Procedures and Setups for the Measurement of Bifacial Solar Devices


The precise measurement of the illuminated current-voltage (IV) characteristics is of central importance for solar cell and module manufacturers. While the procedure for the measurement of conventional monofacial solar devices is well-defined in standards [1], discussions regarding the measurement of bifacial devices are still ongoing. This article gives an overview of the measurement procedures and setups currently available or under development.

 

1. Procedures for Measurement of Bifacial Solar Devices

Different procedures for the measurement of the electrical power (current-voltage (IV) characteristics) of bifacial solar devices have been reported in literature. These are either based on both-sided illumination of the device or on front-side illumination only.

 

1.1. Indoor Measurements with Single-sided Illumination

In production line environments, measurements of conventional solar cells and modules are standardly carried out with single-sided illumination. Therefore, several different procedures based on single-sided illumination have been transferred to bifacial solar devices. The most critical point is the consideration of the additional current that would be generated by illumination of the rear side.

Many manufacturers of bifacial solar modules [2‑5] not only specify the front IV characteristics of the modules under standard testing conditions (STC) in their data sheets, but also the characteristics for additional rear side power gains of typically 5 to 25 %. They do not give details though, whether these have been determined by measurement, extrapolation or calculation with advanced methods.

In addition to front STC parameters, Teppe et al. also state rear IV characteristics at STC for bifacial solar cells [6]. They then use the front and rear characteristics to determine the power of the solar cells under bifacial illumination: The measured rear power is linearly interpolated to 200 W/m2 and added to the measured front STC power. This approach can only be assessed as rough approximation as the power of solar devices in general is not a linear function of the irradiance.

Singh et al. introduced a more advanced method for the characterization of bifacial solar devices based on measuring the front and rear IV characteristics at STC, likewise under single-sided illumination [7, 8]. As further input, their method requires the additional determination of the series resistance Rs or the pseudo fill factor of the device. Assuming a linear dependence between current and irradiance, their method then numerically simulates the power of bifacial devices under bifacial illumination. They thereby noted that it is important to apply a black cover to the non-illuminated side to minimize the influence of stray light.

An extension of this approach is the so-called equivalent irradiance (GE) method [9] for which no additional Rs determination is required (see Figure 1).

                 

Figure 1: Schematic description of the equivalent irradiance (GE) method [9].

 

In addition to measurements of front and rear IV characteristics at STC under single-sided illumination, further front-side measurements at higher irradiances GEi are performed. GEi is thereby given by the short-circuit current bifaciality coefficient Isc,r/Isc,f:

with Isc,r and Isc,f standing for the rear and front STC short-circuit currents, respectively. At least three measurements with different rear-side irradiances GRi are to be conducted and the results interpolated to GR = 100 and 200 Wm-2. A spatial rear irradiance non-uniformity of 2 % or less is recommended by the authors. It is thereby important to apply non-reflective covers to the non-illuminated side, as the reflectance of the measurement chuck can significantly influence the measured current [10‑12].

For reporting of the measurement results of the bifacial device, the front and rear STC parameters as well as the interpolated power at GR = 100 and 200 Wm-2 need to be specified [9]. For bifacial solar cells, it is furthermore important to specify the rear contacting scheme in the report, since the conductance of the measurement chuck can significantly influence the measured IV characteristics [13].

 

1.2. Indoor Measurements with Both-sided Illumination

As further measurement procedure which is very close to operation conditions of the bifacial device, the IV characteristics can also be determined from measurements with both-sided illumination [9, 14]. The front side irradiance thereby shall be set to 1000 Wm‑2 and –similar to the GE method– three different rear irradiances are to be applied. The power of the bifacial device shall then be interpolated to rear irradiances of 100 and 200 Wm‑2. In addition to the front and rear STC parameters, these power values shall be specified in the measurement report.

The authors note that it is important to use non-reflective masks around the solar cell or the solar module to omit interreflections between the two light sources. A non-uniformity of 5 % or better is recommended for the rear illumination.

One company has already adopted this measurement procedure and specified front and rear STC parameters as well as parameters determined under bifacial illumination conditions in their data sheet [15].

 

1.3. Outdoor Measurements with Both-sided Illumination

The electrical power of the bifacial devices can furthermore be measured outdoors under both-sided illumination [9, 16]. Similar to indoor measurements, the front and rear IV characteristics at STC (determined with a non-reflective cover on the non-illuminated side) and the interpolated powers at rear irradiances of 100 and 200 Wm-2 shall be reported. A spatial rear non-uniformity of 5 % or better is recommended.

In a recent study, Deline et al. investigated the application of conventional standard outdoor measurement conditions to bifacial solar devices [16]. These conditions are defined in standard IEC 60904-3 and comprise –besides spectral distribution of AM1.5 and front irradiance of 1000 Wm-2– light bare soil as ground coverage [17]. This corresponds to an albedo of 0.21 [16]. The authors investigated the influence of further conditions not defined in the IEC standard on the rear irradiance and on its spatial homogeneity. They recommended applying single, separated modules at a ground clearance height of at last 1 m for the measurements. Furthermore, by varying the ground coverage, rear irradiances between 0 and close to 300 Wm-2 were realized.

 

2. Setups for Measuring Bifacial Solar Devices

Several setups for the measurement of bifacial devices are commercially available or have been developed at research institutes. In the following, these setups are presented in more detail.

 

2.1. Setups with One Light Source for Single-sided Illumination

For measurement procedures based on single-sided illumination, conventional setups can be applied. However, as these measurements are performed at irradiances of up to 1200 Wm-2, these setups may need to be upgraded. Various companies offering solar simulators have adapted their solar simulators to meet these requirements [9, 27, 28].

 

2.2. Setups with Two Different Light Sources for Both-sided Illumination

The measurement approach most close to operation of bifacial devices includes the illumination from both the front and the rear side. Several companies offer solar simulators with two light sources for indoor measurements of bifacial solar cells and modules:

The company h.a.l.m. provides setups with two light sources for measuring large-area bifacial solar cells with both-sided flash illumination (see Figure 2) [14]. The light sources can be operated either independently or in synchronized mode. 

 

Figure 2: Schematic of the solar simulator offered by h.a.l.m. for measuring bifacial solar cells with two independent light sources [14].

 

The company Eternal Sun commercially offers solar simulators for full-size modules with one steady-state light source for front and a flash source for rear illumination, respectively [18].

Furthermore, the company Neonsee provides setups with dual light sources for both-sided illumination of large-area solar cells, operated in flash illumination [19].

In addition to equipment manufacturers, several setups are applied by research institutes: The institute Gumi Electronics & Information Technology Research Institute (GERI) in Korea operates a setup for both-sided illumination of solar cells using a Xenon lamp for the front and an LED array for rear illumination [20].

As further research institute, the International Solar Energy Research Center (ISC) in Constance, Germany, applies a solar simulator with two different steady-state light sources [21].

 

2.3. Setups with One Light Source for Both-sided Illumination

As alternative to setups with two different light sources, facilities with one light source and mirrors are in use for both-sided illumination of bifacial devices. Two mirrors are thereby applied to deflect the light of the solar simulator to the front and the rear of the bifacial device, which is mounted in parallel to the direction of light incidence (Figure 3). This approach has first been reported by [22] for small solar cells and transferred to large-area solar cells in 2008 [23]. The irradiance on the rear side of the bifacial device can thereby be reduced by inserting neutral density filters into the rear light path. Furthermore, the rear mirror can be replaced by different diffuse reflectors to simulate different albedo conditions [24].

                    

Figure 3: Schematic of a two-mirror setup for both-sided illumination of bifacial solar cell tester with one light source [23].

 

Several research groups use this kind of setup for single large-area solar cells [25], for mini modules of four large-area solar cells [24] and for full-size bifacial modules [26] (Figure 4).

In principle, these setups are applicable for indoor and outdoor measurements, though almost exclusively indoor results have been published so far.

To the authors‘ knowledge, these setups are not commercially available so far but are developed and applied by research institutes.

Figure 4: Picture of two-mirror setup for the measurement of full-size bifacial modules [26].

 

3.  Outlook

There are several promising procedures and setups for the measurement of bifacial solar devices available. Although first experiments already dealt with comparing the different procedures [16, 26, 29, 30], the compatibility of these procedures has not been clarified conclusively. Comprehensive and fast investigations are therefore urgently needed to allow for the fast market introduction of bifacial solar devices.

 

4. References

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