- Email: [email protected]
Heat reduction of concentrator photovoltaic module using high radiation coating
Surface & Coatings Technology 215 (2013) 472–475
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Heat reduction of concentrator photovoltaic module using high radiation coating Kensuke Nishioka a,⁎, Yasuyuki Ota a, Kazuyuki Tamura b, Kenji Araki b a b
Faculty of Engineering, University of Miyazaki, 1-1, Gakuen Kibanadai Nishi, Miyazaki 889-2192, Japan Daido Steel Co., Ltd., 9 Takiharu, Minami-ku, Nagoya 457-8712, Japan
a r t i c l e
i n f o
Available online 5 November 2012 Keywords: Concentrator photovoltaic Thermal radiation Heat release Temperature
a b s t r a c t A thermal radiation layer was coated on the aluminum chassis of a concentrator photovoltaic (CPV) module. The temperature of the solar cell in the CPV module with the thermal radiation coating was approximately 10 °C lower than that of the module without the thermal radiation coating. The uniformity of the temperature distribution in the CPV module was considerably improved. The thermal radiation coating acted not only as a thermal radiation layer but also as a thermal conduction layer. The open-circuit voltage of the CPV module with thermal radiation coating was 0.5 V higher than that of the module without the coating during the period evaluated. The conversion efficiency of the CPV module with thermal radiation coating was 0.5% higher than that of the module without the coating. © 2012 Elsevier B.V. All rights reserved.
Multijunction solar cells have been attracting increasing attention for application in concentrator photovoltaic (CPV) systems owing to their high conversion efficiency [1–3]. Multijunction solar cells consisting of InGaP, InGaAs, and Ge diodes are recognized as super-high-efficiency solar cells and are used for space applications. A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell has delivered a conversion efficiency of 41.1% at 454 suns (454 kW/m2, AM1.5D) [4]. Light concentration is one of the important issues for the development of an advanced photovoltaic (PV) system using high-efficiency solar cells. High-efficiency multijunction cells under high light concentration have also been investigated for terrestrial applications [5–8]. It is considered that the temperature of solar cells considerably increases under light-concentrating operations, and the conversion efficiency of solar cells decreases with increasing temperature [9–11]. It is therefore very important to reduce the cell temperature in CPV modules. In this study, a thermal radiation layer was coated on the aluminum chassis of a CPV module and the effect was evaluated.
CPV module by a spray coating method. The thickness of the layer was 30 μm. The thermal radiation layer consisted of acrylate resin and inorganic fillers. The fillers were selected to radiate the heat, particularly in the temperature range from 40 to 100 °C, which is the main range of operating temperature for the CPV module. The thermal emissivity of the layer coated on aluminum is 0.95, whereas that of aluminum is approximately 0.02–0.1. The triple-junction solar cells were arrayed on the aluminum chassis. Fig. 2 shows a cross-sectional diagram depicting the arrangement from the solar cell to the aluminum chassis. The solar cell was connected to a copper ribbon electrode using a high-thermal-conductivity solder. In order to retain the insulation quality, an aluminum alloy was adopted, and the copper ribbon was applied on it with an insulation layer. In order to detect the temperature of the solar cell in the CPV module, temperature sensors (Pt100) were embedded just below the solar cell. A CPV module was fabricated by connecting 25 lens–cell pairs in series. The current–voltage (I–V) characteristics were measured using an I–V curve tracer (EKO, MP-160). The modules with and without a thermal radiation coating were evaluated at the University of Miyazaki (Miyazaki, Japan). The evaluation was carried out from 13:00 to 17:00 h on September 23, 2011.
2. Experimental procedure
3. Results and discussion
Fig. 1 shows a schematic diagram of the CPV module (area: 1005 mm× 1005 mm). The CPV module consisted of 25 pairs of Fresnel lenses (200 mm× 200 mm), a InGaP/InGaAs/Ge triple-junction solar cell (7 mm× 7 mm), and an aluminum chassis. The concentration ratio of the CPV module was 820 times. A thermal radiation layer [Pelcool (R), PELNOX Ltd.] was coated on the aluminum chassis of a
Fig. 3(a) shows the direct normal irradiance (DNI). The weather during the experiment period was clear. Fig. 3(b) shows the ambient temperature and wind speed. The wind speed was low and there were no significant changes. During the evaluation, the meteorological conditions were stable. Fig. 4 shows the temperature of the solar cell at the center of the CPV module. The cell temperature of the CPV module with the thermal radiation coating was approximately 10 °C lower than that in the CPV module without the thermal radiation coating. The cell temperature
1. Introduction
⁎ Corresponding author. Tel./fax: +81 985 58 7774. E-mail address: [email protected] (K. Nishioka). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.09.064
K. Nishioka et al. / Surface & Coatings Technology 215 (2013) 472–475
473
a 1200 Fresnel Lens
2
DNI (W/m )
1000
Solar cell
Aluminum Chassis
800 600 400
Fig. 1. Schematic diagram of a CPV module. The CPV module consisted of 25 pairs of Fresnel lens (200 mm × 200 mm) and a InGaP/InGaAs/Ge triple-junction solar cell (7 mm × 7 mm).
ð1Þ
b
16:00:00
17:00:00
10
30
o
25
8
20 6 15 4 10 2
5 0 13:00:00
14:00:00
15:00:00
16:00:00
0 17:00:00
Time Fig. 3. Meteorological conditions during measurement; (a) direct normal irradiance, (b) ambient temperature and wind speed. The arrows in the figure show the corresponding vertical axis.
with the thermal radiation coating was approximately 0.5 V higher than that of the module without the coating during the test period. The I–V characteristics of the solar cell are expressed by qV I ¼ I 0 exp −1 −Isc ; nD kT
ð2Þ
where Isc, I0, q, nD, k, and T are the short-circuit current, saturation current, elementary charge, diode ideality factor, Boltzmann constant, and absolute temperature, respectively [12].
100 90
o
Cell Temperature ( C)
where σ, T1, and T2 are the Stefan–Boltzmann constant (5.67 × 10 − 8 W/m 2·K 4), the absolute temperature (K) of the chassis, and the absolute temperature (K) of the environment, respectively. It was found that the radiation heat increased with increasing emissivity, and the high emissivity of the thermal radiation layer enhanced the heat radiation from the chassis to the environment. Fig. 5 shows the difference between the cell temperature at the center (Tcenter) and that at the corner (Tcorner). The cell temperature at the center is highest because the central cell is enclosed by other cells. In the CPV module, the solar cell that receives high energy light behaves as a heat source. A large value of the difference (Tcenter −Tcorner) means that the temperature distribution in the CPV module is not uniform. When the temperature distribution is not uniform, the chassis of the CPV module will become distorted owing to the local elevation in temperature. A partial misalignment between the optical system and the solar cell occurs, and the output characteristics of the CPV module decrease. The value of Tcenter − Tcorner for the CPV module without the coating reached 14.5 °C at time 13:25 h. On the other hand, that for the CPV module with the coating was reduced to 5.1 °C, which shows that the uniformity in the temperature distribution for the CPV module was considerably improved. It is considered that the coating acted not only as a thermal radiation layer but also as a thermal conduction layer. Fig. 6 shows the open-circuit voltage (Voc) of the CPV module with and without the thermal radiation coating. Voc of the CPV module
15:00:00
80 70 60 50 40 13:00:00
Without thermal radiation coating With thermal radiation coating
14:00:00
15:00:00
16:00:00
17:00:0
Time Fig. 2. Cross-sectional diagram showing the arrangement from the solar cell to aluminum chassis.
Wind Speed (m/s)
Q 12
4 4 ¼ A1 ε1 σ T 1 −T 2 ;
14:00:00
Time
Ambient Temperature ( C)
for the CPV module without the coating reached 93.1 °C at 13:16 h. On the other hand, that for the CPV module with the coating decreased to 82.6 °C. The effect of the high-radiation layer was remarkable. When we define a chassis [area: A1 (m 2), emissivity: ε1] and an environment [area: infinite], the radiation heat from the chassis to the environment Q12 (W) is given by
200 13:00:00
Fig. 4. Temperature of solar cell at the center of the CPV module.
474
K. Nishioka et al. / Surface & Coatings Technology 215 (2013) 472–475
16
1.00 With thermal radiation coating Without thermal radiation coating
12
0.95
Voc / Voc (25 oC)
Tcenter - Tcorner ( oC)
14
10 8 6 4
0.85
Without thermal radiation coating With thermal radiation coating
2 0 13:00:00
0.90
14:00:00
15:00:00
16:00:00
0.80 60
17:00:0
65
70
From Eq. (2), Voc (I = 0) is given by nD kT I ln sc þ 1 : q I0
ð3Þ
From Eq. (3), the temperature characteristic of saturation current (I0) markedly influences the temperature characteristic of Voc. The saturation current density (J0) is given by J 0 ¼ qni
2
80
85
Tave. ( C)
Fig. 5. Difference between the cell temperature at the center (Tcenter) and that at the corner (Tcorner).
V oc ¼
75 o
Time
! De Dh þ ; NA W p ND W n
ð4Þ
where ni is the intrinsic carrier concentration, NA and ND are the acceptor and donor concentrations, respectively, Wp and Wn are the thicknesses of the p and n neutral regions, respectively, and De and Dh are the diffusion constants of electrons and holes, respectively [13]. J0 strongly depends on T through its proportionality to the square of ni, which is expressed by 2 3=2 2 3 me mh exp −Eg =kT ; ni ¼ 4M c M v 2πkT=h
ð5Þ
where Mc and Mv are the number of equivalent minima in the conduction and valence bands, respectively, h is Planck's constant, and ⁎ m⁎ e and mh are the effective masses of electrons and holes, respectively [14]. From Eqs. (3)–(5), it is found that the decrease in Voc with increasing temperature arises mainly from the change in ni. The value of J0
Fig. 7. Temperature dependence on normalized Voc of CPV module.
increases exponentially with decreasing 1/T, and Voc decreases linearly with increasing T. Fig. 7 shows the temperature dependence on the normalized Voc of the CPV module. Voc was normalized with Voc at 25 °C. The temperature was the average temperature of the 25 cells in the CPV module. Voc decreases linearly with increasing temperature. The data for the CPV module with the thermal radiation coating existed in the low temperature range owing to the heat-release effect of the coating. As shown in Fig. 4, the cell temperature of the CPV module with the thermal radiation coating was 10 °C lower than that of the module without thermal radiation coating. Eventually, the temperature range of the CPV module with the thermal radiation coating was decreased by 10 °C. Fig. 8 shows the conversion efficiency of the CPV module with and without thermal radiation coating. The conversion efficiency of the CPV module with the thermal radiation coating was 0.5% higher than that of the module without the coating. The temperature coefficient of the conversion efficiency of the CPV cell has been reported as approximately − 0.05%/°C [15]. As shown in Fig. 4, the cell temperature of the CPV module with the thermal radiation coating was reduced by 10 °C. The improved value of 0.5% is therefore reasonable. A high-efficiency CPV module can be achieved with a combination of cell and module technologies. The conversion efficiency of the triple-junction solar cell for concentration approached the theoretical limit and any further improvement required a longer time [16,17]. A previous technology for CPV modules had used expensive fins as heat sinks for heat radiation [18]. In this study, a new simple coating technology for handling heat radiation was developed. By adopting the thermal radiation coating for the CPV module fabrication, the module efficiency was easily improved.
72
24 Without thermal radiation coating With thermal radiation coating
23
Eff. (%)
Voc (V)
71
70
69
22
21 Without thermal radiation coating With thermal radiation coating
68 13:00:00
14:00:00
15:00:00
16:00:00
17:00:0
Time Fig. 6. Open-circuit voltage of CPV module with and without thermal radiation coating.
20 13:00:00
14:00:00
15:00:00
16:00:00
17:00:0
Time Fig. 8. Conversion efficiency of CPV module with and without thermal radiation coating.
K. Nishioka et al. / Surface & Coatings Technology 215 (2013) 472–475
4. Conclusion A thermal radiation layer was coated on the aluminum chassis of a CPV module. The cell temperature of the CPV module with the thermal radiation coating was approximately 10 °C lower than that of the module without the thermal radiation coating. The cell temperature for the CPV module without coating reached 93.1 °C at time 13:16 h. On the other hand, that for the CPV module with coating decreased to 82.6 °C. The effect of the high-radiation layer was remarkable, and the uniformity of the temperature distribution in the CPV module was considerably improved. It is considered that the thermal radiation coating acted not only as a thermal radiation layer but also as a thermal conduction layer. The layer assisted with the thermal diffusion in the chassis. Voc of the CPV module with thermal radiation coating was 0.5 V higher than that of the module without the coating during the test period. The conversion efficiency of the CPV module with thermal radiation coating was 0.5% higher than that of the module without the coating.
Acknowledgments A part of this work was supported by the incorporated administrative agency New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI), Japan.
475
References [1] M. Yamaguchi, Sol. Energy Mater. Sol. Cells 75 (2003) 261. [2] M. Yamaguchi, T. Takamoto, K. Araki, Sol. Energy Mater. Sol. Cells 90 (2006) 3068. [3] T. Takamoto, M. Kaneiwa, M. Imaizumi, M. Yamaguchi, Prog. Photovoltaics Res. Appl. 13 (2005) 495. [4] W. Guter, J. Schone, S.P. Philipps, M. Steiner, G. Siefer, A. Wekkeli, E. Welser, E. Olive, A.W. Bett, F. Dimroth, Appl. Phys. Lett. 94 (2009) 223504. [5] J. Jaus, A.W. Bett, H. Reinecke, E.R. Weber, Prog. Photovoltaics Res. Appl. 19 (2011) 580. [6] K. Nishioka, T. Takamoto, T. Agui, M. Kaneiwa, Y. Uraoka, T. Fuyuki, Sol. Energy Mater. Sol. Cells 90 (2006) 57. [7] K. Araki, M. Kondo, H. Uozumi, N.J. Ekins-Daukes, T. Egami, M. Hiramatsu, Y. Miyazaki, M. Yamaguchi, Sol. Energy Mater. Sol. Cells 90 (2006) 3320. [8] K. Araki, T. Yano, Y. Kuroda, Opt. Express 18 (2010) A53. [9] K. Nishioka, T. Hatayama, Y. Uraoka, T. Fuyuki, R. Hagihara, M. Watanabe, Sol. Energy Mater. Sol. Cells 75 (2003) 665. [10] K. Nishioka, N. Sakitani, K. Kurobe, Y. Yamamoto, Y. Ishikawa, Y. Uraoka, T. Fuyuki, Jpn. J. Appl. Phys. 42 (2003) 7175. [11] K. Nishioka, T. Takamoto, T. Agui, M. Kaneiwa, Y. Uraoka, T. Fuyuki, Sol. Energy Mater. Sol. Cells 85 (2005) 429. [12] S.M. Sze, in: Physics of Semiconductor Devices, Wiley-Interscience Publication, New York, 1981, p. 793. [13] P. Antognetti, G. Massobrio, in: Semiconductor Device Modeling with SPICE, McGraw-Hill, New York, 1988, p. 6. [14] A.L. Fahrenbruch, R.H. Bube, in: Fundamentals of Solar Cells, Acadmic Press, Inc., New York, 1983, p. 238. [15] G. Segev, G. Mittelman, A. Kribus, Sol. Energy Mater. Sol. Cells 98 (2012) 57. [16] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Prog. Photovoltaics Res. Appl. 20 (2012) 606. [17] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Prog. Photovoltaics Res. Appl. 19 (2011) 565. [18] S.K. Natarajan, M. Katz, T.K. Mallick, J. Renewable Sustainable Energy 4 (2012) 011601.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Heat reduction of concentrator photovoltaic module using high radiation coating Kensuke Nishioka a,⁎, Yasuyuki Ota a, Kazuyuki Tamura b, Kenji Araki b a b
Faculty of Engineering, University of Miyazaki, 1-1, Gakuen Kibanadai Nishi, Miyazaki 889-2192, Japan Daido Steel Co., Ltd., 9 Takiharu, Minami-ku, Nagoya 457-8712, Japan
a r t i c l e
i n f o
Available online 5 November 2012 Keywords: Concentrator photovoltaic Thermal radiation Heat release Temperature
a b s t r a c t A thermal radiation layer was coated on the aluminum chassis of a concentrator photovoltaic (CPV) module. The temperature of the solar cell in the CPV module with the thermal radiation coating was approximately 10 °C lower than that of the module without the thermal radiation coating. The uniformity of the temperature distribution in the CPV module was considerably improved. The thermal radiation coating acted not only as a thermal radiation layer but also as a thermal conduction layer. The open-circuit voltage of the CPV module with thermal radiation coating was 0.5 V higher than that of the module without the coating during the period evaluated. The conversion efficiency of the CPV module with thermal radiation coating was 0.5% higher than that of the module without the coating. © 2012 Elsevier B.V. All rights reserved.
Multijunction solar cells have been attracting increasing attention for application in concentrator photovoltaic (CPV) systems owing to their high conversion efficiency [1–3]. Multijunction solar cells consisting of InGaP, InGaAs, and Ge diodes are recognized as super-high-efficiency solar cells and are used for space applications. A metamorphic Ga0.35In0.65P/Ga0.83In0.17As/Ge triple-junction solar cell has delivered a conversion efficiency of 41.1% at 454 suns (454 kW/m2, AM1.5D) [4]. Light concentration is one of the important issues for the development of an advanced photovoltaic (PV) system using high-efficiency solar cells. High-efficiency multijunction cells under high light concentration have also been investigated for terrestrial applications [5–8]. It is considered that the temperature of solar cells considerably increases under light-concentrating operations, and the conversion efficiency of solar cells decreases with increasing temperature [9–11]. It is therefore very important to reduce the cell temperature in CPV modules. In this study, a thermal radiation layer was coated on the aluminum chassis of a CPV module and the effect was evaluated.
CPV module by a spray coating method. The thickness of the layer was 30 μm. The thermal radiation layer consisted of acrylate resin and inorganic fillers. The fillers were selected to radiate the heat, particularly in the temperature range from 40 to 100 °C, which is the main range of operating temperature for the CPV module. The thermal emissivity of the layer coated on aluminum is 0.95, whereas that of aluminum is approximately 0.02–0.1. The triple-junction solar cells were arrayed on the aluminum chassis. Fig. 2 shows a cross-sectional diagram depicting the arrangement from the solar cell to the aluminum chassis. The solar cell was connected to a copper ribbon electrode using a high-thermal-conductivity solder. In order to retain the insulation quality, an aluminum alloy was adopted, and the copper ribbon was applied on it with an insulation layer. In order to detect the temperature of the solar cell in the CPV module, temperature sensors (Pt100) were embedded just below the solar cell. A CPV module was fabricated by connecting 25 lens–cell pairs in series. The current–voltage (I–V) characteristics were measured using an I–V curve tracer (EKO, MP-160). The modules with and without a thermal radiation coating were evaluated at the University of Miyazaki (Miyazaki, Japan). The evaluation was carried out from 13:00 to 17:00 h on September 23, 2011.
2. Experimental procedure
3. Results and discussion
Fig. 1 shows a schematic diagram of the CPV module (area: 1005 mm× 1005 mm). The CPV module consisted of 25 pairs of Fresnel lenses (200 mm× 200 mm), a InGaP/InGaAs/Ge triple-junction solar cell (7 mm× 7 mm), and an aluminum chassis. The concentration ratio of the CPV module was 820 times. A thermal radiation layer [Pelcool (R), PELNOX Ltd.] was coated on the aluminum chassis of a
Fig. 3(a) shows the direct normal irradiance (DNI). The weather during the experiment period was clear. Fig. 3(b) shows the ambient temperature and wind speed. The wind speed was low and there were no significant changes. During the evaluation, the meteorological conditions were stable. Fig. 4 shows the temperature of the solar cell at the center of the CPV module. The cell temperature of the CPV module with the thermal radiation coating was approximately 10 °C lower than that in the CPV module without the thermal radiation coating. The cell temperature
1. Introduction
⁎ Corresponding author. Tel./fax: +81 985 58 7774. E-mail address: [email protected] (K. Nishioka). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.09.064
K. Nishioka et al. / Surface & Coatings Technology 215 (2013) 472–475
473
a 1200 Fresnel Lens
2
DNI (W/m )
1000
Solar cell
Aluminum Chassis
800 600 400
Fig. 1. Schematic diagram of a CPV module. The CPV module consisted of 25 pairs of Fresnel lens (200 mm × 200 mm) and a InGaP/InGaAs/Ge triple-junction solar cell (7 mm × 7 mm).
ð1Þ
b
16:00:00
17:00:00
10
30
o
25
8
20 6 15 4 10 2
5 0 13:00:00
14:00:00
15:00:00
16:00:00
0 17:00:00
Time Fig. 3. Meteorological conditions during measurement; (a) direct normal irradiance, (b) ambient temperature and wind speed. The arrows in the figure show the corresponding vertical axis.
with the thermal radiation coating was approximately 0.5 V higher than that of the module without the coating during the test period. The I–V characteristics of the solar cell are expressed by qV I ¼ I 0 exp −1 −Isc ; nD kT
ð2Þ
where Isc, I0, q, nD, k, and T are the short-circuit current, saturation current, elementary charge, diode ideality factor, Boltzmann constant, and absolute temperature, respectively [12].
100 90
o
Cell Temperature ( C)
where σ, T1, and T2 are the Stefan–Boltzmann constant (5.67 × 10 − 8 W/m 2·K 4), the absolute temperature (K) of the chassis, and the absolute temperature (K) of the environment, respectively. It was found that the radiation heat increased with increasing emissivity, and the high emissivity of the thermal radiation layer enhanced the heat radiation from the chassis to the environment. Fig. 5 shows the difference between the cell temperature at the center (Tcenter) and that at the corner (Tcorner). The cell temperature at the center is highest because the central cell is enclosed by other cells. In the CPV module, the solar cell that receives high energy light behaves as a heat source. A large value of the difference (Tcenter −Tcorner) means that the temperature distribution in the CPV module is not uniform. When the temperature distribution is not uniform, the chassis of the CPV module will become distorted owing to the local elevation in temperature. A partial misalignment between the optical system and the solar cell occurs, and the output characteristics of the CPV module decrease. The value of Tcenter − Tcorner for the CPV module without the coating reached 14.5 °C at time 13:25 h. On the other hand, that for the CPV module with the coating was reduced to 5.1 °C, which shows that the uniformity in the temperature distribution for the CPV module was considerably improved. It is considered that the coating acted not only as a thermal radiation layer but also as a thermal conduction layer. Fig. 6 shows the open-circuit voltage (Voc) of the CPV module with and without the thermal radiation coating. Voc of the CPV module
15:00:00
80 70 60 50 40 13:00:00
Without thermal radiation coating With thermal radiation coating
14:00:00
15:00:00
16:00:00
17:00:0
Time Fig. 2. Cross-sectional diagram showing the arrangement from the solar cell to aluminum chassis.
Wind Speed (m/s)
Q 12
4 4 ¼ A1 ε1 σ T 1 −T 2 ;
14:00:00
Time
Ambient Temperature ( C)
for the CPV module without the coating reached 93.1 °C at 13:16 h. On the other hand, that for the CPV module with the coating decreased to 82.6 °C. The effect of the high-radiation layer was remarkable. When we define a chassis [area: A1 (m 2), emissivity: ε1] and an environment [area: infinite], the radiation heat from the chassis to the environment Q12 (W) is given by
200 13:00:00
Fig. 4. Temperature of solar cell at the center of the CPV module.
474
K. Nishioka et al. / Surface & Coatings Technology 215 (2013) 472–475
16
1.00 With thermal radiation coating Without thermal radiation coating
12
0.95
Voc / Voc (25 oC)
Tcenter - Tcorner ( oC)
14
10 8 6 4
0.85
Without thermal radiation coating With thermal radiation coating
2 0 13:00:00
0.90
14:00:00
15:00:00
16:00:00
0.80 60
17:00:0
65
70
From Eq. (2), Voc (I = 0) is given by nD kT I ln sc þ 1 : q I0
ð3Þ
From Eq. (3), the temperature characteristic of saturation current (I0) markedly influences the temperature characteristic of Voc. The saturation current density (J0) is given by J 0 ¼ qni
2
80
85
Tave. ( C)
Fig. 5. Difference between the cell temperature at the center (Tcenter) and that at the corner (Tcorner).
V oc ¼
75 o
Time
! De Dh þ ; NA W p ND W n
ð4Þ
where ni is the intrinsic carrier concentration, NA and ND are the acceptor and donor concentrations, respectively, Wp and Wn are the thicknesses of the p and n neutral regions, respectively, and De and Dh are the diffusion constants of electrons and holes, respectively [13]. J0 strongly depends on T through its proportionality to the square of ni, which is expressed by 2 3=2 2 3 me mh exp −Eg =kT ; ni ¼ 4M c M v 2πkT=h
ð5Þ
where Mc and Mv are the number of equivalent minima in the conduction and valence bands, respectively, h is Planck's constant, and ⁎ m⁎ e and mh are the effective masses of electrons and holes, respectively [14]. From Eqs. (3)–(5), it is found that the decrease in Voc with increasing temperature arises mainly from the change in ni. The value of J0
Fig. 7. Temperature dependence on normalized Voc of CPV module.
increases exponentially with decreasing 1/T, and Voc decreases linearly with increasing T. Fig. 7 shows the temperature dependence on the normalized Voc of the CPV module. Voc was normalized with Voc at 25 °C. The temperature was the average temperature of the 25 cells in the CPV module. Voc decreases linearly with increasing temperature. The data for the CPV module with the thermal radiation coating existed in the low temperature range owing to the heat-release effect of the coating. As shown in Fig. 4, the cell temperature of the CPV module with the thermal radiation coating was 10 °C lower than that of the module without thermal radiation coating. Eventually, the temperature range of the CPV module with the thermal radiation coating was decreased by 10 °C. Fig. 8 shows the conversion efficiency of the CPV module with and without thermal radiation coating. The conversion efficiency of the CPV module with the thermal radiation coating was 0.5% higher than that of the module without the coating. The temperature coefficient of the conversion efficiency of the CPV cell has been reported as approximately − 0.05%/°C [15]. As shown in Fig. 4, the cell temperature of the CPV module with the thermal radiation coating was reduced by 10 °C. The improved value of 0.5% is therefore reasonable. A high-efficiency CPV module can be achieved with a combination of cell and module technologies. The conversion efficiency of the triple-junction solar cell for concentration approached the theoretical limit and any further improvement required a longer time [16,17]. A previous technology for CPV modules had used expensive fins as heat sinks for heat radiation [18]. In this study, a new simple coating technology for handling heat radiation was developed. By adopting the thermal radiation coating for the CPV module fabrication, the module efficiency was easily improved.
72
24 Without thermal radiation coating With thermal radiation coating
23
Eff. (%)
Voc (V)
71
70
69
22
21 Without thermal radiation coating With thermal radiation coating
68 13:00:00
14:00:00
15:00:00
16:00:00
17:00:0
Time Fig. 6. Open-circuit voltage of CPV module with and without thermal radiation coating.
20 13:00:00
14:00:00
15:00:00
16:00:00
17:00:0
Time Fig. 8. Conversion efficiency of CPV module with and without thermal radiation coating.
K. Nishioka et al. / Surface & Coatings Technology 215 (2013) 472–475
4. Conclusion A thermal radiation layer was coated on the aluminum chassis of a CPV module. The cell temperature of the CPV module with the thermal radiation coating was approximately 10 °C lower than that of the module without the thermal radiation coating. The cell temperature for the CPV module without coating reached 93.1 °C at time 13:16 h. On the other hand, that for the CPV module with coating decreased to 82.6 °C. The effect of the high-radiation layer was remarkable, and the uniformity of the temperature distribution in the CPV module was considerably improved. It is considered that the thermal radiation coating acted not only as a thermal radiation layer but also as a thermal conduction layer. The layer assisted with the thermal diffusion in the chassis. Voc of the CPV module with thermal radiation coating was 0.5 V higher than that of the module without the coating during the test period. The conversion efficiency of the CPV module with thermal radiation coating was 0.5% higher than that of the module without the coating.
Acknowledgments A part of this work was supported by the incorporated administrative agency New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI), Japan.
475
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