Method and apparatus for maximizing energy storage system efficiency and longevity

Abstract

Method and apparatus for maximizing energy storage system efficiency and longevity, comprising: at least one battery; a plurality of inverters, coupled to the battery, for converting DC power to AC power; and a controller, coupled to the plurality inverters, for controlling an operational state of each of the inverters in the plurality of inverters to minimize overall power losses of a battery energy storage system.

Description

METHOD AND APPARATUS FOR MAXIMIZING ENERGY STORAGE SYSTEM EFFICIENCY AND LONGEVITYBACKGROUNDField

[0001] Embodiments of the present invention generally relate to energy storage systems and, in particular, to a method and apparatus for maximizing energy storage system efficiency and longevity.Description of the Related Art

[0002] A distributed energy generation system typically comprises a plurality of energy generators (e.g., solar panels, wind turbines, etc.), one or more power converters (e.g., optimizers, microinverters, inverters, etc.), and a service panel to connect the system to loads and / or a utility power grid. For a solar system, the solar panels are arranged in an array and positioned to maximize solar exposure. Each solar panel or small groups of panels may be coupled to a power converter (so-called micro-inverters) or all the solar panels may be coupled to a single inverter via DC-DC optimizers. The inverter(s) convert the DC power produced by the solar panels into AC power. The AC power is coupled to the service panel for use by a facility (e.g., home or business), supplied to the power grid, and / or coupled to an optional storage element such that energy produced at one time is stored for use at a later time. Other forms of distributed energy generators include wind turbines arranged on a so-called wind farm. Storage elements (energy storage systems) may be one or more of batteries, fly wheels, hot fluid tank, hydrogen storage or the like. The most common storage element is a battery pack (i.e., a plurality of battery cells) having at least one bidirectional inverter coupled to the service panel to supply the batteries with DC power as well as allow the batteries to discharge through the inverter to supply AC power to the facility when needed.

[0003] The battery energy storage systems (BESS) may comprise a battery pack coupled to a plurality of inverters such that the inverters may share the power conversion load to ensure the BESS is always ready to produce the maximum amount of power. However, by continuously operating the inverters, there are significantpower losses. Such continuous power losses also reduce the life expectancy of the inverters and BESS as a whole.

[0004] Therefore, there is a need for a method and apparatus for controlling BESS inverters to minimize losses and maximize efficiency and longevity.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] So that the manner in which the above recited features of the present invention can be understood in detail, a particular description of the invention, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0006] FIG. 1 depicts a block diagram of battery energy storage system (BESS) in accordance with at least one embodiment of the invention;

[0007] FIG. 2 depicts a block diagram of a control system for the BESS of FIG. 1 in accordance with at least one embodiment of the invention;

[0008] FIG. 3 depicts a state diagram of a method that is performed upon executing an inverter control software in accordance with an embodiment of the invention.DETAILED DESCRIPTION

[0009] Embodiments of the present invention comprise apparatus and methods for maximizing battery energy storage system (BESS) efficiency and longevity. Embodiments of the invention utilize an inverter controller to activate and deactivate a plurality of inverters to reduce inverter produced power losses within the BESS. A BESS comprises a BESS controller and one or more batteries coupled to a plurality of bidirectional inverters, i.e., the inverters convert DC battery power to AC power to supply energy to loads (discharge mode) and convert AC power from an energy source to DC power to charge the batteries (charge mode). Each inverter has a power loss that is a function of the amount of power being processed, temperature and theactive / inactive state. As described in detail below, the controller controls the active and inactive state of each inverter to reduce overall system losses while continuing to meet the BESS’ energy supply requirements.

[0010] FIG. 1 depicts a block diagram of a BESS 100 in accordance with at least one embodiment of the invention. The BESS100 comprises one or more batteries 102-1 , 102-2, ... 102-N (collectively, batteries 102), a plurality of bidirectional inverters 106A and 106B, and a controller 108. The batteries 102 couple DC power on a DC wiring network 104 to / from the inverters 106A and 106B. Although two inverters are shown, in other embodiments, more than two inverters may be used. The inverters 106A and 106B are connected in parallel. An AC wiring network 110 couples the AC power terminal 1 12 to / from the inverters 106A and 106B. The controller 108 is connected to both the inverters 106A and 106B.

[0011] When operating in the discharge mode, the batteries 102 supply DC power to the inverters 106A and 106B and the inverters 106A and 106B convert the DC power into AC power at the AC power terminal 112. When operating in the charge mode, the AC power terminal 112 couples AC power to the inverters 106A and 106B and the inverters convert the AC power to DC power to charge the batteries 102. The controller 108 receives operational parameters from a battery management unit (not shown) (BMU) and controls the active and inactive states of the inverters 106A and 106B to facilitate reducing overall power losses of the BESS. The controller 108 may be part of the BMU or a standalone computing device. A detailed description of the controller’s operation is provided with reference to FIGs. 2 and 3 below.

[0012] FIG. 2 depicts a block diagram of the controller 108 in accordance with an embodiment of the invention. The controller 108 comprises at least one processor 200, support circuits 202 and memory 204. The at least one processor 200 may be any form of processor or combination of processors including, but not limited to, central processing units, microprocessors, microcontrollers, field programmable gate arrays, graphics processing units, and the like. The support circuits 202 may comprise well-known circuits and devices facilitating functionality of the processor(s). Thesupport circuits 202 may comprise one or more of, or a combination of, power supplies, clock circuits, communications circuits, cache, and / or the like.

[0013] The memory 204 comprises one or more forms of non-transitory computer readable media including one or more of, or any combination of, read-only memory or random-access memory. The memory 204 stores software and data including, for example, inverter control software 206, and data 208. The inverter control software 206 may be software that, when executed by the processor(s) 200, is capable of controlling the inverters as described below with reference to FIG. 3 in accordance with various embodiments of the invention. In one embodiment, the controller 108 is a general purpose computer that, when executing the inverter control software 206 becomes a specific purpose computing device, specifically, an inverter controller.

[0014] The data 208 may include, but is not limited to, power being processed by each inverter 210, temperature of the inverters 212, transient power requirement information 214 and inverter state 216. Some or all of the data may be provided by the BMU. Alternatively, some of the data may be measured by or provided directly to the controller 108 using sensors within the BESS.

[0015] FIG. 3 depicts a state diagram 300 of the operation of the controller in accordance with an embodiment of the invention. The overall BESS power loss is a function of the sum of the losses of each inverter and can be represented as:P sysjoss — P|OSS_A(PA,TA,SA) + P|OSS_B(PB,TB,SB) + . . . where:PIOSS_A(PA,TA,SA) is the power loss of inverter A which is a function of the power being produced by inverter A, the temperature of inverter A and the operational state of inverter A; andPIOSS_B(PB,T B,SB) is the power loss of inverter B which is a function of the power being produced by inverter B, the temperature of inverter B and the operational state of inverter B.

[0016] By controlling the inverter state (inactive or active) and the amount of power being processed by each inverter, the controller minimizes the overall power loss of the BESS. The controller also must facilitate handling of transient power requirements that arise when the BESS operates as a backup power source or operates in an off- grid mode. Typically, when a transient level of power being needed, a single inverter can operate at two times its rated power output while other inverters are being switched from inactive to active state.

[0017] In an optional embodiment, the controller may also activate one or more inverters to self-heat the inverter. Cold temperature limits the batteries’ ability to charge and discharge properly. As such, activating one or more inverters produces heat within the BESS to warm the batteries to a favorable operating temperature.

[0018] The depicted embodiment of the state diagram 300 is indicative of a BESS that utilizes two inverters. It will be clear from the following description that a state diagram of similar structure may be used for a BESS with more than two inverters.

[0019] The state diagram 300 comprises node 302 where none of the inverters are active, node 310 where one of the inverters is active and one of the inverters is inactive, and node 308 where both inverters are active. Within node 310, there are two nodes representing two possible states where inverter 106A is active and inverter 106B is inactive and vice versa. The inactive and active states may be swapped as indicated by arrows 312 and 314. The transition paths between nodes (states) are represented by paths 1 , 2, 3, and 4.

[0020] Upon system boot up, both inverters will be inactive at node 302. When the BMU informs the controller that (1 ) power is required for a load or (2) that a transient level of power is needed, the inverter state will transition from node 302 to node 310 and one of the inverters will be active and one will be inactive.

[0021] At node 310, the state changes along path 2 to node 308 if (1 ) the load requirements are greater than the power rating of the active inverter or (2) a transient level of power is still needed or (3) the system power loss is minimized with the second inverter being active, or (4) inverter self-heating is required, and a delay of X secondshas elapsed. The delay forms activation / deactivation hysteresis to prevent excessive state transitions as well as ensures that a transient power need exceeds a predefined period before activation of at least one additional inverter.

[0022] Furthermore, at node 310, the state changes along path 3 to node 302, where both inverters are inactive, if (1) the load requirements are less than or equal to zero watts and (2) there is no transient power needed.

[0023] At node 308, the state changes along path 4 to node 310 if (1 ) the load requirements are less than the power rating of the active inverters or (2) a transient level of power is no longer needed or (3) the system power loss is minimized with a single inverter being active, or (4) inverter self-heating is no longer required using both inverters.

[0024] At node 310, to increase the longevity of the inverters, the inverter states are swapped when a temperature difference between the inverters reaches a threshold level. The temperature difference should be large enough to prevent excessive swap transitions along paths 312 and 314.

[0025] In a system with more than two inverters, at least one inverter may be switched to an active state while other inverters are left or switched to an inactive state. When transient power is needed, at least one additional inverter may be activated. When the need for transient power has passed, at least one inverter may be deactivated. A delay (hysteresis) may be used in activating and / or deactivating any of the at least one inverters to ensure excessive activation and deactivation cycling does not occur.

[0026] Consequently, through the use of inverter state control, the BESS is operated with minimal power losses generated by the inverters and extended inverter longevity. In addition, the controller uses the inverter temperature and available power producing capacity to optimize against thermal and load distribution. Through independent control loading and power production of each inverter, the controller can reduce the temperature rise and control daily maximum and minimum temperatures; thus, reducing thermal cycling induced stress and improving the longevity of theinventers. The control of temperature fluctuations also reduces temperature induced mechanical stresses on components of the inverters.

[0027] Here multiple examples have been given to illustrate various features and are not intended to be so limiting. Any one or more of the features may not be limited to the particular examples and embodiments presented herein, regardless of any order, combination, or connections described. In fact, it should be understood that any combination of the features and / or elements described by way of example above are contemplated, including any variation or modification which is not enumerated, but capable of achieving the same. Unless otherwise stated, any one or more of the features may be combined in any order.

[0028] As above, figures are presented herein for illustrative purposes and are not meant to impose any structural limitations, unless otherwise specified. Various modifications to any of the structures shown in the figures are contemplated to be within the scope of the invention presented herein. The invention is not intended to be limited to any scope of claim language.

[0029] Where “coupling” or “connection” is used, unless otherwise specified, no limitation is implied that the coupling or connection be restricted to a physical coupling or connection and, instead, should be read to include communicative couplings, including wireless transmissions and protocols.

[0030] Any block, step, module, or otherwise described herein may represent one or more instructions which can be stored on a non-transitory computer readable media as software and / or performed by hardware. Any such block, module, step, or otherwise can be performed by various software and / or hardware combinations in a manner which may be automated, including the use of specialized hardware designed to achieve such a purpose. As above, any number of blocks, steps, or modules may be performed in any order or not at all, including substantially simultaneously, i.e., within tolerances of the systems executing the block, step, or module.

[0031] Where conditional language is used, including, but not limited to, “can,” “could,” “may” or “might,” it should be understood that the associated features orelements are not required. As such, where conditional language is used, the elements and / or features should be understood as being optionally present in at least some examples, and not necessarily conditioned upon anything, unless otherwise specified.

[0032] Where lists are enumerated in the alternative or conjunctive (e.g., one or more of A, B, and / or C), unless stated otherwise, it is understood to include one or more of each element, including any one or more combinations of any number of the enumerated elements (e.g. A, AB, AB, ABC, ABB, etc.). When “and / or” is used, it should be understood that the elements may be joined in the alternative or conjunctive.

[0033] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

Claims:

1. Apparatus for minimizing power losses in a battery energy storage system comprising: at least one battery; a plurality of inverters, coupled to the battery, for converting DC power to AC power; and a controller, coupled to the plurality inverters, for controlling an operational state of each of the inverters in the plurality of inverters to minimize the overall power losses of the battery energy storage system.

2. The apparatus of claim 1 , wherein the operational state of each of the inverters in the plurality of inverters is either active or inactive.

3. The apparatus of claim 2, wherein, when power is required for a load, the controller activates at least one inverter and deactivates at least one other inverter.

4. The apparatus of claim 3, wherein, when power is required for the load that exceeds the power available from the at least one active inverter, the controller activates at least one additional inverter.

5. The apparatus of claim 4, wherein, before the at least one additional inverter is activated, the controller delays activation for a period of time.

6. The apparatus of claim 2, wherein, when transient level of power is needed, the controller activates the at least one inverter and activates at least one additional inverter if the need for a transient level of power remains for a predefined amount of time.

7. The apparatus of claim 2, wherein, when a temperature difference between the plurality of inverters exceeds a threshold, the inverter states are swapped, where theat least one active inverters are deactivated and the at least one deactivated inverters are activated.

8. The apparatus of claim 2, wherein the controller activates at least one inverter in the plurality of inverters to self heat the at least one active inverter and heat the at least one battery.

9. The apparatus of claim 1 wherein the overall power losses of the battery energy storage system are a function of the sum of losses of each inverter, where the losses of each inverter are a function of an amount of power being produced by the inverter, a temperature of the inverter and the operational state of the inverter.

10. A method for minimizing power losses in a battery energy storage system, comprising: controlling an operational state of each inverter in a plurality of inverters to minimize the overall power losses of the battery energy storage system.11 . The method of claim 10, wherein the operational state of each of the inverters in the plurality of inverters is either active or inactive.

12. The method of claim 11 , wherein, when power is required for a load, controlling further comprises activating at least one inverter and deactivating at least one other inverter.

13. The method of claim 12, wherein, when power is required for the load that exceeds the power available from the at least one active inverter, controlling further comprises activating at least one additional inverter.

14. The method of claim 13, wherein, before the at least one additional inverter is activated, controlling further comprises delaying activation for a period of time.

15. The method of claim 11 , wherein, when transient level of power is needed, controlling further comprises activating the at least one inverter and activating at least one additional inverter if the need for a transient level of power remains for a predefined amount of time.

16. The method of claim 11 , wherein, when a temperature difference between the plurality of inverters exceeds a threshold, swapping the inverter states, where the at least one active inverters are deactivated and the at least one deactivated inverters are activated.

17. The method of claim 11 , wherein controlling further comprises activating at least one inverter in the plurality of inverters to self heat the at least one active inverter and heat an at least one battery within the battery energy storage system.

18. The method of claim 10 wherein the overall power losses of the battery energy storage system are a function of the sum of losses of each inverter, where the losses of each inverter are a function of an amount of power being produced by the inverter, a temperature of the inverter and the operational state of the inverter.

19. A method for minimizing power losses in a battery energy storage system, comprising: controlling an operational state of each inverter in a plurality of inverters to minimize the overall power losses of the battery energy storage system, wherein the operational state of each of the inverters in the plurality of inverters is either active or inactive; when power is required for a load, activating at least one inverter and deactivating at least one other inverter; when power is required for the load that exceeds the power available from the at least one active inverter, activating at least one additional inverter; and before the at least one additional inverter is activated, delaying activation for a period of time.

20. The method of claim 19, wherein, when transient level of power is needed, controlling further comprises activating the at least one inverter and activating at least one additional inverter if the need for a transient level of power remains for a predefined amount of time.

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