Hybrid system with multiple energy storage devices

1. A drive system comprising:

a load;

a direct current (DC) link electrically coupled to the load;

an energy storage system comprising a first energy storage device and a second energy storage device, wherein the first energy storage device and the second energy storage device are arranged in series;

a bi-directional DC-DC converter electrically coupled to the DC link and to the energy storage system, the bi-directional DC-DC converter being connected to a node located between the first energy storage device and the second energy storage device and to a node connecting the second energy storage device to the DC link;

a switch positioned between the second energy storage device and the DC link, the switch being operable in a closed position and an open position to selectively connect and disconnect the second energy storage device to and from the DC link, respectively; and

a controller configured to control operation of the bi-directional DC-DC converter and the switch based on power requirements of the load and based on voltage levels of the first and second energy storage devices.

2. The drive system of claim 1 wherein the second energy storage device comprises an ultracapacitor; and

wherein the controller is configured to control a charging and discharging of the ultracapacitor to maintain the ultracapacitor at a nominal voltage level, the nominal voltage level comprising a voltage level between a demand charge threshold of the ultracapacitor comprising a minimum amount of stored energy for providing transient power from the ultracapacitor and a demand discharge threshold of the ultracapacitor comprising a highest stored energy of the ultracapacitor that allows for energy absorbing.

3. The drive system of claim 2 wherein the controller is configured to:

cause the ultracapacitor to discharge when a voltage thereof is higher than the demand discharge threshold, so as to bring the voltage of the ultracapacitor down to the nominal voltage level; and

cause the ultracapacitor to be charged when a voltage thereof is lower than the demand charge threshold, so as to bring the voltage of the ultracapacitor up to the nominal voltage level.

4. The drive system of claim 3 wherein the controller controls operation of the bi-directional DC-DC converter and actuation of the switch between the closed position and the open position to maintain the ultracapacitor at the nominal voltage level.

5. The drive system of claim 2 wherein the controller is configured to:

determine a load voltage requirement of the load;

compare the load voltage requirement to a nominal DC link voltage value, the nominal DC link voltage value equivalent to a sum of the ultracapacitor nominal voltage level and a nominal voltage value of the first energy storage device;

cause the switch to operate in the open position if the load voltage requirement is less than the nominal DC link voltage value; and

cause the switch to operate in the closed position if the load voltage requirement is greater than the nominal DC link voltage value.

6. The drive system of claim 5 wherein, when the voltage of the ultracapacitor is higher than the demand discharge threshold, the controller causes the ultracapacitor to discharge to provide at least one of a load power and a first energy storage device recharging power, with the providing of the at least one of the load power and the first energy storage device recharging power being based on the comparison of the load voltage requirement to a nominal DC link voltage value.

7. The drive system of claim 1 wherein the switch comprises one of a contactor-type mechanical switch and an electrical switch, with the switch controlling current flow in one direction or in both directions therethrough.

8. The drive system of claim 1 wherein the first energy storage device comprises a battery.

9. An energy system for providing power to a load, the energy system comprising:

an energy storage system electrically coupleable to the load through a DC link, the energy storage system comprising:

a first energy storage device; and

a second energy storage device arranged in series with the first energy storage device, the second energy storage device being coupled to the DC link;

a bi-directional DC-DC converter electrically coupled to the DC link and to the energy storage system, with the bi-directional DC-DC converter connecting the first energy storage device to the DC link such that power to and from the first energy storage device is selectively routed through the bi-directional DC-DC converter and with the bi-directional DC-DC converter positioned such that power to and from the second energy storage device bypasses the DC-DC converter;

a switch positioned between the second energy storage device and the DC link, the switch being operable in a closed position and an open position to selectively connect and disconnect the second energy storage device to and from the DC link, respectively; and

a controller configured to control operation of the bi-directional DC-DC converter and the switch based on power requirements of the load and based on voltage levels of the first and second energy storage devices.

10. The energy system of claim 9 wherein the bi-directional DC-DC converter is connected to a node located between the first energy storage device and the second energy storage device and to a node connecting the second energy storage device to the DC link.

11. The energy system of claim 9 wherein the first energy storage device is positioned on a low voltage side of the energy system and the second energy storage device is positioned on a high voltage side of the energy system, with the bi-directional DC-DC converter boosting voltage from the first energy storage device for providing to the DC link and with the second energy storage device being selectively coupled directly to the DC link via the switch.

12. The energy system of claim 9 wherein the switch comprises one of a contactor-type mechanical switch and an electrical switch, with the switch controlling current flow in one direction or in both directions therethrough.

13. A drive system comprising:

a load;

a direct current (DC) link electrically coupled to the load;

an energy storage system comprising a first energy storage device and a second energy storage device;

a bi-directional DC-DC converter positioned such that the first energy storage device is connected to the DC link through the bi-directional DC-DC converter and the second energy storage device is coupled to the DC link;

a switch positioned between the second energy storage device and the DC link, the switch being operable in a closed position and an open position to selectively connect and disconnect the second energy storage device to and from the DC link, respectively; and

a controller configured to control operation of the DC-DC converter and the switch based on power requirements of the load and based on voltage levels of the first energy storage device and the second energy storage device.

14. The drive system of claim 13 further comprising a DC link capacitor coupled to the DC link, and wherein the first and second energy storage devices of the second energy storage system comprise a battery and an ultracapacitor, respectively.

15. The drive system of claim 14 wherein the controller is configured to maintain the ultracapacitor at a nominal voltage level that is between a demand charge threshold of the ultracapacitor comprising a minimum amount of stored energy for providing transient power from the ultracapacitor and a demand discharge threshold of the ultracapacitor comprising a highest stored energy of the ultracapacitor that allows for energy absorbing, wherein maintaining the ultracapacitor at the nominal voltage level comprises:

causing the ultracapacitor to discharge when a voltage thereof is higher than the demand discharge threshold, so as to bring the voltage of the ultracapacitor down to the nominal voltage level; and

causing the ultracapacitor to be charged when a voltage thereof is lower than the demand charge threshold, so as to so as to bring the voltage of the ultracapacitor up to the nominal voltage level.

16. The drive system of claim 15 wherein the controller is configured to:

determine a load voltage requirement of the load;

compare the load voltage requirement to a nominal DC link voltage value, the nominal DC link voltage value equivalent to a sum of the ultracapacitor nominal voltage level and a nominal voltage value of the battery;

cause the switch to operate in the open position if the load voltage requirement is less than the nominal DC link voltage value; and

cause the switch to operate in the closed position if the load voltage requirement is greater than the nominal DC link voltage value.

17. The drive system of claim 15 wherein, when the voltage of the ultracapacitor is higher than the demand discharge threshold, the controller causes the ultracapacitor to discharge to provide at least one of a load power and a battery recharging power, with the providing of the at least one of the load power and the battery recharging power being based on the comparison of the load voltage requirement to a nominal DC link voltage value.