CALIBRATION PROCEDURES STATE OF THE ART

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1 CALIBRATION PROCEDURES STATE OF THE ART H. Ossenbrink European Commission, DG Joint Research Centre, Institute for Environment and Sustainability, Renewable Energies Unit, Ispra, Italy ABSTRACT Solar Cell Calibration plays an important role for determining the efficiency of photovoltaic devices under defined conditions. Laboratories involved in PV characterisation developed a number of calibration methods and procedures, which progress towards decreasing uncertainty, and worldwide acceptance. This paper gives an overview about the methods used, their specific features and applicability. The overview is limited to reference devices, and does not discuss standard performance measurements on PV devices. 1. INTRODUCTION The most visible figure-of-merit of photovoltaic solar cells is their efficiency. Since first solar cells flew in space, measurement of efficiency and power delivered under defined conditions was a key to improve solar cells, to understand parameters influencing their performance and to forecast their economic value. Calibration is today understood as the process, to determine the short-circuit current of photovoltaic devices under reference conditions with respect to device temperature, irradiance level and spectral distribution. As it would be economically not feasible, to calibrate all solar cells or modules in production with the required precision and uncertainly, use is made of reference cells, which, once thoroughly calibrated, serve as comparison in a simplified measurement procedure of short circuit current of the device under test. Calibration of reference devices in turn can make use of sophisticated methods, which developed in particular since the use of solar cells in terrestrial applications. A key element in calibration is the reference to standard test conditions (STC), which define a solar cell junction temperature of 25 C, an irradiance exposed to of 1000 W/m 2, and a tabular solar spectral distribution of Air Mass 1.5, which corresponds to a sun elevation of 37 at sea level. Whilst temperature and irradiance can be established either in the laboratory or under natural conditions, or at least corrected for, the reference solar spectral distribution is neither to simulate with sufficient match, nor can it be found routinely under natural sunlight. Almost all calibration methods have their own, characteristic approach to take this problem into account. In contrast to PV solar cells, thermopile detectors, which measure heat generated by the incident light, are spectrally almost unselective, representing practically a black body, and calibration of such devices can be performed with less sophisticated methods. A set of thermopile detectors are also the primary reference for the World Radiometric Reference, established by the World Meteorological Organization, and serve as a traceability standard to determine irradiance in the magnitude of solar irradiance. 2. RELEVANCE In today s terrestrial PV markets, the sales price of PV modules is almost entirely related to the maximum power it can deliver at STC. Competition between manufacturer is largely based on the offered unit price per Watt. The maximum power of the modules leaving the factory is established within the quality control systems in place, and refers either to calibrated reference modules, or to calibrated reference cells placed together with the device under test in the measurement plane of a solar simulator. Assuming the world-wide sales of PV modules for 2003 being 500 MW, a global calibration uncertainty of ±3% for the production module would be the equivalent of over- or underestimating the delivered modules by ±15MW. Assuming average sales price of 4 EUR per Watt, this uncertainty translates to ±60 Mio EUR sales volume. Moreover, as profit margins in the whole commercialisation chain are most probably less than 10%, calibration uncertainty can be the decisive factor on profitability of a PV enterprise. 3. REQUIREMENTS 3.1. Uncertainty The basic requirement, which all calibration methods developed so far strive to minimise, is overall calibration uncertainty. The major components of uncertainty are: 1. Bias error. This is a shift of calibration values caused by factors not corrected for, or by poor experimental design. Typical bias errors are for instance drift or ageing of electronic components or misalignment to the light source. 2. Precision error: This includes all uncertainties of subsystems, such as Voltmeters and Analogue to Digital converters, or temperature controls. 3. Random error: basically this error is can caused by the random nature of atmosphere or noise in the signal conditioning. 4. Repeatability: Almost any experiment, when repeated shows slight variations, which are beyond the pure random error. It can be caused by slight changes in the experimental set-up, by the measurement sequence or procedure, or even by human factors. Repeatability can never be better than the random error.

2 Even though uncertainty can be broken down in the above factors which can be estimated or measured, a control of uncertainty can only established by repeated intercomparison campaigns (Round-Robins) involving as many methods and operators as possible [5], [6], [7] Costs There are not to many calibration laboratories in the world, which routinely are engaged in solar cell calibration. It is in all their interest to contain costs for equipment purchase and set-up as low as possible Duration The time required to execute a complete calibration including data analysis and verification is not only the major factor for customer satisfaction, but the associated labour costs may well limit calibration throughput to very few cells per year, worsening the economic balance of a calibration laboratory quite considerably Availability Ideally, a calibration task shall be executed immediately after receipt of the test devices. Outdoor methods, however, rely on actual weather conditions, which often are variable. Also, outdoor measurements should not lead to another bias error caused by climatic conditions, which are difficult to correct for, such as humidity or excessive atmospheric aerosol content Universal Applicability Solar reference cells may incorporate different photovoltaic technologies, which might interfere with specific features of the calibration method. Typical examples are slow response to fast light-pulses or chopped light, or impossibility to control the junction temperature in case of thin-film reference devices. 4. METHODS AND STANDARDS In the following, the variety of calibration methods will be briefly described, with reference to published standards Thermopile Detectors Thermopile detectors serve to determine solar irradiance regardless of spectral distribution. They have in common that solar irradiance is converted into heat in a black absorber. The temperature rise in relation to a reference temperature is proportional to the incoming irradiation. Temperatures are measured by a number of thermocouples connected in series ( Pile ) in order to increase the thermovoltage measured to practical levels of some millivolts. Thermopile detectors are different by their field of view: Pyranometers have a hemispheric field of view, allowing measurement of solar irradiance form the whole sky, whilst pyrheliometers apply collimation to restrict the field of view to typically 5. The apparent angle of the sun s disc is in comparison Pyrheliometers consequently need to be mounted a tracking platform ensuring alignment with the sun s position. A classification of these instruments is published as ISO Pyrheliometers ISO describes the calibration method for pyrheliometers, to be performed under natural sunlight. Both the test and reference pyrheliometer a mounted on a sun-tracking platform, and the values read. The standard pre-scribes minimum irradiance levels as well as a data analysis procedure. This measurement method and its standard has a very high for the whole chain of calibration, as the reference pyrheliometer can also be one of the primary or secondary references in use world-wide within the World Radiometric Reference. Such primary references are Practical Absolute Cavity Radiometers (PACRAD), which absorb incident light on a ideal blackbody, realised as an precision aperture in front of a cavity, which houses a usually cone-shaped, highly absorptive surface. A second, identical cavity within the same case is shielded from sunlight and electrically heated to track the temperature of the first cavity, which is exposed to sunlight. The electrical energy required for heating of the reference cavity divided by the aperture area of the first cavity establishes the irradiance value. Uncertainty of the method depends to a large extent on the time-constant of the instruments involved and the corresponding reading intervals where a compromise has to be found between precision and atmospheric fluctuations. Figure 1 one shows measured uncertainties of the Joint Research Centre s Cavity Radiometers, established in a number of intercomparisons with the World Radiometric Reference. WRR Factor Initial Calibration World Radiometric Reference (WRR) Factors PMO PMO Initial Calibration Re-Calibration IPC-IX (2000) Year Figure 1. Uncertainty and calibration factors relative to the World Radiometric Reference in a 10-yr period of the two JRC Reference Cavity Radiometers. Uncertainties ar less ±0.2% Pyranometers Calibration of Pyranometers with their hemispheric field of view is referenced to pyrheliometers and requires a set-up, which accounts for the diffuse irradiance absorbed. All test methods apply for this purpose a shading device aligned such that no direct irradiance from the sun can fall onto the pyranometers thermopile surface. The shading device is constructed such that its geometry

3 (diameter and distance to the pyranometer) matches the receiving geometry of the pyrheliometer. After reading the diffuse irradiation value of the pyranometer under shaded condition, the shade is removed and a reading of the total irradiance is taken. The difference between total and diffuse irradiance corresponds to the direct irradiance as determined by the pyrheliometer and established the calibration value of the pyranometer. Calibration uncertainty is much influenced by the design of the shading device, and the linearity of the pyranometer under test. ISO describes the calibration methods, taking into account to set-ups: one where the pyranometer under test is mounted co-planar with the pyrheliometer, and a second where the pyranometer is mounted fixed horizontally, and the shadowing device follows the sun. The first method is mechanically simpler, but may need corrections for ground albedo effects. There are a number of methods describing transfer of calibration between reference and field pyranometers, such as in ISO an ASTM Silicon Solar Cells Different from thermopiles, solar cells are spectrally sensitive. Photons with wavelengths greater than 1050 nm are not absorbed at all in silicon cells, and for wavelength below this band-gap limit the quantum efficiency varies. Consequently, even if solar irradiance is constant as measured by a thermopile detector, which integrates over all wavelengths, the short circuit current of a solar cell may vary with the relative spectral distribution. The spectral distribution varies with atmospheric conditions, and even if the standard sun elevation of 37 defining Airmass 1.5 can be found at least twice a day, the variability of the diffuse and direct component of sunlight may bias the measurement results. However, simulated sunlight is even more difficult to match with the standard Air Mass 1.5 spectral distribution, and requires in any case thorough correction of spectral mismatch Simulated Sunlight This method requires a spectroradiometer to determine the spectral distribution of the light source during the calibration. The short-circuit current of the reference solar cell is measured under the simulator s light together with the spectral irradiance. Together with the spectral responsivity data of the reference cell measured along the method described under 4.2.3, a spectral mismatch calculation can be performed to correct for the differences between the simulator and the tabulated standard spectral spectral distribution. The absolute irradiance during calibration can be derived either from the absolute irradiance calibration for each wavelength of the spectroradiometer, or from an intermediate transfer thermopile, limited by filters to the wavelength range of the spectroradiometer. The advantage of the method is that it can be applied for any solar cell technology, in particular when a steadystate simulator is used. Major uncertainty source is the temperature control of the solar cell, and the calibration uncertainty of the spectroradiometer. For a full description of the method, refer to ISO/DIS Natural Sunlight Direct Sunlight Method. This method follows in principle the calibration of pyrheliometers, described in above, with the test pyrheliometer replaced by the reference solar cell. The solar cell is mounted with an aperture close matching the geometry of the reference pyrheliometer. In addition, spectral irradiance of the direct sunlight is measured, in order to perform a spectral mismatch correction, which requires the spectral responsivity of the solar cell. If meteorological conditions are good and repeatable, the mismatch factor can be small (<2%) and most of all, a constant for different measurement campaigns. In this case, the spectroradiometer can be omitted, or replaced by the measurement of selective wavelengths, which allow a model to compute the actual spectral irradiance. One of the principal disadvantages of this method is on one hand the limited field of view of the reference cell which might not correspond to the later use, the other being the fact that the spectral irradiance of the direct sun light is different from the tabulated standard IEC Furthermore, practical considerations limit this method to reference solar cells smaller than about 25 cm 2, as the collimator system required for larger cells becomes rather unhandy. In taking readings one needs to consider the settling time constants for the reference pyrheliometer are at least three orders of magnitude slower than of a solar cell Global Sunlight Method. This method is an adoption of the above described pyranometer calibration method The silicon reference cell is mounted on a sun-tracking device together with a pyranometer and pyrheliometer. The pyranometer is shaded such to measure only the diffuse component of sunlight, the direct one is given by the pyrheliometer readings. The method avoids the principal disadvantage of the direct sunlight method, and might benefit from the closer match of the actual spectral distribution with the standard reference spectrum. If atmospherical conditions are carefully selected, one can achieve high repeatability even without the measurement of the spectral irradiance. Fig.2. Set-Up for calibration comparison of the global and direct calibration method. Mounted are: Collimator for reference cell, uncollimated reference cell, pyranometer as control, shadowed pyranometer for diffuse irradiance, two pyrheliometers and two cavity radiometers (from left).

4 Spectral Responsivity Calibrations Absolute Responsivity. Spectral responsivity calibrations are based on the fact, that the calibration value of a solar cell can be computed for any reference solar spectral distribution, if the absolute responsivity for each wavelength interval is known. All methods for spectral responsivity are based on short-circuit current measured of solar cells exposed to near monochromatic light of known wavelength. This can be achieved by applying band-pass filters of a monochromator between the light source and the solar cell. However, as irradiance of filtered light is low, it needs to be insured that the solar cell is linear in its short circuit, as the number of photons converted per unit of time and surface area is up to three orders of magnitude smaller than at exposure to natural sunlight. The application of additional white bias light up to 1000 W/m 2 can resolve this problem, but requires then in addition temperature control of the cell to be calibrated. The absolute, monochromatic irradiance can be determined either when both the light source level and the absorptivity of the monochromator or filters are known, or by switching the cell under test with a reference detector calibrated for selected wavelengths. This method is described as Differential Spectral Responsivity Method in ISO/DIS Relative Responsivity. If the uncertainty of the absolute responsivity from the previous described methods is insufficient, the measurements can be verified by one or more of the previous natural sunlight or solar simulator methods, as in the spectral mismatch calculation only relative responsivity is required. 5. TRACEBILITY ISSUES According to international agreements, all calibration value of commercial significance must refer to SI-Units. In particular, calibration laboratories accredited according to ISO need to maintain an unbroken chain of traceability. IEC refers to the World Radiometric Reference as the primary standard, as the SI radiometric scale is demonstrated only for irradiance levels far below natural sunlight. Comparison [4] have shown meanwhile the equivalence of both radiometric references. An international standard is in preparation, IEC PWI, which shall define procedures for establishing the traceability of the calibration of photovoltaic reference devices. It will define requirements for calibration procedures used in maintenance of the traceability chain, such as uncertainty, and list some of methods in use. 6. UNCERTAINTY LIMITS All calibration procedures need to establish and regularly confirm their uncertainty limits either through verification measurements or Round-Robin intercomparisons.. References [1], [2] and [3] are examples of such uncertainty estimates. From intercomparisons held in recent years, uncertainties between ±1.0% and ±2.0% are realistic and worldwide achievable. A current Round-Robin focuses on the establishment of a World Photovoltaic Scale (WPVS), which would consist in a set of standardised reference cells compared repeatingly, which shall serve the calibration laboratories as a primary laboratory reference [8]. 7. CONCLUSIONS Calibration methods in use today have evolved to better uncertainties, which are consistently near ±1.5%. The different methods, when carefully applied, have shown in various intercomparisons that the calibration factors achieved are in good agreements. The establishment of the World Photovoltaic Scale, together with a new international standard on traceability of calibrations to SI units will underpin fair and transparent commercialisation of photovoltaic technology on all the world markets. 8. ACKNOWLEDGENTS The author wishes to thank W. Zaaiman of JRC, Italy for much of the material referenced in this publications, and K. Emery of NREL, USA for many fruitful discussions. 9. REFERENCES 9.1. Selected Standards Publications Solar Spectra and Light Simulation IEC Measurement principles for terrestrial photovoltaic (PV) devices with reference spectral irradiance data. IEC Solar Simulator requirements. ISO Reference solar spectral irradiance at ground at different receiving conditions Part1: Direct normal and hemispherical solar irradiance for Air Mass 1.5., ASTM 891 Tables for Terrestrial Direct Normal Solar Spectral Irradiance for Air Mass 1.5 ASTM 892 Tables for Terrestrial Solar Spectral Irradiance for Air Mass 1.5 for a 37 Tilted Surface. ASTM 927 Specification for Solar Simulation for Terrestrial Photovoltaic Testing Calibrations (Thermopiles) ISO 9059 Calibration of field pyrheliometers by comparison to a reference pyrheliometer, 1990 ISO 9060 Specification and classification of instruments for measuring hemispherical solar and direct radiation, ISO 9846 Solar Energy - Calibration of a pyranometer using a pyrheliometer. ISO 9847 Solar Energy - Calibration of field pyranometers by comparison to a reference pyranometer. ASTM 816 Method for Calibration of Secondary Reference Pyrheliometers and Pyrheliometers for Field Use

5 ASTM 824 ASTM 913 ASTM 941 ASTM 1144 Method for Transfer of Calibration from Reference to Field Pyranometers Method for Calibration of Reference Pyranometers with Axis Vertical by the Shading Method Test Method for Calibration of Reference Pyranometers with Axis Tilted by the Shading Method Test Method for Calibration of Non- Concentrator Terrestrial Photovoltaic Primary Reference Cells under Direct Irradiance Calibrations (Solar Cells)) IEC Requirements for reference solar cells. IEC Procedures for establishing the traceability of the calibration of photovoltaic reference devices. (PWI: Proposed Work Item) IEC Requirements for reference solar modules. IEC Computation of spectral mismatch error introduced in the testing of a photovoltaic device. IEC Guidance for the measurement of spectral response of a photovoltaic (PV) device. ISO/DIS15387 Space systems Single-junction space solar cells Measurement and calibration procedures ASTM 1144 Test Method for Calibration of Non- Concentrator Terrestrial Photovoltaic Primary Reference Cells under Direct Irradiance ASTM 1039 Method for Calibration and Characterisation of Non-Concentrator Terrestrial Photovoltaic Reference Cells under Global Irradiation ASTM 1040 Specification for Physical Characteristics of Non-Concentrator Terrestrial Photovoltaic Reference Cells ASTM 1125 Test Method for Calibration of Primary Non-Concentrator Terrestrial Photovoltaic Reference Cells using a Tabular Spectrum ASTM 1362 Test Method for Calibration of Non- Concentrator Photovoltaic Secondary Reference Cells Electrical Performance Measurements IEC Procedure for temperature and irradiance corrections to measured I-V characteristics of crystalline silicon photovoltaic IEC IEC ASTM 973 ASTM Literature Measurement of photovoltaic currentvoltage characteristics. Determination of the equivalent cell temperature (ECT) of photovoltaic (PV) devices by the open-circuit voltage ASTM 1021 Methods for Measuring the Spectral Response of Photovoltaic Cells Test Method for Determination of the Spectral Mismatch Parameter between a Photovoltaic Device and a Photovoltaic Reference Cell Test Method for Determination the Linearity of a Photovoltaic Device with respect to the Test Parameter [1] K. Whitfield and C.R. Osterwald, Procedure for the determination of Photovoltaic Module Outdoor Electrical Performance, Prog. Photovolt: Res. Appl.2001; 9: [2] J.S. Henzing and W.H. Knap, Uncertainty in pyranometer and pyrheliometer measurements at KNMI in De Bilt, TR-235, ISBN: , [3] W. Zaaiman and A. Realini, Photovoltaic Device Calibration Intercomparison Test Report,TNI.98.18, European Commission, Joint Research Centre [4] J. Romero, N.P.Fox and C. Froehlich: Improved comparison of the World Radiometric Reference and the SI radiometric scale. Metrologia, 1995/96, 32, [5] The results of the 1984/1985 Round Robin calibration of reference solar cells for the Summit Working Group on Technology, Growth and Employment, EUR, European Commission, Joint Research Centre 0613, [6] The results of the PEP 87 Round Robin Calibration of Reference Solar Cells and Modules, PTB Bericht, PTB-Opt-31, ISBN , [7] The results of the PEP 93 Intercomparison of Reference Cell Calibrations and Newer Technology Performance Measurements, NREL/TP , March [8] The results of the First World Photovoltaic Scale Recalibration, NREL/TP , March 2000.