Fuel cell system with a multi-stream heat exchanger and its method of operation

A method of operating a fuel cell system, the fuel cell system comprising:

• a fuel cell comprising an anode side and a cathode side and a proton exchange membrane therein between for transport of hydrogen ions from the anode side to the cathode side through the membrane during operation;
• a cathode gas supply, for example air supply, for supplying oxygen gas to the cathode side, the cathode gas comprising oxygen gas,
• a cathode gas heating system, for example an air heating system, for increasing the temperature of the oxygen gas prior to supplying the cathode gas to the cathode side;
• a first cooling circuit for circulating first coolant through the fuel cell for adjusting the temperature of the fuel cell with the first coolant;
• a reformer comprising a catalyser enclosed by reformer walls and configured for catalytic conversion of fuel to syngas, wherein the reformer is conduit-connected to the anode side of the fuel cell for provision of syngas to the fuel cell;
• an evaporator configured for evaporating liquid fuel and conduit-connected to the reformer for provision of the evaporated fuel to the reformer;
• a liquid fuel supply conduit-connected to the evaporator for providing liquid fuel to the evaporator;
• a reformer burner for heating the catalyser inside the reformer by heat transfer through the reformer walls;
• characterised in that the fuel cell system comprises a first heat exchange unit or a second heat exchange unit or both,
• wherein the first heat exchange unit is a first multi-stream heat exchange module and provided as a first single unit with a first housing and comprising at least three flow paths for simultaneous exchange of thermal energy between the fluids in the three flow paths, the three flow paths comprising a first flow path, a second and a third flow path through the first heat exchange unit, wherein the first flow path is for the first coolant and part of the first cooling circuit, wherein the second flow path is for the cathode gas, for example air, and constitutes the cathode gas heating system, for example air heating system, and the third path is for flow of the fuel and constitutes the evaporator, wherein the first flow path is in thermal connection with the second and the third flow paths for simultaneous transfer of thermal energy from the first coolant inside the housing to the cathode gas for heating the cathode gas and from the first cool ant to the fuel for evaporating the fuel during its flow through the first heat exchange unit when the fuel cell system is in power producing operation;
• wherein the second heat exchange unit is a second multi-stream heat exchange unit and provided as a second single unit with a second housing and comprising at least three flow paths, the three flow paths comprising a fourth, a fifth and a sixth flow path through the second heat exchanger unit, wherein the fourth flow path is for a second coolant different from the first cool ant and part of a second cooling circuit, different and flow-separated from the first cooling circuit;
• wherein the fifth flow path is for the first coolant and part of the first cooling circuit, and the sixth flow path is for at least one of:
• the exhaust gas from the cathode side and the flue gas from the reformer burner;
• wherein the fourth flow path is in thermal connection with the fifth and the sixth flow paths for simultaneous transfer of thermal energy from the first coolant inside the housing to the second coolant for reducing the temperature of the first coolant, and from the exhaust gas to the second coolant for condensing the water of the exhaust gas in the sixth flow path prior to leaving the fuel cell system through an exhaust when the fuel cell system is in power producing operation;
• wherein the method comprises during power producing operation of the fuel cell system at least one of:
• providing flow of the first coolant through the first flow path, flow of the cathode gas through the second flow path, and flow of the fuel through the third flow path, and causing simultaneous transfer of thermal energy from the first coolant to the cathode gas for heating the cathode gas and to the fuel and evaporating the fuel during flow through the first heat exchange unit when the fuel cell system is in power producing operation;
• and
• providing flow of the second coolant through the fourth flow path, flow of the first coolant through the fifth flow path, flow through the sixth flow path of at least one of:
• the exhaust gas from the cathode side and the flue gas from the reformer burner;
• and causing simultaneous transfer of thermal energy from the first coolant to the second coolant for reducing the temperature of the first coolant, and from the exhaust gas to the second coolant for condensing the water of the exhaust gas when the fuel cell system is in power producing operation.