DESC:FIELD OF INVENTION
The present invention relates to a system for rapid starting-up of fuel cells, and in particular, to the starting-up of Solid Oxide Fuel cells (hereinafter abbreviated as SOFC).
BACKGROUND
As compared to the traditional combustion technologies that burn fuel, fuel cells undergo a chemical process to convert hydrogen-rich fuel into electricity. Fuel cells do not need to be periodically recharged like batteries, but instead continue to produce electricity as long as a fuel source is provided. Fuel cells are characterized based on their electrolyte material. SOFC is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. The SOFC has a solid oxide or ceramic electrolyte.
Solid Oxide Fuel Cells (SOFCs) are considered as one of the most promising technologies for very high-efficiency electric energy generation. The maximum efficiency of SOFC system can be as high as 90% depending upon the operating condition and configuration.
However, in order to avail the maximum efficiency from SOFCs, several factors being important, need to be optimized. The duration of the start-up is, in fact, a major factor that must be considered since the rapid system response is an important factor in determining the feasibility of SOFC based power units. Start-up may also significantly affect the life span of the system due to thermal stresses on all components.
Prior to starting the SOFC system electricity production, all fuel system components have to be heated up above the dew point before the recirculation of the anode off-gas with high steam content can be initiated. Otherwise, water could condensate to the system components (e.g. reformer catalysts and recirculation blowers), which would have detrimental effects on their operation.
In general, it requires several hours for the initial heating of SOFC. Conventionally, air is heated by electrical heater and it further heats the stack and other components in the system. Once, the system gets heated up to a particular temperature, fuel is used to heat the system and elevate the temperature to the required range, i.e., 650oC and above. Generally, the share of thermal mass of stack in the SOFC system is as low as 8% of the system depending on the size, capacity, and design of the system. The initial heating is primarily done by convection using hot air at low velocity. This heating is complemented by radiative heating in the system. But the radiative heating is only partial while the convective heating is the main phenomenon. So, the heating in such a case is time-consuming and so the star-up of the SOFC system is delayed.
In this regard, one implementation discloses a solid oxide fuel cell with a metal support including a buffer area which is formed on an outer side of a power generating area in an in-plane direction. A pore in the metal support in the buffer area is filled with a material with a thermal conductivity lower than that of a formation material of the metal support. If a thermal conductivity of the buffer area of a metal support is lower than a thermal conductivity of a power generating area, it is possible to make small a temperature gradient in the end portion of the power generating area of the metal support. As a result, at rapid start-up, the heat does not easily conduct from the power generating area to the outer side. This decreases a temperature difference at an end portion of a power generation cell facing the power-generating area and thus, reducing a thermal stress to be generated in the end portion of the power generation cell. However, it does not use radiative heating for the purpose of heating the module to cause rapid start up and so the mass of stack heated in said arrangement is comparatively large and so the heating process is less efficient.
Further, Bossel, Ulf. (2102). “A grove Fuel Cell Event Rapid start-up SOFC modules” discloses a compact 200W SOFC module characterized by its rapid startup capability. The module running on hydrogen and reformate from hydrocarbon fuels or alcohols raises the stack temperature 700oC in less than 5 minutes by electric heaters placed in all bipolar plates. However, said module faces challenges in preventing leakages by placement of the stack in a hermetically sealed container. Further, it becomes difficult for said module to reduce heat losses by suspending the stack on the four thin-walled gas supply and discharge tubes. Another challenge associated with the module is that it is required to conduct direct monitoring in the center of the stack.
Furthermore, another development in the start-up strategy for an SOFC based Auxiliary Power Unit under Transient Conditions reveals that rapid start-up significantly affects the life span of the system due to the thermal stresses on all system components. Therefore, a proper balance must be struck between a fast response and the costs of owning and operating the system so that start-up or any other transient process can be accomplished in a short time as possible and consuming a minimum fuel.
Furthermore, Y.M. Barzi et al (2009) “Numerical analysis of start-up operation of a tubular solid oxide fuel cell” further reveals that an average start-up time is around 2.5 hours with a maximum heat-up rate of 30oC. Also, the highest temperature gradients occur right at the beginning of the start-up phase. Thus, it is known that the probability of a cell failure after a certain point decreases, if the cell can withstand the initial increase in thermal stress.
In particular, there is a need for a system for rapid starting-up of the fuel cell by reducing the heating time of the fuel cell system.
SUMMARY
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended for determining the scope of the invention.
In an embodiment of the present disclosure, a fuel cell system for starting-up a fuel cell stack module is disclosed. The fuel cell system includes a heating unit adapted to heat at least one surface of a fuel cell stack. The heating unit is positioned at a first predefined clearance from the at least one surface of the fuel cell stack. Further, the fuel cell system includes a covering member positioned around the heating unit. A second predefined clearance is defined between at least one surface of the heating unit and the covering member. The fuel cell system includes at least one sheet positioned at a surface from among a plurality of surfaces of the fuel cell stack, wherein the surface is provided with a fuel inlet and an oxidant inlet for the fuel cell stack.
In another embodiment of the present disclosure, the heating unit is a radiative heater adapted to perform radiative heating of a predefined thermal mass of the fuel cell stack through the at least one surface of the fuel cell stack.
In yet another embodiment of the present disclosure, the predefined thermal mass is around 1/20th of a total thermal mass of the fuel cell stack.
In another embodiment of the present disclosure, the first predefined clearance is in a range of 0 to 20 mm.
In yet another embodiment of the present disclosure, a starting-up time of the fuel cell system depends on the first predefined clearance between the heating unit and the at least one surface of the fuel cell stack.
In another embodiment of the present disclosure, the covering member is formed of one of a high temperature alloy material and a non-metallic material.
In yet another embodiment of the present disclosure, the covering member is adapted to form an oxide layer on one of surfaces of the covering member, when a predefined thermal mass of the fuel cell stack is heated by the heating unit.
In another embodiment of the present disclosure, the at least one sheet is formed of a high electrically insulating material.
In yet another embodiment of the present, the at least one sheet is positioned on the surface that is in contact with electrode terminals of the fuel cell stack.
In another embodiment of the present disclosure, the at least one sheet is adapted to avoid mixing of the fuel and the oxidant in the fuel cell stack.
To further clarify the advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Figure 1 illustrates the arrangement of a heating unit in a fuel cell stack module of a fuel cell system, according to an embodiment of the present disclosure;
Figure 2 is a graphical representation of the relation between the starting-up time of the fuel cell stack module in the fuel cell system with a change in the surface temperature of the heating unit for different stack masses, according to an embodiment of the present disclosure; and
Figure 3 is a graphical representation of the relation between the starting-up time of the fuel cell stack module in the fuel cell system with the surface temperature of the heating unit for different stack masses and heating sides, according to an embodiment of the present disclosure.
Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have necessarily been drawn to scale. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.
OBJECTIVES OF THE INVENTION
The main objective of the present disclosure is to provide a system for the rapid starting-up of the fuel cell by reducing the initial heating time of the stack and other components in the fuel cell system.
Another advantage of the present disclosure is to use at least one radiative heater in the proximity of the stack walls to enable a significant rise in the temperature within a fraction of minutes.
Yet another advantage of the present disclosure is to perform heating of the POX, TOX, and stack in parallel which leads to heating of the stack by using power from the battery for small systems. The same concept can also be extended for higher capacity SOFC systems.
Yet another advantage of the present disclosure is that the heating by the proposed solution will ensure uniformity and constant heat flux at outermost surfaces. A uniform temperature at the surface and uniform heat flux will ensure minimum thermal shock.
To further clarify advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings.
DETAILED DESCRIPTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.
Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
Figure 1 illustrates an arrangement of a heating unit 104 in a fuel cell stack module 100 of a fuel cell system, according to an embodiment of the present disclosure. The plurality of fuel cells may interchangeably and individually be referred to as the fuel cell stack and the fuel cell. In an embodiment, the fuel cell system may be embodied as an electric generator that uses fuels, such as Hydrogen and Hydrocarbons, to generate electricity. For instance, the fuel cell system may be adapted to convert chemical energy associated with the fuels into electrical energy which is further used as electricity. In an embodiment, the fuel cell system may be implemented in a vehicle including, but not limited to, electric vehicles, for driving the vehicle using the generated electrical energy. In another embodiment, the fuel cell system may also be implemented in other commercial applications, including but not limited to Range extenders in electric vehicles, critical backup in drones, servers, medical services, military applications, and telecom towers.
In an embodiment, the heating time of a stack 102 and the components in the fuel cell system is substantially reduced by reducing a net thermal mass to be heated. In another embodiment, the heating time is substantially reduced by performing parallel heating of only the required thermal mass. In yet another embodiment, the heating time is substantially reduced by performing radiative heating of the stack 102 in a closed cavity.
In an embodiment, the fuel cell system for starting up the fuel cell stack module 100 may comprise a fuel cell stack module 100. The fuel cell stack module 100 may further comprise one or more fuel cells along with fuel, water, air management, and other required components. The other required components in the fuel cell stack module 100 may be selected depending on the application of the fuel cell system accommodating the fuel cell stack module 100. The one or more fuel cells in the fuel cell stack module 100 are preferably SOFCs arranged in one of a series and a parallel connection depending on the application of the fuel cell system. The SOFCs in the fuel cell stack module 100 may be one of a tubular SOFC and a planar SOFC. In tubular SOFCs, a cathode tube is fabricated first with a porosity of 30-40% to permit rapid transport of the reactant and product gases to cathode/ electrolyte interface where the reactions occur. The electrolyte is applied to the cathode tubes by electrochemical vapour deposition. In planar SOFCs, the anode, the electrolyte, and the cathode form thin, flat layers sintered together and then separated by bipolar plates similar to design of other types of fuel cells. The fuel cell stack module 100 is integrated into a complete fuel cell system to be used in different applications.
In one embodiment, the fuel cell stack module 100 may have a single fuel cell. In another embodiment, the fuel cell stack module 100 may have a plurality of fuel cell stack. The fuel cell stack generates electricity from electrochemical reaction . The fuel cell stack is connected in either series to provide the required voltage or in parallel to provide the required current. Said configuration of the connected fuel cell stack may be referred to as the stack 102. The stack 102 feeds the cathodes with oxygen and the anodes with the fuel. The fuel cells may be connected to each other through an interconnecting means. The interconnecting plates provide an electrically conductive path and transfer oxygen ions from cathode to anode side.
Referring to Figure 1, the fuel cell system for starting-up the fuel cell stack module 100 may comprise the heating unit 104 near one or more surfaces 105 of the stack. In an embodiment, the heating unit 104 may be adapted to heat at least one surface of a fuel cell stack 102. The heating unit 104 is positioned at a first predefined clearance from the at least one surface of the fuel cell stack 102. The heating unit 104 may further comprise a heater and preferably a radiative heater, a heat regulator, a control unit, etc. In an embodiment, the heating unit 104 is a radiative heater adapted to perform radiative heating of a predefined thermal mass of the fuel cell stack 102 through the at least one surface of the fuel cell stack 102.
The heating unit 104 may preferably be positioned in close proximity to the fuel cell stack module 100. The heating unit 104 is configured to heat a pre-defined thermal mass of the fuel cell stack module 100. In one of the embodiments of the present disclosure, the pre-defined thermal mass of the fuel cell stack module 100 may be around 1/20th of the total thermal mass (about 13 kg for 1 kW stack) of the fuel cell stack module 100.
The heating unit 104 may heat the thermal mass of stack 102 from one or more sides. In an embodiment of the present disclosure, the heating unit may perform heating from four sides by overcoming engineering challenges. An example of addressing such engineering challenges includes arranging the entry of electrode terminal as well as entries of fuel and air on one side. The heating from one or more sides of the heating unit 104 ensures uniformity and constant heat flux at outermost surfaces. Such constant and uniform heating of the thermal mass of stack 102 prevents large thermal stress at the end portion of the fuel cell stack module 100 with a relatively low temperature. These thermal stresses, if not prevented, can break (damage the seal) the fuel cell stack module 100.
In an embodiment of the present disclosure, the heating unit 104 is positioned with respect to the fuel cell stack module 100 such that a first predefined clearance 106 is defined between the one or more surfaces of the fuel cell stack module 100 and the one or more surfaces of the heating unit 104. In a preferred embodiment of the present disclosure, the first predefined clearance 106 may lie in the range of 0 to 20 mm. However, the range of the first predefined clearance 106 may vary depending on the size and configuration of the product or the final system. The first predefined clearance 106 affects a starting-up time of the fuel cell system. In an embodiment, the starting-up time of the fuel cell system depends on the first predefined clearance 106 between the heating unit 104 and the at least one surface of the fuel cell stack 102.
Further, referring to Figure 1, the fuel cell system for starting-up a fuel cell stack module 100 may comprise a covering member 108 positioned around the heating unit 104. The covering member 108 may preferably be positioned in a manner such that a second predefined clearance is defined between the one or more outer surfaces of the heating unit 104 and the covering member 108. In an embodiment, the covering member 108 may be positioned around the heating unit 104. A second predefined clearance may be defined between at least one surface of the heating unit 104 and the covering member 108. The covering member 108 surrounding the heating unit 104 is preferably an inconel sheet/ plate, a high temperature alloy material or a non-metal with similar properties. In an embodiment, the covering member 108 is formed of a high electrically insulating material.
Such materials used in preparing the covering member 108 are less prone to oxidation-corrosion and are well suited for service in extreme environments subjected to pressure and heat. In an embodiment, the covering member 108 is adapted to form an oxide layer on one of surfaces of the covering member, when a predefined thermal mass of the fuel cell stack 102 is heated by the heating unit 104. The covering member 108 is adapted to form a layer of thick, stable, and passivating oxide layer on its own surface when the pre-defined thermal mass is heated by the heating unit 104 in the fuel cell system. The layer of thick, stable, and passivating oxide layer formed on the surface of the covering member 108 surrounding the heating unit 104 protects the surface from further attacks.
In an embodiment of the present disclosure, the starting-up system for the fuel cell stack module 100 may comprise one or more sheets 110 positioned at one of the surfaces of the fuel cell stack module 100 in the fuel cell system. In an embodiment, the fuel cell system includes at least one sheet 110 positioned at a surface from among a plurality of surfaces of the fuel cell stack. The surface is provided with a fuel inlet and an oxidant inlet for the fuel cell stack. The at least one sheet 110 may interchangeably be referred to as the sheet 110, without departing from the scope of the present disclosure. Preferably, the sheet 110 is a high electrically insulating material, like mica of variable thickness. The sheet 110 is positioned at such surfaces of the fuel cell stack module 100 which are in contact with fuel and/or air entries. The sheet 110 may be positioned at the surfaces of the fuel cell stack module 100 in contact with electrode terminals (112a, 112b). The sheet 110 is adapted to avoid mixing of fuel and oxidant in the fuel cell stack module 100 of the fuel cell system.
Figure 2 is a graphical representation of the relation between the starting-up time of the fuel cell stack module 100 in the fuel cell stack system with the change in the surface temperature of the heating unit 104 for different stack masses, according to an embodiment of the present disclosure. Said relation has been derived with the first predefined clearance 106 being 10 mm. However, those skilled in the art will appreciate that said data can even be derived for a different range of first predefined clearance 106. The illustrated data has been derived considering only a selected range of change in the surface temperature of the heating unit 104. However, the same is not limited to the stipulated range of change in temperature of the heating unit 104.
Referring to figure 2, when the first predefined clearance 106 is kept 10 mm and the change in the surface temperature of the heating unit 104 is kept constant, the starting-up time of the fuel cell stack module 100 in the fuel cell system increases with increase in the mass of the stack 102. For example, the first predefined clearance 106 is kept at 10 mm and the temperature of the heating unit 104 is kept at 1200 oC . In such an example, the starting-up time of the fuel cell stack module 100 in the fuel cell system is 10 minutes, when the mass of the stack 102 being heated is around 4 Kilograms. In another example, under conditions similar to previous example, the starting-up time of the fuel cell stack module 100 in the fuel cell system is 20 minutes, when the mass of the stack 102 being heated is around 10 Kilograms.
Figure 3 is a graphical representation of the relation between the starting-up time of the fuel cell stack module 100 in the fuel cell system with the surface temperature of the heating unit 104 for different stack masses and the heating sides, according to an embodiment of the present disclosure. Said relation has been derived for the stack masses of 13.6 kg with one side heating, stack mass of 4.08 kg with one side heating, stack mass of 2.72 kg with 2 sides heating, stack mass of 9.52 kg with 1 side heating, stack mass of 2.72 kg with 1 side heating, and stack mass of 2.72 kg with 4 sides heating. For example, for the stack mass of 2.72 kg with 1 side heating, the starting-up time of the fuel cell stack module 100 is around 15 minutes with the surface temperature of the heating unit 104 being 1000 oC. However, when the same mass of stack 102 is heated from 4 sides, the starting-up time of the fuel cell stack module 100 is less than 5 minutes. The illustrated data has been derived considering only a selected range of stack mass and heating sides of the heating unit 104. However, the same is not limited to the stipulated ranges.
While specific language has been used to describe the present subject matter, any limitations arising on account thereto, are not intended. As would be apparent to a person in the art, various working modifications may be made to the method in order to implement the inventive concept as taught herein. The drawings and the foregoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment.
,CLAIMS:1. A fuel cell system for starting-up a fuel cell stack module, the fuel cell system comprising:
a heating unit adapted to heat at least one surface of a fuel cell stack, wherein the heating unit is positioned at a first predefined clearance from the at least one surface of the fuel cell stack;
a covering member positioned around the heating unit, wherein a second predefined clearance is defined between at least one surface of the heating unit and the covering member; and
at least one sheet positioned at a surface from among a plurality of surfaces of the fuel cell stack, wherein the surface is provided with a fuel inlet and an oxidant inlet for the fuel cell stack.

2. The fuel cell system as claimed in claim 1, wherein the heating unit is a radiative heater adapted to perform radiative heating of a predefined thermal mass of the fuel cell stack through the at least one surface of the fuel cell stack.

3. The fuel cell system as claimed in claim 2, wherein the predefined thermal mass is around 1/20th of a total thermal mass of the fuel cell stack.

4. The fuel cell system as claimed in claim 1, wherein the first predefined clearance is in a range of 0 to 20 mm.

5. The fuel cell system as claimed in claim 4, wherein a starting-up time of the fuel cell system depends on the first predefined clearance between the heating unit and the at least one surface of the fuel cell stack.

6. The fuel cell system as claimed in claim 1, wherein the covering member is formed of one of a high temperature alloy material and a non-metallic material.

7. The fuel cell system as claimed in claim 1, wherein the covering member is adapted to form an oxide layer on one of surfaces of the covering member, when a predefined thermal mass of the fuel cell stack is heated by the heating unit.

8. The fuel cell system as claimed in claim 1, wherein the at least one sheet is formed of a high electrically insulating material.

9. The fuel cell system as claimed in claim 8, wherein the at least one sheet is positioned on the surface that is in contact with electrode terminals of the fuel cell stack.

10. The fuel cell system as claimed in claim 9, wherein the at least one sheet is adapted to avoid mixing of the fuel and the oxidant in the fuel cell stack.