Energy storage system for photovoltaic energy and method of storing photovoltaic energy

Abstract

An energy system for renewable energy applications includes a renewable energy source, a bidirectional inverter connected an AC bus and a DC bus, an energy storage unit connected to the bidirectional DC/DC converter, and a control system comprising one or more controllers coupled to the bidirectional inverter and the bidirectional DC/DC converter. The bidirectional inverter is connected to the renewable energy source and a bidirectional DC/DC converter through the DC bus. The system is configured to capture low power of a photovoltaic (PV) array, energy typically lost to inverter clipping, and through the utilization of ramp rate control.

1 . A power system for capturing low voltage energy from a power source, the power system comprising:
 the power source coupled to a DC bus;   a DC/DC power converter coupled to the DC bus and an energy storage device;   a power inverter coupled to the DC bus and an AC bus; and   a control system, the control system comprising one or more controllers configured to:
 monitor a voltage produced by the power source; 
 determine whether the power source is producing a voltage greater than a first predetermined threshold; 
 determine whether the power source is producing a voltage less than a second threshold when it is determined that the power source is producing a voltage greater than the first predetermined threshold; 
 when it is determined that the power source is producing a voltage between the first threshold and the second threshold:
 control the DC/DC power converter to operate in an MPPT mode and store energy generated by the power source in the energy storage device; and 
 control the power inverter not to operate in an MPPT mode; and 
 
 when it is determined that the power source is producing a voltage greater than or equal to the second threshold:
 control the power inverter to operate in an MPPT mode and supply the energy generated by the power source to a power grid through the AC bus; and 
 control the DC/DC power converter not to operate in an MPPT mode. 
 
   
     
     
         2 . The power system of  claim 1 , wherein the first predetermined threshold is equal to expected losses in the DC/DC power converter. 
     
     
         3 . The power system of  claim 1 , wherein the power inverter has a wake up voltage being a voltage magnitude that a voltage at the DC bus must reach for the power inverter to be operational, and the second predetermined threshold is equal to the wake up voltage of the power inverter. 
     
     
         4 . The power system of  claim 1 , wherein the voltage produced by the power source is monitored continuously, and the control system continuously controls the DC/DC power converter and the power inverter to transition between operating in an MPPT mode and not operating in an MPPT mode. 
     
     
         5 . The power system of  claim 1 , wherein in determining whether the power source is producing a voltage less than the second predetermined threshold, the control system is further configured to monitor the voltage at the DC bus. 
     
     
         6 . The power system of  claim 5 , further comprising a sensor sensing the voltage at the DC bus and transmitting the sensed voltage to the power system. 
     
     
         7 . A power system for capturing clipped energy from a power source, the power system comprising:
 the power source coupled to a DC bus;   a DC/DC power converter coupled to the DC bus and an energy storage device;   a power inverter coupled to the DC bus and an AC bus; and   a control system, the control system comprising one or more controllers configured to:
 monitor an output power of the power inverter; 
 compare the output power of the power inverter to a predetermined threshold; 
 when the output power of the power inverter is greater than the predetermined threshold, control the DC/DC power converter to store output power of the power source that exceeds the predetermined threshold in the energy storage. 
   
     
     
         8 . The power system of  claim 7 , wherein the predetermined threshold is a maximum power rating of the power inverter. 
     
     
         9 . The power system of  claim 7 , wherein the output power of the power inverter is monitored continuously, and the control system continuously controls the DC/DC power converter and the power inverter to transition between storing and not storing output power of the power source in the energy storage. 
     
     
         10 . A power system for selectively dispatching energy from a power source, the power system comprising:
 the power source coupled to a DC bus;   a DC/DC power converter coupled to the DC bus and an energy storage device;   a power inverter coupled to the DC bus and an AC bus; and   a control system, the control system comprising one or more controllers configured to:
 monitor parameters external to the power system; and 
 selectively control the DC/DC power converter to store power generated by the power source in the energy storage in accordance with the monitored parameters. 
   
     
     
         11 . The power system of  claim 10 , wherein the parameters external to the power system comprise:
 a PV energy pricing signal for energy supplied to a power grid through the AC bus; and   a curtailment signal for ceasing or reducing an amount of energy supplied to the power grid.   
     
     
         12 . The power system of  claim 11 , wherein the DC/DC power converter stores power generated by the power source in the energy storage when a price in the PV energy pricing signal is below a predetermined threshold. 
     
     
         13 . The power system of  claim 12 , wherein the DC/DC power converter supplies energy stored in the energy storage to the power grid through the power inverter when a price in the PV energy pricing signal is equal to or greater than the predetermined threshold. 
     
     
         14 . The power system of  claim 10 , wherein the parameters external to the power system are monitored continuously, and the control system continuously controls the DC/DC power converter and the power inverter to transition between storing and not storing output power of the power source in the energy storage. 
     
     
         15 . A power system for controlling a ramp rate, the power system comprising:
 a power source coupled to a DC bus;   a DC/DC power converter coupled to the DC bus and an energy storage device;   a power inverter coupled to the DC bus and an AC bus; and   a control system, the control system comprising one or more controllers configured to:
 monitor an output power of the power inverter and a rate of change of the output power of the power inverter; 
 compare the rate of change of the output power of the power inverter with a pre-defined ramp rate; and 
 control the DC/DC converter to charge or discharge the energy storage when the rate of change of the output power of the power inverter differs from the pre-defined ramp rate by more than a predetermined amount. 
   
     
     
         16 . The power system of  claim 15 , wherein the output power of the power inverter and a rate of change of the output power of the power inverter is monitored continuously, and the control system continuously controls the DC/DC power converter to charge or discharge the energy storage until the rate of change of the output power of the power inverter no longer differs from the pre-defined ramp rate by more than the predetermined amount. 
     
     
         17 . The power system of  claim 15 , wherein the DC/DC power converter supplies power to the energy storage when the rate of change of the output power of the power inverter is greater than the pre-defined ramp rate by more than the predetermined amount. 
     
     
         18 . The power system of  claim 15 , wherein the DC/DC power converter discharges power from the energy storage to a power grid through the power inverter when the rate of change of the output power of the power inverter is less than the pre-defined ramp rate by more than the predetermined amount.