1.3 Experiment

A digitized infrared (IR) image of an object can be directly related to the temperature distribution across the object and have also been to study surface temperature in PV modules by various authors [15][16]. It is difficult to directly measure temperature distribution of solar cells immersed in liquid because silicone oil may absorb infrared radiation to a certain extent. System configuration was changed to put solar cells above liquid. Then an IR thermographic analysis can be used to obtain temperature distribution of solar cells. It was performed with an Hy-2001G camera (measurement range -20 to 500°C, accuracy ±2% of range, thermal sensitivity 0.07°C and uncooled focal plane array detector). IR pictures were taken from the front of solar cells. Fluid enters the channel from one corner of solar cell to opposite corner. The geometric concentration ratio of system is 70 and the cell type is polysilicon solar cell.

1.4 Results

The variation of temperature distribution of solar cells with different fluid velocity is depicted in Fig.9 and Fig.10. It is evident that temperature distribution is highly no-uniform on the solar cell surface due to fluid flow and there are large temperature differences especially under concentrated sunlight from inlet and outlet. Local high temperature in Fig.9 and Fig.10 seems relevant to imperfection of crystal lattice and flow channel. Franklin et al [17] present that the both non-uniform light distribution and temperature distribution further cause the reduction of solar cell efficiency. Further research is needed to investigate the effects of no-uniform temperate distribution on the solar cell performance. ANSYS (finite element analysis program) is used to predict the temperature distribution at the same operational parameters of Fig.9 and Fig.10. The results are shown in Fig.11 and Fig.12 that are similar to infrared image (Fig.9 and Fig.10).

2. Conclusions

The steady-state thermal model is used to predict the average of solar cells immersed in silicone oil. Results show that the solar cells temperature increase with the increase of irradiance and inlet fluid temperature. This kind of immersion operation of solar cells is suitable for photovoltaic concentrator because they can provide an effective method of cooling solar cell under concentrated sunlight. Cooling of force convection is more effective method than free convection, but it may increase system cost. The IR thermographic analysis is used to obtain the temperature distribution of solar cell. Further studies are needed to probe the effects of no-uniform temperature distribution on the solar cell performance and find effective method to reduce no-uniform temperature distribution.

NOMENCLATURE

A area(m2)

E electricity(W)

G irradiance(Wm"2)

T temperature(K)

h heat transfer coefficient(Wm"2K"1)

c specific heat(Jkg"1K"1)

m mass flow rate(kgs-1)

v wind velocity(ms-1)

w fluid velocity(ms-1)

Greek letters
a	absorptance
E	emissivity
1	conversion efficiency
5	Stefan-Boltzmann constant(Wm’2K’4)
P	reflectance
Subscripts
a	ambient
g	transparent cover
c	solar cells
f	liquid
o	outlet
i	inlet
s	sky
w	wind

[1]

Fig.1 Cross-sectional view of immersion system

1 h gc

1 h gc

Fig.2 Thermal network for PV system of Fig.3 Thermal network of PV system of

irradiance(W/m2)

Fig.4 Temperature vs. irradiance T=293K, m=0.9kg/s, Ta=298K

forced convection cooling free convection cooling

inlet fluid temperature(K)

Fig.5 Temperature vs. inlet liquid temperature m=0.9kg/s, G=1000W/m2 Ta=298K

SHAPE * MERGEFORMAT

Temperature(K)

Fig.6 Temperature vs. mass flow rate Ti=293K, G=1000W/m2, Ta=298K

irradiance(W/m2) Fig.8 Temperature vs. irradiance Ta=298K

inlet fluid

Fig.9 infrared image of solar cells w=0.4m/s Ta=26°C Ti=25°C G=600W/m2

iutlet fluid

120°C I—- 1

Fig.10 infrared image of solar cells w=0.1m/s Ta=26°C Ti=25°C G=600W/m2

. 20°C

inlet fluid

20°C

120°C I—- 1

outlet fluid

ambient temperature(K) Fig.7 Temperature vs. ambient temperature G=1000W/m2

irradiance(W/m2)

SHAPE * MERGEFORMAT

outlet fluid inlet fluid Fig.11 predicted temperature distribution w=0.4m/s Ta=26°C Ti=25°C G=600W/m2

N outlet fluid inlet fluid Fig.12 predicted temperature distribution w=0.1m/s Ta=26°C Ti=25°C G=600W/m2

*4