Study on Open and Closed Chemical Thermal Energy Storage

Technology with Low-regeneration Temperature Hongzhi LIU

Candidate for the Degree of Doctor

Supervisor: Katsunori Nagano

Division of Human Environmental Systems

Background

Large amount of low temperature industrial waste heat

(< 100ºC) is discharged to the atmosphere. On the other

hand, the solar thermal energy collected by collectors is

also a kind of low temperature heat source. Therefore, low

temperature (< 100ºC) advanced heat recovery technology

needs to be prompted to reduce fossil fuel consumption

and the impact of utilizing fossil fuel energy on

environment and to increase the proportion of utilize

renewable energy in energy consumption sector. Solid-gas

chemical thermal energy storage technology is one of the

most important heat storage technologies to save fossil

fuel, and encourage using renewable energies and reduce

the emission of greenhouse gases to achieve a clean and

sustainable energy society profitable for humankind due to

plenty of advantages, such as high thermal energy storage

density, small heat loss and potential to supply cooling

and heating at the same time. However, the low heat and

mass transfer problems of the chemical materials is one of

the obstacles for developing chemical thermal energy

storage technology. The low-regeneration temperature

material should be developed to store low temperature

industrial waste heat or solar energy. Another problem

with chemical heat storage material would be the

degradation of the material after several cycles use.

Therefore, low-regeneration temperature material with

high stability is expected to be developed, moreover,

suitable chemical thermal energy storage systems are also

hoped to be built and evaluated.

Principle of thermal energy storage technology

Sorption and thermochemical storage systems are

based on performing a reversible chemical reaction (or

desorption), which allows absorption of heat in the

decomposition (desorption) process, an endothermic

process.

Fig. 1 Processes involved in a chemical energy storage

cycle: charging, storing and discharging [2, 3]

Fig. 2 Principle of open thermochemcial energy storage

[4, 5]

Fig. 3 Operation principle of closed thermochemcial

energy storage [4, 5]

A B

Storing

C A B

Heat

Charging (endothermic)

A B C

Discharging (exothermic)

Heat

saturated

warm air

high temperature heat

source

heat release

(Tmedium)

hot air

cold wet air

(ambient)

dry warm air

heat release

(Tmedium)

des

orp

tio

n

adso

rpti

on

high

temperature

heat source

heat release

(Tmedium)

heat release

(Tmedium)

desorption

adsorption

water vapor

water vapor

condensation

evaporation

Low

temperature

heat source

charging

discharging

A reverse synthesis reaction is exothermic process to

release the stored heat in endothermic process. Sorption

and thermochemical storage systems use a reverse physic-

chemical reaction to store energy, and C is the compound.

With heat supply, C can be dissociated into products A

and B, which can be stored separately. C will be formed

with a heat release when A and B bound together [1] (cf.

Eq. 1).

C + heat A + B 1

In general, the following three main processes are

included in a thermal energy storage system: charging,

storing and discharging. The three processes are illustrated

for thermochemical energy storage in Fig.1 [2].

Thermochemical storage systems can be divided into

open and closed systems [4]. The open system is based on

the sorption processes to release heat and desorption

process to store heat. Closed systems work with a closed

working fluid circuit isolated from the atmosphere [6].

In an open sorption thermal energy storage system, air

is working as the heat and mass transfer medium flowing

into the solid thermal energy storage materials, where the

air can contact directly with the material (cf. Fig. 2) [7].

Gaseous working fluid of open system is directly

discharged to the environment and operated at

atmospheric pressure [8]. Normally, only water is possible

to be used as the candidate of the working fluid. The

working solid materials are required to be non-toxic and

non-flammable in open systems. In open thermal energy

storage systems, the heat charging process and heat

discharging process are separated to maintain low heat

loss [4]. In view of the environmental impact and low-

temperature heat sources (< 100ºC), water vapor sorption

on solid materials is promising as a kind of thermal energy

storage method [9-11].

In closed thermal energy storage system, the

components cannot be exposed to the atmosphere. As can

be seen in Fig. 3, in heat release process, water vapor can

be used to combine with the working material and the

operation pressure of the working fluid can be adjusted.

The thermal energy transferred to or from the closed

system needs to use heat exchanger. Compared to the

open thermal energy storage system, the heat storage

density is lower for the closed system due to the water

vapor has to be stored at the same time as the working

adsorbent [7]. However, closed systems are able to supply

higher output temperatures for heating applications than

open system, but usually requires higher temperatures

during the heat charging process than the open system [4].

Meanwhile, closed systems can be used to supply low

temperatures for space cooling [12].

Objectives of this research

There are several objectives of this thesis, which are

listed as follows:

(1) The first objective is to develop a suitable material

for a chemical thermal energy storage system with high

heat storage density by considering the above mentioned

problems of the chemical thermal energy storage.

(2) The second objective is to develop an open

sorption thermal energy storage system, which can be

utilized to recover low temperature waste heat (< 100ºC)

discharged from industrial sector and solar energy.

(3) The third objective in this research is to design an

optimizing closed chemical thermal energy storage system

to supply both heat and cold heat.

Chapter 2 Development of composite material made

from CaCl2 for an open chemical thermal energy

storage system

Fig. 4 Image of a WSS honeycomb unit

Fig. 5 Open sorption thermal energy storage

experimental setup

A chemical thermal energy storage material for an

open chemical thermal energy storage system by

impregnating CaCl2 into mesopores of Wakkanai

Siliceous Shale (WSS), which was built into a honeycomb

（Wakkanai siliceous shale (WSS) 80%, binder 20%）

100 mm

100 mm

36 cells/cm2

200 mm

232 cells/inch2

honeycomb

aluminum flow meter

thermo &hygrometer

CaCl2

fan

hot air producervalve valve

valvevalvethermostat chamber

glass wool

insulation

layer

T thermocouples

inlet outlet

Test section

Thermometry point of the test filter

inlet outlet

200mm

10 40

No.1 No.2 No.3 No.4 No.5 No.6

19080 120 160

data loggerstainless casing

structure (10 cm (width) × 10 cm (length) × 20 cm

(height)) with 36 cells/cm2 shown in Fig. 4 to ensure a

large contact area and low pressure loss. The thickness of

the wall of each cubic cell is 0.28 mm. The original

ceramic material is denoted as WSS, and the other three

CaCl2-supported composite ceramic samples are denoted

as WSS + 2.2 wt% CaCl2, WSS + 13.0 wt% CaCl2, and

WSS + 22.4 wt% CaCl2.

The honeycomb structure thermal energy storage

medium is installed in an open thermal energy storage

system illustrated in Fig. 5, and it can be regenerated at

80ºC showing a high thermal energy storage density at the

same time. The outlet temperature of air flowing through

the filters supported with different amounts of CaCl2 is

shown in Fig. 6.

We assume that for an actual system, an air

temperature difference of greater than 15ºC is required for

waste heat recovery with a long temperature duration of 5-

8 hr for different applications. In this case, WSS is not

suitable for use as a sorption thermal energy storage

material because of its low outlet air temperature, and

WSS + 2.2 wt% CaCl2 is also inappropriate for short

durations of maintaining a high outlet air temperature. In

other words, a honeycomb filter supported by 13.0 wt%

CaCl2 can supply air at temperatures exceeding 40ºC for

longer than 5 hr, and the WSS + 22.4 wt% CaCl2 can

provide a high temperature for almost 9 hr.

Chapter 3 Numerical simulation of an open sorption

thermal energy storage system using composite

sorbent built into a honeycomb structure

In order to decide the best operational condition of the

developed open thermal energy storage system in Chapter

2, a numerical model is created shown in Fig. 7.

The simulation results can approximately predict the

experimental values. Using the simulation program,

optimal operating conditions are selected as follows: Ta,in

= 25°C or 30°C, RHa,in = 95%, fa = 3.0 m3/h, L = 20 or 25

cm for a heat release duration of ten hours.

A realistic application shown in Fig. 8 is proposed and

simulated, which can supply air with a temperature greater

than 40°C for 14 hours, and can be regenerated by the

exhaust heat released from the kerosene-fueled blower

within ten hours during the daytime. After incorporating

the developed open chemical thermal energy storage

system, half of the exhaust heat generated by a kerosene-

fueled blower can be recovered.

Fig. 6 Time change of the inlet and outlet air

temperatures (Regeneration temperature: 80ºC; Humid

air flow rate: 2.0 m3/h)

Fig. 7 The one-dimensional transient model illustration

Fig. 8 (a) Previous paint drying system; (b) improved

incorporated with the developed sorption thermal energy

storage unit

20

25

30

35

40

45

50

20

25

30

35

40

45

50

0 100 200 300 400 500 600

Tain

(ºC

)

Taout(º

C)

Time (min)

T T T T T T T T

aout of WSS

aout of WSS + 13.0 wt% CaCl2

ain of WSS

ain of WSS + 13.0 wt% CaCl2

aout of WSS + 2.2 wt% CaCl2

aout of WSS + 22.4 wt% CaCl2

ain of WSS + 2.2 wt% CaCl2

ain of WSS + 22.4wt% CaCl2

L

L

1.59 mm

L

z

Taout

xaout

ua dz

δs

K Ta

Air

Solid

(a)

(b)

Solid δs

h ρa

ρs

da

1.31 mm

Tain

xain

ua

Ts

dz

ωa =f(Ta,xa) = ωe

h

δs

K

Ta

Ts

da

Air

Solid

δs

xa ωa

ω

ωs ka ks

Outside air

Paint

drying

booth

Kerosene

(100)

Exhaust gas (37), 185

kW

Kerosene

fueled blower

Outside air

Paint

drying

booth

(a)

Kerosene

fueled blower

Exhaust gas

(37), 185 kW

Kerosene

(100)

Paint

drying

booth

Outside

air

Hot air (63)

316kW

Thermal energy

storage unit

(b)

Kerosene

fueled blower

Hot air (63)

316kW

Paint

drying

booth

Outside

air

Hot air

(40 – 60) ºC

Water spray heat

exchanger

Gas to gas heat exchanger

Thermal energy

storage unit

Chapter 4 A composite material made from

mesoporous siliceous shale impregnated with LiCl for

an open sorption thermal energy storage system

Chapter 4 develops a new composite material by

impregnating 9.6 wt% LiCl into WSS in order to get a

wider and lower regeneration temperature range. The

WSS + 9.6 wt% LiCl shows the same sorption amount

with WSS + 22.4 wt% CaCl2 shown in Fig. 9, but The

WSS + 9.6 wt% LiCl can be regenerated at 60ºC, and it

shows higher volumetric heat storage density than the

WSS + 22.4 wt% CaCl2 when the outlet and inlet air

temperature difference is 20ºC at the same regeneration

temperature due to lower desorption activation energy (cf.

Fig. 10). The maximum outlet air temperature flowing out

of the WSS + 9.6 wt% LiCl is less affected by the humid

air flow rate in heat release process due to higher sorption

rate when the sorption amount is small. At last, the WSS +

9.6 wt% LiCl is stable when it is regenerated at 60ºC

during the tested hundreds of sorption/desorption cycles

shown in Fig. 11, which indicates that this material can be

used for a rather long time.

Chapter 5 Composite material made from LiCl for

low-regeneration closed sorption air cooler system

A basic research of the composite material

impregnated with LiCl by determining the isobaric and

isosteric sorption chart of the composite material/water

working pair in a closed thermal energy storage system,

specific heat, and activation energy for desorption. A

small scale sorption air cooler by using the developed

composite material is built illustrated in Fig. 12.

A high inlet and outlet air temperature difference is

observed In Fig. 13 due to the rapid evaporation of the

water inside the evaporator. The effective cooling power

qC was calculated by the temperature difference of the

inlet and outlet air temperature and mass flow amount of

the air according to Eq. 2. Then the mass and volumetric

specific cooling power qSC and qVC can be obtained by the

ratio between the effective cooling power and the filling

amount of the composite material, the volume of the

reactor, respectively.

, ,( )C a a p a in a outq Q C T T 2

/SC Cq q m 3

/VC Cq q V 4

A relatively high value of the specific cooling power

and a high cooling COP are obtained from the

experimental results, which indicates a good perspective

of improving the performance of this developed sorption

cooler.

Fig. 9 Comparison between the sorption amount of WSS

+ 9.6wt% LiCl and that of WSS + 22.4 wt% CaCl2 for

both closed system and open system

Fig. 10 Linear dependence between ln(β/TP

2) versus 1/TP

of water desorbed from the three tested materials

Fig. 11 Stability of WSS + 9.6 wt% LiCl (Desorption

process: 60ºC, RH 5%; sorption process: 25ºC, RH 95%)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60 80 100

So

rpti

on

am

ou

nt ω

(g/g

)

Relative humidity RH (%)

Closed system result of WSS

Closed system result of WSS+9.6 wt% LiCl

Closed system result of WSS+22.4 wt% CaCl₂Open system result of the WSS+9.6 wt% LiCl

Open system result of the WSS+22.4 wt% CaCl₂

-12.0

-11.5

-11.0

-10.5

-10.0

-9.5

-9.0

-8.5

-8.0

2.4 2.6 2.8 3 3.2 3.4

ln(dβ/T

p2)

1000/T (1/K)

WSS

WSS + 9.6 wt% LiCl

WSS + 22.4 wt% CaCl₂

E=83.8 kJ/molE=87.7 kJ/molE=124.7 kJ/mol

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 50 100 150 200 250

Times of repetition

Sorption process of WSS + 9.6 wt% LiCl

Deorption process of WSS + 9.6 wt% LiCl

So

rpti

on

am

ou

nt ω

(gH

2O/g

sam

ple)

Chapter 6 Development of coated type reactor for a

closed chemical heat pump system

A larger scale closed chemical thermal energy storage

lab-scale prototype using the developed composite

material in Chapter 5 was built, and the schematic

experimental setup is shown in Fig. 14.

A cooling experimental cycle (Tinev= 12°C, Tinre= 30°C,

Tincon= 30°C, Tinre= 80°C) is presented in Fig. 15 in the

Clapeyron diagram of WSS + 40 wt % LiCl tested in our

previous study, which relates the equilibrium pressure and

temperature at fixed water vapor content. The

experimental cycle is plotted on the isosteric chart of WSS

+ 40 wt% LiCl measured by a thermogravimetry.

The tests were carried out according to the

experimental condition described previously. Fig. 16

shows the behavior of the average temperature of

composite material and inlet and outlet of the heat

exchanger on the reactor side during 100 minutes of

testing. The first cycle is good heat transfer can be

indicated by the small difference of the composite

temperature and the external heat transer fluid. The water

temperature insider the condenser/evaporator and inlet and

outlet of the heat exchanger on the condenser/evaporator

side during 100 minutes of testing are shown in Fig. 17.

The obvious temperature increase of the water in

regeneration process due to water vapor condensation can

be observed. At the same time, a long duration of water

temperature decrease during the sorption process can also

be detected.

Fig. 12 Schematic of the testing sorption cooler

Fig. 13 Reactor and water temperature changes with time

during heat release process

Fig. 14 Schematic diagram of experimental setup

Fig. 15 Cooling cycle on the Clapeyron diagram of WSS

+ 40 wt % LiCl

E

T

T

T T F

Vacuum pump

Reactor

Condenser/

evaporatorInsulation box

Fan

Rib

bo

n h

eater

T Thermocouple

F Flow meter

E Power measure

Air

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60

Flo

w r

ate

(m3/h

)

Tem

per

atu

re (

ºC)

Time (min)

Inlet air temperature

Outlet air temperature

Reactor temperature

Water temperature

Air flow rate

T

Tp

p

TT

F

F

T

T

3

1

4

5 6

7

1. reactor

2. condenser/evaporator

3. circulating thermostatic bath

4. circulating thermostatic bath

5. circulating thermostatic bath

6. circulating thermostatic bath

7. corrugated type heat exchanger coated with reactant

8. corrugated tube heat exchanger

2

8

T

p

F

Vacuum gauge

Thermocouple

Flow meter Vacuum pump

Solenoid valve

Three way valve

10

100

20 30 40 50 60 70 80 90 100 110

P [

hP

a]

T ( C)

water

80 70 60 50 40 30 20 10 5 2 1 (wt%)

Fig. 16 Temperature of the composite material, and inlet

and outlet temperatures of the heat exchanger on reactor

side changes with time

Fig. 17 Temperature of the water inside

condenser/evaporator, and inlet and outlet temperatures

of the heat exchanger on condenser/evaporator side

changes with time

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Consumption, Vol. 234, Springer Netherlands, 2007, pp.

429-444.

20

30

40

50

60

70

80

10 30 50 70 90 110

Tem

per

ature

(ºC

)

Time (min)

Reactor side heat exchanger inlet temperature

Reactor side heat exchanger outlet temperature

Material temperature

10

15

20

25

30

35

40

10 30 50 70 90 110

Tem

per

ature

(ºC

)

Time (min)

Condenser/evaporator side heat exchanger inlet temperature

Condenser/evaporator side heat exchanger outlet temperature

Water temperature