Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2002-02-13
2004-07-20
Kalafut, Stephen J. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000, C429S006000
Reexamination Certificate
active
06764782
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electrical isolation system for a fuel cell stack and to a method of operating a fuel cell stack.
BACKGROUND OF THE INVENTION
Fuel cell stacks comprise a plurality of fuel cells connected in series and/or in parallel. There are many different designs of fuel cells, some of which operate at extremely high temperatures and others of which operate at relatively low temperatures. Fuel cells, which operate at relatively low temperatures, tend to be preferred for use as power plants in vehicles. There are various types of low temperature fuel cells. One frequently used type of fuel cell for vehicle applications is the so-called PEM fuel cell (Proton Exchange Membrane). In a fuel cell of this kind, an anode electrode and a cathode electrode both coated with catalyst material are separated by a synthetic membrane and the assembly comprising the two electrodes separated by the membrane, frequently called an MEA (membrane electrode assembly) is enclosed between two conductive plates referred to as bipolar plates. In a fuel cell stack a plurality of fuel cells are arranged side by side so that each bipolar plate (apart from the end plates of the stack) is associated with two adjacent fuel cells. The bipolar plates are provided at their sides facing the electrodes with passages or channels which enable hydrogen to be fed to the anode electrode of one fuel cell and oxygen in the form of air to be fed to the cathode electrode of a neighboring fuel cell. When the fuel cell is in operation the protons delivered by the hydrogen migrate through the membrane and combine with the oxygen to form water and generate electricity. When a plurality of fuel cells are arranged in a stack, the bipolar plates serve as a separator between adjacent fuel cells, that is to say the bipolar plate has at one side passages for directing hydrogen to the anode of one fuel cell and at the other side passages for directing air to the cathode of an adjacent fuel cell and keeps these gas flows separated.
In the operation of such fuel cells, heat is generated and provision is made for cooling the fuel cells. This cooling is effected by incorporating cooling passages into the bipolar plates through which a coolant flows. Thus, the bipolar plates have a separating function in that they separate adjacent fuel cells. At the same time, they are connected together electrically, in series and/or in parallel, in order to connect them into a power circuit by which the electricity generated by the fuel cell can be extracted. A typical PEM cell produces an output voltage of about 0.9 V. In a typical fuel cell stack there are a sufficient number of fuel cells to produce a relatively high operating voltage, typically in the range from 100 to 400 V. Fuel cells with high operating voltages are the subject of stringent safety requirements, particularly when liquid coolants are used to cool the fuel cell stack. Previous attempts to meet these requirements have focused on trying to achieve complete isolation of the coolant circuit involving radiators, pumps, tubes as well as complete isolation of the fuel cell stack itself. Attempts have also been made to use non-conductive liquids as the coolant, which is intended to prevent dangerous voltage levels at the fuel cell stack being transmitted by the coolant to the radiator and other components which would prevent a serious safety hazard.
The electrical isolation of large components, such as radiators, is however not very practical in a vehicle or in any other system due to size constraints and problems associated with the blocking of cooling air. The use of non-conductive coolants (for example oil) has significant disadvantages because the physical properties of such coolants, such as heat capacity, heat conductivity, and viscosity, are restricted. Moreover, such non-conductive coolants pose an environmental problem since there is always the danger of leakage, for example if connections fail or in the event of accident damage Moreover, there are particular problems in operating such coolants at low temperatures. Such disadvantageous properties adversely affect the system power density the radiator size and the power required to drive radiator fans and coolant pumps.
Because of these disadvantages attention has been paid to using water plus anti-freeze based coolants for liquid cooling. However, it is important to use a coolant with a relatively low conductivity. As explained above the bipolar plates of the fuel cells of the fuel cell stack are connected electrically in series and/or parallel and the liquid coolant flows in parallel through the bipolar plates. Thus, if the liquid coolant is conductive it effectively represents a ground fault of the bipolar plates, which is clearly undesirable.
Liquid coolants are available with a relatively low conductivity favorable for use in fuel cells. However, there is always the danger, in the practical use of a fuel cell system, that someone could add the wrong coolant to the system. The liquid coolants used in fuel cell stacks are also critical from the point of view that they must be designed to avoid corrosive and electrolytic effects, which could lead to long-term deterioration of the fuel cell stack.
Moreover, it is known that liquid coolants deteriorate in use over a longer period of time.
In addition to the aforementioned problems there is also a general problem with fuel cell stacks in as much as faults can occur which lead to a deterioration or failure of the isolation of the fuel cell stack, which could lead to dangerous situations. Such dangerous situations could be particularly acute if the vehicle has been involved in an accident or if some other malfunction has taken place which impairs the quality of the isolation.
SUMMARY OF THE INVENTION
In view of the above mentioned problems, it is an object of the present invention to provide an electrical isolation system for a fuel cell stack and a method of operating a fuel cell stack such that the quality of the electrical isolation can be continuously monitored and such that safety measures can be taken in the event of faulty isolation to prevent damage to the fuel cell stack and associated components and to prevent dangerous situations due to inadequate electrical isolation.
Moreover, it is a further object of the present invention to make available an electrical isolation system and a method of the above named kind which can be implemented at relatively low cost and which operates reliably.
It is a yet further object of the present invention to provide an electrical isolation system and a method of the above named kind which enables a realistic approach to be taken to considerations such as deterioration of the liquid coolant which necessarily occurs over a period of time, with it being possible to ensure the liquid coolant is changed on time before deterioration has reached a critical level.
In order to satisfy these objects there is provided, in accordance with the present invention, an electrical isolation system for a fuel cell stack comprising a plurality of fuel cells connected in series and a coolant circuit for cooling said fuel cells in operation using a liquid coolant having a restricted electrical conductivity, said fuel cell stack being associated with a chassis having a chassis ground and comprising a plurality of coolant passages for said fuel cells, said coolant passages being connected in parallel and/or in series and said coolant circuit comprising an inlet for feeding said liquid coolant into said stack and into said coolant passages, an outlet for removing said liquid coolant from said stack after flow through said coolant passages, a radiator provided as a heat exchanger to cool said liquid coolant and having an inlet and an outlet, a first coolant flow line connecting said radiator outlet to said fuel cell stack inlet, a second coolant flow line connecting said stack outlet to said radiator inlet and a pump for circulating liquid coolant in said coolant circuit, wherein said coolant circuit comprises a plurality of con
Hinz Hartmut
Raiser Stephen
Brooks Cary W.
General Motors Corporation
Kalafut Stephen J.
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