PERFORMANCE CHARACTERISTICS OF THE SOLAR

PARABOLIC TROUGH COLLECTOR WITH HOT WATER

GENERATION SYSTEM

by

Amirtham VALAN ARASU and Samuel Thambu SORNAKUMAR

Original scientific paperUDC: 536.331.:662.997

BIBLID: 0354-9836, 10 (2006), 2, 167-174

The per for mance of a new par a bolic trough col lec tor hot wa ter gen er a tionsys tem with a well mixed hot wa ter stor age tank is in ves ti gated in the pres -ent work. The stor age tank wa ter tem per a ture is in creased from 35 °C at9.30 h to 73.84 °C at 16.00 h when no en ergy is with drawn from the stor agetank. The av er age beam ra di a tion dur ing the col lec tion pe riod is 699 W/m2.The use ful heat gain, the col lec tor in stan ta neous ef fi ciency, the en ergygained by the stor age tank wa ter and the ef fi ciency of the sys tem as a wholeare found to fol low the vari a tion of in ci dent beam ra di a tion as these pa ram -e ters are strongly in flu enced by the in ci dent beam ra di a tion. The value ofeach of those pa ram e ters is ob served to be max i mum around noon, when the in ci dent beam ra di a tion is max i mum.

Key words: par a bolic trough col lec tor, so lar hot wa ter gen er a tion, so larstor age sys tem, hot wa ter stor age tank, per for mance char ac -ter is tics of solar system

Introduction

Cur rently, so lar par a bolic trough col lec tor (PTC) is em ployed for a va ri ety ofap pli ca tions such as power gen er a tion [1], in dus trial steam gen er a tion [2], and hot wa terpro duc tion [3]. Par a bolic trough col lec tor is pre ferred for steam gen er a tion be cause hightem per a tures can be ob tained with out any se ri ous deg ra da tion of the col lec tor’s ef fi -ciency [4]. So lar ther mal power plants based on PTC are pres ently the most suc cess fulso lar tech nol o gies for elec tric ity gen er a tion, as showed by the So lar Elec tric Gen er a tionSys tems (SEGS) plant at Kramer Junc tion in Cal i for nia, USA [5]. A fea si bil ity study forthe use of PTC in a ho tel for hot wa ter pro duc tion was re ported by Kalogirou and Lloyd[3]. It was shown that PTC could be more cost ef fec tive than the con ven tional flat platecol lec tors. De sign and sim u la tion anal y sis of a PTC hot wa ter gen er a tion sys tem has been done by Valan Arasu and Sornakumar [6]. The sim u la tion anal y sis shows good agree -ment with the test re sults re ported by Valan Arasu and Sornakumar [7]. The pres ent work fo cuses on the per for mance study of a new PTC with hot wa ter gen er a tion sys tem.

167

Description of the parabolic trough collectorwith hot water generation system

In the pres ent work, a new par a bolic trough col lec tor sys tem, which has been de -vel oped for hot wa ter gen er a tion, is pre sented in fig. 1. The PTC sys tem for hot wa tergen er a tion in cludes a PTC, a hot wa ter stor age tank (HWST) of well-mixed type, and acir cu lat ing pump. The pa rab ola of the pres ent col lec tor with a rim an gle of 90° is very ac -cu rately con structed of fi ber glass. A flex i ble so lar re flec tor ma te rial from Clear DomeSo lar, San Diego, Cal., USA (SOLARFLEX foil) with a reflectance of 0.974 [8] is usedin the pres ent work. The so lar re ceiver is made of a cop per tube, a glass en ve lope and rub -ber cork seals at both ends of the glass en ve lope. The cop per tube is coated with a heat re -sis tant black paint and is sur rounded by a con cen tric glass cover with an an nu lar gap of0.5 cm. The rub ber corks are in cor po rated to achieve an air-tight en clo sure. Wa ter fromthe stor age tank is pumped through the cop per tube, where it is heated and then flowsback into the stor age tank. The PTC ro tates around the hor i zon tal north/south axis totrack the Sun as it moves through the sky dur ing the day. The axis of ro ta tion is lo cated atthe fo cal axis. The track ing mech a nism con sists of a low speed 12 V D. C. mo tor and anem bed ded elec tronic con trol sys tem. The in put sig nals to the con trol sys tem are ob tainedfrom light de pend ent re sis tors. The pump for main tain ing the forced cir cu la tion is op er -

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Figure 1. Parabolic trough collector with storage tank

ated by an on-off con trol ler (dif fer en tial ther mo stat), which senses the dif fer ence be -tween the tem per a ture of the wa ter at the out let of the col lec tor (Tfo) and the stor age tankwa ter (Tl). The pump is switched on when ever this dif fer ence ex ceeds a cer tain value andoff when it falls be low a cer tain value. In the pres ent work, the dif fer en tial tem per a turecon trol ler value (Tfo – Tl) is set as +2 °C. It is as sumed that the wa ter in the stor age tank isal ways well mixed and con se quently is at a uni form tem per a ture, Tl, which var ies onlywith time. The spec i fi ca tions of the PTC sys tem are de tailed in tab. 1.

Table 1. Parabolic trough collector system specifications

Items Value

Collector aperture 0.8 m

Collector length 1.25 m

Rim angle 90°

Focal distance 0.2 m

Receiver diameter 12.8 mm

Glass envelope diameter 22.6 mm

Concentration ratio 19.89

Water flow rate 0.7-1.0 l/min.

Storage tank capacity 35 li tres

Tank material Stain less steel

Tank insulation material Glass wool

Insulation Thickness 5 cm

Water pump 367.65 W

Performance testing of the parabolic troughcollector hot water generation system

The per for mance of the new PTC hot wa ter gen er a tion sys tem is de ter mined byob tain ing val ues of col lec tor in stan ta neous ef fi ciency and the sys tem ef fi ciency for dif -fer ent com bi na tions of in ci dent ra di a tion, am bi ent tem per a ture and in let wa ter tem per a -ture. The col lec tor wa ter out let tem per a ture (Tfo), am bi ent tem per a ture (Ta) and stor agetank wa ter tem per a ture (Tl) were re corded with the help of PT 100 – re sis tance tem per a -ture de vice (RTD) sen sors. The so lar beam ra di a tion in ten sity was mea sured by apyrheliometer and the mass flow rate of wa ter by a rotameter. The wind speed was mea -sured by a vane type an e mom e ter. All pa ram e ters were mea sured as a func tion of time

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Valan Arasu, A., Sornakumar, T.: Performance Characteristics of the Solar Parabolic ...

over a 30 min utes pe riod and un der steady-state or quasi-steady-state con di tions. Thetime re sponse of the par a bolic trough so lar col lec tor has been de ter mined in or der to eval -u ate the tran sient be hav iour of the col lec tor, and to se lect the proper time in ter vals for thequasi-steady-state or steady-state con di tion. When ever tran sient con di tions ex ist, theequal ity de fined by eq. (1) does not gov ern the ther mal per for mance of the col lec tor,since part of the so lar en ergy ab sorbed is used for heat ing up the col lec tor and its com po -nents or part of the en ergy lost re sults in cool ing the col lec tor. The time con stant of thecol lec tor is de ter mined ac cord ing to ASHRAE stan dard 93 [9] as 67 s [7].

The ther mal ef fi ciency, h of a con cen trat ing col lec tor op er at ing un der steady--state con di tions can be de scribed by [10, 11]:

h =×

=-

×

Q

A I

mc T T

A Iu p fo fi& ( )

(1)

In the pres ent work, the col lec tor sys tem is op er ated un der closed loop mode.Hence, Tfi is taken as Tl. The rate of en ergy gained by the wa ter in the stor age tank for atime in ter val of 1 hour is given by:

Qs = mwcp(Tlt – Tli) (2)

The hourly ef fi ciency of the PTC hot wa ter stor age sys tem is es ti mated by thefol low ing equa tion:

hss

h

w p lt li

h

Q

A I

m c T T

A I=

×=

-

×

( )(3)

Results and discussions

The vari a tion of col lec tor wa ter out let tem per a ture, Tfo, and stor age tank wa tertem per a ture, Tl, with time on one day, viz. April 30, 2005 is shown in fig. 2. The col lec torwa ter tem per a ture in creases pro gres sively with time, which var ies from 9.30 to 16.00 h,In dian Stan dard Time (IST), as the wa ter is recirculated through a hot wa ter stor age tankof ca pac ity 35 litres. The mass flow rate of wa ter through the col lec tor is 1.0 lpm. Thestor age tank wa ter tem per a ture in creases steadily from an ini tial tem per a ture of 35 °C at9.00 h and touches a max i mum value of 73.84 °C at 16.00 h, as no en ergy is with drawnfrom the stor age tank dur ing the col lec tion pe riod. At any in stant, the col lec tor wa ter tem -per a ture is greater than the stor age tank wa ter tem per a ture.

The vari a tion of beam ra di a tion, I, and use ful heat gain, Qu, with time is shownin fig. 3. It is seen that a fairly smooth vari a tion of beam ra di a tion with the max i mum(747 W/m2) oc curs around noon. The use ful heat gain first in creases, reaches a peakvalue around noon and then de creases. This is due to the fact that the use ful heat gain isstrongly in flu enced by the in ci dent beam ra di a tion and there fore fol lows its vari a tion.

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The col lec tor in stan ta neous ef fi ciency is com puted us ing eq. (1). Plot show ingthe vari a tion of the col lec tor in stan ta neous ef fi ciency with time is given in fig. 4. It willbe noted that the gen eral pat tern of vari a tion of ef fi ciency over a day is the same as that ofthe use ful heat gain be cause the value of ef fi ciency de pends on both the in ci dent beam ra -di a tion and the useful heat gain.

The vari a tion of hourly beam ra di a tion and en ergy gained by the stor age tankwa ter per hour with time is pre sented in fig. 5. The hourly beam ra di a tion in creases tillnoon and then de creases. The en ergy gained per hour, Qs, by the stor age tank wa ter startsin creas ing steadily and touches the peak around noon and then de creases. This ef fect is

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Valan Arasu, A., Sornakumar, T.: Performance Characteristics of the Solar Parabolic ...

Fig ure 2. Vari a tion of col lec torwa ter tem per a ture, Tfo , and stor -age tank wa ter tem per a ture, Tl ,with time

Figure 3. Variation of beamradiation, I, and useful heat gain,Qu , with time

due to the fact that the en ergy gained by the stor age tank wa ter is di rectly pro por tional tothe use ful heat gain across the col lec tor with time. The vari a tion of hourly ef fi ciency ofthe PTC hot wa ter stor age sys tem is plot ted against time in fig. 6. The hourly ef fi ciencyin creases till noon and then de creases.

Conclusion

In the pres ent work, the per for mance of a new par a bolic trough col lec tor with hotwa ter gen er a tion sys tem is in ves ti gated through ex per i ments over one full day in sum merpe riod. The sys tem op er ates un der closed loop mode by re cir cu lat ing the wa ter through a

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Figure 5. Variation of hourlybeam radiation, Ih, and energygained, Qs, with time

Figure 4. Variation of collectorinstantaneous efficiency with time

well mixed hot wa ter stor age tank. No hot wa ter is with drawn from the stor age tank to theload dur ing the col lec tion pe riod. The vari a tion of col lec tor wa ter out let tem per a ture andthe stor age tank wa ter tem per a ture are mea sured be tween 9.30 and 16.00 h. The stor agetank wa ter tem per a ture is in creased from 35 °C at 9.30 h to 73.84 °C at 16.00 h. The av er -age beam ra di a tion dur ing the col lec tion pe riod is 699 W/m2. The use ful heat gain, the col -lec tor in stan ta neous ef fi ciency, the en ergy gained by the stor age tank wa ter, and the ef fi -ciency of the sys tem as a whole are eval u ated on hourly ba sis. All these pa ram e ters arestrongly in flu enced by the in ci dent beam ra di a tion and found to fol low its vari a tion. Themax i mum value of each of those pa ram e ters is ob served around noon, when the in ci dentbeam ra di a tion is at its peak.

Nomenclature

A – collector aperture area, [m2]cp – specific heat capacity of water, [Jkg–1K–1]I – beam or direct radiation, [Wm–2]Ih – hourly beam or direct radiation, [kJm–2h–1]&m – mass flow rate of water through the collector, [kgs–1] mw – mass of water in the storage tank, [kg]Qu – useful heat gain, [W]Qs – energy stored in the storage tank, [kJh–1]Tfi – collector water inlet temperature, [K] Tfo – collector water outlet temperature, [K]Tl – storage tank water temperature, [K]Tli – initial storage tank water temperature, [K]Tlt – storage tank water temperature after 1 hour time interval, [K]h – thermal efficiency of the collector, [%]hs – hourly efficiency of the PTC hot water storage system, [%]

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Figure 6. Variation of systemefficiency with time

References

[1] Lippke, F., Di rect Steam Gen er a tion in Par a bolic Trough So lar Power Plants: Nu mer i cal In -ves ti ga tion of the Tran sients and the Con trol of a Once-Through Sys tem, Jour nal of So lar En -ergy En gi neer ing, 118 (1996), Feb., pp. 9-14

[2] May, E. K., Murphy, L. M., Per for mance Ben e fits of the Di rect Gen er a tion of Steam inLine-Fo cus So lar Col lec tors, Jour nal of So lar En ergy En gi neer ing, 105 (1983), May, pp.126-133

[3] Kalogirou, S., Lloyd, S., Use of So lar Par a bolic trough Col lec tors for Hot Wa ter Pro duc tionin Cy prus. A fea si bil ity study, Re new able En ergy, 2 (1992), 2, pp. 117-124

[4] Kalogirou, S., Lloyd, S., Ward, J., Eleftheriou, P., De sign and Per for mance Char ac ter is tics of a Par a bolic-Trough So lar-Col lec tor Sys tem, Ap plied En ergy, 47 (1994), pp. 341-354

[5] Co hen, G., Kear ney, D., Im proved Par a bolic Trough So lar Elec tric Sys tem Based on theSEGS Ex pe ri ence, Pro ceed ings, ASES An nual Con fer ence So lar 94, 1994, pp. 147-150

[6] Valan Arasu, A., Sornakumar, T., De sign, and Sim u la tion Anal y sis of a Par a bolic Trough So -lar Col lec tor Hot Wa ter Gen er a tion Sys tem, International En ergy Jour nal, 6 (2005), 2, pp.13-25

[7] Valan Arasu, A., Sornakumar, T., Per for mance Char ac ter is tics of Par a bolic Trough So larCol lec tor Sys tem for hot Wa ter Gen er a tion, In ter na tional En ergy Jour nal, 7 (2006), 2, pp.137-145

[8] ***, ClearDome So lar Sys tems Heat ing and Cook ing Prod ucts, San Diego Cal., USA, 2003,www.cleardomesolar.com

[9] ***, ASHRAE Stan dard 93. , Method of Test ing to De ter mine the Ther mal Per for mance ofSo lar Col lec tors. Amer i can So ci ety of Heat ing, Re frig er at ing and Air-Con di tion ing En gi -neers, At lanta, GA, USA, 1986

[10] Duffie, J. A., Beckman, W. A., So lar En gi neer ing of Ther mal Pro cesses (2nd ed.), John Wileyand Sons, New York, USA, 1991

[11] Sukhatme, S. P., So lar En ergy – Prin ci ples of Ther mal Col lec tion and Stor age (2nd ed.), TataMcGraw-Hill Pub lish ing Com pany Lim ited, New Delhi, 1999

Authors' address:

A. Valan Arasu, T. Sornakumar Faculty of Mechanical EngineeringThiagarajar College of EngineeringMadurai – 625 015, India

Corresponding author (A. Valan Arasu):E-mail: a_valanarasu@yahoo.com

Paper submitted: September 15, 2005Paper revised: January 31, 2006Paper accepted: June 12, 2006

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