Fuel cell minimum fuel recycle with maximum fuel utilization

1. A method of operating a fuel cell power plant which comprises providing a flow of fuel recycle gas having a substantially constant quantity of recycle fuel therein at all levels of power output, said flow of fuel recycle gas having a sufficient quantity of recycle fuel therein at a minimum level of power output to avoid fuel starvation within said fuel cell power plant at said minimum level of power output.

2. A method of operating a fuel cell power plant, comprising:

providing a minimal flow of fresh fuel from a source to said fuel cell power plant at a minimum power output level; end

providing a flow of recycle fuel gas which is substantially constant at all levels of power output of said fuel cell stack, said flow including sufficient recycle fuel gas at said minimum power output level to prevent, together with said minimal flow of fresh fuel, fuel starvation within said power plant.

3. A fuel cell power plant, comprising:

a source of fuel providing a minimal flow of fresh fuel to said fuel cell power plant at a minimum power output level; and

means providing a flaw of fuel recycle gas which is substantially constant at all levels of power output of said fuel cell stack, said flow including sufficient recycle fuel at said minimum power output level to prevent, together with said minimal flow of fresh fuel, fuel starvation within said power plant.

4. A power plant according to claim 3 wherein:

said means providing a flow of fuel recycle gas comprises an ejector designed to choke at the pressure of fuel provided by said source with a flow rate of combined fresh fuel from said source and recycle fuel to satisfy the fuel requirement of said fuel cell power plant at said minimum power output level.

5. A power plant according to claim 3 wherein:

said means providing a flow of fuel recycle gas comprises an electrochemical hydrogen pump with a constant current generator providing current across said electrochemical hydrogen pump of a magnitude to provide said sufficient quantity of recycle fuel gas flow.

6. A fuel cell power plant operable at power output levels between a full rated power output level and a minimum power output level, comprising:

a plurality of fuel cells, each having a fuel inlet and a fuel outlet, said fuel cells arranged in one or more stacks selected from (a) one stack comprising the first and last stack in said fuel cell power plant or (b) a series of stacks arranged in serial fuel flow relationship such that every stack except a last stack in said series provides fuel to a stack next subsequent thereto in said series, and every stack except a first stack in said series receives fuel from a next preceding one of said stacks in said series, each stack having a fuel inlet and a fuel outlet;

a source of fuel for providing fresh fuel to the fuel inlet of at least one of said stacks; and

a passive fuel recycle gas pump providing recycle fuel gas flow from the fuel outlet of at least one of said stacks to the fuel inlet of one or more of said stacks in substantially constant quantity, at all of said power output levels, said substantially constant quantity being sufficient to prevent fuel starvation of said fuel cells when operating near maximum fuel utilization at minimum power.

7. A power plant according to claim 6 wherein:

said passive fuel recycle gas pump is an ejector with its primary inlet connected to said source, its secondary inlet connected to said fuel outlet of at least one of said stacks, and its outlet connected to said fuel inlet of at least one of said stacks, and is designed to choke at the pressure of fuel provided by said source with a flow rate of combined fresh fuel from said source and recycle fuel to satisfy the fuel requirement of said one or more stacks at said minimum power output level.

8. A power plant according to claim 7 further comprising:

at least one pressure regulator, the output of each pressure regulator feeding the fuel inlet of a corresponding stack, each pressure regulator providing substantially no fuel to any of said one or more of said stacks at said minimum power output level and providing all fuel required to satisfy the fuel requirement of said one or more stacks at power output levels above said minimum power output level in excess of the fuel required to satisfy the fuel requirement at said minimum power output level.

9. A power plant according to claim 6 wherein:

said passive fuel recycle gas pump is an electrochemical hydrogen pump, connected between said fuel outlet of at least one of said stacks and said fuel inlet of at least one of said stacks, with a constant current generator providing current across said electrochemical hydrogen pump of a magnitude to provide said sufficient quantity of recycle fuel gas flow.

10. A power plant according to claim 9 further comprising:

at least one pressure regulator, the output of each said pressure regulator feeding the fuel inlet of a corresponding stack, one of said pressure regulators connected to said first stack and providing to said first stack, at said minimum power output level, sufficient fuel to satisfy, when combined with said recycle fuel gas, the fuel requirement of said one or more stacks at said minimum power output level.

11. A power plant according to claim 10 wherein:

there is a series of fuel cells arranged in said serial fuel flow relationship; and

any of said stacks other than said first stack is provided no fuel by a corresponding one of said pressure regulators at said minimum power output level.

12. A power plant according to claim 10 wherein:

at power output levels other than said minimum power output level, all of the fresh fuel requirement of said one or more stacks in excess of that required at said minimum power level is provided by said pressure regulators.

13. A power plant according to claim 6 wherein:

there is only one stack.

14. A power plant according to claim 6 wherein:

there are two stacks.