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Electrical characterization method for bifacial photovoltaic modules
Solar Energy Materials & Solar Cells 127 (2014) 136–142
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Electrical characterization method for bifacial photovoltaic modules Jai Prakash Singh a,b,n, Armin G. Aberle a,b, Timothy M. Walsh a a b
Solar Energy Research Institute of Singapore, National University of Singapore, Singapore 117475, Singapore Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore
art ic l e i nf o
a b s t r a c t
Article history: Received 2 December 2013 Received in revised form 10 April 2014 Accepted 16 April 2014
We present a method to characterize bifacial photovoltaic (PV) modules for simultaneous front and rear side illumination (i.e., bifacial illumination). The method uses standard monofacial (i.e., single-sided) indoor current–voltage measurements on the front and rear sides of the bifacial module, while covering the other side with a non-reflecting black cover. To define the performance under bifacial illumination, new bifacial parameters (short-circuit current, open-circuit voltage, fill factor and efficiency) are calculated using a one-diode model of the PV module. Assuming linear current response of PV modules for different illuminations, the method numerically simulates the performance of bifacial PV modules for various combinations of front and rear side irradiances. The method is useful in predicting the behavior of the bifacial PV module under bifacial illumination, without actually measuring the module under these conditions. Using a silicon wafer based bifacial PV module, we demonstrate experimentally that the method can predict the performance to within 1% variation with actual measurements. & 2014 Elsevier B.V. All rights reserved.
Keywords: Bifacial PV module STC measurement Bifacial illumination Bifacial efficiency
1. Introduction To reduce the cost of solar power generation, the main focus areas of the photovoltaic (PV) industry are improving the efficiency, upscaling of production volumes, and low-cost materials for solar cells and modules. All these approaches deal with the $/Wpeak cost of PV systems. However, the ultimate aim of all cost reduction approaches is to reduce the cost of solar generated energy, i.e. $/kWh. As solar cell efficiency increases, more advanced fabrication processes are required to further improve the efficiency. Thus, the complexity and cost of the device also increases, which results in diminishing returns [1]. In addition to the efficiency improvement, one possible approach to reduce the cost of solar electricity is to increase the performance and energy yield of the PV systems for the same kWpeak installation capacity. Bifacial PV modules can produce additional energy by converting solar energy to electrical energy from both sides of the module. This capability of converting light from either side of the module into electricity makes bifacial modules suitable to reduce the cost of PV electricity. The bifacial modules can enhance the power density (power per unit area on the front surface) and thus arearelated costs such as land, cabling, installation structure etc. can be reduced [2]. Many researchers are exploring the energy gain potential of bifacial PV modules as compared to monofacial n Corresponding author at: Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Block E3A, #06-01, Singapore 117574, Singapore. E-mail address: [email protected] (J.P. Singh).
http://dx.doi.org/10.1016/j.solmat.2014.04.017 0927-0248/& 2014 Elsevier B.V. All rights reserved.
modules in various installation configurations and climate conditions [3–5]. To assess their performance and quality, PV modules are measured in an indoor environment under standard test conditions (STC) as defined by the International Electrotechnical Commission (IEC) [6]. For conventional monofacial modules, standards have been adopted to rate the modules in terms of output power and efficiency under STC [7]. Besides the performance assessment, these standards help manufacturers/users to sell/buy the modules according to the price per watt power output under STC. Since bifacial PV modules operate under simultaneous front and rear side illumination (i.e., bifacial illumination) in real-world conditions, it is advisable to characterize this module type for bifacial illumination. At present, bifacial module manufacturers suffer with the problem of standard indoor measurements and quoting the price per watt of the bifacial solar cells and modules. In the absence of standards, most bifacial PV module manufacturers report the front side monofacial electrical parameters under STC and tabulate the efficiency/power with a linear addition of front and rear side efficiencies for particular rear side irradiance conditions [8–10]. However, since PV module efficiency/power does not vary linearly with the irradiance, the simple addition of front and rear side efficiencies will introduce inaccuracy in power estimation. While estimating the bifacial performance, the nonlinear behavior of PV modules with irradiance should be taken into account. As a possible solution for accurately measuring the current–voltage (I–V) parameters under bifacial illumination, Ohtsuka et al. and Elder et al. presented solar simulators with simultaneous bifacial illumination capability [11,12]. However,
J.P. Singh et al. / Solar Energy Materials & Solar Cells 127 (2014) 136–142
I sc f
Nomenclature Symbol description (unit) x Gf
Gr ηbi Pbi
V oc bi I sc bi FF bi ℛIsc ηf
I sc r
Irradiance ratio (dimensionless) Irradiance on the front side of the module (W/m2) Irradiance on the rear side of the module (W/m2) Efficiency of bifacial module for simultaneous front and rear side illumination (bifacial illumination) (%) Output power of bifacial module for bifacial illumination (W) Open-circuit voltage of bifacial module for bifacial illumination (V) Short-circuit current of bifacial module for bifacial illumination (A) Fill factor of bifacial module for bifacial illumination (%) Relative current gain (dimensionless) Module efficiency measured at STC for front side illumination only (%)
because of the resources and time utilization, it is neither practical nor feasible to measure the bifacial module for all possible combinations of front and rear side illuminations. In addition to this, the set-up requires additional components (such as mirrors and multiple reference sensors). In this paper, we focus on addressing the above mentioned issues for bifacial module characterization for bifacial illumination. The proposed method requires only a standard monofacial indoor measurement set-up to measure the front and rear side of the module separately under STC. Then, a one-diode equivalent model for PV modules is used to calculate the performance parameters (electrical parameters) of bifacial PV modules for bifacial illumination. To estimate the bifacial module performance, characteristic curves for bifacial parameters are plotted for various front and rear side irradiance conditions.
V oc f V oc r FF f FF r pFF I 0m
Rsh P Rs P Rs'
137
Short-circuit current measured for front side illumination of the module at STC (A) Short-circuit current measured for rear side illumination of the module at STC (A) Open-circuit voltage measured for front side illumination of module at STC (V) Open-circuit voltage measured for rear side illumination of module at STC (V) Fill factor measured for front side illumination of the module at STC (%) Fill factor measured for rear side illumination of the module (%) Pseudo fill factor (FF of the module considering no series resistance effect) (%) Module one-diode model parameter equivalent to dark saturation current in solar cell one-diode model (A) Lumped shunt resistance of the module (Ω) Resistive power loss (W) Relative resistive power loss (dimensionless)
modules under bifacial illumination can be defined as follows: P bi ¼ I sc bi V oc bi FF bi
ð2Þ
I sc bi V oc bi FF bi Amodule ðGf þ Gr Þ
ð3Þ
ηbi ¼
where Amodule is the module area (front surface only) and I sc bi ; V oc bi ; FF bi ; ηbi and P bi are the electrical parameters corresponding to the bifacial illumination. Therefore, in order to calculate the bifacial efficiency ðηbi Þ, we need to know I sc bi ; V oc bi and FF bi for bifacial illumination. To calculate the four bifacial electrical parameters ðI sc bi ; V oc bi ; FF bi and ηbi Þ, we require standard monofacial indoor measurements as described in the following sections. 2.1. Monofacial indoor measurements of bifacial modules
2. I–V characterization of bifacial modules: the method To estimate and characterize the I–V parameters of a bifacial module under bifacial illumination, we start by defining the term irradiance ratio in the following way: x ¼ Gr =Gf
ð1Þ
where Gf is the irradiance on the front side and Gr is the irradiance on the rear side of the bifacial PV module. Note that irradiance ratio is related to – but not exactly the same as – albedo. Albedo is defined as the fraction of solar irradiance reflected from the ground, whereas irradiance ratio is the ratio of rear and front irradiance in the module plane. Now, to define the equivalent performance of a bifacial module under bifacial illumination, we assume that the I sc of the bifacial module varies linearly with front and rear irradiance, and a bifacial module can be considered as a standard monofacial module operating at a current which is equal to the sum of the current generated from both sides of the module, i.e. once the carriers have been generated, it makes no difference to the module from which side the light entered the module [11,13]. As the irradiance conditions on the front and rear sides of the bifacial module change, the electrical parameters (I sc , V oc , FF, power and efficiency) of the module will also change. Power and efficiency of bifacial
To characterize both the front and the rear side of a bifacial module, it should be separately measured from each side. A standard monofacial indoor measurement set-up can be used to separately measure the I–V characteristic of the front and rear side of the module under STC (1000 W/m2, module temperature of 25 1C), by appropriately installing the module in the tester. During these measurements it is important to cover the opposite side (i.e., the non-illuminated side) with a black-cover, to ensure that no stray light (for example due to reflection from the wall/measurement structure) can enter the module. In real practice, due to nonzero reflectance of the cover, the light entering to the cell gap area and the light passing through the module active area is reflected back at rear side cover which affects the module current measurement. Thus for measuring I–V characteristics of a bifacial module, one should ensure that a black-cover (with zero or extremely low reflectance) is used, especially when measuring the rear side I–V characteristics of the module. From the monofacial front and rear side I–V measurements we calculate the bifacial parameters, as described in subsequent sections. 2.2. Calculation of Isc bi With the assumption of linear current response under varying irradiance conditions, we can calculate the resultant module current under bifacial illumination. When a bifacial PV module is
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illuminated with a front-side irradiance of Gf and a rear-side irradiance of Gr ¼ xGf , the total short-circuit current will simply be the sum of the two currents:
monofacial measurements of the module's I–V curves under frontonly and rear-only conditions are available.
I sc bi ¼ I sc f þ xI sc r ¼ ℛIsc I sc f
2.4. Calculation of FFbi
ð4Þ
where ℛIsc is the gain in short-circuit current relative to monofacial front-side only illumination and given by the following equation: ℛIsc ¼
I sc f þ x:I sc r I sc r ¼ 1þx ; I sc f I sc f
ð5Þ
The I sc f and I sc r are both measured under STC conditions, assuming that no stray light enters the module. Here ℛIsc is the factor by which the bifacial module current increases under bifacial illumination compared to monofacial front-only illumination. 2.3. Calculation of Voc bi Since a PV module is composed of a number of solar cells connected in series, it is reasonable to consider a PV module following the one-diode characteristics with lumped parameters describing the behavior of the diode [14]. The one-diode model to describe the module I–V characteristics can be written as follows: V þIRmod V þ IðVÞRmod IðVÞ ¼ I sc I 0m exp ð6Þ 1 K mV T Rsh where I 0m and Km are the module parameters equivalent to the saturation current and the diode ideality factor in the one-diode model of a solar cell and Rmod and Rsh are the lumped parasitic series resistance and shunt resistance of the module, respectively. For sufficiently high irradiance, the shunt leakage term in Eq. (6) can be neglected, giving the following equation: V þIRmod IðVÞ ¼ I sc I 0m exp 1 ð7Þ K mV T
PV modules have finite series resistance arising from the resistance of solar cells, resistance of ribbons for cell interconnection, contact resistance, and other resistive effects. It is well known that series resistance causes resistive power loss in solar cells and modules and that the fill factor of PV devices reduces with an increase in the resistive losses. The resistive power loss in a PV module is proportional to the square of the current. Hence the resistive losses in a bifacial module under bifacial illumination are expected to be higher than those of front-only illumination. Thus, there will be a net reduction in the FF of the bifacial PV module when illuminated from both sides compared to singlesided illumination. Similar to our previous work for bifacial solar cells, to calculate the bifacial FF (FFbi) for a bifacial module, we calculate the relative resistive losses using two different approaches, and then equate the two [15]. The first approach uses Ohm's law to calculate the relative resistive losses. The second approach considers the change in the module FF due to the additional rear-side irradiance. In the first approach, the resistive loss (due to series resistance) for a bifacial module with series resistance Rmod, is given by the following equation: P Rs ¼ I 2 Rmod ;
ð14Þ
where I is the module current. Using this relation, the additional resistive loss due to bifacial operation of the module can be calculated. From Eq. (4), the module current with bifacial illumination will increase by a factor equal to the current gain ðℛIsc Þ. Thus, the relative increase in power loss with additional current generation due to the bifacial operation is given by the following equation: I 2bi Rmod I 2f Rmod
To find the relation between V oc and I sc from Eq. (7), we consider open-circuit conditions (i.e., I¼ 0): V oc 1 ð8Þ I sc ¼ I 0m exp K mV T
P 0Rs ¼
Eq. (8) has two unknowns, I 0m and Km. Now, writing Eq. (8) for front and rear side I–V measurements performed on the bifacial PV module, we get the following equations: V oc f I sc f ¼ I 0m exp 1 ð9Þ KmV T
which can be simplified to
V oc r 1 I sc r ¼ I 0m exp K mV T
ð10Þ
Solving Eqs. (9) and (10) for the unknown parameter Km gives the following equation: K mV T ¼
V oc r V oc f lnðI sc r =I sc f Þ
ð11Þ
Similarly, we can write Eq. (12) for the bifacial illumination. V ð12Þ I sc bi ¼ I 0m exp oc bi 1 K mV T Now using Eqs. (9), (11) and (12), V oc bi can be calculated as follows: V oc bi ¼ V oc f
ðV oc r V oc f ÞlnðℛIsc Þ þ lnðI sc r =I sc f Þ
ð13Þ
Eq. (13) provides the bifacial V oc of the module for bifacial (i.e., simultaneous front and rear side) illumination, provided that
¼
I 2f Rmod ðℛIsc :I f Þ2 Rmod I 2f Rmod I 2f Rmod
;
P 0Rs ¼ ℛ2Isc 1:
ð15Þ
ð16Þ
For simplification, we have assumed that the module operating current changes in a similar way with irradiance as the shortcircuit current does. This assumption is valid for PV modules with reasonably good FF (475%).We checked the validity of this assumption by performing PV module simulations with the circuit simulation software LTSpice. In the second approach, we consider the change in power loss and hence module FF because of the change in module operating current. If pFF is the pseudo FF of the module considering no series resistance loss, then the power loss due to the series resistance for front side, rear side, and bifacial illumination can be written as follows: P Rs f ¼ ðpFF FF f ÞV oc f I sc f
ð17Þ
P Rs r ¼ ðpFF FF r ÞV oc r I sc r
ð18Þ
P Rs bi ¼ ðpFF FF bi ÞV oc bi I sc bi
ð19Þ
where P Rs f ; P Rs r and P Rs bi are the resistive losses due to the front, rear, and bifacial illuminations, respectively. Here, we assume that pFF remains the same for the change in irradiance under consideration. The relative increase in loss under
J.P. Singh et al. / Solar Energy Materials & Solar Cells 127 (2014) 136–142
bifacial illumination with respect to front-only illumination can then be written as P Rs' ¼
ðpFF FF bi ÞV oc bi I sc bi ðpFF FF f ÞV oc f I sc f ðpFF FF f ÞV oc f I sc f
ð20Þ
Comparing the relative resistive losses in Eqs. (16) and (20) from both approaches, we get ℛ2Isc 1 ¼
ðpFF FF bi ÞV oc bi I sc bi ðpFF FF f ÞV oc f I sc f ðpFF FF f ÞV oc f I sc f
ð21Þ
Eq. (21) has two unknowns, pFF and FF bi . The pFF of the module can be calculated using the front and rear I–V parameters measured under STC. Writing Eq. (21) for rear side STC measurement, we get:
I sc r I sc f
2 1 ¼
ðpFF FF r ÞV oc r I sc r ðpFF FF f ÞV oc f I sc f ðpFF FF f ÞV oc f I sc f
ð22Þ
After simplifying Eq. (22), we get the following value for pFF: pFF ¼
ðI sc r =I sc f ÞFF f FF r ðV oc r =V oc f Þ ðI sc r =I sc f Þ ðV oc r =V oc f Þ
ð23Þ
Inserting this result into Eq. (21) gives the following equation: V oc f FF bi ¼ pFF ℛIsc ð24Þ ðpFF FF f Þ V oc bi Eq. (24) gives the bifacial FF under bifacial illumination, for the case of known front-only and rear-only illumination of the bifacial PV module. Now, with the calculation of V oc bi , I sc bi and FF bi for the bifacial PV module, the module power and efficiency under bifacial illumination can be calculated using Eqs. (2) and (3). It is emphasized that this method of calculating the bifacial parameters of the bifacial module requires only standard I–V measurements under single-sided illumination conditions.
3. Indoor bifacial module measurements and characterization To characterize a bifacial PV module for bifacial illumination with indoor measurements, we chose a commercially available large-size silicon wafer based bifacial PV module. The module consists of 96 silicon wafer solar cells with an area of 110 cm2 each. The measurements on this module were performed at the Solar Energy Research Institute of Singapore (SERIS), using a flash type solar simulator (SunSim3b LS, PASAN). This measurement set-up is a commercially available standard solar simulator which is routinely used to measure monofacial PV modules. As explained earlier, while measuring one side of the module, the other side was covered with a black cloth1 to ensure that no stray light could enter the module. The front-side I–V curve of the bifacial module was measured under STC (1000 W/m2, 25 1C module temperature). Its rear-side I–V curve was measured under a low irradiance of 200 W/m2 (and a module temperature of 25 1C), since in realworld outdoor conditions the rear side of the bifacial module is generally under low irradiance conditions. Table 1 shows the measured electrical parameters and the measurement conditions for the selected bifacial PV module. 1 The black-cloth which we used is standard black velvet, for which the measured weighted average reflectance (corresponding to AM1.5) is 3.5%. We have calculated that for modules with standard cell gap, the effect on module current is minimal and can be neglected.
139
3.1. Comparison of simulated and measured I–V parameters To demonstrate the method for bifacial parameter extraction, the I–V parameters obtained with our numerical simulation method were compared with I–V parameters measured in an indoor environment. It would, of course, be ideal to measure the bifacial module under bifacial illumination conditions. However, because of the unavailability of such a solar simulator, we measured the bifacial module under 5 different front-only and rear-only illumination conditions, as shown by the symbols in Figs. 1 and 2. Also shown in Fig. 1 (solid lines) are the simulated results. As can be seen, all simulated parameters are in good agreement with the measured ones. The difference between the simulated and measured V oc is less than 0.2%, for the efficiency it is less than 0.1% (absolute). At high irradiance, the simulated FF is slightly lower than the measured FF, whereby the maximum deviation is approximately 1% (relative). As discussed earlier in the paper, certain assumptions were made to simplify the model. We have assumed that the operating current of the module varies in same fashion as the module I sc . However for the modules with low FF (high series resistance), this assumption will overestimate the module operating current, and hence the resistive losses, especially for high irradiance conditions as described by Eq. (24). In turn the simulated FF will be lower for high irradiance conditions. For the investigated module, the measured FF under STC (front side) was 72.9%, which is a fairly low value for present-day modules. The results shown in Figs. 1 and 2 confirm the effectiveness of the proposed method for predicting the module performance at various intensity levels on the front and rear side, for single-sided illumination. 3.2. Bifacial module characterization for bifacial illumination To characterize the investigated bifacial PV module under bifacial illumination, the module was measured under monofacial indoor conditions as explained in the previous section. Then the simulated bifacial parameters were calculated using the method discussed in Section 2. Figs. 3–6 show the characteristic curves with the variation of numerically simulated bifacial I–V parameters for the bifacial module. To predict the module behavior under varying irradiance conditions for front and rear sides, characteristic curves are plotted for front side irradiance varying from 100 to 1100 W/m2, with the irradiance ratio varying from 0 to 0.6. These plots cover most of the irradiance conditions which would be encountered outdoors throughout the day. From Fig. 3 it can be observed that the variation in bifacial Voc of the module with irradiance is similar in fashion for different rear to front side illuminations. As expected, the bifacial Voc is higher for higher front and rear side irradiance conditions. The bifacial FF changes with the module irradiance conditions due to change in resistive losses as shown in Fig. 4. With the increase in front and rear side irradiance, the bifacial FF decreases in a nonlinear fashion. Thus, to ensure good bifacial performance, a bifacial PV module with good bifaciality (current response from front and rear side) and operating under high rear-side irradiance conditions, should have good FFf measured under STC. One of the most important characteristic curves is the bifacial efficiency plot. Fig. 5 shows that for relatively low irradiance, the bifacial efficiency is low because of the change in major loss mechanism under low light conditions (low light behavior of PV modules) [16]. As can be seen from Fig. 5, the bifacial efficiency shows a broad peak at intermediate irradiances, whereby the peak positions shift to lower front side irradiances for increasing Gr/Gf ratios. This
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Table 1 Front and rear side electrical parameters of the silicon wafer based bifacial PV module. Measurement type
I sc (A)
V oc (V)
FF
Front side under STC Rear side at irradiance of 200 W/m2 and 25 1C module temperature
3.68 0.62
67.37 59.92
72.86 78.86
I
66
I
4
I
64
Isc [A]
module Voc [V]
Power (W)
η (%)
180.7 29.5
14.90 12.16
5
68
62 V
oc-front
60
measured
V
simulated
V
measured
oc-rear oc-rear
I
3
sc-front sc-front sc-rear sc-rear
simulated measured simulated measured
2
simulated
V
oc-front
58
1
56 100 200 300 400 500 600 700 800 900 1000 1100
0 100 200 300 400 500 600 700 800 900 1000 1100
irradiance [W/m2]
irradiance [W/m2]
16
module efficiency [%]
80
78
module FF [%]
(%)
76
74
FF FF
72
FF FF
front front rear rear
simulated measured simulated measured
70 100 200 300 400 500 600 700 800 900 1000 1100
15 14 13 12
efficiency
11
efficiency efficiency
10
efficiency
front front rear rear
simulated measured simulated measured
9 100 200 300 400 500 600 700 800 900 1000 1100
irradiance [W/m2]
irradiance [W/m2]
Fig. 1. Measured I–V parameters (symbols) of the bifacial module, for single-sided illumination from front and rear as described in Table 1. (a) Voc, (b) Isc, (c) FF, and (d) efficiency. Also shown (solid lines) are the simulated results.
70 Power Power
module Power [W]
160
Power Power
120
front front rear rear
simulated 68
measured
simulated
module Voc [V]
200
measured
80
66 64 62
Gr /G f = 0.0 Gr /G f = 0.2
60
40
Gr /G f = 0.4
58 0 100
200
300
400
500
600
700
800
900 1000 1100
irradiance [W/m2]
56 100
Gr /G f = 0.6 200
300
400
500
600
700
front side illumination
800
900
1000 1100
[W/m2]
Fig. 2. Measured and simulated power of the bifacial module for front and rear side illumination (single-sided illumination).
Fig. 3. Simulated V oc of the bifacial module for bifacial illumination.
behavior is expected because the module resistive loss is caused by the total current response from the front and rear side of the module. The maxima in the bifacial efficiency plot can be used to
optimize a bifacial module for a particular location (front side irradiance) and installation condition (rear side irradiance). The plot for power under bifacial illumination is shown in Fig. 6. It can
80
30
78
25
76
power gain [%]
module FF [%]
J.P. Singh et al. / Solar Energy Materials & Solar Cells 127 (2014) 136–142
74
Gr/G f = 0.0
72
Gr/G f = 0.2
20
15
10
Gr/G f = 0.4
70
G r /G f = 0.1
Gr/G f = 0.6 68 100
200
300
400
141
5 500
600
700
800
900
G r /G f = 0.2 G r /G f = 0.3
1000 1100 0 100
front side illumination [W/m2]
200
300
400
500
600
700
800
900
1000
1100
front side illumination [W/m2]
Fig. 4. Simulated FF of the bifacial module for bifacial illumination.
Fig. 7. Simulated power gain from a bifacial module as compared to the monofacial module of similar type. 15.2
condition can be estimated using these characteristic plots. If we fix the Gr/Gf ratio for a certain installation, it is easier to compare bifacial PV modules with different technologies and from different manufacturers for their performance under bifacial illumination.
bifacial efficiency [%]
14.8
14.4
14.0
13.6
Gr/Gf = 0.0
13.2
Gr/Gf = 0.4
Gr/Gf = 0.2 4. Conclusion
Gr/Gf = 0.6 12.8 100
200
300
400
500
600
700
800
900 1000 1100
front side illumination [W/m2] Fig. 5. Simulated efficiency of the bifacial module for bifacial illumination.
300
Gr /G f = 0.0 Gr /G f = 0.2
module power [W]
250
Gr /G f = 0.4 200
Gr /G f = 0.6
150
100
50
0 100
200
300
400
500
600
700
800
900
1000
1100
front side illumination [W/m2] Fig. 6. Simulated power of the bifacial module for bifacial illumination.
be used to determine the module power under varying irradiance conditions throughout the day. One more interesting plot is the power gain for various Gr/Gf ratios, as shown in Fig. 7. From this plot it is easy and straight-forward to estimate the power gain potential of a bifacial module technology as compared to a monofacial module of the same technology for a particular installation. The bifacial module performance and energy yield (for known temperature behavior) for a particular installation and irradiance
The method proposed in this work provides the bifacial performance of a bifacial PV module without actually measuring it under bifacial illumination conditions. A bifacial PV module is a complex device, and hence certain assumptions are necessary to simplify the corresponding one-diode model. However, using a silicon wafer based bifacial module, we have shown that these assumptions do not significantly affect the results and that the module's simulated output power agrees to within 1% with the measured power. In our study, measured and simulated parameters were compared for various monofacial front and rear illuminations, rather than bifacial illumination, under an indoor environment. The measurement of bifacial modules for bifacial performance is time consuming and requires additional resources. The presented method addresses these issues and formulates a model to predict the bifacial performance of bifacial modules using standard monofacial indoor measurements, similar to those used for monofacial modules. The measurements can be performed in an industrial environment using a solar simulator that is routinely used for monofacial modules. However, one should ensure that while measuring one side, the other side (nonilluminated side) of bifacial module is covered with a black-cover (with zero or extremely low reflectance). The presented method can be useful to standardize the bifacial module I–V characterization via the inclusion of one additional parameter, the rear-side power (or efficiency) gain.
Acknowledgments The Solar Energy Research Institute of Singapore (SERIS) is sponsored by the National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB).
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[9] 〈http://www.prismsolar.com/products.php〉 (accessed 20.09.13). [10] 〈http://www.advancedsolarphotonics.com/solar-energy-products/monocyr stalline-pv-modules/bifacial-series-glass-to-glass-photovoltaic-module〉 (accessed 20.09.13). [11] H. Ohtsuka, M. Sakamoto, M. Koyama, K. Tsutsui, T. Uematsu, Y. Yazawa, Characteristics of bifacial solar cells under bifacial illumination with various intensity levels, Prog. Photovoltaics: Res. Appl. 9 (2001) 1–13. [12] A. Edler, M. Schlemmer, J. Ranzmeyer, R. Harney, Flasher setup for bifacial measurements, bifiPV workshop, Konstanz, 2012. [13] K.R. McIntosh, C.B. Honsberg, S.R. Wenham, The impact of rear illumination on bifacial solar cells with floating junction passivation, in: Proceedings of the 2nd World Conference and Exhibition on Photovoltaic Solar Energy Conversion, Vienna, 1998, pp. 1515–1518. [14] J.A. del Cueto, S.R. Rummel, Comparison of diode quality plus other factors in polycrystalline cells and modules from outdoor and indoor measurements, Conference Record of the 31st IEEE Photovoltaic Specialists Conference, 2005, pp. 511–514. [15] J.P. Singh, T.M. Walsh, A.G. Aberle, A new method to characterize bifacial solar cells, Prog. Photovoltaics: Res. Appl. (2012), http://dx.doi.org/10.1002/ pip.2341. [16] N.H. Reich, W.G.J.H.M. van Sark, E.A. Alsema, R.W. Lof, R.E.I. Schropp, W.C. Sinke, W.C. Turkenburg, Crystalline silicon cell performance at low light intensities, Sol. Energy Mater. Sol. Cells 93 (2009) 1471–1481.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Electrical characterization method for bifacial photovoltaic modules Jai Prakash Singh a,b,n, Armin G. Aberle a,b, Timothy M. Walsh a a b
Solar Energy Research Institute of Singapore, National University of Singapore, Singapore 117475, Singapore Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore
art ic l e i nf o
a b s t r a c t
Article history: Received 2 December 2013 Received in revised form 10 April 2014 Accepted 16 April 2014
We present a method to characterize bifacial photovoltaic (PV) modules for simultaneous front and rear side illumination (i.e., bifacial illumination). The method uses standard monofacial (i.e., single-sided) indoor current–voltage measurements on the front and rear sides of the bifacial module, while covering the other side with a non-reflecting black cover. To define the performance under bifacial illumination, new bifacial parameters (short-circuit current, open-circuit voltage, fill factor and efficiency) are calculated using a one-diode model of the PV module. Assuming linear current response of PV modules for different illuminations, the method numerically simulates the performance of bifacial PV modules for various combinations of front and rear side irradiances. The method is useful in predicting the behavior of the bifacial PV module under bifacial illumination, without actually measuring the module under these conditions. Using a silicon wafer based bifacial PV module, we demonstrate experimentally that the method can predict the performance to within 1% variation with actual measurements. & 2014 Elsevier B.V. All rights reserved.
Keywords: Bifacial PV module STC measurement Bifacial illumination Bifacial efficiency
1. Introduction To reduce the cost of solar power generation, the main focus areas of the photovoltaic (PV) industry are improving the efficiency, upscaling of production volumes, and low-cost materials for solar cells and modules. All these approaches deal with the $/Wpeak cost of PV systems. However, the ultimate aim of all cost reduction approaches is to reduce the cost of solar generated energy, i.e. $/kWh. As solar cell efficiency increases, more advanced fabrication processes are required to further improve the efficiency. Thus, the complexity and cost of the device also increases, which results in diminishing returns [1]. In addition to the efficiency improvement, one possible approach to reduce the cost of solar electricity is to increase the performance and energy yield of the PV systems for the same kWpeak installation capacity. Bifacial PV modules can produce additional energy by converting solar energy to electrical energy from both sides of the module. This capability of converting light from either side of the module into electricity makes bifacial modules suitable to reduce the cost of PV electricity. The bifacial modules can enhance the power density (power per unit area on the front surface) and thus arearelated costs such as land, cabling, installation structure etc. can be reduced [2]. Many researchers are exploring the energy gain potential of bifacial PV modules as compared to monofacial n Corresponding author at: Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Block E3A, #06-01, Singapore 117574, Singapore. E-mail address: [email protected] (J.P. Singh).
http://dx.doi.org/10.1016/j.solmat.2014.04.017 0927-0248/& 2014 Elsevier B.V. All rights reserved.
modules in various installation configurations and climate conditions [3–5]. To assess their performance and quality, PV modules are measured in an indoor environment under standard test conditions (STC) as defined by the International Electrotechnical Commission (IEC) [6]. For conventional monofacial modules, standards have been adopted to rate the modules in terms of output power and efficiency under STC [7]. Besides the performance assessment, these standards help manufacturers/users to sell/buy the modules according to the price per watt power output under STC. Since bifacial PV modules operate under simultaneous front and rear side illumination (i.e., bifacial illumination) in real-world conditions, it is advisable to characterize this module type for bifacial illumination. At present, bifacial module manufacturers suffer with the problem of standard indoor measurements and quoting the price per watt of the bifacial solar cells and modules. In the absence of standards, most bifacial PV module manufacturers report the front side monofacial electrical parameters under STC and tabulate the efficiency/power with a linear addition of front and rear side efficiencies for particular rear side irradiance conditions [8–10]. However, since PV module efficiency/power does not vary linearly with the irradiance, the simple addition of front and rear side efficiencies will introduce inaccuracy in power estimation. While estimating the bifacial performance, the nonlinear behavior of PV modules with irradiance should be taken into account. As a possible solution for accurately measuring the current–voltage (I–V) parameters under bifacial illumination, Ohtsuka et al. and Elder et al. presented solar simulators with simultaneous bifacial illumination capability [11,12]. However,
J.P. Singh et al. / Solar Energy Materials & Solar Cells 127 (2014) 136–142
I sc f
Nomenclature Symbol description (unit) x Gf
Gr ηbi Pbi
V oc bi I sc bi FF bi ℛIsc ηf
I sc r
Irradiance ratio (dimensionless) Irradiance on the front side of the module (W/m2) Irradiance on the rear side of the module (W/m2) Efficiency of bifacial module for simultaneous front and rear side illumination (bifacial illumination) (%) Output power of bifacial module for bifacial illumination (W) Open-circuit voltage of bifacial module for bifacial illumination (V) Short-circuit current of bifacial module for bifacial illumination (A) Fill factor of bifacial module for bifacial illumination (%) Relative current gain (dimensionless) Module efficiency measured at STC for front side illumination only (%)
because of the resources and time utilization, it is neither practical nor feasible to measure the bifacial module for all possible combinations of front and rear side illuminations. In addition to this, the set-up requires additional components (such as mirrors and multiple reference sensors). In this paper, we focus on addressing the above mentioned issues for bifacial module characterization for bifacial illumination. The proposed method requires only a standard monofacial indoor measurement set-up to measure the front and rear side of the module separately under STC. Then, a one-diode equivalent model for PV modules is used to calculate the performance parameters (electrical parameters) of bifacial PV modules for bifacial illumination. To estimate the bifacial module performance, characteristic curves for bifacial parameters are plotted for various front and rear side irradiance conditions.
V oc f V oc r FF f FF r pFF I 0m
Rsh P Rs P Rs'
137
Short-circuit current measured for front side illumination of the module at STC (A) Short-circuit current measured for rear side illumination of the module at STC (A) Open-circuit voltage measured for front side illumination of module at STC (V) Open-circuit voltage measured for rear side illumination of module at STC (V) Fill factor measured for front side illumination of the module at STC (%) Fill factor measured for rear side illumination of the module (%) Pseudo fill factor (FF of the module considering no series resistance effect) (%) Module one-diode model parameter equivalent to dark saturation current in solar cell one-diode model (A) Lumped shunt resistance of the module (Ω) Resistive power loss (W) Relative resistive power loss (dimensionless)
modules under bifacial illumination can be defined as follows: P bi ¼ I sc bi V oc bi FF bi
ð2Þ
I sc bi V oc bi FF bi Amodule ðGf þ Gr Þ
ð3Þ
ηbi ¼
where Amodule is the module area (front surface only) and I sc bi ; V oc bi ; FF bi ; ηbi and P bi are the electrical parameters corresponding to the bifacial illumination. Therefore, in order to calculate the bifacial efficiency ðηbi Þ, we need to know I sc bi ; V oc bi and FF bi for bifacial illumination. To calculate the four bifacial electrical parameters ðI sc bi ; V oc bi ; FF bi and ηbi Þ, we require standard monofacial indoor measurements as described in the following sections. 2.1. Monofacial indoor measurements of bifacial modules
2. I–V characterization of bifacial modules: the method To estimate and characterize the I–V parameters of a bifacial module under bifacial illumination, we start by defining the term irradiance ratio in the following way: x ¼ Gr =Gf
ð1Þ
where Gf is the irradiance on the front side and Gr is the irradiance on the rear side of the bifacial PV module. Note that irradiance ratio is related to – but not exactly the same as – albedo. Albedo is defined as the fraction of solar irradiance reflected from the ground, whereas irradiance ratio is the ratio of rear and front irradiance in the module plane. Now, to define the equivalent performance of a bifacial module under bifacial illumination, we assume that the I sc of the bifacial module varies linearly with front and rear irradiance, and a bifacial module can be considered as a standard monofacial module operating at a current which is equal to the sum of the current generated from both sides of the module, i.e. once the carriers have been generated, it makes no difference to the module from which side the light entered the module [11,13]. As the irradiance conditions on the front and rear sides of the bifacial module change, the electrical parameters (I sc , V oc , FF, power and efficiency) of the module will also change. Power and efficiency of bifacial
To characterize both the front and the rear side of a bifacial module, it should be separately measured from each side. A standard monofacial indoor measurement set-up can be used to separately measure the I–V characteristic of the front and rear side of the module under STC (1000 W/m2, module temperature of 25 1C), by appropriately installing the module in the tester. During these measurements it is important to cover the opposite side (i.e., the non-illuminated side) with a black-cover, to ensure that no stray light (for example due to reflection from the wall/measurement structure) can enter the module. In real practice, due to nonzero reflectance of the cover, the light entering to the cell gap area and the light passing through the module active area is reflected back at rear side cover which affects the module current measurement. Thus for measuring I–V characteristics of a bifacial module, one should ensure that a black-cover (with zero or extremely low reflectance) is used, especially when measuring the rear side I–V characteristics of the module. From the monofacial front and rear side I–V measurements we calculate the bifacial parameters, as described in subsequent sections. 2.2. Calculation of Isc bi With the assumption of linear current response under varying irradiance conditions, we can calculate the resultant module current under bifacial illumination. When a bifacial PV module is
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illuminated with a front-side irradiance of Gf and a rear-side irradiance of Gr ¼ xGf , the total short-circuit current will simply be the sum of the two currents:
monofacial measurements of the module's I–V curves under frontonly and rear-only conditions are available.
I sc bi ¼ I sc f þ xI sc r ¼ ℛIsc I sc f
2.4. Calculation of FFbi
ð4Þ
where ℛIsc is the gain in short-circuit current relative to monofacial front-side only illumination and given by the following equation: ℛIsc ¼
I sc f þ x:I sc r I sc r ¼ 1þx ; I sc f I sc f
ð5Þ
The I sc f and I sc r are both measured under STC conditions, assuming that no stray light enters the module. Here ℛIsc is the factor by which the bifacial module current increases under bifacial illumination compared to monofacial front-only illumination. 2.3. Calculation of Voc bi Since a PV module is composed of a number of solar cells connected in series, it is reasonable to consider a PV module following the one-diode characteristics with lumped parameters describing the behavior of the diode [14]. The one-diode model to describe the module I–V characteristics can be written as follows: V þIRmod V þ IðVÞRmod IðVÞ ¼ I sc I 0m exp ð6Þ 1 K mV T Rsh where I 0m and Km are the module parameters equivalent to the saturation current and the diode ideality factor in the one-diode model of a solar cell and Rmod and Rsh are the lumped parasitic series resistance and shunt resistance of the module, respectively. For sufficiently high irradiance, the shunt leakage term in Eq. (6) can be neglected, giving the following equation: V þIRmod IðVÞ ¼ I sc I 0m exp 1 ð7Þ K mV T
PV modules have finite series resistance arising from the resistance of solar cells, resistance of ribbons for cell interconnection, contact resistance, and other resistive effects. It is well known that series resistance causes resistive power loss in solar cells and modules and that the fill factor of PV devices reduces with an increase in the resistive losses. The resistive power loss in a PV module is proportional to the square of the current. Hence the resistive losses in a bifacial module under bifacial illumination are expected to be higher than those of front-only illumination. Thus, there will be a net reduction in the FF of the bifacial PV module when illuminated from both sides compared to singlesided illumination. Similar to our previous work for bifacial solar cells, to calculate the bifacial FF (FFbi) for a bifacial module, we calculate the relative resistive losses using two different approaches, and then equate the two [15]. The first approach uses Ohm's law to calculate the relative resistive losses. The second approach considers the change in the module FF due to the additional rear-side irradiance. In the first approach, the resistive loss (due to series resistance) for a bifacial module with series resistance Rmod, is given by the following equation: P Rs ¼ I 2 Rmod ;
ð14Þ
where I is the module current. Using this relation, the additional resistive loss due to bifacial operation of the module can be calculated. From Eq. (4), the module current with bifacial illumination will increase by a factor equal to the current gain ðℛIsc Þ. Thus, the relative increase in power loss with additional current generation due to the bifacial operation is given by the following equation: I 2bi Rmod I 2f Rmod
To find the relation between V oc and I sc from Eq. (7), we consider open-circuit conditions (i.e., I¼ 0): V oc 1 ð8Þ I sc ¼ I 0m exp K mV T
P 0Rs ¼
Eq. (8) has two unknowns, I 0m and Km. Now, writing Eq. (8) for front and rear side I–V measurements performed on the bifacial PV module, we get the following equations: V oc f I sc f ¼ I 0m exp 1 ð9Þ KmV T
which can be simplified to
V oc r 1 I sc r ¼ I 0m exp K mV T
ð10Þ
Solving Eqs. (9) and (10) for the unknown parameter Km gives the following equation: K mV T ¼
V oc r V oc f lnðI sc r =I sc f Þ
ð11Þ
Similarly, we can write Eq. (12) for the bifacial illumination. V ð12Þ I sc bi ¼ I 0m exp oc bi 1 K mV T Now using Eqs. (9), (11) and (12), V oc bi can be calculated as follows: V oc bi ¼ V oc f
ðV oc r V oc f ÞlnðℛIsc Þ þ lnðI sc r =I sc f Þ
ð13Þ
Eq. (13) provides the bifacial V oc of the module for bifacial (i.e., simultaneous front and rear side) illumination, provided that
¼
I 2f Rmod ðℛIsc :I f Þ2 Rmod I 2f Rmod I 2f Rmod
;
P 0Rs ¼ ℛ2Isc 1:
ð15Þ
ð16Þ
For simplification, we have assumed that the module operating current changes in a similar way with irradiance as the shortcircuit current does. This assumption is valid for PV modules with reasonably good FF (475%).We checked the validity of this assumption by performing PV module simulations with the circuit simulation software LTSpice. In the second approach, we consider the change in power loss and hence module FF because of the change in module operating current. If pFF is the pseudo FF of the module considering no series resistance loss, then the power loss due to the series resistance for front side, rear side, and bifacial illumination can be written as follows: P Rs f ¼ ðpFF FF f ÞV oc f I sc f
ð17Þ
P Rs r ¼ ðpFF FF r ÞV oc r I sc r
ð18Þ
P Rs bi ¼ ðpFF FF bi ÞV oc bi I sc bi
ð19Þ
where P Rs f ; P Rs r and P Rs bi are the resistive losses due to the front, rear, and bifacial illuminations, respectively. Here, we assume that pFF remains the same for the change in irradiance under consideration. The relative increase in loss under
J.P. Singh et al. / Solar Energy Materials & Solar Cells 127 (2014) 136–142
bifacial illumination with respect to front-only illumination can then be written as P Rs' ¼
ðpFF FF bi ÞV oc bi I sc bi ðpFF FF f ÞV oc f I sc f ðpFF FF f ÞV oc f I sc f
ð20Þ
Comparing the relative resistive losses in Eqs. (16) and (20) from both approaches, we get ℛ2Isc 1 ¼
ðpFF FF bi ÞV oc bi I sc bi ðpFF FF f ÞV oc f I sc f ðpFF FF f ÞV oc f I sc f
ð21Þ
Eq. (21) has two unknowns, pFF and FF bi . The pFF of the module can be calculated using the front and rear I–V parameters measured under STC. Writing Eq. (21) for rear side STC measurement, we get:
I sc r I sc f
2 1 ¼
ðpFF FF r ÞV oc r I sc r ðpFF FF f ÞV oc f I sc f ðpFF FF f ÞV oc f I sc f
ð22Þ
After simplifying Eq. (22), we get the following value for pFF: pFF ¼
ðI sc r =I sc f ÞFF f FF r ðV oc r =V oc f Þ ðI sc r =I sc f Þ ðV oc r =V oc f Þ
ð23Þ
Inserting this result into Eq. (21) gives the following equation: V oc f FF bi ¼ pFF ℛIsc ð24Þ ðpFF FF f Þ V oc bi Eq. (24) gives the bifacial FF under bifacial illumination, for the case of known front-only and rear-only illumination of the bifacial PV module. Now, with the calculation of V oc bi , I sc bi and FF bi for the bifacial PV module, the module power and efficiency under bifacial illumination can be calculated using Eqs. (2) and (3). It is emphasized that this method of calculating the bifacial parameters of the bifacial module requires only standard I–V measurements under single-sided illumination conditions.
3. Indoor bifacial module measurements and characterization To characterize a bifacial PV module for bifacial illumination with indoor measurements, we chose a commercially available large-size silicon wafer based bifacial PV module. The module consists of 96 silicon wafer solar cells with an area of 110 cm2 each. The measurements on this module were performed at the Solar Energy Research Institute of Singapore (SERIS), using a flash type solar simulator (SunSim3b LS, PASAN). This measurement set-up is a commercially available standard solar simulator which is routinely used to measure monofacial PV modules. As explained earlier, while measuring one side of the module, the other side was covered with a black cloth1 to ensure that no stray light could enter the module. The front-side I–V curve of the bifacial module was measured under STC (1000 W/m2, 25 1C module temperature). Its rear-side I–V curve was measured under a low irradiance of 200 W/m2 (and a module temperature of 25 1C), since in realworld outdoor conditions the rear side of the bifacial module is generally under low irradiance conditions. Table 1 shows the measured electrical parameters and the measurement conditions for the selected bifacial PV module. 1 The black-cloth which we used is standard black velvet, for which the measured weighted average reflectance (corresponding to AM1.5) is 3.5%. We have calculated that for modules with standard cell gap, the effect on module current is minimal and can be neglected.
139
3.1. Comparison of simulated and measured I–V parameters To demonstrate the method for bifacial parameter extraction, the I–V parameters obtained with our numerical simulation method were compared with I–V parameters measured in an indoor environment. It would, of course, be ideal to measure the bifacial module under bifacial illumination conditions. However, because of the unavailability of such a solar simulator, we measured the bifacial module under 5 different front-only and rear-only illumination conditions, as shown by the symbols in Figs. 1 and 2. Also shown in Fig. 1 (solid lines) are the simulated results. As can be seen, all simulated parameters are in good agreement with the measured ones. The difference between the simulated and measured V oc is less than 0.2%, for the efficiency it is less than 0.1% (absolute). At high irradiance, the simulated FF is slightly lower than the measured FF, whereby the maximum deviation is approximately 1% (relative). As discussed earlier in the paper, certain assumptions were made to simplify the model. We have assumed that the operating current of the module varies in same fashion as the module I sc . However for the modules with low FF (high series resistance), this assumption will overestimate the module operating current, and hence the resistive losses, especially for high irradiance conditions as described by Eq. (24). In turn the simulated FF will be lower for high irradiance conditions. For the investigated module, the measured FF under STC (front side) was 72.9%, which is a fairly low value for present-day modules. The results shown in Figs. 1 and 2 confirm the effectiveness of the proposed method for predicting the module performance at various intensity levels on the front and rear side, for single-sided illumination. 3.2. Bifacial module characterization for bifacial illumination To characterize the investigated bifacial PV module under bifacial illumination, the module was measured under monofacial indoor conditions as explained in the previous section. Then the simulated bifacial parameters were calculated using the method discussed in Section 2. Figs. 3–6 show the characteristic curves with the variation of numerically simulated bifacial I–V parameters for the bifacial module. To predict the module behavior under varying irradiance conditions for front and rear sides, characteristic curves are plotted for front side irradiance varying from 100 to 1100 W/m2, with the irradiance ratio varying from 0 to 0.6. These plots cover most of the irradiance conditions which would be encountered outdoors throughout the day. From Fig. 3 it can be observed that the variation in bifacial Voc of the module with irradiance is similar in fashion for different rear to front side illuminations. As expected, the bifacial Voc is higher for higher front and rear side irradiance conditions. The bifacial FF changes with the module irradiance conditions due to change in resistive losses as shown in Fig. 4. With the increase in front and rear side irradiance, the bifacial FF decreases in a nonlinear fashion. Thus, to ensure good bifacial performance, a bifacial PV module with good bifaciality (current response from front and rear side) and operating under high rear-side irradiance conditions, should have good FFf measured under STC. One of the most important characteristic curves is the bifacial efficiency plot. Fig. 5 shows that for relatively low irradiance, the bifacial efficiency is low because of the change in major loss mechanism under low light conditions (low light behavior of PV modules) [16]. As can be seen from Fig. 5, the bifacial efficiency shows a broad peak at intermediate irradiances, whereby the peak positions shift to lower front side irradiances for increasing Gr/Gf ratios. This
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Table 1 Front and rear side electrical parameters of the silicon wafer based bifacial PV module. Measurement type
I sc (A)
V oc (V)
FF
Front side under STC Rear side at irradiance of 200 W/m2 and 25 1C module temperature
3.68 0.62
67.37 59.92
72.86 78.86
I
66
I
4
I
64
Isc [A]
module Voc [V]
Power (W)
η (%)
180.7 29.5
14.90 12.16
5
68
62 V
oc-front
60
measured
V
simulated
V
measured
oc-rear oc-rear
I
3
sc-front sc-front sc-rear sc-rear
simulated measured simulated measured
2
simulated
V
oc-front
58
1
56 100 200 300 400 500 600 700 800 900 1000 1100
0 100 200 300 400 500 600 700 800 900 1000 1100
irradiance [W/m2]
irradiance [W/m2]
16
module efficiency [%]
80
78
module FF [%]
(%)
76
74
FF FF
72
FF FF
front front rear rear
simulated measured simulated measured
70 100 200 300 400 500 600 700 800 900 1000 1100
15 14 13 12
efficiency
11
efficiency efficiency
10
efficiency
front front rear rear
simulated measured simulated measured
9 100 200 300 400 500 600 700 800 900 1000 1100
irradiance [W/m2]
irradiance [W/m2]
Fig. 1. Measured I–V parameters (symbols) of the bifacial module, for single-sided illumination from front and rear as described in Table 1. (a) Voc, (b) Isc, (c) FF, and (d) efficiency. Also shown (solid lines) are the simulated results.
70 Power Power
module Power [W]
160
Power Power
120
front front rear rear
simulated 68
measured
simulated
module Voc [V]
200
measured
80
66 64 62
Gr /G f = 0.0 Gr /G f = 0.2
60
40
Gr /G f = 0.4
58 0 100
200
300
400
500
600
700
800
900 1000 1100
irradiance [W/m2]
56 100
Gr /G f = 0.6 200
300
400
500
600
700
front side illumination
800
900
1000 1100
[W/m2]
Fig. 2. Measured and simulated power of the bifacial module for front and rear side illumination (single-sided illumination).
Fig. 3. Simulated V oc of the bifacial module for bifacial illumination.
behavior is expected because the module resistive loss is caused by the total current response from the front and rear side of the module. The maxima in the bifacial efficiency plot can be used to
optimize a bifacial module for a particular location (front side irradiance) and installation condition (rear side irradiance). The plot for power under bifacial illumination is shown in Fig. 6. It can
80
30
78
25
76
power gain [%]
module FF [%]
J.P. Singh et al. / Solar Energy Materials & Solar Cells 127 (2014) 136–142
74
Gr/G f = 0.0
72
Gr/G f = 0.2
20
15
10
Gr/G f = 0.4
70
G r /G f = 0.1
Gr/G f = 0.6 68 100
200
300
400
141
5 500
600
700
800
900
G r /G f = 0.2 G r /G f = 0.3
1000 1100 0 100
front side illumination [W/m2]
200
300
400
500
600
700
800
900
1000
1100
front side illumination [W/m2]
Fig. 4. Simulated FF of the bifacial module for bifacial illumination.
Fig. 7. Simulated power gain from a bifacial module as compared to the monofacial module of similar type. 15.2
condition can be estimated using these characteristic plots. If we fix the Gr/Gf ratio for a certain installation, it is easier to compare bifacial PV modules with different technologies and from different manufacturers for their performance under bifacial illumination.
bifacial efficiency [%]
14.8
14.4
14.0
13.6
Gr/Gf = 0.0
13.2
Gr/Gf = 0.4
Gr/Gf = 0.2 4. Conclusion
Gr/Gf = 0.6 12.8 100
200
300
400
500
600
700
800
900 1000 1100
front side illumination [W/m2] Fig. 5. Simulated efficiency of the bifacial module for bifacial illumination.
300
Gr /G f = 0.0 Gr /G f = 0.2
module power [W]
250
Gr /G f = 0.4 200
Gr /G f = 0.6
150
100
50
0 100
200
300
400
500
600
700
800
900
1000
1100
front side illumination [W/m2] Fig. 6. Simulated power of the bifacial module for bifacial illumination.
be used to determine the module power under varying irradiance conditions throughout the day. One more interesting plot is the power gain for various Gr/Gf ratios, as shown in Fig. 7. From this plot it is easy and straight-forward to estimate the power gain potential of a bifacial module technology as compared to a monofacial module of the same technology for a particular installation. The bifacial module performance and energy yield (for known temperature behavior) for a particular installation and irradiance
The method proposed in this work provides the bifacial performance of a bifacial PV module without actually measuring it under bifacial illumination conditions. A bifacial PV module is a complex device, and hence certain assumptions are necessary to simplify the corresponding one-diode model. However, using a silicon wafer based bifacial module, we have shown that these assumptions do not significantly affect the results and that the module's simulated output power agrees to within 1% with the measured power. In our study, measured and simulated parameters were compared for various monofacial front and rear illuminations, rather than bifacial illumination, under an indoor environment. The measurement of bifacial modules for bifacial performance is time consuming and requires additional resources. The presented method addresses these issues and formulates a model to predict the bifacial performance of bifacial modules using standard monofacial indoor measurements, similar to those used for monofacial modules. The measurements can be performed in an industrial environment using a solar simulator that is routinely used for monofacial modules. However, one should ensure that while measuring one side, the other side (nonilluminated side) of bifacial module is covered with a black-cover (with zero or extremely low reflectance). The presented method can be useful to standardize the bifacial module I–V characterization via the inclusion of one additional parameter, the rear-side power (or efficiency) gain.
Acknowledgments The Solar Energy Research Institute of Singapore (SERIS) is sponsored by the National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB).
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