Abstract:
According to the invention there is provided a gaseous product generator including:
       at least one rechargeable metal-air cell operable to provide the gaseous product; and   a manifold arrangement having an inlet structure allowing a feed atmosphere to be introduced to the metal-air cell and an outlet structure for transporting the gaseous product to an intended point of use thereby to provide an environmentally enhancing function.

Description:
BACKGROUND TO THE INVENTION 
       [0001]    This invention relates to a gaseous product generator and an associated method of generating a gaseous product, with particular, but by no means exclusive, reference to applications on-board aircraft. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    It is well known that aircraft require sources of oxygen for passenger and crew use in the event of loss of cabin pressure, and also nitrogen gas (or at least an oxygen depleted air supply) for inerting fuel tanks so as to lessen the risk of explosions. There are a number of systems which have been used for one or both of these purposes. Hollow fibre membrane (HFM) technology has been used to produce nitrogen enriched air for fuel tank inerting purposes. The HFM process involves oxygen separation from a high pressure air source through polymeric fibres. Advantages are that the separation process requires no moving parts. However, the technique has no inherent gas storage capability, and does not generate an oxygen supply. Conversely, ceramic membrane (CM) technology is used for oxygen production, but not the production of nitrogen enriched air. Again, there are no moving parts associated with the gas separation step. The total air liquefaction of oxygen and nitrogen technique (TALON) utilises liquefaction and air distillation columns to provide oxygen and liquefied nitrogen enriched air. In the related system for aircraft fuel tank inerting (SAFTI), liquefied nitrogen enriched air only is produced. These systems are complex, and require moving parts. Pressure swing adsorption (PSA) uses zeolite molecular sieves for oxygen or nitrogen production. There is no inherent storage capability, and the system is inflexible in that once configured a system will only produce the maximum amount of oxygen. In general, the systems are heavy and consume relatively large amounts of energy. 
       OBJECTS OF THE INVENTION 
       [0003]    The present invention, in at least some of its embodiments, provides oxygen generation and/or oxygen depleted air generation in a relatively simple system. Additionally, the present invention has an inherent oxygen storage capability, and there are no moving parts associated with the gas separation step. 
       SUMMARY OF THE INVENTION 
       [0004]    According to a first aspect of the invention there is provided a gaseous product generator including: 
         [0005]    at least one rechargeable metal-air cell operable to provide the gaseous product; and 
         [0006]    a manifold arrangement having an inlet structure allowing a feed atmosphere to be introduced to the metal-air cell and an outlet structure for transporting the gaseous product to an intended point of use thereby to provide an environmentally enhancing function. 
         [0007]    Preferably, the rechargeable metal-air cell is a rechargeable lithium-air cell. Other forms of metal-air cells, such as an aluminium-air cell or a zinc-air cell may be used. 
         [0008]    Typically, a rechargeable metal-air cell includes an anode formed from an electropositive metal, an air cathode, and an electrolyte in communication with the anode and also in communication with the air cathode. The air cathode has a surface which is in communication with the feed atmosphere, possibly through an oxygen permeable interface structure such an oxygen permeable membrane. The cell may include one or more separators, such as a separator positioned between the anode and the electrolyte and/or a separator positioned between the air cathode and electrolyte. Generally, the anode is in conductive communication with an anode current collector. 
         [0009]    Metal-air cells of this type are well known as rechargeable electrochemical cells which are used to provide a supply of electrical energy. The fundamental physical process behind the storage and subsequent production of electrical energy is a following reversible chemical reaction wherein gaseous oxygen is absorbed by the metal-air cell when it is discharged, and oxygen is released by the metal-air cell when it is charged. The present inventor has realised that oxygen production and/or oxygen absorption associated with the use of metal-air cells can be exploited for useful purposes. In particular, oxygen produced while a metal-air cell is charging and/or an oxygen depleted feed atmosphere produced while a metal-air cell is discharging can be utilised in environmentally enhancing functions. 
         [0010]    Preferably, the manifold arrangement includes an oxygen supply system for supplying oxygen generated by charging the metal-air cell to an intended point of use. The oxygen supply system may include at least a portion of the outlet structure in the form of an oxygen outlet, and may further include an oxygen inlet structure for introducing an oxygen containing gas stream to the metal-air cell whilst oxygen is generated by charging the metal-air cell. The oxygen can be supplied for breathing purposes in a number of possible application areas, such as in aircraft, buildings or medical applications. In aircraft, the oxygen can be supplied continuously, or as an additional, on-demand source of oxygen, for example during an emergency. 
         [0011]    Additionally, or alternatively, at least a portion of the outlet structure of the manifold may be in the form of an oxygen depleted feed atmosphere outlet for transporting an oxygen depleted feed atmosphere generated by discharging the metal-air cell to an intended point of use. The oxygen depleted feed atmosphere can be used for a variety of purposes in which it is desirable to provide an essentially inert atmosphere, such as inerting fuel tanks and in other fire or explosion prevention or fire fighting applications. Other applications are as a nitrogen gas generator, as a source of gas for use in inflating devices such as safety devices and flotation devices, and as a propellant for other media. 
         [0012]    Typically, the feed atmosphere is air. 
         [0013]    In preferred embodiments, the manifold arrangement includes one or more valves for controlling the supply of gas to the metal-air cell and/or the transportation of the gaseous product from the metal-air cell. Advantageously, the generator further includes a valve control system for controlling the operation of the valves. 
         [0014]    The generator may utilise a single metal-air cell. Preferably, however, the generator includes a plurality of metal-air cells and a metal-air cell control system which is configured to control the operation of the metal-air cells so that, when one metal-air cell is charging, at least one other metal-air cell is discharging. 
         [0015]    The valve control system and the metal-air cell control system may be provided as separate systems, but preferably they form part of a single control system which controls the metal-air cells so that a desired scheme of cell discharging/charging is provided and controls the valves to ensure that the correct flow of gas is achieved. A microprocessor based control system may be utilised. 
         [0016]    Advantageously, the rechargeable metal-air cell includes an elongate air cathode which, either wholly or in combination with the manifold arrangement, defines an elongate passageway along which gases can flow during charging and discharging of the metal-air cell. In this way, it is possible to provide a large air cathode surface area whilst minimising any restriction to the flow of gases. Preferably, the metal-air cell is in the form of a tubular structure having an inner wall formed by the elongate air cathode, wherein the elongate passageway is an interior bore of the tubular structure defined by the elongate air cathode. For the avoidance of doubt, a feature such as an oxygen permeable membrane which is positioned on the surface of the air cathode side of the metal-air cell, is considered to be part of the air cathode. 
         [0017]    The elongate air cathode may be corrugated or fluted. 
         [0018]    Whilst the primary purpose of the present invention is the generation of a gaseous product, it is advantageous that energy recovery is possible during discharging of the metal-air cell. The actual energy recovered during discharge of the metal-air cell may be stored, or used for a desired purpose, such as charging another cell, operating equipment such as valves, pumps etc, or other purposes. 
         [0019]    Although it is considered advantageous that the present invention can be used as on-off demand system, it is possible to store the gaseous product by any suitable means, such as by liquefaction. 
         [0020]    According to a second aspect of the invention there is provided an aircraft having a gaseous product generator, in which the gaseous product generator includes: 
         [0021]    at least one rechargeable metal-air cell operable to provide the gaseous product; and 
         [0022]    a manifold arrangement having an inlet structure allowing a feed atmosphere to be introduced to the metal-air cell and an outlet structure for transporting the gaseous product to an intended point of use on-board the aircraft thereby to provide an environmentally enhancing function. 
         [0023]    The inlet structure of the manifold arrangement may be configured to admit air from outside of the aircraft to the metal-air cell as the feed atmosphere. 
         [0024]    The gaseous product may be oxygen produced by charging the metal-air cell, and the manifold arrangement may transport the oxygen to a pre-determined location on-board the aircraft for breathing purposes. 
         [0025]    The gaseous product may be an oxygen depleted atmosphere produced by discharging the metal-air cell, and the manifold arrangement may transport the oxygen depleted atmosphere to a pre-determined location on-board the aircraft for atmosphere inerting purposes. The manifold arrangement may transport the oxygen depleted atmosphere to a fuel tank for inerting purposes. 
         [0026]    According to a third aspect of the invention there is provided a method of generating a gaseous product including the steps of; 
         [0027]    providing one or more rechargeable metal-air cells; 
         [0028]    operating the metal-air cells to generate a gaseous product; and 
         [0029]    transporting the gaseous product to an intended point of use thereby to provide an environmentally enhancing function. 
         [0030]    The metal-air cell may be disposed on an aircraft, and the environmentally enhancing function may be provided on-board the aircraft. 
         [0031]    The gaseous product may be oxygen, the metal-air cell being charged in order to provide the oxygen, and the environmentally enhancing function may be the provision of oxygen in a pre-determined location for breathing purposes. 
         [0032]    The gaseous product may be an oxygen depleted atmosphere, the metal-air cell being discharged in order to provide the oxygen depleted atmosphere, and the environmentally enhancing function may be the inerting of an atmosphere in a pre-determined location. 
         [0033]    According to a fourth aspect of the invention there is provided the use of at least one rechargeable metal-air cell to generate a gaseous product which is transported to an intended point of use thereby to provide an environmentally enhancing function. 
         [0034]    Whilst the invention has been described above, it extends to any inventive combination of the features set out above, or in the following description, drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    Embodiments of gaseous product generators in accordance with the invention will now be described with reference to the accompanying drawings, in which:— 
           [0036]      FIG. 1  is a schematic diagram of an embodiment of the invention; 
           [0037]      FIG. 2  is a cross section view of a metal-air cell having an enhanced cathode surface area; 
           [0038]      FIG. 3  shows cross sectional views of alternative metal-air cells having enhanced cathode surface areas; and 
           [0039]      FIG. 4  is a semi-schematic diagram of an aircraft equipped with a gas generator of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0040]    Reversible lithium-air cells are secondary electrochemical cells which utilise the following reversible electrochemical reaction: 
         [0000]      2Li+O 2           Li 2 O 2    
         [0041]    The Li-air cell absorbs oxygen on discharge, and gives up oxygen on charge via the formation of lithium peroxide. The present inventor has realised that Li-air cells and other metal-air cells might be employed as oxygen generators and/or as a means of generating oxygen depleted air for a variety of purposes. 
         [0042]    In order to function as a gas generator, the Li-air cell is provided with an appropriate manifold arrangement enabling gas to be supplied and removed from the Li-air cells required.  FIG. 1  is a schematic diagram of a gas generator, shown generally at  10 , which comprise a Li-air cell  12 , an air inlet  14 , an outlet for oxygen depleted air  16 , an inlet for an oxygen supply system  18 , and an outlet for the oxygen supply system  20 . Each of these gas conducting lines has an associated valve  14   a ,  16   a ,  18   a ,  20   a . The operation of the valves can be controlled by an appropriate control system  22 . The valve sequence for a single cell operating as a gas generator is as shown in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 valve sequence for single metal-air cell 
               
             
          
           
               
                 Cell Condition 
                 Air IN 
                 O 2  IN 
                 Inert OUT 
                 O 2  OUT 
               
               
                   
               
               
                 Charging 
                 CLOSED 
                 OPEN 
                 CLOSED 
                 OPEN 
               
               
                 Discharging 
                 OPEN 
                 CLOSED 
                 OPEN 
                 CLOSED 
               
               
                   
               
             
          
         
       
     
         [0043]    Although dependent on the precise application envisaged, in many situations it is advantageous to utilise two or more Li-air cells in a generator. In this way, at least one cell can be charging whilst the other is discharging. An appropriate control system can be used to operate the cell in a co-ordinated manner. It is envisaged that even systems which utilise a plurality of metal-air cells can be relatively lightweight. 
         [0044]    Enhanced efficiency can be obtained if the Li-air cell is arranged in some form of tubular structure in order to maximise the surface area of the air cathode whilst providing minimal restriction to the gas flow.  FIG. 2  shows such an arrangement, wherein a metal-air cell, shown generally at  24 , is in the form of a tube wherein the air cathode  26  forms the inner wall of the tube, and thus defines an inner gas channel along which gas can flow. The metal-air cell is arranged in a concentric manner with the anode  28  acting as the outer wall of the tubular structure, with an electrolyte/separator structure  30  being disposed concentrically between the air cathode  26  and anode  28 . The metal-air cell shown in  FIG. 2  is of elongate form in order to provide an inner cathode surface area. 
         [0045]      FIG. 3   a  shows a related embodiment of a metal-air cell, depicted generally at  32 , in which the metal-air cell again is in the form of a tubular structure. The air cathode  34  forms an inner wall of the tubular structure and defines an inner passageway along which gases can flow. The anode  36  acts as an outer wall of the structure, and an electrolyte/separator  38  is disposed between the anode  36  and air cathode  34 . In the embodiment shown in  FIG. 3   a , the components of the metal-air cell, in particular the air cathode  34 , are corrugated, in order to provide an enhanced cathode surface area. 
         [0046]      FIG. 3   b  shows a further embodiment of a metal-air cell, depicted generally at  40 , in which the metal-air cell again is in the form of a tubular structure. In this embodiment, the air cathode  42  forms an outer wall of the tubular structure, and the anode  44  acts as an inner wall of the structure. An electrolyte/separator  46  is disposed between the anode  44  and the air cathode  42 . In the embodiment shown in  FIG. 3   b , the anode  44  defines an inner passageway. However, it may be possible to provide embodiments in which there is no inner passageway. The metal-air cell  40  is disposed in a conduit  48  which is part of the generator device comprising the metal-air cell  40 . Thus, in the embodiments shown in  FIG. 3   b , it is the outer wall of the tubular structure formed by the air cathode  42  and the walls of the conduit  48  which define a passageway along which gases can flow in order to interact with the air cathode  42 . A plurality of metal-air cells might be disposed in a conduit in this manner.  FIG. 3   c  shows a further embodiment of a metal-air cell, depicted generally at  50 , in which the metal-air cell  50  is disposed against a wall  52  of the manifold arrangement, with these two structures acting together to form and define an inner passageway along which gases can flow. In the embodiments shown in  FIG. 3   c , the metal-air cell  50  is of domed or substantially hemispherical cross-sectional form, wherein the air cathode  54  forms an inner wall of the domed structure and thereby is in communication with gases flowing along the inner passageway. The anode  56  acts as an outer wall of the structure, and an electrolyte/separator  58  is disposed between the anode  56  and the air cathode  54 . It will be appreciated that the manifold arrangement and the metal-air cell can take many different cross-sectional forms in order to provide a combination which defines an inner passageway along which gases can flow. 
         [0047]      FIG. 4  is a schematic diagram of an aircraft  60  having a plurality of jet engines  62  and a plurality of fuel tanks  64 . An air bleed  66  is taken from an engine compressor of one of the jet engines and is used as a feed gas supply to a gas generator  68  of the invention. The bleed air may be cooled and compressed as necessary prior to introduction into the gas generator  68 . Oxygen produced by the gas generation  68  is circulated around the aircraft  60  via an oxygen supply system  70 . Oxygen depleted air is introduced into the fuel tank  64  using an oxygen depleted air supply system  72 . Additional gas generators and/or air bleeds might be provided as required.