Parallel DC power sources with different characteristics

• US 6,583,523 B1
• Filed: 08/09/2000
• Issued: 06/24/2003
• Est. Priority Date: 08/09/2000
• Status: Expired due to Fees

• Current Assignee: Inverters Unlimited, Inc.
• Original Assignee: Inverters Unlimited, Inc.
• Inventors: Bhate, Suresh K.
• Primary Examiner(s): Sircus, Brian
• Assistant Examiner(s): Polk, Sharon A.
• Application Number:
  US09/635,378
• Time in Patent Office: 1,049 Days
• Field of Search: 307/48, 307/86, 320/101, 320/140
• US Class Current: 307/86
• CPC Class Codes: H02J 1/10 Parallel operation of dc sources
• CPC Class Codes: Y02P 80/20 using renewable energy

Abstract

A method and structure for electric power management that economically satisfies peak power demand and immediately satisfies a sudden substantial increase in power demand. An electric power management system comprises a first circuit element in parallel with a second circuit element, and a load in parallel with the first circuit element. The first circuit element includes a first direct current (DC) energy source in series with a first diode that prevents current backflow into the first energy source. The first energy source may comprise a conventional energy source such as fossil fuel, natural gas, hydroelectric power, etc. Alternatively, the first energy source may comprise an alternative energy source such as, inter alia, fuel cells, solar cells, wind power, and biomass power. The second circuit element includes a second DC energy source in series with a second diode that prevents current backflow into the second energy source, wherein said current backflow could be high enough to damage the second energy source. The second energy source, such as a battery, can respond instantaneously to a sudden demand in power. When the open circuit voltage (V₂₀) of the second energy source is less than the terminal voltage (V₁) of the first energy source, the first energy source alone supplies power to the load. When V₂₀≧V₁, the first energy source and the second energy source collectively supply power to the load. The preceding relationships between V₂₀ and V₁ hold during both steady state and transient operation of the electric power management system.

Claims

1. A power management system for supplying electric power to a load, comprising:

a first electrical circuit element that includes a first direct current (DC) energy source in series with a first rectifier that prevents current backflow into the first energy source; and

a second electrical circuit element that includes a second DC energy source in series with a second rectifier that prevents current backflow into the second energy source, wherein the second electrical circuit element is in parallel with the first electrical circuit element, wherein an open circuit voltage V₁₀ of the first energy source exceeds an open circuit voltage V₂₀ of the second energy source, wherein V₂₀ is set to a value that depends on a power variable selected from tho group consisting of a first reference power level P_LREF, a second reference power level P_1REF, and a combination thereof, wherein P_LREF is a function of a load power representation P_L(t), wherein t denotes time, and wherein P_1REF is a function of a power output of the first energy source.

2. The power management system of claim 1, wherein the power variable is P_LREF.

3. The power management system of claim 2, wherein P_LREF=P_LAVE+Z P_LSD, wherein P_LAVE is a time average of P_L(t) subject to a weighting function W(t), wherein P_LSD is a standard deviation of P_L(t) subject to the weighting function W(t), and wherein Z is a real number such that Z≧

0.

4. The power management system of claim 3, wherein Z is about equal to 0.

5. The power management system of claim 1, wherein the power variable is P_1REF.

6. The power management system of claim 5, wherein the function of the power output of the first energy source is a rated power P_1R, wherein P_1REF=HP_1R, and wherein H is between about 0.25 and about 1.

7. The power management system of claim 6, wherein H is about equal to 1.

8. The power management system of claim 1, wherein the first energy source includes a fuel coil.

9. The power management system of claim 1, wherein the second energy source includes a battery.

10. The power management system of claim 9, further comprising a battery charger coupled to the battery for charging the battery, wherein the battery charger is in parallel with the battery and in parallel with the first energy source, and wherein the battery charger is capable of receiving a charging current from the first energy source when the first energy source is otherwise outputting a power that is less than a rated power of the first energy source.

11. The power management system of claim 1, further comprising a DC to DC converter in series with the second energy source, wherein the DC to DC converter prevents an output current of the first energy source from exceeding a maximum current, and wherein the maximum current is a function of an output voltage V_OUT of the DC to DC converter.

12. The power management system of claim 11, further comprising a signal processor coupled to the DC to DC converter, wherein the signal processor dynamically controls V_OUT.

13. The power management system of claim 12, further comprising:

a comparator coupled to the signal processor; and

a current transducer coupled to the comparator, wherein the current transducer is in series with the first energy source, wherein the current transducer measures the output current of the first energy source and communicates the measured output current to the comparator, wherein the comparator generates a comparison of the measured output current with a setpoint current and communicates the comparison to the signal processor, wherein the signal processor acts upon the comparison in accordance with an algorithm and determines a new value for V_OUT, and wherein the signal processor sends a signal to the DC to DC converter which causes V_OUT to be set to the new value.

14. The power management system of claim 1, wherein the first energy source includes a fuel cell, and wherein the second energy source includes a battery.

15. A power management system for supplying electric power to a load, comprising:

a first electrical circuit element that includes a first direct current (DC) energy source in series with a first rectifier that prevents current backflow into the first energy source; and

a second electrical circuit element that includes a second DC energy source in series with a second rectifier that prevents current backflow into the second energy source, wherein the second electrical circuit element is in parallel with the first electrical circuit element, wherein an open circuit voltage V₁₀ of the first energy source exceeds an open circuit voltage V₂₀ of the second energy source, wherein the second energy source includes a battery, and wherein the second rectifier is a silicon controlled rectifier that is utilized to maintain an output voltage of the battery between a lower voltage and a higher voltage.

16. The power management system of claim 15, wherein V₂₀ is between 50 volts and 60 volts, and wherein the higher voltage is about equal to V₂₀, and wherein the lower voltage is about 4 to 10 volts below the higher voltage.

17. The power management system of claim 15, wherein the first energy source includes a fuel cell.

18. The power management system for supplying electric power to a load, comprising:

a first electrical circuit element that includes a first direct current (DC) energy source in series with a first rectifier that prevents current backflow into the first energy source;

a second electrical circuit element that includes a second DC energy source in series with a second rectifier that prevents current backflow into the second energy source, wherein the second electrical circuit element is in parallel with the first electrical circuit element; and

a DC to DC converter in series with the second energy source, wherein the DC to DC converter prevents an output current of the first energy source from exceeding a maximum current, wherein the maximum current is a function of an output voltage V_OUT of the DC to DC converter, wherein an open circuit voltage V₂₀ of the second energy source is set to a value that depends on a power variable selected from the group consisting of a first reference power level P_1REF, a second reference power level P_1REF, and a combination thereof, wherein P_LREF is a function of a load power representation P_L(t), and wherein t denotes time, wherein P_1REF is a function of a power output of the first energy source.

19. The power management system of claim 18, wherein the open circuit voltage V₂₀ of the second energy source exceeds an open circuit voltage V₁₀ of the first energy source.

20. The power management system of claim 18, wherein the first energy source includes a fuel cell.

21. The power management system of claim 18, wherein the second energy source includes a battery.

22. The power management system of claim 18, wherein the first energy source includes a fuel cell, and wherein the second energy source includes a battery.

23. A method for supplying electric power to a load, comprising:

forming a first electrical circuit element that includes a first direct current (DC) energy source in series with a first rectifier that prevents current backflow into the first energy source; and

forming a second electrical circuit element that includes a second DC energy source in series with a second rectifier that prevents current backflow into the second energy source, wherein the second electrical circuit element is in parallel with the first electrical circuit element, wherein an open circuit voltage V₁₀ of the first energy source exceeds an open circuit voltage V₂₀ of the second energy source, wherein V₂₀ is set to a value that depends on a power variable selected from the group consisting of a first reference power level P_LREF, a second reference power level P_1REF, and a combination thereof, wherein P_LREF is a function of a load power representation P_L(t), wherein t denotes time, and wherein P_1REF is a function of a power output of the first energy source.

24. The method of claim 23, wherein the power variable is P_LREF.

25. The method of claim 24, wherein P_LREF=P_LAVE+Z P_LSD, wherein P_LAVE is a time average of P_L(t) subject to a weighting function W(t), wherein P_LSD is a standard deviation of P_L(t) subject to the weighting function W(t), and wherein Z is a real number such that Z≧

0.

26. The method of claim 25, wherein Z is about equal to 0.

27. The method of claim 23, wherein the power variable is P_1REF.

28. The method of claim 27, wherein the function of the power output of the first energy source is a related power P_1R, wherein P_1REF=HP_1R, and wherein H is between about 0.25 and about 1.

29. The method of claim 28, wherein H is about equal to 1.

30. The method of claim 23, wherein the first energy source includes a fuel cell.

31. The method of claim 30, wherein the first energy source includes a battery.

32. The method of claim 23, wherein the second energy source includes a battery.

33. The method of claim 32, further comprising coupling a battery charger to the battery for charging the battery, wherein the battery charger is in parallel with the battery and in parallel with the first energy source, and wherein the battery charger is capable of receiving a charging current from the first energy source when the first energy source is otherwise outputting a power that is less than a rated power of the first energy source.

34. The method of claim 32, wherein the second rectifier is a silicon controlled rectifier that is utilized to maintain an output voltage of the battery between a lower voltage and a higher voltage.

35. The method of claim 34, wherein V₂₀ is between 50 volts and 60 volts, and wherein the higher voltage is about equal to V₂₀, and wherein the lower voltage is about 4 to 10 volts below the higher voltage.

36. The method of claim 23, further comprising placing a DC to DC converter in series with the second energy source, wherein the DC to DC converter prevents an output current of the first energy source from exceeding a maximum current, and wherein the maximum current is a function of an output voltage V_OUT of the DC to DC converter.

37. The method of claim 36, further comprising coupling a signal processor to the DC to DC converter, wherein the signal processor dynamically controls V_OUT.

38. The method of claim 37, further comprising:

coupling a comparator to the signal processor; and

coupling a current transducer to the comparator, wherein the current transducer is in series with the first energy source, wherein the current transducer measures the output current of the first energy source and communicates the measured output current to the comparator, wherein the comparator generates a comparison of the measured output current with a setpoint current and communicates the comparison to the signal processor, wherein the signal processor acts upon the comparison in accordance with an algorithm and determines a new value for V_OUT, and wherein the signal processor sends a signal to the DC to DC converter which causes V_OUT to be set to the new value.

39. The method of claim 23, wherein the first energy source includes a fuel cell, and wherein the second energy source includes a battery.

40. A method for supplying electric power to a load, comprising:

forming a first electrical circuit element that includes a first direct current (DC) energy source in series with a first rectifier that prevents current backflow into the first energy source;

forming a second electrical circuit element that includes a second DC energy source in series with a second rectifier that prevents current backflow into the second energy source, wherein the second electrical circuit element is in parallel with the first electrical circuit element; and

placing a DC to DC converter in series with the second energy source, wherein the DC to DC converter prevents an output current of the first energy source from exceeding a maximum current, wherein the maximum current is a function of an output voltage V_OUT of the DC to DC converter, wherein an open circuit voltage V₂₀ of the second energy source is set to a value that depends on a power variable selected from the group consisting of a first reference power level P_LREF, a second reference power level P_1REF, and a combination thereof, wherein P_LREF is a function of a load power representation P_L(t), and wherein t denotes time, wherein P_1REF is a function of a power output of the first energy source.

41. The method of claim 40, wherein the open circuit voltage V₂₀ of the second energy source exceeds an open circuit voltage V₁₀ of the first energy source.

42. The method of claim 40, wherein the first energy source includes a cell.

43. The method of claim 40, wherein the first energy source includes a battery.

44. The method of claim 40, wherein the first energy source includes a fuel cell, and wherein the second energy source includes a battery.