Study on the application of glass ceramics based on vanadium dioxide as a bypass device in solar cells

doi:10.21203/rs.3.rs-5301493/v1

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Study on the application of glass ceramics based on vanadium dioxide as a bypass device in solar cells

https://doi.org/10.21203/rs.3.rs-5301493/v1

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The work explored the possibility of using glass critical thermistors based on vanadium dioxide to improve reliability and prevent electrothermal overloads in photovoltaic components of solar cells. Glass-ceramic materials based on vanadium dioxide and vanadium phosphate glass V₂O₅-P₂O₅ were used as a thermistor element, which abruptly change the value of electrical resistance by 1.5–2 orders of magnitude at a temperature of about 70°C. It has been established that the thermistor structures under consideration can function as bypass elements in solar modules. Such elements react directly both to the presence of overvoltage and to temperature increase in individual photovoltaic cells with increased resistance, and thus provide increased reliability of solar array as a whole.

vanadium dioxide

glass ceramics

critical thermistor

photovoltaic cell

One of the main reasons for the significant deterioration in the reliability of photovoltaic components of solar panels is the occurrence of electrical overloads and corresponding overheating. The physical reasons for such situations may be shading of individual solar photocells, damage to them and other components of solar panels by corrosion during operation, hidden manufacturing defects, or the results of insulation degradation under the influence of the environment [1–3].

To date, a number of methods and means have been proposed to prevent the occurrence of electrical overloads: bypass diodes, resistor dividers, active bypass transistor switches [4–9].

The most common technical solution to these problems is the inclusion of additional elements – bypass diodes. However, the practice of installing such diodes parallel to a chain of solar cells has shown that “hot spots” still arise, and this leads to an accelerated deterioration of the characteristics of solar cells. Bypass diodes are most effective in preventing hot spots in short cell sequences, but such designs are not used in modern panels for economic reasons [4–7]. On the other hand, they, like other previously mentioned circuit solutions, react mainly to electrical overloads (usually overvoltage). An increase in temperature is not an input-controlled parameter for them and leads to consequences similar to overheating of the protected elements. From this point of view, the development of protection elements that react directly to temperature increases is relevant.

One of the possible solutions to this problem is the use of thermistors based on materials with a metal-semiconductor phase transition (MSPT), considered theoretically in [10]. These solid-state elements have an abrupt dependence of electrical conductivity upon reaching the corresponding phase transition temperature Т_t.

If such a thermistor is connected in parallel to a photocell (PV cell) with increased resistance (shaded or damaged), a significant voltage drop occurs across it. This voltage is also applied to the parallel-connected thermistor, which can lead to its heating and transition to a highly conductive state. Thus, shunting of the inactive PV cell occurs, similar to the operation of a bypass diode.

In addition, when constructively supplementing a separate photovoltaic cell with a bypass thermistor element that is in thermal contact with it, the shunting effect can occur at voltage values lower than the voltage that provides the MSPT of a separate thermistor, since the latter can additionally be heated due to the heat generated by the PV cell.

It should be noted that such thermistors are widely used in the implementation of automatic control devices, switches and power limiters, thermoelectric converters of electrical and optical signals, etc. [11] However, information about their use in solar cells is not provided in the literature. In this regard, it is relevant to experimentally substantiate the possibilities and prospects of using critical thermistors based on vanadium dioxide to prevent overheating in the circuits of photovoltaic solar array systems.

The paper presents the results of experimental studies of the influence of electrical overvoltage and temperature on the electrical characteristics of a separate photovoltaic cell connected in parallel to a thermistor based on vanadium dioxide and the solar module as a whole.

Glass-ceramic materials based on vanadium dioxide and vanadium phosphate glass (VPG) of the V₂O₅-P₂O₅ system, described in [12], were used as an active thermistor element. Thanks to the MSPT in vanadium dioxide, these elements exhibit a sharp change in thermophysical, electrical and magnetic properties. The phase transition temperature is Т_t ≈ 68°C. At temperatures below T_t, such glass-ceramic materials are semiconductor materials. Above T_t their electrical properties are close to those of the metal phase. The change in electrical resistance of glass ceramics based on VO₂ during MSPT reaches several orders of magnitude [12].

Figure 1a shows the temperature dependence of the resistance of a glass ceramic sample based on vanadium dioxide. When obtaining this dependence, the rate of temperature increase did not exceed 1°С /min, and the temperature measurement error was no more than ± 0.5°С.

A characteristic feature of this dependence is a sharp, one and a half to two orders of magnitude, change in the resistance value in the temperature region of 70°C. This increase in resistance is associated with the MSPT in VO₂.

Electronic devices based on such materials can operate at high currents. This allows the creation of elements known as threshold switches and critical thermistors. Critical thermistors, combining the properties of a thermal relay and a thermistor with a negative temperature coefficient of resistance, can, in particular, be used for effective protection against overheating of electronic devices [11, 13].

Figure 1b shows the current-voltage characteristic of the sample. In this dependence one can see another feature of such materials – the presence of hysteresis. Switching a sample from a low-conducting state to a high-conducting state occurs at a higher voltage than the reverse switching.

The vanadium dioxide glass ceramic sample used in this work as a critical thermistor had a resistance at room temperature of about 70 Ohms, and after switching to a highly conductive state – up to 0.5 Ohm.

The studies used PV cell samples made of polycrystalline silicon with geometric dimensions of 150×50 mm². The kinetics of the dependences of the current I(t) through the load resistance R_load, the voltage drop U(t) and the temperatures of the photoelectric T_ph(t) and thermistor T_th(t) elements (Fig. 2a) or a combined structure based on them T(t) (Fig. 2b) was studied when switching on the external direct voltage drop U_ext from 0 to 5–15 V. The measuring circuit shown in Fig. 2 was applied.

The load role was performed by the resistor R_load. The time counting during the measurement of relaxation dependencies was carried from the moment the voltage U_ext was applied.

The voltage, which in real cases is created in the photovoltaic module (series connection of active PV cells) and applied to the (inactive, shaded) photocell with high resistance, was simulated by an external constant voltage source U_ext.

Temperature measurements were made with thermocouples. In this case, some problems arose related to its gradient along the surface of the samples of both the PV cell and the thermistor element (its dimensions were 10 mm in diameter and 1 mm in thickness), as well as to the influence of the formation of high conductivity channels during MSPT in the latter [14–15]. As a result, the thermocouples gave readings of some average, usually underestimated, temperature. In this regard, for the thermistor element, the temperature was calculated from the value of its resistance using the dependence in Fig. 1a.

To measure the current-voltage characteristic I(U) and the power curve P(U), the well-known measuring circuit of a voltmeter-ammeter was used [8]. In these measurements, the role of loading was performed by a variable resistor. The light source role was performed by a solar radiation simulator providing AM1.5 conditions. For studies at elevated temperatures, the model of the solar array module was placed in a heated thermostat, and after each measurement it was cooled to room temperature in free mode.

4.1. The influence of a bypass thermistor on the electrical characteristics of a high-resistance photocell

The kinetic dependences of the current through the load resistance (I) and voltage (U) at a parallel connection of a reverse-biased single-crystalline silicon PV cell and a VO₂ thermistor thermally insulated from it, and the corresponding dependences of their temperatures T_ph and T_th are shown in Fig. 3.

The dependences of temperature T_{ph, th}(t) and current I(t) of the structure under study show that they increase sharply in the initial sections, which correlates with observations for reverse-biased PV cells in the pre-breakdown mode [16].

This corresponds to an increase in power dissipation for both the PV cell and thermistor. An increase in temperature leads to a decrease in the voltage drop U across such a parallel connection due to a decrease in its electrical resistance and redistribution of the external voltage U_ext between this connection and the load resistance R_load connected in series with it (Fig. 2).

During the relaxation process, the temperature of the bypass thermistor increases to values corresponding to T_t. In this case, the temperature of the photocell first increases and then decreases. As a result, a stable state is achieved with constant values of current (I_st), voltage drop (U_st) and temperatures (T_st) of the PV cell and thermistor. In this case, the voltage drop change and increase in PV cell temperature are insignificant. In such way its thermal and electrical protection is implemented.

The dependence of the steady-state values of the studied characteristics on the external voltage U_ext is shown in Fig. 4.

In the range of small U_ext, the thermistor does not heat up to the MSPT temperature. At the same time, the bypass current created by it increases slightly, the voltage on the PV cell increases, and the temperatures of both mentioned elements also increase. At voltages U_ext sufficient to heat the thermistor to the MSPT temperature, its resistance sharply decreases. This leads to the appearance of a corresponding significant bypass current, a significant decrease in voltage and temperature of the PV cell.

In accordance with the above, a decrease in the overvoltage applied to the parallel connection of the PV cell and thermistor and an increase in the current through it are associated with a critical decrease in electrical resistance when the thermistor heats up. As can be seen from the research results presented here, the steady-state values of the current through the bypass thermistor can reach significant values. This value is determined by the internal resistance of the power source and the load resistance connected in series (Fig. 2). The internal resistance of the active (illuminated) PV solar module (which in the case under consideration is modeled by the voltage source U_ext) is small (up to several Ohms [17, 18]. Thus, the resistance R_load can be considered as the main current-limiting resistive element.

Figure 5a shows the effect of this resistance on the kinetics of current change through an inactive PV cell with a high resistance and a bypass thermistor.

As can be seen, with an increase in R_load, there is an increase in the duration of the transient process caused by the MSPT in the thermistor. At large values of load resistance the phase transition is not implemented at all. Accordingly, with increasing R_load, a decrease in the steady-state current through the load resistance I_st and thermistor temperature is detected, which leads to an increase in the voltage drop U_st across such a PV cell with a high resistance (Fig. 5b).

4.2. Electrical characteristics of a high-resistance photovoltaic cell in thermal contact with a thermistor

It should be noted that the considered design option for protecting the solar module from the presence of PV cell with increased resistance using a separate bypass thermistor is based on the self-heating of the said resistor due to the electrical power allocated to it. That is, for it, as for bypass diodes, the input parameters are electrical quantities (current or overvoltage).

However, the basic property of a thermistor to respond to an increase in temperature makes it possible to implement designs whose input parameters can be thermal factors. One of the options for such design is a PV cell structure with a layer of a critical thermistor based on vanadium dioxide in thermal contact [10].

The kinetic dependences of the load resistor current I, the voltage drop across the PV cell U and the temperature of such a two-layer structure when the electronic p–n junction of the PV cell is reverse biased (i.e. has a high resistance) are shown in Fig. 6.

In general, the analyzed patterns are similar to those obtained for the case using a separate thermistor element, discussed earlier. At sufficient voltages to heat the thermistor layer to temperature T_t, there is an increase in the steady-state load resistance current and a decrease in the voltage drop across the PV cell. The main differences include the fact that the dependence of temperature on time T(t), almost common for the PV cell and the thermistor layer, has a non-decreasing form. The external voltage at which such a process of current bypass through a heated to the MSPT temperature thermistor can be realized is much lower. The reason for this is that an increase in the temperature of the thermistor is achieved both due to self-heating and due to heat transfer from the heated photocell.

As you can see, the dependences of the steady current I_st through the load resistor and the voltage drop U_st across the PV cell on the external voltage U_ext, presented in Fig. 7, are similar to those considered for the structure photocell – separate thermistor. However, the PV cell temperature does not decrease after the thermistor transition to a low-conductivity state because of the thermal contact between the photocell and the thermistor layer.

4.3. The influence of the thermistor element on the photovoltaic characteristics of the solar module

Critical thermistors are able to provide bypassing the electrical current of the solar module past the passive (shaded or faulty) PV cell with high resistance. It is also important to ensure that the connection of the bypass thermistor element does not affect the operation of a properly functioning module.

Figure 8 shows the characteristics of a solar module model consisting of four PV cell samples connected in series, when using thermistor bypass elements as electrothermal protection and without them. At room temperature (17°С), the current-voltage characteristic and power curve of the model were measured without and with the connection of thermistor bypass elements, which at such temperatures are in a low-conducting (insulating) state.

As can be seen from Fig. 8 (curves 1, 2) the presence of the indicated bypass elements in the electrical circuit of the solar module model does not affect its photoelectric characteristics if all its PV cells are in good working order.

Figure 9 shows the characteristics of a model of the same solar module without and in the presence of a failure, that is, when one of the structures photocell–thermistor has a temperature above the point of the thermistor transition to a highly conductive (metallic) state.

In practice, this state can be achieved both by increasing the temperature of the specified circuit and by heating it with an electric current. As can be seen, such a failure (overheating) of individual elements leads to the same results as when using mechanical short-circuiting (throwing out the corresponding element or elements from the series circuit). The result is the same – a reduction in the generated voltage and power.

The results of the conducted studies allow us to conclude that protection elements based on critical thermistors with MSPT can function as bypass elements in photovoltaic modules to protect them from electrical and thermal overloads or damaged photovoltaic cells.

The thermistor elements under consideration ensure the flow of current in the modules, bypassing passive PV cells with high resistance, and reacting at the same time

to the presence of overvoltage on such PV cells (similar to bypass diodes) and/or;
to high temperature in them, which is undesirable when operating photovoltaic cells.

The presented results can be considered as the basis for the further development of a promising method of protecting photovoltaic elements in solar modules from electrothermal overloads.

However, some remarks are necessary. The maximum operating temperature of the most common silicon photovoltaic cells lies in the range of 60–85°C [19]. For the option with thermal contact PV–thermistor, at high temperatures (exceeding the temperature of the resistance jump in glass ceramics), the module will operate in bypass mode. Thus, this temperature must correlate with the range of permissible operating temperatures of the PV and the feasibility of the PV operation at such temperatures. It should be noted that, depending on the composition of the glass ceramics, the temperature of the resistance jump in the thermistor can vary slightly within the range of 65–80°C [12]. In this regard, for photovoltaic solar cells, the maximum operating temperature of which exceeds the specified limit temperatures, the selection and creation of thermistor materials with a higher temperature of electrical conductivity jump are important.

The issue of reducing the temperature of the thermistor and its transition to a passive state after shading is certainly important for the use of the bypassing element under consideration in the switching mode. It should be noted that during the reverse transition of the PV to the active (illuminated) highly conductive state, the voltage drop across it (and, of course, across the bypassing thermistor) will decrease sharply, because the power generated by a series connection of n PVs is almost evenly distributed between them, and each of them accounts for 1/n^th of it. It is possible that for bypass thermistors that allow process implementation, this voltage will not be enough to maintain them in a heated state.

In general, the question of the thermistor behavior after the PV transition to the active state, its connection with the structure and method of connecting the modules deserves a separate detailed study.

This work was supported by the Ministry of Education and Science of Ukraine (the research project No. 0124U000524 "Functional materials based on crystals, glasses and nanocomposites of complex oxides").

The authors have no relevant financial or non-financial interests to disclose.

All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by V. R. Kolbunov, A. S. Tonkoshkur, A. Yu. Lyashkov and S. V. Mazurik. The first draft of the manuscript was written by V. R. Kolbunov, S. F. Lyagushyn and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

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No competing interests reported.

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You are reading this latest preprint version

Study on the application of glass ceramics based on vanadium dioxide as a bypass device in solar cells

Status:

Version 1

Abstract

Figures

1 Introduction

2 Electrical properties of vanadium dioxide thermistor

3 Research methodology

4. Results and discussions

4.1. The influence of a bypass thermistor on the electrical characteristics of a high-resistance photocell

4.2. Electrical characteristics of a high-resistance photovoltaic cell in thermal contact with a thermistor

4.3. The influence of the thermistor element on the photovoltaic characteristics of the solar module

5. Conclusion

Declarations

References

Additional Declarations

Status:

Version 1