Patent Application
20130171538
The present invention concerns Cell stacks, in particular Solid Oxide Fuel Cell (SOFC) stacks or Solid Oxide Electrolysis Cell (SOEC) stacks, where the flow directions of the cathode gas relative to the anode gas internally in each cell are combined so that each cell has counter-flow as well as co-flow. Further a plurality of gas streams can be merged after passing a first part of each cell, before the gas stream is again split and carried to the second part of each cell.
In the following, the invention is explained in relation to SOFC. Accordingly, in the SOFC the cathode gas is an oxidation gas and the anode gas is a fuel gas. The invention can, however, also be used for other types of cells such as SOEC as already mentioned or even Polymer Electrolyte fuel cells (PEM) or Direct Methanol Fuel Cells (DMFC).
An SOFC comprises an oxygen-ion conducting electrolyte, a cathode where oxygen is reduced and an anode where hydrogen is oxidised. The overall reaction in an SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water. The operating temperature for an SOFC is in the range 550 to 1000° C., preferably circa 650 to 850° C. An SOFC delivers in ordinary operation a voltage of approximately 0.8V. To increase the total voltage output, the fuel cells are assembled in stacks in which the fuel cells are electrically connected via interconnector plates.
In order to produce the required hydrogen, the anode usually possesses catalytic activity for the steam reforming of hydrocarbons, particularly natural gas, whereby hydrogen, carbon dioxide and carbon monoxide are generated. Steam reforming of methane, the main component of natural gas, can be described by the following equations:
CH4+H20⇄CO+3H2
CH4+CO2⇄2CO+2H2
CO+H20⇄CO2+H2
During operation an oxidant such as air is supplied to the solid oxide fuel cell in the cathode region. Fuel such as hydrogen is supplied in the anode region of the fuel cell. Alternatively a hydrocarbon fuel such as methane is supplied in the anode region where it is converted to hydrogen and carbon oxides by the above reactions. Hydrogen passes through the porous anode and reacts at the anode/electrolyte interface with oxygen ions generated on the cathode side and conducted through the electrolyte. Oxygen ions are created in the cathode side as a result of the acceptance of electrons from the external circuit of the cell.
Interconnects serve to separate the anode and cathode sides of adjacent cell units and at the same time enable current conduction between anode and cathode. Interconnects are usually provided with a plurality of channels for the passage of anode gas (fuel) on one side of the interconnect and cathode gas (oxidant gas) on the other side. The flow direction of the anode gas is defined as the substantial direction from the anode gas inlet region to the anode gas outlet region of a cell unit. Likewise the flow direction of the cathode gas is defined as the substantial direction from the cathode gas inlet region to the cathode gas outlet region of a cell unit. Thus, internally a cell can have co-flow if the anode gas flow direction is substantially the same as the cathode gas flow direction or cross-flow if the anode gas flow direction is substantially perpendicular to the cathode gas flow direction or counter-flow if the anode gas flow direction is substantially opposite to the cathode gas flow direction.
Conventionally, the cells are stacked on top of each other with complete overlap resulting in a stack with for instance co-flow having all anode gas and cathode gas inlets on one side of the stack and all anode gas and cathode gas outlets on the opposite side. Due to the overall exothermicity of the electrochemical process, the outlet gases leave at a higher temperature than the inlet temperature. When combined in an SOFC stack operating at for instance 750° C. a significant temperature gradient across the stack is generated. These temperature gradients across the stack are to some extent necessary for the cooling of the stack, since the air cooling is proportional to the temperature gradient, but large thermal gradients induce thermal stresses in the stack which are highly undesirable and they entail difference in current density and electrical resistance. Therefore the problem of thermal management of an SOFC stack exists: to reduce thermal gradients enough to avoid unacceptable stresses but have sufficiently large thermal gradients to be able to cool the stack with said gasses.
The thermal gradients necessary to cool the stack set a limit to the electrical effect which a cell stack can obtain. A high average cell temperature is advantageous with regard to electric effect, and the average cell temperature should be as close to the maximum limit for the materials of the cell and of cell stack. But since temperature gradients are necessary to cool the stack, the average cell temperature is in reality lower than this maximum limit temperature. The area specific resistance (ASR) of the cell is higher with a low average cell temperature, thus the electrical effect is lowered with a lowering average cell temperature.
U.S. Pat. No. 6,830,844 describes a system for thermal management in a fuel cell assembly, particularly for preventing temperature gradients of above 200° C. across the cathodes by periodically reversing the air flow direction across the cathode, thereby alternating the supply and exhaust edges of the cathodes.
U.S. Pat. No. 6,803,136 describes a fuel cell stack with a partial over-lap in between the cells comprising the stack resulting in an overall spiral configuration of the cells. The cells are angularly offset to one another which provides ease of mani-folding and thermal management.
It is an object of the present invention to provide a cell stack, particularly a solid oxide fuel cell stack with a high average cell temperature which results in a minimised ASR, and thereby to improve the cell and cell stack electrical effect as compared to state of the art cell stacks.
It is a further object of the present invention to provide a cell stack, particularly a solid oxide fuel cell stack where the cell outlet temperature is close to or equal to the maximum temperature of the cell with respect to the cell and stack components durability, whereby the cooling efficiency of the cathode gas and anode gas is improved.
It is yet another object of the present invention to provide a cell stack, particularly a solid oxide fuel cell stack with networking of the anode gas where the anode gas streams from each cell in the cell stack are collected and mixed after passing a part of each cell and then split and passed over the remaining part of each cell, whereby fuel utilisation is optimized.
It is an object of the present invention to provide a cell stack, particularly a solid oxide fuel cell stack, where the flow on each cell is changed from counter-flow on a first part of the cell to co-flow on a second part of the cell of the anode gas relative to the cathode gas on each cell, which optimises the heat distribution on each cell and in the cell stack as a whole.
It is a further object of the present invention to provide a cell stack, particularly a solid oxide fuel cell stack, where the counter-flow/co-flow area ratio can be varied to fit the specific desired operating point.
These and other objects are solved by the invention.
Accordingly, we provide a cell stack, in particular a solid oxide fuel cell stack comprising a plurality of planar cells arranged in layers on top of each other in planes parallel to each other, in which each cell unit comprises an anode, an electrolyte and a cathode, and where the anode and cathode of adjacent cells are separated from each other by an interconnect. Each cell of the cell stack has at least one anode gas inlet region, at least one cathode gas inlet region; at least one anode gas outlet region and at least one cathode gas outlet region. The anode part of each cell in the stack or the cathode part is divided into at least a first part and a second part. In any of these two cases, each cell is thus divided into at least a first part and a second part. Each cell in the cell stack has counter-flow of the anode gas flow direction relative to the cathode gas flow direction in the first part of the cell; and co-flow of the anode gas flow direction relative to the cathode gas flow direction in the second part of the cell.
In state of the art fuel cells, a choice between the advantages of counter-flow and co-flow respectively would have to be made, the advantages being:
Counter-flow advantage: High average cell temperature resulting in a low area specific resistance (ASR) of the cells.
Co-flow advantage: The outlet temperature of anode and cathode gas is nearly as high or equal to the maximum temperature of the stack (which is not possible for counter-flow). This maximises the cooling efficiency of cathode gas and anode gas. On the system level this gain means low parasitic loss to gas blowers and a reduced heat exchanger capacity requirement.
But in the flow pattern according to this invention, where the first part of the cell operates in counter-flow and the second part of the cell operates in co-flow, the advantages of counter-flow and co-flow are combined. Hence a low ASR and a maximised cooling efficiency of the reactant gasses are achieved in combination in each of the particular cells in a cell stack, since both flow geometries are achieved simultaneously. This is particularly the case when operating with a reforming i.e. cooling anode gas. The high average cell temperature is achieved by the optimised heat distribution in the cell and thus in the stack. This optimised heat distribution also means that the internal stresses are lowered because the differential temperature of the stack is lowered. Otherwise, internal stresses demand considerable compression forces to the stack and may lead to damage of the stack.
The actual ratio between the counter-flow area and the co-flow area of each cell in the stack can be varied depending on the type of anode gas, the operation parameters, the stack-specific characteristics etc. to achieve the best operation efficiency especially with regard to ASR and cooling efficiency. In an embodiment of the invention, the counter-flow area of the cell is circa 80% of the active area of the cell whereas the co-flow area is circa 20% of the active area of the cell. In further embodiments of the invention the counter-flow area is between 85% and 70% of each cell or between 95% and 50% of each cell.
It is possible to apply these principles of the described counter-flow/co-flow cell to the cathode side of the cell as well as the anode side of the cell. I.e. either the cathode gas flows substantially in one direction whereas the anode gas flows first in a substantially opposite direction of the cathode gas and thereafter in the substantially same direction as the cathode gas or; the anode gas flows substantially in one direction whereas the cathode gas flows first in a substantially opposite direction of the anode gas and thereafter in the substantially same direction as the anode gas. But changing flow directions of the anode gas (the fuel) is usually simpler due to smaller flow rates of the anode gas relative to the cathode gas.
Thus, in one embodiment of the invention, the cathode gas flows in a first direction across the substantially entire electro-chemically active cathode part of each of the plurality of cells in the cell stack, and the anode gas flows in a second direction substantially opposite the first direction across a first anode part of each cell and then flows substantially in said first direction across a second anode part of each cell.
In an embodiment of the invention, the afore mentioned flow pattern is obtained by locating the cathode gas inlet region near a first edge of the cell and the cathode gas outlet region near a second edge of the cell. It is understood that the substantial entire active area of the cell is located between said first and said second edge of the cell. The cathode gas thus enters through the inlet region near the first cell edge, flows over the active area of the cathode in a first direction and then exits through the cathode gas outlet near the second cell edge.
On the opposite side of the cell, viz. on the anode region, the anode gas enters through the first anode gas inlet region located between said first and second edge of the cell. The anode gas then performs the first anode gas flow pass where it flows in a second direction substantially opposite said first direction before it exits through the first anode gas outlet region located near the first edge of the cell. Hence in this first anode gas flow pass across a first part of the cell, the anode and cathode gas have counter-flow. After exiting through the first anode gas outlet region, the anode gas is carried through an internal or external ducting/manifolding and re-enters the anode part of the cell through the second anode gas inlet region which is also located between the first and the second edge of the cell. The anode gas then performs a second flow pass in the first direction from the second anode gas inlet region across a second part of the cell and exits through the second anode gas outlet region located near the second edge of the cell. Hence, on this second part of the cell, the anode gas and cathode gas flow in co-flow.
The cell may have an internal or external gas manifolding or a combination of the two, as is well described in the state of the art. In one embodiment, the cathode gas may be carried to and from the cathode parts of the cells in a stack by an external manifolding, such that the cathode gas inlet region is actually almost the entire part of the first edge of the cells. The anode gas may in this embodiment be carried to and from the anode parts of the cells in a stack by an internal manifolding, such as holes in the interconnects.
In an embodiment of the invention, the intermediate anode gas streaming out of each cell in a cell stack is networked. Here “intermediate” means the gas having performed the first flow pass, but before having performed the second flow pass. The anode gas exiting each cell via the first anode gas outlet region is collected in the manifolding and mixed to a substantially homogeneous intermediate common anode gas mix, before it is split and carried further to each second anode gas inlet region of each cell in the stack. This networking of the intermediate anode gas streams compensates for differences in the cell fuel utilisation and therefore provides a higher overall fuel utilisation.
In a further embodiment of the invention, a combination of counter-flow and co-flow over each cell is provided by other means than plural gas inlet and outlet regions. Instead the gas flow channels of the interconnects are adapted to provide a gas flow in substantially a first direction on a first face of the interconnect and a gas flow in a second, substantially opposite direction on a first part of the second face of the interconnect as well as a gas flow in said first direction on a second part of the second face of the interconnect. This embodiment can also be adapted such that either the anode gas or the cathode gas only flows in one direction. Again, as the cathode gas flow is generally larger than the anode gas flow, it can be advantageous to construct the interconnect gas flow channels such that the cathode gas flows substantially in only one direction, whereas the anode gas flows in substantially two directions considering for instance pressure loss.
1. A cell stack comprising a plurality of cells adapted to operate as fuel cells or electrolysis cells and stacked on top of each other to form a plurality of cell layers in the cell stack, each of said cells comprising
When exiting the first anode gas outlet region, the partly reacted anode gas from each of the like cells in a cell stack is collected (not shown) and mixed into a common intermediate anode gas stream, and subsequently this gas stream is again split and re-enters each fuel cell via the second anode gas inlet region 107. The anode gas performs a second flow pass across a second anode part of the cell 103 in a substantially first gas flow direction 110 and finally exits via the second anode gas outlet region 109 formed simply by the edge of the cell where the exhausted anode gas mixes with the exhausted cathode gas. On the opposite, not visible cathode surface of the cell, the cathode gas is flowing in a substantially first gas flow direction 110 across the substantially entire active area of the cell, i.e. across both the first and the second part of the cell from the cathode gas inlet region 104 to the cathode gas outlet region 105. Hence, since the first gas flow direction is opposite the second gas flow direction, it can be seen from
On the opposite, not visible, cathode side of the cell, the cathode gas enters through the cathode gas inlet region 204 passes across the substantially entire part of the active cathode surface of the cell in a first gas flow direction 210 and finally exits through the cathode gas outlet region 205. The cathode gas inlet and outlet regions are both formed as the first and second edge of the cell by means of external manifolding (not shown) as known in the art. Accordingly, the anode gas flows in a second gas flow direction across a first part and a first gas flow direction across a second part of the cell, and as the cathode gas flows in a first gas flow direction across both the first and the second part of the cell, the cell has a counter-flow across the first and a co-flow over the second part of the cell with respect to anode and cathode gas.
The chart diagram in
This combined flow pattern is clear from the temperature profile shown in
From position “0” and onwards to circa position “3.5” the cell temperature rises steadily as the cathode gas is heated by the electro-chemical reactions in the cell. Thereafter the cell temperature drops steadily as the first anode inlet region is approached, because the entering anode gas is relative cold and thus lowers the resulting temperature of the cell. The local lowest resulting temperature at the position near the first anode gas inlet (circa position “5.5”) is, however, not as low as the lowest resulting temperature near position “0”, again because the anode gas flow is smaller than the cathode gas flow and therefore has a relatively lower influence on the resulting temperature. This far, from position “0” to circa position “5.5”, the cell temperature profile corresponds to a state of the art counter-flow cell.
Near position “5.5” the anode gas having exited the cell via the first anode gas outlet region near position “0” re-enters the cell via the second anode gas inlet region and performs a second flow pass. Now, the anode gas flows in the first gas flow direction, hence, from circa position “5.5” to circa position “8” the cell has a co-flow of the anode gas relative to the cathode gas. The consequence is clearly seen on the temperature graph: From circa position “5.5” to “8” the temperature increases rapidly as both the anode gas and the cathode gas have already been pre-heated during the flow pass on the first part of the cell. After a first steep increase to approximately the level of the cathode gas and the anode gas temperature when entering the second part of the cell, the resulting temperature increases more steadily all the way to the second edge of the cell near position “8”, where the resulting temperature has reached its maximum temperature which is near or equal to the maximum allowable temperature of the stack considering the durability of the arrangement. This second part of the temperature profile (when disregarding the initial steep temperature increase) corresponds to the temperature profile of a state of the art co-flow cell. The initial steep temperature increase is among other caused by a considerable horizontal thermal conduction in the inter-connect which has the effect that the large amount of heat developed in the pure hydrogen region (viz. the co-flow part) is spread to the inlet area of the Co-flow part.
In
For a solid oxide fuel cell stack, the power density for Co-flow, Counter-flow and combined counter-co-flow is calculated under the following conditions:
Fuel type: Pre-reformed natural gas
Fuel utilisation: 86%
Air utilisation: 25% (for case 1-3, variable in case 4-5)
Air and fuel inlet temperatures: 700° C.
Maximum temperature of stack: 825° C.
Cell type: 1 (LSM cathode)
Under these specific conditions, a split of the cell area into a first area of 66% of the substantial, total, active area for counter-flow, and 34% of the substantial total active area for co-flow is close to an optimum according to the invention, when maximising power density. The combined counter-co-flow case is compared to pure counter- and pure co-flow state of the art stacks:
At the same air utilisation, a significant power density increase is obtained with the combined counter-co-flow geometry with the consequence of a slightly lower cell voltage causing a drop in the electrical efficiency of 10% compared to counter-flow.
Changing the operation specification to a fixed cell voltage of 757 mV (meaning fixed electrical efficiency), the current density of the counter-flow and co-flow stacks can be increased, if the air utilisation is decreased. The effect of this is represented by case 4 and 5.
Compared to a counter-flow having same efficiency and power density as the combined flow geometry, the air flow of the combined flow case is 40% lower, which e.g. minimises the blower power and heat exchanger costs. This is also of major importance, because the blower constitutes the largest parasitic loss in most systems.
Comparing the combined counter-co-flow case with the co-flow case, the combined case is clearly the better choice. Having the same electrical efficiency (cell voltage), the power density of the co-flow stack is 22% lower, and the air flow is a little higher.
The most common range for cell area splitting with typical operating conditions, a reforming fuel and a specific cell type is 20 to 40% co-flow area.
In the case of e.g. larger cells or faster reforming, the outlet temperature of the counter-flow part becomes lower, and the co-flow part has to constitute a larger part of the total cell area. This is a general rule in the case of changed operating parameters (air utilisation, fuel utilisation, inlet and outlet temperatures, and fuel composition); if the counter-flow part outlet is relatively low, or the Nernst potential in the co-flow part is low, the co-flow part must constitute a larger part of the cell e.g. 40 to 50%.
Conversely it is also possible to use the same operating parameters and cell characteristics to increase the counter-flow part outlet temperature or Nernst potential of the co-flow part to a relatively high level. In that case the co-flow part may constitute only a small part of the cell area e.g. 5 to 20%. A small co-flow part is also relevant in the case of two stacks in serial connection, as the co-flow stack can be operated at a lower cell voltage, generating more heat.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/005898 | 9/28/2010 | WO | 00 | 3/14/2013 |
