Vehicular fuel cell cooling system

A cooling system for a vehicular fuel cell utilizes packet pumps to electrically isolate the fuel from a grounded radiator. Fluid in a packet pump is transported from an inlet port to an outlet port in discrete packets. Because these packets are physically separated from one another, electricity does not flow through the fluid from the inlet port to the outlet port. Packet pumps include peristaltic pumps and external gear pumps.

TECHNICAL FIELD

This disclosure relates to the field of fuel cell cooling systems. More particularly, the disclosure pertains to a system to electrically isolate a fuel cell from a vehicle radiator.

BACKGROUND

Fuel cells produce electrical power from a fuel, such as hydrogen, and an oxidizer such as airborne oxygen. Fuel cells may be used to propel vehicles by using the electrical energy to power one or more electrical motors to rotate the vehicle wheels. A fuel cell vehicle produces less pollution and less carbon dioxide than vehicles powered by internal combustion engines, particularly if hydrogen is used as the fuel. Fuel cells have advantages over batteries including the ability to refill a fuel tank in less time than would be required to recharge batteries.

FIG. 1schematically illustrates a cooling system for a vehicular fuel cell. Fuel cell10includes alternating anode layers and cathode layers12and14. When provided with fuel and oxidizer, these layers generate a high voltage between a negative terminal16and positive terminal18. One or more inverters (not shown) convert the direct current (DC) voltage difference between terminals16and18into a three phase alternating current (AC) voltage between the terminals of one or more electrical motors (not shown) to propel the vehicle. Heat is produced as a by-product of generation of electrical power. To dissipate this heat, cooling fluid is circulated around the anode and cathode layers12and14and through a radiator20. Electrical motor22drives a pressure pump24to force the cooling fluid through fuel cell10, coolant line26, radiator20, and coolant line28.

Radiator20is grounded to vehicle structure. The high voltage electrical system is intended to float with respect to the vehicle structure. In other words, the system is designed to electrically isolate terminals16and18from the vehicle structure. Toward that end, coolant lines26and28are fabricated from non-conductive materials. However, unless the coolant liquid is also non-conductive, the coolant itself provides a potential electrical connection. Water has desirable properties as a coolant and the electrical conductivity can be made relatively low, but not zero, by de-ionizing. Known non-conductive fluids have less desirable properties as coolants. The impedance of a conductive path is proportional to the length of the path and inversely proportional to the cross sectional area. In order to increase the impedance of the coolant paths between fuel cell10and radiator20, coolant lines26and28are long and have a small diameter. Packaging these long lines requires considerable space in the vehicle. Also, the pressure required to force fluid through the circuit also increases with line length and decreases with cross sectional area. Therefore, the dimensions that improve electrical isolation increase the pump pressure required to achieve a desired flow rate.

SUMMARY OF THE DISCLOSURE

Packet pumps, such as peristaltic pumps or certain types of gear pumps, transmit fluid from a pump inlet to a pump outlet in discrete volumes which may be called packets. The packets may be physically separated from one another within the pump. If the packets are physically separated by an electrically insulating material, then the pump provides an electrical isolation function in addition to providing the pumping function.

A fuel cell powered vehicle includes a fuel cell, a radiator, and at least one isolator. The radiator may be grounded to the vehicle whereas neither the positive nor the negative terminals of the fuel cell are grounded to the vehicle. The isolator includes a first and a second isolator port and is configured to electrically isolate the first isolator port from the second isolator port while permitting flow of an electrically conductive fluid, such as water, between the first and second isolator ports. The isolator may be a packet pump that, in addition to providing electrical isolation, also forces the fluid to flow between the isolator ports. For example, the isolator may be a peristaltic pump, gear pump, rotary vane pump, etc. The first isolator port is fluidly connected to a cooling port of the fuel cell, for example by a tube. The second isolator port is fluidly connected to radiator port of the radiator, for example by a tube. Since the isolator provides electrical isolation, tubes connecting the isolator ports to the fuel cell and the radiator are not necessarily electrical insulators and need not be artificially long. The cooling loop includes a fluid return path from the radiator to the fuel cell. The fluid return path may include a second isolator. The two isolators may both be pumps driven by a common shaft or by shafts that are driveably connected to one another.

A cooling system, suitable for an electrically charged heat source such as a fuel cell, includes a radiator and first and second packet pumps. One of the packet pumps has an outlet port fluidly connected to an inlet port of the radiator. The other packet pump has an inlet port fluidly connected to an outlet port of the radiator. The packet pumps may be, for example, peristaltic pumps having a tube connecting a pump inlet to a pump outlet, a rotor and at least one roller. The rollers compress the tube at a compression point, separating the fluid at the pump inlet from the fluid at the pump outlet to decrease any electrical path conductivity via the fluid between the pump inlet and the pump outlet. The rollers are attached to the rotor such that the compression point moves as the rotor turns, pushing the fluid in front of the compression point and drawing fluid from behind the compression point. In some embodiments, a second peristaltic pump is integrated with the first peristaltic pump such that they share a common rotor and rollers.

DETAILED DESCRIPTION

FIG. 2schematically illustrates a cooling system for a vehicular fuel cell using isolators to increase impedance as opposed to long coolant lines. To dissipate this heat, cooling fluid is forced through fuel cell10and radiator20by peristaltic pumps30and32. Peristaltic pumps30and32are driven by a common shaft34which is driven by electric motor36. Alternatively, the shafts of pumps30and32could be driveably connected to one another. Two shafts are driveably connected if rotation of one shaft forces the other shaft to rotate at a proportional speed. The inlet of pump30is connected to a first cooling port of fuel cell10by coolant line38and the outlet of pump30is connected to a first radiator port of radiator20by coolant line40. Similarly, the inlet of pump32is connected to a second radiator port of radiator20by coolant line42and the outlet of pump32is connected to a second coolant port of fuel cell10by cooling line44. In addition to forcing the coolant through the coolant loop, peristaltic pumps30and32provide increased electrical isolation in the coolant loop. Therefore, coolant lines38,40,42, and44need not be artificially long and do not necessarily need to be fabricated from non-conductive material. AlthoughFIG. 2shows peristaltic pumps in both the forward and the return fluid lines, use of a peristaltic pump in only one or the other of the forward and return lines provides some of the advantage.

FIG. 3illustrates a peristaltic pump such as pumps30and32inFIG. 2. A pump housing50includes a semi-circular surface52. A flexible tube54having an inlet port56and an outlet port58rests against the surface52. Rotor60supports a number of rollers62which rotate with respect to the rotor. At any given rotor position, at least one of the rollers compresses the flexible tube54against the surface52. As rotor60turns, fluid in front of the compression point is forced to move through the tube toward outlet port58. Fluid behind the compression point is drawn away from inlet port56. The rollers are spaced apart from one another such that, as the rotor turns, a roller begins compressing the tube near the inlet port before another roller stops compressing the tube near the outlet port. Multiple peristaltic pumps, such as30and32, may share a common housing, rotor, and rollers and have separate tubing.

At the position shown inFIG. 3, a discrete volume of fluid between compression points, called a packet, is separated from the remainder of the fluid. Therefore, electricity cannot flow via the fluid among inlet port56, packet64, and outlet port58. If the tubing is made of non-conductive material, then inlet port56is substantially electrically isolated from outlet port58. Even if the rollers do not completely separate adjacent packets, the drastically reduced cross sectional area of the tubing at the compression point creates very high electrical impedance. The electrical impedance between inlet port56and outlet port58can be further increased by increasing the number of rollers62to create multiple packets of electrically isolated fluid.

In some embodiments, one or both of the peristaltic pumps30and32may be replaced by other types of packet pumps such as the external gear pump illustrated inFIG. 4. A packet pump is a device that separates the fluid into discrete packets and forces the packets of fluid from an inlet port to an outlet port. An external gear pump includes a housing70defining an inlet port72and an outlet port74. Two meshing gears76and78rotate within the housing. One of the gears is typically driven by a shaft that extends out of the housing while the other gear is driven by the meshing action of the gears. The gear teeth80closely approach a semi-circular surface82of the housing through a portion of each revolution. Packets of fluid84are forced from the inlet port to the outlet port as the gears76and78rotate. A seal is created between the meshing teeth to prevent the fluid from flowing from the outlet port74to the inlet port72between the gears.

Notice that the packets of fluid84between gear teeth and semi-circular surface82are separated from the remainder of the fluid. Notice also that the fluid in the inlet port72is separated from the fluid in the outlet port74by meshing gear teeth. Therefore, current flow via the fluid between the inlet port72and outlet port74is substantially reduced or eliminated. If the housing70and the gears76and78are made of non-conductive material, then inlet port72is electrically isolated from outlet port74. Even if the interface between meshing gear teeth and between the gear teeth and housing70are not perfect, the cross sectional area of any conductive path is drastically reduced creating very high electrical impedance.

The packet pumps described above provide two functions: forcing the fluid to flow and electrically isolating the inlet and outlet ports. In some embodiments, the pumping function may be provided by only one of the two devices30and32or by another device such as a pressure pump. In such embodiments, one or both of devices30and32may provide only the isolation function. An isolator is a device that provides electrical isolation while permitting fluid flow, but not necessarily forcing the flow. A device such as the external gear pump ofFIG. 4functions as an isolator when neither gear is driven by an external shaft. In such an embodiment, the gears rotate in response to a pressure drop between the inlet port and the outlet port.