FIELD OF THE INVENTION

The present invention relates to a method of operating an energy storage system or ESS with optimal efficiency; and more particularly to, the method of operating the ESS with at least two unit-BESSs, with optimal efficiency, that distributes power.

BACKGROUND OF THE INVENTION

An energy storage system or ESS refers to a system for using energy effectively by storing remaining energy in a power grid and using the energy at a required time. In general, it may contribute to optimization of power quality through load leveling and frequency control by storing idle power at a time of encountering less load like nighttime and using the power at a time of encountering much load like daytime.

The ESS is expected to perform a core function of converting now actively developed new and renewable energy sources into high quality of power and then linking them to a power network. In particular, it is a system required to link a wind power generation system and a solar power generation system with high output volatility to the power grid.

A principle of storing energy is to receive electric energy from a power grid, store it as ionized, kinetic, physically compressional, and chemical energy, convert it to electric energy at a required time, and supply it to a power grid. Medium and large ESSs may be classified into various types. A lithium-ion battery is a typical secondary battery with a separators and electrodes between anode and cathode in which lithium ions store and discharge energy while moving. Yet, it is less economical but thanks to good output characteristics and great efficiency, it has been recently widely used. Particularly, a method of forming a whole ESS with one battery pack and a plurality of unit-BESSs composed of lithium ion batteries for one destination site makes up for its disadvantages of being less economical and having a shorter life of the lithium-ion battery.

To distribute charged/discharged power of the whole ESS comprising multiple unit-BESSs, a different solution from such ESS with one battery pack, however, is needed.

DETAILED EXPLANATION OF THE INVENTION

Technical Problem

An object of the present invention is to provide a method of operating an ESS with optimal efficiency that optimizes output distribution by considering efficiency of a PCS and that of a battery under different situations. More specifically, it is to propose an algorithm for optimizing output distribution by taking into account efficiency of a PCS and that of a battery under each of different situations because the efficiency of the PCS changes by up to 12% depending on its output and that of the battery changes by up to 25% depending on its SOC.

In particular, another object of the present invention is to provide a method of operating an ESS composed of a plurality of unit-BESSs with optimal efficiency to optimize output distribution of each unit BESS for one destination site.

The other object of the present invention is to provide a method of making efficiency of SOC and output based on efficiency data of a PCS and a battery, linearizing them by each section, finding an energy optimization point depending on input/output command values and the number of operating units for operation.

Means of Solving the Problem

A method of operating an ESS with optimal efficiency in accordance with one example embodiment of the present invention may comprise steps of: collecting charge/discharge efficiency data of a PCS; collecting charge/discharge efficiency data of a battery depending on current state of charge of the battery; creating charge/discharge efficiency data of a unit BESS including the PCS and the battery by using the collected data; determining optimal charge/discharge levels of at least two unit-BESSs included in the ESS by using charge/discharge efficiency data of the at least two unit-BESSs to satisfy commanded input/output power values of the whole ESS at a current point of time; and charging or discharging the at least two unit-BESSs depending on the determined optimal charge/discharge power values.

Herein, the step of creating charge/discharge efficiency data of a unit BESS may include steps of: dividing a current-SOC section of batteries included in the unit-BESSs into multiple current-SOC subsections; and drawing a relationship between target-SOC of the battery and charge/discharge efficiency of the unit-BESSs by combining a relationship between the target-SOC of the battery and charge/discharge efficiency of the battery with that between the target-SOC of the battery and charge/discharge efficiency of the PCS in charge of the battery with respect to the individual current-SOC subsections.

Herein, the step of creating charge/discharge efficiency data of a unit BESS may further include steps of: dividing a charge/discharge efficiency level section in the relationship between the target-SOC of the battery and the charge/discharge efficiency of the unit-BESSs into multiple subsections; and linearizing the relationship between the target-SOC of the battery and the charge/discharge efficiency of the unit-BESSs in the multiple subsections.

Herein, at the step of determining optimal charge/discharge levels of at least two unit-BESSs, optimal charge/discharge levels of the at least two unit-BESSs may be determined by using the linearized relationships between the target-SOC of the battery and the charge/discharge efficiency of the unit-BESSs in the multiple subsections with respect to the individual current-SOC subsections corresponding to the current-SOC of the battery at a specific point of time.

Herein, the step of determining optimal charge/discharge levels of at least two unit-BESSs may determine optimal charge/discharge levels in accordance with equations as shown below.

Herein, a step of receiving input/output power values of the whole ESS at a current point of time and obtaining information on current charge/discharge levels of batteries equipped in the at least two unit-BESSs at the current point of time may be further included.

Herein, after the step of charging or discharging the unit-BESSs, a step of receiving input/output power values of the whole ESS at a next time and obtaining information on current charge/discharge levels of batteries equipped in the at least two unit-BESSs at the next time may be performed and return to the step of determining optimal charge/discharge levels of at least two unit-BESSs.

A method of operating an ESS with optimal efficiency in accordance with another example embodiment of the present invention, as a method of creating charge/discharge efficiency data of each of multiple unit-BESSs in an ESS equipping the multiple unit EBSSs composed of PCSs and batteries, may comprise steps of: collecting charge/discharge efficiency data of the PCS; collecting charge/discharge efficiency data of a battery depending on current-SOC of the battery; dividing a current-SOC section of the collected charge/discharge efficiency data of the battery into multiple current-SOC subsections; drawing a relationship between target-SOC of the battery and charge/discharge efficiency of the unit-BESSs by combining a relationship between the target-SOC of the battery and charge/discharge efficiency of the battery with that between the target-SOC of the battery and charge/discharge efficiency of the PCS in charge of the battery with respect to the individual current-SOC subsections; dividing a charge/discharge efficiency level section in the relationship between the target-SOC of the battery and the charge/discharge efficiency of the unit-BESSs into multiple subsections; and linearizing the relationship between the target-SOC of the battery and the charge/discharge efficiency of the unit-BESSs in the multiple subsections.

Herein, after the step of linearizing, it may further perform a step of forming a table by parameterizing the linearized straight line.

Effects of the Invention

In accordance with the embodiment of the present invention, a method of operating an ESS with optimal efficiency with the aforementioned configuration has an advantage of optimizing output distribution in consideration of efficiency of a PCS and a battery by situation.

In addition, the present invention has an advantage of saving internal output of a battery and improving usage of a whole ESS by using an optimal distribution algorithm and optimizing efficiency in consideration of efficiency.

Besides, the present invention has an advantage of optimizing output distribution for individual BESSs which are equipped in the ESS at a destination site.

Furthermore, the present invention has a benefit of being capable of flexibly handling against any removal of a PCS due to its breakdown upon adding a new PCS when a PMS performs the algorithm.

Moreover, the present invention also gives an operator of an ESS effective operation and profit approximately 10% more than the existing ones.

