Method and Apparatus for Storing and Depleting Energy

Abstract

A method to control storage into and depletion from multiple energy storage devices. The method enables an operative connection between the energy storage devices and respective power converters. The energy storage devices are connectible across respective first terminals of the power converters. At the second terminals of the power converter, a common reference is set which may be a current reference or a voltage reference. An energy storage fraction is determined respectively for the energy storage devices. A voltage conversion ratio is maintained individually based on the energy storage fraction. The energy storage devices are stored individually with multiple variable rates of energy storage through the first terminals. The energy storage is complete for the energy storage devices substantially at a common end time responsive to the common reference.

Description

BACKGROUND

1. Technical Field

Embodiments herein relate to storage of electrical energy (e.g., in a bank of multiple batteries) from electrical power sources, and power delivery (e.g., from the bank of multiple batteries) to an electrical network.

2. Description of Related Art

Load balancing of electrical power refers to various techniques to store excess electrical power during low demand periods for subsequent release when demand for electrical power increases. Storage of electrical energy from the electrical network may be performed within and/or outside the electrical network. For example, the storage of electrical energy may involve the customer and/or be performed on customer premises. For example, a storage electrical heater stores thermal energy during the evening, or at night when electricity is available at lower cost, and releases the heat during the day. Replenishing energy during off peak times may require incentives for consumers to participate, usually by offering cheaper rates for off peak electricity.

Load balancing may include customer owned energy storage/delivery systems operating at times independently from the electrical network and/or working in concert with the electrical network. Customer owned energy storage/delivery systems may include various energy sources including wind turbines, photovoltaic arrays, and/or fuel cells which may be independent and/or integrated with the battery storage. Customer owned energy storage/delivery systems may also be utilized to sell and deliver electricity back to the electrical network during peak demand on the electrical network.

BRIEF SUMMARY

According to features of the embodiments, various methods are provided to control storage into and depletion from multiple energy storage devices. Energy storage devices are operatively connected to respective power converters. The energy storage devices are connectible across respective first terminals of the power converters. At the second terminals of the power converter, a common reference is set which may be a current reference or a voltage reference. An energy storage fraction is determined respectively for the energy storage devices. A voltage conversion ratio of the power converter is maintained individually based on the energy storage fraction. Energy is stored individually with multiple variable rates of energy storage through the first terminals. The energy storage is complete for the energy storage devices substantially at a common end time responsive to the common reference. Energy from the energy storage devices is depleted individually with multiple variable energy depletion rates through the first terminals. The depleted energy is complete for the energy storage devices substantially at a common end time which is responsive to the common reference. The energy storage devices may include energy converters between electrical energy and at least one other form of energy.

According to features of the embodiments, various methods are provided for control of charging and discharging of multiple batteries. The batteries are connectible across respective first terminals of the power converters. At the second terminals of the power converter a common reference may be a current reference or a voltage reference. A battery charge fraction may be determined respectively for the batteries. A voltage conversion ratio, of the power converter, may be individually maintained based on the battery charge fraction. The batteries may be charged individually with multiple variable charging powers through the first terminals. The charging may be complete for the batteries substantially at a common end time responsive to the common reference. The batteries may be discharged individually with multiple variable discharging powers through the first terminals. The discharging may be complete for the batteries substantially at a common end time responsive to the common reference.

While charging, the voltage conversion ratio may be maintained substantially proportional to the amount of additional charge that may be stored in the respective batteries. While discharging, the voltage conversion ratio may be maintained substantially proportional to the remaining available charge in the respective batteries.

The method further enables connection of the DC terminals of an AC/DC inverter to the second terminals and enables connection of the AC terminals of the AC/DC inverter to an AC electrical network source of power. The AC/DC inverter may be configured to set the common reference through the DC terminals. A central controller may be attached to the AC/DC inverter to provide a value for the common reference. The common reference may be a current reference or a voltage reference.

With the second terminals of the power converters serially connected, the common reference may be set as a current reference. With the second terminals of the power converters parallel connected, the common reference may be set as a voltage reference.

The batteries may form a first bank of multiple batteries and a second bank of multiple batteries like the first bank may be located in different geographic location from the first bank. The first and the second bank may share the common reference and consequently share same common end times for charging and discharging.

According to features of the embodiments, various systems may be provided which control charge and discharge of multiple batteries including multiple power converters each including first terminals and second terminals. The first terminals are connectible to the batteries and the second terminals are connectible to a direct current (DC) source of power. At the second terminals of the power converter a common reference may be set either a current reference or a voltage reference. Each of the power converters includes a controller operatively attached to the power converter. The controller includes a fuel gauge operatively attached to the respective battery. The fuel gauge with the controller are operable to determine a battery charge fraction of the battery. The controller may be operable to maintain a voltage conversion ratio based on the battery charge fraction. The batteries are fully charged substantially at a common end time responsive to the common reference. The batteries are fully discharged substantially at a common end time responsive to the common reference.

The common reference may be set as a current reference by a serial connection of the second terminals of the power converters. The common reference may be set as a voltage reference by a parallel connected of the second terminals of the power converters.

An AC/DC inverter includes AC terminals and DC terminals has the AC terminals connectible to an AC electrical network source of power. The DC terminals may be operatively attached to the second terminals. The AC/DC inverter may include a control portion configured to set the common reference through the DC terminals.

A central controller may be operatively attachable to the AC/DC inverter to provide a value for the common reference.

A first multiple of batteries form a first bank and a second multiple of batteries form a second bank like the first bank. The first bank and second bank may not be collocated. while sharing the common reference. The first and the second bank share the common end times for charging and discharging responsive to the common reference.

Various systems may be provided or controlling energy storage into and energy depletion from multiple energy storage devices. Multiple power converters include first terminals and second terminals and the energy storage devices are connectible across respective first terminals of the power converters. A common reference may be set at the second terminals, wherein the common reference may be current reference or a voltage reference. An energy gauge may be configured to determine an energy storage fraction respectively for the energy storage devices. The power converters maintain individually a voltage conversion ratio, based on said energy storage fraction. Energy may be stored in the energy storage devices individually with a multiple variable rates of energy storage through the first terminals. The energy storage may be complete for the energy storage devices substantially at a common end time responsive to the common reference.

In some variations, energy may be depleted from the energy storage devices jointly and/or individually. Where the energy may be depleted using multiple variable energy depletion rates and/or a constant rate. The energy may also be depleted through one or more terminals such as the first terminals. The energy depletion may also be configured to occur through one power converters. The rates of energy depletion through any one terminal and/or power converter may be independent and/or dependent of the variable rates of energy depletion through any other terminal and/or power converters. In some embodiments, the energy depletion for the energy storage devices is substantially at a common end time. The common end time may be achieved through suitable mechanisms such as using a common reference (e.g., a digital and/or analog reference). For example, a first rate of depletion of a first energy storage device may be accelerated as compared to a second rate of depletion of a second energy storage device so that all of the energy storage devices reach a depleted state substantially at a common end time. This may be accomplished via one or more local and/or remote controllers which may be coupled to one or more of the energy storage devices and/or configured to periodically and/or continually monitor the energy storage devices in order to control the depletion of these devices. Other feedback control mechanisms may also be utilized. These may be controlled locally and/or networked to a remote location and controlled in conjunction with other energy storage devices at various locations. The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 shows a hybrid power generating system, according to a feature of the embodiments.

FIG. 2 shows an implementation of an energy bank shown in FIG. 1, according to a feature of the embodiments.

FIG. 3 shows more details of the direct current (DC) to DC converter shown in FIG. 2, according to a feature of the embodiments.

FIG. 3a illustrates a buck plus boost converter according to a feature of the embodiments.

FIG. 4 shows a method applied to the implementation of the energy bank shown in FIG. 2, according to a feature of the present invention embodiments.

FIGS. 5-6 show other implementations of the energy bank shown in FIG. 1, according to various embodiments.

FIG. 7 shows a method for energy storage according to one or more embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to features of the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The features are described below to explain the embodiments by referring to the figures.

Before explaining features of the embodiments in detail, it may be understood that the embodiments are not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The embodiments are capable of other features or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Aspects of the embodiments are directed to charging and discharging of energy storage devices. The storage of energy and depletion of energy of two or more energy storage devices takes into account the amount of stored energy relative to the capacity of energy storage in the energy storage device. According to an aspect of the embodiments, the energy storage devices may be batteries and respective end times of charging and discharging of two or more of the batteries may be remotely set responsive to a common voltage or current reference. All the batteries may be fully charged or fully emptied at the same time which may be controllable remotely using the common voltage or current reference. The energy storage and energy depletion may thereby be balanced to prevent some of the energy storage devices to be empty when other energy storage devices still contain energy.

Aspects of the embodiments are directed to controlling energy charge/discharge between any number of energy storage devices, e.g. batteries. Balance and control of charge/discharge of batteries may use different types of batteries, different capacitors and/or other types of energy storage devices in the same system.

It should be noted that the embodiments, by non-limiting example, alternatively be configured to include different energy sources such as wind turbines, hydro turbines, fuel cells; mechanical energy storage such as a flywheel, gas pressure, spring compression, pumping water and/or lifting mass against gravity; and chemical energy storage such as battery and fuel cell.

The terms “alternating current (AC) network”, “AC power supply” and “power network” as used herein are used interchangeably and refer to an AC power source. The AC power source typically supplies power to domestic, industrial, infrastructure, or facility-based processes separately thereto or in addition to an AC electrical network provided from an electricity utility company. The AC power source may be derived from an AC generator or the output of a direct current (DC) to AC inverter. A DC input of the DC to AC inverter may transfer electrical energy sourced from for instance a photovoltaic array, fuel cells, batteries and/or DC generator.

Referring now to the drawings, FIG. 1 shows a power generation and storage system 10, according to a feature of the embodiments. An alternating current (AC) electrical network 12 may be connected to a multiple electrical power sources generally AC generation units 16 and a number of energy banks 14 connected to AC electrical network 12 at respective nodes A and B. AC electrical network 12 may be shown in FIG. 1 as a single phase electrical network but alternative embodiments of the embodiments may be configured with a 3 phase electrical network. AC electrical network 12 may be part of a public AC electrical network network or a private AC electrical network. AC generation units 16 may include AC power produced from wind turbines, hydro turbines, fuel cells, super-conducting flywheel, and capacitors, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and/or photovoltaic panel arrays. The power may be produced by AC generators and/or DC/AC inverters or a combination thereof such as combined AC outputs of micro-inverters. Energy banks 14 in general may provide the capability to store excess AC power from electrical network 12 and/or store AC power during periods of time when the cost of producing AC power may be relatively inexpensive. Energy banks 14 in general may provide the capability to provide previously stored power onto electrical network 12 when demanded.

One or more local and/or remote controllers such as central controller 110 may be operatively attached to energy banks 14 to determine how and when energy banks 14 may provide or receive AC power to/from electrical network 12. Central controller 110 may monitor the status of electrical network 12 via sensor 18 with respect to voltage, current, phase angle, power factor, real power, apparent power and/or reactive power. By and/or, it is meant that these items may be used in any combination and/or subcombination.

An energy bank 14 may be physically located with an AC generation unit 16 and both energy bank 14 and AC generation unit 16 may provide the capability to source/sink AC power onto/from a local electrical network associated with the AC generation unit 16. The local electrical network may be loaded with the power demands of a factory for example. The local electrical network may disconnect from electrical network 12 and rely entirely on power delivered from the AC generation unit 16 and/or the energy bank 14. The energy bank 14 may be comprised of one or more subbanks 14a, 14b, etc., which may or may not be collocated.

Reference is now made to FIG. 2, which shows an exemplary implementation 14a of energy bank 14, according to embodiments. Multiple energy storage devices, e.g. batteries 20 may be connected to first terminals 260 of respective power converter modules 202. At first terminals 260 (reference denoted for one of batteries 20), voltage across battery 20 while charging may be denoted as V_c and voltage across battery 20 while discharging may be denoted V_bat. In general, the battery voltages V_bat for batteries 20 may be different, as are charging voltages V_c for batteries 20. Second terminals 262 of power converters 202 are connected together in a serial string 350. Alternatively, a parallel connection may also be utilized. Each energy storage device may include one or more batteries and or battery cells connected in series, parallel or combinations thereof. Further, each energy storage device may be variously configured to include other energy storage devices, e.g., fuel cell, capacitor, etc., in addition to and/or instead of the described battery. Further, control systems may be used in each energy source individually, and/or across several energy sources, and/or distributed locally and/or remotely.

Power converter modules 202 may include bi-directional direct current (DC) to DC converters. The DC voltage across second terminals 262 of one power converter module 202 may be denoted as voltage V. Power converter modules 202 may convert (to a high efficiency) power V×I_Ref from second terminals 262 to a power V_C×I_Bat at first terminals 260 used to charge a battery 20 on first terminals 260 or to discharge battery 20 by converting power from battery 20 (I_Bat×V_Bat) on first terminals 260 to a power V×I_Ref on second terminals 262. The power V×I_Ref on second terminals 262 used to discharge a battery 20 provides (to a high efficiency) power onto electrical network 12 via DC/AC inverter 200. Serial string 350 may be connected in parallel across DC terminals W and X of inverter 200. AC terminals Y and Z of inverter 200 connect to electrical network 12 at nodes A and B.

Inverter 200 may be configured to be bi-directional, to convert alternating current (AC) power on AC terminals Y and Z to a DC power on DC terminals W and X for charging or convert DC power on DC terminals W and X to an AC power on terminals Y and Z for discharging batteries 20 and providing the power to electrical network 12. Inverter 200 may be controlled by and/or monitored by central controller 110, for instance by power line communications or by wireless communications.

In alternative embodiments of the embodiments, power converter module 202 may include a DC/AC bi-directional micro-inverter and then inverter 200 may not used. Where each power converter module 202 may be a bi-directional switching micro-inverter, string 350 connects directly across AC electrical network 12 at nodes A and B. String 350 connected across AC electrical network 12 at nodes A and B allows conversion of a portion of AC 12 to a DC voltage V_C which may be used to charge a respective battery 20. Conversely the DC voltage V_Bat of a battery 20 may be converted to a portion of AC 12, thereby discharging a respective battery 20.

In various embodiments of 14a, a first multiple of batteries 20 may form a first bank of batteries and a second multiple of batteries 20 may form a second bank that is similar or different from the first bank. The first bank and second bank may not be collocated, but may share the common reference I_ref.

Reference is now made to FIG. 3 which shows details of the direct current (DC) to DC converter 202 shown in FIG. 1, according to embodiments. Power converter module 202 may include a fuel gauge 302 attached to sensor 310 which may monitor the voltage and current on both the first terminals 260 of converter circuit 300. Fuel gauge 302 may be controlled by one or more controllers 306 (e.g., a microprocessor, ASIC, FPGA, linear feedback control system, CPU, logic, firmware, and/or other suitable device) according to an algorithm (e.g., an algorithm stored in memory 304), may further monitor; the temperature, internal resistance, state of charge and/or state of discharge of each respective battery 20. Batteries 20 may also be different types of batteries with respective battery characteristics stored in memory 304. The controller may control power converter 300 to charge and discharge battery 20 based on any suitable parameters such as sensor data, the stored battery characteristics, and/or dynamic feedback mechanisms.

Power converter circuit 300 may be variously configured. For example, it may be a DC-DC converter, and DC-AC converter, an AC-AC converter and/or an AC-DC converter. In some embodiments, power converter 300 is a DC-DC converter configured to include a buck stage followed by a boost stage or a boost stage followed by a buck stage. Further details of power converter circuit 300 may be now made by reference also to FIG. 3a which illustrates a buck plus boost converter 300 according to a feature of the embodiments. Buck plus boost converter 300 has a buck circuit 320. An inductor 328 and common rail 329 connect buck circuit 320 to boost circuit 322. Buck circuit 320 has a low side buck MOSFET GA, connected between common rail 329 and one side of inductor 328 and one side of a high side buck MOSFET GC, the other side of a high side buck MOSFET GC connects to one terminal of first terminals 260. A capacitor C₁ may be shunt connected across first terminals 260. Boost circuit 322 has a low side boost MOSFET GB, connected between common rail 329 and the other side of inductor 328 and one side of a high side boost MOSFET GD. The other side of a high side boost MOSFET GD connects to one terminal of second terminals 262. A capacitor C₂ may be shunt connected across second terminals 262. The symmetry of converter 300 may be such that depending on how MOSFETs GA, GB, GC and GD are driven and controlled, converter 300 may be a buck stage followed by a boost stage or a boost stage followed by a buck stage. In other variations, the buck circuit and boost circuit each have respective inductors in place of common inductor 328. Further variations may configure GA, GB, GC, and GD as switches (e.g., semiconductor switches). In various examples, switches GA, GB, GC, and GD are controlled by control circuitry that may include one or more programmable pulse width modulators, controllers, and other logic. In various embodiments, the control circuitry may be one or more separate devices, disposed locally and/or remotely, and/or may be integrated within controller 306.

Reference is now also made to FIG. 4 which shows a method 401, according to a feature of the embodiments. In step 403, batteries 20 may be connected to first terminals 260 of respective converter modules 202. Second terminals 262 may be connected together in series to form string 350. In other embodiments, step 403 may include the second power interfaces 262 of the power converter modules 202 being connected together in parallel as further described herein.

In step 405 current reference I_Ref, that may be the current in serial string 350 may be set to a programmed current by inverter 220. The current value of current reference I_Ref may be programmed within the control circuitry of inverter 220 via central controller 110. In step 407, a battery charge fraction may be determined for each battery 20 by fuel gauge 302, microprocessor 306 and stored in memory 304. The battery charge fraction may be defined herein by the fraction: the charge stored in battery 20 divided by the charge capacity of battery 20. The charge capacity of a battery in the first case when battery 20 is new is known and stored in memory 304 of fuel gauge 302. Monitoring by fuel gauge 302 in subsequent charging and discharging cycles of batteries 20, may allow the charge capacity and/or battery charge fraction to be updated in step 407 as well as providing further information for the setting current reference I_Ref in step 405 also.

In alternate embodiments, Battery charge fraction (BC), may be determined, for example, by integrating the discharge and charge current (I_Bat) over a duration to calculate the change of charge in the battery. The change in charge may then be subtracted from the charge capacity to determine remaining charge stored in the battery. Step 407 may further include updating the setting of current reference, I_Ref, based on the charge fraction and/or charge capacity.

In step 409, each battery 20 may be charged individually assuming that each power converter 300 may be substantially 100% efficient, with a charging power (P_charge):

$P_{\mathrm{charge}}=E\times C\times \left(1-\mathrm{BC}\right)=V\times I_{\mathrm{Ref}}C=\frac{V_C}{t_c}$

where C=charge factor of the battery 20 (volts per hours)

• • V_C=voltage at first terminals 260.
  • t_c=charging time.
  • V=voltage at second terminals 262.
  • E=charge storage capacity of a battery 20 (ampere-hours).
  • BC=battery charge fraction of the battery 20.
  • I_Ref=reference current through second terminals 262 of power converter 300
  • I_Bat=current charging battery 20.

Re-arranging and solving for charging time t_c

$t_c=\frac{E\times V_C\times \left(1-\mathrm{BC}\right)}{V\times I_{\mathrm{Ref}}}$

In the equation for t_c above it can be seen that according to a feature of the embodiments, if converters 300 maintain individually a voltage conversion ratio (V_c/V) proportional to the reciprocal of E·(1−BC), the amount of additional charge that may be stored in battery 20, then t_c may be fully determined by the current reference I_Ref. Consequently, for multiple batteries, charging may be completed for batteries 20 substantially at a common end time t_c which may be responsive to current reference I_Ref.

In step 411, each battery 20 may be discharged individually assuming that each converter 300 may be substantially 100% efficient, with a power (P_discharge):

$P_{\mathrm{discharge}}=E\times D\times \mathrm{BC}=V{I}_{\mathrm{Ref}}D=\frac{V_{\mathrm{Bat}}}{t_d}$

where D=discharge factor of the battery 20.

• • V_Bat=Voltage of battery 20.
  • t_d=discharging time
  • E=charge storage capacity of a battery 20 (ampere-hour)
  • BC=battery charge fraction of battery 20.
  • V=voltage between second terminals 262 of power converter 202.
  • I_ref=reference current through second terminals 262 of power converter 202.

Re-arranging and solving for charging time t_d

$t_d=\frac{E\times V_{\mathrm{Bat}}\times \mathrm{BC}}{V\times I_{\mathrm{Ref}}}$

In the equation for t_d above, it can be seen that according to a feature of the embodiments, if converters 202 maintain individually a voltage conversion ratio (V_bat/V) proportional to the reciprocal of E·BC, the remaining available charge in battery 20, then t_d may be fully determined by the current reference I_Ref. Consequently, for multiple batteries, discharging may be completed for batteries 20 substantially at a common end time td which may be responsive to current reference I_Ref.

In step 413, by virtue of each battery 20 being charged or discharged individually at respective first terminals 260 according to steps 409 or 411 respectively, batteries 20 are substantially fully charged or substantially fully discharged at substantially the same time.

A data history may be logged in memory 304 and include type of battery 20, the state of charge or discharge of respective batteries 20, along with a data of the present state of charge or discharge of respective batteries 20. Storage of the data history with the data for a battery 20 which may include an charge storage capacity, a charge stored, a charge factor, a battery charge fraction, a battery charge and a battery discharge factor of respective batteries 20. The charge storage capacity of a battery 20 may include the usable capacity of charge available from battery 20, the portion of battery 20 which may be empty and rechargeable and an unusable capacity which can no longer be recharged because of deterioration of battery 20 with usage over time. The battery charge fraction may be defined by the energy stored in battery 20 divided by the energy capacity of battery 20

In general, the charge or discharge current hat depends on amount of time to charge or discharge the batteries 20. The amount of time to charge or discharge the batteries 20 may be based on the common current reference (I_Ref) and/or an end time of charging or discharge for example which may be at six am in the morning for example. Depending on the time from when batteries 20 are desired to be charged or discharge, the amount of time to charge or discharge the batteries 20 may be determined based on the common current reference (I_Ref). If the time from when batteries 20 are desired to be charged or discharge is nine in the evening, then the amount of time to charge or discharge the batteries 20 is 9 hours. Alternatively a shorter amount of time for charging or discharging with the amount of time to charge or discharge the batteries 20 may be selected based on the common current reference (I_Ref). In general, the greater the level of the common current reference (I_Ref) means a shorter period of time to charge or discharge batteries 20. The initiation of the charging or discharging of batteries 20 may be controlled by central controller 110 to inverter 200 and converter 202. The level of the current reference (I_Ref) may be set by central controller 110 and controlled by inverter 200.

Reference is now made to FIG. 5 which shows an energy bank implementation 14b, according to a feature of the embodiments. Multiple batteries 20 are connected to first terminals 260 of respective power converters 202. The second terminals 262 of power converters 202 are connected in parallel to give a parallel connection. Where converters 202 are bidirectional DC to AC converters, the parallel connection may be made directly across AC 12 at nodes A and B. Where power converter modules 202 include bidirectional DC to DC converters 300, the parallel connection may be made directly across terminals W and X of a bidirectional inverter 200 and terminals Y and Z of inverter 200 are connected across AC 12 at nodes A and B.

The parallel connection gives serves as a common voltage reference (V_Ref) for converters 202. Power converters 202 may convert power from V_Ref×I_C to a power V_C×I_Bat used to charge a battery 20 on the first terminals 260 or convert power from battery 20 (I_Bat×V_Bat) on the first terminals 260 to a power I_C×V_Ref on the second terminals 262. The power I_C×V_Ref on the second terminals 262 used to discharge a battery 20 provides power onto electrical network 12 via inverter 200. Both inverter 200 and converters 202 include all of the features described in the description of FIG. 2 above.

Reference is now made again to method 401 applied to energy bank implementation 14b shown in FIG. 5, according to a feature of the embodiments.

Method 401 shows batteries 20 in an energy bank 14b may be charged or discharged according to a common voltage reference (V_Ref). By way of example, where five batteries 20 are to be charged, it is assumed that each of the five batteries 20 are of the same type with an open circuit voltage of 25 volts and charge/discharge rating of 10 ampere hours, AC 12 and therefore the common voltage reference (V_Ref) may be 240 volts root mean square (RMS) or 240 volts DC on each converter 202.

In step 403, batteries 20 are connected to first terminals 260 of respective converters 202. Second terminals 262 are connected together in parallel to the terminals W and X of inverter 200.

In step 405 voltage reference V_Ref may be set to be constant by inverter 220 via central controller 110 for the purpose of charging batteries 20. Consequently as a result of voltage reference V_Ref set constant, each current (I_C) through second terminals 262 of each converter 202 will be different because of respective usable capacity of energy available from a battery 20 at any point in time will be different. In step 407, a battery charge fraction may be determined for each battery 20. Monitoring by fuel gauge 302 in subsequent charging and discharging cycles of batteries 20, allows the battery charge fraction to be determined in step 407 as well as providing further information for the setting voltage reference V_Ref in step 405 also.

In step 409, each battery 20 may be charged individually, assuming that each converter 202 may be substantially 100% efficient, with a power (P_charge). The charging powers (P_charge) on each of first terminals 260 according to the equation above for P_charge are therefore responsive to the remaining energy desired to fully charge the respective battery 20 and responsive to voltage reference V_Ref.

In step 411, each battery 20 may be discharged individually assuming that each converter 202 may be substantially 100% efficient, with a power (P_discharge). The discharging powers (P_discharge) on each of first terminals 260 according to the equation above for P_discharge, are therefore responsive to the remaining energy desired to fully discharge the respective battery 20 and responsive to voltage reference V_Ref. The charging powers (P_charge) on each of first terminals 260 according to the equation above for P_charge are therefore responsive to the remaining energy desired to fully charge the respective battery 20 and responsive to voltage reference V_Ref.

In step 413, by virtue of each battery 20 being charged or discharged individually at respective first terminals 260 according to steps 409 or 411 respectively, batteries 20 are substantially fully charged or substantially fully discharged at substantially the same time.

The method 401 and system components described above balance and control charge/discharge of batteries 20 which can use different types of batteries with different capacities along with other types of energy storage in the same system. By using method 401, the system 10 controls the energy storage between and in energy banks 14. Within an energy bank 14, the energy may be spread in a balanced way that prevents part of an energy bank 14 to be partly empty when the other part still contains energy. In a similar way between energy banks 14, the energy may be spread in a balanced way that prevents one or more energy banks 14 to be partly empty when one or more energy banks 14 still contains energy. Therefore, batteries 14 are balanced so that if one battery 20 may be connected, after number of charges and discharges the battery 20 will get to the same charge fraction as the others. The situation that a battery 20 will be empty before another battery 20 or that a battery 20 may be full or empty before all the other batteries 20 are discharged or charged may be also avoided.

In still further embodiments, Battery charge fraction (BC), may be determined, for example, by integrating the discharge and charge current (I_Bat) over a duration to calculate the change of charge in the battery. The change in charge may then be subtracted from the charge capacity to determine remaining charge stored in the battery. Step 407 may further include updating the setting of current reference, I_Ref, based on the charge fraction and/or charge capacity. In step 409, each battery 20 may be charged individually and independently based on the current reference (I_Ref) set in step 405, and based on controlling the voltage conversion ratio (r=Vc/V) and charging time period (t_c). For example, the amount of charge (Qc) put into the battery is given by:

Qc=I _Bat *t _c   (eq. 1).

The charging current (I_Bat) can be determined from the efficiency of the power converter (e), and the input reference voltage as follows:

Pin=(V*I_Ref)=(e*P_charge)=e*(V_c*I_Bat)   (eq. 2).

Solving equation 2 for IBat results in:

I _Bat=(V*I_Ref)/(V_c*e)=(1/e)*(V/V_c)*I_Ref   (eq. 3).

Substituting I_Bat from equation 3 into equation 1 and replacing (V_c/V) with the voltage conversion ratio (r) controlled by controller 306 and/or controller 110 results in the following relationship.

Q=(1/e)*(1/r)*I_Ref*t_c   (eq. 4).

Thus, in various embodiments, the amount of charge (Q) into the battery may be controlled based on controlling (e.g., with controller 306 and/or controller 110) the voltage conversion ratio (r), the current reference (I_Ref) at the second power interface 262, and the charging time period (t_c). The voltage conversion ratio may, for example, be controlled in a buck plus boost converter by varying the duty cycle of the switching in the converter.

In certain variations, for the common reference I_Ref, step 409 may include charging each battery 20 independently by controlling conversion ratio (r) and the charging time period (t_c) to a percentage (p) of the remaining capacity (Q_c) in the battery, given by:

Q _c =E*(1−BC)=(1/e)*(1/r)*I_Ref*t_c   (eq. 5),

where:

• • E=charge storage capacity of battery 20 (ampere-hours).
  • BC=battery charge fraction of battery 20.

Various embodiments include each power converter 300 maintaining (e.g., with controller 306 and/or controller 110) individually and independently a respective voltage conversion ratio (r) proportional to the reciprocal of the remaining capacity (Q_c) in the respective battery 20:

r=1/(p*Q_c)=1/(p*E*(1−BC))   (eq. 6),

where:

• • p=the percentage of remaining capacity Q_c to charge over t_c.

By incorporating equation 6 into equation 5, we find that charge only depends on I_Ref in the following manner:

E*(1−BC)=(1/e)*(p*E*(1−BC))*I_Ref*t_c   (eq. 7),

which simplifies to:

t _c=(p/e)*(1/I_Ref)   (eq. 8).

Thus, in this embodiment, the charge time (t_c) for each battery 20 may be inversely proportional to reference current I_Ref in response to each power converter 300 being controlled individually an independently such that voltage conversion ratio of the respective power converter is inversely proportional the remaining capacity (Q_c) of the respective battery 20. In various embodiments in which the percentage (p) and converter efficiency (e) are the same or approximately the same across the multiple converters module 202 in string 350, the respective batteries 20 will be charged to full capacity (E) in the same time t_c, since all of the converters have the same reference I_Ref. Note that this is the case regardless of whether each battery has the same remaining capacity or a different remaining capacity compared to other batteries.

In these embodiments, the charge time may be the same regardless of whether each battery has the same remaining capacity or a different remaining capacity compared to other batteries, because the charging current I_Bat for each battery is variable and controlled based on the individual batteries remaining capacity.

Efficiency (e) may be estimated in some variations to be 100% in controlling the conversion ratio r. In other variations, actual efficiency may be determined for each power converter module 202 and stored in memory 304, in controller 306 or in controller 110. Controller 306 and controller 110 may control each power converter 300 based on the actual efficiency of each converter. Actual efficiency may be a predetermined value provided by a manufacture or may be determined, e.g., by testing. Actual efficiency may include multiple efficiency values for each converter depending on operating conditions, and control of each converter may be based on the multiple efficiency values at multiple respective operating conditions (e.g., buck mode, boost mode, duty cycles, temperature, current, voltage, duty cycle, etc.) Operating conditions may be measured values taken during charging and discharging operations.

In some variations, percentage (p) may be a value pre-set value from zero to one (i.e., 0% to 100%). In other variations, percentage (p) may be a value stored in each memory 304, or in a memory or register within controller 110. In certain variations, p may be set to have different predetermined values for one or more of the batteries 20 so that each battery is charged to predetermined different rates and levels. Percentage p may also be set to compensate for differences in converter efficiencies (e) (e.g., set each battery p/e to a common predetermined value) and differences in battery conditions (e.g., environmental conditions).

Reference is now made to FIG. 6 which shows an energy bank implementation 14c, according to an feature of the embodiments. The energy bank implementation 14c may be the same as energy bank implementation 14b shown in FIG. 5 except electrical connections to terminals of batteries 20 are replaced with electrical connections to terminals of energy storage devices 21. Similarly batteries 20 shown in FIG. 2 may also be replaced with energy storage devices 21. Energy storage devices 21 may be elector-mechanical devices such as a DC motor and/or generator for example. The DC motor and/or generator when acting as a motor may be capable of converting electrical energy available on the electrical connections of the motor to a mechanical energy. The mechanical energy may be used, for instance to pump water against gravity to a holding pool. The water from the holding pool may be released later to produce electricity when the energy storage device 21 serves as an electrical generator and/or turbine. The electrical generator and/or turbine, therefore converts the mechanical energy of water flow over the turbine to electricity, thereby depleting the stored energy in the holding pool. Other electro-mechanical devices for energy storage and/or energy depletion may include springs, weights, compressors to compress gas into a sealed tank, cavities or sealed underground caves for later release, fly wheels conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. Energy storage devices 21 may be electrochemical devices such as a fuel cell or a battery. Energy storage devices 21 may also be electrostatic devices such as a capacitor. Energy storage devices 21 may be a combined electro-thermal system which may use molten salt to store solar power and then dispatch that power as desired. The electro-thermal system pumps molten salt through a tower heated by the sun's rays and/or heat which may be taken away from a photovoltaic array too. The pumping of the molten salt may be via electricity from the photovoltaic array and/or a mains electricity electrical network. Insulated containers store the hot salt solution and when desired, water may then be added to the stored molten salt to create steam which may be fed to turbines to generate electricity.

Various embodiments may intermix and combine the different types of energy storage devices within an energy bank 14. Different energy banks 14 within system 10 may include different combinations of energy storage devices. In the embodiments described above with energy storage devices replacing batteries 20, the above description of battery charge fraction (BC) with respect to batteries is equivalent to an energy storage fraction of the energy storage device. That is, energy storage fraction is the fraction of energy stored in the energy storage device divided by the energy storage capacity of the energy storage device Further, charging and discharging with respect to battery 20 is equivalent to storing and depleting energy with respect to the energy storage device.

Reference is now made to FIG. 7 which shows a method 701 for energy storage, according to an feature of the embodiments. In the discussion of the method steps 703-713 of method 701 that follows, the term “energy storage” to an energy storage device 21 may be used. Steps 403-413 of FIG. 4, may considered as analogous or equivalent in principle to steps 703-713 where the energy storage fraction may be equivalent to a battery charge fraction and storing and depletion of energy may be equivalent to charging and discharging of batteries respectively.

In step 703, an operative connection between the energy storage devices 21 and respective power converters 202 may be made. The operative connection allows energy conversion by energy storage devices 21 (for example electrical energy to mechanical energy and vice versa) so that electrical energy may flow to and from converters 202.

In step 705 voltage reference V_Ref may be set constant (with respect to FIG. 6) or current reference I_Ref to be set constant (with respect to FIG. 2) by inverter 220 via central controller 110. Consequently as a result of voltage reference V_Ref or I_Ref set constant, each current through second terminals 262 of each converter 202 will be different because of respective usable capacity of energy available from an energy storage device 21 at any point in time will be different.

In step 707, an energy storage fraction may be determined for each energy storage device 21. The energy storage fraction may be energy stored in an energy storage 21 divided by the energy capacity of an energy storage 21. A circuitry like fuel gauge 302 and memory 304 attached to microprocessor 306 may be used to provide information with respect the energy storage capacity, monitoring and measuring of energy storage 21 and depletion of an energy storage 21. The circuitry may then control buck+plus converter 300 by maintaining a voltage conversion ratio of converter 300 based on the energy storage fraction.

In steps 709 and 711 for energy storage and energy depletion respectively, steps 709 and 711 are applied to FIG. 2 with energy storage devices 21 instead of batteries 20. In step 709, the power desired for energy storage in energy storage 21 may be expressed by the same equation used for the charging of batteries 20, described above. In step 709 energy may be stored in each energy storage 21 individually with multiple variable rates of energy storage through respective connections at first terminals 260.

Similarly, in step 711, the power desired for energy depletion from energy storage 21 may be expressed by the same equation used for the discharging of batteries 20, described above. In step 711 energy may be depleted from each energy storage 21 individually, with multiple variable rates of energy depleted through respective connections at first terminals 260.

In step 713, by virtue of each energy storage device 21 having energy stored or depleted individually at respective first terminals 260 according to steps 709 or 711 respectively, energy storage devices 21 have substantially a full amount of energy stored or substantially fully depleted at substantially the same time.

Although selected features of the embodiments have been shown and described, it is to be understood the embodiments are not limited to the described features. Instead, it is to be appreciated that changes may be made to these features without departing from the principles and spirit of the embodiments, the scope of which is defined by the claims and the equivalents thereof.

Claims

1. A method comprising: determining: a first state of charge (SOC) of a first energy storage device coupled to a first power converter, anda second SOC of a second energy storage device coupled to a second power converter; andcontrolling, based on the first and the second SOCs, the first and the second power converters to transfer first power between the first energy storage device and an electrical network and transfer second power between the second energy storage device and the electrical network such that the first energy storage device and the second energy storage device reach respective predetermined charge states at a common time.

2. The method of claim 1, wherein controlling the first and the second power converters further comprises: transferring the first power at a first rate and the second power at a second rate;controlling the first rate to be greater than the second rate based on the first SOC being higher than the second SOC;controlling the first rate to be less than the second rate based on the first SOC being lower than the second SOC; andcontrolling the first rate to be substantially equal to the second rate based on the first SOC being substantially equal to the second SOC.

3. The method of claim 1, wherein, in a discharging mode, controlling the first and the second power converters comprises transferring, to the electrical network, the first power from the first energy storage device and the second power from the second energy storage device.

4. The method of claim 1, wherein, in a charging mode, controlling the first and the second power converters comprises transferring, from the electrical network, the first power to the first energy storage device and the second power to the second energy storage device.

5. The method of claim 1, wherein controlling the first and the second power converters further comprises controlling a voltage conversion ratio of each of the first and the second power converters.

6. The method of claim 1, wherein a first output of the first power converter and a second output of the second power converter are connected in a serial string; and wherein controlling the first and the second power converters further comprises providing a common voltage reference to the first and second power converters.

7. The method of claim 6, wherein controlling the first and the second power converters further comprises regulating a total voltage across the serial string.

8. The method of claim 1, wherein a first output of the first power converter and a second output of the second power converter are connected in a parallel arrangement; and wherein controlling the first and the second power converters further comprises providing a common current reference to the first power converter and second power converter.

9. The method of claim 8, wherein controlling the first and the second power converters further comprises regulating a total current flowing through the parallel arrangement.

10. The method of claim 1, wherein controlling the first and the second power converters further comprises the first power converter or the second power converter being in a boost mode.

11. The method of claim 1, wherein controlling the first and the second power converters further comprises the first power converter or the second power converter being in a buck mode.

12. The method of claim 1, wherein controlling the first and the second power converters further comprises the first power converter or the second power converter being in a cascaded buck plus boost mode.

13. The method of claim 1, wherein the first power converter or the second power converter comprises a non-isolating power converter.

14. A system comprising: a first energy storage device;a second energy storage device;a first power converter coupled to the first energy storage device;a second power converter coupled to the second energy storage device; anda controller configured to, determine a first state of charge (SOC) of the first energy storage device and a second SOC of the second energy storage device; andcontrol, based on the first and the second SOCs, the first and the second power converters to transfer first power between the first energy storage device and an electrical network and transfer second power between the second energy storage device and the electrical network such that the first energy storage device and the second energy storage device reach respective predetermined charge states at a common time.

15. The system of claim 14, wherein the controller is configured to control the first and the second power converters by: transferring the first power at a first rate and the second power at a second rate;controlling the first rate to be greater than the second rate based on the first SOC being higher than the second SOC;controlling the first rate to be less than the second rate based on the first SOC being lower than the second SOC; andcontrolling the first rate to be substantially equal to the second rate based on the first SOC being substantially equal to the second SOC.

16. The system of claim 14, wherein the controller is configured to control the first and the second power converters by controlling a voltage conversion ratio of each of the first and the second power converters.

17. The system of claim 14, wherein a first output of the first power converter and a second output of the second power converter are connected in a serial string; and wherein the controller is configured to control the first and the second power converters by providing a common voltage reference to the first and second power converter.

18. The system of claim 17, wherein the controller is further configured to control the first and the second power converters by regulating a total voltage across the series string.

19. The system of claim 14, wherein a first output of the first power converter and a second output of the second power converter are connected in a parallel arrangement; and wherein the controller is configured to control the first and the second power converters by providing a common current reference to the first and second power converters.

20. The system of claim 19, wherein the controller is further configured to control the first and the second power converters by regulating a total current flowing through the parallel arrangement.