JP2005203398A - Photovoltaic power generation system, and short-circuiting failure suppression method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the occurrence of failure in a photovoltaic power generating system caused by a partial shade, and to improve durability in a field in a solar cell in which a back reflection layer comprises Ag, Cu, or Au. <P>SOLUTION: The photovoltaic power generation system having a photovoltaic element containing silver, copper, or gold at least on a back electrode has a means for applying a voltage that is equal to or higher than the oxidation-reduction potential of metal for composing the back electrode, and lower than a forward on-voltage Va of a parasitic diode of the photovoltaic element as a forward bias voltage to the photovoltaic element at night. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a system and method for suppressing a failure of a photovoltaic power generation system caused by a partial shade.

  Photovoltaic power generation using photovoltaic cells with photovoltaic elements has great potential as an alternative energy source to solve various environmental problems caused by using existing power generation methods such as thermal power generation, hydroelectric power generation, and nuclear power generation. ing.

  In recent years, photovoltaic power generation using sunlight, which is clean energy, as an energy source has an extremely small influence on the global environment, and is gradually spreading both in Japan and overseas.

  Conventionally, monocrystalline or polycrystalline silicon has been often used as a material for forming a photovoltaic layer of a solar cell used for photovoltaic power generation. However, these solar cells require a lot of energy and time for crystal growth, and after that, complicated processes are required, so that the mass production effect is difficult to be achieved and it is difficult to provide them at a low price.

  For this reason, the cost of the power generation system per Wh is higher than that of the existing power generation method, and as a result, it is an adverse effect of the spread of solar power generation.

In order to solve this problem, so-called thin film semiconductor solar cells using compounds such as amorphous silicon, CdS, and CuInSe ₂ have been actively researched and developed. In these solar cells, it is only necessary to form a photovoltaic layer made of as many semiconductor layers as necessary on an inexpensive substrate such as glass or stainless steel, and the manufacturing process is relatively simple, and the cost can be reduced. However, since the conversion efficiency of the thin-film solar cell is lower than that of the crystalline silicon solar cell, it has not been widely spread so far as a solar cell for power generation.

  As one of the methods for improving the conversion efficiency of the thin-film solar cell, a method of providing a back reflecting layer has been conventionally known.

  The back surface reflection layer is provided in order to increase the light reflectance of the substrate surface on which the photovoltaic layer composed of a thin film semiconductor layer is formed. The back reflective layer has a function of reflecting incident sunlight that has not been absorbed by the photovoltaic layer, returning it to the photovoltaic layer again, and promoting absorption. Since this back surface reflection layer also serves as an electrode on the substrate surface, a metal material having good reflectivity and conductivity such as gold (Au), silver (Ag), copper (Cu), aluminum (Al), etc. is preferably used. For example, see Patent Document 1 (Japanese Patent Laid-Open No. 06-085299).

In particular, Au and Ag are most preferable as a constituent material of the back reflective layer because they exhibit a reflectance of 95 to 98% in almost all wavelength regions of visible light. Cu has a very low reflectivity of about 60% in a wavelength region shorter than a wavelength of 600 nm, but shows a high reflectivity almost inferior to Ag in a wavelength region of about 600 to 1000 nm. Further, since light having a wavelength of 600 nm or less is almost absorbed when passing through a photovoltaic layer made of a semiconductor, it is sufficient that the metal layer has a high reflectance with respect to light in this range. However, it can be said that Cu is advantageous in terms of cost compared with Ag and Au. Al is a metal that is easy to use in terms of cost, but in a relatively long wavelength region of about 600 to 1000 nm, which is important as a reflective layer, the reflectance is about 85 to 90%, which is considerably lower than Ag. However, in recent years, studies have been made on the use of Ag, Cu, or Au for the back reflective layer.
Japanese Patent Laid-Open No. 06-085299

  However, when a thin film photovoltaic device using Ag, Cu, or Au having a high reflectance as a back reflecting layer is used for a long time in an outdoor environment in order to improve conversion efficiency, the film quality of the photovoltaic layer of the photovoltaic device It was found that sometimes the shunt resistance decreased with the poor one, and as a result, the conversion efficiency decreased.

  As a typical event, we explained the phenomenon of shunt resistance drop from winter to spring in an amorphous silicon solar cell module (semiconductor layer thickness of about 300 nm) using Ag as the back reflective layer, which was installed in a snowy area for several years. To do.

  In order to analyze the cause of the decrease in the shunt resistance of the solar cell module, a location where the shunt resistance of the solar cell module is decreased is specified. When a current distribution is applied in the forward direction of the photovoltaic device, the temperature distribution of the surface temperature imaging camera (Mitsubishi Electric thermal imager IR-5120C, hereinafter abbreviated as IR camera) using far infrared rays is used. The short-circuit location can be identified from the difference. When the short-circuited portion is examined with an optical microscope, a spot having a metallic luster with a diameter of about 10 to 30 μm can be found. Further, when this spot is analyzed with an X-ray microanalyzer (EMAX-5770 manufactured by Horiba, Ltd., hereinafter abbreviated as XMA), an Ag signal that is significantly stronger than the surroundings is detected at this spot portion. From this, it is considered that Ag of the back surface reflection layer is deposited on the surface for some reason. As a result, a short circuit occurs between the back surface reflection layer of the photovoltaic element and the surface electrode, and one of the output currents that should be originally taken out to the outside. It is surmised that the efficiency of the photovoltaic element is reduced due to the loss of the portion.

  This solar cell module has intrusion of moisture due to snow accumulation, and it is considered that a part of the solar cell module was not exposed to sunlight (partial shade) for a long time.

  Usually, in order to ensure long-term reliability as a solar cell, it is necessary to seal a photovoltaic element used as a solar cell with a highly transparent sealing material. A film, EVA resin or the like is used. Although it depends on the installation environment of the solar cell, it is difficult to completely prevent the permeation of moisture by long-term outdoor exposure, resulting in deterioration of the sealing material. If the surface material is made of glass, moisture can be prevented from permeating from the surface, which can be improved. However, since the infiltration of moisture from the bonding edge of the glass and photovoltaic element cannot be suppressed, the effect is not perfect. .

  In a solar power generation system, when sunlight is incident in a partial shade state where a part of the solar cell module constituting the system is shielded from light due to snow or the like, the shielded photovoltaic element is reverse-biased by the system voltage. It will be.

  FIG. 7 shows an equivalent circuit of the solar cell module, in which 701 is a photovoltaic element, 702 is a parasitic diode, 703 is a photovoltaic current source, 704 is a shunt resistor (parallel resistor), and 705 is a series resistor (series resistor). , 706 are bypass diodes.

  Usually, a solar cell module has a bypass diode 706 connected in parallel with the photovoltaic element 701 in order to protect the photovoltaic element 701. As a result, the reverse bias voltage applied to the photovoltaic element 701 is relaxed by the bypass diode 706. However, since the ON resistance of the bypass diode 706 is not 0, a reverse bias voltage remains between the positive and negative electrodes of the photovoltaic element 701, and a reverse bias voltage is applied for about 0.7 to 1.5V. Become.

  The voltage is higher than the oxidation-reduction potential of the metal material constituting the back reflective layer, and is sufficient to cause a chemical reaction.

  On the other hand, under a situation where the humidity is sufficient and a voltage higher than the voltage described above is applied, Ag, which is a constituent material of the back surface reflection layer, causes a phenomenon called ion migration (hereinafter abbreviated as migration). It is known.

  From the above, it is considered that the failed solar cell module is caused by migration of Ag which is a constituent material of the back surface reflection layer.

  Such a decrease in shunt resistance due to migration is considered to occur similarly in Cu and Au, in which migration tends to occur in addition to Ag in the back surface reflection layer.

  The present invention provides a photovoltaic power generation system and method that suppresses short-circuit failure of a solar cell module that uses Ag, Cu, or Au as a back surface reflection layer, based on the above-described mechanism of decrease in photoelectric conversion efficiency due to migration. Objective.

  The photovoltaic power generation system of the present invention is a photovoltaic power generation system having a photovoltaic element containing silver, copper, or gold at least on the back electrode, and the back electrode is used as a forward bias voltage for the photovoltaic element at night. It is characterized by having means for applying a voltage in a range not less than the oxidation-reduction potential of the constituent metal and less than the forward ON voltage Va of the parasitic diode of the photovoltaic element.

  The photovoltaic power generation system short-circuit failure suppressing method of the present invention is a photovoltaic power generation system having a photovoltaic element containing silver, copper, or gold on at least a back electrode, and the photovoltaic element is used as a forward bias voltage at night. A voltage in a range not less than the oxidation-reduction potential of the metal constituting the back electrode and less than the forward ON voltage Va of the parasitic diode of the photovoltaic element is applied.

  According to the solar power generation system short-circuit failure suppressing method of the present invention, in the solar cell in which the back reflection layer is made of Ag, Cu, or Au, the short-circuit failure of the solar power generation system can be suppressed and the durability in the field can be improved. .

  An embodiment of a method for suppressing a short-circuit fault in a solar power generation system according to the present invention will be described with reference to FIGS.

  FIG. 1 is a schematic diagram of a photovoltaic power generation system according to the present invention.

  The solar cell module 101 is connected to the inverter system 103 via the backflow prevention diode 102. In FIG. 1, four solar cell modules 101 are connected in parallel. The solar cell module 101 may have a form in which a plurality of solar cell modules called strings are connected in series. The series-parallel number of the solar cell modules 101 may be designed according to each system.

  A system for applying a forward bias voltage to the solar cell module 101 at night is a bias power source 105 and a switching unit 104. At night, that is, when there is no solar radiation, the switching means 104 is turned ON and the bias voltage application from the bias power source 105 is switched. When there is solar radiation, the switching unit 104 is turned off, the bias power supply 105 is disconnected, and the bias voltage application is stopped.

  Note that the forward bias voltage referred to here refers to the forward voltage with respect to the parasitic diode of the photovoltaic element in the solar cell module 101.

  If the metal ions from the back electrode material such as Ag have moved to the front electrode side, they are moved again to the back electrode side by applying a forward bias voltage from the bias power source 105. However, since the bias voltage is not applied to the photovoltaic device that has already undergone sufficient migration and is short-circuited, it cannot be recovered. Therefore, it is necessary to simultaneously operate the performance deterioration suppressing system of the present invention from the time of installing the solar power generation system.

  It is desirable that the solar cell module 101 to be installed has the same shunt resistance value in the dark state of the individual photovoltaic elements constituting the solar cell module 101 as much as possible. This is because, when a forward bias voltage is applied from the bias power source 105, an equal forward bias voltage can be applied to the individual photovoltaic elements constituting the solar cell module 101. When the shunt resistance value varies, the forward bias voltage applied to each photovoltaic element also varies in proportion. This causes variations in the short-circuit failure suppression effect. Photovoltaic elements that are stably produced at a solar cell manufacturer's mass production plant have no problem because the shunt resistance value is also controlled and its variation is suppressed, but it is produced for more active management. The photovoltaic elements 101 may be manufactured using photovoltaic elements having the same shunt resistance value by ranking the photovoltaic elements obtained by ranking.

  The method of measuring the shunt resistance in the dark state can be obtained from the inclination by placing the photovoltaic element or the solar cell module 101 in a state where no light such as a dark room is incident and measuring the IV characteristics. In a simple manner, it can also be obtained from the leakage current value when a voltage that does not turn on the parasitic diode described above is applied.

  Note that the voltage at which the diode is turned on is a forward applied voltage that becomes a turning point where a current suddenly flows between the anode and the cathode of the diode. This will be described later, and will be described in detail here. Omitted.

  As the switching means 104, a diode or an electromagnetic relay can be used.

  FIG. 2 shows a case where a diode 204 is used for the switching means 104. When the solar radiation disappears and the output voltage of the solar cell module 201 becomes equal to or lower than the bias voltage set in advance in the bias power source 205, current flows from the bias power source 205 via the diode 204, and a forward bias voltage application state from the bias power source is eventually observed. become. When there is solar radiation, the output voltage of the solar cell module 201 is higher than that of the bias power source 205, so that no current flows from the diode 204 and the application of the forward bias from the bias power source is released.

  The set voltage of the bias power source 105 in FIG. 1 is determined by the series number of solar cell modules 101, that is, the output voltage and the starting voltage of the inverter 103. In each photovoltaic power generation system, the number of series solar cell modules 101 varies, and the setting voltage of the bias power source 105 is also a setting voltage corresponding to each. For this reason, it is necessary to set the output voltage of the bias power source 105 at the same time when the photovoltaic power generation system is installed. It is also effective to save the space to incorporate the function of the bias power source 105 in the inverter 103 in advance.

  The lower limit value of the set voltage of the bias power source 105 is determined by the type of metal material used for the back electrode layer and the series number of photovoltaic elements constituting the solar cell module 101. The set voltage must be greater than or equal to (the oxidation-reduction potential of the metal used in the back electrode layer) × (the number of photovoltaic elements constituting the solar cell module 101 in series). This is because the oxidation-reduction reaction does not occur at a voltage lower than the lower limit. The redox potential of the metal material used for the back electrode layer is about 0.3V for Cu, 0.8V for Ag, and about 1.5V for Au.

  The upper limit value of the set voltage of the bias power source 105 may be less than (the voltage at which the parasitic diode of the photovoltaic element described above is turned on: Va) × (the number of photovoltaic elements constituting the solar cell module 101 in series). . This is because the object can be achieved with a very small amount of power if the voltage is less than this voltage value. The voltage Va at which the parasitic diode of the photovoltaic element is turned on can be obtained by the following procedure.

  The dark state IV characteristics of the solar cell module 101 or a single photovoltaic element constituting the solar cell module 101 are obtained.

  As shown in FIG. 6, Va draws an asymptotic line near the point where the IV characteristic of the photovoltaic element sufficiently rises, and is a voltage at the intersection of the asymptotic line and the horizontal axis. It is clear from the graph of FIG. 6 that the IV characteristic rises rapidly when the applied voltage is Va or higher.

  Va varies depending on the type of photovoltaic element, but the voltage value is usually about 0.4 to 0.9 V for a crystalline silicon photovoltaic element and 0.5 to 0 for a single amorphous silicon photovoltaic element. .7V or so. FIG. 6 shows an example of a triple type amorphous silicon photovoltaic device, and Va is about 2V. As described above, in the case of a stacked photovoltaic element, Va increases in proportion to the number of stacked layers.

  When a voltage equal to or higher than the voltage Va is applied to the photovoltaic element, a large current flows through the photovoltaic element and power consumption occurs. In the present invention, such a large electric power is not necessary, and it is sufficient that the parasitic diode of the photovoltaic element constituting the solar cell module 101 is not turned on and only a minute electric power necessary for the oxidation-reduction reaction is required. The forward bias voltage applied to the power element is preferably as low as possible below Va.

  In addition, in the case of an amorphous silicon solar cell, it is known that the conversion efficiency deteriorates due to the forward current injection. When the forward bias voltage exceeds Va, a large current flows to accelerate the deterioration of the conversion efficiency. Become.

  In the case of the configuration of FIG. 2, if (Va) × (the number of photovoltaic elements in series) is equal to or higher than the starting voltage of the inverter 103, the inverter 103 operates and causes a malfunction. Needless to say, the voltage is set lower than (Va) × (the number of photovoltaic elements in series) and lower than the starting voltage of the inverter 103.

  As the bias power source 105, a commercially available DC stabilized power source, a battery, or the like can be used. In the short-circuit fault suppression system of the photovoltaic power generation system of the present invention, as described above, voltage is applied in a region where the parasitic diode of the photovoltaic element is OFF, so that a large amount of power is not necessary and a small power supply facility may be used. There are also benefits.

  The set voltage of the bias power source 105 is most preferably obtained by measuring the nighttime IV characteristics of the photovoltaic power generation system after the installation as a photovoltaic power generation system and using the same method as Va described above. Actual measurement is very difficult in terms of equipment, such as the need for a power supply. Therefore, one solar cell module 101 or a single photovoltaic element constituting the solar cell module 101 is selected, Va is obtained from the IV characteristic in the dark state, and this is multiplied by the series number of photovoltaic elements. It can also be substituted.

  FIG. 3 shows a case where a switch that can be controlled by an external signal such as an electromagnetic relay 304 is used for the switching means 104. In this case, in order to control the switching timing of the electromagnetic relay 304, it is necessary to detect the presence of solar radiation. For simplicity, a means 309 for monitoring the output voltage of the solar cell module 301 may be provided for detection. In addition, the photosensor 306 may be separately installed to detect solar radiation. Alternatively, a calendar means (not shown) may be provided without using the optical sensor 306, and the opening / closing time of the electromagnetic relay 304 may be programmed in advance to open / close the electromagnetic relay 304 at a time when the solar radiation is sufficiently reduced.

  Opening / closing control of the electromagnetic relay 304 is performed by a control device 308 including a sequencer and a computer. The output voltage of the solar cell module 301 and the signal from the optical sensor 306 (the presence or absence of solar radiation) are monitored, and the electromagnetic relay 304 is opened and closed. In this case, the standby power can be reduced by simultaneously opening and closing the main power source of the bias power source 305.

  In addition, as shown in FIG. 4, another electromagnetic relay 407 linked to the electromagnetic relay 404 may be added to completely disconnect the solar cell module 401 from the inverter 403. With the system configuration of FIG. 4, the bias power source 405 can set a voltage of (Va) × (the number of photovoltaic elements in series) regardless of the starting voltage value of the inverter 403, and further prevents malfunction of the inverter 403. it can.

  By using the embodiment as described above, a forward bias voltage can be applied to the solar cell module 101 at night when the photovoltaic power generation system is not operating, and the solar cell module 101 in which migration is in progress. Short circuit failure of the photovoltaic power generation system can be suppressed.

1 is a schematic diagram of a photovoltaic power generation system according to an embodiment of the present invention. It is a block diagram of the solar energy power generation system which is one Embodiment of this invention. It is a block diagram of the solar energy power generation system which is one Embodiment of this invention. It is a block diagram of the solar energy power generation system which is one Embodiment of this invention. It is a block diagram of the conventional solar power generation system. It is a graph for setting the forward bias voltage of the present invention. It is an equivalent circuit diagram of a solar cell module.

Explanation of symbols

101, 201, 301, 401, 501 Solar cell module 102, 202, 302, 402, 502 Backflow prevention diode 103, 203, 303, 403, 503 Inverter 104 Switching means 204 Switching diode 105, 205, 305, 405 Bias power supply 304, 407 Electromagnetic relay 306, 406 Optical sensor 308, 408 Control device 309, 409 Voltage monitoring means 701 Photovoltaic element 702 Parasitic diode 703 Photovoltaic current source 704 Shunt resistance (parallel resistance)
705 series resistance (series resistance)
706 Bypass diode

Claims (2)

  In a photovoltaic power generation system having a photovoltaic element containing silver, copper or gold at least on the back electrode, as a forward bias voltage on the photovoltaic element at night, the oxidation-reduction potential of the metal constituting the back electrode is equal to or higher than A photovoltaic power generation system comprising means for applying a voltage lower than a forward ON voltage Va of a parasitic diode of the photovoltaic element.   In a photovoltaic power generation system having a photovoltaic element containing silver, copper or gold at least on the back electrode, as a forward bias voltage on the photovoltaic element at night, the oxidation-reduction potential of the metal constituting the back electrode is equal to or higher than A method for suppressing a short-circuit fault in a photovoltaic power generation system, wherein a voltage lower than a forward ON voltage Va of a parasitic diode of the photovoltaic element is applied.

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