Steam generator for a fuel cell system

Abstract

The steam generator for the fuel cell system includes a heat exchanger 34 with an internal heat exchange surface, a water inlet pipe 46, and a dripper head 52 with a flow path connected to the pipe 46. The head 52 extends inside the heat exchanger above the heat exchange surface to deliver water downward onto the heat exchange surface for conversion to steam. The head 52 has outlet holes 56 arranged along the flow path, and between adjacent outlet holes 56, the head has a stepped profile on at least its lower surface to prevent droplets from adjacent holes from coalescing. The fuel inlet pipe can have a section coaxially mounted on a portion of the pipe 46. The section of the fuel inlet pipe can surround a portion of the water inlet pipe. In a fuel cell system, the steam generator can include a fuel inlet pipe and a combined steam and fuel outlet, with a reformer connected directly or indirectly downstream from the steam generator.

Description

[Technical field]

[0001] The present invention relates to a steam generator for a fuel cell system, a fuel cell system including a steam generator or steam fuel heater, and a method of using a steam generator to generate steam or heated steam and fuel. [Background technology]

[0002] A fuel cell system typically includes a fuel cell stack, a reformer for fully or partially reforming an inlet fuel (to allow hydrogen or syngas (reformate) to be fed to the fuel cell stack), and a steam generator for delivering steam and fuel to the reformer. The reformate, when fed to the stack along with an oxidant, undergoes electrochemical reactions on either side of the electrochemically active layers of the fuel cells in the fuel cell stack, respectively, producing both heat (hot exit gases) and electricity.

[0003] In many fuel cell systems, the heat generated is used, via a heat exchanger, to operate both a steam generator and a reformer.

[0004] In some fuel cell systems there is no separate reformer; instead, reforming occurs within the stack.

[0005] The amount of electricity generated from the fuel cell stack needs to be controllable. To increase the electrical output, more fuel or reformate needs to be fed to the fuel cell stack, or the operating conditions within the stack need to be changed (e.g., to increase its efficiency). With respect to the former approach, it is important to ensure that the fuel or reformate is fed without excessive pressure pulsations, at a consistent ratio of fuel to steam, and without wide temperature variations. Moreover, it is preferable that the reformate or fuel is fed at a controllable rate (which can be varied on demand) with minimal time lag. To achieve this, the reformer (if present) needs to be fed with heated fuel and steam with minimal pressure pulsations and without time lag. Thus, the present invention relates to the provision of an improved steam generator to achieve a controllable and homogenous delivery of the fuel / steam mixture to the reformer or fuel cell stack, to best enable minimal fluctuations in stack voltage due to fuel-side composition or pressure changes. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2015 / 136295 Brochure Summary of the Invention [Means for solving the problem]

[0007] According to the present invention there is provided a steam generator for a fuel cell system, the steam generator comprising: a heat exchanger having at least one internal heat exchange surface; A water inlet pipe; a dripper head including a flow passage fluidly connected to a water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water down onto the heat exchange surface for conversion to steam; Including, The dripper head includes a plurality of outlet holes spaced along a length of the flow path; Between adjacent outlet holes, the dripper head has a stepped profile on at least its lower surface when viewed in cross section to prevent droplets from adjacent holes from coalescing, providing a steam generator.

[0008] By "a stepped profile of at least its lower surface," it is meant that at least the lower half of the dripper head has a stepped surface at different heights. The stepped surface or step (e.g., the level depicted by a corner or shoulder) promotes surface tension effects that can prevent the droplet from spreading beyond the stepped surface, e.g., the droplet will not tend to climb upwards due to the attractive force against it as it is held as a droplet by surface tension.

[0009] The corners or shoulders may include convex corners or shoulders, and preferably sharp corners or shoulders. They may be formed by the intersection of two (preferably planar) surfaces: a lower surface (the radially outer surface) and a rising surface (e.g., a surface extending "upward" from the lower surface toward the axis of the dripper head (droplets will not tend to climb onto the lower surface due to gravity tending to keep them on the lower surface)).

[0010] The lower surface may have an inner shoulder (closest to the hole) and an outer shoulder (facing away from the hole). It may then have two convex corners. As the water droplet exits the hole, it will form against the inner shoulder and may grow up on the lower surface before reaching the outer shoulder. However, the droplet will not tend to spread much beyond the outer shoulder due to a rising surface extending upward from that outer shoulder that forms a gravitational barrier to such progression.

[0011] In other embodiments, the hole can be in the lower surface, whereby there can be a single convex shoulder on the side of the lower surface (facing away from the hole). The single convex shoulder can be annular about the hole, or it can be one of a pair of shoulders on either side of the hole, spaced apart along the axis of the dripper head.

[0012] In practice, the or each convex intersection, corner or shoulder can be rounded, but it is preferred that it is substantially angular (i.e. sharp). However, the intersection can be more or less rounded. The angularity or sharpness of the intersection (i.e. of the corner or shoulder) can be defined by the radius of curvature of the intersection / corner / shoulder in cross section, or by its perceived sharpness (i.e. looking like a line of intersection, not a curved corner (in cross section)). Ideally, it will have a radius in cross section of less than 0.5 mm, if it can be perceived as having a given radius. However, instead, a more rounded intersection, corner or shoulder can be provided (e.g. a radius (in cross section) of up to half the diameter or the longest width of the hole with which it is associated).

[0013] Alternatively, it may have a radius of up to 40% of the step height when measured perpendicular to the dripper head axis (or parallel to the cavity axis), but more preferably the radius of an intersection, corner or shoulder will be no more than 20% of that height, or even no more than 10% or 5% of that height.

[0014] The step surface can be of a selected size and shape to allow droplet growth to an appropriate size so that steam generation is controlled and responsive, while preventing excessive droplet size, thus preventing or minimizing the possibility of any coalescence between droplets from neighboring holes, and this can be controlled by requiring a selected range of water flow rates and selected hole sizes.

[0015] In use, the holes are preferably selectively positioned laterally along the dripper head or within the heat exchanger and positioned above each heat exchange surface onto which it is desired that droplets be dropped, i.e., the holes are positioned such that the heat exchange surface is below the holes, thus allowing the droplets to drip directly onto the heat exchange surface.

[0016] The holes are provided for presenting droplets of water, and as such typically have a diameter of at least 0.2 mm, more preferably at least 0.3 mm. The droplets thereby formed will be larger than the hole diameter and therefore typically are at least 0.5 mm wide, more typically at least 1 mm wide, as they fall under gravity from the holes towards the heat exchange surface for conversion to steam. The droplets are not a spray of atomized water, nor are the droplets steam. (The head with the outlet holes is a dripper head, after all, not an atomizer or spray nozzle, whereby the water drips therefrom under the influence of gravity as a liquid rather than a gas, and as a drip rather than a spray.)

[0017] The stepped profile between adjacent outlet holes can be at least two stepped surfaces at different heights above the heat exchange surface. Typically, adjacent outlet holes are provided on stepped surfaces respectively, with at least one stepped surface between them, i.e. at different heights above the heat exchange surface. Typically, adjacent outlet holes are provided on stepped surfaces respectively, at the same height above the heat exchange surface.

[0018] In addition to convex intersections, corners, or shoulders, there may be concave corners or intersections that define an upper surface extending generally parallel to the axis of the dripper head between adjacent holes. Similarly, the concave corners, like the convex intersections, corners, or shoulders, may be sharp or angular or radiused, either with the same angularity or radius in cross section relative to neighboring convex intersections, corners, or shoulders, or with a different radius or angularity in cross section, but preferably within similar radius limits.

[0019] In most instances, the length of the upper surface extends between the opposing raised surfaces of adjacent steps of adjacent cavities in the dripper head, the extension defining a gap (usually >2 mm or >3 mm) between the opposing raised surfaces of the adjacent steps.

[0020] The upper surface may include one or more planar surfaces, or it may be curved about the axis of the dripper head.

[0021] Preferably, the height of each step is measured vertically (perpendicular to the axis of the dripper head) from its upper surface to its lower surface. If the upper or lower surface is curved about the axis of the dripper head, the height is measured through a sagittal plane of the dripper head extending through the centerline of each hole. Preferably, the height is greater than the diameter of the hole. More preferably, the height is at least two or three times the diameter of the hole.

[0022] The length of the gap between the opposing raised surfaces of adjacent steps of adjacent holes, measured parallel to the axis of the dripper head at the mid-height of the raised surfaces (halfway between the upper and lower surfaces), is preferably equal to or greater than the height of the shorter of the two steps at the ends of the gap, although preferably the two steps have the same height. Again, this may be measured in the sagittal plane if the surface has a curvature about the axis of the dripper head. More preferably, the gap is at least two or three times the height of the shorter of the two steps at the ends of the gap.

[0023] These dimensions for the steps result in a profile and spacing between the stepped surfaces that tends to minimize or eliminate coalescence of droplets between adjacent holes, such that droplets will tend to fall vertically downward from the dripper head below the holes in use.

[0024] Typically, the dripper head is elongated with a flow passage including holes axially spaced along its length.

[0025] In one embodiment, adjacent outlet holes are each provided on a stepped surface that forms part of a structure that projects outwardly toward the underlying heat exchange surface. The stepped surface can optionally be of a size and shape selected to encourage the growth of droplets of an appropriate size.

[0026] The structure may include a sub-pathway that extends laterally away from the flow path and leads to an outlet hole above the step surface. The sub-pathway may be threaded to receive a hollow screw with a bore of a diameter selected to provide a desired cross-section and therefore flow rate. Thus, the hole size may be varied or selected depending on the use of a particular hollow screw.

[0027] Adjacent outlet holes may be provided in respective protruding structures, the protruding structures being integrally formed with the dripper head and separated from one another by grooves or channels.

[0028] Alternatively, the structure can be a peg, i.e., cylindrical in shape with a generally flat bottom with a hole in the bottom, and a shoulder around the edge of the bottom.

[0029] Alternatively, each outlet hole may be separated from an adjacent outlet hole by at least one baffle projecting outwardly relative to the outlet hole. The term baffle is used to mean a projecting structure or (e.g. lateral) partition, which restricts the flow of the liquid, for example to prevent it from spreading in a particular direction. The baffle can be a narrow or wide structure or partition with corners / shoulders as defined above.

[0030] By providing isolation of adjacent holes, the water droplets from the exit holes provided into the heat exchanger can be of a controlled size and positioned in the correct location (above the heat exchange surface) (all within acceptable parameters), ensuring that evaporation of the water stream onto the heat exchange surface occurs in a continuous and steady regime, reducing or minimizing pressure pulsations in the steam-fuel stream output from the heated steam (and, optionally, fuel gas) outlet port of the heat exchanger.

[0031] As previously explained, the controlled water droplet size is preferably at least 0.5 mm in diameter (i.e., the preferred minimum droplet size), more preferably at least 1 mm in diameter, i.e., having a diameter (at its widest point) of at least 0.5 mm or more preferably at least 1 mm, as it falls under the influence of gravity towards the heat exchange surface.

[0032] Preferably, each outlet hole is separated from an adjacent hole by a respective associated pair of baffles disposed axially along the dripper head on either side of each hole, the baffles preferably being spaced apart to define a gap therebetween of at least 2 mm to allow water to collect therein and form droplets.

[0033] In embodiments where the outlet holes are spaced sufficiently close together, the dripper head can include outlet holes and baffles (or steps) that are alternately spaced at axial intervals spaced apart along the dripper head such that only a single baffle or step is provided between adjacent holes. However, preferably, two spaced baffles are present between adjacent holes to create a series of steps and also create spaces or gaps between adjacent baffles and holes. This helps to further prevent drips from adjacent outlet holes from merging together or otherwise interacting with each other and forming pools of liquid in the heat exchanger (if water could pool, it would evaporate more slowly, which would result in a delayed response of the steam flow rate from the steam generator under transient inlet flow conditions). It can also cause droplets to drip away from the intended heat exchange surface. It can also allow water to collect during or after a shutdown event. The reason is that it will not evaporate easily enough in advance, which may cause difficulties when restarting the fuel cell system, or even local oxidation or deposits (after evaporation), which will result in maintenance problems. Therefore, a steady dripping of droplets from each outlet is intended, rather than coalescence of water in the dripper head between neighboring outlet holes (which may lead to irregular or uncontrolled dripping and thus excessive stagnation of the heat exchanger rather than the desired (controlled) water droplets).

[0034] Pooling can be especially problematic if it occurs toward the base of the heat exchanger (where the hot gas first enters the heat exchanger) because it can result in unstable water boiling, resulting in undesirable (and significant) pressure pulsations in the steam outflow. Therefore, it is important to avoid large water droplets. This is especially true because, due to gravity, any unboiled water will naturally collect at the base of the heat exchanger.

[0035] The protruding structure, baffle, or step that projects outwardly relative to the outlet hole can be an integral or fitted structure. It preferably defines a shoulder (e.g., arc-shaped) that is laterally spaced from the outlet hole. It may be formed from a flange or washer, or a groove in the wall of the dripper head, with the outlet hole positioned in a recess formed by the groove.

[0036] The dripper head can be machined so that grooves are cut into the walls of the dripper head, or structures can be fabricated on or integrally formed on the dripper head.

[0037] The structures can extend partially around the perimeter or periphery of the dripper head, or can extend entirely, making it less likely that water will pool across the multiple exit holes because each drop is contained both vertically and horizontally.

[0038] Preferably, the outlet holes extend radially relative to the longitudinal axis of the dripper head, and ideally they face downwards.

[0039] The dripper head can be a removable or replaceable component, which preferably is screw-fit onto or into a connector at the end of the water inlet pipe.

[0040] The dripper head generally includes two or more outlet holes at axial intervals spaced apart along the dripper head, each outlet hole being isolated from an adjacent hole by a respective associated pair of partitions, each associated pair of partitions being disposed axially along the dripper head on either side of each hole, such that at least two partitions are provided between adjacent holes. By providing two partitions between adjacent holes, water will not pool beyond the outer edge of the partition in a manner that interacts with a corresponding pool from an adjacent hole, and the gap between adjacent partitions separates any such pooling.

[0041] The heat exchange surface can be a plate or tubular surface of the heat exchanger. There can additionally be fins or other non-flat members extending into the heat exchanger, typically extending from the plate or tubular surface in the heat exchanger. For example, there can be lanced offset fin arrangements in the heat exchanger. These help to avoid any straight path in the water / steam path of the steam generator. Thus, multiple sites and surfaces are presented in the heat exchanger for heat transfer to the water as it descends, so that evaporation of the water occurs before stagnation can occur at the base of the heat exchanger, even if the water is not quickly evaporated when it hits the heat exchange surface just below the exit hole.

[0042] Preferably, the steam generator also has a fuel inlet pipe to allow fuel to be fed into the steam generator for mixing with the steam, such that the exiting steam becomes heated steam fuel gas suitable for either feeding a reformer or feeding directly to a fuel cell stack. Preferably, gaseous fuel is fed into the steam generator together with water, which may surround the water inlet pipe.

[0043] The droplets from the dripper head form at a desired or controllable rate from the outlet holes. They can be controlled by the water feed pressure or by a flow control valve, which can be variable so that different steam delivery rates can be realized from the steam generator. The water is still liquid water when it arrives at the outlet holes, i.e. there is active control of conditions to avoid premature steam formation (i.e. before dripping from the dripper nozzle onto the heat exchange surface). The present invention aims to provide controlled drips of a certain regularity and size from the outlet holes of the dripper head to achieve a desired steam output from the steam generator. Thus, the water arrives at the outlet holes as liquid water and leaves the dripper head as droplets of liquid water that fall under gravity onto the heat exchange surface below the dripper head. Thus, the present invention does not vent the steam or atomized spray from the dripper head or outlet holes, but instead drips droplets that will fall under the influence of gravity.

[0044] The exit holes are provided at spaced axial locations along the lower half or surface of the dripper head, so that gravity helps to avoid droplets becoming trapped between the exit holes.

[0045] The partitioning structures on either side of the exit aperture may form a gap around the entire periphery of the dripper head, preferably it is an annular gap.

[0046] At least one of the partitioning structures may have an annular configuration about the dripper head.

[0047] Both partitioning structures may have an annular configuration around the dripper head on either side of at least one of the exit holes.

[0048] The partitioning structure may have a height extending in a radially outward direction relative to a longitudinal axis of the dripper head that is greater than a width of the partitioning structure.

[0049] The partitioning structure or baffle may comprise a separate component mounted above the dripper head.

[0050] The dripper head can include a tubular end section and the pair of partitioning structures can include a pair of washers mounted on the tubular end section, one proximal to the exit hole and the other distal to the exit hole with respect to the tubular end section. This simple structure is very cost effective.

[0051] Preferably, the washer is press fit onto the tubular end section, which minimizes the gap between the outside of the tubular end section and the inside of the washer, thus minimizing the opportunity for water to leak between them, which could promote water pooling between adjacent outlet holes.

[0052] Preferably, the pair of partitioning structures are integrally formed with the tubular end section, for example as an integral flange or shoulder.

[0053] The structure may define parallel sidewalls and, during use of the steam generator, water is pumped from the outlet aperture between the parallel sidewalls.

[0054] In some embodiments, the parallel sidewalls are perpendicular to the longitudinal axis of the nozzle.

[0055] Typically, only one exit hole is provided between each pair of parallel sidewalls.

[0056] In some embodiments, the dripper head is a one-piece construction.

[0057] To allow water to be pumped through the dripper head, the dripper head has a central bore that is fluidly connected to both the water inlet pipe and the exit port. Since the end is preferably not an exit port, the dripper head typically has a closed distal end.

[0058] Where the structure is formed by a transverse cut or groove in the side of the dripper head and the exit hole is at the base of the transverse cut or groove, the partition can define arc-shaped shoulders on either side of the exit hole.

[0059] The sidewalls need not be parallel, and in some embodiments they are outwardly tapered and wider on the outside.

[0060] The dripper head may have a hexagonal section along at least a portion of its length and preferably has a threaded bore at its proximal end to facilitate attachment of the nozzle to the end of the water inlet pipe.

[0061] Preferably, the steam generator has a steam outlet connected to a reformer of the fuel cell system.

[0062] Preferably, the vapor outlet is at or near the bottom of the heat exchanger and the dripper head is mounted within the heat exchanger at or near the top of the heat exchanger.

[0063] Preferably, the fuel is combined with steam in a steam generator.

[0064] In some embodiments, the steam generator is a combined steam generator and fuel heater for a fuel cell system (eg, a steam fuel heater in which steam and fuel are mixed and heated).

[0065] Preferably, the fuel cell system is a solid oxide fuel cell system. Preferably, it has at least one stack of fuel cells therein. Preferably, at least one of the stacks is formed by a metal-supported solid oxide fuel cell. The solid oxide fuel cell (or stack thereof) can be as described in WO2015136295, the entire contents of which are incorporated herein by purely reference.

[0066] In some embodiments, the fuel inlet pipe has a section that is coaxially mounted to a portion of the water inlet pipe. Preferably, the section of fluid (fuel) inlet pipe surrounds a portion of the water inlet pipe. Preferably, this arrangement is provided at the point of entry of both pipes into the steam generator.

[0067] The sections and portions may be located at or adjacent to the outer wall of the heat exchanger or may extend the fuel and water flows through the outer wall of the heat exchanger. Typically, the water inlet pipe is mounted perpendicular to the outer wall of the heat exchanger. Similarly, the fuel inlet pipe is typically mounted perpendicular to the outer wall of the heat exchanger.

[0068] Combining the fuel and water inlets allows a suitable arrangement to be provided.

[0069] According to this aspect of the invention, there is provided a steam generator for a fuel cell system, the steam generator comprising: a heat exchanger having at least one internal heat exchange surface; A water inlet pipe; A fuel inlet pipe; a dripper head fluidly connected to the water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water down onto the heat exchange surface for conversion to steam; Including, A steam generator is provided in which the fuel inlet pipe has a section that surrounds a portion of the water inlet pipe at or adjacent to the outer wall of the heat exchanger and extends through the outer wall of the heat exchanger such that the section of the fuel inlet pipe, and the fuel therein during use, acts to insulate the surrounded portion of the water inlet pipe from the heat of the heat exchanger.

[0070] This steam generator may also have the same features as the steam generator of the first aspect of the invention.

[0071] The present invention also provides a fuel cell system comprising a fuel cell stack, a fuel inlet pipe, a water inlet pipe, an air or oxidant inlet pipe, a combined steam generator and fuel heater connected to the fuel inlet pipe and the water inlet pipe, an optional reformer connected to the combined steam generator and fuel heater to provide hydrogen or syngas to the fuel cell stack, and a heat exchanger for directly or indirectly taking heated fluid from the outlet of the fuel cell stack, the heat being used to heat at least one internal heat exchange surface of the heat exchanger, the at least one internal heat exchange surface being used to generate steam from water from the water inlet pipe via a dripper head, the combined steam generator and fuel heater being a steam generator as defined above and including both the fuel inlet pipe and the water inlet pipe, and the fuel being heated by mixing with the steam in the steam generator.

[0072] The heated fluid can be the fuel electrode gas or the air electrode gas from the stack.

[0073] According to another aspect of the invention, the use of the above fuel cell system to minimize stack voltage fluctuations due to fuel-side composition or pressure changes in the fuel / vapor mixture entering the fuel cell stack of the fuel cell system is disclosed.

[0074] The present invention also provides a method of generating steam using a steam generator as defined above, wherein water is delivered to a heat exchange surface through a water inlet pipe and a dripper head, after which the water is converted to steam. Preferably, the method uses a dripper head as defined above to minimize pressure pulsations in the steam produced through controlling the droplet size from the dripper head.

[0075] Additionally, this may be accomplished by using (eg, by selective positioning) a dripper head with a stepped profile on its underside to position the drips above the heat exchanger surface.

[0076] In accordance with the present invention, there is also provided a method of minimizing pressure pulsations in a steam generator during steam generation, the steam generator including a heat exchanger with at least one internal heat exchange surface, a water inlet pipe, and a dripper head including a flow path fluidly connected to the water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water droplets down onto the heat exchange surface for conversion to steam, the dripper head including a plurality of outlet holes spaced along a length of the flow path, between adjacent outlet holes the dripper head has a stepped profile on at least its underside when viewed in cross section, the method including the step of adjusting a flow rate of the water to a selected flow rate, whereby the stepped profile (e.g., in conjunction with a selected hole size) prevents droplets from adjacent holes from coalescing.

[0077] The stepped profile may have a corner or shoulder as defined above.

[0078] Where the dripper head has a longitudinal axis, a cross section for the stepped profile is through a vertical sagittal plane of the dripper head and extends through the longitudinal axis.

[0079] The stepped profile provides rising surfaces over which water droplets tend not to rise, such that water droplets on adjacent rising surfaces do not interact and therefore cannot coalesce.

[0080] Preferably, the dripper head holes are positioned vertically above the respective heat exchange surfaces of the heat exchangers for rapid evaporation of the droplets as they land on those heat exchanger surfaces.

[0081] Optionally, at the same time, fuel is pumped into the heat exchanger through a fuel inlet pipe, where the fuel mixes with the steam in the heat exchanger and is heated.

[0082] In yet a further aspect, a steam generator for a fuel cell system, the steam generator comprising: a heat exchanger having at least one internal heat exchange surface; A water inlet pipe; a dripper head fluidly connected to the water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water down onto the heat exchange surface for conversion to steam; Including, A steam generator is provided in which the dripper head includes a plurality of lateral exit apertures spaced axially along the dripper head, each aperture being isolated from an adjacent aperture by at least one structure that projects outwardly relative to the lateral exit apertures.

[0083] In yet another aspect, a steam generator for a fuel cell system, the steam generator comprising: a heat exchanger having at least one internal heat exchange surface; A water inlet pipe; a dripper head including a flow passage fluidly connected to a water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water down onto the heat exchange surface for conversion to steam; Including, the dripper head includes a plurality of outlet holes spaced along a length of the flow path, and the steam generator is configured such that, in use, water emerges from the dripper head outlet holes as water droplets, rather than as steam or a spray, which droplets fall under gravity towards a heat exchange surface for conversion to steam; Between adjacent outlet holes, the dripper head has a stepped profile on at least its lower surface when viewed in cross section to prevent droplets from adjacent holes from coalescing, providing a steam generator.

[0084] Water droplets from the dripper head will fall under the influence of gravity, while steam (which is a gas or atomized spray) (which acts like a gas due to the small size of the gas particles and the gas associated with the spray particles) will tend to remain suspended or will be powered by the ejection force from the exit hole (i.e., the flow of gas associated with the spray) rather than being primarily influenced by gravity.

[0085] The present invention also provides a method of operating a steam generator for a fuel cell system, the steam generator comprising: a heat exchanger having at least one internal heat exchange surface; A water inlet pipe; a dripper head including a flow passage fluidly connected to a water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water down onto the heat exchange surface for conversion to steam; Including, the dripper head includes a plurality of outlet holes spaced apart along a length of the flow path, and between adjacent outlet holes the dripper head has a stepped profile, when viewed in cross section, on at least its underside to prevent droplets from adjacent holes from coalescing; The method is: Delivering water to a dripper head through a water inlet pipe; dripping water from the dripper head through the outlet holes as water droplets, rather than as steam or a spray, the water droplets dripping onto a heat exchange surface for conversion to steam; The present invention provides a method comprising:

[0086] In some embodiments, the steam generator forms part of the fuel cell system and generates steam that is supplied to a fuel cell inlet in the fuel cell system.

[0087] The method of the present invention is intended to ensure that the water is still liquid water when it arrives at the exit orifice and when it drips onto the heat exchange surface. Thus, in a preferred embodiment, the method actively controls the conditions upstream of the dripper head to avoid premature steam formation. For example, the steam generator can include a control system configured to deliver liquid water to the exit orifice at a suitable temperature and mass flow rate such that the liquid water forms droplets that fall under gravity. For example, this can include a controller in the steam generator in operative communication with sensors (e.g., temperature / pressure / flow sensors) and actuators (e.g., control valves and / or mass flow controllers) to achieve this. Thus, the present invention can obtain controlled drips of suitable regularity and size. The liquid water arrives at the exit orifice as liquid water and leaves the dripper head as a series of liquid water droplets that fall under gravity onto the heat exchange surface, which is then converted to steam by the heat at the heat exchange surface. Thus, the present invention prevents the water from exiting the dripper head as steam. The present invention also prevents water from emerging from the dripper head as an atomized spray (eg, small droplets that do not fall under the influence of gravity).

[0088] The inventors have recognized that the dripper head should provide droplets of water that form discontinuously above the dripper head and then grow until they are large enough to descend or fall under the influence of gravity. This is different from steam, which likewise differs from an atomized spray, in which tiny droplets appear continuously in the gas flow as a precisely mixed spray of water particles and a supporting gas flow, and the tiny particles are suspended in the gas flow (usually air), and therefore they do not tend to fall under the influence of gravity.

[0089] The method of the present invention uses multiple parameters to ensure that droplets are presented. First, the outlet holes are provided as dripper holes, not spray holes, and therefore have the appropriate size and shape for dripping, not spraying. Furthermore, the dripper head operates such that the water pressure (mass flow delivery rate), water temperature, surrounding heat exchanger air temperature, and air pressure all work together to generate water droplets that fall under gravity. Thus, the method avoids the generation of air-atomized water spray, as well as premature steam generation in the dripper head and water inlet pipe, so that liquid water does not vaporize before it has a chance to fall as droplets from the dripper head.

[0090] These and other features of the invention will now be described in more detail, purely by way of example, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0091] [Figure 1] 1 is a diagram illustrating a schematic of a fuel cell system including a steam generator according to the present invention; [Diagram 2] FIG. 1 is a diagram illustrating a first embodiment of a steam generator for a fuel cell system. [Diagram 3] FIG. 13 illustrates an alternative dripper head arrangement connected to a steam generator. [Figure 4] FIG. 4 shows the dripper head arrangement of FIG. 3 in more detail. [Figure 5a] 1A-1C are schematic partial cross-sectional views through two alternative steam generators showing details of the dripper heads and water and fuel delivery; [Figure 5b] 1A-1C are schematic partial cross-sectional views through two alternative steam generators showing details of the dripper heads and water and fuel delivery; [Figure 6] FIG. 13 illustrates a further dripper head arrangement. [Figure 7] FIG. 13 illustrates a further dripper head arrangement. [Figure 8a] 8A and 8B are a perspective view and a cross-sectional view of the dripper head of FIG. 7. [Figure 8b] 8b shows a corresponding view of a variation of the dripper head of FIG. 8a. [Figure 8c] 8b shows a corresponding view of a variation of the dripper head of FIG. 8a. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0092] Referring initially to Figure 1, a basic fuel cell system (e.g., a solid oxide fuel cell system) is illustrated generally. As can be seen, the fuel cell system 10 includes a stack 12 of fuel cells 14.

[0093] The fuel cell has an electrochemically active layer and a stack with fuel and oxidant flow paths on either side of the electrochemically active layer to allow an electrochemical reaction to occur across the electrochemically active layer, producing both electricity 16 and heat from the cell (and thus the stack), the heat being in the form of heated outgas 18, which will be a combination of the fuel electrode gases and air electrode gases exiting the stack exhaust.

[0094] The heated outgas 18 can be used by the fuel cell system 10 for several purposes. One of these is to preheat the oxidant (usually air 27) that is fed into the stack 12 using an air heater (first heat exchanger 20). The oxidant is usually air, but other oxidant gases can be used (e.g., oxygen-rich gases, etc.). Typically, for solid oxide fuel cells operating at temperatures between 400° C. and 600° C., the oxidant 27 will be heated by the first heat exchanger 20 and output at approximately 300° C. to 500° C. As shown, this can be accomplished using high-grade heat from the cathode (oxidant) offgas 18a after it leaves the stack 12.

[0095] The heated outgas 18 can also be used to preheat the fuel 26 and water 28 to produce a heated steam and fuel mixture 30. That is, after losing some heat in the air heater 20, the cathode (oxidant) offgas 18b can then transfer lower heat to the fuel 26 and water 28. The resulting heated steam and fuel mixture 30 will typically be fed to the reformer 24, although in some fuel cell systems the reforming is carried out in the stack 12. The reforming process will likely be carried out partially in the reformer 24 and then continued in the stack 12. This is because the output from the reformer 24 is generally only partially reformed to syngas 25 (hydrogen and carbon monoxide), which is also mixed with unreformed fuel and steam.

[0096] For the purpose of producing the steam and fuel mixture 30, a steam generator 22 (second heat exchanger 34) is provided, which receives both fuel 26 and water 28 at the inlet end, the water 28 being converted to steam in the steam generator 22, the fuel 26 being entrained in the steam or mixed with the steam and heated so that the mixture 30 can be output as a heated steam and fuel gas mixture 30. For a solid oxide fuel cell operating at a temperature between 400°C and 600°C, the steam and fuel mixture will be heated by the second heat exchanger 22 and output at approximately 200°C to 500°C (usually 250°C to 350°C). The heated steam and fuel gas mixture 30 is then fed into the reformer 24, which can also be heated by the outgas 18. The reformer 24 then outputs a reformate (or syngas) mixture 25, which includes at least hydrogen and carbon monoxide. The syngas, along with heated air 32 from the first heat exchanger 20, can then be fed into the stack 12 and subjected to electrochemical processing by the fuel cells 14 in the fuel cell stack 12.

[0097] In some fuel cell systems, instead, steam is produced first and then the fuel is mixed into it separately, perhaps in a reformer or in a separate mixing chamber, which can have an additional heat exchanger.

[0098] In a typical fuel cell system, fuel is supplied as methane, propane, or ethanol.

[0099] Additionally, the water provided to create the steam can be tap water or, more preferably, distilled water, since distilled water will form less deposits upon evaporation.

[0100] The above is just one arrangement of heat exchangers in a fuel cell system. The heated anode off-gas can also pass through the heat exchangers for transfer of heat to the incoming gases, and the heat exchangers can be arranged in other orders. Additional heat exchangers can also be present in the fuel cell system to ensure that the heated air and syngas are fed into the stack at the proper temperature, as well as to ensure that the heat provided to the first and second heat exchangers is adequate for the operation of the air heater from the steam generator, and also for the reformer.

[0101] 2, there is shown a schematic example of the steam generator 22. As can be seen, the steam generator 22 includes a heat exchanger section 34 with inlet and outlet ports, including a combined fuel and water inlet port 36, a heat supply fluid inlet port 38, a heat supply fluid outlet port 40, and a heated steam and fuel gas outlet port 42.

[0102] A combined fuel and water inlet port 36 may be mounted through a manifold on the side of the heat exchanger section 34, preferably with an insulated gasket to avoid leakage across the joint and also provide a thermal shield from the heat exchanger section 34 to the manifold. The gasket may be a thermiculite gasket.

[0103] Upstream of the combined fuel and water inlet port 36, this embodiment has a T-junction 44 with a flow-through water inlet pipe 46 and a side-port fuel inlet pipe 48. Due to the T-junction, the water inlet and the fuel inlet can be combined into a single inlet port 36, as will be explained in more detail with reference to later embodiments.

[0104] As shown, the heat exchanger 34 is a counter-flow heat exchanger. This is the preferred arrangement. In the arrangement shown, the combined fuel and water inlet port 36 is mounted toward the top of the heat exchanger section 34, and the heated steam and fuel gas outlet port 42 is positioned toward the bottom of the heat exchanger section 34, such that liquid water enters the top of the heat exchanger section and exits as steam from the bottom of the heat exchanger section 34, while a heat supply inlet port 38 is provided at the bottom of the heat exchanger section 34 and a heat supply outlet port 40 is provided at the top of the heat exchanger section 34. Having this counter-flow of heat supply to the direction of flow of water and steam production provides efficient operation of the heat exchanger function within the heat exchanger section 34.

[0105] 3 and 4 (enlarged view), an alternative dripper head arrangement for the steam generator 22 is shown. In this embodiment, a T-junction 44 with a side-ported fuel inlet pipe 48 is further upstream from the steam generator, such that a longer length of water inlet pipe 26 is surrounded by a concentric outer fuel inlet pipe 48. The heat exchanger section 34 can still have the same counter-flow arrangement, with the ports in the same locations as before, and the dripper head at the top of the steam generator. However, the figure also shows heated interior plates 50 of the heat exchanger 34, which are heated by the throughflow of heated outgas 18. These heated interior plates 50 provide the initial heat exchange surface inside the heat exchanger section 34, onto which water 28 (supplied into the heat exchanger 34 via a combined fuel and water inlet port 36) can be pumped, typically by dripping.

[0106] As can be seen from FIG. 4, the dripper head 52 is provided at the distal end of the water inlet pipe 46. It extends into the heat exchanger portion 34 and positions outlet holes 56 (better seen in a later embodiment) above the plate 50 onto which the water drips. In this embodiment, the outlet holes extend laterally (perpendicular to the axis of the dripper head). To enable the dripper head 52 to be so mounted within the steam generator 22, the plates 50 have apertures 54 therethrough into which the dripper head 52 is inserted. However, the dripper head 52 could simply be mounted above the upper wall of the plate 50.

[0107] As can be seen, in this embodiment, the apertures 54 in the plate 50 are sized to be larger than the dripper head 52, both for ease of insertion and to prevent direct contact between the plate 50 and the dripper head 52. Such direct contact would allow conductive heat transfer from the plate 50 to the dripper head 52, which could result in overheating of the dripper head 52 and eventual boiling of the water in the dripper head 52 prior to dripping from the outlet holes 56 of the dripper head 52, which could create a stall in the water flow through the dripper head 52, which in turn could create a pressure change in the steam output from the steam generator 22, with the possibility of internal deposits in the dripper head (due to evaporation of water in an enclosed space) that could eventually build up and block the dripper head. All of these symptoms would be undesirable. Thus, the arrangement of the present invention is designed to minimize the occurrence of such overheating. This in turn minimizes the risk of blockages and minimizes any pressure changes or pressure spikes or troughs (because stopping the water flow would cause the pressure to drop, while resuming the flow could create excess water into the system, thus creating a spike in the steam pressure).

[0108] The present invention aims to deliver liquid water over a plate (or other such heated heat exchange surface that may be provided) by attempting to ensure that the water remains liquid (i.e., not vapor) all the way to the inside of the heat exchanger and until it drips out of the dripper head.

[0109] In this embodiment, the dripper head 52 has outwardly protruding structures on either side of the outlet holes 56 and a closed distal end. The outwardly protruding structures separate the two downwardly facing outlet holes 56 and prevent or minimize the risk of water pooling as the water drips from the outlet holes 56 onto the plate 50 below the holes 56. Thus, the water will tend to drip or pump directly down as a controlled drip onto the plate 50 below each outlet hole 56.

[0110] In this embodiment, there are two plates 50 and there are two holes 56. Other embodiments can have more plates than holes, for example if not all plates are for receiving a water drip feed. Preferably, the holes are selectively positioned relative to the internal heat exchange surface of the steam generator to optimize steam generation.

[0111] 5a and 5b, which are schematic partial cross-sectional views showing two alternative dripper heads 52 inserted into the top of the heat exchanger portion 34 of the steam generator. Each figure shows only a partial section of the heat exchanger portion 34, and the two heated plates 50 can again be seen. Additionally, the dripper heads 52 can be seen along with the exit holes 56, which are now clearly visible on the lower half of the elongated dripper head.

[0112] The outlet holes 56 are fluidly connected to the internal channel 58 of the water inlet pipe 46 such that the incoming water 28 can feed from the water supply through the internal channel 58 of the water inlet pipe 46 and exit through the outlet holes in the dripper head 52 and onto the plate 50. Because the plate 50 is hot, when the water lands on the plate 50 it will begin to heat and evaporate, forming steam, and the steam (and any liquid water) will be entrained down through the heat exchanger section 34 towards the heated steam and fuel gas outlet port 42 (not shown in this view). As it passes down through the heat exchanger section 34, the water or steam can travel through fins or other interwoven or non-flat members 60, preferably the water-fuel flow path has multiple interruptions and the flow direction generally changes due to the internal fins, increasing the heat exchange area for evaporation, which ultimately drives a thoroughly mixed outlet stream. Also, these members or fins 60 will be at high temperatures (usually from the heat of the plate 50 to which they are connected). This arrangement therefore ensures a large contact area for the evaporation of water and thus rapid steam formation. It also provides for consistent maintenance of steam pressure within the steam generator, such that any pressure pulsations that occur as a result of water dripping onto the heated surface can be maintained at a low level at the steam outlet 42.

[0113] This preferred labyrinth flow path arrangement, and by dripping water onto a plate (or other such heat exchange surface within the heat exchanger section) (preferably from holes located at selected locations relative to such plate (e.g., directly above)), optimizes the ability of the steam generator 22 to quickly and easily temporarily change its steam production rate. This can be accomplished by simply varying the flow rate of water (and fuel) into the heat exchanger, while ensuring that the water is still fully vaporized by the outlet 42.

[0114] Additionally, the fins or uneven members 60 in the heat exchanger 34 encourage the steam to expand in multiple directions as it is produced, thus helping to even out any pressure changes in the heat exchanger section 34, thus again providing low pressure pulsations. This has benefits because high pressure pulsations in the heat exchanger 34 would result in the reformer 24 receiving a fluctuating steam / fuel mixture (potentially with unevenly mixed steam and fuel), which would affect the efficiency of the reformer 24, which may also affect the operation of the respective fuel cells.

[0115] Thus, the present invention seeks to provide low pressure pulsations to produce uniform and consistent steam and fuel output from the steam generator 22 into the reformer 24. Ultimately, the exiting stream 30 to be received by the fuel reformer 24 should be a superheated steam-fuel mixture that does not contain liquid water.

[0116] The dripper head 52 is designed to drip consistently onto the plate 50 rather than forming stagnant water or large droplets. To this end, the dripper head has a stepped profile between adjacent holes, with sharp corners between the steps (i.e., steps of different heights) configured such that surface tension effects prevent droplets from adjacent holes from coalescing. Notably, structures may be provided on either side of each outlet hole 56 of the dripper head 52 to isolate the different outlet holes from one another. In the embodiment of FIG. 5a, the structures include outwardly extending flanges 62. The outwardly extending flanges 62 are integrally formed on the dripper head 52, which in turn is integrally formed on the water inlet pipe 46.

[0117] The structures 62 are continuous ring flanges that extend around (the entire) periphery (or circumference) of the water inlet pipe 46. They are spaced axially away from the outlet holes 56 by approximately an outlet hole diameter, although other distances are possible as well. They also extend laterally outward from the outlet holes 56 a distance between 1 and 2 hole diameters, although other lengths are possible as well.

[0118] In the embodiment of Figure 5b, the structure also includes an outwardly extending flange 62 that is integrally formed on dripper head 52, which in turn is integrally formed on water inlet pipe 46. However, in this embodiment, the structure extends only around the underside of the periphery of water inlet pipe 46. If holes are provided only on the underside of the dripper head, gravity and surface tension effects may mean that only a partial flanged structure may be required to confine the droplets, whereas holes provided on the upper half of the dripper head may not need to be confined by a continuous ring structure.

[0119] In the embodiment shown in both Figures 5a and 5b, there are two such flanges 62 provided for each outlet hole 56, and there are two such outlet holes 56. Thus, there are four such flanges 62. The flanges 62 extend perpendicular to the central axis of the water inlet pipe 46 (or, more specifically, the dripper head 52). In this embodiment, the flange 62 closest to the distal end of the dripper head 52 is spaced from the distal end, and the distal end is closed.

[0120] On the exterior of the heat exchanger section 34, surrounding the water inlet pipe 46 is a tee manifold 64. The tee manifold 64 covers a portion of the water inlet pipe 46 and may be attached to the outer wall of the heat exchanger section 34 by any conventional means, although preferably with a gasket to provide a tight seal thereagainst. The gasket may be a thermal insulating gasket (such as a Thermiculite gasket, for example) that can minimize conductive heat transfer from the sides of the heat exchanger 34 to the tee manifold 64.

[0121] The T-manifold 64 has an internal passageway with a larger diameter than the water inlet pipe 46 (or the portion thereof that extends therethrough). It therefore provides an annular gap between the inner wall of the T-manifold and the outer wall of the portion of the water inlet pipe 46. This annular gap provides a flow path for the fuel 26. The fuel 26 enters the T-manifold 64 through a side branch 66. This arrangement allows the fuel to enter the heat exchanger section 34 as a shroud around the water inlet pipe 46 and then mix with the steam generated in the heat exchanger section 34.

[0122] Covering the water inlet pipe 46 with the fuel 26 gives the effect of suspending the water inlet pipe away from any solid surfaces close to the heat exchanger, and the water inlet pipe can only be attached at the left end. This prevents any direct conduction of heat from the structure of the heat exchanger 34 into the water inlet pipe 46. This helps to keep the water in the water inlet pipe 46 in a liquid state until it is dripped from the dripper head 52. The reason is that the heat from the heat exchanger section 34 will first need to raise the temperature of the fuel 26 before it can raise the temperature of the water 28 in the water inlet pipe 46, but that fuel 26 continues to be refreshed from the cooler fuel supply while the steam generator 22 is operating. As such, with this arrangement, water exiting the exit holes 56 during operation of the steam generator 22 will tend to drip the water out of the exit holes 56 rather than venting the steam, thus minimizing the occurrence of steam blockage in the water inlet pipe 46 or dripper head 52 (which may otherwise occur, resulting in pressure fluctuations), and it also reduces or eliminates evaporation deposits occurring in the dripper head 52 or water inlet pipe 46 (which may be difficult to clean out and may lead to failure of the steam generator 22). The junction, i.e., the beginning of the gas shroud, may be varied from the previous FIG. 3 (e.g., showing a longer length covered).

[0123] With this arrangement of the steam generator 22, the water 28 and fuel 26 are mixed inside the heat exchanger 234 as the water boils and converts to steam, rather than being mixed before entering the heat exchanger 234. This can have an advantage over premixing the fuel and water, in that the relatively dry fuel gas flowing through the same passage through the heat exchanger helps to evaporate the water into the gas stream.

[0124] Referring now to Figure 6, another embodiment of the invention is shown. Similar to that of Figure 5a, but in this embodiment, rather than an integral flange 62, a pair of washers 72 are fitted over the pipes to be positioned on either side of each downwardly facing outlet hole 56, with a gap 78 between the pair of washers which forms a recessed section for the washers. These washers 72 may be press fitted over the free end of the water inlet pipe 46.

[0125] As can be seen in FIG. 6, when water flows out of the outlet hole 56, water droplets 74 form, but due to surface tension, the droplets 74 are held in and spread around the gaps between adjacent washers 72 on either side of the outlet hole 56. In particular, surface tension effects mean that the droplets do not spread around the protruding outer shoulders of the washers into the adjacent gaps 78. On the other hand, in the absence of the washers 72, the droplets 74 can spread along the water inlet pipe 46, which keeps the droplets 74 local to the outlet hole 56. Also, because the droplets of adjacent outlet holes 56 are held apart and do not combine or mix, their size can be controlled. As a result, the water droplets 74 cannot combine to form larger water droplets, which may tend to generate increased pressure pulsations in the steam generator if the water were to sit on the exchange surfaces of the heat exchanger rather than steadily evaporating. Also, by ensuring that the droplets grow in the desired location relative to the underlying heat exchange surface, this can prevent them from falling in a random manner, for example, they may fall on the side of the plate and thus land on a different part of the heat exchange surface, and then (if large enough) potentially run down through the heat exchanger and lodge at the base of the heat exchanger, thus creating the potential for greater pressure pulsations towards the outlet of the heat exchanger section 34 or the heated steam and fuel gas outlet ports 42 (because the hottest part is at the bottom, where the heated fluid enters the heat exchanger).

[0126] Although the non-flat members 60 in the heat exchanger 34 will tend to resist such liquid pooling, avoiding larger water droplets from the dripper head 52 remains an important feature of the present invention, since water pooling can then be substantially eliminated, thus desirably resulting in lower pressure pulsations in the steam generator.

[0127] 7, another embodiment of the dripper head 52 is shown. In this embodiment, the dripper head 52 is a replaceable dripper head that can be fitted over the end of the water inlet pipe 46. This can be done by the threaded end of the water inlet pipe 46 and an internally threaded hole in the proximal end of the dripper head 52.

[0128] To facilitate tightening of dripper head 52 onto the threads, the cross section of dripper head 52 can be hexagonal, like a nut, at least in a portion of the dripper head (herein, at its proximal end). The hexagonal cross section allows a wrench to tighten the nozzle onto the water inlet pipe 46.

[0129] In this embodiment, the outlet holes 56 are again provided on the underside of the dripper head, each with a structure on either side thereof, which in this embodiment is an arc-shaped flange 76 with sharp corners. This dripper head 52 may be fabricated by cutting or molding a portion of the sidewall of the dripper head 52. Thus, shoulders on either side of the outlet holes 56 are provided to contain the water droplets and resist lateral spreading. In this embodiment, the flanges do not extend all around the circumference of the dripper head 52, but they can still perform the same function of preventing droplets from joining together from adjacent outlet holes 56, as shown in FIG. 6.

[0130] In this embodiment, further transverse cuts are provided: one between the two exit holes 56 to create a recess 78 between the two arc-shaped flanges 76 located between the exit holes 56, and two more outside the other two "outer" arc-shaped flanges 76 to define the outer sides of the arc-shaped flanges 76.

[0131] Thus, the four arc-shaped flanges 76 each positioned in a pair on either side of a respective one of the outlet holes 56 have a gap 78 between the pair, the inner gap helping to ensure that water droplets between adjacent outlet holes cannot coalesce because to do so they would have to bridge the gap.

[0132] 8a to 8c, which show three alternative embodiments of a dripper head, each including a perspective view of the underside of the dripper head on the left and a cross-sectional view of the head (rotated through 180 degrees) in its operative orientation with the aperture oriented downwards on the right. Figure 8a shows dripper head 52 as described with reference to Figure 7, with outlet aperture 56 provided with structure on each side thereof, which in this embodiment are integrally formed arc-shaped flanges 76.

[0133] FIG. 8b shows a further dripper head 152, which is a variant of the dripper head 52 described with reference to FIGS. 7 and 8a. The dripper heads 52 and 152 are substantially similar and only the differences shall be described. Instead of arc-shaped flanges, the dripper head 152 of this variant is provided with transverse pipe sections 176 leading away from the main fluid path. The outlet holes 56 are provided at the ends of each transverse pipe section. Each transverse pipe section 176 extends laterally or radially downwards relative to the dripper head 152, to a depth similar to that of the arc-shaped flange 76 of the dripper head 52. The ends of the transverse pipe sections 176 are much larger than the diameter of the outlet holes and present flat end surfaces or steps with recessed steps between the pipe sections or sharp corners protruding relative to the sections 178. The flat end surface on which the droplet forms can be selected to be of an appropriate size to support the growing droplet, while gravity and surface tension effects prevent the droplet from spreading laterally around sharp corners, thus preventing the droplet from filling gaps and merging with adjacent droplets in the (higher) recessed section 178. The lateral pipe section 176 is shown as circular in cross-section, although other cross-sections (e.g., square, pentagonal, or hexagonal) can also be used.

[0134] FIG. 8c shows a view of a further dripper head 252, which is a further variant of the dripper head 52. The dripper heads 52, 152 and 252 are substantially similar and only the differences shall be described. Instead of an arc-shaped flange, the dripper head 252 of this variant is provided at its underside with an integral curved or arc-shaped surface 276, with an arc-shaped vertical edge surface 277 defining a sharp corner. The downwardly facing arc-shaped surface 276 may be a continuation of the generally cylindrically shaped periphery of the sidewall of the dripper head 252 and may be formed by cutting or molding a peripheral portion of the sidewall of the dripper head 252. Again, lateral side channels off the main channel lead to the outlet holes 56 provided in the arc-shaped surface 276, on which droplets may form. Thus, the outlet holes 56 are provided in downwardly-projecting sections 276 of the dripper head 252 that are defined by sharp corners and are again separated by recessed (higher) sections 278 which, through the effects of gravity and surface tension, prevent water droplets from adjacent outlet holes from bridging the gaps 278 and merging.

[0135] As shown in FIG. 8c, the outlet hole 56 of the dripper head 252 has a larger diameter compared to that of the dripper heads 52 and 152. The larger diameter facilitates customization of the dripper head 252 by use of a screw thread 57 provided in the outlet hole 56 of the dripper head 252. The screw thread 57 allows for customization of the dripper head outlet hole by insertion of a grub screw with a bore, the bore size being customizable to control the flow rate. Similarly, the screw thread can be used to add a pipe section similar in shape and function to the pipe section 176 to position and adjust the diameter of the outlet hole 56. Alternatively, the outlet hole 56 of the dripper head 252 can be a hole of a similar diameter to the outlet hole shown in FIG. 8a and FIG. 8b.

[0136] Accordingly, the present invention has been described above purely by way of example and modifications of detail may be made thereto within the scope of the claims appended hereto. [Explanation of symbols]

[0137] 10. Fuel Cell System 12 Stack 14 Fuel Cell 16 Electrical Output 18 Heated outgassing 18a Outgas flow path 20 First heat exchanger 22 Steam Generator 24 Reformer 25 Syngas 26 Fuel 27 Air 28 water 30 Heated steam and fuel mixture 32 Heated Air 34 Second Heat Exchanger 36 Water inlet port 38 Heat supply inlet port 40 Heat supply outlet port 42 Heated steam and fuel gas outlet ports 44 T-junction 46 Water inlet pipe 48 Fuel inlet pipe 50 heated inner plate 52 Dripper Head 54 Aperture 56 Exit hole 57 Screw Thread 58 Internal Channels 60 Finn 62 Flange 64 T-manifold 66 Side Branch 72 Washer 74 water droplet 76 Arc-shaped flange 78 Recessed Section 152 Dripper Head 176 Horizontal Pipe Sections 178 Recessed Section 252 Dripper Head 276 Arc-shaped Surface 277 Arc-shaped edge 278 Recessed Section

Claims

1. 1. A steam generator for a fuel cell system, the steam generator comprising: a heat exchanger having at least one internal heat exchange surface; A water inlet pipe; a dripper head including a flow passage fluidly connected to the water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water down onto the heat exchange surface for conversion to steam; The dripper head includes a plurality of outlet holes spaced along a length of the flow channel; wherein, between adjacent outlet holes, the dripper head has a stepped profile on at least its lower surface when viewed in cross section to prevent droplets from adjacent holes from coalescing.

2. 10. The steam generator of claim 1, wherein the stepped profile between adjacent outlet holes includes at least two stepped surfaces at different heights above the heat exchange surface.

3. 3. The steam generator of claim 2, wherein the adjacent outlet holes are each provided on a stepped surface, with at least one stepped surface between them, i.e. at a different height above the heat exchange surface.

4. 4. The steam generator of claim 3, wherein the adjacent outlet holes are each provided on a stepped surface forming part of a structure that projects outwardly toward the heat exchange surface below, the stepped surface optionally being of a size and shape selected to encourage growth of droplets of an appropriate size.

5. 5. The steam generator of claim 4, wherein the adjacent outlet holes are provided in respective protruding structures, the protruding structures being integrally formed with the dripper head and separated from one another by grooves or channels.

6. 4. The steam generator of claim 1, wherein each outlet hole is separated from an adjacent outlet hole by at least one baffle that projects outwardly relative to the outlet hole.

7. 7. The steam generator of claim 6, wherein each outlet hole has a respective pair of baffles associated therewith that are disposed axially along the dripper head on either side of each hole to isolate it from adjacent holes.

8. The steam generator of claim 7 , wherein the pair of baffles includes parallel sidewalls.

9. 9. The steam generator of claim 6, wherein the baffle is an integral structure of the dripper head.

10. 10. The steam generator of claim 6, wherein the baffle extends partially around a periphery or perimeter of the dripper head.

11. The steam generator of claim 10 , wherein the baffle defines an outwardly projecting arc-shaped shoulder.

12. 10. The steam generator of claim 6, wherein the baffle extends around the entire periphery or perimeter of the dripper head.

13. 9. The steam generator of claim 6, wherein the baffle comprises a separate component mounted on the dripper head.

14. 14. The steam generator of claim 13, wherein the dripper head includes a tubular end section and the baffle includes a pair of washers mounted on the tubular end section, one of each pair proximal to its exit hole and the other of each pair distal to its exit hole.

15. 15. The steam generator of claim 1, wherein the dripper head is a removable or replaceable component.

16. 16. The steam generator of claim 1, wherein the outlet holes are provided at spaced axial positions along a lower half or surface of the dripper head.

17. 17. The steam generator of claim 1, further comprising a fuel inlet pipe, the steam generator configured to combine steam and fuel inside the steam generator during use.

18. 18. The steam generator of claim 17, wherein the fuel inlet pipe has a section coaxially mounted within a portion of the water inlet pipe.

19. 20. The steam generator of claim 18, wherein the section of the fuel inlet pipe surrounds a portion of the water inlet pipe.

20. A steam generator according to any one of claims 1 to 16, a fuel inlet pipe; the fuel inlet pipe has a section surrounding a portion of the water inlet pipe at or adjacent to the outer wall of the heat exchanger and extends through the outer wall of the heat exchanger such that the section of the fuel inlet pipe and the fuel therein during use act to insulate the surrounded portion of the water inlet pipe from the heat of the heat exchanger.

21. A fuel cell system comprising a steam generator described in any one of claims 1 to 20.

22. 22. The fuel cell system of claim 21, wherein the steam generator includes a fuel inlet pipe and a combined steam and fuel outlet, and a reformer is connected directly or indirectly downstream of the steam generator in the fuel cell system.

23. 21. A fuel cell system comprising a fuel cell stack, a fuel inlet pipe, a water inlet pipe, an air or oxidant inlet pipe, a steam generator, an optional reformer connected to the steam generator for providing hydrogen or syngas to the fuel cell stack, and a heat exchanger for taking directly or indirectly a heated fluid from an outlet of the fuel cell stack, the heat being used to heat at least one internal heat exchange surface of the heat exchanger, the at least one internal heat exchange surface being used to generate steam from water from the water inlet pipe via a dripper head, the steam generator being as defined in any one of claims 1 to 20.

24. 24. Use of a fuel cell system according to any one of claims 21 to 23 to minimise stack voltage fluctuations due to fuel-side composition or pressure changes in the fuel / vapor mixture entering a fuel cell stack of said fuel cell system.

25. A method of generating steam using a steam generator described in any one of claims 1 to 20, wherein water is delivered to the heat exchange surface through the water inlet pipe and the dripper head and converted to steam, and optionally, fuel is simultaneously delivered into the heat exchanger, where the fuel is mixed with the steam and heated.

26. 1. A method of minimizing pressure pulsations in a steam generator during steam generation, the steam generator including a heat exchanger with at least one internal heat exchange surface, a water inlet pipe, and a dripper head including a flow passage fluidly connected to the water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water droplets down onto the heat exchange surface for conversion to steam, the dripper head including a plurality of outlet holes spaced along a length of the flow passage, the dripper head having a stepped profile on at least its underside when viewed in cross section between adjacent outlet holes, the method including the step of adjusting a flow rate of water to a selected flow rate, whereby the stepped profile prevents droplets from adjacent holes from coalescing.

27. 1. A method of operating a steam generator for a fuel cell system, the steam generator comprising: a heat exchanger having at least one internal heat exchange surface; A water inlet pipe; a dripper head including a flow passage fluidly connected to the water inlet pipe, the dripper head extending inside the heat exchanger above the heat exchange surface for delivering water down onto the heat exchange surface for conversion to steam; the dripper head includes a plurality of outlet holes spaced apart along a length of the flow path, and between adjacent outlet holes, the dripper head has a stepped profile, when viewed in cross section, on at least its underside to prevent droplets from adjacent holes from coalescing; The method comprises: delivering water to the dripper head through the water inlet pipe; dripping water from the dripper head through the outlet holes as water droplets, and not as steam or a spray, the water droplets dripping onto the heat exchange surface for conversion to steam; A method comprising:

28. 28. A method according to any one of claims 25 to 27, wherein the steam generator forms part of a fuel cell system and generates steam which is fed to a fuel cell inlet in the fuel cell system.

Images