Actual optical and thermal performance of PV-modules - ScienceDirect

ELSEVIER

Solar Energy Materials and Solar Cells 41/42 (1996) 557-574

Actual optical and thermal performance of PV-modules S. Krauter, R. Hanitsch University of Technology Berlin, Electrical Machines Institute, D-10587 Berlin, Germany

Abstract Actual efficiency of photovoltaic generators is often lower than predicted by standard test conditions (STC) or standard operating conditions (SOC). This is mainly caused by an underestimation of reflection losses and solar cell temperature in the module. To get more accurate results in predicting the actual performance of PV-modules, the parameters influencing incoming (optical parameters) and outgoing power flow (electrical and thermal parameters) were investigated by simulation and some verifying experiments at the University of New South Wales and the Australian desert.

1. Optical parameters In order to precisely represent the actual optical conditions in the module, a model for the encapsulation of the cell was developed which determines the insolation reaching the cell from sun and sky irradiance. This was done by modelling the optical processes occurring outside and inside the enc~/psulation (direct and diffuse irradiance, reflection, absorption). 1.1. Modelling o f irradiance

As an input for the three slab optical system of the solar module encapsulation a sophisticated modelling of the irradiance onto the module was carried out. The reflection losses at a PV-module were modeled as a function of incidence angle, state of polarization, spectral irradiance, and the optical properties of the encapsulation materials. 0927-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-0248(95)001 43-3

558

S. Krauter, R. Hanitsch / Solar Energy Materials and Solar Cells 4 1 / 4 2 (1996) 557-574 1500-

J

Ts = Ts = Ys = 7s =

o

~

.~..,1000-

~., 5 0 0 -

0

90*= 70*= 50*= 30*= *=

500

1000

~50C

2000

AM AM AM AM AM

1.00 1.06 1.31 2.00 5.76

2500

Wavelength [rim] Fig. 1. Interpolated beam spectra for various Ts-

1.1.1. Direct irradiance The direct irradiance is unpolarized and the angle of incidence is derived from the astronomical parameters at the time of day and day of the year and geographical location [1]. The actual terrestrial spectrum of the sun was interpolated from the CIE-spectra [2]: The CIE-spectra are given for four discrete air masses only (AM 1, AM 1.5, AM 2, AM 5.6). Intermediate values of sun elevation angles % have been computed by exponential interpolation. The calculated results at Ts = 10° - 90° are shown in Fig. 1. 1.1.2. Diffuse irradiance The diffuse irradiance was modeled as a non-homogenous illuminated half-sphere at the sky over the module. The spatial distribution of the sky sphere was modelled according to DIN 5034 part 2 [3]. A contour plot of the irradiance levels of a projection of the sky dome on the horizontal at a sun elevation of 30 ° is shown in Fig. 2. Because each scattering process causes a different state of polarization of the scattered as well as of the transmitted component of a ray, the diffuse irradiance is polarized (Fig. 3). Because the reflection losses at the PV-module depend on the state of polarization towards the plane of incidence (see below), this effect has also been accounted for. Each wavelength-band of the spectrum [2] of each point at the sky-dome (at a spatial distance of 15°) was ray traced through the module encapsulation according to the angle of incidence and state of polarization. 1.2. Optical parameters of the encapsulation 1.2.1. Reflective losses under realistic conditions In earlier work, estimations of reflective losses at the surface of PV-modules have been based on normal incidence and amount to 2 - 4 % of the insolation. This is correct only for tracking systems without any diffuse insolation (as in space). For non-tracking systems direct insolation is perpendicular to the module only twice a year. At other times the reflected component increases according to the FRESNEL equations (see

S. Krauter, R. Hanitsch / Solar Energy Materials and Solar Cells 4 1 / 4 2 (1996) 557-574

559

N l

,

,

i

,

,

Fig. 2. Non-isotropic distribution of diffuse irradiance (according to DIN 5034) for a sun elevation of 30 ° and clear sky in W m-2 sr- I (projection from sky dome).

below). The different encapsulation layers cause multiple reflections inside and among the slabs (see also scheme in Fig. 4). Due to optical dispersion of the materials the transmittance also depends on the incoming spectrum.

/ i:

~X".".' r'k ..

/

: '" ' .... ,~..

~ ' ,.." . " .

•" ;

~.' x~ _..'.,,'

:

:.'k

;I~.'~-~"

",

~ '.."

: :';--~"

wl_xi_~.-DL,W~,( ,:k

i ~

". . . . . . . . . . ..~....~,

/

. . ~ \ ...." \

r'~.'-..'

,~'v

4

/--0 -

!\

\~ ~ : ............. \

........\

.~,~...'

~~x~..'."..'.f .4_ ~

[-~-~-..'~.'~

•

"i~k

:..

"~".-z-I

- ,,,.'-Z ~..." ~.-~.....~ ....... -m~

7

..

', x '.....$-'~]...........I.->.........

Fig. 3. Degree of polarization (in %) and directions of planes of polarization (arrows) of diffuse irradiance at a sun elevation of 30* (projection from the sky dome).

560


i Fig. 4. Ray tracing through the module layers.

The abbreviation ARC in Fig. 4 stands for the anti-reflective-coating of the solar cell, it consists of TiO 2 or another optical material to bridge over the refractive indices of silicon and front cover. EVA is the pottant ethylene-vinyl-acetate which serves as a mechanical, thermal and optical interface between the solar cells and the glass cover. 1.2.2. Non-normal incidence

In general, incidence of insolation is non-normal. The reflectance can be calculated by the FRESNEL equations (see, e.g., Born and Wolf [4]) as a function of angle of incidence 0in (see Fig. 5). The components of the direction of polarization parallel (11) or perpendicular (_1_) to the angle of incidence are calculated separately from each other. Normalized reflection (Rip R . ) and transmission (TII, T±) are given as: tan2( 0in -- 0out) RII = tan2(0in + 0°ut) A R±

sin2( 0in -- 0out) sin2(0in + 0°ut) ,

(TII= 1 -RII ) A (T± = 1 - R ± ) .

(1)

(2)

The angle of refraction is given as: Oout= arcsin( ~1° sin Oin) •

Fig. 5. Incidence on a surface at 0m.

(3)

S. Krauter, R. Hanitsch / Solar Energy Materials and Solar Cells 41/42 (1996) 557-574

561

1.2.3. Modelling of a plane slab Incident insolation has to pass through two optical boundary layers and sustain attenuation by absorption inside the material. The reflection at the lower boundary is not lost completely, but bounced up and forward through the slab at decreasing intensity (Fig. 6). In order to establish a common nomenclature, each layer is marked by an index " k " , the upper medium by the index " k - 1" and the lower medium by " k + I ". The angle of incidence at the transition from medium " k - 1" to the medium " k " is marked by Ok, the angle of refraction by Ok+ 1. The optical transitions are denoted by the indices of media in the order of the radiation passing through them " k , k + 1" (e.g., T01). The transmitted parts i of the irradiance on slab k are marked as Wk,i, the reflected ones as Pk,r A distinction between the planes of polarization is not made any more in this or the following sections in order to keep the formulas simple. So for the generally used variables R and T, the specific components RII, R ± and TII, T± are to be inserted. The incident irradiance is also normalized to E = 1. The attenuation of a ray E k after a passage through a slab k, is determined by the coefficient of absorption ak(A) of the material, its thickness d k and the incidence angle 0 k on the considered slab k: =exp -

kcos0 k]-

(4)

Therefore, reflection losses Rot occur at the incident boundary surface of slab 1 and Rt2 on its lower surface, while the remaining transmitted part consists of: Tt, t

=

T0tTl2exp - a t

.

(5)

The internal reflection Rt2 is computed the same as Rot using Eq. (1), as well, the new incidence angle 0 2 for the lower layer 2 according to Eq. (2). The internal reflection at the boundary 1-2 passes through layer 1 again, and is attenuated accordingly. At the boundary 0-1 this ray is refracted once more, while a component T~0 passes into medium 0. The reflected component Rt0 reaches boundary 1-2 attenuated, where another part Tt2 penetrates layer 2. ( -3atdt ) TI'2 ~" T ° l R t 2 R l ° T t 2 e x p cosO'----~ " (6)

n o

T1z"~ n2

-* TI, 1

Tl2"~ - - ~ ' TI, 2

Fig. 6. Transmission through a plane slab.

562


Summing up all transmitted fractions of the layer 1:

[ _Otldl~ o¢ [

[

)]i-I COSO1

(7)

This infinite series is a geometrical one and can be summarized:

[ -- °¢ldl ro,r,2exp ) 1 -

Rl2 Rl0ex p

(8)

cos01

Because radiation is being absorbed in the slab, the reflectivity p of a slab has to be computed explicitly, because O ¢ 1 - r. The reflected components Pl,i of a slab 1 are to be calculated (first one trivial Ol,l R01) as: =

( -2a'dl ) Pl,2 TmRt2Tloexp cos01 ~ =

Pl.3 = TolR22RloTloexp PI.i>I =

,

(9)

1o)

cos0-----~ '

TolRI21R{o2Tmexp

c°s01

(11)

This infinite series is a geometrical series again: TmRl2Tl0exp( pl =Rol + 1 -

--201dlcosOl)

(_2aldl) RloRl2ex p cos01

•

(12)

1.2.4. Internal transmission and reflection Knowledge of the internal transmission ¥ is necessary to determine the transmission of multiple layer systems, for example Yj: the transmittance of slab 1, when it is illuminated from reflections coming out of slab 2. To distinguish internal transmittance and internal reflectance from the external ones, a bar over the variable is used.

[ --aldt Tl°exp~ c ° - - ~ 1 ) 1 -

RloRl2ex p

(13)

cos01

At the boundary between slab 2 and slab 1, T2~ is neglected because it has been already


563

accounted for by the net reflectivity of the lower slabs. Internal reflectivity ~ is set up the same way: RloTt2exp( -cos012°qdt ) p l ~1 -

Rl2R,0ex p

( _2Crldl ) . cos0"---~

(14)

The transition from slab 2 to slab 1 (T21) is also neglected. 1.2.5. Transmittance through two slabs

An optical system consisting of slab 1 and slab 2 now will be examined (Fig. 7). In addition to the internal reflections inside each slab there are also reflections over 2 slabs (between the upper boundary of the other slab and the lower boundary of the lower slab) to be considered. In order to keep the picture simple, the rays and their infinite series are summed up as slab transmittances z and slab reflectances p. This is done in Fig. 7 and is marked by bold arrows.The following fraction outlines the most direct way into slab 3:

(15)

z'r2 T12,1 "~- TI 2

It should be mentioned that for the combination of the slab transmittances rl and r 2 the reflection at the boundary 1-2 has been taken into account in r~ as well as in r 2. So T~2 has to be compensated once in r,2. Accordingly this has to be done for further combinations of slabs rk+ i. The reflected part ~l "( P2 - R21)" T~-21 from inside layer 2 enters slab 1 and is reflected at the boundary 1-0 back into slab 1 by losing ¥1. From slab 1 the fraction Pl reaches slab 2.The fraction ~ can be treated in the same way as the direct incoming part and, therefore, has the same attenuation z 2 . T~-2~ as that in slab 2 when it enters slab 3. This results in: r12,2 =

p2 - Rl2 T122

)r2

( 16)

n o

nl

~-Rz~

n2 n3

- r - r2

-r,2.,=

Fig. 7. Transmission through a two slab system.

564

s. Krauter, R. Hanitsch/ Solar Energy Materials and Solar Cells 41/42 (1996) 557-574

Again a fraction /92 - RI2 is reflected from slab 2 into slab 1. Summarized, the total transmittance is:

TIT2~[(D2-R12)P1] i-I "2"12

Tl 2

i=,[

¥~

,

(17)

TIT2 7"12 = T12 -- ( P 2 - - R I 2 ) P l "

(18)

The inner reflectivity P21 of the slab system is: ~'272 Pl

P21 = P2 +

(19)

TI2 -- PI( P2 -- RI2) '

while ¥2 can be expressed according to Eq. (13). 1.2.6. Transmittance through three slabs

The module encapsulation is now considered as three slabs of different optical properties. The upper two-slab-system is treated by its input and output properties as a single slab system and therefore the same procedure as described above in chapter 1.2.5 could be used recursively: z I is substituted by 7-12, ~'2 by 7-3, Tl2 by TEa, SO the first fraction of 7-~23 could be written as: TI2T 3

T123.1 =

(20)

T23

For the combination of the slab transmittances 7-12 and 7-3 the reflection at boundary 2 - 3 has been taken into account in 7-12 as well as in r 3. So T23 has to be compensated once in 7123. The reflected part 71z. ( P3 - R23)" T231 from inside layer 3 enters slab system I - 2 and is reflected at the boundary l - 0 back into slab system 12 by losing ¥12. From slab system I - 2 the fraction ~ 21 reaches slab 3. The fraction P21 can be treated in the same way as the direct incoming part and therefore has the same attenuation 7-3" T~ I as that in slab 3 when it enters slab 4. This results in: Tt23, 2 =

r12( P3 -- R23) P21T3 T23

(21)

nI n2 n3

n4

TI23A

7"12a,2

Fig. 8. Transmission through a three slab system.

S. Krauter, R. Hanitsch / Solar Energy Materials and Solar Cells 41/42 (1996) 557-574

565

Again a fraction P3-223 is reflected from slab 3 into slab system 1-2. Summarized, the total transmittance is: TI2T 3 pc

( P3 -- R 2 3 ) P 2 1

7"123= T23 i= ]1 3"123

:

-T23

]i-I

]

(22)

T12T3 T23 -- ( 03 --R23) P21

(23)

This "1"123 consists of the components 1-12311 and TI23 j_ which are to be multiplied by the components of the according directions of polarization. 1.3. Optical transmittance 1.3.1. Transmittance o f a real encapsulation Using the model for an optical three-slab-system mentioned above, the transmittances for real modules and their dependence on the refractive indices of the two front slabs have been investigated. A plot of the transmittance as a function of incident angle for a simulation of a real module (PQ 4 0 / 5 0 supplied by ASE, former Telefunken Systemtechnik in Wedel, Germany) at a wavelength of 800 nm (max. spectral efficiency) is shown in Fig. 9.

1.3.2. Variation o f n I and n 2 The dependency of the transmittance on the refractive indices of the two front slabs has also been examined. The results for the simulations to determine the relative irradiance reaching the cell as a function of the refractive indices nj and n 2 (n 3 = 2.30, n 4 = 3.69) at the top slabs are shown in Fig. l0 for 0 = 0 ° and unpolarized insolation. An increase of the angle of incidence shows a shift of the maximum transmittance towards lower refractive indices, especially of the top slab. This is caused by the increasing reflection losses at the air boundary layer (n o = 1.00) to the upper front layer

10

]

I

. . . . .

I

[

]

q

r

0.8-

parallel " ' '~" . . . . . . . . . polanz .... e ..... perpendicular polarized -unpolarized air :n=l) /

8 .=_

0.6-

"'-.

\ \\\ ~ \

"'.. ~ \\ ~

2\

"\

\

\\

~

t- 0 . 4 -

I-0.2 0.0 0

10

20

,30

40

50

60

70

80

angle of incidence in d e g r e e s

Fig. 9. Transmission of encapsulation (PQ 40/50).

90

566

S. Krauter, R. Hanitsch/ Solar Energy Materials and Solar Cells 41 /42 (1996) 557-574 /,/

/'>~

~~ /

,

2.1o-/> ~ 2.00-

/

~ ~

x=

19o'

fl)

1.70-

.

,

\

//

~

%

1.5o:

._

,

// ~ t / ; 8 8 /

2.20://

--,.,o-.

/mzAk.-4

1.202 1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

refractive index n

Fig. 10. Contour of transmittance as a function of the refractive indices of the two top layers n I and n z (n 3 = 2.30, n o = 3.69). * : standard modules.

n L, w h i l e the losses at the l o w e r boundaries increase t o a m u c h smaller extend due reduced i n c i d e n c e angles resulting f r o m the p r e v i o u s angle o f refraction.

to

the

1.3.3. Comparison o f different models A s s h o w n in Fig. 11, the usually used n o r m a l incidence g i v e s only a peak v a l u e o f the real transmittance o f the encapsulation s y s t e m o n c e during a day, especially if only the a i r / g l a s s transition is regarded.

1

0.~5

............ -- ~.~ -- - - -.~.'~..--.- --..-~---=~-- .

0.9

.

.

.

.

...............

.......... . . . . . . . . . .

J.......

........

8

"~ I-"

o.6

..........

0.75

....

, ....

~

..~I

......

........ l',~

ozr

,, 4

V 6

;,,,,

.... 10

, .... 12

~ ...... 14

~J 16

.... 18

, 20

time of day

Fig. 11. Transmittance of an encapsulation of a standard PV module (PQ 40/50) over a day (March 21 at 34°S) calculated with different models.


,

5C~

567

C

~ 4C ~ 3C

•

r~r~r~h

90

r r~OOtJle olOVU"~

Fig. 12. Reflection losses at direct irradiance (Site: 34.5°N, Module: PQ 40/50, fixed southward).

1.3.4. Accumulated reflection losses over a day

The model described above was also used for a simulation over one day for a standard module repeating the calculation each 15 minutes with their corresponding incident angles and spectra. The total reflection losses of a three layer encapsulation of a standard module (PQ 40/50) during a day are 15.5% of the incoming global radiation for an adequate module elevation angle (see Figs. 12 and 13). For high elevation angles (50°-90°), the reflection losses increase up to 42.5% for direct radiation. Polarization of diffuse radiation lowers

50i 30 "~ 20 "6 ~o

"')~?h

9o

~

~/rflodul e e

Fig. 13. Reflection for diffuse irradiance (Site: 34.5°N, Module: PQ 40/50, fixed southward).

568

S. Krauter, R. Hanitsch/ Solar Energy Materials and Solar Cells 41/42 (1996) 557-574

the diffuse incidence by 0.5-5% [5] and the influence of optical dispersion is around 1% [6,7].

2. T h e r m a l p a r a m e t e r s

PV modules show reduced voltage and efficiency at elevated cell temperatures (by 0.4-0.5% K-~ for crystalline silicon solar cells [8]). The cell temperature at a certain heat flow density t)heat A- l (absorbed irradiance minus generated PV power density) is determined by the heat conductivity of the encapsulation and by the heat transfer to the environment which consists of convective heat transfer and the heat radiation exchange among module and sky or ground. A sketch of the incoming and the outgoing energy flows of a PV-module is given in Fig. 14. 2.1. Thermal process inside the PV-module

After absorption of the incoming radiation in the solar cell a portion of it is converted into electricity and diverted. The remaining heat flow gets from the cell through the encapsulation to the surface of the module (steady state heat flow) or increases the temperature of the module (non-steady state heat flow). At conventional standard modules the thermal capacity is relatively low, so a simple steady state heat flow could be used. 2.2. Heat dissipation by convection

Convective heat flows can not be treated in a closed mathematical model and, therefore, have to be computed by iteration or approximation. The convective heat transfer coefficient h c is a bulky function of the actual air (TA) and module surface temperature (TM), air viscosity, thermal conductivity and heat capacity, characteristic length and elevation angle of the module and finally speed and direction of wind. The models used have been based on the extensive work by Mehl [9].

% Qh e a t

qooow,

: hoITM-

r- trans.

Q,o .on

Fig. 14. Balance of energy flows at a PV module

A

S. Krauter, R. Hanitsch/ Solar Energy Materials and Solar Cells 41/42 (1996) 557-574

569

56¢,.)

0) t--

52

J Q)

o. 50

E

[I-E] 200 Watt. horizontally mounted

4a ,n~. 2,o0,wQt,t. ve~!~.,,~ ~..,~.~.t;d, ... , . . . . . . . . . , .... , 0

10

20

30

40

50

60

70

80

90

m o d u l e elevotion in degrees

Fig. 15. Cell temperature at a heat flow of 200 W in a M 55 module (400 W m -2 ) for Tambient = Tsky = 24.9°C, natural convection.

Results of measurements are plotted in Fig. 15. The elevation angle of the module has little influence on the natural convective heat transfer coefficient in the 20°-80 ° elevation angle range ( < 0.2%). However, at almost horizontal position (00-20 ° angle of module elevation) the energy output of the module decreases by approximately 0.7% due to elevated cell temperature. Horizontal, instead of vertical mounting of a standard module increases efficiency by 0.3%. Heating was done by forcing an electrical power dissipation in the cells (using cells as diodes in forward direction). To minimize radiation exchange the tests were carried out in a huge hall with the wall temperatures (~___A Tsky ) equal to air temperature TA.

2.3. Heat radiation exchange The total radiative heat flow from the module 0r~d to the environment consists of the components of the radiation exchange of the front (F) and rear sides (R) of the module with the sky and the ground (G): t)Fsky, ORsky, 0FC and 0Re. These components are functions of the temperature of the module surface (T F, TR), the temperature of the ground Tc, the equivalent sky temperatures Tsky [10-12] and the according emissivities e. Also these components are determined by the so called "radiation shape factor" (or " v i e w factor") ~0;., the radiative surface area A of the module and the Stefan-Boltzmann constant tr ~5.67- 10- 8 W m - 2 K - 4 ) [13]. In general this can be expressed as follows [13]: Grad =

EQij = E ij

i jl

°r~iEjAi~°ij(Ti4:

Tj4)

~"

EorEic'jAi~ij(Ti 4 ij

Tj4) •

(24)

570


The approximation above is exact for (1 - e/)(1 - e~)~u~j i ,~: 1. Applied on the elevation angle 7M of the module [6]: arad = 10"[ aFeF('sky( T 4 - Ts~y)(1 + sin( 9 0 ° - 7M), + ' c ( T 4 - Td)(1 - sin( 9 0 ° - 7M)), -I-aR'R('sky(ZR4 - T~ay)(1- COSTM), -t- e~(T~ - T~)(1 -~t-COS')/M))].

(25)

3. Simulation

For every solar position during a day, each point on the sky sphere was described by its specific spectrum, polarization and incidence angle towards the module surface. All components were ray-traced separately until their absorption in the solar cell. After summing of all absorbed components the balance of energy flows was carded out. Then power dissipation by electrical load, heat transfer to the front and back surface of the module, heat dissipation by radiation exchange with the ground and the sky, and natural and forced convection was simulated [14]. The quite interesting results from a simulation of the module efficiency during a day are shown in Fig. 16. Explanations for characteristics of the shape are given in the figure.

3.1. Verification The model used for the determination of the cell temperature was verified by measurements at" the Arid Zone Research station of the University of New South Wales in the Australian desert [14]. For the simulation a constant wind speed of 2 m / s was 0,13

i

i

i

i

i

i

i

I

i

i

i

d l f ~ rodloflononly, low coil tomporc~ures

>. 0.12 o tO~

-~ o.1 EO

o.lo~ "0 0

o.o9

0.08

4~

~ ~ ~ ~ 1'o I'! 1'2 1'~ 1~ 1'5 1'6 1'~ 18 1'9 2o time

of day

Fig. 16. PV moduleefficiencyduring a day (June 21 at 34.5°N, PQ 40/50 by ASE, r/STC= 0.10).

S. Krauter, R. Hanitsch / Solar Energy Materials and Solar Cells 41 / 4 2 (1996) 557-574

571

51)

(.~

40

~

30

cl. E~ 20

..........

[ i J ! J

.............

i

11

12

/

10

6

7

8

9

10

•

T_cell, predicted T_cell, measured T ambient in simulation

o

T-ambient, measured

13

14

15

16

17

18

time of clay

Fig. 17. Measured cell temperaturesversus simulation at M a r c h elevation: 30°).

4 in F o w l e r s G a p ( 3 1 ° 5 ' S , 1 4 1 ° 4 0 ' E , m o d u l e

used as input and the ambient temperature as shown in Fig. 17. It can be seen that the predicted cell temperature follows accurately the actual measured one with a peak deviation of 3K, which occurred only when the actual wind speed was quite different from 2 m / s . The time dependence of PV module efficiency shown in Fig. 16 has been validated by Van den Berg et al. [15] for times from 7 to 17 hrs. The efficiency measurements around dawn, which have the greatest uncertainty, require more sensitive measuring equipment and a precise, maximum-power-point tracker due to the low intensity levels. Additional results will be published after better measurements are obtained with improved equipment.

3.2. Optical improvements The total reflection losses can be lowered from 15.5% to 11.4% for optimal matching of refractive indices (n I = 1.32 and n 2 = 1.75); with real materials (e.g., low refractive glass at n I = 1.43 and poly-carbonate (PC) at n 2 = 1.60), an achievement of up to 12.9% is possible, which means a gain in the daily produced energy of 3% (see [6,7]). An improvement of transmittance and corresponding daily generated electrical energy in the 5% range could also be achieved by V-groove structured surfaces [16-18].

3.3. Thermal improvements A tilt of the module azimuth towards east allows a better performance in the morning when lower ambient temperatures occur (optimum at 35 ° with a gain in energy output of 0.4%).

572

S. Krauter, R. Hanitsch / Solar Energy Materials and Solar Cells 4 1 / 42 (1996) 557-574

70-

60-

°0C_50_

..~ 40-

00~000"0 ""

20

o 10-

oo '

thermal enhanced PV-module conventional PV-Module, directmounted I

'

6

~

'

I

7

'

8

I

'

9

I

'

I

10

'

I

11

'

12

I

'

13

+

'

I

14

'

I

15

'

16

I

'

I

17

'

18

I

19

solar time of clay Fig. 18. Cell temperatures of a convectional PV module compared to TOEPVIS module during a day for hot add conditions.

A gain of 2.6% was achieved by attaching a small water tank (12.3 liter) to the backside of a module to increase the total heat capacity. During a day the minimum efficiency is then delayed two hours from the time of maximum insolation at lower peak temperatures. A latent heat storage film is under development and should allow an improvement of 8-10%. 3.4. New modules

Continuous studies led us to the TOEPVIS-module (Thermal and Optical Enhanced PV module with Integrated Stand). Here the thermal capacity (a water tank made out of

0.10O@.~..o-,o..+,.o.+.o+.o..+o .,0"

-o,.+..+..

_.~ 0.08-

E o.08-

"

0.04.

Lu

0.02. O0

thermal

enhonced

convenfior'~l

0.00

•

6

,

7

•

,

8

-

,

il

.

,

10

•

,

11

.

,

12

PV-module

PV--moOule •

,

13

•

,

14

.

(I~10/40) ,

15

•

,

16

•

,

17

•

18

Solo" time of day Fig. 19. Efficiency of TOEPVIS module compared to a conventional module within a hot day.


573

Fig. 20. TOEPVIS-2 prototype made of acrylic.

recycled polyethylene (PE)) was enlarged in such a way that the tank also serves as the module stand. The cell temperatures are lowered significantly, which results in a gain of energy output of 11-14% (see Fig. 18, Fig. 19). A photo of the prototype module is shown in Fig. 20. Together with an encapsulation with better matched refractive indices and a partial surface structure [17], an increased electrical energy output of 18% under arid conditions is possible. Relative to the cost of a conventional PV installation with framing, mounting, installation and foundations, the TOEPVIS installation will cost only 60-80%. In total the TOEPVIS module will allow a cost reduction of 10-15%, plus an increased output, so a total gain of about 30% could be achieved.

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S. Krauter, R. Hanitsch / Solar Energy Materials and Solar Cells 41 / 4 2 (1996) 557-574

4. C o n c l u s i o n The m o d e l and the simulation p r o g r a m d e v e l o p e d a l l o w us to predict optical and thermal p e r f o r m a n c e under realistic operating conditions, and they are p r o m i s i n g tools for e v a l u a t i n g n e w P V p o w e r plants with the aim o f increasing efficiency.

Acknowledgements The authors are grateful for the support o f Martin Green, S m a r t W e n h a m and their team f r o m the U N S W Centre for P h o t o v o l t a i c D e v i c e s and S y s t e m s during the measurem e n t phase in Australia.

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