WO1992006514A1

WO1992006514A1 - Metal air batteries

Info

Publication number: WO1992006514A1
Application number: PCT/GB1991/001672
Authority: WO
Inventors: Robert Moore
Original assignee: Alupower-Chloride Limited
Priority date: 1990-09-28
Filing date: 1991-09-27
Publication date: 1992-04-16
Also published as: AU8651291A; GB9021221D0

Abstract

A metal air battery comprises a plurality of cells (6), each including a metal anode and an air cathode (8), a fan (102) arranged to draw air from the atmosphere and to blow it through or past the air cathodes, an alkaline electrolyte reservoir (2) divided into two portions by a weir (50) and a single electrolyte pump (58) whose inlet communicates with one of the said portions and which is arranged to pump electrolyte through the cells whence it returns to the other of the said portions, whereby, in use, electrolyte thus returned flows over the weir back to the said first portion of the reservoir. The weir (50) is of tubular shape, whereby the said one portion of the reservoir is very much smaller than the other, and is of variable height and is connected to a motor (52) arranged to move it between an inoperative height above the normal electrolyte level and an operative height below the normal electrolyte level whereby the said one portion of the reservoir is empty when the battery is not in use but when the battery is to be activated the weir is lowered to its operative height to permit electrolyte to flow into the said one portion of the reservoir from the said other portion of the reservoir.

Description

METAL AIR BATTERIES

The present invention relates to metal air batteries and is particularly concerned with aluminium air batteries. Such batteries include a plurality of cells, each of which comprises one or more metal anodes, one or more air cathodes, e.g. an air permeable catalyst:, through which, in use, air is blown, and an alkaline electrolyte, e.g. potassium hydroxide.

Such batteries may be maintained in an inactive state for extended periods of time and then activiated rapidly to produce their maximum output by circulating the electrolyte over the anodes and blowing air over the catalyst of the cathodes. Such batteries are therefore particularly suitable for standby applications and have a very high energy density per unit weight because one of the reactants, namely oxygen, is not stored within the battery but is simply extracted from the atmosphere, when required.

Figure 1 of the accompanying drawings is a diagrammatic perspective partly cut-away view of a known type of metal air battery. The battery comprises an outer housing 2, extending horizontally within which is a support panel 4 which carries a plurality of metal air cells 6 arranged in two rows side by side. Each cell includes an air cathode 8, comprising one or more sheets of air-permeable, catalytically active material sealed to an outer frame 10, and a metal anode, e.g. of aluminium with appropriate additives. The cathodes are connected in series, as also are the anodes, to form the battery. The interior of each cell communicates via a respective downwardly extending tube 12, which passes through the panel 4, with an electrolyte supply manifold 14 which is divided into two longitudinally by a partition 16. At its top, each cell communicates via an overflow weir (not shown) with an electrolyte return pipe 18 which also passes through the panel 4. Each half of the supply manifold 14 communicates via two pipes 20*with a manifold 22 which is connected to three submersible electrolyte pumps 24.

The lower portion of the housing 2 constitutes an electrolyte reservoir which is divided into two portions of different size by a dividing wall 26 which constitutes an overflow weir. The cells are situated above the larger portion of the electrolyte reservoir and the smaller portion accommodates the pumps 24.

Adjacent the cells is a perforated air distributor wall 28 whose interior communicates with spaces between the cells through appropriately positioned apertures (not shown) and with the outlet of an air fan 30 carried by the wall 28. The fan is connected to be powered by electricity generated by the battery. In use, air is blown between the cells past the cathodes 8 and then flows through the electrolyte return ports 18 into the electrolyte reservoir. It then flows over the electrolyte, typically 5M potassium hydroxide, and entrains the hydrogen which is produced and then flows out to atmosphere through a condenser 32 through which air is blown by fans 34. The condenser removes water vapour from the discharged air whilst any entrained droplets of electrolyte are coalesced by a mist curtain 36 suspended between the underside of the panel and the upper edge of the partition 26.

In use, the electrolyte is cooled by means of a heat exchanger 38 through which electrolyte is pumped from the smaller portion of the electrolyte reservoir by a submerged pump 40. The electrolyte is cooled by air blown through the heat exchanger 38 by a fan 42 and is then returned to the electrolyte reservoir.

Prior to use, potassium hydroxide electrolyte is poured into the electrolyte reservoir up to generally similar levels on both sides of the partition 26. When the battery is required to commence producing power, the pumps 24 are activated by a small auxiliary battery and electrolyte is caused to flow into the manifold 12, through the cells and then back into the larger portion of the reservoir. As electricity generation commences, the pumps and all the fans are powered by excess power produced by the battery and the auxiliary battery is no longer required. The electrolyte leaving the cells carries with it solid aluminium hydroxide formed by reaction with the anodes which progressively accumulates at the bottom of the larger portion of the reservoir to form a substantial layer 44. The level of electrolyte in the larger portion of the reservoir progressively rises until it reaches the top of the partition 26 and it then flows over the partition into the small portion of the reservoir and is pumped continuously around the battery. The partition 26 prevents particles of aluminium hydroxide from being drawn into and thus clogging the pumps 24. Batteries of the type described above have a very high energy density and are capable of producing a surprisingly high power output for a protracted period of time. They can be activated after many years of inactivity and are thus particularly suitable for standby applications. t However, they suffer from a number of problems. Thus the electrolyte pumps are exposed at all times to the highly caustic electrolyte and it is found that this can tend to cause failure of the pumps after an extended period of time. Thus a standby battery of this type which has been inactive for many years may then fail to operate on the one occasion on which it is required. It is therefore a first object of the present invention to provide a metal air battery in which the pump is not normally exposed to the electrolyte and is only so exposed when the battery is actually required to produce power and is thus not subject to failure due to corrosion of the electrolyte pump.

According to a first aspect of the present invention there is provided a metal air battery comprising a plurality of cells, each including a metallic anode and an air cathode, a fan arranged to draw air from the atmosphere and to blow it through or past the air cathodes, an alkaline electrolyte reservoir divided into two portions by a weir, electrolyte pumping means whose inlet communicates with one of the said portions and which is arranged to pump electrolyte through the cells whence it returns to the other of the said portions, whereby, in use, electrolyte thus returned flows over the weir back to the said first portion of the reservoir wherein the weir is of variable height and is connected to an actuator arranged to move it between an inoperative height above the normal electrolyte level and an operative height below the normal electrolyte level whereby the said one portion of the _treservoir is empty when the battery is not in use but when the battery is to be activated the weir is lowered to its operative height to permit electrolyte to flow into the said one portion of the reservoir from the said other portion of the reservoir.

The weir may be generally similar to that in the known battery, i.e. in the form of a partition of adjustable height extending across the electrolyte reservoir. It is, however, preferred that it is in the form of an open ended cylinder or tube of adjustable height. The inlet of the electrolyte pump may communicate with the interior of this tube but it is preferred that the tube itself constitutes the inlet to the electrolyte pump. In this embodiment the portion of the electrolyte reservoir which is separated from the remainder of it by the weir is thus very much smaller than in the known battery and when the battery is required to produce power, the weir, that is to say the inlet tube of the electrolyte pump is simply lowered to a level at which electrolyte in the reservoir flows into it and operation then commences. In practice, the actuator is likely to be a simple electric motor, which is connected to be powered by the auxiliary battery used for the initial few minutes of operation of the electrolyte pump. It will be appreciated that when the known battery described above is not in operation, the electrolyte within the reservoir is in contact with the atmosphere via various routes, e.g. via the cells and the air fan, via the air outlet and condenser and via apertures in the support panel which communicates with that portion of the housing which accommodates the condenser and electrolyte cooler and is not sealed from the atmosphere. This is no problem in the short term, but over the extended period of many years for which the battery may remain inactive, the electrolyte can dry out to the extent that its volume is significantly reduced and the concentration of alkali thus significantly increased. Further, it is found that the electrolyte becomes progressively carbonated due to the presence of carbon dioxide in the air and this degrades the performance of the battery.

It is therefore a further object of the invention to provide a metal air battery in which the electrolyte is substantially sealed from the atmosphere when the battery is not in operation whereby the electrolyte is not degraded even over a prolonged period of storage.

According to a further aspect of the present invention there is provided a metal air battery comprising a plurality of cells, each including a metallic anode and an air cathode, an alkaline electrolyte reservoir divided into two portions by a weir, electrolyte pumping means whose inlet communicates with one of the said portions and which is arranged to pump electrolyte through the cells whence it returns through an electrolyte return path to the other of the said portions, whereby, in use, electrolyte thus returned flows over the weir back to the said first portion of the reservoir, a fan arranged to draw air from the atmosphere through an air inlet and to blow it along an air inlet pathway which passes through or past the air cathodes into the reservoir and then out to atmosphere through 'an air outlet pathway wherein the air outlet pathway includes an outlet valve which is arranged normally to be closed but to be open when the battery is producing power and the air inlet pathway between the air inlet and the reservoir includes an inlet valve which is also arranged normally to be closed but to be open when the battery is producing power.

The inlet and outlet valves may be motorised valves or simple spring loaded valves arranged normally to be closed and to seal the electrolyte reservoir but to open under the air pressure exerted by the fan when t a battery is in use. It is, however, preferred that the weir is movable between an inoperative high position when the battery is producing no power and an operative low position when the battery is producing power and that when it is in the high position it forms a seal with the seat member and constitutes the outlet valve. In the most preferred embodiment the movable weir is tubular and the air outlet pathway includes an aperture formed in a seat member and the tubular weir is arranged to form a seal with the seat member around the aperture when at its inoperative height, whereby the tubular weir has two quite separate functions. It is preferred that a portion of the air inlet pathway is constituted by the electrolyte return path and the air inlet valve is situated in the electrolyte return path and that the air inlet valve is a float valve which is arranged to be moved from the closed to the open position by the fall in the electrolyte level in the reservoir which inherently occurs when battery operation commences and electrolyte is withdrawn from the reservoir. The electrolyte in the reservoir is thus sealed "from the atmosphere at all times when the battery is not in operation and is thus not subject to atmospheric degradation.

Further features and details of the invention will be apparent from the following description of one specific embodiment of battery which is given by way of example with reference to Figures 2 to 4 of the accompanying drawings, in which:-

Figure 2 is a perspective partly exploded and partly cut away view of an aluminium air battery in accordance with the invention from which various components have been omitted for the sake of clarity;

Figure 3 is a perspective view of the otorised weir and pump unit; and

Figure 4 is a sectional elevation of one of the float valves controlling the air inlet to the electrolyte reservoir;

Those components which are substantially the same as those shown in Figure 1 are designated with the same reference numerals. The battery comprises an outer housing 2 which is again divided into two portions, one above the other, by a support panel, which is not a unitary construction, and which is generally designated 4. Supported on the panel 4 are 24 aluminium air cells 6 which are arranged in two parallel rows and electrically connected in series.

* The lower portion of the housing 2 constitutes an electrolyte reservoir but no fixed overflow weir extends across it. This is replaced by a tubular weir 50 of bellows construction whose height may be varied by a motor 52 mounted on a pump support plate 54. The interior of the tubular weir 50 communicates with a pump chamber 56 within which is an electrolyte pump 58, the upper portion of which is visible in Figures 2 and 3. The pump is carried by the pump support plate and connected to an electric motor 60. The outlet 62 of the pump 58 is connected to the interior of the cells.

The electrolyte reservoir is thus divided into two portions by a weir in the usual manner but the weir is constituted by a tube 50 and the portion downstream of the wt .r is thus only a very small proportion of the total volume of the reservoir. The weir is movable vertically by the motor 52 through an adjustable distance up a shaft 64. When the battery not operational the weir is at a height above the electrolyte level in the reservoir and the interior of the weir and the pump chamber are empty. The pump thus does not spend prolonged periods immersed in the caustic electrolyte and is thus not subjected to failure by corrosion. This means that three electrolyte pumps in parallel are no longer required and that a single pump is sufficient. Furthermore, the pump can be operated dry periodically in order to check that it is still operative without pumping any electrolyte and thus actuating the battery. When battery operation is to commence, the weir 50 is lowered to a level below the electrolyte level and electrolyte flows into the pump chamber and is pumped through the cells in the usual manner. The weir and the pump 60 are initially operated by the usual auxiliary battery but the pump is subsequently operated by the power produced by the aluminium air battery.

The two rows of cells are supported on the panel 4 via a manifold unit 64. This constitutes a one-piece moulded tray of plastics material which defines three parallel manifolds, namely two electrolyte inlet manifolds 66 between which are an electrolyte outlet manifold 68. The inlet manifolds 66 communicate with the outlet 62 of the pump and have a substantially closed upper surface in which a plurality of holes 70 is formed. Each hole 70 communicates with the interior of a respective cell via a respective inlet spigot 72 which depends from the cell and extends into the associated hole. The interior of the cells communicate via overflow weirs with outlet spigots 74 through which electrolyte is returned to the electrolyte outlet manifold 68 which is open topped. The electrolyte is then returned to the electrolyte reservoir by one or more, in this case two, return pipes 76 depending from the underside of the manifold unit 64. The two return pipes 76 pass through respective holes in the support panel 4. Secured to the panel 4 below each such hole is a non-return float valve which is designated 88 in Figure 2 and whose construction is shown in Figure 4. The float valve comprises a tubular body 90 whose lower end is open and is traversed by a pin 92 and at whose upper end is an outwardly extending peripheral flange 94 by which the valve is secured to the panel 4 with the interposition of a sealing gasket 96. At the upper end of the body 90 is an inwardly extending annular shoulder 98. Within the chamber defined by the body 90 between the shoulder 98 and plate 92 is a spherical float 100 which is adapted to form a seal with the shoulder 98. When the battery is not in use, the electrolyte reservoir, is full to a level at which the float 100 is pressed into sealing contact with the shoulder 98 thereby sealing the return path. When the battery is actuated, electrolyte is withdrawn from the reservoir whose level then drops. The float then moves away from the shoulder 90 and the return path is automatically opened.

When the battery is in use, air is blown over the air cathodes by a fan 102 which is driven by the power produced by the battery. After the air has passed through the cells it flows into the electrolyte reservoir through the return valves 88. It then flows over the electrolyte entraining the hydrogen which is produced, through an opening in the support panel 4 and thence out to atmosphere through an outlet in the outer casing, which is not shown for the sake of clarity, via a condenser and filter which is also not shown. In this case the air opening in the support panel 4 is above and coaxial with the movable tubular weir 50 and is in fact formed in the pump support plate 54 which constitutes a portion of the support panel 4. The air outlet pathway is defined downstream of the opening in the support plate 54 by a housing 104, on the top of which the motor 52 is mounted, and a discharge connector 106. At the upper end of the tubular weir 50 is an annular flange 108 which carries a resilient sealing ring 110. As mentioned above, the weir 50 is movable between upper and lower positions and the upper position is arranged to be such that the sealing ring 110 is urged into sealing contact with the underside of the support plate 54 and thus seals the air outlet from the electrolyte reservoir.

When the known battery shown in Figure 1 is not in use, the electrolyte in the reservoir communicates with atmosphere through the electrolyte return ports 18 through which, in use, air flows from the cells into the reservoir and also through the air outlet opening from the reservoir to the atmosphere. However, when the battery in accordance with the present invention is not in use, the air inlet opening(s) to the reservoir are sealed by the float valves 88 and the air outlet opening from the reservoir is sealed by the cooperation of the seal 110 on the weir 50 and the pump support plate. The movable weir thus serves two quite different functions. This means that when the battery is not in use, the electrolyte in the reservoir is not in communication with atmosphere. Water loss from, and carbonation of, the electrolyte are thus reduced whereby the battery may remain unused for many years without the electrolyte being significantly degraded. If a battery in accordance with the invention is to be used as a standby battery to power e.g. a telephone exchange in the event of a failure of the normal power supply, it may remain unused for many years. During this time the float valves 88 are retained closed by the electrolyte in them and the movable weir is retained in its upper position whereby the KOH electrolyte in the reservoir is isolated from the atmosphere and the electrolyte pump is not in contact with the electrolyte and is thus not subject to corrosion. If emergency power is required, the battery controller issues signals to move the weir 50 to its lower position below the normal electrolyte level and to commence operation of the electrolyte pump both of which are powered by the small auxiliary battery which normally forms part of such metal air batteries. The electrolyte flows, as indicated by the arrows in Figure 3, into the pump chamber 56 and is then pumped to the cells. Power generation by the battery then commences and some of the power which is produced is used to power the electrolyte pump and also the fan 30 which blows air through the cells. The other auxiliary equipment such as condensers, heat exchangers and the like described above in connection with Figure 1 is also provided and operated by power produced by the battery. As the initial electrolyte leaves the cells the valve floats 100 are depressed and the electrolyte and air pathway to the electrolyte reservoir is opened. The electrolyte returns to the reservoir and the air flows over it and out through the air outlet, which is open due to the lowering of the weir, to atmosphere. Aluminium hydroxide particles accumulate in the bottom of the reservoir and are prevented from being drawn into the electrolyte pump by the weir which remains a significant distance above the bottom of the reservoir.

Claims

1. A metal air battery comprising a plurality of cells, each including a metallic anode and an air cathode, a fan arranged to draw air from the atmosphere and to blow it through or past the air cathodes, an alkaline electrolyte reservoir divided into two portions' by a weir, electrolyte pumping means whose inlet communicates with one of the said portions and which is arranged to pump electrolyte through the cells whence it returns to the other of the said portions, whereby, in use, electrolyte thus returned flows over the weir back to the said first portion of the reservoir characterised in that the weir (50) is of variable height and is connected to .a motor (52) arranged to move it between an inoperative height above the nc ιal electrolyte level and an operative height below tne normal electrolyte level whereby the said one portion of the reservoir (2) is empty when the batterv is not in use but when the battery is to be actⁱvated the weir (50) is lowered to its operative he: t to permit electrolyte to flow into the said one portion of the reservoir from the said other portion of the reservoir.

2. A battery as claimed in claim 1 characterised in that the electrolyte pumping means is a single electrolyte pump (58) .

3. A battery as claimed in claim 1 or claim 2 in which the weir comprises a tubular member (50) whose interior communicates with the inlet of the pumping means (58) .

4. A battery as claimed in claims 2 and 3 in which the electrolyte pump includes an inlet tube which is constituted by the weir (50) .

5. A metal air battery comprising a plurality of cells, each including a metallic anode and an air cathode,' an alkaline electrolyte reservoir divided into two portions by a weir, electrolyte pumping means whose inlet communicates with one of the said portions and which is arranged to pump electrolyte through the cells whence it returns through an electrolyte return path to the other of the said portions, whereby in use, electrolyte thus returned flows over the weir back to the said first portion of the reservoir, -a fan arranged to draw air from the atmosphere through an air inlet and to blow it along an air inlet pathway which passes through or past the air cathodes into the reservoir and then out to atmosphere through an air outlet pathway characterised in that the air outlet pathway includes an outlet valve (54,110) which is arranged normally to be closed but to be open when the battery is producing power and that the air inlet pathway between the air inlet and the reservoir includes at least one inlet valve (88) which is also arranged normally to be closed but to be open when the battery is producing power.

6. A battery as claimed in claim 5 characterised in that the weir (50) is movable between an inoperative high position when the battery is producing no power and an operative low position when the battery is producing power and that when it is in the high position it forms a seal with the seat member (54) and constitutes the outlet valve.

7. A battery as claimed in claim 6 characterised in that the air outlet pathway includes an aperture formed in the seat member (54) and the tubular weir (50) is arranged' to form a seal with the seat member (54) around the aperture when at its inoperative height.

8. A battery as claimed in any one of claims 5 to 7 characterised in that a portion of the air inlet pathway is constituted by the electrolyte return path and the or each air inlet valve (88) is situated in the electrolyte return path.

9. A battery as claimed in claim 8 characterised in that the air inlet valve is a float valve (88) which is arranged to be moved from the closed to the open position by the fall in the electrolyte level in the reservoir (2) which occurs when battery operation commences.