heat exchanger with heat storage capability

8/2/2019 Heat Exchanger with Heat Storage Capability

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SEMINAR REPORT

ON

HEAT PIPE HEAT EXCHANGER WITH LATENT HEAT STORAGE

Presented by

SENTHIL.R (M070196ME)

THERMAL SCIENCES

Department of Mechanical Engineering

NATIONAL INSTITUTE OF TECHNOLOGY CALICUT

CALICUT -673601, KERALAWINTER 2007-08


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CERTIFICATE

This is to certify that this seminar report entitled Heat pipe heat exchanger

with latent heat storage is a bonafide record of the seminar presented by

SENTHIL .R, Reg. No M070196ME during the winter semester 2007-2008 in

partial fulfillment of the requirement for the award of M. Tech degree in

Mechanical Engineering by the National Institute of Technology, Calicut.

Faculty in charge of Seminar

Dr.A.Ramaraju Dr. C.B.Sobhan

Professor Professor

Department Of Mechanical Engineering

National Institute Of Technology, Calicut


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ACKNOWLEDGEMENT

I would like to express my sincere gratitude to the faculty of The

Department of Mechanical Engineering for their kind cooperation to my seminar

topic. I especially express my gratitude to Dr.A.Ramaraju and Dr. C.B.Sobhan

for their able guidance on the subject.

I also wish to thank all my friends for their kind co-operation


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CONTENTS

PAGE NO:

ABSTRACT 4

1. INTRODUCTION 6

2. ENERGY STORAGE METHODS 6

3. PHASE CHANGE MATERIALS 9

4. EXPERIMENTAL PROCEDURE AND SETUP 15

5. RESULTS AND DISCUSSIONS 16

5.1. Charging only operation performance 16

5.2. Discharging only operation performance 25

5.3. Simultaneous charging/discharging performance 30

6. CONCLUSION 31

REFERENCES 32


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ABSTRACT

A new thermal storage system, a heat pipe heat exchanger with latent heat storage, isdiscussed. The new thermal system, a heat exchanger which have a bunch of heat pipes, have a

provision for energy storage and may operate in three basic different operation modes, the

charging only, the discharging only and the simultaneous charging/discharging modes, which

makes the system suitable for various time and/or weather dependent energy systems. In this

section, the basic structure, the working principle and the design concept are briefly discussed.

An brief introduction about the phase change materials, which stores thermal energy as latent

heat, were also discussed. Extensive experimental results are presented of the charging only and

discharging only operations and the effects of the inlet temperature and the flow rate of the cold/

hot water were also discussed. The results show that the heat exchanger performs the designed

functions very well and can both store and release the thermal energy efficiently.


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Nomenclature

mc Mass of cold fluid in kg.

mh Mass of hot fluid in kg.

Cp Specific heat of the fluid.

Ti Initial temperature of the fluid

Tf Final temperature of the fluid

Tm Melting point of PCMQ Amount of heat transfer

Th Temperature of hot fluidTc Temperature of cold fluid

T pcm Temperature of PCM

h m Change in enthalpy of the medium

ar Fraction of PCM melted

Rw Thermal conduction resistance of the heat pipe wall

Rhh Convection resistance between the hot water and the heat pipe wall

Rhc Convection resistance between the heat pipe wall and the cold water

RHP Axial thermal resistance of the heat pipe

Rfh Thermal resistance between the heat pipe and the working fluid in hot fluid

Rch Thermal resistance between the heat pipe and the working fluid in cold fluid

Rh Resistance on hot fluid side

Rc Resistance on cold fluid side


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1.INTRODUCTION

The storage of energy in suitable forms, which can conventionally be converted into the required

form, is a present day challenge to the technologists. Energy storage not only reduces the

mismatch between supply and demand but also improves the performance and reliability of

energy systems and plays an important role in conserving the energy. It leads to saving of

premium fuels and makes the system more cost effective by reducing the wastage of energy and

capital cost. For example, storage would improve the performance of a power generation plant

by load leveling and higher efficiency would lead to energy conservation and lesser generation

cost. Latent thermal energy storage system that could be used for smoothing daily load profiles

consisted of narrow vertical parallel plates of PCM separated by rectangular flow passages. In

order to enhance the heat transfer process between the PCM and the working fluid, variouscapsules packed bed latent heat storage systems have also been proposed.

2. ENERGY STORAGE METHODS

Types of energy storage methods are given below.

The different forms of energy that can be stored include mechanical, electrical and

thermal energy. One of prospective techniques of storing thermal energy is the application of

phase change materials (PCMs). The use of a latent heat storage system using phase change

materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-

energy storage density and the isothermal nature of the storage process.

2.1. Mechanical energy storage

Mechanical energy storage systems include gravitational energy storage or pumped hydropower

storage (PHPS), compressed air energy storage (CAES) and flywheels. The PHPS and CAES

technologies can be used for large-scale utility energy storage while flywheels are more suitablefor intermediate storage. Storage is carried out when inexpensive off-peak power is available,

e.g., at night or weekends. The storage is discharged when power is needed because of

insufficient supply from the base-load plant.


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2.2. Electrical storage

Energy storage through batteries is an option for storing the electrical energy. A battery is

charged, by connecting it to a source of direct electric current and when it is discharged, the

stored chemical energy is converted into electrical energy. Potential applications of batteries are

utilization of off-peak power, load leveling, and storage of electrical energy generated by wind

turbine or photovoltaic plants. The most common type of storage batteries is the lead acid and

NiCd.

2.3. Thermal energy storage

Thermal energy storage can be stored as a change in internal energy of a material as sensible

heat, latent heat and thermochemical or combination of these. An overview of major technique of

storage of solar thermal energy is shown in Fig. 1 .

Fig. 1. Different types of thermal energy storage .

2.3.1. Sensible heat storage: In sensible heat storage (SHS), thermal energy is stored by raising

the temperature of a solid or liquid. SHS system utilizes the heat capacity and the change in

temperature of the material during the process. The amount of heat stored depends on the

specific heat of the medium, the temperature change and the amount of storage material.


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Amount of heat energy stored is given by, Q = mCp(Tf - Ti)

The sensible heat storage capacity of some selected solidliquid materials is shown in Table 1 .

Table 1: list of solidliquid materials for sensible heat storage

2.3.2. Latent heat storage: Latent heat storage (LHS) is based on the heat absorption or release

when a storage material undergoes a phase change from solid to liquid or liquid to gas or vice

versa. The storage capacity of the LHS system with a PCM(Phase Change Material) medium is

given by

Q = m[C sp(Tm - Ti) + a mh m + C lp(Tf - Tm)]

2.4. Thermochemical energy storage .

Thermochemical systems rely on the energy absorbed and released in breaking and reforming

molecular bonds in a completely reversible chemical reaction. In this case, the heat stored

depends on the amount of storage material, the endothermic heat of reaction, and the extent of

conversion.

Q = a r m hr

Amongst above thermal heat storage techniques, latent heat thermal energy storage is

particularly attractive due to its ability to provide high-energy storage density and itscharacteristics to store heat at constant temperature corresponding to the phase-transition

temperature of phase change material (PCM). Phase change can be in the following form: solid

solid, solidliquid, solidgas, liquidgas and vice versa.


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In solidsolid transitions, heat is stored as the material is transformed from one

crystalline to another. These transitions generally have small latent heat and small volume

changes than solid liquid transitions. Solidsolid PCMs offer the advantages of less stringent

container requirements and greater design flexibility. Solidgas and liquidgas transition through

have higher latent heat of phase transition but their large volume changes on phase transition are

associated with the containment problems and rule out their potential utility in thermal-storage

systems. Large changes in volume make the system complex and impractical. Solidliquid

transformations have comparatively smaller latent heat than liquidgas. However, these

transformations involve only a small change in volume. Solidliquid transitions have proved to

be economically attractive for use in thermal energy storage systems. PCMs themselves cannot

be used as heat transfer medium. A separate heat transfer medium must me employed to transfer

energy with heat exchanger between one fluid and the PCM. The heat exchanger to be used hasto be designed specially, in view of the low thermal diffusivity of PCMs and volume changes.

Any latent heat energy storage system therefore, possess at least following three

components:

(i) a suitable PCM with its melting point in the desired temperature range,

(ii) a suitable heat exchange surface, and

(iii) a suitable container compatible with the PCM.

The development of a latent heat thermal energy storage system hence, involves the

understanding of three essential subjects: phase change materials, containers materials and heat

exchangers.

3. PHASE CHANGE MATERIALS

Phase change materials (PCM) are Latent heat storage materials. The thermal energy transfer occurs when a material changes from solid to liquid, or liquid to solid. This is calleda change in

state, or Phase. Initially, these solidliquid PCMs perform like conventional storage

materials, their temperature rises as they absorb heat. Unlike conventional (sensible) storage

materials, PCM absorbs and release heat at a nearly constant temperature. They store 514 times

more heat per unit volume than sensible storage materials such as water, masonry, or rock. A


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large number of PCMs are known to melt with a heat of fusion in any required range. However,

for their employment as latent heat storage materials these materials must exhibit certain

desirable thermodynamic, kinetic and chemical properties which are as follows:

3.1. Thermal properties

(i) Suitable phase-transition temperature.

(ii) High latent heat of transition.

(iii) Good heat transfer.

Selecting a PCM for a particular application, the operating temperature of the heating or

cooling should be matched to the transition temperature of the PCM. The latent heat should be as

high as possible, especially on a volumetric basis, to minimize the physical size of the heat store.

High thermal conductivity would assist the charging and discharging of the energy storage.

3.2. Physical properties

(i) Favorable phase equilibrium.

(ii) High density.

(iii) Small volume change.

(iv) Low vapor pressure.

Phase stability during freezing melting would help towards setting heat storage and high

density is desirable to allow a smaller size of storage container. Small volume changes on phasetransformation and small vapor pressure at operating temperatures to reduce the containment

problem.

3.3. Kinetic properties

(i) No supercooling.

(ii) Sufficient crystallization rate.

3.4. Chemical properties(i) Long-term chemical stability.

(ii) Compatibility with materials of construction.

(iii) No toxicity.

(iv) No fire hazard.


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3.5. Economics

(i) Abundant.

(ii) Available.

(iii) Cost effective.

Low cost and large-scale availability of the phase change materials is also very important.

Some popular phase change materials are given below.

Table 2 : List of phase change materials and their properties

One major issue that needs to be addressed is that most phase-change materials (PCM)

with high energy storage density have an unacceptably low thermal conductivity and hence heat

transfer enhancement techniques are required for any latent heat thermal storage (LHTS)

applications. To increase the heat transfer rate in the heat exchanger, between the fluid and PCM,

it uses a set of heat pipes. This equipment is what we call as heat pipe heat exchanger.

In a latent heat thermal storage (LHTS) system, during phase change the solidliquid

interface moves away from the heat transfer surface. During this process, the surface heat flux

decreases due to the increasing thermal resistance of the growing layer of the molten/ solidified

medium. In the case of solidification, conduction is the sole transport mechanism, and in the case


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of melting, natural convection occurs in the melt layer and this generally increases the heat

transfer rate compared to the solidification process. This decrease of the heat transfer rate calls

for the usage of proper heat transfer enhancement techniques in the LHTS systems. So some fins

are attached to the surface of the heat pipe.

A new thermal storage system, a heat pipe heat exchanger with latent heat storage system

may operate in three basic different operation modes, the charging only, the discharging only and

the simultaneous charging/discharging modes, which makes the system suitable for various time

dependent energy systems.Latent TES is receiving more and more attention because of its large

energy storage density and the significant reduction in storage volume and, most importantly, the

isothermal behavior during the charging and discharging process compared with sensible heat

storage systems. Hence, latent thermal energy storage is widely used in the conversion andutilization of renewable energies, and in various heat recovery systems. we can see, all these

latent thermal energy storage devices use the wall of the heat transfer fluid passage as the heat

transfer surface of the PCM. This means the heat transfer area on the PCM side is completely

determined by the heat transfer area on the working fluid side, although these two heat transfer

areas may not be equal. However, as we know, most PCMs are poor heat conduction media, and

therefore, the dominant thermal resistances in the heat transfer process between the PCM and the

working fluid is on the PCM side. Therefore, according to heat transfer theory, the most efficient

way for improving the heat transfer process is to enhance the heat transfer on the PCM side.

Various methods for PCM thermal conductivity enhancement have been proposed and studied by

many researchers. Some of the most common methods are attaching fins to heat transfer walls,

dispersing metal particles or rings or carbon fibers of high conductivity into PCMs, etc.

However, the most direct and also the most efficient way is to increase the heat transfer area on

the PCM side. This is usually impossible or results in a large increase in the pressure drop of the

working fluid and a large decrease in the effective PCM storage volume for conventional latent

thermal energy storage systems due to the increased length of the flow passages of the workingfluid. This difficulty may be removed by introducing heat pipes with longitudinal into the

thermal energy storage unit.


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A heat pipe heat exchanger with latent heat thermal storage has many advantages over the

above mentioned conventional devices. Because the heat transfer areas on the hot fluid side, the

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

can be set at any desired value, at least theoretically. This is one of the most important features of

the heat pipe heat exchanger with latent heat thermal energy storage.

Fig. 2. A heat pipe heat exchanger with latent heat thermal storage

Fig. 2 presents the systematic configuration of a heat pipe heat exchanger with latent heat

thermal storage. The heat exchanger consists mainly of four parts. The hot fluid flow passage (6),

the PCM chamber (7) and the cold fluid flow passage (5) are connected by a number of heat

pipes (3). The phase change material (8) is stored in the PCM chamber. In order to enhance the

heat transfer process, annular fins made of pure copper are attached to the heat pipes. As one can

see from the figure, the sizes of the hot fluid flow passage, the PCM chamber and the cold fluid

passage can be designed independently, which presents one of the major advantages over other latent heat thermal storage systems.

The charging only mode is where the hot fluid flows through the hot fluid flow passage,

the heat is transferred through the heat pipes to the PCM to melt the PCM and the energy is

stored in the PCM as the latent and/or sensible heat. Under this operation mode, the evaporation


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section of the heat pipes is the part in the hot fluid flow passage, and the condensation section is

the part in the PCM chamber. The discharging only mode is where the cold fluid flows through

the cold fluid flow passage and receives the heat that is extracted from the PCM by the heat

pipes. Under this operation mode, the evaporation section of the heat pipes is the part in the PCM

chamber, and the condensation section is the part in the cold fluid flow passage. The

simultaneous charging and discharging mode is where both the hot and cold fluids flow through

their corresponding flow passages. There are two possible sub-operation modes under this mode:

Fluid to fluid heat transfer with discharging heat from the PCM is when both the hot fluid and

the PCM release heat to the cold fluid. Under this mode, both parts of the heat pipes, that in the

hot fluid flow passage and that in the PCM chamber, may play the role of the evaporator of the

heat pipes, and the part of the heat pipes in the cold fluid flow passage is the condenser. Fluid to

fluid heat transfer with charging heat to the PCM is when the hot fluid releases heat to both thecold fluid and the PCM. Under this mode, both parts of the heat pipes, that in the cold fluid flow

passage and that in the PCM chamber, act as the heat pipe condenser, and the part of the heat

pipes in the hot fluid flow passage is the heat pipe evaporator. In addition, there is still another

possible operation mode theoretically in which the state of the PCM is unchanged, and the heat

released from the hot fluid is all transferred to the cold fluid through the heat pipes.

In order to study the performance of this kind of heat pipe exchangers and its feasibility,

we considered heat exchanger of dimensions 1000 x 500 x 120 mm. Five gravity heat pipes run

through the hot fluid flow passage, the PCM chamber and the cold fluid flow passage. The heat

pipes are 28 mm in external diameter and 950 mm in length and are made of pure copper. The

working fluid of the heat pipes is acetone. Because of the variable evaporator size that is needed

for realizing the functions of the heat exchanger, the amount of working fluid is much bigger

than that in conventional heat pipes. In order to enhance the heat transfer processes,

circumferential copper fins 27 mm long and 0.4 mm thick are used for the PCM chamber, and

the same fins with a length of 14 mm are used for the hot and cold passages. The fin pitch is 5mm. Fig. 3 depicts the dimensions of the heat exchanger and the locations of the thermocouples

in the PCM. The PCM used is an industrial paraffin wax. The melting point is 52.1 0C, and the

latent heat is 132.4 kJ/kg. The quantity of PCM used in the heat exchanger is 25.1 kg, and thus,

the estimated energy storage in the form of latent heat is about 3300 kJ.


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Fig.3. Dimensions of the heat pipe heat exchanger and the thermocouple distribution in thePCM.

4. EXPERIMENTAL SETUP AND PROECDURE

In order to evaluate the performance of our heat pipe heat exchanger with latent heat storage, an

experimental system was set up, as shown in Fig. 4 . The system consists of the heat pipe heat

exchanger (2), a low temperature bath (10) that can provide water of a temperature as low as50C, a high temperature bath (1) that can provide water of a temperature as high as 95 0C, a HP

34970A data logger (3), two LZB-15 flow meters (5), a personal computer (4), two circulation

pumps (8) and several valves (6) that are used for controlling the flow rate and direction. There

are 48 T-type thermocouples (7) in total. To measure the water temperatures at the outlets and

the inlets of the heat exchanger, 2 thermocouples are used. Forty thermocouples are used to

measure the temperature distribution of the PCM. Their locations are given in Fig. 3 . In Fig. 3 (a),

the horizontal locations of the five thermocouples that are at the same vertical position are 14,

31, 41, 48 and 55 mm from the axis line of the heat pipe, the vertical locations are the same as

that indicated in Fig. 3 (b). Allthe pipings in the experimental system are well insulated by

applying a porous polythene insulator of a thickness of 80 mm.


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Fig. 4. Schematic diagram of the experimental system .

Prior to starting the charging, discharging or charging/discharging experiments of the

heat exchanger, the PCM in the PCM chamber is heated or cooled by circulating the cold water

or the hot water to the desired uniform temperature. After that, the charging only experiments

were started by turning off the cold water loop (the cold water flow passage was also emptied)

and turning on the hot water loop. The discharging only experiments were started by turning off

the hot water loop (the hot water flow passage was also emptied) and turning on the cold water

loop. The simultaneous charging and discharging experiments were realized by turning on both

the hot and cold water loop. The experiments were performed with different inlet temperatures

and different flowrates of the circulation water and the performance of heat pipe heat exchanger

is analyzed here.

5. REULTS AND DISCUSSIONS

5.1. Charging only operation performance

5.1.1. Performance of the heat pipes

Although the heat pipes used in the unit have the same structure and working principles as

conventional heat pipes, their operation modes are quite different from the conventional ones.

They have much more working fluid than the conventional heat pipes, and the evaporation and

condensation areas are variable according to their applied working conditions. Therefore, it is of

basic importance to make sure that these heat pipes do work and provide the functions for which


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they are designed. The experiments shows that the heat pipes used in our heat exchangers can

function properly and effectively. Fig. 5 depicts the measured wall temperature variation of the

heat pipe with time with a hot water flow rate of 3.33 kg/min and a hot water inlet temperature of

800C, and Fig. 6 presents the heat pipe wall temperature distribution along the axial direction of

the heat pipe at different times.

Fig.5. Heat pipe wall temperature variation with time at different vertical positions z = 140, 280,

420, 560 and 720 mm at 14 mm from the axis of the heat pipe.

Fig.6. Wall temperature profile along the heat pipe length at various times.

Charging only mode: TPCM,0 = 28.5 0C, Th = 80 0C, mh = 3.33 kg/min.


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From these two figures, we can see that during the early stage of the charging process of

the heat exchanger, the heat transferred from the hot water to the heat pipe evaporator is mainly

used to heat the walls of the heat pipes and, thus, to raise the temperature of the heat pipes, which

explains the rapid raise of the heat pipe wall temperature during this period. However, as soon as

the wall temperature is higher than the PCM temperature, some of the heat is transferred to heat

the PCM that surrounds the heat pipe, and this part of the heat increases with the heat pipe wall

temperature. Therefore, the wall temperature increase rate slows as the process continues.

Actually, the wall temperature approaches a constant as soon as the preheating period of the

PCM (from its initial temperature of 28.5 0C to its melting point) ends and melting starts. It

should also be noted that during the whole process, the temperature difference across the length

of the heat pipe is very small, usually less than 1 0C, which is the typical characteristic of heat

pipes and proves its excellent temperature leveling ability and fast transient thermal response.

5.1.2. Melting curves and phase change interfaces

Fig. 7 is one group of the typical melting curves at different radial positions. During the initial

period of heating, the PCM absorbs and stores the energy transferred by the heat pipe from the

hot water in the form of sensible heat. This heat is used to raise the temperature of the PCM

gradually to its melting point. As soon as TC6 (the wall temperature of the heat pipe) is higher

than the melting point, the melting process starts. Before melting takes place, the heat transfer

through the PCM is pure conduction, and the temperature increases almost linearly with time.

Because of the low thermal conductivity of the PCM, the temperature near the heat pipe

increases very quickly. However, after the temperature of the PCM reaches its melting point and

the melting process starts, the temperature increase rate of the PCM is significantly slowed. The

heat absorbed by the phase change interface is equal to the energy stored as latent heat plus the

heat transferred to its neighbor region. It is this mechanism that causes the different trends of

temperature variations at the different locations. For instance, the temperature of thermocouple

TC10 almost increases linearly with time during the whole process, which is quite different fromTC6 that has an apparent constant temperature period. This is because TC10 is located at the

symmetrical position of the two neighboring heat pipes, which is an adiabatic surface.


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Fig. 7. PCM temperature versus time at the different radial positions from the axis of the heat pipe at z = 140 mm. Charging only mode: TPCM,0 = 28.4 0C, Th = 80.1 0C, mh = 2.50 kg/min.

Therefore, the heat transferred from the inner side to it is all stored and, thus, raises its

temperature. TC7 is the only thermocouple that is located in the region of the fins. From Fig. 6 ,

one can see the temperature difference between TC6 and TC7 is much smaller than the

temperature difference of the other thermocouples. This proves that the fins attached to the heat

pipe enhanced the heat transfer process between the heat pipe and the PCM effectively.

Fig. 8. Temperature distribution of the PCM at 90 min. Charging only mode: TPCM,0 = 28.4 0C,

Th = 80.1 0C, mh = 2.50 kg/min.


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Fig. 8 shows the temperature distribution of the PCM at 90 min under the experimental

conditions of a hot water inlet temperature of 80.1 0C, a hot water flow rate of 2.5 kg/min and the

initial PCM temperature of 28.4 0C. From this figure, we can find that the temperature variation

along the axial direction of the heat pipe is much smaller than that along the radial direction,

which again proves that the heat pipe has a very good temperature leveling ability and a very

small thermal resistance in the axial direction. It can also be seen from the figure that within the

influence region of the fins, the temperature profile is much more uniform than in the other

region, both in the radial and the axial directions, the slopes of the temperature surface along the

radial direction and the axial direction between r = 14 and 31 mm are much smaller than that of

the other regions. This again proves that the fins are effective in enhancing the heat transfer

process.

Fig. 9. Phase interface position at different times. Charging only mode:

TPCM,0 = 28.5 0C, Th = 80 0C, mh = 3.33 kg/min.

Fig. 9 depicts the liquidsolid interfaces at various times. Using linear interpolation, theinterface position is deduced from the temperature measurements on the assumption that the

phase change takes place at a single melting temperature (52.1 0C). The shape of these liquid

solid interfaces generally agrees with our common knowledge: the region that is near the hot

water passage (larger z region) melts faster than the region far away from the hot water passage

(smaller z region). It can also be seen from Fig. that the distance in the radial direction between


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any two neighboring interfaces generally decreases with r. Since the time intervals between any

two neighboring interfaces are equal, therefore, this fact proves that the interface migration

velocity decreases with r and, thus, with time. This, in reality, reflects the fact that as the process

proceeds, the melted region increases and so does the radius of the melted region around the heat

pipe. The same increment of melted region in the r direction at larger r needs more PCM to be

melted than at smaller r. Actually, by applying a simple energy balance analysis to the melting

front, one can prove that the interface migration velocity is simply in inverse proportion to r if a

constant heating power is presumed. Therefore, the reduction in the interface migration velocity

mainly results from this geometrical effect, and hence, although the PCM melting rate should

decrease with time due to the decrease of the temperature difference between the heat pipe wall

temperature and the PCM, this reduction should be smaller than that in the interface migration

velocity.

5.1.3. Influence of the hot water inlet temperature

As one may expect, the hot water inlet temperature should have a very strong influence on the

charging operation processes. Therefore, a large number of experiments were conducted to study

this influence.

Fig. 10. Influences of the hot water inlet temperature on the charging only process:

(At TC6, TPCM,0 = 28.3 0C, mh = 2.50 kg/min).


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Figs. 10 and 11 summarize some of the typical results and depict the influences of the

hot water inlet temperature on the history of the heat pipe wall temperature and on the PCM

temperature variation, respectively. It is shown from these figures that the inlet temperature of

the hot water has a very strong and direct influence. This is because, under the same initial

temperature and flow rate conditions, the overall heat transfer coefficient from the hot water to

the PCMis basically a constant, and therefore, the heat flow from the hot water to the PCM (via

the heat pipes) is directly proportional to the temperature difference between the hot water and

the PCM.

Fig. 11. Influences of the hot water inlet temperature on the charging only process:

PCM temperature at TC9 (TPCM,0 = 28.3 0C, mh = 2.50 kg/min).

Since the initial PCM temperature is the same in these experiments, therefore the heat flow is

indirect proportion to the inlet temperature of the hot water to a great extent. We may further

conclude the melting completion time should, thus, also decrease directly with the inlet

temperature increase. Our experimental results prove this deduction: the time for completion of melting for the inlet temperature of 70 0C is 251 min, for 80 0C, this value is reduced to 149 min

and for 90 0C, itis only 121 min. The melting completion time of the hot water inlet temperature

of 90 0C is, thus, only 48% of that of 70 0C, which means a reduction of 52% (note that the initial

inlet temperature difference, that is, the inlet temperature of the hot water minus the initial PCM


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temperature was changed from 41.7 to 61.7 0C by increasing the inlet temperature from 70 to

900C, which means an increase of 48% in the initial temperature difference).

5.1.4. Influences of the hot water flow rate

Increasing the hot water flow rate will enhance the heat transfer process between the hot water

and the wall of the evaporator section of the heat pipe, and therefore, the hot water flow rates

should also influence the charging processes.

Fig. 12. Influences of the hot water flow rate on the charging only process:

heat pipe wall temperature at TC6(TPCM,0 = 28.2 _C, Th = 80.1 _C).

Fig. 13. Influences of the hot water flow rate on the charging only process:

PCM temperature at TC9 (TPCM,0 =28.2 _C, Th = 80.1 _C).


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Figs. 12 and 13 depict the influences of the hot water flow rate on the heat pipe wall

temperature and on the PCM temperature, respectively. From these two figures, we can see the

flow rate does produce a significant influence on the process, and both the temperature of the

heat pipe wall and the temperature of the PCM increase monotonously with the flow rate. As the

flow rate increases from 0.83 to 3.33 kg/min, the melting completion time is reduced from 189 to

144 min, which indicates a reduction of 24%. Apparently, compared with the inlet temperature,

the influences of the flow rate are much weaker. This may be explained as follows. We all know

from basic heat transfer theory that increasing the flow rate can only improve the convection heat

transfer between the hot water and the wall of the evaporator section of the heat pipe, and the

thermal resistance of this convection heat transfer process is less important than the thermal

conduction resistance of the PCM due to the very small thermal conductivity of the PCM.

Furthermore, according to convection heat transfer theory, the convection heat transfer coefficient is directly proportional to the nth power of the flow rate, where n is a constant less

than unity and within 0.4 and 0.8 for our unit under the experimental flow conditions. This also

contributes to the weak effects of the flow rate on the process.

From Figs. 12 and 13 , we can also note that the influence of the flow rate is less apparent

in the initial period than in the longer time period. As has been mentioned earlier, the PCM has a

small thermal conductivity, and thus, the solid PCM in the initial state should present a very

large thermal resistance. Therefore, this thermal resistance is the dominant one in the overall heat

transfer process from the hot water to the PCM. Thus, reducing the less important thermal

resistance of the convection heat transfer process between the hot water and the heat pipe wall

will not significantly improve the overall heat transfer process. However, as the process

proceeds, more and more PCM is melted and natural convection within the melted PCM

gradually plays a role, and this results in a decrease in the thermal resistance of the PCM. Of

course, the decrease in the thermal resistance of the PCM increases the relative importance of the

convection thermal resistance between the heat pipe wall and the hot water in the overall heat

transfer proc ss, which results in a more apparent influence of the flow rate compared with that in

the initial period.


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5.2. Discharging only operation performance

The discharging only operation experiments were conducted under various conditions. The inlet

temperature of the cold water was from 10 to 30 0C, and the flow rate of the cold water was from

0.83 to 3.33 kg/min. As has been already mentioned in the previous section, in order to perform

the discharging only operation experiments, the PCM was first heated to a given temperature

(usually well higher than the melting point of the PCM) by circulating the hot water for 3 to 4

hours. Then, after the PCM reached its given uniform temperature and the whole system was

steady, the hot water circulation was stopped. As soon as the hot water was evacuated from the

hot water passage, the cold water was started to circulate in the cold water loop and the

experiment starts.

5.2.1. Solidification curves and discharging characteristicsThe discharging only operation is actually a solidification process of the PCM that results from

the heat pipe cooling. In this operation mode, the section of the heat pipes that is buried in the

PCM is the evaporator.

Fig. 14. PCM temperature vs time at different radial positions from the axis of the heat pipe at

z=140 mm:discharging only mode TPCM,0 = 76 0C,Tc = 17 0C,mc = 1.67 kg/min).


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Fig. 14 presents a group of typical solidification curves that were obtained in various

discharging only mode experiments. From this figure, we can see that the PCM was cooled very

quickly from the liquid to the solid state, and therefore, the mode is a typical solidification

process. The solidification curves can be divided into three different regions, the initial region,

the solidification region and the cooling region. In the initial region, the liquid PCM is cooled to

its melting point, and the heat recovered by the cold water is, therefore, mainly the sensible heat

of the liquid PCM. Since the sensible heat is much smaller than the latent heat, the decreasing

rate of the PCM temperature is faster in this period than in the other periods. After that, when

solidification takes place, the process gets into the second stage, and the temperature of the PCM

decreases much slower than in the initial period due to the latent heat releasing effect.

Of course, after the solidification of the PCM is completed, the heat recovered by the cold water

is again the sensible heat of the PCM, and this certainly speeds up the decreasing of the PCMtemperature.

It should also be noted that the temperature difference between TC10 and TC6 first

increases and then decreases with time. To show this more clearly, Fig. 15 depicts the

temperature profile along the radial direction at various times. At the very beginning of the

process, the PCM is in the liquid state, and therefore, the effective conductivity of the PCM is

well enhanced by the natural convection within the PCM. This and the initial uniform

temperature certainly causes a uniform temperature distribution along the radial direction in the

early stage of the process. However, as the process proceeds, solidification of the PCM finally

commences. Solidification of the PCM not only restrains the natural convection but also

produces a solid PCM layer of low thermal conductivity on the heat pipe. Therefore, the

temperature gradient in the radial direction increases with time. This tendency continues until the

process approaches its final steady state as the PCM temperature approaches the cold water

temperature. After that, the temperature gradient in the radial direction decreases with time,

which means, as we can understand, that the system will finally acquire its new steady state witha new uniform temperature distribution after a long enough running time.


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Fig. 15. PCM temperature profiles in the radial direction at the different times. Discharging only

mode: TPCM,0 =76 _C, Tc = 17 _C, mc = 1.67 kg/min.

From Fig. 15 , one can see that the temperature profile of the finned region (near the heat

pipe wall,) in the radial direction is more even than that in the region r > 31 mm. This proves that

the fins on the heat pipes do enhance the heat transfer process as expected. Fig. 16 displays the

temperature distribution of the PCM at 60 min, which further proves the effectiveness of the fins

in enhancing the heat transfer process.

Fig. 16. Temperature distribution of the PCM at 60 min. Discharging only mode:

TPCM,0 = 76 _C, Tc = 17 _C,mc = 2.50 kg/min.


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5.2.2. Influences of the inlet temperature and the flow rate of the cold water.

As in the charging only operation, the inlet temperature and the flow rate of the cold

water will affect the performance of the heat pipe heat exchanger. Figs. 17 depicts the influences

of the cold water inlet temperature on the discharging process.

Fig. 17. Influences of cold water inlet temperature on the discharging only : heat pipe wall

temperature atTC6 (TPCM,0 = 75.7 0C, mc = 2.5 kg/min).

It can be seen from these figures that the inlet temperature of the cold water has an

important influence. The reason for this is the same as for the charging only operation as it is

stated in Section 4.1.3. That is, under the same initial temperature and flow rate conditions, the

overall heat transfer coefficient from the cold water to the PCM is basically a constant, and

therefore, the heat flow from the PCM to the cold water (via the heat pipes) is directly

proportional to the temperature difference between the PCM and the cold water temperature.

Lowering the inlet temperature of the cold water means increasing the temperature difference

and, therefore, enhances the whole heat transfer process. The solidification completion time

should, thus, also decrease with decreasing inlet temperature.

For example, the time for completion of solidification for the inlet temperature of 25 0C is

155 min, for 17 0C, this value is reduced to 132 min and for 10 0C, it is only 118 min. The

solidification completion time of the cold water inlet temperature of 10 0C is, thus, only 76% of

that of 25 0C, which means a reduction of 24% (note that the initial inlet temperature difference,


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i.e., the initial PCM temperature minus the inlet temperature of the cold water was changed from

50.7 to 65.7 0C by decreasing the inlet temperature from 25 to 10 0C, which means an increase of

30% in the initial temperature difference).

Fig. 18. Influences of the cold water flow rate on the discharging only process: heat pipe wall

temperature at TC6(TPCM,0 = 75.5 0C, Tc = 25 0C).

Figs. 18 present the influences of the cold water flow rate on the heat pipe wall temperature and

on the PCM temperature, respectively. From these two figures, we can see the flow rate also

produces a certain influence on the process, and both the temperature of the heat pipe wall and

the temperature of the PCM decrease monotonously with the flow rate. As the flow rate increases

from 0.83 to 3.33 kg/min, the solidification completion time is reduced from 177 to 146 min,

which indicates a reduction of 17.5%. It is worthwhile to mention that the influence of the inlet

temperature and the flow rate is stronger on the charging only process than on the discharging

only process. This is because of the so called sequence effects of insulation materials1 and the

restrained natural convection. As time elapses, the natural convection effect is becoming weaker

in the discharging only mode, whereas this effect is becoming stronger in the charging only

mode. Furthermore, the PCM that is in solid state has a smaller effective conductivity than that in

the liquid state. Therefore, the equivalent thermal resistance on the PCM side, which is always


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the dominant term of the overall thermal resistance of the heat transfer process from the hot or

cold water to the PCM, is bigger during the discharging only operation than the charging only

operation. It is due to this increased PCM side thermal resistance of the discharging only

operation compared with that of the charging only operation that results in its less sensitive

reaction to the change of the flow rate and the inlet temperature.

5.3 Simultaneous charging/discharging modes.

The resistance circuit for Simultaneous charging/discharging modes can be drawn as shown in

the fig. 19.

Fig. 19. Equivalent thermal circuit for the heat pipe exchanger with latent heat storage.

Analyzing the above circuit, we arrive at the condition for classifying whether the given

simultaneous charging/discharging process is a fluid to fluid heat transfer process with charging

heat to the PCM or the PCM is releasing heat, and therefore, the process is a fluid to fluid heat

transfer process with discharging heat from the PCM.

Where,

If the above given equation is satisfied then the PCM is receiving heat, and therefore, the

process is a fluid to fluid heat transfer process with charging heat to the PCM. Otherwise, if


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equation is not satisfied, then the PCM is releasing heat, and therefore, the process is a fluid to

fluid heat transfer process with discharging heat from the PCM.

6. CONCLUSION

Using heat pipes as the heat transfer elements that run through the hot fluid passage, the PCM

chamber and the cold fluid passage, a new latent heat thermal storage system has been

developed. It has many advantages over other thermal energy storage devices. The heat transfer

surface areas for the hot fluid, for the PCM and for the cold fluid may be designed

independently, which permits one to enhance the overall heat transfer process more efficiently by

the rational design of each heat transfer surface. The system can be operated in different modes:

the charging only, the discharging only and the simultaneous charging/discharging modes. Thismore flexible operation makes it suitable for systems of time and/or weather dependent energy,

especially solar energy and other renewable energies. The experimental results on the charging

only mode and the discharging only mode of the system show that the new device performs the

designed functions very well. It can both store and release the thermal energy efficiently.

Therefore, the device can be used as a conventional system in which the charging and

discharging are operated independently. Experimental show that the inlet temperature of the

cold/hot fluid has a stronger influence on the discharging/charging process than the flow rate.


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REFERENCES

1. Zhongliang Liu , Zengyi Wang, Chongfang Ma, An experimental study on heat transfer

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

and discharging only modes, Energy Conversion and Management 47 (2006) 944966

2. Zhongliang Liu , Zengyi Wang, Chongfang Ma, An experimental study on heat transfer

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

and discharging only modes, Energy Conversion and Management 47 (2006) 967991

3. Atul Sharma, V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage

withphase change materials and applications, Renewable and Sustainable Energy

Reviews 1 (2007)

4. H. Hagens , F.L.A. Ganzevles , C.W.M. van der Geld, M.H.M. Grooten , Air heat

exchangers with long heat pipes: Experiments and predictions, Applied thermal

engineering,27 (2007) 24262434

heat exchanger with heat storage capability

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