T) air heating collector

PERCAMON

Renewable Energy 16 (1999) 725-730

SYSTEM PERFORMANCE

STUDIES ON A PHOTOVOLTAIClTHERMAL HEATING COLLECTOR

(PV/T) AIR

H.P.Garg and R.S.Adhikari

Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi-l 10 016, INDIA

ABSTRACT A computer simulation model is presented for the analysis of a solar photovolta.ic/thermal (PVIT) hybrid collector with air as heat transfer fluid and algorithm for making quantitative prediction regarding the performance of the system is described. Thermal efficiency curves for the solar PV/T hybrid collectors corresponding to various type of absorbers have been derived. In order to appreciate the model, numerical calculations have been made for evaluating the system performance corresponding to typical climate of Delhi, India. 0 1998 Elsevier Science Etd. All rights reserved.

KEYWORDS Photovolta.ic/Thermal collector, simulation model, system performance.

INTRODUCTION Hybrid photovoltaic/thermal energy conversion is a relatively new and promising technology for the production of both thermal and electrical energy simultaneously. A review of literature illustrates that a number of theoretical as well as experimental studies have been reported on PVlT systems with air and liquid as heat transfer fluid. A detailed study (IT Power, 19%) on PV hybrid concept has been carried out by IT Power Ltd. and New Castle Photovoltaics Applications Centre, commissioned by the E.C. Joint Research Centre at ISPRA. Studies on hybrid PVlT collectors have also been made at Indian Institute of Technology, New Delhi in collaboration with All India Council of Technical Education (AICTE), New Delhi. Very recently, the present authors (Garg and Adhihri, 1997a,b) have carried out detailed simulation studies on conventional PVlT air heating collectors. In the present investigation, a computer simulation model is presented for the analysis of a solar photovoltaic/tbermal (PV/T) hybrid collector with air as heat transfer fluid and algorithm for making quantitative prediction regarding the performance of the system is described. It has been assumed that the photovoltaic cell efficiency can be represented as a linear decreasing function of its temperature. Thermal efficiency curves for the solar PV/T hybrid collectors corresponding to various type of absorbers have been derived. In order to appreciate the model, numerical calculations have been made for evaluating the system performance corresponding to typical climate of Delhi, India 096@1481/99/S--see front matter 0 1998 Elsevier Science Ltd. All rights reserved. PII: SO960-1481(98)00263-8

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A schematic configuration along with associated energy transfer mechanism for a conventional PVlT air heating collector is shown in fig. 1. The physical model is composed of a transparent cover, a metallic black absorbing surface and a well insulated rear plate. In this configuration inlet air is passed through the passage between absorber and rear plate. The absorber plate is coated with black paint and photovoltaic cells are pasted over it. The adhesive used for pasting photovoltaic cells is characterized by high thermal conductive and good electric insulating material. Photovoltaic cells are pasted over one meter wide absorber plate at equal distance in equal number of rows and columns per unit area of the absorber surface.

-!“p-g

Fig. 1.

Schematic configuration of a conventional PVlT air heating collector alongwith associated energy transfer networks.

The energy balance equations for various components of the system can be written as follows:

dTg _ MC “1 Lc + hp-gflp ggdt-

- Tg) + hs_g(Ts -Tg) - hg-a(Tg

- Ta)

(1)

MsCsx

= o2 fc

- hs-p ps

- TP) - hs-g (T, -Tg)

- WT,)

(2) UT,) = (l-q)“s

?s AR 1~

where Absorber Plate M’

C

P

dTP_ “3

PT--

1~ + hsmp (Ts - TP) - hp-g (TP -Tg)

- hp-b fl,

- Tb) - hp-f (Tp - Tf) (3)

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Rear dTb Mb cb dt=

hp_b (Tp - Tb) - hb-f (Tb -Tf) - Ub (Tb - Ta) (4)

Workinu Fluid (air)

dTf

Mf Cfx

= - m Ct$To - Ta) + hp-f (Tp -Tf) + hb-f (Tb - Tf) (3 TO

+

Tj

Tf = (--I-) where Fraction of energy absorbed by different components of the system are defined as: cy1 = (I-Rg)og o2 = (1-Rg)( 1-og& a3 = (1 -Rg)( 1-olg)(l -(Y& 1 -AR)cx~

(6) where AR is the ratio of the area covered by photovoltaic cells to collector area. CALCULATION

PROCEDURE

Various differential equations developed in the simulation model have been solved for evaluating the temperatures of different system components, using a numerical method, based on Runga-Kutta formulas. The various heat transfer coefficients were calculated using the relation given in Duffe and Beckman (1991) and Tan and Charters (1969). Performance Parameters Various performance parameters of the system can be calculated as follows: Thermal efficiency m Cf(To - Ti) ?t = IC (7) Photovoltaic cell efficiency (Floreschetez,

1975)

‘IS = nr[l - Br(T, - Tr)l (8) & = (T,* - Tr)-’ where and T,* is the photovoltaic cell temperature at which its efficiency drops to zero. efficrency ue = ‘IS AR(1 -Rgl)(l-Rg2)(1 -agl)(l -agz) (9) Efticiw 9T = ‘It + ?e (10)

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NUMERICAL RESULTS AND DISCUSSION For the appreciation of the developed model, calculations have been made for a PV/T hybrid collector with air as heat transfer fluid. The efficiency of photovoltaic cell at the reference temperature is assumed to be equal to 10%. It has been assumed that system is operated under the condition of continuous airflow throughout day and night. This operational condition corresponds to the system application for continuous space heating. A list of relevant themo-physical parameters used for the calculations have been given in Table 1. Table 1.

List of Themo-physical Parameters

Parameter

Value

Parameter

Value

oo

0.90

Ma

1.5 KglmL

as

0.90

MS

8.5 KglmL

era

0.04

Mo

8.5 Kg/m&

R,

0.04

Mf

0.0647 Kg/m2

EP

0.lO(Selective Coating)

0.9O(Black Paint)

840.0 J/Kg “C

cg

c

lo.10

IC,

500.0 J/Kg “C

T-,

I270 “C

IC,

500.0 J/Kg “C

Fig.2 shows the thermal efficiency curve of a PVlT air heating collector for a fixed duct depth, collector length, mass flow rate and different photovoltaic cell densities 0 and 10x10 /m2, corresponding to the absorber with and without selective coating. The results of the simulation model presented in the figures corresponds to the value of solar irradiance and ambient temperature to be equal to 800 W/m2 ‘C and 25 OC respectively. The values of emissivity for the normal black paint and selective coating absorber have been assumed to be 0.9 and 0.1 respectively.

301 ,002

,007

.01x) (Tl-7d*i(

Fig. 2.

.ono % m’ I W)

.0220

.027

-.002

.w7

&20 nI’TdqL(

.0,70

a220

oc n? I WI

Thermal efficiency curves for a PVlT hybrid air heating collector

The two different values of solar cell densities basically represent the absorber without solar cells (conventional air heater) and the absorber fully covered with solar cells respectively. It has been observed that the thermal performance of selective coated absorber is better than the normal black paint absorber. The thermal performance without photovoltaic cells is better than when absorber is fully covered with photovoltaic cells for both kind of absorbers. This behaviour is on account of the fact that as number of solar cells on the absorber plate increases, a relatively larger fraction of the incident solar irradiance is converted to electrical energy, thus reducing the converted thermal energy fraction and hence the reduction in thermal efficiency.

s ‘7

729

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In order to appreciate the simulation model, numerical calculations have been carried out for a PV/T hybrid air heating collector corresponding to typical climate of Delhi, India. The evaluations have been carried out for the design configuration of the hybrid PV/T air heating collector which corresponds to a duct length of 2 m. and duct depth of 5 cm. with a fixed cell density of (6 x 6)/n?. The values of air mass flow rate chosen are in the range 25-1.50 Kg/h m*. The results have been presented for the months of January and June in subsequent figures. It is to be mentioned that the months of January and June represent the typical winter and summer conditions in Delhi. Fig. 3 represents the hourly variation of outlet temperature with different air mass flow rate both the winter and summer conditions. It has been observed that for both the winter and summer days, the higher flow rate resulted lower air temperature increment (To_Ti).

12

2

4

6

2

I2

OQnQQl4Qanrs2O2r222s2~ Tim

-

i-m

-

i.OO*m.’

*m

*’

Fig. 3.

I

2

0

4

6

6

-

iwo~r’

-

&UC

7

8

eQnQQurnnms20?r22222 Tlnu Of thm Da7 wwd

of tha Da7 hurl

ll#n ma

-

iSSlw?l~

-

a00

wh

m’

+

tiowmm’

+

i,.owhw’

Hourly variation of the outlet temperature for a PV/T air heating collector typical winter and summer days in Delhi.

for

The hourly variation of photovoltaic cell efficiency for different air mass flow rate are shown in Fig.4. The higher mass flow results a lower cell temperature. As a consequence, higher cell efficiencies for higher mass flow rates have been observed.

16.0 34.0 13.0 (2.0 19.0 10.0 0.0 8.0 7.0 2.0 6.0 (2

3

4

6

2

I8

0onQoum*nQo202l22222. Tlm0fthaoa7b(ou~

Fig. 4.

12a462722QnQQYImffmm~~~~2. Tk

et WI4 D42 wwrl

Hourly variation of the photovoltaic cell efficiency for a PVlT collector for typical winter and summer days in Delhi.

hybrid air heating

Figure 5 shows the system performance hybrid PV/T air heating collectors over a period of one year. The variation of daily system efficiencies alongwith ambient air temperature and daily global irradiance over a collector surface (all averaged over the month) are depicted in the figure.

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JAN PCS MAR APN YAV NN AIt Monfh Of fk

Fig. 5.

AlJO HP

CCT NOV DEC

”

mu

li; Ii* Osmtim aftkr0~ Annual variation of system efficiency alongwith the ambient temperature and daily solar irradiance over the year (all averaged over months).

REFERENCES Duffie, J.A. and W.A. Beckman (1991). Solar engineering of Thermal Processes, Wiley, New York. Florschuetz, L.W. (1975). On heat rejection from terrestrial solar cell arrays with sunlight concentration, IEEE PhotovoltaicSpecialisrs Conference Records, May 1975, 318-326. Garg, H.P. and R.S. Adhikari (1997a). Conventional hybrid photovoltaiclthermal (PVIT) air heating collectors: steady state simulation, Renewable Energy, 11(3), 363-385. Garg, H.P. and R.S. Adhikari (1997b). Transient simulation of a conventional hybrid photovoltaiclthermal (PV/T) air heating collector, Int. J. Energy Research,. (in press). IT Power Ltd. (1995). Hybrid Photovoltaic/Thermal Concepts, Final Report Produced for EC Joint Research Centre, ISPRA, June 1995. Tan, H.M. and W.W.S. Charters (1969). Effect of thermal entrance region on turbulent forced convective heat transfer for an asymmetrically heated rectangular duct with uniform heat flux, Solar Energy , 12, 513-516. Acknowledgement This work is supported by All India Council of Technical Education (AICTE), New Delhi, India. Nomenclature A=area. m2 b =duct depth, m B=duct width, m C =specific heat, J/Kg ‘C E=electrical energy produced by hotovoltaic cell, W h=heat transfer coefficient, W/m 1 ‘C I = solar irradiance, W/m2 L=collector length, m M=mass, Kg m=mass flow rate, Kg/h m2 N=cell number, per m i.e. number of cells per metre along length and width (cell density=N x N /m2) R = reflectivity T = temperature, OC t=time, sec. U=he.at loss coefficient, W/m* OC

Greek letters (Y=absorptivity, n =efticiency t =emissivity

fraction of energy absorbed

Subscripts a=ambient b=rear plate c =collector f =working fluid(air) g = transparent cover i =inlet 0 =outlet p=absorber plate r = reference s = solar cell