Hybrid Electrical Energy Storage Systems - Massoud … · Hybrid Electrical Energy Storage Systems....

Naehyuck Changand

Massoud Pedram

Seoul National UniversityUniversity of Southern California

Hybrid Electrical Energy Storage Systems

Hybrid Electrical Energy Storage Systems

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Outline

Electrical energy storage systemsEnergy storage elementsCharge management circuitsHybrid Electrical Energy Storage (HEES) systemsHEES optimization examples

Amortized system cost optimizationMaximum power transfer trackingRate capacity effect mitigation

Conclusions

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Electrical Energy Storage Systems

DefinitionAn electrical energy storage (EES) systems use various forms of energy such as chemical, kinetic, or potential energy to store energy that will later be converted to electricity

Electrical energy has higher grade of energy qualityConverting electrical energy to other types of energy is efficient and easy but the opposite is not trueElectrical energy has a greater capacity to feedback and control lower grades of energy quality

ExamplesSupercapacitor: electrical energyBattery: chemical energyFlywheel: kinetic energyCAES (compressed air energy storage): potential energy

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EES system functionalitiesPower supplyEnergy managementPower quality controlPeak shaving

Ranges of EES systemsPower supplies for portable electronics (a few Wh or smaller)Uninterruptible power supplies (hundreds of Wh)Grid-scale EES (GWh or larger)

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Conventional EES systems mostly employ a single type of EES elementFocus is on the EES element characteristicsTry to achieve higher performance by using a more efficient EES element

Conventional EES systems have a rather simple architectureCharge/discharge circuitSingle power pathStraightforward charge/discharge policy

EES element: a storage device such as a battery and a supercapacitorEES system: an integrated system including EES element, control electronics, and control policy

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Conventional EES Systems

ArchitectureA single type of EES elementA single power path from input, to the storage, and to output

The input and output may be tied together in some configurationsThe power path may be bidirectional in some cases

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BatteryPremium Power PowerBlock 150Zinc-Flow battery cells

Examples of EES Systems, I

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FlywheelSun Microsystems, ColoradoData center UPS’s

Examples of EES Systems, II

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Examples of EES Systems, III

CAES (compressed air energy storage)Iowa Stored Energy Park, Iowa (planned)

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Performance Metrics for EES systems

Cycle efficiency, cost per unit capacity, energy density, power capacity, cycle life, and environmental impact including end-of-life disposal costNo single type of EES element can simultaneously fulfill all the desired characteristics of a modern EES system

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Performance Metrics

Capital costCost per unit of delivered energy ($/kWh) or per unit of output power ($/kW)Initial investment for EES system installationLower capital cost is generally against to other performance metrics

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Performance Metrics, cont’d

Charging efficiencyRatio of energy delivered from the energy source to the EES element

Discharging efficiencyRatio of energy delivered from the EES element to the load

Cycle efficiencyProduct of charging and discharging efficienciesSignificantly affected by charging/discharging profiles and ambient conditionsExamples includes rate capacity effect and charge recovery effect

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Cycle lifeThe capacity or SOH (state of health) tends to gradually deteriorate due to irreversible physical and chemical changesDefined as the number of cycles an EES element can perform before its capacity drops to a specific percentage (60-80% typically) of its initial full-charged capacityStrongly dependent on the depth of discharge (DoD) before that element is rechargedLonger cycle life EES elements

Electrical, mechanical or thermal technologiesSupercapacitor, flywheel, thermal energy storage (TES) and cryogenic energy storage (CES), etc.

Shorter cycle life of batteriesDue to unavoidable chemical deteriorations

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Power and energy densityRated output power divided by the volume (or mass) of the EES elementEnergy density is the stored energy divided by the volume (or mass)Metal-Air batteries have the highest energy density (1-10kWh/kg)Ordinary batteries, TES and CES have medium energy densitySupercapacitor and flywheel have the lowest energy density but the highest power densityA HEES system can take advantage of the energy density of a primary source for long operational time and the power density of a secondary source for power requirements

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Self-discharge rateHow quickly an EES element will lose its energy when it simply sits on the shelfDetermined by the inner structure and chemistry, as well as ambient temperature and humidityOrdinary batteries can store energy reliably up to tens of daysLithium primary batteries can be stored 10 years on the shelfFlow batteries have very small self-discharge rate and thus are suitable for long-term energy storageSupercapacitors have large self-discharge rate and are useful for short-term energy storage for a maximum of several hours or a couple of days

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EES Elements: Commercial Off-the-Shelf

Lead-acid batteryShort cycle life (500-800 cycles)Low energy density (30-50 Wh/kg)Poor low temperature performance and thus a thermal management system is requiredRelatively high power density (75-300 W/kg), able to supply high surge currentsMedium energy efficiency (70-90%)Low cost (100-200 $/kWh)Suitable in motor vehicles to provide the high current demand for automobile starter motorsUsed in large-scale commercial energy management systems

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EES Elements, cont’d

NiCd batteryRelatively high energy density (40-60 Wh/kg or 50-150 Wh/L)Moderate cycle life (as high as 2,000 cycles)Relatively high self-discharge rate (10% per month)High cost (800-1,000 $/kWh)Undesirable environmental effect due to the use of toxic heavy metal

Specific energy 40–60 Wh/kg

Volumetric energy density 50–150 Wh/L

Specific power 150 W/kg

Charge/discharge efficiency 70%–90%

Self-discharge rate 10%/month

Cycle durability 2,000 cycles

Nominal cell voltage 1.2 V

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NiMH batteryEnergy density (60-80 Wh/kg) is twice as large as lead-acid batteries and 40% higher than that of NiCd batteriesCheaper to purchase than Li-ion batteriesSuffer from the memory effect, although much less pronounced than that in the NiCd batteriesRelatively high self-discharge rate (0.5-1% per day)High power density (250-1,000 W/kg)Due to the high power density, these batteries are widely used in high current drain consumer electronics, such as digital cameras with LCDs and flashlightsWidely used in hybrid electric vehicles such as the Toyota Prius, Honda Insight, Ford Escape Hybrid, etc.

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Li-ion batteryHigh energy density (100-250 Wh/kg or 250-360 Wh/L)High efficiency (almost 100%)Long cycle life (as high as 10,000 cycles)No memory effectLow self-discharge rate (0.1-0.3% per day)High cost (above 600 $/kWh)In many portable electronic devices and growing in military, electric vehicle, and aerospace applications

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SupercapacitorHigh cycle efficiency (almost 100%) and long cycle life (up to 1,000,000 cycles)Significantly high volumetric power density (above 100,000 W/kg) but low energy density (2.5-15 Wh/kg)Large self-discharge rate compared with ordinary batteries

About 20% energy may be lost per day

Suitable for energy storage with frequent charging/discharging cycles or periodic high current pulses. May be used in battery-supercapacitor hybrid systemsTerminal voltage variation whereby the terminal voltage is linearly proportional to its SOC (state of charge)

May affect conversion efficiency in the power converters connected to the supercapacitors

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Metal-Air batteryThe anode is made of pure metal and cathode is connected to an inexhaustible source of airThe highest energy density (theoretically more than 10 kWh/kg) and are relatively inexpensive to produceEnvironmentally friendlyCurrently suffer from the low efficiency of the required electrical recharging process

Many manufacturers offer refuel units where the consumed metal is replaced mechanically

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Emerging technologiesCompressed air energy storage

Medium efficiency (70%)

Cryogenic energy storage (CES)No environmental burden (because of the use of liquid nitrogen or liquid air)Relatively high energy density (150-250 Wh/kg)Low capital cost and relatively long storage periodLow efficiency currently (40-50%)

http://www.matternetwork.com

http://www.engineering.leeds.ac.uk

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Performance Comparison of EES Elements

EES Elements Capital cost ($/kWh) Cycle efficiency Cycle life

Lead-acid 100-200 70-90% 500-800

NiCd battery 800-1,000 70-90% 2,000

NiMH battery 450-1,000 66% 500-1,000

Li-ion battery 600-2,500 >90% 1,000-10,000

NaS battery 300-500 87% 2,500

Metal-Air battery 10-60 <50% 100-300

Supercapacitor 20,000-50,000 >90% 500,000-1,000,000

Flywheel 1,000-5,000 >90% 20,000+

High-temperature TES 30-60 30-60% -

CES 3-30 40-50% -

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Performance Comparison, cont’d

EES Elements Self-discharge per day Energy density Power density

Lead-acid 0.1-0.3% 30-50 Wh/kg 75-300 W/kg

NiCd battery 0.2-0.6% 40-60 Wh/kg 150-300 W/kg

NiMH battery 0.5-1% 60-80 Wh/kg 250-1,000 W/kg

Li-ion battery 0.1-0.3% 100-250 Wh/kg 250-340 W/kg

NaS battery ~20% 150-240 Wh/kg 150-230 W/kg

Metal-Air battery Very small 1-10 kWh/kg -

Supercapacitor 20-40% 2.5-15 Wh/kg 100,000+ W/kg

Flywheel 100% 10-30 Wh/kg 400-1,500 W/kg

High-temperature TES 0.05-1% 80-200 Wh/kg -

CES 0.5-1% 150-250 Wh/kg 10-30 W/kg

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Life time of each EES element

Total energy cost per day

Total amortized cost per daySum of the purchase price, setup/disposal costs, and energy usage cost per day

EES System: Amortized Cost

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Switching Mode DC-DC Converter

Buck (step down) converterLower the output voltage compared to the input voltageLC filtering of the switching output Average output voltage is determined by the power switch’s turn-on duty ratio

Boost (step up) converterIncrease the output voltage compared to the input voltageAdd the energy stored in the inductor capacitorAverage output voltage is determined by the switch turn-off duty ratio

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DC-DC Converter, cont’d

Inverted Buck-Boost converterInverted outputLow efficiency due to presence of the diodeSimple and easy to control

4-switch Buck-boost converterNon-inverted outputHigh efficiency Complex controlSynchronous switches replace the freewheeling diodeSelectively operated as a buck or a boost converter according to the input and output voltages

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Constant Current Charger

Linear chargerCurrent regulator using power MOSFET in saturation modePrecise current control without noiseLow efficiency due to the IR loss in the MOSFET

Switching mode chargerCurrent feedback buck-boost converterProvide basic charging protocol and protection featuresSwitching noise70-80 % conversion efficiency

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Constant current charger using high efficiency buck-boost converter moduleLinear technology LTM4607 buck-boost converter micro module

4-switch buck-boost topologyOn-chip power switchesVery high conversion efficiency: ~98%

Charger, cont’d

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Charger, cont’d

Constant current charger using high efficiency buck-boost converter moduleVoltage and current feedback loop modificationsSafe and efficient charging operation

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Charge Management Circuits

Battery/suepercapacitor chargerPower converter specialized for charging batteries or supercapacitorsCharge management for various cell configurations and multiple chemistryCombined charging schemes (e.g., constant voltage + constant current) for reliability and efficiency

Charger efficiency issueGenerally, batteries are charged by the AC power, and so conversion efficiency has not been a critical issueWhen a renewable power source such as a solar cell or fuel cell is used, power conversion efficiency should be taken into consideration

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Charge Management Circuits: Summary

Bridge diode AC-DC rectifierMake DC to AC with diode bridge

Voltage regulatorZener-diode regulator

Regulate the voltage using breakdown voltage

Voltage regulator using OP-ampRegulate the voltage using the feedback loop

Three-terminal voltage regulatorWidely used in common DC power supplying circuit

Switching mode DC-DC converterBuck DC-DC converter

Step down the voltage according to the switching duty cycle, Dt

Boost DC-DC converterStep up the voltage according to the switching duty cycle, (1- Dt)

H-bridge DC-AC inverterMake DC to AC by alternatively switching the H-bridge

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Charge Management Circuits, cont’d

Power converters and efficiencyEES systems involve AC-DC and DC-AC conversionsLow conversion efficiency directly results in a high operational costDesigning the power converters should consider operating cost (efficiency) as well as the implementation costThe energy efficiency of the power converters are affected by

Design such as switch resistance, inductor and transformer performanceOperational conditions such as input voltage, output voltage, load current, switching frequencyEnergy efficiency of an EES element is often measured under the best condition

1/25 C discharge rateBest temperature conditionWith brand new batteries

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Sources of power loss in a switching regulatorESR (Equivalent Series Resistance) of each component

I2R power dissipation in switch, inductor and etc.Dependent on the load current

Parasitic capacitances of MOSFET switching gatePower dissipation to drive two switchesIndependent of load current

Control circuitryPower dissipation for PWM control and miscellaneous circuitsIndependent of load current

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Conversion efficiency of a DC-DC converter by a power MOSFET configuration

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Power efficiency of LTC3414 and TPC62320

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Switching charger power dissipationSum of the conduction, switching, and controller power losses

Conduction lossIR loss in the MOSFET switches, inductor, and ESR of the bulk capacitorProportional to the equivalent resistance of the components and output current

Switching lossMOSFET switch gate drive lossProportional to the input voltage, switching frequency, and gate capacitance

Controller lossStatic power dissipated by the switch controller

General characteristicsThe charger efficiency decreases when the input and output voltage difference is largeBoost converters are less efficient than buck converters

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Switching chargersSwitching chargers transfer energy from a power source to an EES elementThe charging efficiency is not constant, but depends on the input voltage, output voltage, and output currentExample: efficiency and power consumption of a Linear Technology LTC3531


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Charging efficiency variation in supercapacitor chargingCharging efficiency variation should be considered when the input/output voltages are not constant

Example: input from solar cell and output to supercapacitor

Conventional MPPT (maximum power point tracking) techniques are not necessarily energy efficient here

Maximizing the charger input power may not maximize the charger output current


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Monitoring and controlBMS (battery management system)

Maintains stability and sources of important information for the charge management policiesEstimates the SOC and SOH, controls the voltage, current, and cell balances (distributing SOC evenly across the bank), and provides diagnostics functionality

CAN, FlexRay network for monitoringSCADA (supervisory control and data acquisition) system for HCI

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Hybrid Electrical Energy Storage (HEES) Systems

MotivationComputer memory hierarchy

No single type of memory, device can achieve short access time, high density, balanced read/write performance, low cost per bit, low power consumption, non-volatility, etc.

Concept of hybrid EES systemsEach EES element has different characteristics such as cycle efficiency, leakage current, cycle life, storage cost, and volumetric energy density, power rating, and so onNo single type of EES element can simultaneously fulfill all the desired characteristicsExploit the advantages of each EES element and hide its disadvantagesHybrid EES systems consist of both low unit cost (e.g., lead-acid batteries) and high unit cost (e.g., supercapacitors) EES elementsShould be minimized by allocating the amounts of low-cost vs. high-cost EES elements while meeting other performance constraintsDifferent types of energy storage elements can be organized in an appropriate storage hierarchy and reconfigured on the fly

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HEES System Architecture

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HEES System Architecture, cont’d

To provides enhanced cycle efficiency, extended cycle life for each element, increased energy storage capacity, and output power rating

Heterogeneous EES elementsGeneralized power migration busesHigh-efficiency power convertersOptimal charge management policies

Logically hierarchical, physically flat structure to shorten the charge transfer pathsThe architecture is closer to scratch pad memory rather than cache memory in that:

Both allocation and replacement policies are requiredFlat physical structure (cache is hierarchical)

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HEES System Architecture, cont’d

Charge migration path can be a multiple power bus or a crossbar networkAllows optimal simultaneous migration operations

The optimal power bus voltage for each migration is a function of the EES elements and their SOCA shared power bus requires the same voltage level of all the power converters for simultaneous migration operations

Reconfigurability for enhanced cycle efficiencyEach EES bank consists of multiple homogeneous EES elements to meet certain voltage, power and capacity requirements maintaining balancing

Charging/discharging efficiency is a strong function of SOC as well as voltage and current levelsA fixed array structure cannot always guarantee desirable cycle efficiencyFor example, a supercapacitor bank may have very low terminal voltage with low SOC

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HEES System: Charge Allocation (charging)

Charge allocation problem is to determine the destination EES banks that maximize the charging efficiencyCharging efficiency is dependent on

Type and SOC of the bankVoltage and current characteristics of the power sourceDegree of MPTT (maximum power transfer tracking) for the power source, and so forth

The most efficient EES bank changes over time as SOC changes during charging When the SOC of the destination EES has a significant change during the charging process, reconfiguration of the internal connections may be desirable

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HEES System: Charge Replacement (discharge)

Charge replacement is to determine the most efficient EES banks which are capable of supplying electrical energy to a given load demandDischarging efficiency is dependent on

Rate capacity effectPower ratingTerminal voltage, and so on

The most efficient EES bank changes over time as SOC changes during discharging

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HEES System: Charge Migration

Each EES element has different self-discharge rate

Long-term energy storage necessitates charge migration among various EES banks

Only a part of EES banks can accommodate high current charge/discharge demand

Such banks may have very high self-discharge rates - supercapacitor banksRemaining charge in the high-leakage bank may be transferred to low-leakage banks for long-term storage

Charge migration is expensive because of charge loss during both discharge and recharge processes

Should be carefully performed by predicting the future load demand as well as energy supplied by the external power source

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HEES System: Joint Charge Optimization

Charge allocation, replacement and migration cannot be optimized separately because of their sometimes conflicting natures as well as the strong couplings among them

For example charge migration may become inefficient in a system designed for optimal charge allocation

Thus the joint optimization of allocation, replacement and migration is a challenging, yet critical, problem

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Amortized system cost optimization in a HEES system for large-scale energy storageMaximum power transfer tracking in a solar energy harvesting systemRate capacity effect mitigation in a system supporting widely varying load currents

Examples of Optimizing HEES Systems

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Amortized cost optimization exampleDaily and weekly energy usage profiles

Example I: Amortized System Cost Optimization

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Amortized cost optimization exampleAssumptions

Installation and disposal costs are included in the cost per capacityEnergy cost is 10 ¢/kWh

Two energy storage elements


Criteria Battery Supercapacitor

Cyclelife 2,000 100,000

Cost per capacity ($/kWh) 1,000 40,000

Cycle efficiency 80% 100%

Self discharge rate (%/day) 0.1% 15%

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Amortized cost optimization exampleConventional EES approaches

Battery-only EESSuffer from low cycle efficiency and short cycle life

Supercapacitor-only EESNot suitable for long-term storage

HEES approachUse batteries for weekly energy storage

Large capacitance: low cost-per-capacity requiredLong storage duration: low self-discharge rate required

Use supercapacitors for daily energy storageFrequent charge/discharge: high cycle efficiency and long cycle life required


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Amortized cost optimization example

Low storage cost due to battery cycle life enhancement using supercapacitors as daily energy storageLow energy cost due to high cycle efficiency of supercapaictors


Cost ($/week) Battery-only EES Supercapacitor-only EES Hybrid EES

Storage cost 160.3 181.6 153.3

Energy cost 40.1 45.4 38.3

Total cost 200.4 227.0 191.6

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Solar energy harvesting is a promising method for self-sustainable systemsHarvested energy is necessary to be stored in an EES system for stable power supply

Continuous operation in cloudy weather or at nightA rechargeable battery and/or supercapacitor is usedEnergy transfer involves energy loss

Example II: Solar Energy Harvesting System

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Optimal solar energy harvesting systemMaximizing the harvested energy at the minimum costEnergy conversion efficiency should be maximized

Charger conversion efficiencySignificantly affected by the state of the energy source (PV module) and energy storage (supercapacitor)The environment and system state change over time, and maximizing the system-wide efficiency is non-trivialPrevious battery-based energy harvesting techniques may not be efficient for supercapacitors

No attempt to find the system-wide optimal design considering the energy source and storage for supercapacitors

For a given energy harvesting requirement, the minimum size of a PV cell array and the minimum capacity of a battery are typically considered

Solar Energy Harvesting Example, cont’d

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Comparison between batteries and supercapacitors (revisit)Supercapacitors are suitable for frequently charged and discharged energy storageSignificant voltage change, which may result in poor efficiency, should be overcome


Battery Metric Supercapacitor

High Energy density Low

Low Power density High

Low Cycle life High

Low Cycle efficiency High

Small Self-discharge rate Large

Large Negative impact on environment Small

Low Cost High

Small Voltage change Large

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Maximum power point tracking (MPPT) techniqueMaximizes the output power from the PV moduleDoes not guarantee the maximum energy accumulationCharger output power by the MPPT technique may be less than that by the MPTT technique

Three design considerationsPV cell configurationCharger selectionSupercapacitor configuration


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PV module configuration: voltage and current characteristics depend on series and parallel configuration

More PV cells in series: higher voltageMore PV cells in parallel: higher current

The minimum number (cost-optimal) of cells and their energy-optimal configuration should be derived


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MPTT problem statement and solutionGiven

Energy requirementSolar irradiance profileCharger efficiency dataUnit PV cell characteristic

FindOptimal PV module configuration (number of connected cells)Optimal supercapacitor capacitance

SolutionApproximate harvested energy for given configurationIteratively increase the number of PV cells

Charger design may also be optimization object to increase the energy efficiency (e.g., power switch, switching frequency, etc.)


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Charger selectionCharger maximum output and current ratingIR loss and gate drive loss

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Supercapacitor capacitance considerationSmaller capacitance: higher voltageLarger capacitance: lower voltage

Energy-optimal capacitance should be derivedCost of the supercapacitance is determined by the given energy requirement, not its capacitance


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Maximum charge power surfaceThe MPTT method maximizes the power into the supercapacitor at all timesThe maximum power transferred into the supercapacitor is a function of the irradiance level and the supercapacitor voltage


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Example III: High Efficiency for Variable Loads

Discharge with different constant current Discharge with constant and pulsed current

Rate capacity effect in batteriesUndeliverable charge according to the battery operating condition

Energy impactPower loss in the battery caused by battery internal resistance

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Variable Load Support Example, cont’d

A typical load profile for military radio systemsAn EES system may be designed to be capable of handling a (variable) load of average power between 5 and 50 watts, which may also contain significant (up 100-watts) transient spikesThe EES system must handle both positive and negative transient load power pulses

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Parallel connectionConnect the battery and supercapacitor in parallelStraightforward solutionFiltering voltage fluctuationGodfrey Sikha and Branko N. Popov, Performance optimization of a battery–capacitor hybrid system, Journal of Power Sources 134 (2004) 130–138

Voltage and current responseParallel connection

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Constant-current charger-based hybrid architectureConstant-current charger

Separate the battery current from the load current Widely used for battery and supercapacitor managementSimple circuit implementation

Rate-capacity effect reductionMitigate current fluctuation

Converter efficiency Supercapacitor terminal voltage should be maintained in a high-efficiency range

Constant current hybridVoltage and current response

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Energy density of the storage elementsLi-ion batteries: 250 ~ 300 Wh/LSupercapacitor: 10 ~ 20 Wh/ L

Conversion efficiencyDepends on input/output voltage and current

Supercapacitor capacitances Small capacitance results in large supercapacitor voltage variation and low charger/regulator efficiencyLarge capacitance increases volume of the system

Parallel connection

Constant current hybrid

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Controller board

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BatteryLi-ion GP2N1051 cell2SP1 pack8.4 V 350 mAh

SupercapacitorNessCap ESHSR-0010C04S array2.5F, 10.8V

7.7 % deliverable energy gain

Parallel connection

Constant current hybrid

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Voltage and current waveformThe HEES system architecture shows smaller current fluctuation compared with the parallel connection

Parallel connection Constant current hybrid

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Conclusions

Initial work on hybrid electrical energy storage (EES) systems by using a combination of various EES elements

No single EES element can fulfill all requirements of a modern EES system

Architectural consideration of the hybrid EES system based on the computer memory hierarchy (scratch pad memory) concept

Setup the concept of key EES management operationsCharge allocation, replacement, and migration

HEES optimization examplesPeak shavingMaximum power transfer trackingRate capacity mitigation

Future workSystematic (mathematical) approaches for holistic optimization of a HEES system

Simultaneous optimization of charge allocation, replacement and migration processes

Hybrid Electrical Energy Storage Systems - Massoud … · Hybrid Electrical Energy Storage Systems....

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