Hybrid Electrical Energy Storage
Systems
Naehyuck Chang
and
Massoud Pedram
Seoul National University
University of Southern California
Outline
Electrical energy storage systems
Energy storage elements
Charge management circuits
Hybrid Electrical Energy Storage (HEES) systems
HEES optimization examples
Amortized system cost optimization
Maximum power transfer tracking
Rate capacity effect mitigation
Conclusions
2
Electrical Energy Storage Systems
Definition
An 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 quality
Converting electrical energy to other types of energy is efficient and easy but the
opposite is not true
Electrical energy has a greater capacity to feedback and control lower grades of
energy quality
Examples
Supercapacitor: electrical energy
Battery: chemical energy
Flywheel: kinetic energy
CAES (compressed air energy storage): potential energy
3
Electrical Energy Storage Systems
EES system functionalities
Power supply
Energy management
Power quality control
Peak shaving
Ranges of EES systems
Power supplies for portable electronics (a few Wh or smaller)
Uninterruptible power supplies (hundreds of Wh)
Grid-scale EES (GWh or larger)
4
Electrical Energy Storage Systems
Conventional EES systems mostly employ a single type of EES element
Focus is on the EES element characteristics
Try to achieve higher performance by using a more efficient EES element
Conventional EES systems have a rather simple architecture
Charge/discharge circuit
Single power path
Straightforward charge/discharge policy
EES element: a storage device such as a battery and a supercapacitor
EES system: an integrated system including EES element, control electronics, and control policy
5
Conventional EES Systems
Architecture
A single type of EES element
A single power path from input, to the storage, and to output
The input and output may be tied together in some configurations
The power path may be bidirectional in some cases
6
Examples of EES Systems, I
Battery
Premium Power PowerBlock 150
Zinc-Flow battery cells
7
Examples of EES Systems, II
Flywheel
Sun Microsystems, Colorado
Data center UPS’s
8
Examples of EES Systems, III
CAES (compressed air energy storage)
Iowa Stored Energy Park, Iowa (planned)
9
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 cost
No single type of EES element can simultaneously fulfill all the desired
characteristics of a modern EES system
10
Performance Metrics
Capital cost
Cost per unit of delivered energy ($/kWh) or per unit of output power ($/kW)
Initial investment for EES system installation
Lower capital cost is generally against to other performance metrics
11
Performance Metrics, cont’d
Charging efficiency
Ratio of energy delivered from the energy source to the EES element
Discharging efficiency
Ratio of energy delivered from the EES element to the load
Cycle efficiency
Product of charging and discharging efficiencies
Significantly affected by charging/discharging profiles and ambient conditions
Examples includes rate capacity effect and charge recovery effect
12
Performance Metrics, cont’d
Cycle life
The capacity or SOH (state of health) tends to gradually deteriorate due to
irreversible physical and chemical changes
Defined 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 capacity
Strongly dependent on the depth of discharge (DoD) before that element is
recharged
Longer cycle life EES elements
Electrical, mechanical or thermal technologies
Supercapacitor, flywheel, thermal energy storage (TES) and cryogenic energy storage (CES),
etc.
Shorter cycle life of batteries
Due to unavoidable chemical deteriorations
13
Performance Metrics, cont’d
Power and energy density
Rated output power divided by the volume (or mass) of the EES element
Energy 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 density
Supercapacitor and flywheel have the lowest energy density but the highest power
density
A 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
14
Performance Metrics, cont’d
Self-discharge rate
How quickly an EES element will lose its energy when it simply sits on the shelf
Determined by the inner structure and chemistry, as well as ambient temperature
and humidity
Ordinary batteries can store energy reliably up to tens of days
Lithium primary batteries can be stored 10 years on the shelf
Flow batteries have very small self-discharge rate and thus are suitable for long-term
energy storage
Supercapacitors have large self-discharge rate and are useful for short-term energy
storage for a maximum of several hours or a couple of days
15
EES Elements: Commercial Off-the-Shelf
Lead-acid battery
Short cycle life (500-800 cycles)
Low energy density (30-50 Wh/kg)
Poor low temperature performance and thus a thermal management system is
required
Relatively high power density (75-300 W/kg), able to supply high surge currents
Medium energy efficiency (70-90%)
Low cost (100-200 $/kWh)
Suitable in motor vehicles to provide the high current demand for automobile starter
motors
Used in large-scale commercial energy management systems
16
EES Elements, cont’d
NiCd battery
Relatively 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
17
EES Elements, cont’d
NiMH battery
Energy density (60-80 Wh/kg) is twice as large as lead-acid batteries and 40% higher
than that of NiCd batteries
Cheaper to purchase than Li-ion batteries
Suffer from the memory effect, although much less pronounced than that in the NiCd
batteries
Relatively 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 flashlights
Widely used in hybrid electric vehicles such as the Toyota Prius, Honda Insight, Ford
Escape Hybrid, etc.
18
EES Elements, cont’d
Li-ion battery
High 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 effect
Low self-discharge rate (0.1-0.3% per day)
High cost (above 600 $/kWh)
In many portable electronic devices and growing in military, electric vehicle, and
aerospace applications
19
EES Elements, cont’d
Supercapacitor
High cycle efficiency (almost 100%) and long cycle life (up to 1,000,000 cycles)
Significantly high volumetric power density (above 100,000 W/kg) but low energy
density (2.5-15 Wh/kg)
Large self-discharge rate compared with ordinary batteries
About 20% energy may be lost per day
Suitable for energy storage with frequent charging/discharging cycles or periodic high
current pulses. May be used in battery-supercapacitor hybrid systems
Terminal voltage variation whereby the terminal voltage is linearly proportional to its
SOC (state of charge)
May affect conversion efficiency in the power converters connected to the supercapacitors
20
EES Elements, cont’d
Metal-Air battery
The anode is made of pure metal and cathode is connected to an inexhaustible
source of air
The highest energy density (theoretically more than 10 kWh/kg) and are relatively
inexpensive to produce
Environmentally friendly
Currently suffer from the low efficiency of the required electrical recharging process
Many manufacturers offer refuel units where the consumed metal is replaced mechanically
21
EES Elements, cont’d
Emerging technologies
Compressed air energy storage
Medium efficiency (70%)
Cryogenic energy storage (CES)
No environmental burden (because of the use of liquid nitrogen or liquid air)
Relatively high energy density (150-250 Wh/kg)
Low capital cost and relatively long storage period
Low efficiency currently (40-50%)
http://www.engineering.leeds.ac.uk
http://www.matternetwork.com
22
Performance Comparison of EES Elements
EES Elements
Lead-acid
Capital cost ($/kWh)
Cycle efficiency
Cycle life
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
300-500
87%
2,500
10-60
<50%
100-300
20,000-50,000
>90%
500,000-1,000,000
1,000-5,000
>90%
20,000+
High-temperature TES
30-60
30-60%
-
CES
3-30
40-50%
-
NaS battery
Metal-Air battery
Supercapacitor
Flywheel
23
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
~20%
150-240 Wh/kg
150-230 W/kg
Very small
1-10 kWh/kg
-
20-40%
2.5-15 Wh/kg
100,000+ W/kg
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
NaS battery
Metal-Air battery
Supercapacitor
Flywheel
24
EES System: Amortized Cost
Life time of each EES element
Total energy cost per day
Total amortized cost per day
Sum of the purchase price, setup/disposal costs, and energy usage cost per day
25
Switching Mode DC-DC Converter
Buck (step down) converter
Lower the output voltage
compared to the input voltage
LC filtering of the switching
output
Average output voltage is
determined by the power switch’s
turn-on duty ratio
Boost (step up) converter
Increase the output voltage
compared to the input voltage
Add the energy stored in the
inductor capacitor
Average output voltage is
determined by the switch turn-off
duty ratio
26
DC-DC Converter, cont’d
Inverted Buck-Boost converter
Inverted output
Low efficiency due to presence of
the diode
Simple and easy to control
4-switch Buck-boost converter
Non-inverted output
High efficiency
Complex control
Synchronous switches replace the
freewheeling diode
Selectively operated as a buck or a
boost converter according to the
input and output voltages
27
Constant Current Charger
Linear charger
Current regulator using power
MOSFET in saturation mode
Precise current control without
noise
Low efficiency due to the IR loss
in the MOSFET
Switching mode charger
Current feedback buck-boost
converter
Provide basic charging protocol
and protection features
Switching noise
70-80 % conversion efficiency
28
Charger, cont’d
Constant current charger using high efficiency buck-boost converter module
Linear technology LTM4607 buck-boost converter micro module
4-switch buck-boost topology
On-chip power switches
Very high conversion efficiency: ~98%
29
Charger, cont’d
Constant current charger using high efficiency buck-boost converter module
Voltage and current feedback loop modifications
Safe and efficient charging operation
30
Charge Management Circuits
Battery/suepercapacitor charger
Power converter specialized for charging batteries or supercapacitors
Charge management for various cell configurations and multiple chemistry
Combined charging schemes (e.g., constant voltage + constant current) for reliability
and efficiency
Charger efficiency issue
Generally, batteries are charged by the AC power, and so conversion efficiency has
not been a critical issue
When a renewable power source such as a solar cell or fuel cell is used, power
conversion efficiency should be taken into consideration
31
Charge Management Circuits: Summary
Bridge diode AC-DC rectifier
Make DC to AC with diode bridge
Voltage regulator
Zener-diode regulator
Regulate the voltage using breakdown voltage
Voltage regulator using OP-amp
Regulate the voltage using the feedback loop
Three-terminal voltage regulator
Widely used in common DC power supplying circuit
Switching mode DC-DC converter
Buck DC-DC converter
Step down the voltage according to the switching duty cycle, Dt
Boost DC-DC converter
Step up the voltage according to the switching duty cycle, (1- Dt)
H-bridge DC-AC inverter
Make DC to AC by alternatively switching the H-bridge
32
Charge Management Circuits, cont’d
Power converters and efficiency
EES systems involve AC-DC and DC-AC conversions
Low conversion efficiency directly results in a high operational cost
Designing the power converters should consider operating cost (efficiency) as well as
the implementation cost
The energy efficiency of the power converters are affected by
Design such as switch resistance, inductor and transformer performance
Operational conditions such as input voltage, output voltage, load current, switching
frequency
Energy efficiency of an EES element is often measured under the best condition
1/25 C discharge rate
Best temperature condition
With brand new batteries
33
Charge Management Circuits, cont’d
Sources of power loss in a switching regulator
ESR (Equivalent Series Resistance) of each component
I2R power dissipation in switch, inductor and etc.
Dependent on the load current
Parasitic capacitances of MOSFET switching gate
Power dissipation to drive two switches
Independent of load current
Control circuitry
Power dissipation for PWM control and miscellaneous circuits
Independent of load current
34
Charge Management Circuits, cont’d
Conversion efficiency of a DC-DC converter by a power MOSFET configuration
35
Charge Management Circuits, cont’d
Power efficiency of LTC3414 and TPC62320
36
Charge Management Circuits, cont’d
Switching charger power dissipation
Sum of the conduction, switching, and controller power losses
Conduction loss
IR loss in the MOSFET switches, inductor, and ESR of the bulk capacitor
Proportional to the equivalent resistance of the components and output current
Switching loss
MOSFET switch gate drive loss
Proportional to the input voltage, switching frequency, and gate capacitance
Controller loss
Static power dissipated by the switch controller
General characteristics
The charger efficiency decreases when the input and output voltage difference is
large
Boost converters are less efficient than buck converters
37
Charge Management Circuits, cont’d
Switching chargers
Switching chargers transfer energy from a power source to an EES element
The charging efficiency is not constant, but depends on the input voltage, output
voltage, and output current
Example: efficiency and power consumption of a Linear Technology LTC3531
38
Charge Management Circuits, cont’d
Charging efficiency variation in supercapacitor charging
Charging 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
39
Charge Management Circuits, cont’d
Monitoring and control
BMS (battery management system)
Maintains stability and sources of important information for the charge management policies
Estimates 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 monitoring
SCADA (supervisory control and data acquisition) system for HCI
40
Hybrid Electrical Energy Storage (HEES) Systems
Motivation
Computer 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 systems
Each EES element has different characteristics such as cycle efficiency, leakage current, cycle
life, storage cost, and volumetric energy density, power rating, and so on
No single type of EES element can simultaneously fulfill all the desired characteristics
Exploit the advantages of each EES element and hide its disadvantages
Hybrid EES systems consist of both low unit cost (e.g., lead-acid batteries) and high unit cost
(e.g., supercapacitors) EES elements
Should be minimized by allocating the amounts of low-cost vs. high-cost EES elements while
meeting other performance constraints
Different types of energy storage elements can be organized in an appropriate storage
hierarchy and reconfigured on the fly
41
HEES System Architecture
42
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 elements
Generalized power migration buses
High-efficiency power converters
Optimal charge management policies
Logically hierarchical, physically flat
structure to shorten the charge
transfer paths
The architecture is closer to scratch
pad memory rather than cache memory
in that:
Both allocation and replacement policies are
required
Flat physical structure (cache is hierarchical)
43
HEES System Architecture, cont’d
Charge migration path can be a multiple power bus or a crossbar network
Allows optimal simultaneous migration operations
The optimal power bus voltage for each migration is a function of the EES elements and their
SOC
A shared power bus requires the same voltage level of all the power converters for
simultaneous migration operations
Reconfigurability for enhanced cycle efficiency
Each 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
levels
A fixed array structure cannot always guarantee desirable cycle efficiency
For example, a supercapacitor bank may have very low terminal voltage with low SOC
44
HEES System: Charge Allocation (charging)
Charge allocation problem is to determine
the destination EES banks that maximize the
charging efficiency
Charging efficiency is dependent on
Type and SOC of the bank
Voltage and current characteristics of the
power source
Degree 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
45
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 demand
Discharging efficiency is dependent on
Rate capacity effect
Power rating
Terminal voltage, and so on
The most efficient EES bank changes over
time as SOC changes during discharging
46
HEES System: Charge Migration
Each EES element has different selfdischarge 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 selfdischarge rates - supercapacitor banks
Remaining 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
47
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
48
Examples of Optimizing HEES Systems
Amortized system cost optimization in a HEES system for large-scale energy
storage
Maximum power transfer tracking in a solar energy harvesting system
Rate capacity effect mitigation in a system supporting widely varying load
currents
49
Example I: Amortized System Cost Optimization
Amortized cost optimization example
Daily and weekly energy usage profiles
50
Example I: Amortized System Cost Optimization
Amortized cost optimization example
Assumptions
Installation and disposal costs are included in the cost per capacity
Energy cost is 10 ¢/kWh
Two energy storage elements
Criteria
Battery
Supercapacitor
Cyclelife
2,000
100,000
Cost per capacity ($/kWh)
1,000
40,000
Cycle efficiency
80%
100%
Self discharge rate (%/day)
0.1%
15%
51
Example I: Amortized System Cost Optimization
Amortized cost optimization example
Conventional EES approaches
Battery-only EES
Suffer from low cycle efficiency and short cycle life
Supercapacitor-only EES
Not suitable for long-term storage
HEES approach
Use batteries for weekly energy storage
Large capacitance: low cost-per-capacity required
Long storage duration: low self-discharge rate required
Use supercapacitors for daily energy storage
Frequent charge/discharge: high cycle efficiency and long cycle life required
52
Example I: Amortized System Cost Optimization
Amortized cost optimization example
Cost ($/week)
Battery-only EES
Supercapacitoronly 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
Low storage cost due to battery cycle life enhancement using supercapacitors as
daily energy storage
Low energy cost due to high cycle efficiency of supercapaictors
53
Example II: Solar Energy Harvesting System
Solar energy harvesting is a promising method for self-sustainable systems
Harvested energy is necessary to be stored in an EES system for stable power
supply
Continuous operation in cloudy weather or at night
A rechargeable battery and/or supercapacitor is used
Energy transfer involves energy loss
54
Solar Energy Harvesting Example, cont’d
Optimal solar energy harvesting system
Maximizing the harvested energy at the minimum cost
Energy conversion efficiency should be maximized
Charger conversion efficiency
Significantly 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 systemwide efficiency is non-trivial
Previous 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
55
Solar Energy Harvesting Example, cont’d
Comparison between batteries and supercapacitors (revisit)
Supercapacitors are suitable for frequently charged and discharged energy storage
Significant 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
56
Solar Energy Harvesting Example, cont’d
Maximum power point tracking (MPPT) technique
Maximizes the output power from the PV module
Does not guarantee the maximum energy
accumulation
Charger output power by the MPPT technique may
be less than that by the MPTT technique
Three design considerations
PV cell configuration
Charger selection
Supercapacitor configuration
57
Solar Energy Harvesting Example, cont’d
PV module configuration: voltage and current characteristics depend on series
and parallel configuration
More PV cells in series: higher voltage
More PV cells in parallel: higher current
The minimum number (cost-optimal) of cells and their energy-optimal
configuration should be derived
58
Solar Energy Harvesting Example, cont’d
MPTT problem statement and solution
Given
Energy requirement
Solar irradiance profile
Charger efficiency data
Unit PV cell characteristic
Find
Optimal PV module configuration (number of connected cells)
Optimal supercapacitor capacitance
Solution
Approximate harvested energy for given configuration
Iteratively 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.)
59
Solar Energy Harvesting Example, cont’d
Charger selection
Charger maximum output and current rating
IR loss and gate drive loss
60
Solar Energy Harvesting Example, cont’d
Supercapacitor capacitance consideration
Smaller capacitance: higher voltage
Larger capacitance: lower voltage
Energy-optimal capacitance should be derived
Cost of the supercapacitance is determined by the given energy requirement,
not its capacitance
61
Solar Energy Harvesting Example, cont’d
Maximum charge power surface
The MPTT method maximizes the power into the supercapacitor at all times
The maximum power transferred into the supercapacitor is a function of the
irradiance level and the supercapacitor voltage
62
Example III: High Efficiency for Variable Loads
Rate capacity effect in batteries
Undeliverable charge according to the battery operating condition
Energy impact
Power loss in the battery caused by battery internal resistance
Discharge with different constant current
Discharge with constant and pulsed current
63
Variable Load Support Example, cont’d
A typical load profile for military radio systems
An 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 100watts) transient spikes
The EES system must handle both positive and negative transient load power pulses
64
Variable Load Support Example, cont’d
Parallel connection
Connect the battery and supercapacitor in parallel
Straightforward solution
Filtering voltage fluctuation
Godfrey Sikha and Branko N. Popov, Performance optimization of a battery–capacitor
hybrid system, Journal of Power Sources 134 (2004) 130–138
Parallel connection
Voltage and current response
65
Variable Load Support Example, cont’d
Constant-current charger-based hybrid architecture
Constant-current charger
Separate the battery current from the load current
Widely used for battery and supercapacitor management
Simple circuit implementation
Rate-capacity effect reduction
Mitigate current fluctuation
Converter efficiency
Supercapacitor terminal voltage should be maintained in a high-efficiency range
Voltage and current response
Constant current hybrid
66
Variable Load Support Example, cont’d
Energy density of the storage elements
Li-ion batteries: 250 ~ 300 Wh/L
Supercapacitor: 10 ~ 20 Wh/ L
Conversion efficiency
Depends on input/output voltage and
current
Supercapacitor capacitances
Small capacitance results in large
supercapacitor voltage variation and low
charger/regulator efficiency
Parallel connection
Large capacitance increases volume of the
system
Constant current hybrid
67
Variable Load Support Example, cont’d
Controller board
68
68
Variable Load Support Example, cont’d
Battery
Li-ion GP2N1051 cell
2SP1 pack
8.4 V 350 mAh
Supercapacitor
NessCap ESHSR-0010C0
4S array
2.5F, 10.8V
Parallel connection
7.7 % deliverable energy
gain
Constant current hybrid
69
Variable Load Support Example, cont’d
Voltage and current waveform
The HEES system architecture shows smaller current fluctuation compared
with the parallel connection
Parallel connection
Constant current hybrid
70
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 operations
Charge allocation, replacement, and migration
HEES optimization examples
Peak shaving
Maximum power transfer tracking
Rate capacity mitigation
Future work
Systematic (mathematical) approaches for holistic optimization of a HEES system
Simultaneous optimization of charge allocation, replacement and migration processes
71