DK177168B1 - Procedure for diagnosing solar module module failures - Google Patents

Abstract

There is provided a method for fault diagnosis on a solar module in which electrical potentials are checked within the solar module to provide the possibility for carrying out the fault diagnosis even when the solar module is flot exposed to 5Ufl light. Specifically the solar cell module is excited by both a DC BIAS and an AC voltage over a wide frequency range, and the impedance of the solar cell module is measured as a function of the frequency response.

Description

DK 177168 B1

Method for fault diagnosis on solar modules

FIELD OF THE INVENTION

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The present invention relates to a method for fault diagnosis on a solar module in which electric parameters are measured within the solar module and in the materials constituting a solar power system (wires, soldering etc.) to provide the possibility for carrying out the fault diagnosis even when the solar module is not exposed to sun light. Specifically solar 10 cell the modules are excited by both a DC BIAS and an AC voltage over a wide frequency range, and the impedance of the modules is measured as a function of the frequency response and the DC BIAS.

15 BACKGROUND OF THE INVENTION

Determining the characteristics of the performance of solar cells is essential to improve their optimization for sunlight energy conversion. Impedance spectroscopy (IS) is a valuable tool in many areas of materials science and electrical device technology, and it 20 has been applied to highly efficient commercial monocrystalline silicon solar cells as well as other solar cell types.

The analysis of experimental results shows that in Si solar cells it is possible to separate the physical components of the capacitance, as well as to monitor the variation of the 25 different internal resistances over different conditions of bias voltage and illumination.

The IS technique is based on analysing the electrical response of a material, to an oscillating electromagnetic (EM) field. The EM field is applied as an alternating current (AC) or voltage at two electrical terminals in contact with the material. Typically one is 30 interested in analysing the impedance in a wide frequency range. IS is widely applied in a broad class of materials systems and devices, including inorganic, organic and biological systems. In solar cell science and technology the most commonly applied frequency technique is admittance spectroscopy.

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It should be remarked that impedance and admittance are reciprocal functions, so that they give exactly the same information. However, by tradition admittance spectroscopy denominates a special method that operates at reverse voltage and evaluates the energy levels of the majority of carrier traps (in general, all those that cross the Fermi level) as 5 well as trap densities of states.

In contrast to this, in electrochemistry one is usually more interested in injecting electronic charge into the electrode, and the term generally adopted is electrochemical impedance spectroscopy (EIS). In solar cells it is clearly important to perform frequency analysis in the 10 reverse region of the diode characteristics, since this probes the selectivity of the contacts.

By exploring the forward bias range, both in dark and under illumination with different light intensities, a variety of properties can be separately investigated, including transport in the photoactive layer, contacts, bulk and surface capacitance, etc. This approach has been amply used in recent years for dye-sensitized solar cells (DSC) and organic solar cells, while 15 there are only a few works to date on solid state devices, such as those based on nanocrystalline/amorphous Si, thin-film, CdTe/CdS, GaAs/Ge, and CdS/Cu(ln,Ga)Se2 solar cells.

WO 2011/032993 A1 describes a method for characterising at least one solar cell module 20 and monitoring changes over time, including changes attributable to faults. The method comprises applying an AC voltage with an amplitude of 10 V to 2 kV to the solar cells in a broad frequency range from 1 kHz to 2 MHz, and measuring the impedance as a function of the frequency. Changes in the impedance spectrum with respect to earlier recorded impedance spectra are detected. Computing means calculates measuring values from the 25 detected changes, and measuring results from different points in time are stored. WO 2011/032993 A1 does not envisage the use of a DC bias and evaluate the measurements in terms of fitting a physical model to the measured data. The present invention provides a method for the detection of material-specific changes in interconnected solar modules that temporally evolves in the processed materials of the modules and their interconnections, 30 effected by outside and inner causes, before the occurrence of larger damages are recognizable. This is not possible with the system in WO 2011/032993 A1, wherein even small variations in daylight that would otherwise seem invisible to the eye, are captured by the string of solar cells resulting in a continuous variation in photo-voltage. The present invention constitutes an improvement over WO 2011/032993 A1, wherein the reliability of 35 the fault diagnosis in solar cell systems having multiple solar cell modules is improved.

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It is an object of the present invention to provide a more reliable method for fault diagnosis of solar cell modules.

SUMMARY OF THE INVENTION

Specifically the present invention provides a method for diagnosing failure modes within a 10 solar cell system including one or more solar cell modules, said method comprising the steps: i) applying with a power potentiostat a constant electric potential or current in the form of a DC signal across/through the solar cell modules, said potential being in the range of -1000 to +1000 Volts DC (potentiostatic mode) or a DC current typically in the range of 15 5-10 Amperes (galvanostatic mode); ii) applying in addition to the DC BIAS of i) an AC voltage, while scanning a frequency range from 1Hzto 10 MHz to achieve an impedance spectrum; iii) comparing the impedance spectrum with a control impedance spectrum recorded from an intact solar cell system or a previously measured impedance spectrum on a subunit of 20 the entire solar power system; and iv) diagnosing faults by detection of signifincant changes in model parameters, when numerically fitting a physical model to measured electrical data.

A remarkable feature with respect to the impedance spectrum of semiconductor solar cells 25 is that it depends totally on the operating point (I,V). Especially the impedance varies drastically at operating points in the vicinity of the maximum power point (VMPP , lMPP). A potentiostat thus allows that impedance data can be measured at different operating points, which makes detailed analysis involving large series of different spectra possible. It furthermore ensures that measurements can be done at similar conditions from day to 30 day. The latter is essential for comparison of different spectra obtained during the life cycle of a PV installation.

The procedure for data analysis is to import the measured impedance data as a function of BIAS and temperature Z( f,V, T ) and use the data either in the form (phase angle, total impedance, frequency) = (|Ζ|, θ, f^j or to convert the data format: (real part, imaginary part, frequency) = (Zreah Zjmag, f). A data point in this set is then typically considered as a vector in the complex plane.

4 DK 177168 B1 5 By using a physical model, which is an equivalent circuit model containing the solar cell parameters, and by employing a “complex nonlinear least square” CNLS mathematical method for fitting the model to the data, the resultwill be a list of parameters. The model is a mathematical function, and this function (equation) is deduced when the applied equivalent circuit is analysed. Complex solar module connections will have a complex 10 equivalent circuit and less complex systems, e.g. simple strings of modules, will have a less complex equivalent circuit. The specific model used for analysis is therefore tailored to the specific system, or group of systems. The result of the automated fitting procedure is stored and used for comparison when a new spectrum is recorded at a later time.

15 The physical model will depend on the system, but typically equivalent circuit models based on transmission lines will be employed for analysis of impedance spectra. The result of the automated fitting procedure is stored and used for comparison when a new spectrum is recorded at a later time.

20 The impedance spectrum and hence the model parameters that can be deduced, will have a characteristic frequency and a strong DC potential dependence. Normally features in the impedance spectrum arising from leeds, wires, soldering, and other metallic (ohmic) materials will show at the highest frequencies while features arising from the semiconductor materials in the solar cells will show at medium frequencies. It is essential 25 that impedance features (observed as curve/peak shapes in either a Nyquist or a Bode type plot) will depend heavily on DC potential, illumination and temperature, and comparison of data is therefore based data obtained under the same physical conditions.

The present invention provides a method for the detection of material-specific changes in 30 interconnected solar modules that temporally evolves in the processed materials of the modules and their interconnections, effected by outside and inner causes, before the occurrence of larger damages are recognizable.

DK 177168 B1 5

Also a regular quality control of the produced modules and their installation in a solar generator must be possible. A group of interconnected solar cell modules are subjected to both DC and AC voltage over a great frequency range and the impedance is recorded as a function of the frequency response measured. These measurements are repeated at even 5 intervals. From one measurement to the next the change of the measurement data is an indication for changes in the used materials or interconnection of the solar generator. The DC power potentiostat may also serve to keep the energy yield high by boosting the voltage of a number of solar cells.

10 In response of the inner and outside inductances, capacitances and resistances the ageing condition produce a characteristic frequency response of the impedance (impedance spectrum), for a given design of the devices. These outside and inner influences are e.g. the UV irradiation, temperature, temperature changes, the concentration of humidity and duration thereof.

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In particular according to the present invention the course of the impedance as function of an AC and DC BIAS in series or parallel, • applied between the positive and the negative pole on a module, • or modules interconnected in matrix and/or interconnected module strings in direct 20 contact • or at the DC voltage side of corresponding inverters in a far frequency range measured.

In particular are also provided that the impedance for the characterization of resultant 25 changes and for the early recognition of malfunctions such as corrosion, contact problems, and de-lamination, all of which decrease the efficiency, lifetime, and maintenance of the system.

The system consisting (the invention) of a central database and measurement device (C 30 and B in figure 1) may be installed in existing systems before or after the DC to AC power inverter.

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BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the system for analysis in a schematic form.

5 Fig. 2 shows the device that is used to characterise a series of solar units.

DETAILED DESCRIPTION OF THE INVENTION

10 In the following the present invention is described in more detail.

Referring to Fig. 1 there is shown the system for analysis in a schematic form. A, A' and A" is a sequence of subunits of a larger photovoltaic power system (typically A will be a so called string consisting of typically 7-10 modules), B is the characterization device 15 containing a DC potentiostat capable of operating both in potentiostatic or galvanostatic mode, an AC frequency generator, a frequency response analyser (FRA) and a computer with network access to other computers in the system, C is a central database (CDB) that can store and analyse data. The units A, B and C are meant to automatically interact based on a computer control carried out in CDB computer system. Connections between 20 A and B (including switches) shows that one B device may characterise a series of subunits by using a switch.

Fig. 2 shows the main characterization device which consist of a) a DC power potentiostat, b) an AC frequency generator, c) a frequency response analyser and d) an integrated 25 control computer that can control the measurement, store data and communicate with mainly central data base (see figure 1).

The method of the present invention preferably involves the following aspects: 30 1) Scanning routine at a certain electric potential set either by the power potentiostat or by illumination, on a subunit of the the solar power system (typically a string) which is an electrically isolated subunit of the whole photovoltaic system.

2) The result from the scanning routine is converted to a suitable data format and transmitted to the CDB for further analysis.

DK 177168 B1 3) The instruction to perform a scan can be a part of a planned routine or it can be based on the event where the analysis procedure identifies a significant difference between collected data sets. The latter will automatically initiate further data collection, in depth analysis and potentially and alarm will be generated.

5 4) Instructions from the CDB to perform a scan or initiate some other action (an action that preserves high energy yields or protects the system) is based on previously collected data as well as special control algorithms installed on the CDB main computer.

5) The measurement device (B in figure 1) will help right after installation of solar modules, in finding subunits that does not perform as expected and help to clarify what needs to be 10 done in order to restore the photovoltaic power plant.

6) After some period of time there will typically occur descent in performance which is either slowly induced by ageing effects or more rapidly occurring after for instance thunder storms or theft of modules. The method plays an essential role in detecting slowly induced changes as well as measuring the effect of more abrupt phenomenon.

15 7) Slowly induced material degradation and perfromance loss, is treated based on certain algorithms that will identify problematic areas and help to rationalize maintenance.

8) A user friendly system interface, intended for service and maintenance is an integral part of the invention.

9) Abrupt changes at the power plant will be translated into alarm messages, and a work 20 plan for restoring energy production will be generated.

10) The system (invention) is able to monitor and evaluate photovoltaic power systems, and hence the system is furthermore intended as a financial forecasting tool that can be used for evaluating new types of solar cells with unknown life cycles.

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Claims (3)

8 DK 177168 B1 A method for diagnosing faults in a solar cell system comprising multiple solar modules, comprising the steps performed during the day and potentially also in the dark: i) applying an electrical BIAS in the form of a DC signal over the solar cell modules with a potentiostat, in the range of -1000 to +1000 volts DC in potentiostatic mode or 5-10 amps in galvanostatic mode; ii) in addition to the DC voltage from step i) applying an AC voltage while scanning 10 at a frequency range from 1 Hz to 10 MHz with an AC amplitude of up to 1000 volts AC; (iii) comparing the impedance spectrum with a control impedance spectrum recorded from an intact photovoltaic system or previously recorded in the photovoltaic system; a physical model for the data and stores the parameters derived from this modeling routine. The method of claim 1, wherein deviations in the recorded spectrum in the frequency range 1Hz-10MHz are automatically detected by a computer program and used for further analysis of the system. The method of claim 1, wherein the physical model is based on an equivalent solar cell circuit containing circuit elements relevant to the characterized system. 25 DK 177168 B1 A "A" - / - B _ / _ A _ C Figure 1 DK 177168 B1 B Figure 2