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The performance of silicon solar cells operated in liquids
Applied Energy 86 (2009) 1037–1042
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
The performance of silicon solar cells operated in liquids Yiping Wang a, Zhenlei Fang a,*, Li Zhu b, Qunwu Huang a, Yan Zhang a, Zhiying Zhang a a b
School of Chemical Engineering and Technology, Tianjin University, 92# Nankai District, Tianjin 300072, PR China School of Architecture, Tianjin University, China
a r t i c l e
i n f o
Article history: Received 14 March 2008 Received in revised form 21 August 2008 Accepted 28 August 2008 Available online 16 October 2008 Keywords: Liquid immersion cooling of solar cells Simulated sunlight Series and shunt resistance
a b s t r a c t Better performance can be achieved when the bare silicon solar cells are immersed into liquids for the enhanced heat removing. In this study, the performance of solar cells immersed in liquids was examined under simulated sunlight. To distinguish the effects of the liquid optic and electric properties on the solar cells, a comparison between immersion of the solar module and the bare solar cells was carried out. It was found that the optic properties of the liquids can cause minor efficiency changes on the solar cells, while the electric properties of the liquids, the molecular polarizable and ions, are responsible for the most of the changes. The bare solar cells immersed in the non-polar silicon oil have the best performance. The accelerated life tests were carried out at 150 °C high temperature and under 200 W/m2 ultraviolet light irradiation, respectively. It was found that the silicon oil has good stability. This study can give support on the cooling of the concentrated photovoltaic systems by immersing the solar cells in the liquids directly. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The concentrated photovoltaic (CPV) system is seen as a promising method to lower the cost of green power generation. Under 100-sun concentration, for example, one square centimeter of solar cell area produces the same electricity as 100 cm2 would without concentration. The use of concentration, therefore, enables the replacement of the more expensive semiconductor area with cheaper materials (e.g., lenses or mirrors). The use of concentration, however, requires that the module use a dual-axis tracking system, in addition to providing an efficient heat removal mechanism. Still, the savings in the semiconductor area and the higher output due to the use of the higher cell efficiency make the use of CPV modules more economical. Cooling of the solar cells is one of the main concerns when designing CPV systems. There are generally two ways, passive cooling and active cooling. A wide variety of passive cooling options are available, which is more suitable for single cell and linear geometries. The simplest ones involve solids of high thermal conductivity and an array of fins or other extruded surface to suit the application [1–3]. Complex systems involve phase changes and various methods for natural circulation [4]. For densely packed cells, the active cooling is the only feasible solution which forces the water or other mediums by pump in pipes fixed at the back of the modules [5–7]. However, the heat dissipation rate is limited by the thermal resistance of the bonds connected to the cells. * Corresponding author. Tel.: +86 15922089752; fax: +86 22 27404771. E-mail addresses: [email protected], [email protected] (Z. Fang). 0306-2619/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2008.08.020
With the immersion of the bare cells in liquids, the thermal resistances of the bonds are eliminated and the front surfaces of the cells are also changed to be the heat transfer areas. So the solar cells will work at lower temperature and corresponding higher conversion efficiency will be obtained. The detailed research of the liquids optic and electric properties on the performance of the solar cells is of great importance. Russell [8] has put the bare cells into an elongated tube filled with the non-conductive liquids and found that the refractive index is suitable for concentrating the incident radiation onto the cell. Christian [9] has immersed the bare cells in a coolant with an extremely low electric conductivity and the operations were carried out in the boiling range of the coolant. The gas bubbles rise from one side of a cell and pass at the rear side of another cell located above it, and the coolant is circulated mechanically. So the solar cell temperature can be effectively cooled down under the concentrated radiation. Tanaka [10] has pointed out that the dielectric liquids could work as a medium for optical concentration and increase the efficiency of the solar cells. Ugumori [11] carried out the study under the continuous wave laser, and outlined that except an efficient light collection by means of refraction and inner reflection, the photocurrent increase is due to the adsorption of the molecules polarizable which can reduce the carrier recombination at the surface of the cells. Abrahamyan’s work [12] showed that with the dielectric liquid films, the efficiency of the silicon solar cells could be 40–60% higher than the reference value. It is mainly determined by the decrease of the surface recombination velocity and the light reflection. It is good to see some work has already been done on the immersion of the bare cells in liquids, but the effects of different
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Y. Wang et al. / Applied Energy 86 (2009) 1037–1042
liquids on the bare cells are still not fully developed, neither in real data nor in theoretical analyses. In this study, the performance analysis of both the bare solar cells and solar module in air and in liquid media, respectively, is conducted. The changes of the maximum power and fill factor of the solar cells are analyzed. At last, the liquid, in which the solar cells have the highest efficiency, is chosen for the accelerated life test. 2. Experimental Fig. 1 illustrates the diagram of the immersion of the bare solar cells and solar module in liquids. The iodine–tungsten lamp was employed as the solar simulator with the intensity of 999 W/m2, measured by a pyranometer (EPPLY Model PSP). Eleven single crystalline solar cells, with an area of 2.5 105 m2, were connected in series for the immersion of the bare solar cells test. Six solar cells were connected in series and encapsulated by EVA, which were used for the immersion of solar module test. The solar cells and the solar module were both placed at the bottom of the vessel. Considering the properties of the solar cells and the cooling requirement, the liquid selected should meet the under requirements. (1) The liquid should have good heat transfer performance. (2) The absorption of the sunlight by the liquids should match with the spectral response of the solar cells. (3) The liquid should be nontoxic and have good chemical stability. (4) The liquid should be economic. In the work, six liquids were chosen, which include the polar ethanol and glycerin, the non-polar benzene and silicon oil, and the inorganic distilled water and tap water. The thickness of the liquid cover film was adjusted to detect the relevant consequences. Considering the absorptivity by the cover films [13], the thickness
of 3, 6 and 9 mm were selected in the experiment. The platinum thermo-resistances were attached to the cells to measure the temperature. The variable resistance box is applied to measure the solar cells performance and the double exponential solar cell theoretical model is utilized to fit the experimental data of solar cell and solar module I–V characteristics. The accelerated life experiment includes the ultraviolet light test and the high temperature experiment, which employed the high pressure UV–Hg lamp and thermostatic drying chamber, respectively. 3. Results and discussion 3.1. Effects of different liquids on the solar module Table 1 lists the data of the short circuit current (Isc) and the open circuit voltage (Voc) of the solar module, and also their variations under different conditions. All of the measurements were performed at the same solar cell temperature. The light intensity onto the module can be affected by its cover film. The results show that the Voc of the module is almost not changed. Whereas the module manifests certain changes in the Isc and the thicker cover films cause the lower Isc. The more changes on the Isc are mainly because the Isc is proportional to the light intensity and the Voc is the natural logarithm of the light intensity. The maximum power (Pmax) and fill factor (FF) of the solar module is 3.654 103 W and 0.711 in air, respectively. The variations of them in all the liquids are shown in the range of 2.05% to 4.46% and 0.14% to 2.53%, respectively. The solar module immersed in the glycerin, benzene and distilled water showed the similar results. It is concluded that the performance changes of the solar module have no direct relationship with the different types or kinds. As the liquids are indirectly contacted with the bare solar cells, the electric properties of the liquids almost can not have an impact on the cells performance. It is indicated that the optic prop-
a
b Simulated sunlight
Silicon solar cell
Simulated sunlight Silicon solar cell
Liquid Anode Cathode
Liquid
Cathode
Anode
solar Thermal resistance cell module
Thermal resistance
Fig. 1. Schematic of the solar cells and the solar module immersed in liquids system.
Table 1 The short current (Isc) and the open circuit voltage (Voc) of the solar module under different conditions and their changes between in air and in different liquids Medium (m)
Isc (mA)
Voc (V)
Air
1.9097
2.69
Isc changes (%) –
Voc changes (%) –
Ethanol
0.003 0.006 0.009 0.003
1.8926 1.8823 1.8610 1.9455
2.70 2.70 2.70 2.70
0.89 1.43 2.55 1.87
0.37 0.37 0.37 0.37
Silicon oil
0.006 0.009 0.003
1.9378 1.9029 1.9601
2.70 2.71 2.72
1.47 0.36 2.64
0.37 0.74 1.10
Tap water
0.006 0.009
1.8901 1.8454
2.72 2.71
1.03 3.37
1.10 0.74
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Y. Wang et al. / Applied Energy 86 (2009) 1037–1042
erties of the liquids have a minor effect on the solar cells performance. It has been pointed out that the liquids could enhance the light collection, but there were no detailed explanations [11,12]. When the module was immersed in air, part of the radiation coming at low angles is reflected, due to the cosine response, which causes the incident light intensity less than its due value. When the module was immersed in liquids, the refraction caused by immersing the module in liquids can partly rectify the cosine response. But the light intensity at low angles is very low under the simulated sunlight and therefore the light lost due to the cosine response is very small. Abrahamyan et al. [12] have studied the I–V characteristic of one solar cell (2 103 m2) under the diffused solar irradiation and two cells in parallel under the direct irradiation. The results indicated that the rate of the photocurrent increase under the diffuse radiation is more than that under the direct radiation. In our opinion, the main reason for the phenomenon is that the liquid cover films can partly correct the cosine response. Table 2 lists the refractive indexes of the different media. The refractive index of the liquid (n1) lies between that of air (n0) and that of glass (n2). When the cells are immersed in the air, the reflectivity R0 of the glass is
R0 ¼
ðn0 n2 Þ2
ð1Þ
ðn0 þ n2 Þ2
With the liquid cover film, the reflectivity Rl changes to be
Rl ¼
r21 þ r22 þ 2r 1 r2 cos 2h 1 þ r 21 þ r 22 þ 2r 1 r2 cos 2h
ð2Þ
where
r1 ¼
n0 n1 n1 n2 2pn1 d1 ; r2 ¼ ;h ¼ n0 þ n1 n1 þ n2 k
When d1 ¼ 4k obtained as
Rmin ¼
k n1
ð3Þ
; (k=1, 3, 5, 7,. . .), the minimum reflection Rmin can be
2 2 n1 n0 n2 n21 þ n0 n2
ð4Þ
where d1 is the thickness of the liquid film (m) and k is the wavelength (m). When the refraction index of the liquid (n1) was introduced into the Eq. (2), it can be deduced that Rmin 6 Rl 6 R0. So the reflectivity with the cover film is less than that without the cov-
Table 2 Refractive indexes of the media Medium
Air
Ethanol
Silicon oil
Tap water
Glass
Refractive index
1.000
1.361
1.397
1.332
1.500
er film. However, it is difficult to adjust the thickness of the cover film to the nano-scale to get the minimum reflectivity. Based on the Lambert Law, the absorbance by the liquid cover film can be written as
A ¼ lg
I0 ¼ K1b I
ð5Þ
where A is the absorbance, I0 is the incident radiance (W m2), I is the transmitted radiance (W m2), K1 is the constant of proportionality, and b is the path length of the cover film (m). The transmitted radiance I can be expressed as
I ¼ I0 10K 1 b
ð6Þ
With increasing the thickness of the cover film, the intensity of the transmitted light will be decreasing. In addition, considered the anti-reflection by the cover film, it is also hard to acquire an optimized thickness of the cover film in practice. 3.2. Effects of different liquids on the solar cells Table 3 lists the Isc and Voc of the solar cells, and also their variations under different conditions. All the measurements were carried out at the same solar cell temperature. The solar cells performance shows a regular changes with the electric properties of the liquids changing. With the same thickness of the cover film, the Isc of the solar cells in the weak electrolyte of tap water increases the most, comes after in the polar ethanol and then the non-polar silicon oil, but the Voc changes in a reverse way. The Pmax and FF of the cells in air is 6.88 103 W and 0.726, respectively. Fig. 2 illustrates the I–V characteristic and dark characteristic curves of the cells in ethanol. With the 3, 6 and 9 mm ethanol cover films, the Pmax increases 1.89%, 0.29% and 0.29%, respectively, but the FF decreased in the percentage of 8.68, 8.68 and 9.09, respectively. The dark characteristic curves show that the reverse current in ethanol is a little higher than that in air at the same voltage. So the Voc and FF of the solar cells in ethanol make a certain decrease under illumination. Fig. 3 illustrates the I–V characteristic and dark characteristic curves of the cells in silicon oil. The increase of Isc is less than that of in ethanol. However, the Pmax increases 3.78%, 3.49% and 4.07%, respectively, and the FF change 0.14%, 0.55% and 2.20%, respectively. The dark characteristics of the cells show that the reverse current is almost the same as that of in air. So the Voc and FF of the solar cells in silicon oil is almost the same as that in air under illumination. Fig. 4 illustrates the I–V characteristic and dark characteristic curves of cells in tap water. The Pmax decreases 42.59%, 48.26% and 52.03%, respectively, and the FF decreases 45.04%, 49.17% and 50.55%, respectively. The dark characteristic curves show that the increase of the reverse current in darkness is larger than that in
Table 3 The short current (Isc) and the open circuit voltage (Voc) of the solar cells under different conditions and their changes between in air and in different liquids Medium (m)
Isc (mA)
Voc (V)
Air
1.8407
0.515
Isc changes (%) –
Voc changes (%) –
Ethanol
0.003 0.006 0.009 0.003
2.0625 2.0420 2.0420 1.9113
0.513 0.510 0.509 0.511
12.05 10.94 10.94 3.83
0.39 0.97 1.16 0.78
Silicon oil
0.006 0.009 0.003
1.9004 1.9009 2.3527
0.514 0.514 0.421
3.24 3.27 27.81
0.19 0.19 18.25
Tap water
0.006 0.009
2.3992 2.3649
0.402 0.389
30.34 28.49
21.94 24.47
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2.0
2.0
Current (mA)
Current (mA)
Dark characteristic curves in darkness
1.5 0.003 m EtOH film 0.006 m EtOH film 0.009 m EtOH film Non-liquid film
1.0 0.5
1.5 0.003 m EtOH film 0.006 m EtOH film 0.009 m EtOH film Non-liquid film
1.0 0.5
I-V characteristic curves under illumination
0.0
0.0 0
1
2
3
4
5
6
0
1
2
Voltage (V)
3
4
5
6
5
6
5
6
Voltage (V)
Fig. 2. The I–V and dark characteristic curves of the solar cells immersed in ethanol.
2.0
2.0
1.5
1.5
Current (mA)
Current (mA)
Dark characteristic curves in darkness
0.003 m silicon film 0.006 m silicon film 0.009 m silicon film Non-liquid film
1.0 0.5
1.0 0.003 m silicon film 0.006 m silicon film 0.009 m silicon film Non-liquid film
0.5
I-V characteristic curvesun derillumination
0.0
0.0 0
1
2
3
4
5
0
6
1
2
3
4
Voltage (V)
Voltage (V)
Fig. 3. The I–V and dark characteristic curves of the solar cells immersed in silicon oil.
2.5
4
I-V characteristic curves under illumination
Current (mA)
Current (mA)
Darkcharacteristic curves in darkness
2.0 1.5 1.0
0.003 m H2O film 0.006 m H2O film
0.5
3 0.003 m H2O film
2
0.006 m H2O film 0.009 m H2O film Non-liquid film
1
0.009 m H2O film Non-liquid fim
0
0.0 0
1
2
3
4
5
Voltage (V)
6
0
1
2
3
4
Voltage (V)
Fig. 4. The I–V and dark characteristic curves of the solar cells immersed in tap water.
all the other conditions. So the Voc and FF of the solar cells in tap water decrease the most under illumination. The performance of the cells in the glycerin, benzene and distilled water is similar as that illustrated above. The reverse current in darkness has increased most in tap water, comes after in polar ethanol, and then non-polar liquid silicon oil, which is the same as the Isc changing under illumination. And the changes of the reverse current in darkness can illustrate the dark current changes for the solar cells under illumination. It is indicated that the changes of the solar cells performance are related with the electric properties of the liquids and the solar cells immersed in the nonpolar silicon oil have the best performance. Tadaki’s research [11] suggested that the increase of the short current in the polar liquids was more than that in the non-polar liquids, and also the same to the reverse current in darkness, which
is consistent with our results. The changes of the Voc and FF have not been given in his research. However, our research shows that these factors are changed in contrary with the Isc. 3.3. Accelerated life test The stability is also very important when using liquids as the encapsulant materials. In general, the pottant is ethylene vinyl acetate (EVA) and the potential degradation of an encapsulated PV cell is complex, because different thermo-chemical reactions can occur at the polymer/superstrate and polymer/M/SiO2 interfaces as well as in the polymer [14]. The solar cells immersed in silicon oil have the best performance and at the same time the silicon oil is able to perform several unique properties that can be retained over wide temperature
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range: high chemical stability, good weather resistance and low surface tension. These unique physical and chemical performances make silicon oil well suited for applications that experience large temperature swings and long service lifetimes. The high temperature and ultraviolet radiation are the most important two factors affecting the life time of the modules. The high temperature experiment at 150 °C has been carried out and its duration is 200 h. The ultraviolet radiation experiment, with the intensities of 200 W/m2, has also been carried out and its duration is 120 h. Fig. 5 is the transmittance of the silicon oil after the 150 °C high temperature and ultraviolet experiment, measured by the UV–Vis spectrophotometer. The spectral response of the silicon solar cells ranges from 400 nm to 1100 nm. Compared with the transmittance before the experiment, the transmittance of the nitrogen seal and air seal after high temperature experiment has decreased 1.53% and 1.78%, respectively, and the transmittance of the nitrogen seal and air seal after ultraviolet experiment has decreased 0.57% and 0.59%, respectively. The performance of the solar cells has almost not changed between before and after the experiment. It is indicated that the silicon oil has good tolerance on high temperature and ultraviolet radiation. 3.4. Liquids properties influence on the solar cell performance The changes of the solar cells performance are more than those of the solar module when they are immersed in liquids. It is thought that the electric properties of the liquids play an important role on this phenomenon. Based on the physical model of the solar cell, the theoretical analysis is carried out. The double exponential model is a detailed physic-mathematical approach to express the I–V characteristics of the solar cell, where the diffusion current and recombination current are represented by two diodes with different exponential behavior and become more closely related to the physical phenomena. The model is mainly expressed as
qðV þ IRs Þ I ¼ IL I01 exp 1 A1 KT qðV þ IRs Þ V þ IRs 1 I02 exp A2 KT Rsh
ð7Þ
100
95 N2
90
85
300
Air
400
500
600
700
Wavelength (nm)
800
Rs
900
dV dI I¼0
Rsh
ð9Þ
dV dI I¼Isc
ð10Þ
It shows that, at the 0 V in Figs. 3–5, the slopes of the I–V curves in liquids are less than those in air, especially in ethanol and tap water. So the Rsh of the cells has decreased and it can reduce the Voc and FF of the cells, but it has a relative small effect on the Isc. However, the slopes of the I–V curves at the zero current in all the liquids are almost the same as that in air except in the tap water. The molecular polarizable and ions in the liquids could be attracted to the front and back surface of the cells, when the cells work in the liquids. The electric fields both at the front and the back surfaces are formed and their directions are identical to the built-in electric field in the cell. For the silicon solar cell is indirect transition materials, the surface recombination at the back surface is also very important. So the electric fields at both surfaces can suppress the surface recombination. However, the electric fields can also cause electric leakage, which can result in the decrease of the Rsh. The non-polar and polar molecular have different permanent electric moment l, so different intensities of the electric fields are formed with different liquids. It can form the highest electric fields in the tap water for the ions in the liquids, comes after the polar liquids and last the non-polar liquids. And the intensities of the fields have an influence on the suppression extent on surface recombination and the rate decrease of the Rsh. Considering both the effects by the liquids, the solar cell immersed in non-polar liquids have the best performance. 3.5. Liquids direct cooling method in CPV system The study mainly focused on selecting the suitable liquids to directly cool the solar cells. The results showed that the non-polar liquid has beneficial influence on the solar cell performance. However, some other factors should be considered for selecting the most suitable liquids at specific conditions, such as the solar cell working temperature range, heat dissipation performance and the cost of the liquids. In our future study, we will pay more attention to the heat transfer performance of this new cooling method. In its practical application, the bare solar cells are fixed in the liquids directly. It can form circulation flow by the extra power, so the solar cells can be cooled by both the cell’s surfaces. This method can eliminate the thermal resistance of the dielectric layer between the solar cell and radiator which is the main thermal
100
95
N2
90
Air
85
150 ˚C high temperature experiment
ð8Þ
Deduced from the Eq. (7), the Rs and Rsh can be expressed as
Transmisttance (%)
Transmisttance (%)
where IL is the photo-current, I01 exponential term is the Shockley diffusion current which includes the electronic conduction phenomena in the quasi-neutral region of the junction and the I02 exponential term corresponds to the carrier recombination through deep levels in the space-charge region of the junction and in the device surface. In addition, the model includes the series and the shunt resistances, Rs and Rsh, respectively, and other classical diffusion and recombination diode ideality factors, A1 and A2 respectively.
Conventionally; Rs =Rsh 1
300
Ultraviolet radiation experiment
400
500
600
700
Wavelength (nm)
Fig. 5. The transmittance of the silicon oil after the accelerated life test.
800
900
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Y. Wang et al. / Applied Energy 86 (2009) 1037–1042
resistance in the traditional cooling ways. So the heat generated by the cells can dissipate effectively. The solar cell temperature can be reduced and its efficiency is improved. So it can lower the cost of the systems and supply stability cooling method in the CPV systems. 4. Conclusion The goal of this work is to analyze the effects of liquid categories and the thickness of the cover films on the power output of the bare silicon solar cells under simulated sunlight. The comparison of the I–V characteristics of the solar module immersed in air and liquids shows that the cover films on the solar cell model can work as an anti-reflection film, but they can also absorb part of light. About these two factors, the absorptivity by the cover film plays an important role, especially with the thicker films. The effects of different liquids on the characteristics of the solar cells have been investigated and the results show that the Isc have increased and the Voc have decreased when the solar cells immersed in different kinds of liquids. The ions and molecules polarizable in liquids can form electric fields at the cell’s front and back surfaces, which are the same direction with the built-in electric field and they can suppress the surface recombination. However, the electric fields can also cause electric leakage and decrease the Rsh of the solar cell. The intensities of the electric fields can determine the rate of the suppression on surface recombination and also the decrease rate of the Rsh. In conclusion, the solar cells immersed in the nonpolar silicon oil have the best performance and the silicon oil also has good stability for the encapsulation of the solar cells. Acknowledgements The authors would like to thank Prof. Martin Green (University of New South Wales) for the fruitful discussions in SWC2007, and
also thank Post-doctoral Wei Tian (University of Calgary, Canada) for critical reading of the manuscript. The research is supported by Tianjin Technological Development Program Project of China (05YFGZSF02800 and 06YFSZSF04600) and the Key Research Program of the National ‘‘Eleventh Five-year Plan” of China (2006BAA04B03-03). References [1] Minano JC, Gonzalez JC, Zanesco I. Flat high concentration devices. In: 24th IEEE PVSC, vol. 1. Hawaii; 1994. p. 1123–26. [2] Araki K, Uozumi H, Yamaguchi M. A simple passive cooling structure and its heat analysis for 500x concentrator PV module, In: 29th IEEE PVSC, 2002. p. 1568–71. [3] Luque A, Sala G, Arboiro JC, Bruton T, Cunningham D, Mason N. Some results of the EUCLIDES photovoltaic concentrator prototype. Prog Photovoltaics Res 1997;5(3):195–212. [4] Akbarzadeh A, Wadowski T. Heat pipe-based cooling systems for photovoltaic cells under concentrated solar radiation. Appl Therm Eng 1996;16:81–7. [5] Tilford CL, Sinton RA, Swanson RM, Crane RA, Verlinden P. Development of a 10 kW reflective dish PV system. In: 23rd IEEE PVSC, 1993. p. 1222–27. [6] Lasich JB. Cooling circuit for receiver of solar radiation. Patent WO02080286; 2002. [7] Verlinden PJ, Terao A, Smith DD, McIntosh K, Swanson RM, Ganakas G, et al. Will we have a 20%-efficient (PTC) photovoltaic system? In: Proceedings of the 17th european photovoltaic solar energy conference; 2001. [8] Russell CR. Optical concentrator and cooling system for photovoltaic cells. Patent US 4052228; 1981. [9] Christian KH. Cooling photovoltaic (PV) cells during concentrated solar radiation in specified arrangement in coolant with as low electric conductivity as possible. Patent DE 19904717; 2000 [in German]. [10] Tanaka K. Solar energy converter using a solar cell in a shallow liquid layer. Patent US 6583349B2; 2003. [11] Ugumori T, Ikeya M. Efficiency increase of solar cells operated in dielectric liquid. Jpn Appl Phys 1981;20:77–80. [12] Abrahamyan YA, Serago VI, Aroutiounian VM, Anisimova ID, Stafeev VI, Karamian GG, et al. The efficiency of solar cells immersed in liquid dielectrics. Sol Energy Mater Sol Cells 2002;73:367–75. [13] Muaddi JA, Jamal MA. Spectral response and efficiency of a silicon solar cell below water surface. Sol Energy 1992;49:29–33. [14] Czanderna AW, Pern FJ. Encapsulation of PV modules using ethylene vinylacetate copolymer as a pottant: a critical review. Sol Energy Mater Sol Cells 1996;43:101–81.
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
The performance of silicon solar cells operated in liquids Yiping Wang a, Zhenlei Fang a,*, Li Zhu b, Qunwu Huang a, Yan Zhang a, Zhiying Zhang a a b
School of Chemical Engineering and Technology, Tianjin University, 92# Nankai District, Tianjin 300072, PR China School of Architecture, Tianjin University, China
a r t i c l e
i n f o
Article history: Received 14 March 2008 Received in revised form 21 August 2008 Accepted 28 August 2008 Available online 16 October 2008 Keywords: Liquid immersion cooling of solar cells Simulated sunlight Series and shunt resistance
a b s t r a c t Better performance can be achieved when the bare silicon solar cells are immersed into liquids for the enhanced heat removing. In this study, the performance of solar cells immersed in liquids was examined under simulated sunlight. To distinguish the effects of the liquid optic and electric properties on the solar cells, a comparison between immersion of the solar module and the bare solar cells was carried out. It was found that the optic properties of the liquids can cause minor efficiency changes on the solar cells, while the electric properties of the liquids, the molecular polarizable and ions, are responsible for the most of the changes. The bare solar cells immersed in the non-polar silicon oil have the best performance. The accelerated life tests were carried out at 150 °C high temperature and under 200 W/m2 ultraviolet light irradiation, respectively. It was found that the silicon oil has good stability. This study can give support on the cooling of the concentrated photovoltaic systems by immersing the solar cells in the liquids directly. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The concentrated photovoltaic (CPV) system is seen as a promising method to lower the cost of green power generation. Under 100-sun concentration, for example, one square centimeter of solar cell area produces the same electricity as 100 cm2 would without concentration. The use of concentration, therefore, enables the replacement of the more expensive semiconductor area with cheaper materials (e.g., lenses or mirrors). The use of concentration, however, requires that the module use a dual-axis tracking system, in addition to providing an efficient heat removal mechanism. Still, the savings in the semiconductor area and the higher output due to the use of the higher cell efficiency make the use of CPV modules more economical. Cooling of the solar cells is one of the main concerns when designing CPV systems. There are generally two ways, passive cooling and active cooling. A wide variety of passive cooling options are available, which is more suitable for single cell and linear geometries. The simplest ones involve solids of high thermal conductivity and an array of fins or other extruded surface to suit the application [1–3]. Complex systems involve phase changes and various methods for natural circulation [4]. For densely packed cells, the active cooling is the only feasible solution which forces the water or other mediums by pump in pipes fixed at the back of the modules [5–7]. However, the heat dissipation rate is limited by the thermal resistance of the bonds connected to the cells. * Corresponding author. Tel.: +86 15922089752; fax: +86 22 27404771. E-mail addresses: [email protected], [email protected] (Z. Fang). 0306-2619/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2008.08.020
With the immersion of the bare cells in liquids, the thermal resistances of the bonds are eliminated and the front surfaces of the cells are also changed to be the heat transfer areas. So the solar cells will work at lower temperature and corresponding higher conversion efficiency will be obtained. The detailed research of the liquids optic and electric properties on the performance of the solar cells is of great importance. Russell [8] has put the bare cells into an elongated tube filled with the non-conductive liquids and found that the refractive index is suitable for concentrating the incident radiation onto the cell. Christian [9] has immersed the bare cells in a coolant with an extremely low electric conductivity and the operations were carried out in the boiling range of the coolant. The gas bubbles rise from one side of a cell and pass at the rear side of another cell located above it, and the coolant is circulated mechanically. So the solar cell temperature can be effectively cooled down under the concentrated radiation. Tanaka [10] has pointed out that the dielectric liquids could work as a medium for optical concentration and increase the efficiency of the solar cells. Ugumori [11] carried out the study under the continuous wave laser, and outlined that except an efficient light collection by means of refraction and inner reflection, the photocurrent increase is due to the adsorption of the molecules polarizable which can reduce the carrier recombination at the surface of the cells. Abrahamyan’s work [12] showed that with the dielectric liquid films, the efficiency of the silicon solar cells could be 40–60% higher than the reference value. It is mainly determined by the decrease of the surface recombination velocity and the light reflection. It is good to see some work has already been done on the immersion of the bare cells in liquids, but the effects of different
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liquids on the bare cells are still not fully developed, neither in real data nor in theoretical analyses. In this study, the performance analysis of both the bare solar cells and solar module in air and in liquid media, respectively, is conducted. The changes of the maximum power and fill factor of the solar cells are analyzed. At last, the liquid, in which the solar cells have the highest efficiency, is chosen for the accelerated life test. 2. Experimental Fig. 1 illustrates the diagram of the immersion of the bare solar cells and solar module in liquids. The iodine–tungsten lamp was employed as the solar simulator with the intensity of 999 W/m2, measured by a pyranometer (EPPLY Model PSP). Eleven single crystalline solar cells, with an area of 2.5 105 m2, were connected in series for the immersion of the bare solar cells test. Six solar cells were connected in series and encapsulated by EVA, which were used for the immersion of solar module test. The solar cells and the solar module were both placed at the bottom of the vessel. Considering the properties of the solar cells and the cooling requirement, the liquid selected should meet the under requirements. (1) The liquid should have good heat transfer performance. (2) The absorption of the sunlight by the liquids should match with the spectral response of the solar cells. (3) The liquid should be nontoxic and have good chemical stability. (4) The liquid should be economic. In the work, six liquids were chosen, which include the polar ethanol and glycerin, the non-polar benzene and silicon oil, and the inorganic distilled water and tap water. The thickness of the liquid cover film was adjusted to detect the relevant consequences. Considering the absorptivity by the cover films [13], the thickness
of 3, 6 and 9 mm were selected in the experiment. The platinum thermo-resistances were attached to the cells to measure the temperature. The variable resistance box is applied to measure the solar cells performance and the double exponential solar cell theoretical model is utilized to fit the experimental data of solar cell and solar module I–V characteristics. The accelerated life experiment includes the ultraviolet light test and the high temperature experiment, which employed the high pressure UV–Hg lamp and thermostatic drying chamber, respectively. 3. Results and discussion 3.1. Effects of different liquids on the solar module Table 1 lists the data of the short circuit current (Isc) and the open circuit voltage (Voc) of the solar module, and also their variations under different conditions. All of the measurements were performed at the same solar cell temperature. The light intensity onto the module can be affected by its cover film. The results show that the Voc of the module is almost not changed. Whereas the module manifests certain changes in the Isc and the thicker cover films cause the lower Isc. The more changes on the Isc are mainly because the Isc is proportional to the light intensity and the Voc is the natural logarithm of the light intensity. The maximum power (Pmax) and fill factor (FF) of the solar module is 3.654 103 W and 0.711 in air, respectively. The variations of them in all the liquids are shown in the range of 2.05% to 4.46% and 0.14% to 2.53%, respectively. The solar module immersed in the glycerin, benzene and distilled water showed the similar results. It is concluded that the performance changes of the solar module have no direct relationship with the different types or kinds. As the liquids are indirectly contacted with the bare solar cells, the electric properties of the liquids almost can not have an impact on the cells performance. It is indicated that the optic prop-
a
b Simulated sunlight
Silicon solar cell
Simulated sunlight Silicon solar cell
Liquid Anode Cathode
Liquid
Cathode
Anode
solar Thermal resistance cell module
Thermal resistance
Fig. 1. Schematic of the solar cells and the solar module immersed in liquids system.
Table 1 The short current (Isc) and the open circuit voltage (Voc) of the solar module under different conditions and their changes between in air and in different liquids Medium (m)
Isc (mA)
Voc (V)
Air
1.9097
2.69
Isc changes (%) –
Voc changes (%) –
Ethanol
0.003 0.006 0.009 0.003
1.8926 1.8823 1.8610 1.9455
2.70 2.70 2.70 2.70
0.89 1.43 2.55 1.87
0.37 0.37 0.37 0.37
Silicon oil
0.006 0.009 0.003
1.9378 1.9029 1.9601
2.70 2.71 2.72
1.47 0.36 2.64
0.37 0.74 1.10
Tap water
0.006 0.009
1.8901 1.8454
2.72 2.71
1.03 3.37
1.10 0.74
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erties of the liquids have a minor effect on the solar cells performance. It has been pointed out that the liquids could enhance the light collection, but there were no detailed explanations [11,12]. When the module was immersed in air, part of the radiation coming at low angles is reflected, due to the cosine response, which causes the incident light intensity less than its due value. When the module was immersed in liquids, the refraction caused by immersing the module in liquids can partly rectify the cosine response. But the light intensity at low angles is very low under the simulated sunlight and therefore the light lost due to the cosine response is very small. Abrahamyan et al. [12] have studied the I–V characteristic of one solar cell (2 103 m2) under the diffused solar irradiation and two cells in parallel under the direct irradiation. The results indicated that the rate of the photocurrent increase under the diffuse radiation is more than that under the direct radiation. In our opinion, the main reason for the phenomenon is that the liquid cover films can partly correct the cosine response. Table 2 lists the refractive indexes of the different media. The refractive index of the liquid (n1) lies between that of air (n0) and that of glass (n2). When the cells are immersed in the air, the reflectivity R0 of the glass is
R0 ¼
ðn0 n2 Þ2
ð1Þ
ðn0 þ n2 Þ2
With the liquid cover film, the reflectivity Rl changes to be
Rl ¼
r21 þ r22 þ 2r 1 r2 cos 2h 1 þ r 21 þ r 22 þ 2r 1 r2 cos 2h
ð2Þ
where
r1 ¼
n0 n1 n1 n2 2pn1 d1 ; r2 ¼ ;h ¼ n0 þ n1 n1 þ n2 k
When d1 ¼ 4k obtained as
Rmin ¼
k n1
ð3Þ
; (k=1, 3, 5, 7,. . .), the minimum reflection Rmin can be
2 2 n1 n0 n2 n21 þ n0 n2
ð4Þ
where d1 is the thickness of the liquid film (m) and k is the wavelength (m). When the refraction index of the liquid (n1) was introduced into the Eq. (2), it can be deduced that Rmin 6 Rl 6 R0. So the reflectivity with the cover film is less than that without the cov-
Table 2 Refractive indexes of the media Medium
Air
Ethanol
Silicon oil
Tap water
Glass
Refractive index
1.000
1.361
1.397
1.332
1.500
er film. However, it is difficult to adjust the thickness of the cover film to the nano-scale to get the minimum reflectivity. Based on the Lambert Law, the absorbance by the liquid cover film can be written as
A ¼ lg
I0 ¼ K1b I
ð5Þ
where A is the absorbance, I0 is the incident radiance (W m2), I is the transmitted radiance (W m2), K1 is the constant of proportionality, and b is the path length of the cover film (m). The transmitted radiance I can be expressed as
I ¼ I0 10K 1 b
ð6Þ
With increasing the thickness of the cover film, the intensity of the transmitted light will be decreasing. In addition, considered the anti-reflection by the cover film, it is also hard to acquire an optimized thickness of the cover film in practice. 3.2. Effects of different liquids on the solar cells Table 3 lists the Isc and Voc of the solar cells, and also their variations under different conditions. All the measurements were carried out at the same solar cell temperature. The solar cells performance shows a regular changes with the electric properties of the liquids changing. With the same thickness of the cover film, the Isc of the solar cells in the weak electrolyte of tap water increases the most, comes after in the polar ethanol and then the non-polar silicon oil, but the Voc changes in a reverse way. The Pmax and FF of the cells in air is 6.88 103 W and 0.726, respectively. Fig. 2 illustrates the I–V characteristic and dark characteristic curves of the cells in ethanol. With the 3, 6 and 9 mm ethanol cover films, the Pmax increases 1.89%, 0.29% and 0.29%, respectively, but the FF decreased in the percentage of 8.68, 8.68 and 9.09, respectively. The dark characteristic curves show that the reverse current in ethanol is a little higher than that in air at the same voltage. So the Voc and FF of the solar cells in ethanol make a certain decrease under illumination. Fig. 3 illustrates the I–V characteristic and dark characteristic curves of the cells in silicon oil. The increase of Isc is less than that of in ethanol. However, the Pmax increases 3.78%, 3.49% and 4.07%, respectively, and the FF change 0.14%, 0.55% and 2.20%, respectively. The dark characteristics of the cells show that the reverse current is almost the same as that of in air. So the Voc and FF of the solar cells in silicon oil is almost the same as that in air under illumination. Fig. 4 illustrates the I–V characteristic and dark characteristic curves of cells in tap water. The Pmax decreases 42.59%, 48.26% and 52.03%, respectively, and the FF decreases 45.04%, 49.17% and 50.55%, respectively. The dark characteristic curves show that the increase of the reverse current in darkness is larger than that in
Table 3 The short current (Isc) and the open circuit voltage (Voc) of the solar cells under different conditions and their changes between in air and in different liquids Medium (m)
Isc (mA)
Voc (V)
Air
1.8407
0.515
Isc changes (%) –
Voc changes (%) –
Ethanol
0.003 0.006 0.009 0.003
2.0625 2.0420 2.0420 1.9113
0.513 0.510 0.509 0.511
12.05 10.94 10.94 3.83
0.39 0.97 1.16 0.78
Silicon oil
0.006 0.009 0.003
1.9004 1.9009 2.3527
0.514 0.514 0.421
3.24 3.27 27.81
0.19 0.19 18.25
Tap water
0.006 0.009
2.3992 2.3649
0.402 0.389
30.34 28.49
21.94 24.47
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2.0
2.0
Current (mA)
Current (mA)
Dark characteristic curves in darkness
1.5 0.003 m EtOH film 0.006 m EtOH film 0.009 m EtOH film Non-liquid film
1.0 0.5
1.5 0.003 m EtOH film 0.006 m EtOH film 0.009 m EtOH film Non-liquid film
1.0 0.5
I-V characteristic curves under illumination
0.0
0.0 0
1
2
3
4
5
6
0
1
2
Voltage (V)
3
4
5
6
5
6
5
6
Voltage (V)
Fig. 2. The I–V and dark characteristic curves of the solar cells immersed in ethanol.
2.0
2.0
1.5
1.5
Current (mA)
Current (mA)
Dark characteristic curves in darkness
0.003 m silicon film 0.006 m silicon film 0.009 m silicon film Non-liquid film
1.0 0.5
1.0 0.003 m silicon film 0.006 m silicon film 0.009 m silicon film Non-liquid film
0.5
I-V characteristic curvesun derillumination
0.0
0.0 0
1
2
3
4
5
0
6
1
2
3
4
Voltage (V)
Voltage (V)
Fig. 3. The I–V and dark characteristic curves of the solar cells immersed in silicon oil.
2.5
4
I-V characteristic curves under illumination
Current (mA)
Current (mA)
Darkcharacteristic curves in darkness
2.0 1.5 1.0
0.003 m H2O film 0.006 m H2O film
0.5
3 0.003 m H2O film
2
0.006 m H2O film 0.009 m H2O film Non-liquid film
1
0.009 m H2O film Non-liquid fim
0
0.0 0
1
2
3
4
5
Voltage (V)
6
0
1
2
3
4
Voltage (V)
Fig. 4. The I–V and dark characteristic curves of the solar cells immersed in tap water.
all the other conditions. So the Voc and FF of the solar cells in tap water decrease the most under illumination. The performance of the cells in the glycerin, benzene and distilled water is similar as that illustrated above. The reverse current in darkness has increased most in tap water, comes after in polar ethanol, and then non-polar liquid silicon oil, which is the same as the Isc changing under illumination. And the changes of the reverse current in darkness can illustrate the dark current changes for the solar cells under illumination. It is indicated that the changes of the solar cells performance are related with the electric properties of the liquids and the solar cells immersed in the nonpolar silicon oil have the best performance. Tadaki’s research [11] suggested that the increase of the short current in the polar liquids was more than that in the non-polar liquids, and also the same to the reverse current in darkness, which
is consistent with our results. The changes of the Voc and FF have not been given in his research. However, our research shows that these factors are changed in contrary with the Isc. 3.3. Accelerated life test The stability is also very important when using liquids as the encapsulant materials. In general, the pottant is ethylene vinyl acetate (EVA) and the potential degradation of an encapsulated PV cell is complex, because different thermo-chemical reactions can occur at the polymer/superstrate and polymer/M/SiO2 interfaces as well as in the polymer [14]. The solar cells immersed in silicon oil have the best performance and at the same time the silicon oil is able to perform several unique properties that can be retained over wide temperature
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range: high chemical stability, good weather resistance and low surface tension. These unique physical and chemical performances make silicon oil well suited for applications that experience large temperature swings and long service lifetimes. The high temperature and ultraviolet radiation are the most important two factors affecting the life time of the modules. The high temperature experiment at 150 °C has been carried out and its duration is 200 h. The ultraviolet radiation experiment, with the intensities of 200 W/m2, has also been carried out and its duration is 120 h. Fig. 5 is the transmittance of the silicon oil after the 150 °C high temperature and ultraviolet experiment, measured by the UV–Vis spectrophotometer. The spectral response of the silicon solar cells ranges from 400 nm to 1100 nm. Compared with the transmittance before the experiment, the transmittance of the nitrogen seal and air seal after high temperature experiment has decreased 1.53% and 1.78%, respectively, and the transmittance of the nitrogen seal and air seal after ultraviolet experiment has decreased 0.57% and 0.59%, respectively. The performance of the solar cells has almost not changed between before and after the experiment. It is indicated that the silicon oil has good tolerance on high temperature and ultraviolet radiation. 3.4. Liquids properties influence on the solar cell performance The changes of the solar cells performance are more than those of the solar module when they are immersed in liquids. It is thought that the electric properties of the liquids play an important role on this phenomenon. Based on the physical model of the solar cell, the theoretical analysis is carried out. The double exponential model is a detailed physic-mathematical approach to express the I–V characteristics of the solar cell, where the diffusion current and recombination current are represented by two diodes with different exponential behavior and become more closely related to the physical phenomena. The model is mainly expressed as
qðV þ IRs Þ I ¼ IL I01 exp 1 A1 KT qðV þ IRs Þ V þ IRs 1 I02 exp A2 KT Rsh
ð7Þ
100
95 N2
90
85
300
Air
400
500
600
700
Wavelength (nm)
800
Rs
900
dV dI I¼0
Rsh
ð9Þ
dV dI I¼Isc
ð10Þ
It shows that, at the 0 V in Figs. 3–5, the slopes of the I–V curves in liquids are less than those in air, especially in ethanol and tap water. So the Rsh of the cells has decreased and it can reduce the Voc and FF of the cells, but it has a relative small effect on the Isc. However, the slopes of the I–V curves at the zero current in all the liquids are almost the same as that in air except in the tap water. The molecular polarizable and ions in the liquids could be attracted to the front and back surface of the cells, when the cells work in the liquids. The electric fields both at the front and the back surfaces are formed and their directions are identical to the built-in electric field in the cell. For the silicon solar cell is indirect transition materials, the surface recombination at the back surface is also very important. So the electric fields at both surfaces can suppress the surface recombination. However, the electric fields can also cause electric leakage, which can result in the decrease of the Rsh. The non-polar and polar molecular have different permanent electric moment l, so different intensities of the electric fields are formed with different liquids. It can form the highest electric fields in the tap water for the ions in the liquids, comes after the polar liquids and last the non-polar liquids. And the intensities of the fields have an influence on the suppression extent on surface recombination and the rate decrease of the Rsh. Considering both the effects by the liquids, the solar cell immersed in non-polar liquids have the best performance. 3.5. Liquids direct cooling method in CPV system The study mainly focused on selecting the suitable liquids to directly cool the solar cells. The results showed that the non-polar liquid has beneficial influence on the solar cell performance. However, some other factors should be considered for selecting the most suitable liquids at specific conditions, such as the solar cell working temperature range, heat dissipation performance and the cost of the liquids. In our future study, we will pay more attention to the heat transfer performance of this new cooling method. In its practical application, the bare solar cells are fixed in the liquids directly. It can form circulation flow by the extra power, so the solar cells can be cooled by both the cell’s surfaces. This method can eliminate the thermal resistance of the dielectric layer between the solar cell and radiator which is the main thermal
100
95
N2
90
Air
85
150 ˚C high temperature experiment
ð8Þ
Deduced from the Eq. (7), the Rs and Rsh can be expressed as
Transmisttance (%)
Transmisttance (%)
where IL is the photo-current, I01 exponential term is the Shockley diffusion current which includes the electronic conduction phenomena in the quasi-neutral region of the junction and the I02 exponential term corresponds to the carrier recombination through deep levels in the space-charge region of the junction and in the device surface. In addition, the model includes the series and the shunt resistances, Rs and Rsh, respectively, and other classical diffusion and recombination diode ideality factors, A1 and A2 respectively.
Conventionally; Rs =Rsh 1
300
Ultraviolet radiation experiment
400
500
600
700
Wavelength (nm)
Fig. 5. The transmittance of the silicon oil after the accelerated life test.
800
900
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resistance in the traditional cooling ways. So the heat generated by the cells can dissipate effectively. The solar cell temperature can be reduced and its efficiency is improved. So it can lower the cost of the systems and supply stability cooling method in the CPV systems. 4. Conclusion The goal of this work is to analyze the effects of liquid categories and the thickness of the cover films on the power output of the bare silicon solar cells under simulated sunlight. The comparison of the I–V characteristics of the solar module immersed in air and liquids shows that the cover films on the solar cell model can work as an anti-reflection film, but they can also absorb part of light. About these two factors, the absorptivity by the cover film plays an important role, especially with the thicker films. The effects of different liquids on the characteristics of the solar cells have been investigated and the results show that the Isc have increased and the Voc have decreased when the solar cells immersed in different kinds of liquids. The ions and molecules polarizable in liquids can form electric fields at the cell’s front and back surfaces, which are the same direction with the built-in electric field and they can suppress the surface recombination. However, the electric fields can also cause electric leakage and decrease the Rsh of the solar cell. The intensities of the electric fields can determine the rate of the suppression on surface recombination and also the decrease rate of the Rsh. In conclusion, the solar cells immersed in the nonpolar silicon oil have the best performance and the silicon oil also has good stability for the encapsulation of the solar cells. Acknowledgements The authors would like to thank Prof. Martin Green (University of New South Wales) for the fruitful discussions in SWC2007, and
also thank Post-doctoral Wei Tian (University of Calgary, Canada) for critical reading of the manuscript. The research is supported by Tianjin Technological Development Program Project of China (05YFGZSF02800 and 06YFSZSF04600) and the Key Research Program of the National ‘‘Eleventh Five-year Plan” of China (2006BAA04B03-03). References [1] Minano JC, Gonzalez JC, Zanesco I. Flat high concentration devices. In: 24th IEEE PVSC, vol. 1. Hawaii; 1994. p. 1123–26. [2] Araki K, Uozumi H, Yamaguchi M. A simple passive cooling structure and its heat analysis for 500x concentrator PV module, In: 29th IEEE PVSC, 2002. p. 1568–71. [3] Luque A, Sala G, Arboiro JC, Bruton T, Cunningham D, Mason N. Some results of the EUCLIDES photovoltaic concentrator prototype. Prog Photovoltaics Res 1997;5(3):195–212. [4] Akbarzadeh A, Wadowski T. Heat pipe-based cooling systems for photovoltaic cells under concentrated solar radiation. Appl Therm Eng 1996;16:81–7. [5] Tilford CL, Sinton RA, Swanson RM, Crane RA, Verlinden P. Development of a 10 kW reflective dish PV system. In: 23rd IEEE PVSC, 1993. p. 1222–27. [6] Lasich JB. Cooling circuit for receiver of solar radiation. Patent WO02080286; 2002. [7] Verlinden PJ, Terao A, Smith DD, McIntosh K, Swanson RM, Ganakas G, et al. Will we have a 20%-efficient (PTC) photovoltaic system? In: Proceedings of the 17th european photovoltaic solar energy conference; 2001. [8] Russell CR. Optical concentrator and cooling system for photovoltaic cells. Patent US 4052228; 1981. [9] Christian KH. Cooling photovoltaic (PV) cells during concentrated solar radiation in specified arrangement in coolant with as low electric conductivity as possible. Patent DE 19904717; 2000 [in German]. [10] Tanaka K. Solar energy converter using a solar cell in a shallow liquid layer. Patent US 6583349B2; 2003. [11] Ugumori T, Ikeya M. Efficiency increase of solar cells operated in dielectric liquid. Jpn Appl Phys 1981;20:77–80. [12] Abrahamyan YA, Serago VI, Aroutiounian VM, Anisimova ID, Stafeev VI, Karamian GG, et al. The efficiency of solar cells immersed in liquid dielectrics. Sol Energy Mater Sol Cells 2002;73:367–75. [13] Muaddi JA, Jamal MA. Spectral response and efficiency of a silicon solar cell below water surface. Sol Energy 1992;49:29–33. [14] Czanderna AW, Pern FJ. Encapsulation of PV modules using ethylene vinylacetate copolymer as a pottant: a critical review. Sol Energy Mater Sol Cells 1996;43:101–81.