FUEL CELL SYSTEM
The present invention relates to fuel cells and in particular to proton-exchange
membrane type fuel cells in which hydrogen is supplied to the anode side of the fuel cell,
oxygen is supplied to the cathode side of the fuel cell and water by-product is produced
at and removed from the cathode side of the fuel cell.
Such fuel cells typically comprise a proton exchange membrane (PEM) sandwiched
between two porous electrodes, together comprising a membrane-electrode assembly
( EA). The MEA itself is conventionally sandwiched between: (i) a cathode diffusion
structure having a first face adjacent to the cathode face of the MEA and (ii) an anode
diffusion structure having a first face adjacent the anode face of the MEA. The second
face of the anode diffusion structure contacts an anode fluid flow field plate for current
collection and for distributing hydrogen to the second face of the anode diffusion
structure. The second face of the cathode diffusion structure contacts a cathode fluid
flow field plate for current collection, for distributing oxygen to the second face of the
cathode diffusion structure, and for extracting excess water from the MEA. A plurality of
such fuel cells are conventionally layered in a series configuration to form a fuel cell
stack.
A fuel cell stack may be conveniently disposed within a housing or other supporting
frame or structure which may also provide support and/or protection for other
components of a fuel cell system. The other components of the fuel cell system may
include various elements, such as one or more fans for forced ventilation of the cathode
diffusion structure to deliver oxygen, a fuel delivery system, fuel and air flow monitoring
systems, a temperature monitoring system, a cell voltage monitoring system, and
electronics for providing control functions to the fuel cell stack.
One additional component commonly used in a fuel cell system is a hydrogen detector
for detecting hydrogen leaks from the fuel cell stack or from a supporting fuel delivery
system. A hydrogen leak detector can be an important feature for safe operation of a
fuel cell stack but adds cost and complexity to the construction of the fuel cell system.
Further, a hydrogen leak detector may successfully detect the presence of a hydrogen
leak, but does not actively assist in dealing with escaped gas; rather the leak detector is
conventionally used to trigger an alarm condition and/or to shut down the fuel cell and/or
fuel supply.
It is an object of the present invention to provide an improved system for detecting
hydrogen leaks in a fuel cell system.
According to one aspect, the present invention provides a fuel cell system comprising: a
fuel cell including at least one cathode and a cathode conduit for passage of oxidant gas
through or over the cathode; a housing containing said fuel cell and defining a plenum
around the fuel cell; a ventilation system configured to force air from the plenum into the
cathode conduit; and a control system configured to monitor the fuel cell voltage and to
detect a drop in voltage attributable to the presence of hydrogen in the cathode conduit.
The housing may define a plenum confining all faces of the fuel cell except a cathode
exhaust face thereof, the cathode exhaust face including a downstream end of the
cathode conduit. The ventilation system may comprise a fan disposed in a wall of the
housing configured to blow air into the plenum. The housing may be configured such
that the primary exit path for air in the plenum is via the cathode conduit. The system
may include a plurality of fuel cells formed into one or more stacks within said housing.
The control system may comprise: a cell voltage monitoring system for determining the
actual voltage of the fuel cell or of one or more cells in said fuel cell stack; a processor
for receiving inputs indicative of the operating conditions of the fuel cell or fuel cell stack
and determining therefrom an expected voltage of the one or more fuel cells being
monitored; and a comparator for determining whether the difference between the actual
voltage and the expected voltage exceeds a predetermined threshold indicative of a
predetermined level of hydrogen in the cathode conduit. The housing may comprise at
least one air inlet for passage of ambient air into the plenum, the system further
including a fan disposed downstream of the cathode conduit and configured to pull air
from the plenum into the cathode conduit and exhaust the air therefrom.
Embodiments of the present invention will now be described by way of example and with
reference to the accompanying drawings in which:
Figure 1 shows a fuel cell system comprising a fuel cell stack disposed within a
housing; and
Figure 2 shows a schematic diagram of the functional components of a hydrogen
leak detection system incorporated into the fuel cell system of figure 1.
With reference to figure 1, a fuel cell system 1 comprises a fuel cell stack 2 disposed
within a housing 3 . The housing 3 contains the fuel cell stack 2 and defines an air space
or plenum 4 around the fuel cell stack. The fuel cell stack 2 includes a plurality of cells 5
layered together in a conventional stack configuration so that the output voltages of
successive cells 5 in the stack 2 can be coupled in series to provide a stack output of
any desired voltage. The fuel cell stack 2 may be of conventional construction to include
anode channels for delivering fuel to the anode sides of membrane-electrode
assemblies in the stack, and cathode channels for delivering oxidant to the cathode
sides of the membrane-electrode assemblies in the stack. The fuel cell stack 2 may be
of the open cathode type such that oxidant is delivered to the cathode side of the MEA
by forced ventilation at substantially atmospheric pressure through the cathode
channels. The stack 2 comprises a number of faces including a cathode inlet face 6
into which cathode air is blown and a cathode exhaust face 7 from which cathode air is
expelled. The housing 3 preferably does not entirely enclose the fuel cell 2, but leaves
the cathode exhaust face 7 exposed to ambient air on the exterior of the housing 4 as
shown in figure 1. The housing may be sealed to the edges of the stack 2 to prevent air
from passing around the edge of the stack.
The housing 3 includes an aperture 8 and an associated fan 9 therein which, during
operation of the fuel cell stack, is configured to force air into the plenum 4 thereby
slightly raising the pressure of the air in the plenum so that it is forced into the cathode
inlet face 6, passes through cathode conduits in the fuel cell stack 2 and is expelled via
the cathode exhaust face 7. While the air is conveyed through the cathode conduits, it
provides oxidant gas that passes through or over the cathode surfaces of the fuel cells in
the stack, and carries the water by-product out of the cathode to the exhaust face 7 .
The fan 9 may also be positioned to blow the air over any supporting electronics 15
within the housing, which serves to cool circuit devices and thereby to preheat the air
passing into the fuel cell. Other forms of ventilation system to generate appropriate air
flows may be used, e.g. blowers, compressors or the like. The expression "fan" is
intended to encompass all such generators of air flows.
Fuel cell stacks are typically made from tens or hundreds of layers including membraneelectrode
assemblies, diffusion layers, fluid flow plates and gaskets sealing the various
layers. Imperfect sealing of the layers, or degradation of materials used over time, can
result in escape of hydrogen gas. Therefore, in many applications, it is desirable to
monitor for escape of hydrogen gas from the stack 2 . In the design of figure 1, it will be
recognised that an escape of hydrogen from the stack 2 will primarily result in the
hydrogen leaking into the plenum 4 . In addition, any hydrogen escape from a supporting
fuel delivery system also contained in the housing 3 will result in the hydrogen leaking
into the plenum 4 . The inventors have recognised that it is not necessary to provide a
separate hydrogen detector within the plenum 4 because the fuel cell stack itself, in
conjunction with a suitable control system, may be used to detect any hydrogen leak.
Any hydrogen leaking into the plenum 4 will be force-ventilated through the cathode inlet
face 6 and into the cathode conduits of the fuel cell stack. Hydrogen present at the
cathode face of the MEA of a fuel cell results in a drop in voltage and loss of efficiency of
the fuel cell. This voltage drop can be detected by careful monitoring of the fuel cell
using a control system as described below. In addition, a hydrogen leak in the close
vicinity of the inlet to the housing, but not necessarily within the housing, could also be
drawn in to the housing by the fan 9 and result in the hydrogen being force-ventilated
through the cathode conduits.
As shown in figure 2 , the fuel cell stack 2 includes at least two cell voltage monitoring
terminals 20 which can be used to detect the voltage of a cell or a series of cells or even
the whole stack. Fuel cells may have cell voltage monitoring terminals on many cells in
the stack, or on selected groups of cells connected in series. The cell or stack voltage(s)
is / are passed to a cell voltage monitoring circuit 2 1 and logged. A processor 22
monitors the fuel cell voltage or voltages and is configured to detect a drop in cell or
stack voltage attributable to the presence of hydrogen in the cathode conduit.
To do this, the processor 22 may operate to detect an unexpected change in cell or
stack voltage output and trigger a detection condition in the event of an unexpected fall
in cell or stack voltage. Alternatively, the processor may operate to compare an actual
cell or stack voltage output with an expected cell or stack voltage output, given prevailing
operating conditions for the cell or stack. To do this, the processor 22 may be provided
with a plurality of inputs 23 corresponding to sensed operating conditions of the fuel cell,
cells or stack. These operating conditions may include such parameters as temperature,
fuel flow, electrical load, cathode output humidity, local ambient humidity, fuel cell age,
atmospheric air pressure, recent operational history etc. The processor may use the
inputs to determine an operating condition which can be used to determine an expected
voltage by way of an appropriate algorithm or look-up table. The processor may include
a comparator for determining whether the difference between the actual voltage
measured and the expected voltage derived from the operating conditions of the fuel cell
exceeds a certain amount or threshold that would be indicative of a predetermined level
of hydrogen in the cathode conduit, e.g. arising from a hydrogen leak into the plenum.
The extent to which the processor requires knowledge of some or all of the operating
conditions identified above will depend on the sensitivity of hydrogen detection required.
The cathode of a fuel cell can be extremely sensitive to the presence of hydrogen such
that a substantial drop in the cell voltage, possibly even to near-zero, can occur with only
a few parts per million (ppm) of hydrogen present in the cathode air flow.
A further benefit of the closed configuration of the housing 3 and ventilation system of
figures 1 and 2 for hydrogen detection is that any leaking hydrogen that does escape,
and that is detected by the fuel cell, will be to some extent depleted by the reaction
taking place on the cathode side of the PEM. This will be exhausted safely as water
from the cathode exhaust. Thus, not only can the leaking hydrogen be conveniently
detected, but it can be partially or fully rendered harmless by the fuel cell itself.
Various changes to the exemplary fuel cell system described in connection with figures 1
and 2 could be made.
The cathode exhaust face 7 of the stack 2 could also be contained within the housing 3
but be directly coupled to the exterior of the housing by a suitable ventilation duct or
other air flow conduit.
The fan 9 could be positioned on or adjacent to the downstream cathode exhaust face 7
of the fuel cell stack and be arranged to pull air from the plenum 4 via the cathode
conduit. In such a configuration, the aperture 8 in the housing 3 would preferably be
sized to ensure that the exhaust fan is strong enough to maintain a slight negative
pressure in the plenum sufficient to ensure that sufficient hydrogen from any significant
escape will be drawn through the cathode conduit. The aperture 8 could comprise a
number of apertures distributed around the housing for smoother air flow.
It is preferable that the housing 3 is relatively airtight to ensure that hydrogen leaking
from the fuel cell stack 2 is captured within the housing and is then forced into the
cathode conduits of the fuel cell. However, it will be understood that complete
airtightness is not essential. The degree of closure of the housing required will be
determined in part by such factors as the strength of the fan and the airflows necessary
through the stack and the sensitivity of the stack to detecting small concentrations of
hydrogen in a larger air flow. Thus, all that is required is that the housing 3 offers
sufficient containment to the plenum 4 to ensure that a sufficient proportion of any
hydrogen leaking from the stack 2 is captured for forced ventilation through the cathode
conduit for reliable detection. In a preferred arrangement, the housing and ventilation
system is configured such that the primary (i.e. dominant) exit path for air in the plenum
4 is via the cathode conduit.
The housing 3 can contain any size of fuel cell or fuel cell stack and multiple stacks
could share a common housing.
Other embodiments are intentionally within the scope of the accompanying claims.
CLAIMS
1. A fuel cell system comprising:
a fuel cell including at least one cathode and a cathode conduit for passage of
oxidant gas through or over the cathode;
a housing containing said fuel cell and defining a plenum around the fuel cell;
a ventilation system configured to force air from the plenum into the cathode
conduit; and
a control system configured to monitor the fuel cell voltage and to detect a drop
in voltage attributable to the presence of hydrogen in the cathode conduit.
2 . The fuel cell of claim 1 in which the housing defines a plenum confining all faces
of the fuel cell except a cathode exhaust face thereof, the cathode exhaust face
including a downstream end of the cathode conduit.
3. The fuel cell system of claim 1 in which the ventilation system comprises a fan
disposed in a wall of the housing configured to blow air into the plenum.
4 . The fuel cell system of claim 3 in which the housing is configured such that the
primary exit path for air in the plenum is via the cathode conduit.
5. The fuel cell system of claim 1 comprising a plurality of said fuel cells formed into
one or more stacks within said housing.
6. The fuel cell system of claim 1 or claim 5 in which the control system comprises:
a cell voltage monitoring system for determining the actual voltage of the fuel cell
or of one or more cells in said fuel cell stack;
a processor for receiving inputs indicative of the operating conditions of the fuel
cell or fuel cell stack and determining therefrom an expected voltage of the one or more
fuel cells being monitored; and
a comparator for determining whether the difference between the actual voltage
and the expected voltage exceeds a predetermined threshold indicative of a
predetermined level of hydrogen in the cathode conduit.
7 . The fuel cell system of claim 1 in which the housing comprises at least one air
inlet for passage of ambient air into the plenum, the system further including a fan
disposed downstream of the cathode conduit and configured to pull air from the plenum
into the cathode conduit and exhaust the air therefrom.