Integrated recuperation loop in fuel cell stack

Details Integrated recuperation loop in fuel cell stack Integrated recuperation loop in fuel cell stack

2002-01-03

2004-06-29

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S010000, C429S006000, C429S006000, C429S006000, C429S006000

Reexamination Certificate

active

06756144

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a fuel cell system which uses the waste heat from hot exhaust coming out of fuel cell stacks to increase the overall fuel cell efficiency. More particularly, the present invention focuses on a method and apparatus to simplify the heat exchange to recuperate the high temperature gas at the outlet of a fuel cell without the complexity of an external recuperator or heat exchanger. The current invention may also eliminate a recuperator from a fuel cell system while maintaining the heat recuperation efficiency to increase the system reliability and maintainability. In effect, the current invention replaces complicated and costly heat exchanger or multiple heat exchangers and associated complex piping and ducting.
Fuel cells electrochemically react fuels with oxidants to generate electricity. The key components in a fuel cell include a cathode material, an electrolyte material, and an anode material. The electrolyte is a non-porous material sandwiched between the cathode and anode materials. The fuel and oxidant fluids are typically gases and are continuously passed through separate passageways. A fuel gas can be hydrogen, a short chain hydrocarbon, or a gas containing desired chemical species in some form. An oxidant may be an oxygen containing gas, or quite commonly air. Reactant gases, fuels and oxidants, are typically pre-heated before being fed to electrolyte. Electrochemical conversion occurs at or near the three-phase boundaries of each electrode (cathode and anode) and the electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.
One of the common constructions of fuel cells is a solid oxide fuel cell (SOFC) that uses solid electrolytes for power generation. Solid electrolytes are either ion conducting ceramic or polymer membranes. In the former instance, the electrolyte is typically made of a ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons, which ensures that the electrons must pass through the external circuit to do useful work. With such an electrolyte, the anode is oftentimes made of nickel/YSZ cermet and the cathode is oftentimes made of doped lanthanum manganite.
SOFCs of various construction geometries have been designed. These include the tubular, segmented cells in series, and planar geometries. These various constructions are described in “Ceramic Fuel Cells” by N. Q. Minh, Journal of the American Ceramic Society, 76, p. 563, 1993.
Sometimes, a planar construction resembles a cross-flow heat exchanger in a cubic configuration. The planar cross flow fuel cell is built from alternating flat single cell membranes (which are trilayer anode/electrolyte/cathode structures) and bipolar plates (which conduct current from cell to cell and provide channels for gas flow into a cubic structure or stack). The bipolar plates are oftentimes made of suitable metallic materials. The cross-flow stack is manifolded externally on four faces for fuel and oxidant gas management.
A radial or co-flow design is another popular design in fuel cell construction. An annular or circular shaped anode and cathode sandwich an electrolyte therebetween. Annular or circular shaped separator plates sandwich the combination of anode, cathode, and electrolyte. A fuel manifold and an oxidant manifold respectively direct fuel and oxidant to a central portion of the stack so that the fuel and oxidant can flow radially outward from the manifolds.
Regardless of the particular fuel cell configuration, the electrochemical reaction between the fuel and oxidant produces electric energy, spent fuel and oxidant exhaust. Quite often, the exhaust gas from a fuel cell is the original reactant gas which has been depleted of the particular migrating species in ionic form as a result of the electrochemical reaction. This conversion of fuel and oxidant to electricity in a fuel cell also produces heat, particularly at high current/power densities, which is removed to maintain the fuel cell at an efficient operating temperature.
Conventional thermal management in a fuel cell forces a cooling medium, either a liquid or gaseous coolant stream, through the fuel cell assembly. Coolants, such as water or air, are used for fuel cell heat exchange depending on the operating temperature of the fuel cell. The coolant enters a fuel cell at a temperature near the operating temperature of the fuel cell. When the coolant passes through the fuel cell, the waste energy from the electrochemical reaction in the fuel cell is carried away by the heat capacity of the coolant. The volume flow of the coolant is closely related to the temperature rise of the cooling medium that is determined by the constraints associated with thermal stress of the components in the fuel cell such as ceramic cells in an SOFC. The heat exchange system typically incorporates mechanical components to facilitate the heat transfer to the cooling medium. Flow channels are routinely employed to keep coolant, fuel and oxidant in their separate passage. In many cases, the cooling medium is also a reactant, e.g., air in an SOFC. The effectiveness of heat exchange is, thus, dependent on the available heat transfer surfaces and efficiency of radiation and convection.
To achieve higher voltages for specific applications, the individual electrochemical cells are connected together in series to form a stack. To achieve higher currents, individual cells are connected in parallel. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. The electrical interconnect oftentimes also provides for passageways which allow oxidant fluid to flow past the cathode and fuel fluid to flow past the anode, while keeping these fluids separated. Also typically included in the stack are ducts or manifolding to conduct the fuel and oxidant into and out of the stack.
In a traditional fuel cell system design, hot exhaust gas from the fuel cell is fed to the heat exchanger, and preheated reactant gas is received from the heat exchanger through insulated piping. For high temperature fuel cells such as SOFCs and molten carbonate fuel cells, costly alloy materials are often used for such piping. The heat exchanger and piping also require considerable installation and maintenance expense, particularly in a multi-stack fuel cell. An effective heat exchange system in a multi-stack fuel cell requires some piping surfaces at each individual stack to maintain an optimal operating temperature for each and overall fuel stack efficiency. One of the problems encountered by a fuel cell design in this aspect can be illustrated by the following example:
When a circulating coolant is used to circulate through the passageways in the fuel assembly, a pump may be required to circulate the coolant. Furthermore, connecting tubes, an expansion tank, radiator, thermal and/or other controls may be necessary to properly complete the heat exchange system. In addition to the added expense and the complexity of integrating the circulating coolant system with the multi-stack fuel assembly, other issues are the need of electricity to operate the pump, the pump is subject to mechanical failure, coolant may become contaminated or ionized resulting in electric short circuits or shunts in the fuel assembly, and the coolant may leak into the fuel cell reaction areas and/or freeze-up. This is a major challenge to a fuel cell thermal designer from the standpoint of weight, cost, structure integrity, maintenance and reliability.
Many have attempted to provide a simpler solution to the heat exchange requirement in a fuel cell assembly. Of particular interest are the following references:
U.S. Pat. No. 3,595,699 focuses on an active cooling method to maintain fuel cell temperature by electrically monitoring the current when it is incr

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