Winter School on RES,

at Afyon Karahisar University

16-18 January 2013

Prof. Socrates Kaplanis

President of the TEI of Patras

Dr. Eleni Kaplani

http://solar-net.teipat.gr

kaplanis@teipat.gr

ICAROS : Man and Nature

Temperature effect on PV cell performance

The PV cell technology provides extremely beautiful products which are systematically explored and integrated into any business or system.

They attract interest due to the direct conversion of solar radiation into current, (Power) and.Heat !

although not produced as such.

However,

Sun & Heat affect its Target !!!

Temperature effect on PV cell performance:

A solar roof affected by Tc in the RES Lab. TEI Patras

PV cell ageing due to Tc

PV cell browning due to hot spotsTc

PV array placed almost horizontally on a roof.

Tc effect ?

Temperature effect on PV Winter School

(i,V) characteristic curve of a PV-cell

The characteristics of a diode at dark and under illumination are given by the

superposition of the two lines. This (i,V) provides the current i= iph- I0.

Temperature effect on PV Winter School at Afyon Karahisar University

We call characteristic curve of a PV-cell the one which represents the current i delivered by the

PV-cell versus the voltage, V, across the

resistance, which is connected to its terminals.

The characteristic curve can be understood theoretically if we design the electric equivalent

circuit of a PV-cell, which is a current generator

connected in parallel with a diode.

A simple electric equivalent circuit of a PV cell

1eIii kT

qV

0ph

1

I

iln

q

kTV

0

ph

oc

General type current conservation

sh

sDphRDph

R

irVIiiiIii

sh

The 2 diode model

1eI1eIi

Iii

T

s

T

s

2V

irV

r

V

irV

0ph

Dph

Studying (i-V) and PV-cell

performance

Question: Does the characteristic i-V depend on the temperature of the PV cell?

Answer: Yes.

Comment: isc slightly increases with Tc, while VOC decreases as Tc increases.

isc and Voc dependence on Tc

The dependence on temperature has to be taken into account for the various PV-sizing problems and accurate calculations.

isc increases slightly with Tc, according to:

VOC decreases, as TC

increases according to:

per PV-cell, or for ns PV-cells in series

C

V102.3

dT

dV0

3OC

104sc

sc

K103dT

di

i

1

C

Vn102.3

dT

dV0s

3OC

Various expressions on PV cell basic quantities

i-V characteristic :

T108.7

VirVexp1ii

5

ocssc

300T1031CA0.034i 4sc

T300

C0.06log0.631.25V 10oc

srAC0.051300T0.00060.8FF

100%0.1CA

FFVi ocsc

100%0.1CA

Vi

Reference systems to study PV cell performance

N.O.C.T.(Normal Operating Cell Temperature)

In order to design a PV-generator and especially to estimate the installed power or the peak power, Pm, (Wp) there should be a reference system on which the power delivered by the PV-generator will be estimated.

As the PV-panel performance depends on solar insolation and temperature or on environmental parameters, there are two-reference systems in use.

S.T.C. reference system

(S.T.C.)- Standard Test Conditions

This implies that for PV-cell tests, the environmental conditions should be:

Solar Intensity on the PV-cell: 103 W/m2 or 100 mW/cm2

Temperature of the PV-cell: TC=250C, and

Solar spectrum AM1.5 (air mass),

Another set of conditions is the so-called Standard Operating Conditions (S.O.C.)

These conditions have been proposed in order to determine

the peak power Pm or Wp, as they approach better the

reality.

S.O.C

These conditions are: Solar insolation on the PV-cell: IT=800 W/m

2 , the

ambient temperature, Ta=20 0C, and wind velocity VW

Problem

A PV-cell operates in an environment Ta =30 0C

with solar intensity, on it: IT=800 W/m2.

Estimate Tc.

Solution:

From the theory of heat transfer as IT is partially absorbed by the PV-cell its temperature increases above Ta.

It is proven that Tc satisfies the relationship:

Twac IhTT

CCCW

Km

m

WC 000

02

2

0 54243003.080030

W

KmhW

02

03.0

Problem

A PV-panel consists of 34 cells in series.

When IT=700W/m2; Ta=34

0C;

isc=3A; Voc=20.4V; Pmax=45.9W;

PV-cell characteristics under S.T.C. NOCT=430C.

Find : isc, Voc, Pmax under the above conditions.

Steps:

1.

We assume that isc does not change dramatically with TC, as Voc does

2. Estimate ,

Put, NOCT=430 C, Ta=340 CTC=54.120 C

AA 1.27.03 sci

8.0

7.0

20:

0

CNOCT

TTT aCC

Estimate new Voc under field conditions

VoltsVT

dT

dVV C

oc

oc 1.18)2512.54(34103.24.20)25(30 0oc,54.12V

75.04.203

9.45

VA

W

ocsc

m

ocsc

mm

Vi

P

Vi

ViFF

Why do we estimate FF ?

FF does not change dramatically with TC; we assume it constant

. Estimate the corrected Pmax

WVoltsACVm

WiFFP ocsccondnew 5.281.181.275.0)12.54(700

0

2max,

621.09.45

5.28

W

W

That is, the PV-panel operates at 62.1% of its peak level.

PV cell efficiency change with Tc vs C

The rate of change of PV-efficiency with T is given by:

dT

FFd

FF

1

dT

id

i

1

dT

dV

V

1

dT

d

1 sc

sc

oc

oc

Voc vs Concentration with Tc as parameter

Efficiency vs C with Tcas parameter

Power decreases by 0.4% for every 10C increase

Experimental results with transient

consideration

The change in Voc and isc does not take

place in a dt of time, but it is based on the

law of energy balance which has an

inherent exp term.

That is to be discussed later in these

lectures

Voc transient vs t at natural air environment;

starts from 0.579 to 0.525.

time constant =40 s

-100

0

100

200

300

400

500

600

700

0 50 100 150 200 250 300 350

Tasi

Transient isc vs t at natural air flow environment,

isc starts from 0.375 and levels out at 0.397 A.

Time constant =40 s

-50

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350

Reyma

PV cell transient temperature profile natural

heat flow at IT= 103 W/m2 time constant = 40 s

16

21

26

31

36

41

46

51

0 50 100 150 200 250 300 350

Tc

Ta

The same as before with forced air flow

to extract heat

0

100

200

300

400

500

600

700

0 100 200 300 400 500

Tasi

The same IT as before for forced air

flow

-50

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500

Reyma

PV cell temperature profile at same IT with forced air

flow. The same time constant; lower Tc ; lower losses

16

18

20

22

24

26

28

30

0 100 200 300 400 500

Tc

Ta

Voc profile at IT higher than before; the temp.

effect is higher; almost the same; no fan used. Voc started from 0.603 V down to 0.496 V

-100

0

100

200

300

400

500

600

700

0 200 400 600 800 1000

Tasi

isc time profile. The temp. effect is higher than for lower

IT. It starts from 0.795A and levels out at 0.871A; no fan

-200

0

200

400

600

800

1000

0 200 400 600 800 1000

Reyma

Temperature profile vs time. Time constant, , almost the same. Temp. is now much higher; no fan used

16

26

36

46

56

66

76

0 200 400 600 800 1000

Tc

Ta

Voc time profile; fan is used to extract heat off. It

starts from 0.601V and levels at 0.550 V. Time

constant the same

-100

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600

Tasi

isc time profile for heat extraction conditions.

Values start from 0.797 A and level down at

0.843 A

-100

0

100

200

300

400

500

600

700

800

900

0 100 200 300 400 500 600

Reyma

PV cell Temperature time profile with fan used to

cool down the cell. The effect and the changes are

obvious

16

21

26

31

36

41

46

0 100 200 300 400 500 600

Tc

Ta

Measured Voc vs temperature in K for a c-Si cell at

various. Please, determine dVoc/dTc

VocMEAS vs Tc

0,3

0,35

0,4

0,45

0,5

0,55

0,6

0,65

290 310 330 350 370 390 410

120

100

90

80

70

60

50

40

30

20

Voc profile measured and predicted for the

same as before c-Si PV cell

60 Voc vs t

0,48

0,5

0,52

0,54

0,56

0,58

0,6

0 50 100 150 200

PREDICT

MEASURED

The BIPV performance model

The temperature developed in a PV cell working in field conditions, or integrated into a building shell, strongly affects the PV cell efficiency.

It plays a significant role in the PV cell performance and the overall annual yield.

The temperature of the PV cell depends on the solar radiation intensity on the cell, IT (W/m

2), the ambient air temperature, Ta (C), the cell technology and structure, the geometry of the PV array in the field; that is, the tilt angle and the orientation, as well as its surrounding.

The performance model

The latter might have the form of:

PV panels placed across a wall, like facades, roofs or shadow hangers, or

a PV array in free environment.

Both cases above are very important to investigate, either for the design of buildings with solar technology elements integrated into their structure, or for a more accurate estimation of the energy to be delivered by a PV generator in a period of time.

On the other hand, the wind strongly affects the PV

module temperature but not considered here.

It is necessary to distinguish between natural convection of heat, (air flow is caused by the

difference in temperatures, between the PV cell

and its environment), or forced air flow, due to

wind. Also, the wind direction matters.

The effect of the angle of inclination on the PV efficiency, as it is associated to the temperature

profile developed on the PV cell is our concern.

As the PV module may convert only a small part of the radiation into power, eg. 15% in the

case of c- Si cells, the rest of the radiation,

minus the reflected part, is dissipated, finally,

into heat.

This causes the PV cell temperature to raise above the ambient air temperature.

Provided the conditions of the PV system and its surrounding do not change fast, then,

steady state conditions prevail and the thermal capacities of the PV system may be neglected.

Energy Balance Equation

Thus, we may consider that approximately, the 85% of the solar radiation on the PV cell is converted into heat which finally flows to both PV sides, with an assumption of equal rates. This heat is then passed into the environment. The expression which may hold for the energy balance for steady state conditions takes the form:

()* = pv*IT + hpv,f * (Tpv,f Ta )+ hpv,b *(Tpv,b Ta) (1)

Giving a value to () almost 1, setting hpv,f = hpv,b =h and Tpv,f = Tpv,b = Tpv , eq(1) may take a simple

approximate form:

(1- pv )* IT = h* 2*(Tpv-Ta) (2)

Then, eq(2) may be written:

Tpv = Ta + 0.5 *(1-pv)* IT/ hpv (3)

For pv = 15% and hpv=10 W/m2K for plane

surfaces out in the open air, with a negligible wind

velocity, less than 1m/s, eq(3) gives:

Tpv= Ta + 0.0425* IT

If we set hpv =15 W/m2K, which is a case with

some wind prevailing , or a case that the air

flow past the PV back surface gets turbulent,

then instead of 0.0425 m2K/W the coefficient

becomes 0.028 m2K/W.

Such figures derived from the simple analysis above are expected when studying the effect of

IT on Tpv for cases of PV panels, outdoors.

We may, thus, well assume that:

Tpv= Ta + * IT (5)

may be a function dependent on many parameters, like:

the solar radiation spectrum, or correspondingly the clearness index, Kt,

the inclination angle, , as hpv generally depends on this angle,

the type of flow, that is natural or forced flow, and

the pattern of air flow past the PV panel, ie laminar or turbulent, which determines the value of hpv

It is important to design experiments to determine the PV panel performance, its maximum power, Pm and pv, associated to the IT and the value, as parameters.

The coefficient (Km2/W) might well be assumed as a function of the inclination of the module, , where =f(.) or it is a function of cos()., when calculating the Grashof or the Nusselt numbers,

All these lead to the conclusion that even with the same solar radiation intensity on two PV panels, which have different inclination angles, one may get different results for Pm, pv and Tpv.

A PV generator serving as a shading

device, too.

A PV generator in a free space outdoors.

Experimental set up

A system may be built to collect and manage data and measure the PV module performance equipped with:

A data logger An i-V characteristic portable system , developed and

built for this project

A Pyranometer Kipp & Zonen, type CMP3 and temperature sensors: thermocouple T ( Cu-Cons).

Additionally, for cross-check reasons, a portable infra-red thermometer, type Mikron M90 series was used. The sensors were placed on the back side of the PV module directly on the Tedlar foil.

A sensor for the ambient air temperature, Ta The i-V curves were obtained and analysed for all

cases examined.

Results

Tpv, Ta, and IT were measured, for various values from 160 to 750.

Pm and pv were determined vs IT for predetermined values of the angle of inclination, . Each measurement lasted for a period of time, let the system reach its steady state and get a whole performance profile of a range of IT values under such conditions.

The i-V curves were obtained, so that Pm was determined. Then, the specific efficiency, Pm/IT, was calculated.

1st case: the PV panel as a hang over or

shading device

The temperature difference between the PV panel and the ambient defined by, Tdif =Tpv- Ta,

increased, in general, linearly with IT, for any

angle.

Generally, Tdif decreases for the same IT on the PV panel, as the angle increases;

However, for an inclination, , equal 470 there appears that Tdif is higher than expected.

This behaviour is attributed to partial blockage in the air flow for this type of PV array set up.

This general trend for Tdif was expected, as the PV surface heat transfer coefficient,

hpv, increases as takes higher values.

It is, thus, expected that for the same value of IT , the highest Tdif value is for the

horizontal position.

Therefore, as gets smaller the Power should relatively decrease and eventually

the efficiency of the PV panel.

Tdif vs IT, on a PV panel inclined over a window.

The results are given for various inclination

angles.

Pm vs IT, for various inclination angles. For the

same IT, Pm increases as angle increases, till the 40o . The PV array acts as a shading device

. Relative efficiency vs IT for a PV array at an inclined position in front of a window. For the same value of IT the efficiency is higher as the inclination

increases. This trend reverses for high values; see curves corresponding to 47o and 75o

The relative efficiency, Pm/IT, decreases with IT

The effect on the PV panel efficiency by an increase of IT, is finally negative. In all cases, but one, the relative efficiency decreases with IT, instead of increasing with the solar radiation intensity. This is due to the stronger negative effect the temperature has on Pm and consequently on the efficiency.

Conclusively, the negative effect of temperature, dpv/d, is absolutely higher than the positive effect of dpv/dI.

On the contrary, for high values, eg for an inclination =75, the above figure shows that for low IT

2nd Case: The PV generator placed in a free

environment outdoors.

The measurements obtained are rationally interpreted, without any exceptionas the previous case.

Tdif increases with IT and takes larger values for small . In this case, Tdif takes lower values compared to case 1,

However, as the natural air flow at the back side of the PV surface changes to turbulent, due to the increase in the Grashof number, at about 400 -450, a larger drop in Tdif occurs.

On the contrary, in high values, the flow of the air passed by the PV panel turns again laminar or the layers glide over the back PV surface.

. Tdif, vs IT for various inclination angles of the PV array placed in a free environment, outdoors.

Values of coefficient

Analysis of the data, taken from the experiments show that coefficient which appears takes values, which follow the pattern:

0.025 for angles around 35o-40o,

while, it increases for lower or higher than 40o angle values, reaching 0.050 for free space environment and

0.065 for the case the PV panel is placed in front of the window.

Pm vs IT for various inclination angles. The

figure stands for a PV array positioned in a free

space, outdoors.

. Relative efficiency, Pm/IT vs IT for various inclination angles. The figure holds for a PV array

placed in a free space, outdoors.

Analysis of Results Examine the behaviour and the rate of change

of the efficiency pv over the Tdif for various inclinations and at different IT levels, in field

conditions.

Choose the curves representing the inclination angles 470 and 260, inter-related with the ones

for IT values: 700, 800, 900 W/m2.

The result is that the relative change of the efficiency over the Tdif for both the inclination

angles of the PV panel, (dpv/d)/pv, takes values from about 0.40%/0C to -0.50%/0C.

It is important to notice that the estimation of the relative change of the efficiency with

respect to temperature, (dpv/d)/pv , was tried for either:

a chosen solar irradiation level on the PV panel: 700, 800, 900 W/m2 at two different angles, or for

a specific angle of inclination, 470 or 260, in a range of IT values.

The same analysis was followed to obtain (dpv/d)/pv values using data corresponding to the curves for the angles 470 and 160, as in apropriate figures.

This analysis gave a higher value for (dpv/d)/pv. The relative change, (dpv/d)/pv, was estimated to about -1.0%/0C,

which denotes the effect that the inclination angle has in the PV performance.

he estimation of (dpv/d)/pv from the data was achieved with two approaches:

1st Approach

From a figure the (dpv/dI)/pv value was determined for an inclination angle. Then, from

the first figure dTdif/dI was determined for the

same .

Hence, (dpv/dI)/(dT/dI) /pv was estimated. The results provide that the relative change of pv per oC, is of the order of -0.5% to -0.6%/ oC for

=47o, while for = 36o the result is

-0.95%/oC and for =26o it is about 1.0%/ oC

Those results hold for the range of IT from 700 1000 W/m2

2nd Approach

In this mode, there was selected an IT value and two angles of inclination.

The effect of the angle of inclination associated to a value of IT, which changes during the day, along with the effect from the Tdif , which differs with , shows that the relative decrease in the efficiency, when the angle changes from 47o to 26o takes values:

-0.44% at 700 W/m^2 .

--0.50% at 800 W/m^2

-0.47 % at 900 W/m^2

When we analyze data for the same coefficient but for a change in from 47o to 16o ,then the relative decrease is doubled.

The integrated relative change of (dpv/dIT)/pv over a range of IT that is [(dpv/dIT)/pv]

1. for the same angle of inclination, , is obtained according to the following analysis:

[(dpv/dI)/pv]*=

[V-1oc*(dVoc/dT)*(dT/dI)+

i-1sc*(disc/dT)*(dT/dI)+

FF-1*(dFF/dT)*(dT/dI)]*

The estimation of the expression ( dpv/dI)/pv)I using from literature the values of the rate of change of Voc , isc , FF and

the rate Tpv changes with IT ,

taken from the figure, provides a theoretical value of -3.7 3.8 % for the range of measurements from 800 to 1000 W/m2

On the other hand, the analysis of the experimental data provided in the figures above give a value of -7% at 470 and 5.56 % at 160 for the same range 800 to 1000 W/m2

A new Book on RES

Renewable Energy Systems: Theory, Innovations and Intelligent Applications

Editors: Socrates Kaplanis and Eleni Kaplani (Technological Educational Institute of Patras, Greece)

Book Description: This book aims to provide a friendly and comprehensive

tool in the study of the key issues of Renewable Energy

Systems,

in order to gain a deeper insight in this broad field

through thematic investigations, and, finally,

to become able to design competitive innovations and

intelligent applications of Renewable Energy Systems in

the domestic, agricultural and industrial sectors.

This work is a collaborative attempt to elaborate useful technical information from many countries around the

world concerning the efficient and effective use and

management of Renewable Energy Systems,

either autonomous or hybrids, and

to deliver theoretical and experimental analysis in Renewable Energy Systems issues,

with numerous exercises, extended problems and case studies, simulation models and algorithms,

(Imprint: Nova Publishers, NY)