Thermal conductivity enhancement in a latent heat storage system

Thermal conductivity enhancement in a latent heat storage system

Eman-Bellah S.Mettawee

a,*

,Ghazy M.R.Assassa

b

a

Department of Solar Energy,National Research Center of Egypt,Box 31,Shoubra Garden,Postal Code 11241,Cairo,Egypt

b

Department of Mechanical Engineering,Faculty of Engineering at Shoubra,Zagazig University,Egypt

Received 16August 2005;received in revised form 17November 2006;accepted 27November 2006

Available online 29December 2006

Communicated by:Associate Editor Peter Lund

Abstract

Latent heat storage systems especially those employing organic materials have been reported to exhibit a rather slow thermal response.This is mainly due to the relatively low thermal conductivity of organic latent heat materials.In this study,experiments were carried out to investigate a method of enhancing the thermal conductivity of para?n wax by embedding aluminum powder in it.The size of the aluminum powder particles was 80l m.The tested mass fractions in the PCM-aluminum composite material were 0.1,0.3,0.4,and 0.5of aluminum.The used mass fraction in the experimental work was 0.5.

The experiments were conducted by using a compact PCM solar collector.In this collector,the absorber-container unit performed the function of absorbing the solar energy and storing the phase change material (PCM).The solar energy was stored in the PCM and was discharged to cold water ?owing in pipes located inside the PCM.Charging and discharging processes were carried out.The propagation of the melting and freezing fronts was studied during the charging and the discharging processes.The time wise temperatures of the PCM were recorded during the processes of charging and discharging.The solar intensity was recorded for the charging process.It was found that the charging time was reduced by approximately 60%by adding aluminum powder in the wax.In the discharging process,exper-iments were conducted for di?erent water ?ow rates of 9–20.4kg/h.It was found that the useful heat gained increased when adding alu-minum powder in the wax as compared to the case of pure para?n wax.The heat transfer characteristics were studied.ó2006Elsevier Ltd.All rights reserved.

Keywords:Solar energy;Storage;Latent heat;Composite material;Thermal conductivity

1.Introduction

Latent heat storage using liquid–solid phase change to reduce storage volumes of solar systems has gotten wide-spread attention.Salt hydrates are used as they have large latent heat and high density.However,they show several disadvantages after repeated cycling.Because of incongru-ent melting,segregation may occur;moreover,large subco-oling e?ects hamper their application.Decomposition and corrosion problems can limit the lifetime for some salt hydrate stores.These problems have led to a renewed

attention to organic phase change materials (PCMs),such as para?n and fatty acids.The drawbacks mentioned do not occur in these https://www.360docs.net/doc/b311854785.html,paring to salt hydrate organic PCMs have lower volumetric thermal energy den-sity for storage and show in general a melting range instead of ?xed melting temperature.Moreover,they have a low thermal conductivity.The latter may reduce the heat extraction rate of the storage during the solidi?cation cycle.Cabeza et al.(2002)discussed three methods to enhance the heat transfer in a cold storage working with water/ice as PCM.The three methods wear addition of stainless steel pieces,copper pieces (both have been proposed before)and a new PCM-graphite composite material.The PCM-graphite composite material showed an increase in heat ?ux bigger than with any of the other techniques.Chow

0038-092X/$-see front matter ó2006Elsevier Ltd.All rights reserved.doi:10.1016/j.solener.2006.11.009

*

Corresponding author.Tel.:+20122792307;fax:+2025830978.E-mail address:emanbellah@https://www.360docs.net/doc/b311854785.html, (E.-B.S.Mettawee).

https://www.360docs.net/doc/b311854785.html,/locate/solener

Solar Energy 81(2007)

839–845

et al.(1996)proposed two enhancement techniques to the thermal conductivity of the phase change materials (PCMs).The baseline case utilized LiH encapsulated in a thin SS304container.The?rst enhancement technique used LiH encapsulated in smaller containers of various shapes contained in a bigger cylindrical container?lled with Li.The second enhancement technique focused on a metal/phase-change material(M/PCM)composite.Specif-ically,5%Ni was added uniformly to LiH so that the mix-ture had the same volume as the baseline storage unit.The enhancement techniques were analyzed numerically and their e?ectiveness was assessed for both constant surface heat?ux and constant surface temperature conditions. Mehling(2000)described the?rst store built with new com-posite material and discussed experimental results.In his new method,the phase change material was put into a por-ous graphite matrix to form a composite material that com-bines the large heat storage capacity of the phase change material and the high thermal conductivity of the graphite matrix.The experiments were performed in two di?erent experimental setups.The?rst setup was used to character-ize the heat store itself.The second setup was used to test the performance of the heat store in a heating system where the heat store was supposed to cut peaks in the heating demand.The results showed that the maximum power sup-plied by the heating system could be increased by a factor of three with the new heat store.Min et al.(2001),(EG) prepared exfoliated graphite as a composite based on par-a?n,styrene–butadiene–styrene(SBS).In that composite, para?n undergoes solid–liquid phase change in the SBS network,and there was no leakage of it even in the state of melting.The composite exhibited high thermal conduc-tivity and nearly80%of the latent heat of fusion per unit mass of the para?n.The M/PCM composite gave the best enhancement in thermal conductivity.Velraj et al.(1999) discussed various heat-transfer enhancement methods for latent heat thermal storage(LHTS)systems.They con-ducted three di?erent experiments to augment heat transfer and reported their observations.Zhongliang et al.(2005) did experimental work to study the performance of a ther-mal storage unit using stearic acid as the heat storage med-ium.They designed a new type of?n to enhance the thermal response of the stearic acid.The experimental results showed that the?n could improve the heat transfer of the melting process of the thermal storage unit greatly. The equivalent thermal conductivity of the PCM could be augmented by a factor up to three.The analysis of their experimental results showed that the enhancement mecha-nism of the?n was attributed to its ability to improve both heat conduction and natural convection very e?ectively.

In the present study the addition of aluminum-powder to para?n wax,to improve the thermal conductivity,was investigated experimentally.This study was done using a compact solar collector.The heat transfer characteristics of charging process were discussed.The melting and the freezing contour were detected.A comparative study between the results using pure para?n wax and that using composite of para?n and aluminum powder is represented.

2.Experimental setup and procedures

An experimental model of one sector of a compact solar collector was designed and built to investigate the charging and the discharging processes.The volume and the con-struction of each sector were selected according to the input solar energy during the charging process.A sche-matic cross-section of the experimental apparatus consists of a steel absorber-container unit of1.3m in length and the detailed features are shown in Fig.1.The absorber plate was tilted45°(for winter optimization).A copper pipe of0.012m inner diameter and1.35m long was embed-ded inside the PCM to carry the heat transfer?uid inside the collector.Due to the geometry of the container,the copper pipe was located at the same distance from con-

Nomenclature

A surface area of each melt sector,m2

a width of each melt sector,m

C p speci?c heat of cooling water,J/kg°C

h m,t average heat transfer coe?cient,W/m2

h x,t local heat transfer coe?cient,W/m2

m water mass?ow rate,kg/h m2

m c mass of compound,kg

m f mass of aluminum powder,kg

m m mass of pure para?n wax,kg

n normal direction of the melt surface

k f thermal conductivity of aluminum powder, W/m K

k m thermal conductivity of pure para?n wax, W/m K

T ia average of the inlet water temperature,°C T l temperature of liquid PCM,°C

T oa average of the outlet water temperature,°C T m melting temperature of PCM,°C

T s temperature of solid PCM,°C

T W absorber wall temperature of,°C

t time,s

Q u useful heat gained,W

V c volume of compound,m3

V m volume of pure para?n wax,m3

V f volume of aluminum powder,m3

d thickness of PCM melt layer,m

q density,kg/m3

g d the mean daily e?ciency

k latent heat of fusion,kJ/kg

840 E.-B.S.Mettawee,G.M.R.Assassa/Solar Energy81(2007)839–845

tainer walls to have the same thickness of PCM around it, as shown in Fig.1.A0.05mm thick glass wall layer was placed over the cover to provide thermal insulation for only the discharging process.The physical properties of the new compound were calculated as follows:

(a)The thermal conductivity of the compound is de?ned

as

k c?k m v mtk f v f

(b)The density of the compound is de?ned as

q

c

?q m v mtq f v f

(c)The speci?c heat of the compound is de?ned as

C p

c ?C p

m

m mtC p

f

m f

where v f=V f/V c is the volume fraction of aluminum powder;v m=V m/V c is the volume fraction of wax;

m f=M f/M c is the mass fraction of aluminum pow-der;m m=M m/M c is the mass fraction of wax.

Physical properties of the composite,para?n wax,and aluminum powder are shown in Table1.

In the charging process water,inlet and outlet valves are closed.The charging process starts when the absorber plate is exposed to the solar radiation.Solar radiation was mea-sured by a silicon cell pyranometer(SOL-A-METER MARK)at the same inclination of the absorber plate with accuracy3%.The temperature distribution along the sur-face of the absorber plate was measured.Solar radiation increases the temperature of the PCM contained in the test section.The PCM melts when its temperature reaches the melting point.The temperature distribution in the radial direction of the PCM was measured at time intervals of 15min using20thermocouples as shown in Fig.5.They were calibrated within the accuracy of±1.5%based on Extech Model MT310.In addition,solar intensity and the ambient air temperature were measured.The charging process ends when the PCM completely melted.

The discharging process starts when a polyurethane layer covers the absorber plate.The water inlet and outlet valves are opened.The water inlet and outlet temperatures were measured with two thermocouples inserted inside the water pipe at its inlet and outlet.The temperature distribu-tion inside the PCM around the water pipe was measured with twelve thermocouples.The inlet temperature of water was approximately kept constant(Ti=30±2°C).The mass?ow rate ranged from9to20.4kg/h(from0.15to 0.34lit/min for an absorber area of0.2m2with uncertainty of3%).

3.Results and dissection

3.1.Aluminum mass fraction selection

Heating curves of para?n wax and composite material of0.1,0.3,0.4,and0.5mass fraction of aluminum are shown in Fig.2.From the?gure,one can see that the heat transfer rate is increased of composite as compared with that of para?n wax.The heat transfer rate increases as mass fraction of aluminum increases.However,the change of the heat transfer is very small for0.3,0.4,and0.5mass fraction of aluminum.The time for complete melting of pure para?n wax and0.1mass fraction of aluminum is nearly one and half of0.3,0.4,and0.5mass fraction of alu-minum.The selected mass fraction of aluminum is0.5.

Table1

Physical properties

Compound Para?n wax Aluminum

Latent heat of fusion,k266kJ/kg266kJ/kg

Melting temperature,T m53.5°C53.5°C

Solid phase density844kg/m3810kg/m32700kg/m3

Liquid phase density814kg/m3780kg/m3

Thermal conductivity 4.09W/m K0.21W/m K207W/m K

Speci?c heat 2.476kJ/kg K 2.5kJ/kg K0.896kJ/kg K

Mesh size80l m

E.-B.S.Mettawee,G.M.R.Assassa/Solar Energy81(2007)839–845841

3.2.Charging process

3.2.1.Temperature distribution

The experimental results shown in Fig.3represent the variation of the temperature of the PCM VS time as well as the solar radiation during the charging process.The?g-ure shows a comparison,for reference,between the results of the new composite and results of pure para?n wax at some selected points.It is found that the temperature of the composite increases gradually with high temperature gradient due to the high thermal conductivity of the solid composite.The?gure indicates also that,for para?n wax,the temperature of the thermocouples increases grad-ually in the solid state with low temperature gradient. Fig.3indicates that for thermocouple No.1,the maximum melting time for the composite is approximately3h,while that for pure wax is5h.

3.2.2.Solid–liquid interface

The position of the phase change interface versus time for both cases of pure para?n wax and composite is shown in Fig.4.The daily average of the incident solar radiation

842 E.-B.S.Mettawee,G.M.R.Assassa/Solar Energy81(2007)839–845

during this experiment was750W/m2.The?gure shows that the melting rate for compound is greater than that for pure para?n wax.The complete melting time for the composite is about5h,however,for para?n wax it is about7h.The PCM starts to melt at a layer adjacent to the absorber plate due to the direct contact with the absor-ber plate.As the layer thickness increases and the natural convection grows;heat is no longer transported directly across the melt layer,but it is carried by natural convection in liquid PCM to the interface of the solid PCM.For each advanced time the melt layer thickness at the upper portion is greater than the lower one.This behavior may be attrib-uted to the used geometry of the test section and to the enhancement of the free convection.

3.2.3.Heat transfer characteristics

During the charging process,the average heat transfer coe?cient h m,t at any time may be obtained as

h m;t?

X

h x;t?A x;t

.X

A x;te1TThe surface area A x,t is calculated by multiplying the sur-face width(a)by the length of the wax container.The local heat transfer coe?cient,h x,t,may be evaluated from the

E.-B.S.Mettawee,G.M.R.Assassa/Solar Energy81(2007)839–845843

local thermal energy balance at the melting surface as shown in Fig.5.An instantaneous energy balance at the interface may be written as

k s o T s o n x ;t àk l o T l o n

x ;t ?q L d d d t x ;t e2T

This energy balance is used to determine the local heat

transfer coe?cient h x ,t ,which is de?ned by:àk l

o T l

o n x ;t ?h x ;t eT W àT m Te3TThen from Eqs.(1)and (2)local heat transfer coe?cient h x ,t at any location x and at any time,t is determined from:

h x ;t ?q L d d d t x ;t tk s o T s

o n

x ;t !,eT W àT m Te4Twhere (d d /d t )x ,t is measured from the experimental data as

shown in Fig.5and (o T s /o n )x ,t is calculated as

o T s o n x ;t ?o T x o x y x ;t ào T y

o y x

x ;t "#,

2e5TThe average heat-transfer coe?cient along the interface and the solar radiation for both pure para?n wax and new composite as functions of time are compared in Fig.6.Inspection of the ?gure reveals that the trend of the average heat-transfer coe?cient is similar for both cases with higher values for composite than that for pure wax.In the case of composite,the melting rate increases due to the increase of the thermal conductivity for both li-quid and solid PCM.3.3.Discharge process

3.3.1.Temperature distribution

During the discharge process,the temperature distribu-tion of the PCM indicated by the 12thermocouples ?xed around the water pipe had been measured at equal inter-vals.Fig.7shows the temperature distribution at a selected

thermocouple number p1for both para?n wax and composite for water ?ow rate of 15kg/h.Inspection of

844 E.-B.S.Mettawee,G.M.R.Assassa /Solar Energy 81(2007)839–845

the?gure shows that heat transfer rate of composite is increased for solidi?cation process as compared with that of para?n wax.This because the thermal conductivity is higher than that of para?n wax,so that heat transfer is enhanced in the conduction dominated solidi?cation process.

3.3.2.Solid–liquid interface

During the discharge process,solid–liquid interface could be found using the readings of the thermocouples ?xed around the water pipe.Fig.8a and b,show the solid–liquid interface contours as a function of time using 9kg/h water mass?ow rate for para?n wax and compos-ite.At the start of the discharge process,the solidi?cation shapes for the composite and pure para?n are non-homog-enous around the water tube due to the e?ect of the natural convection at the upper portion.After a while,the e?ect of conduction becomes dominate.High thermal conductivity of the composite makes the shapes more homogenous than for para?n wax.More expected the solidi?cation process faster for composite than for para?n wax.

3.3.3.The mean daily e?ciency

The mean daily e?ciency g d is calculated by the formula:

g d?MC peT oaàT iaT=AGe6Twhere M is the total mass of water that heated during the discharge process and G is the total incoming solar radia-tion during the charging process.The mean daily e?ciency g d was determined for di?erent water mass-?ow rate and for di?erent solar radiation.It ranged from32%to54.8%for para?n wax;however,it ranged from82%to94%for com-posite.The increase of the daily e?ciency is due to the de-crease in melting time and the increase in the useful heat gains that given by the following classical equation:Q

u

?máC páeT outàT inTe7TFig.9shows a comparison of a time wise(Q u)for pure par-a?n wax and compound for the same water?ow rate of 15kg/h.The?gure shows that the useful heat gained is greater for composite than that for pure wax.This is due the increase of the heat transfer because of the higher ther-mal conductivity of the composite.

4.Conclusions

Para?n wax is attractive for use in solar heat storage.It has good latent heat and is stable.Embedding aluminum powder in the wax enhances its low thermal conductivity. The experimental study presented in this paper suggests the following conclusions:

1.The charging time decreased by60%for composite than

pure para?n wax.

2.For pure para?n wax the solid–liquid pro?les formed

through melting are approximately similar in shape dur-ing the charging and discharging processes irrespective of the time.In the case of composite,they are similar for the charging process but di?erent for the discharging process except at the end of the process.

3.The useful heat gained increases as the aluminum pow-

der is added to the para?n wax.

4.During the charging process,the average heat-transfer

coe?cient has greater value for composite than for pure para?n.

5.The mean daily e?ciency for para?n wax is from32%

to54.8%;while it is from82%to94%for composite. References

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