Patent Application
20080085437
The present invention relates to a heat and humidity exchanger. The heat and humidity exchanger includes a cartridge comprising a pleated water-permeable membrane. Flow field elements are disposed within the folds of the pleated membrane to direct the flow of fluid streams across opposing surfaces of the membrane between at least one inlet and outlet on each side. In addition to defining the fluid flow paths within the folds, the flow field elements provide support for the membrane and control the pleat spacing within the cartridge. The flow field elements improve the flow distribution of the fluids across the membrane surfaces, improve the heat and humidity exchange and reduce the pressure drop, therefore allowing the exchanger to be more compact. The heat and humidity exchanger is particularly suitable for fuel cell applications and energy recovery ventilator (ERV) applications.
Heat and humidity exchangers (also sometimes referred to as humidifiers) have been developed for a variety of applications, including building ventilation (HVAC), medical and respiratory applications, gas drying, and more recently for the humidification of fuel cell reactants for electrical power generation. Many such devices involve the use of a water-permeable membrane via which moisture and, provided there is a temperature differential across the membrane, heat is transferred between fluid streams flowing on opposite sides of the membrane.
Planar plate-type heat and humidity exchangers use membrane plates that are constructed of planar, water-permeable membranes (for example, Nafion®, cellulose, polymers or other synthetic membranes) supported with a separator material and/or frame. The plates are typically stacked, sealed and configured to accommodate intake and exhaust streams flowing in either cross-flow or counter-flow configurations between alternate plate pairs, so that humidity (and often heat) is transferred via the membrane. Other types of exchangers such as hollow tube and enthalpy wheel humidifiers typically use high cost materials and are generally less compact than plate-type devices. Hollow tube humidifiers also have the disadvantage of high pressure drop, and enthalpy wheels tend to be unreliable because they have moving parts.
A heat recovery ventilator (HRV) is a mechanical device that incorporates a heat exchanger with a ventilation system for providing controlled ventilation into a building. The heat exchanger heats or cools the incoming fresh air using the exhaust air. Devices that also exchange moisture between the two air streams are generally referred to as Energy Recovery Ventilators (ERVs), sometimes also referred to as Enthalpy Recovery Ventilators. An ERV removes excess humidity from, or adds humidity to, the ventilating air that is being brought into a building. The two primary reasons to install an ERV are energy savings and improving indoor air quality in buildings.
In order for buildings to have good indoor air quality they require an exchange of the stale indoor air with fresh outdoor air. In hot and humid climates, energy is wasted when the cooled air from the building is exhausted. In an ERV the exhaust air can be used to cool the warmer air being brought in from the outside, reducing the need for air conditioning. The energy consumption of air conditioners can also be reduced (for example, by as much as 30-50%) by having an ERV remove water vapor from the incoming air, reducing the load on the system cooling the air. If buildings are too humid, ERVs will lower humidity levels, reducing the likelihood of mould, bacteria, viruses, and fungi which cause sickness, absenteeism and lost productivity. On the other hand, in cold dry climates, energy is wasted when the warm air from the building is exhausted, plus there can be an additional issue of the incoming air stream being too dry. As well as transferring heat from the exhaust air to the incoming air, ERVs can be used to recycle water vapor from the exhaust air stream, raising humidity levels, thereby reducing skin irritation and respiratory symptoms caused by dry air.
Thus, in ERVs heat and humidity is transferred between the exhaust air stream of a building and the intake air stream of that building. ERVs typically comprise an enclosure, pumps or fans to move the air streams, ducting, as well as filters, control electronics and other components. The key component in the ERV which transfers the heat and humidity between the air streams is called the core. The two most common types of ERVs are those based on rotating enthalpy wheel devices and those based on planar membrane plate-type devices, both mentioned above. Enthalpy wheel ERVs (also known as energy wheels) typically have a cylindrical honeycomb core that is coated with desiccant. A motor rotates the cylinder, transferring the heat and humidity between the intake and exhaust air streams that are typically directed in a counter-flow configuration through opposite ends of the core. Generally the size and depth of the wheel, along with its rotation speed, will determine the degree of energy recovery. Planar plate-type ERV cores use layers of static plates that are sealed and configured to accommodate the intake and exhaust streams flowing in either cross-flow or counter-flow configurations between alternate plate pairs.
In fuel cell applications, various approaches have been used to increase the humidity of reactant gas streams supplied to a solid polymer fuel cell, such as a proton-exchange membrane (PEM) fuel cell. For example, some conventional systems humidify a reactant gas stream by flowing the reactant gas stream and liquid water on opposite sides of a water-permeable membrane. Water from the liquid stream is transferred through the membrane, thereby humidifying the reactant gas stream. Such liquid water-to-gas humidifiers are commonly used in solid polymer fuel cell systems in which water is used as a cooling fluid, as the cooling water is a convenient source of water for the humidifier. Other conventional approaches for humidification of fuel cell reactant gas streams include the injection of water vapor or atomized water droplets into the reactant stream, without the use of a membrane-type humidifier. Enthalpy wheels have also been used.
It has been shown that in some cases the reactant gas supply streams for solid polymer fuel cells can be heated and humidified using heat generated by the fuel cell and water vapor from the fuel cell exhaust. For example, the heat and water vapor in the oxidant exhaust stream can be used to heat and humidify an incoming reactant supply stream (typically the oxidant) by flowing the inlet stream and the fuel cell oxidant exhaust stream on opposite sides of a water-permeable membrane in a gas-to-gas humidity exchanger. Such an approach is described in U.S. Pat. Nos. 6,106,964; 6,416,895; and 6,783,878. Different membrane-based humidity exchanger constructions can be used for this application, including jelly roll configurations and tube bundle configurations, as well as the planar membrane plate-type designs mentioned above. In the latter, discrete sheets of water-permeable membrane are sandwiched between pairs of plates. Aligned openings in the stacked plates and membranes form internal fluid manifolds for supplying and exhausting the reactant streams to fluid passages that can be formed in the surfaces of plates to direct and distribute the streams across the respective membrane surface.
A benefit of planar plate-type heat and humidity exchanger designs for fuel cell, ERV and other applications, is that they are readily scaleable as the quantity (as well as the dimensions) of the modular membrane plates can be adjusted for different end-use applications. However, in this type of device there is a large number of joints and edges that need to be sealed. As a result such devices can be labor intensive and expensive to manufacture. Also their durability can be limited, with potential delamination of the membrane from the frame and failure of the seals resulting in leaks and poor performance and cross-over contamination (leakage between streams). In ERV applications existing planar plate-type ERV cores typically do not produce the total enthalpy exchange required and they are typically not durable in cold weather climates.
Another approach to heat and humidity exchanger design is to incorporate a pleated water-permeable material in the exchanger. For example, U.S. Pat. No. 4,040,804 describes a heat and moisture exchanger for exchanging heat and moisture between incoming and outgoing air for room ventilation. The exchanger consists of a pleated sheet of water-permeable paper. Air is directed in one direction along the pleats on one side of the pleated paper, and the return air flows in the opposite direction along the pleats on the other side of the pleated paper. The ends of the pleated cartridge are closed by dipping them in wax or a castable potting compound which adheres to the paper. The pleats are separated or spaced, and air passages between the folds are provided, by adhering grains of sand to the pleated paper.
A humidifying apparatus with a similar pleated design, but this time for fuel cell applications, is described in U.S. Patent Application Publication No. 2007/0007674. The apparatus includes a pleated structure comprising a humidifying membrane with a gas-permeable reinforcing material superimposed on one or both surfaces thereof. The reinforcing material imparts a self-supporting property to the pleated structure and maintains the pleat pitch or separation between the membrane pleats, as well as allowing the introduction of gas into the inner portion of the pleated structure. The reinforcing material is a netted or porous sheet material, for example, a woven or non-woven fabric, or preferably a resin or metal net. Secured to the pleated structure around a periphery thereof is a frame.
A primary advantage of such pleated designs is that the manufacturing is simplified. Established continuous pleating manufacturing processes can be used. This allows pleated membranes to be fabricated at lower cost than the planar membrane plates used in conventional membrane plate-type devices, where each layer has to be assembled from discrete pieces of membrane, a support material and frame, and then the layers assembled and sealed together. The fact that there are fewer seals required reduces the potential for leaks, so pleated membrane heat and humidity exchangers also tend to be less prone to failure.
Although the reinforcing material used in the pleated humidifying apparatus, described in the aforementioned patent application, allows introduction of gas into the inner portion of the pleated structure, it does not provide controlled or directional gas flow distribution over the membrane surface. Furthermore, the fluid flow paths through or across such materials tend to be quite tortuous and turbulent, so the flow can be quite restricted and the pressure drop across the apparatus can be significant.
Performance can be improved, and the required heat and humidity exchanger size can therefore be reduced, by enhancing flow distribution across one or both membrane surfaces, controlling the relative flow paths of the two streams on opposite sides of the membrane and reducing the pressure drop across the exchanger, and/or using a membrane with improved water transport and other properties.
In one aspect of the invention, a pleated membrane cartridge comprises a pleated water-permeable membrane having a plurality of folds on each side thereof. At least some of the folds on at least one side of the pleated membrane have a flow field element disposed therein. In preferred embodiments the cartridge also comprises a perimeter seal around the perimeter of the pleated membrane.
In another aspect, a heat and humidity exchanger for transferring heat and moisture between a first fluid stream and a second fluid stream comprises a housing and pleated cartridge enclosed within the housing. The housing has a first inlet port, a first outlet port, a second inlet port and a second outlet port. The pleated cartridge comprises a pleated water-permeable membrane. First flow field elements are disposed in some or all of the folds on one side of the pleated membrane for directing the first fluid stream within the folds, from the first inlet port to the first outlet port. Optionally, second flow field elements are disposed in some or all of the folds on the other side of the pleated membrane for directing the second fluid stream within the folds, from the second inlet port to the second outlet port.
The flow field elements define a plurality of discrete fluid flow channels within the folds of the pleated membrane cartridge. In preferred embodiments the flow field elements are individual discrete structures that are disposed in each of the folds. They can be attached to the membrane or not. The membrane is water-permeable, typically substantially gas impermeable, can be pleated and is suitable for use in fuel cell applications, ERV applications, or the particular end-use application. In addition to more conventional water-permeable membranes, porous membranes that contain one or more hydrophilic additives or coatings can be used.
The heat and humidity exchanger can be used in fuel cell applications. In an embodiment of a solid polymer fuel cell system, the first inlet port is connected to a reactant exhaust port of a solid polymer fuel cell, and the second outlet port is connected to a reactant inlet port of a solid polymer fuel cell. The reactant exhaust port is typically the cathode-side (oxidant) exhaust port and the reactant inlet port is typically the cathode-side (oxidant) inlet port, but either or both ports can be on the (fuel) anode-side. The heat and humidity exchanger can also be used in an energy recovery ventilator (ERV) for transferring heat and humidity between air streams entering and exiting a building.
In another aspect of the invention, a method for transferring humidity between a first fluid stream and a second fluid stream comprises flowing the first and second fluid streams on opposite sides of the pleated membrane cartridge described above. Heat is also transferred between the first and second fluid streams if there is a temperature difference between them. In an embodiment of a fuel cell application, the first fluid stream is a reactant supply stream for fuel cell and the second fluid stream is reactant exhaust stream from a fuel cell. In an embodiment of an ERV application the first fluid stream is an intake air stream for a building and the second fluid stream is an exhaust air stream for a building.
In another aspect of the invention, an ERV core comprises a porous membrane with at least one hydrophilic additive. In a corresponding method for transferring heat and humidity between an intake air stream and an exhaust air stream of a building, the intake and exhaust air streams are directed on opposite sides of the ERV core comprising a porous membrane with at least one hydrophilic additive.
a-c illustrate flow field elements of similar designs but with varying size and numbers of channels.
a illustrates an example of a flow field element with an array of linear channels.
a illustrates a flow field element that is tapered in a direction that would be oriented parallel to the fold of the pleat in a pleated membrane cartridge.
The present invention relates to a heat and humidity exchanger comprising a pleated membrane cartridge that is enclosed in a housing. The cartridge comprises a pleated water-permeable membrane, sealed around the perimeter, and flow field elements that are disposed within the folds (pleats) on at least one side of the pleated membrane. Folds on the other surface of the water-permeable membrane can also have flow field elements disposed therein. The membrane fold wraps around and generally contacts both faces of the flow field element. When there are flow field elements on both sides of the membrane, the flow field elements are effectively interleaved on opposite sides of the pleated membrane.
The flow field elements are for directing fluid streams across the surface of the membrane that is in contact with the element. The open-channels of the flow field element offer less restriction to fluid flow than the netting or meshes used to support the membrane in existing pleated membrane-type heat and humidity exchangers. They also provide more controlled and uniform distribution of the fluid stream over the membrane surface. This improves heat and humidity transfer and reduces pressure drop allowing the exchanger to be more compact for a given humidification capability. In addition, the flow field elements can provide support for the pleated structure, in particular against differential pressures occurring across the membrane from one side to the other. They also provide a convenient way to control or define the spacing between adjacent pleats, and assist in maintaining a consistent pleat pitch (with little or no bunching of pleats) in the membrane cartridge—this can also result in improved performance and a more compact device.
The present heat and humidity exchanger design, comprising a pleated water-permeable membrane cartridge with flow field elements, is particularly suitable for fuel cell applications where it can be used to humidify a reactant stream supplied to a fuel cell. The drier reactant supply stream is directed across one side of the pleated cartridge and a moist stream is directed across the other side, so that water and typically heat is transferred to the supply stream via the water-permeable membrane. The moist stream can be an exhaust stream from the fuel cell, and is preferably the oxidant exhaust stream.
The present pleated membrane cartridge with flow field elements can also be used as the core in energy recovery ventilators (ERVs) for transferring heat and humidity between air streams entering and exiting a building. This is accomplished by flowing the streams on opposite sides of the pleated membrane cartridge. The membrane allows the heat and moisture to transfer from one stream to the other while substantially preventing the air streams from mixing or crossing over.
The present pleated membrane cartridge with flow field elements can also be used in gas drying applications. Another possible application is removing humidity from a contained area occupied by humans (for example, a military tank) without exchanging gases. The present design may also be useful in other applications where moisture and/or heat need to be transferred between fluid streams without significant gas cross-over occurring.
In embodiments where heat and humidity exchanger 10 is coupled to the cathode side of an operating fuel cell system (not shown in
Heat and humidity exchanger 10 can also be used in an ERV application for exchanging heat and humidity between air streams being directed into and out of a building (not shown in
The pleated membrane cartridge can be fabricated using established pleating manufacturing processes. Because the pleating process is typically a continuous process, the pleated elements can be fabricated at lower cost than planar membrane plates.
The perimeter of the pleated membrane is sealed to prevent the fluids from leaking between the pleated membrane cartridge and the interior of the housing, and from one side of the cartridge to the other. For example, the cartridge can comprise a gasket or seal disposed around the perimeter of the pleated membrane. Additional gaskets or seals can be disposed against the interior of the housing between the inlet and outlet ports to prevent short-circuiting of the fluids between where the pleated cartridge and the housing meet. Such sealing features are not shown in the simplified illustration of
Two possible flow configurations for directing the fluid streams across the pleated membrane cartridge are illustrated in
In
In
Flow field elements can be disposed within some or all of the folds of the pleated membrane cartridge in any of the embodiments illustrated in
In the embodiment illustrated in
a-c shows how a flow field element of one basic design can be scaled for different applications.
A disadvantage of having flow field elements with a U-shaped flow channel design on both sides of the pleated membrane as shown in
Embodiments with the flow configuration shown in
As will be clear from the foregoing description, a common flow field element design can be used throughout the pleated membrane cartridge (so that the flow field elements used on the wet side of the membrane are the same as those used on the dry side). Alternatively different flow field element designs can be employed within a single cartridge. For example, flow field elements used on one side of the pleated membrane can have a different flow field path or channel pattern than those used on the other side of the pleated membrane. Similarly, flow field elements used on one side of the membrane can be of one thickness, and the flow field elements used on the other side of the membrane can be of a different thickness. Either of these variations from one side of the membrane to the other could allow, for example, for preferential pressure drop compensation for the wet side fluid flow based on viscosity and volumetric flow differences in the wet and dry fluid streams. There could be other differences between the flow field elements used on the wet side and dry side of the membrane. The designs and relative orientation of the separators on opposite sides of the membrane can also be selected to improve the structural support provided to the membrane.
Similarly, even on one side of the membrane different flow field elements can be used in different folds. This approach could be used, for example, to balance flow distribution across the cartridge if there would otherwise be uneven fluid supply to the different pleats from the supply manifold or header.
The flow field element is typically a planar structure that is essentially the same thickness across its entire area. However, in some embodiments its thickness can vary. In some such embodiments it is a tapered structure. For example, it can be tapered in the direction that is oriented parallel to the direction of the fold, so that it is thicker in the inlet region and thinner in the outlet region or vice versa. For example, it may be advantageous if the flow field element is thicker (with a correspondingly larger flow channel cross-sectional area) where the stream is wetter, and thinner where the stream is drier. An example of such a tapered flow field element 600 is shown in
The flow field element can be fabricated from a polymer, such as polypropylene, or any other suitable material. The material can be fluid impermeable or porous. Porous materials may allow some fluid access to occur even where the flow field element actually contacts the membrane. The material can be rigid or flexible, but preferably has structural properties that allow the flow field element to provide a desirable degree of membrane support and pleat pitch consistency as described above. The flow field element can be fabricated using a variety of conventional techniques including, but not limited to, injection molding, stamping and possibly extrusion. In some embodiments the flow field elements could be printed or formed directly on one or both sides of the membrane, for example, prior to pleating. This would reduce or eliminate the need for interconnecting regions between ribs, and the flow field element would not need to be able to be a self-supporting (free-standing) structure. Another approach would be to form the flow field element or channel pattern in the membrane material itself so it is integral to the membrane. An example would be an extruded membrane with ribs on at least one side serving as linear flow channels.
In preferred embodiments, the flow field elements are discrete structures that are inserted into individual folds in the pleated membrane. These can be introduced into the folds (on one or both sides of the membrane) during the membrane pleating process or inserted afterwards, either manually or as part of an automated manufacturing or assembly process. They can be bonded or attached to the membrane or not.
In alternative embodiments, multiple flow field elements could be formed in one piece or be joined together. Such multiple flow field elements could be layered or laminated against one or both sides of the unpleated membrane (with or without actual bonding) and pleated along with it, or they could be inserted into a series of adjacent folds after the membrane has been pleated. Again they can be bonded or attached to the membrane or not.
The channel design of the flow field element provides a valuable and convenient way of controlling the flow distribution within the heat and humidity exchanger. The illustrated embodiments show just a few examples of the types of flow field channel designs or patterns that can be used in a flow field element. As with fuel cell reactant flow field plates, a wide variety of channel designs can be used in the heat and humidity exchanger flow field element. For example, the flow field element need not have a symmetrical flow channel pattern. The channels do not have to be predominantly linear channels (although typically these will provide a shorter path and therefore lower pressure drop). The flow field element does not necessarily define a U-shaped flow path for the fluid. The channel width, depth or cross-sectional area can vary along its length. There can be more than one inlet and/or outlet region per flow field element. There are many other possible variations, however the design of the flow field element preferably seeks to enhance heat and humidity exchanger performance (that is affected by flow distribution, pressure drop, and active area access, among other things) while taking into account the desired structural functions of the flow field element (including membrane support, pleat spacing), as well as other factors including manufacturability, cost, and durability.
The design parameters of the pleated membrane component also affect the performance of the heat and humidity exchanger. For example, the pleat geometry and aspect ratio (pleat length, width, pitch, and so forth) affect the performance of the exchanger. These parameters can be adjusted and selected, in combination with the design of the flow field element(s), to give the desired heat and humidity exchanger performance, pressure drop, and exchanger volume, for a particular application.
In combination with careful selection of the pleated membrane and flow field element design parameters mentioned above, operational parameters can also be adjusted and selected to give desirable heat and humidity exchanger performance. The flow field elements described herein allow operational parameters such as residence time and diffusivity of water vapor in the streams in the heat and humidity exchanger to be controlled more readily and accurately than with conventional homogeneous membrane support materials, such as netting or meshes.
In contrast to conventional membrane support materials, flow field elements are engineered components that can be carefully designed to improve heat and humidity exchange across the membrane, and thereby make the humidity exchanger more volume efficient. Nonetheless, they are typically simple to manufacture and incorporate into the pleated membrane heat and humidity exchanger.
Heat and humidity exchangers of the design described herein are particularly suitable as gas-to-gas humidity exchangers, where moisture is transferred from a wetter to a drier gas stream via the water-permeable membrane. However, the heat and humidity exchangers could be used with a liquid stream (for example, liquid water or some kind of aqueous stream) on one or both sides, or with a stream that has two-phase flow. Thus, references to “fluid” herein generally refer to a gas stream, but are also intended to encompass liquid streams, or streams in which there is two-phase flow.
The exchanger housing can be constructed from a hard polymer material (typically a rigid plastic) or from a metal such as aluminium. The housing has at least two inlet ports and at least two exhaust ports. Typically there is a single inlet and outlet port for the wet stream and, and a single inlet and outlet port for the dry stream, although in some embodiments there could be multiple ports for some or all of the streams; for example, there could be additional ports to further reduce the pressure drop across the exchanger.
The pleated membrane can be any type of a membrane that is water-permeable and suitable for use in fuel cell applications, ERV applications, or the particular end-use application, provided that it can be pleated. In addition to more conventional water-permeable membranes, porous membranes with a thin film coating that substantially blocks gas flow but allows humidity exchange can be used. Also porous membranes that contain one or more hydrophilic additives or coatings can be used as the pleated heat and humidity exchanger membrane. Porous membranes with hydrophilic additives or coatings have desirable properties for use in heat and humidity exchangers generally, and in particular for use in heat and humidity exchangers with a pleated membrane cartridge. Examples of porous membranes with hydrophilic additives include silica-filled polyethylene (PE) from Entek, Duramic or NSG; silica-filled PVC from Amersil; silica-filled PEEK from SiM; and PFSA (perfluorosulfonic acid) coated composite membranes from Fumatech. These types of membranes have favourable heat and humidity transfer properties, are inexpensive, mechanically strong, dimensionally stable, easy to pleat, are bondable to gasket materials such as polyurethane, are resistant to cold climate conditions, and have low permeability to gas cross-over when wet or dry. The ratio of hydrophilic additive to polymer is important. There needs to be enough additive to allow water transfer but also adequate polymer to provide the membrane with strength and durability.
For ERV applications, porous membranes with hydrophilic additives have been found to offer advantages over conventional ERV membrane materials even in conventional membrane plate-type designs. Testing of membrane samples and ERV cores has revealed that porous membranes with hydrophilic additives generally provide better heat and humidity transfer. They are also more durable than desiccant-coated paper-based membranes that are commonly used in ERV applications, particularly when exposed to high levels of condensation (high saturation) and under freeze-thaw conditions.
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.
| Number | Date | Country | |
|---|---|---|---|
| 60847937 | Sep 2006 | US | |
| 60929937 | Jul 2007 | US |
