Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
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Thermal Storage System to Provide Highly-efficientElectric Power Resilience in the Era of Renewable Energy

KENJIRO YAMAMOTO*1 KAZUHIRO DOMOTO*2
MAKOTO TOBO*3

TSUTOMU KAWAMIZU*4

TAKAHIRO YAMANA*5

YUJI OTA*6

To enhance electric power resilience (robustness to endure a significant and sudden
unbalance between supply and demand while regulating reserve capabilities) in line with theincreasing use of renewable energy, thermal storage systems are incorporated into the turbine
bypass system, etc., of thermal power plant systems so that waste heat during startup or themismatched heat between boiler and turbine/generator outputs under extremely low loadconditions can be stored. This stored heat can generate electricity by releasing its energy to thepre-boiler and/or boiler during high-load operation, which results in energy savings of around 2%
or more. By utilizing the significant amount of latent heat of phase change materials (PCMs :applying melting/solidification processes) or by increasing the temperature difference of sensible
heat storage materials such as molten salts and water, thermal energy storage equipment can bemade compact, thereby making it possible to be installed on the premises of a power plant.
We are currently working on the development of such a system to realize its practicalapplication at a price per electric capacity comparable to battery storage systems.

|1. Introduction
According to the prediction based on published data such as the Japan’s Energy Statistics by
the Japanese Agency for Natural Resources and Energy, Japan’s power generation breakdown byenergy source in 2030 is considered to be as follows: 22-24% for renewable energy, 26% for coal,
27% for LNG, 3% for oil and 20-22% for nuclear energy. Renewable energy will become one of
the major power sources, as its share is expected to increase from 17% in 2017. In addition, mostrenewable energy power generation systems are either photovoltaic (PV) or wind power. Theirelectricity supply to the power grid fluctuates considerably depending on the natural environment(e.g., insolation or wind conditions). Under such circumstances, increasing/decreasing the power
supply from thermal power plants or water pumping/releasing from pumped-storage hydroelectric
power plants are now key to balancing the supply and demand of electricity. This is also the casewith European countries, despite their being advanced in terms of introducing renewable energy
applications(1). Figure 1 gives a balancing example by Kyushu Electric Power Co., Inc., which has
the highest renewable energy application rate in Japan(2). This power company operates a coal-fired
power generation unit at the minimum output level, and allows the GTCC (LNG) to run with theminimum number of axis. In spite of making full use of the regulating capability of all the thermal
power plants, there was a case of a company limiting the connection of PV output to the power grid*1

Authority of Technology, Boiler Business Strategy Planning Department, Boiler Technology Integration Division,*2

Deputy Director, Boiler Business Strategy Planning Department, Boiler Technology Integration Division, Mitsubishi Hitachi*3Engineering Manager, Nagasaki Plant Engineering Department, Project Management Division, Mitsubishi Hitachi Power*4

Research Manager, Heat Transfer Research Department, Research & Innovation Center

Mitsubishi Hitachi Power Systems,Ltd.
Power Systems,Ltd.
Systems,Ltd.
*5

Heat Transfer Research Department, Research & Innovation Center*6

Engineering Manager, CIS Department, ICT Solution Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
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when the amount of power it generated reached 60% of the demand in their supply area .
If the renewable electricity supply continues to increase, thermal power plants need to reduce
the output likelihood in daytime by (1) increasing the number of shutdowns, (2) operating the unit
at a lower output level than the current minimum level and (3) preferably improving the ramp rate
to respond to sudden changes in insolation or wind conditions.
Under such circumstances, in accordance with Japan’s 5th Strategic Energy Plan in 2018, in
which renewable energy is expected to become a major power source in the future, national
research and development agency (NEDO ： the New Energy and Industrial Technology
Development Organization) considers “heat energy storage (thermal energy storage)” to be a
critical technology and is promoting its social implementation.
This report presents the feasibility of the function to provide electric power resilience withhigh efficiency by applying thermal energy storage systems at thermal power plants. Utilizing thehigh heat capacity of the latent heat of PCMs or increasing the temperature difference of sensible
heat storage materials such as molten salts and water can make such a system compact. This report
also introduces the current situation regarding the development of this system applying metallic
PCMs and molten salts as thermal energy storage media, the former of which can further improve
the responsiveness to output changes owing to its quick heat charging/discharging capability.

Figure 1 Electricity supply and demand balancing with increasing use of renewable energy(example of Kyushu Electric Power Co., Inc.)|2. Thermal storage system to enhance electric power resilience when
renewable energy supply is significant
2.1

Thermal storage system concept

Thermal power plants normally operate boilers that feed heat corresponding to the electric
generation output at the generator terminal, which is the sum of the sending-end output (electricity
for supply) and auxiliary consumption (equal to roughly 5% of the rated generator capacity). Whenelectric power demand and supply are balanced by shutdown/startup or extremely low-load
operation in which the turbine/generator output is lower than the minimum boiler output, heat loss
during startup or that caused by an output mismatch between the minimum output of boiler andturbine/generator output is released as waste heat to the condenser from the turbine bypass system,
etc., and finally it is disposed of in the sea or atmosphere. This mismatch happens because, in
coal-fired power generation units, it is necessary to keep the boiler output at around 15% or higher
to realize a stable coal pulverization process in the mills and stable combustion in the boiler
combustion furnace.
Figure 2 shows schematic diagram of a thermal power plant system in which the thermal
storage system is incorporated into the turbine bypass system, etc., and also illustrates the concept

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how to utilize it. To make maximum use of waste heat without causing loss, it is important for thethermal storage system to have the capability of storing the latent heat of steam as well as
low-temperature sensible heat of water. As indicated by the h-s diagram in Figure 3(a), it is very
desirable to use thermal energy storage equipment that can store nearly as much heat as the heat
drops utilized by turbine cycle power generation. By utilizing the stored heat for electric generation
under high-load operating conditions, where turbine efficiency is high, a substantial energy-saving
effect can be obtained compared with low-load power generation (Figure 3(b)).
The thermal storage system consists of heat exchangers containing thermal energy storage
materials with different thermal energy storage temperatures, piping, valves and control units, as
shown in Figure 2(a). In the process of heat charging, steam or water from the turbine bypass
system and separator drain system are introduced to the heat exchanger, and disposed of in thecondenser after the heat is stored. On the other hand, in the process of heat discharging, the fluid(steam or water) in the pre-boiler and boiler are heated in accordance with the subranges of theoperable temperature range to minimize the impact on the steam cycle (patent pending).

Figure 2 Schematic diagram of fluid system with thermal energy storage and conceptual
diagram for utilization

Figure 3 h-s diagram for steam water and relationship between turbine load and heatconsumption rate

2.2

Operation of thermal storage system

Five operational patterns using the aforementioned thermal storage system are presented
hereunder.
First pattern is a case in which the thermal power generation unit is shutdown during daytime
when there is sufficient renewable electricity supply and the unit is started up in the evening tomeet the demand until the morning when the renewable energy supply returns. The amount of
mismatched heat between the boiler and turbine/generator outputs is usually disposed of as waste
heat from the turbine bypass system (TB valve) and separator drain system (WDC, abbreviation of

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water separator drain control, valve) to the condenser during startup. Figure 4 is the heat balancechart of a 700-MW unit during startup (2 hours) after 8 hours shutdown. Waste heat to thecondenser without being fed to the turbine accounts for 12.5% of the fuel heat input. As illustrated
in Figure 5(a), this waste heat is stored (indicated by the red shaded areas) and the stored heat isdischarged to be used for power generation (the blue shaded areas). This energy saving is notpossible with a battery storage system or pumped-storage hydroelectric power generation, but isonly enabled by employing a thermal storage system.
In Figure 5(b), the boiler output is reduced to the minimum level (15%), while theturbine/generator output is set at 10%, from which auxiliary consumption (5%) is subtracted, andthe remaining 5% is transmitted to the grid. Waste heat equivalent in amount to the output
mismatch of 5% is stored via the turbine bypass system. The stored heat is discharged in the samemanner as in (a).
Figure 5(c) represents a thermal energy storage pattern in which, with the minimum boiler
output of 15%, the turbine/generator output reduced to 5% is used for auxiliary consumption (5%)
resulting in no electricity supply to the grid while the generator circuit breaker closed. The waste
heat equivalent to the mismatch of 10% is stored via the turbine bypass system. As there is no
power left to be transmitted to the grid, the renewable electricity supply can be increased.
Illustrated in Figure 5(d) is an operational pattern in which all the heat produced at theminimum boiler output of 15% is stored and power for auxiliary consumption (5%) is provided
from the grid, while the turbine/generator are placed out of operation (i.e., boiler single operation)and the generator circuit breaker is open. As the boiler remains in a warmed-up condition, thesynchronization of the turbine/generator and an increase in electricity output can be realized more
rapidly than in the case of a plant shutdown. By receiving the power from the grid to the power
plant, the renewable electricity supply can be further increased.
In Figure 5(e), in addition to (d), externally-provided power is also used for the electricheater that is used to heat thermal energy storage materials, and the renewable electricity supply
can be further increased by this amount.

Figure 4

Heat balance chart of 700-MW unit during startup (2 hours) after 8 hours shutdown

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
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Figure 5 Operational patterns of thermal power plant with use of thermal storage system

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2.3

Estimation of energy-saving effect and economic efficiency

Table 1 summarizes the energy-saving effects of the aforementioned five operational
patterns. When controlling the amount of power supplied from the thermal power plant by shutting
down the plant for 8 hours, lowering the electricity output to the grid or receiving electricity fromthe grid during daytime while utilizing thermal energy storage as in the case of Figures 4(a) to 4(e),an additional energy-saving effect of 2-6% can be achieved in comparison with a battery storage
system or pumped-storage hydroelectric power plant. As mentioned in Figure 3(b), this is because
the stored heat can be used under high-load conditions with a high turbine efficiency. From theperspective of energy usage, the efficiencies of conversion to electric power in a thermal energy
storage system, battery storage system and pumped hydroelectric storage system are estimated to
be 90%, 85% and 70%, respectively. These values correspond to the approximate ratio among heatcharging and discharging efficiency, power charging and discharging efficiency andpumped-storage efficiency. With the increase of renewable energy, thermal power plants will often
be operated under low-load conditions with the low turbine efficiency. Therefore, the thermalenergy storage system can be an effective method to offset the deviation from the target efficiencyof thermal power generation provided as the benchmark by the Energy Conservation Act of Japan.
Furthermore, from the perspective of utilizing energy of waste heat for power generation, which isotherwise released into the sea or atmosphere from the condenser, the benchmark index will be
improved by as much waste heat as is used based on the calculation formula as defined by the
Agency.
Table 1 Energy-saving effect estimation
Thermal energy storage patternFigure 5(a)Thermal energy storage of waste heat during
plant shutdown/startup (it is nearly equal to12.5% of heat input during startup)*1
Figure 5(b)5% thermal energy storage by lowering theminimum turbine output*2
Figure 5(c)10% thermal energy storage with no power
supply to grid with synchronization *2
Figure 5(d)15% thermal energy storage by placing
turbine generator out of operation *2
Figure 5(e)15% thermal energy storage by placing
turbine generator out of operation (+powerreceived from girdexternally by α%)*2

Evaluation index

Fuel heat input

Stored thermalenergyIn the case of storing as electricity bytransmitting it instead of storing asthermal energy
PumpedBattery storage*3hydroelectric
storage*4

1.9%

Applicable only tothermal heat storage

2.9%(1.8%)

1.1%(Baseline)

Baseline

6.2%(3.9％)

2.3%(Baseline)

Baseline

10.0%(6.2%)

3.8%(Baseline)

Baseline

10.0%+α(6.2%+α)

3.8%+α(Baseline)

Baseline*(In general) With regard to the numerical values given in the table above, the stored energy reduction due toaging degradation is not considered in any storage methods.
*1 Based on the hypothetical operating conditions of startup (2 hours) in DSS mode (8 hours of shutdown time
during daytime), followed by operations at 50% load for 2 hours and at 20% load for about 12 hours (average
daily load percentage: approx. 15%).
*2 Based on the hypothetical operating conditions of 8 hours of thermal energy storage time, followed byoperations at 50% load for 6 hours and at 20% load for about 10 hours (average daily load percentage: approx.
20%).
*3 The battery energy conversion efficiency of 85.7% is taken into consideration (with the assumption of
AC-to-DC conversion 95% × charge/discharge 95% × DC-to-AC conversion 95%).
*4 The general efficiency of pumped-storage hydroelectric power generation of 70% is taken into consideration.

|3. Development of thermal energy storage equipment
3.1

Thermal energy storage materials

The first example of a successfully-applied thermal energy storage material in the field of
power generation was water used in the Ruths accumulator(4) to cope with peak load in Europe in
1916. Since then, various studies have been conducted, and PCM application technologies are also
under development(5)-(9). In order to have variation of the range of the dischargeable temperature as
described in Section 2.1, from the perspective of minimizing the impact on the steam/water cycle
during heat discharge, it is desirable that the temperature range of the thermal energy storage
equipment can be set for multiple conditions by employing several types of PCMs with different

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melting points. Figure 6(a) is a schematic diagram that shows the principle of the PCM heatcharging/discharging process. The use of metallic PCMs enables quick charging/discharging ofheat and reduction in the equipment size by taking advantage of their high heat capacities andconductivities. Size reduction can also be achieved using liquid sensible heat storage materials suchas molten salts, oils and water, because the difference in temperatures between pre- andpost-storage is increased. With these liquid sensible heat storage materials, heatcharging/discharging processes can also be accelerated by increasing the flow rate in the heatexchanger. Figure 6(b) compares the sizes of metallic PCMs, molten salt and concrete (sensible
heat storage). The degree of size reduction is determined by the reciprocal of heat capacity. Figure7(a) presents an example of a thermal storage system with a PCM, while Figure 7(b) is an example
of a thermal storage system in which a liquid sensible heat storage material is used. Table 2 lists
some of the thermal energy storage material candidates for each of the temperature subranges.
Since some materials are highly susceptible to corrosion, if this is the case, anti-corrosion measuresfor equipment in the thermal storage system are required.

Figure 6 Principle of PCM mechanism and volumetric comparison under the conditions inwhich stored heat amounts are the same

Figure 7 Thermal storage system examples

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Table 2 Examples of thermal energy storage materials, physical properties and thermal
indices (Values are approximate)
Thermal energy
storage materialHigh
temperature

MediumtemperatureLow
temperature

3.2

Al basedZn based

Melting
point
℃
From
≈560
From
≈380

Densitykg/m3

Latentheat
kJ/kg

Storablethermalenergy(latent
heat)
MJ/m3

Specificheat
kJ/KgK

Storablethermalenergy(sensible
heat)
MJ/m3K

Heat
conducti
vity
W/mK

≈2500

≈500

≈1300

≈1.0

≈2.5

≈160

≈6600

≈140

≈920

≈0.5

≈3.3

≈100

NaNO3

≈310

≈2000

≈180

≈360

≈1.7

≈3.4

≈0.5

Mineral oil

―

≈770

―

―

≈2.6

≈2.0

≈0.1

Water

―

≈1000

―

―

≈4.2

≈4.2

≈0.5

Erythritol

≈120

≈1500

≈340

≈510

≈1.0

≈1.5

≈0.7

StoragemethodexampleLatentheat
Latentheat
Sensibleheat
Latentheat
Sensibleheat
Sensibleheat
Latentheat

Heat exchanger and heat transfer analysis

Figure 8 describes a heat transfer analysis model of the system in Figure 7(a) with a heatexchanger using a PCM. Supposing that an Al-based PCM with a latent heat range of 560°C to
580°C is used as the thermal energy storage material, the heat exchanger has a heat transfer tube of
20m in length. The dynamic behavior of the steam temperature at the tube outlet and the amount ofheat charged/discharged were predicted. The typical analysis results are given in Figure 9. In thecase of charging heat from steam at a temperature of 600°C (Figure 9(a)), the temperature of steam
at the tube outlet falls down to approximately 570°C in the beginning, as the storage of thermalenergy sequentially starts from upstream. As the temperature of the upstream PCM increases, thesteam temperature at the tube outlet also increases. In about 2100 seconds (35 minutes), the twotemperatures (i.e., PCM and steam at the tube outlet) will reach a level almost as high as the steam
temperature at the tube inlet, which means the completion of heat charging process. At the time ofcompletion, the PCM temperature at the outlet will be as high as or higher than the latent heatrange, i.e., 560-580°C. To the contrary, when stored heat is discharged to steam at a temperature of
500°C (Figure 9(b)), the steam temperature at the tube outlet is elevated to approximately 565°C inthe beginning. As the PCM temperature decreases as a result of heat discharge, the steam
temperature at the tube outlet also decreases. In about 1200 seconds (20 minutes), the PCM
temperature becomes almost as low as the beginning temperature level of the inlet steam (500°C),
completing the heat discharging process. These results indicate that the use of a metallic PCM canrealize the function of quick heat charge and discharge, which is equal or superior to systems using
molten salts, etc.

Figure 8 Thermal energy storage heat exchanger and heat transfer analysis modelMitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
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Figure 9 Example of heat transfer analysis using a thermal energy storage heat exchanger
with a metallic PCM(Dynamic behavior of outlet temperatures, etc.)

3.3

Evaluation of the impact on steam/water cycle during heat discharge

Transient response simulation of heat discharge to high-pressure feedwater heater systemfrom heat storage equipment with a medium temperature range (300°C class) was conducted toconfirm no occurrence of problems in the power plant steam/water cycle. Figure 10 gives theresults. The behaviors of the generator output, etc., were examined under the conditions in which
15% of the flow rate of high-pressure feedwater was bypassed to the thermal energy storage heatexchanger during the operation at 100% rated load. A transient change in plant behavior isobserved soon after the start of heat discharge, which is, however, not problematic to the extent that
all the process values remain in their respective permissible ranges. Further examination has also
confirmed that after the completion of heat discharge (i.e., [PCM outlet temperature] < [feed water
heater outlet temperature]), there is no significant impact on plant behavior even in the case ofcontinued water flow, and the rated output can be maintained.
Simulation will also be planned to examine the transient response to heat discharge during
operation in the high temperature range (400-600°C class) and determine the bypass ratio thatcauses no problematic behavior in terms of the temperature of the boiler heat transfer surface.
As for the heat charging process, operation is conducted according to the concepts that quick
valve manipulation should be avoided at the start of heat charge and the steam that has passed
through the thermal energy storage heat exchanger is damped to the condenser, so we havetherefore concluded that the transient response of the thermal energy storage system has no impact
on plant behavior.

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Figure 10 Transient response simulation results during heat discharge (discharge of heat tohigh-pressure feedwater heater system)

3.4

Development of thermal storage systems and their practical application

In selecting thermal energy storage materials, high heat capacity and high thermal
conductivity are desirable properties. However, it is also necessary to ensure the feasibility of theentire system by considering the corrosion potential, corrosion prevention measures and prices.
The reliability of corrosion resistance and coating material soundness (against cracking/detachment and self-healing property, etc.) must be ensured for thermal energy storage equipment,
particularly so if corrosion resistant materials such as metallic PCMs are used for the heatexchanger. With a view to pilot-scale demonstration test in the future, we have started conducting
basic tests at our Research & Innovation Center to realize societal implementation in 2025.

|4. Prototype design and economic evaluation
The prototype design to incorporate the thermal storage system into an existing thermal
power plant was made and its economic evaluation was roughly conducted. Figure 11(a) is aschematic representation with the assumption of using a thermal energy storage material with thesame heat capacity as molten salt, which indicates that the system can be successfully installed on
the premises of an existing plant. With the use of metallic PCM as a thermal energy storage
material, the system can fit in even more compactly. Figure 11(b) compares the price per resultant
electricity output of the thermal storage system with those of a battery storage system and apumped-storage hydroelectric power plant (the value of pumped-storage was estimated very
roughly based on a certain assumption about construction cost as a reference). The estimation
results show that it is possible to set a price as low as or lower than a battery storage system ifdevelopment is carried out regarding thermal energy storage systems. It is also worth evaluating
whether the economic feasibility of thermal storage systems is further improved by integrating theenergy-saving effect (about 2-6%) mentioned in Section 2.3. We will continue to work on thedevelopment and practical application to realize its social implementation.

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Figure 11

Thermal storage system price estimation|5. Conclusion
We reported the result of feasibility study when a thermal storage system is incorporated intoa thermal power plant as a measure of providing electric power resilience in the era of the increasinguse of renewable energy. This can be realized by installing thermal energy storage equipment in theturbine bypass system and separator drain system. Not only improving the plant’s output
regulating/reserve capabilities, but also energy-saving effect of about 2-6% can be produced. This
energy-saving effect is superior to that of battery storage systems or pumped-storage hydroelectric
power generation. With some development regarding thermal energy storage systems, it is feasiblethat the thermal energy storage system will be made compact enough to be installed on the premisesof an existing thermal power plant. From the perspective of economic evaluation, setting a price as
low as or lower than a battery storage system is considered possible.

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