BACKGROUND

The present disclosure relates generally to frequency response systems configured to add or remove electricity from an energy grid, and more particularly to a frequency response controller that determines optimal power setpoints for a battery power inverter in a frequency response system.

Increased concerns about environmental issues such as global warming have prompted an increased interest in alternate clean and renewable sources of energy. Such sources include solar and wind power. One method for harvesting solar energy is using a photovoltaic (PV) field which provides power to an energy grid supplying regional power.

Availability of solar power depends on the time of day (sunrise and sunsets) and weather variables such as cloud cover. The power output of a PV field can be intermittent and may vary abruptly throughout the course of a day. For example, a down-ramp (i.e., a negative change) in PV output power may occur when a cloud passes over a PV field. An up-ramp (i.e., a positive change) in PV output power may occur at sunrise and at any time during the day when a cloudy sky above the PV field clears up. This intermittency in PV power output presents a problem to the stability of the energy grid. In order to address the intermittency of PV output power, ramp rate control is often used to maintain the stability of the grid.

Ramp rate control is the process of offsetting PV ramp rates that fall outside of compliance limits determined by the electric power authority overseeing the grid. Ramp rate control typically requires the use of an energy source that allows for offsetting ramp rates by either supplying additional power to the grid or consuming more power from the grid. Stationary battery technology can been used for such applications. Stationary battery technology can also be used for frequency regulation, which is the process of maintaining the grid frequency at a desired value (e.g. 60 Hz in the United States) by adding or removing energy from the grid as needed. However, it is difficult and challenging to implement both ramp rate control and frequency regulation simultaneously.

Ramp rate control and frequency regulation both impact the rate at which energy is provided to or removed from the energy grid. However, ramp rate control and frequency regulation often have conflicting objectives (i.e., controlling PV ramp rates vs. regulating grid frequency) which the same battery from being used for ramp rate control and frequency regulation simultaneously. Additionally, conventional ramp rate control and frequency regulation techniques can result in premature degradation of battery assets and often fail to maintain the state-of-charge of the battery within an acceptable range. It would be desirable to provide solutions to these and other disadvantages of conventional ramp rate control and frequency regulation techniques.

SUMMARY

One implementation of the present disclosure is an electrical energy storage system. The system includes a battery configured to store and discharge electric power to an energy grid, a power inverter configured to use battery power setpoints to control an amount of the electric power stored or discharged from the battery, the battery power setpoints comprising at least one of frequency regulation power setpoints and ramp rate control power setpoints, and a controller. The controller is configured to use a battery life model to generate the battery power setpoints for the power inverter. The battery life model includes one or more variables that depend on the battery power setpoints.

In some embodiments, the controller is configured to generate the frequency regulation power setpoints based on a frequency of the energy grid, generate the ramp rate control power setpoints based on a power output of a photovoltaic field, and combine the frequency regulation power setpoints and the ramp rate control power setpoints to generate the battery power setpoints.

In some embodiments, the one or more variables that depend on the battery power setpoints include at least one of a temperature of the battery, a state-of-charge of the battery, a depth of discharge of the battery, a power ratio of the battery, and an effort ratio of the battery.

In some embodiments, the controller is configured to determine a relationship between each of the variables in of the battery life model and one or more variables manipulated by the controller.

In some embodiments, the controller is configured to use the battery life model to estimate an amount of battery degradation that will result from the battery power setpoints.

In some embodiments, the controller is configured to use the estimated amount of battery degradation to estimate a cost of the battery degradation that will result from the battery power setpoints.

In some embodiments, the controller is configured to estimate an amount of frequency response revenue that will result from the battery power setpoints and generate the battery power setpoints as a function of both the estimated cost of battery degradation and the estimated amount of frequency response revenue that will result from the battery power setpoints.

In some embodiments, the battery life model is a parametric model. The parametric model may include a regression coefficient for each of the one or more variables in the battery life model. The controller may be configured to perform a curve fitting process to determine values for the regression coefficients.

Another implementation of the present disclosure is a method for operating an electrical energy storage system. The method includes storing electric power in a battery and discharging the stored electric power to an energy grid. The method may further include using battery power setpoints to control an amount of the electric power stored or discharged from the battery. The battery power setpoints may include at least one of frequency regulation power setpoints and ramp rate control power setpoints. The method may further include using a battery life model to generate the battery power setpoints. The battery life model may include one or more variables that depend on the battery power setpoints.

In some embodiments, the method my further include generating the frequency regulation power setpoints based on a frequency of the energy grid, generating the ramp rate control power setpoints based on a power output of a photovoltaic field, and combining the frequency regulation power setpoints and the ramp rate control power setpoints to generate the battery power setpoints.

In some embodiments, the one or more variables that depend on the battery power setpoints include at least one of a temperature of the battery, a state-of-charge of the battery, a depth of discharge of the battery, a power ratio of the battery, and an effort ratio of the battery.

In some embodiments, the method may further include determining a relationship between each of the variables in of the battery life model and one or more variables manipulated by a controller.

In some embodiments, the method further includes using the battery life model to estimate an amount of battery degradation that will result from the battery power setpoints.

In some embodiments, the method further includes using the estimated amount of battery degradation to estimate a cost of the battery degradation that will result from the battery power setpoints.

In some embodiments, the method further includes estimating an amount of frequency response revenue that will result from the battery power setpoints and generating the battery power setpoints as a function of both the estimated cost of battery degradation and the estimated amount of frequency response revenue that will result from the battery power setpoints.

In some embodiments, the battery life model is a parametric model. The parametric model includes a regression coefficient for each of the one or more variables in the battery life model. In some embodiments, the method further includes performing a curve fitting process to determine values for the regression coefficients.

Another implementation of the present disclosure is a controller for an electrical energy storage system. The controller includes a processing circuit. The processing circuit is configured to use a battery life model to generate battery power setpoints. The battery life model comprising one or more variables that depend on the battery power setpoints. The processing circuit is further configured to use the battery power setpoints to control an amount of electric power stored or discharged from a battery. The battery power setpoints comprising at least one of frequency regulation power setpoints and ramp rate control power setpoints.

In some embodiments, the processing circuit is further configured to generate the frequency regulation power setpoints based on a frequency of the energy grid, generate the ramp rate control power setpoints based on a power output of a photovoltaic field, and combine the frequency regulation power setpoints and the ramp rate control power setpoints to generate the battery power setpoints.

In some embodiments, the one or more variables that depend on the battery power setpoints include at least one of a temperature of the battery, a state-of-charge of the battery, a depth of discharge of the battery, a power ratio of the battery, and an effort ratio of the battery.

In some embodiments, the processing circuit is further configured to use the battery life model to estimate an amount of battery degradation that will result from the battery power setpoints. In some embodiments, the processing circuit is further configured to use the estimated amount of battery degradation to estimate a cost of the battery degradation that will result from the battery power setpoints.

Those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the detailed description set forth herein and taken in conjunction with the accompanying drawings.

