< Optical simulations of bifacial PV cells and modules

Challenges of energy rating of bifacial and BIPV modules


In order to achieve the European targets of reduction of greenhouse gas emissions of at least 40% by 2030 compared to 1990 levels and 32% share of renewable energy sources in the consumed energy by 2030, new power plants need to be installed which will involve mainly wind and solar energy systems [1]. In this regard, bifacial modules which are already a common technology choice for new utility-scale solar plants thanks to their higher performance compared to mono-facial devices, may play an important role in future solar PV deployment. Besides these power plants, the building sector has a great potential not only for PV electricity generation by means of rooftop and BIPV systems [2] but also for the reduction of energy consumption [3]. Besides, BIPV does not compete for land resources with other uses such as agriculture unlike utility scale plants and allows a better match between energy demand and generation. These advantages in terms of energy generation and land use requirements could result in an increased presence of these devices in the future solar PV market with the subsequent need for standards like those applicable to mono-facial devices.

In August 2018, after more than 20 years under development the IEC Standard series 61853 "Photovoltaic (PV) module performance testing and energy rating" was finalized with the publication of the last two parts (Parts 3 and 4). Focused on mono-facial modules, the standard serves as a tool to compare the performance of different PV technologies under the same climatic conditions, or to compare the behaviour of a specific technology under different climatic regions. Therefore, it can help PV manufacturers, installers and investors to choose the most suitable PV technology for a given location.

1. Energy yield and Energy rating

It is important at this point to clearly distinguish between energy yield and energy rating since these two concepts are often confused and even used synonymously.

Energy yield is the measurement of or an estimate of the energy produced by a specific PV system, constructed in a particular way with specific inclination and azimuth (orientation) angles and located in a particular location. It requires specific data of the mounting type (fixed, tracking system, open rack or building integrated) and the location, since for the prediction of the energy yield it is necessary to account for the presence of shadows, dust or snow. Therefore it requires several years of data, typically at least 10 years, in order to have a representative estimate for that particular system and location. This long term performance estimate requires also taking into consideration the PV module's degradation. The units of the energy yield are kWh per unit of time, normally year or the PV system lifetime.

On the contrary, energy rating is a simplified measure of how a given module type will perform in a generic site with a reference climate and it is normally calculated considering a single year of data. It is intended to enable a performance comparison between devices or technologies, but it does not provide an accurate yield estimation as it does not consider site specific aspects, nor soiling or module degradation.

The energy rating performance comparison is not based on the estimated energy yield (kWh) but on the so called "Climate Specific Energy Rating" parameter. The CSER (Eq. 1) is the ratio of the calculated energy yield of a PV module under certain climate conditions to the energy yield that would have been obtained if the module efficiency would have been the one measured under Standard Test Conditions (STC: module temperature of 25 °C, 1000 W/m2 of in-plane irradiance with a spectral content as defined in the IEC 60904-3 [4]). The CSER is a dimensionless value defined as:

                                                     (Eq. 1)

where Emod,year is yearly energy output (kWh/year), Gref is equal to 1000 W×m-2 as the irradiance applied at STC to measure the maximum power at STC (Pmax, STC). Hp is the yearly global in-plane irradiation (kWh×m-2 ×year) which is provided in Part 4 of the standard for each reference climatic region.

 

 

2. IEC 61853 Photovoltaic (PV) module performance testing and energy rating

The IEC 61853 standard is based on three components: a set of meteorological datasets which represent the climatic conditions PV installations are most likely to encounter worldwide, various mathematical models to estimate the PV power output taking into consideration various effects to simulate real working conditions different from the STC, and a set of parameters that are the input data to the said estimation models. All these aspects are described in the four parts that form the standard series [5-8].

2.1. IEC 61853-1. Irradiance and temperature performance measurements and power rating

Part 1 describes the methodology to measure the PV module power at maximum power of operation (Pmax) at various levels of irradiance and module temperature. These measurements can be obtained outdoors or indoors using solar simulators. The set of measuring points are shown in Table 1. The irradiance spectral distribution should correspond to AM1.5 as defined in [4].

Table 1. Irradiance and temperature levels to obtain the power matrix of the device under consideration

Module temperature (°C)

Irradiance (W×m-2)

100

200

400

600

800

1000

1100

15

NA

25

50

NA

NA

75

NA

NA

NA

(NA: not applicable)

2.2. IEC 61853-2. Spectral responsivity, incidence angle and module operating temperature measurements

The methodology used to estimate the PV module output takes into consideration various effects to model its performance under conditions different from the STC used to define the reported Pmax in the module's datasheet. Part 2 of the standard describes the methodology to evaluate and quantify those effects which include shallow-angle reflectivity, spectral response and module working temperature.

The output of this part of the standard include the parameter used to model the angle of incidence (AOI) effect (ar), and the coefficients required to estimate the PV module's operating temperature under different levels of in-plane irradiance, ambient temperature and wind conditions (u0 and u1). In addition to these, the spectral responsivity of the module (SR), needed for the calculation of the spectral mismatch, should be measured according to the procedure described in IEC 60904-8.

2.3. IEC 61853-3. Energy rating of PV modules

Part 3 of the standard contains the complete methodology to calculate the PV module energy rating under different standard climatic profiles. The models applied estimate for every hour of a complete reference year the energy output (Wh) used to estimate the Climate Specific Energy Rating (CSER) parameter. The full methodology is illustrated in the simplified flow chart of Figure 1 and concatenates various models so as to calculate for every hour two sets of data: on the one hand, the effectively absorbed in-plane irradiance corrected for the angle of incidence and spectral effects and on the other hand the temperature reached by the module.

The outcome of this part of the standard is the CSER parameter calculated for the PV module under analysis for the six climatic regions defined in Part 4.

Figure 1: Simplified schematic of the steps used within IEC 61853-3 to calculate CSER.

 

2.4. IEC 61853-4. Standard reference climatic profiles

In part 4 six reference climatic datasets are used to describe the most representative working conditions that PV installations will encounter worldwide. They are described as: tropical humid, subtropical arid, subtropical coastal, temperate coastal, high elevation above 3000m and temperate continental. They contain hourly values for one year of several variables, including broadband and spectrally resolved irradiance, ambient temperature and wind speed. The datasets also provide the solar elevation angle and the angle of incidence between the sun and the normal to the module's surface. This is assumed to be installed in a free standing open rack, facing the equator with an inclination angle of 20°.

Figure 2 contains the CSER calculated for four PV devices of different technologies in the six reference climates and shows that the energy output will be expected to vary significantly in different climates, and between technologies. This is just an illustrative example and should not be used to generalise the performance of specific commercially available modules.

Figure 2: CSER calculated at the six reference climatic regions for four PV technologies.

 

3. Possible extension of the IEC 61853 standard series to bifacial and BIPV devices

The scope of the IEC 61853 is limited at the moment to mono-facial and single-junction devices. The impact of the extension to BIPV and bi-facial modules on the normative reference can be summarized in Table 2. The main considerations when assessing the specific considerations required in order to create an extended energy rating procedure are:

  • Mounting scenarios: at present the standard considers the modules located facing the equator and with an inclination angle of 20°. However, the current application trends of bi-facial and BIPV include vertical east-west racks and other installation options. The need to add a new set of mounting scenarios should be verified.
  • New input datasets:

o    The reflected irradiance is not included at present in the defined standard climatic profiles and therefore not considered in the energy rating procedure. However, the albedo should be defined and provided for use in applications involving bi-facial modules, either on stand-alone systems or integrated into the built environment.

o    The in-plane irradiance values and the angle of incidence between the solar direct radiation and the module's surface would also need to be provided for the new set of mounting scenarios representative of the current trend of bi-facial and BIPV installation options.

  • Spectral correction factor: to introduce a new definition including the spectral response at the rear of the module and the spectral albedo beneath the module. The reflected irradiance is not included in the current version of the standard.
  • Power matrix: a new definition and methodology on how to measure the power matrix for bi-facial modules needs to be discussed and agreed.
  • Thermal dynamics:

o    Higher working temperatures are often found in BIPV devices as compared to free standing modules. The power matrix described in Part 1 (see Table 1) might be extended with additional points covering higher temperatures.

o    The methodology defining the thermal coefficients used when estimating the working temperature of BIPV modules (coefficients u0 and u1 described in IEC 61853-2 [6]) should be investigated and validated.

o    Patterned textures, aesthetic coatings or special foils with non-uniform appearance might induce thermal gradients along the PV module. Further investigations are required in order to assess the potential use of more suitable thermal models.

  • Angular correction factor: to define a new correction factor accounting for non-uniform modules appearance when applied to patterned textures, aesthetic coatings or special foils. The new angular loss function should establish a simple analytical model that allows the computation of the beam and diffuse in-plane irradiance.

Table 2. List of normative references susceptible to be modified in order to extend the scope of the IEC 61853 and to determine the energy rating of BIPV and bi-facial modules

 

60904 (I-V)

61853-1 (Matrix)

61853-2 (SR)

61853-2 (AOI)

61853-2 (NMOT)

61853-3 (CSER)

61853-4 (Clim. Prof.)

BiFi

 [1]

BIPV

[1] The standard IEC 60904-1-2: Measurement of current-voltage characteristics of bifacial photovoltaic devices is expected to be published by 02.2019

 

REFERENCES

[1] Huld T., Bodis K., Pinedo Pascua I., Dunlop E., Taylor N., Jäger-Waldau A. (2018). The rooftop potential for PV systems in the European Union to deliver the Paris Agreement. European Energy Innovation. Spring 2018. Pages 12-17

[2] Bodis K., Huld T., Pinedo Pascua I., Taylor N., Jäger-Waldau A. (2017). Technical potential of rooftop photovoltaics in EU member states, regions and cities. JRC Technical Reports, European Commission, Ispra. JRC110353.

[3 The European Parliament and the Council of the European Union, European Commission (2010): Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings (recast). 2010. Pages. 13–35. http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex%3A32010L0031 (accessed on 25th January 2019)

[4] IEC 60904-3. Photovoltaic devices – Part 3: Measurement principles for terrestrial photovoltaic (PV) solar devices with reference spectral irradiance data. Edition 3.0 International Electrotechnical Commission (2016).

[5] IEC 61853-1. Photovoltaic (PV) module performance testing and energy rating – Part 1: Irradiance and temperature performance measurements and power rating. Edition 1.0 International Electrotechnical Commission (2011).

[6] IEC 61853-2. Photovoltaic (PV) module performance testing and energy rating – Part 2: Spectral responsivity, incidence angle and module operating temperature measurements. Edition 1.0 International Electrotechnical Commission (2016).

[7] IEC 61853-3. Photovoltaic (PV) module performance testing and energy rating – Part 3: Energy rating of PV modules. Edition 1.0 International Electrotechnical Commission (2018).

[8] IEC 61853-4. Photovoltaic (PV) module performance testing and energy rating – Part 4: Standard reference climatic profiles. Edition 1.0 International Electrotechnical Commission (2018).