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 apparatus that currently employs a mechanical vapour recompression or refrigerant/absorption solvent cooling or heat pumping system.
In this specification the term ‘heat pump’ refers to 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 an intended application. The term ‘heat pump’ may be 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.
Chlorofluorocarbons (CFCs e.g. CFC 11, CFC 12) and hydrochlorofluorocarbons (HCFCs eg HCFC 22, HCFC 123) are stable, of low toxicity and non-flammability 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The CFCs and HCFCs have been superseded in new air conditioning, refrigeration and heat pump equipment by hydrofluorocarbons (HFCs eg HFC 134a, HFC 125, HFC 32) either as pure fluids or as blends.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 potentials. 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.
According to the present invention there is provided A heat pump device comprising:
The invention provides a heat pump device in which a temperature difference is established by inducing cyclical expansion and compression pulses in a working fluid vapour or gas which passes through an adsorbent porous solid located in one or more heat exchangers.
The device may be specifically arranged so that heat is emitted at an elevated temperature at a first one location or end of the heat exchanger and taken in at a lower temperature at a second location or end of the heat exchanger.
In use of the device the working fluid enters the adsorbent solid at one end under elevated pressure and is removed at lower pressure or by suction at the second end.
The device may also include a heat transfer fluid.
In a preferred embodiment the device includes a heat transfer fluid, means for passing the heat transfer fluid in thermal contact with the heat exchanger, arranged so that the flow of heat transfer fluid removes or adds heat from or to the heat exchanger.
Preferably the direction of flow of the heat transfer fluid changes with the compression and expansion pulses of the working fluid.
More preferably the direction of flow of the heat transfer fluid reverses in coordination with the compression and expansion pulses of the working fluid.
In a particularly preferred embodiment the reversal of direction of flow of the heat transfer fluid is synchronized with said pulses.
The frequency of the cyclical motion of the working fluid may be the same as the frequency of cyclical motion of the heat transfer fluid.
The working fluid can be a vapour, gas or liquid preferably a vapour or gas. In the first part of the cycle the heat transfer fluid is caused to move in use from the low temperature end towards the higher temperature end of the heat exchanger to remove heat from the hot end of a heat exchanger and reject it into a suitable heat sink. In a second part of the cycle the heat transfer fluid, is caused to move in an opposite direction from the higher temperature end towards the lower temperature end of the heat exchanger where it is cooled before entering the refrigerated volume. To achieve a cyclical operation the heat transfer fluid may be caused to oscillate forwards and backwards along the heat exchanger with same frequency as the cyclical compression and expansion, that is pulsing, of the working fluid. When the working fluid is being admitted to the porous solid under compression the heat transfer fluid moves in the opposite direction to the working fluid. When the working fluid is being removed from the porous under suction the heat transfer fluid moves in the same direction as the working fluid.
The means for inducing pulses in the working fluid may be a positive displacement compressor.
Alternatively the means for inducing pulses in the working fluid includes a valve switching system and a compressor. Preferably the valve switching system alternately connects the body of adsorbent material to high and low pressure reservoirs of the working fluid.
Optionally but preferably the device comprises a further heat exchanger adapted to remove heat of compression from the working fluid vapour or gas before it contacts the adsorbent.
The hot and cold temperatures generated depend upon the specific applications for which the embodiments of this invention are used, in particular whether refrigeration or air conditioning is required. In this context air conditioning is to be understood 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.
Preferably the temperature gradient comprises a relatively high temperature at the inlet and a relatively low temperature at the outlet.
In an alternative embodiment the working fluid is a blend of a relatively strongly adsorbed fluid and a relatively weakly adsorbed fluid. These fluids do not strongly interact with each other. The more weakly adsorbed fluid may serve to sweep the more strongly adsorbed from the adsorbent during suction and carries it to the adsorbent during compression. Examples of such combinations are carbon dioxide/nitrogen and carbon dioxide/argon with activated porous carbon as the adsorbent. A preferred embodiment the blend is a combination of a relatively strongly adsorbed gas, especially ammonia or carbon dioxide with one or both of the light gases hydrogen and helium. The light gases have high thermal conductivies compared to the heavier working fluid gases or vapours and thus improve heat transfer to and from the adsorbent during the adsorption and desorption of the heavier working fluid. A combination of helium and carbon dioxide is especially preferred because the blend is non-flammable.
In a further embodiment of this invention a blend of two or more working fluids is selected such that one adsorbs more strongly than the other. When the heat pump is operating under a relatively light load the less strongly adsorbed fluid is at a higher concentration in the circulating fluid than its concentration in the blend originally introduced into the unit. Conversely the concentration of the more strongly adsorbed component in the mixture remaining on the adsorbent is greater than the original blend concentration. When the heat pump is operating under a heavy load the concentration of the circulating fluid contains a greater proportion of the more strongly adsorbed fluid and the composition of the circulating fluid approaches that of the loaded blend. By using a blend with these properties the heat pump can adapt its operation to changing loads and thus maximize its energy efficiency. An example of such a blend is a combination of propane and carbon dioxide used with a porous carbon adsorbent.
For single room air conditioning the heat transfer fluid will be generally be air. In cooling mode the cold temperature will be generally about 5 to about 15° C. while hot temperatures will be generally about 35 to about 60° C. Cooling powers will typically range from about 3 kW to about 100 kW. In heating mode the output temperature to the room will be typically be about 20 to about 30° C. and input temperatures from the outside air typically about 2 to about 15° C. Heating powers will typically be about 4 to about 150 kW. Some devices may be designed simply to provide heating and may generally be described as heat pumps, although this is a more restricted use of the term than that used in this specification.
For the air conditioning of large buildings with multiple rooms such as hotels and office blocks the heat transfer fluid can be water which maybe piped through each room where air will be blown over the cold water piping to provide the required cooling. This system is analogous to conventional chiller installations. The temperature of the water fed into the system will be typically at about 5-10° C. and the water returning to the device will be typically at about 10 to 15° C. In heating mode the temperature of the water leaving the device will be about 25 to about 40° C., while the return water will be typically about 15 to about 30° C. Cooling powers typically range from 50 kW to 10 MW. Chillers can also be used in process industries, for example cooling condenser water in distillation equipment.
In one embodiment of the invention the device is used to provide refrigeration typically at temperatures down to about −30° C. In this application it is preferable to use a relatively low freezing point heat transfer liquid. In a further embodiment of this invention equipment performance is optimised by carrying out the heat pumping process over two or more stages. This approach is especially advantageous for temperatures below about −20° C. Although carbon dioxide is a good refrigerant for Rankine cycle heat pumps with condenser temperatures lower than about 0° C., it has a critical temperature of 31° C. and a high critical pressure of 72 bar. For this reason it is unattractive for use in heat pumps where the heat output temperature is above 0° C.
An especially preferred embodiment comprises a conventional heat Rankine cycle stage wherein carbon dioxide is the working fluid and which pumps heat from a temperature in the range generally about −55 to about −10° C. and reject it at a temperature generally in the range about −20 to about 0° C. to the low temperature side of a second stage device employing the present invention. This second stage built according to this invention rejects heat at temperatures of about 35 to about 70° C. while operating at maximum pressures in the range about 10 to about 30 bar, typical of current HFC based refrigeration equipment.
The working fluid may be selected from any chemically stable fluid that can be reversibly adsorbed onto and desorbed from a suitable porous solid. Preferred fluids include carbon dioxide, air and nitrogen. The working fluid may be a fluorocarbon or a mixture of fluorocarbons boiling between −140° C. and 40° C., preferably between −90° C. and 0° C., more preferably between −90° C. and −20° C. CFCs, HCFCs and EFCs are acceptable in those territories where their use is permitted but 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 is acceptable 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 with low global warming potentials relative to CO2 preferably less than 150, more preferably less than 100 and most preferably less than 10. Preferred compounds are fluorinated olefins. More preferred are fluoroolefins containing a trifluorovinyl group. Even more preferred are fluoro-olefins containing at least one hydrogen atom. Especially preferred are fluoro-propenes and their blends. Where fluorinated compounds are not acceptable, especially preferred working fluids are CO2 and N2 which combine low environmental impact with low toxicity and non-flammability.
In the literature the term ‘porous solid’ is 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.
Firstly, the internal surface area of a preferred porous solid is greater than about 10 m2g−1, more preferably greater than about 100 m2g−1, most preferably greater than about 1000 m2g−1.
Secondly, the void space in a preferred porous solid is distributed between a combination of macro-, meso- and micro-pores. The porous solid has at least 10% of its void volume in the form of micropores with diameters less than about 2 nm, at least 5% of its void volume in the form of mesopores with diameters less than about 50 nm.
Thirdly a preferred porous solid is capable of reducing the pressure of the working fluid vapour or gas in contact with it, i.e. it adsorbs the working fluid.
Fourthly, 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.
Fifthly, a preferred 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, for example fumed silica, granular silica or aerogel silica, including granular, monolith and flexible blanket aerogels may be used. Natural or artificial glasses, ceramics or molecular sieves may be used. Carbons which may be used include granules, monoliths, fabrics, aerogels and membranes. Examples of porous carbons suitable for this invention are described in PCT/GB01/04222 the disclosure of which is incorporated into the specification by reference. Various organic materials including resorcinol-formaldehyde foams or aerogels, polyurethane, polystyrene or other polymers in the form of foams and aerogels. Polymers of intrinsic porosity in which the tailored pore sizes are created by the 3-dimensional linking of appropriate precursor molecules with constrained geometries are also suitable for this invention. A range of composite materials are acceptable, including silica-carbon composites.
Porous materials may be made by blowing polymeric foams, and by sol-gel processes for manufacture of porous ceramics, silica or other mineral aerogels or organic aerogels. Organic materials for example coconut and coal, may be pyrolysed, and then further processed for example by treating with steam, to produce activated carbons. Polymer aerogels may be pyrolysed to produce carbon aerogels. Hydrocarbons may be pyrolysed to produce carbon membranes. Molecular sieves and carbon black or by plasma processes such as the APNEP (Atmospheric Pressure Non-Equilibrium Plasma) system developed by CTech Ltd. Carbon based materials, such as activated carbons derived from biomass precursors, e.g. coconut shell, 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, or 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 porous materials may be obtained by thermolysis, for example production of fumed silica from silicon tetrachloride using an oxy-hydrogen flame or by plasma processes. Organic-inorganic precursors may be processed by thermolysis to produce molecular sieves. Natural mineral hydrates may be thermalised, for example vermiculite and perlite.
In one embodiment of this invention a heat pumping device comprises an adsorbent porous solid blend whose the properties vary between the high and low temperature ends of the adsorbent tube beds.
In a further embodiment of this invention the porous solid is selected such that its permeability to gas flow along the tube is sufficiently low to allow a pressure gradient to be generated along the tube during compression and during suction. The permeability of the adsorbent can be controlled in various ways. For example the range of particle sizes can be selected according to the length and cross-sectional area of the tube and the working fluid flow rate to give the desired pressure gradient. Alternatively the porous solid particles may be compressed into a monolith with a suitable binder to give the required pressure gradient.
The gas or vapour working fluid may be matched to preferred porous solids 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, celluloses), metal sponges (e.g. Ni, Ti, Fe), and metals or metal complexes supported on organic polymers or carbon.
Not all these gases and their combinations with the available porous solids may be appropriate. Although the CFCs and the HCFCs continue to be manufactured and used in the developing world their phase-out under the Montreal Protocol is already occurring. In territories when continued use of these chlorinated fluids is still legal then they can 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 is not currently being considered, then their use in the present invention is acceptable, but not preferred because their global warming potentials are much higher than some of the other gases listed above. SO2 is not preferred because of its toxicity.
Hydrocarbons can be coupled with carbon, alkylated silica or an organic polymer, especially a hydrocarbon polymer such as polystyrene. Although more preferred than the halogenated fluids and SO2, hydrocarbons are restricted to applications where the appropriate precautions can be taken against their marked flammability hazard, for example in large industrial applications or low-inventory, hermetically-sealed systems such as domestic refrigerators. A further disadvantage is that the enthalpy changes associated with the sorption/desorption of hydrocarbons is less than for more polar gases, notably CO2, SO2 and NH3. In some territories hydrocarbons are also disliked because any leak to the atmosphere where exposure to sunlight generates “photo-chemical smog”.
Hydrogen is readily sorbed 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. Like hydrocarbons hydrogen reacts with atmospheric hydroxyl radicals that play a key role in removing naturally-emitted hydrocarbons as well as man-made pollutants such as HFCs. Increased hydrogen emissions can thus indirectly increase globally warming.
Ammonia can be used with carbon, silica or with an organic polymer. It is suitable for applications where its toxicity and flammability can be controlled, for example large commercial and industrial applications or low inventory, hermetically sealed domestic applications.
The most preferred working 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 would be very small. By obtaining carbon dioxide from a natural source, such as biomass fermentation, any gas emitted from the device would have zero contribution to global warming. 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° C. Especially preferred are substances where 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.
For very low temperature refrigeration involving temperatures below −55° C. carbon dioxide is not practical because its triple is −56.7° C. For sub −55° C. temperatures N2 is preferred as a working fluid with adsorbent, such as activated carbon. The preferred heat transfer fluid is the atmosphere within refrigerated enclosure, which in many cases will be air. This design will provide cooling in the range −130° to −40° C. and will reject heat in the range −55 to −25° C. to a higher temperature stage.
In further embodiment of this invention blends of gases or vapours can be employed provided that they do not chemical react. Thus a hydrocarbon such as propane can be mixed with carbon dioxide.
Preferably the temperature changes generated when the fluid reversibly adsorbs and desorbs should be greater than 5° C. and more preferably greater than about 10° C.
Seven important parameters may contribute to the temperature change:
(a) the integrated heat of adsorption (IHA) measured between the lowest and highest pressure between which the adsorbent operates;
(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, expressed in units of kJ/mol, 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 just below approximately 2 bar at the heat rejection temperature is advantageous in that it 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. It does require the use of high volume throughput compressor such as a centrifugal compressor and this is especially suited to large water chillers for example 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 such that the pressure of the adsorbent at the lowest working pressure of the device is not less than approximately 1 bar to prevent the ingress of atmospheric gases which are not significantly adsorbed by the porous solid. Preferably the IHA is chosen such that in a given application the lowest operating pressure is not less than 1.5 bar.
A special advantage of the present invention is that it allows even relatively small temperature changes below 5° C. induced by fluid adsorption and desorption to generate the required substantial temperature differences between the ends of the adsorbent tubes, for example a difference of 35° which is required to generate the cold and hot air temperatures of 10° C. and 45° C. typically required for 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° C. and more preferably greater than 10° C. 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 kJ/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 of the porous solid (USA) 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). But 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 porous solids with high densities provided this does not affect the other adsorbent physical properties discussed above. 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. The optimum maximum pressure depends upon the pressure/adsorption properties of the porous solid. For the combination of CO2/activated carbon the optimum working pressure is generally around 20 bar.
The thermal conductivity of the adsorbent is important. Porous solids, especially in particulate or granular form, have low thermal conductivities consequently heat transfer during adsorption and desorption limits the cycle time of the beds. In a preferred embodiment the heat pump comprises one or more adsorbent tubes which are long in comparison to their width or diameter and are adapted for progressive removal or addition of heat from one end to the other end. The ratio of tube length to diameter should be preferably greater than about 5:1, more preferably greater than about 10:1 and most preferably greater than about 20:1.
To improve the thermal conductivities of adsorbents they can advantageously be composed partially or entirely from heat conducting materials. The latter may include graphite, preferably as flakes, fibres or foams; metal mesh, powder, wire or fibres, preferably comprising high thermal conductivity metals such as copper and aluminium; organic polymers with high thermal conductivities, such as polyaniline and poly-pyrrolidine or mixtures thereof. Such polymers, in at least one chemical form, generally have good thermal and electrical conductivities.
In a preferred embodiment of this invention such thermally conducting polymers containing basic nitrogen atoms and constituting at least a proportion of the porous solid are used. Such a porous solid may also contribute to the adsorption of carbon dioxide. Compressing the porous solid into a monolith also improves thermal conductivity.
Table 1 lists examples of various adsorbents and their thermal conductivities. This demonstrates that the addition of a heat conducting additive substantially improves the thermal conductivity of an adsorbent.
Preferred adsorbents 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 adsorbent heat exchanger configuration also influences the ease with which heat is transferred to and from the porous solid. An important requirement is to maximise the heat exchange without increasing the thermal capacity of the metal components of the heat exchanger so that temperature changes do not fall below the preferred value of 5° C.
The invention is further described by means of example but not in any limitative sense with reference to the accompanying drawings of which:
a to 13g are views of a third device in accordance with the invention; and
In this embodiment the heat transfer fluid is a liquid constrained to flow in an external duct concentric to the adsorbent tube as shown in
In another embodiment of this invention the heat transfer liquid flows through a tube within the absorption tube as shown in
In one embodiment of this invention the gas flow is advantageously constricted to generate a pressure gradient along the adsorbent tube. This can be achieved by in various ways, used alone or in various combinations. For example one method involves the use of solid heat exchanger fins in the form of unperforated discs perpendicular to the axes of the adsorbent and liquid tubes providing small gaps between their edges and the inner wall of the adsorbent tube through which the working fluid is constrained to pass. In a second method gaps between the fins and the inner wall are sealed by polymer gaskets, but the fins are perforated by small holes which restrict the gas flow. By varying the numbers of fins employed, the size of the gap between their edges and/or the diameters and number of the perforations the desired pressure gradient can be achieved.
In a further embodiment the inner wall of the adsorbent tube is provided with a low thermal conductivity liner. This may inhibit the flow of heat from the adsorbent to the wall of the adsorbent tube. 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 magnitude of the temperature changes. The liner can also serve as container for the adsorbent and heat transfer liquid tube 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, the metals generally favoured when high thermal conductivities are preferred. 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 as shown in
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 a liquid duct as shown in
In a further embodiment of this invention the heat transfer fluid is a gas, preferably air. This may be caused to flow along the outer surface of the adsorbent tube. To provide good heat transfer the outer surface of the tube is fitted with longitudinal or spiral fins. Heat transfer from the adsorbent to internal wall of an adsorbent tube may be advantageously promoted by perforated metal plates or discs of metal mesh or fibre which are preferably located perpendicular to the tube axis. For optimum heat transfer these should in close contact with the inner wall. These discs can also serve to constrict the working fluid gas flow to generate a pressure gradient along adsorbent tube. For example if perforated metal plates are used the number of plates and the diameter and number of the perforations can be selected to give the desired pressure gradient.
Good heat transfer between the adsorbent and the heat transfer fluid is clearly desirable for optimum performance. Metal adsorbents which can be used with hydrogen are especially advantageous in that they have much higher thermal conductivities than non-metallic materials such as carbon, zeolites and silica gel.
The choice of the fluid/adsorbent combination may depend upon a number of factors whose values must be selected to provide an optimum performance for a given application and the design of the adsorbent heat exchangers. The adsorbent can be contained in tubes as described above. The adsorbent can be contained in sets of tubes in parallel, each set simultaneously undergoing compression or suction. Alternatively the adsorbent can be contained in sets of tubes in series connected by pipes. While the pressure drop across a single tube can be small a substantial pressure gradient can be established across the tube series by incorporating restrictions to the gas flow in the connecting pipes. In another embodiment of this invention the adsorbent is contained within a pair of plates sealed their edges and equipped with inlet and outlets at opposites ends of the plates. Heat transfer fluid removes or adds heat by flowing over the external faces of the plates. This mode of construction produces an adsorption plate heat exchanger. Sets of these plate heat exchangers can assembled in parallel into a module such that the heat transfer fluid flows between each pair of heat exchangers.
In a preferred embodiment of this invention a device comprises a working fluid, a positive displacement compressor driven by source of mechanical power, and a porous adsorbent solid through which pulses of compressed working fluid are able to expand. Cooling is produced by desorption of working fluid from the porous solid by reducing the pressure of the gas in contact with one end the solid while pressure induced adsorption of the working fluid by the solid at the other end 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 should be comparable with the latent heats of vaporisation of CFC, HCFC, HFC, hydrocarbon and ammonia working fluids presently used for conventional Rankine Cycle based devices which they are intended to replace.
One configuration for the device is shown in
The device operates in a cycle described by the following steps starting from the state where the suction stroke of the reciprocating compressor has just been completed and the compression stroke is just about to start, i.e. bottom dead centre.
For the device to operate successfully in the mode described it is important that the pressure of the gas at any point in the porous solid connecting 11.7 and 11.9 oscillates about a mean value so that working fluid travels through the solid via a series of adsorptions and desorptions induced by the compressor. This process will provide the major contributions to the enthalpy changes in 11.7 and 11.9. To optimize the performance of the device the external air stream should also oscillate along the heat exchanger as shown in
The reciprocating compressor shown in
Various compressor designs can be used, provided they are configured to deliver pulses of compressed gas at the hot end of the adsorption bed and remove pulses of expanded gas at the cold end.
Porous solids, such as activated carbons, have excellent adsorption capabilities for vapours and gases, but are poor thermal conductors requiring long cycle times, e.g. >1 minute, to enable heat to be taken in during the suction phase and heat to be rejected during the compression phase. If operated in this manner when the heat pump is being used for cooling it will only supply cold during half of its operating cycle. This limitation is overcome in a further preferred embodiment of this invention containing two beds operating 1800 out of phase, such that as one bed is under going suction/desorption while the other is undergoing compression/adsorption. This embodiment, employing a pair of adsorption tubes, is illustrated in
In a further preferred embodiment of the device more than one pair of adsorbent heat exchangers are used so that when the pressure is being equalized between one pair of heat exchangers the compressor continues moving working fluid between the two members of a second pair of heat exchangers. This arrangement has the advantage of providing effective continuous heat pumping.
In a further embodiment of this invention the adsorbent beds are cooled and heated by a circulating liquid.
A suitable heat transfer liquid requires a combination of properties which are determined by its intended application. For air conditioning air is an attractive option. When liquids are employed they preferably have low viscosities to minimise pumping energy. 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 considered. 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 that 100° C., more preferably greater than 130° C. and even more preferably greater than 200° C. Most preferably the liquid should be non-flammable. Preferred liquids include those already known to the industry as secondary refrigerants. These materials include water, brines, 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° C. compositions with wide liquid ranges are require, while retaining the desirable properties of flash points greater than 100° C. and normal boiling points greater than the highest temperature. Preferred substances include esters and ethers containing 3 or more carbon atoms which 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 as lower freezing points, higher boiling points, lower viscosities or higher flash points. If such additives were to be used alone they would not be preferred, but are acceptable when used in mixtures where they constitute less than 50% of composition by mass.
To avoid adverse environmental impacts 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.
Number | Date | Country | Kind |
---|---|---|---|
050795308 | Apr 2005 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/GB2006/001482 | 4/21/2006 | WO | 00 | 5/5/2008 |