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Influence of spectrum and latitude on the annual optical
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performance of a dielectric based BICPV system
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In preparation for
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Solar Energy
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2015
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Hasan Baig*, Eduardo F. Fernández, Tapas K. Mallick
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Environment and Sustainability Institute
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University of Exeter, Penryn, Cornwall TR10 9FE, UK
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* Corresponding author: Hasan Baig
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Tel: 01326 259467
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Email: [email protected]
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Abstract
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The optical analysis of a concentrating photovoltaic system plays an important role in
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determining its overall performance. Typically, ray tracing with a standard AM 1.5 source
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spectrum is utilized to carry out this process. However, this does not represent the actual
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operating conditions experienced by the device. The solar spectrum changes depending on the
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time and geographic location. In this work, we propose and demonstrate a procedure to include
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the changing solar spectrum whilst predicting the annual performance of a building integrated
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concentrating photovoltaic system. Using a statistical approach a frequency of occurrence of
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the different solar spectrum is estimated for different locations and utilized for carrying out the
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annual performance prediction of the system. It is found that using the standard spectrum
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underestimates the actual system performance. The highest optical efficiency of 79.8% was
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observed for Kemi when considering the actual spectrum values. This value was found to be
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1.6% higher than that obtained using AM 1.5D spectrum. An average difference of 1.25% was
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found in the annual performance of the system when evaluated for six different geographic
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locations.
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1. Introduction
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The design and analysis of the optical element plays an important role in determining the
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overall performance of a Building Integrated Concentrating Photovoltaic (BICPV) system.
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Several BICPV systems have been studied in the last few years [1-7]. The optical efficiency
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and the acceptance angle [2, 6] of the optical element are typically used as the parameters to
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measure its effectiveness in focusing the incident light on the solar cell. All the possible optical
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losses such as reflection, transmission, absorption occurring within the concentrator are
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accounted for while evaluating the optical efficiency of the system. However, there exist no
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established standards or methods for carrying out the optical analysis of these types of systems.
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Typically, the optical analysis is performed using ray tracing techniques based on either in-
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house algorithms [6-9] or commercial codes [2, 10-12]. Researchers have used standard
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monochromatic and polychromatic sources to perform the optical analysis of these systems.
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Using monochromatic light source both dielectric [4, 7, 9] and reflective type BICPV systems
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have been studied earlier[3, 12]. This approach was however found to limit to account for the
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spectral response of the optical elements under study, causing a mismatch in the experimental
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and numerically estimated values [6]. An improvement for this methodology was suggested
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recently [1, 10], where the use to this methodology is to use a standard solar energy spectrum
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like AM1.5G to bring the modelling results closer to the experiments in a standard testing
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environment.
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The incoming spectral irradiance and its distribution received by earth are affected by several
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time varying atmospheric parameters. In most of the earlier studies, the theoretical optical
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efficiency has been investigated without considering the solar spectrum weighted optical
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properties of the components [8, 9, 13-17]. An estimation of the spectral impact on optical
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efficiency will help in the accurate prediction of the diurnal/monthly/annual performance of
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BICPV systems for different geographic location having different atmospheric conditions. This
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is very essential because a good solar cell under standard conditions could become an average
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solar cell when operating under a solar concentrator and turn worse under actual operating
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conditions [18]. The analysis of the spectral impact on the performance of Photovoltaic (PV)
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[19-22] and High Concentrating Photovoltaic (HCPV) [23-25] devices has been widely
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addressed by several authors. However, the influence of spectral changes on the performance
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of BICPV systems has not been undertaken and still remains unknown. Incorporating this effect
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will allow better understanding and accurate evaluation of the performance of BICPV systems
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under real operating conditions.
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The intent of this study is to determine the long term performance of a BICPV system under
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varying spectral conditions and provide guidance on modelling such systems. Different sun
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angles are utilized to determine the annual optical efficiency of the system and its yearly yield
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output. Six different geographical locations are used for estimating the annual output of the
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system which gives us an understanding of the range of incident angles under which the system
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can work optimally.
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2. System Description
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The system under study essentially consists of a concentrator element, an encapsulation
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material and a solar cell, which can be sandwiched between a typical double-glaze window.
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The concentrator element used in this study is a truncated dielectric asymmetric compound
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parabolic concentrator (DiACPC), designed with a range of acceptance angles 0°-55° [6, 7].
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This concentrator is designed to be used with crystalline-Si solar cells of 6mm width as shown
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in Figure 1. The CPC profiles of acceptance half angles (0o&33o) are generated. Dielectric
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material is introduced within this profile which eventually reduces the acceptance angles
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because of the untruncated structure.
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Figure 1 Design of the DiACPC used for building integration [7]
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The dielectric profiles are truncated by 68% to achieve the acceptance half angles (0o&55o).
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The concentration ratio of the unit is 2.8×, and is made of clear polyurethane material which
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has good transmission and dielectric properties. The concentrator is collecting the radiation
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because of total internal reflection at the parabolic surfaces of CPC profile. Laser Grooved
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Buried Contact type solar cells having a dimensions of 6 mm X 115 mm are utilized in the
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system. Further details about the system manufacture, its environmental impacts and indoor
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performance may be found here [6, 7, 26]. A systemic precaution has been taken to minimize
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the light escaping from the material interfaces and proper optical coupling has been done to
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eliminate air gaps [17].
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The encapsulation material needs to have very good transmission properties in the UV-visible
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wavelength range, to minimize the optical losses. However for CPV applications, especially
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for dielectric concentrating systems, the encapsulation material should also have good adhesion
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properties, to hold the dielectric concentrators on top of the solar cells. Figure 2 shows the
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prototype of the BICPV system under study. The module is made with 28 solar cells; 2 parallel
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strings of 14 solar cells
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Figure 2 A prototype of the BICPV system
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The optical transmission of the different optical elements is evaluated using a PerkinElmer UV-
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Vis-NIR spectrophotometer. Figure 3 shows the spectral transmission of the polyurethane
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material and the Sylguard used in the manufacture of the system. A considerable transmission
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loss can be observed in the range of 300-400 nm of the spectrum by the polyurethane material.
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Once the incident light gets refracted through the optical concentrator and the encapsulant
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material it is incident on the solar cells where it is converted into electricity. The spectral
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responsivity is used as a measure to account for the current produced per unit incident power.
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Typically, spectral response (S) is calculated using the short circuit current measured at the
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external contacts of the device, which is usually same as the generated photocurrent. External
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quantum efficiency (EQE) can be measured by spectral response of the solar cell using the Eq.
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(1) [27].
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QE ( ) 
qS ( )
 hc
(1)
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The external quantum efficiency of the solar cell and the module is used to evaluate the effect
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of incorporating the concentrator and the encapsulation with the solar cell. An EQE analysis of
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the bare, encapsulated crystalline solar cell and the prototype CPV has been undertaken to
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understand the change in spectral response of the module as shown in Figure 3 using a Bentham
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PVE300 PV Characterization System.
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Figure 3 Optical transmission of the concentrator and encapsulant material with the EQE of the solar cell
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The EQE of the bare cell and CPV units were measured using a Bentham spectral response set-
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up at the Environmental and Sustainability Institute, University of Exeter, UK. The EQE
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measurement set-up is shown in Figure 4 . The spectral response of the solar cell was scanned
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for a range of wavelengths. Light from a source was directed through a monochromator and
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filter before illuminating the sample under test. The set-up was first initialized with a calibrated
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photodiode, before measuring the spectral response of the cell. The spectral response was
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measured under a biased light from a xenon lamp.
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Figure 4 The spectral response set-up at Environment and Sustainability Institute (ESI), University of Exeter
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Due to the absorption of the concentrator material, the EQE of the CPV module drops to zero
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at 400nm while the EQE of the both bare cell and encapsulated cell is found to be 73-74%. The
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decrease in EQE of the prototype CPV module over the range of 400 nm to 1100 nm is due to
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the optical losses in the concentrator and optical elements of the CPV module. The transmission
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of the concentrator material is found to be very good within the range of 420nm to 1100nm
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with a maximum transmittance of 89.6%.
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3. Methodology
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In order to better understand the modelling procedure the overall methodology is presented in
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the flowchart as shown in Figure 5. Based on the geographic location of the system and the
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time of the year we evaluate the sun position and input it to the SMARTS program. This is
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used to estimate the solar irradiance and the spectrum for different time of the day. We them
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estimate the frequency of occurrence of these spectral irradiance for the entire year. Using the
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spectral irradiance data, the optical geometry and the solar cell spectral response the optical
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simulation is setup to estimate the optical efficiency corresponding to different air mass of the
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spectral irradiance. The optical efficiency for diffuse radiation is also estimated during this
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process. Finally the annual performance of the system is evaluated integrating the frequency of
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the different AM values. This cycle is repeated for different times of the year and geographical
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locations.
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Figure 5 Flowchart showing the methodology followed for estimating the annual output of the BICPV system
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3.1 Simulation of the spectral direct irradiance
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In addition to using a standard AM1.5D multi-wavelength spectrum the present study uses
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different values of air mass (AM) based on the angle of incidence. It is known that with diurnal
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variation and sun positions both incident angle and air mass varies continuously. Therefore it
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is essential to vary the spectrum of incident radiation for each incident angle while modeling
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the performance of the BICPV system.
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The incident spectrum is affected by different atmospheric parameters such as the air mass
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(AM), aerosol optical depth (AOD) and precipitable water (PW). Among these parameters, the
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air mass has proven to be the main reason of spectral changes. Because of this, the use of AM
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as a parameter to evaluate the spectral impact on the performance of PV and HCPV devices is
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widely used and accepted as a good approximation [24, 28-32]. The AM can be calculated as
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a function of Sun’s zenith angle (z) [33] as shown in Eq.(2).
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AM 
1
Cosz  0.45665 z
0.07
(96.4836  z ) 1.697
(2)
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Figure 6 shows the variation of air mass with solar zenith angle, z (90-altitude angle). This
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continuous variation of spectrum with incident angle provides a highly accurate solar spectrum
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weighted transmission/reflection values, which eventually implies higher accuracy in optical
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efficiency. Once the value of AM is available at each particular zenith angle, the direct spectral
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distribution was accurately simulated with the Simple Model of the Atmospheric Radiative
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Transfer of Sunshine (SMARTS) [34, 35].
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Figure 6 Variation of air mass with the zenith angle
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Figure 7 shows example of few of the simulated solar irradiance spectra. As can be seen, the
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increase of AM reduces the incoming irradiance. However, the attenuation produced is larger
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in the UV region of the short-wave spectrum.
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Figure 7 Effect of air mass on the spectral direct irradiance incident on the system
3.2 Optical Analysis
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The optical analysis was carried out by using a polychromatic source of light with desired
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intensity and spectrum. The light from the source impinges on the concentrator undergoes TIR
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and exits the concentrator as shown in Figure 8. A detector is placed along the exit aperture
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which mimics the solar cell and is used to predict the outgoing flux. For each incident angle,
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an optical simulation is carried out determining the energy of the exiting rays reaching the solar
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cell. It is known that during diurnal variation of sun position, the sun angles continuously
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changes.
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Figure 8 Basic system configuration showing the different components and the total internal reflection (TIR)
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For a BICPV system installed vertically on the south facing wall, the incident angle exhibit two
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components while changing the sun position i.e., the longitudinal and the transverse
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component. Both the components vary simultaneously for diurnal variation of sun position.
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However, in this optical analysis only the effect of the transverse component of solar incident
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angle has been consider since the effect of the longitudinal component is found to be very
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minimal for line-axis concentrator while the axis of the concentrator is in east-west direction
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[8]. The most important element while carrying the optical analysis is the source of the
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spectrum utilized for carrying out the analysis.
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Using the different spectrum obtained using the SMARTS calculator, a series of simulations
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are performed at different incidence angles corresponding to the different AM values. The
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variation of the optical efficiency of the system under different incoming spectrum is compared
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with the standard AM1.5D spectrum and the monochromatic spectrum reported earlier [7] as
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shown in Figure 9.
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Figure 9 Optical efficiency of the concentrator under standard AM1.5D, monochromatic and varying spectrum
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It may be clearly seen that the module performance is different under the actual spectral
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conditions as compared to the standard reference of AM1.5D. At smaller incidence angles the
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optical efficiency of the system is higher compared to AM 1.5D. The optical efficiency of this
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kind of system with incident spectrum has already been reported by the same authors [20]. For
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both the cases of spectrum (actual spectrum and AM1.5D), the decrease in efficiency with
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increase in incident angle is due to the higher partial reflection from the aperture surface of the
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concentrator. Since the concentrator is designed for range of acceptance angles 55°, the sudden
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drop in optical efficiency can be observed after incident angle 55°. The dielectric concentrator
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does not collect most of the radiation incident with an angle higher than 55º. Ray tracing
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simulation shows that the radiation incident with angles higher than 55º either escape from
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parabolic surface or reflect back through the aperture surface after multiple reflection within
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the concentrator. It may be clearly seen that the module performance is different under the
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actual spectral conditions as compared to the standard reference of AM1.5D and the
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monochromatic spectrum. As mentioned earlier this change in optical efficiency occurs due to
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the change in spectrum of the incident direct irradiance for each incident angle. As it can be
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noticed that the optical efficiency is higher for smaller incident angles which occur during
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morning and evening hours. For smaller incident angles (signifies higher air mass) the spectrum
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has a bit red rich, which is the primary reason for enhancement of the optical efficiency. As it
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can be seen that the concentrator material has higher absorption for the wavelength below
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400nm (as shown in Figure 3), so red shift of the incident spectrum has reduce the absorption
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losses in the optical materials. For higher incident angles (i.e. for less air mass), the difference
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in optical efficiency due to actual spectrum and AM1.5D decreases. This difference in the
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optical efficiency can have a significant effect on the predicted output performance of the
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system which will be discussed in the later sections.
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3.3 Impact of diffuse irradiation
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The diffuse part of solar radiation can contribute significantly to the performance of a dielectric
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concentrator. A study of angular acceptance shows that a wide range of incident rays can be
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accepted by the designed concentrators, especially by DiACPC-55, even outside the acceptance
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half angles. Therefore, diffuse radiation can still contribute to the optical efficiency, even
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though rays are incident at angles out of the defined acceptance range. Three different angular
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distributions of solar insolation are possible: isotropic, cosine and hybrid Gaussian are
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employed to estimate the optical performance of solar concentrators. From the angular
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acceptance study [36], it is observed that the designed concentrator can only accept 40-46% of
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the total diffuse radiation, which reduces the average energy flux at the receiver. Study shows
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that the DiACPC-55 can collect 44.86% of the diffuse solar energy respectively, considering
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all the possible losses within the concentrators.
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4. Annual performance of the system at different locations
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In this section, the expected annual influence of the spectral solar radiation variations at six
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different locations on the performance of the BICPV module is evaluated. The sites considered
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in this study are: Madrid (40.4), Paris (48.85), Exeter (50.72), Edinburgh (55.95), Aberdeen
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(57.15), and Kemi (64.73). Thus, it is possible to analyze the system in a wide range of
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operating conditions.
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4.1 Estimation of the annual optical efficiency
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The procedure to estimate the annual optical efficiency is analogous to the one presented in
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[33] to quantify the annual spectral losses of PV and HCPV devices. Firstly, the value of θ is
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estimated every minute during daylight for a whole year at each site considered. Based on this
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data, the frequency distribution of θ (P (θ)) at each location is obtained. After that, the direct
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solar irradiance spectrum is simulated for 200 numbers of θ values equally distributed between
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0o and 90o using Eq.(2) and SMARTS. The optical efficiency η(θ) of the LCPV system is
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simulated for 200 solar spectra at each site following the procedure described in the previous
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section. Finally, the annual optical efficiency is obtained using Eq.(3).
annual 
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 P( ) ( )d
 P( )d
(3)
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The procedure to estimate annual optical efficiency by considering only the zenith angle at
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each location is equivalent to the one commented above. The frequency distribution of θ (P
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(θ)) at each location is also obtained from the values of θ previously computed every minute
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during daylight for a whole year. However, in this case, the optical efficiency of the BICPV
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system is obtained for 200 θ values considering only the AM1.5D reference spectrum following
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the procedure also described in the previous section. To analyze the impact of the spectral
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variation under different climatic conditions six different cities were selected and calculations
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of the annual optical efficiencies of the system were carried out. The cities considered in this
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study are: Madrid (40.4), Paris (48.85), Exeter (50.72), Edinburgh (55.95), Aberdeen (57.15),
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and Kemi (64.73). The cities are in geographic locations from low latitude to high latitude to
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represent sensitivity of the performance of the system. Finally, the annual optical efficiency at
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each location as a function of the zenith angle for the reference spectrum is also obtained using
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Eq.(3).
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Figure 10 shows the variation of the optical efficiency obtained for the different location under
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varying actual spectra and AM1.5D respectively. The highest optical efficiency of 79.8% was
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observed for Kemi when considering the actual spectrum values. This value was found to be
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1.6% higher than that obtained using AM 1.5D spectrum. The lowest optical efficiency of 68.8
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% was seen in the case of Madrid which was 1.35 % higher than that obtained using AM1.5D.
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The variation in the optical efficiency is particularly because of its geographic location. When
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comparing the optical efficiency values for all the different cities it was found that the AM1.5D
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under predicts the optical efficiency of the system. It was observed that the difference in optical
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efficiency increases with increase in the latitude. This can be explained to the progressive red
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shift of the incident spectra with latitude since the AM values also increases with latitude.
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Finally, an average difference of 1.5% in the estimated optical efficiency were found.
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Figure 10 Annual optical efficiencies under direct irradiance of the system at seven different locations
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4.2. Annual Energy Output
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As in any other type of power source, the prediction of the annual energy output is the key
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parameter for evaluating the performance and profitability of the system [37, 38]. As
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commented in the previous sub-section, the annual optical efficiency of the BICPV module is
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affected by the time-varying direct spectral component of the radiation. Hence, it is appropriate
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to evaluate the impact of these variations on the annual energy harvested. The energy yield of
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photovoltaic devices can estimated by using either indirect or direct methods[39]. Among
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them, the direct method based on the efficiencies of the system discussed and adapted to
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concentrator technology in [40] has been used. The annual energy output per unit of solar cell
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(Eoutput,annual) is defined as:
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𝐸𝑜𝑢𝑡𝑝𝑢𝑡,𝑎𝑛𝑛𝑢𝑎𝑙 = 𝐺𝑎𝑛𝑛𝑢𝑎𝑙 ·CR ·𝜂𝑠𝑦𝑠𝑡𝑒𝑚
(4)
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being Gannual the total or global annual irradiation on the plane of the panel, CR the
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concentration ratio and ηsystem the efficiency of the system defined as:
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𝜂𝑠𝑦𝑠𝑡𝑒𝑚 = 𝜂𝑐𝑒𝑙𝑙 · 𝜂𝑜𝑝𝑡 (5)
being ηcell the efficiency of the solar cells and ηopt the optical efficiency of the concentrator.
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As previously discussed, the optical efficiency of the BICPV module is different under the
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direct and diffuse components of the radiation. Bearing this in mind, Eq. (5) must to be
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rewritten as:
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𝜂𝑠𝑦𝑠𝑡𝑒𝑚,𝑑𝑖𝑟𝑒𝑐𝑡 = 𝜂𝑜𝑝𝑡,𝑑𝑖𝑟𝑒𝑐𝑡 · 𝜂𝑐𝑒𝑙𝑙
(6)
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𝜂𝑠𝑦𝑠𝑡𝑒𝑚,𝑑𝑖𝑓𝑓𝑢𝑠𝑒 = 𝜂𝑜𝑝𝑡,𝑑𝑖𝑓𝑓𝑢𝑠𝑒 · 𝜂𝑐𝑒𝑙𝑙
(7)
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and the contribution to the annual energy output of both components of the radiation as:
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𝐸𝑜𝑢𝑡𝑝𝑢𝑡,𝑎𝑛𝑛𝑢𝑎𝑙,𝑑𝑖𝑟𝑒𝑐𝑡 = 𝐺𝑑𝑖𝑟𝑒𝑐𝑡,𝑎𝑛𝑛𝑢𝑎𝑙 · 𝜂𝑜𝑝𝑡,𝑑𝑖𝑟𝑒𝑐𝑡 · 𝐶𝑅 · 𝜂𝑐𝑒𝑙𝑙
(8)
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𝐸𝑜𝑢𝑡𝑝𝑢𝑡,𝑎𝑛𝑛𝑢𝑎𝑙,𝑑𝑖𝑓𝑓𝑢𝑠𝑒 = 𝐺𝑑𝑖𝑓𝑓𝑢𝑠𝑒,𝑎𝑛𝑛𝑢𝑎𝑙 · 𝜂𝑜𝑝𝑡,𝑑𝑖𝑓𝑓𝑢𝑠𝑒 · 𝐶𝑅 · 𝜂𝑐𝑒𝑙𝑙
(9)
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Thus, the annual energy output of the system is finally given by:
𝐸𝑜𝑢𝑡𝑝𝑢𝑡,𝑎𝑛𝑛𝑢𝑎𝑙 = 𝐸𝑜𝑢𝑡𝑝𝑢𝑡,𝑎𝑛𝑛𝑢𝑎𝑙,𝑑𝑖𝑟𝑒𝑐𝑡 + 𝐸𝑜𝑢𝑡𝑝𝑢𝑡,𝑎𝑛𝑛𝑢𝑎𝑙,𝑑𝑖𝑓𝑓𝑢𝑠𝑒
(10)
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Using the procedure above, the annual energy yield of the BICPV system is estimated at the
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six locations considered. The input annual direct and diffuse irradiations on the plane of the
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module are obtained from the PVGIS data source[41], see table 1. Figure 11 shows the results
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considering the optical efficiencies estimated in the previous sub-section and plotted in figure
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10. As expected, the annual energy output is under predicted with the reference spectrum, and
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spectral gains are always found considering the actual spectrum. In particular, a difference
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ranging from 1% to 1.5% in the annual energy output at the sites studies has been found. It can
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be also seen that Madrid is the location with the highest output (336 kWh/m2 using the actual
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spectrum and 332 kWh/m2 using the reference spectrum). This can be explained since, although
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Madrid presents the lowest optical efficiencies, it has by far the highest Gdirect,annual value. On
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the contrary, Paris shows the lowest energy output (334 kWh/m2 using the actual spectrum and
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331 kWh/m2 using the reference spectrum). Although this location presents a similar Gdirect,annual
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value to the rest of the sites, the lower optical efficiencies lead to a higher reduction in the
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energy yield. It is also important to note that, although Gannual clearly decreases with latitude,
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the energy output shows an almost constant performance due to the increase of the optical
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efficiency of the BICPV module with latitude, as shown in figure 10. Moreover, an almost
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constant energy output spectral gain of around 3 kWh/m2 has been found when considering the
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actual spectrum at all the sites analyzed.
City (Latitude)
Gdirect,annual
Gdiffuse,annual
Gannual
(kWh/m2)
(kWh/m2)
(kWh/m2)
Madrid (40.40)
1065
275
1340
Paris (48.85)
623
311
934
Exeter (50.72)
635
300
935
Edinburgh (55.95)
606
288
895
Aberdeen (57.15)
593
275
868
Kemi (64.73)
636
214
850
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Table 1. Values of the direct and diffuse irradiations used to estimate the annual energy output
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of the BICPV system obtained from PVGIS for the six locations considered. The total or global
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irradiation is also shown.
336
337
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Figure 11 Annual Energy output of the BICPV system based on the standard AM 1.5 spectrum and the actual
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As commented above, the relative annual spectral impact was ranging from 1% to 1.5%. This
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clearly suggests that the spectral changes occurring in the environment influence the
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performance of the BICPV module. In order to have sense of the magnitude of this impact, it
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is appropriate to compare the results obtained in this section with other similar studies. Table
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2 shows the spectral impact on the energy output of several types of PV and HCPV systems
carrying spectrum.
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estimated by other authors. As the air mass is the parameter with the largest spectral impact,
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table 2 only shows studies concerning latitudes within the range of this study. Based on this
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table, it may be concluded that the BICPV system shows a similar spectral influence to c-Si
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and CIGS devices. This can explained because they have a similar and wide absorption band
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able to absorb low energetic photons. So, they are not so affected by the air mass changes since
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the maximum attenuation is located in the short-wave UV region of the spectrum (high
350
energetic photons), as shown in figure 7. This also explains why the BICPV system always
351
present spectral gains compared with the other systems shown in table 2. As commented, the
352
optical devices used to concentrate the incoming light produce important transmission losses
353
at short wavelengths, see figure 3. So, the progressive red shift of the spectrum with the air
354
mass increases the performance of the system under real operating conditions. It is also worth
355
to mention the significant different found between the BICPV and HCPV systems also based
356
on optical devices. This can be understood due to the fact that HCPV devices use multi-junction
357
solar cells which are strongly influenced by the input spectrum.
System
Latitude
Spectral impact
Reference
BICPV
40.4 to 64.7
1% to1.5%
Section 4.2.
HCPV
40 to 65
-4.9% to -11.5%
[42]
c-Si
40.4 to 48.8
-0.6% to 1.4%
[43, 44]
a-Si
40.4 to 48.8
-0.4% to 3.4
CIGS
40.4 to 48.8
-1% to 0.6
358
Table 2. Annual spectral impact on the energy yield of the BICPV system and other PV and
359
HCPV systems.
360
5. Conclusion
361
While prior research efforts have focused primarily on the indoor performance modelling of
362
such BiCPV systems. This study lays down foundation for a modelling procedure to predict
363
the optical and annual performance of a BICPV module under actual outdoor conditions.
364
Optical efficiency is an important parameter for predicting the overall performance of the
365
system. Detailed optical analysis is carried out under different spectral conditions and results
366
compared with standard AM 1.5D spectrum used. Results show the importance of the optical
367
efficiency and its dependence on the spectrum. An analysis of the annual influence of the
368
spectral variation on the performance of the BICPV system has been conducted for Madrid,
369
Paris, Exeter, Edinburgh, Aberdeen, and Kemi. The research highlighted the importance of
370
using the spectral dependent optical efficiency while analyzing the annual performance of the
371
system. The annual performance of the system is little affected due to the varying spectrum.
372
The highest optical efficiency of 79.8% was observed for Kemi when considering the actual
373
spectrum values. This value was found to be 1.6% higher than that obtained using AM 1.5D
374
spectrum. A difference of about 1.25 % in the optical efficiency was seen in most of the sites
375
considered under the study. The difference in the output is a yearly average ranging between
376
1% and 1.5% for different sites. This gives us a better understanding of the impact the spectral
377
variations can have on a given BICPV system. Despite this, the spectral dependence of BICPV
378
systems can be considered low and similar to other conventional PV devices such as CIGS and
379
c-Si. Thus, the energy output of these type of devices can be predicted with a low margin of
380
error without considering the impact of the spectral variations, in a first approximation.
381
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