Power slope targeting for DC generators

• US 20040021445A1
• Filed: 07/31/2002
• Published: 02/05/2004
• Est. Priority Date: 07/31/2002
• Status: Active Grant

• Current Assignee: Sustainable Living Systems
• Original Assignee: Sustainable Living Systems
• Inventors: Harris, Brent Earle
• Granted Patent: US 7,087,332 B2
• Time in Patent Office: Days
• US Class Current: 320/136
• CPC Class Codes: G05F 1/67 to the maximum power available from a generator, e.g. from solar cell

Abstract

A simple feedback control loop, in conjunction with an improved maximum power point tracking intermediate controller, can be used ensure efficient operation of a power generator. The improved maximum power point tracking controller operates the generator at its maximum allowable power point. A power output of the generator is measured and compared to a power output setpoint. Operating characteristics of the generator are then adjusted to cause the maximum allowable power point and measured power output to approximate the power output setpoint. Although applicable to all types of generators, this is particularly beneficial in fuel cell generator systems and other systems where damage to generator components can occur if operated above a maximum allowable power output level. In other systems, the maximum allowable power output may approach or equal a maximum power point (or maximum possible power point).

Claims

1. A method of efficiently operating a fuel cell, said method comprising:

determining a maximum allowable power output of the fuel cell by determining a point on a power curve of the fuel cell at which additional power output may cause damage to the fuel cell;

measuring a power output from the fuel cell;

adjusting one or more electrical parameters of the fuel cell to cause an actual power output of the fuel cell to approximate the maximum allowable power output.

2. A method according to claim 1, wherein the adjusting one or more parameters of the fuel cell comprises controlling a voltage of the fuel cell.

3. A method according to claim 1, wherein determining the maximum allowable power output of the fuel cell comprises varying a voltage level of the fuel cell while measuring a power output thereof.

4. A method according to claim 1, wherein determining the maximum allowable power output and adjusting one or more parameters of the fuel cell to obtain the maximum allowable power output comprises repeatedly decreasing a voltage level of the fuel cell until the measured power output begins to decrease at a rate greater than the target slope then increasing the voltage level of the fuel cell until the measured power output begins to increase at a rate less than the target slope.

5. A method according to claim 1, wherein determining the maximum allowable power output comprises tracing a polarization curve of the fuel cell for a current set of operating conditions and identifying a point on the polarization curve having a target slope that corresponds to the maximum allowable power output.

6. A method according to claim 1, wherein determining the maximum allowable power output comprises analyzing DC voltage and current ripple of the fuel cell.

7. A method according to claim 1, wherein adjusting one or more parameters of the fuel cell to approximate the maximum allowable power output comprises controlling a rate of fuel input into the fuel cell to control the power output thereof.

8. A method according to claim 7, wherein controlling the rate of fuel input into the fuel cell comprises increasing the rate of fuel input to increase power output and decreasing the rate of fuel input to decrease power output.

9. A method of operating a fuel cell, said method comprising:

identifying a maximum allowable power point for the fuel cell; and

operating the fuel cell with a power output near the maximum allowable power point.

10. A method of operating a fuel cell according to claim 9, wherein identifying a maximum allowable power point comprises determining a target slope of a power curve corresponding to a point above which damage to the fuel cell may result from increased power output.

11. A method of operating a fuel cell according to claim 10, wherein operating the fuel cell with a power output near the maximum allowable power point comprises measuring a slope of the power curve at a present operating point;

comparing the measured slope to the target slope; and

adjusting one of the fuel cell operating parameters to cause the measured slope to approximate the target slope.

12. A method of operating a fuel cell according to claim 11, wherein adjusting one or more characteristics of the fuel cell comprises adjusting the voltage of the fuel cell.

13. A method of efficiently operating a fuel cell system, said method comprising:

determining the operating characteristics of the fuel cell system;

identifying a maximum allowable power point for the fuel cell system using the operating characteristics of the fuel cell system; and

operating the fuel cell system at or about its maximum allowable power point.

14. A method according to claim 13, further comprising measuring a power output from the fuel cell system.

15. A method according to claim 14, further comprising controlling a rate of fuel delivery to the fuel cell system to control a power output of the fuel cell system.

16. A method according to claim 14, wherein the maximum allowable power point comprises a power output level above which damage would result to the fuel cell system.

17. A method according to claim 14, wherein the maximum allowable power point corresponds to a target slope on a power curve for a given set of operating characteristics.

18. A fuel cell system control circuit, comprising:

a power measuring device for measuring a power output from a fuel cell system;

a comparison circuit for comparing the measured power output determined based on a maximum allowable power point with a power setpoint; and

a fuel flow controller configured to control a feed rate of reactants to the fuel cell based on a difference between the measured power output and the power setpoint.

19. A fuel cell system control circuit according to claim 18, wherein the maximum allowable power point corresponds to a power level above which damage may result to the fuel cell.

20. A fuel cell system control circuit according to claim 18, further comprising a power slope targeting controller configured to receive a power slope target corresponding to a maximum allowable power point.

21. A fuel cell system control circuit according to claim 18, wherein the comparison circuit comprises an output power controller configured to produce a control signal based on a power output error corresponding to a difference between the measured power output and the power output setpoint.

22. A fuel cell system control circuit according to claim 21, wherein the fuel flow controller operates in response to the control signal.

23. A method of controlling a generator, said method comprising:

evaluating generator characteristics to determine a target slope on a curve representing generator characteristics;

using the target slope to determine a maximum allowable power point for a set of operating conditions, wherein the maximum allowable power point represents a power output above which point the generator may become damaged or impaired; and

operating the generator to generate a power output approximating the maximum allowable power point.

24. A method according to claim 23, wherein operating the generator to generate a power output approximating the maximum allowable power point comprises measuring a power output from the generator, generating a control signal in response to a difference between the measured power output and the maximum allowable power point, and adjusting generator characteristics in response to the control signal to cause the measured power output to approach the maximum allowable power point.

25. A method according to claim 23, wherein the generator is a fuel cell system and wherein operating the generator to generate a power output approximating the maximum allowable power point comprises measuring a power output from the fuel cell system, comparing the power output from the fuel cell system with the maximum allowable power point, generating a control signal based on a difference between the measured power output and the maximum allowable power point, and operating a flow controller responsive to the control signal to increase or decrease fuel flow to cause the measured power output to approach the maximum allowable power point.

26. A method according to claim 25, wherein determining the maximum allowable power point for the fuel cell system comprises analyzing fuel cell system characteristics to determine a point along the power curve above which damage to the fuel cell system may result from a further increase in the voltage of the fuel cell system.

27. A method according to claim 23, wherein the curve is a power curve or a polarization curve.

28. A generator comprising:

a power generating device;

a power measuring device configured to measure a power output from the power generating device;

a power slope targeting controller configured to determine a maximum allowable power point based on a power slope target for the generator;

a comparator configured to compare the measured power output with power output setpoint and to generate a control signal based on a difference between the power output setpoint and the measured power output; and

a power controller configured to control the power output from the generator in response to the control signal from the comparator.

29. A generator according to claim 28, wherein the power generating device comprises a fuel cell.

30. A generator according to claim 29, wherein the power controller comprises a flow controller configured to control a flow rate of fuel into the fuel cell based on the control signal.

31. A generator according to claim 30, wherein the flow controller is configured to increase the flow rate of fuel into the fuel cell when the control signal indicates that the measured power output is below the power output setpoint.

32. A generator according to claim 30, wherein the flow controller is configured to decrease the flow rate of fuel into the fuel cell when the control signal indicates that the measured power output is above the power output setpoint.

33. A fuel cell generator system comprising:

a fuel cell;

a power output measuring device for measuring a power output from the fuel cell;

a power control system that operates the fuel cell at its maximum power point determined based on the fuel cell operating conditions

a comparator for comparing the measured power output with a power output setpoint; and

a flow controller for controlling a flow rate of fuel into the fuel cell based on a control signal from the comparator.

34. A fuel cell generator system according to claim 33, wherein the fuel cell generator system is connected to a grid.

35. A fuel cell generator system according to claim 33, wherein the maximum power point is a maximum allowable power point, and wherein the maximum allowable power point comprises a power level above which damage may result to the fuel cell.

36. A fuel cell generator system according to claim 33, wherein the flow controller is configured to increase the flow rate of fuel into the fuel cell when the measured power output is below the power output setpoint.

37. A fuel cell generator system according to claim 33, wherein the flow controller is configured to decrease the flow rate of fuel into the fuel cell when the measured power output is above the power output setpoint.

38. A fuel cell generator system according to claim 33, wherein the maximum power point corresponds to a maximum allowable power point for a given set of operating conditions, and wherein the maximum allowable power point is determined by identifying a target slope on a power curve for a standard set of operating conditions corresponding to a maximum allowable power point for the standard set of operating conditions and by determining a point on a power curve for the given set of operating conditions that corresponds to the target slope.