performance evaluation of a helix tube colar collector system

8/9/2019 Performance Evaluation of a Helix Tube Colar Collector System

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INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2007; 31:11691179Published online 29 May 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/er.1327

SHORT COMMUNICATION

Performance evaluation of a helix tube solar collector system

Kritsada Boonchom1,3, Anusorn Vorasingha2, Nipon Ketjoy3,

Chaluay Souvakon4 and Theerachai Bongkarn2,*,y

1Faculty of Science and Technology, ChiangMai Rajabhat University, ChiangMai 50300, Thailand2Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand

3School of Renewable Energy, Naresuan University, Phitsanulok 65000, Thailand4Faculty of Science and Technology, Uttaradit Rajabhat University, Uttaradit 53000, Thailand

SUMMARY

In this paper we evaluated the performance of a helix tube solar collector. This helix tube collectorincreases the absorbed radiation of diffuse and ground reflection. The inner and outer diameters of thehelix tube are 8 and 12 mm, respectively. The helix tube was designed as a copper tube which is coated witha nanocarbon composite. A cylindrical glass, length 1.2 m, outer diameter 1.397 101 m and thickness6.4 103 m, was used as a cover. The performance of the collector was in Phitsanulok province, Thailand,which is located at latitude 16.7838N with an average wind velocity of 3 m s1. Solar absorbance was 0.94and the IR emission of nanocarbon was 0.15. The heat transfer fluid was Paratherm MGTM oil at 328C andthe oil mass flow rate was 9.5 kg h1. These were used to evaluate the performance of the collector. Theperformance of the helix tube collector and the temperature difference of oil (inlet and outlet) were 50.32%

and 47.438C, respectively. Copyright# 2007 John Wiley & Sons, Ltd.

KEY WORDS: helix tube solar collector; nanocarbon composite; the performance of collector; heat transfer fluid

1. INTRODUCTION

Hot fluid is an essential requirement in industry as well as in the domestic sector (Kalogirou,

2004). The solar collector is a device which absorbs incoming solar radiation, converts it into

heat, and transfers this heat to a fluid (usually air, water, or oil) which flows through the

collector. Solar collectors are categorized in two ways: non-concentrating or stationary and

*Correspondence to: Theerachai Bongkarn, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand.yE-mail: [email protected]

Contract/grant sponsor: ChiangMai Rajabhat University, ThailandContract/grant sponsor: EPPO, Energy Ministry, ThailandContract/grant sponsor: Naresuan University, Thailand

Received 15 October 2006Accepted 15 October 2007Copyright # 2007 John Wiley & Sons, Ltd.


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concentrating (sun tracking system). The various types of stationary solar collectors are

flat-plate collectors (FPCs), evacuated tube collectors (ETCs) and compound parabolic

collectors (CPCs) and various types of concentrating, such as parabolic trough collectors

(PTCs), parabolic dish and central receivers. Solar thermal applications can be categorized by

temperature in two ways. Low temperatures (both active and passive) are used to heat and coolbuildings, to heat water for domestic and industrial uses, to heat swimming pools, to power

refrigerators, to desalinate water for drinking purposes and to dry agriculture material. High

temperatures are used to operate engines to generate electricity, etc.

For low-temperature applications, the important part is the solar collector (FPC or ETC).

The components of a collector such as the covering glass and the absorbing surface were studied

to improve their efficiency. Antireflective coatings and surface texture techniques were used to

develop the transmittance of the cover glass. High-temperature-resistant transparent insulating

materials (FPC and CPC) were used as cover glass. They showed a higher efficiency as collectors

than conventional glass (Schweiger, 1997; Benzet al., 1998). Teixeiraet al. (2001) found that the

level of absorbance (a) and emittance (e) of the absorber surface was directly affected by the

FPC efficiency. Characteristics of the spectral solar selected: 0:85a50:95 and 0:055e50:2 were

found in nanocomposite materials (Oelhafen and Schuler, 2005). Katzen et al. (2005) prepared athin film of porous silica and nanosized carbon with an absorbance of a 0:94 and an IRemittance of e 0:15: In this study, the helix tube, coated with a nanocarbon compositereceiver, was designed for a high absorption of solar radiation and a low loss of thermal

radiation. The performance of the collector was evaluated by thermal analysis based on the Thai

climate in Phitsanulok province. The result was also compared with a traditional FPC.

2. HELIX TUBE SOLAR COLLECTOR

A good collector should have high solar radiation absorption and low thermal loss radiation.

So, the interior surface and vacuum envelope were chosen to increase absorbed radiation and

reduce thermal loss. A helix tube solar collector was designed as shown in Figure 1. It consists ofa vacuum cylindrical cover glass and copper helix tube.

The glass solar collectors, which transmit as much as 90% of the incoming shortwave solar

radiation but transmitting virtually none of the long-wave radiation emitted outward by the

absorber plate, were used as the cover glass for the collector. The cylindrical cover glass has an

outer diameter of 1.397 101 m, a length of 1.2 m and a thickness of 6.4 103 m. The

incidence transmittance of a normal cover glass collector is 0.88. The cover glass was sealed with

Aluminum plate

Oil

outlet

Helix tube

Vacuum chamberOil

inlet

Cover glass

Figure 1. Schematic diagram of helix tube solar collector.

K. BOONCHOM ET AL.1170

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2007; 31:11691179

DOI: 10.1002/er


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aluminium plate and was evacuated. The helix copper was coated with a nanocarbon composite.

A thin film of nanocarbon composite has excellent optical parameters (a 0:94 and e 0:15).The helix copper tube has an outer diameter 0.012 m, inner diameter 0.008 m and receiver area

(AR) approximate 0.5 m2 (surface area of helix tube).

3. THERMAL ANALYSIS OF COLLECTOR

The bases of the thermal analysis of the collectors are considered to be the collectors thermal

efficiency. The performance of a solar collector is defined as the ratio of the useful energy

delivered to the energy incident on the collector aperture (Duffie and Beckman, 1980; Kreith and

Kreider, 1978). In a steady state, the performance of a solar collector (Z) can be written as

Z Qu

ItAR1

The performance of a solar collector is described by the energy balance that indicates the

distribution of incident solar energy into useful energy gain, thermal losses, and optical losses.

The useful energy output of the collector is the difference between the absorbed solar radiation

and the thermal loss. The useful energy output of the collector is defined by this equation:

Qu ARFRSULTi Ta 2

FR as collector heat removal factor:

FR mCp

ARUL1 exp

ARULF0

mCP

3

F0 as the collector efficiency factor:

F0 Uo

UL4

UL is the overall collector loss coefficient. It is the sum of the top, bottom and edge losscoefficient

UL Ut Ub Ue 5

The thermal network of the collector can be written as shown in Figure 2.

The overall collector thermal resistance can be written as

RL 1

hr:Coahc:CoaACo

1

hr:RCoAR6

The overall collector loss coefficient can be written as

UL 1

RL

AR 7

The heat transfer from the outer surface of tube to the fluid in the tube includes the tube wall.

Thus, the heat transfer from the tube to the fluid is the overall heat transfer coefficient ( Uo)

which is calculated from the base on the outside diameter of the helix tube.

Uo 1

RLR4R5

AR 8

A HELIX TUBE SOLAR COLLECTOR SYSTEM 1171


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The wind heat transfer coefficient as

hc:Coa or hw Nuk

L 9

The radiation coefficient from cover tube to ambient is

hr:Coa eCosT2CoT

2sTCo Ts 10

The radiation coefficient from receiver tube (helix tube) to cover tube is

hr:RCo sT2R T

2CoTRTCo

1eR

eR1

1eCo

eCo

AR

ACo

11

The isotropic diffuse sky model, hourly based, was used to calculate the absorbed radiations

which can be written as this equation:

S IbRbtab Idtad1cos b

2

rgIb Idtag

1cos b

2

12

1 cos b=2 and 1cos b=2 are the view factors from the collector to the sky and fromcollector to the ground. The subscripts b, d and g represent beam, diffuse and ground,

respectively.

4. RESULT AND DISCUSSION

4.1. The absorbed radiation

The absorbed radiation was evaluated hourly by using the isotropic diffuse sky model which is

based on the monthly average daily total radiation %H as seen in Table I (Department of

Energy Development and Promotion and Silapakorn University, 1999). The absorbed radiation

hr:CoaACo

hc :Coa ACoR1 =

1R2 =

1

R3 = 1

S

QuhfiAinside tube

R51

=

Tfi

r

r

R4tubediameterinner

tubediameterouter

2Lk

ln

=

Ta

h r:RCo AR

Figure 2. Thermal network of the helix tube solar collector.



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of helix tube solar collector was calculated from Equation (12) and can be written asS IbRbtab0:5 IdtadrgIb Idtag0:5 13

Equation (13) consists of terms such as beam radiation, diffuse radiation and ground reflects

radiation. Beam radiation angle b 08was a half of beam radiation in Equation (13) because

the aperture area was located on the top of helix tube only. The maximum diffuse radiation

value was onlyIdtadbecause the helix tube could only receive diffuse radiation from the whole

of the northern hemisphere. The tilted angle (b) from ground reflection radiation term as if

b 1808 and the helix tube also received the ground reflection radiation on the bottom side

only.

This means 1cos b=2; in terms of ground reflection radiation, was equal to 0.5. Theabsorbed radiation of the FPC was calculated from Equation (13) with a tilted angle of

16.7838N

S IbRbtabIdtad1 cos16:783

2

rgIb Idtag

1cos16:783

2

14

Moreover, from Equation (12), the value of transmittance (t) of the glass and the absorbance (a)

of the thin film of nanocarbon also effected the absorption of radiation. The absorbed radiation

of the helix tube solar collector and the FPC was evaluated from Equations (13) and (14),

respectively (Table II). The absorbed radiation of the helix tube is lower than the FPC.

However, from Equation (5), the overall collector loss coefficient ( UL) of the FPC on the bottom

side (Ub) and the edge side (Ue) was higher than the helix tube collector.

4.2. Heat transfer coefficient

The heat transfer coefficient of the helix tube collector was considered from a thermal network

which consisted of three parts:

* The heat transfer coefficient from cover tube to ambient (hc;Coa or hw).* The heat transfer coefficient from cover tube to ambient (hr;Coa).* The heat transfer coefficient from receiver tube (helix tube) to cover tube (hr;RCo).

Table I. Monthly average daily total radiation.

MonthBeam radiation

(MJm2)Diffuse radiation

(MJm2)Total radiation

(MJ m2)

1 9.6 6.1 15.7

2 11 6.7 17.73 12.4 7.5 19.94 13.3 7.9 21.25 14.1 8.1 22.26 13.5 8.2 21.77 10.9 8.3 19.28 13.8 7.9 21.79 10 7.9 17.9

10 9.6 7.1 16.711 9.5 6.2 15.712 9.8 5.8 15.6



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The heat transfer coefficient of the collector was studied under these conditions: ambient

temperature at 308C, cover temperature at 508C, wind velocity 05 m s1, emissive of glass (e) at

0.88 and the average temperature 408C.

Thehwcan be calculated from Equation (9). Nu in this equation is the Nusselt number which

relates to Reynolds number (Re) shown in Equation (16).

Nu 0:40 0:54Re0:52 for 0:1> Re >1000 15

andNu 0:30Re0:6 for 1000> Re > 50 000

Re was calculated from this equation

RerVD

m 16

If the value of Re is interval 100050 000 then the Nusselt number can be calculated by:

Nu 0:3Re0:6:The value of Re and Nu are shown in Table III. The results ofhwwith severalwind velocities showed in Figure 3. The hr;Coa andhr;RCo were evaluated form Equations (10)

and (11), respectively. The hr;Coa is equal 6.14 W m2 K1. The hr;RCo with difference wind

velocity were also demonstrated in Figure 3. The hwincreased rapidly and thehr;RCo increased

slightly with increasing wind velocity. These may be the hwandhr;RCotransferred in the air and

vacuum, respectively.

4.3. The overall heat transfer coefficient

The overall heat transfer coefficient (Uo) consists of the overall collector loss coefficient, heat

transfer coefficient of conduction and heat transfer coefficient of convection.

The overall collector loss coefficient (UL) can be calculated from the equation

UL AR

hr:Coahc:CoaAc

AR

hr:RCoAR

117

The heat transfer coefficient of conduction and the heat transfer coefficient of convection were

calculated based on the outside helix tube. The overall heat transfer coefficient ( Uo) is shown in

the following equation:

Uo 1

UL

router diameter tubeln router diameter tube

rinner diameter tube

k

router diameter tube

hfirinner diameter tube

2664

3775

1

18

Table II. The absorbed radiation of helix tube collector and FPC on tilted angle 16.7838N.

Time (5:16) I total (Wm2) Sflat plate (W m2) Shelix (Wm2)

9.0010.00 859.90 526.80 418.8810.0011.00 1033.66 732.75 575.71

11.0012.00 1128.46 859.93 679.6012.0013.00 1128.46 859.93 679.6013.0014.00 1033.66 732.75 575.7114.0015.00 859.90 526.80 418.88



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The heat transfer coefficient of fluid (hfi) in Equation (18) can be calculated by using the physical

properties of Paratherm MGTM oil (food grade) at 328C: viscosity 0.00422 kg m1 s1, thermal

conductivity 0.14056W m1 K1, specific heat capacity 2268.41 J kg1 K1 and mass flow rate of

oil as 9.5kg s1. The Reynolds number was calculated from Equation (16) (Re 99:60). Thisvalue is lower than 2200 thus Nu 3:7: The heat transfer coefficient of the oil was65.01Wm2 K1. The UL and Uo with the different wind velocities are shown in Figure 4.

The Uo was lower than the UL because the heat transfer fluid reduced the difference in

temperature between the cover glass and the helix tube.

4.4. Performance of collector

The heat gain input of the collector is calculated from a total incident radiation (Table II) with a

receiver area (0.5 m2). The heat gain output was calculated with a wind velocity of 0 and 3 m s1.

The heat gain results are shown in Figure 5.

Table III. The heat transfer coefficients of convectionwith different wind velocity.

vw (m s1) Re Nu

0 0 0

1 7219.55 61.982 14 439.09 93.943 21 658.64 119.814 28 878.18 142.385 36 097.73 162.78

Figure 3. Heat transfer coefficient with various wind velocities.



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The performance of the collector can be calculated from Equation (1) and is shown in

Table IV. The performance of the helix tube solar collector with vwat 3m s1 is lower thanvwat

0 m s1. This result is related to the overall heat loss coefficient. Both the helix tube and the FPC

Figure 4. The overall collector loss coefficient and the overall heat transfer coefficient.

0

100

200

300

400

500

600

Time 9.00-10.00 10.00-11.00 11.00-12.00 12.00-13.00 13.00-14.00 14.00-15.00

Heatgain(W)

Heat

gain input

Heat gain

output at

vw=0 m/s

Heat gain

output at

vw=3 m/s

Figure 5. Heat gain input and output related to time.



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were considered at a wind velocity of 3 m s1. The average performance of the helix tube

collector at a working time (9.0014.00) is 48.34%. The performance of a good FPC can be

calculated from (Kalogirou, 2004)

Z 0:7926:65 DT

Itotal

0:06

DT2

Itotal

19

The performance of FPC calculated from (25) is shown in Table V. The average performance of

the flat plate calculated at working time (9.0014.00) with vwat 3m s1 was 41.71%, which was

lower than the helix tube collector. The performance of the helix tube was increased by the

increase in the absorbed radiation. It is different from the FPC because the heat loss coefficient

of the flat plate increased with the increase in absorbed radiation.

5. CONCLUSION

The performance of the helix tube collector was evaluated based on the monthly average daily

total radiation in Phitsanulok province, Thailand, with an average wind velocity of 3 m s1. The

maximum performance radiation of the helix tube collector was higher than the flat plate by

11.03%. Moreover, the average performance of helix tube solar collector at working time

(10.0014.00) was higher than the flat plate collector by 6.63%. The result also indicated that the

Table IV. The performance of helix tube solar collector.

Time Qu (W) Ti (K) To (K) DT(K) Performance (%)

vw 0 m s1 9.0010.00 190.06 305.15 336.90 31.75 44.21

10.0011.00 262.34 305.15 348.98 43.83 50.76

11.0012.00 310.22 305.15 356.97 51.82 54.9812.0013.00 310.22 305.15 356.97 51.82 54.9813.0014.00 262.34 305.15 348.98 43.83 50.7614.0015.00 190.06 305.15 336.90 31.75 44.21

vw 3 m s1 9.0010.00 172.86 305.15 334.03 28.88 40.21

10.0011.00 239.68 305.15 345.19 40.04 46.3711.0012.00 283.93 305.15 352.58 47.43 50.3212.0013.00 283.93 305.15 352.58 47.43 50.3213.0014.00 239.68 305.15 345.19 40.04 46.3714.0015.00 172.86 305.15 334.03 28.88 40.21

Table V. The performance of flat plate collector.

Time DT (8C)Performance

of flat plate (%)

9.0010.00 28.88 51.0510.0011.00 40.04 44.1311.0012.00 47.43 39.2912.0013.00 47.43 39.2913.0014.00 40.04 44.1314.0015.00 28.88 51.05



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thin film nanocarbon composite coated on the helix tube significantly improved performance.

This reveals a good capability of the helix tube collector to convert solar energy to heat for solar

thermal applications.

NOMENCLATURE

AR receiver area (m2)

Cp specific heat capacity (kJ kg1 K1)

FR collector heat removal factor

F0 collector efficiency factor%H monthly average daily total radiation (MJ m2)

hr;Coa the heat transfer coefficient from cover tube to ambient (W m2 K1)

hc;Coa or hw the heat transfer coefficient from cover tube to ambient (W m2 K1)

hr;RCo the heat transfer coefficient from receiver tube (helix tube) to cover tube

(W m2 K1)

Ib beam hourly irradiation (W m

2

)Id diffused hourly irradiation (W m2)

It total hourly irradiation (W m2)

K extinction coefficient

k thermal conductivity (W m1 K1)

L thickness or length space (m)

m mass flow rate (kg s1)

Nu Nusselt number

Qu the useful energy (W J s1)

R thermal resistance (m2 K W1 m2)

Re Reynolds number

RL thermal resistance of the collector loss (m2 K W1 m2)

S absorbed radiation (W)T temperature (K)

Ub the bottom loss coefficient (W m2 K1)

Ue the edge loss coefficient (W m2 K1)

Uo overall heat transfer coefficient (W m2 K1)

UL collector overall heat loss coefficient (W m2 K1)

Ut the top loss coefficient (W m2 K1)

vw wind velocity (m s1)

Greek letters

a absorbance

b tilted angleg surface azimuth angle

d declination

e emittance

Z performance of collector

y incident angle



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yZ zenith angle

m absolute viscosity

r density

rg ground reflectance

t transmittancef latitude

o hour angle

Subscripts

a ambient

b beam radiation

c convection

Co cover tube

d diffuse radiation

g ground reflect radiation

i inleto outlet

R receiver

r radiation

w wind

fi fluid inlet

ACKNOWLEDGEMENTS

The authors are grateful to the School of Renewable Energy, Faculty of Science, Naresuan University,

Faculty of Science and Technology, ChaingMai Rajabhat University and Ministry of Energy, Thailand,for financial support. Acknowledgement is also to Don Hindle for his helpful comments and editing of themanuscript.

REFERENCES

Benz N, Hasler W, Hetfleish J, Tratzky S, Klein B. 1998. Flat-plate solar collector with glass TI. Proceedings of Eurosun,Portoroz, Slovenia.

Department of Energy Development and Promotion and Silapakorn University. 1999. Solar Energy Map of Thailand.Jirangrath Printing: Bangkok, Thailand.

Duffie JA, Beckman WA. 1980. Solar Technology (2nd edn). Wiley: New York, U.S.A.Kalogirou SA. 2004. Solar thermal collectors and applications. Progress in Energy and Combustion Science 30:231295.Katzen D, Levy E, Mastai Y. 2005. Thin films of silicacarbon nanocomposites for selective solar absorbers. Applied

Surface Science 248:514517.

Kreith F, Kreider JF. 1978. Principles of Solar Engineering. McGraw-Hill: New York, U.S.A.Oelhafen P, Schuler A. 2005. Nanostructured materials for solar energy conversion. Solar Energy 79:110121.Schweiger H. 1997. Optimisation of solar thermal absorber elements with transparent insulation. Ph.D. Thesis,

Universitat Politecnica de Catalunya, Spain.Teixeira V, Sousa E, Costa MF, Nunes C, Rosa L, Carvalho MJ, Collares-Pereira M, Roman E, Gago J. 2001.

Spectrally selective composite coatings of CrCr2O3 and MoAl2O3 for solar energy applications. Thin Solid Films392:320326.



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performance evaluation of a helix tube colar collector system

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