such phenomena from one full-module L-I-V curve is problematic. ... Keywords: concentrator photovoltaic; CPV; test method; L-I-V curve; shutter array technique; ...
Shutter array technique for real-time non-invasive extraction of individual channel responses in multi-channel CPV modules John P.D. Cook, Mark D. Yandt, Michael Kelly, Jeffrey F. Wheeldon, Karin Hinzer, and Henry Schriemer SUNLAB, University of Ottawa, Ottawa, Canada ABSTRACT Concentrator photovoltaic (CPV) solar energy systems use optics to concentrate direct normal incidence (DNI) sunlight onto multi-junction photovoltaic (MJPV) cells fabricated from III-V compound semiconductors on germanium substrates. The MJPV receiver, which integrates cell and bypass diode, is then mated with its concentrating optic to form a channel, and several such channels form a CPV module, in which the receivers are connected electrically in series. The two ends of the module receiver string are brought out to a single pair of electrical connections, at which point the lightcurrent-voltage (L-I-V) response of the entire module can be tested. With commercial CPV modules commonly sealed against outdoor exposure, there are no other accessible test points, and field installation on trackers further complicates access to performance data. There are many physical phenomena influencing module performance, and in early development and commercialization some of these may not yet be completely under control. Unambiguous diagnosis of such phenomena from one full-module L-I-V curve is problematic. Simple, fast test methods are needed to develop more detailed information from full-module on-tracker testing, without opening up modules in the field. We describe a test protocol, using a simple optical shutter array constructed to fit mechanically over the module. When module L-I-V curves are recorded for each of various combinations of open and closed shutters, the information can be used to identify one or more anomalous channels, and to further identify the kind of anomaly present, such as optical misalignment, conductor failure, series or shunt resistance, and so on. Simulated results from anomaly models can be compared with the measured results to identify the anomalous behaviour. Results herein are compared with direct singlechannel measurements to verify the technique. The L-I-V response curves were obtained in continuous real time, an approach found to be more helpful than single-shot capture in understanding field response. A triangular wave function generator is used to drive the DC power supply, and a four-channel digital sampling oscilloscope displays and stores the real time response. Where modules exhibit unstable or intermittent response under certain conditions, this is immediately obvious in real-time display. Keywords: concentrator photovoltaic; CPV; test method; L-I-V curve; shutter array technique; continuous dynamic display; solar energy
1. INTRODUCTION The concentrator photovoltaic (CPV) technique seeks to improve the photovoltaic (PV) solar energy value proposition compared to flat panel technologies by exploiting (a) the logarithmic dependence of semiconductor photovoltaic device voltage on light intensity, and (b) the high photoconversion efficiency of multi-junction tandem III-V compound semiconductor photovoltaic devices, now routinely exceeding 40%. Although such devices are more costly, very much smaller device areas are viable since concentration factors of 500–1000 are routinely attainable with simple, inexpensive optics. Each CPV channel typically consists of concentrating optics, a photovoltaic cell-carrier assembly (CCA), a bypass Schottky diode, a passive heat radiator, and electrical interconnections. A CPV module carries an array of such channels, connected optically in parallel and electrically in series, and mechanically supported and protected inside a weatherproof housing through which electrical access is achieved with two feedthroughs. An array of CPV modules are mounted on a solar tracker, and optically aligned to collect maximum direct normal incidence insolation throughout the day, and connected electrically in series to a power inverter which converts direct current (DC) to alternating current (AC) delivering power to the grid. Like all solar photovoltaic technologies, performance, low cost and long-term reliability (typically 20 years) are the mission-critical design drivers, and effective test methods for all relevant performance and reliability phenomena are essential to successful technology development.
Photonics North 2013, Pavel Cheben, Jens Schmid, Caroline Boudoux, Lawrence R. Chen, André Delâge, Siegfried Janz, Raman Kashyap, David J. Lockwood, Hans-Peter Loock, Zetian Mi, Eds., Proc. of SPIE Vol. 8915, 891509 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2037346 Proc. of SPIE Vol. 8915 891509-1 Downloaded From: http://spiedigitallibrary.org/ on 04/03/2014 Terms of Use: http://spiedl.org/terms
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Figure 1. SUNRISE installation at the Canadian Centre for Housing Technology facility, National Research Council campus, Ottawa, Canada.
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Figure 2. L-I-V curves measured on two 6-channel CPV modules after approximately 18 months field operation. Ambient temperature is noted. Module ‘A’ has degraded; module ‘B’ is in good condition.
SUNLAB recently partnered with industrial developers of CPV photovoltaic cells, modules and trackers, and with the Canadian Centre for Housing Technology (CCHT), to establish the SUNRISE demonstrator tracker facility at the National Research Council of Canada campus on Montréal Rd., Ottawa (Figure 1). This facility operates a twodimensional (2D) solar tracker carrying 33 CPV modules connected in two strings, each module containing 6 channels based on Fresnel lens optics and 1 cm2 triple-junction photovoltaic (3JPV) cells mounted on alumina carriers attached to an aluminum heatsink rail. Early prototype modules initially demonstrated approximately 21% AC efficiency, with a stable string voltage. However after about 4 months in operation, the string voltage began to gradually decline from about 215 V DC, restabilizing around 160 V DC after 18 months. No cause could be identified arising from insolation, tracker alignment, soiling, cabling, inverter operation or grid connect, and eventually module degradation was declared. Early speculation regarding degradation mechanism centred on thermal conductivity changes in the 3JPV die attach and CCA attach, leading to hotter 3JPV operation and hence lower open-circuit voltage (VOC). In failure analysis and reliability studies it is always desirable to obtain as much information as possible before disrupting a system in operation, and before risking further artificial secondary damage. With CPV modules on tracker, one practicable test is a full-module light-current-voltage (L-I-V) response. However, this is not necessarily completely
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informative, since various faults from the several channels can combine in ways which are not easily distinguished. Figure 2 shows L-I-V curves from a degraded module ‘A’ and a known-good module ‘B’. The reduction in VOC and maximum-power-point voltage (VMPPT) are obvious. The good module ‘B’ can be accurately simulated (dashed line) with a 2-diode model, but the modeled degradations which might reproduce the module ‘A’ response are arbitrary and not necessarily unique. A technique to measure individual channel responses on-tracker on-sun would be valuable. De Bernardez and Buitrago have shown a shutter technique1 for estimating the dark I-V curve of an individual element of a monocrystalline silicon photovoltaic panel, and extract ideality factors and saturation currents with a view to tracking reliability degradation. Alers et al. have developed at technique2 using partial shuttering and differential L-I-V curves to sense shunt resistance in a selected individual element of polysilicon photovoltaic panels. This work will apply shutter technique to CPV modules on-tracker on-sun, to extract several performance parameters of a selected individual CPV cell carrier assembly.
2. EXPERIMENT & RESULTS 2.1 Shutter Array Technique
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In Figure 3 we show the 2-diode model response (dashed lines) of a known-good 6-channel CPV module in which incident illumination has been interrupted in various combinations of individual channels. A very regular characteristic set of responses is obtained. This is reproduced very well in L-I-V curves from a known-good module measured ontracker on-sun (coloured lines) using mechanical shutters to interrupt insolation in individual channels, and the uniformity of the 6 channels is obvious. The forward turn-on of the photovoltaic diodes (bottom right) and of the bypass diodes (top left) is clearly seen and effectively modeled. This “shutter array frame” apparatus is simple and inexpensive, as shown in Figure 4. The frame can easily be placed over the input aperture array of the CPV modules, and the shutters can be operated for two adjacent modules for a single placement of the frame. We use the word “shutter” to distinguish from “shading” or “shadowing”, which refer to interruption of insolation arising from adjacent PV arrays or from nearby objects, or from soiling.
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Figure 3. Measured (solid) and modeled (dashed) L-I-V curves for a 6-channel known-good CPV module using shutter array. With all shutters closed, the curve (a) is obtained, and curve (g) arises with all shutters open. The intermediate curves are for (b) any single shutter open; (c) any two shutters open; and so on (d–f). The bypass diode forward turn-on is seen top left, and the photovoltaic device turn-on is seen lower right.
Figure 5 shows a set of L-I-V curves obtained for module ‘C’. The red curve is the full module response, showing a degraded VOC and fill factor (FF). When one shutter at a time was opened to the sun, the other L-I-V curves were recorded. Immediately it is clear that the 6 channels are substantially dissimilar, and that while channels 2, 3, and 6 are responding nearly normally, channels 1, 4, and 5 are degraded. These responses can be modeled semi-quantitatively (dashed lines) by empirically adding to the model appropriate amounts of shunt resistance.
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To verify that the individual channel responses are valid with shutters, we removed this module from the tracker, opened it up, and added direct electrical connections to each channel, then replaced the device on-sun and recorded individual channels directly, with results shown in Figure 6. Direct channel measurements agree well with the modeled curves from Figure 6. This affords confidence that the shutter array frame technique has safely deconvoluted the individual channel responses from the full module response.
Figure 4. View of 6-panel shutter frame placed over a CPV module on tracker, with shutter #2 open and other shutters closed. An instrumentation panel is visible at left, carrying (top to bottom) an Eppley pyroheliometer, a global radiometer, the tracker sun sensor, and a tracker alignment jig.
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Figure 5. CPV module ‘C’ after approximately 18 months of field operation, measured with single-shutter-open technique (solid lines), and modeled (dashed lines). Overall, the module VOC appears substantially reduced (a) compared to known-good module ‘B’; channels 2, 3, and 6 show normal response; channel 5 response suggests mild electrical shunt resistance; channels 1 and 4 suggest heavily shunted response; (b) shows model all closed.
So far we have assumed that the bypass diodes are all intact and functioning properly, because this is fundamental to the “single-shutter-open” configuration (SSO) which has the advantage that the deconvoluted single-channel L-I-V curves are easy to interpret. However, if a bypass diode is faulty, closing the shutter on that channel risks damaging the photovoltaic device since the full voltage of the module string is impressed across one channel and that current may not be completely shunted by the faulty bypass diode. This drives the PV diode into hard reverse bias, and eventually into avalanche and permanent damage. It is possible to avoid this by using “single-shutter-closed” configurations (SSC), where single shutters are at first only partially closed while observing the “continuous dynamic L-I-V display” (discussed in section 2.2 below), in order to identify the faulty bypass diodes by their abnormal response in negative voltage bias;
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Figure 6. Direct measurements made on individual channels in module ‘C’, and compared with the same models as Figure 5.
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Figure 7. Shutter frame measurements and models made on module ‘A’, using single-shutter-closed technique. Curves are: (a) all shutters open; (b) all shutters closed; (c) channel 3 or channel 5 closed; (d) channel 6 closed; (e) channel 4 closed; (f) channel 1 or 2 closed.
no shutter is ever closed far enough to risk the damage just described. Once the faulty bypass diodes are identified, SSC can be applied to the remaining channels without risk. We return to module ‘A’ and apply the SSC configurations, with the result in Figure 7. Again the red curves are fully illuminated (a) or completely dark (b), and 5 channels contribute to each of the intermediate curves. With a little practice it is possible to assess the normal and abnormal channels by inspection, and to estimate equivalent parasitics to model the types of degradation contributing to each (dashed lines). In this case channel 2 is slightly affected, and channels 3 and 5 are heavily affected; modeling with added shunt resistance in those channels (Figure 8) reproduces the curves well (Figure 7). Without further investigation it is impossible to determine exactly where the shunt resistance arises, whether in the photovoltaic diode, the bypass diode, or elsewhere in the CCA. Further investigation was done by opening up the module on the bench, and it was found by mechanical probing that the component attach material (conductive epoxy) for the bypass diodes in channels 3 and 5 had degraded, leading to intermittent electrical connection. We now speculate that in operation these faulty bypass circuits allowed very large reverse bias to develop on the 3JPV device if that channel was even momentarily obstructed, and such events repeated over months eventually did enough damage to enough 3JPV
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Figure 10. Hysteresis arising on a single CCA measured on the bench at low repetition rates. Slowest scan rate (0.069 V/s) has lowest VOC, increasing scan rate leads to progressively higher VOC.
devices to reduce the total module string voltage. This is a quite different degradation mechanism from our early speculation. Recently a new generation of CPV modules was installed, with redesigned 3JPV die attach and CCA attach and improved optics, which demonstrated about 26% DC module efficiency and stable VOC over a 12-month period. 2.2 Continuous Dynamic Response Technique In the preceding section we referred to the “continuous dynamic L-I-V display”. This was achieved with the following field apparatus (Figure 9). A continuous triangular-wave function generator was used to drive a DC power amplifier (KEPCO model 20-10M) providing up to 200 W and 40 Vp-p, and capable of sinking up to 100 W of CPV module power. This was connected from the ground-level test station over a 10 m force-sense cable bundle up to the CPV module riding on the tracker overhead. Power was carried on dual AWG12 cables, and module voltage sense was done with lighter gauge cable. Parasitic cable resistance effects were eliminated by the force-sense technique except for the relatively short module access cables and the module interior wiring. Current was monitored by developing a voltage
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Figure 11. Hysteresis arising at high repetition rates, measured on the same CCA on the bench (left), and modeled with SPICE (right). Arrows show direction of scan. Loops open as scan rate increases from 690 V/s to 6900 V/s.
across a 0.2 Ω precision sense resistor, Rsense, with a small temperature coefficient of resistance. Module voltage-and current-sense signals were applied to a fast digitizing oscilloscope (Agilent model DSO6014A) with 4 differential inputs and arithmetic capability, and its X-Y display capability was used to continuously visualize the L-I-V curve. The oscilloscope also has a data capture mode, where a single trigger sweep can be digitized and written to a permanent file. Hysteresis was observed in L-I-V curves acquired with this apparatus, arising from two sources: (a) time-dependent junction temperature differential compared to the main thermal mass (module heatsink rail), and (b) reactive parasitics, mainly inductive, in the CPV module and the force-sense cable bundle. In 3JPV devices the temperature dependence of voltage is known, and though it is weaker by an order of magnitude compared to silicon devices it is still measurable, and serves as a useful junction temperature thermometer. The 3JPV device temperature hysteresis arises as the L-I-V curve is swept from V = 0 through VMPPT and on to VOC, because near VMPPT about 38% of the incident optical power is being converted to electrical power and immediately removed from the 3JPV device allowing it to cool, and then to warm up again as the sweep moves on to VOC. Figure 10 shows that at very slow repetition rates around 0.01 Hz (0.069 V/s slew rate) there is no hysteresis. The thermal time constant between the 3JPV die and its supporting carrier is of the order of 1 s, and so a hysteresis opens up at similar repetition rates (0.69 V/s). At still higher repetition rates the hysteresis gradually closes again (69 V/s) as the scan cycle time becomes much shorter than the CCA thermal time constant and the 3JPV die temperature approaches some average value. As the repetition rate increases even further, the hysteresis reopens (Figure 11) due to inductive current phase lag arising from the module wiring (above 690 V/s slew rate). There is an optimum repetition rate where hysteresis effects are negligible, at approximately 10 Hz, which is convenient for continuous dynamic display. SPICE modeling of this apparatus with lumped parasitics of R = 20 mΩ, L = 2 µH was effective in reproducing observed behaviour (Figure 11).
3. CONCLUSIONS The shutter array L-I-V acquisition technique with continuous dynamic display is inexpensive to establish, relatively easy to operate in the field, and permits the field researcher a more prompt and detailed interaction with the CPV modules under test, compared to single-shot L-I-V curve acquisition, without demounting modules or breaking into them. In particular:
Rapid intermittent changes in device response are immediately obvious;
Faulty channels can be identified quickly by shutter manipulation, and some preliminary assessment of the nature of the fault may be possible, including abnormal optical throughput, parasitic resistance change or equivalent, and device changes;
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Faulty bypass diode response can be detected by partial shutter closure without risk of further damage on full shutter closure;
Tracker motion effects on module and channel response can be studied in real time;
Some investigation of dynamic thermal environment of individual cells can in principle be done using hysteresis, providing the possibility of detecting faulty die attach or carrier attach.
We have used the term “shutter array frame technique” to refer specifically to deliberate quantitative interruption of channel insolation during testing, as compared to “shadowing” or “shading” which refer more generally to insolation interruption arising from other trackers or other nearby objects or soiling.
REFERENCES [1] De Bernardez, L., & Buitrago, R. H., “Dark I–V curve measurement of single cells in a photovoltaic module,” Progress in Photovoltaics: Research and Applications, 14(4), 321-327 (2006). [2] Alers, G. B., Zhou, J., Deline, C., Hacke, P. and S. R. Kurtz, "Degradation of individual cells in a module measured with differential IV analysis," Progress in Photovoltaics: Research and Applications 19(8), 977-982 (2011).
ACKNOWLEDGEMENTS This work was supported by the National Sciences and Engineering Research Council of Canada, the National Research Council of Canada, the Business Development Bank of Canada, and the Ontario Centres of Excellence. The staff of the Canadian Center for Housing Technology, particularly M. Swinton, M. Armstrong and F. Szadkowski, provided excellent technical support for field activities, and Dr. C. Popescu of Spectra-Nova assisted with test apparatus development. Devices and technical support were provided by Cyrium Technologies and OPEL Solar.
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