Field of the invention

The present invention relates to the field of heat-mass exchangers, in particular of the type of so called "packed-bed reactors", and to a method of heat-mass transfer between a gas flow and a liquid desiccant flow.

Background of the invention

Heat-mass exchanger typically provides a double functionality, i.e. causing a concentration change of components in the fluids and causing a temperature change of the fluids going through it. As such it is e.g. used for the functionality of dehumidification and/or cooling of a gass flow. They are also referred to as "absorbers", and many different types of absorbers exist. They may be part of a larger system, such as e.g. an air conditioning system, etc..

One type of heat-mass exchangers is based on a packed-bed absorber. US 5,351,497A describes a heat mass exchanger based on a packed-bed absorber, the system disclosed in US 5,351,497A being shown in FIG. 1 - prior art. The packed-bed absorber consists of a porous bed of contact medium that is flooded with desiccant. As the desiccant flows down through the bed, it comes in contact with the water-vapor-containing process gas that can be flowing either down, up, or across the bed. The desiccant-which, by definition, has a strong affinity for water vapor-absorbs water vapor from the process gas. During dehumidification, heat is released as the water vapor condenses and mixes with the desiccant. This heat will equal the latent heat of condensation for water plus the chemical heat of mixing between the desiccant and water. At desiccant concentrations typical of a liquid-desiccant gas conditioner, the chemical heat of mixing will be about an order of magnitude smaller than the latent heat of condensation, or smaller.

There is a continuous drive for rendering heat mass transfer systems as efficient as possible. The greater part of the actual heat mass exchangers based on transfer in a packed bed between salt solution and gas, have a mass rate of salt solution that is similar to the gas flow rate. In such exchangers, the ratio of the hygroscopic capacity of the salt to the gas flow is typically in the order of 10 ² . This large difference in hygroscopic capacity gives a high vapor pressure difference and/or temperature difference in the bed between gas and salt solution, resulting in exergy destruction. The higher exergy destruction in the bed compromise the possibility to develop an efficient dehumidification device.

Although some heat mass exchangers are known where both flows have similar hygroscopic capacity, such exchangers typically are developed for air conditioning systems where air needs to be dried and cooled, therefore they need to rely on internal cooling devices rendering the cooling systems expensive.

There is a continuous quest for efficient humidifiers/dehumidifiers, i.e. for humidifiers/dehumidifiers which are exergy efficient.

Solutions have been developed to increase the contact surface and/or to increase the transfer coefficients by ensuring a low contact angle. Non-structured volumes consisting of ring shaped material have been used. These volumes have a high porosity. The ring shaped materials are relatively expensive. Another solution that has been provided is the use of a structured volume coated with materials to reduce the contact angle of the fluid and hence having an increased wetted and effective contact surface.

German patent application DE10141525 describes a heat mass exchange reactor wherein the structure comprises surfaces on a double plate, which are coated with a thin layer of small solid-state particles so that there are clearances and cavities between the particles.

US patent US5351497 describes an air-conditioning process using an internally-cooled, liquid- desiccant absorber through which the air to be conditioned, the coolant and the liquid desiccant are passed. Cooling air is passed though the absorber in intimate contact with the cooling water to promote evaporative cooling of the cooling water.

Those solutions are relatively expensive and/or not useful for dehumidification and/or not efficient. There is still need for improved heat mass transfer systems. Summary of the invention

It is an object of embodiments of the present invention to provide a good mass exchanger creating an intense contact between a gas flow and a working fluid, and a good method of humidifying/dehumidifying through heat and mass transfer.

In particular, it is an object of embodiments of the present invention to provide an exergy efficient heat and mass transfer system between a gas and a liquid desiccant, and an exergy efficient method of heat and mass transfer between a gas and a liquid desiccant.

It is an advantage of embodiments of the present invention that an exergy efficient mass exchanger between salt solution and gas is obtained with little or no droplet formation. It is an advantage of embodiments of the present invention that obtained mass exchangers can be used as a component in an exergy efficient dehumidification device (a vapor heat pump).

It is an advantage of embodiments of the present invention that these can be made at low cost.

The mass exchanger may be a heat-mass exchanger, although embodiments of the present invention are not limited thereto.

This objective is accomplished by a method and device according to embodiments of the present invention.

The present invention relates to a heat mass transfer device for humidifying or dehumidifying a gas flow, the device comprising:

- a gas inlet for receiving a gas to be humidified or dehumidified, and a gas outlet for providing the humidified or dehumidified gas;

- a heat mass transfer unit arranged in fluid connection with the gas inlet and the gas outlet, and comprising a structure being a reticulated structure or a stack of individual objects, the structure having a structural porosity of at least 60% through which the gas flow can pass, the structure having a surface density accessible for the gas flow of at least 100 m ² /m ³ , the structure being contactable with a hygroscopic fluid;

- a supply unit configured for supplying said hygroscopic fluid for contacting said structure. In some embodiments, a stack of individual objects is used. These objects may be three dimensional objects.

In some embodiments, a reticulated structure may be used.

It is an advantage of a device as specified above, in that a highly efficient mass-transfer (in particular water exchange) can take place between the gas flow and the hygroscopic fluid, even at a moderate temperature difference and a moderate vapor concentration difference between the gas and the hygroscopic fluid.

It is an advantage of both a reticulated structure as well as the stack of objects that the fluid, if sprayed in droplets, is broken up and spread out quickly over the material surface of the structure. As such the fluid cannot form droplets as these droplets are broken up flowing down at the nodes where the nets are joined.

It is an advantage of a device that the mass transfer can take place under improved exergy- efficiency, in particular because the porosity and the surface density characteristics of the structure allow a good temperature and heat transfer even if the hygroscopic capacity of the gas flow and the hygroscopic capacity of the hygroscopic fluid are in the same order of magnitude.

It is an advantage of a device having a structural porosity of at least 60% in that it allows a gas to pass through (for example from bottom to the top of the device), while at the same time allowing a hygroscopic fluid to pass through (for example from the top to the bottom of the device), thus acting as a contact medium. In this way the gas and the fluid can come into contact, and moisture content can be exchanged between the gas and the liquid.

It is an advantage of this device that it can be used in combination with an exergy efficient regeneration / reconcentration technique. Such efficient regeneration systems need hygroscopic solutions that are at the output of the heat mass exchanger substantially diluted compared to the solution at the input (+10% or more weight increase due to water absorption), as delivered by the device of the current invention. It is to be noticed that when similar hygroscopic capacities occur for both flows, this implies a diluted salt solution whereas if the hygroscopic capacity of the salt is significantly larger, the salt solution is only little diluted. Together, it results in an exergy efficient dehumidification device (also known as a vapor-heat pump). The coefficient of performance COP might be 7 or higher, so highly more efficient than the current state of the art.

It is an advantage of a device with a structure having a surface density of at least 100 m ² /m ³ , in that it provides a very large total "contact surface" accessible for the gas flow, in a very compact manner, thus resulting in a smaller device, occupying less space. In other words, the surface area is high for a low volume.

It is an advantage that the supply unit keeps the "surface segments" wetted with the hygroscopic liquid. It is an advantage that the supply unit may function time-continuously or in a periodic manner, or in a time-interleaved way.

The surface density accessible for gas-flow may, in some embodiments, be at least 300 m ² /m ³ , or at least 350 m ² /m ³ , or at least 400 m ² /m ³ . The structural porosity may in some embodiments be at least 60%, or at least 75%, or at least 90%. It is an advantage that a highly porous material may be used ensuring a low pressure drop for the gas flowing through it.

The reticulated structure may comprise a material that is a hydrophilic material.

Where in embodiments of the present invention reference is made to a material that is hydrophilic reference to a material wherein the contact angle for water on that material is less than 90°, for example around 77°, or for example less than 45° or for example less than 15°. For example in the case of PU material, the average contact angle is about 77°, but there is a large hysteresis of 35° to 83°. The large hysteresis results in the fact that salt solution sprayed on the material spreads over the surface and that further flow over the material adds less to the transfer (because the contact angle changes from 35° to 83°). For materials where the contact angle is lower, also the flow downstream the material adds to the transfer. It is an advantage that there is good "wetting" of the structure material, hence there is a reduced tendency to drop formation of the hygroscopic liquid, but instead the liquid will spread out, and form a film over a larger surface, which is beneficial for an improved mass/heat transfer between the gas and the hygroscopic liquid, hence an improved (de)humidification. The hydrophilic nature thus may assist in generating a good spreading of the fluid over the different parts of the structure.

The structure (141) may comprise or may consist of an impregnable material.

It is an advantage that the structure comprises a material that is impregnable with a hygroscopic fluid, e.g. because the material has (material) pores, because such a surface shows a good wetting of the surface with the hygroscopic fluid, so that a good transfer of moisture can take place between the gas and the fluid. An impregnable material shows an even better wetting than a material which is not impregnable, because of the capillary effect of said pores, which keeps the surface wetted. In this way an efficient (de)humidification can take place. The structure may be a reticulated structure being a reticulated foam.

It is an advantage of embodiments of the present invention that a reticulated foam can easily be manufactured with existing techniques. It is an advantage of a reticulated structure that a very high interaction path length for the fluid to interact with the gas flow is obtained, whereby the path is a continuous path which cannot be broken up by a particular interaction of individual members. It is an advantage of a reticulated structure that the mass and heat transfer coefficient of the gas is higher due to very small tubes of the foam, hence enhancing the mass and heat transfer of the heat mass transfer device. It is an advantage of a reticulated structure to have low pressure drop for the air combined with high heat and mass transfer. It is an advantage of reticulated structure that the formation of droplets is avoided, instead, the reticulated structure will provide break up the formation of droplets and a thin film over the ribs as to provide a long path for the fluid through the whole structure.

The reticulated structure may be a reticulated PU foam.

It is an advantage of embodiments of the present invention that the structure is readily available on the market.

The reticulated structure may be made of a porous material. The structure may be a stack of individual objects, the individual objects being long fibers, the fibers having a plurality of different orientations in the stack.

The length of the fibres may be at least one to two times the height of the structure.

It is an advantage of stacking fibers that can be good wetted during the distribution of the salt solution, and combined with the very low mass transfer resistance to the gas flow due to their small radius, it results in high mass transfer. It is also an advantage of using a stack of individual elements over e.g. a system with metal parallel tubes or plates for example, in that the material cost for the structure can be low.

The fibers may be randomly oriented.

The hygroscopic fluid may be a hygroscopic salt solution. Examples of hygroscopic solutions salt solutions are CaC and LiCI. Furthermore hygroscopic mixtures also may be used, such as for example Klimat as available from Solvay.

The structure may have a height of at least 1cm and smaller than 50 cm, even smaller than 10cm.

The heat mass transfer system may comprise a regenerator adapted for regenerating the hygroscopic fluid, the regenerator being connected to a second input of the device for providing regenerated hygroscopic fluid, and connected to a second output of the device for receiving hygroscopic fluid that has passed through the structure.

The connection between the regenerator and the heat mass transfer device can be direct. The supply unit may be configured for supplying said hygroscopic fluid such that a substantial part of the surface of the structure (for example but not limited to in the order of at least 20%) is contacted with said hygroscopic fluid.

The structure may comprise pore openings providing access to the pores, the pore openings having an average diameter of at least 1mm.

The hygroscopic capacity for the gas flow and liquid flow may be equal or substantially equal. The ratio of the hygroscopic capacity of the liquid flow and the hygroscopic capacity of the gas flow over the structure is a value in the range of 0.10 to 10.0.

The reticulated structure may be an open cell structure.

The present invention also relates to a system comprising a heat mass transfer device as described above. The system may be configured for processing a gas flow having a low flow rate of at least 0,1 m/s. The gas flow rate may be smaller than 2 m/s.

The present invention also relates to a system comprising a heat mass transfer device as described above, the system being a vapor heat pump. The present invention furthermore relates to a method of humidifying or dehumidifying a gas, the method comprising

contacting a hygroscopic fluid with a structure being a reticulated structure or a stack of individual objects, the structure having a structural porosity of at least 60% through which the gas flow can pass, the structure having a surface density accessible for the gas flow of at least 100 m ² /m ³ , and

passing a flow of said gas through the structure for inducing a heat mass transfer between the gas flow and the hygroscopic fluid.

It is noted that the supply of hygroscopic fluid may occur on a continuous or discontinuous basis, both in terms of quantity and of time. In particular, the supply of hygroscopic fluid can be time-continuous or time-interleaved.

Passing a flow of said gas may comprise providing a gas flow with a flow rate of 0,1 m/s. The gas flow rate may be smaller than 2 m/s.

The method may comprise selecting a flow rate of the hygroscopic fluid such that the ratio of the hygroscopic capacity of the hygroscopic liquid flow and the hygroscopic capacity of the gas flow over the structure is a value in the range of 0.10 to 10.0 .

It is an advantage that the hygroscopic capacity of the gas flow and of the hygroscopic capacity of the hygroscopic liquid is in the same order of magnitude, (and not a factor of about 100 different, as in some prior art embodiments) as this results in a high exergy efficiency. In particular embodiments, the ratio of the hygroscopic capacity of salt stream to air flow may be a value in the range of 0.2 to 10, or even 0.5 to 5. This provides an even higher exergy efficiency. The present invention also relates to the use of a heat mass transfer device as described above for humidifying or dehumidifying a gas flow using a hygroscopic fluid.

The present invention also relates to the use of a heat mass transfer device as described above for industrial and/or agricultural applications.

The present invention also relates to the use of a heat mass transfer device as described above for applications requiring a gas flow having a volume flow rate of at least 0,05m ³ /s.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. Brief description of the drawings

FIG. 1 shows a prior art liquid desiccant absorber in connection with a boiler functioning as regenerator of the desiccant.

FIG. 2 shows an example of a heat mass transfer device according to an embodiment of the present invention

FIG. 3 illustrates how a contact angle of a liquid on a surface is measured.

FIG. 4 shows a schematic representation of an example of a reticulated structure, in this case open cell PU foam, which can be used as structure in embodiments according to the present invention.

FIG. 5 illustrates the salt solution to gas mass ratio (M sola) and the salt to gas mass ratio (MRsa) for CaC in the case equal hygroscopic capacities as can be used in embodiments of the present invention. These hygroscopic capacities are calculated for the temperatures as presented by the adiabatic line in the drawing.

FIG. 6 illustrates the behavior of a salt solution and air throughout a matrix in a counter flow arrangement, as can be used in embodiments of the present invention.

FIG. 7 illustrates different matrix materials as can be used in embodiments according to the present invention.

FIG. 8 illustrates a pressure drop for a matrix of 5cm of different matrix materials at frontal air velocity (full line for PU foam, dotted line for natural fiber), as can be used in embodiments according to the present invention.

FIG. 9 illustrates a graph indicating the extrapolated dehumidification performance of 5cm PU foam, illustrating features of embodiments according to the present invention.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Any reference signs in the claims shall not be construed as limiting the scope.

In the different drawings, the same reference signs refer to the same or analogous elements.

Detailed description of illustrative embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination. In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

In the present invention, the word "porosity" is used in its usual meaning. "Porosity" or "void fraction" is a measure of the void (i.e., "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0 and 100%.

Where in the present invention reference is made to a structure with a "structural porosity" relates to the porosity induced by the holes in the netted structure or the spaces in between the different elements. Where in the present invention reference is made to a "material porosity", reference is made to the porosity of the porous material used for the individual elements or the porosity of the porous material used for the netted structure.

The term "hygroscopy" has its usual meaning, as the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically "changed" somewhat, by an increase in volume, stickiness, or other physical characteristic of the material, as water molecules become "suspended" between the material's molecules.

In the present invention, "hygroscopic fluid" and "desiccant liquid" are used as synonyms, both meaning that the fluid or liquid has a reduced vapor pressure above it and as a consequence has a high affinity for water, and is capable of taking up and retaining water from its surrounding. In the present invention, "equilibrium" like in "equilibrium vapor pressure" or "equilibrium relative humidity" stands for the gas state in thermodynamic equilibrium with the hygroscopic fluid.

In the present invention, the term "hygroscopic capacity", is used to express the degree in which a substance is hygroscopic. The latter may be expressed as the mass of absorbed water per mass of substance (gas, hygroscopic solution) and per unit change of the equilibrium relative humidity in an adiabatic process.

In this invention the "hygroscopic capacity of a flow" is the multiplication of the hygroscopic capacity of the substance with the mass flow rate of the substance taking into account the conditions of the flow at the respective input.

The expression "exergy efficiency" of a process (also known as the "second-law efficiency" or "rational efficiency") is the ratio of the exergy change of a flow going from an initial state to a desired state to the exergy delivered to the process. It is an indication of the efficiency of a process taking the second law of thermodynamics into account.

In the context of the present invention, the expression "surface density (of a contact medium) accessible for a gas flow" means the ratio of the total contact-surface area of that contact medium (e.g. the surface of the walls forming the structural voids mentioned above) to the volume filled by that contact medium, and is expressed in m ² per m ³ .

In the context of the present invention, the expression "effective surface density (of a contact medium) for mass transfer" means the ratio of the total surface area where mass transfer occurs between the hygroscopic liquid on the contact medium and the gas flowing through the medium to the volume filled by that contact medium, and is expressed in m ² per m ³ .

In the present invention, the expression "contact angle" has its usual meaning, as being the angle, conventionally measured through the liquid, where a liquid/vapor interface meets a solid surface, as illustrated in FIG. 3.

Where in embodiments of the present invention reference is made to "pore diameter" it is defined as for the diameter of a sphere or the diameter of a three dimensional channel fitting in the pore. It will be understood that where reference is made to an average pore diameter of a structure having for example n pores and a pore volume of A, , it is defined as the diameter of n identical pores resulting in the same pore volume A.

Where in embodiments of the present invention reference is made to a "diameter of a pore opening" it is defined as the diameter of circular opening towards a pore, I having the same surface as the imaginary plane connecting neighbouring pores of the structure. It will be understood that where reference is made to an average pore opening of a structure having for example n pores and a total pore opening surface of A, it is defined as the diameter of n identical pores openings resulting in the same total pore opening surface A .

In reticulated structures the pore diameter and the diameter of the pore openings is hence well defined. In structures being a stack of individual elements, the pore diameter and the diameter of the pore openings can be defined by the average pore diameter and diameter of the pore openings as above.

Although the present invention will be illustrated mainly by using a salt solution as an example of a liquid desiccant, and air as an example of gas, the invention is not limited thereto, and is also applicable to other hygroscopic fluids and other gases.

Although the present invention will be illustrated for the application of dehumidifying a gas, the invention can also be used to humidify a gas or to dry a hygroscopic salt solution.

Although the basic working principles of heat-mass exchanger using such a reactor are known in the art, the inventors of the present invention found that the majority of existing heat mass exchangers (e.g. using a salt solution for (de)humidifying air) have a mass rate (expressed in kg/s) of the salt solution that is similar to the mass rate (expressed in kg/s) of the air flow. They realized that in such exchangers, the hygroscopic capacity of the salt solution flow is typically about a factor 50 to 500 higher, e.g. 100 to 200 higher, than the hygroscopic capacity of the air flow. This large difference in hygroscopic capacity provides a high partial vapor pressure difference and/or temperature difference in the interface between the air and the salt solution. While such a large difference is well suited for "fast vapor exchange", it also results in a relatively large amount of exergy destruction. The present invention is concerned with the problem of exergy efficiency and provide solutions improving the exergy efficiency of the heat- mass transfer system, i.e. to reduce the loss of exergy in the processes.

For exergy efficiency in packed bed reactors based on interaction between a gas flow and a hygroscopic fluid, the hygroscopic salt solution typically shows a decrease of exergy due to its hydration (decrease of salt concentration) and the air shows an increase of exergy due to its dehydration and heating up. Embodiments of the present invention advantageously use a reduced (e.g. minimal) driving force in the diffusion processes. This is achieved in embodiments of the present invention by reducing on the one hand the difference in vapor pressure (partial pressure) between the gas flow and the hygroscopic salt solution and on the other hand by reducing the difference in temperature between the salt solutions, the contact medium and the gas flow.

Although a low temperature difference, and a low difference in vapor concentration (partial pressure) in principle also decreases the diffusion rate and the heat transfer rate. This was addressed in embodiments of the present invention by a high effective contact surface, a high mass transfer coefficient and a high heat transfer coefficient. This is achieved by (1) ensuring that the contact surface is wetted with a thin film of the hygroscopic salt solution only, (2) there is a low convective resistance between the gas flow and the thin film due to the use of a reticulated structure or a stack of individual objects. Due to this combination the rate of transfer is comparable to the prior art solutions, moreover the gas-liquid interface is very thin ensuring that a large amount of the liquid volume is actively participating in the transfer. In other words, by reducing the salt solution flow rate to a flow rate with comparable hygroscopic capacity as the gas flow and maximizing the effective contact surface in combination with high mass and heat transfer coefficients, substantially the same amount of heat-mass transfer can be achieved, but with a higher exergy efficiency.

In the present invention also the parasitic energy consumption are reduced: the lower flow rate for salt solutions and the lower pressure drop over the matrix results in lower pumping power.

This results in dehumidifying device for a gas flow with a higher exergy efficiency.

FIG. 2 shows an embodiment of a heat-mass exchanger 100 according to an aspect of the present invention. The heat-mass exchanger is a device suitable for dehydrating and heating gas and for humidifying a hygroscopic salt solution, or vice versa. The heat-mass transfer device 100 may also be called an absorber, or humidifier or dehumidifier.

The device 100 has a gas inlet 120 for receiving a gas to be humidified or dehumidified, and a gas outlet 122 for providing the humidified or dehumidified gas. The device 100 further comprises a mass transfer unit 140, arranged in fluid connection with the gas inlet 120 and the gas outlet 122.

The "mass transfer unit" may e.g. be a compartment, or a chamber where mass- transfer takes place between the gas and the hygroscopic liquid. It is noted that the compartment is not called "heat or mass transfer unit" because at least mass is exchanged, e.g. water content, and it is left open whether also heat is exchanged, although in practice that might well the case. The mass transfer unit 140 comprises a structure 144, forming or being a medium with a structural porosity of at least 60%, or at least 75%, or at least 90%. The void space in the medium forms channels through which the gas flow can pass in several directions. The pore diameter of the medium is preferably on average at least 1 mm. The pore diameter thereby can be defined as indicated above. According to some embodiments, the diameter of the pore openings, i.e. the diameter of the openings towards the pores, on average may be at least lmm. The diameter of the pore opening may be as indicated above. A structure with such a structural porosity allows passage of the gas flow at a moderate pressure loss. During operation, the structure 141 acts as the contact medium, i.e. material to which the hygroscopic fluid adheres, while the gas flow can pass through the structure. The structure 141 has a surface density accessible for the gas flow of at least 100 m ² /m ³ , which is relatively large. According to embodiments of the present invention, the structure may be a reticulated structure but alternatively may be a stack of individual objects that are arranged for having the structural porosity and surface density as identified above. Such objects may be three dimensional objects. The objects may for example be fibers. An advantage of using a reticulated structure is that it allows providing a higher porosity, a larger contact area and a better break down of fluid flows counteracting the formation of droplets. Furthermore, it may provide a firm structure that is resistant to clothing.

The device 100 further comprises a supply unit configured to supply said hygroscopic fluid 142 to said structure 150. As a minimum, the supply unit may comprise a tube segment with at least one opening or at least one nozzle or the like, for providing the hygroscopic fluid. In the embodiment of FIG. 2 the supply unit comprises a plurality of nozzles 147 for evenly spreading the hygroscopic fluid over the structure. But the supply unit may also include further components, such as e.g. a reservoir for collecting the hygroscopic fluid, and/or a pump system, etc., none of which are shown in the drawings.

The supply unit keeps the surface, e.g. surface segments, wetted with the hygroscopic liquid. The supply unit may function time-continuously or in a periodic manner, or in a time- interleaved way.

The hygroscopic fluid may e.g. be a salt solution, in particular a non-saturated salt- solution. The hygroscopic fluid may be sprayed on the structure material. The gas flow and the liquid flow may be arranged in a so called counter-flow manner. Preferably the hygroscopic fluid is thereby sprayed at an upper part of the device onto the structure, and flows down due to gravitation, whereas the gas is preferably entering the device at a lower part of the device, underneath the structure, and leaving the device at an upper part of the device, after passing through the structure material. It is noted that a counter-flow arrangement of the gas and the hygroscopic liquid typically provides a higher exergy efficiency than a cross flow. However, a counter-flow arrangement is not absolutely required, and the invention would also work with a cross-flow arrangement, or even with a parallel flow arrangement.

The structure 100 may further comprise a perforated floor, or a grating or a strainer, or the like, where the structure 141 is resting upon. Underneath the strainer or grating or perforated floor 144, or the like, an air distribution matrix could be foreseen, for distributing the gas over the bottom of the structure. Also provisions for draining the salt solution could be foreseen (not shown).

One of the key elements of embodiments of the present invention is the structure used to act (during use) as the "contact medium" (also referred to as "matrix material") for allowing an exergy efficient heat-mass transfer between the gas and the hygroscopic fluid.

The main characteristics of the structure are: "1) a structural porosity of at least 60%, "2) a surface density of at least 100 m ² /m ³ , and (3) comprising or consisting of a material where the hygroscopic fluid is spreading itself over and flowing downwards in a thin film of maximum 0,3mm preferably 0,05mm. The latter may e.g. be a salt solution.

In one set of embodiments, the structure is a reticulated structure, which may also be referred to as a netted structure. One example thereof is an open cell foam. Such reticulated structure may be a PU foam, a wood fiber / plastic composite foam, etc. FIG. 4 illustrates an example of such a reticulated structure.

In another set of embodiments, the structure is a stack of objects, the objects typically being 3D objects. One particular example thereof are long i.e. in length at least once the height of the structure fibers such as cocos fiber or other natural fibers. The stacking may be performed randomly.

In an embodiments, the structure 141 may comprise or consist of a plurality of stacked individual elements. The individual elements should be non-planar, and may have an irregular shape. Then also the structure, and more particularly the voids thereof, and the channels formed by those voids, have an irregular shape. Then also the path of the gas flow through the matrix is not straight, but is continually changing direction.

The structure has a porosity (referred to herein as "structural porosity") of at least 60% through which the gas flow can pass, e.g. in the direction indicated by the arrow. In preferred embodiments of the present invention, the individual elements in additional also have a material porosity (not visible in the drawing), e.g. pores with a diameter of e.g. 0.1 mm or less, with a capillary effect, and helps to keep large parts of the surface wetted, even parts where the salt solution would normally not arrive if only gravitation and the gas flow are taken into account. The elements in the stack may all have the same size and shape, or may have different sizes and shapes. It is to be noticed that where reference is made to material porosity, reference is made to the porosity of the material the structural elements are made of, not to the pores between the structural elements.

These elements are wetted with a thin layer of hygroscopic liquid, and the gas comes into contact with these wetted surfaces. This results in a low mass transfer resistance in the gas flow, or a high mass transfer coefficient. Wetting may be rendered more easy by making/selecting the structure to be hydrophilic. Hydrophilic materials typically have a low contact angle for liquids and in particular for water of less than 90°. FIG. 3 illustrates how a contact angle of a liquid on a surface is typically measured. In the example shown, the contact angle Θ is about 50°. The smaller the contact angle, the better the "wetting" of the surface, and in the context of the present invention, the larger the effective surface density , so that the diffusion can take place over a larger total interface are, resulting in an improved heat and mass exchange. In embodiments of the present invention, the contact angle of the hygroscopic fluid on the surface of the structure material is lower than 90°, e.g. around 77°, e.g. below 45°, e.g. below 15°, which means that there is good "wetting" of the material of the structure, so that the hygroscopic liquid will spread out over the surface rather than forming individual droplets. This is beneficial for an improved mass transfer between the gas and the hygroscopic liquid, hence an improved (de)humidification can take place.

The gas flow can pass through said structure 141 by means of a pressure drop between the gas inlet 120 and the gas outlet 122. The hygroscopic fluid can pass through said structure by means of gravitational force. Besides having voids forming channels, the structure 141 also provides surface area(s), whereto the hygroscopic fluid can adhere, so as to form a thin film. In some embodiments, the material is chosen to be impregnable. By choosing a material which is impregnable by the hygroscopic fluid, such as e.g. wood, a film of hygroscopic liquid is created, rather than droplets. This increases the total surface area through which the diffusion between the hygroscopic liquid and the gas has to take place.

Using a slightly higher salt concentration e.g. provides a somewhat drier gas. So salt concentration in chosen especially but not limiting in function of the desired humidity of the output gas. However, when using a structure as specified above as the contact medium, the humidity of the incoming gas, and the concentration of the salt in the hygroscopic fluid can vary over relative large ranges, while operating at good, e.g. increased exergy conditions as compared to the prior art.

As indicated above, the heat mass exchanger may be used in a larger system, e.g. an air-conditioning system for buildings, industrial, agricultural applications.

In one aspect, the present invention also relates to a method for humidifying or dehumidifying a gas. The method thereby typically comprises the steps of contacting a hygroscopic fluid with a structure being a reticulated structure or a stack of individual objects, the structure having a structural porosity of at least 60% through which the gas flow can pass, the structure (141) having a surface density accessible for the gas flow of at least 100 m ² /m ³ , and passing a flow of said gas through the structure for inducing a mass transfer between the gas flow and the hygroscopic fluid. In certain embodiments, the method may comprise controlling the flow rate such that the ratio of the hygroscopic capacity of the liquid flow and the hygroscopic capacity of the gas flow over the structure is a value in the range of 0.20 to 10.0 , e.g. in a range 0.5 to 5. Other features and advantages may be as expressed by the functionality of components of an exchanger as described in the first aspect.

In one exemplary embodiments, the following steps may be performed, whereby reference is made to FIG. 2. The method comprises the steps of passing a flow of gas, e.g. air through a structure 144. The method also comprises, before and/or during operation, supplying a hygroscopic fluid 142, e.g. a hygroscopic salt solution to said structure 150. The structure, e.g. reticulated PU foam, thus may be submerged or sprayed in a salt solution before placing them in the device 100, but that is not absolutely necessary, and they may also be placed in the device in unwetted state, which is of course much more practical.

In the example shown in FIG. 2, the hygroscopic fluid is sprayed on top of the structure by means of a plurality of nozzles. The salt solution ending at the bottom of the reservoir is removed by an outlet (not shown), and is preferably regenerated, so that it can be reused. The regeneration itself may or may not be part of a method according to the present invention. For the purpose of dehumidification, the combination of embodiments of the present invention with an efficient regeneration method will result in an efficient dehumidification device. During operation of the device, the parameters of the gas flow (e.g. mass flow rate, temperature and humidity) can typically not be altered, as they are determined by external factors, and are therefore typically considered as given parameters (within a certain range). The parameters of the structure 141, such as e.g. the material, stack thickness, etc, and the type of hygroscopic fluid, are typically determined during the design stage, and are normally fixed for the lifetime of the apparatus. What can be tuned during operation however, are the parameters of the hygroscopic fluid, such as e.g. the flow rate, the salt concentration at the inlet, etc. According to embodiments of the invention, these parameters are chosen as a function of an optimization between exergy efficiency and dehumidification capacity. Absolute maximization would require that these parameters are constantly adapted. Nevertheless it was found that a structure 141 having the characteristics as described above (related to porosity and surface density) provide a very good exergy efficiency of at least 25% for an input gas flow within a relatively large temperature range and humidity range. In another aspect, the present invention also relates to the use of a heat-mass transfer device as described in the first aspect for humidifying or dehumidifying a gas flow using a hygroscopic fluid.

In another aspect, the present invention relates to the use of such a heat-mass transfer device as part of a vapor heat pump. Such a vapor heat pump may be a device, advantageously highly efficient, that transforms energy of vapor in the gas (latent heat) to sensible heat, thereby using some, advantageously small, exergy inputs. In such a vapor heat pump there is at the input a) the wet air at a low temperature and b) some other exergy input e.g. in the form of electricity, at the output there is dry air at a higher temperature. Such a vapor heat pump can exist of the heat mass exchanger as described in the present invention and a regeneration unit for the diluted salt solution e.g. a mechanical vapor compression unit. Such a vapor heat pump has the following working schema: the input of the vapor heat pump is humid wet air and some other exergy input. This humid air together with a concentrated salt solution coming from the regeneration unit enters the heat mass exchanger. The output of the heat mass exchanger is dry air and diluted salt solution. The dry air goes out of the vapor heat pump. The diluted salt solution enters the regeneration unit together with some other exergy input. The output of the regeneration is concentrated salt solution what will be used as input for the heat mass exchanger as described. EXAMPLE:

In a particularly advantageous embodiment, the heat-mass exchanger 100 has the following combination of features: a high surface density preferably more than 200 m ² /m ³ ; a (structural) porosity preferably more than 60%; a not organized structure 141 consisting of elements with physical (irregular) shapes and size leading to the high (structural) porosity, e.g. reticulated PU foam. The material has surface characteristics (e.g. pores) leading to low contact angles of the hygroscopic fluid, and causing the fluid to spread over the wetted surface thereby forming a thin surface layer and keeping the material wetted, thus causing a high effective surface density. The material may have a certain roughness, and irregular form. Preferably preferential paths for the salt solution and for the gas through the volume are avoided. Such system is preferably used in a counter-flow arrangement with the ratio of the hygroscopic capacity of the liquid flow and the hygroscopic capacity of the gas flow over the structure is a value in the range of 0.5 to 5. The salt solution at the output may be substantially diluted compared to the input (in this case more than 10% gain of weight due to water absorption). This results in the vapor pressure of the gas output similar to the equilibrium vapor pressure of the salt solution in the upper part of the matrix as presented in Fig. 6. (and vice versa). Such a system has a high exergy efficiency of more than 20%. The same device 100 can selectively be used for humidification or for drying. In the latter case, gas to be humidified absorbs water from the salt solution on the matrix.

The main advantages of such a system are: Good transfer characteristics due to low resistances and high effective contact surface; a compact system (the thickness of the structure can be as low as e.g. 10 cm); reduced energy required to pump around the salt solution (less mass flow is required); reduced ventilation power required due to the low pressure drop; use of a cheap material (e.g. reticulated PU foam); the material is abundantly available; use of organic material (environmental friendly, not toxic; e.g. cocos fibers); use of material that is easy to recycle or to replace; a good exergy efficiency. By way of illustration, embodiments of the present invention not being limited thereto, some simulation results illustrate the advantage of embodiments of the present invention.

By way of illustration, some results are further discussed below. FIG. 5 illustrates the salt solution to gas mass ratio (M sola) and the salt to gas mass ratio (MRsa) for CaC in the case equal hygroscopic capacities as can be used in embodiments of the present invention. The air is at 20°C and RH on the x-axis stands for relative humidity. FIG. 6 illustrates the behavior of a salt solution and air throughout a matrix in a counter flow arrangement, as can be used in embodiments of the present invention. Ta is temperature air, Ts is temperature of the salt solution, RHa is relative humidity of air, RHs is the equilibrium relative humidity of the salt solution. The x-axis indicates the place in the matrix expressed in m with the bottom 0. The left y-axis indicates the temperatures of air and salt solution, and the right y-axis indicates the relative humidity of the air and salt solution.

Two exemplary test results illustrate embodiments of the present invention. Tests have been done in a prototype version with a matrix frontal area of 0.14m ² , which is small compared to the targeted application but an appropriate surface for testing. The matrix itself constituted of PU foam (FIG. 7 on the left, corresponding with FIG. 4) and a natural fibre (FIG. 7 on the right). The PU foam has a diameter of the pore opening between l-5mm and a pore diameter of about 5mm. Its porosity is around 97% and its surface density 360 m ² /m ³ . The stack of natural fibers consisted of fibers with a thickness of about 1mm. The fibers touch each other at some places and leave pore openings at other of around 5mm, the pores have random shapes but its diameter is at least in the order of 1mm. The porosity is around 93% and the surface density is about 300m ² /m ³ . The fibers form a totally random stack.

These materials were tested under the following conditions:

• the input air is around 21°C and 86%RH (relative humidity). Under this conditions, the air has a hygroscopic capacity of 0.0052kg per kg of input air.

· the input salt solution has an equilibrium relative humidity of around 48%, and has a temperature of 21°C. Its hygroscopic capacity is around 1.14kg per kg of salt solution. The variable testing conditions were

• A layer thickness of 5 and 10 cm

• An air velocity of around 0.4 and 0.8 m/s

· A Mass ratio of salt solution to air of around 1/100 and 1/200. This results in a ratio of hygroscopic capacities of salt solution flow divided by air flow (MR x hygroscopic capacity of salt solution /hygroscopic capacity of air) of respectively 2.2 and 1.1, or -as targeted to allow for maximum efficiency- the hygroscopic capacities are almost equal. During the experiments we measured

· The temperatures of the salt solutions and air flows at in/output.

• The RH of the in/output airflow

• The density of the in/output salt solution.

• The volumetric air flow Input conditions PU-foam Natural fiber u _a (m/s) H M R a/r dehum ΠΕΧΗΜΕΧ a/r dehum ΠΕΧΗΜΕΧ

(m) (kg/kg) (1/s) (W/m ² ) (-) (1/s) (W/m ² ) (-)

0.4 5 1/100 8.3 1866 0.63 5.9 1681 0.57

0.4 5 1/200 8.3 1554 0.63 6.2 1485 0.61

0.8 5 1/100 13.6 3754 0.51 9.5 3045 0.48

0.8 5 1/200 11.7 2976 0.50 10.0 2661 0.51

0.4 10 1/100 3.9 2013 0.55 4.7 2259 0.57

0.4 10 1/200 3.5 1730 0.55 4.4 1906 0.59

0.8 10 1/100 6.1 3523 0.43 7.2 3813 0.45

0.8 10 1/200 5.3 3034 0.40 6.7 3386 0.44

Table: performance for PU foam and natural fiber as matrix material in a heat mass transfer system calculated from experimental values. Variables: u _a stands for air velocity, H stands for height of matrix, MR stands for mass ratio of salt solution to air, a/r is the volumetric combined heat/mass transfer coefficient expressed in 1/s, dehum is the realized dehumidification in the testing per m ² frontal area, ΠΕΧΗΜΕΧ is the exergy efficiency.

Based on this 9 measured variables, following performance variables could be calculated:

• a/r: The volumetric combined heat/ mass transfer coefficient expressed in 1/s. The heat and mass transfer in the matrix can be calculated based on this value.

• dehum: The dehumidification in the experiment expressed per m ² frontal area (W/m ² )

• nEX: The exergy efficiency of the dehumidification process. This values are calculated by dividing the exergy value of dehumidified air with the exergy input of salt solution and ventilation.

The conclusions are

• A higher air velocity results in higher volumetric heat and mass transfer coefficient (a/r). • A higher MR results often in an enhancement of the volumetric heat and mass transfer coefficient (a/r). This can be contributed to the better wetting of the matrix material by the longer spraying.

• A higher packing height does not or not much enhance the mass/heat transfer. This is due to the fact that the spraying of the salt solution on the matrix results in a top surface that is covered with a salt solution layer. During the rinsing down of the salt solution, the covering of the surface will be less resulting from the formation of preferential channels. This explains that a higher packing height do not or nor much result in higher heat mass transfer. A higher packing height, however, does results in an increased pressure drop. As a result, the system becomes less efficient.

• PU foam has better mass transfer characteristics (a/r, dehum) than natural fibers. The pressure drop of PU-foam and natural fibers are presented in the graph below and are sufficiently low due to the high porosity and large pore diameters/openings, allowing the air to circulate through the bed with low exergy losses. In FIG. 8, the pressure drop is shown for 5 cm of two matrix materials at frontal air velocity (full line for PU foam, dotted line for natural fiber). FIG. 9 illustrates the extrapolated dehumidification performance of 5cm PU foam. Dehumidification capacity (D) of 5cm PU foam sprayed with a salt solution of 48%eqRH and RH input air of 87.5%. msal stands for 0.001217 kg/s/m ² of the salt solution. MR stands for mass ratio between salt solution and air. Values calculated based on extrapolation of experimental results.

From the graph can be concluded

• For a given MR, hence a given ratio of hygroscopic capacities, is dehumidification almost linear to air velocity.

• Dehumidification can be up to 5000W per m ² of surface material for lm/s air velocity.

Of course more concentrated salt solution, more humid air and a higher air velocity can result in an even higher dehumidification.

Furthermore, from these curves, at an air flow rate of 0,2m/s the capacity per m ² frontal area is 1000W, for realistic applications of about lm ² , this allows to dehumidify 720m ³ per hour. Using embodiments of the present invention as dehumidifier, the outgoing air typically is more dry and more warm than the incoming air. REFERENCES:

100 heat mass transfer device

120 input gas flow

122 output gas flow

140 heat mass transfer unit

141 structure, e.g. reticulated PU foam

142 hygroscopic fluid

143 spray elements

144 strainer

145 hygroscopic fluid output

146 hygroscopic fluid input

147 plurality of nozzles