an experimental study on heat transfer characteristics of...

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Energy Conversion and Management 47 (2006) 944–966

An experimental study on heat transfer characteristicsof heat pipe heat exchanger with latent heat storage.Part I: Charging only and discharging only modes

Zhongliang Liu *, Zengyi Wang, Chongfang Ma

Key Laboratory of Enhanced Heat Transfer and Energy Conservation,

Ministry of Education and Key Laboratory of Heat Transfer and Energy Conversion,

Beijing Education Commission, College of Environmental and Energy Engineering,

Beijing University of Technology, Beijing 100022, PR China

Received 23 October 2004; received in revised form 23 May 2005; accepted 10 June 2005Available online 27 July 2005

Abstract

A new thermal storage system, a heat pipe heat exchanger with latent heat storage, is reported. The newsystem may operate in three basic different operation modes, the charging only, the discharging only andthe simultaneous charging/discharging modes, which makes the system suitable for various time and/orweather dependent energy systems. In this part of the paper, the basic structure, the working principleand the design concept are briefly introduced. Extensive experimental results are presented of the chargingonly and discharging only operations, and the effects of the inlet temperature and the flow rate of the cold/hot water were also investigated. The results show that the heat exchanger performs the designed functionsvery well and can both store and release the thermal energy efficiently.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Latent heat storage; Heat pipe; Heat exchanger; Heat transfer; Experimental

0196-8904/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.enconman.2005.06.004

* Corresponding author. Tel.: +86 10 67391917/67392566; fax: +86 10 67391983.E-mail address: [email protected] (Z. Liu).

mailto:[email protected]

Nomenclature

f accumulated or extracted heat fractionfs relative phase change rate, s�1

mc mass flow rate of cold water, kg/minmh mass flow rate of hot water, kg/minQ heat supplied, recovered, stored or lost, MJr radial coordinate from heat pipe axis line, mmt time, minT temperature, �CTc inlet temperature of cold water, �CTh inlet temperature of hot water, �CTPCM,0 initial temperature of phase change material (PCM), �Cz vertical coordinate from upper separation plate of the phase change material

chamber, mm

Z. Liu et al. / Energy Conversion and Management 47 (2006) 944–966 945

1. Introduction

A proper energy storage unit cannot only improve the total efficiency of the energy conversionand utilization system but also help to balance or regulate the mismatch between energy supplyand energy demands in quantity, location and time. For example, the availability of solar energydepends largely on time, weather condition and latitude, the electricity demands vary with time.Therefore, the energy originally from solar energy and off peak electricity needs to be stored.There are many thermal energy storage (TES) methods [1] such as sensible TES, latent TES,chemical TES and, more recently, ionic liquid TES [2]. Latent TES is receiving more and moreattention because of its large energy storage density and the significant reduction in storage vol-ume and, most importantly, the isothermal behavior during the charging and discharging processcompared with sensible heat storage systems. Hence, latent thermal energy storage is widely usedin the conversion and utilization of solar and other renewable energies, various heat recovery sys-tems, off peak electricity storage, air conditioning and heat pump systems.

Because of the importance of thermal energy storage in energy systems, various latent thermalenergy storage devices have been developed. Shamsundar and Srinivasan [3] gave a description ofa heat exchanger with latent thermal energy storage that uses an array of cylindrical tubes as thefluid passage and outside of the tubes is the phase change material (PCM). A numerical analysiswas performed of the unit by using a two dimensional formulation. Banaszek et al. [4] designed aspiral thermal energy storage unit and investigated its phase change characteristics. Later [5] theygave a numerical simulation of the unit and compared with their experimental results. Brousseauand Lacroix [6] proposed a multi-layer latent thermal energy storage system that could be used forsmoothing daily load profiles. The unit consisted of narrow vertical parallel plates of PCM sep-arated by rectangular flow passages. In order to enhance the heat transfer process between thePCM and the working fluid, various capsules packed bed latent heat storage systems have alsobeen proposed. Ismail and Henrıquez [7] designed a latent thermal energy storage system that

946 Z. Liu et al. / Energy Conversion and Management 47 (2006) 944–966

consisted of spherical capsules filled with PCM placed inside a cylindrical tank fitted with a work-ing fluid circulation system to charge and discharge the storage tank. Guo et al. [8] investigateda latent heat storage system in which the cool storage tank is filled with rectangular enclosurescontaining PCM.

There are still many other types of thermal energy storage units in the literature. However, aswe can see, all these latent thermal energy storage devices use the wall of the heat transfer fluidpassage as the heat transfer surface of the PCM. This means the heat transfer area on the PCMside is completely determined by the heat transfer area on the working fluid side, although thesetwo heat transfer areas may not be equal. However, as we know, most PCMs are poor heat con-duction media, and therefore, the dominant thermal resistances in the heat transfer process be-tween the PCM and the working fluid is on the PCM side. Therefore, according to heat transfertheory, the most efficient way for improving the heat transfer process is to enhance the heattransfer on the PCM side. Various methods for PCM thermal conductivity enhancement havebeen proposed and studied by many researchers. Some of the most common methods are attach-ing fins to heat transfer walls, dispersing metal particles or rings or carbon fibers of high con-ductivity into PCMs, etc. [9–13]. However, the most direct and also the most efficient way isto increase the heat transfer area on the PCM side. This is usually impossible or results in a largeincrease in the pressure drop of the working fluid and a large decrease in the effective PCM stor-age volume for conventional latent thermal energy storage systems due to the increased length ofthe flow passages of the working fluid. This difficulty may be removed by introducing heat pipesinto the thermal energy storage unit. Horbaniuc et al. [14] presented the idea of a heat pipe heatexchanger with latent thermal storage and provided a theoretical analysis of the unit to study theinfluence of the number of longitudinal fins attached to the heat pipe on the total solidificationtime of the PCM. However, no experimental data were provided. Lu et al [15] proposed a similardevice for wind power heating systems as early as in 1992, although sensible heat thermal stor-age material was used. A heat pipe heat exchanger with latent heat thermal storage has manyadvantages over the above mentioned conventional devices. Because the heat transfer areason 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 isone of the most important features of the heat pipe heat exchanger with latent heat thermal en-ergy storage.

In this paper, a new heat pipe exchanger with latent heat thermal energy storage was designedand manufactured. The performance of the unit under various possible operation modes wasinvestigated experimentally.

2. The heat pipe heat exchanger with latent heat thermal energy storage

Fig. 1 presents the systematic configuration of a heat pipe heat exchanger with latent heat ther-mal storage. The heat exchanger consists mainly of four parts. The hot fluid flow passage (6), thePCM 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 transferprocess, annular fins made of pure copper are attached to the heat pipes. As one can see from thefigure, the sizes of the hot fluid flow passage, the PCM chamber and the cold fluid passage can be

Fig. 1. A heat pipe heat exchanger with latent heat thermal storage: The systematic configuration (1) Hot fluid in; (1 0)hot fluid out; (2) cold fluid in; (2 0) cold fluid out; (3) heat pipes; (4) annular fins; (5) cold fluid flow passage; (6) hot fluidflow passage; (7) PCM chamber; (8) PCM; (9) upper separation and (10) Lower separation.


designed independently, which presents one of the major advantages over other latent heatthermal storage systems. As Horbaniuc et al. [14] has already pointed out, this unique thermalenergy storage system has three different operation modes. The charging only mode is wherethe hot fluid flows through the hot fluid flow passage, the heat is transferred through the heatpipes to the PCM to melt the PCM and the energy is stored in the PCM as the latent and/or sen-sible heat. Under this operation mode, the evaporation section of the heat pipes is the part in thehot fluid flow passage, and the condensation section is the part in the PCM chamber. The dis-charging only mode is where the cold fluid flows through the cold fluid flow passage and receivesthe heat that is extracted from the PCM by the heat pipes. Under this operation mode, the evap-oration section of the heat pipes is the part in the PCM chamber, and the condensation section isthe part in the cold fluid flow passage. The simultaneous charging and discharging mode is whereboth the hot and cold fluids flow through their corresponding flow passages. There are two pos-sible sub-operation modes under this mode: Fluid to fluid heat transfer with discharging heat fromthe 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, mayplay the role of the evaporator of the heat pipes, and the part of the heat pipes in the cold fluidflow passage is the condenser. Fluid to fluid heat transfer with charging heat to the PCM is whenthe hot fluid releases heat to both the cold fluid and the PCM. Under this mode, both parts of theheat pipes, that in the cold fluid flow passage and that in the PCM chamber, act as the heat pipecondenser, 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 thePCM is unchanged, and the heat released from the hot fluid is all transferred to the cold fluid

Table 1Main parameters of the heat pipes

Type of heat pipe Gravity drivenManufacturer Nanjing Shengnuo Heat Pipes Ltd.Pipe and fin material CopperPipe internal diameter 24 mmPipe wall thickness 2.0 mmWorking fluid Acetone, 129 gWorking temperature 0–100 �CWicking material –Pipe length 950 mmType of fins CircumferentialFin thickness 0.4 mmFin length 27 mm/14 mmFin pitch 5 mm


through the heat pipes. As it has been noted by Horbaniuc et al. [14], the simultaneous chargingand discharging operation mode presents another major advantage of the unit and gives a veryflexible operation to adapt to fluctuations in the energy supply and consumption. Therefore,the unit is most suitable for solar energy and other time dependent energy systems.

In order to study the performance of this kind of heat pipe exchangers and its feasibility, wedesigned and manufactured a prototype of the heat exchanger. The heat exchanger is of dimen-sions of 1000 · 500 · 120 mm. Five gravity heat pipes run through the hot fluid flow passage, thePCM chamber and the cold fluid flow passage. The heat pipes are 28 mm in external diameter and950 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 ex-changer, the amount of working fluid is much bigger than that in conventional heat pipes. Inorder to enhance the heat transfer processes, circumferential copper fins 27 mm long and0.4 mm thick are used for the PCM chamber, and the same fins with a length of 14 mm are usedfor the hot and cold passages. The fin pitch is 5 mm. Table 1 lists the detailed parameters of theheat pipes. Fig. 2 depicts the dimensions of the heat exchanger and the locations of the thermo-couples in the PCM. More detailed structure and design methods are available in Refs. [16] and[17]. The PCM used is an industrial paraffin wax (52#). The melting point is 52.1 �C, and the latentheat is 132.4 kJ/kg by differential scanning calorimeter (DSC) analysis. The quantity of PCM usedin the heat exchanger is 25.1 kg, and thus, the estimated energy storage in the form of latent heat isabout 3300 kJ.

3. Experimental setup and procedure

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. 3. The system consists of the heat pipe heatexchanger (2), a low temperature bath (1 0) that can provide water of a temperature as low as 5 �C,a high temperature bath (1) that can provide water of a temperature as high as 95 �C, a HP34970A data logger (3), two LZB-15 flow meters (5), a personal computer (4), two circulation

TC16 TC17 TC18 TC19 TC20




TC1 TC2 TC3 TC4 TC5

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730

130

140

T C 3 5 T C 3 6 T C 3 7

T C 3 8 T C 3 9 T C 4 0

T C 3 2 T C 3 3 T C 3 4

T C 2 9 T C 3 0 T C 3 1

T C 2 6 T C 2 7 T C 2 8

120

160

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140

730

130

140

a

b

Fig. 2. Dimensions of the heat pipe heat exchanger and the thermocouple distribution in the PCM. (a) The horizontallocations of the five thermocouples at each vertical position are 14, 31, 41, 48 and 55 mm from the axis of the heat pipe,respectively and (b) the horizontal locations of the three thermocouples at each vertical position are 41, 48 and 55 mmfrom the axis of the heat pipe, respectively.


1

1´

66 5

8

6

8

6

5

3

7

4

2

Cold water loop

hot water loop

Fig. 3. Schematic diagram of the experimental system. (1) High temperature bath; (1 0) low temperature bath; (2) heatpipe heat exchanger with latent heat storage; (3) HP data logger; (4) personal computer; (5) flow meter; (6) valves; (7)thermocouples and (8) circulation pump.


pumps (8) and several valves (6) that are used for controlling the flow rate and direction. There are48 T-type thermocouples (7) in total. To measure the water temperatures at the outlets and theinlets of the heat exchanger, 2 thermocouples are used at each outlet or inlet and are placed insidemixing cups. Forty thermocouples are used to measure the temperature distribution of the PCM.Their locations are given in Fig. 2. In Fig. 2(a), the horizontal locations of the five thermocouplesthat are at the same vertical position are 14, 31, 41, 48 and 55 mm from the axis line of the heatpipe, the vertical locations are the same as that indicated in Fig. 2(b). The horizontal locations ofthe three thermocouples that are at the same vertical position in Fig. 2(b) are 41, 48 and 55 mmfrom the axis line of the heat pipe. All the thermocouples are carefully calibrated, and the esti-mated error of temperature is 0.1 �C. In order to reduce the uncertainties in temperature differencemeasurements and, thus, minimize the error in estimating the heat flow rate of the water flows, thetemperature difference of the water between the outlet and the inlet is measured directly by con-necting the thermocouple at the outlet and the one at the inlet in series. The flow rate of circula-tion water is measured by LZB-15 flow meters with an accuracy of 2%. The uncertainty for heatflow deduction from the measured temperatures and flow rates are estimated to be smaller than8.7%, among which the largest contribution is from the temperature difference measurements ofthe hot and/or cold water. In order to reduce environmental influences and heat losses, the heatexchanger and all the pipings in the experimental system are well insulated by applying a porouspolythene insulator of a thickness of 80 mm.

Prior to starting the charging, discharging or charging/discharging experiments of the heat ex-changer, the PCM in the PCM chamber is heated or cooled by circulating the cold water or thehot water to the desired uniform temperature. After that, the charging only experiments werestarted by turning off the cold water loop (the cold water flow passage was also emptied) and turn-ing on the hot water loop. The discharging only experiments were started by turning off the hotwater loop (the hot water flow passage was also emptied) and turning on the cold water loop. Thesimultaneous charging and discharging experiments were realized by turning on both the hot andcold water loop. The experiments were performed with different inlet temperatures and differentflow rates of the circulation water. In this part of the paper, only the experiments of the charging


only and the discharging only operation modes are presented. The results for the simultaneouscharging and discharging operation modes are reported in the second part of this paper.

4. Experimental results and discussions

4.1. Charging only operation performance

4.1.1. Performance of the heat pipesAlthough the heat pipes used in the unit have the same structure and working principles as con-

ventional heat pipes, their operation modes are quite different from the conventional ones. Theyhave much more working fluid than the conventional heat pipes, and the evaporation and conden-sation areas are variable according to their applied working conditions. Therefore, it is of basicimportance to make sure that these heat pipes do work and provide the functions for which theyare designed. Our experiments show that the heat pipes used in our heat exchangers can functionproperly and effectively. Fig. 4 depicts the measured wall temperature variation of the heat pipewith time with a hot water flow rate of 3.33 kg/min and a hot water inlet temperature of 80 �C,and Fig. 5 presents the heat pipe wall temperature distribution along the axial direction of the heatpipe at different times. From these two figures, we can see that during the early stage of the charg-ing process of the heat exchanger, the heat transferred from the hot water to the heat pipe evap-orator is mainly used to heat the walls of the heat pipes and, thus, to raise the temperature of theheat 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 heatis transferred to heat the PCM that surrounds the heat pipe, and this part of the heat increaseswith the heat pipe wall temperature. Therefore, the wall temperature increase rate slows as theprocess continues. Actually, the wall temperature approaches a constant as soon as the preheating

0 50 100 150 200t (min)

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T (

˚C)

TC1, 140 mmTC6, 280 mmTC11, 420 mmTC16, 560 mmTC21, 720 mm

z

Fig. 4. Heat pipe wall temperature variation with time at different vertical positions z = 140, 280, 420, 560 and 720 mmat 14 mm from the axis of the heat pipe. Charging only mode: TPCM,0 = 28.5 �C, Th = 80 �C, mh = 3.33 kg/min.

100 230 360 490 620 750z (mm)

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T (

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30 min60 min90 min

120 min150 min210 min

Fig. 5. Wall temperature profile along the heat pipe length at various times. Charging only mode: TPCM,0 = 28.5 �C,Th = 80 �C, mh = 3.33 kg/min.


period of the PCM (from its initial temperature of 28.5 �C to its melting point) ends and meltingstarts. It should also be noted that during the whole process, the temperature difference across thelength of the heat pipe is very small, usually less than 1 �C, which is the typical characteristic ofheat pipes and proves its excellent temperature leveling ability and fast transient thermal response.

4.1.2. Melting curves and phase change interfacesExtensive experiments were performed to test the performance of the heat pipe exchanger for

the charging only operation mode, and thus, many melting curves of the PCM were obtained.Fig. 6 is one group of the typical melting curves at different radial positions. During the initialperiod of heating, the PCM absorbs and stores the energy transferred by the heat pipe fromthe hot water in the form of sensible heat. This heat is used to raise the temperature of thePCM gradually to its melting point. As soon as TC6 (the wall temperature of the heat pipe) isequal to or 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 lin-early with time. Because of the low thermal conductivity of the PCM, the temperature near theheat pipe increases very quickly. However, after the temperature of the PCM reaches its meltingpoint and the melting process starts, the temperature increase rate of the PCM is significantly slo-wed. The heat absorbed by the phase change interface is equal to the energy stored as latent heatplus the heat transferred to its neighbor region. It is this mechanism that causes the differenttrends of temperature variations at the different locations. For instance, the temperature of theoutermost thermocouple TC10 almost increases linearly with time during the whole process,which is quite different from TC6 that has an apparent constant temperature period. This is be-cause TC10 is located at the symmetrical position of the two neighboring heat pipes, and we allknow that a symmetrical surface in a symmetrical heat transfer system is also an adiabatic surface.Therefore, the heat transferred from the inner side to it is all stored and, thus, raises its temper-

T (

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75

TC6, 14TC7, 31TC8, 41TC9, 48TC10, 55

r, mm

Fig. 6. PCM temperature versus time at the different radial positions r = 14, 31, 41, 48 and 55 mm from the axis of theheat pipe at z = 140 mm. Charging only mode: TPCM,0 = 28.4 �C, Th = 80.1 �C, mh = 2.50 kg/min.


ature. TC7 is the only thermocouple that is located in the region of the fins. From Fig. 6, one cansee the temperature difference between TC6 and TC7 is much smaller than the temperature differ-ence of the other thermocouples, for example, at 10 min, the temperature difference between TC6and TC7 is 0.8 �C, that between TC7 and TC8 is 3.1 �C, that between TC8 and TC9 is 3.8 �C andthat between TC9 and TC10 is 2.2 �C; at 120 min, the corresponding temperature differences are0.3, 3.4, 5.9 and 4.2 �C. This proves that the fins attached to the heat pipe enhanced the heat trans-fer process between the heat pipe and the PCM effectively.

Fig. 7 shows the temperature distribution of the PCM at 90 min under the experimental con-ditions of a hot water inlet temperature of 80.1 �C, a hot water flow rate of 2.5 kg/min and the

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Fig. 7. Temperature distribution of the PCM at 90 min. Charging only mode: TPCM,0 = 28.4 �C, Th = 80.1 �C,mh = 2.50 kg/min.


initial PCM temperature of 28.4 �C. From this figure, we can find that the temperature variationalong 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 verysmall thermal resistance in the axial direction. It can also be seen from the figure that withinthe influence region of the fins, the temperature profile is much more uniform than in the otherregion, both in the radial and the axial directions, the slopes of the temperature surface alongthe radial direction and the axial direction between r = 14 and 31 mm are much smaller than thatof the other regions. This again proves that the fins are effective in enhancing the heat transferprocess.

Fig. 8 depicts the liquid–solid interfaces at various times. Using linear interpolation, the inter-face position is deduced from the temperature measurements on the assumption that the phasechange takes place at a single melting temperature (52.1 �C). The shape of these liquid–solid inter-faces generally agrees with our common knowledge: the region that is near the hot water passage(the larger z region) melts faster than the region far away from the hot water passage (the smaller zregion). It can also be seen from Fig. 8 that the distance in the radial direction between any twoneighboring interfaces generally decreases with r. Since the time intervals between any two neigh-boring interfaces are equal, therefore, this fact proves that the interface migration velocity de-creases with r and, thus, with time. This, in reality, reflects the fact that as the processproceeds, the melted region increases and so does the radius of the melted region around the heatpipe. The same increment of melted region in the r direction at larger r needs more PCM to bemelted than at smaller r. Actually, by applying a simple energy balance analysis to the meltingfront, one can prove that the interface migration velocity is simply in inverse proportion to r ifa constant heating power is presumed. Therefore, the reduction in the interface migration velocitymainly results from this geometrical effect, and hence, although the PCM melting rate should

20 25 30 35 40 45 50 55r (mm)

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z (m

m)

t (min)60728496108120

Fig. 8. Phase interface position at different times. Charging only mode: TPCM,0 = 28.5 �C, Th = 80 �C, mh = 3.33 kg/min.

0 44 88 132 176 220t (min)

0

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1

f

Fig. 9. Accumulated heat fraction versus time during the charging only operation: TPCM,0 = 28.4 �C, Th = 80.1 �C,mh = 3.33 kg/min.


decrease with time due to the decrease of the temperature difference between the heat pipe walltemperature and the PCM, this reduction should be smaller than that in the interface migrationvelocity. Fig. 9 presents the accumulated heat fraction variation with time. The accumulated heatfraction f is equal to the heat accumulated at a given time to the total heat needed for melting allthe PCM. The accumulated heat here includes both the sensible and the latent energy stored in thecharging process and is calculated from the enthalpy differences of the hot water between the inletand the outlet. From this figure, we may find that the accumulated heat fraction and time presenta basically linear relationship within the testing period, which indicates a nearly constant PCMmelting rate, especially in the later period of the process.

4.1.3. Influences of the hot water inlet temperatureAs 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 studythis influence. Figs. 10 and 11 summarize some of the typical results and depict the influences ofthe hot water inlet temperature on the history of the heat pipe wall temperature and on the PCMtemperature variation, respectively. It is shown from these figures that the inlet temperature of thehot water has a very strong and direct influence. This is because, under the same initial temper-ature 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 heatpipes) is directly proportional to the temperature difference between the hot water and the PCM.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 con-clude the melting completion time should, thus, also decrease directly with the inlet temperatureincrease. Our experimental results prove this deduction: the time for completion of melting for theinlet temperature of 70 �C is 251 min, for 80 �C, this value is reduced to 149 min and for 90 �C, it

0 50 100 150 200 250t (min)

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85

T (

˚C)

Inlet temp.90 °C80 °C70 °C

Fig. 10. Influences of the hot water inlet temperature on the charging only process: heat pipe wall temperature at TC6(TPCM,0 = 28.3 �C, mh = 2.50 kg/min).

0 50 100 150 200 250t (min)

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T (

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Inlet temp.90 °C80 °C70 °C

Fig. 11. Influences of the hot water inlet temperature on the charging only process: PCM temperature at TC9(TPCM,0 = 28.3 �C, mh = 2.50 kg/min).


is only 121 min. The melting completion time of the hot water inlet temperature of 90 �C is, thus,only 48% of that of 70 �C, which means a reduction of 52% (note that the initial inlet temperaturedifference, that is, the inlet temperature of the hot water minus the initial PCM temperature waschanged from 41.7 to 61.7 �C by increasing the inlet temperature from 70 to 90 �C, which meansan increase of 48% in the initial temperature difference).


4.1.4. Influences of the hot water flow rateIncreasing 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 ratesshould also influence the charging processes. Figs. 12 and 13 depict the influences of the hot waterflow rate on the heat pipe wall temperature and on the PCM temperature, respectively. From

t (min)

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0 40 80 120 160 200 24025

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Flowrate3.33 kg/min2.50 kg/min1.67 kg/min0.83 kg/min

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).

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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).


these two figures, we can see the flow rate does produce a significant influence on the process, andboth the temperature of the heat pipe wall and the temperature of the PCM increase monoto-nously with the flow rate. As the flow rate increases from 0.83 to 3.33 kg/min, the melting com-pletion 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. Thismay be explained as follows. We all know from basic heat transfer theory that increasing the flowrate can only improve the convection heat transfer between the hot water and the wall of the evap-orator section of the heat pipe, and the thermal resistance of this convection heat transfer processis less important than the thermal conduction resistance of the PCM due to the very small thermalconductivity of the PCM. Furthermore, according to convection heat transfer theory [18], the con-vection heat transfer coefficient is directly proportional to the nth power of the flow rate, where nis a constant less than unity and within 0.4 and 0.8 for our unit under the experimental flow con-ditions. This also contributes to the weak effects of the flow rate on the process. From Figs. 12 and13, we can also note that the influence of the flow rate is less apparent in the initial period than inthe 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. There-fore, this thermal resistance is the dominant one in the overall heat transfer process from the hotwater to the PCM. Thus, reducing the less important thermal resistance of the convection heattransfer process between the hot water and the heat pipe wall will not significantly improve theoverall heat transfer process. However, as the process proceeds, more and more PCM is meltedand natural convection within the melted PCM gradually plays a role, and this results in a de-crease in the thermal resistance of the PCM. Of course, the decrease in the thermal resistanceof the PCM increases the relative importance of the convection thermal resistance between theheat pipe wall and the hot water in the overall heat transfer process, which results in a moreapparent influence of the flow rate compared with that in the initial period.

4.2. Discharging only operation performance

The discharging only operation experiments were conducted under various conditions. Theinlet temperature of the cold water was from 10 to 30 �C, and the flow rate of the cold waterwas from 0.83 to 3.33 kg/min. As has been already mentioned in the previous section, in orderto perform the discharging only operation experiments, the PCM was first heated to a given tem-perature (usually well higher than the melting point of the PCM) by circulating the hot water for 3to 4 hours. Then, after the PCM reached its given uniform temperature and the whole system wassteady, the hot water circulation was stopped. As soon as the hot water was evacuated from thehot water passage, the cold water was started to circulate in the cold water loop and the experi-ment starts.

4.2.1. 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 thePCM is the evaporator. Fig. 14 presents a group of typical solidification curves that were obtainedin various discharging only mode experiments. From this figure, we can see that the PCM wascooled very quickly from the liquid to the solid state, and therefore, the mode is a typical solid-

t (min)

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TC6, 14TC7, 31TC8, 41TC9, 48TC10, 55

r, mm

Fig. 14. PCM temperature versus time at the different radial positions r = 14, 31, 41, 48 and 55 mm from the axis of theheat pipe at z = 140 mm: discharging only mode (TPCM,0 = 76 �C, Tc = 17 �C, mc = 1.67 kg/min).


ification process. The solidification curves can be divided into three different regions, the initialregion, the solidification region and the cooling region. In the initial region, the liquid PCM iscooled to its melting point, and the heat recovered by the cold water is, therefore, mainly the sen-sible heat of the liquid PCM. Since the sensible heat is much smaller than the latent heat, thedecreasing rate of the PCM temperature is faster in this period than in the other periods. Afterthat, when solidification takes place, the process gets into the second stage, and the temperatureof 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 isagain the sensible heat of the PCM, and this certainly speeds up the decreasing of the PCM tem-perature. It should also be noted that the temperature difference between TC10 and TC6 first in-creases and then decreases with time. To show this more clearly, Fig. 15 depicts the temperatureprofile along the radial direction at various times. At the very beginning of the process, the PCMis in the liquid state, and therefore, the effective conductivity of the PCM is well enhanced by thenatural convection within the PCM. This and the initial uniform temperature certainly causes auniform temperature distribution along the radial direction in the early stage of the process. How-ever, as the process proceeds, solidification of the PCM finally commences. Solidification of thePCM not only restrains the natural convection but also produces a solid PCM layer of low ther-mal conductivity on the heat pipe. Therefore, the temperature gradient in the radial direction in-creases with time. This tendency continues until the process approaches its final steady state as thePCM temperature approaches the cold water temperature. After that, the temperature gradient inthe radial direction decreases with time, which means, as we can understand, that the system willfinally acquire its new steady state with a new uniform temperature distribution after a longenough running time.

From Fig. 15, one can see that the temperature profile of the finned region (near the heat pipewall, the region of r = 14–31 mm on the abscissa of Fig. 15) in the radial direction is more even

10 20 30 40 50 60r (mm)

5

15

25

35

45

55

65

T (

˚C)

30 min60 min90 min120 min

150 min210 min270 min

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.


than that in the region r > 31 mm. This proves that the fins on the heat pipes do enhance the heattransfer 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. 17 presents the extracted heat fraction variation with time. The extracted heat here includesboth the sensible and the latent energy discharged in the discharging process. By comparing thiswith the accumulated heat fraction during the charging only process that is depicted in Fig. 9, wecan see their remarkable difference. As has been pointed out earlier, the accumulated heat fractionpresent a basically linear relationship with time during the charging only operation, and the phase

100

230

360490

620750

z (mm)

10

1928

3746

55

r (mm)

40

45

50

55

60

40

45

50

55

60

T (˚C) T (˚C)

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.

0 60 120 180 240 300t (min)

0

0.2

0.4

0.6

0.8

1

f

0.0

0.3

0.6

0.9

1.2

1.5

fτ

fτ,10-2/min

f

Fig. 17. Extracted heat fraction versus time during the discharging only operation: TPCM,0 = 76 �C, Tc = 17 �C,mc = 2.5 kg/min.


change rate is nearly a constant, especially in the later period of the process. However, in the dis-charging only operation, the phase change rate decreases very quickly with time. To show this, aso-called relative phase change rate fs is also depicted in Fig. 17. Here fs is defined by the followingequation,

fs ¼ofot

fs was calculated simply from the f � s experimental data by the forward differencing methodand the unit of fs is 10�2/min in Fig. 17. The differences in phase change rate variation trendswith time between the charging and the discharging process are a result of the different heattransfer resistance and natural convection effects. The charging only operation is actually a melt-ing process. As the process proceeds, more and more solid PCM is melted, and therefore, theeffective thermal conductivity of the PCM increases with time due to the enhanced natural con-vection effect. Hence, the thermal resistance during the charging only operation decreases withtime. This certainly compensates the temperature decrease effect that would result in a large de-crease of the phase change rate and restrains decreasing the phase change rate and, thus, pre-sents a nearly constant phase change rate during the late period of the melting process.However, the discharging only operation is a process of solidification, and the solid phasePCM on the heat pipes increases with time. Thus, the natural convection is restrained by thediminishing liquid PCM space. The increased solid state PCM plus the restrained natural con-vection will certainly result in the increase of the thermal resistance of the process with time.This explains the reduced phase change rate in the late period of the solidification. As onemay understand, due to the rapid decrease of the temperature difference between the PCMand the heat pipe, the phase change rate for both the charging only and the discharging onlymodes decreases with time during their initial period, and therefore, they show a similar varia-tion trend in this period.


4.2.2. Influences of the inlet temperature and the flow rate of the cold waterAs 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. 18 and 19 depict the influences of thecold water inlet temperature on the discharging process. It can be seen from these figures that theinlet temperature of the cold water has an important influence. The reason for this is the same as forthe charging only operation as it is stated in Section 4.1.3. That is, under the same initial temper-

0 50 100 150 200 250 300t (min)

20

30

40

50

60

70

80

T(º

C)

Inlet temp.10 ˚C17 ˚C25 ˚C

Fig. 18. Influences of the cold water inlet temperature on the discharging only process: heat pipe wall temperature atTC6 (TPCM,0 = 75.7 �C, mc = 2.5 kg/min).

0 50 100 150 200 250 300t (min)

20

30

40

50

60

70

80

T (

˚C)

Inlet temp.101725

˚C˚C˚C

Fig. 19. Influences of the cold water inlet temperature on the discharging only process: PCM temperature at TC9(TPCM,0 = 75.7 �C, mc = 2.5 kg/min).

t (min)

T (

˚C)

0 80 160 240 32030

40

50

60

70

80


Fig. 20. Influences of the cold water flow rate on the discharging only process: heat pipe wall temperature at TC6(TPCM,0 = 75.5 �C, Tc = 25 �C).


ature and flow rate conditions, the overall heat transfer coefficient from the cold water to the PCMis basically a constant, and therefore, the heat flow from the PCM to the cold water (via the heatpipes) is directly proportional to the temperature difference between the PCM and the cold watertemperature. Lowering the inlet temperature of the cold water means increasing the temperaturedifference and, therefore, enhances the whole heat transfer process. The solidification completiontime should, thus, also decrease with decreasing inlet temperature. For example, the time for com-pletion of solidification for the inlet temperature of 25 �C is 155 min, for 17 �C, this value is reducedto 132 min and for 10 �C, it is only 118 min. The solidification completion time of the cold waterinlet temperature of 10 �C is, thus, only 76% of that of 25 �C, which means a reduction of 24% (notethat the initial inlet temperature difference, i.e., the initial PCM temperature minus the inlet tem-perature of the cold water was changed from 50.7 to 65.7 �C by decreasing the inlet temperaturefrom 25 to 10 �C, which means an increase of 30% in the initial temperature difference).

Figs. 20 and 21 present the influences of the cold water flow rate on the heat pipe wall temper-ature and on the PCM temperature, respectively. From these two figures, we can see the flow ratealso produces a certain influence on the process, and both the temperature of the heat pipe walland the temperature of the PCM decrease monotonously with the flow rate. As the flow rate in-creases from 0.83 to 3.33 kg/min, the solidification completion time is reduced from 177 to146 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 stron-ger 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

1 For a cylindrical system, if there are two different thermal insulation materials that should be used for insulation,then a better insulation result can be obtained by placing the one of smaller thermal conductivity closest to the cylinderbeing insulated. This is an obvious conclusion of the analysis of the one dimensional composite cylindrical conductionsystem.

0 50 100 150 200 250 300 350t (min)

30

40

50

60

70

80

T (

˚C)


Fig. 21. Influences of the cold water flow rate on the discharging only process: PCM temperature at TC9(TPCM,0 = 75.5 �C, Tc = 25 �C).


elapses, the natural convection effect is becoming weaker in the discharging only mode, whereasthis effect is becoming stronger in the charging only mode. Furthermore, the PCM that is in solidstate has a smaller effective conductivity than that in the liquid state. Therefore, the equivalentthermal resistance on the PCM side, which is always the dominant term of the overall thermalresistance of the heat transfer process from the hot or cold water to the PCM, is bigger duringthe discharging only operation than the charging only operation. It is due to this increasedPCM side thermal resistance of the discharging only operation compared with that of the charg-ing only operation that results in its less sensitive reaction to the change of the flow rate and theinlet temperature.

5. Concluding remarks

Using heat pipes as the heat transfer elements that run through the hot fluid passage, the PCMchamber 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 areasfor the hot fluid, for the PCM and for the cold fluid may be designed independently, which per-mits one to enhance the overall heat transfer process more efficiently by the rational design of eachheat transfer surface. The system can be operated in different modes: the charging only, the dis-charging only and the simultaneous charging/discharging modes. This more flexible operationmakes it suitable for systems of time and/or weather dependent energy, especially solar energyand other renewable energies. The experimental results on the charging only mode and the dis-charging only mode of the system show that the new device performs the designed functions verywell. It can both store and release the thermal energy efficiently. Therefore, the device can be usedas a conventional system in which the charging and discharging are operated independently.


Extensive experimental investigations have been conducted of the influences of the various oper-ation parameters on the performance of the unit, and the results show that the inlet temperatureof the cold/hot fluid has a stronger influence on the discharging/charging process than the flowrate. The results for the simultaneous charging/discharging mode will be reported in the secondpart of this paper.

Acknowledgements

This work is supported by the Key Project No. G2000026306 of The National FundamentalResearch and Development Program, The Ministry of Science and Technology of The People�sRepublic of China, The National Key Technologies R & D Program Project No.2003BA808A19-7, Chinese National Natural Science Foundation Project No. 50276001 andBeijing Natural Science Foundation Project No. 3032007.

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