Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June. 2009)36

Development of Grid-stabilization Power-storage Systemwith Lithium-ion Secondary Battery

TADASHI GENGOYOSHINORI KOBAYASHITSUTOMU HASHIMOTOMASAAKI MINAMITETSURO SHIGEMIZU

KATSUAKI KOBAYASHI

Mitsubishi Heavy Industries, Ltd., (MHI) is developing a large-capacity lithium-ion
secondary battery in a joint effort with Kyushu Electric Power Co., Inc. Although a conventional
battery of this type offers both the advantages of large capacity and long life, internal resistancecauses the battery temperature to increase while charging or discharging at a high current.
Thus, the charge and discharge rates are limited when the battery is used in thegrid-stabilization power-storage system of a renewable-energy source (such as wind power orphotovoltaic power). We have optimized the battery materials and design to lower the internal
resistance, and have obtained a battery that permits 2C charge and discharge. Furthermore, wehave developed a 100-kWh-class grid-stabilization power-storage unit, enabling a rated
operation of 100 kW, by connecting these batteries in series and in parallel.

|1. Introduction
Renewable energy, such as wind power, photovoltaic power, and biomass fuel power, arecurrently being introduced worldwide. The United States has announced that upwards of 40 billion
dollars will be invested in a Green New Deal Policy, and their efforts have been focused on thedevelopment of renewable energies. In Japan also, a policy is in place to double the power exercise
price of photovoltaic power generation for household use (amongst other things), and it is predictedthat the percentage of generated power furnished by renewable energies will increase gradually.
Unfortunately, renewable energies extracted from natural sources, such as wind andphotovoltaic power, are subject to power fluctuations caused by weather conditions. Thesefluctuations can be reduced by introducing various technologies into the latest high capacity windturbines. However, when a source of renewable energy capable of generating a large amount ofpower is applied to a grid with limited receiving capability, a system to suppress power fluctuations
is required, and in this case, stabilization by storing excess energy in a large capacity secondarybattery is considered to be a promising technology. Though grid stabilization of renewable energy
using various kinds of batteries has been studied both domestically and abroad, battery
performance, cost and other issues remain.
In this paper, we present the lithium-ion secondary battery and grid-stabilization
power-storage unit produced by the technology development research of Grid-stabilization
Power-storage Systems, a project of the New Energy and Industrial Technology Development
Organization (NEDO) that is being jointly carried out with Kyushu Electric Power Co., Inc.

|2. Development of the lithium-ion secondary battery2.1

Features of the battery
Generally speaking, a lithium-ion secondary battery has the following features:・ High voltage (nominal discharge voltage: 3.6 to 3.8 V),
・ High energy density,Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June. 2009)37

・ No memory effect,
・ Wide usable temperature range,
・ Maintenance-free operation,
・ High efficiency, and rapid rate of charge and discharge, and・ Excellent storage characteristics.
A lithium-ion secondary battery with the above features is considered to be the mostpromising candidate for configuring a high-voltage large-capacity grid-stabilization power-storage
facility. However, cost and safety issues remain, since the material cost is higher than that of a leadbattery, and a means is required to assure the safe operation of the control circuit and structure.
MHI is currently developing a large-capacity high-safety lithium-ion secondary battery to addressthese problems1.
Figure 1 shows the battery reaction principle and a structural drawing of the battery. In thetype of lithium-ion battery generally used for notebook-sized personal computers (and other suchdevices), the main stream reels off the electrode in a cylindrical spiral, whereas in the MHI battery,films of electrode-active material are formed on both faces of the metal foil (made of acurrent-collecting material), the film is cut into an angular shape, and the cathode and anode plates
are alternately laminated. The electrode plate design offers the advantage that the heat generated bycharging and discharging is readily transferred through the current correcting foil.
Moreover, heat dissipation is easily accomplished with an angular laminated battery, since
the heat transfer velocity in the direction parallel to the electrode face is larger by a factor of ten
than the velocity in the normal direction. In addition, low-cost high-safety lithium manganese oxide
is used as the main component of the active material of the positive electrode.

Figure 1 Principle of the lithium-ion secondary battery, and structure of the MHI battery

2.2

Improvement of the battery

Although the conventional MHI battery has a large capacity, its internal resistance is high,resulting in an undesirable temperature increase if charging and discharging are carried out at ahigh current, such as 1C (a current capable of charging or discharging the rated capacity within onehour). In particular, the temperature rises readily during discharge, up to the state of a low charging
rate (hereinafter referred to as SOC).
Presumably the reason for this is that the resistance due to the electrode reaction of lithium
manganese oxide (a positive electrode active material) becomes high in the low-voltage region. A
grid-stabilization power-storage system must keep pace with the rapid fluctuations of wind power
generation, and it is therefore desirable that high-current charging and discharging is possible.
The maximum power targeted by the battery is largely affected by the system configuration. The
objective of this research is the development of a battery that permits charging and discharging at acurrent of 1C or more, in order to configure a grid-stabilization power-storage facility in which
input and output of the entire capacity can be accomplished within one hour.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June. 2009)38

Table 1 compares the performance of an MHI conventional battery with that of the battery
developed in FY 2008. For the FY 2008 battery, the internal resistance was reduced by optimizing
the electrode design and the composition of the electrode active materials. As a result, although thebattery capacity was reduced to 344 Wh, the internal resistance was approximately halved at SOC
50 %, and hence the power characteristic was greatly improved.
Table 1 Performance of lithium ion secondary battery (single cell)
Performance

Conventional battery

Battery developed in FY 2008

Battery dimensions(mm)

W 116 X D 66.5 X H 175

Battery weight(kg)

2.8

2.8

Operation voltage range(V)

3.1 - 4.15

2.7 - 4.2

Average voltage

(V)

3.8

3.7

Battery capacity(Ah)

105 (C/3)

93 (1C)

Energy capacity(Wh)

399 (C/3)

344 (1C)

Internal resistance(mΩ)

1.2 (SOC50％)

0.59 (SOC50％)

Figure 2 shows the change in the capacity plotted with respect to current. Since theinternal resistance was lower over the entire operational voltage range, the capacity was nearly
constant when charging or discharging at high current. A capacity of 344 Wh was obtained at 1C,and even at 2C, the capacity was 338 Wh.

Figure 2 Capacity change versus current|3. Development of the grid-stabilization power-storage facility
3.1Specifications

In Japan, dispersed power sources with large fluctuations in voltage, frequency, etc. (such as
photovoltaic power and wind power) that are interconnected with the electric power grid mustsatisfy the Technical Requirement Guidelines2 stipulated by the Ministry of Economy, Trade, andIndustry. To this end, any functions/devices that are required to stabilize the grid interconnection
must be added/installed; we believe that the most promising approach is grid stabilization with apower-storage system.
However, if this system is going to be able to compensate the rated electric power whenpower generation from a renewable-energy source is interrupted, it must possess electric power at
the same level as a wind farm or megawatt-class solar power unit. In this research, we studied thespecifications of a 100-kWh-class grid-stabilization power-storage unit as a basic unit device in agrid-stabilization power-storage system.
Figure 3 shows a control block diagram of a 100-kWh-class unit battery, and Table 2 lists
the basic specifications.
If cells of the type discussed in the preceding paragraphs are used, 336 cells are necessary toobtain 110 kWh or more, including a margin of 10 %. When a 100-kWh-class unit is configured
with a single row of cells connected in series, the maximum voltage of the row, determined by(upper voltage limit of cell)  (number of cells), exceeds the maximum input voltage of the charge
and discharge device, as well as the withstand voltages of the cell monitoring unit (CMU) and thebattery management unit (BMU).

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June. 2009)39

Figure 3 Block diagram of a 100-kWh-class power-storage unit
Table 2 Basic specifications of the 100 kWh class unit
Cell box

Outer shape(per box)(mm)
Number of cells

Panel specification

W1,500×D800×H2,000
112 cells (series connection)

Rated current(A)

95 (1C)

Allowable current(A)

190 (２C)

Rated capacity

(kWh)

39

Specification ofRated voltage

(V)

210

discharge

alternating

Frequency

(Hz)

60

devicecurrent portion

Unit charge andNumber of phases
Rated capacity

3 phases and 3 wires(kVA)

Instantaneous maximum capacity150225 (10 seconds)(kVA)
Specification of direct

Number of connected cells

current portion

336 cells(112 cells in series X 3 rows in parallel)

Rated electric power(ｋW)

50 kW bidirection chopper X 3

Accordingly, we adopted a configuration with three rows connected in parallel, each row
having 112 cells connected in series (28 module cells connected in series were contained in a cellbox). The input/output response speed to operation commands was 50 ms, taking into account both
the data communication speed and the response speed of the inverter. Furthermore, the unit
charge/discharge device was both a power source and a load, capable of inputting electric powerwith a complicated waveform to our 100-kWh-class unit battery, or of outputting it from the samebattery, in order to stabilize a renewable source of energy.

3.2

Development of 100-kWh-class unit

To maintain its stability and lifespan, the lithium-ion secondary battery must be controlledwith a monitoring unit. In particular, to avoid over-charging and over-discharging, a voltage
balance circuit is often installed to monitor the voltage of each individual cell and prevent voltage
deviations between cells. In this research, we studied three voltage balance methods: resistancebypass, charge power source, and simultaneous use of resistance bypass and charge power source.
Since the resulting balance performances of the three methods were almost equivalent, we adopted
the resistance bypass method, in which the cost and size of the CMU are minimum, and thespecifications are common to those for vehicles.
In the case of a large-capacity power-storage system, it is necessary to connect multiple cells
in series and in parallel. In particular, in a cell group in which many cells of low internal resistanceare connected in series and in parallel, when an unlikely occurence of cell failure causes a voltage
drop at a particular cell, a large current may flow into a parallel connection from the remaining
cells. In order to avoid this phenomenon, the current and voltage were controlled at the point whereseries cells are connected in parallel.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June. 2009)
40Figure 4 shows comparative results of the configurations of multi-parallel connections
investigated in this research. We selected a multi-parallel connection-chopper method, which cancontrol the current in each row and reduce the number of circuit parts (such as the number of gridinterconnction protection devices, etc.), after considering three different methods (multi-paralleland series cells, multi-parallel choppers, and multi-parallel inverters), with the objective ofobtaining an MW-class system. This method was therefore adopted for the unit charge anddischarge device to be fabricated.

Figure 4 Configurations of various series and parallel connections, and evaluation results

Figure 5 shows a system block diagram of the entire test facility, and Figure 6 presents anexternal view of a 100-kWh-class unit. A series cell box contained 112 cells, and a unit wascomposed of three series cell panels. In order to reduce the installation space, the cells wereinstalled on the shelves of multiple stages, and the electrical room was allocated to the top stage
because of the heat it generates.
Figure 7 shows the results of the charge and discharge tests. At a cell charge and discharge
rate of 1C (95 A), the charge energy capacity was 119.6 kWh, while the discharge energy capacity
was 109.4 kWh. Thus the design enabled a 100-kWh-class capacity. In future research, we intend
to carry out a life evaluation via a cycle test, simulating a complicated electric power waveformfrom a wind turbine.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June. 2009)41

Figure 5 Block diagram of the entire 100-kWh-class unit

Figure 6 100-kWh-class grid-stabilization Figure 7 Operational results
power-storage unit
100-kWh-class unit cellsforthe|4. Conclusion
As the number of renewable-energy power-generation systems continues to increase, there
will be an increased need for grid-stabilization power-storage systems. Such a storage system
requires a large secondary battery offering both high performance and low cost. The lithium-ion
secondary battery affords more advantages than other batteries in respect to energy density, powerdensity, workable temperature range, maintainability, etc. It will undoubtedly become the battery ofchoice for grid-stabilization power-storage systems owing to future cost reductions resulting fromenlargement and mass production.
MHI has commercialized the processes of wind power generation and photovoltaic powergeneration, and is developing a grid-stabilization power-storage system with a high-performancesecondary battery.
In this research, we developed a 100-kWh-class power storage unit using a battery composedof large 350-Wh-class cells, and confirmed its initial performance for possible application togrid-stabilization power-storage systems for wind farms and megawatt-class solar power units. In
future work, we intend to carry out a life evaluation via cycle tests with patterns simulating bothwind turbine and photovoltaic power generation, including practical assessments of safety andreliability. In addition, we are planning an early product commercialization of the large system byfurther improving and reducing the cost of the cells and electric power converters.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 2 (June. 2009)
42References
1.

2.

Hashimoto T. et al., Development of Lithium Ion Battery and Grid Stabilization Technology for
Renewable Energy Using Secondary Battery System, Mitsubishi Heavy Industries Technical Review Vol.
44 No. 4 (2007)
Technical Requirement Guidelines by the Japan’s Ministry of Economy, Trade, and Industry (Oct. 2004)

Authors
Tadashi Gengo
General Manager
New Energy Systems Department
Power Systems Headquarter

Yoshinori Kobayashi
Manager
New Energy Systems Department
Power Systems Headquarter

Tsutomu Hashimoto
Manager
New Energy Systems Department
Power Systems Headquarter

Masaaki Minami
Nagasaki Plant Engineering
Department
Nagasaki Shipyard and Machinery
Works

Tetsuro Shigemizu
Manager
Nagasaki Research andDevelopment Center, TechnicalHeadquarters

Katsuaki Kobayashi
Nagasaki Research and Development
Center, Technical Headquarters