14.4 CO₂ independently flows from bed 14.3 to bed 14.5 because of the pressure difference existing between them. 6. Gas desorbing from 14.3 pre-cools the water before it is cooled further by 14.2.

7. Gas adsorbing in 14.5 pre-heats the water before it heated further by 14.4.

8. The operation of the system proceeds until it achieves the condition summarised in Table 3. Table 3

Bed Pressure Temperature (⁰C)

4.2 Low 20

14.3 Intermediate 35

14.4 High 30

14.5 Intermediate 15

When this condition is reached as detected by sensors monitoring pressure and temperature, or after set time interval the valves are switched to the positions shown in Figures 14c and 14d.

Stage 2

1. The compressor sucks CO₂ from bed 14.5 which cools as a result of gas desorption and thus cooling the water to 5 ⁰C.

2. The cold water enters heat exchanger 14.12 where it cools the room and its temperature rises to 15 ⁰C.

3. The gas is compressed it onto the bed 14.3 where it is adsorbed raising the temperature of the water to 45 ⁰C. 4. The hot water enters heat exchanger 14.13 where it is cooled to 35 ⁰C.

5. Simultaneously with the compressor-driven transfer of gas from 14.5 to 14.3 CO₂ independently flows from bed 14.4 to bed 14.2 because of the pressure difference existing between them.

6. Gas desorbing from 14.4 pre-cools the water before it is cooled further by 14.5. 7. Gas adsorbing in 14.2 pre-heats the water before it heated further by 14.3.

8. The operation of the system proceeds until it achieves the condition summarised in Table 1.

At this point the valves are switched back to the positions shown in Figure 14a and 14b to complete the cycle.

This design has several advantages. The pressure equalisation stage of the cycle operates simultaneously with the suction/compression stage with the system arranged so that beds undergoing also equalisation contribute to the heat/cooling of the water stream. In the two bed system with the stages operating successively shown in Figure 6 the compressor is either idling on "open" circuit or is temporally switched off and then back on when compression is required. In either case energy is consumed without delivering a useful heat pumping effect. The arrangement indicated in Figure 14 allows the compressor to operate continuously providing heat pumping throughout the cycle.

The four bed system has described above uses water as an intermediary heat transfer fluid and in air conditioning industry terminology can be described as a chiller. The system can be readily designed with air as the heat transfer fluid. If the primary use of the system is heat, rather than cool, a room then it can be described as a heat pump.

By designing the system to reach lower temperatures below O⁰C, with, say water/ethylene glycol as the heat transfer fluid then it can be used for refrigeration.

In an alternative embodiment, a bypass valve may be provided in parallel with the compressor, e.g. between the valves 14.6 and 14.8, allowing for dual modes of operation in accordance with the first and second aspects of this invention. In this case the valves 14.9 and 14.7 may be arranged so that direct flow between the valves 14.9 and 14.7 is prevented. Furthermore, a controllable flow rate valve, e.g. a needle valve, may be located between the valves 14.9 and 14.7. In this way flow between the valves 14.9 and 14.7 may be turned on and off or may be regulated.

A further embodiment of this invention incorporates six adsorption beds as shown in Figure 15. The system comprises a compressor 15.1; a heat exchanger 15.2 to remove the heat of compression; six adsorption beds 15.3 to 15.8; and six three-way valves 15.9 to 15.14 controlling the gas flow direction between the beds. The system further comprises an air or water circuit for transferring heat to and from the adsorption beds. Figure 15 shows a water circuit comprising heat exchangers 15.16 to 15.18 and 15.20 to 15.22 in contact with the adsorption beds. Heat exchanger 15.15 removes heat from the room or space being air conditioned. Heat exchanger 15.19 rejects heat the environment. Pump 15.25 serves to circulate the water (or other heat transfer liquid such as a glycol). Two three-way valves 15.23 and 15.24 enable the direction of the water flow through the water circuit to be reversed as required.

The device operates in a similar fashion to the 4-bed system shown in Figure 14 with the addition of a third pair of beds. The advantage of this design is that enables smoother temperature profiles ("glides") along the sets of warm and cold beds thus improving energy efficiency. The invention is further described by means of examples, but not in any limitative sense.

Examples

Example 1

The apparatus shown in Figures 16a and 16b was used to pump heat using a compressor driven adsorption system constructed in accordance with this invention. The system comprised:

two adsorbent beds 1 and 2; a CO₂ cylinder 3; a valve 4 to allow CO₂ to be charged to the system;

a diaphragm compressor 5; two 3 -way valves 6 and 7, which allow the two beds to be switched between adsorption and desorption; air flow inlets 16 and 19; air flow outlets 18 and 20; 4-way air switching valve 17; gas buffer vessel 21 to prevent over-pressurization; pressure gauge PIl to read the maximum pressure system pressure; pressure gauge PI2 to read compressor suction pressure; connecting lines 9, 10, 11, 12, 13, 14, and 15.

Each adsorbent bed comprised a central copper heat exchanger tube (internal diameter (OD) 16.7 mm, external diameter 19, and length 250 mm) fixed concentrically with a brass outer tube ( OD 57mm; ID 54 mm; length 220 mm) by two brass end caps soldered to the outer tube. Each bed was also fitted with pipes at each end to allow the CO₂ to enter and leave. The heat exchanger area of the inner tube was extended by a spiral of 31 copper wire loops soldered to the outer-surface of the tube. The wire diameter was the 0.6 mm, and the diameter of the spiral winding was 44 mm. A further looped spiral copper wire was soldered inside the tube. This configuration provided an effective method for transferring heat to and from the air streams and the carbon adsorbent

The adsorbent was a standard, commercial 30-70 mesh activated carbon designed for CO₂ adsorption and supplied by Chemviron Carbon. The two adsorbent beds 1, 2 were charged with carbon adsorbent, and purged with carbon dioxide to remove air.

The valves 6 and 7 were Swagelok 0.25 inch 3-way stainless valves. Compressor 5 was an oil- free KNF Neuberger N 035.2 ANE twin headed diaphragm compressor with the heads connected in series and capable of pumping gas at 30 standard litre/minute with a maximum operating pressure of 1 barg. Connecting lines mainly nylon tubing of diameter 0.25 inch. Short lengths of 0.25 inch stainless steel tubing were either side of the 20 micron filter 24 incorporated to prevent any dust from the adsorption beds entering the compressor. Figure 16a shows Bed 1 in adsorptive mode (hot) while Figure 16b shows Bed 2 in adsorptive mode (hot). The different positions of the air valve 17 position allows warm and cool air to always leave via exit 18 and exit 20 respectively, whichever bed was hot or cold thus providing the continuous supply of warmed air and cooled air.

In a typical experiment the system was pressurized to, 2 barg, from carbon dioxide cylinder 3. After filling, the system was isolated at valve 4, and the cylinder was disconnected.

The compressor 5 was switched on, with the three-way valves 6, 7 in the positions shown in Figure 16a. Carbon dioxide was drawn from bed 2 through line 8, valve 6, lines 9 and 10 into the compressor 5, and then through lines 11, 12, valve 7 and line 13 into bed 1. The carbon dioxide could not return into the system through line 14 which was closed by valve 6. Bed 2 was closed by valve 7 on line 15. Carbon dioxide was thus removed from bed 2, and pressurized onto bed 1. The heat generated by the exothermic adsorption of the gas in bed 1 was removed by the air flow 16, with the airflow valve 17 in the position shown, to exit 18. Similarly, the endothermic desorption of gas lowered the temperature of bed 2 cooling air flow 19 which left the rig by exit 20. The temperatures of the warm air stream at 18 and 20 were measured periodically with thermocouples.

After a specific time interval or upon reaching a specific minimum temperature difference between the two air streams the three-way valves 6, 7 and airflow valve 17 were operated near-simultaneously, thus switching the system to the condition shown in Figure 16b. Bed 1 was initially at higher pressure than bed 2 so that gas spontaneously flowed rapidly from 1 to 2 through the compressor. When the pressures between the two beds had equalized the compressor started to suck gas from bed 1 and compressed it onto bed 2. During this process the temperature of bed 1 dropped as the result of CO₂ desorption, while the temperature of bed 2 increased because of gas adsorption.

Although the compressor continued to drive gas in the same sense before and after the valve positions had been changed, the operation of the adsorbent tubes was reversed. Carbon dioxide was drawn from tube 1 through line 14, valve 6, lines 9 and 10, through the compressor

5, and then through lines 11, 12, valve 7 and line 15 to tube 2. In this configuration, line 8 was closed at valve 6, and line 13 was closed at valve 7. Also the air flow was changed by the new position of the air valve 17, so that the exothermic heat of adsorption, now generated in tube 2, was removed by air flow 19, but still exited at 18, providing a continuous heated air flow at this point. Similarly, the heat required for the endothermic gas desorption in tube 1 was supplied by air flow 16, which now emerged from exit 20, providing a continuous cooled air flow at this point.

When the heat had decreased to a low level, the two three-way valves 6 and 7, as well as the airflow valve 17, were returned to positions shown in Figure 16a, and the process was repeated.

Example 2

The test rig described in Example 1 was operated with the supply air temperature at

22.4 ⁰C and with a switching time of 4 minutes. The pressure of each bed was swung between -0.1 bara and 4 bara. The cold and hot air streams were maintained at average temperatures of 15.1 ⁰C and 30.0 ⁰C respectively. Figure 17 provides the exit temperature/time plots for the two air streams.

Example 3

The test rig described in Example 1 was operated with the supply air temperature at

20.05 ⁰C and with a switching time of 10 minutes. The pressure of each bed was swung between -0.1 barg and 3-3.5 barg. The cold and hot air streams were maintained at average temperatures of 13.2 ⁰C and 27.5 ⁰C respectively. Figure 18 provides the exit temperature/time plots for the two air streams.

Example 4

The test rig described in Example 1 was operated with the supply air temperature at

21.6 ⁰C and switching the valves when the cold air stream reached 15 ⁰C. The maximum pressure of each bed was 4 barg. The cold and hot air streams were maintained at average temperatures of 14.0 ⁰C and 28.8 ⁰C respectively. Figure 19 provides the exit temperature/time plots for the two air streams.

A preferred adsorption bed design, is shown in Figures 19(a) to (c). Figure 19(a) shows a face view of the bed; Figure 19(b) shows a side view; Figure 19(c) shows a detail of the adsorbent in the bed. Each absorption bed comprises two or more rows of tubes (19.1), with each row containing at least three tubes. External fins (19.2) enhance the heat transfer to and from the external heat transfer fluid (eg water or air) flow (19.3). A header pipe (19.4) connects all the pipes in a row allowing carbon dioxide to be fed simultaneously to each tube in a row. A footer pipe (19.5) allows carbon dioxide to be removed simultaneously from each tube in a row. Compression line (19.6) allows gas to be fed simultaneously to each header pipe. Suction line (19.7) allows gas to be exhausted simultaneously from each footer pipe. A valve (19.8) controls the entry of gas into the bed. A valve (19.9) controls the removal of gas from the bed. A micro- porous carbon adsorbent (19.13) is in thermal contact with the inner walls of the tubes. For clarity only a representative number of fins are shown.

In Figure 19(b) the valve (19.8) is open and (19.9) is closed so that CO₂, represented by solid block arrow, is being compressed on to the bed. The water flow represented by the solid block arrow (19.3) is removing heat from the bed generated by CO₂ adsorption on the microporous carbon. If valve (19.8) is closed and valve (19.9) is open then CO₂ will be sucked from the bed. The dotted block arrow (19.11) represents the CO₂ direction and the dotted block arrow (19.12) represents the water flow which has been reversed.

A preferred design for the adsorbent is shown in Figure 19(c). This comprises a monolith (19.13) with one of more channels (19.14) parallel to the long axis of a tube to facilitate CO₂ flow (19.15) along the tube. A thermally conductive means for ensuring good thermal contact between the adsorbent and the inner walls of the tubes, for example a thermally conducting paste, (13.16) which contains graphite. The tube containing the adsorbent (19.17) is selected to withstand a maximum working pressure of 20 bar gauge and is preferably a good conductor of heat. Aluminium and copper are especially preferred as the materials for (19.17). The CO₂ flowing down these channels enters the adsorbent radially through pores. For ease of assembly a tube may contain several monoliths. The especial advantage of this embodiment is that it allows the bed to operate with substantially the same pressure over the adsorbent at each point in the cycle, since the combination of the compression, suction, header footer and channels will ensure the pressure drop across the bed is low preferably less than about 0.25 bar.

In a preferred embodiment of the present invention the multi-tube adsorption bed shown in Figure 19 is incorporated in a heat pump having the 4-bed/2 array design shown in Figure 20. This provides an essentially continuous heat pumping effect. The heat pump device shown in Figure 20 is a circulating water chiller which is cooling a room. The device comprises four adsorption beds (20.1, 20.2, 20.3 and 20.4). An oil free compressor and a (20.5) heat exchanger (20.6) which removes heat from the room (20.7); Heat exchanger (20.8) rejects heat to the external atmosphere; A water pump (20.9) circulates water through adsorption beds, (20.6) and (20.7) 3-way valves (20.10) and (20.11) allow the water flow through the adsorption beds to be periodically reversed; Valves (20.12, 20.13, 20.14, 20.15) control the direction of flow of CO₂ in the device. Adjustable needle valve (20.16) controls the CO₂ flow rate between pairs of beds. Associated pipe work in the water and gas circuits are also shown. The block arrow (20.17) represents the removal of heat from (20.7) by (20.6); Block arrow (20.18) represents the rejection of heat to the external atmosphere by (20.8). The pipes shown as solid lines represent the CO₂ circuit; the pipes shown as dotted lines represent the water circuit.

A special feature of the device shown in Figure 20 during operation is that each multi- tube absorption bed will have a permanent temperature profile imposed on it which will always be in the same direction. The operation of the device is shown in Figure 21 in which the beds are represented by simple rectangles on which hare superimposed the temperature profiles during various stages in the cycle. The numbering of the components in Figure 21 correspond to their equivalents in Figure 20. In Figure 21 (a) and (b) the full stepped line within the adsorption bed rectangles represent the temperature profile at the beginning and end of each stage. The dotted stepped line represents the extreme change in the temperature profile during the stage. The doubled headed arrows indicate that the temperature moves between the two sets of stepped lines during each stage of the cycle.

The operation of the device is described the following sequence:

1. In Figure 21 (a) the valves controlling the water and CO₂ flows are set in the positions shown. The direction of the water flow is indicated by the dotted arrows while the CO₂ flow direction is shown by the simple line arrows.

2. Compressor (21.5) pumps CO₂ from bed (21.1) to bed (21.4). Initially the two beds are at the same pressure Pl .

3. During pumping process gas desorbs from bed (21.1) so its temperature profile drops. Water flowing through this bed is progressively cooled and enters heat exchanger (21.7) where it cools the room (21.7) and the water itself becomes heated. When heat being transferred to the bed from the water exceeds the heat being removed by the gas desorption the temperature profile starts to rise and returns to its original position Tl. This represents the end of this stage of the cycle. The gas pressure over the bed is now at P2, which is necessarily lower than the starting pressure Pl. 4. During gas pumping the temperature gradient across bed (21.4) rises so water flowing through this bed is progressively heated and enters heat exchanger (21.8) where it rejects heat to the external ambient air. When heat transferred to the bed from the water exceeds the heat generated by gas desorption the temperature profile having reached T4 starts to drop and returns to its original valve T3. This represents the end of this stage of the cycle. The gas pressure over the bed is then at P3, which is necessarily higher than Pl.

5. Beds (21.2) and (21.3) are initially at pressures P3 and P2 respectively causing CO₂ to flow through the control valve (21.16) desorbing from (21.2) and adsorbing on (21.3) valve (21.16) is set so that the time taken for the gas pressures in the two beds to equalise is essentially the same time taken to pump gas from (21.1) to (21.4).

6. Bed (21.2) cools as gas desorbs so its temperature profile drops from T3 and cools water returning from heat exchanger (21.8). When the rate of heat transferred to the bed from the water exceeds the rate of heat generated by gas desorption the temperature profile having reached T5 starts to drop and returns to its original position T3. This represents the end of this stage of the cycle. The gas pressure over the bed is Pl.

7. Bed (21.3) heats as gas is adsorbed which thus heats the water entering it from heat exchanger (21.6). The temperature profile increases from Tl to

T6 until the rate of heat transferred to the water is greater than the rate of heat generation from gas adsorption. The temperature profile having reached T6 then starts to drop and returns to its original position Tl. This represents the end of this stage of the cycle. The pressure over the bed is Pl.

The overall effect of operation of the device is the transfer of heat via the water circuit from the heat exchanger (21.6) to heat exchanger (21.8). At the end of the stage bed (21.3) is in the same state of temperature and pressure as (21.1) was at the beginning and bed (21.4) is in the same state of temperature and pressure as (21.2).

The positions of the valves controlling the water and CO₂ flows are now set to the positions shown in Figure 21(b). The second stage of the cycle is essentially the same as the first described above but with beds (21.1) and (21.4) interchanged and beds (21.2) and (21.3) interchanged; the direction of the water flow is reversed. When the second stage of the cycle has been completed the device is in the same stage as at beginning of the first stage.

Operating conditions will vary depending upon the desired temperature in the room and the external temperature. Typically water will enter (20.6) at 3⁰C to 1O⁰C and leave 7⁰C to 12⁰C. The external temperature will be typically 25⁰C to 35⁰C.

In a further preferred embodiment of this invention the heat transfer fluid is air. Figure

22 shows a device based on multi-tube absorption beds. The operation of the device is anlogous to that shown in Figures 20 and 21.

A continuous adsorption heat pump device a 4-bed/2 array system was constructed as shown in Figure 22. The system comprised an oil-free KNF diaphragm compressor model N 145 1.2AN18, 22.5 ; eight 2-way gas valves EP13PV40/30E from Beta Ltd (UK), 22.9 to 22.19; water pump KAG M42X30/1 from Fluid-o-Tech Ltd. UK 22.17; water flow meter "grey transducer" from Titan Enterprises Ltd (UK) 22.7; four water valves ACL Type 32E 22.18 to 22.21; flat plate heat exchangers from Alfa-Laval CB14-10H 22.6 and 22.8; control circuitry to periodically switch the valves; and a data logger Grant Squirrel 2040 series to record temperatures, pressures and flow rates. Each adsorption bed comprised an outer copper tube 105 mm long, outer diameter 56.09 mm, wall thickness 1.626 mm. An inner copper heat exchange tube carried a water flow along the axis of the outer tube, outer diameter 12 mm, wall thickness 2 mm, to which was soldered a continuous spiral of copper wire loops which reached to ~ 2 mm of the containing tube. A rod wrapped with copper wire was located within the inner tube to promote water turbulence. Brass end caps were used to hold the inner and outer tubes together. Inlet tubes at either end of the outer tube curved to allow the entry and exit of CO₂ 980 g of 30/40 mesh granulated carbon absorbent obtained from Chemviron Carbon Ltd (UK) formulated for CO₂ adsorption occupied the volume between the inner and outer tubes and between the copper wire loops. A filling port in brass end cap to allowed the introduction of the carbon. Wire gauze filters at the CO₂ were used at the entry and exit points to prevent the carbon granules entering the connecting pipe work. In Figure 22 the dotted connecting lines represent the water circuit while the solid connecting lines represent the CO₂ circuit.

The device was operated with the 2-way valves set as shown in Figure 22(a) for the first part of the cycle. The white circles represent open valves and the black circles closed valves. For the second half of the cycle valves were set as shown in Figure 22(b). When the CO₂ valves were operated the water flow was also reversed by operating valves (22.18 to 22.21) as shown in Figure 22(a) and (b). The time between switching operations was set to 6 minutes. The water flow rate through the beds and the ducts (22.6a and 22.8a) was set to 1 L/m and 0.97 L/m through ducts (22.8b and 22.6b).

During the first half of the cycle CO₂ was pumped from bed (22.1) to bed (22.4) by the compressor. The initial pressures of the two beds were 1.9 bara. The maximum pressure reached in (22.4) when it reached its maximum pressure after pumping for 6 minutes was 4.05 bara. The minimum pressure reached in (22.1) after suction for 6 minutes was 0.56 bara.

At the beginning of this 6 minute period the gas pressures in (22.2) and (22.3) were 4.05 bara and 0.56 bara respectively. After 6 minutes these had essentially become equal by gas flow through the needle valve (22.22) and reached a common pressure of 1.91 bara.

The maximum temperature of the water leaving the higher temperature heat exchanger

22.8a was 25 ⁰C. The minimum temperature of the water leaving the lower temperature heat exchanger 22.6a was 14.5 ⁰C.

To initiate the second half of the cycle the open valves were closed, the closed valves opened and the water flow reversed so that the compressor now pumped gas from (22.3) to (22.2) while the gas flowed from (22.4) to (22.1) via the needle valve (22.3).

Water from the mains flowing through the channels (22.6b) and (22.8b) of heat exchangers (22.6) and (22.8) as shown provided a heat source and heat sink respectively for the device.

The average cooling Coefficient of Performance (COP) was calculated by dividing the heat removed from the water flowing through heat exchanger (22.6a) during one complete cycle by the difference between the heat transferred to the water in heat exchanger (22.8a) minus the heat removed from the water flowing through heat exchanger (22.6a). The value obtained was

3.3.

The average heating COP was calculated by dividing the heat transferred to the water flowing through heat exchanger (22.8a) during one complete cycle by the difference between the heat transferred to the water in heat exchanger (22.8a) minus the heat removed from the water flowing through heat exchanger (22.6a). The value was 4.3.

Claims

1. A heat pump including first and second adsorption beds, each bed including a porous solid, the first bed having a first heat exchanger and the second bed having a second heat exchanger;

a circuit for working fluid communicating between the beds;

a compressor adapted to cause working fluid to flow within the circuit between the beds;

the working fluid comprising a reactive gas capable of adsorption and desorption by the porous solid;

the first heat exchanger including a first conduit for a heat transfer fluid;

the second heat exchanger including a second conduit for a heat transfer fluid;

valve means for switching flows of heat transfer fluid through the first and second heat exchangers; and

a number of adsorption beds, the number being 2N wherein N is a positive integer and wherein each bed has an inlet and an outlet;

the heat exchangers being arranged in a plurality of arrays, each array comprising an input for heat from a flow of the heat transfer fluid or an output for heat to the heat transfer fluid;

wherein the heat exchangers of each array provide a temperature glide to the respective flow of heat transfer fluid.

2. A heat pump including first and second adsorption beds, each bed including a porous solid, the first bed having a first heat exchanger and the second bed having a second heat exchanger;

a circuit for working fluid communicating between the beds;

valve means for switching flows of heat transfer fluid through the first and second heat exchangers; a compressor connected between the valve means adapted to cause working fluid to flow within the circuit between the beds;

the working fluid comprising a reactive gas capable of adsorption and desorption by the porous solid;

the first heat exchanger including a first conduit for a heat transfer fluid;

the second heat exchanger including a second conduit for a heat transfer fluid; and

a bypass valve connected between the valve means in parallel to the compressor and adapted when opened to allow flow between the beds bypassing the compressor.

3. A heat pump as claimed in claim 1 or 2, wherein the reactive gas is selected from the group consisting of:

hydrocarbons, ammonia, hydrogen, HFCs, fluoro-iodides, unsaturated fluorinated compounds containing 2 to 6 carbon atoms, fluoro-olefins containing trifluorovinyl groups, CO₂, nitrogen and mixtures thereof.

4. A heat pump as claimed in claim 3 wherein the reactive gas is carbon dioxide.

5. A heat pump as claimed in any preceding claim, wherein the porous solid is selected from the group consisting of:

silica, glasses, ceramics, molecular sieves, activated carbons, microporous organic polymers, organometallic polymers, polymers of define intrinsic porosity and mixtures thereof.

6. A heat pump as claimed in claim 5, wherein the porous solid is activated carbon.

7. A heat pump as claimed in any preceding claim, wherein the working fluid comprises a reactive gas and an additional gas.

8. A heat pump as claimed in claim 7 wherein the working fluid is a mixture of CO₂ and an additional gas having a higher thermal conductivity than CO₂,

9. A heat pump as claimed in claim 8 wherein the ratio of thermal conductivity of the additional gas to the thermal conductivity of CO₂ is 1.5 or greater.

10. A heat pump as claimed in claim 9 wherein the ratio is about 5 or greater.

11. A heat pump as claimed in claim 10 wherein the ratio is about 8 or greater.

12. A heat pump as claimed in any of claims 8 to 11, wherein the additional carrier gas is selected from He,Ne,H_2> D₂DH and mixtures thereof.

13. A heat pump as claimed in claim 12, wherein the working fluid includes helium and the reactive gas is carbon dioxide.

14. A heat pump as claimed in claim 13, wherein the working fluid comprises helium and the reactive gas is carbon dioxide.

15. A heat pump as claimed in any preceding claim wherein the device is an air conditioning unit and the heat transfer fluid is air.

16. A heat pump as claimed in any of claims 1 to 8, wherein the heat transfer fluid comprises water or other secondary refrigerant.

17. A heat pump as claimed in any preceding claim wherein the porous solid has an internal surface area greater than about 10m²g^"' .

18. A heat pump as claimed in claim 17 wherein the porous solid has an internal surface area greater than about 100 m²g^"'.

19. A heat pump as claimed in any preceding claim wherein the porous solid has an internal surface area greater than about 1000 m²g^"1.

20. A heat pump as claimed in any preceding claim wherein the porous solid has at least 10% of its void volume in the form of micropores with diameters less than about 2 nm.

21. A heat pump as claimed in any preceding claim wherein the porous solid has at least 5% of its void volume in the form of mesopores with diameters less than about 50 nm.

22. A heat pump as claimed in any preceding claim wherein the adsorbent composite comprises an adsorbent composite comprising an adsorbent and a heat conducting material.

23. A heat pump as claimed in claim 22 wherein the heat conducting material is selected from the group consisting of: graphite flakes, fibres or foams, metal mesh, powder, wire or Fibres; and mixtures thereof.

24. A heat pump as claimed in claim 22 or 23 wherein the heat conducting material comprises polyaniline or polypyrrolidine.

25. A heat pump as claimed in any preceding claim wherein the adsorbent or adsorbent composite has a thermal conductivity greater than about 0.5 w/(m.K).

26. A heat pump as claimed in claim 25 wherein the adsorbent or adsorbent composite has a thermal conductivity greater than about 5 w/(m.K).

27. A heat pump as claimed in claim 26 wherein the adsorbent or adsorbent composite has a thermal conductivity greater than about 50 w/(m.K).

28. A heat pump as claimed in any preceding claim wherein the adsorbent or adsorbent composite is contained in a container having a low thermal conductivity liner.

29. A heat pump as claimed in claim 28, wherein the container comprises a tube including a low thermal conductivity liner, sleeve or coating.

30. A heat pump as claimed in claim 28 or 29, wherein the liner, sleeve or coating comprises a container for the adsorbent.

31. A heat pump as is claimed in any of claims 1 to 27 wherein the first or second bed comprises a conduit containing an array of tubes containing porous solid through which working fluid can pass, wherein heat transfer fluid passes in contact with the array, heat being transferred between the working fluid, adsorbent and heat transfer fluid through outer surfaces of the tubes.

32. A heat pump as claimed in claim 31 wherein a temperature gradient is maintained in use across the array in the direction of flow of the heat transfer fluid.

33. A heat pump as claimed in claim 31 and 32 wherein the array comprises parallel tubes perpendicular to the direction flow of the heat transfer fluid.

34. A heat pump as claimed in any of claims 31 to 33 wherein the direction of flow of the heat transfer fluid in a conduit is unchanged during each cycle of heating or cooling of the bed.

35. A heat pump as claimed in any of claims 31 to 34 wherein the working fluid is introduced or removed simultaneously from each tube of a bed.

36. A heat pump as claimed in any of claims 31 to 35 wherein each tube comprises an outer wall and a body of porous material within the tube in thermal contact with the wall and one or more channels extending longitudinally of the tube.

37. A heat pump as claimed in Claim 36 wherein a channel extends axially of the tube.

38. A heat pump as claimed in any of claims 31 to 37 wherein the working fluid in the channel enters the porous adsorbent in a radial direction in use.

39. A heat pump as claimed in any of claims 31 to 38 wherein the pressure of working fluid over the absorbent is substantially constant across the bed.

40. A heat pump as claimed in any of claims 31 to 39 wherein the heat pump comprises 4 beds arranged in 2 arrays.

41. A heat pump as claimed in any of claims 31 to 40 wherein the reactive gas comprises CO₂ and the heat transfer medium is water or air.

42. A heat pump as claimed in any preceding claim wherein the heat transfer fluid is a single phase and wherein the temperature difference between each heat exchanger and the external single phase heat transfer liquid is not more than 20⁰C.

43. A heat pump as claimed in claim 42 wherein the temperature difference is not more than 12⁰C.

44. A heat pump as claimed in any of claims 1 to 30, wherein heat transfer fluid undergoes evaporation during heat removal from the adsorption bed.

45. A heat pump as claimed in claim 44, wherein the heat transfer fluid undergoes condensation during supply of heat to the adsorption bed.

46. A heat pump as claimed in any of claims 1 to 30, wherein heat is transferred to or from the adsorption bed by a combination of a gaseous heat transfer fluid and a liquid heat transfer fluid.

47. A heat pump as claimed in any of claims 1 to 30, wherein the heat transfer fluid is humid air and wherein water condenses from the humid air during the desorption of the reactive gas.

48. A heat pump as claimed in any of claims 1 to 30, wherein the heat transfer fluid is humid air and wherein latent heat or condensation of condensed water is recovered by evaporation during the heating phase.

49. A heat pump as claimed in any of claims 1 to 30, wherein the heat transfer fluid is water or humid air and wherein the water freezes during desorption of the working fluid.

50. A heat pump as claimed in any preceding claim wherein the compressor has a variable power drive.

51. A heat pump as claimed in claim 50 wherein the compressor is a diaphragm compressor.

52. A heat pump as claimed in claim 1 or any claim dependent on claim 1, wherein the temperature increases or decreases from one end of an array to the other.

53. A heat pump as claimed in claim 1 or any claim dependent on claim 1, wherein the temperature glide maintains a temperature difference between an exchanger and a heat transfer medium in contact with the exchanger and wherein the temperature difference below a predetermined value.

54. A heat pump as claimed in claim 1 or any claim dependent on claim 1, including a circuit adapted so that working fluid is pumped in use from the low temperature bed at an end of a first array to a high temperature bed at an end of a second array and wherein working fluid is allowed to pass from a high temperature bed of the first array to a lower temperature bed of the second array.

55. A heat pump as claimed in claim 54, wherein working fluid is allowed to flow from a high temperature bed of the first array to an intermediate temperature bed of the second array and wherein working fluid is allowed to flow from an intermediate temperature bed of the first array to a lower temperature bed of the second array.

56. A heat pump as claimed in claim 1 or any claim dependent on claim 1, wherein during repeated cycles of operation the working fluid cascades alternately between beds of one array and beds of the other array from the highest temperature bed of one array to the lowest temperature bed of the other array.

57. A heat pump as claimed in claim 1 or any claim dependent on claim 1, wherein when the lowest temperature bed is depleted to a predetermined extent the arrays are caused to change in function from heating to cooling or from cooling to heating and wherein the flows of heat transfer fluid are reversed.

58. A heat pump as claimed in claim 1 or any claim dependent on claim 1, wherein four heat exchangers are arranged in two arrays.

59. A heat pump as claimed in claim 1 or any claim dependent on claim 1, wherein six heat exchangers are arranged in two arrays.