Ts-3 the effective length of solar parabolic concentrating collector

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1-023 (O) The Joint International Conference on “Sustainable Energy and Environment (SEE)” 1-3 December 2004, Hua Hin, Thailand

67

The Effective Length of Solar Parabolic Concentrating Collector

Manoon Pidhuwan1, Sombat Teekasap

2and Joseph Khedari

1

1 Energy Technology Department, School of Energy and Materials, King’s Mongkut University of Technology Thonburi, Bangkok,Thailand

2

Engineering Faculty, Southeast Asia University, Nongkham, Bangkok, Thailand.

Abstract: This paper presents the effective length of solar parabolic concentrating collector. The system is fabricated with localmaterials; using the stainless steel sheet of regular size (1.2 x 2.4 m) as parabolic reflector and carbon steel pipe as absorber. The

collector is fixed at low latitude of Bangkok, 13.4 ° N and 100.3 °E. Result of the test showed that even through the system is simple

with low investment cost, both of performance and efficiency was satisfy. From experiments, the temperature of heating media

(thermal oil) was quite steady after media flow proceeds at about 3/4 of total length and so forth at on flow rates. This parameter ishelpful for optimizing others similar system.

Keywords: Parabolic, Effective Length, Solar, Concentrating, PCC.

1. INTRODUCTION

In the past decades, researchers and engineers tried to find

the best solution for utilizing solar energy, which has a lot of parameters to be considered such as collector geometry,latitude, solar time, solar collecting types and etc. This paper presents geometry of parabolic concentrating collector; PCC

with focus on the effective length. The local materials are usedfor fabricate the PCC system. Consequently, the experimentalhas shown an optimal length of PCC. The system consists of

mainly 3 parts: -Firstly, absorber and collector, the parabolic concentrating

collector materials used was a regular size of stainless steel

304 sheet of 1200 x 2400 mm and 3 mm of thickness.Fabricating under the basis of parabolic equation for reflecting

direct solar ray to the absorber with a radius of 573 mm andwidth of 992 mm.

The absorber materials was seam less carbon steel pipe,

with the inside diameter of 40 mm, and the outside diameter of 48 mm and 2920 mm in length.

Secondly, heating collection system, shell and tube heatexchanger is selected for this experiment with thermal oil as a

working fluid in carrying heat from absorber and relief theheat to water.

Finally, driving and measurement unit, the thermal oil is

driven by gear pump. The thermocouple type ‘K’ is installed 5 positions along the absorber for measuring thermal oil

temperature and 2 positions to measure the inlet and outlettemperature of water.

The experimental has done at the low latitude of Bangkok

as 13.44 ° N and longitude 100.34 °E, and collector is placedon East-West.

2. DESIGN

The collector is designed with simple parabolic equationand merged of solar radiation method in order to optimization

the fabrication with local material.

Parabolic Collector

According to the size limitation of stainless steel sheet,

width 1.2 m and long 2.4 m, and rim angle is 180°, that makes

the focus line is in placed the cord line.The simple parabolic equation can apply to solve above

condition, where x is an axial to place a curve, y is a center line of focal, R is a radius of parabolic curve, and f is a focal

line, which shown in Fig. 1Consideration the simple parabolic equation [4]:

x2 = 2 R y (1)

f = R / 2 (2)

Fig. 1 The schematic of parabolic.

The sheet is width of 1200 mm, it is a curve of R, and

focus point ‘f’ is a half of R, therefore, α is:

R cos α = R / 2 (3)

cos α = (R/2)(1/R)

α = 60°

Thus, a curve length is 1200 mm with α 60°, it canreconsider to find R in length as:

b2π= 2 π R (4)

R = [b (1800)] / [2 π α]

Then finding the concentration ratio ‘C’, using a ratio

between collecting area; Aa and absorbing area; Ar :

C = Aa / Ar (5)

Aa, the solar collecting area is calculated by the project area

of collector, and Ar will consider in topic of absorber.

a / 2 = R sin α (6)

a = 2 R sin α

Where ‘a’ is only the width of collector, which isreconsider to the width of absorber, the shading of radiation:

Absorber

The absorber is designed according to the limitation of the

collector, which considering parameters and common practice,such as, piping, working fluid velocity, fabrication and alsoheat loss. Thereby, the absorber is fabricated by the seamless

R

α

f (0,R/2)

cc (0,R)

y

xa/2

R / 2




68

carbon steel pipe, with the inside diameter of 40 mm, outside

diameter 48 mm and 2920 mm in length to provide the solar time between 10:30 and 14:00 hour.

Fig. 2 The width of focus line.

The width of absorber, ‘D’, is defined by:

D = 2 r r sin 0.267 (7)

Furthermore, the collector is fixed; therefore, the altitude

angle will be affected to the image of all the times. It moves

for 10° per day. Since the pipe is very long, so that two strips

of carbon steel is attached to the tube for increasing thestrength of absorber. Strip has the width of 25 mm of each.

Thus, overall width of absorber ‘w’ is 98 mm [(25x2) + 48]

Fig. 3 The absorber pipe.

D’ develop = r r sin θ (8)

Thus the concentration ratio is modified to:

C = [(a–w) L] / [w L] (9)

Where ‘L’ is a both of length, collector and absorber pipe.

The refection coefficient of collector is approximated rangeof 0.8 to 0.85 [9] by polished technique. And the absorber pipe

is coated with black colour and insulated with foiled fiberglasson upper part.

Copper tube pipeline and globe valve is selected for flow

rate adjustment.

Fig. 4 The schematic of collector.

3. EXPERIMENTAL

The collector is fixed, placed on East-West location,

therefore any testing will done on instantaneous technique, thedata was be kept into the data logger, and testing under theconditions are following:- Clear sky

- Wind speed of 3 to 5 m/s

- Ambient temperature 26 to 32 °C

- Humidity ratio 55 to 70 %

- Latitude 13.44 ° N and Longitude 100.34 °E

- Time 10:30 to 14:00 hour Furthermore, it is necessary to adjust a declination angle

‘δ’ for everyday and also keep duct cleaning before test.

Fig. 5 The location of collector.

Testing time is separated for 2 periods; first course is doneon November to December 2000, and another done on

February 2001. The measuring temperature was done at 5 positions on the absorber pipe for thermal oil, and 2 positionsfor inlet and outlet water temperature in the heat exchanger.

T1 T2 T3 T4 T5

600 600 600 600260 260

Inlet

Outlet

Fig. 6 The position of temperature measurement.

y

x

a/2

16'D/2

(φr+16')

φr

(φr-16')

θ

r r

w

D'

340

970

992

260

14.5

230

600

286.45R 573

120 ฐ

2 8 6. 4 5




69

Thermal oil is controlled by globe valve, which is divided

of 5 flow rates, 0.295, 0.345, 0.415, 0.515 and 0.685 kg/min.The solar energy will be prepared with a data from‘Meteorological department’ in order to reference. The dry

block instrument type calibrates the thermocouple K type for measuring temperature.

4. RESULTS

Consideration of 5 thermal oil flow rates 0.295, 0.345,

0.415, 0.515 and 0.685 kg/min. The maximum solar energy onto the collector was 880 W/m2, and it creates energy of 338.72W.

The result is divided for 2 parts; heating performance andLMTD in term of dimension concern, which briefs the heating

performance shortly following:

Fig. 7 The relation of efficiency and energy of thermal oilflow rate at 0.415 kg/min.

The maximum and average efficiency of system is 25.2 %

and 21.9 %, respectively. The flow rate given maximumefficiency is 0.415 kg/min, while maximum temperature in

working fluid is 75.8 °C by flow rate 0.295 kg/min.

The absorber pipe is a major heat loss, concerning to theR e, and the environmental, wind has a minor affection heat

loss.Reconsideration to the major heat loss, not only the R e, the

pipe wing is similar a heat sink instead the heat storage, cause

the diameter of absorber pipe is bigger than calculation fromtheoretical.

The next interesting, is the relation of log mean

temperature difference; LMTD, that is called effective length.Most of experiments, the curves of LMTD are similar, even

though the thermal oil velocity is changed from low to high,the characteristic curves are still the same.

y = 20.576Ln(x) +35.973

R2 = 0.8566

0

10

20

30

40

50

60

70

80

1 2 3 4 5

Pip e su r f a ce

Thermal o i l

Log

Fig. 8 The characteristic curve of LMTD of thermal oil flow

rate 0.295 kg/min.

y = 20.285Ln(x) +32.093R2 = 0.9822

0

10

20

30

40

50

60

70

80

1 2 3 4 5

P ip e su r f a ce

Thermal o i l

Log

Fig. 9 The characteristic curve of LMTD of thermal oil flowrate 0.345 kg/min.

y = 1 7.33Ln(x) +32.057

R2 = 0.9849

0

10

20

30

40

50

60

70

80

1 2 3 4 5

P ip e su r f a ce

Thermal o i l

Log


y = 11 .745Ln(x) +32.419

R2 = 0.9494

0

10

20

30

40

50

6070

80

1 2 3 4 5

P ip e su r f a ce

Thermal o i l

Log


y = 8.1 635Ln(x) +32.623

R2 = 0.8729

0

10

20

30

40

50

60

70

80

1 2 3 4 5

P ip e su r f a ce

Thermal o i l

Log

Fig. 12 The characteristic curve of LMTD of thermal oil flow

rate 0.685 kg/min.

y = - 0 . 0 0 5 6 x3

+ 0 . 1 2 4 8 x2

+ 8 . 4 7 7 5 x + 1 6 6 . 5 2

R2

= 0 . 9 8 3 3

0

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

1 0 : 3 0 1 1 : 0 0 1 1 : 3 0 1 2 : 0 0 1 2 : 3 0 1 3 : 0 0 1 3 : 3 0 1 4 : 0 0

T i m e

E n e r g y ( W )

0

5 0

1 0 0

E f f i c i en c y ( % )

E n e r g y f r o m c o l l e c t o r

E n e r g y o n c o l l e c lt o r

E f f i c i e n c y

P o l y




70

The maximum and minimum LMTD in term of average is

33.85 °C and 23.54 °C, respectively. However, in case of calculating at 4th position, LMTD is decreased during 3–4 %,

while the cost of collector and absorber is cut off 25 %.

5. CONCLUSION

The collector and absorber can be reduced for 1/4 of overall length with temperature can be reduced at average of 3.253 %. However, this experimental is only tested on one

curve of low concentration ratio, which is changed to another curves, if the concentration ratio will be higher, the LMTD

probably changed. The availability will be reconsider to findan optimization again. Whatsoever, the trend should be as

same as the low concentration ratio.

ACKNOWLEDGEMENTS

The authors would like to express our gratitude to Assoc.

Prof. Dr. Thanakom SOONTORNCHAINACKSAENG, for his ideas during the study. Also for Somchai Karnchang Co.,

Ltd., for it generosity was providing technician and facilitiesfor the test unit fabrication.

REFERENCES

[1] Collares-Pereira, M. (1991) High Concentration Two-Stage Optics for Parabolic Trough Solar Collectors with

Tubular Absorber and Large Rim Angle, Solar Energy, 47, pp. 457-66.

[2] Thomas, A. and Satish A. (1994) Design Data for The

Computation of Thermal Loss in The Receiver of AParabolic Trough Concentrator, Energy Converse, 35, pp.

555-68.[3] Ge, X. (1996) Solar Collection Assembly of Solar Industry

Heating and Generating Systems with Low Cost and High

Efficiency, Dept. of Thermal Science and EnergyEngineering. Institute of Applied Solar Energy.

[4] Duffie, J. A. and Beckman, W. A. (1990) Solar EnergyThermal Process. Wiley, New York.

[5] Rabl, A. (1976) Comparison of Solar Concentrators, Solar

Energy, pp. 18, 93.[6] Wieder, S. (1982) An Introduction to Solar Energy for

Scientists and Engineers. Fairleigh Dickinson University,John Wiley & Sons.

[7] Meinel Aden B. and Meinel, Marjorie P. (1979) Applied

Solar Energy an Introduction. Optical Sciences Center,University of Arizona, Addision-Wesley PublishingCompany.

[8] Holman, J.P. (1992) Heat Transfer. 7th ed. McGraw-Hill.[9] Agnihotri, O.P. and Gupta, B.K. (1981) Solar selective

Surfaces. John Wiley & Sons.

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Ts-3 the effective length of solar parabolic concentrating collector