Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1993-03-19
1995-03-07
Niebling, John
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
204150, 204196, 204197, 204231, 204DIG4, 429119, H01M 600
Patent
active
053954912
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to sea water cells or batteries which use oxygen dissolved in seawater as oxidant, like for instance cells described in PCT applications Nos. PCT/N089/00040 and PCT/N090/00045. The invention relates to a method for preventing sea water cells from being destroyed by biofouling.
Such sea water cells consist of an anode made from an electronegative alloy based e.g. on magnesium, zinc, aluminum or lithium and a cathode which is a more or less inert current conductor. The most common materials in the cathode are materials which are resistant to sea water, as e.g. copper, stainless steel, titanium or carbon. A cathode may also be covered with a catalyst which catalyses the reduction of oxygen. Sea water contains little oxygen, about 10 g/m.sup.3. As a consequence oxygen reducing sea water cells must have a very open structure to insure sufficient flow of fresh sea water through the cell. Sea water cells are usually connected in parallel because the cells share the sea water as a common electrolyte. The common electrolyte would result in shortcircuiting currents via the sea water in series connected batteries. A DC/DC converter transforms the low voltage of the sea water cells (1 to 2 V) up to a more suitable value, as e.g. 28 V.
A great problem with sea water cells which must be opera rive over longer periods of time (months or years), is the biological growth on the cathode. This leads to increased resistance against transport of oxygen to the surface of the cathode,--resulting in reduced performance.
It is well known that copper and copper alloys are insertsitive to biological growth in sea water, because copper ions which are released by corrosion of the surface, have biocidic properties. It is for this reason that copper and copper compounds are used in growth preventing paint and antifouling, for subsea use.
The good properties of copper in sea water and the biocidic action of copper ions are the reason why copper is used as cathodes in sea water cells. Cells with copper cathodes have a lower cell voltage (about 1 V) than what is possible with other materials which are covered with a catalytic layer as e.g. catalyzed stainless steel (about 1.6 V), but this is compensated for in many applications by the absence of biological growth. The antifouling properties of the copper does, however, depend on the rate of corrosion of the copper. As the potential on a copper cathode is reduced when the current density on the surface is increased, the corrosion rate will decrease and thereby also the ability to prevent biological growth. For a light buoy this is not a problem because the cathode potential usually has time to increase sufficiently when the cell is unloaded between the light flash periods, so that the average corrosion is high enough. Another solution is of course to reduce the load on the cell and thereby prevent growth, or oversize the cell for the particular application.
The object of the present invention is to optimize the use of sea water cells of the mentioned type. The main features of the invention are defined in the accompanying claims. By using this invention there is obtained a substantial increase of the lifetime of sea water cells.
Above mentioned and other features and objects of the present invention will clearly appear from the following detailed description of embodiments of the invention taken in conjunction with the drawings, where
FIG. 1 shows a diagram for a sea water cell plant,
FIG. 2 shows the cross section of a cell without copper cathode, and
FIG. 3 shows a principle diagram for water flow through a cell having an auxiliary electrode.
The method described in connection with FIG. 1 should be used in order to obtain optimum utilization of a cell. A sea water battery 1 is connected, over a DC/DC converter 2,--which converts the low cell voltage from the sea water battery, e.g. 1.0 V, to a more suitable voltage, e.g. 28 V,--in order to charge an accumulator 3.
If a greater growth protection is desired than what can be obtained by free corrosion of a coppe
REFERENCES:
patent: 3520790 (1970-07-01), Araki et al.
patent: 3661742 (1972-05-01), Osborn et al.
patent: 4306952 (1981-12-01), Jansen
Den Norske Stats Oljeselskap A.S.
Niebling John
Phasge Arun S,.
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