Performance Analysis of a Double-pass Photovoltaic-Thermal PVT Solar Collector With CPC and Fins

8/10/2019 Performance Analysis of a Double-pass Photovoltaic-Thermal PVT Solar Collector With CPC and Fins

1/13

Performance analysis of a double-passphotovoltaic/thermal (PV/T) solar collector

with CPC and fins

Mohd. Yusof Hj. Othman*, Baharudin Yatima

,Kamaruzzaman Sopianb, Mohd. Nazari Abu Bakara

aCenter for Applied Physics Studies, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor, MalaysiabDepartment of Mechanical and Material Engineering, Universiti Kebangsaan Malaysia,

43600, Bangi, Selangor, Malaysia

Received 10 November 2003; accepted 31 October 2004

Available online 25 May 2005

Abstract

The use of PV/T in combination with concentrating reflectors has a potential to significantly

increase power production from a given solar cell area. A prototype double-pass photovoltaic-

thermal solar air collector with CPC and fins has been designed and fabricated and its performance

over a range of operating conditions was studied. The absorber of the hybrid photovoltaic/thermal

(PV/T) collector under investigation consists of an array of solar cells for generating electricity,

compound parabolic concentrator (CPC) to increase the radiation intensity falling on the solar cells

and fins attached to the back side of the absorber plate to improve heat transfer to the flowing air.

Energy balance equations have been developed for the various nodes of the system. Both thermal and

electrical performance of the collector are presented and discussed.

q 2005 Elsevier Ltd. All rights reserved.

Keywords:Photovoltaic-thermal (PV/T); Solar collector; CPC; Fin

0960-1481/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.renene.2004.10.007

Renewable Energy 30 (2005) 20052017

www.elsevier.com/locate/renene

* Corresponding author.
http://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renene


2/13

Nomenclature

A Surface area (m2)

C Specific heat (J kgK1 KK1)

d Gap loss correction

h Heat transfer coefficient (WmK2 KK1)

hf Fin height (m)

kf Thermal conductivity of the fin

L Collector length (m)

m 8 Mass flow rate (kg sK1 mK2)

Nu Nusselt number

P Solar cell packing factor

Re Reynolds numberS Solar irradiance (WmK2)

T Temperature (8K)

U Heat loss coefficient (WmK2 KK1)

wf Fin thickness (m)

W Collector width (m)

Subscripts

ab(T) Top absorber surface

ab(B) Bottom absorber surface

a Ambient

B Beam radiationbp Back plate

c Convective

D Diffuse radiation

f1 Working fluid (air) at first channel

f2 Working fluid (air) at second channel

r Radiative

R Reflector

S Solar cell

tot Total

g Glass coverp Absorber plate

Greek letters

a Absorptivity

h Efficiency

t Transmitivity

3 Emissivity

q Acceptance half-angle

r Reflectivity

Mohd.Y.Hj. Othman et al. / Renewable Energy 30 (2005) 200520172006


3/13

1. Introduction

Solar energy is a clean energy which has the potential to meet a significant proportion

of the worlds energy needs. It can be broadly classified into two systems; thermal energysystem which converts solar energy into thermal energy and photovoltaic energy system

which converts solar energy into electrical energy. The vital component in solar energy

system is the solar collector. In the thermal system, the collector is heated by the sun and

the heat is then transferred to a working fluid. In the photovoltaic system, the collector is

comprised of photovoltaic cells which converts the solar radiation into electrical energy.

Normally, these two collection systems are used separately. It has been shown that these

systems can be combined to form hybrid photovoltaic/thermal (PV/T) system. The term

PV/T refers to solar thermal collectors that use PV cells as an integral part of the absorber

plate. The system generates both thermal and electrical energy simultaneously. The

number of the photovoltaic cells in the system can be adjusted according to the local loaddemands. A number of simulation as well as experimental studies have been reported on

the photovoltaic-thermal (PV/T) system. Among the first, Kern and Russell[1], gave the

concept of PV/T collector using water or air as the heat removal fluid. Florschuetz[2],

suggested an extension of the HottelWhillier model for the analysis of PV/T system and

Raghuraman[3]presented numerical methods predicting the performance of liquid and air

PV/T flat plate collectors. Cox and Raghuraman[4], performed computer simulations on

air type hybrid system. Bhargava et al. and Prakash[5,6]reported the effect of air mass

flow rate, air channel depth, length and fraction of absorber plate area covered by solar

cells (packing factor).

The collector was essentially a single-pass air heater with the air flow channel between

two metallic plates. The upper plate was painted black and the solar cells were pasted

directly on the top surface. The adhesive material must be of a special kind such that it is

thermally conducting but electrically insulating. Sopian et al. [7] successfully

demonstrated the improved performance of a double-pass collector over the single-pass

collector due to efficient cooling of the photovoltaic cells.

An important component that determines the cost of the hybrid system is the solar cell.

The solar cell area required for a photovoltaic-thermal air heater should generate sufficient

energy to run the system. This is the amount of energy required to circulate the air through

the collector. A promising cost reduction technique would be the use of sun lightconcentrator. Several concentrators have been discussed in the literature. Garg et al. [8]

presented the first simulation study of the single-pass photovoltaic-thermal air heater with

plane reflector. Garg and Adhikari [9,13]reported the performance analysis of a hybrid

photovoltaic-thermal collector with integrated CPC troughs. Both cases indicated that the

total efficiency with reflector is slightly higher compared with the systems without

concentrators. Due to the increased solar radiation, the average plate and solar cell

temperatures increased. Since the collector with reflector operated at a higher temperature,

the electrical efficiency also decreased.

In this work, a new design of a double-pass photovoltaic-thermal air collector with

compound parabolic concentrator and fins was studied. The collector design concept isshown inFig. 1. Air enters through the upper channel formed by the glass cover and the

photovoltaic panel and is heated directly by the sun. Next it enters the lower channel

Mohd.Y.Hj. Othman et al. / Renewable Energy 30 (2005) 20052017 2007


4/13

formed by the back plate and the photovoltaic panel. The compound parabolic

concentrators, concentrate solar radiation onto the PV cells. The fins on the back of the

photovoltaic panel increases the heat transfer to the air and enhances the efficiency of the

system.

2. Theoretical analysis

A steady-state one-dimensional analysis in closed-form solutions is obtained. This

involves steady-state energy balance equations in various nodes of the system. The

thermal schematic model of the double-pass PV/T solar air collector is shown in Fig. 2.

To simplify the analysis, the following assumptions have been made.

i. Steady state of energy transfer has been achieved.

ii. Capacity effects of the glass cover, solar cells and back plate have been neglected.

iii. The temperatures of the glass cover, solar cells and plates are vary only in the direction

of working fluid flow.

iv. The CPC is ideal and free from fabrication errors and any beam of incident radiation

falls within the acceptance angle can reach the receiver with the help of parabolic

reflector.

Fig. 1. The schematic model of a double-pass photovoltaic thermal solar collector with CPC and fins.

Fig. 2. The thermal schematic model of the double-pass PV/T with CPC and fins.



5/13

v. The average number of reflection for radiation passing through CPC inside the

acceptance-half angle is treated constant.

vi. The side losses from the system are negligible.

2.1. Energy balance

The energy balance equations can be written as follows

(a) The glass cover.

agStotCR1Ctgrpr2 nR

ZhrgsTgKTsChcgwTgKTwChcgf1TgKTf1 ChrpgAabT

ACTgKTp (1)

where

nZAR=ASEd: average number of reflection for radiation passing through CPC inside

the acceptance-half angle.

StotZSBCSD: total radiation

CRZ 1sin q: concentration ratio of CPC[12]Ed: the fraction of radiation emitted by truncated CPC which goes to the solar cell.

(b) The inlet air stream between the glass cover and the collector plate.

m 8Cf1

WdTf1

dxZhcgf1TgKTf1Chcpf1

AabT

ACTpKTf1 (2)

(c) The collector plate which consists of the solar cells, CPC and fins.

tgapSUCRrnRd 1C

~np~ng~n2 ~nR

CR

1KPCtgapvSUPCRr

nRd

! 1C~npv~ng~n

2 ~nR

CR

1Khpv Zhcpf1

AabT

AcTpKTf1Chrpg

AabT

AcTpKTg

Chcpf2AabB

AchpTpKTf2Chrpbp

AabB

AcTKTbp 3

where SUZSBCSD=CR

hpZ1KAfin

AabB1Khf: fin effectiveness

hfZtanhmhf

mhf: fin efficiency

mZ

2hckfwf

1

2

d: gap loss correction.



6/13

(d) The outlet air stream between the fined absorber plate and the back plate.

Km 8Cf2

W

dTf2

dx

Zhcpf2AabB

Ac

cpTpKTf2 Chcbpf2TbpKTf2 (4)

(e) The back plate.

hcbpf2Tf2KTbpChrpbpAabB

ACTpKTbp ZUbTbpKTa (5)

2.2. Analytical solution

The variables Tg, Tp and Tbp can be eliminated from Eqs. (2) and (4) by substituting

Eqs. (1)(3) into them. The following two linear first-order differential equations are

obtained.

dTf1dx

ZA1Tf1CA2Tf2CA3 (6)

dTf2dx

ZB1Tf1CB2Tf2CB3 (7)

where A1, A2, A3, B1, B2 and B3 are the constants obtained through algebraic

manipulations. The Eqs. (6) and (7) can be solved by using operational method in

closed-form solutions with the following boundary conditions.

Tf1xZ0 ZTa

Tf1 ZTf2

Hence, the temperatures of the fluid in both channels as a function of distance in flow

direction can be obtained (x-direction).

Tf1x Z1

B1C1e

D1xD1KB2CC2eD2xD2KB2KB2

B1A3KA1B3

A1B2KB1A2

KB3

(8)

Tf2x ZC1eD1xCC2e

D2xCB1A3KA1B3

A1B2KB1A2

(9)

where

C1, C2: the constants obtained by applying the boundary conditions into Eqs. (8)

and (9).

D1, D2: the roots obtained from the second-order differential operator equation.

2.3. Heat transfer coefficients

The radiative and convective heat transfer coefficient for parallel plates are obtained

from Ong[14]. Malik and Buelow[10]obtained the ratio of the Nusselt number for rough



7/13

and smooth duct for the flat sheet type as follow,

Nurough

Nusmooth

Z1:101C8!10K6 ReK5!10K11 Re2 10; 000!Re!50; 000

This correlation can be utilized to calculate the convective heat transfer between the

absorber plate and the back plate as well as the glass cover.

2.4. Performance parameters

The performance parameters of the hybrid PV/T system are obtained in terms of the

solar cell efficiency and the thermal efficiency. The solar cell efficiency depends on the cell

temperature as given by[2].

hpv Zhref1K0:0054TpavKTref

where href is the reference efficiency of solar cell at TrefZ25 8C and the mean cell

temperature Tpav is obtained by integrating the function Tp(x) as follow,

Tpav Z

xZL

xZ0 TpxdxxZL

xZ0 dx

The instantaneous thermal efficiency of the PV/T system is given by[11].

hthermal

Z m 8CfToKTidtCR

Stotdt

The combined photovoltaic-thermal efficiency of the system is the sum of photovoltaic

and thermal efficiency of the system[7].

hpvt Z

m 8CfToKTidtC

PEdt

CR

StotdtZhthChpv

where the instantaneous electrical energy generated, PE in the double-pass photovoltaic-

thermal air heater with CPC and fins is written as:

PE ZtgSUPhpvapvdCRrR n 1C~n

pv~n

g~n2 n

RCR

3. Experimental setup

The schematic diagram of the experimental setup is shown in Fig. 3. The basic

components of the setup are as follows. (a) The double-pass photovoltaic/thermal solar

collector. (b) The air flow measurement system. (c) The temperature measurement system.(d) The wind speed measurement system. (e) The current and voltage measurement

system. (f) The solar radiation measurement system. (g) The data acquisition system.



8/13

The collector dimensions are 0.85 m!1.22 m (W!L). The height of the upper channel is

16.5 cm. The height of the lower channel can be varied from 30 to 120 mm. The total area

covered by solar cells is 0.38 m2. CPC with concentration ratio of 1.86 is used as a reflector

and located parallel to the air flow. The collector was tested at steady state operation under

indoor conditions to determine their electrical and thermal efficiency for various operating

temperatures. Twenty-three tungsten halogen lamps each rated at 500 W were used tosimulate solar radiation during the test. The intensity of the incoming solar radiation was

measured by Eppley pyranometer installed parallel to the collector plane. The wind speed

was measured by micro response anemometer. Ambient temperature and other

temperature at several positions of the system as at input (Ti), output (To), photovoltaic

cell (Tpv), air in the duct (Tf), etc were measured by k-type thermocouple. The air flow

sensing element was of the vane type probe head and connected direct to the data logger.

4. Results and discussion

The mathematical model have been made for PV/T air heating system with CPC and

fins. An algorithm has been developed to predict the working fluid (air) temperature at

both channels as a function of the absorber length. Various thermo-physical parameter

used in the calculation have been summarized in Table 1.

4.1. Electrical performance

During the tests, the photovoltaic electrical output was connected to a load simulating

real system operation. The maximum power pointPmaxwas determined from the collectedcurrent (I) and voltage (V) data. When measuring currentvoltage characteristics, (see

Fig. 4) electrical losses become visible. These losses are indicated by decrease in fill factor

Fig. 3. Experimental setup of the double-pass photovoltaic/thermal solar collector.



9/13

at high currents and a decrease in open-circuit voltage at elevated cell temperatures. The

fill factor decreases from 0.54 at radiation intensity of 400 W/m2 to 0.42 at radiation

intensity of 700 W/m2 and mass flow rate of 0.027 kg/s. The module saturation current

causes the open-circuit voltage to drop, whereas the short-circuit current increases slightlywith increasing temperature at the same radiation level. As can be seen in Fig. 5, the

electrical maximum power of the collector increases with the radiation intensity but the

open-circuit voltage drops slightly with the radiation intensity due to increase in cell

temperature at constant mass flow rate.

4.2. Photovoltaic/thermal performance

Fig. 6 shows the maximum rise in air temperature (T0KTi)max is seen to drop as

expected with increasing mass flow rate.Fig. 7shows the effect of radiation intensity onthe temperature rise at various flow rates. The temperature rise is proportional to the

radiation intensity at a specific mass flow rate.Fig. 8shows the effect of the mass flow rate

Table 1

Thermo-physical parameters

Parameter Value Parameter Value

ag 0.06 rp 0.05

ap 0.95 rg 0.02

apv 0.9 rR 0.9

3g 0.94 CR 1.85

3p 0.9 P 0.44

tg 0.9 n 0.6

Fig. 4. Variation of IV curve on solar radiation at mass flow rate of 0.027 kg/s (d1Z16.5 cm, d1Z3 cm, TiZTaZ

3132 8C).



10/13

on the efficiencies (photovoltaic, thermal and combined photovoltaic/thermal) of the

collector. In connection withFig. 6,it is obvious that as the air temperature rise gets less

and less with increasing mass flow rate, the collector thermal efficiency getscorrespondingly higher and higher due to the decrease in the average temperatures of

the absorber plate and the glass cover, thereby reducing the top losses. Increasing the flow

rate will increase the heat transfer coefficient between the channel walls and the working

fluid, resulting in a lower mean photovoltaic cells temperature. This will increase

Fig. 5. Power against voltage at mass flow rate of 0.027 kg/s (d1Z16.5 cm, d2Z3 cm,TiZTaZ3132 8C).

Fig. 6. Variation of maximum air temperature rise with air specific mass flow rate at radiation level of 500 W/m2

(d1Z16.5 cm, d2Z3 cm, TiZTaZ3132 8C).



11/13

the electrical efficiencies of the collector. However the enhancement in the value of

electrical efficiency is remarkably very small as compared to the increase in the value of

mass flow rate. Nevertheless from Fig. 8 it shows some contribution of the electrical

efficiency to the total efficiency of the system. The combined efficiency varies from 39 to

70% at mass flow rate of 0.0150.16 kg/s and radiation intensity at 500 W/m2.It has been observed that the experimental results were slightly higher than as predicted

in mathematical model. This characteristic may be due to the effect of IR radiation

released by tungsten halogen lamps during the test. It has been seen in Fig. 8that the IR

Fig. 7. The effect of solar radiation on the temperature rise at various mass flow rate (d1Z16.5 cm,d2Z3 cm,TiZ

TaZ3132 8C).

Fig. 8. The effect of mass flow rate on efficiencies at solar radiation of 500 WmK2 (d1Z16.5 cm,d2Z3 cm,TiZ

TaZ3132 8C).



12/13

radiation gave an impact to the electrical performance of the collector and reduced to 50%

of the actual electrical energy produced meanwhile the thermal performance increased by

10% of the predicted value.

5. Conclusions

The developed steady state model predicts the thermal and electrical performance of a

PV/T collector with CPC and fins The prediction results agreed with the results obtained

from the experiment in solar laboratory. Nevertheless some correction should be made due

to the effect of IR radiation during the test. In general, results show that electricity

production in a PV/T hybrid module decreases with increasing temperature of the air flow.

This implies that the air temperature should be kept as low as possible. On the other hand,

the system should deliver hot air for other purposes. A trade off between maximizing

electricity production and producing hot air of useful temperatures is thus necessary. The

simultaneous use of hybrid PV/T, CPC and fins has a potential to significantly increase in

power production and reduce the cost of photovoltaic electricity.

Acknowledgements

The authors would like to thank the Ministry of Science, Technology and the

Environment Malaysia for sponsoring this work under project IRPA 02-02-02-0007-EA109.

References

[1] Kern Jr. EC, Russell MC. Combined photovoltaic and thermal hybrid collector system. Proceedings of 13th

IEEE Photovoltaic Specialist 1978 pp. 11537.

[2] Florschuetz LW. Extension of the HottelWhillier model to the analysis of combined photovoltaic thermal

flat collector. Solar Energy. 1979;22:3616.

[3] Raghuraman P. Analytical predictions of liquid and air photovoltaic/thermal flat plate collector

performance. J Solar Energy Eng 1981;103:2918.[4] Cox CH, Raghuraman P. Design considerations for flat-plate photovoltaic/thermal collectors. Solar Energy

1985;35:22745.

[5] Bhargava AK, Garg HP, Agarwal RK. Study of a hybrid solar system-solar air heater combined with solar

cell. Solar Energy 1991;31(5):4719.

[6] Prakash J. Transient analysis of a photovoltaic-thermal solar collector for co-generation of electricity and

hot air/water. Energy Conv Manage 1994;35(11):96772.

[7] Sopian K, Yigit KS, Liu HT, Kakac S, Veziroglu TN. Performance analysis of photovoltaic thermal air

heaters. Energy Conv Manage 1996;37(11):165770.

[8] Garg HP, Agarwal RK, Bhargava AK. The effect of plane booster reflectors on the performance of a solar air

heater with solar cells suitable for a solar dryer. Energy Conv Manage 1991;35(6):54354.

[9] Garg HP, Adhikari RS. 2000. Hybrid photovoltaic/thermal utilisation systems. Final report submitted to All

India Council of Technical Education. New Delhi, India

[10] Malik MAS, Buelow FH. Heat transfer characteristic of a solar dryer. UNESCO Congress. Sun Service

Mankind, Paris 1973;55.



13/13

[11] Garg HP, Adhikari RS. Performance analysis of a hybrid photovoltaic/thermal (PV/T) collector with

integrated CPC troughs. Int J Energy Res 1999;23:1295304.

[12] Rabl A. Optical and thermal properties of compound parabolic concentrator. Solar Energy 1976;18:

497511.

[13] Garg HP, Adhikari RS. Transient simulation of conversional hybrid photovoltaic/thermal (PV/T) air heating

collectors. Int J Energy Res 1998;22:54762.

[14] Ong KS. Thermal performance of solar air heaters: mathematical model and solution procedure. Solar

Energy 1995;55(2):93109.


Documents

Performance Analysis of a Double-pass Photovoltaic-Thermal PVT Solar Collector With CPC and Fins