Energy matrices of U-shaped evacuated tubular collector (ETC) integrated with compound parabolic concentrator (CPC)

Energy matrices of U-shaped evacuated tubular collector (ETC) integrated with compound parabolic concentrator (CPC)

Solar Energy 153 (2017) 531–539 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Energy mat...

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Solar Energy 153 (2017) 531–539

Contents lists available at ScienceDirect

Solar Energy journal homepage: www.elsevier.com/locate/solener

Energy matrices of U-shaped evacuated tubular collector (ETC) integrated with compound parabolic concentrator (CPC) R.K. Mishra a,⇑, Vihang Garg b, G.N. Tiwari c a

Department of Mechanical Engineering, Hi-Tech Institute of Engineering and Technology, Ghaziabad, UP, India Department of Computer Science, Hi-Tech Institute of Engineering and Technology, Ghaziabad, UP, India c Bag Energy Research Society (BERS), Sodha Bers Complex, Varanasi, UP, India b

a r t i c l e

i n f o

Article history: Received 18 January 2017 Received in revised form 6 May 2017 Accepted 2 June 2017

Keywords: Evacuated tubular collector CPC Exergy Energy matrices

a b s t r a c t This paper deals with the detailed thermal modeling of U-shaped evacuated tubular collector (ETC) integrated with compound parabolic concentrator (CPC). Such type of system is very useful in the preheating process for very high temperature industrial applications. The theoretical model have also been experimentally validated. The energy matrices like Energy pay back time (EPBT), Energy production factor (EPF) and Life cycle conversion efficiency (LCCE) for evacuated tubular collectors connected in series without and with concentrator have been evaluated and compared in this paper. For six number of collectors connected in series, the maximum difference water temperature between inlet and outlet in ETCCPC combination is found to be 24 °C, whereas it is about 17 °C in ETC without CPC. A good agreement between theoretical and experimental results has been observed. The annual thermal energy gain is found to be 1461.63 kW h and 1859.66 kW h for ETC and ETC-CPC system, respectively. The annual exergy gain is found to be 137.5 kW h and 165.9 kW h for ETC and ETC-CPC system, respectively. EPBT is increased by 6.2% and 11.6% and EPF is decreased by 5.7% and 25% on energy and exergy basis respectively for 20 years life of the system when CPC is integrated with ETC. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Now a day evacuated tubular collectors are quite easily available in the market and are used to supply the hot water. A major reason behind China’s dominant position in the solar water heater (SWH) industry in the world over recent years has been its technological choice of evacuated tube instead of flat plate collectors (Tiwari and Mishra, 2012; Mishra and Tiwari, 2013a). Through continuous innovations in research and development, manufacturing, and marketing, China has ensured that evacuated tube SWHs satisfy consumer needs with low initial costs and short payback periods. SWH companies have also been able to generate considerable profits even without financial incentives from the government. Preliminary investigations of economic advantages of evacuated tube SWHs mainly resulting from innovations in China, imply that Chinese companies should continue to develop ways to minimize the cost of evacuated tube SWHs and to improve their quality rather than follow developments in industrialized countries where flat plate SWH technologies dominate (Qiu et al., 2015). Together with the significant advantage over the flat plate collector, the ETC have some ⇑ Corresponding author. E-mail address: [email protected] (R.K. Mishra). http://dx.doi.org/10.1016/j.solener.2017.06.004 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.

disadvantages like utilization of vary low area and having low optical efficiency factor. Buttinger et al. (2010) have developed a nontracking and low-concentrating collector for the economical supply of solar process heat at temperatures between 120 and 150 °C. A compound parabolic concentrator (CPC) integrated evacuated tubular collector using air as working fluid is designed to provide hot air at high and moderate temperature. Liu et al. (2013) experimentally investigated that solar collector integrated with open thermosyphon has a better performance in terms of thermal efficiency. Two truncated CPC and the U-shape evacuated tube together, have been developed and tested by Li et al. (2013a). They suggested that such types of CPC solar collectors are feasible for a wide range of intermediate temperature applications. They also suggested that the daily thermal efficiencies of the 3  and the 6  CPC collectors can reach 40% and 46% at the collection temperature of 200 °C, respectively. Steady state analysis of an ETC fixed along the focal line of a compound parabolic concentrator has been developed by Sawhney et al. (1984). They studied the effect of vacuum leak on the performance of such collectors. The University of Chicago Solar Energy Group is continuing a program and commitment to develop an advanced evacuated solar collector integrating with a non imaging compound parabolic concentrator (ICPC) into the design. The Group is trying to develop a high

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Nomenclature ETC CPC A Aa Ar Cf dx e F’ FR h L I(t) Ib _f m N R r

evacuated Tubular Collector compound Parabolic Concentrator area, m2 aperture area, m2 receiver area, m2 specific heat of fluid, J/kg K elemental length, m root mean square percent deviation flat plate collector efficiency factor flow rate factor heat transfer coefficient, W/m2 K total length of the tube, m total solar radiation, W/m2 beam Radiation, W/m2 mass flow rate of water in, kg/s number of evacuated tubular collectors outer radius of the glass tube, m radius of U-shaped copper tube, m

efficiency, low cost solar collector for supplying solar thermal energy at temperatures up to 250° (O’Gallagher et al., 1990). Mills et al. (1986) compared thermal energy gain and relative cost of specular reflectors of the fixed CPC type. The reflectors were designed with a cylindrical evacuated tubular absorber, and a gap is allowed between reflector and absorber to accommodate the tubular glass envelope and evacuated space. An East-West reflector design of concentration ratio > 1.4 is suggested at temperatures above 100 °C. Development of the cost-effective single-pass evacuated tubular collector (SPETC) for the solar processes heat is a particular topic of active research. A novel SPETC with a symmetrical compound parabolic concentrator (CPC) has been introduced, experimentally investigated and theoretically analyzed. These results suggested that the novel SPETC is feasible for industrial process heat and solar cooling system combined with the adsorption chiller or the desiccant wheel (Li et al., 2013b). Liu et al. (2014) have presented a low-cost all-glass evacuated tubular solar steam generator with simplified CPC. Steam at temperature more than 200 °C at pressure ranging from 0.10 to 0.55 MPa can be produced by such collector. A novel minichannel-based solar collector consists of a U-shaped flattube absorber with and without a concentrator having a selective coating on its external surface have been analyzed by Sharma and Diaz (2011). Collares-Pereira et al. (1995) described and tested, a new type of CPC collector both optically and thermally, in different configurations. The results were compared with the energy delivered by other collector types like flat pate collector and evacuated tubular collectors. A new generation of advanced evacuated tube solar collectors capable of delivering efficient high temperature performance without tracking or tilt adjustment is being developed as an alternative to tracking collectors for temperatures up to and even exceeding 250 °C by Winston et al. (1986). RoblesOcampo et al. (2007) have designed and made an original waterheating planar collector and a set of reflecting planes and concluded that the estimated overall solar energy utilization efficiency for the system related to the direct radiation flux is of the order of 60%, with an electric efficiency of 16.4%. Conventional evacuated tubular collector is suitable only for thermosyphon process and passive heating of water. Mishra et al. (2015) studied and developed the theoretical model for a modified design of evacuated ETC. In that design a U-shaped copper tube with fin is inserted inside the tube of ETC to make evacuated tubular collector suitable for the force mode (active heating of water) for high temperature industrial applications.

Ut,pa

overall heat transfer coefficient from absorber plate to ambient through glass cover, W/m2 K

Greek letters a absorptivity s transmittivity gt instantaneous thermal efficiency q reflectivity Subscript a c f fi fo g p

ambient collector fluid inlet fluid outlet fluid glass plate

In the present paper, the theoretical model for evacuated tubular collectors connected in series, developed by Mishra et al. (2015) have been experimentally validated. The thermal modeling of evacuated tubular collector (ETC) integrated with compound parabolic concentrator (CPC) have been performed which is very useful in the preheating processes for very high temperature industrial applications. The theoretical model have also been experimentally validated. The experimental setups for ETC and ETC-CPC system have been installed at Hi-Tech Institute of Engineering and Technology, Ghaziabad, India. This paper also deals with the energy matrices for evacuated tubular collectors connected in series with and without concentrator. 2. System description In order to increase the thermal performance of U-shaped evacuated tubular collector (Mishra et al., 2015), the effective receiver area should be increased, this can be done by integrating ETC with a compound parabolic concentrator and a proposed design of same has been shown in Fig. 1. Figure shows a schematic diagram of an evacuated tubular collector (ETC) integrated with compound parabolic concentrator (ETC-CPC) with Aa > Ar, where Aa is the aperture area and Ar is receiver area. ETC’s are fixed along the focal line of a compound parabolic concentrator to receive the maximum amount of solar radiation after reflection from the reflector. Such N number of ETC-CPC collector combinations are connected in series for high temperature applications (Fig. 2). 3. Thermal analysis 3.1. Evacuated tubular collectors without CPC and connected in series Following Mishra et al. (2015), the outlet temperature from Nth collector for N number of collectors connected in series can be written as:

T foN ¼

ðAF R asÞ1 ð1  K NK Þ ðAR F R U L Þ1 ð1  K NK Þ IðtÞ þ T a þ K NK T fi _ _ f Cf ð1  K K Þ mf C f ð1  K K Þ m

ð1Þ

3.1.1. Useful thermal energy gain The rate of useful heat output from N-ETC connected in series can be given as:

Q_ u;N ¼ ðasÞeff IðtÞ  ðUAÞeff ðT fi  T a Þ

ð2Þ

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Solar radiations

Evacuated glass tube

U-shape copper tubes

CPC

Fig. 1. Elevated view of a CPC integrated ETC.

Inlet, Tfi

Tfo,2= Tfi,3, Tfo,r-1= Tfi,r

Tfo,1= Tfi,2

Tfo,r= Tfi,r+1, Tfo,N-2= Tfi,N-1 Tfo,N-1= Tfi,N

Outlet, Tfo,N

Fig. 2. CPC integrated ETC connected in series.

3.1.2. Instantaneous thermal efficiency After calculating the rate of useful thermal energy gain (Q_ u;N ) from Eq. (2), an instantaneous thermal efficiency (gi ) of evacuated tubular collector can be defined as follows,

Q_ u;N gi ¼ N  Ac  IðtÞ

the exergy extracted by the working fluid for N number of ETC connected in series (E_ ex;N ), in W is given by

ðT þ 273Þ _ f cf ðT fo;N  T fi Þ  m _ f cf ðT a þ 273Þ ln fo E_ ex;N ¼ m ðT fi þ 273Þ

ð4Þ

ð3Þ

where Ac is the area of evacuated tubular collector. 3.1.3. Exergy gain The exergy analysis is based on the second law of thermodynamics. Following Bejan(1978) and Yunus and Micheal (2008),

qas2 Ib Aa ½Rate of solar radiation available on ETC

¼

3.2. Evacuated tubular collectors integrated with CPC and connected in series (Fig. 2) The energy balance equation for ETC-CPC combination can be given as:

½F 0 hpf ðT p  T f Þ þ U t;pa ðT p  T a ÞAr ½Rate of thermal energy transferred from the blackedn plate to the fluid þrate of thermal energy loss from the plate to ambient through the glass

ð5Þ

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Hence,

and

_ f Cf m

dT f dx

½Rate of thermal energy carried away by the flowing fluid

Tp ¼

  Aa Ar

þ F 0 hpf T f þ U t;pa T a

Tp  Tf ¼

qas2 Ib

  Aa Ar 0

T fo1 ¼ PF 1 ðasÞeff Ar F r  Define;

1

Ar F r U L ¼ KK _ f Cf m

T fo1 ¼  U t;pa ðT f  T a Þ

hpf ðT p  T f Þ ¼

  hpf U :h A qas2 Ib a  0 t;pa pf ðT f  T a Þ Ar ðF hpf þ U t;pa Þ ðF hpf þ U t;pa Þ 0

F 0 hpf ðT p  T f Þ ¼ PF 1 qas2 Ib

  Aa  U L ðT f  T a Þ Ar

ð7Þ

where

PF 1 ¼

F 0 hpf F 0 U t;pa :hpf and U L ¼ 0 0 ðF hpf þ U t;pa Þ ðF hpf þ U t;pa Þ

    dT f Aa  U L ðT f  T a Þ ¼ 2R PF 1 qas2 Ib dx Ar

PF 1 ðasÞeff Ar F r Ar F r U L Ib þ T þ K K T fo1 _ f Cf _ f Cf a m m

ð11Þ

PF 1 ðasÞeff Ar F r ð1  K 2K Þ Ar F r U L ð1  K 2K Þ Ib þ T þ K 2K T fi _ f Cf _ f C f ð1  K K Þ a ð1  K K Þ m m

ð12Þ

In a similarly way, outlet temperature from 3rd collector can be obtained as:

T fo3 ¼

PF 1 ðasÞeff Ar F r ð1  K 3K Þ Ar F r U L ð1  K 3K Þ Ib þ T þ K 3K T fi _ f Cf _ f C f ð1  K K Þ a ð1  K K Þ m m

ð13Þ

In a similar way, outlet temperature from Nth collector can be obtained as:

Putting values from Eq. (7) in Eq. (6) we get,

_ f Cf m

ð10Þ

Substituting the value of Tfo1 from Eq. (10) into Eq. (11) we get,

T fo2 ¼

or,

PF 1 ðasÞeff Ar F r Ar F r U L Ib þ T þ K K T fi _ f Cf _ f Cf a m m

In a similar way,

½F hpf þ U t;pa  T fo2 ¼

or,

  Ib Ta Ar F r U L T fi þ Ar F r U L  þ 1 _ f Cf _ f Cf _ f Cf m m m

Therefore,

½F 0 hpf þ U t;pa 

From here,

ð6Þ

½Rate of available energy to the fluid

where Aa is the aperture area and Ar = 2pRL is the receiver area. R is the radius of the outer glass tube of evacuated tubular collector and L is the length of blackened absorber plate below the glass tube. The ’dx’ is elemental length along the length of blackened tube inside the glass tube. From Eq. (5),

qas2 Ib

F 0 hpf ðT p  T f Þ2pRdx

¼

dx

ð8Þ

T foN ¼

PF 1 ðasÞeff Ar F r ð1  K NK Þ Ar F r U L ð1  K NK Þ Ib þ T þ K NK T fi _ f Cf _ f C f ð1  K K Þ a ð1  K K Þ m m

ð14Þ

Define,

qas2

  Aa ¼ ðasÞeff Ar

3.2.1. Useful thermal energy gain The rate of useful heat output from N-ETC connected in series can be given as:

On integrating Eq. (8) under boundary conditions: at x = 0; Tf = Tfi and at x = L; Tf = Tfo1 we get,

 T fo1 ¼

PF 1 ðasÞeff þ Ta UL



N

    2RLU L 2RLU L 1  exp  þ T fi exp  _ f Cf _ f Cf m m ð9Þ

Tfo1 is the outlet temperature from one (first) collector unit. From Eq. (9)



T fo1 ¼



PF 1 ðasÞeff Ib m _ f Cf 2RLU L T a nU L þ  1  exp  _ f Cf _ f Cf _ f Cf UL m m m      _ f Cf m 2RLU L 2RLU L þ T fi exp   1  exp  _ f Cf _ f Cf UL m m

Define,

   _ f Cf m 2RLU L ¼ Fr 1  exp  _ f Cf U L Ar m or,



    2RLU L Ar F r U L ¼ 1 exp  _ f Cf _ f Cf m m

ð1  K K Þ _ f C f ðT foN  T fi Þ ¼ PF 1 ðasÞeff Ar F r Ib Q_ u;N ¼ m ð1  K K Þ þ PF 1 ðasÞeff Ar F r ¼ PF 1 ðasÞeff Ar F r

ð1  K NK Þ _ f C f T fi T a þ ðK Nk  1Þm ð1  K K Þ

ð1  K NK Þ _ f C f ð1  K NK ÞT fi I b  ½m ð1  K K Þ

 PF 1 ðasÞeff Ar F r

ð1  K NK Þ Ta ð1  K K Þ

_ f Cf As: Ar F r U L ¼ ð1  K K Þm Hence,

ð1  K NK Þ _ f C f ½ð1  K NK ÞT fi  ð1  K K Þ Ib  m Q_ u;N ¼ PF 1 ðasÞeff ð1  K K Þ 

ð1  K NK Þ T a ð1  K K Þ

¼ PF 1 ðasÞeff

ð1  K NK Þ _ f C f ½ð1  K NK ÞðT fi  T a Þ Ib  m ð1  K K Þ

¼ PF 1 ðasÞeff

ð1  K NK Þ ð1  K NK Þ I b  Ar F r U L ðT fi  T a Þ ð1  K K Þ ð1  K K Þ

ð15Þ

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4. Experimental setup and observations

Inlet

Photographs of the complete experimental set ups installed at Hi-Tech Institute of Engineering and Technology, Ghaziabad, India for six numbers of evacuated tubular collector (ETC) connected in series without and with compound parabolic concentrator (ETCCPC) are shown in the Figs. 3 and 4 respectively. A valve has been used to control the mass flow rate of the water flowing inside the copper tubes. To start with the experiment, the mass flow rate of the water inside the tube has been adjusted at a value 0.01 kg/s with the help of the valve at inlet. The following parameters are measured at an interval of 60 min during the experimentations: 1. 2. 3. 4. 5.

Solar intensity on inclined ETC Ambient temperature Inlet water temperature Outlet water temperature Air velocity above ETC surface.

ETC fitted with U-shape copper tube

compound parabolic concentrator

Instruments and their description, used for taking observations have been listed in Table 1. The ambient temperature, inlet and outlet water temperatures are measured with the help of mercury in glass type thermometer. A solarimeter is used to measure the solar intensity falling on the inclined ETC surface. Anemometer is used to measure the average velocity of air above the surface of collectors. The hourly variations of various solar intensity, inlet and outlet temperatures for six number of evacuated tubular collectors connected in series for a typical day in the month of March 16, 2016 are given in Table 2 and for ETC-CPC connected in series for a typical day in the month of July 07, 2016 are given in Table 3.

5. Embodied energy consumption Embodied energy is defined as: ‘‘the quantity of energy required by all of the activities associated with a production process, including the relative proportions consumed in all activities upstream to the acquisition of natural resources and the share of energy used in making equipments and in other supporting functions i.e. direct energy plus indirect energy”, Treloar (1994). The Breakup of embodied energy of each components of fabrication of evacuated tubular collector without and with CPC has been tabulated in

Inlet

Fig. 4. Photograph of the experimental set up for ETC-CPC with six number of collectors.

Table 1 Description of various instruments. Sr. No.

Instrument

Operating range

Least count

Particular

1 2

Thermometer Solarimeter

0–150 °C 0–1000 W/m2

0.5 °C 20 W/m2

5

Anemometer

0.4–30 m/s

0.1 m/s

Mercury in glass Calibrated against pyranometer Low friction ballbearing vane

Table 4. The performance of a thermal system is computed using three basic matrices. These are the energy payback time (EPBT), the energy production factor (EPF) and the life cycle conversion efficiency (LCCE) (Mishra and Tiwari, 2013b). 5.1. Energy pay back time (EPBT)

EPBT ¼

Embodied Energy ðEin Þ Annual Energy Output ðEout Þ

Outlet

Fig. 3. Photograph of the experimental set up for ETC with six number of collectors.

ð16aÞ

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Table 2 Hourly variations of solar intensity and various temperatures on March 06, 2016 for six numbers of ETC connected in series. Time (h)

I(t) (W/m2)

Ta (°C)

Tin (°C)

Tout (°C)

DT (°C)

8 9 10 11 12 13 14 15 16 17

280 480 680 800 860 860 800 660 500 260

15 16 16 17 18 20 23 26 27 29

17 19 20 20 21 23 25 27 29 31

20 25 29 34 38 39 40 41 38 35

05 06 09 14 17 16 15 14 09 04

6. Statistical analysis Chapra and Canale (1988) have given expressions for correlation coefficient (r) and root mean square percent deviation (e) for comparing the theoretical results with experimental results as,

P P P n X i Y i  ð X i Þð Y i Þ ffi q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Correlation coefficient ðrÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P P P P n X 2i  ð X i Þ2 n Y 2i  ð Y i Þ2 ð17aÞ where Xi and Yi are theoretical and experimental values respectively. n is the number of observations. For r > 0 a positive linear relationship exists. r < 0 a negative linear relationship exists. r = 0 implies no linear relationship between theoretical and experimental results. and Root mean square percent deviation

Table 3 Hourly variations of solar intensity and various temperatures for six numbers of ETCCPC connected in series on July 07, 2016. Time (h)

I(t) (W/m2)

Ta (°C)

Tin (°C)

Tout (°C)

8 9 10 11 12 13 14 15 16 17

260 500 720 940 880 720 680 400 260 200

26 28 29 31 33 34 34 31 31 28

28 30 31 33 35 36 36 33 33 30

30 37 41 47 48 47 47 40 36 31

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ðei Þ2 ðeÞ ¼ n where ei ¼

or

va ¼

1 EPBT

ð16bÞ

If va ? 1, for EPBT = 1 the system is worthwhile otherwise it is not worth from energy point of view. and (ii) On life time basis,

vL ¼

Eout  T Ein

ð16cÞ

5.3. Life cycle conversion efficiency (LCCE)

/ðtÞ ¼

Eout  T  Ein Esol  T

i

 100

The variation of solar intensity and ambient temperature for typical days in the month of March and July 2016 are shown in Figs. 5 and 6 respectively. The values of design parameters of ETC and ETC-CPC system are given in the Table 5. The mass flow rate _ is kept at a constant value of 0.01 kg/s. Outlet water tempera(m) ture from sixth number of ETC and ETC-CPC combination is evaluated by solving Eqs. (1) and (14) using MATLAB 13.0 software at a _ = 0.01 kg/s), for the design parameters constant mass flow rate (m given in Table 5. The hourly variations of theoretical and experimental results of outlet water temperature for ETC and ETC-CPC combinations are shown in Figs. 7 and 8 respectively. From figures a good agreement between theoretical values and experimental results has been observed. The correlation coefficient (r) and root mean square percent deviation (e) are also evaluated using Eq. (17a) and (17b) respectively. The values of r and e are also shown in respective figures. Eqs. (2) and (15) have been used to calculate the useful thermal energy gain for ETC and ETC-CPC connected in series. The monthly variation of overall thermal energy gain for evacuated tubular collector connected in series (N = 6 and _ f ¼ 0:01 kg=s) without and with compound parabolic concentram tor (CPC) is shown in Fig. 9. Annual overall thermal energy gain is obtained as 1461.63 kW h for ETC and 1859.66 kW h when CPC are integrated with the ETC. Maximum thermal energy gain is obtained in summer months for both ETC and ETC-CPC systems due to the higher availability of solar radiation in these months.

(i) On annual basis

Eout Ein

X i Y i Xi

7. Results and discussion

5.2. Energy production factor (EPF)

va ¼

h

ð17bÞ

ð16dÞ

where Esol is the total solar input (radiation) over the life time (T years) of the system.

Table 4 Breakup of embodied energy of different component for ETC and ETC-CPC system. Material

Specific energy (MJ/kg)

Glass 14 Copper 94.82 Mild steel support structure 27.27 Plastic tank 101.4 CPC 261.8 In welding In Brazing In creating vacuum Total Energy content (embodied energy) (Without CPC) Total Energy content (embodied energy) (With CPC)

Weight (kg)

Energy content (MJ)

Energy content (kW h)

16 8 15 10 8

224 3792.8 409 1014 2094 53.1 44 426 5962.9 8056.9

62.22 1053.55 113.61 281.66 581.67 14.75 12.22 118.33 1657 2238

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1000

It

35

Ta

30

Ambient Temperature, oC

Solar Intensity, I(t) (W/m2)

900 800

25

700 600

20

500 400

15

300

10

200 5

100 0

0 8

9

10

11

12

13

14

15

16

17

Time, hour Fig. 5. Hourly variation of solar intensity and ambient temperature for March 06, 2016 for Ghaziabad, India.

Ta

1000

40

900

35

800

30

700

25

600 20

500

15

400

10

300 200

Ambient Temperature, Ta (oC)

Solar Intensity, I(t) (W/m2)

I(t)

5 8

9

10

11

12

13

14

15

16

17

Time (hour) Fig. 6. Hourly variation of solar intensity and ambient temperature for July 07, 2016 for Ghaziabad, India.

Table 5 Design parameters of ETC and ETCCPC.

50

Theoretical

r=0.981; e=6.59

Experimental

Parameters

Values

Aa Ar r R L Cf F0 hp,f hi ho PF1 Lg Kg Ut p,a vo

0.22 m2 0.055 m2 0.012 m 0.0275 m 2m 4190 J/kg K 0.986 100 W/m2 °C 5.8 W/m2 °C 9.5 W/m2 °C 0.9789 0.005 m 1.09 W/m °C 2.1 W/m2 °C 1.0 m/s 0.95 0.80 0.85

s a q

Minimum is obtained in December month due to the less availability of solar radiation. The overall exergy gain have been calculated for both the systems by using Eq. (4). The monthly variation of

Outlet temperature, OC

45 40 35 30 25 20 15 10 8

9

10

11

12

13

Time, hour

14

15

16

17

Fig. 7. Hourly variation of theoretical values and experimental results for ETC _ = 0.01 kg/s). (N = 6 and m

overall exergy gain for evacuated tubular collector connected in _ f ¼ 0:01 kg=s) without and with compound series (N = 6 and m parabolic concentrator (CPC) is shown in Fig. 10. Annual exergy gain is obtained as 137.5 kW h and 165.9 kW h ETC and ETC-CPC

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Outlet Temperature, oC

60

Theoritical

Table 6 Energy pay back time considering annual energy and exergy of the system.

r=0.95; e=6.80

Experimental

50

Energy pay back time (EPBT) (year)

40

Basis

ETC

ETC-CP

Energy Exergy

1.13 12.09

1.20 13.49

30 20

1.13 years and 1.20 years for ETC and ETC-CPC systems respectively. On exergy basis EPBT is calculated to be 12.09 years and 13.49 years respectively for ETC and ETC-CPC systems (Table 6). Eq. (16c) has been used for calculating the energy production factor (EPF) on annual basis. The calculated values of EPF on life time basis for 10, 15 and 20 years life of the system are given in Table 7. It has been observed that EPF is decreased by 5.7% and 25% on energy and exergy basis respectively for 20 years life of the system when CPC is integrated with ETC. Eq. (16d) has been used for calculating the life cycle conversion efficiency (LCCE) of the system on life time basis for T = 10, 15 and 20 years (Esol = 7446 kW h), considering annual thermal energy and exergy of the system. The results have been summarised in Table 7.

10 0 8

9

10

11

12

13

14

15

16

17

Time, hour Fig. 8. Hourly variation of theoretical and experimental results for ETC-CPC _ = 0.01 kg/s). combination (N = 6 and m

system. Eq. (16a) has been used to evaluate the energy pay back time (EPBT) on the annual basis, considering annual energy and exergy of the system. EPBT in terms of energy is found to be

250.00

Without CPC

With CPC

Thermal energy gain, kWh

Without CPC=1461.63 kWh With CPC = 1859.66 kWh 200.00

150.00

100.00

50.00

0.00 Jan

Feb March April May

June

July

Aug

Sept

Oct

Nov

Dec

Month of the year Fig. 9. Monthly variation of overall thermal energy gain.

25.00

Without CPC

With CPC Without CPC=137.5 kWh With CPC = 165.9 kWh

Exergy gain, kWh

20.00

15.00

10.00

5.00

0.00 Jan

Feb March April May

June

July

Aug

Sept

Oct

Month of the year Fig. 10. Monthly variation of overall exergy gain.

Nov

Dec

539

R.K. Mishra et al. / Solar Energy 153 (2017) 531–539

Table 7 Energy production factor (EPF) and life cycle conversion efficiency (LCCE) on life time basis for T = 10, 15 and 20 years, considering annual thermal energy and exergy of the system. System

Energy basis Life of the system (years) 10

ETC ETC-CPC

Exergy basis Life of the system (years) 15

20

10

15

20

EPF

LCCE

EPF

LCCE

EPF

LCCE

EPF

LCCE

EPF

LCCE

EPF

LCCE

8.82 8.31

0.33 0.66

13.23 12.46

0.35 0.56

17.64 16.62

0.36 0.45

0.83 0.74

0.07 0.04

1.24 1.11

0.09 0.14

1.65 1.48

0.14 0.15

8. Conclusions On the basis of present studies following conclusions have been drawn:  There is a good agreement between theoretical and experimental results for outlet water temperature of evacuated tubular collectors have been observed (r = 0.981 and 0.9; e = 6.59 and 6.80 for ETC without and with CPC)  The annual thermal energy gain is found to be 1461.63 kW h and 1859.66 kW h and annual exergy gain is found to be 137.5 kW h and 165.9 kW h respectively for ETC and ETC-CPC system for six number of collectors connected in series.  Energy pay back time (EPBT) is increased by 6.2% and 11.6% and Energy production factor (EPF) is decreased by 5.7% and 25% on energy and exergy basis respectively for 20 years life of the system when CPC is integrated with ETC.

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