Experimental study of the heat transfer characteristics of a new-type flat micro-heat pipe thermal storage unit

Accepted Manuscript Experimental study of the heat transfer characteristics of a new-type flat micro-heat pipe thermal storage unit Y.H. Diao, S. Wang, Y.H. Zhao, T.T. Zhu, C.Z. Li, F.F. Li PII:

S1359-4311(15)00629-8

DOI:

10.1016/j.applthermaleng.2015.06.070

Reference:

ATE 6763

To appear in:

Applied Thermal Engineering

Received Date: 13 January 2015 Accepted Date: 25 June 2015

Please cite this article as: Y.H. Diao, S. Wang, Y.H. Zhao, T.T. Zhu, C.Z. Li, F.F. Li, Experimental study of the heat transfer characteristics of a new-type flat micro-heat pipe thermal storage unit, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.06.070. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Experimental study of the heat transfer characteristics of a

2

new-type flat micro-heat pipe thermal storage unit

3

Y.H. Diao∗, S. Wang, Y.H. Zhao, T.T. Zhu, C.Z. Li, F.F. Li

4

The Department of Building Environment and Facility Engineering, The College of

5

Architecture and Civil Engineering, Beijing University of Technology, No.100

6

Pingleyuan, Chaoyang District, Beijing 100124, China

RI PT

1

7

Abstract

SC

8

In this study, the heat transfer characteristics of a new-type flat micro-heat pipe

10

thermal storage unit, which uses a moderate-temperature paraffin as heat storage

11

phase change material, were investigated. The basic structure, working principle, and

12

design concept of the thermal storage unit were discussed. Experiments were

13

conducted to study the charging and discharging heat transfer processes of the

14

thermal storage unit under different inlet temperatures and flow rates for the cold/hot

15

heat transfer fluid (HTF). Experimental results show that the performance of the

16

thermal storage unit was steady and efficient during heat charging and discharging,

17

and the average charging and discharging powers of the thermal storage unit were

18

approximately 658 and 894 W, respectively, under specific experimental parameters.

19

Keywords: new-type flat micro-heat pipe, paraffin, phase change material, thermal

20

storage unit

AC C

EP

TE D

M AN U

9

21 22 23 24 ∗

Corresponding author. Tel.: +86 010 67391608-802; fax: +86 010 67391608-802 E-mail address: [email protected] (Y.H. Diao)

1

ACCEPTED MANUSCRIPT Nomenclature Flat micro-heat pipe thermal storage unit

LHTES

Latent heat thermal energy storage

HTF

Heat transfer fluid

PCM

Phase change material

TSB

Thermal storage block

SDHW

Solar domestic hot water

RI PT

FMHPTSU

∆hm

Latent heat capacity (kJ/kg)

∆T

Temperature difference (K)

2

1. Introduction

out

ste

s

Solid

TE D

1

M AN U

SC

Dimensional variables Subscripts cp Specific heat capacity (kJ/kg·K) cold Cold HTF m Mass (kg) hot Hot HTF Q Energy (kJ) i Initial t Time (s) m Melting T Temperature (K) f Final K Heat conductivity coefficient (W/(m·K)) l Liquid in Inlet out Outlet Greek letters

Solar thermal energy for domestic hot water heating is one of the most effective

4

and efficient areas of alternative energy exploitation. The use of phase change

5

materials (PCMs) in latent heat thermal energy storage (LHTES) can reduce the

6

volume and weight of storage because of their high storage density and can

7

overcome major obstacles in the further deployment of solar thermal energy [1].

8

LHTES has high performance, reliability, high storage density, and nearly constant

9

temperature energy delivery. Moreover, LHTES stores/releases energy when

10

materials undergo a phase change. These materials are called PCMs and have

11

recently been the focus of important practical interest because of their high energy

AC C

EP

3

2

ACCEPTED MANUSCRIPT 1

storage density during phase change within a narrow temperature range [2]. However,

2

low thermal conductivity in most PCMs is the main factor that influences the heat

3

transfer performance of thermal storage. To solve the above problem, most heat transfer enhancement techniques have

5

concentrated on improving charging and discharging rates [3, 4]. Many scholars

6

have designed different thermal storage structures. Chiu and Martin [5] studied the

7

thermal performance and heat transfer characteristics of a fin-tube heat exchanger for

8

heat storage that uses stored heat to reduce the load where heating and cooling

9

energies are generated. Rathod et al. [6] conducted an experimental study to enhance

10

the thermal performance in shell-tube latent heat storage unit using longitudinal fins

11

on the PCM side of the HTF tube. Avci et al. [7] proposed a horizontal

12

shell-and-tube type of LHTES system and experimentally investigated the charging

13

and discharging of the PCM. Rathod et al. [8] assembled a shell-and-tube type of

14

heat exchanger using paraffin wax as the PCM. Shon et al. [9] analyzed the thermal

15

behavior of a fin-tube heat exchanger filled with solid PCM to improve the heat

16

storage rate of an automotive coolant waste heat recovery system. Shamsundar et al.

17

[10] described a heat exchanger with an LHTES that uses an array of cylindrical

18

tubes as fluid passage channels and analyzed the two-dimensional phase change

19

process of salt or other PCMs as storage medium. Al-Abidi et al. [11] investigated

20

heat transfer enhancement using internal and external fins for PCM melting in a

21

triplex tube heat exchanger (TTHX) through simulation calculation.

AC C

EP

TE D

M AN U

SC

RI PT

4

3

ACCEPTED MANUSCRIPT A heat pipe thermal storage unit has many advantages over the abovementioned

2

conventional devices. For example, heat transfer areas on the hot fluid, cold fluid,

3

and PCM sides can be designed independently, and the PCM side heat transfer area

4

can be set to any desired value. Faghri [12, 13] described the use of miniature heat

5

pipes in small LHTES modules and applied for two US patents. Wang et al. [14]

6

utilized a thermal energy storage device in which passive control is adopted to

7

eliminate the drawbacks observed in conventional thermal storage systems. The most

8

important characteristic of the device is that no pump and electromagnetic valve are

9

required. Liu et al. [15] established the dynamic characteristics model of a heat

10

storage device with a heat pipe and analyzed the effects of the inlet temperature of

11

the heat pipe medium and the initial temperature of the PCM on the PCM thickness

12

and temperature, total heat storage capacity, and heat storage rate of the heat storage

13

device during the charging process. Liu et al. [16] designed a heat pipe heat

14

exchanger for energy storage. Recently, Bogdan et al. [17] developed a mathematical

15

model to study the solidification within a longitudinal finned heat pipe latent heat

16

thermal storage system. Hussein [18] investigated the efficiency of a wickless heat

17

pipe flat plate solar collector with a cross-flow heat exchanger using theoretical and

18

experimental methods. The results indicated that the number of wickless heat pipes

19

has a significant effect on the efficiency of the collector. Christopher et al. [19]

20

presented embedded heat pipes that enhance LHTES. The above literature indicates

21

that heat pipe thermal storage unit is a new heat storage method.

AC C

EP

TE D

M AN U

SC

RI PT

1

4

ACCEPTED MANUSCRIPT In this study, we adopted a new-type flat micro-heat pipe, which has many

2

advantageous characteristics, including lightness, compactness, efficiency, and a

3

large heat exchange area with a given volume, as the main heat transfer element in

4

thermal storage unit; a US patent has been applied for this new-type flat micro-heat

5

pipe [20]. It is a structural innovation in comparison with the traditional thermal

6

storage unit. To study the actual performance of the thermal storage unit, we

7

conducted an experimental investigation on its charging and discharging

8

performances under various experimental parameters. The mass flow rate is varied

9

from 1.5 L/min to 2.5 L/min, the temperature of the hot HTF is varied from 75 °C to

10

90 °C, and the temperature of the cold HTF is varied from 10 °C to 20 °C.

11

Experimental results illustrate the heat transfer processes of the new-type flat

12

micro-heat pipe, phase change behavior of the PCM, and energy storage capacity of

13

the thermal storage unit. The effects of HTF flow rate and inlet temperature on

14

charging and discharging are also presented.

SC

M AN U

TE D

EP

15

RI PT

1

2. Experimental system, procedure, and calculation method

17

2.1. Heat storage material

18

AC C

16

The PCM is selected based on its phase change temperature range and the

19

operating temperatures of the SDHW system. Paraffin is known to be an interesting,

20

chemically stable, and nontoxic material without regular degradation and has high

21

latent heat storage capacities with a narrow temperature range. In this study, 18.7 kg

5

ACCEPTED MANUSCRIPT 1

of paraffin is used as the PCM. Table 1 outlines the main thermophysical properties

2

of paraffin.

3

2.2. New-type flat micro-heat pipe thermal storage unit (FMHPTSU) In this study, the new-type flat micro-heat pipe is selected as the core component

5

of the thermal storage unit. Acetone is selected as the working medium for the

6

new-type flat micro-heat pipe. The configuration size and cross-section of the

7

new-type flat micro-heat pipe is shown in Fig. 1(c). The heat transfer mechanism of

8

the heat pipe is that the working medium evaporates when the evaporator section

9

absorbs heat from the hot source. Then, the vapor rises to the condenser section and

10

is condensed. Meanwhile, heat is transferred to the cold source. After condensation,

11

the fluids flow back into the evaporator section through the heat pipe microchannels.

12

To study the performance and feasibility of this new-type FMHPTSU, a thermal

13

storage unit with dimensions of 740 × 86 × 460 mm is designed and manufactured.

14

The schematic of the new-type FMHPTSU is shown in Fig. 1(a) and the interior

15

structure is shown in Fig. 1(b). As shown in Fig. 1(f), two new-type flat micro-heat

16

pipes and two longitudinal flat fins are mechanically combined to form the thermal

17

storage block (TSB). The cross-section dimension of the new-type flat micro-heat

18

pipe with a length of 710 mm is shown in Fig. 1(c). The cross-section dimension of

19

the longitudinal flat fin with length of 410 mm is shown in Fig. 1(d). The thermal

20

storage unit mainly consists of five TSBs. Four hetero-shaped pipes [740 mm in

21

length, with the cross-section dimension shown in Fig. 1(e)] are connected to the

22

bottom and top surfaces of these new-type flat micro-heat pipes as fluid passage

AC C

EP

TE D

M AN U

SC

RI PT

4

6

ACCEPTED MANUSCRIPT channels of the thermal storage unit. The hetero-shaped pipes are also mechanically

2

connected to the heat pipes. Hot HTF transfer heat into the thermal storage unit

3

through the bottom of the hetero-shaped pipe and cold HTF absorb heat from the

4

thermal storage unit through the top of the hetero-shaped pipe. Conductive silicone

5

is used to cover the clearance between the new-type flat micro-heat pipe and the flat

6

fin to reduce thermal contact resistance. Furthermore, the new-type flat micro-heat

7

pipe, flat fin, and hetero-shaped pipe are all made of pure aluminum, and the

8

container is made of a 3 mm corrosion-resistant plate. The thermal storage unit and

9

all the exposed surfaces of the experimental system are well insulated with

M AN U

SC

RI PT

1

10

polystyrene materials that have a thermal conductivity of λ = 0.038 W/m•K.

11

2.3. Experimental setup

Fig. 2 shows the schematic of the experimental setup, which consists of the

13

new-type FMHPHSU, a charging loop, and a discharging loop. The following

14

equipment was used: low- and high-temperature baths with a sensitivity of ± 0.1 °C

15

to provide the desired temperature of the HTF; two circulation pumps to provide

16

circulating power; four valves to adjust the HTF flow rate; two turbine flow meters

17

with a sensitivity of 0.05 L/min to measure the HTF flow; and four K-type

18

thermocouples with a accuracy of 0.1 °C to measure the HTF temperatures at the

19

inlets and outlets of the thermal storage unit, which was calibrated by a precision

20

thermometer. An Agilent data collector and a computer were used to record and

21

process the experimental data.

AC C

EP

TE D

12

7

ACCEPTED MANUSCRIPT In this study, 22 K-type thermocouples are used to measure the inside temperature

2

of the thermal storage unit. Fig. 3 shows the coordinate and thermocouple

3

distributions inside the thermal storage unit. Six thermocouples (T1–T6) are

4

arranged on the surface of the new-type flat micro-heat pipe. The position of the

5

measuring point is shown in Fig. 3(a). In the PCM, the thermocouples are arranged

6

in different positions in the y- and z-directions. Fig. 3(a) also shows the

7

thermocouple distribution

8

z-direction. Fig. 3(b) shows the thermocouple distribution (T16–T20) between the

9

ribs in the y-direction. To calculate the heat loss of the container wall, two

10

thermocouples (T21 and T22) are arranged on the inside and outside surfaces of the

11

thermal insulation material, as shown in Fig. 3(c).

12

2.4. Experimental procedure

13

In this study, the charging and discharging process is independent. Prior to starting

14

the charging/discharging experiment, the PCM in the container is heated or cooled

15

by circulating hot and cold HTF, respectively, to ensure a constant PCM initial

16

temperature. For the charging mode, the high-temperature bath is opened to heat the

17

hot HTF to the required inlet temperature, the circulation pump is turned on, and the

18

valves are adjusted to the required flow rate; data are collected at the same time. For

19

the discharging mode, the low-temperature bath is opened to cool the cold HTF to

20

the required inlet temperature, the circulation pump is turned on, and the valves are

21

adjusted to the required flow rate; data are collected at the same time. The

22

experiments were performed with different inlet temperatures and flow rates of the

RI PT

1

AC C

EP

TE D

M AN U

SC

(T7–T15) along the inner surface of the container in the

8

ACCEPTED MANUSCRIPT circulation HTF, and the experimental parameters are outlined in Table 2. Meanwhile,

2

Fig. 4 shows that the Stefan number increases linearly with an increase in hot HTF

3

temperature during charging and a decrease in cold HTF temperature during

4

discharging. This relationship is shown in Eq. (1):

Ste =

5

c p ∆TSte ∆hm

, (1)

RI PT

1

where ∆TSte = Tin ,hot − Tm is the charging experiment temperature difference and

7

∆TSte = Tm − Tin,cold is the discharging experiment temperature difference.

SC

6

In all the experiments in this paper, the charging process starts when the average

9

temperature of the PCM is at 25 °C and ends at 70 °C. The discharging process starts

M AN U

8

10

when the average temperature of the PCM is at 80 °C and ends at 30 °C.

11

2.5. Stored energy

14

TE D

13

During charging/discharging, the actual energy storage capacity of the PCM is estimated by using Eq. (2) as follows:

Qstored = m c p , s (Tm − TPCM ,i ) + ∆hm + c p ,l (TPCM , f − Tm )  , (2)

EP

12

where m is the mass of the PCM in the thermal storage unit ( m = 18.7 kg), cP , s

16

and cP ,l is the specific heat of the solid and liquid PCM, respectively, Tm is the

17

melting temperature of the PCM, TPCM ,i and TPCM , f are the initial and final

18

temperatures of the PCM in the system, respectively. Moreover, we assume that all

19

the PCM melts and reaches the final temperature.

20

3. Results and discussion

21

3.1. Repeatability

AC C

15

9

ACCEPTED MANUSCRIPT The first experiment shows the repeatability of the experimental procedure prior to

2

the actual experiment. The experimental system is charged three times using the

3

same experimental parameters. The three results are compared to evaluate the

4

repeatability of the experimental system. Fig. 5 shows the temperature profiles

5

measured using the thermocouples that are placed on the surface of the heat pipe (T2)

6

and inserted into the PCM (T11) and the inlet and outlet (T23 and T24) of the

7

thermal storage unit.

SC

RI PT

1

Based on the results shown in Fig. 5, the experimental setup clearly produces

9

repeatable results. This finding indicates that these experiments can be run with the

10

same operating system parameters ( TPCM ,i = 25 °C, Tin ,hot = 80 °C, and flow rate =

11

2.0 L/min) with the thermocouples at different measuring point locations. These

12

results can be merged to obtain more data points for the same experimental

13

parameters and provide a clear diagram of the overall transient heat transfer

14

characteristics for the experimental setup.

15

3.2. Performance of the heat pipes

EP

TE D

M AN U

8

To evaluate the performance of the new-type flat micro-heat pipe used in the

17

thermal storage unit, Fig. 6(a) shows the measured variation of the wall temperature

18

of the new-type flat micro-heat pipe with time at a hot HTF flow rate of 2.0 L/min

19

and inlet temperature of 80 °C. The experimental results indicate that the new-type

20

flat micro-heat pipe used in the thermal storage unit is steady and effective. At the

21

beginning of the thermal storage unit charging process, the hot HTF flows through

22

the hetero-shaped tube. Then, heat is transferred from the hot HTF to the heat pipe.

AC C

16

10

ACCEPTED MANUSCRIPT The evaporation section of the heat pipe absorbs the heat, which activates the system

2

to run rapidly. As such, the temperature of the entire heat pipe increases rapidly.

3

When the wall temperature of the heat pipe is higher than the melting temperature of

4

the PCM, the PCM begins to melt and store a large amount of latent heat. Therefore,

5

the increase rate of the wall temperature changes slowly as the process continues [as

6

shown in Fig. 6(a), the experiment was conducted from 30 min to 50 min]. Fig. 6(b)

7

shows the wall temperature variations of the new-type flat micro-heat pipe with time

8

at a cold HTF flow rate of 2.0 L/min and inlet temperature of 15 °C. The cold HTF

9

flows through the hetero-shaped tube at the beginning of the thermal storage unit

10

discharging process. Then, the heat is transferred from the heat pipe to the cold HTF.

11

The condensation section of the heat pipe releases heat and refluxes rapidly. As such,

12

the temperature of the heat pipe decreases rapidly. When the experiment has been

13

underway for 10 min, the wall temperature of the heat pipe is lower than the melting

14

temperature of the PCM, and the PCM begins to solidify and releases latent heat.

15

Then, the decrease rate of the wall temperature slows gradually, as shown in Fig.

16

6(b).

SC

M AN U

TE D

EP

AC C

17

RI PT

1

Fig. 7 shows the wall temperature distribution along the z-direction of the

18

new-type flat micro-heat pipe at various times. The wall temperature of the heat pipe

19

becomes nearly constant as soon as the PCM preheating period ends. The

20

temperature difference along the length direction of the heat pipe is small (usually

21

less than 1 °C), which is a typical characteristic of heat pipes, thus proving its

22

excellent temperature leveling ability and fast transient thermal response. Moreover, 11

ACCEPTED MANUSCRIPT 1

this characteristic is also possible as the new-type flat micro-heat pipe works steadily

2

and normally in the heat transfer process.

3

3.3. Energy storage In the study, energy storage is calculated and analyzed based on the average

5

temperature of paraffin during the charging/discharging mode. For charging, the

6

initial average temperature of the PCM is 25 °C, and the final average temperature of

7

the PCM is 70 °C. For discharging, the initial average temperature of the PCM is

8

80 °C, and the final average temperature of the PCM is 30 °C. Fig. 8(a) shows the

9

actual energy storage during the charging process ( TPCM ,i = 25 °C, TPCM , f = 70 °C,

10

Tin ,hot = 80 °C, and flow rate = 2.0 L/min) and the discharging process ( TPCM ,i = 80 °C,

11

TPCM , f = 30 °C, Tin ,cold = 15 °C, and flow rate = 2.0 L/min). According to the

12

difference between TPCM ,i and TPCM , f of the PCM, the actual energy storages of the

13

paraffin are 5919 and 6173 kJ for the charging and discharging modes, respectively.

14

In addition, the obtained charging and discharging times are 150 and 115 min,

15

respectively. The data indicate that the actual energy storages are unequal for the

16

charging and discharging processes because the charging and discharging values of

17

TPCM ,i and TPCM , f are different. Fig. 8(b) shows the amount of thermal energy

18

transferred in thermal storage unit per second (thermal power) during charging

19

( TPCM ,i = 25 °C, TPCM , f = 70 °C, Tin ,hot = 80 °C, and flow rate = 2.0 L/min) and

20

discharging ( TPCM ,i = 80 °C, TPCM , f = 30 °C, Tin ,cold = 15 °C, and flow rate =

21

2.0 L/min) modes. The average charging and discharging powers of the heat

22

exchanger are approximately 658 and 894 W, respectively. In this study, part of the

AC C

EP

TE D

M AN U

SC

RI PT

4

12

ACCEPTED MANUSCRIPT heat is absorbed by the wall of the thermal storage unit, flat fin, and so on, during the

2

charging and discharging processes. Thus, the practical charging power should be

3

greater than the calculated value and the discharging power should be smaller than

4

the calculated value.

5

3.4. Charging experiment

RI PT

1

The charging experiments were conducted under the following operating

7

parameters: hot HTF inlet temperatures of 75, 80, 85, and 90 °C and hot HTF flow

8

rates of 1.5, 2.0, and 2.5 L/min (as shown in Table 2). All the initial experimental

9

parameters of the charging experiments conducted are initially consistent. The

10

charging experiment begins when the initial average temperature of the PCM is

11

25 °C and ends when the PCM average temperature reaches 70 °C.

12

3.4.1. Temperature profile inside the PCM

TE D

M AN U

SC

6

The system is charged many times under different operating parameters. Thus, we

14

obtained various PCM temperature profiles from the experiments. In this experiment,

15

the system is charged using 80 °C hot HTF with a flow rate of 2.0 L/min.

EP

13

Fig. 9 shows the temperature profiles over time throughout the charging process,

17

which were measured at different positions in the y-direction at the middle of the

18

heat storage section. As shown in Fig. 9, the PCM temperature of T16 increases,

19

whereas the temperature increase of T17-T19 is delayed, although such delay effect

20

is not obvious. The temperature of T20 shows the slowest increase, and the delay

21

effect is obvious. For example, the following temperatures are recorded for T16-T20

22

within 30 min: T16, 56.6 °C; T17, 53.4 °C; T18, 52.9 °C; T19, 52.2 °C; and T20,

AC C

16

13

ACCEPTED MANUSCRIPT 44.1 °C. The high temperature of T16 is mainly caused by its placement on the

2

surface of the flat fin against the heat pipes. Therefore, the temperature of T16 is

3

approximately equal to the heat pipe temperature, and its temperature increase is

4

considerably large. T17-T19 measure the PCM temperature between the rib of the

5

flat fin, moving gradually away from the heat pipes. The temperature of the fin bed

6

can be considered to be approximately equal to that of the fin along the rib height

7

direction. The distance of the rib of the aluminum fin is about 6 mm. Thus, the

8

increase in rib temperature is slightly higher than the temperature increase of

9

T17-T19. Unlike the temperature increase of T16, that of T17-T19 is delayed,

10

although the delay effect is not obvious. Different from the temperature points

11

discussed above, the PCM temperature at T20 has an evident time delay because T20

12

is far from the heat pipe and fin, thereby causing large heat transfer thermal

13

resistance. Thus, the rate of the temperature change is relatively slow. This result

14

indicates that the fin plays a significant role in the charging process. Fig. 9 also

15

shows that heat conduction is the main heat transfer mechanism in the solid PCM

16

during the initial stage. Unlike convective heat transfer, heat conduction is not

17

subject to the effects of gravity. Therefore, the temperature increases slowly in the

18

initial stage.

SC

M AN U

TE D

EP

AC C

19

RI PT

1

Fig. 10 shows the temperature profiles over time throughout the charging process,

20

which were measured at different positions in the z-direction on the internal surface

21

of the container. The charging process can divided into three stages, namely, initial,

22

melting, and liquid stages. Before the temperature reaches the melting point, heat 14

ACCEPTED MANUSCRIPT transfer relies mainly on heat conduction in the stage. The thermal resistance at the

2

PCM heat transfer side is larger and plays a leading role in the heat transfer

3

Therefore, all measuring points of the PCM temperature change tend to be the same.

4

With the increase of temperature in the PCM, the melting stage began when the

5

PCM temperature reaches melting point. The PCM at the top of the container melts

6

faster than the PCM at the middle and bottom of the container as the melting stage

7

continues. It is mainly because the effect of natural convection which causes more

8

melted PCM to move upward to the top of the container. Thus, the PCM temperature

9

at the top of the container increases rapidly. In the liquid stage, along with the

10

accumulation of charging, the temperature gradient of PCM in the z-direction is

11

reduced and due to natural convection. Therefore, we conclude that the natural

12

convection has a significant role in the charging process.

13

3.4.2. Effect of hot HTF inlet temperature on charging

TE D

M AN U

SC

RI PT

1

For the heat transfer process, use a high-temperature heat source is more effective

15

than the use of a low-temperature heat source to heat the same material. Therefore,

16

in this study, the inlet temperature of the hot HTF should strongly influence the

17

charging processes of the new-type FMHPTSU under the same conditions. As such,

18

many experiments are conducted to investigate this effect.

AC C

19

EP

14

The system is charged at an HTF flow rate of 2.0 L/min and various inlet

20

temperatures (75, 80, 85, and 90 °C) to study the effect of inlet temperature on

21

charging. Figs. 11 and 12 show the influences of hot HTF inlet temperature during

22

the charging process on the temperatures of the heat pipe wall and PCM. As 15

ACCEPTED MANUSCRIPT illustrated in Figs. 11 and 12, the final temperature of the PCM increases and the

2

melting time decreases with the increase in hot HTF inlet temperature. In other

3

words, the hot HTF inlet temperature has a significant and direct influence on the

4

melting process of the new-type FMHPHTS. As illustrated in Fig. 12, the charging

5

completion time (that is, the temperature of T20 exceeds 70 °C) for the hot HTF inlet

6

temperatures of 75 °C, 80 °C, 85 °C, and 90 °C are 255, 140, 103, and 90 min,

7

respectively. We can thus conclude that a high inlet temperature leads to a short

8

charging time. This finding can be explained as follows. At the initial stage, the

9

thermal resistance at the PCM heat transfer side is large and performs an important

10

function in the heat transfer process, which leads to the influence of the hot HTF

11

temperature being unapparent. Thus, the trend of temperature variation is almost the

12

same at this stage. At the early melting stage, the thermal resistance at the PCM heat

13

transfer side decreases gradually with the increase in liquid PCM. During the heat

14

transfer process at the actual melting stage, the influence of the hot HTF temperature

15

on the charging process gradually increases. Upon the complete melting of the PCM,

16

the influence of the hot HTF temperature becomes more evident than that at the

17

previous stage. In general, the thermal resistance at the PCM heat transfer side

18

changes gradually, along with the different trends in the phase change transition

19

stage at inlet temperatures of 75 °C, 80 °C, 85 °C, and 90 °C.

20

3.4.3. Effect of flow rate on charging

AC C

EP

TE D

M AN U

SC

RI PT

1

21

The system is charged at 80 °C HTF and various flow rates (1.5, 2.0, and

22

2.5 L/min) to study the effect of flow rate on the charging process. The flow rates 16

ACCEPTED MANUSCRIPT during charging directly affect the melting rate of the PCM. Figs. 13 and 14 depict

2

the expected influences of the hot HTF flow rate on the heat pipe wall and PCM

3

temperatures, respectively. From these two figures, we observed that the hot HTF

4

flow rate influences the charging process, and the temperatures of the heat pipe wall

5

and PCM increase proportionally with the flow rate. As illustrated in Fig. 14, the

6

charging completion times (when the temperature of T20 exceeds 70 °C) for the flow

7

rates of 1.5, 2.0, and 2.5 L/min are 165, 145, and 135 min, respectively. The

8

completion time is obviously reduced to 12% for the hot HTF flow rate from 1.5

9

L/min to 2.0 L/min and to 6.9% for the hot HTF flow rate from 2.0 L/min to 2.5

10

L/min. In other words, the influence of flow rate on the charging process is not

11

obvious and gradually decreases with the increase in the hot HTF flow rate. This

12

result is mainly attributable to the important effect of flow rate change on the

13

convective heat transfer between the hot HTF and the hetero-shaped pipe wall.

14

However, the convective heat transfer between the PCM and TSB plays a lead role in

15

the heat transfer process during charging. Thus, the effect of flow rate change is not

16

decisive.

17

3.5. Discharging experiment

SC

M AN U

TE D

EP

AC C

18

RI PT

1

Discharging experiments were conducted under the following operating

19

parameters: cold HTF inlet temperatures of 10, 15, and 20 °C and cold HTF flow

20

rates of 1.5, 2.0, and 2.5 L/min (as shown in Table 2). All the initial experimental

21

parameters of the discharging experiments conducted are initially consistent. The

17

ACCEPTED MANUSCRIPT 1

discharging experiment starts when the initial average temperature of the PCM is

2

80 °C and ends when the PCM average temperature reaches 30 °C.

3

3.5.1. Temperature profiles inside the PCM The system is discharged many times under different operating parameters. Thus,

5

various PCM temperature profiles can be obtained from the experiments. The system

6

is discharged at a cold HTF temperature of 15 °C and a flow rate of 2.0 L/min.

RI PT

4

From Figs. 15 and 16, we observe that the PCM temperature during discharging

8

also can divided into three stages, namely, initial, solidification, and cooling stages.

9

At the initial stage, a large temperature difference is observed between the

10

high-temperature PCM and low-temperature cold HTF. A lot of sensible heat is

11

released from PCM to the cold HTF and the temperature changes more quickly in

12

the stage. For Fig.16, it can be found that the PCM exist certain temperature gradient

13

temperature measuring point in z-directions, it's means that the natural convection

14

has slight influence on heat transfer in initial stage. When the PCM temperature is

15

close to the solidification temperature, the solidification stage begins. Subsequently,

16

the PCM releases latent heat and the PCM temperature slowly changes and becomes

17

stable. As shown in Fig. 16, the temperature distribution exhibits the same trend at

18

this stage, and the temperatures are approximately equal in the z-direction. This

19

outcome indicates the lack of natural convection caused by the temperature

20

difference. The solidification process is completed when the PCM temperature is less

21

than the melting temperature. Fig. 16 shows that the PCM temperature decreases

AC C

EP

TE D

M AN U

SC

7

18

ACCEPTED MANUSCRIPT 1

slowly during the cooling stage. PCM is in a solid state at this stage; thus, no natural

2

convection occurs as heat conduction plays a lead role.

3

3.5.2. Effect of cold HTF inlet temperature We have observed from the discharging experiments that the cold HTF inlet

5

temperature have a certain influence on the performance of the thermal storage unit.

6

Therefore, many discharging experiments must be conducted under different

7

operating conditions. Figs. 17 and 18 show the influence of the cold HTF inlet

8

temperature on the discharging process. These figures show that the inlet

9

temperature of the cold HTF has a obvious influence on the discharging process.

10

Figs. 17 and 18 show that at the initial and solidification stages, a large amount of

11

heat from the PCM is transferred to the cold HTF because the PCM has substantial

12

sensible heat and latent heat. At these two stages, the discharging capacity of the

13

thermal storage unit mainly depends on the PCM side. Thus, the effect of the cold

14

HTF inlet temperature is not obvious, and the trend of temperature change is almost

15

uniform. At the cooling stage, the heat transfer process depends on heat conduction

16

while the influence of the PCM temperature on the PCM thermal conductivity is

17

extremely small in the PCM solid state. Thus, the overall heat transfer coefficient

18

from the cold HTF to the PCM can be considered to be nearly constant. The

19

temperature difference between the PCM and the cold HTF is the main driving force

20

for the heat transfer process from the PCM to the cold HTF. The lower cold HTF can

21

generate a large temperature difference to accelerate the release of heat and reduce

22

the discharging time. Therefore, the effect of the cold HTF inlet temperature is

AC C

EP

TE D

M AN U

SC

RI PT

4

19

ACCEPTED MANUSCRIPT obvious. As illustrated in Fig. 18, the discharging completion times (when the

2

temperature of T20 is below 25 °C) for the cold HTF inlet temperatures of 20 °C,

3

15 °C, and 10 °C are 235, 145, and 120 min, respectively. A low inlet temperature

4

clearly leads to a short discharging time.

5

3.5.3. Effect of flow rate on discharging

RI PT

1

The system is discharged at a cold HTF temperature of 15 °C and various flow

7

rates (1.5, 2.0 and 2.5 L/min) to study the effect of flow rate on the heat transfer

8

characteristics of the thermal storage unit.

M AN U

SC

6

Figs. 19 and 20 show the influence of the cold HTF flow rate on the heat pipe wall

10

and the PCM temperatures. The two figures also indicate the influence of flow rate

11

on the discharging process. As illustrated in Fig. 20, the discharging completion

12

times (when the temperature of T20 is below 25 °C) for the flow rates of 1.5, 2.0,

13

and 2.5 L/min are 155, 142, and 135 min, respectively. The completion time is

14

reduced to 8.4% for the cold HTF flow rate from 1.5 L/min to 2.0 L/min and to 4.9%

15

for the cold HTF flow rate from 2.0 L/min to 2.5 L/min. This result illustrates that

16

the influence of the cold HTF flow rate on the discharging process is also not

17

obvious. The main reason is that the flow rate change has an important effect on the

18

convective heat transfer between the hetero-shaped pipe wall and the cold HTF.

19

However, the convective heat transfer between the PCM and the TSB plays a lead

20

role in the heat transfer process during discharging. Thus, the effect of flow rate

21

change is not decisive.

22

4. Conclusion

AC C

EP

TE D

9

20

ACCEPTED MANUSCRIPT The results of the experimental study on the new-type FMHPTSU under various

2

experimental parameters show that the new-type flat micro-heat pipe works steadily

3

and efficiently in the charging and discharging processes and significantly improves

4

the heat transfer efficiency of the thermal storage unit.

RI PT

1

The effect of natural convection has a significant role in the charging process as

6

more melted PCM is observed at the top of the container during charging. However,

7

natural convection is weakened significantly during the entire discharging process.

8

Knowledge about natural convection patterns in thermal storage units facilitates the

9

design of the system to maximize the energy storage rate by enhancing heat transfer.

10

Simultaneously, under certain experimental conditions, the charging and discharging

11

times are 150 and 115 min, respectively, and the average charging and discharging

12

powers of the thermal storage unit are approximately 658 and 894 W, respectively.

M AN U

Moreover,

the

TE D

13

SC

5

hot/cold

HTF

temperatures

significantly

influence

the

charging/discharging processes. The increase of the hot HTF temperature results in

15

significantly fast melting. The decrease in the cold HTF temperature results in

16

significantly fast solidification. The hot/cold HTF flow rate also influences the

17

charging/discharging processes, but this influence of the hot/cold HTF flow rate on

18

the charging/discharging process is not obvious. In future work, we will optimize the

19

charging mode and structure size of the thermal storage unit, and use it in solar

20

thermal applications.

21

References

AC C

EP

14

21

ACCEPTED MANUSCRIPT 1

[1] F. Agyenim, N. Hewitt, P. Eames, M. Smyth. A review of materials, heat transfer

2

and phase change problem formulation for LHTESS, Renewable and Sustainable

3

Energy Reviews 2010 (14) 615–628. [2] M. Medrano, M.O. Yilmaz, M. Nogués, I. Martorell, J. Roca, L.F. Cabeza.

5

Experimental evaluation of commercial heat exchangers for use as PCM thermal

6

storage systems. Applied Energy 2009 (86) 2047–2055.

RI PT

4

[3] B. Zalba, J.M. Marín, L.F. Cabeza, H. Mehling. Review on thermal energy

8

storage with phase change: materials, heat transfer analysis and applications.

9

Applied Thermal Engineering 2003(23) 251–283.

M AN U

SC

7

[4] H. Mehling, L.F. Cabeza. Phase change materials and their basic properties. In:

11

Paksoy H, editor. Thermal energy storage for sustainable energy consumption

12

2003 257–277.

TE D

10

[5] J.N.W. Chiu, V. Martin. Submerged finned heat exchanger latent heat storage

14

design and its experimental verification. Applied Energy 2012 (93) 507–516.

15

[6] M.K. Rathod, J Banerjee. Thermal performance enhancement of shell and tube

16

latent heat storage unit using longitudinal fins. Applied Thermal Engineering

AC C

17

EP

13

2015 (75) 1084–1092.

18

[7] M. Avci, M.Y. Yazici. Experimental study of thermal energy storage

19

characteristics of a paraffin in a horizontal tube-in-shell storage unit. Energy

20

Conversion and Management 2013 (73) 271–277.

22

ACCEPTED MANUSCRIPT 1

[8] M.K. Rathod, J. Banerjee. Experimental investigations on latent heat storage unit

2

using paraffin wax as phase change material. Experimental Heat Transfer, 2014

3

(27) 40–55. [9] J. Shon, H. Kim, K. Lee. Improved heat storage rate for an automobile coolant

5

waste heat recovery system using phase-change material in a fin-tube heat

6

exchanger. Applied Energy 2014 (113) 680–689.

RI PT

4

[10] N. Shamsundar, R. Srinivasan. Analysis of energy storage by phase change with

8

an array of cylindrical tubes. In: Proceedings of ASME Winter Annual Meeting.

9

USA: San Francisco 1978 35–40.

M AN U

SC

7

[11] A.A. Al-Abidi, S. Mat, K. Sopian, M.Y. Sulaiman. Internal and external fin heat

11

transfer enhancement technique for latent heat thermal energy storage in triplex

12

tube heat exchangers [J]. Applied Thermal Engineering, 2013(53) 147–156.

TE D

10

[12] A. Faghri. US Patent No. 5000252, 1991.

14

[13] A. Faghri. US Patent No. 4976308, 1990.

15

[14] J.C. Wang, S.J. Lin, S.L. Chen. Charge and discharge characteristics of a

16

thermal energy storage device. Experimental Heat Transfer, 2005 (18) 45–60.

17

[15] X. Liu, G.Y. Fang, Z. Chen. Dynamic charging characteristics modeling of heat

18

storage device with heat pipe. Applied Thermal Engineering 2011 (31)

AC C

19

EP

13

2902–2908.

20

[16] Z.L. Liu, Z.Y. Wang, C.F. Ma. An experimental study on heat transfer

21

characteristics of heat pipe heat exchanger with latent heat storage. Part I:

23

ACCEPTED MANUSCRIPT 1

charging only and discharging only modes. Energy Conversion and Management

2

2006 (47) 944–966. [17] H. Bogdan, D. Gheorghe, P. Aristotel. Mathematical models for the study of

4

solidification within a longitudinally finned heat pipe latent heat thermal storage

5

system. Energy Conversion and Management 1999 (40) 1765–1774.

RI PT

3

[18] H.M.S. Hussein. Theoretical and experimental investigation of wickless heat

7

pipes flat plate solar collector with cross flow heat exchanger. Energy

8

Conversion and Management 2007 (48) 1266–1272.

M AN U

SC

6

9

[19] C.W. Robak, T.L. Bergman, A. Faghri. Enhancement of latent heat energy

10

storage using embedded heat pipes. International Journal of Heat and Mass

11

Transfer 2011 (54) 3476–3484.

[20] Y. H. Zhao, K. R. Zhang, Y. H. Diao. US Patent No. 0203777, 2011.

13

[21] E.M. Robynne, G. Dominic. Experimental study of the phase change and energy

14

characteristics inside a cylindrical latent heat energy storage system: Part 1

15

consecutive charging and discharging. Renewable Energy 2014 (62) 571–581.

EP

19

AC C

16

TE D

12

20

Fig. 3. Thermocouple distribution on the new-type flat micro-heat pipe and in the

17 18

21 22 23

Figure caption

Fig. 1. New-type FMHPTSU Fig. 2. Schematic of the experimental system

PCM of the thermal storage unit Fig. 4. Stefan number of different hot and cold HTF temperatures during charging and discharging

24

ACCEPTED MANUSCRIPT 1

Fig. 5. Temperature profiles over time for three charging experiments measured at

2

different temperature measuring points during the charging mode ( TPCM ,i = 25 °C,

3

Tin ,hot = 80 °C, and flow rate = 2.0 L/min)

Fig. 6. Wall temperature variation of the new-type flat micro-heat pipe with time

5

at different z-direction positions in the heat storage section: (a) charging mode

6

( TPCM ,i = 25 °C, Tin ,hot = 80 °C, and flow rate = 2.0 L/min) and (b) discharging mode

7

( TPCM ,i = 80 °C, Tin ,cold = 15 °C, and flow rate = 2.0 L/min)

RI PT

4

Fig. 7. Wall temperature distribution along the z-direction of the new-type flat

9

micro-heat pipe at various times: (a) charging mode ( TPCM ,i = 25 °C, Tin ,hot = 80 °C,

10

and flow rate = 2.0 L/min) and (b) discharging mode ( TPCM ,i = 80 °C, Tin ,cold = 15 °C,

11

and flow rate = 2.0 L/min)

M AN U

SC

8

Fig. 8. (a) Actual energy storage and (b) power during the charging ( TPCM ,i = 25 °C,

13

TPCM , f = 70 °C, Tin ,hot = 80 °C, and flow rate = 2.0 L/min) and discharging ( TPCM ,i =

14

80 °C, TPCM , f = 30 °C, Tin ,cold = 15 °C, and flow rate = 2.0 L/min) modes

TE D

12

Fig. 9. PCM temperature profiles over time measured at different positions in the

16

y-direction at the middle of the heat storage section: charging mode ( TPCM ,i = 25 °C,

17

Tin ,hot = 80 °C, and flow rate = 2.0 L/min)

EP

15

Fig. 10. PCM temperature profiles over time measured at different positions in the

19

z-direction on the internal surface of the container: charging mode ( TPCM ,i = 25 °C,

20

Tin ,hot = 80 °C, and flow rate = 2.0 L/min)

21 22 23 24

AC C

18

Fig. 11. Influence of hot HTF inlet temperature during the charging process on the heat pipe wall temperature at T3 ( TPCM ,i = 25 °C and flow rate = 2.0 L/min) Fig. 12. Influence of hot HTF inlet temperature during the charging process on the PCM temperature at T20 ( TPCM ,i = 25 °C and flow rate = 2.0 L/min)

25

ACCEPTED MANUSCRIPT 1 2 3 4

Fig. 13. Influence of hot HTF flow rate during the charging process on the heat pipe wall temperature at T3 ( TPCM ,i = 25 °C and Tin ,hot = 80 °C) Fig. 14. Influence of hot HTF flow rate during the charging process on the PCM temperature at T20 ( TPCM ,i = 25 °C and Tin ,hot = 80 °C) Fig. 15. PCM temperature over time at different positions in the y-direction at the

6

middle of the heat storage section: discharging mode ( TPCM ,i = 80 °C, Tin ,cold = 15 °C,

7

and flow rate = 2.0 L/min)

RI PT

5

Fig.16. PCM temperature over time at different positions in the z-directions on the

9

internal surface of the container: discharging mode ( TPCM ,i = 80 °C, Tin ,cold = 15 °C,

14 15 16 17 18 19 20 21 22

M AN U

13

the heat pipe wall temperature at T3 ( TPCM ,i = 80 °C and flow rate = 2.0 L/min) Fig. 18. Influence of cold HTF inlet temperature during the discharging process on the PCM temperature at T20 ( TPCM ,i = 80 °C and flow rate = 2.0 L/min)

TE D

12

Fig. 17. Influence of cold HTF inlet temperature during the discharging process on

Fig. 19. Influence of cold HTF flow rate during the discharging process on the heat pipe wall temperature at T3 ( TPCM ,i = 80 °C and Tin ,cold = 15 °C) Fig. 20. Influence of cold HTF flow rate during the discharging process on the

EP

11

and flow rate = 2.0 L/min)

PCM temperature at T20 ( TPCM ,i = 80 °C and Tin ,cold = 15 °C)

AC C

10

SC

8

Tables

Table 1 Thermophysical properties of the paraffin used in the study Table 2 List of experiments conducted and experimental parameters used

23

26

ACCEPTED MANUSCRIPT Tables

RI PT

Table 1 Thermophysical properties of the paraffin used in the study Property Value Melting/solidification temperature [ ] 58 Latent heat capacity [kJ/kg] 198.8 Thermal conductivity [W/(m·K)]—solid/liquid 0.22/0.27 Specific heat [kJ/(kg·K)]—solid/liquid 2.60/2.66

AC C

EP

TE D

M AN U

SC

Table 2 List of experiments conducted and experimental parameters used Temperature Flow rate (water side) Process HTF inlet ( ) PCM at start ( ) Volumetric flow rate (L/min) Repeatability 80 25 2.0 90 25 1.5 2.0 2.5 85 25 1.5 2.0 2.5 Charging 80 25 1.5 2.0 2.5 75 25 1.5 2.0 2.5 10 80 1.5 2.0 2.5 Discharging 15 80 1.5 2.0 2.5 20 80 1.5 2.0 2.5

1

ACCEPTED MANUSCRIPT Figures (a) Cold HTF

RI PT

Hetero-shape pipe New-type flat micro-heat pipe

Flat fin

M AN U

740

Hot HTF

TE D

(b)

(d)

AC C

EP

(c)

SC

86

1

460

ACCEPTED MANUSCRIPT (e)

SC

RI PT

(f)

AC C

EP

TE D

M AN U

Fig. 1. New-type FMHPTSU

Fig. 2. Schematic of the experimental system

2

ACCEPTED MANUSCRIPT (c)

(a)

EP

TE D

M AN U

SC

RI PT

(b)

Fig. 3. Thermocouple distribution on the new-type flat micro-heat pipe and in the PCM of the thermal storage unit 0.48

Stefon

0.40 0.36 0.32

0.64 0.62 0.60

Stefon

0.44

0.66

AC C

(a) Charging

0.58 0.56 0.54

0.28

0.52

0.24

0.50

0.20

0.48

75

80

T (? )

85

90

(b) Discharging 10

15

20

T (? )

Fig. 4. Stefan number of different hot and cold HTF temperatures during charging and discharging 3

ACCEPTED MANUSCRIPT 90

70 60 50 40 T2Exp1 T11Exp1 T23Exp1 T24Exp1

30 20

0

30

60

T2Exp2 T11Exp2 T23Exp2 T24Exp2

90

RI PT

Temperature(

)

80

T2Exp3 T11Exp3 T23Exp3 T24Exp3

120

150

SC

Time(min)

Fig. 5. Temperature profiles over time for three charging experiments measured at different

M AN U

temperature measuring points during the charging mode ( TPCM ,i = 25 °C, Tin ,hot = 80 °C, and flow rate = 2.0 L/min)

80 (a) Charging

)

70

TE D

Temperature (

60 50 40

EP

30

0

30

60

90

Time(min)

AC C

20

T2 T3 T4 T5

4

120

150

ACCEPTED MANUSCRIPT 90 T2 T3 T4 T5

70 60 50 40 30 (b) Discharging

20

0

20

40

60

80

100

120

SC

Time(min)

RI PT

Temperature (

)

80

Fig. 6. Wall temperature variation of the new-type flat micro-heat pipe with time at different

M AN U

z-direction positions in the heat storage section: (a) charging mode ( TPCM ,i = 25 °C, Tin ,hot = 80 °C, and flow rate = 2.0 L/min) and (b) discharging mode ( TPCM ,i = 80 °C, Tin ,cold = 15 °C, and flow rate = 2.0 L/min)

80

50 40

30min 60min 90min 120min 150min

EP

Temperature (

60

TE D

)

70

30

AC C

(a) Charging

20

T2

T3

T4

5

T5

ACCEPTED MANUSCRIPT 70 30 min 60 min 90 min 120 min 150 min

(b) Discharging

)

60

40 30

RI PT

Temperature (

50

20 10

T4

T3

T5

SC

T2

Fig. 7. Wall temperature distribution along the z-direction of the new-type flat micro-heat pipe at

M AN U

various times: (a) charging mode ( TPCM ,i = 25 °C, Tin ,hot = 80 °C, and flow rate = 2.0 L/min) and (b) discharging mode ( TPCM ,i = 80 °C, Tin ,cold = 15 °C, and flow rate = 2.0 L/min) 7000 (a)

2000 1000

TE D

3000

EP

Energy storage (KJ)

5000 4000

6173 (KJ)

5919 (KJ)

6000

0

Charging

Discharging

AC C

1000

900

(b)

894 (W)

800

Power (W)

700

658 (W)

600 500 400 300 200 100

0

Charging

Discharging

6

ACCEPTED MANUSCRIPT Fig. 8. (a) Actual energy storage and (b) power during the charging ( TPCM ,i = 25 °C, TPCM , f = 70 °C, Tin ,hot = 80 °C, and flow rate = 2.0 L/min) and discharging ( TPCM ,i = 80 °C, TPCM , f = 30 °C, Tin ,cold = 15 °C, and flow rate = 2.0 L/min) modes

RI PT

80

60 50

SC

Temperature ( )

70

40

20

0

30

M AN U

30

60

90

T20 T19 T18 T17 T16

120

150

Time (min)

Fig. 9. PCM temperature profiles over time measured at different positions in the y-direction at the middle of the heat storage section: charging mode ( TPCM ,i = 25 °C, Tin ,hot = 80 °C, and flow rate = 2.0

TE D

L/min)

80

Initial stage

Liquid stage

)

EP

70

Melting stage

T7 T8 T9 T10 T11 T12 T13 T14 T15

50

AC C

Temperature (

60

40 30 20

0

30

60

90

120

150

Time (min)

Fig. 10. PCM temperature profiles over time measured at different positions in the z-direction on the internal surface of the container: charging mode ( TPCM ,i = 25 °C, Tin ,hot = 80 °C, and flow rate = 2.0 L/min)

7

ACCEPTED MANUSCRIPT 90

70 60 50 Inlet temp. 75 80 85 90

40 30 20

0

30

60

90

120

150

SC

Time (min)

RI PT

Temperature (

)

80

Fig. 11. Influence of hot HTF inlet temperature during the charging process on the heat pipe wall

M AN U

temperature at T3 ( TPCM ,i = 25 °C and flow rate = 2.0 L/min)

90

70 60 50 40 30 0

30

EP

20

TE D

Temperature ( )

80

Inlet temp. 75 80 85 90

60

90

120

150

Time (min)

AC C

Fig. 12. Influence of hot HTF inlet temperature during the charging process on the PCM temperature at T20 ( TPCM ,i = 25 °C and flow rate = 2.0 L/min)

8

ACCEPTED MANUSCRIPT 80

)

70

50 40

Flow rate 1.5 L/min 2.0 L/min 2.5 L/min

30 20

RI PT

Temperature (

60

0

30

60

90

120

150

SC

Time (min)

Fig. 13. Influence of hot HTF flow rate during the charging process on the heat pipe wall

M AN U

temperature at T3 ( TPCM ,i = 25 °C and Tin ,hot = 80 °C)

80

)

70

50

TE D

Tempreature (

60

40 30

0

30

EP

20

Flow rate 1.5 L/min 2.0 L/min 2.5 L/min

60

90

120

150

Time (min)

AC C

Fig. 14. Influence of hot HTF flow rate during the charging process on the PCM temperature at T20 ( TPCM ,i = 25 °C and Tin ,hot =80 °C)

9

ACCEPTED MANUSCRIPT 90 T16 T17 T18 T19 T20

70 60 50 40 30 20 10 0

30

60

90

120

SC

Time (min)

RI PT

Temperature (

)

80

Fig. 15. PCM temperature over time at different positions in the y-direction at the middle of the heat

M AN U

storage section: discharging mode ( TPCM ,i = 80 °C, Tin ,cold = 15 °C, and flow rate = 2.0 L/min)

90 Initial stage

Solidification stage

70 60 50 40 30 0

EP

20

Cooling stage

TE D

Temperature (

)

80

30

60

90

T7 T8 T9 T10 T11 T12 T13 T14 T15

120

Time (min)

AC C

Fig.16. PCM temperature over time at different positions in the z-directions on the internal surface of the container: discharging mode ( TPCM ,i = 80 °C, Tin ,cold = 15 °C, and flow rate = 2.0 L/min)

10

ACCEPTED MANUSCRIPT 90 Inlet temp. 10 15 20

70 60 50 40 30 20 10

0

30

60

90

120

SC

Time (min)

RI PT

Temperature (

)

80

Fig. 17. Influence of cold HTF inlet temperature during the discharging process on the heat pipe wall

M AN U

temperature at T3 ( TPCM ,i = 80 °C and flow rate = 2.0 L/min)

90

Inlet temp. 10 15 20

70 60

TE D

Temperature ( )

80

50 40 30

EP

20

0

20

40

60

80

100

120

Time (min)

AC C

Fig. 18. Influence of cold HTF inlet temperature during the discharging process on the PCM temperature at T20 ( TPCM ,i = 80 °C and flow rate = 2.0 L/min)

11

ACCEPTED MANUSCRIPT 90 Flow rate 1.5 L/min 2.0 L/min 2.5 L/min

70 60 50 40

RI PT

Temperature ( )

80

30 20 0

20

40

60

80

100

120

SC

Time (min)

Fig. 19. Influence of cold HTF flow rate during the discharging process on the heat pipe wall

M AN U

temperature at T3 ( TPCM ,i = 80 °C and Tin ,cold = 15 °C)

90

Flow rate 1.5 L/min 2.0 L/min 2.5 L/min

70 60

TE D

Temperature (

)

80

50 40 30

EP

20

0

30

60

90

120

150

Time (min)

AC C

Fig. 20. Influence of cold HTF flow rate during the discharging process on the PCM temperature at T20 ( TPCM ,i = 80 °C and Tin ,cold = 15 °C)

12

ACCEPTED MANUSCRIPT Highlights

A thermal storage unit using new-type flat micro-heat pipes was presented.



The performance of the thermal storage unit was investigated experimentally.



The thermal storage unit has good performance in the charging/discharging

RI PT



AC C

EP

TE D

M AN U

SC

process.