WO2023224492A1

WO2023224492A1 - An isolated fuel cell system

Info

Publication number: WO2023224492A1
Application number: PCT/NO2023/050115
Authority: WO
Inventors: Kristian EIKELAND HOLMEFJORD
Original assignee: Corvus Energy AS
Priority date: 2022-05-19
Filing date: 2023-05-19
Publication date: 2023-11-23
Also published as: NO20220590A1

Abstract

Described herein is a fuel cell system comprising: a module assembly comprising a fuel cell stack including at least one fuel cell; a hot side cooling circuit comprising at least one hot side conduit for transferring cooling fluid into, through and out of the module assembly to transfer heat away from the fuel cell stack; and at least one resistive component for electrically isolating all conductive components of the hot side cooling circuit and the module assembly from a system ground.

Description

An Isolated Fuel Cell System

The present invention relates to a fuel cell system, and in particular to a fuel cell system wherein the module assembly, including the fuel cell stack, and a hot side cooling circuit are electrically isolated from a system ground.

Fuel cell systems generally contain a plurality of single cells, placed adjacent one another and forming a fuel cell stack having a layered structure. Each of the cells within the stack is able to generate electrical energy from chemical energy via redox (reduction-oxidation) reactions when supplied with oxygen and fuel. Fuel cells are often used to power vehicles, for example, or to provide a reliable emergency backup power source, or a primary or secondary power source on seafaring vessels.

The oxygen and the fuel, which may be hydrogen, are continuously fed to the fuel cells of the stack while the system is in operation. The cells each comprise an anode, a cathode, and a layer of a selected electrolyte within which ions can flow between the two. It is this flow of ions which allows for transfer of charge between the cathode and anode, thereby creating an electrical current which can be transferred through an external load circuit.

There are many different types of fuel cells currently in use, and these use different compounds for the fuel, or different configurations and materials for the anode, cathode, and the electrolyte. One specific type of fuel cell, most often used for powering vehicles, is the proton exchange membrane (PEM) fuel cell. PEM fuel cells generally comprise two opposing plates forming a housing and providing pathways for fuel and oxygen to flow through the cell. Between the two plates are the cathode and the anode, which sandwich a polymer electrolyte membrane, configured to allow protons, but not electrons, to pass through. Electrons are stripped from the hydrogen at the anode and these travel through the external load circuit and on to the cathode to provide the electric current through the system. The remaining protons travel through the polymer electrolyte membrane to the cathode, where they react with the incoming oxygen and the electrons travelling from the external load circuit to form water, which is removed from the system. Various other materials, such as catalysts for splitting the hydrogen and the oxygen entering the cell, can be included.

Efficient operation of the system depends on the fuel cell stack being maintained at a desired temperature, and for this reason fuel cell stacks are also typically provided with cooling circuits to carry cooling fluid via the stack, and sometimes through an external heat exchanger such as a radiator, to transfer heat from the stack to the surrounding environment. Parts of the system in direct contact with the stack may become electrically charged, and may have the potential to cause a great deal of harm to a person coming into contact with these parts. To prevent this, one or more of the electrically conductive components within the cooling circuit running through the cell stack are coupled to a system ground. This traditional configuration is shown in figure 1 , which illustrates a prior art system comprising a module assembly including the fuel cell stack and a cooling circuit, wherein the module assembly ground is coupled to system ground.

Water as the cooling fluid is a fairly common choice, since this ensures a functional cooling method for a fuel cell, but this also introduces a conductive path between DC+ to ground and DC- to ground if the conductive cooling liquid is in contact with grounded pipes or heat exchangers. In systems using PEM fuel cells, and in particular within larger systems comprising multiple fuel cells connected in parallel, the low resistance between any DC conductors and ground (a result of these components being grounded) can result in a number of technical challenges. One example of a challenge with this type of system is the difficulty in carrying out effective ground fault monitoring. This low resistance (which is in the kQ range) will dominate measurements used to detect ground faults in the load circuit. As a result, when a ground fault does occur, which will lower the resistance in another part of the circuit, this will not be so easily detected. Another issue is leakage of charge through the ground connection, with a negative effect on the efficiency of the system being one possible consequence. An example of a typical system with status monitoring is described in US-A- 2002/0192521 , which relates to an isolation system for a fuel cell stack. The system is designed for use in a vehicle, and various conductive parts of the housing of the stack and the cooling circuit are coupled to the system ground. A resistance between a fuel cell of the stack and system or chassis ground is measured to monitor the status of the system.

According to a first aspect of the present invention, there is provided a fuel cell system comprising: a module assembly comprising a fuel cell stack including at least one fuel cell; a hot side cooling circuit comprising at least one hot side conduit for transferring cooling fluid into, through and out of the module assembly to transfer heat away from the fuel cell stack; and at least one resistive component for electrically isolating all conductive components of the hot side cooling circuit and the module assembly from a system ground.

The electrical isolation of both the module assembly and components of the hot side cooling circuit results in a fuel cell system which is efficient to run. The configuration of the system also means that more effective ground fault monitoring can be carried out. In embodiments, the fuel cell assembly comprises a module support structure within which the fuel cell stack is contained and supported. This support structure can take the form of a housing or a frame for containing or supporting the other components of the module assembly, including the fuel cell stack. This module support structure, being part of the module assembly, will also be isolated from system ground. The module assembly (including the module support structure if present) may be electrically coupled to a module ground, but in such a case the module ground is isolated from system ground, which is in contrast to the system shown in figure 1. The hot side conduit may be configured to transfer cooling fluid through the fuel cell stack within the module assembly.

Electrically isolating the conductive components of the hot side cooling circuit using the one or more resistive components refers to the fact that the resistance introduced by these components is high enough that there is substantially no current flow from any part of the hot side cooling circuit or module assembly through to system ground. Preferably, the resistance provided by the at least one resistive component is in the MQ (Megaohm) range, and may be equal to or greater than 1MQ, preferably greater than 5MQ, and ideally greater than 10MQ. The entirety of these components of the system can thus be considered as electrically isolated from the system ground. The resistive component can comprise a non-conductive cooling fluid in contact with the hot side conduit for transferring heat (but not electrical current) away from the hot side circuit and/or can comprise a solid structure in contact with the hot side cooling circuit and/or the module assembly.

In embodiments, the resistance due to the resistive component is at least 1MQ.

In embodiments, the cold side cooling fluid is oil.

In embodiments, the hot side cooling circuit is configured to transfer heat away from the fuel cell stack and through a heat exchanger. The hot side cooling circuit can in some cases carry cooling fluid in a closed loop through the module assembly where heat is transferred to the cooling fluid, through the heat exchanger where heat is removed from the cooling fluid, and back to the module assembly again.

In an embodiment, the hot side cooling circuit comprises a heat exchanger through which the hot side conduit is configured to transfer the cooling fluid. The hot side cooling conduit is therefore configured to transfer the cooling fluid via a heat exchanger after the cooling fluid has travelled through the module assembly. The heat exchanger is then the primary means by which heat is transferred away from the fuel cell stack. In an embodiment, the fuel cell system comprises the heat exchanger. The heat exchanger can be a radiator, a series of pipes, or a portion of the hot side conduit having a larger surface area than the rest of the conduit for efficient transfer of heat away from the hot side cooling fluid.

Parts of a more extensive cooling system may be coupled to system ground as described below in a case where a cold side cooling circuit, as well as the hot side cooling circuit, runs through the heat exchanger. However, the part of the cooling system carrying fluid through the fuel cell stack itself (the hot side cooling circuit), will always be electrically isolated from system ground. There is not necessarily a corresponding cold side closed-loop cooling circuit included as part of the system, and the hot side cooling circuit may act to transfer heat directly to the environment, or to cold side cooling fluid which flows past the heat exchanger from and to the surroundings. This can be so, for example, if the heat exchanger through which the hot side conduit runs transfers energy to the surroundings, such as to surrounding air. The heat exchanger in this case may be a radiator.

System ground refers to the neutral point of a connected system including surfaces which can be touched by a person without current flowing through them to the earth (without them receiving an electrical shock). System ground may be represented by the hull or body of a ship and connected surfaces, or the body of a vehicle and connected surfaces, for example. Generally, any surfaces of a vehicle or area (floors, walls, doors, and so on) which are coupled to system ground can be contacted safely by a person.

In embodiments, the module assembly comprises a module support structure for containing the fuel cell stack, wherein the module support structure is isolated from system ground. The module support structure represents a supporting frame, and in some cases a surrounding housing, to support elements of the fuel cell stack and related components of the fuel cell assembly. In some cases, the module assembly may include a housing which entirely surrounds and encloses the fuel cell, but in general the support structure will comprise a frame of some sort for holding the fuel cell stack and other components of the module assembly in position.

In embodiments, the module assembly comprises a frame or housing configured to contain and support the fuel cell stack, and to hold the fuel cell stack in position.

In embodiments, the at least one resistive component comprises one or more components of the heat exchanger. The required high electrical resistance is therefore provided by elements of the heat exchanger itself. This is an efficient method for electrically isolating the fuel cell assembly and hot side cooling circuit from other parts which will most often extend further from the stack such as the conduits of any cold side cooling circuit.

In embodiments, the system comprises a cold side cooling circuit comprising at least one cold side conduit for transferring cold side cooling fluid away from the heat exchanger (usually directing cold side cooling fluid into, through and out of the heat exchanger), and the at least one resistive component is arranged to electrically isolate components of the hot side cooling circuit from components of the cold side cooling circuit within the heat exchanger. The cold side cooling circuit can be coupled to system ground, so that the danger of electrical shocks when coming into contact with this part of the system is eliminated. Proper, simple, monitoring of ground faults in the load circuit can be achieved as a result of the isolation, as explained above. The cold side cooling fluid is electrically isolated from the hot side cooling fluid, and this isolation is achieved by one or more components of the heat exchanger as the one or more resistive components.

In embodiments, the resistive component comprises the cold side cooling fluid, which is non-conductive. The non-conductive cold side cooling fluid may be air, and in some cases, this can be directed past the heat exchanger either within a conduit which forms a closed-loop, or from and to the surroundings (i.e. in an open conduit). In some cases, the non-conductive cold side cooling fluid can be oil. In some cases, this can be carried within a closed-loop circuit through the heat exchanger and can itself be cooled in a second heat exchanger.

In embodiments, the heat exchanger comprises a heat exchanger housing within which a hot side conduit of the hot side cooling circuit flows in order to transfer heat away from the hot side cooling fluid. In embodiments, the hot side conduit forms a tortuous path through the housing of the heat exchanger. The conduit thus bends and twists to increase the surface area of the conduit in order to maximise cooling. In embodiments, the hot side conduit is arranged to carry hot side cooling fluid in both directions along the housing. This conduit may in such a case run from an inlet at one end of the housing along the housing to a second end, and back again to the first end. The outlet may be at the first end, or the conduit may double back for a third pass (or more than three passes) through the housing. The housing may be elongated, and the first and second ends may be located at each end of a longitudinal axis of the housing so that the hot side conduit runs along the housing in a longitudinal direction. In embodiments, the hot side conduit runs adjacent to, contains, or runs within a cold side cooling conduit, so that heat is transferred from the cooling fluid in the hot side conduit to the cooling fluid in the cold side conduit across an interface. The hot side conduit may run within the cold side cooling conduit in some cases as mentioned. For example, the cold side cooling fluid may fill the rest of the housing, and may surround the hot side conduit(s) running therethrough. In embodiments, the hot side conduit within the heat exchanger comprises a plurality of conduits (such as tubes or pipes) carrying fluid in the same direction through the heat exchanger. These multiple conduits may direct fluid from one end to another and then back again one or more times, as described above. This further increases the surface area available for cooling.

In embodiments, the at least one resistive component comprises a non-conductive coating applied to surfaces within the heat exchanger. In embodiments, the non- conductive coating is applied to surfaces of the hot side conduit within the heat exchanger, such as walls of the hot side conduit. The non-conductive coating may be applied to all interfaces or walls which are contacted by both the hot side cooling fluid and the cold side cooling fluid. This may be a wall of the hot side cooling circuit around which cold side cooling fluid flows, and the coating may be applied to one or both sides of this wall. Whatever the configuration of the heat exchanger, the choice of the surface or surfaces to which the coating is applied is generally made so as to prevent the flow of electrical current from elements of the hot side conduit to elements of the cold side conduit or the surrounding environment.

The coating may be applied on one or both sides of the interface between the hot side conduit and either a cold side conduit or the surrounding environment. Generally, the interface will also represent a physical barrier which prevents mixing of the hot side cooling fluid and a cold side cooling fluid, whilst allowing transfer of heat from the hot side cooling fluid to the cold side cooling fluid or the surroundings. This physical barrier or interface can be coated with the non-conductive paint to prevent flow of current/electricity from the hot side cooling fluid and/or the hot side conduit to the cold side cooling fluid, and/or a cold side conduit if present. The coating can be applied only on one side of the interface (either the hot side or the cold side) or on both the hot and cold sides of the interface. This coating, which is not electrically conductive (has a high resistivity), is applied so as to prevent the flow of current from conductive parts of the hot side conduit and/or the fluid therein to adjacent parts of the heat exchanger. Heat is transferred away from the hot side cooling fluid within the heat exchanger, but there is no path for electrical current to flow away from the hot side conduit and the fluid therein inside the heat exchanger. The structure of the heat exchanger does not in this case need to be altered significantly and additional resistors and the like are not required. The isolating resistance for the module housing and the hot side cooling circuit components can be provided solely by the non-conductive coating.

The conductive coating may be an anti-corrosion coating. In embodiments, the non- conductive coating is an anti-corrosion paint. Anti-corrosion paints prevent the flow of ions (which is linked to corrosion), and so are also effective in preventing the conduction of electricity. This type of paint has not been put to such a use before, but its application to conduits of the heat exchanger will be extremely effective as a means for isolating the fuel cell assembly and hot side cooling circuit from system ground. Such paints are cheap, as well as being easy to produce and apply. The anti-corrosive/non-conductive coating may comprise ceramic particles. In some cases, the ceramic particles can be combined with a polymer binder. The paint may be solvent based (i.e. containing a solvent such as butyl acetate or xylene).

In embodiments, at least one conductive component of the cold side cooling circuit is coupled to system ground. It may be that coupling one component to system ground ensures that all of the conductive components are coupled to system ground, or if some components are electrically isolated from each other, each separate conductive component can be separately connected to ground to prevent electrical shocks from contact with the cold side cooling circuit.

In embodiments, the system comprises an outer enclosure for surrounding the module assembly (including the module support structure if present). The outer enclosure is electrically isolated from the module assembly and hot side cooling circuit and can be coupled to system ground in order to prevent electric shocks when contacting the enclosure. This represents a simple way to minimise the risk of electric shocks from the non-grounded module assembly. Personnel are able to touch the enclosure and current does not flow through this from the module assembly, so that the enclosure provides protection against contact with the parts contained within it. The enclosure may substantially completely enclose and surround the module assembly within, although there will be inlets and outlets provided for at least air, fuel, water, and cooling fluid. The structure of the enclosure will be such that physical (or electrical) contact with the module assembly within is prevented as far as possible. The enclosure may also completely surround the hot side cooling circuit and the heat exchanger if present. Alternatively, part of the hot side conduit can extend outward of the enclosure and/or the heat exchanger can be located outward of the enclosure. If parts of the hot side cooling circuit and/or the heat exchanger extend outward of the enclosure, physical barriers can be included to prevent contact with these parts. Although these are not necessarily present, additional barriers to prevent contact with the hot side cooling circuit will help to improve the safety of the system.

In embodiments, the outer enclosure is electrically isolated from the module assembly (representing a module ground) by non-conductive elements positioned between the outer surfaces of the module assembly and the inner surface of the outer enclosure. In embodiments, the non-conductive elements are formed of rubber or some other isolating material. The non-conductive parts may comprise spacers placed between the module assembly and the enclosure to prevent direct contact between the two. In some cases, the entire enclosure may be formed of a non- conductive material. The enclosure can be closed, or can be partially open, such as at one or each end.

In embodiments, the hot side cooling circuit is a closed circuit. A pump or another means may be provided to move the hot side cooling fluid through the circuit, from the pump, through the module assembly via the fuel cells of the stack to cool them, through the heat exchanger, where heat is removed from the cooling fluid, and back to the pump. The pump may operate continuously when the fuel cells are in operation.

In embodiments, the module assembly comprises a local control unit for receiving control signals and controlling operation of the fuel cell stack. This local control unit will generally be coupled to module ground and will be located within the module assembly support structure near to the fuel cell stack itself. In embodiments, the system also comprises a main control unit in communication with the local control unit for sending the control signals to the local control unit, and the local control unit and main control unit are electrically isolated from one another. The main control unit will usually be positioned externally to the module assembly to make this electrical isolation simpler. The main control unit will generally be coupled to system ground and the local control unit will be coupled to a module ground. A conductive path between the two units would therefore provide a path for current to flow between system ground and module ground. To prevent this, the two control units can be galvanically isolated from one another by ensuring that there is no conductive wiring extending all of the way between the two, i.e. no path for current to flow between the two units. The isolation can be achieved using, for example, a transformer located in the signal path between the local and main control units, or via the use of optical fibers or other (non-electrically conductive) means to transfer the control signals between the two units.

In embodiments, the module assembly comprises core components for power generation. These will also be located within the module support structure if present. In embodiments, the at least one fuel cell is a proton exchange membrane fuel cell.

According to a second aspect of the present invention, there is provided a fuel cell system comprising: a module assembly comprising a fuel cell stack including at least one fuel cell; a hot side cooling circuit comprising at least one hot side conduit for transferring hot side cooling fluid through the module assembly and to a heat exchanger to transfer heat away from the fuel cell stack; and a cold side cooling circuit for transferring a non-conductive cold side cooling fluid through the heat exchanger, such that heat is transferred within the heat exchanger from the hot side cooling fluid to the cold side cooling fluid but all conductive components of the hot side cooling circuit and module assembly are electrically isolated from a system ground. The non-conductive cold side cooling fluid is therefore a resistive component for electrically isolating all conductive components of the hot side cooling circuit and the module assembly from the system ground.

In embodiments, the hot side cooling fluid is a water-based fluid. In embodiments, the cold side cooling fluid is oil.

In embodiments, one or more of the hot side cooling circuit and the cold side cooling circuits are closed loops passing through the heat exchanger.

In embodiments, the system comprises a third cooling circuit for transferring cooling fluid through a second heat exchanger to transfer heat away from the cold side cooling fluid. The cold side cooling circuit also therefore passes through this second heat exchanger. The third cooling circuit may be configured to transfer seawater through the second heat exchanger as the cooling fluid therein.

In embodiments, at least one conductive component of the cold side cooling circuit is coupled to system ground. In embodiments, the system comprises an outer enclosure for surrounding the module assembly, wherein the outer enclosure is electrically isolated from the module assembly.

In embodiments, the outer enclosure is electrically isolated from the module assembly by non-conductive elements positioned between the outer surface of the module assembly and the inner surface of the outer enclosure.

In embodiments, the module assembly comprises a module support structure configured to contain and support the fuel cell stack.

In embodiments, the at least one fuel cell is a proton exchange membrane fuel cell.

In embodiments, the module assembly comprises a local control unit, the system comprises a main control unit external to the module assembly for sending control signals to the local control unit, and the local control unit and main control unit are electrically isolated from one another.

Embodiments of the present invention will now be described, by way of example only, with reference to the following diagrams wherein:

Figure 1 shows a circuit diagram for a prior art fuel cell module with cooling circuit;

Figure 2 illustrates components of a fuel cell system;

Figure 3 shows a circuit diagram for a fuel cell module and cooling circuit, where the module housing and hot side cooling circuit are isolated from system ground;

Figure 4A illustrates an outer enclosure;

Figure 4B illustrates a module assembly including a module support structure; Figure 4C shows an enclosure with a module assembly installed;

Figure 4D illustrates a possible means for closing the enclosure with the module assembly mounted therein; and

Figure 5 shows a possible configuration for a heat exchanger.

Figure 2 illustrates the structure of a fuel cell system comprising a module assembly 4 including a module support structure 6, representing the module assembly boundary. The components of the fuel stack 8 are held within the module support structure. The stack itself comprises multiple fuel cells, which may be arranged so as to share intervening walls as well as two end plates and an electrode adjacent each end plate which can be used to couple the stack to a load. An insulating layer is usually positioned between each of the electrodes and the adjacent end plate. A housing 12 of the fuel cell stack 8, which can include the end plates, surrounds and supports the cells of the stack and is itself positioned within the support structure if present. The voltage may vary for the different parts of the stack, but the components of the stack are generally associated with very high voltages and contact with these parts of the system should be avoided. The module assembly 4 in this case also comprises a local control unit 10 for receiving control signals and controlling operation of the stack, and DC/DC converter 11 , which is also associated with the fuel cell stack. The housing 12 of the fuel cell stack is permeable to both air and hydrogen, as shown. The local control unit 10 can be in communication with a main or external control unit (not shown in the figure) to transfer control signals to the module assembly, as described above. The main and local control units can be electrically isolated from one another to prevent current flowing between the two using a transformer or by coupling the units for signal transfer using optical fibers or other non-conductive means.

The hot side cooling circuit 16 is shown in figure 2, forming a closed loop within which hot side cooling fluid flows driven by a pump 18. This runs through the fuel cells of the stack to transfer heat away from the stack itself. Although not always the case, one or more additional cooling circuits can be included, and these can function to transfer heat away from electronic components of the fuel cell stack other than the fuel cells themselves. The hot side cooling circuit 16 can be completely contained within an outer enclosure 2 (shown in figure 4A), or can extend outward of this in some cases. The hot side cooling circuit is only partly contained within the inner module assembly support structure 6. The heat exchanger 24 is generally either located within the outer enclosure but outside of the module assembly support structure, or is located outside of both, with the hot side conduit extending through the walls of the outer enclosure.

As mentioned above, the traditional fuel cell system shown in figure 1 includes an electrical coupling between the module support structure 6 or module ground 20 and the system ground 22. Such a system has a low resistance (in the kQ range) between one or more conductors within the module assembly and hot side cooling circuit and ground (i.e., parts of the hot side cooling circuit, the fuel cell module assembly and any module assembly housing and auxiliaries are all grounded). Connecting multiple systems of this type in parallel will further lower the resistance, meaning that proper monitoring of ground faults is made even more difficult. A traditional system is also associated with some leakage of current through the ground connection, which reduces efficiency overall.

In the system described herein, in contrast to a traditional setup, the module housing of the fuel cell stack is electrically decoupled or is electrically isolated from the system ground 22 (i.e. main ship ground if located on a marine vessel). This means that the usual low conductor-to-ground resistance can be eliminated, which allows for effective ground fault monitoring and more efficient operation, among other things.

In one example, electrical isolation of the cooling circuit and module housing can be achieved by electrically isolating the fuel cell module from its surroundings and including a portion of the heat exchanger 24 (or another device) having a very high electrical resistance, so that the current to system ground is effectively zero. Any current within conductive components of the hot side cooling circuit and the module assembly itself will also therefore be prevented from running to ground.

In Figure 3, which shows a circuit diagram for a system implementing additional isolation of the module assembly 4 and hot side cooling circuit from ground, an electrically isolating heat exchanger 24 is added. A possible configuration for the heat exchanger 24 will be described below, but this component effectively introduces a very high resistance which prevents current flowing from the hot side cooling circuit to system ground. The hot side cooling circuit comprises one or more hot side conduit(s) carrying cooling fluid both through the heat exchanger 24 and into the module assembly 4, where it passes through or around the fuel cells of the stack. In this system, the conductor to ground resistance can be increased significantly, allowing for multiple fuel cell systems to be connected in parallel.

A potential disadvantage of the apparatus is an increased risk of electrical shocks when handling or using the system, due to the fact that the module assembly 4 and the hot side cooling circuit 16 are not coupled to system ground 22. In order to mitigate these risks, the module assembly and components of the hot side cooling circuit, as well as any auxiliaries, are housed within an outer enclosure 2 which is not electrically conductive, or which is electrically isolated from the components contained within it. These components will include at least the module assembly and sometimes also the whole of the hot side cooling circuit 16 and the heat exchanger 24. The heat exchanger 24 can also be located outward of the outer enclosure 2 as mentioned above, in which case the hot side conduit will pass twice through the enclosure on the way to and from the heat exchanger. If the heat exchanger, or another part of the hot side conduit, is located outward of the outer enclosure, it can be sensible to include additional protection means, such as a physical barrier, to prevent contact with these parts.

This additional outer enclosure will help to prevent personnel accessing and touching parts of the system which are not coupled to system ground, including the module assembly, during operation. An example configuration for an enclosure 2 of this type is shown in figure 4A. In this case ends of the enclosure 2 are open, which allows access to the module and any other components within. An example configuration for a module assembly 4, including the fuel cell stack 8 within (contained within fuel cell stack housing 12), is shown in figure 4B. Here the module support assembly 6 is formed as a frame which surrounds and provides support to the internal components including the stack. The module assembly may be smaller in length than the enclosure, however, meaning that it is necessary to reach into the enclosure to contact the module assembly, and accidental contact is unlikely. The outer enclosure can be gas-tight, which helps to ensure that the elements contained within it are properly physically isolated and cannot be contacted by personnel nearby, or by other objects which could cause a ground fault. The module assembly is placed within the enclosure, along with all or a section of the conduit(s) carrying hot side cooling fluid. Additional inlets and outlets are provided to the enclosure for oxygen, hydrogen, and either hot or cold side cooling fluid depending on the position of the heat exchanger, as shown in figure 4C. Once the module assembly is in place, the enclosure can be closed by way of one or more caps, doors, or plates, as shown in figure 4D. This closure may render the enclosure gas-tight in some cases, as mentioned above. Clearly the inlets and outlets for the various gases, cold side cooling fluid, and fuel, will still be present to allow materials to pass from inside the enclosure to outside (or vice-versa) for operation of the fuel cell stack, but the enclosure may be sealed around these inlets and outlets.

Rubber sheets, fittings, or tubing can be positioned between the enclosure 2 and the module support structure 6 in order to physically separate the module assembly from the enclosure. These rubber components have high resistivity, and can effectively electrically isolate the support structure of the module assembly from the surrounding outer enclosure.

As mentioned, heat is carried away from the fuel cells of the stack via a heat exchanger 24 which both efficiently transfers heat and electrically isolates the hot side cooling circuit 16 from a cold side cooling circuit 28. The hot side cooling circuit comprises one or more conduits 26, which may be pipes or tubes as shown, and which carry the hot side cooling fluid around the components of the fuel cell. Within the module assembly, elements of the fuel cell stack may form channels which carry the cooling fluid through and along the walls of the stack itself, as close as possible to the anodes and cathodes of the fuel cells which require cooling. There may be a plurality of channels within the fuel cell stack, and fewer or only one channel carrying cooling fluid from the heat exchanger 24 into and out of the module assembly 4 through the support structure 6. Whatever the structure of the pathways for the cooling fluid, the hot side cooling circuit functions to circulate hot side cooling fluid through or around the hot parts of the fuel cell stack and back out towards the heat exchanger, within which heat is transferred away from the hot side cooling fluid within the hot side conduits. In most cases, this cooling fluid will flow back to the fuel cells again after losing heat as it passes through the heat exchanger, so that the hot side cooling circuit represents a closed loop. The hot side conduits 26 (and the cold side conduits 32 if present) may be formed at least partly from metal or other conductive components.

The hot side cooling fluid may be water. The cooling fluid for both hot and cold side cooling circuits can comprise a water-based fluid, which may or may not include additives. There may be no additives in the cooling fluid on the cold side, but the hot side cooling fluid will generally include at least additives for reducing conductivity of the fluid. The cold side cooling circuit 28 may comprise a cooling loop which passes through a second heat exchanger through which salt water, such as sea water, is passed around the conduits of the cooling circuit. Seawater is obviously a convenient cooling fluid for use on a seafaring vessel, but it is corrosive, so there will usually only be one such heat exchanger located on board any vessel. This will have a very solid structure (formed of stainless steel or similar) in order to resist corrosion as far as possible. One example uses a water-based cooling fluid as the hot side cooling fluid, and oil as the cold side cooling fluid. The oil is non-conductive and can provide the resistance required to prevent flow of current between the hot side and cold side cooling fluid within the heat exchanger (can act as the resistive component). Optionally, an additional cooling circuit can be added to further cool the cold side cooling fluid (the oil), for example using seawater, possible in a second heat exchanger.

The heat exchanger 24, one possible configuration of which is shown in figure 5, comprises a housing 30 through which fluid circulating within the hot side cooling circuit 16, and fluid flowing within the cold side cooling circuit 28, can pass. Within the heat exchanger a section of the one or more hot side conduits 26 and a section of the one or more cold side conduits 32 run adjacent one another to allow for transfer of heat between the two. The hot side conduit(s) carries fluid which has been heated within the fuel cell through the heat exchanger and the fluid within the cold side conduit(s) is heated within the heat exchanger and carries this energy away for transfer to the environment.

In this case the housing 30 of the heat exchanger itself forms the walls of the cold side conduit 32. The housing therefore includes at least one inlet 34 for cooling fluid of the cold side cooling circuit and at least one outlet 36 for cooling fluid of the cold side cooling circuit. This cold side cooling fluid fills the housing and surrounds a number of smaller hot side conduits 26 carrying cooling fluid of the hot side cooling circuit from and back to the module assembly. The hot side cooling fluid passes through the heat exchanger from at least one inlet 38 at a first end 40 of the housing to a second end 42, and then turns and travels back to the first end and to an outlet 44. The extension of the length of the part of the hot side conduit(s) 26 located within the heat exchanger 24, due to the double pass through the housing, increases the surface area of the hot side conduit within the heat exchanger. The same is true of the division of a single hot side conduit into a plurality of smaller hot side conduits within the heat exchanger in the example shown in the figure. Both features (each of which can be separately implemented) maximizes the transfer of heat away from the hot side cooling fluid. The hot side conduit may direct fluid back and forth along the housing more than twice, such as three or more times.

The conduit(s) of the hot side and cold side cooling conduits will usually share one or more walls 46 within the heat exchanger, across which heat is transferred. These walls provide an interface between the fluid within the hot side cooling circuit and the fluid within the cold side cooling circuit across which heat can be transferred, but through which electrical current cannot flow. These shared wall sections can be formed from, or coated or covered with a non-conductive material which allows for the transfer of heat from the fluid within the hot side conduit to the fluid within the cold side conduit, but does not conduct electricity. This then electrically isolates the hot side conduit(s) and the cold side conduit(s) within the heat exchanger from one another. This in turn electrically isolates the whole of the hot side cooling circuit, which is not coupled to system ground, from the cold side cooling circuit, which is coupled to system ground. The non-conductive coating introduces a high electrical resistance between the two cooling circuits, and represents the resistive component of the heat exchanger in this case. The cold side cooling fluid may pass around the hot side conduit, but may not itself be contained within a conduit. This may be the case if heat is transferred directly to the environment in a radiator, for example. The same means can be used to isolate the hot side cooling circuit from the surrounding environment within the heat exchanger in such a case. The resistive component of the heat exchanger can also be implemented in a different way, for example by use of a non-conductive cooling fluid on the cold side.

The non-conductive coating may cover the outer surface of the hot side conduits 26, the inner surface of these conduits, or both. In the example shown, the hot side conduit runs within a larger cold side conduit delimited by the housing 30 of the heat exchanger, which provides for optimum heat transfer. This may not, however, always be the case. The cold side conduit section(s) can run within a larger hot side conduit section in some cases, or the two can run side by side. The coating can comprise a paint which is traditionally used to prevent corrosion (an anti-corrosion paint), but which can also function to prevent electricity from travelling from the components of one cooling circuit to the other. The paint can comprise corrosion protection pigments and a binder or other inert fillers. In some cases, the paint comprises ceramic particles combined with a polymer binder, and may be solvent based (i.e. containing a solvent such as butyl acetate or xylene). An example of a coating developed for corrosion protection, and a candidate for use as a non-conductive coating within heat exchanger 24, is described in Zang, D., Xun, X., 2020, ‘Ceramics Coated Metallic Materials: Methods, Properties and Applications’, in M. Mhadhbi (ed.), Advanced Ceramic Materials, IntechOpen, London.

10.5772/intechopen.93814. The thickness of the non-conductive coating can be between 2pm and 50pm, preferably between 5pm and 20pm, and can be applied by spraying, dipping, painting, or by similar methods.

Isolation from system ground can also be achieved using a non-conductive cooling fluid (air is commonly used, but various oils are also a possibility), rather than or in addition to a non-conductive coating. In this case the mechanism works by cooling a conductive cooling fluid within the hot side cooling circuit in a heat exchanger with non-conductive cooling fluid on the cold side (e.g. using a radiator as the heat exchanger and air as the cold side cooling fluid). If the system is to be employed on a ship, however, the best option is to use ordinary water (with impurities, i.e. conductive) in grounded steel pipes on the cold side, which can in turn be cooled using seawater in some cases. In order to provide high electrical resistance between the conductive fuel stack coolant on the hot side and (also conductive) grounded water on the cold side, the heat exchanger must provide the necessary electrical resistance. This can be done using a non-conductive coating as described above.

Claims

1. A fuel cell system comprising: a module assembly comprising a fuel cell stack including at least one fuel cell; a hot side cooling circuit comprising at least one hot side conduit for transferring cooling fluid into, through and out of the module assembly to transfer heat away from the fuel cell stack; and at least one resistive component for electrically isolating all conductive components of the hot side cooling circuit and the module assembly from a system ground.

2. The fuel cell system of claim 1 , wherein a resistance due to the resistive component is at least 1MQ.

3. The fuel cell system of any of claims 1 and 2, wherein the hot side cooling circuit comprises a heat exchanger through which the hot side conduit is configured to transfer the cooling fluid.

4. The fuel cell system of claim 3, wherein the at least one resistive component comprises one or more components of the heat exchanger.

5. The fuel cell system of claim 4, wherein the resistive component comprises a non-conductive coating applied to surfaces within the heat exchanger.

6. The fuel cell system of claim 5, wherein the non-conductive coating is applied to walls of the hot side conduit within the heat exchanger.

7. The fuel cell system of any of claims 5 and 6, wherein the non-conductive coating is an anti-corrosion paint.

8. The fuel cell system of any of claims 3 to 7, wherein the system comprises a cold side cooling circuit comprising at least one cold side conduit for transferring cold side cooling fluid away from the heat exchanger, and the resistive component is arranged to electrically isolate components of the hot side cooling circuit from components of the cold side cooling circuit.

9. The fuel cell system of claim 8, wherein the resistive component comprises the cold side cooling fluid, which is non-conductive.

10. The fuel cell system of claim 9, wherein the cold side cooling fluid is oil.

11 . The fuel cell system of any of claims 8 to 10, wherein at least one conductive component of the cold side cooling circuit is coupled to system ground.

12. The fuel cell system of any of claims 1 to 11 , comprising an outer enclosure for surrounding the module assembly, wherein the outer enclosure is electrically isolated from the module assembly.

13. The fuel cell system of any of claims 1 to 12, wherein the outer enclosure is electrically isolated from the module assembly by non-conductive elements positioned between the outer surface of the module assembly and the inner surface of the outer enclosure.

14. The fuel cell system of any of claims 1 to 13, wherein the module assembly comprises a module support structure configured to contain and support the fuel cell stack.

15. The fuel cell system of any of claims 1 to 14, wherein the hot side cooling circuit is a closed circuit.

16. The fuel cell system of any of claims 1 to 15, wherein the at least one fuel cell is a proton exchange membrane fuel cell.

17. The fuel cell system of any of claims 1 to 16, wherein the module assembly comprises a local control unit, the system comprises a main control unit external to the module assembly for sending control signals to the local control unit, and the local control unit and main control unit are electrically isolated from one another.