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Optical characterization and durability of immersion cooling liquids for high concentration III-V photovoltaic systems
Solar Energy Materials and Solar Cells 174 (2018) 124–131
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Optical characterization and durability of immersion cooling liquids for high concentration III-V photovoltaic systems
MARK
⁎
Xinyue Hana, , Yongjie Guoa, Qian Wanga, Patrick Phelanb a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287-6106, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: High concentrating photovoltaic Immersion cooling liquids Optical transmittance Durability Multi-junction solar cells
The optical transmittance and durability of several immersion cooling liquids that could be used for high concentration III-V photovoltaic systems are investigated. Firstly, the optical transmittance of the liquid candidates divided into three categories: synthetic oil, silicone oil and mineral oil are determined based on a double optical path-length transmittance method. The normalized photocurrent density of GaInP/GaInAs/Ge triple-junction solar cells when illuminated by the solar spectral irradiance filtered by each liquid is reported. Results show that the liquid candidates exhibit superior transmittance for the UV and visible wavelengths of interest, whereas they display some absorption bands in the 1200–1800 nm spectrum and thus the photocurrent of the bottom subcell Ge decreases. This is not an issue since the bottom subcell normally produces excess current. Then, the optical transmittance of the liquid candidates is monitored during exposure to UV light, damp heat and high-temperature accelerated aging tests. The average transmittance for the wavelengths of interest is introduced to quantify changes in the optical transmittance of immersion cooling liquids after being subjected to the accelerated aging tests. Results from the accelerated aging tests indicate that dimethyl silicone oil, white oils A/B/ C and C14 n-alkane are suitable for immersion cooling of multi-junction solar cells whose average transmittance losses are less than the total degradation value of 5% allowed for CPV modules under qualification tests in the IEC62108 standard. The optimum liquid for immersion cooling multi-junction solar cells is found to be dimethyl silicone oil due to its high transmittance for the wavelengths of interest and its loss in average transmittance over each exposure period is always less than 0.5%.
1. Introduction High concentrating photovoltaic systems applying III-V multi-junction solar cells have been attracting a tremendous amount of attention from the scientific community and industrial developers due to the record efficiencies of III-V multi-junction solar cells, which have increased significantly over the past several years up to 46.0% (AM1.5D, 508 suns) [1–4]. Even though III-V multi-junction cells have shown superior performance, more than 50% of the absorbed incident sunlight which cannot be converted into electricity is still dissipated as heat within the cells. It is well known that the efficiencies of solar cells decrease as the cell operation temperature increases. Therefore, the cooling system is very important for concentrating photovoltaic (CPV) applications, especially for high CPV systems with densely packed cells [5]. A significant amount of research on CPV cooling has been carried out, as summarized by Royne et al. [5] and Micheli et al. [6]. Further, a review of the literature on cooling systems for CPV systems concluded that effective uniform cooling is highly important [7]. In fact, in our ⁎
Corresponding author. E-mail address: [email protected] (X. Han).
http://dx.doi.org/10.1016/j.solmat.2017.08.034 Received 18 May 2017; Received in revised form 2 August 2017; Accepted 29 August 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
previous study, we proposed direct liquid immersion cooling for high CPV systems with densely packed solar cells and demonstrated that it is one of the promising solutions for effective uniform CPV cooling [8]. In addition, Han et al. discussed the optical and electrical effects of candidate dielectric liquids on silicon concentrator solar cells [9,10]. The high heat dissipation capability of direct liquid immersion cooling for dish high CPV systems and linear CPV systems with silicon solar cells have been confirmed by several authors [11–13]. There is still, however, not adequate experimental research on which to choose an appropriate immersion cooling liquid for multijunction solar cells. Besides exhibiting high optical transmittance for the spectral response region of multi-junction solar cells, the durability of appropriate immersion cooling liquids needs to be established since it is extremely significant for CPV practical applications. As a matter of fact, for any optical material with potential application in CPV systems used for various functions ranging from concentrating optics to homogenizers, encapsulants and even heat dissipation, aging tests are necessary before real-world applications, to demonstrate the reliability
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α0 λ τ
Nomenclature c d h IAM1.5D Jnp q R1 R2 T x
light speed, m/s thickness of cuvette wall, cm Planck's constant direct solar spectral irradiance at AM1.5, W m−2 nm−1 normalized photocurrent density electron charge, C reflectance at interface between air and cuvette wall reflectance at interface between liquid and cuvette wall measured transmittance cuvette path-length, cm
Abbreviations CPV CPV/T EQE HCPV NIR PV UV VIS
Greeks α
absorption coefficient of cuvette wall, cm−1 wavelength, nm transmittance of liquid
concentrating photovoltaic concentrating photovoltaic/thermal external quantum efficiency high concentrating photovoltaic near infrared photovoltaic ultraviolet visible
absorption coefficient of liquid, cm−1
good heat transfer properties, but also include additional optical and electrical properties. According to the required properties [19] and the available physical properties of the liquids shown in Table 1 [20–23], seven liquids were identified as potential immersion cooling liquids for high concentration III-V photovoltaic systems: Therminol VP-1, dimethyl silicone oil, three types of white oils, C14 n-alkane and C16 isoalkane. In fact, white oil is also called paraffin oil. Therminol VP-1 is one member of the synthetic oil particularly developed as heat transfer oil by the company called Solutia [20]. Three types of white oil, C14 nalkane and C16 iso-alkane fall into the category of mineral oil. Depending on the refining process, three types of white oils are selected: food grade white oil, cosmetic grade white oil and industrial grade white oil (referred to as white oils A/B/C throughout the paper). According to the data described in Table 1, almost all of them exhibit good heat transfer properties. Among them, Therminol VP-1 has the highest density of 1060 kg/m3, whereas mineral oils have lower densities ranging between 764 and 880 kg/m3. White oils have higher specific heat in comparison with other liquids. All the selected liquids except for C16 iso-alkane have similar thermal conductivities. The viscosities of C14 nalkane and C16 iso-alkane are lower, when compared with Therminol VP-1, dimethyl silicone oil and white oils. With respect to the electrical properties of the liquids, Therminol VP-1, dimethyl silicone oil and some mineral oil manufacturers provide dielectric constant data. However, in terms of the optical properties of the liquids, the available information is the refractive indices and transparency in the visible spectrum. For use as an immersion coolant, the optical transmittance is much more significant than as a conventional heat transfer fluids. Therefore, the optical transmittance of these liquids must first be measured for comparison with the spectral response region of multijunction solar cells. In addition, the durability of the liquid candidates is also one of the key factors when choosing a liquid for high concentration III-V photovoltaic applications. Finally, cost is a further consideration for more economical system. In fact, the seven liquid candidates are commonly-used commercial inexpensive heat transfer fluids. Generally, synthetic oil is the most expensive heat transfer fluid,
of the optical properties. Outdoor aging tests are time-consuming and thus accelerated aging tests are usually carried out [14]. For example, the optical durability of Fresnel concentrators were studied by subjecting them to accelerated aging in an ultraviolet (UV) environmental chamber [15]. In order to analyze the reliability of silicone and ethylene vinyl acetate (EVA) encapsulants for PV modules, McIntosh et al. discussed the effect of damp heat and UV exposure on the optical properties of the encapsulants [16]. Victoria et al. examined the transmittance of several fluids that could be used for immersing concentrator optics, both before and after accelerated UV radiation exposure [17]. Looser et al. achieved the most suitable fluid as a spectral absorption filter for CPV/T systems by analyzing the impact of high temperature and UV light accelerated aging tests on the transmittance of the fluids [18]. In this paper, several different cheap, clear, and easily available liquid candidates for immersion cooling multi-junction solar cells have been examined in terms of their optical transmittance based on a double optical path-length transmittance method. In addition, an analysis of the normalized photocurrent density ( Jnp ) of triple-junction solar cells in the presence of each immersion cooling liquid has been conducted. To maintain a relatively constant power output of the HCPV systems with densely packed multi-junction solar cells, the immersion cooling liquids should withstand for long term operation and environmental exposure without much degradation. Thus, the influence of accelerated aging tests on the optical transmittance of immersion cooling liquids for multi-junction solar cells is investigated. The average transmittance is adopted to quantify the changes in the optical transmittance of immersion liquids after being submitted to the accelerated aging tests. 2. Investigated immersion cooling liquids for III-V cells Since immersion liquids for cooling III-V solar cells under high concentration perform the dual purpose of heat transfer and optical adaptation, the requirements for the liquid properties not only include Table 1 Physical properties of the candidates for immersion cooling liquids [20–23]. Properties
Therminol VP-1
Dimethyl silicone oil
White oil A/B/C
C14 n-alkane
C16 iso-alkane
Color Density (kg m−3) Specific Heat (J kg−1 K−1) Thermal Conductivity(W m−1 K−1) Viscosity(103 Pa s) Boiling point(°C) Refractive index Dielectric constant
Clear 1060 1570 0.14 2.63 257 1.65 3.35
Clear 915 1758 0.12 4.58 170 1.40 2.60
Clear 880/822/816 2130 0.15 6.78/2.30/2.45 255–276 1.48 2.20
Clear 764 1578 0.14–0.15 1.60 254 1.43 2.03
Clear 767 N/A N/A 1.94 211 1.42 N/A
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transmittance over a majority of the spectrum. The average transmittance calculated from the spectral transmittance achieved by this work and published values [24,26,27] shown in Fig. 1 are 76.8%, 77.8% and 78.0%, respectively. The deviations between this work and ref. [24], refs. [26,27] are ± 1% and ± 1.2%, respectively. Based on these results, we can conclude that there is a good agreement between the optical transmittance obtained from this work and the published values.
followed by silicone oil, with mineral oil being the cheapest [19]. 3. Initial optical transmittance of liquids 3.1. Experimental method Optical transmittance measurements were conducted using a LAMBDA 950 UV-Visible-Near Infrared (UV-VIS-NIR) spectrophotometer (Perkin-Elmer Inc.). The measurement resolution of the instrument is ≤0.05 nm in the UV/VIS and ≤0.20 nm in the NIR. The liquids were enclosed in a high UV-transmitting quartz cuvette with optical path-lengths of 5 mm and 10 mm. All the measurements were made twice and the average was calculated. The transmittance was measured from 200 to 2500 nm over 5-nm intervals at ambient temperature, 25 °C, as this encompasses more than 98% of the solar energy reached on the earth's surface. However, the transmittance for a given optical path-length cuvette produced by the spectrophotometer is not the exact value of the liquid sample. Both the reflection at each interface (air-cuvette wall, cuvette wall-liquid) and the absorption of the cuvette walls should be considered in the wavelengths of interest when interpreting the transmittance measurements. In reality, our previous study developed a methodology to determine the optical transmittance of liquids using a spectrophotometer and photometry correction [24]. But one of the critical procedures for conducting the photometry correction requires using glass of the same material as the cuvette and twice its thickness, but identifying such glass can be difficult. Here, instead a double optical path-length transmittance method is proposed to achieve the optical transmittance of the liquid candidates, as described in the following. Based on the Beer-Lambert law, the optical transmittance of liquid τ (λ ) is related to its absorption coefficient α (λ ) and to its optical pathlength x as:
τ (λ ) = exp(−α (λ )⋅x )
3.2.2. Initial optical transmittance of candidate liquids Through the methodology proposed above, the initial optical transmittance of the selected liquids for immersion cooling of III-V multi-junction solar cells is presented in Fig. 2. AM1.5 direct solar spectral irradiance is also shown in Fig. 2 for reference. Fig. 2a shows that Therminol VP-1 has limited light transmission for wavelengths lower than 300 nm. But it is highly transparent from 400 nm to 1100 nm and display strong absorption bands at 1150 nm, 1400 nm and again at 1600–1750 nm. In Fig. 2a, the optical transmittance of dimethyl silicone oil is also depicted. It exhibits high optical transmittance until the NIR spectrum (1100 nm). From there, it displays transmittance troughs around 1200 nm, 1400 nm and again 1700–1850 nm. C-H bonds included in both Therminol VP-1 and dimethyl silicone oil can be used to explain the transmittance troughs mentioned above [28]. Observing Fig. 2b and Fig. 2c, the optical transmittance of white oils A/B/C, C14 n-alkane and C16 iso-alkane is very similar to each other. All of them show more than 95% transmittance between 400 and 1100 nm. After that, their transmittance fluctuates with significant transmittance valleys at around 1200 nm, 1400 nm and again at 1700–1800 nm. The transmittance cut-off is at about 2250 nm. Evidently, compared with C14 n-alkane and C16 iso-alkane, the three white oils perform well in the optical transmittance of the UV spectrum. Further, the food grade white oil namely white oil A, shows higher transmittance in the highly energetic wavelengths than the other two white oils. We also report on the normalized photocurrent density ( Jnp ) of multi-junction solar cells. This parameter can be used to evaluate the effect of liquid transmittance on the photocurrent density of solar cells when illuminated by the solar spectral irradiance filtered by each liquid. Jnp is defined by [24,29]:
(1)
where λ is the wavelength. However, the measured transmittance T (λ ) using the spectrophotometer is the total value of the liquid layer and two cuvette glass slabs, immersed in air. At each interface (air-cuvette wall, cuvette wall-liquid) Fresnel losses take place and some light in the wavelengths of interest is absorbed by the cuvette walls. With the assumption of no scattering, negligible coherent effects [25] and single reflections at the interfaces, the total transmittance T (λ ) can be written as [24]:
T (λ ) = τ (λ )(1 − R1)2 (1 − R2)2 exp(−2α w⋅d )
∫λ
Jnp = [
/[
(2)
τ (λ ) EQE (λ ) IAM1.5D (λ )(qλ /hc ) dλ]
∫λ
λ2
1
EQE (λ ) IAM1.5D (λ )(qλ / hc ) dλ]
(4)
where τ (λ ) represents the optical transmittance of the investigated liquid, EQE (λ ) the external quantum efficiency (EQE) of the solar cell used, IAM1.5D (λ ) stands for the AM1.5 direct irradiance spectrum, q the
where R1 is the reflection at the air-cuvette wall interface, R2 the reflectance at the cuvette wall-liquid interface, α w the absorption coefficient of the cuvette wall, and d the cuvette wall thickness. Eq. (2) indicates that the measured transmittance T (λ ) is proportional to the optical transmittance of the liquid τ (λ ) . Therefore, with air as the reference, if we obtain two measured transmittance T1 (λ ) and T2 (λ ) at two different optical path-lengths x1 and x2 , the absorption coefficient of the liquid α (λ ) can be determined from:
α (λ ) = [−1/(x2 − x1)]ln[T2 (λ )/ T1 (λ )] = [−1/ Δx ]ln[T2 (λ )/ T1 (λ )]
λ2
1
(3)
Finally, according to the absorption coefficient of liquid α (λ ) obtained above, the optical transmittance of the liquid τ (λ ) at any optical pathlength x can be easily determined through Eq. (1). 3.2. Results and discussion 3.2.1. Experimental validation Using the double optical path-length transmittance method described in the previous section, the optical transmittance of water is presented in Fig. 1. The results from this work are shown in relation to the published values [24,26,27]. This method provides accurate optical
Fig. 1. Optical transmittance of water with 10 mm thickness.
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In this work, only a triple-junction solar cell manufactured by Tianjin Lantian Solar Tech Co., Ltd is analyzed. The triple-junction solar cell is made of a GaInP top subcell (EQE between 350 and 660 nm), a GaInAs middle subcell (EQE between 660 and 890 nm), and a Ge bottom subcell (EQE between 890 and 1800 nm) [30]. Through Eq. (4), the normalized photocurrent density ( Jnp ) of the top, middle, and bottom subcells, and the complete triple-junction solar cell when AM1.5D reference irradiance is filtered by each liquid are presented in Table 2. We found that the photocurrent losses of both the top and middle subcells are considerably lower than that of the bottom subcell for a liquid thickness of 10 mm. This phenomenon can be attributed to the optical transmittance of the liquids presented in Fig. 2. All the selected liquids exhibit superior transmittance for the wavelengths of the top and middle subcells in the UV and VIS spectrums. However, the optical transmittance of these liquids shows some decrease over the wavelength range of 1200–1800 nm. This implies that the part of the spectrum absorbed in the Ge bottom subcell will be reduced and thus its generated current will be decreased when the liquids are used. In fact, the Ge bottom subcell of the GaInP/GaInAs/Ge material combination is known to generate a large excess current under AM1.5D reference irradiance, which limits the cell performance [31]. Therefore, this will not be a problem for the complete triple-junction solar cells if the current loss caused by the reduction in transmittance does not make the Ge bottom junction as the current limiting subcell. In addition, Jnp of the complete triple-junction solar cell indicates that the smallest power loss for such solar cells would be observed with Therminol VP-1 immersion, followed by dimethyl silicon oil, with the largest loss occurring with white oil C immersion. In summary, all of these liquids were selected for further durability research based on the optical properties analysis of the liquid candidates in their initial state. The accelerated aging tests include a UV degradation test, a damp heat test, and low- & high-temperature tests. 4. Durability analysis of liquid candidates 4.1. Accelerated aging tests CPV modules are exposed to extremely strict environmental conditions of UV radiation, humidity, heat, etc. [32]. One way a CPV module with liquid immersion cooling can fail an accelerated aging test is when the optical transmittance of its immersion liquid reduces significantly with age. Thus, long-term stable optical properties of immersion cooling liquids for multi-junction solar cells must be demonstrated over the lifetime of the solar cell. For CPV technology, IEC62108 norm is the first developed standard and many kinds of qualification tests are designed to guarantee the durability and reliability of CPV modules and assemblies [33,34]. In order to predict the durability of the selected liquid candidates, accelerated aging tests are described in this section to illustrate how the optical transmittance of liquids is affected by UV light, damp heat and temperature exposure. The three aging tests are considered to be the most likely to cause degradation on the optical transmittance of immersion cooling liquids. The optical transmittance of the liquid candidates is monitored before and after exposure to the following conditions: (a) UV conditioning test: UV exposure with an intensity of 56.8 kWh/m2 in the wavelength range below 400 nm, while holding the liquid samples at 60 °C; (b) Damp heat test: 1400 h at 85% relative humidity and 85 °C; and (c) Temperature test: firstly, constant temperature of 65 °C for 700 h, and followed by constant temperature of 100 °C for 300 h. In addition, it should be noted that our accelerated aging tests do not replicate the qualification standard for CPV module. We don’t follow the exact procedures described in the IEC62108 norm, nor do we utilize the complete module, which is a very costly process. Instead, we are concerned with the relative changes in optical transmittance of liquids, before and after, under stringent test conditions. The three exposures of these tests exceed those required by the IEC62108 standard
Fig. 2. Optical transmittance of (a) Therminol VP-1, dimethyl silicone oil, (b) white oils A/B/C and (c) C14 n-alkane, C16 iso-alkane, as well as AM1.5D solar spectral irradiance.
electron charge, c the speed of light, h Planck's constant, and λ1 to λ2 is the corresponding spectral response wavelength range of each subcell or multi-junction solar cell. 127
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Table 2 Normalized photocurrent density (Jnp) of the top, middle, and bottom subcells, and the entire 3-junction solar cell when AM1.5D reference irradiance is filtered by each liquid. Liquid species
Liquid thickness
Top (350–660 nm) GaInP Jnp
Middle (660–890 nm) GaInAs Jnp
Bottom (890–1800 nm) Ge Jnp
3-junction CPV cell (350–1800 nm) GaInP/GaInAs/Ge Jnp
τ (λ ) = 1 Therminol VP-1 Dimethyl silicone oil White oil A White oil B White oil C C14 n-alkane C16 iso-alkane
0 mm 10 mm 10 mm 10 mm 10 mm 10 mm 10 mm 10 mm
1 0.980 0.992 0.997 0.998 0.980 0.974 0.995
1 0.988 0.992 0.993 0.996 0.982 0.982 0.995
1 0.904 0.857 0.785 0.792 0.783 0.795 0.796
1 0.950 0.934 0.905 0.909 0.896 0.900 0.910
Fig. 3, it can be clearly observed that the UVA 340 fluorescent lamp provides much more irradiance than the AM1.5D solar spectrum in the wavelength range below 365 nm. In addition, the UV irradiance in terms of power in the wavelength range below 400 nm specified by the UV conditioning test in IEC62108 standard is quantified in Table 3. It can be seen that the UV exposure in this case exceeds the duration required by the IEC 62108's UV conditioning test. Since Therminol VP-1 samples exhibit a color shift from clear to orange-brown (noticeable through visual inspection since 41.5 h UV exposure) possibly caused by the degradation of the synthetic oil component, we only consider the other six liquid candidates in the following section. Fig. 4 describes the optical transmittance measured for the six liquid samples as a function of wavelength and UV exposure time, showing that optical transmittance get worse within the wavelength range below 400 nm for mineral oils, and below 300 nm for dimethyl silicone oil. No distinct effect of degradation, however, is detected for any of the liquid samples in the VIS wavelengths. Regarding the IR wavelengths, a slight decrease in measured transmittance is observed in wavelengths 1400–1700 nm for the five mineral oils. On the contrary, dimethyl silicone oil remains stable in the NIR wavelengths. As mentioned above, the wavelength range used in the GaInP/GaInAs/Ge triple-junction solar cells is 350–1800 nm. For dimethyl silicone oil, the photocurrent of the complete triple-junction solar cells will remain almost constant after 1400 h UV conditioning test. For the mineral oils, in most cases, the photocurrents of both the top and bottom subcells will have slight decreases after UV exposure whereas the middle subcell photocurrent will remain unaltered. A calculation of the average transmittance for the spectral response region (350–1800 nm) of the specified GaInP/GaInAs/Ge triple-junction solar cell revealed the biggest losses from reductions in the optical transmittance (see Fig. 5). UV exposure time-dependent transmittance losses were observed for the five mineral oils. The transmittance losses of white oils A/B/C, C14 n-alkane and C16 iso-alkane exposed to UV degradation test with respect to the values before the test are 4.66%, 3.81%, 3.57%, 1.97% and 5.74%, respectively. Dimethyl silicone oil exhibited a transmittance loss of 0.42%, after 1400 h UV exposure and a slight increase after 800 h UV exposure, which is consistent with the findings of Everett et al. [35] and Eltermann et al. [36]. But the root cause for that has not been provided.
Fig. 3. Spectral irradiance of the UVA 340 lamp used and the AM1.5D solar spectrum.
[33]. 4.2. Measurement For each liquid under each exposure, we prepared two samples and characterized their optical transmittance as a function of exposure time through the LAMBDA 950 spectrophotometer mentioned above. Furthermore, two duplicate tests were conducted for every set of transmittance measurements and the averaged values were analyzed. In addition to the optical transmittance characterization, the visual appearance of the samples was examined every day. 4.3. Results and discussion 4.3.1. UV exposure In this case, the promising liquids were contained in the high UVtransmitting quartz tubes and were aged for 1400 h in a UV weathering chamber equipped with UVA 340 fluorescent lamps, while maintaining the liquid samples at 60 °C. Fig. 3 presents the spectral irradiance of the UVA 340 fluorescent lamp provided by the manufacturer. Also included is the spectral irradiance of the AM1.5 direct normal solar spectrum. In Table 3 UV irradiance within the wavelength range below 400 nm. Requirement for UV exposure
AM1.5D spectrum
UVA 340 lamp
Standard
λ (nm)
E (kWh/m2)
P (W/m2)
treq (h)
treq (days)
P (W/m2)
t (h)
E (kWh/m2)
IEC 62108
＜400
50
30.5
1639
273
40.6
1400
56.8
Note: the table provides the requirements of the UV degradation test in the IEC62108 standard in terms of wavelength range λ and accumulated UV dosage E. It also provides the incident light power P for the AM1.5D spectrum, the required time treq to satisfy the IEC62108 standard and the equivalent time treq(days) in real conditions (where treq(h) = 6 × treq(days) [32]). Further, the table provides the incident light power P for the UVA 340 lamp, the aging time t, and the accumulated UV dosage E.
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Fig. 5. Average transmittance of liquid samples before and after UV exposure (56.8 kWh/ m2, 60 ± 5 °C).
Fig. 4. Optical transmittance of liquid samples before and after UV exposure (56.8 kWh/ m2, 60 °C).
4.3.2. Damp heat exposure In this test, the damp heat exposure exceeds the aging time required by the official standard IEC62108 to identify the durability of the liquid candidates for immersion cooling multi-junction solar cells. Fig. 6 plots the effect of long-term damp heat exposure on the selected six immersion cooling liquids. It can be seen that all the liquid samples exhibit that the degradation mainly occurs at UV wavelengths, but the degree of change is much more distinctive than the results obtained from UV exposure. Both white oil A and white oil B show higher transmittance losses in the UV spectrum than the other four tested liquids. Further, the transmittance in the VIS and NIR ranges is affected minimally except for white oil B in this test duration. While Victoria et al. [17] suggested that the discrepancy in behavior of white oils A, B and C could be explained by the refining process adopted to achieve them. Obviously, the content of impurities in white oil C is lower than in both white oil A and B, remaining almost stable of the samples after 1400 h damp heat exposure. The average transmittance of the liquid samples before and after the damp heat test (1400 h at 85% relative humidity and 85 °C) is shown in Fig. 7. It highlights that the transmittance losses of both dimethyl silicone oil and white oil C after damp heat exposure are less than 0.5%. White oil A and White oil B are more strongly influenced and their transmittance losses are 1.34% and 3.42%, respectively, while C14 nalkane and C16 iso-alkane degraded less, with 0.77% for C14 n-alkane and less than 1% for C16 iso-alkane. Moreover, with visual inspection, we did not detect any change in color over the test duration for all the tested liquids.
Fig. 6. Optical transmittance of liquid samples before and after the damp heat test (1400 h at 85% relative humidity and 85 °C).
aging tests have been performed. Based on the experimental results of liquids acquired from the UV conditioning test, the liquid candidates except for Therminol VP-1 were sealed in borosilicate glass containers and exposed for a period of 700 h to 65 °C and followed by to 100 °C for 300 h. Since the operating temperature of the selected triple-junction solar cells is ranges from −40 °C to 100 °C [30], the high-temperature (100 °C) exposure would show the maximum effect on the liquids. The aim of the low-temperature exposure is to see whether the optical properties of the liquids can remain stable under the typical operating temperature of CPV modules [38]. The effect of temperature exposure for a total period of 1000 h on the liquid candidates for cooling III-V cells is illustrated in Fig. 8. Near identical results are obtained for the liquid samples between the damp heat test and the temperature test. The major change in optical transmittance of all the tested liquids after the temperature test is limited to
4.3.3. Temperature exposure It is well known that temperature is one of the main environmental factors of PV module degradation in almost all identified degradation modes [37]. To simulate thermal degradation under real conditions and thus the optical transmittance loss during the immersion cooling liquid lifetime, low-temperature followed by high -temperature accelerated 129
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are very positive. The transmittance losses of all the tested liquids except for C16 iso-alkane under UV exposure are less than the total degradation value of 5% allowed for CPV modules under the qualification tests in the IEC62108 standard. Since the average transmittance loss of C16 iso-alkane after being subjected to the UV test is over 5% (see Fig. 10), it has been deemed not suitable for immersion cooling III-V multi-junction solar cells. The effect of UV degradation on the average optical transmittance of each liquid is much larger than those from the damp heat test and temperature tests. In addition, it can be observed in Fig. 10 that dimethyl silicone oil presents a degradation of less than 0.5% in average optical transmittance over the period of each accelerated aging test. The difference in behavior of dimethyl silicone oil and mineral oils has to do with their respective covalent bonds. The covalent silicon bonds in dimethyl silicone oil has a larger dissociation energy of 451 kJ/mol than that (347 kJ/mol) of the covalent carbon bonds in mineral oils [17]. Therefore, the results from the accelerated aging tests indicate that the optimum liquid for III-V cells immersion cooling application is dimethyl silicone oil, followed by C14 n-alkane. They also give confidence for the reliable performance of white oils A/ B/C.
Fig. 7. Average transmittance of liquid samples before and after the damp heat test (1400 h at 85% relative humidity and 85 °C).
5. Conclusions Three groups of common and inexpensive liquids encompassing seven materials have been studied in terms of their optical transmittance and durability for immersion cooling III-V multi-junction solar cells under high concentration. A double optical path-length transmittance method is proposed for determining the optical transmittance of the liquid candidates. Further, the normalized photocurrent density ( Jnp ) is used to characterize the effect of liquid transmittance on the photocurrent density of GaInP/GaInAs/Ge triple-junction solar cells when illuminated by the solar spectral irradiance filtered by each liquid. Results show that all the liquids have promising initial optical transmittance for the wavelengths of interest and are selected for further accelerated aging tests to study their durability. The accelerated aging tests include a UV conditioning test, a damp heat test, and a lowtemperature test at 65 °C followed by a high-temperature test at 100 °C. Results from the accelerated aging tests indicate that Therminol VP-1 and C16 iso-alkane are considered unsuitable for CPV application as they suffered a serious degradation after UV exposure. The other five liquid candidates were demonstrated as appropriate to be utilized for high concentration III-V photovoltaic systems. Their average transmittance losses are less than the total degradation value of 5% allowed for CPV modules under the qualification tests in the IEC62108 standard. The optimum liquid for immersion cooling of multi-junction solar cells is found to be dimethyl silicone oil, due to its high transmittance for the Fig. 8. Optical transmittance of liquid samples before and after low- and high-temperature tests (700 h at 65 °C & 300 h at 100 °C).
shorter wavelengths in the UV spectrum. The much more significant deterioration of transmittance in the UV region is observed for white oil B and white oil A. For immersion cooling triple-junction solar cells applications, in most cases, only the photocurrent of the top subcell would decrease. Quantitatively, the average transmittance losses are 0.22%, 1.38%, 2.54%, 0.95%, 0.76% and 1.26% for white oils A/B/C, C14 n-alkane and C16 iso-alkane subjected to the temperature degradation test with respect to their initial values (see Fig. 9). During the temperature test period, no change was detected visually in terms of the liquid color. Finally, the relative changes in the average optical transmittance of each liquid after accelerated aging tests are summarized in Fig. 10, which is related to the power losses that the CPV module would manifest due to degradation. As a matter of fact, the results shown in Fig. 10
Fig. 9. Average transmittance of liquid samples before and after low- and high-temperature tests (700 h at 65 °C & 300 h at 100 °C).
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Fig. 10. Average transmittance losses of liquid samples after exposure to all the accelerated aging tests.
spectral response wavelengths of triple-junction cells, and it was the most stable under the accelerated aging tests. Acknowledgements This research is supported by the National Natural Science Foundation of China (51776091, 51306077), Training Project for Young Teachers of Jiangsu University (5521130006) and Oversea Study Scholarship from China Scholarship Council (201608695005). References [1] G. Zubi, J.L. Bernal-Agustín, G.V. Fracastoro, High concentration photovoltaic systems applying III–V cells, Renew. Sustain. Energy Rev. 13 (2009) 2645–2652. [2] Z. Wang, H. Zhang, D. Wen, W. Zhao, Z. Zhou, Characterization of the InGaP/ InGaAs/Ge triple-junction solar cell with a two-stage dish-style concentration system, Energy Convers. Manag. 76 (2013) 177–184. [3] O.Z. Sharaf, M.F. Orhan, Thermodynamic analysis and optimization of denselypacked receiver assembly components in high-concentration CPVT solar collectors, Energy Convers. Manag. 121 (2016) 113–144. [4] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, D.H. Levi, A.W.Y. HoBaillie, Solar cell efficiency tables (version 49), Prog. Photovolt. Res. Appl. 25 (2017) 3–13. [5] A. Royne, C.J. Dey, D.R. Mills, Cooling of photovoltaic cells under concentrated illumination: a critical review, Sol. Energy Mater. Sol. Cells 86 (2005) 451–483. [6] L. Micheli, N. Sarmah, X. Luo, K.S. Reddy, T.K. Mallick, Opportunities and challenges in micro-and nano-technologies for concentrating photovoltaic cooling: a review, Renew. Sustain. Energy Rev. 20 (2013) 595–610. [7] H.M. Bahaidarah, A.A. Baloch, P. Gandhidasan, Uniform cooling of photovoltaic panels: a review, Renew. Sustain. Energy Rev. 57 (2016) 1520–1544. [8] X. Han, Q. Wang, J. Zheng, J. Qu, Thermal analysis of direct liquid-immersed solar receiver for high concentrating photovoltaic system, Int. J. Photoenergy 2015 (2015) 1–9. [9] X. Han, Y. Wang, L. Zhu, Electrical and thermal performance of silicon concentrator solar cells immersed in dielectric liquids, Appl. Energy 88 (2011) 4481–4489. [10] X. Han, Y. Wang, L. Zhu, The performance and long-term stability of silicon concentrator solar cells immersed in dielectric liquids, Energy Convers. Manag. 66 (2013) 189–198. [11] L. Zhu, R.F. Boehm, Y. Wang, C. Halford, Y. Sun, Water immersion cooling of PV cells in a high concentration system, Sol. Energy Mater. Sol. Cells 95 (2011)
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Optical characterization and durability of immersion cooling liquids for high concentration III-V photovoltaic systems
MARK
⁎
Xinyue Hana, , Yongjie Guoa, Qian Wanga, Patrick Phelanb a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287-6106, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: High concentrating photovoltaic Immersion cooling liquids Optical transmittance Durability Multi-junction solar cells
The optical transmittance and durability of several immersion cooling liquids that could be used for high concentration III-V photovoltaic systems are investigated. Firstly, the optical transmittance of the liquid candidates divided into three categories: synthetic oil, silicone oil and mineral oil are determined based on a double optical path-length transmittance method. The normalized photocurrent density of GaInP/GaInAs/Ge triple-junction solar cells when illuminated by the solar spectral irradiance filtered by each liquid is reported. Results show that the liquid candidates exhibit superior transmittance for the UV and visible wavelengths of interest, whereas they display some absorption bands in the 1200–1800 nm spectrum and thus the photocurrent of the bottom subcell Ge decreases. This is not an issue since the bottom subcell normally produces excess current. Then, the optical transmittance of the liquid candidates is monitored during exposure to UV light, damp heat and high-temperature accelerated aging tests. The average transmittance for the wavelengths of interest is introduced to quantify changes in the optical transmittance of immersion cooling liquids after being subjected to the accelerated aging tests. Results from the accelerated aging tests indicate that dimethyl silicone oil, white oils A/B/ C and C14 n-alkane are suitable for immersion cooling of multi-junction solar cells whose average transmittance losses are less than the total degradation value of 5% allowed for CPV modules under qualification tests in the IEC62108 standard. The optimum liquid for immersion cooling multi-junction solar cells is found to be dimethyl silicone oil due to its high transmittance for the wavelengths of interest and its loss in average transmittance over each exposure period is always less than 0.5%.
1. Introduction High concentrating photovoltaic systems applying III-V multi-junction solar cells have been attracting a tremendous amount of attention from the scientific community and industrial developers due to the record efficiencies of III-V multi-junction solar cells, which have increased significantly over the past several years up to 46.0% (AM1.5D, 508 suns) [1–4]. Even though III-V multi-junction cells have shown superior performance, more than 50% of the absorbed incident sunlight which cannot be converted into electricity is still dissipated as heat within the cells. It is well known that the efficiencies of solar cells decrease as the cell operation temperature increases. Therefore, the cooling system is very important for concentrating photovoltaic (CPV) applications, especially for high CPV systems with densely packed cells [5]. A significant amount of research on CPV cooling has been carried out, as summarized by Royne et al. [5] and Micheli et al. [6]. Further, a review of the literature on cooling systems for CPV systems concluded that effective uniform cooling is highly important [7]. In fact, in our ⁎
Corresponding author. E-mail address: [email protected] (X. Han).
http://dx.doi.org/10.1016/j.solmat.2017.08.034 Received 18 May 2017; Received in revised form 2 August 2017; Accepted 29 August 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
previous study, we proposed direct liquid immersion cooling for high CPV systems with densely packed solar cells and demonstrated that it is one of the promising solutions for effective uniform CPV cooling [8]. In addition, Han et al. discussed the optical and electrical effects of candidate dielectric liquids on silicon concentrator solar cells [9,10]. The high heat dissipation capability of direct liquid immersion cooling for dish high CPV systems and linear CPV systems with silicon solar cells have been confirmed by several authors [11–13]. There is still, however, not adequate experimental research on which to choose an appropriate immersion cooling liquid for multijunction solar cells. Besides exhibiting high optical transmittance for the spectral response region of multi-junction solar cells, the durability of appropriate immersion cooling liquids needs to be established since it is extremely significant for CPV practical applications. As a matter of fact, for any optical material with potential application in CPV systems used for various functions ranging from concentrating optics to homogenizers, encapsulants and even heat dissipation, aging tests are necessary before real-world applications, to demonstrate the reliability
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α0 λ τ
Nomenclature c d h IAM1.5D Jnp q R1 R2 T x
light speed, m/s thickness of cuvette wall, cm Planck's constant direct solar spectral irradiance at AM1.5, W m−2 nm−1 normalized photocurrent density electron charge, C reflectance at interface between air and cuvette wall reflectance at interface between liquid and cuvette wall measured transmittance cuvette path-length, cm
Abbreviations CPV CPV/T EQE HCPV NIR PV UV VIS
Greeks α
absorption coefficient of cuvette wall, cm−1 wavelength, nm transmittance of liquid
concentrating photovoltaic concentrating photovoltaic/thermal external quantum efficiency high concentrating photovoltaic near infrared photovoltaic ultraviolet visible
absorption coefficient of liquid, cm−1
good heat transfer properties, but also include additional optical and electrical properties. According to the required properties [19] and the available physical properties of the liquids shown in Table 1 [20–23], seven liquids were identified as potential immersion cooling liquids for high concentration III-V photovoltaic systems: Therminol VP-1, dimethyl silicone oil, three types of white oils, C14 n-alkane and C16 isoalkane. In fact, white oil is also called paraffin oil. Therminol VP-1 is one member of the synthetic oil particularly developed as heat transfer oil by the company called Solutia [20]. Three types of white oil, C14 nalkane and C16 iso-alkane fall into the category of mineral oil. Depending on the refining process, three types of white oils are selected: food grade white oil, cosmetic grade white oil and industrial grade white oil (referred to as white oils A/B/C throughout the paper). According to the data described in Table 1, almost all of them exhibit good heat transfer properties. Among them, Therminol VP-1 has the highest density of 1060 kg/m3, whereas mineral oils have lower densities ranging between 764 and 880 kg/m3. White oils have higher specific heat in comparison with other liquids. All the selected liquids except for C16 iso-alkane have similar thermal conductivities. The viscosities of C14 nalkane and C16 iso-alkane are lower, when compared with Therminol VP-1, dimethyl silicone oil and white oils. With respect to the electrical properties of the liquids, Therminol VP-1, dimethyl silicone oil and some mineral oil manufacturers provide dielectric constant data. However, in terms of the optical properties of the liquids, the available information is the refractive indices and transparency in the visible spectrum. For use as an immersion coolant, the optical transmittance is much more significant than as a conventional heat transfer fluids. Therefore, the optical transmittance of these liquids must first be measured for comparison with the spectral response region of multijunction solar cells. In addition, the durability of the liquid candidates is also one of the key factors when choosing a liquid for high concentration III-V photovoltaic applications. Finally, cost is a further consideration for more economical system. In fact, the seven liquid candidates are commonly-used commercial inexpensive heat transfer fluids. Generally, synthetic oil is the most expensive heat transfer fluid,
of the optical properties. Outdoor aging tests are time-consuming and thus accelerated aging tests are usually carried out [14]. For example, the optical durability of Fresnel concentrators were studied by subjecting them to accelerated aging in an ultraviolet (UV) environmental chamber [15]. In order to analyze the reliability of silicone and ethylene vinyl acetate (EVA) encapsulants for PV modules, McIntosh et al. discussed the effect of damp heat and UV exposure on the optical properties of the encapsulants [16]. Victoria et al. examined the transmittance of several fluids that could be used for immersing concentrator optics, both before and after accelerated UV radiation exposure [17]. Looser et al. achieved the most suitable fluid as a spectral absorption filter for CPV/T systems by analyzing the impact of high temperature and UV light accelerated aging tests on the transmittance of the fluids [18]. In this paper, several different cheap, clear, and easily available liquid candidates for immersion cooling multi-junction solar cells have been examined in terms of their optical transmittance based on a double optical path-length transmittance method. In addition, an analysis of the normalized photocurrent density ( Jnp ) of triple-junction solar cells in the presence of each immersion cooling liquid has been conducted. To maintain a relatively constant power output of the HCPV systems with densely packed multi-junction solar cells, the immersion cooling liquids should withstand for long term operation and environmental exposure without much degradation. Thus, the influence of accelerated aging tests on the optical transmittance of immersion cooling liquids for multi-junction solar cells is investigated. The average transmittance is adopted to quantify the changes in the optical transmittance of immersion liquids after being submitted to the accelerated aging tests. 2. Investigated immersion cooling liquids for III-V cells Since immersion liquids for cooling III-V solar cells under high concentration perform the dual purpose of heat transfer and optical adaptation, the requirements for the liquid properties not only include Table 1 Physical properties of the candidates for immersion cooling liquids [20–23]. Properties
Therminol VP-1
Dimethyl silicone oil
White oil A/B/C
C14 n-alkane
C16 iso-alkane
Color Density (kg m−3) Specific Heat (J kg−1 K−1) Thermal Conductivity(W m−1 K−1) Viscosity(103 Pa s) Boiling point(°C) Refractive index Dielectric constant
Clear 1060 1570 0.14 2.63 257 1.65 3.35
Clear 915 1758 0.12 4.58 170 1.40 2.60
Clear 880/822/816 2130 0.15 6.78/2.30/2.45 255–276 1.48 2.20
Clear 764 1578 0.14–0.15 1.60 254 1.43 2.03
Clear 767 N/A N/A 1.94 211 1.42 N/A
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transmittance over a majority of the spectrum. The average transmittance calculated from the spectral transmittance achieved by this work and published values [24,26,27] shown in Fig. 1 are 76.8%, 77.8% and 78.0%, respectively. The deviations between this work and ref. [24], refs. [26,27] are ± 1% and ± 1.2%, respectively. Based on these results, we can conclude that there is a good agreement between the optical transmittance obtained from this work and the published values.
followed by silicone oil, with mineral oil being the cheapest [19]. 3. Initial optical transmittance of liquids 3.1. Experimental method Optical transmittance measurements were conducted using a LAMBDA 950 UV-Visible-Near Infrared (UV-VIS-NIR) spectrophotometer (Perkin-Elmer Inc.). The measurement resolution of the instrument is ≤0.05 nm in the UV/VIS and ≤0.20 nm in the NIR. The liquids were enclosed in a high UV-transmitting quartz cuvette with optical path-lengths of 5 mm and 10 mm. All the measurements were made twice and the average was calculated. The transmittance was measured from 200 to 2500 nm over 5-nm intervals at ambient temperature, 25 °C, as this encompasses more than 98% of the solar energy reached on the earth's surface. However, the transmittance for a given optical path-length cuvette produced by the spectrophotometer is not the exact value of the liquid sample. Both the reflection at each interface (air-cuvette wall, cuvette wall-liquid) and the absorption of the cuvette walls should be considered in the wavelengths of interest when interpreting the transmittance measurements. In reality, our previous study developed a methodology to determine the optical transmittance of liquids using a spectrophotometer and photometry correction [24]. But one of the critical procedures for conducting the photometry correction requires using glass of the same material as the cuvette and twice its thickness, but identifying such glass can be difficult. Here, instead a double optical path-length transmittance method is proposed to achieve the optical transmittance of the liquid candidates, as described in the following. Based on the Beer-Lambert law, the optical transmittance of liquid τ (λ ) is related to its absorption coefficient α (λ ) and to its optical pathlength x as:
τ (λ ) = exp(−α (λ )⋅x )
3.2.2. Initial optical transmittance of candidate liquids Through the methodology proposed above, the initial optical transmittance of the selected liquids for immersion cooling of III-V multi-junction solar cells is presented in Fig. 2. AM1.5 direct solar spectral irradiance is also shown in Fig. 2 for reference. Fig. 2a shows that Therminol VP-1 has limited light transmission for wavelengths lower than 300 nm. But it is highly transparent from 400 nm to 1100 nm and display strong absorption bands at 1150 nm, 1400 nm and again at 1600–1750 nm. In Fig. 2a, the optical transmittance of dimethyl silicone oil is also depicted. It exhibits high optical transmittance until the NIR spectrum (1100 nm). From there, it displays transmittance troughs around 1200 nm, 1400 nm and again 1700–1850 nm. C-H bonds included in both Therminol VP-1 and dimethyl silicone oil can be used to explain the transmittance troughs mentioned above [28]. Observing Fig. 2b and Fig. 2c, the optical transmittance of white oils A/B/C, C14 n-alkane and C16 iso-alkane is very similar to each other. All of them show more than 95% transmittance between 400 and 1100 nm. After that, their transmittance fluctuates with significant transmittance valleys at around 1200 nm, 1400 nm and again at 1700–1800 nm. The transmittance cut-off is at about 2250 nm. Evidently, compared with C14 n-alkane and C16 iso-alkane, the three white oils perform well in the optical transmittance of the UV spectrum. Further, the food grade white oil namely white oil A, shows higher transmittance in the highly energetic wavelengths than the other two white oils. We also report on the normalized photocurrent density ( Jnp ) of multi-junction solar cells. This parameter can be used to evaluate the effect of liquid transmittance on the photocurrent density of solar cells when illuminated by the solar spectral irradiance filtered by each liquid. Jnp is defined by [24,29]:
(1)
where λ is the wavelength. However, the measured transmittance T (λ ) using the spectrophotometer is the total value of the liquid layer and two cuvette glass slabs, immersed in air. At each interface (air-cuvette wall, cuvette wall-liquid) Fresnel losses take place and some light in the wavelengths of interest is absorbed by the cuvette walls. With the assumption of no scattering, negligible coherent effects [25] and single reflections at the interfaces, the total transmittance T (λ ) can be written as [24]:
T (λ ) = τ (λ )(1 − R1)2 (1 − R2)2 exp(−2α w⋅d )
∫λ
Jnp = [
/[
(2)
τ (λ ) EQE (λ ) IAM1.5D (λ )(qλ /hc ) dλ]
∫λ
λ2
1
EQE (λ ) IAM1.5D (λ )(qλ / hc ) dλ]
(4)
where τ (λ ) represents the optical transmittance of the investigated liquid, EQE (λ ) the external quantum efficiency (EQE) of the solar cell used, IAM1.5D (λ ) stands for the AM1.5 direct irradiance spectrum, q the
where R1 is the reflection at the air-cuvette wall interface, R2 the reflectance at the cuvette wall-liquid interface, α w the absorption coefficient of the cuvette wall, and d the cuvette wall thickness. Eq. (2) indicates that the measured transmittance T (λ ) is proportional to the optical transmittance of the liquid τ (λ ) . Therefore, with air as the reference, if we obtain two measured transmittance T1 (λ ) and T2 (λ ) at two different optical path-lengths x1 and x2 , the absorption coefficient of the liquid α (λ ) can be determined from:
α (λ ) = [−1/(x2 − x1)]ln[T2 (λ )/ T1 (λ )] = [−1/ Δx ]ln[T2 (λ )/ T1 (λ )]
λ2
1
(3)
Finally, according to the absorption coefficient of liquid α (λ ) obtained above, the optical transmittance of the liquid τ (λ ) at any optical pathlength x can be easily determined through Eq. (1). 3.2. Results and discussion 3.2.1. Experimental validation Using the double optical path-length transmittance method described in the previous section, the optical transmittance of water is presented in Fig. 1. The results from this work are shown in relation to the published values [24,26,27]. This method provides accurate optical
Fig. 1. Optical transmittance of water with 10 mm thickness.
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In this work, only a triple-junction solar cell manufactured by Tianjin Lantian Solar Tech Co., Ltd is analyzed. The triple-junction solar cell is made of a GaInP top subcell (EQE between 350 and 660 nm), a GaInAs middle subcell (EQE between 660 and 890 nm), and a Ge bottom subcell (EQE between 890 and 1800 nm) [30]. Through Eq. (4), the normalized photocurrent density ( Jnp ) of the top, middle, and bottom subcells, and the complete triple-junction solar cell when AM1.5D reference irradiance is filtered by each liquid are presented in Table 2. We found that the photocurrent losses of both the top and middle subcells are considerably lower than that of the bottom subcell for a liquid thickness of 10 mm. This phenomenon can be attributed to the optical transmittance of the liquids presented in Fig. 2. All the selected liquids exhibit superior transmittance for the wavelengths of the top and middle subcells in the UV and VIS spectrums. However, the optical transmittance of these liquids shows some decrease over the wavelength range of 1200–1800 nm. This implies that the part of the spectrum absorbed in the Ge bottom subcell will be reduced and thus its generated current will be decreased when the liquids are used. In fact, the Ge bottom subcell of the GaInP/GaInAs/Ge material combination is known to generate a large excess current under AM1.5D reference irradiance, which limits the cell performance [31]. Therefore, this will not be a problem for the complete triple-junction solar cells if the current loss caused by the reduction in transmittance does not make the Ge bottom junction as the current limiting subcell. In addition, Jnp of the complete triple-junction solar cell indicates that the smallest power loss for such solar cells would be observed with Therminol VP-1 immersion, followed by dimethyl silicon oil, with the largest loss occurring with white oil C immersion. In summary, all of these liquids were selected for further durability research based on the optical properties analysis of the liquid candidates in their initial state. The accelerated aging tests include a UV degradation test, a damp heat test, and low- & high-temperature tests. 4. Durability analysis of liquid candidates 4.1. Accelerated aging tests CPV modules are exposed to extremely strict environmental conditions of UV radiation, humidity, heat, etc. [32]. One way a CPV module with liquid immersion cooling can fail an accelerated aging test is when the optical transmittance of its immersion liquid reduces significantly with age. Thus, long-term stable optical properties of immersion cooling liquids for multi-junction solar cells must be demonstrated over the lifetime of the solar cell. For CPV technology, IEC62108 norm is the first developed standard and many kinds of qualification tests are designed to guarantee the durability and reliability of CPV modules and assemblies [33,34]. In order to predict the durability of the selected liquid candidates, accelerated aging tests are described in this section to illustrate how the optical transmittance of liquids is affected by UV light, damp heat and temperature exposure. The three aging tests are considered to be the most likely to cause degradation on the optical transmittance of immersion cooling liquids. The optical transmittance of the liquid candidates is monitored before and after exposure to the following conditions: (a) UV conditioning test: UV exposure with an intensity of 56.8 kWh/m2 in the wavelength range below 400 nm, while holding the liquid samples at 60 °C; (b) Damp heat test: 1400 h at 85% relative humidity and 85 °C; and (c) Temperature test: firstly, constant temperature of 65 °C for 700 h, and followed by constant temperature of 100 °C for 300 h. In addition, it should be noted that our accelerated aging tests do not replicate the qualification standard for CPV module. We don’t follow the exact procedures described in the IEC62108 norm, nor do we utilize the complete module, which is a very costly process. Instead, we are concerned with the relative changes in optical transmittance of liquids, before and after, under stringent test conditions. The three exposures of these tests exceed those required by the IEC62108 standard
Fig. 2. Optical transmittance of (a) Therminol VP-1, dimethyl silicone oil, (b) white oils A/B/C and (c) C14 n-alkane, C16 iso-alkane, as well as AM1.5D solar spectral irradiance.
electron charge, c the speed of light, h Planck's constant, and λ1 to λ2 is the corresponding spectral response wavelength range of each subcell or multi-junction solar cell. 127
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Table 2 Normalized photocurrent density (Jnp) of the top, middle, and bottom subcells, and the entire 3-junction solar cell when AM1.5D reference irradiance is filtered by each liquid. Liquid species
Liquid thickness
Top (350–660 nm) GaInP Jnp
Middle (660–890 nm) GaInAs Jnp
Bottom (890–1800 nm) Ge Jnp
3-junction CPV cell (350–1800 nm) GaInP/GaInAs/Ge Jnp
τ (λ ) = 1 Therminol VP-1 Dimethyl silicone oil White oil A White oil B White oil C C14 n-alkane C16 iso-alkane
0 mm 10 mm 10 mm 10 mm 10 mm 10 mm 10 mm 10 mm
1 0.980 0.992 0.997 0.998 0.980 0.974 0.995
1 0.988 0.992 0.993 0.996 0.982 0.982 0.995
1 0.904 0.857 0.785 0.792 0.783 0.795 0.796
1 0.950 0.934 0.905 0.909 0.896 0.900 0.910
Fig. 3, it can be clearly observed that the UVA 340 fluorescent lamp provides much more irradiance than the AM1.5D solar spectrum in the wavelength range below 365 nm. In addition, the UV irradiance in terms of power in the wavelength range below 400 nm specified by the UV conditioning test in IEC62108 standard is quantified in Table 3. It can be seen that the UV exposure in this case exceeds the duration required by the IEC 62108's UV conditioning test. Since Therminol VP-1 samples exhibit a color shift from clear to orange-brown (noticeable through visual inspection since 41.5 h UV exposure) possibly caused by the degradation of the synthetic oil component, we only consider the other six liquid candidates in the following section. Fig. 4 describes the optical transmittance measured for the six liquid samples as a function of wavelength and UV exposure time, showing that optical transmittance get worse within the wavelength range below 400 nm for mineral oils, and below 300 nm for dimethyl silicone oil. No distinct effect of degradation, however, is detected for any of the liquid samples in the VIS wavelengths. Regarding the IR wavelengths, a slight decrease in measured transmittance is observed in wavelengths 1400–1700 nm for the five mineral oils. On the contrary, dimethyl silicone oil remains stable in the NIR wavelengths. As mentioned above, the wavelength range used in the GaInP/GaInAs/Ge triple-junction solar cells is 350–1800 nm. For dimethyl silicone oil, the photocurrent of the complete triple-junction solar cells will remain almost constant after 1400 h UV conditioning test. For the mineral oils, in most cases, the photocurrents of both the top and bottom subcells will have slight decreases after UV exposure whereas the middle subcell photocurrent will remain unaltered. A calculation of the average transmittance for the spectral response region (350–1800 nm) of the specified GaInP/GaInAs/Ge triple-junction solar cell revealed the biggest losses from reductions in the optical transmittance (see Fig. 5). UV exposure time-dependent transmittance losses were observed for the five mineral oils. The transmittance losses of white oils A/B/C, C14 n-alkane and C16 iso-alkane exposed to UV degradation test with respect to the values before the test are 4.66%, 3.81%, 3.57%, 1.97% and 5.74%, respectively. Dimethyl silicone oil exhibited a transmittance loss of 0.42%, after 1400 h UV exposure and a slight increase after 800 h UV exposure, which is consistent with the findings of Everett et al. [35] and Eltermann et al. [36]. But the root cause for that has not been provided.
Fig. 3. Spectral irradiance of the UVA 340 lamp used and the AM1.5D solar spectrum.
[33]. 4.2. Measurement For each liquid under each exposure, we prepared two samples and characterized their optical transmittance as a function of exposure time through the LAMBDA 950 spectrophotometer mentioned above. Furthermore, two duplicate tests were conducted for every set of transmittance measurements and the averaged values were analyzed. In addition to the optical transmittance characterization, the visual appearance of the samples was examined every day. 4.3. Results and discussion 4.3.1. UV exposure In this case, the promising liquids were contained in the high UVtransmitting quartz tubes and were aged for 1400 h in a UV weathering chamber equipped with UVA 340 fluorescent lamps, while maintaining the liquid samples at 60 °C. Fig. 3 presents the spectral irradiance of the UVA 340 fluorescent lamp provided by the manufacturer. Also included is the spectral irradiance of the AM1.5 direct normal solar spectrum. In Table 3 UV irradiance within the wavelength range below 400 nm. Requirement for UV exposure
AM1.5D spectrum
UVA 340 lamp
Standard
λ (nm)
E (kWh/m2)
P (W/m2)
treq (h)
treq (days)
P (W/m2)
t (h)
E (kWh/m2)
IEC 62108
＜400
50
30.5
1639
273
40.6
1400
56.8
Note: the table provides the requirements of the UV degradation test in the IEC62108 standard in terms of wavelength range λ and accumulated UV dosage E. It also provides the incident light power P for the AM1.5D spectrum, the required time treq to satisfy the IEC62108 standard and the equivalent time treq(days) in real conditions (where treq(h) = 6 × treq(days) [32]). Further, the table provides the incident light power P for the UVA 340 lamp, the aging time t, and the accumulated UV dosage E.
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Fig. 5. Average transmittance of liquid samples before and after UV exposure (56.8 kWh/ m2, 60 ± 5 °C).
Fig. 4. Optical transmittance of liquid samples before and after UV exposure (56.8 kWh/ m2, 60 °C).
4.3.2. Damp heat exposure In this test, the damp heat exposure exceeds the aging time required by the official standard IEC62108 to identify the durability of the liquid candidates for immersion cooling multi-junction solar cells. Fig. 6 plots the effect of long-term damp heat exposure on the selected six immersion cooling liquids. It can be seen that all the liquid samples exhibit that the degradation mainly occurs at UV wavelengths, but the degree of change is much more distinctive than the results obtained from UV exposure. Both white oil A and white oil B show higher transmittance losses in the UV spectrum than the other four tested liquids. Further, the transmittance in the VIS and NIR ranges is affected minimally except for white oil B in this test duration. While Victoria et al. [17] suggested that the discrepancy in behavior of white oils A, B and C could be explained by the refining process adopted to achieve them. Obviously, the content of impurities in white oil C is lower than in both white oil A and B, remaining almost stable of the samples after 1400 h damp heat exposure. The average transmittance of the liquid samples before and after the damp heat test (1400 h at 85% relative humidity and 85 °C) is shown in Fig. 7. It highlights that the transmittance losses of both dimethyl silicone oil and white oil C after damp heat exposure are less than 0.5%. White oil A and White oil B are more strongly influenced and their transmittance losses are 1.34% and 3.42%, respectively, while C14 nalkane and C16 iso-alkane degraded less, with 0.77% for C14 n-alkane and less than 1% for C16 iso-alkane. Moreover, with visual inspection, we did not detect any change in color over the test duration for all the tested liquids.
Fig. 6. Optical transmittance of liquid samples before and after the damp heat test (1400 h at 85% relative humidity and 85 °C).
aging tests have been performed. Based on the experimental results of liquids acquired from the UV conditioning test, the liquid candidates except for Therminol VP-1 were sealed in borosilicate glass containers and exposed for a period of 700 h to 65 °C and followed by to 100 °C for 300 h. Since the operating temperature of the selected triple-junction solar cells is ranges from −40 °C to 100 °C [30], the high-temperature (100 °C) exposure would show the maximum effect on the liquids. The aim of the low-temperature exposure is to see whether the optical properties of the liquids can remain stable under the typical operating temperature of CPV modules [38]. The effect of temperature exposure for a total period of 1000 h on the liquid candidates for cooling III-V cells is illustrated in Fig. 8. Near identical results are obtained for the liquid samples between the damp heat test and the temperature test. The major change in optical transmittance of all the tested liquids after the temperature test is limited to
4.3.3. Temperature exposure It is well known that temperature is one of the main environmental factors of PV module degradation in almost all identified degradation modes [37]. To simulate thermal degradation under real conditions and thus the optical transmittance loss during the immersion cooling liquid lifetime, low-temperature followed by high -temperature accelerated 129
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are very positive. The transmittance losses of all the tested liquids except for C16 iso-alkane under UV exposure are less than the total degradation value of 5% allowed for CPV modules under the qualification tests in the IEC62108 standard. Since the average transmittance loss of C16 iso-alkane after being subjected to the UV test is over 5% (see Fig. 10), it has been deemed not suitable for immersion cooling III-V multi-junction solar cells. The effect of UV degradation on the average optical transmittance of each liquid is much larger than those from the damp heat test and temperature tests. In addition, it can be observed in Fig. 10 that dimethyl silicone oil presents a degradation of less than 0.5% in average optical transmittance over the period of each accelerated aging test. The difference in behavior of dimethyl silicone oil and mineral oils has to do with their respective covalent bonds. The covalent silicon bonds in dimethyl silicone oil has a larger dissociation energy of 451 kJ/mol than that (347 kJ/mol) of the covalent carbon bonds in mineral oils [17]. Therefore, the results from the accelerated aging tests indicate that the optimum liquid for III-V cells immersion cooling application is dimethyl silicone oil, followed by C14 n-alkane. They also give confidence for the reliable performance of white oils A/ B/C.
Fig. 7. Average transmittance of liquid samples before and after the damp heat test (1400 h at 85% relative humidity and 85 °C).
5. Conclusions Three groups of common and inexpensive liquids encompassing seven materials have been studied in terms of their optical transmittance and durability for immersion cooling III-V multi-junction solar cells under high concentration. A double optical path-length transmittance method is proposed for determining the optical transmittance of the liquid candidates. Further, the normalized photocurrent density ( Jnp ) is used to characterize the effect of liquid transmittance on the photocurrent density of GaInP/GaInAs/Ge triple-junction solar cells when illuminated by the solar spectral irradiance filtered by each liquid. Results show that all the liquids have promising initial optical transmittance for the wavelengths of interest and are selected for further accelerated aging tests to study their durability. The accelerated aging tests include a UV conditioning test, a damp heat test, and a lowtemperature test at 65 °C followed by a high-temperature test at 100 °C. Results from the accelerated aging tests indicate that Therminol VP-1 and C16 iso-alkane are considered unsuitable for CPV application as they suffered a serious degradation after UV exposure. The other five liquid candidates were demonstrated as appropriate to be utilized for high concentration III-V photovoltaic systems. Their average transmittance losses are less than the total degradation value of 5% allowed for CPV modules under the qualification tests in the IEC62108 standard. The optimum liquid for immersion cooling of multi-junction solar cells is found to be dimethyl silicone oil, due to its high transmittance for the Fig. 8. Optical transmittance of liquid samples before and after low- and high-temperature tests (700 h at 65 °C & 300 h at 100 °C).
shorter wavelengths in the UV spectrum. The much more significant deterioration of transmittance in the UV region is observed for white oil B and white oil A. For immersion cooling triple-junction solar cells applications, in most cases, only the photocurrent of the top subcell would decrease. Quantitatively, the average transmittance losses are 0.22%, 1.38%, 2.54%, 0.95%, 0.76% and 1.26% for white oils A/B/C, C14 n-alkane and C16 iso-alkane subjected to the temperature degradation test with respect to their initial values (see Fig. 9). During the temperature test period, no change was detected visually in terms of the liquid color. Finally, the relative changes in the average optical transmittance of each liquid after accelerated aging tests are summarized in Fig. 10, which is related to the power losses that the CPV module would manifest due to degradation. As a matter of fact, the results shown in Fig. 10
Fig. 9. Average transmittance of liquid samples before and after low- and high-temperature tests (700 h at 65 °C & 300 h at 100 °C).
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Fig. 10. Average transmittance losses of liquid samples after exposure to all the accelerated aging tests.
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