Patent Application
20140255804
The present invention relates generally to the regeneration of a catholyte solution in a redox fuel cell, and more particularly, to the regeneration of a catholyte solution in a redox fuel cell by way of atomisation. There is thus provided a method of regenerating a catholyte solution in a redox fuel cell, a regeneration zone in a redox fuel cell, a redox fuel cell comprising such a regeneration zone, and use of the regeneration zone.
The subject of regeneration technology is well known within the field of fuel cells and it is a well understood technique to use the generation of small diameter air bubbles in a catholyte flow to achieve this purpose. Indeed, this technique works well with catholyte solution bubble formation in which the air:liquid ratio is in the region of 4:1 (wherein the oxidant is air for example). However, problems are encountered when the air:liquid ratio increases because it then becomes difficult to maintain a foam. In order to generate and maintain foams, such as polyoxometallate foams, at a much higher air:liquid ratio, it would be necessary to incorporate surface tension modifiers. This is undesirable because it adds to the complexity of the system and also unnecessarily increases the cost. Additionally, surface tension modifiers can interact with the membrane material in the fuel cell stack and cause loss of performance. Additionally, at an air:liquid ratio of about 7:1, the foam ceases to behave as a fluid (as, for instance, one can observe with foam on washing up liquid or shaving foam) which is highly undesirable.
From the discussion that is to follow, it will become apparent how the present invention as defined in the claims addresses the aforementioned deficiencies and provides numerous additional advantages not hitherto contemplated or possible with prior art techniques and instructions.
A desirable air:liquid ratio for second generation catalysts in automotive applications is about 20:1 to 30:1. As discussed, it is difficult, if not impossible, to maintain a foam at this air:liquid ratio using prior art techniques.
According to a first aspect of the present invention, there is encompassed a method of regenerating a catholyte solution in a redox fuel cell, comprising the steps of:
The method addresses the problems mentioned above in that atomisation of the catholyte solution enables the production of catholyte solution droplets having the requisite surface area and being able to sustain it for a period long enough to achieve the appropriate mass transfer of oxidant (for instance, oxygen) from the catholyte. This is made possible by the formation of a mist of fine droplets. Foam formation at air:liquid ratios of about 30:1 is possible due to the atomisation of the catholyte solution which causes the individual droplets to be spaced by a significantly smaller amount than droplets having a larger size and not formed by atomisation, for instance. A small droplet size may be advantageous because it may minimise the diffusion distance for oxygen within the droplet and thus reduces the required time of flight to achieve the required mass transfer. In at least some embodiments, the spacing between droplets may not be critical as oxygen diffusion in air is fast and, therefore, not limiting.
It may be that the method comprises the step of providing multiple droplet formation means for catholyte solution atomisation. Provision of multiple droplet formation means may have the effect of enhancing the efficiency of the atomisation process. It may be advantageous to inject in multiple points in order to achieve a more uniform distribution of droplets within the reaction chamber.
The droplet formation means may be constituted by injection means.
The droplet formation means may include spray nozzles, spinning discs, acoustically driven oscillators, and micro-fluidic driven oscillators. Of course, it will be appreciated that other forms of injection means also provide the equivalent desired effect.
It may be that the oxidant stream flow directs the fine droplets. The oxidant stream flow may deter the fine droplets from impacting the reaction vessel of the redox fuel cell so that time of flight of the droplets is sufficient to accomplish required mass transfer.
Smaller droplets often require a shorter time of flight and may, therefore, be more easily directed by the oxidant stream flow. The average oxidant:liquid ratio of the fine droplets may be from 10:1 to 50:1, more particularly 20:1 to 30:1. The average oxidant:liquid ratio of the fine droplets may be 30:1. Below a ratio of 10:1 the spacing may become unacceptably low. Above a ratio of 50:1 the air flow rate and blower consumption may become undesirably high.
The spacing between the fine droplets may be about 1.5 times the average diameter of the fine droplets. Spacing in this region enables foam formation at the requisite oxidant:liquid ratio. The driving parameter, for instance, may be the amount of oxidant necessary to regenerate the catholyte. As an example, an efficient system may be considered one in which the air:liquid ratio is as described and at least 50% of the oxidant (oxygen) is consumed. As a higher proportion of the oxygen is consumed, the rate of mass transfer may slow down. The spacing, therefore, may be a simple geometric calculation based on spheres at a given air:liquid ratio.
The average droplet diameter size may range from 1 to 100 μm. More particularly, the average droplet diameter size may range from 1 to 5 μm. Where the droplet size is larger, for instance, the mass transfer may be slower due to the time for diffusion in the liquid phase. Larger droplets may be feasible in a large scale system using conventional spray tower technology.
The oxidant may comprise air. The oxidant may comprise oxygen. By use of oxygen, for example, mass transfer rates may be higher thus reducing the time of flight and reactor volume. Of course, the oxidant may comprise a combination of air and additional oxygen to allow, for instance, membrane enrichment of oxygen in air.
The separation means may comprise a cyclone separator. More particularly, the separation means may comprise a spiral cyclone separator. On achieving a sufficient time of flight, coalescence of the droplets may be accomplished by use of a cyclone or other suitable means.
The catholyte solution may comprise a catalyst, such as a polyoxometallate (POM).
In another aspect of the present invention, there is provided a regeneration zone in a redox fuel cell, comprising:
According to a further aspect of the present invention, there is envisaged a redox fuel cell comprising:
According to another aspect, the present invention provides use of the regeneration zone as specified herein in a redox fuel cell for regeneration of a catholyte solution.
The present invention will now be described more particularly, by way of example, with reference to the accompanying drawings, in which:
Referring first to
By contrast, it can be seen from
Referring now to
In accordance with the present invention, it was seen to increase the air:liquid ratios in the catholyte solution by dividing the liquid into fine droplets in an air (oxidant) stream flow, rather than generating finely divided bubbles, as performed in conventional techniques.
In use, air was drawn through the atomiser system 1 by way of a vacuum pump 9, shown connected at the top of the cube-like sealed sample port 5. A small sample of fully reduced polyoxometallate was sprayed into the test chamber 3 (on the right hand side), and the mist of fine droplets were then separated and collected in the sealed sample port 5. The droplets pass via the conduit 7. The redox potential of the separated mist is then measured.
The mean droplet size obtainable from such an atomiser is 60 microns with water. The pressure generated by the hand pump is of the order of 8 bar. At this pressure, the energy required to pump 18 litres per minute of polyoxometallate would be of the order of 200 W of hydraulic power and with a pump efficiency of 70%, 280 W of electrical energy would be required. Since there would be minimal energy required to circulate the air (unlike mixers/ejectors), for instance) it does not have to be pressurised and the parasitic power is not out of the question.
Having conducted the test, a sufficient amount of polyoxometallate was collected in order to obtain a redox measurement. The corresponding redox potentials and the redox dates are as follows:
The change in vanadium reduction is equal to the change measured in a single path through the fuel cell stack. It is envisaged that the time of flight of the droplets still in the system would be approximately less than one second. Reducing the air flow velocity would increase the time of flight and consequent regeneration.
The cyclone separator worked particularly well with very high air: droplet ratios.
Referring now to
In use, hydrogen is catalysed on the anode 13 in a conventional fashion. However, unlike conventional technology, the electron and proton are absorbed into a catholyte solution containing redox catalyst systems, which flow continuously from the stack to an external regenerator, generally indicated 25. In the regenerator 25, the catholyte comes into contact with air. The electron, proton and oxygen from air reacts to form water, which exits the regenerator 25 as vapour along the arrow indicated 27. The regenerated catholyte then flows back into the cell 11 by means of pump 29.
In this embodiment, the regenerator 25 (regeneration zone) comprises droplet formation means for catholyte solution atomisation operable to generate and mists of fine droplets; means 21 for supplying an oxidant stream to the cathode 15 region of the cell 11; means for feeding the mists of fine droplets in to the oxidant stream; means for regulating the oxidant stream flow so that time of flight of the droplet is sufficient to accomplish required mass transfer; and separation means for separating the list of fine droplets from the oxidant stream.
In this way, a catholyte solution having an air:liquid ratio of 30:1 can be achieved whilst the mixture can advantageously continue to behave as a well-mixed fluid.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1110357.9 | Jun 2011 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2012/051410 | 6/19/2012 | WO | 00 | 5/28/2014 |
