WO2023191718A2

WO2023191718A2 - Apparatus for characterization of photovoltaic modules

Info

Publication number: WO2023191718A2
Application number: PCT/SG2023/050206
Authority: WO
Inventors: Mauro PRAVETTONI
Original assignee: National University Of Singapore
Priority date: 2022-03-29
Filing date: 2023-03-29
Publication date: 2023-10-05
Also published as: WO2023191718A3

Abstract

Apparatus for characterization of photovoltaic modules and spectral responsivity testing tools are described. An apparatus for characterization of photovoltaic modules comprises: a solar tracker configured to support a photovoltaic module under test and to orientate the photovoltaic module under test to be normal to a solar radiation direction; a monitoring system configured to be electrically coupled to the photovoltaic module under test and monitor an electrical output of the photovoltaic module under test; a reference solar cell arranged on the solar tracker and configured to measure incident solar radiation; an enclosure surrounding the solar tracker and the reference solar cell, the enclosure comprising a retractable cover movable between a closed position and an open position, wherein, when the apparatus is in use, in the closed position the photovoltaic module under test is covered by the retractable cover and in the open position, the photovoltaic module under test is directly exposed to solar radiation.

Description

APPARATUS FOR CHARACTERIZATION OF PHOTOVOLTAIC MODULES

TECHNICAL FIELD

The present disclosure relates to electrical characterization of solar photovoltaic modules and in particular to an apparatus for characterization of solar photovoltaic modules.

BACKGROUND

Large area and long pulse or continuous solar simulators have raised general interest for the last decade in the field of solar module metrology: new commercial photovoltaic (PV) modules based on high-efficiency c-Si are now booming in the market and require stable irradiation of above 100ms to be tested accurately. Other promising materials such as organic PV (OPV) or perovskite solar cells need even longer illuminations up to several minutes, and large area modules based on these materials are expected to be commercially available by the end of the current decade. The solar simulator manufacturers are struggling to improve the time duration of their instruments to provide the best tool to the industry and research centers, while keeping the highest classification in terms of spatial uniformity and spectral match to standard AM1.5 spectral irradiance. Nowadays, only few manufacturers worldwide can provide large area solar simulators with light pulses longer than 100ms or continuous and classified better than Class B (allowing measurement accuracy capabilities better than 10%): the cost of such equipment easily exceeds SGD 1 million, while the old generation pulsed simulators with less than 50ms are expected to be phased out soon.

However, recent advances in outdoor testing methodologies have demonstrated that precise outdoor current voltage (l-V) measurements of PV modules are possible in a wide range of irradiances and temperatures, and under various weather conditions. A certain diffuse earlier skepticism towards the capability of performing accurate characterization of PV devices using the natural solar irradiation has therefore been revised. Nevertheless, the combination of outdoor testing capability with the benefits of indoor laboratory environment and with the most advances characterization tools is still missing. SUMMARY

According to a first aspect of the present disclosure, an apparatus for characterization of photovoltaic modules is provided. The apparatus comprises: a solar tracker configured to support a photovoltaic module under test and to orientate the photovoltaic module under test to be normal to a solar radiation direction; a monitoring system configured to be electrically coupled to the photovoltaic module under test and monitor an electrical output of the photovoltaic module under test; a reference solar cell arranged on the solar tracker and configured to measure incident solar radiation; an enclosure surrounding the solar tracker and the reference solar cell, the enclosure comprising a retractable cover movable between a closed position and an open position, wherein, when the apparatus is in use, in the closed position the photovoltaic module under test is covered by the retractable cover and in the open position, the photovoltaic module under test is directly exposed to solar radiation.

The apparatus of the present disclosure provides a measurement laboratory for accurate electrical characterization of solar photovoltaic modules. It has applications in the solar module industry, solar energy research centres and utility scale power plants, for which it is designed to represent the testing station for the operation and maintenance (O&M) activities of the plant.

The retractable cover may be dome, which may be implemented as a clamshell dome.

The retractable cover may be arranged to expose the photovoltaic module under test to a 2TT steradian aperture when in the open position.

The retractable cover may be translucent. This allows the operator observing the position of the solar disc and determining the air mass value for the given testing conditions on a graduation scale.

In an embodiment, the retractable cover is provided with a scale indicating air mass values corresponding to solar disc positions on the retractable cover. In an embodiment, the apparatus further comprises a laser indicator configured to project an indication of the orientation of the photovoltaic module under test onto the retractable cover when the retractable cover is in the closed position.

In an embodiment, the apparatus further comprises a second reference solar cell arranged on the solar tracker with a shading disk and configured to measure diffuse solar radiation.

In an embodiment, the apparatus further comprises an air conditioning system configured to maintain a temperature of the photovoltaic module under test within a test temperature range.

In an embodiment, the test temperature range is 20°C to 30°C.

In embodiments, the apparatus is provided as a laboratory building which hosts measurement instruments, air-conditioning unit and monitoring station, topped with the fully automated dome with a 2TT steradian aperture.

In an embodiment, the solar tracker is a two-axis solar tracker.

In an embodiment, the apparatus further comprises a linear mount configured to move the solar tracker between a retracted position and an extended position, wherein in the retracted position, the photovoltaic module under test is within the enclosure and in the extended position, the photovoltaic module under test is above the enclosure.

In an embodiment, the apparatus further comprises a spectral responsivity testing tool comprising a bandpass filter and configured to select a wavelength range of solar radiation incident on a selected cell of the photovoltaic module under test.

In an embodiment, the spectral responsivity testing tool further comprises an optical chopper configured to modulate the solar radiation incident on a selected cell of the photovoltaic module under test and the monitoring system further comprises a lock-in amplifier configured to separate an AC signal corresponding to the modulated solar radiation from the electrical output of the photovoltaic module under test. In an embodiment, the spectra! responsivity testing tool further comprises a neutral beam splitter and a photodiode configured to measure an intensity of the solar radiation within the selected wavelength range in an embodiment, the selected wavelength range has a full width at half maximum of less than 15nm.

According to a second aspect of the present disclosure, a spectral responsivity testing tool for characterization of photovoltaic modules is provided. The spectral responsivity testing tool comprising a bandpass filter and configured to select a wavelength range of solar radiation incident on a selected cell of a photovoltaic module under test.

In an embodiment, the spectral responsivity testing tool further comprises an optical chopper configured to modulate the solar radiation incident on a selected cell of the photovoltaic module under test.

In an embodiment, the spectral responsivity testing tool further comprises a neutral beam splitter and a photodiode configured to measure an intensity of the solar radiation within the selected wavelength range.

In an embodiment, the selected wavelength range has a full width at half maximum of less than 15nm.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the present invention will be described as non-limiting examples with reference to the accompanying drawings in which:

FIG.1 shows an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention;

FIG.2 shows components of an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention; FIG.3 shows markings on the retractable cover of an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention;

FIG.4A and FIG.4B show an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention with a retractable cover respectively in a closed position and an open position;

FIG.5A and FIG.5B are block diagrams showing monitoring systems of an apparatus for characterization of photovoltaic modules according to embodiments of the present invention;

FIG.6A shows a spectral responsivity testing tool for measuring spectral responsivity of a module photovoltaic module under test according to an embodiment of the present invention;

FIG.6B shows a spectral responsivity testing tool according to an embodiment of the present invention;

FIG.7 is a bar graph showing the results of a comparison between typical uncertainty contributions for indoor current-voltage (l-V) characterization of large-area PV modules and the uncertainty contributions reasonably achievable in outdoor measurements, with state-of-the-art equipment;

FIG.8A to FIG.8D are charts showing an example of range of testing parameters for an older generation and the newest generation PV modules in a typical sunny day in Singapore; and

FIG.9A and FIG.9B are charts showing the effect of various corrections from outdoor measurements to STC in Singapore, for two test samples measured at fixed tilt.

DETAILED DESCRIPTION The sun is the ultimate resource of energy for the Earth: with the exception of nuclear energy, all other known sources of energy are directly or indirectly solar energy. Energy from the sun is irradiated to the Earth as electromagnetic waves, or photons: the distribution of photon energy received per unit time, wavelength and area is the solar spectral irradiance (the solar spectrum). Due to absorption and scattering of light from the atmosphere, the solar spectrum varies with the position of the sun and with the weather condition. Variations of the sun elevation cause in fact changes in absorption and scattering, which is roughly a function of the thickness of atmosphere that the sun-ray crosses before reaching the Earth: the parameter Air Mass (AM) is therefore conventionally introduced to refer to different solar spectra, being AMO (“air mass zero”) the solar spectrum outside the atmosphere (in first approximation equal to the spectrum of a black body at the temperature of the sun); AM1 (“air mass one”) the solar spectrum when the sun disc has the maximum elevation (a= 90 deg, when the sun is at the Zenith) and therefore the sun-ray crosses perpendicularly the atmosphere, and has the minimum optical path through it to reach the irradiated detector; and maximum AM at sunrise and sunset.

The presence of clouds enhances absorption and diffusion of photons, not only varying the level of total irradiance (energy per unit time and per unit area) reaching the ground, but also changing the distribution of spectral irradiance (energy per unit time, per unit area and per unit wavelength): given the same position of the sun in the sky, the spectral irradiance is different in clear-sky conditions, partially cloudy, cloudy or overcast conditions, respectively.

It is often useful to distinguish between the direct and diffuse components of the global spectral irradiance, where: the direct component is the portion of spectral irradiance that comes in a straight line from the sun at its current position in the sky to the detector; and the diffuse component is the difference between the global and the direct components. (In clear-sky conditions the diffuse component is relatively minimum than in partially cloudy, cloudy or overcast conditions, in the latter case being the dominant component of the global spectrum).

Spectral irradiance measurement method is described for example in CIE 038-1977 CIE 038-1977, “Radiometric and Photometric Characteristics of Materials and Their Measurement (E) (F) (G)”: in PV metrology it is usually performed with a spectrometer. There are a variety of spectrometers on the market for outdoor applications, depending on whether they are fast or slow-response and on the detector technology (CMOS, CCD or arrays of photodiodes; c-Si or InGaAs).

Solar irradiance is harvested by a solar module, which generates electrical power via photovoltaic (PV) effect. In embodiments of the present disclosure, the testing sample is electrically characterized via measuring its l-V characteristic curve at Standard Test Conditions (STC: 1000Wrrr² global irradiance, 25°C cell temperature, and AM1.5 spectral irradiance IEC 60904-3:2019, “Photovoltaic devices - Part 3: Measurement principles for terrestrial photovoltaic (PV) solar devices with reference spectral irradiance data”, Ed. 4.0. ), according to the international standard IEC 60904-1 : IEC 60904-1 :2020, “Photovoltaic devices - Part 1 : Measurement of photovoltaic currentvoltage characteristics”, Ed. 3.0

PV modules of different technologies and/or bill of materials respond differently to the solar spectrum: the spectral responsivity (SR) describes which part of the solar spectrum is absorbed by the testing module and converted to electricity and in which amount. Its measurement is described in IEC 60904-8:2014. “Photovoltaic devices - Part 8: Measurement of spectral responsivity of a photovoltaic (PV) device”, Ed. 3.0. As the global irradiance is recorded by a reference detector, its spectral responsivity needs also to be known to evaluate the effect of spectral mismatch between the testing spectrum and standard AM1.5 and between the reference detector and the testing device, according to their SR: the spectral mismatch correction is calculated according to IEC 60904-7:2019, “Photovoltaic devices - Part 7: Computation of the spectral mismatch correction for measurements of photovoltaic devices”, Ed. 4.0.

SR is a characteristic of the photovoltaic material and of the encapsulants: it thus represents a sort of identity card of the testing device. Silicon (Si) PV has SR ranging from 300 to 1100 nm, with a long tail to 1200 nm, typical of indirect bandgap semiconductors. Commercial homojunction crystalline silicon (c-Si) modules, have typically a higher responsivity in the UV than heterojunction (HJT) Silicon. Cadmiumtelluride (CdTe), a direct semiconductor, has a sharp cut-off in its SR at 900 nm; CIGS (Copper Indium Gallium Selenide), another “second generation” PV material, has similar SR than c-Si, though generally lower than c-Si. In general, the SR cut-off wavelength depends on the bandgap of the semiconductor (or semiconductor alloys): spectral mismatch represents an important source of uncertainty in PV metrology and the proposed kit to measure SR using the natural solar source is one of the novel features of the present disclosure.

The proposed measurement equipment may be implemented at module level with an approach similar to that described by Y. Tsuno, Y. Hishikawa, and K. Kurokawa, “A Method for Spectral Response Measurements of Various PV Modules”, Proc. 23rd European Photovoltaic Solar Energy Conference and Exhibition, 1 -5 September 2008, Valencia, Spain, 2723-2727, doi: 10.4229/23rdEUPVSEC2008-4C0.2.3 and by M. Pravettoni, K. Anika, R. Galleano, H. Mullejans and E. D. Dunlop, “An Alternative Method for Spectral Response Measurements of Large-area Thin Film Photovoltaic Modules”, Prog. Photovolt: Res. Appl. 20(4), 416-422 (2012), commercialized for example by Enlitech with an indoor equipment of about 2x3 m² footprint.

Electrical characterization of PV modules at STC requires careful control of temperature (set to 25°C) and irradiance (set to 1000Wrrr²). Temperature transient can be kept under control with air conditioning, accurate flow of conditioned air towards the testing module surface and module temperature monitoring; irradiance is directly monitored with the irradiance detector. Deviations from the STC values of both temperature and irradiance can be corrected knowing the curve correction factors, which can also be measured directly according to IEC 60891 :2009, “Photovoltaic devices - Procedures for temperature and irradiance corrections to measured l-V characteristics”, Ed. 2.0.

FIG.1 shows an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention. As shown in FIG.1 , the apparatus 100 forms a fully-equipped laboratory for electrical characterization of PV modules that takes advantage of the natural solar irradiance as an ideal spectrum, large area and continuous irradiation. The apparatus 100 comprises an enclosure 102 with a retractable cover 104. The retractable cover 104 may be in the form of a dome and is movable between an open position and a closed position. A photovoltaic module under test 110 is supported by a solar tracker 120 within the enclosure 102. When the retractable cover 104 is moved to the open position, solar radiation is incident on the photovoltaic module under test 110 which allows electrical characterization of the photovoltaic module under test 110.

FIG.2 shows components of an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention. As shown in FIG.2, the apparatus comprises a first reference cell detector 122 coupled to the solar tracker 120 and configured to detect global irradiance. A second reference cell detector 124 is coupled to the solar tracker 120 and configured to detect diffuse components of solar radiation, by means of a shading disk 125. A laser indicator 128 in the form of a laser pointer is coupled to the solar tracker 120 and configured to project an indication of the orientation of the solar tracker 120 and therefore the photovoltaic module under test 110 onto the retractable cover 104.

A test station 130 is located within the controller. An air conditioning system 140 is provided in the enclosure 102 and arranged to generate cool air 142 to cool the interior of the enclosure 102 and to maintain the photovoltaic module under test 110 within a test temperature range.

During the aperture operation, a thermal gradient is expected to occur in the inner laboratory. Power supply to the air-conditioning system should provide enough thermal stability to guarantee the photovoltaic module under test does not vary in temperature within a reasonable range (25 ± 5)°C, providing a thermal uncertainty contribution of around ±1.5% for most of the commercial modules. Alternatively, the air-conditioning system may be configured to maintain the temperature within the range (25 ± 3)°C. Flow of conditioned air should be design to protect thermal inertia.

The solar tracker 120 is a two-axis tracker having a tracking accuracy: <1 deg. The elevation of the module can be extended to operating position with a motorized linear stage when the dome is fully open. The 2-axis solar tracker uses well-known algorithms to align to the angle of the sun.

The total irradiance is monitored by the first reference cell detector 122 which is a calibrated silicon reference cell mounted on the solar tracker 120 and aligned to the photovoltaic module under test. The second reference cell detector 124 is covered with a shading disc 125, which allows the direct irradiance to be subtracted from the global incident irradiance to measure the diffuse solar radiation.

The temperature of each of the reference cells is set and controlled by a Peltier cooling element on the back of the reference cell to keep it constantly at the standard temperature of 25°C.

The cell temperature for the photovoltaic module under test is inferred from the reading of four Pt1 OO sensors placed on the back of the module (centre, central-bottom, central-left and top-right cell positions). Thermal instability is kept under control by the flow of air of the air conditioning system 140.

FIG.3 shows markings on the retractable cover of an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention. As shown in FIG.3, a graduated AM scale is provided on the retractable cover. In this embodiment, the retractable cover may be formed as a translucent dome. The graduated AM scale comprises markings at AM1 150, AM1.5 152 and AM2 152. The laser indicator 128 described above with reference to FIG.2 can be used to pre-align the photovoltaic module under test with a specific sun position on the graduated AM scale of the translucent dome. The scale allows also an accurate prediction of the best timing to perform measurements at AM1.5, or gives indication on the spectral mismatch according to the sun elevation.

FIG.4A and FIG.4B show an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention with a retractable cover respectively in a closed position and an open position.

As shown in FIG.4A, the retractable cover is in a closed position 104A. In this configuration, the solar tracker 120 may be moved a retracted position 121 A by a telescopic linear mount. This allows preparation of the photovoltaic module under test to be carried out by an operator. As shown in FIG.4B, the retractable cover is in an open position 104B. The solar tracker is moved to a position above the fixed part of the enclosure by moving the telescopic linear mount to an extended position 121 B. This position allows the photovoltaic module under test to be exposed to a 2TT steradian field of view of solar radiation. As shown in FIG.4B, the photovoltaic module under test 110, the first reference cell detector 122 and the second reference cell detector 124 are all directly exposed to solar radiation to allow characterization of the photovoltaic module under test 110.

As shown in FIG.4A and FIG.4B, the retractable cover may be formed as a clamshell dome structure.

The translucent dome should be made on a material that guarantees protection of the inner laboratory from the atmospheric agent. Additionally, the translucent material may be protected from ageing effects such as soiling, yellowing, and scratching. A double layer dome could be implemented, in which the outer part, more robust and more opaque, is able to protect the inner translucent part during periods of non-operation.

FIG.5A is a block diagram showing a monitoring system of an apparatus for characterization of photovoltaic modules according to an embodiment of the present invention. The monitoring system 500 monitors the power discharged from the photovoltaic module under test 120. As shown in FIG.5A, the monitoring system 500 comprises a waveform generator 502, which triggers the electronic load 506 to start measurement, and a data acquisition module 510.

The waveform generator 502 controls the voltage ramp applied by the electronic load 506 to the terminals of the photovoltaic module under test, that can be swept from 0 to the open-circuit voltage \/oc (direct V ramp) or from \/oc to 0 (reverse V ramp): the duration and the shape of the sweep pulse can be varied to allow for accurate measurements of slow-response devices. The electric power generated from the photovoltaic module under test during operations is discharged through the calibrated electronic load 506. The current generated by the photovoltaic module under test 120 (read through a shunt box), the applied voltage (read from the photovoltaic module under test 120 with a 4-point contact), the instant total irradiance (read by the calibrated reference cell 112 through a shunt) and the diffuse irradiance (read by the calibrated reference cell 112 through a shunt) is recorded by the data acquisition module 510. Measurement is triggered when the seashell dome if fully opened.

In some embodiments, a multichannel c-Si/lnGaAs monochromator spectrometer, able to measure spectral irradiance from 250 to above 1200 nm, is applied in order to cover the SR range of most of the commercially available solar modules. Measurement of the solar spectrum simultaneously with solar module testing allows to minimize the uncertainty contribution due to spectral mismatch and, at the same time, to control the spectral variation with AM and weather conditions.

FIG.5B is a block diagram showing the operation when the apparatus is used with a spectral responsivity testing tool 200 as described below with reference to FIG.6A and FIG.6B. As shown in FIG.5B, the monitoring system 520 comprises a lock-in amplifier 504 to filter AC signals from background DC signals, and the filtered signals are passed to the data acquisition module 510.

FIG.6A shows a spectral responsivity testing tool for measuring spectral responsivity of a module photovoltaic module under test according to an embodiment of the present invention. As shown in FIG.6A, the spectral responsivity testing tool 200 is placed over a target photovoltaic cell of the photovoltaic module under test 120.

FIG.6B shows a spectral responsivity testing tool according to an embodiment of the present invention. As shown in FIG.6B, the spectral responsivity testing tool 200 is a collimating tube in which natural direct light 220 from the sun enters a slit 202 and is filtered by a bandpass filter 204 to be selected from an available set box, covering the range between 300 and 1200 nm of central wavelengths (CW), with full-width halfmaximum (FWHM) of not more than 15 nm. The quasi-monochromatic light 222 is then split by a neutral beam splitter 206: the reflected beam 224 is recorded from a silicon photodiode 208 by a photodiode trans impedance amplifier (current-to-voltage converter); the transmitted beam 224 is chopped by an optical chopper 210, before exiting the tube and being driven towards a selected cell 112 of the photovoltaic module under test. The non-selected cells are irradiated by global irradiance; the selected cell is therefore current-limiting and the AC signal from the spectral responsivity testing tool 200 can be filtered from the background DC signal with a lock- in amplifier as shown in FIG.5B. A set of additional bias light can be implemented to illuminate the selected cell.

The relative SR of the testing module is then obtained as

where A_ceu _ref and A_ceu _testare the reference photodiode and testing module cell area, respectively; I_Sc,testW ^and hc,refW ^are the short-circuit current values measured on the testing module (AC signal from the lock-in amplifier) and on the reference photodiode (DC signal from the trans impedance amplifier), respectively; SR_ref,absW is the calibrated absolute SR of the reference photodiode.

The SR_{test rei} (A) obtained can be measured on different cells of the same testing module to detect for current mismatch between cells. The absolute SR, SR_{test abs}(A), can be obtained as

the correction factor k is defined as

where I_SC,STC is the short-circuit current of the photovoltaic module under test at STC and N_p is the number of cells in parallel of the photovoltaic module under test.

FIG.7 shows the results of a comparison between typical uncertainty contributions for indoor current-voltage (l-V) characterization of large-area PV modules and the uncertainty contributions in outdoor measurements, with state-of-the-art equipment. Uncertainty analysis was performed to compare the typical accuracy of indoor measurements with the expected accuracy of outdoor measurements at the state-of- the-art technology of measurement equipment. A large-area PV module of 2.4x 1 .1 m², Pmax,sTc = 530 W, Si (p-type, n-type or HJT) was considered; indoor measurement is assumed with a typical Class A (±2% spatial uniformity over the test area) tunnel simulator of 100 ms pulse duration. Outdoor measurement was assumed to be performed with the capability of keeping the module at (25 ± 5)°C (or, with appropriate temperature correction able to correct the measurement temperature to that range); outdoor irradiance was assumed in the range of (1000 ± 100) W/m², with ±1 % uncertainty in temperature setting (due to the accuracy in curve correction).

No spectral correction was assumed. With current know-how in testing laboratories, spectral correction is usually performed, giving rise to an uncertainty contribution due to spectral mismatch (between the test sample and the reference device, and between the testing source and AM1.5g) which is typically in the range of 0-0.5%. When no correction is available, indoor measurements can have an additional uncertainty contribution up to ±2%; in outdoor measurements, under a range of air masses from AM1 to AM2, the uncertainty is expected to be <1 %.

Thermal drift represents the largest source of uncertainty contribution in outdoor measurements (affecting Voc and Pmax measurements) and needs to be carefully controlled with air conditioning unit and environment insulation when the dome is not in operating mode. Irradiance setting is the second-largest uncertainty contribution in outdoor measurements (affecting Isc and Pmax). With the apparatus of the present disclosure, it is mitigated by a careful choice of the tilt of the two-axis tracker.

With state-of-the-art equipment, measurement uncertainty in outdoor measurements is <5%, negligibly larger than what can be achieved indoors on the large-area module under investigation and with a Class A simulator. The uncertainty can be further reduced by lowering the thermal drift and irradiance setting contributions, which is the goal of the apparatus of the present disclosure.

Preliminary measurements were conducted on two test samples: multi-crystalline silicon (multi-Si p-type, referred to as “mod 1”) and silicon heterojunction (HJT, “mod 2”). These represent a selection of state-of-the-art PV module technologies, from an older generation (mod 1 ) to the newest generation PV modules (mod 2).

FIG.8A to FIG.8D show the range of testing parameters for an older generation (mod 1 ) and the newest generation PV modules (mod 2), measured in Singapore in a typical sunny day. FIG.8A shows the range of module temperature; FIG.8B shows the range of in-plane radiance; FIG.8C shows the range of angle of incidence; and FIG.8D shows the range of air mass.

Measurements of Isc were performed at a fixed tilt on a range of module temperatures (T), in-plane irradiance (G), angles of incidence (AOI) and air masses (AM), as shown in FIG.8A-D. The range of irradiance is wide and can be mitigated to (1000 ± 100) W/m² when measurements will be performed on the 2-axis tracker. The range of temperature is acceptable, but the setpoint is high and will be reduced to (25 ± 5)°C in the apparatus of the present disclosure. The range of air masses (AM) is representative of the test site in Singapore; the range of angles of incidence is at the highest side and will be reduced when measurements will be performed on the 2-axis tracker.

FIG.9A and FIG.9B show the effect of various corrections from outdoor measurements to STC in Singapore, for two test samples measured at fixed tilt. FIG.9A shows results for Module 1 : a p-type multi-Si glass-back sheet module; FIG.9B shows results for Module 2, a Silicon heterojunction (HJT) glass-back sheet module.

The results in FIG.9A and FIG.9B display the range of outdoor measurements of ISC before any correction (box&wisker plot at the left of each chart) compared with the reduction in the spread of results after performing: spectral mismatch corrections; angle of incidence corrections; irradiance correction; and temperature correction. After all correction, the spread of results is around ±10%, with interquartile range (IQR) of about ±5%, which is slightly larger than the results of the uncertainty analysis of FIG.7, where the uncertainty contribution for Isc was about ±3%. But it should be noticed that the range of measurement data analysed here is wider than that considered in FIG.7, particularly for what relates to the irradiance setting contribution (there: 1000 ± 100 W/m²; here: 700 ±500 W/m²). The apparatus for characterization of photovoltaic modules described in this disclosure combines the benefits of the observatory domes used in astronomy with the most advanced measurement techniques for the electrical characterization of solar modules in a unique solar testing laboratory for solar PV devices.

Testing and calibration laboratories may benefit from the invention of the apparatus for characterization of photovoltaic modules that overcomes the limitations of solar simulators. As quality improvements and tightening revenues in the solar market is pushing solar module manufacturers to look for more accurate characterization of their products, the invention offers a tool that may be attractive for many.

Universities and research centers, in particular those located in certain geographical regions (South East Asia, Middle East, desert regions of South America), may benefit from the invention, which is potentially cost competitive with respect to conventional solar simulators for indoor application. The apparatus for characterization of photovoltaic modules provides a unique laboratory for the education of young researchers in the field of solar energy and to enhance the development of new materials and material properties. It will also assist accurate reliability studies for long term monitoring under harsh weather conditions: the apparatus for characterization of photovoltaic modules in fact can be deployed near remote testing sites (a “laboratory- on-site” concept), with the advantage of a robust and reliable metrology laboratory for intermediate precision studies and laboratory comparisons.

The same “laboratory-on-site” concept will also be attractive for the Operation and Maintenance (O&M) routine of utility scale power plants: the next two decades will see most of the gigawatt of power generation under installation nowadays facing reliability problems. The need of module retesting and module replacement will soon be urgent and the apparatus for characterization of photovoltaic modules as “laboratory-on-site” will supply to the need of a quick and accurate diagnosis for the suspected faulty modules. Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the art that many variations of the embodiments can be made within the scope and spirit of the present invention.

Claims

1 . An apparatus for characterization of photovoltaic modules, the apparatus comprising: a solar tracker configured to support a photovoltaic module under test and to orientate the photovoltaic module under test to be normal to a solar radiation direction; a monitoring system configured to be electrically coupled to the photovoltaic module under test and monitor an electrical output of the photovoltaic module under test; a reference solar cell arranged on the solar tracker and configured to measure incident solar radiation; an enclosure surrounding the solar tracker and the reference solar cell, the enclosure comprising a retractable cover movable between a closed position and an open position, wherein, when the apparatus is in use, in the closed position the photovoltaic module under test is covered by the retractable cover and in the open position, the photovoltaic module under test is directly exposed to solar radiation.

2. The apparatus according to claim 1 , wherein the retractable cover is a dome.

3. The apparatus according to claim 2, wherein the retractable cover is a clamshell dome.

4. The apparatus according to any preceding claim, wherein the retractable cover is arranged to expose the photovoltaic module under test to a 2TT steradian aperture when in the open position.

5. The apparatus according to any preceding claim, wherein the retractable cover is translucent.

6. The apparatus according to claim 5, wherein the retractable cover is provided with a scale indicating air mass values corresponding to solar disc positions on the retractable cover.

7. The apparatus according to claim 5 or claim 6, further comprising a laser indicator configured to project an indication of the orientation of the photovoltaic module under test onto the retractable cover when the retractable cover is in the closed position.

8. The apparatus according to any preceding claim, further comprising a second reference solar cell arranged on the solar tracker and configured to measure diffuse solar radiation.

9. The apparatus according to any preceding claim, further comprising an air conditioning system configured to maintain a temperature of the photovoltaic module under test within a test temperature range.

10. The apparatus according to claim 9, wherein the test temperature range is 20°C to 30°C.

11. The apparatus according to any preceding claim, wherein the solar tracker is a two-axis solar tracker.

12. The apparatus according to any preceding claim, further comprising a linear mount configured to move the solar tracker between a retracted position and an extended position, wherein in the retracted position, the photovoltaic module under test is within the enclosure and in the extended position, the photovoltaic module under test is above the enclosure.

13. The apparatus according to any preceding claim, further comprising a spectral responsivity testing tool comprising a bandpass filter and configured to select a wavelength range of solar radiation incident on a selected cell of the photovoltaic module under test.

14. The apparatus according to claim 13, wherein the spectral responsivity testing tool further comprises an optical chopper configured to modulate the solar radiation incident on a selected cell of the photovoltaic module under test and the monitoring system further comprises a lock-in amplifier configured to separate an AC signal corresponding to the modulated solar radiation from the electrical output of the photovoltaic module under test.

15. The apparatus according to claim 13 or claim 14, wherein the spectral responsivity testing tool further comprises a neutral beam splitter and a photodiode configured to measure an intensity of the solar radiation within the selected wavelength range.

16. The apparatus according to any one of claims 13 to 15, wherein the selected wavelength range has a full width at half maximum of less than 15nm.

17. A spectral responsivity testing tool for characterization of photovoltaic modules, the spectral responsivity testing tool comprising a bandpass filter and configured to select a wavelength range of solar radiation incident on a selected cell of a photovoltaic module under test.

18. The spectral responsivity testing tool according to claim 17, further comprising an optical chopper configured to modulate the solar radiation incident on a selected cell of the photovoltaic module under test.

19. The spectral responsivity testing tool according to claim 17 or claim 18, further comprising a neutral beam splitter and a photodiode configured to measure an intensity of the solar radiation within the selected wavelength range.

20. The spectral responsivity testing tool according to any one of claims 17 to 19, wherein the selected wavelength range has a full width at half maximum of less than 15nm.