This application claims priority under 35 U.S.C. 119 to Japanese Patent Application No. 2004-269340, filed Sep. 16, 2004, the entire disclosure of which is hereby incorporated herein by reference.
The present invention relates to a fuel cell system equipped with a coolant channel for cooling the fuel cell stack and, more particularly, to control of the coolant conductivity.
A typical fuel cell system is equipped with, for example, a fuel cell comprised of a stack structure with multiple layering of fuel cells (power generation units). By supplying an oxidizer gas, such as air, to an oxidizer electrode and a fuel gas, such as hydrogen, to a fuel electrode of each cell, generated output is obtained by electrochemically reacting oxygen in the air and hydrogen through an electrolyte membrane. There are great expectations for putting this kind of fuel cell system into practical use, for example as a power source for automobiles, and research and development towards practical application is currently thriving.
In a fuel cell system, such as the one described above, some sort of a cooling mechanism is required in order to maintain the correct operating temperature (about 80° C.), because the fuel cell stack generates heat during power generation. A cooling mechanism with a structure that cools the fuel cell by providing a circulatory supply of coolant to the fuel cell stack through a coolant channel that is connected to the fuel stack is common.
In regards to a fuel cell system equipped with a cooling mechanism that provides a circulatory supply of coolant to the fuel cell stack, deterioration of the coolant conductivity becomes a problem. By the process of repeated circulatory supply of the coolant to cool the fuel cell stack, the coolant conductivity gradually increases, because of the elution of metallic ions from each of the parts used in the coolant channel. When the coolant conductivity exceeds a specified value, the conductivity becomes a factor in shortening the life of the fuel stack cell. Also, when the coolant conductivity is high, this may cause a so-called liquid junction, causing the problem of wasteful consumption of generated output.
In order to avoid the problems associated with an increase of coolant conductivity, various fuel cell systems utilize a deionization unit to decrease conductivity within the coolant channel. In particular, the fuel cell systems pass the coolant through the deionization unit when the coolant conductivity is high.
Some systems measure the conductivity of the circulating coolant that is supplied to the fuel cell stack, and accordingly control the flow rate of the coolant to the deionization unit. In other words, the systems attempt to extend the life of the ion-exchange resin of the deionization unit by controlling the flow of the coolant to the deionization unit with the amount depending on the coolant's level of conductivity, and preventing the constant flow of coolant to the deionization unit.
The present invention is directed to a fuel cell system that sufficiently reduces the ion elution in the fuel cell system, making possible the extension of the life span of the ion-exchange resin in the deionization unit.
For example, a fuel cell system is described that includes a fuel cell stack, a coolant channel which cools said fuel cell stack, a conductivity detector which detects the conductivity of the coolant flowing in the coolant channel, a conductivity reducer which reduces the coolant conductivity, and a conductivity regulator which regulates the reduction amount of the conductivity by the conductivity reducer. By means of a fuel cell system with a structure such as this, the present invention, regulates the reduction amount of the conductivity by the conductivity reducer, so that the conductivity regulator maintains the conductivity range of the coolant at or below the allowance limit value and at or above the specified value.
In the fuel cell system of the present invention, it is recognized that the higher the level of the coolant's conductivity (given that it is within the range that does not cause harm to the fuel cell stack), then the lower the level of ion elution rate of the fuel cell system to the coolant. Thus, the described fuel cell system maintains the conductivity of the coolant supplied to the fuel cell stack for circulation at the highest level possible at or above a specified value, within the range allowed by the fuel cell stack. Consequently, it is possible to effectively suppress the increase in the coolant conductivity and simultaneously restrain the ion elution from the fuel cell system to a minimum. In the case of using a deionization unit with ion-exchange resin as a conductivity reducer, this results in extension of the life of the ion-exchange resin of the deionization unit.
In one embodiment, a fuel cell system comprises a fuel cell stack, a conductivity meter that detects a conductivity of a coolant that cools the fuel cell stack, and a conductivity reducer that reduces the conductivity of the coolant. The fuel system further includes a valve to control the flow of the coolant into the conductivity reducer, wherein the valve can prevent the flow of the coolant through the conductivity reducer. The conductivity controller regulates the amount the conductivity is reduced by adjusting the valve.
In another embodiment, a method comprises detecting a conductivity of a coolant for a fuel cell stack, and adjusting a valve to control the flow of the coolant through a conductivity reducer and maintain the conductivity within a conductivity range at or above a specified value and at or below an allowance limit value of the fuel cell stack.
In another embodiment, a fuel cell system comprises a fuel cell stack, means for detecting a conductivity of a coolant that that cools the fuel cell stack, means for reducing the conductivity of the coolant, means for adjusting the flow of the coolant through the reducing means, and means for controlling the conductivity of the coolant within a range selected to have a reduced conductivity-time gradient by controlling the conductivity of the coolant within a range selected to have a reduced conductivity-time gradient by controlling the adjusting means to block the flow of the coolant.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
The exemplary embodiment of the fuel cell system is comprised of a fuel cell stack 1 that generates electricity by the supply of a fuel gas (such as hydrogen) and an oxidizer gas (such as air), and also of a cooling system that provides a circulatory supply of coolant into fuel cell stack 1 for cooling purposes. The cooling system is comprised of a pump 2 that supplies coolant to the fuel cell stack 1, a deionization unit 3 that eliminates the ions dissolved in the coolant, and a radiator 4 that regulates the temperature of the coolant.
Each of the following parts is connected by coolant lines 5: the cooling pump 2 and the deionization unit 3, the deionization unit 3 and the fuel cell stack 1, the fuel cell stack 1 and the radiator 4, and the radiator 4 and the pump 2. Also, between the pump 2 and the fuel cell stack 1, a bypass line 6 is provided that bypasses the deionization unit 3, and a three-way valve 7 is provided at the meeting point of bypass line 6 and the coolant line 5, which is the latter part of the deionization unit 3.
Also, in one embodiment of the fuel cell system, the cooling system is equipped with a conductivity meter 8 that detects the conductivity of the coolant flowing in the coolant lines 5, a temperature sensor 9 that measures the temperature of the coolant, and a controller 10 that controls the flow rate of the coolant passing through the deionization unit 3 by operating the three-way valve 7, based on the detected value from the conductivity meter 8 and measured value from the temperature sensor 9. Each of these mentioned above, the controller 10, the conductivity meter 8, the temperature sensor 9, and the three-way valve 7, are connected to each other by control lines 11.
In the example embodiment of the fuel cell system as described above, the coolant lines 5 correspond to the coolant channel, the conductivity meter 8 corresponds to a conductivity detection means, and the deionization unit 3 corresponds to a conductivity reducer. Also, in this fuel cell system, the controller 10 operates the three-way valve 7 based on the coolant conductivity detected by the conductivity meter 8 and the coolant temperature measured by the temperature sensor 9, and controls the flow rate of coolant passing through the deionization unit 3.
In one embodiment, controller 10 regulates the reduction amount of conductivity by the deionization unit 3 so that the conductivity of the circulating coolant that is supplied by the cooling system to the fuel cell stack 1 is maintained within the conductivity range, at or under the allowance limit value for the fuel cell stack 1 and at or above a specified value. In some instances, controller 10 may adjust the valve 7 to completely prevent flow of the coolant through the deionization unit 3, thereby maintaining the conductivity above the specified value and below the allowance limit. In some embodiments, the allowance limit value may be set based on a safety factor offset from a permissible limit for the fuel cell stack 1. A specific description of the conductivity control of the coolant, which is characteristic to this fuel cell system in this example embodiment, is described below.
This means that by maintaining the coolant conductivity at a certain high level, the increase in conductivity, in another words, the elution of ions into the coolant, can be suppressed. Therefore, the example embodiment of the fuel cell system regulates the reduction amount of conductivity by the deionization unit 3, within the range of the allowable conductivity (allowance limit value) c4 or lower for the fuel cell stack 1. As a result, the conductivity has a smaller gradient than the gradient at the time of low conductivity when the gradient of time change of the conductivity reaches a maximum. For this reason, controller 10 maintains the coolant conductivity at a relatively high level where the gradient is smaller. The allowable limit conductivity c4 for the fuel cell stack 1 can be determined by the insulation resistance of the fuel cell system or the corrosiveness of each of the parts where coolant flows.
In the example embodiment of the fuel cell system, it is possible to reduce the ion elution amount from the fuel cell system that must be eliminated in the deionization unit 3, and extend the life span of the ion-exchange resin in the deionization unit 3 by using the controls mentioned above. In general, the “life span” of the ion-exchange resin refers to the period during which the total ion-exchange equivalent of the ion-exchange resin is used up by absorbing ions from the coolant.
To confirm the effectiveness of the present invention, a case using the conventional control techniques of maintaining a low coolant conductivity and a case using the control technique of the present invention of maintaining a relatively high conductivity were performed. The life span of the ion-exchange resin in the deionization unit 3 for each case was compared. During the tests, the coolant temperature was to be maintained at a steady 80° C. and the fuel cell system was structured so that the relationship of the ion elution rate would be similar to the one in
In addition, the deionization unit 3 was given the capability to reduce only 50% of the coolant conductivity flowing into the deionization unit 3, and the ion equivalent per unit conductivity in the coolant was set to 1 meq/(μS/cm), the ion equivalent that can be eliminated by the ion-exchange resin in the deionization unit 3 was set to 100 meq, the allowable conductivity determined by the insulation resistance of the fuel cell system was set to 15 μS/cm, and the duration of the coolant circulating once in the cooling system of the fuel cell system was set to 1 minute.
As shown in
The above was explained under the condition that the coolant temperature was at a steady 80° C. However, the actual coolant temperature is subject to change according to the operating conditions. In the case where the coolant temperature changes, in order to more effectively extend the life of the ion-exchange resin as stated above, it may be preferable to set the target conductivity at a low level while the coolant temperature is high, and set the target conductivity at a high level while the coolant temperature is low. This is because the lower the temperature, the higher the allowed conductivity, generally.
In the described fuel cell system, the reduction amount of the conductivity by the deionization unit 3 is regulated by controlling valve 7. Valve 7 may, for example, be closed to block coolant from flowing through deionization unit 3 and completely blocking deionization unit 3 from coolant line 5 in order to maintain a relatively high level (that is within the range of at or under the allowable limit value accepted by the fuel cell stack 1) of conductivity of the circulating coolant that is supplied to the fuel cell stack 1 by the cooling system. Therefore, it is possible to effectively suppress the increase in coolant conductivity while keeping the ion elution from the fuel cell system to coolant at a minimum, and to extend the life span of the ion-exchange resin in the deionization unit 3. Also, at this time, by setting the coolant conductivity in the range that will not cause leakage of electricity or influence the corrosion of parts, it is possible to operate safely and extend the life of each part that comprises the fuel cell system.
Moreover, when the temperature of the coolant is low, the target conductivity is set high to decrease the reduction amount of conductivity by the deionization unit 3, and when the temperature of coolant is high, the target conductivity is set low to increase the reduction amount of conductivity by the deionization unit 3. This extends the life span of the ion-exchange resin in the deionization unit 3, by suppressing the ion elution in the most suitable condition according to the temperature requirement of the coolant.
Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.
Number | Date | Country | Kind |
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2004-269340 | Sep 2004 | JP | national |