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SiO2 optical thin-film filter for hybrid solar power systems
Applied Energy 92 (2012) 298–306
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Investigation of a broadband TiO2/SiO2 optical thin-film filter for hybrid solar power systems Chunhui Shou a, Zhongyang Luo a,⇑, Tao Wang a, Weidong Shen b, Gary Rosengarten c, Wei Wei a, Cheng Wang a, Mingjiang Ni a, Kefa Cen a a b c
State Key Laboratory of Clean Energy Utilization, Zhejiang University, China State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, China School of Mechanical and Manufacturing Engineering, University of New South Wales, Australia
a r t i c l e
i n f o
Article history: Received 22 February 2011 Received in revised form 28 August 2011 Accepted 17 September 2011 Available online 1 December 2011 Keywords: Solar power Hybrid system Spectrally selective filter Optical thin-film Broadband
a b s t r a c t Using the technology of spectral selectivity to integrate different solar power generators in a hybrid system is a feasible way to improve the optical-electric efficiency. This paper presents an 82-layer broadband optical interference thin-film filter matching with crystalline silicon photovoltaic cells, which using TiO2 and SiO2 as fabrication materials and can be used in hybrid solar power systems like photovoltaic–thermoelectric generator (PV–TEG) systems. The design, optimization and fabrication process of the filter is described, high reflectance from 400 nm to 1100 nm as well as high transmittance from 1100 nm to 2500 nm over the broadband of solar spectrum are obtained. The classical electron beam evaporation plant is used to fabricate the filter. Four different incidence angles’ optical performances of the sample filter are measured which agree well with the numerical simulation results. The electrical characteristics of a typical Silicon photovoltaic cell using the fabricated sample filter are measured. An average efficiency increase of 3.24% for the solar cell with respect to the solar energy it receives can be obtained due to the filter. A calculation model for a hybrid PV–TEG system using this thin-film filter is proposed and the benefits of the filter for hybrid solar power systems are demonstrated. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The terrestrial solar radiation spectrum covers a broad band of wavelengths. Typical photovoltaic (PV) solar cells with a given band-gap can utilize only a certain portion of this spectrum depending on the cells’ spectral response. A photon with energy below the band-gap is unable to contribute to the electrical current; it will be converted to heat, which decreases the cells’ efficiency. A photon with energy above the band-gap can be converted by giving up the energy in excess of the band-gap as heat to the cells. Only photons with energy at or close to the solar cells’ band-gap energy provide the most efficient photovoltaic conversion [1]. More than 50% of the incident solar energy is converted to heat for a common PV module [2]. It is better to integrate PV cell with other energy converter in a proper combination to form a hybrid system, which means weakness of one converter can be overcome by the other type [3,4]. Using concentrators to focus a large area of solar radiation onto a PV cell of smaller surface area reduces the amount of PV materials
⇑ Corresponding author. Address: Zheda Road 38, Hangzhou 310027, China. Tel.: +86 571 8795 2440; fax: +86 571 8795 1616. E-mail address: [email protected] (Z. Luo). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.09.028
required to convert the same quantity of solar energy to electrical energy compared to a standard one-sun PV cells [5]. As high conversion efficiency (27.6% [6]) for concentrator Si solar cells is possible now, solar concentrator systems have drawn increasing interest. However, with increasing concentration levels, there are more photons with band-gap energy mismatch. One traditional way to solute this problem is using photovoltaic/thermal (PVT) hybrid system, which utilizes otherwise the wasted heat. As PV used as a thermal absorber in PVT [2], it is overheated easily, and the efficiency of the PV conversion is reduced. An optimal way of using a solar power system with concentrator is to divide the solar radiation by preventing the unwanted irradiation wavelengths from reaching the cells thus utilizing only the photons that best match the cells’ band-gap. There are generally three methods to accomplish this optimal way of solar power utilization [7]. The first approach uses thermophotovoltaic (TPV) [8] or luminescent concentrators [9] to shift the solar spectrum toward wavelengths better matched to the selected solar cells. The second approach stacks PV cells of varying band-gap from highest to lowest to get good efficiency with absorbing a larger part of the solar spectrum [10]; the third approach uses a beam splitter to separate the incident radiation into some selected spectrum to drive photovoltaic and photo thermal conversion [11–13].
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Nomenclature C E EQE FF h I P q Q SR T V
optical concentration solar irradiance, W/m2 external quantum efficiency fill factor of solar cell Planck constant, m2 kg/s current of solar cell, A power, W electric charge, C heat loss, W/m2 spectral responsivity temperature, °C voltage of solar cell, V
Greek symbols temperature factor of Si cell b temperature coefficient e hemispherical surface emissivity c concentration factor of Si cell g energy conversion efficiency k wavelength, nm m frequency, Hz q reflectance of the filter r Stephan–Boltzmann constant, W/m2 K4
a
As in a solar concentrator system, especially with a high concentration ratio, it is wiser to produce electricity with useful thermal energy, rather than abandoning this heat. Due to the rapid progress in the field of optical devices [14–17], the beam splitting approach was taken up by researchers [11–13,18–22]. Though the design and manufacture of optical filters for the solar concentrator systems faces many challenges such as dealing with the broad range of wavelengths available in the solar spectrum, it is well-established that the use of dielectric optical thinfilm interference filters to split the solar light with high energy-flux is the preferred option. An optical dielectric thin-film interference filter can be defined as a series of zero absorption thin parallel layers of alternate high and low refractive index materials, attached to a substrate like glass. As illustrated in Fig. 1, the filter reflects and transmits the selected spectrum regions, based on the interference of light between the individual thin layers. There are plenty of reports focusing on the design of these solar spectrum filters [12,13,15,17,18,21–23]. Osborn’s group had proposed a 12-layer sample optical multilayer filter consisting six quarterwave pairs of TiO2 and SiO2 for hybrid solar PV systems [21], aim to get a high reflectance from 600 nm to 1000 nm. Imenes’ group focus on the design and optimization process, two kinds of good performance filter for a hybrid photovoltaic/thermal central receiver system
s
transmittance of the filter
Subscripts 0 refers c refers conv refers cool refers CPV refers el refers g refers in refers m refers oc refers ph refers PV refers rad refers ref refers s refers sc refers sub refers SYS refers TEG refers
to to to to to to to to to to to to to to to to to to to
ambient PV cell convection from the cell surface cooling system concentration photovoltaic conversion electrical output of solar cell cover glass of solar cell input data on the solar cell data of solar cell at maximum power point open circuit incident photon photovoltaic conversion radiation from the cell surface reference value spectral short circuit substrate of solar cell hybrid solar power system thermoelectric generator
[18] are calculated. However, to produce multilayers with a measured optical performance that is as close as possible to that designed is difficult as manufacturing thick coatings requires suitable materials and an advanced deposition technique. Currently there are only a few studies concerned with the fabrication or experimental utilization of these filters with PV cells. In this paper, a reliable and practical filter with good efficiency performance will be presented. A wider selectivity wave range than most filter will be used, which can match more commercial crystalline silicon cells’ spectral response range than before. A more feasible design structure which can commonly be used for fabrication in the optical filter industry should be obtained. The reminder of this paper is structured as follows. First, in Section 2, the design process of an optical thin-film filter that can be applied to hybrid solar systems like photovoltaic–thermoelectric generator (PV–TEG) are described and the performance characteristics of designed stacks are showed. In addition, the needle numerical method is used to optimize the optical properties of filter. Then, in Section 3 the fabrication procedure is provided and the optical properties of sample filter are measured. In Section 4, the performance of a typical Si cell using this filter is presented. In Section 5, a calculation model has been developed to analyze the properties of the hybrid PV–TEG system using the beam splitting thinfilm interference filter. Furthermore, under realistic assumptions, a discussion of the performances of PV–TEG systems using this thin-film filter is reported. And the works are summarized in Section 6 as a conclusion.
2. Design of the solar beam filter 2.1. Filter strategy
Fig. 1. Schematic diagram of the thin-film interference filter.
Photovoltaic conversion is highly spectrally dependent, so solar cell performance as a function of wavelength is required to optimally match a cell response to the characteristics of the design of the beam filter. Fig. 2 shows part of the irradiation spectra for an air mass of 1.5 (AM 1.5) [24] and the fraction of this radiation that can be absorbed by a bulk Si device [25], where a value of 1100 nm
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Fig. 2. AM 1.5 spectrum with the reflecting window and the wavelengths that can be absorbed by a Si device.
is used as the cut-off response. The spectral distribution of the AM 1.5 shown in Fig. 2 spans the wavelength region from 300 nm to 2500 nm and contains overwhelming majority of the total incident solar energy. The wavelength selectivity of the filter is in the region from 400 nm to 1100 nm, which contains about 76% of the total solar energy, and corresponds to high efficiency PV conversion. Reflecting this segment of energy (illustrated in Figs. 2 and 3) to the solar cell, the rest of the energy is simultaneously transmitted to either a thermoelectric generator (TEG) or a thermal power generator (TG). Fig. 3 is a basic system schematic of the spectrally selective approach with TEG which will be discussed in the latter section. 2.2. The numerical methods of design The optical thin-film solar filter has performances that depend on the number of layers of coatings, the refractive index and thickness of each layer. All the characteristics useful can be found by numerical methods, which also can easily take into account the properties of real materials. Some of these methods need a starting design which can be any typical analytical solution, and then the designed is refined by adjusting parameters like the thickness and the refractive index of the layers to minimize a merit function [18]. Some methods like global optimization do not need a starting design and can search for the solution in the whole allowed region
Fig. 3. Basic system schematic of the spectrally selective approach.
automatically. However, for the complex broadband multilayer designs, this method requires a large number of layers and a very long time for calculating. A synthesis design method like the needle method can reduce the number of layers [26]. In this paper, a series of stacks mading up by a number of symmetrical periods as 0.5HL0.5H will be used to get the excepted high reflectance over particular spectrum range. H and L are respectively, a quarterwave layer of high index material and low index material. Then the needle method will be used to optimizate it and make it feasible for fabrication. The region of the solar spectrum that needs to be reflected (400–1100 nm as shown in Fig. 2) is very broad for filter design and fabrication. A consequence of the broad wavelengths band is the highly complex requirements of the coatings, which can be so thick that they may be easily damaged by internal stresses. As a feasible solution of this, two series of stacks at each side of the substrate will be deposited to reduce the number of the layers and the total physical thickness of each stack. The structure diagram of this is shown in Fig. 4. The design process should be performed for a specified design angle of incidence, which is assumed as 45° for the hybrid solar system shown in Fig. 3. The front surface of the substrate is coating with stacks as 0.7(0.5HL0.5H)50.8(0.5HL0.5H)40.9(0.5HL0.5H)4 (0.5HL0.5H)4, and the back surface coating with stacks as 0.8(0.5HL0.5H)40.89(0.5HL0.5H)40.9(0.5HL0.5H)4. Central wavelength is set as 580 nm for the front coating and 1000 nm for the back coating. TiO2 and SiO2 are used as the high index material and low index materials, which have negligible absorption in the spectral range of interest. These two materials are commonly used for coating and can manage with high thermal load, which would suitable for fabrication and further using in the optical industry. The optical constants H and L at different wavelengths are listed in Tables 1 and 2. 2.3. The optimization of the designed stacks The needle optimization method works by adding new zerothickness layers onto a coating design, and using a local optimization methods to improve the new design. If the zero-thickness layer has been placed in the correct position, the local optimization will force the new layers to grow. The needle optimization method implemented in the thin film design software TFCalc (Software Spectra Inc.) is used here to design and analyze the thin-film filter for hybrid solar power systems. The design angle of incidence is set as 45°. The optimization begins with 38-layer starting design stacks as Sub/ 0.7(0.5HL0.5H)50.8(0.5HL0.5H)40.9(0.5HL0.5H)4(0.5HL0.5H)4/Air
Fig. 4. Diagram of the designed thin-film filter structure and a simply illustration of the interference of the lights reflected from a random layer’s two sides.
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C. Shou et al. / Applied Energy 92 (2012) 298–306 Table 1 Optical constants of TiO2 film at different wavelengths. Wavelength (nm)
400
430
450
500
550
600
700
800
900
Index N Index K
2.3970 0.0028
2.3132 0.0028
2.2737 0.0028
2.2108 0.0027
2.1779 0.0027
2.1607 0.0027
2.1478 0.0026
2.1465 0.0026
2.1487 0.0026
Table 2 Optical constants of SiO2 film at different wavelengths. Wavelength (nm)
350
400
450
500
550
600
650
700
900
1000
1562
Index N Index K
1.472 0
1.467 0
1.463 0
1.459 0
1.455 0
1.452 0
1.450 0
1.446 0
1.437 0
1.434 0
1.429 0
on the front surface of the substrate, and 27-layer stacks as Sub/ 0.8(0.5HL0.5H)40.89(0.5HL0.5H)40.9(0.5HL0.5H)4/Air on the back surface. Air is incident and exit medium. Sub means BK7 glass substrate. After setup the stack formula and the parameters for the environment and materials, the design target of the front stacks is set as reflectance above 90% from 400 to 670 nm and transmittance above 90% from 1100 to 2500 nm. Simultaneously, reflectance above 90% from 670 to 1100 nm and transmittance above 90% from 1100 to 2500 nm is the target for the back stacks. The optimization results of the filter structure are shown in Fig. 5. The front stacks consists of 52 layers with a total thickness of 4320 nm, and the back stacks consists of 30 layers with a total thickness of 4330 nm. The reflectance curve of the optimized filter as a function of wavelength is comparing with the corresponding reflectance characteristics of the two starting design stacks shown in Fig. 6. The optimized front stacks have a maximum reflectance of 97% at 636 nm, while the back coating has a maximum reflectance of 98.2% at 929 nm. Compared to the discrete multilayer filter in Ref. [18], a wider high reflectance range (nearly 200 nm) and less number of layers (reduced by 25 layers) are obtained, which means more types of silicon cells’ response range can be matched and more feasible for the fabrication. While keeping similar high performance from 600 nm to 1100 nm, much better transmittance from 1100 nm to 2500 nm than the relevant one are provided by this filter.
Fig. 6. Reflectance characteristics of designed front and back stacks comparing with the optimized filter at an incident angle of 45°.
The filter was fabricated by classical electron beam evaporation plant (Balzers BAK600). Coatings were deposited at both sides of
100 mm 100 mm BK7 glass substrate illustration as Fig. 4. The background vacuum was 2:0 103 Pa while the pressure of oxygen for TiO2 deposition was kept 1.8 102 Pa. Meanwhile, the deposition temperature on substrate was kept at 300 °C during the process. The deposition rates of TiO2 and SiO2 are respectively 0.3 nm/s and 1 nm/s controlled by a quartz sensor. During the deposition of the filter, the indirect monochromatic optical monitoring in transmission was used to control the thickness of each layer. The sample filter fabricated is shown in Fig. 7. The measurements of optical performance in the spectral range from 400 nm to 2500 nm were performed with a resolution of
Fig. 5. Refractive index profiles of the filter. (The substrate thickness is not scaled in the figure.)
Fig. 7. Sample of the fabricated broadband TiO2/SiO2 optical thin-film filter.
3. Fabrication of the designed filter
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Fig. 10. All the data were measured in sunny and cloudless weather. The temperature of the cell during the testing process was controlled around 35–40 °C. There are clear increases in short circuit current and the output powers as the incident power density on the solar cell becomes higher than 2200 W/m2. The open-circuit voltages follow a similar behavior which is not so obvious. From the figures it is evident that filter strongly increases the short circuit current and the maximum output power of a PV cell given the similar incident power density compared with the regular one. And the maximum output power with the filter increases faster than the one without. As the incident power density becomes higher, these effects become more obvious. On the other hand, this filter does not affect the open-circuit voltage (except the change in (a)) very much in all measured conditions. The energy-conversion efficiency of solar cell is defined by, Fig. 8. Measured reflectance of the filter with different incident angles (0°, 15°, 30° and 45°).
2 nm on a spectrophotometer (Shimada UV310). Fig. 8 shows the measured results of the filter with an angle of incidence of 0°, 15°, 30° and 45°, respectively. The experimental results shown in Fig. 8 agree very well with those of the numerical simulation. There is small difference of the low reflectance part mainly owing to the variation of thickness and optical constants of materials used between the deposition and the design. Near the boundary of selectivity region from 1100 nm to 1250 nm, there is an undesired shoulder with a relatively high reflectance. For a long pass filter, the accumulated deposition errors will result in the distortion of optical properties near the cut-off region. Therefore, the shoulder appears in the final optical performance of the filter after the deposition on the front and rear sides. Fortunately, the solar radiation energy in the spectral region from 1100 nm to 1250 nm is low so the decay in the transmission will not have a large adverse effect on the performance of the filter. The high reflectance over the stop band 400–1100 nm in Fig. 8 ensures the filter is well match to silicon photovoltaic cells. There is a variation in the filter performances as a function of incident angle. The boundary shifts to longer wavelengths as the angle is varied from 45° to 0°, due to the optical length becoming shorter. The ripples over 400– 1100 nm are no longer visible at the angle of 0° because the polarization effect disappears at the normal incidence. Compared to 12layer filter reported in Ref. [21], nearly two times wider selective band of spectrum is obtained. Much smoother curve of better performance data is shown over the whole selectivity range by the fabricated sample filter. 4. The measured performance of a Si cell with and without the filter In order to investigate the filter’s usefulness for PV Si cells, the performance of a commercial crystalline Si cell Si–N (UXUAN Inc.) are tested with and without the filter. Radiation data is collected from a 30A-P Thermopile head (OPHIR OPTRONICS Inc.), which has a flat spectral response for irradiation over a broadband spectral range from 150 nm to 6000 nm. The electrical characteristic of cell is measured by a Keithley 2440 source measurement unit. The temperature of the cell is observed via T type thermocouples with an Agilent 34970A Data Acquisition/Switch Unit. The characteristics of the cell are measured outside under the natural sun light with Fresnel lenses at the latitude of Zhejiang University HangZhou, China. The results of the experimental current–voltage characteristics and output power with the different incidence power density are showed in Fig. 9. The PV conversion efficiencies are reported in
g¼
Pm V m Im ¼ Pin Pin
ð1Þ
where Pm is the cell’s maximum electric output power and Pin is the input power of the solar irradiation on the cell’s surface. Vm, Im are the voltage and current at maximum power point. By the comparison of the efficiency, using the filter brings an average efficiency increase of 3.24% for the solar cell, while the highest increase is around 5% with incident power density at approximately 3000 W/m2. As the Si–N cell is commercial crystalline silicon which is not designed for concentrated sunlight the efficiency decreases as the incident power density becomes higher. The principal reasons for the reduction in cell efficiency are the grid series resistance losses that rise fast under concentration, and current crowding near the junctions [5,27]. Better efficiency performance of the PV systems could be expected, if silicon concentrator cells are used with the filter. 5. Efficiency calculation of hybrid PV–TEG systems 5.1. Model description On the basis of the experimental data reported in Sections 2 and 3, the expected total conversion efficiency for the combined PV– TEG hybrid system using the optical thin-film filter can be calculated. The schematic of the spectrally selective PV–TEG system is shown in Fig. 3. The filter’s performance is shown in Fig. 7. The solar TEG efficiency can be assumed as 8% [11], which is constant over the whole solar spectrum. Four kinds of different silicon cells are investigated as the PV part of the hybrid system: (1) commercial crystalline silicon cell SSRC-C50 (SSRC Inc.); (2) commercial crystalline silicon cell Si–N; (3) heterostructure solar cell based on p-type crystalline silicon (HJ + Cr + ZnO) [28]; (4) backside point-contact silicon concentrator solar cell II-3B [29]. There are two types of solar cell spectral response data used in this paper to get the spectral conversion efficiency of cells. The first one is the external quantum efficiency (EQE); the second is the spectral responsivity (SR). The EQE can be defined as the current obtained outside the device per incoming photon [1], and it gives information on the current that a cell will produce when illuminated by a particular wavelength. In Fig. 11 the EQE curves over 400–1100 nm of the three silicon cells are shown [28]. The SR describes the sensitivity of the cell to incident radiation of different wavelengths. The data of SR of the concentrator solar cell is shown in Fig. 12 in terms of ‘the outputs short-circuit current per unit incident power’ [29]. The EQE and SR can be used to express the spectral conversion efficiency gPV,k in Eq. (2), as a ratio of the spectral output power Pk and the spectral incident photon power PPV,k:
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(a)
(b)
(c)
(d)
Fig. 9. Measured current–voltage characteristics and output powers with and without the filter using different incident power density respectively.
The efficiency of the hybrid system of three silicon cells will be calculated as well as the efficiency of the backside point-contact silicon solar cell II-3B under concentration with and without the filter. For the hybrid PV–TEG systems working without concentration, the weighted standard cell efficiency which the photovoltaic conversion actually uses can be given by,
R k2
qk gPV;k Ps;k dk gPV;work ¼ k1R k4000 qk Ps;k dk k280
ð3Þ
Fig. 10. Comparison of the Si–N cell efficiency with and without the filter.
gPV;k ¼
Pk Isc;k V oc FF q V oc FF ¼ ¼ EQE ¼ SR FF V oc P ph;k hmph Pph;k
ð2Þ
where Pk is the product of the spectral short circuit current Isc,k, with the open circuit voltage V1 and the fill factor FF of the solar cell; q is electric charge, hmph is photon energy [11,30]. It is assumed that FF and V1 are not wavelength dependent. The spectral efficiency characteristics as a function of wavelength of the four cells can be calculated and shown in Figs. 11 and 12.
Fig. 11. The EQE and spectral efficiency curves of the three silicon cells (at one-sun AM 1.5).
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Fig. 12. The SR and spectral efficiency curves of the concentrator solar cell (at onesun).
Fig. 13. The efficiency versus incident power density for a silicon solar cell.
a ¼ 1 bref ðT c T ref Þ where qk is the spectral reflectance of the filter over solar spectrum as designed, gPV,k is the spectral efficiencies of solar cells, Ps,k is the spectral radiation power of AM 1.5G [24], k1 to k2 are the portion of wavelengths that cells have a spectral response. Meanwhile, the system conversion efficiency gSYS1 can be calculated with following equation:
R k4000 k280
gSYS1 ¼
sk gTEG Ps;k dk þ R k4000 k280
R k2 k1
qk gPV;k Ps;k dk
Ps;k dk
ð4Þ
where sk is the spectral transmittance of the filter at a particular wavelength as calculated in the design software TFCalc, gTEG is the efficiency of the solar TEG. The weighted standard cell efficiency of II-3B with the filter can be given in the similar way as Eq. (3),
R
k2 qk gPV;k P0s;k dk gref1 ¼ k1R k4000 qk P 0s;k dk k280
ð5Þ
where P0s;k is the spectral beam radiation power of AM 1.5 (Direct & circumsolar) [24], and the cell efficiency without the filter is calculated as,
R
k2 gPV;k P0s;k dk gref2 ¼ k1R k4000 k280
Ps;k dk
ð6Þ
The use of concentrator results in increases of the incident power density and the cell temperature. Ref. [30], the effect of concentration and temperature can be taken into account by multiplying gref1 and gref2 with a concentration factor c and a temperature factor a respectively:
gCPV 1 ¼ gref1 a c
ð7Þ
gCPV 2 ¼ gref2 a c
ð8Þ
ð10Þ
which is from the traditional linear expression for the PV electrical efficiency [31]. Where bref is the temperature coefficient, Tref is the silicon cell’s reference temperature. In this paper, bref is set as 0.0045 [31] and Tref = 25 °C. To get the cell temperature’s varied relationship with incident power density on the solar cell’s surface, a one-dimensional thermal model [32] is used, which calculates the transferred heat and assume the heat flow is primarily directed in the normal direction. The Si cell is idealized, which contain five layers: cover glass, adhesive, cell, solder and substrate. The thicknesses of the various layers can be known from the cell manufactory [29,32]. The schematic of the model is shown in Fig. 14. Based on an energy balance, the input solar energy is equal to the output energy and the relation is given by,
E ¼ Q rad þ Q conv þ Q cool þ Pel
ð11Þ
where E is the input solar irradiance, Qrad is the heat loss by radiation from the surface, Qconv is the heat loss through convection from the surface, Qcool is the thermal energy removed by the cooling system, Pel is the electrical output of the cell. The radiative heat flux is represented as follows:
Q rad ¼ erðT 4g T 40 Þ
ð12Þ
where Tg is the glass surface temperature, T0 is the ambient temperature, e is the surface emissivity and r is the Stephan–Boltzmann
For a constant cell temperature, the typical silicon concentration cell behavior with incident power density as shown in Fig. 13 [29], and curve fitting is used for the solid line. The concentration factor c can be given by,
c ¼ 1 þ a1 X þ a2 X 2 þ a3 X 3 þ . . .
ð9Þ
where X = lnC, C stands for optical concentration [30]. As the efficiency of the photovoltaic conversion is greatly affected by the temperature of the PV cell Tc, the effect of temperature should be calculated. The temperature factor a can be given by,
Fig. 14. Schematic of the heat transfer mechanism of the one-dimensional thermal model of PV cell.
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constant. When the cell temperature is up to 170 °C, this equation is linearized for simplification as follows:
Q rad ¼ 4erT 30 ðT g T 0 Þ
ð13Þ
The convective heat flux is given by
Q rad ¼ 4erT 30 ðT g T 0 Þ
ð14Þ
where Rconv is the thermal resistance of convection and can be obtained from the reference. The heat flux removed by the cooling system is
Q cool ¼
T sub T 0 Rcool
ð15Þ
where Tsub is the substrate temperature, Rcool is the thermal resistance through the cooling system. Fig. 15 shows the simulation results of cell temperature versus illumination for various values of Rcool (Km2/W). Ref. [33] Rcool can be assumed as 0.0002 Km2/W, comparing with the case Rcool = 0.001 Km2/W simultaneously. The electrical efficiency of the hybrid PV–TEG system using concentration silicon cell with the filter can be calculated as:
R k4000
gSYS2 ¼
k280
sk gTEG
P0s;k dk þ R k4000 k280
ac
R k2 k1
qk gPV;k
P0s;k dk
Ps;k dk
Table 3 Comparison of calculation results of the hybrid systems at one-sun.
ð16Þ
where concentration factor c and temperature factor a depend on the incident power density as shown in Fig. 16. It is clear that this concentrator silicon cell efficiency generally increases with illumination intensity before near 100 suns. As the concentration factor is higher than 1 up to 250 suns, an improvement in the efficiency can be expected when fluxes higher than 1000 W/m2. The heating load makes the cell temperature increase with illumination intensity, which would degrade the PV efficiency. If Rcool = 0.001 Km2/ W, the temperature effect is significant when the illumination intensity higher than 15 suns. If Rcool = 0.0002 Km2/W, the cell could work well until 120 suns. 5.2. Results and discussion The calculation results of the PV–TEG systems using three cells in Fig. 11 with the filter are listed in Table 3. gPV is the reported efficiency of each cell given by the manufacturer. There are clearly efficiency gains for the systems use Si–N and SSRC-C50 (2%). However, the efficiency improvement of heterostructure cell system is small due to the high efficiency (gPV) it already has, in addition to the slight mismatch of the filter design spectrum with the
Fig. 15. Cell temperature versus incident power density.
Fig. 16. The concentration factor c and the temperature factor a versus incident power density.
Si–N SSRC-C50 Heterostructure cell
gPV
gSYS1
gSYS1 —gPV
gPV,work
gPV,work–gPV
13.36 14.84 17.0
15.30 16.80 17.97
1.94 1.96 0.97
17.64 19.65 21.15
4.28 4.81 4.15
cell’s EQE curve. It can be observed in the system, the PV cells all work in a better efficiency as gPV,work shown in Table 3. Fig. 17 presents the efficiency performances of concentration PV system (with no filter) and concentration hybrid PV–TEG system with the filter at various optical concentration levels. All the cases following a similar trend: the efficiency increases with the concentration ratio, until a maximum, then decreases with increasing concentration ratio. The system’s cooling thermal resistance has a very large effect on the efficiency. When the cooling thermal resistance is 0.0002 Km2/W, maximum efficiency is much higher and there is a larger concentration range suitable for PV cell compared to the other cases. If system efficiencies are to be kept up to 18%, and cooling thermal resistance of 0.0002 Km2/W is assumed, then the maximum efficiency of 22.2% can be achieved by the concentrated PV system (CPV) at 41 suns. Meanwhile, the hybrid system could give a maximum efficiency of 21.6% at 51 suns. There is a crossing point of
Fig. 17. Hybrid systems’ electrical efficiency versus concentration.
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21.2% at 85 suns, which means: the concentrated PV system could work well from 1 to 85 suns; the hybrid PV–TEG system with the filter can gain more energy and work in a better efficiency at the higher concentration level from 85 suns to 230 suns in this situation. The power efficiency of hybrid system with the filter obtains 1.1% enhancement at 200 suns comparing with the CPV system. If the cooling thermal resistance is set to be 0.001 Km2/W, there would be a max value of 20.9% for the CPV system at 14 suns and a value of 20.4% for the hybrid system at 20 suns. The CPV system can work well from 1 to 35 suns and the hybrid system would be better at higher concentration level until 73 suns. 3.21% efficiency enhancement will be gained by hybrid system with the filter at 150 suns comparing with the CPV system. The higher cooling thermal resistance and concentration ratio are, the better effect of filter on efficiency can be expected. As TEG used in the hybrid system is only one feasible choice of energy convertors, better energy utilization performance can be acquired if other convertors with higher solar-to-electric efficiency are integrated. 6. Conclusion An optical broadband multilayer dielectric films filter for spectral selectivity utilization of solar energy is developed. The optimized design structure of the filter consists of two stacks, 52 layers with a total thickness of 4320 nm on one side and 30 layers with a total thickness of 4330 nm on the other side of a BK7 glass substrate, using TiO2 and SiO2 as design and fabrication materials. Comparing to some existing filters, wider high reflectance range (400–1100 nm) and better transmittance of near infrared are obtained with less layers and simpler structure. A classical electron beam evaporation plant (Balzers BAK600) has been successfully used to fabricate the filter. The measured optical data provided by a spectrophotometer (Shimada UV310) show that the spectral splitting effect behaves very similarly to what is designed. The performance of a solar cell Si–N with a filter is tested; there is a maximum efficiency increase of 5% for the Si cell with respect to the solar radiation it receives with a regular setup. A calculation model include four kinds of Si cells has been set up to predict a hybrid PV–TEG system’s efficiency with the filter. There are nearly two percents of efficiency gains for the hybrid systems with no concentrator comparing with PV systems without the filter. For the system with concentrator, the hybrid PV–TEG system with the filter gains more energy and work in higher efficiency than the CPV system when the illumination intensity level becomes appropriately high. A better improvement effect of the PV cell’s performance due to the filter can be achieved with the higher cooling thermal resistance and concentration ratio. Therefore, this optical broadband thin-film filter for beam splitting is a suitable technology for hybrid PV–TEG system; it is very likely that any real success of this hybrid system will be developed with improvements in optical coating technology and in TEG materials. Such a filter also offers great flexibility to combine various energy convertors with Si PV cells forming high efficiency hybrid solar utilization systems. If energy convertors with higher efficiency than TEG applied in the hybrid systems, like solar thermal power generators, better performance of the systems can be expected. Acknowledgements This work is partially supported by China Postdoctoral Science Foundation (20080441248), Research Foundation of Science and Technology Department of Zhejiang Province, China (2009C21023) and Zhejiang Provincial Natural Science Foundation (Y1090504).
References [1] Wenham S, Green M, Watt M, Corkish R. Applied photovoltaics. Earthscan/ James & James; 2007. [2] Chow TT. A review on photovoltaic/thermal hybrid solar technology. Appl Energy 2010;87(2):365–79. [3] Zhai H, Dai YJ, Wu JY, Wang RZ. Energy and energy analyses on a novel hybrid solar heating, cooling and power generation system for remote areas. Appl Energy 2009;86(9):1395–404. [4] Yang HX, Zhou W, Lou CZ. Optimal design and techno-economic analysis of a hybrid solar-wind power generation system. Appl Energy 2009;86(2): 163–9. [5] Luque A, Andreev V. Concentrator photovoltaics. Springer Verlag; 2007. [6] Green M, Emery K, Hishikawa Y, Warta W. Solar cell efficiency tables (version 36). Prog Photovoltaic 2010;18:346–52. [7] Imenes A, Mills D. Spectral beam splitting technology for increased conversion efficiency in solar concentrating systems: a review. Sol Energy Mater Sol Cells 2004;84:19–69. [8] Coutts T. An overview of thermophotovoltaic generation of electricity. Sol Energy Mater Sol Cells 2001;66:443–52. [9] Van Sark W, Barnham K, Slooff L, Chatten A, Buchtemann A, Meyer A, et al. Luminescent solar concentrators – a review of recent results. Opt Express 2008;16:21773–92. [10] Marti A, Davies PA, Olivan J, Algora C, Terron MJ, Alonso J et al. High-efficiency photovoltaic conversion with spectrum splitting on GaAs and Si cells located in light confining cavities. In: 23rd IEEE photovoltaic specialists conference. Louisville, Ky; 1993. p. 768–73. [11] Kraemer D, Hu L, Muto A, Chen X, Chen G, Chiesa M. Photovoltaic– thermoelectric hybrid systems: a general optimization methodology. Appl Phys Lett 2008;92:243503. [12] Imenes A, Buie D, Mills D, Schramek P, Bosi S. A new strategy for improved spectral performance in solar power plants. Sol Energy 2006;80:1263–9. [13] Hamdy M, Osborn D. The potential for increasing the efficiency of solar cells in hybrid photovoltaic/thermal concentrating systems by using beam splitting. Sol Wind Technol 1990;7:147–53. [14] Dobrowolski J, Browning S, Jacobson M, Nadal M. 2007 Topical meeting on optical interference coatings: manufacturing problem. Appl Opt 2008;47:231–45. [15] Imenes A, McKenzie D. Flat-topped broadband rugate filters. Appl Opt 2006;45:7841–50. [16] Lampert C. Advanced optical materials for energy efficiency and solar conversion. Sol Wind Technol 1987;4:347. [17] Demichelis F, Minetti-Mezzetti E, Perotto V. Optical studies of multilayer dielectric–metal–dielectric coatings as applied to solar cells. Sol Cells 1982;6:323–33. [18] Imenes A, Buie D, McKenzie D. The design of broadband, wide-angle interference filters for solar concentrating systems. Sol Energy Mater Sol Cells 2006;90:1579–606. [19] Segal A, Epstein M, Yogev A. Hybrid concentrated photovoltaic and thermal power conversion at different spectral bands. Sol Energy 2004;76:591–601. [20] Penn J. High concentration spectrum splitting solar collector. Patent US6469241, USA; 2002. [21] Chendo M, Jacobson M, Osborn D. Liquid and thin-film filters for hybrid solar energy conversion systems. Sol Wind Technol 1987;4:131–8. [22] Osborn D, Chendo M, Hamdy M, Luttmann F, Jacobson M, Macleod H, et al. Spectral selectivity applied to hybrid concentration systems. Sol Energy Mater 1986;14:299–325. [23] Demichelis F, Minetti-Mezzetti E, Agnello M, Perotto V. Bandpass filters for thermophotovoltaic conversion systems. Sol Cells 1982;5:135–41. [24] ASTM. G173-03 Standard tables for reference solar spectral irradiances. Derived from SMARTS v. 2.9.2.
Contents lists available at SciVerse ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Investigation of a broadband TiO2/SiO2 optical thin-film filter for hybrid solar power systems Chunhui Shou a, Zhongyang Luo a,⇑, Tao Wang a, Weidong Shen b, Gary Rosengarten c, Wei Wei a, Cheng Wang a, Mingjiang Ni a, Kefa Cen a a b c
State Key Laboratory of Clean Energy Utilization, Zhejiang University, China State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, China School of Mechanical and Manufacturing Engineering, University of New South Wales, Australia
a r t i c l e
i n f o
Article history: Received 22 February 2011 Received in revised form 28 August 2011 Accepted 17 September 2011 Available online 1 December 2011 Keywords: Solar power Hybrid system Spectrally selective filter Optical thin-film Broadband
a b s t r a c t Using the technology of spectral selectivity to integrate different solar power generators in a hybrid system is a feasible way to improve the optical-electric efficiency. This paper presents an 82-layer broadband optical interference thin-film filter matching with crystalline silicon photovoltaic cells, which using TiO2 and SiO2 as fabrication materials and can be used in hybrid solar power systems like photovoltaic–thermoelectric generator (PV–TEG) systems. The design, optimization and fabrication process of the filter is described, high reflectance from 400 nm to 1100 nm as well as high transmittance from 1100 nm to 2500 nm over the broadband of solar spectrum are obtained. The classical electron beam evaporation plant is used to fabricate the filter. Four different incidence angles’ optical performances of the sample filter are measured which agree well with the numerical simulation results. The electrical characteristics of a typical Silicon photovoltaic cell using the fabricated sample filter are measured. An average efficiency increase of 3.24% for the solar cell with respect to the solar energy it receives can be obtained due to the filter. A calculation model for a hybrid PV–TEG system using this thin-film filter is proposed and the benefits of the filter for hybrid solar power systems are demonstrated. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The terrestrial solar radiation spectrum covers a broad band of wavelengths. Typical photovoltaic (PV) solar cells with a given band-gap can utilize only a certain portion of this spectrum depending on the cells’ spectral response. A photon with energy below the band-gap is unable to contribute to the electrical current; it will be converted to heat, which decreases the cells’ efficiency. A photon with energy above the band-gap can be converted by giving up the energy in excess of the band-gap as heat to the cells. Only photons with energy at or close to the solar cells’ band-gap energy provide the most efficient photovoltaic conversion [1]. More than 50% of the incident solar energy is converted to heat for a common PV module [2]. It is better to integrate PV cell with other energy converter in a proper combination to form a hybrid system, which means weakness of one converter can be overcome by the other type [3,4]. Using concentrators to focus a large area of solar radiation onto a PV cell of smaller surface area reduces the amount of PV materials
⇑ Corresponding author. Address: Zheda Road 38, Hangzhou 310027, China. Tel.: +86 571 8795 2440; fax: +86 571 8795 1616. E-mail address: [email protected] (Z. Luo). 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.09.028
required to convert the same quantity of solar energy to electrical energy compared to a standard one-sun PV cells [5]. As high conversion efficiency (27.6% [6]) for concentrator Si solar cells is possible now, solar concentrator systems have drawn increasing interest. However, with increasing concentration levels, there are more photons with band-gap energy mismatch. One traditional way to solute this problem is using photovoltaic/thermal (PVT) hybrid system, which utilizes otherwise the wasted heat. As PV used as a thermal absorber in PVT [2], it is overheated easily, and the efficiency of the PV conversion is reduced. An optimal way of using a solar power system with concentrator is to divide the solar radiation by preventing the unwanted irradiation wavelengths from reaching the cells thus utilizing only the photons that best match the cells’ band-gap. There are generally three methods to accomplish this optimal way of solar power utilization [7]. The first approach uses thermophotovoltaic (TPV) [8] or luminescent concentrators [9] to shift the solar spectrum toward wavelengths better matched to the selected solar cells. The second approach stacks PV cells of varying band-gap from highest to lowest to get good efficiency with absorbing a larger part of the solar spectrum [10]; the third approach uses a beam splitter to separate the incident radiation into some selected spectrum to drive photovoltaic and photo thermal conversion [11–13].
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Nomenclature C E EQE FF h I P q Q SR T V
optical concentration solar irradiance, W/m2 external quantum efficiency fill factor of solar cell Planck constant, m2 kg/s current of solar cell, A power, W electric charge, C heat loss, W/m2 spectral responsivity temperature, °C voltage of solar cell, V
Greek symbols temperature factor of Si cell b temperature coefficient e hemispherical surface emissivity c concentration factor of Si cell g energy conversion efficiency k wavelength, nm m frequency, Hz q reflectance of the filter r Stephan–Boltzmann constant, W/m2 K4
a
As in a solar concentrator system, especially with a high concentration ratio, it is wiser to produce electricity with useful thermal energy, rather than abandoning this heat. Due to the rapid progress in the field of optical devices [14–17], the beam splitting approach was taken up by researchers [11–13,18–22]. Though the design and manufacture of optical filters for the solar concentrator systems faces many challenges such as dealing with the broad range of wavelengths available in the solar spectrum, it is well-established that the use of dielectric optical thinfilm interference filters to split the solar light with high energy-flux is the preferred option. An optical dielectric thin-film interference filter can be defined as a series of zero absorption thin parallel layers of alternate high and low refractive index materials, attached to a substrate like glass. As illustrated in Fig. 1, the filter reflects and transmits the selected spectrum regions, based on the interference of light between the individual thin layers. There are plenty of reports focusing on the design of these solar spectrum filters [12,13,15,17,18,21–23]. Osborn’s group had proposed a 12-layer sample optical multilayer filter consisting six quarterwave pairs of TiO2 and SiO2 for hybrid solar PV systems [21], aim to get a high reflectance from 600 nm to 1000 nm. Imenes’ group focus on the design and optimization process, two kinds of good performance filter for a hybrid photovoltaic/thermal central receiver system
s
transmittance of the filter
Subscripts 0 refers c refers conv refers cool refers CPV refers el refers g refers in refers m refers oc refers ph refers PV refers rad refers ref refers s refers sc refers sub refers SYS refers TEG refers
to to to to to to to to to to to to to to to to to to to
ambient PV cell convection from the cell surface cooling system concentration photovoltaic conversion electrical output of solar cell cover glass of solar cell input data on the solar cell data of solar cell at maximum power point open circuit incident photon photovoltaic conversion radiation from the cell surface reference value spectral short circuit substrate of solar cell hybrid solar power system thermoelectric generator
[18] are calculated. However, to produce multilayers with a measured optical performance that is as close as possible to that designed is difficult as manufacturing thick coatings requires suitable materials and an advanced deposition technique. Currently there are only a few studies concerned with the fabrication or experimental utilization of these filters with PV cells. In this paper, a reliable and practical filter with good efficiency performance will be presented. A wider selectivity wave range than most filter will be used, which can match more commercial crystalline silicon cells’ spectral response range than before. A more feasible design structure which can commonly be used for fabrication in the optical filter industry should be obtained. The reminder of this paper is structured as follows. First, in Section 2, the design process of an optical thin-film filter that can be applied to hybrid solar systems like photovoltaic–thermoelectric generator (PV–TEG) are described and the performance characteristics of designed stacks are showed. In addition, the needle numerical method is used to optimize the optical properties of filter. Then, in Section 3 the fabrication procedure is provided and the optical properties of sample filter are measured. In Section 4, the performance of a typical Si cell using this filter is presented. In Section 5, a calculation model has been developed to analyze the properties of the hybrid PV–TEG system using the beam splitting thinfilm interference filter. Furthermore, under realistic assumptions, a discussion of the performances of PV–TEG systems using this thin-film filter is reported. And the works are summarized in Section 6 as a conclusion.
2. Design of the solar beam filter 2.1. Filter strategy
Fig. 1. Schematic diagram of the thin-film interference filter.
Photovoltaic conversion is highly spectrally dependent, so solar cell performance as a function of wavelength is required to optimally match a cell response to the characteristics of the design of the beam filter. Fig. 2 shows part of the irradiation spectra for an air mass of 1.5 (AM 1.5) [24] and the fraction of this radiation that can be absorbed by a bulk Si device [25], where a value of 1100 nm
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Fig. 2. AM 1.5 spectrum with the reflecting window and the wavelengths that can be absorbed by a Si device.
is used as the cut-off response. The spectral distribution of the AM 1.5 shown in Fig. 2 spans the wavelength region from 300 nm to 2500 nm and contains overwhelming majority of the total incident solar energy. The wavelength selectivity of the filter is in the region from 400 nm to 1100 nm, which contains about 76% of the total solar energy, and corresponds to high efficiency PV conversion. Reflecting this segment of energy (illustrated in Figs. 2 and 3) to the solar cell, the rest of the energy is simultaneously transmitted to either a thermoelectric generator (TEG) or a thermal power generator (TG). Fig. 3 is a basic system schematic of the spectrally selective approach with TEG which will be discussed in the latter section. 2.2. The numerical methods of design The optical thin-film solar filter has performances that depend on the number of layers of coatings, the refractive index and thickness of each layer. All the characteristics useful can be found by numerical methods, which also can easily take into account the properties of real materials. Some of these methods need a starting design which can be any typical analytical solution, and then the designed is refined by adjusting parameters like the thickness and the refractive index of the layers to minimize a merit function [18]. Some methods like global optimization do not need a starting design and can search for the solution in the whole allowed region
Fig. 3. Basic system schematic of the spectrally selective approach.
automatically. However, for the complex broadband multilayer designs, this method requires a large number of layers and a very long time for calculating. A synthesis design method like the needle method can reduce the number of layers [26]. In this paper, a series of stacks mading up by a number of symmetrical periods as 0.5HL0.5H will be used to get the excepted high reflectance over particular spectrum range. H and L are respectively, a quarterwave layer of high index material and low index material. Then the needle method will be used to optimizate it and make it feasible for fabrication. The region of the solar spectrum that needs to be reflected (400–1100 nm as shown in Fig. 2) is very broad for filter design and fabrication. A consequence of the broad wavelengths band is the highly complex requirements of the coatings, which can be so thick that they may be easily damaged by internal stresses. As a feasible solution of this, two series of stacks at each side of the substrate will be deposited to reduce the number of the layers and the total physical thickness of each stack. The structure diagram of this is shown in Fig. 4. The design process should be performed for a specified design angle of incidence, which is assumed as 45° for the hybrid solar system shown in Fig. 3. The front surface of the substrate is coating with stacks as 0.7(0.5HL0.5H)50.8(0.5HL0.5H)40.9(0.5HL0.5H)4 (0.5HL0.5H)4, and the back surface coating with stacks as 0.8(0.5HL0.5H)40.89(0.5HL0.5H)40.9(0.5HL0.5H)4. Central wavelength is set as 580 nm for the front coating and 1000 nm for the back coating. TiO2 and SiO2 are used as the high index material and low index materials, which have negligible absorption in the spectral range of interest. These two materials are commonly used for coating and can manage with high thermal load, which would suitable for fabrication and further using in the optical industry. The optical constants H and L at different wavelengths are listed in Tables 1 and 2. 2.3. The optimization of the designed stacks The needle optimization method works by adding new zerothickness layers onto a coating design, and using a local optimization methods to improve the new design. If the zero-thickness layer has been placed in the correct position, the local optimization will force the new layers to grow. The needle optimization method implemented in the thin film design software TFCalc (Software Spectra Inc.) is used here to design and analyze the thin-film filter for hybrid solar power systems. The design angle of incidence is set as 45°. The optimization begins with 38-layer starting design stacks as Sub/ 0.7(0.5HL0.5H)50.8(0.5HL0.5H)40.9(0.5HL0.5H)4(0.5HL0.5H)4/Air
Fig. 4. Diagram of the designed thin-film filter structure and a simply illustration of the interference of the lights reflected from a random layer’s two sides.
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C. Shou et al. / Applied Energy 92 (2012) 298–306 Table 1 Optical constants of TiO2 film at different wavelengths. Wavelength (nm)
400
430
450
500
550
600
700
800
900
Index N Index K
2.3970 0.0028
2.3132 0.0028
2.2737 0.0028
2.2108 0.0027
2.1779 0.0027
2.1607 0.0027
2.1478 0.0026
2.1465 0.0026
2.1487 0.0026
Table 2 Optical constants of SiO2 film at different wavelengths. Wavelength (nm)
350
400
450
500
550
600
650
700
900
1000
1562
Index N Index K
1.472 0
1.467 0
1.463 0
1.459 0
1.455 0
1.452 0
1.450 0
1.446 0
1.437 0
1.434 0
1.429 0
on the front surface of the substrate, and 27-layer stacks as Sub/ 0.8(0.5HL0.5H)40.89(0.5HL0.5H)40.9(0.5HL0.5H)4/Air on the back surface. Air is incident and exit medium. Sub means BK7 glass substrate. After setup the stack formula and the parameters for the environment and materials, the design target of the front stacks is set as reflectance above 90% from 400 to 670 nm and transmittance above 90% from 1100 to 2500 nm. Simultaneously, reflectance above 90% from 670 to 1100 nm and transmittance above 90% from 1100 to 2500 nm is the target for the back stacks. The optimization results of the filter structure are shown in Fig. 5. The front stacks consists of 52 layers with a total thickness of 4320 nm, and the back stacks consists of 30 layers with a total thickness of 4330 nm. The reflectance curve of the optimized filter as a function of wavelength is comparing with the corresponding reflectance characteristics of the two starting design stacks shown in Fig. 6. The optimized front stacks have a maximum reflectance of 97% at 636 nm, while the back coating has a maximum reflectance of 98.2% at 929 nm. Compared to the discrete multilayer filter in Ref. [18], a wider high reflectance range (nearly 200 nm) and less number of layers (reduced by 25 layers) are obtained, which means more types of silicon cells’ response range can be matched and more feasible for the fabrication. While keeping similar high performance from 600 nm to 1100 nm, much better transmittance from 1100 nm to 2500 nm than the relevant one are provided by this filter.
Fig. 6. Reflectance characteristics of designed front and back stacks comparing with the optimized filter at an incident angle of 45°.
The filter was fabricated by classical electron beam evaporation plant (Balzers BAK600). Coatings were deposited at both sides of
100 mm 100 mm BK7 glass substrate illustration as Fig. 4. The background vacuum was 2:0 103 Pa while the pressure of oxygen for TiO2 deposition was kept 1.8 102 Pa. Meanwhile, the deposition temperature on substrate was kept at 300 °C during the process. The deposition rates of TiO2 and SiO2 are respectively 0.3 nm/s and 1 nm/s controlled by a quartz sensor. During the deposition of the filter, the indirect monochromatic optical monitoring in transmission was used to control the thickness of each layer. The sample filter fabricated is shown in Fig. 7. The measurements of optical performance in the spectral range from 400 nm to 2500 nm were performed with a resolution of
Fig. 5. Refractive index profiles of the filter. (The substrate thickness is not scaled in the figure.)
Fig. 7. Sample of the fabricated broadband TiO2/SiO2 optical thin-film filter.
3. Fabrication of the designed filter
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Fig. 10. All the data were measured in sunny and cloudless weather. The temperature of the cell during the testing process was controlled around 35–40 °C. There are clear increases in short circuit current and the output powers as the incident power density on the solar cell becomes higher than 2200 W/m2. The open-circuit voltages follow a similar behavior which is not so obvious. From the figures it is evident that filter strongly increases the short circuit current and the maximum output power of a PV cell given the similar incident power density compared with the regular one. And the maximum output power with the filter increases faster than the one without. As the incident power density becomes higher, these effects become more obvious. On the other hand, this filter does not affect the open-circuit voltage (except the change in (a)) very much in all measured conditions. The energy-conversion efficiency of solar cell is defined by, Fig. 8. Measured reflectance of the filter with different incident angles (0°, 15°, 30° and 45°).
2 nm on a spectrophotometer (Shimada UV310). Fig. 8 shows the measured results of the filter with an angle of incidence of 0°, 15°, 30° and 45°, respectively. The experimental results shown in Fig. 8 agree very well with those of the numerical simulation. There is small difference of the low reflectance part mainly owing to the variation of thickness and optical constants of materials used between the deposition and the design. Near the boundary of selectivity region from 1100 nm to 1250 nm, there is an undesired shoulder with a relatively high reflectance. For a long pass filter, the accumulated deposition errors will result in the distortion of optical properties near the cut-off region. Therefore, the shoulder appears in the final optical performance of the filter after the deposition on the front and rear sides. Fortunately, the solar radiation energy in the spectral region from 1100 nm to 1250 nm is low so the decay in the transmission will not have a large adverse effect on the performance of the filter. The high reflectance over the stop band 400–1100 nm in Fig. 8 ensures the filter is well match to silicon photovoltaic cells. There is a variation in the filter performances as a function of incident angle. The boundary shifts to longer wavelengths as the angle is varied from 45° to 0°, due to the optical length becoming shorter. The ripples over 400– 1100 nm are no longer visible at the angle of 0° because the polarization effect disappears at the normal incidence. Compared to 12layer filter reported in Ref. [21], nearly two times wider selective band of spectrum is obtained. Much smoother curve of better performance data is shown over the whole selectivity range by the fabricated sample filter. 4. The measured performance of a Si cell with and without the filter In order to investigate the filter’s usefulness for PV Si cells, the performance of a commercial crystalline Si cell Si–N (UXUAN Inc.) are tested with and without the filter. Radiation data is collected from a 30A-P Thermopile head (OPHIR OPTRONICS Inc.), which has a flat spectral response for irradiation over a broadband spectral range from 150 nm to 6000 nm. The electrical characteristic of cell is measured by a Keithley 2440 source measurement unit. The temperature of the cell is observed via T type thermocouples with an Agilent 34970A Data Acquisition/Switch Unit. The characteristics of the cell are measured outside under the natural sun light with Fresnel lenses at the latitude of Zhejiang University HangZhou, China. The results of the experimental current–voltage characteristics and output power with the different incidence power density are showed in Fig. 9. The PV conversion efficiencies are reported in
g¼
Pm V m Im ¼ Pin Pin
ð1Þ
where Pm is the cell’s maximum electric output power and Pin is the input power of the solar irradiation on the cell’s surface. Vm, Im are the voltage and current at maximum power point. By the comparison of the efficiency, using the filter brings an average efficiency increase of 3.24% for the solar cell, while the highest increase is around 5% with incident power density at approximately 3000 W/m2. As the Si–N cell is commercial crystalline silicon which is not designed for concentrated sunlight the efficiency decreases as the incident power density becomes higher. The principal reasons for the reduction in cell efficiency are the grid series resistance losses that rise fast under concentration, and current crowding near the junctions [5,27]. Better efficiency performance of the PV systems could be expected, if silicon concentrator cells are used with the filter. 5. Efficiency calculation of hybrid PV–TEG systems 5.1. Model description On the basis of the experimental data reported in Sections 2 and 3, the expected total conversion efficiency for the combined PV– TEG hybrid system using the optical thin-film filter can be calculated. The schematic of the spectrally selective PV–TEG system is shown in Fig. 3. The filter’s performance is shown in Fig. 7. The solar TEG efficiency can be assumed as 8% [11], which is constant over the whole solar spectrum. Four kinds of different silicon cells are investigated as the PV part of the hybrid system: (1) commercial crystalline silicon cell SSRC-C50 (SSRC Inc.); (2) commercial crystalline silicon cell Si–N; (3) heterostructure solar cell based on p-type crystalline silicon (HJ + Cr + ZnO) [28]; (4) backside point-contact silicon concentrator solar cell II-3B [29]. There are two types of solar cell spectral response data used in this paper to get the spectral conversion efficiency of cells. The first one is the external quantum efficiency (EQE); the second is the spectral responsivity (SR). The EQE can be defined as the current obtained outside the device per incoming photon [1], and it gives information on the current that a cell will produce when illuminated by a particular wavelength. In Fig. 11 the EQE curves over 400–1100 nm of the three silicon cells are shown [28]. The SR describes the sensitivity of the cell to incident radiation of different wavelengths. The data of SR of the concentrator solar cell is shown in Fig. 12 in terms of ‘the outputs short-circuit current per unit incident power’ [29]. The EQE and SR can be used to express the spectral conversion efficiency gPV,k in Eq. (2), as a ratio of the spectral output power Pk and the spectral incident photon power PPV,k:
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(a)
(b)
(c)
(d)
Fig. 9. Measured current–voltage characteristics and output powers with and without the filter using different incident power density respectively.
The efficiency of the hybrid system of three silicon cells will be calculated as well as the efficiency of the backside point-contact silicon solar cell II-3B under concentration with and without the filter. For the hybrid PV–TEG systems working without concentration, the weighted standard cell efficiency which the photovoltaic conversion actually uses can be given by,
R k2
qk gPV;k Ps;k dk gPV;work ¼ k1R k4000 qk Ps;k dk k280
ð3Þ
Fig. 10. Comparison of the Si–N cell efficiency with and without the filter.
gPV;k ¼
Pk Isc;k V oc FF q V oc FF ¼ ¼ EQE ¼ SR FF V oc P ph;k hmph Pph;k
ð2Þ
where Pk is the product of the spectral short circuit current Isc,k, with the open circuit voltage V1 and the fill factor FF of the solar cell; q is electric charge, hmph is photon energy [11,30]. It is assumed that FF and V1 are not wavelength dependent. The spectral efficiency characteristics as a function of wavelength of the four cells can be calculated and shown in Figs. 11 and 12.
Fig. 11. The EQE and spectral efficiency curves of the three silicon cells (at one-sun AM 1.5).
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Fig. 12. The SR and spectral efficiency curves of the concentrator solar cell (at onesun).
Fig. 13. The efficiency versus incident power density for a silicon solar cell.
a ¼ 1 bref ðT c T ref Þ where qk is the spectral reflectance of the filter over solar spectrum as designed, gPV,k is the spectral efficiencies of solar cells, Ps,k is the spectral radiation power of AM 1.5G [24], k1 to k2 are the portion of wavelengths that cells have a spectral response. Meanwhile, the system conversion efficiency gSYS1 can be calculated with following equation:
R k4000 k280
gSYS1 ¼
sk gTEG Ps;k dk þ R k4000 k280
R k2 k1
qk gPV;k Ps;k dk
Ps;k dk
ð4Þ
where sk is the spectral transmittance of the filter at a particular wavelength as calculated in the design software TFCalc, gTEG is the efficiency of the solar TEG. The weighted standard cell efficiency of II-3B with the filter can be given in the similar way as Eq. (3),
R
k2 qk gPV;k P0s;k dk gref1 ¼ k1R k4000 qk P 0s;k dk k280
ð5Þ
where P0s;k is the spectral beam radiation power of AM 1.5 (Direct & circumsolar) [24], and the cell efficiency without the filter is calculated as,
R
k2 gPV;k P0s;k dk gref2 ¼ k1R k4000 k280
Ps;k dk
ð6Þ
The use of concentrator results in increases of the incident power density and the cell temperature. Ref. [30], the effect of concentration and temperature can be taken into account by multiplying gref1 and gref2 with a concentration factor c and a temperature factor a respectively:
gCPV 1 ¼ gref1 a c
ð7Þ
gCPV 2 ¼ gref2 a c
ð8Þ
ð10Þ
which is from the traditional linear expression for the PV electrical efficiency [31]. Where bref is the temperature coefficient, Tref is the silicon cell’s reference temperature. In this paper, bref is set as 0.0045 [31] and Tref = 25 °C. To get the cell temperature’s varied relationship with incident power density on the solar cell’s surface, a one-dimensional thermal model [32] is used, which calculates the transferred heat and assume the heat flow is primarily directed in the normal direction. The Si cell is idealized, which contain five layers: cover glass, adhesive, cell, solder and substrate. The thicknesses of the various layers can be known from the cell manufactory [29,32]. The schematic of the model is shown in Fig. 14. Based on an energy balance, the input solar energy is equal to the output energy and the relation is given by,
E ¼ Q rad þ Q conv þ Q cool þ Pel
ð11Þ
where E is the input solar irradiance, Qrad is the heat loss by radiation from the surface, Qconv is the heat loss through convection from the surface, Qcool is the thermal energy removed by the cooling system, Pel is the electrical output of the cell. The radiative heat flux is represented as follows:
Q rad ¼ erðT 4g T 40 Þ
ð12Þ
where Tg is the glass surface temperature, T0 is the ambient temperature, e is the surface emissivity and r is the Stephan–Boltzmann
For a constant cell temperature, the typical silicon concentration cell behavior with incident power density as shown in Fig. 13 [29], and curve fitting is used for the solid line. The concentration factor c can be given by,
c ¼ 1 þ a1 X þ a2 X 2 þ a3 X 3 þ . . .
ð9Þ
where X = lnC, C stands for optical concentration [30]. As the efficiency of the photovoltaic conversion is greatly affected by the temperature of the PV cell Tc, the effect of temperature should be calculated. The temperature factor a can be given by,
Fig. 14. Schematic of the heat transfer mechanism of the one-dimensional thermal model of PV cell.
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constant. When the cell temperature is up to 170 °C, this equation is linearized for simplification as follows:
Q rad ¼ 4erT 30 ðT g T 0 Þ
ð13Þ
The convective heat flux is given by
Q rad ¼ 4erT 30 ðT g T 0 Þ
ð14Þ
where Rconv is the thermal resistance of convection and can be obtained from the reference. The heat flux removed by the cooling system is
Q cool ¼
T sub T 0 Rcool
ð15Þ
where Tsub is the substrate temperature, Rcool is the thermal resistance through the cooling system. Fig. 15 shows the simulation results of cell temperature versus illumination for various values of Rcool (Km2/W). Ref. [33] Rcool can be assumed as 0.0002 Km2/W, comparing with the case Rcool = 0.001 Km2/W simultaneously. The electrical efficiency of the hybrid PV–TEG system using concentration silicon cell with the filter can be calculated as:
R k4000
gSYS2 ¼
k280
sk gTEG
P0s;k dk þ R k4000 k280
ac
R k2 k1
qk gPV;k
P0s;k dk
Ps;k dk
Table 3 Comparison of calculation results of the hybrid systems at one-sun.
ð16Þ
where concentration factor c and temperature factor a depend on the incident power density as shown in Fig. 16. It is clear that this concentrator silicon cell efficiency generally increases with illumination intensity before near 100 suns. As the concentration factor is higher than 1 up to 250 suns, an improvement in the efficiency can be expected when fluxes higher than 1000 W/m2. The heating load makes the cell temperature increase with illumination intensity, which would degrade the PV efficiency. If Rcool = 0.001 Km2/ W, the temperature effect is significant when the illumination intensity higher than 15 suns. If Rcool = 0.0002 Km2/W, the cell could work well until 120 suns. 5.2. Results and discussion The calculation results of the PV–TEG systems using three cells in Fig. 11 with the filter are listed in Table 3. gPV is the reported efficiency of each cell given by the manufacturer. There are clearly efficiency gains for the systems use Si–N and SSRC-C50 (2%). However, the efficiency improvement of heterostructure cell system is small due to the high efficiency (gPV) it already has, in addition to the slight mismatch of the filter design spectrum with the
Fig. 15. Cell temperature versus incident power density.
Fig. 16. The concentration factor c and the temperature factor a versus incident power density.
Si–N SSRC-C50 Heterostructure cell
gPV
gSYS1
gSYS1 —gPV
gPV,work
gPV,work–gPV
13.36 14.84 17.0
15.30 16.80 17.97
1.94 1.96 0.97
17.64 19.65 21.15
4.28 4.81 4.15
cell’s EQE curve. It can be observed in the system, the PV cells all work in a better efficiency as gPV,work shown in Table 3. Fig. 17 presents the efficiency performances of concentration PV system (with no filter) and concentration hybrid PV–TEG system with the filter at various optical concentration levels. All the cases following a similar trend: the efficiency increases with the concentration ratio, until a maximum, then decreases with increasing concentration ratio. The system’s cooling thermal resistance has a very large effect on the efficiency. When the cooling thermal resistance is 0.0002 Km2/W, maximum efficiency is much higher and there is a larger concentration range suitable for PV cell compared to the other cases. If system efficiencies are to be kept up to 18%, and cooling thermal resistance of 0.0002 Km2/W is assumed, then the maximum efficiency of 22.2% can be achieved by the concentrated PV system (CPV) at 41 suns. Meanwhile, the hybrid system could give a maximum efficiency of 21.6% at 51 suns. There is a crossing point of
Fig. 17. Hybrid systems’ electrical efficiency versus concentration.
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21.2% at 85 suns, which means: the concentrated PV system could work well from 1 to 85 suns; the hybrid PV–TEG system with the filter can gain more energy and work in a better efficiency at the higher concentration level from 85 suns to 230 suns in this situation. The power efficiency of hybrid system with the filter obtains 1.1% enhancement at 200 suns comparing with the CPV system. If the cooling thermal resistance is set to be 0.001 Km2/W, there would be a max value of 20.9% for the CPV system at 14 suns and a value of 20.4% for the hybrid system at 20 suns. The CPV system can work well from 1 to 35 suns and the hybrid system would be better at higher concentration level until 73 suns. 3.21% efficiency enhancement will be gained by hybrid system with the filter at 150 suns comparing with the CPV system. The higher cooling thermal resistance and concentration ratio are, the better effect of filter on efficiency can be expected. As TEG used in the hybrid system is only one feasible choice of energy convertors, better energy utilization performance can be acquired if other convertors with higher solar-to-electric efficiency are integrated. 6. Conclusion An optical broadband multilayer dielectric films filter for spectral selectivity utilization of solar energy is developed. The optimized design structure of the filter consists of two stacks, 52 layers with a total thickness of 4320 nm on one side and 30 layers with a total thickness of 4330 nm on the other side of a BK7 glass substrate, using TiO2 and SiO2 as design and fabrication materials. Comparing to some existing filters, wider high reflectance range (400–1100 nm) and better transmittance of near infrared are obtained with less layers and simpler structure. A classical electron beam evaporation plant (Balzers BAK600) has been successfully used to fabricate the filter. The measured optical data provided by a spectrophotometer (Shimada UV310) show that the spectral splitting effect behaves very similarly to what is designed. The performance of a solar cell Si–N with a filter is tested; there is a maximum efficiency increase of 5% for the Si cell with respect to the solar radiation it receives with a regular setup. A calculation model include four kinds of Si cells has been set up to predict a hybrid PV–TEG system’s efficiency with the filter. There are nearly two percents of efficiency gains for the hybrid systems with no concentrator comparing with PV systems without the filter. For the system with concentrator, the hybrid PV–TEG system with the filter gains more energy and work in higher efficiency than the CPV system when the illumination intensity level becomes appropriately high. A better improvement effect of the PV cell’s performance due to the filter can be achieved with the higher cooling thermal resistance and concentration ratio. Therefore, this optical broadband thin-film filter for beam splitting is a suitable technology for hybrid PV–TEG system; it is very likely that any real success of this hybrid system will be developed with improvements in optical coating technology and in TEG materials. Such a filter also offers great flexibility to combine various energy convertors with Si PV cells forming high efficiency hybrid solar utilization systems. If energy convertors with higher efficiency than TEG applied in the hybrid systems, like solar thermal power generators, better performance of the systems can be expected. Acknowledgements This work is partially supported by China Postdoctoral Science Foundation (20080441248), Research Foundation of Science and Technology Department of Zhejiang Province, China (2009C21023) and Zhejiang Provincial Natural Science Foundation (Y1090504).
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