Uncertainty of PV Module Energy Rating in accordance with IEC 61853

Werner Herrmann, TÜV Rheinland Solar GmbH

Introduction

The energy rating of PV modules is related to its energy yield performance in a specific climate. In contrast to the nominal output power, which is only related to the performance at a single operating point (Standard test Conditions, STC), the energy rating considers the interaction of the PV module characteristics with the climatic conditions at the operating site. With neglect of shading and soiling, the following PV module parameters affect the amount of produced energy: (a) temperature behaviour, (b) low irradiance behaviour, (c) spectral responsivity and (d) angular responsivity.

To be able to compare the energy yield performance of PV modules, the IEC 61853 standard series (Table 1) defines a specific metric, which describes the energy yield performance of PV modules by a single parameter, the Climate Specific Energy Rating (CSER).

	Title
IEC 61853-1: 2011	Photovoltaic (PV) module performance testing and energy rating - Part 1: Irradiance and temperature performance measurements and power rating
IEC 61853-2: 2016	Photovoltaic (PV) module performance testing and energy rating - Part 2: Spectral responsivity, incidence angle and module operating temperature measurements
IEC 61853-3: 2018	Photovoltaic (PV) module performance testing and energy rating - Part 3: Energy rating of PV modules
IEC 61853-4: 2018	Photovoltaic (PV) module performance testing and energy rating - Part 4: Standard reference climatic profiles

Table 1: IEC 61853 standard series

FIGURE 1

Figure 1: Methodology of IEC 61853 series for climate specific energy rating

As shown in Figure 1, the calculation of the Climate Specific Energy Rating (CSER) of PV modules requires the input of various PV module performance parameters (blue boxes) and tabulated reference data (grey boxes) that are documented in IEC 61853-4 (climate data) and IEC 60904-3 (AM1.5 spectral irradiance distribution). The yellow boxes represent data processing steps.

The CSER value is calculated according to the following equation

[math]\displaystyle{ CSER= \frac {E_{MOD,Year}/H_{Year}} {P_{MAX,STC}/G_{REF,STC}} }[/math]

E_MOD,Year is the calculated PV module annual energy yield (IEC 61853-3), H_Year is the annual in-plane solar radiation of the selected reference climate (IEC 61853-4), P_MAX,STC is the PV module nominal output power at STC and G_REF,STC is the STC reference irradiance, which is set to 1000 W/m². The CSER parameter can be interpreted as the DC performance ratio of the PV module. CSER=1 means that the annual average operational efficiency corresponds to the STC efficiency of the PV module. In practice, CSER values differ from 1 and the deviation from 1 indicates annual yield losses or gains. Table 1 gives an overview of all input quantities for CSER calculation with remarks on uncertainty aspects.

Input quantity	Information source	Uncertainty aspects
Reference climates	IEC 61853-4 defines 6 climates: -     Tropical humid -     Subtropical arid -     Subtropical coastal -     Temperate coastal -     Temperate continental -     High elevation Parameters: -     Ambient temperature -     Wind speed at module height -     Sun elevation -     Sun incidence angle -     Global horizontal irradiance -     Direct horizontal irradiance -     Global in-plane irradiance -     Direct in-plane irradiance -     Spectrally resolved global in-plane irradiance	For all parameters an annual time series of hourly averages is given. Spectral irradiance data are presented in a low resolution for 32 discrete wavelength bands (Kato bands [2]) Spectral mismatch calculation requires the calculation of the low resolution AM1.5 spectral irradiance (IEC 60904-3) and relative spectral responsivity of the PV module with the same “Kato” wavelength basis.
(G-T) matrix of PMAX	(G-T) measurement in accordance with IEC 61853-1: PV module temperature: 15°C to 75°C Irradiance: 100 W/m² to 1100 W/m²	Relative PMAX uncertainty [math]\displaystyle{ \frac {\Delta P_{MAX}} {P_{MAX}} }[/math] to be calculated for each (G-T) test condition of the (G-T) matrix.
ar parameter of angular response curve	Angular response measurement in accordance with IEC 61853-2	The uncertainty of the incident angle modifier ΔIAM (θ) increases with the incident angle θ. Δar uncertainty results from the procedure defined in section 5.
Relative spectral responsivity (SR) curve of the PV module	SR measurement is performed in accordance with IEC 60904-8 and related to 25°C device temperature	SR uncertainty is wavelength dependent and follows a bathtub curve. Temperature related shifts of the SR curve are not considered.
Operating temperature	The operating temperature is modelled with two parameters (u0 and u1) that are determined in accordance with IEC 61853-2. These parameters describe the impacts of irradiance (u0) and wind speed (u1).	The u0 and u1 parameters are subject to uncertainty, which is mainly determined by the number of useful data with thermal equilibrium in the monitoring period and the resulting wind speed range [6].

Table 2: Overview of CSER uncertainty sources

With this background, the CSER uncertainty is composed of the following components:

• ΔCSER_(G-T): Uncertainty related to measurement uncertainty of the (G-T) power matrix
• ΔCSER_SMM: Uncertainty related to spectral mismatch uncertainty
• ΔCSER_AR: Uncertainty related to angular response uncertainty
• ΔCSER_TMOD: Uncertainty related to modelling of PV module operating temperature
• ΔCSER_DATA: Uncertainty related to data processing

In the following sections of this paper, the contributions to CSER uncertainty are individually analysed. All CSER calculations were performed with an Excel software developed by TÜV Rheinland. The tool has been validated against others from European research institutes under the work presented in [1].

CSER uncertainty related to interpolation and extrapolation methods

Interpolation and extrapolation of the (G-T) power matrix

The (G-T) power matrix, measured in accordance with IEC 61853-1, does not cover all operating conditions contained in the six reference climates of IEC 61853-4. Extrapolation and interpolation methods for the P_MAX data table must be applied that will introduce uncertainties.

As interpolation and extrapolation methods are not clearly defined in the standard IEC 61853-1, harmonised calculation methods have been developed in the MetroPV project [1]. Figure 2 summarises the recommended procedures.

FIGURE 2

Figure 2: Interpolation and extrapolation methods of the G-T matrix for P_MAX as defined in [1]. Eqn 14 and Eqn 14b refer to the definitions in [1].

Translation of spectrally resolved data to Kato bands

For all reference climates, tabulated spectral irradiances are based on 32 wavelength intervals, which commonly known as Kato bands [2]. Spectral mismatch calculation therefore requires the translation of high-resolution AM1.5 reference spectral irradiance (IEC 60904-3) and PV module spectral responsivity to these intervals (Figures 3 and 4). With regard to minimized uncertainties introduced by the translation, averaging and interpolation methods have been presented in [1].

The robustness of best practice methods has been validated with CSER intercomparison studies, in which European research institutes have processed a given data set of a c-Si PV module (P_MAX (G-T) matrix, angular response, spectral responsivity). Results were published in [1] and have shown that the proposed methods reduce differences DCSER to 0.1 % (rel.).

Result: ΔCSERDATA uncertainty related to interpolation and transition to low resolution can be assumed lower than ±0.1 %

FIGURE 3

Figure 3: Translation of high-resolution AM1.5 spectral irradiance to low resolution AM1.5 Kato spectrum [2]

FIGURE 4

Figure 4: Translation of high-resolution spectral responsivity to low resolution Kato wavelength bands