Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage...

Solar Energy 78 (2005) 416–426

www.elsevier.com/locate/solener

Thermal performance of a solar cooker based on an evacuatedtube solar collector with a PCM storage unit

S.D. Sharma a,*, Takeshi Iwata b, Hiroaki Kitano b, Kazunobu Sagara a,1

a Department of Architectural Engineering, Graduate School of Engineering, Osaka University, 2–1 Yamadaoka, Suita,

Osaka 565-0871, Japanb Department of Architecture, Faculty of Engineering, Mie University, Tsu 514-8507, Japan

Received 6 April 2004; received in revised form 20 July 2004; accepted 2 August 2004

Available online 11 September 2004

Communicated by: Associate Editor Michael Grupp

Abstract

The thermal performance of a prototype solar cooker based on an evacuated tube solar collector with phase change

material (PCM) storage unit is investigated. The design has separate parts for energy collection and cooking coupled by

a PCM storage unit. Solar energy is stored in the PCM storage unit during sunshine hours and is utilized for cooking in

late evening/night time. Commercial grade erythritol was used as a latent heat storage material. Noon and evening

cooking experiments were conducted with different loads and loading times. Cooking experiments and PCM storage

processes were carried out simultaneously. It was observed that noon cooking did not affect the evening cooking,

and evening cooking using PCM heat storage was found to be faster than noon cooking. The cooker performance under

a variety of operating and climatic conditions was studied at Mie, Japan.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Solar energy; Phase change material; Latent heat storage; Evening cooking; Evacuated tube solar collector

1. Introduction

Domanski et al. (1995) and Buddhi and Sahoo (1997)

have studied the use of a phase change material (PCM)

as a storage medium for a box type solar cooker de-

signed to cook in the evening or during non-sunshine

hours. They used stearic acid (melting point 69 �C) as

0038-092X/$ - see front matter � 2004 Elsevier Ltd. All rights reserv

doi:10.1016/j.solener.2004.08.001

* Corresponding author. Tel.: +81 6 6879 7643; fax: +81 6

6879 7646.

E-mail addresses: [email protected], sharma_

[email protected] (S.D. Sharma).1 ISES member.

a PCM for heat storage. Buddhi and Sahoo filled the

PCM below the absorbing plate of the cooker. In such

type of design, the rate of heat transfer from the PCM

to the cooking pot during the discharging mode of the

PCM is slow, and more time is required for cooking

an evening meal.

Sharma et al. (2000) designed and developed a cylin-

drical PCM storage unit for a box type solar cooker to

cook food in the late evening. Since this unit surrounds

the cooking vessel, the rate of heat transfer between the

PCM and the food is higher, and cooking can be faster.

They reported that by using 2.0kg of acetamide (melting

point 82 �C) as a latent heat storage material, a second

ed.

mailto:[email protected],

mailto:[email protected]

Nomenclature

A net absorber area for the tubes (1.82m2 · 2 =

3.64m2)

Cw specific heat of water (=4.18kJ/kg �C)G solar irradiance (MJ)

Ii enthalpy of PCM (kJ/kg)

Ih (ave) average solar irradiance at horizontal sur-

face (W/m2)

Is20 solar irradiance at 20� angle inclined from

the horizontal surface (W/m2)

Is20(ave) average solar irradiance at 20� angle inclinedfrom the horizontal surface (W/m2)

m mass flow rate (lmin�1)

mave average mass flow rate (lmin�1)

Mi mass of PCM (kg)

Mload mass of loaded water (kg)

QPCMStored total heat stored in the PCM (kJ)

QwaterStored total heat gain by water (HTF) (kJ)

Ta ambient air temperature (�C)Ta(ave) average ambient air temperature (�C)Tin water (HTF) temperature at the inlet of the

ETSC (�C)Tload water (loaded) temperature (�C)Tout water (HTF) temperature at the outlet of the

ETSC (�C)Tmaxin maximum water (HTF) temperature at the

inlet of the ETSC (�C)Tmaxout maximum water (HTF) temperature at the

outlet of the ETSC (�C)TPCMmax PCM maximum temperature inside the

storage unit (�C)TPCM(L,T,3 cm) PCM temperature at 3cm radial dis-

tance from left top side (30cm height) of

the PCM storage unit (�C)TPCM(L,C,3 cm) PCM temperature at 3cm radial dis-

tance from left center side (15cm height) of

the PCM storage unit (�C)

TPCM(L,B,3 cm) PCM temperature at 3cm radial dis-

tance from left bottom side (0cm height)

of the PCM storage unit (�C)TPCM(R,T,3 cm) PCM temperature at 3cm radial dis-

tance from right top side (30cm height) of

the PCM storage unit (�C)TPCM(R,C,3 cm) PCM temperature at 3cm radial dis-

tance from right center side (15cm height)

of the PCM storage unit (�C)TPCM(R,B,3 cm) PCM temperature at 3cm radial dis-

tance from right bottom side (0cm height)

of the PCM storage unit (�C)TPCM(B,C,3 cm) PCM temperature at 3cm radial dis-

tance from bottom center of the PCM stor-

age unit (�C)t total time (from Pump ON to Pump OFF)

(minutes)

Vi volume for each point inside PCM storage

unit (m3)

Greek symbols

qs density of PCM at solid phase (=1.48kg/m3)

ql density of PCM at liquid phase (=1.30kg/m3)

qpcm density of PCM (qpcm = (qs + ql)/2 =

1.39kg/m3)

as absorptivity transmittivity product for the

collector (0.81)

Subscript

i The number of thermocouples inside the

PCM

Abbreviations

PCM phase change material

HTF heat transfer fluid (water)

ETSC evacuated tube solar collector

S.D. Sharma et al. / Solar Energy 78 (2005) 416–426 417

batch of food could be cooked if it is loaded before 3:30

PM during winter. They recommended that the melting

temperature of a PCM should be between 105 and

110 �C for evening cooking.

Buddhi et al. (2003) tested acetanilide as a PCM with

a melting point of 118 �C for night cooking in a box type

cooker with three reflectors. Acetanilide was filled in the

cylindrical storage unit and it was reported that by using

4.0kg of acetanilide, food could be cooked up to 8:00

PM.

Morrison and Mills (1987), Balzar et al. (1996) and

Kumar et al. (2001) have used evacuated tube solar col-

lectors (ETSC) for cooking. Schwarzer et al. (2003)

tested a design with a collector having reflectors and

pebbles as thermal storage for cooking. This design

has the possibility of indoor cooking and can incorpo-

rate a baking oven.

No work has been performed on solar cookers with

latent heat storage using ETSC. We tried to develop a

solar cooker with PCM storage based on ETSC. For this

purpose, there is a need to identify a latent heat storage

material with appropriate melting point (>110 �C) for

cooking (Sharma et al., 2000). Erythritol (melting point

118 �C, latent heat of fusion 339.8kJ/kg) was used for

the present set-up. The prototype was fabricated by a lo-

cal manufacturer and installed on the roof of the Satel-

lite Venture Business Laboratory, Mie University, Tsu,

Japan (Longitude 136� 31 0 and Latitude 34�44 0) for test-

ing thermal performance. Experiments were conducted

for different loads and second batch loading times.

Table 1

Specification for the evacuated tube solar collector (single

panel)

Item Specificationa

418 S.D. Sharma et al. / Solar Energy 78 (2005) 416–426

Experimental results are presented for a system load of

5kg, 7kg, 8kg and 10kg water, respectively. The perfor-

mance results are also presented for a range of operating

and climatic conditions.

Manufacturer name Nippon Electric

Glass Co., Japan

Model no. DP6-2800

Number of collector tubes 6

External dimensions (mm) L2972 · W930 · H185

Gross area (m2) 2.76

Net absorber area (m2) 1.82

Weight (kg) 64.0

Tilted angle of absorber plate (degree) 20

Operating pressure (kPa) <490

Vacuum (Pa) <10�3

Net absorber area for single tube (m2) 0.3039

Glass thickness (mm) 2.0

a Specifications supplied by the manufacturer.

2. Experimental set-up and measurements

The schematic diagram of an ETSC with a PCM

storage unit is shown in Fig. 1. It consists of an ETSC,

a closed loop pumping line containing water as the heat

transfer fluid (HTF), a PCM storage unit, cooking unit,

pump, relief valve, flow meter and a stainless steel tube

heat exchanger. The ETSC was procured from Nippon

Electric Glass Co., Japan and its specification is given

in Table 1. Two panels, each containing six collector

tubes, were used. The absorbing plate of the ETSC

was tilted at 20� facing due south.

The other part of the system is the PCM storage unit.

This unit has two hollow concentric aluminum cylinders,

and its inner and outer diameters are 304mm and

441mm, respectively, and 420mm deep with 9mm thick-

Evacuated tube solar collector

Flow

Data logger

PC

Solar irradiance at 20 degree inclinedand horizontal surface

PCM temperatures

Collector inlet and outlet

Temperature

Flow MeterRelie

Fig. 1. Outline of the prototype solar cooker based on ev

ness. The space between the cylinders was filled with

45kg erythritol for use as the PCM. As the phase change

materials have positive volumetric expansion on melt-

Water source

rate valve

Cooking vessel

PCM storage unit

f valve

acuated tube solar collector with PCM storage unit.

420 36

0

60

441

304

60

99

Pressure valve

Water Inlet

Water Outlet

Thermocouples

For PCM Filling

A

A’

60 30 0

Left side Right side

Spiral tube

(a) (b)

Fig. 2. (a) Sectional view of the PCM storage unit. (b) A–A 0 vertical section of PCM storage unit.

Fig. 3. Photograph of the stainless steel tube around PCM heat

storage unit.


ing, the storage unit was not filled completely and space

was left for volumetric expansion during melting of the

PCM. The dimensions of the cooking vessel are

297mm in diameter and 300mm in height, which can

be easily inserted inside the PCM storage unit. Sectional

and vertical views are shown in Fig. 2(a) and (b).

Thirty-four ‘‘T’’ type calibrated thermocouples, accu-

racy ± 0.2 �C, were used to measure the temperature dis-

tribution inside the PCM storage unit, inlet/outlet

temperature of the ETSC, water temperature inside the

cooking vessel and ambient air temperature respectively.

Thermocouples were welded at different positions inside

the PCM storage unit and the inlet/outlet of the ETSC.

The distance between the thermocouples was set at

30mm for obtaining the temperature distributions in a

radial direction within the PCM as shown in Fig. 2(b).

For the water temperature inside the cooking pot, a

thermocouple was inserted through the cooking vessel

lid. For the ambient air temperature, a thermocouple

was kept in the shade with good contact in the air. A

pyranometer was used to measure solar irradiance at

20� inclined with the accuracy of 1.5%. Another pyra-

nometer was used to measure the solar irradiance on a

horizontal surface with the accuracy of 2.5%. All ther-

mocouples and pyranometers were connected with a

data logger to record temperatures and solar irradiance

at intervals of 6s.

A pump (370W) circulates the heated water (HTF)

from the ETSC through the insulated pipes to the

PCM storage unit using a stainless steel tubing (diameter

21.6mm) heat exchanger that wraps around the cooking

unit in a closed loop as shown in Fig. 1. A photograph

of the stainless steel tubing heat exchanger is shown in

Fig. 3. A pump with sufficient power was selected be-

cause of the large frictional resistance in the solar collec-

tor and pipe circuit. During sunshine hours, the heated

water transfers its heat to the PCM and the heat is

stored in the form of latent heat through the stainless

steel tubing heat exchanger. This stored heat is utilized

to cook the food in the evening or when the sun intensity

is not sufficient to cook food. The PCM storage unit, the

piping and pump are all insulated to prevent heat losses.

A valve between the ETSC and storage unit controls the

water flow, while a magnetic type flow-meter, accuracy

0.25%, was used to measure the flow rate of the water

(HTF). A pressure relief valve, operational at a pressure

over 300kPa inside the pipe, was incorporated into the

system to ensure safe operation.


3. Phase change material (PCM)

Various materials available for energy storage in a

temperature range near 80–120 �C have been identified

for solar cooking (Domanski et al., 1995; Kakiuchi

et al., 1998; Sharma et al., 1999). The choice of the mate-

rial is based on the melting temperature, the latent heat

of fusion, density and other considerations such as tox-

icity, corrosiveness and cost. The thermo-physical prop-

erties, thermal kinetic behavior and thermal stability of

erythritol were investigated (Kakiuchi et al., 1998). We

chose commercial grade erythritol (C4H10O4) because

of its melting temperature 118 �C with high heat of fu-

sion 339kJ/kg, its low cost (US$5/kg) and large-scale

availability in the Japanese market. The erythritol used

here was procured from Mitsubishi Chemical Ltd., Ja-

pan; its thermo-physical properties are given in Table

2 and its enthalpy curve in Fig. 4.

4. Experimental results and discussion

Cooking experiments were conducted using the

ETSC and PCM storage unit. Selected results are pre-

sented in the figures to show the phase transition behav-

ior of the PCM, the water temperature inside the

cooking vessel, the heat transfer fluid temperatures and

solar irradiance. Variations of PCM temperatures with

Table 2

Thermo-physical properties of erythritol (Kakiuchi et al., 1998)

Chemical structure C4H10O4

Molecular weight 122.2

Melting point ( �C) 118.0

Heat of fusion (kJ/kg) 339.8

Specific heat

(kJ/kg�C)1.38 at (20�C) and 2.76 at (140�C)

Density (kg/dm3) 1.48 at (20�C) and 1.30 at (140�C)Heat conductivity

(W/m K)

2.64 at (20�C) and 1.17 at (140�C)

0

100

200

300

400

500

600

700

0 20 40 60 80 100 120 140 160 180 200Temperature[C]

Enth

alpy

[kJ/

kg]

Fig. 4. Enthalpy curve of erythritol.

and without load on 2, 3 and 5 September 2002 and

22 August 2003 are shown in Figs. 5–8. PCM tempera-

tures were measured at various positions in the storage

unit to ascertain the phase transition behavior. As seen

from Figs. 5–8 the collected solar energy transferred to

the PCM raises its temperature from the initial temper-

ature to the higher temperature around the melting

point. In the case of a large temperature difference be-

tween the heated water (HTF) and the PCM, the initial

change in temperature is a faster process because of the

high heat transfer rate to the PCM. After this rapid in-

crease, the temperature becomes somewhat constant

during the melting period. Similar shaped heating curves

are reported for other PCMs (Sari and Kaygusuz, 2002;

Hasan, 1994).

A test run was conducted without load on 2 Septem-

ber 2002 with an average flow rate of 6.43 l/min. The

PCM temperature distribution inside the unit is shown

in Fig. 5. The PCM maximum temperature (138 �C)was observed at TPCM(R,C,3 cm). The PCM was charged

at 1:00 PM. All PCM temperatures were more than

110 �C at 3:00 PM, which is higher than the lowest tem-

perature (near 75 �C) required for cooking most types of

food (Domanski et al., 1995; Khalifa et al., 1987).

Fig. 6 shows the variations of PCM temperatures on

3 September 2002. This experiment was undertaken to

see whether the PCM was capable of evening cooking

or not. At 5:00 PM. 10kg of water was loaded inside

the cooking vessel. The initial temperature of the loaded

water was 31.6 �C. From this figure, it can be observed

that TPCM(R,C,3 cm) suddenly dropped from 134.1 �C to

102.1 �C due to the water (1.0kg) filled in the 7mm

gap between the cooking vessel and the PCM storage

unit. At the moment when the water was filled into the

gaps, some of the water was converted into steam. Water

was put into the gap to increase the heat transfer rate

from the PCM unit to the cooking vessel. Results show

that the temperature for the load of 10kg water sharply

increased. From 6:35 PM, the temperature of the load

stayed at 100 �C. It was found that the total time for

evening cooking was 96min and that the PCM storage

unit was able to cook food in the late evening. It was ob-

served that heat transfer from the PCM to the cooking

vessel was higher due to the water that is filled in the

gap. However, this is impractical because of high

heat loss through vaporization of water and risk to the

user.

On 5 September 2002, an experiment was conducted

for noon and evening cooking with a load of 8kg water

and an average flow rate of 8.14 l/min. This experiment

was done to see the effect of noon cooking on evening

cooking. At 10:00 AM, 8.0kg of water was loaded inside

the cooking vessel for noon cooking. Temperatures of

the PCM and loaded water are shown in Fig. 7. Almost

all the PCM was melted except on the bottom. A

maximum temperature of near 130 �C was found at

25

50

75

100

125

150

9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00

Time

Tem

pera

ture

[C]

TPCM (L, T, 3cm) TPCM (L, C, 3cm) TPCM (L, B, 3cm) TPCM (R, T, 3cm)

TPCM (R, C, 3cm) TPCM (R, B, 3cm) TPCM (B, C, 3cm) Tout

Pump ON Pump OFF

Fig. 5. Variation of PCM temperatures, Tout without load on 2 September 2002.

25

50

75

100

125

150

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 19:00Time

Tem

pera

ture

[C]

TPCM (L, T, 3cm) TPCM (L, C, 3cm) TPCM (L, B, 3cm)

TPCM (R, T, 3cm) TPCM (R, C, 3cm) TPCM (R, B, 3cm)

TPCM (B, C, 3cm) TLoad(10kg) Tout

Pump ON Pump OFF

Fig. 6. Variations of PCM temperatures, Tout and Tload (10kg) temperatures for evening cooking on 3 September 2002.


TPCM(R, T,3 cm). At 5:00 PM, 8kg of water was loaded for

evening cooking. Results show that noon and evening

cooking on the same day is possible and daytime cook-

ing does not affect evening cooking. PCM temperatures

were found near 100 �C in the early morning (6:00 AM),

so once the food is cooked it can be kept warm until the

next morning. During this experiment, the heat transfer

rate from the PCM unit to the cooking vessel was found

to be very poor as no water filled in the gap to increase

the heat transfer, and because of the poor insulation

around the PCM storage unit. After this experiment,

urethane foam insulation was added around the PCM

storage unit to prevent heat losses.

We also tried to conduct cooking experiments in Jan-

uary 2003. All PCM temperatures were found to be less

than 100 �C in the winter experiments in spite of good

insulation around the PCM storage unit. A minimum

temperature of near 75 �C was observed inside the

PCM storage unit in winter. Thus, low temperature

cooking may be also possible in January, but the PCM

did not melt in winter for the present set-up under Jap-

anese climatic conditions.

25

50

75

100

125

150

8:00 11:00 14:00 17:00 20:00 23:00 2:00 5:00Time

Tem

pera

ture

[C]

TPCM (L, T, 3cm) TPCM (L, C, 3cm) TPCM (L, B, 3cm)TPCM (R, T, 3cm) TPCM (R, C, 3cm) TPCM (R, B, 3cm)TPCM (B, C, 3cm) Tout Tload (8kg)

Pump ON Pump OFF

Fig. 7. Variations of PCM temperatures, Tout and Tload (8kg) temperatures for noon and evening cooking on 5 September 2002.

25

50

75

100

125

150

7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00

Time

Tem

pera

ture

[C]

TPCM (L, T, 3cm) TPCM (L, C, 3cm) TPCM (L, B, 3cm)TPCM (R, T, 3cm) TPCM (R, C, 3cm) TPCM (R, B, 3cm)

TPCM (B, C, 3cm) Tout Tload (10 kg)

Pump ON Pump OFF

Fig. 8. Variations of PCM temperatures, Tout and Tload (10kg) temperatures for noon and evening cooking on 22 August 2003.


On 10 and 22 August and 4 September 2003, experi-

ments were conducted for noon and evening cooking

with loads of 5, 10 and 7kg water, the average flow rates

were 9.22, 9.32 and 8.97 l/min, respectively. Fig. 8 shows

the evolution of PCM temperatures and a water load of

10kg on 22 August 2003. The maximum PCM tempera-

ture (129 �C) was reached at TPCM(R,C,3 cm) around 1:30

PM. Almost all the PCM was melted except at the bot-

tom of the storage unit. The result shows that PCM tem-

peratures drop significantly due to the heat transfer from

the PCM to the load of 10kg water for evening cooking

at 5:00 PM. It was observed that evening cooking is fas-

ter than noon cooking for the same load, even without

filling the water in the gap to increase the heat transfer.

Cooking experiments with loads of 5 and 7kg water

were conducted on 10 August and 4 September 2003,

and the same phenomenon was observed. PCM temper-

atures, Tin, Tout, average solar irradiance and ambient

air temperature for loads of 5, 7 and 10kg water are pre-

sented in Table 3. Results show that Tmaxout varies from

130 to 135 �C around midday, and is sufficient to charge

the PCM. During the charging period, the maximum

Table

3

PCM

temperatures,Tin,Tout,averagesolarirradiance

andambientairtemperature

fornoonandeveningcooking

Date

10August

2003

22August

2003

4September

2003

Load

5kg(noonandevening)

10kg(noonandevening)

7kg(noonandevening)

Tim

e8:05

(loadin)

12:30

(loadout)

16:35

(loadin)

19:30

(loadout)

9:00

(loadin)

13:30

(loadout)

17:00

(loadin)

20:15

(loadout)

8:00

(loadin)

14:00

(loadout)

17:15

(loadin)

20:15

(loadout)

Tload

24.2

100.1

29.0

94.8

26.6

95.3

27.6

95.4

29.5

95.6

22.7

97.0

Tin

55.5

126.6

••

88.8

131.2

••

72.1

129.6

••

Tout

56.3

129.0

••

90.3

134.5

••

72.7

132.6

••

TPCM(L,C,3cm)

30.3

112.9

114.9

94.9

80.9

119.6

116.4

100.7

71.7

119.7

116.8

104.5

TPCM(R

,C,3cm)

30.2

112.9

115.7

94.0

84.6

127.4

110.7

98.8

70.8

125.2

116.9

102.0

TPCM(B,C,3cm)

30.8

85.3

107.6

90.6

65.8

106.4

110.2

93.8

77.2

104.1

110.6

95.8

Tmaxin

126.6

––

132.4

––

131.5

––

Tmaxout

129.8

––

135.8

––

135.1

––

TPCMmax

125.4

––

131.0

––

130.5

––

Ta

30.9

28.5

33.9

29.6

33.4

29.6

I h(ave)

719

––

724

––

660

––

I s20(ave)

819

––

860

––

792

––

•PumpOFF.


PCM temperature varies from 125 to 130 �C. PCM tem-

peratures are more than 110 �C at the time of evening

cooking.

Fig. 9 shows the evolution of Tout, Tin, ambient air

temperature and solar irradiance (at 20�) for typical win-ter and summer days. Tout reached its maximum of

about 138 �C around 12:45 PM in summer, compared

to 94 �C in winter at around 1:30 PM. Tout in summer

is sufficient for charging the PCM and can store only

sensible heat in winter in the Japanese climate. Solar

irradiance increases to its maximum value (1043W/m2)

at 11:00 AM in summer months and 804W/m2 in winter

months. Results show that solar irradiance and Tout are

appropriate for solar cooking with PCM storage in Jap-

anese summer months.

5. Fraction of incident solar energy stored by PCM

and water

From the measured data, we can get the fraction (f1)

during PCM charging, this is the ratio of the total stored

heat in the PCM (QPCMStored) to the solar irradiance (G),

and is defined as

f1 ¼QPCMStored

G: ð1Þ

The total heat stored in the PCM (QPCMStored) was cal-

culated by the evaluation of the enthalpy inside the

PCM storage unit. On the basis of the measured temper-

ature distribution in the PCM, we evaluated enthalpy

for each point in the PCM storage unit using the enthal-

py curve of erythritol (Kakiuchi et al., 1998). Mass (Mi)

of the PCM at each point was calculated using the

relation Mi = Viqpcm. Total heat stored in the

PCM (QPCMStored) was obtained using the following

relation:

QPCMStored ¼X21i¼1

fMiIðT iÞg: ð2Þ

Solar irradiance (G) was calculated with the following

equation:

G ¼ ðsaÞAZ t

0

Is20 dt: ð3Þ

Fraction (f2) is the ratio of the total stored heat in the

PCM (QPCMStored) to the total heat gain by the heat

transfer fluid �water� (QWaterStored), and is defined as

f2 ¼QPCMStored

QWaterStored

: ð4Þ

Total heat gain by the water (QWaterStored) was obtained

from the following relation:

0

25

50

75

100

125

150

9:00 10:00 11:00 12:00 13:00 14:00 15:00Time

Tem

pera

ture

[C]

200

400

600

800

1000

1200

Sola

r irra

dian

ce (2

0 de

gree

) [W

/m2 ]

Tout (summer) Tin (summer)Ta (summer) Tout (winter)Tin (winter) Ta (winter)Is20 (summer) Is20 (winter)TT

I

Fig. 9. Variation of Tin, Tout, Ta and Is20 with time for a typical day in summer and winter month.


QWaterStored ¼ mCw

Z t

0

ðT out � T inÞdt; ð5Þ

where ‘‘m’’ is the flow rate of the water.

Fraction (f3) is the ratio of the total heat gain by the

heat transfer fluid �water� (QWaterStored) to the solar irra-

diance (G), and is defined as

f3 ¼QWaterStored

G: ð6Þ

Fractions f1, f2 and f3 were calculated with and with-

out load on different days and tabulated in Table 4.

Fraction f3 presents the collector performance. The

ETSC showed good performance and f3 varies between

66 and 77% for loads of 5, 7, 8 and 10kg water and

71% without load. The difference between the incident

solar irradiance and the heat gain by HTF represents

the collector thermal loss. Fraction f2 showed poor per-

formance because of the insufficient heat transfer to the

PCM storage unit and heat losses through the unit. As a

result, fraction f1( = f2 · f3) also shows poor

performance.

There are several methods to enhance heat transfer in

a latent heat thermal storage unit. The use of finned

tubes in thermal storage systems with different configu-

rations has been reported by Morcos (1990) and Velraj

et al. (1999). An embedded carbon fiber brush was also

used to enhance thermal conductivity in the PCM (Fu-

kai et al., 2000). Though our experiments and analysis

indicated that the prototype solar cooker yielded satis-

factory performance in spite of low heat transfer, a mod-

ified design for the heat exchanger in the thermal storage

unit should enhance the rate of heat transfer in our pres-

ent set-up.

6. Conclusion

In the present study, the thermal performance of a

prototype solar cooker based on an ETSC with a

PCM heat storage unit was studied. A simple cylindrical

PCM heat storage unit was designed to store solar en-

ergy during sunshine hours and to cook food in the

evening. The experimental results from the present set-

up allow the following conclusion:

(i) The system is able to cook successfully twice (noon

and evening) in a single day during Japanese sum-

mer months. Noon cooking did not affect evening

cooking, and the evening cooking using the PCM

heat storage was found to be faster than noon

cooking.

(ii) The PCM did not melt in January (winter) in

Japan. In summer, PCM temperatures reached

more than 110 �C at the time of evening cooking.

Hence, erythritol is a promising PCM for solar

cooking.

(iii) Fraction f1 defined as the ratio of stored heat to

solar irradiance showed poor performance. How-

ever, cooking experiments showed that the PCM

storage unit is able to store an adequate amount

of heat for noon and evening cooking and is also

capable to keep PCM temperatures (near 75 �C)until the next morning.

This system is expensive but shows good potential

for community applications. It provides high PCM

temperatures of up to 130 �C without tracking and al-

lows cooking in the shade and also in a conventional

Table

4

ThefractionofincidentsolarenergystoredbyPCM

andwater

Date

Mload(kg)

mave(l/m

in)

QPCMStored(M

J)Q

WaterStored(M

J)G

(MJ)

f 1¼

QPCMStored

G

�� ð%

Þf 2

¼Q

PCMStored

QWaterStored

�� ð%

Þf 3

¼QWaterStored

G

�� ð%

Þ

2September

2002

Noload

6.43

12.3

39.9

56.0

21.9

30.8

71.2

5September

2002

8(twotimecooking)

8.14

10.4

40.9

61.5

16.9

25.4

66.5

10August

2003

5(twotimecooking)

9.22

8.9

40.3

56.2

15.8

22.1

71.7

22August

2003

10(twotimecooking)

9.32

9.3

35.6

46.2

20.1

26.1

77.1

4September

2003

7(twotimecooking)

8.97

8.3

36.3

52.9

15.7

22.9

68.6


kitchen during non-sunshine hours or in the evening.

Therefore, solar cookers based on an ETSC with a

PCM storage unit should help to popularize solar

cookers in Japan, as well as in other regions having

good sun shine.

Acknowledgements

The authors are grateful to the Satellite Venture Busi-

ness Laboratory, Mie University, Japan for providing

the financial support and constant encouragement for

the present study. S.D. Sharma is very grateful to Japan

Society for the Promotion of Science for funding the

post doctoral fellowship for the year 2003–2005.

References

Balzar, A., Stumpf, P., Eckhoff, S., Ackermann, H., Grupp, M.,

1996. A solar cooker using vacuum tube collectors with

integrated heat pipes. Solar Energy 58 (1–3), 63–68.

Buddhi, D., Sahoo, L.K., 1997. Solar cooker with latent heat

storage: Design and experimental testing. Energy Conver-

sion and Management 38 (5), 493–498.

Buddhi, D., Sharma, S.D., Sharma, A., 2003. Thermal perfor-

mance evaluation of a latent heat storage unit for late

evening cooking in a solar cooker having three reflectors.

Energy Conversion and Management 44, 809–817.

Domanski, R., El-Sebaii, A.A., Jaworski, 1995. Cooking during

off-sunshine hours using PCMs as storage media. Energy 20

(7), 607–616.

Fukai, J., Kanou, M., Kodama, Y., Miyatake, O., 2000.

Thermal conductivity enhancement of energy storage media

using carbon fibers. Energy Conversion and Management

41, 1543–1556.

Hasan, A., 1994. Phase change material energy storage system

employing palmitic acid. Solar Energy 52 (2), 143–154.

Kakiuchi, H., Yamazaki, M., Yabe, M., Chihara, S., Teru-

numa, T., Sakata, Y., Usami, T., 1998. A study of erythritol

as phase change material. IEA Annex 10-PCMs and

chemical reactions for thermal energy storage, second

workshop, 11–13, November Sofia, Bulgaria.

Khalifa, A.M.A., Taha, M.M., Akyurt, M., 1987. Design,

development and testing of a new concentrating type solar

cooker. Solar Energy 38 (2), 79–88.

Kumar, R., Adhikari, R.S., Garg, H.P., Kumar, A., 2001.

Thermal performance of a solar pressure cooker based on

evacuated tube solar collector. Applied Thermal Engineer-

ing 21, 1699–1706.

Morrison, G.L., Mills, D.R., 1987. Solar stove for developed

countries. In: Proc. ISES World Congress, Hamburg,

Federal Republic of Germany, pp. 13–18.

Morcos, V.H., 1990. Investigation of a latent heat thermal

energy storage system. Solar Wind Technology 7 (2/3), 197–

202.

Sari, A., Kaygusuz, K., 2002. Thermal performance of palmitic

acid as a phase change energy storage material. Energy

Conversion and Management 43, 863–876.


Schwarzer, K., Maria Eugenia Vieria da Silva, 2003. Solar

cooking system with or without heat storage for

families and institutions. Solar Energy 75, 35–

41.

Sharma, S.D., Buddhi, D., Sawhney, R.L., 1999. Accelerated

thermal cycle test of latent heat storage materials. Solar

Energy 66 (6), 483–490.

Sharma, S.D., Buddhi, D., Sawhney, R.L., Sharma, A., 2000.

Design development and performance evaluation of a latent

heat unit for evening cooking in a solar cooker. Energy

Conversion and Management 41, 1497–1508.

Velraj, R., Seeniraj, R.V., Hafner, B., Faber, C., Schwarzer, K.,

1999. Heat transfer enhancement in a latent heat storage

system. Solar Energy 65, 171–180.

Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage...

Documents

Transcript of Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage...