CLAIM OF PRIORITY

[0001] This patent application claims the benefit of priority, under 35 U.S.C. §

119, of U.S. Provisional Patent Application Serial No. 62/512,869, filed May 31, 2017, titled "ENERGY STORAGE SYSTEM" the entire disclosure of which is incorporated herein by reference.

BACKGROUND

[0002] The worldwide demand for electrical energy has been increasing year by year. Most of the electrical energy demand is met by energy produced from conventional energy sources such as coal and gas. However, in recent years, with the rising global climate change issues, there has been a push for electricity generation by renewable energy resources such as solar power and wind power.

[0003] Wind turbine generators are regarded as environmentally friendly and relatively inexpensive alternative sources of energy that utilize wind energy to produce electrical power. Further, solar power generation uses photovoltaic (PV) modules to generate electricity from the sunlight. Since the intensity of wind and sunlight is not constant the power output of wind turbines and PV modules fluctuate throughout the day. Unfortunately, the electricity demand does not vary in accordance with solar and wind variations.

[0004] An energy storage system may help to address the issue of variability of solar and wind power at a small scale. Essentially, the variable power from solar and wind power plants can be stored in the energy storage system which can then be used at a later time or at a remote location. Energy storage systems may also be charged from a power network and could be used to address the frequency variations, harmonic suppression, voltage support and power quality in the power network. [0005] When PV modules are connected to the energy storage system it is desirable to match the voltage of PV modules with that of batteries in the energy storage system. Therefore, a system and a method that will address the foregoing issues is desirable.

DRAWINGS

[0006] These and other features and aspects of embodiments of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0007] FIG. 1 illustrates a schematic diagram of a conventional energy storage system;

[0008] FIG. 2 illustrates a schematic diagram of an energy storage system in accordance with embodiments;

[0009] FIG. 3 illustrates a schematic diagram of a buck-boost power converter in accordance with embodiments for the system of FIG. 2;

[0010] FIG. 4 illustrates a schematic diagram of a buck-boost power converter in accordance with embodiments for the system of FIG. 2;

[0011] FIG. 5 illustrates a schematic diagram of a buck-boost power converter in accordance with embodiments for the system of FIG. 2;

[0012] FIG. 6 illustrates a schematic diagram of a buck-boost power converter in accordance with embodiments for the system of FIG. 2;

[0013] FIG. 7 illustrates a flowchart for a process to operate the system of FIG.

2 in accordance with embodiments; and

[0014] FIG. 8 illustrates a typical solar energy power generation curve for a renewable energy source. DETAILED DESCRIPTION

[0015] Embodying systems and methods couple two DC sources to one dc/ac inverter while maintaining full control over power flow direction and power sharing among the three terminal system. The two DC sources can specifically feature an overlapping voltage range. There is no need to regulate the voltage onto an intermediary stiff DC bus.

[0016] In accordance with embodiments, bidirectional buck-boost converters can be connected between batteries and a variable-level DC bus connecting a renewable energy power source to the DC/ AC inverter. The buck-boost converters are controlled to regulate the power flow between batteries and the DC bus. The batteries can have a variable DC voltage depending on individual states of charge. Depending on the renewable energy power produced, the grid power extracted, and the battery states of charge, the battery voltages can be higher or lower than the DC bus voltage. The buck- boost converters provide the coupling between these widely tolerant DC bus systems while at the same time providing the regulation of the power flow. The solution can be implemented with/without galvanic isolation providing additional degrees of freedom with respect to equipment grounding.

[0017] FIG. 1 illustrates conventional energy storage system 100, which includes DC bus 102. A plurality of batteries 104 are connected to the DC bus. Each of the plurality of battery can include a plurality of battery cells connected in series and/or parallel. The batteries may get charged from the DC bus, and/or may provide energy to loads 108 connected to the DC bus. Loads 108 can include a car charger, electric drives, lighting loads etc. When a particular load is an alternating current (AC) load a DC-to-AC converter may be used between DC bus 102 and the AC load(s). Energy storage system 100 can also include other components such as a controller, a communication module, and a protection module.

[0018] In some implementations energy storage system 100 may be connected to AC power network 110 via a power network-side inverter 112. The power network can be a consumer, commercial, and/or utility scale power network. In some implementations the energy storage system may also be connected to renewable energy power module 114, which can generate energy from one or more renewable energy generation sources (e.g., photovoltaic (PV) panels, wind turbines, geothermal exchanges, or any other renewable energy generation source). The renewable energy power module 114 is connected to the energy storage system via renewable energy power converter 116.

[0019] By controlling the DC bus voltage, batteries 104 may be charged from power network 1 10 and renewable energy power module 114. Moreover, in implementations batteries 104 may supply power to the power network. Further, renewable energy power converter 116 can be controlled by power network local controller 1 16 such that maximum power is fetched from the renewable energy power module 114.

[0020] FIG. 2 illustrates energy storage system 200 in accordance with embodiments. Energy storage system 200 can include a plurality of battery modules 202A, 202B, . . . , 202N coupled to DC bus 208. Each of battery modules 202A, 202B, . . . , 202N include a respective battery 210, respective power converter 212, and respective local controller 214. In some implementations, each respective battery can be formed from a two or more battery modules stacked in series, where internally each battery module can have a plurality of batteries in series and/or parallel. Each respective power converter 212 couples a respective battery to the DC bus. In accordance with embodiments, power converters 212 can be bidirectional devices that either source and/or sink current from the battery module. In some configurations, a single power converter can be located between the DC bus and more than one (or all) battery modules.

[0021] Power network grid 219 can be in electrical communication with DC bus 208 through power network inverter 218. The power network inverter can either source power to DC bus 208 from the power network, or provide power from the DC bus to power network grid 219. Power network local controller 216 is provided for control of power network inverter 218.

[0022] Renewable energy power module 220 contains one or more renewable energy generators. The renewable energy power module can be coupled to DC bus 208 by direct electrical communication with the DC bus 208 (as shown). Renewable energy power module 220 can provide about its maximum possible power to DC bus 208. The maximum power that can be generated is dependent on factors related to the nature of the renewable energy (e.g., wind speed/duration, ambient temperature, sunlight intensity, etc.). However, the maximum possible power that can be transferred from a solar-sourced renewable energy power module 220 to the DC bus depends on the relationship between the DC bus voltage level and the output voltage of power module 220. For other implementations of a renewable energy power module (e.g., wind, water, geothermal, etc.), a power converter would need to be interposed between the renewable energy power module and the DC bus.

[0023] In accordance with embodiments, the DC bus voltage is controlled to about match the power module output voltage to about maximize power transfer to the DC bus. In the conventional implementation illustrated in FIG. 1, because renewable energy power converter 1 16 provides a buffer between the output terminal of renewable energy power module 1 14 and DC bus 102, the entire DC bus voltage need not be controlled. Thus, the conventional approach is to use renewable energy power converter 1 16 to control only the voltage at the output terminal of the renewable energy power module 1 14.

[0024] With regard to FIG. 2, central controller 220 includes input/output unit

226, through which the central controller is in communication with respective local controllers 214 of respective battery modules, and also in communication with power network local controller 216. In accordance with embodiments to effectuate control of the DC bus voltage, the central controller can provide control command signals tailored to each local controller. These commands can instruct the local controller to operate switches within, and adjust an output voltage of, respective power converters. The communication from central controller to local controllers could be digital communication. In accordance with implementations, communication can be wireless, or wired, and can include various protocols— e.g., RS 232 communication, Bluetooth, WIFI, Zigbee, TCP/IP, power line carrier, etc. Central controller 220 can include memory unit 224 for local memory and/or cache operations. [0025] Central controller 220 and each of local controller 214, 216 can be a control processor implemented as a programmable logic device (e.g., a complex programmable logic device (CPLD), field programmable gate array (FPGA), Programmable Array Logic (PAL), a microcontroller, application-specific integrated circuit (ASIC), etc.).

[0026] Central controller 220 can be in communication with data store 230 across an electronic communication network, or be in direct connection with the data store. The central controller can include processor unit 222 which executes executable instruction 232 to cause the control unit to perform operations disclosed herein.

[0027] Local controller 214, 216 can include a processor unit, memory unit, input/output unit, and executable instructions stored in the memory unit. In some implementations the local controller can also include an analog-to-digital converter to convert received analog signals (from, perhaps, sensors), a user interface (e.g., visual display, printer, etc.) that can indicate current status or other information and parameters. The local controller may also include a digital to analog converter for converting digital signals into analog signals to control the power converters.

[0028] FIG. 3 illustrates a schematic diagram of buck-boost power converter

300 in accordance with embodiments. Buck-boost power converter 300 is bidirectional, and can be used in the energy storage system of FIG. 2. The bidirectional buck-boost converter can either reduce the input voltage or alternatively boost the input voltage. The systems depicted in FIGS. 1-2 include two types of DC sources (i.e., a renewable energy power module and batteries), which are connected to a power converter (i.e., power network inverter 112, 218; or power converter 1 16, 212). The power converter can maintain full control over power flow in/out of battery modules 202A, 202B, 202N, or control to maximize power extraction from renewable power module 1 14 (dependent on the system configuration).

[0029] The battery voltage can vary depending on the renewable energy produced, the power extracted by the power network, and the battery state of charge. In the conventional system of FIG. 1 , where no power converter is connected between the batteries and the DC bus, the DC bus voltage is also variable. This variable voltage could be higher or lower than the voltage at the renewable energy power module output terminal.

[0030] Energy storage system 200 of FIG. 2 does include power converters connected between the batteries and the DC bus. However, the battery voltages could be higher or lower than the DC bus voltage. Therefore, bidirectional buck-boost converter 300 can either reduce or boost the DC bus voltage. The buck-boost converter provides coupling between these widely tolerant DC bus systems, while at the same time providing the regulation of the power flow.

[0031] Buck-boost converter 300 includes an input capacitor 302 and an output capacitor 304. One terminal of capacitor 302 is directedly connected with one terminal of capacitor 304 to form a first node 305. The input voltage (Vin) is applied across the input capacitor 302 and an output voltage (Vout) with reversed polarity is obtained at output capacitor 304. Two semiconductor switches 306, 308 are coupled between the input and output capacitors. In one embodiment, the semiconductor switches 306, 308 include silicon switches or silicon carbide switches. In another embodiment, the semiconductor switches 306, 308 include insulated gate bipolar transistors (IGBTs) or metal oxide semiconductor field effect transistors (MOSFET). An inductor 310 is connected between the first node 305 and a second node 307 formed by interconnection of semiconductor switches 306, 308.

[0032] In operation, when semiconductor switch 306 is turned on, some energy is stored in the inductor 310. Further, when semiconductor switch 308 is turned on the energy in the inductor 310 is discharged into output capacitor 304. By controlling duty cycles of semiconductor switches 306, 308, Vout can be controlled. The term "duty cycle" refers to a ratio between the period a semiconductor switch is on and the period the switch is off.

[0033] When the duty cycle is controlled, the on/off time of the semiconductor switches is controlled. The time period depends on a switching frequency. In certain embodiments, the switching frequency could also be controlled to control output voltage Vout instead of controlling duty cycles. In one embodiment, a pulse width modulation technique may be used to control duty cycle and switching frequency of semiconductor switches. In one embodiment, Vout could be controlled to be higher or lower than input voltage Vin. In another embodiment, Vout could be controlled to be equal to Vin.

[0034] FIG. 4 illustrates a schematic diagram of buck-boost power converter

400 in accordance with embodiments. This buck-boost power converter can be used in the energy storage system of FIG. 2. Similar to FIG. 3, bidirectional buck-boost converter 400 of FIG. 4 includes input capacitor 402, output capacitor 404, an inductor 406 coupled between first node 405 and second node 407. However, semiconductor switches 306, 308 of FIG. 3 are now replaced with switching modules 408, 410. Each of the switching modules 408, 410 includes two semiconductor switches 412, 414 and a diode 416. The diode is connected between interconnection node of semiconductor switches 412, 414 and second node 407.

[0035] The switch voltage rating for semiconductor switches 306, 308 of converter 300 is given by (Vin + Vout) due to output voltage being at reverse polarity. As compared to converter 400, the switch voltage rating of semiconductor switches 412, 414 is (Vin+Vout)/4 because the diodes 416 clamp voltages across the semiconductor switches.

[0036] FIG. 5 illustrates a schematic diagram of buck-boost power converter

500 in accordance with embodiments. This buck-boost power converter can be used in the energy storage system of FIG. 2. In buck-boost converter 500, semiconductor switches 502, 504 are coupled across an input capacitor 506 and semiconductor switches 508, 510 are coupled across an output capacitor 512. An inductor 514 is coupled between a first node 505 and a second node 507. The first node 505 is formed by an interconnection point between semiconductor switches 502, 504 and the second node 507 is formed by an interconnection point between semiconductor switches 508, 510. [0037] As compared with embodiments of Figs. 3 and 4, the output voltage of converter 500 is provided at output capacitor 512 without reversing its polarity. Moreover, since inductor 514 of converter 500 is provided in series to the load current, it also provides a current limiting capability in case of a fault.

[0038] FIG. 6 illustrates a schematic diagram of buck-boost power converter

600 in accordance with embodiments. This buck-boost power converter can be used in the energy storage system of FIG. 2. In buck-boost converter 600, an input inductor 602 is coupled in series between input terminals and a first node 605 formed by an interconnection point of semiconductor switches 604, 606.

[0039] An output inductor 608 is coupled in series between output terminals and a second node 607 formed by an interconnection point of semiconductor switches 610, 612. The terminals of semiconductor switches 604, 606 which are not connected to each other are coupled in parallel to the terminals of semiconductor switches 610, 612 which are not connected to each other. Moreover, a capacitor 614 is also connected between nodes 616, 618 formed by interconnection of semiconductor switches 604, 610 and 606, 612 respectively. Converter 600 also provides output voltage without reversing polarity. Series inductors 602, 608 provide current limiting capability in case of a fault.

[0040] Figure 7 illustrates a flowchart for process 700 to operate energy storage system 200 (FIG. 2) in accordance with embodiments. For purposes of discussion, process 700 is implemented with energy storage system 200 having renewable energy power module 220 configured for solar energy as its source of renewable energy. However, the invention is not so limited. It should be readily understood that process 700 is equally applicable to other types and/or forms of renewable energy power sources.

[0041] Figure 8 illustrates solar energy power generation curve 800 for a renewable energy source (e.g., for discussion purposes a photovoltaic cell or panel). With exposure to solar irradiance, renewable energy power module 220 can generate initial power generation 802. As the intensity of the solar irradiance increases over time, this generated power increases.

[0042] The generated power can be provided to power network inverter 218 via

DC bus 208. The amount of power a power network inverter can sink to power network 219 is represented by inverter power limit 804. Solar irradiance can peak, which causes the renewable energy power module to reach maximum power generation level 808, and subsequently decrease power generation to minimum power generation point 810. Excess power generation region 806 represents the area under curve 800 where the renewable energy power module is generating more power than can be sunk by the power network inverter. This phenomenon is referred to as a DC/ AC ratio— where a ratio of 1 indicates that the inverter can push everything to the grid; a ratio of 1.5 indicates that in the standard case the inverter will be overpowered by at least 50% of the inverter's base rating.

[0043] With regard to FIG. 7, a renewable energy power module begins to generate power by converting its renewable energy input (e.g., solar irradiance) to a voltage output. The renewable energy power module begins to provide its output power, step 702, to DC bus 208 (i.e., initial power generation 802). As output power from the renewable energy power module increases, the generated power can cross inverter power limit 804, step 704. For example, on a sunny day, the power from a solar array can exceed the amount of power that the network inverter can manage.

[0044] Excess generated power (above the inverter power limit 804) can be stored, step 706, in one or more of battery modules 202A, 202B, . . . , 202N to the extent that respective batteries 210 can sink the energy without exceeding respective individual battery state of charge (i.e., the battery can accept the energy charge). For example, if a photovoltaic source is generating more power than the network inverter can manage (excess power generation region 806), that power is directed to the batteries through respective power converters 212. In some implementations, if no battery can absorb additional energy then power generation by the renewable energy power module can be curtailed by reducing or eliminating the amount of output power generated by the renewable energy power module. [0045] As the renewable energy input to the renewable energy power module begins to decrease, step 708, from above to below inverter power limit 804 (e.g., the sunsets or is blocked by clouds), options for use of the battery stored energy are available. In accordance with one embodiment, one or more respective battery 210 can be discharged, step 710, through its respective power converter to provide power to DC bus 208. The amount of energy provided to the DC bus can be up to about the inverter power limit 804. In accordance with another embodiment, a time delay can be instituted, step 712, between the drop in renewable energy power module output and the time when battery discharge begins. This delay can be variable and/or predetermined based on a prediction of future peak demand load levels and timing.

[0046] If power network 219 has an excess of power, step 714, one or more of respective batteries 210 can be charged. The respective batteries store, step 716, excess power network energy sourced to DC bus 208 through power network inverter 218. Particularly, under conditions where the batteries have the capacity to sink additional energy and there is no excess power being generated by the renewable energy power module. In some embodiments, rather than determining if the power network has excess power (step 714), respective batteries 210 can first be charged at their maximum rate. Then remaining power can be pushed to the power network.

[0047] In some implementations, it could be that the batteries are sized such that no power remains to be pushed to the grid. This absence (and/or level) of remaining power can vary throughout the year, dependent on environmental conditions (solar, wind, water, etc.). Dependent on various constraints and outcomes, the control processor can determine which scenario is most beneficial (i.e., charging the batteries with excess power not sourced to power network 219; or first charging the batteries prior to providing power to the network).

[0048] In accordance with some implementations, the charging (steps 706, 716) and discharging (steps 710, 712) of individual respective batteries can be distributed in a balanced manner across the batteries. [0049] In accordance with embodiments where each respective battery 210 has about an identical capacity, the incremental value of adding an additional (last) battery may be fairly low. For example, energy storage capacity rectangle 820, 822, 824, 826, 828, 830 represent the energy storage capacity for six identical batteries. That portion of energy storage capacity rectangle 820, 822, 824, 826, 828, 830 that intersect with excess power generation region 806 is a representation of the incremental storage capacity for each additional battery. The capacity of the battery equated with energy storage capacity rectangle 830 is underutilized compared with the battery equated with energy storage capacity rectangle 820. In accordance with embodiments, different sized batteries can be chosen based on the required capacity. However, a more complex control scheme to direct energy charge/discharge could need to be implemented.

[0050] Constraints on the control scheme can include the rate(s) of charge for each battery (power), and the total capacity (energy). The DC/DC converter is sized such that it is able to provide the maximum charging and discharging capacity needed by the application. The storage capacity rectangles depicted in FIG. 8 are a simplification, as some batteries cannot take a constant charging current until they are at full capacity ("full"). The height of a rectangle is limited by the lower of an individual battery rack converter rating, and the capability of the batteries to accept charge. The width of the rectangle could be optimized to be more tailored to the total available energy in the photovoltaic array— because each battery stack is interfaced to the DC bus via a DC/DC converter, the size of each rack can be chosen to optimize the total storage capacity needed. Thus, effectively (although not necessary) customizing the width of each rectangle to match the energy available to the optimal size. Approaching the point where adding another battery rack or module (smallest incremental unit readily commercially available) would yield diminishing returns.

[0051] In accordance with other implementations, the capacity of the network inverter can also impact the overall energy storage system operating point on energy power generation curve 800. In any of these embodiments, however, more solar power would be used than in more conventional approaches. [0052] When renewable energy power module 220 is not producing power for an extended period (e.g., evenings for a photovoltaic unit), it can be advisable to isolate the renewable energy power module (220) from DC bus 208 (by control of one or more switches). If not isolated, the DC bus (208) can attempt to energize the renewable energy power module. This energization results in a dissipation of valuable battery energy as heat. However, heating the module can be a desired effect— i.e., to melt snow off the modules so that sunshine can reach the cells. Implementing heating the modules is a tradeoff between the energy needed to melt the snow, and the benefit returned by exposing the panels to the sun sooner. Under this scenario, instead of generating electrical power, a photovoltaic module behaves as a load by dissipating electrical power in the form of heat.

[0053] Embodying systems and methods maintain control of energy flow in and out of the battery, while also providing control over extracting maximum or substantial power from a renewable energy source module. Implementations can reduce the number of converters needed, thus, minimizing cost and resistive power losses in the system. Further, with a lower part count the system has a reduced footprint compared to conventional systems.

[0054] In accordance with some embodiments, a computer program application stored in non-volatile memory or computer-readable medium (e.g., register memory, processor cache, RAM, ROM, hard drive, flash memory, CD ROM, magnetic media, etc.) may include code or executable instructions that when executed may instruct and/or cause a controller or processor to perform methods disclosed herein, such as a method of controlling the sourcing and/or sinking of power generated by a renewable energy power module of an energy storage system, as described above.

[0055] The computer-readable medium may be a non-transitory computer- readable media including all forms and types of memory and all computer-readable media except for a transitory, propagating signal. In one implementation, the nonvolatile memory or computer-readable medium may be external memory. [0056] Although specific hardware and methods have been described herein, note that any number of other configurations may be provided in accordance with embodiments of the invention. Thus, while there have been shown, described, and pointed out fundamental novel features of the invention, it will be understood that various omissions, substitutions, and changes in the form and details of the illustrated embodiments, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Substitutions of elements from one embodiment to another are also fully intended and contemplated. The invention is defined solely with regard to the claims appended hereto, and equivalents of the recitations therein.