Abstract:
An energy storage system has a reduced number of capacitors for storing energy such as renewable energy, thereby reducing cost and improving stability of the system. The energy storage system is configured to store power from a power generating unit, and includes: a storage capacitor having a first end electrically coupled to one end of the power generating unit; a secondary battery having a first terminal electrically coupled to a second end of the storage capacitor, and a second terminal electrically coupled to another end of the power generating unit; and a first converter configured to selectively couple the storage capacitor and the secondary battery to a load.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0067871, filed Jul. 14, 2010, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments of the present invention relate to an energy storage system. 
     2. Description of the Related Art 
     Recently, renewable energy is emerging due to the depletion of fossil fuels and environmental issues. The renewable energy uses natural energy such as sunlight, solar heat, wind power, tidal power or geothermal heat, and electricity generating systems mainly using sunlight are being widely and practically applied. 
     A renewable energy generating system is a system that supplies a power of renewable energy generator to a load or a grid. When the power produced by a renewable energy generator is less than the power consumed by a load, all the power available from the renewable energy generator is consumed by the load, and an insufficient power is supplied through a grid. When the power produced by the renewable energy generator is greater than the power consumed by the load, a surplus power that is not consumed by the load among the power produced by the renewable energy generator is supplied to a grid as a reverse flow power. 
     A power storage system is a system that converts power to a physical or chemical energy and stores the energy. The power storage system is connected to the grid, receives power (“night power”) from the grid during night, stores the received power, and uses the energy of the received power during a daytime. Further, an energy storage system supplies an emergency power during blackout, during which electricity is not supplied through the grid. 
     Such an energy storage system combines a renewable energy generating system and a power storage system, and stores the surplus power from the renewable energy generator and night power from the grid in the power storage system. In the energy storage system, power generated by the renewable energy generating system may be stored in the power storage system, or may be provided to the load and/or the grid. 
     SUMMARY 
     Aspects of embodiments according to the present invention are directed toward an energy storage system, which reduces the number of capacitors for storing renewable energy, thereby reducing the cost and securing electrical stability. 
     According to at least one embodiment, an energy storage system is configured to store power from a power generating unit. The energy storage system includes: a storage capacitor having a first end electrically coupled to one end of the power generating unit; a secondary battery having a first terminal electrically coupled to a second end of the storage capacitor, and a second terminal electrically coupled to another end of the power generating unit; and a first converter configured to selectively couple the storage capacitor and the secondary battery to the load. 
     The energy storage system may further include an inverter coupled to the first converter. 
     The energy storage system may further include a controller coupled to the first converter and the inverter, and configured to control an operation of the first converter. 
     The storage capacitor and the secondary battery may be coupled to an output terminal of the power generating unit. 
     The first converter may include first and second switches coupled in series across the storage capacitor and the secondary battery, and the inverter may be coupled to a contact point between the storage capacitor and the secondary battery and a contact point between the first and second switches. 
     The controller may be configured to apply the control signal to the first and second switches to form a path for supplying a power to the load through the storage capacitor or the secondary battery. 
     The controller may be configured to drive the first and second switches complimentarily. 
     The energy storage system may further include a maximum power point tracker coupled to the output terminal of the power generating unit, wherein the storage capacitor and the secondary battery are coupled in series across both ends of the maximum power point tracker. 
     The energy storage system may further include a transformer including a primary winding coupled to the contact point between the storage capacitor and the secondary battery and the contact point between the first and second switches, and a secondary winding coupled to the inverter. 
     The energy storage system may further include a second converter coupled between the secondary winding of the transformer and the inverter, and for transducing an output power of the transformer into an Alternating Current (AC) power to be applied to the inverter or for transducing an output power of the inverter into a Direct Current (DC) power to be applied to the transformer. 
     The energy storage system may further include a link capacitor coupled between the second converter and the inverter in parallel, and for storing the power from the second converter or the inverter. 
     The second converter may include four switches, and the four switches may include transistors or diodes. 
     The power generating unit may be configured to generate a power with one selected from the group consisting of sunlight, solar heat, wind power, tidal power and geothermal heat. 
     The converter may be a bi-directional converter. 
     The inverter may be a bi-directional inverter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this application. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the drawings: 
         FIG. 1  is a block diagram of an energy storage system according to an embodiment; 
         FIG. 2  is a circuit diagram of a controller of an energy storage system according to an embodiment which controls a duty ratio; 
         FIG. 3  is a diagram for describing a circuit configuration when operating a first switch of a first converter in an energy storage system according to an embodiment; 
         FIG. 4  is a diagram for describing a circuit configuration when operating a second switch of a first converter in an energy storage system according to an embodiment; 
         FIG. 5  is a diagram showing characteristics of a voltage and current when a power failure occurs in a grid coupled to an energy storage system according to an embodiment and a power of a solar cell remains; 
         FIG. 6  is a diagram showing characteristics of a voltage and current when a power failure occurs in a grid coupled to an energy storage system according to an embodiment and a power of a solar cell is insufficient; 
         FIG. 7  is a diagram showing characteristics of a voltage and current when a power failure occurs in a grid coupled an energy storage system according to an embodiment and a power of a solar cell is not generated; 
         FIG. 8  is a diagram showing characteristics of a voltage and current when a power is supplied from a battery in the connecting of a grid coupled to an energy storage system according to an embodiment; and 
         FIG. 9  is a diagram showing characteristics of a voltage and current when a power of a solar cell is supplied to a load in the connecting of a grid coupled to an energy storage system according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this application will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     Hereinafter, the configuration and operation of an energy storage system according to embodiments of the present invention will be described in detail. 
       FIG. 1  is a block diagram of an energy storage system according to an embodiment. 
     Referring to  FIG. 1 , an energy storage system  1000  according to an embodiment includes a renewable energy unit (e.g., a power generating unit)  120 , a storage capacitor  130 , a battery  140 , a first converter  150 , a transformer  160 , a second converter  170 , a link capacitor  180 , an inverter  190 , and a controller  200 . 
     The renewable energy unit  120  converts natural energy into electrical energy. That is, the renewable energy unit  120  generates a power with renewable energy such as sunlight, solar heat, wind power, tidal power or geothermal heat. In the present application, the renewable energy unit  120  will be described primarily in reference to a solar cell. However, the present invention is not limited thereto. 
     The renewable energy unit  120  may generate a power during the daytime, for example, when the renewable energy unit  120  includes a solar cell (or solar cells). The renewable energy unit  120  supplies the power, which is generated during the daytime, to the energy storage system  1000 . The energy storage system  1000  supplies the received power to the load  10 , or stores the received power in the battery  140  of the energy storage system  1000 , or provides the received power to a grid  110 , which may be connected to the energy storage system  1000 . 
     Moreover, the renewable energy unit  120  may be coupled to a maximum power point tracker  121  through a relay RL. The maximum power point tracker  121  includes an inductor, a switch, and a diode. The maximum power point tracker  121  detects a voltage and a current at a power point where a power generated by the renewable energy unit  120  is at the maximum. Also, the maximum power point tracker  121  maintains the states of the voltage and current and enables the transfer of the maximum power that may be generated by the renewable energy unit  120 . In the embodiment depicted in  FIG. 1 , the maximum power point tracker  121  includes an inductor, a transistor, and a diode, and operates as a booster converter having an output terminal connected to the storage capacitor  130 . However, other types of converters may be used in other embodiments. The controller  200  determines the output voltage and current of the maximum power point tracker  121  by adjusting the on/off timing of the transistor of the maximum power point tracker  121 . 
     In the above description, the renewable energy unit  120  is described as being connected to the storage capacitor  130  through the maximum power point tracker  121 . However, depending on the type of the energy generator used in the renewable energy unit  120 , the maximum power point tracker  121  may not be used, and the renewable energy unit  120  may be directly connected to the storage capacitor  130  through the relay RL. 
     The storage capacitor  130  and the battery  140  are serially coupled to the output terminal of the maximum power point tracker  121 . That is, the output of the maximum power point tracker  121  is distributed to the storage capacitor  130  and the battery  140 . If a voltage applied across the both ends of the storage capacitor  130  is higher than the withstand voltage of the storage capacitor  130 , the storage capacitor  130  may not operate properly, thus the distribution of the withstand voltage by connecting the capacitors in series may be used. According to an embodiment of the present invention, the storage capacitor  130  and the battery  140  are coupled to the maximum power point tracker  121  in series. The voltage transferred to the storage capacitor  130  is lower than the voltage transferred to the storage capacitor in a case where only the storage capacitor is coupled to the output terminal of the maximum power tracker  121 . Therefore, the number of capacitors that constitute the storage capacitor  130  may be reduced. Consequently, the capacitance of the storage capacitor  130  increases, and thus the number of elements which are coupled to the battery  130  in parallel may be reduced. As a result, the number of desired elements in the storage capacitor  130  may be reduced, when the storage capacitor  130  is serially coupled to the battery  140 . 
     The battery  140  may receive a power from at least one of the grid  110  and the renewable energy  120  and may be charged with the power. Moreover, when the load  10  requires an additional power, for example, when a power supply from the grid  110  is cut off or an amount of power consumption of the load  10  is higher than an amount of power that is supplied from the grid  110  and the renewable energy unit  120 , the battery  140  may be discharged and thereby supply power to the load  10 . 
     The first converter  150  is coupled to the storage capacitor  130  and the battery  140  as shown in  FIG. 1 . The first converter  150  controls the turn on/off of the storage capacitor  130  and the battery  140  to allow the storage capacitor  130  and the battery  140  to be charged/discharged. The first converter  150  may be configured with a bi-directional converter, and may allow the power of the storage capacitor  130  and the power of the battery  140  to be supplied to the load  10  or allow the surplus power of the grid  110  to be supplied to the battery  140 . The first converter  150  is illustrated as a bi-directional converter in  FIG. 1 . However, the present invention is not limited to the embodiment using the bi-directional converter, and substantially the same functions may be implemented, for example, by using a plurality of one-directional converters. 
     In  FIG. 1 , the first converter  150  includes first and second switches Q 1  and Q 2  that are coupled in series. Moreover, one end of the primary winding of the transformer  160  is coupled to a contact point between the first and second switches Q 1  and Q 2 , and the other end of the primary winding is coupled to a contact point between the storage capacitor  130  and the battery  140 . The first and second switches Q 1  and Q 2  operate complimentarily. That is, the second switch Q 2  is turned off when the first switch Q 1  is turned on, but the second switch Q 2  is turned on when the first switch Q 1  is turned off. The first switch Q 1  is turned on to form a path through which the storage capacitor  130  is coupled to the primary winding of the transformer  160 . On the other hand, the second switch Q 2  is turned on to form a path through which the battery  140  is coupled to the primary winding of the transformer  160 . The operations of the first and second switches Q 1  and Q 2  will be described below in more detail. 
     The one end of the primary winding of the transformer  160  is coupled to the contact point between the storage capacitor  130  and the battery  140 , and the other end of the primary winding of the transformer  160  is coupled to the contact point between the first and second switches Q 1  and Q 2 . Moreover, the secondary winding of the transformer  160  is coupled to the second converter  170 . Thus, the storage capacitor  130  and the battery  140  are disconnected from the inverter  190  by the transformer  160 , thereby securing electrical stability. 
     Moreover, the transformer  160  receives a voltage from the storage capacitor  130  or the battery  140  and boosts or steps down the voltage according to a winding ratio, or it receives a voltage from the grid  110  and boosts or steps down the voltage according to a winding ratio. 
     The second converter  170  is coupled to the secondary winding of the transformer  160 . The second converter  170  may be configured in a bridge transistor type including four switches. Also, the control electrode of each of the switches is coupled to the controller  200 , and each of the switches may be turned on/off according to the signal of the controller  200 . The second converter  170  transduces an AC voltage, which is outputted from the secondary winding of the transformer  160 , into a DC voltage. The DC voltage outputted from the second converter  170  may be applied to the inverter  190  through the link capacitor  180 . Moreover, the second converter  170  transduces a DC voltage applied from the inverter  190  into an AC voltage and applies the AC voltage to the secondary winding of the transformer  160 . 
     Moreover, although the second converter  170  is shown and described in reference to a full bridge structure having four transistors, the second converter  170  may be configured with a full-bridge diode that is configured with four diodes and is commonly used. In this case, the second converter  170  operates as a rectifier, i.e., receives the AC voltage from the transformer  160  and rectifies the AC voltage into a DC voltage. 
     The link capacitor  180  is coupled between the second converter  170  and the inductor  190 . The link capacitor  180  is charged to link voltage due to the output voltage of the second converter  170  or the inverter  190 . Therefore, even if the output voltage of the second converter  170 /the inductor  190  fluctuates, the voltage of the inductor  190 /the second converter  170  can be maintained constantly (or substantially constantly). 
     One end of the inverter  190  is coupled to the link capacitor  180 . The inverter  190  may be configured with a bi-directional inverter. The inverter  190  receives the output voltage of the link capacitor  180  and converts the output voltage into an AC voltage suitable for the load  10 . Also, the inverter  190  receives the AC voltage of the grid  110 , converts the AC voltage into a DC voltage through a rectifying operation, and applies the DC voltage to the second converter  170 . Therefore, the AC voltage transduced by the second converter  170  may be transferred to and stored in the battery  140  through the transformer  160 . 
     Moreover, the other end of the inverter  190  is coupled to the grid  110 . The inverter  190  may be configured with four switches and performs voltage conversion according to the turn-on/off of each of the switches. Such a configuration is known to those skilled in the art, and thus its detailed description will be omitted. 
     The controller  200  is coupled to the maximum power point tracker  121 , the first converter  150 , the second converter  170  and the inverter  190 . The controller  200  is coupled to the control electrodes of switches that configure the maximum power point tracker  121 , the first converter  150 , the second converter  170  and the inverter  190 . Thus, the controller  200  controls the turn-on/off of the switches with control signals. 
     Particularly, the controller  200  may control the turn-on/off of the first and second switches Q 1  and Q 2  of the first converter  150  and allow a power to be applied through the first converter  150 . The controller  200  may turn on the first switch Q 1  and allow the power of the storage capacitor  130  to be applied to the load  10 . Also, the controller  200  may turn on the second switch Q 2  and allow the power of the battery  140  to be applied to the load  10 , or may allow the battery  140  to receive a power from the maximum power point tracker  121  or the grid  110  and to be charged with the received power. 
     The following description will be made on an operation where the controller of the energy storage system according to an embodiment controls the first converter  150 . 
       FIG. 2  is a circuit diagram of a controller of an energy storage system according to an embodiment which controls a duty ratio. 
     Referring to  FIG. 2 , the controller  200  includes three operational amplifiers AMP 1 , AMP 2  and AMP 3  that are coupled to the first converter  150 , a feedback circuit of the operational amplifier AMP 1 , and a feedback circuit of the operational amplifier AMP 2 . 
     First, a voltage V Link  that is applied across the ends of the link capacitor  180  is divided by first and second resistors R 1  and R 2  that are serially connected. A voltage that is applied across the second resistor R 2  is applied to the negative terminal (−) of the first operational amplifier AMP 1  as an input voltage, and a reference voltage Vref is applied to the positive terminal (+) of the first operational amplifier AMP 1 . A third resistor R 3 , a first capacitor C 1  and a second capacitor C 2  that form the feedback of the first operational amplifier AMP 1  amplifies a difference between voltages that are applied to the input terminals (+, −) of the first operational amplifier AMP 1 . Thus, the first operational amplifier AMP 1  operates and outputs a voltage corresponding to the voltage difference of the voltage V Link  of the link capacitor  180  with respect to the reference voltage Vref. Accordingly, the higher the voltage V Link  of the link capacitor  180 , the lower the output value. To the contrary, the lower the voltage V Link  of the link capacitor  180 , the higher the output value. 
     The second operational amplifier AMP 2  that is a next stage receives the output voltage of the first operational amplifier AMP 1  through a positive terminal (+). Also, the second operational amplifier AMP 2  receives a current Ip, which flows through the primary winding of the transformer  160 , through a negative terminal (−) and a fourth resistor R 4  that are coupled in series. If the second operational amplifier AMP 2  is an ideal amplifier, the voltage of the negative terminal (−) is the same as that of the positive terminal (+) in operating. Accordingly, the primary winding current Ip may be changed into a voltage signal proportional to it. A fifth resistor R 5 , a third capacitor C 3  and a fourth capacitor C 4  that form the feedback of the second operational amplifier AMP 2  compares the voltage signal with the output voltage of the first operational amplifier AMP 1  to operate and output a voltage difference. Accordingly, the higher the primary winding current Ip, a lower value is outputted. To the contrary, the lower the primary winding current Ip, a higher value is outputted. 
     The third operational amplifier AMP 3  that is a stage next to the second operational amplifier AMP 2  receives the output voltage of the second operational amplifier AMP 2  through a negative terminal (−). Also, the third operational amplifier AMP 3  receives a sawtooth wave having a certain frequency (for example, 50 KHz) through a positive terminal (+). The third operational amplifier AMP 3  compares the sawtooth wave with the output voltage of the second operational amplifier AMP 2  to operate according to the voltage difference. Here, the third operational amplifier AMP 3  does not have a feedback connection, and thus it operates in a saturation region. Therefore, when the output voltage of the second operational amplifier AMP 2  is greater than the sawtooth wave, a positive saturation voltage value is outputted, but when the output voltage of the second operational amplifier AMP 2  is less than the sawtooth wave, a negative saturation voltage value is outputted. 
     The controller  200  uses the output voltage of the third operational amplifier AMP 3  as the control voltage of the second switch Q 2 . The controller  200  inverts the output voltage of the third operational amplifier AMP 3  and uses the inverted voltage as the control voltage of the first switch Q 1 . As a result, the controller  200  determines a duty ratio between the first and second switches Q 1  and Q 2 . 
     Accordingly, the controller  200  may determine the duty ratio between the first and second switches Q 1  and Q 2  that configure the first converter  150  (e.g., bi-directional converter  150 ) by using the voltage V Link  of the link capacitor  180  and the primary winding current Ip. 
     During daytime when the grid  110  is coupled to the energy storage system, all the energy of the renewable energy unit  120  is transferred to the load  10  through the link capacitor  180 . Accordingly, the secondary winding current I L  of the transformer  160  has an average value of 0 A. In this case, the controller  200  determines the output voltage of the second operational amplifier AMP 2  that allows the secondary winding current I L  to become 0 A, and the third operational amplifier AMP 3  compares the determined voltage with the sawtooth wave to determine the duty ratio between the first and second switches Q 1  and Q 2 . 
     When the grid  110  is disconnected, the voltage of the link capacitor  180  should be maintained at a predetermined voltage (for example, 400 V). Thus, the controller  200  compares the voltage V Link  of the link capacitor  180  with the reference voltage Vref to output a voltage, and determines the output voltage of the second operational amplifier AMP 2  that allows the secondary winding current I L  to flow in order for the same voltage as the output voltage to be generated. Moreover, the controller  200  compares the output voltage of the second operational amplifier AMP 2  with the sawtooth wave to determine the duty ratio between the first and second switches Q 1  and Q 2  through the third operational amplifier AMP 3 . 
     The following description will be made on power flow based on the operation of the first converter  150  of the energy storage system according to an embodiment. 
       FIG. 3  is a diagram for describing a circuit configuration when operating the first switch Q 1  of the first converter  150  in the energy storage system according to an embodiment.  FIG. 4  is a diagram for describing a circuit configuration when operating the second switch Q 2  of the first converter  150  in the energy storage system according to an embodiment. 
     Referring to  FIG. 3 , when the first switch Q 1  of the first converter  150  is turned on, a current path that passes through the primary winding of the transformer  160  from the storage capacitor  130  is formed along a path that is indicated by an arrow. Accordingly, the discharge path of the renewable energy unit  120  is formed. 
     Referring to  FIG. 4 , when the second switch Q 2  of the first converter  150  is turned on, a current path that passes through the primary winding of the transformer  160  from the battery  140  is formed along a path that is indicated by an arrow. Accordingly, the discharge path of the battery  140  is formed. Also, the path may operate as the charge path of the battery  140  according to the direction of a current. 
     Moreover, when the first and second switches Q 1  and Q 2  are turned on, the direction of a current that passes through the first switch Q 1  and the primary winding is opposite to the direction of a current that passes through the second switch Q 2  and the primary winding, and thus a current and a voltage that are applied to the primary winding of the transformer  160  have an AC type. Therefore, the transformer  160  may boost a voltage that is applied from the primary winding. 
     Hereinafter, a description on the power flow of the energy storage system according to an embodiment will be divided depending on cases. 
       FIG. 5  shows characteristics of a voltage and current when a power failure occurs in a grid of an energy storage system according to an embodiment and a power output of a solar cell exceeds demand. 
       FIG. 5  illustrates a graph when the voltage V Link  of the link capacitor  180  is 400 V, the voltage of the battery  140  is 200 V, the generation power of the renewable energy unit  120  is 1.6 KW and the consumption power of the load  10  is 1.2 KW. At this point, the average of the current I L  of the inductor  181  is about 3 A, and when multiplying the 3 A and the voltage V Link  of 400 V of the link capacitor  180 , it can be seen that the consumption power of the load  10  is 1.2 KW. In this case, an average current is shown as about −2 A in the primary winding of the transformer  160 . Moreover, the primary winding current Ip may be recognized as the discharge current of the battery  140 , and thus it can be seen that the battery  140  is being charged. 
     Accordingly, when a power failure occurs in the grid  110 , it can be seen through the simulation of  FIG. 5  that a residual power of 400 W which is not transferred to the load  10  by the renewable energy unit  120  is supplied to the battery  140  and the battery  140  is being charged with the supplied power. 
       FIG. 6  shows characteristics of a voltage and current when a power failure occurs in a grid of an energy storage system according to an embodiment and a power output of a solar cell is insufficient to meet demand. 
     In the simulation of  FIG. 6 , the voltage of the battery  140  is 200 V and the consumption power of the load  10  is 1.2 KW, but the generation power of the renewable energy unit  120  is set to 800 W. In this case, an average current is shown as about 2 A in the primary winding of the transformer  160 . 
     Accordingly, when the grid  110  is cut off, a power is transferred from the renewable energy unit  120  to the load  10 . In this case, the power from the renewable energy unit  120  is insufficient to meet the demand of the load  10 . A power of 400 W is supplied from the battery  140  to the load  10  to supplement the power supplied by the renewable energy unit  120 , therefore it can be seen through the simulation of  FIG. 6  that the battery  140  is being discharged. 
       FIG. 7  shows characteristics of a voltage and current when a power failure occurs in a grid of an energy storage system according to an embodiment and a power of a solar cell is not generated. 
     In the simulation of  FIG. 7 , the voltage of the battery  140  is 200 V and the consumption power of the load  10  is 1.2 KW, but the generation power of the renewable energy unit  120  is set to 0 W (for example, at night). In this case, an average current is shown as about 6 A in the primary winding of the transformer  160 . 
     Accordingly, when the grid  110  is cut off and power is not generated by the renewable energy unit  120 , it can be seen through the simulation of  FIG. 7  that the battery  140  supplies 1.2 KW required by the load  10  to the load  10 . 
       FIG. 8  shows characteristics of a voltage and current when a power is supplied from a battery of an energy storage system that is coupled to a grid according to an embodiment. 
     In the simulation of  FIG. 8 , the voltage of the battery  140  is 200 V, a consumption power transferred from the inverter  190  to the load  10  is 2 KW because the load  10  is in a peak state, the power generated by the renewable energy unit  120  is set to 0 W (for example, at 5 p.m. to 10 p.m.), and the grid  110  is set in a connected state. In this case, an average current is shown as about 10 A in the primary winding of the transformer  160 . 
     When the grid  110  is connected and an amount of power is not generated by the renewable energy unit  120 , it can be seen through the simulation of  FIG. 8  that the battery  140  supplies a power of 1.2 KW other than a power supplied from the grid  110  to the load  10  to meet the power demand of the load  10 . 
       FIG. 9  shows characteristics of a voltage and current when a power of a solar cell is supplied to a load of an energy storage system that is coupled to a grid according to an embodiment. 
     In the simulation of  FIG. 9 , the consumption power of the load  10  is 1.2 KW, the power generated by the renewable energy unit  120  is set to 700 W, and the grid  110  is connected. In this case, an average current is shown as about 0 A in the primary winding of the transformer  160 . Also, since the average current of an inductor current I L  is about 1.75 A and the voltage V Link  of the link capacitor  180  is about 400 V, it can been seen through the simulation of  FIG. 9  that all the power of 700 W generated by the renewable energy unit  120  is transferred to the load  10  and the battery  140  is not charged/discharged. 
     The energy storage system according to embodiments of the present invention serially connects the storage capacitor and the battery to the output terminal of the maximum power point tracker and allows the storage capacitor to divide and receive a voltage, and thus the number of elements configuring the storage capacitor can be reduced. 
     Moreover, the energy storage system according to embodiments couples one end of the primary winding to the contact point between the storage capacitor and the battery, couples the other end of the primary winding to the contact point between the first and second switches, includes the transformer having the secondary winding coupled to the rectifier, and disconnects the storage capacitor and the battery from the inverter, thereby securing electrical stability. 
     Exemplary embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims and their equivalents.