FR2832869A1 - Photoelectric panel for battery charging includes combination of series and parallel connections, with voltage converter raising output voltage - Google Patents

Abstract

The photoelectric panel (23) comprises a number of basic photoelectric cells used to charge a low voltage accumulator (11). The voltage from the photoelectric cells is applied via a converter, raising the voltage to a suitable level for charging the accumulator. A minimum number of cells is used so that the voltage of the photoelectric panel is greater than the threshold operating voltage of the power converter. The photoelectric panel (23) comprises a number of basic photoelectric cells connected together in series, connected to a power converter (24) which in turn is used to charge a low voltage accumulator (11). A certain minimum number of cells is used in construction of the panel, such that under a given degree of illumination, the voltage of the photoelectric panel is greater than the threshold operating voltage of the power converter. The cells of the panel may be connected together partially in series and partially in parallel. They are predominantly in parallel, with series connections being used solely to achieve the necessary threshold output voltage. The voltage converter serves to raise the voltage, and operates at a constant frequency. The accumulator to be charged may be an electro-chemical cell, or may be a capacitor or super-capacitor.

Description

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IMPROVEMENT FOR CHARGER
PHOTOVOLTAIC TYPE
The invention relates to an improvement for a photovoltaic type charger. It also relates to the implementation process which consists of dimensioning the photovoltaic panel and of associating with it a step-up converter, so that the load is not penalized in the case where the illumination decreases.

 The field of the charging device according to the invention is that of very low voltage accumulators, that is to say whose nominal voltage is less than or equal to 24 Volts, or any value for which there are very commonly photo-voltaic panels, allowing direct charging of the accumulator.

 More specifically, the invention relates to devices of the photovoltaic type, intended to satisfy the charge of electric accumulator in very variable lighting circumstances. In particular, the devices using a method according to the invention lend themselves to charging the accumulator when the available illumination decreases sharply compared to its nominal value.

 It is known not to use the output of the photovoltaic panels directly, in energy sensing devices. US Patent 4,494,180 cites numerous scientific references to work carried out on the search for an optimal operating point. In Anglo-Saxon literature, this approach is known by the abbreviation MPPT: maximum power point tracking.

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 This involves operating the panel under voltage and current flow conditions that correspond to the maximum power.

Such an optimal point is represented by the reference (48) in FIG. 2 of the cited patent. This patent describes in particular how to optimize the power so as to directly power an engine, without even passing through an accumulator.

 It will be noted that in this patent a DC / DC converter of ratio N is used, as in the present invention, but this is a buck converter and not a booster. This involves varying the apparent charge seen by the panel, so as to remain at the bend of the characteristic (I, U) of the panel, whatever the illumination or the temperature.

 The same concern for adapting the apparent impedance of the load, seen by the panel, will also be found in the older patent US Pat. No. 3,384,806.

 In US Pat. No. 3,696,286, this time it is a question of controlling at least one variable frequency converter by the difference between the voltage delivered by at least one main panel, operating under load, and a reference cell, operating unladen while being placed under the same temperature and lighting conditions as the main panel.

 The converter structure is that of a conventional elevator, but this one explicitly plays the role of power amplifier, and not voltage, as shown in Figure 1 and claimed. The goal is to automatically decrease the power called by the load if the available power of the panel decreases: it is again an adaptation system, type MPPT.

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 This patent provides for the use of several amplifiers, each operating at variable frequencies, to balance the output voltages of panels placed in parallel.

 Patent US-4,695,785 finally describes an MPPT device with regard to variations in lighting and / or temperature (FIG. 4). The device is controlled at maximum available power from the periodic (and brief) measurement of the available short-circuit current. This value is used to act on a switching regulator in order to adapt the power called by the load to the available power.

 It will be noted that the summary of the invention does not mention the raising or lowering nature of the switching regulator. In the description of the preferred embodiment, it is however mentioned (col 4, line 34) that the components are those of a boost converter, which corresponds to FIG. 1, but this characteristic is obviously not not essential since it is clearly stipulated in the description of FIG. 1 (col 7, line 27) that this structure is taken as a current regulator. It is indicated in the following paragraph that other regulating arrangements could just as easily be used as blocking or forward converters.

 In the devices known from the prior art, the aim is exclusively to approximate the working conditions at the elbow of the output characteristic, intensity-voltage curve, of the panel, while the device of the invention is designed to operate preferentially the area in which the photovoltaic panel works as a current generator, i.e. the horizontal part of the intensity-voltage curve, even if the power is not optimal, and the converter

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 the role of the chopper is to maintain the operating point on the elbow of the output characteristic.

 Let us add that the device of the invention is advantageously used for example for supplying DC motors for the temporary and infrequent operation of solar or other protections from the accumulator.

 Thus, the method for dimensioning and using a surface photovoltaic panel S intended for charging a very low voltage accumulator, of nominal value, is characterized in that the panel is chosen so as to deliver, under the conditions nominal illumination, a short-circuit current as high as possible provided that the no-load voltage, lower than the nominal voltage, remains above the operating threshold of a step-up converter interposed between said panel and said accumulator.

 According to an additional characteristic, the panel is chosen so as to deliver, under the nominal lighting conditions, a short-circuit intensity as high as possible as long as the elbow voltage, lower than the nominal voltage, remains above the threshold of operation of a step-up converter interposed between said panel and said accumulator.

 According to another additional characteristic, the panel comprises a single cell, or cells exclusively connected in parallel, and in that in that the converter has a minimum operating voltage lower than the no-load voltage of said panel.

 According to another characteristic, the panel comprises only a single cell, or cells exclusively connected in parallel, and in that

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 the converter has a minimum operating voltage lower than the bend voltage of said panel.

 The invention also relates to the device for implementing the method of the invention, which is characterized in that the step-up converter uses a constant frequency.

 According to an alternative embodiment, the converter comprises active periods and inactive periods, which inactive periods are used to charge using the panel an upstream storage capacitor.

 Other characteristics and advantages of the invention will emerge from the description which follows with reference to the appended drawings which are given only by way of nonlimiting examples.

 Figure 1 is a view illustrating the traditional device.

 Figure 2 is a schematic view of the device according to the invention.

 FIG. 3 is a graphic representation of the load curve of a traditional device and of the device according to the invention.

 Figure 4 is a schematic view of the charging device according to the invention and in particular of its step-up converter.

 FIG. 5 illustrates the operation, and displays the conduction times of the switch.

 FIG. 1 represents a traditional charging device (12) for ensuring the charging of an accumulator (11) of nominal voltage (UA), said device being of the type with photovoltaic panel, comprising a

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 configuration of the cells giving a no-load voltage (UB) greater than the nominal voltage (UA), and this for a nominal illumination (WR).

 FIG. 2 represents a charging device according to the invention.

For the same panel surface, the method according to the invention consists rather in choosing a configuration of the panel cells (23) giving a no-load voltage equal to UB / N for this same nominal lighting value, and interposing between the panel and accumulator (11) a step-up converter element (24) of multiplying factor equal to N, N being normally constant, greater than unity and as large as possible.

 In the preferred embodiment of a device according to the invention, this converter (24) has at least one intermittent operating mode, associated with an upstream storage element for electrical energy.

 A conventional device for charging a nominal voltage accumulator (UA) leads to choosing a panel such that the characteristic (I, U) has a no-load voltage (UB) greater, or even much greater, than (UA), as shown schematically in Figure 3, by the curve (31). To facilitate the description, it will be assumed that this voltage is obtained by placing 4 photovoltaic cells in series.

 At the optimum, as in all the MPPT devices of the prior art, the operating point is such that (UA) corresponds to the elbow (32) of the curve (31).

 Breaking with current practices, the invention recommends, for the same surface of the photovoltaic panel, to favor the value Imax of the short-circuit current, to the detriment of the no-load voltage (UB).

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Compared to a normal design, this voltage (UB) will be divided by N, while the current will be multiplied by the same factor. We then obtain the curve (33). In the example, the 4 cells of the panel are now arranged in parallel.

 The invention does not specifically seek to follow the point (34) of maximum power, but will preferentially use the range (35) in which the panel behaves as a current generator.

 It will be noted that, for large values of N, the characteristic (33) moreover tends to become rectangular, and that it is not possible to remain, in a stable manner, elsewhere than on the range (35). The elbow and no-load tensions then have substantially the same value.

 Of course, this sizing of the panel requires the insertion of a real voltage-raising device so as to allow the battery to charge. To simplify, we will take as multiplicative factor the same value N as previously, but we could just as easily take a different value M, provided that the no-load voltage multiplied by N or by M is greater than the nominal voltage (UA) of l 'accumulator.

 This sizing of the photovoltaic panel may seem very surprising, since it leads to the need for a step-up converter which could very well be dispensed with by normal sizing. However, its interest lies in the ability of the panel to supply current even in circumstances of poor lighting, this current being exploited, whatever its dynamic variation.

 In practice, N can take a value (not necessarily an integer) as large as possible. There is however a limitation

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 technological: it is imperative that the voltage UB / N remains higher than the minimum input voltage of a semiconductor converter or the like.

 Ideally, the design according to the invention amounts to using a photovoltaic panel comprising only cells in parallel (or equivalently, a single cell) for a given total surface.

The converter production technology should therefore have a minimum operating voltage lower than the elbow voltage or at least the no-load voltage of a cell of the photovoltaic panel.

 Figure 4 schematically illustrates a device in which the panel (42) is sized according to the method of the invention for charging an accumulator (11). Other devices, not shown, can be connected in parallel to this accumulator.

 The step-up converter here comprises an inductor (43), a switch (44) controlled by an oscillator (45) whose duty cycle defines the voltage rise, and operating for example at constant frequency, and finally a diode (46). Any other lifting structure known to those skilled in the art would also be suitable.

 In a first simplified version, the diode (47) could be replaced by a conductive wire, and the capacitor (48) could be omitted.

We can also keep one without the other.

 Such an assembly allows for example the charging of a 6 Volt accumulator from a panel which will be chosen to deliver 2.8 Volt when empty. The duty cycle is then chosen at 0.8, which gives a multiplicative factor

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 normal of the converter equal to 5. It can be seen that all the conditions according to the invention are met.

 This assembly is improved by adding the elements (47) and (48), which make it possible to produce an upstream storage element. Furthermore, the converter (45) is provided with an input (451) inhibiting its operation if the voltage on this input becomes less than a given threshold (Ul), and then keeps it inhibited as long as this voltage has not resumed a value at least equal to (U2).

 It should be noted that certain circuits, used under conditions of low supply voltage, spontaneously exhibit this behavior. Linear's LT1613 circuit, which performs all of the booster and oscillator functions, naturally stops working if its voltage drops below about IV, and resumes for a slightly higher voltage, with a hysteresis of 0. , 1 V.

 The assembly with diode and input capacitor is known to those skilled in the art: it avoids directly stressing the panel when the switch is turned on.

 However, it takes on another function here, in association with the input (451), or the intrinsic operation equivalent to this input, that of a relaxation type operation.

 FIG. 5 illustrates such an operation, by visualizing the conduction times of the switch.

 It is a puff operation, delivering a few pulses when the low current delivered by the panel has been able to charge the capacitor enough to reach the threshold voltage (U2), which immediately causes a discharge, until reaching ( IU). In some

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 in the case of a low load current, there are thus only a few pulses (51), ultimately one or two, which are separated by a time (52) decreasing when the current increases. As soon as the current delivered becomes sufficient, the conduction becomes continuous.

 This configuration makes it possible to lose nothing of the electrical charges supplied by the photovoltaic panel.

 In the preferred embodiment, the diode (47) is also not used, so as to avoid an additional voltage drop.

 Finally, use can also be made of a muting input (452) in the event that the output voltage comes to exceed the maximum voltage of the battery at full charge.

 Of course, the invention is not limited to the embodiments described and shown by way of examples, but it also includes all the technical equivalents and their combinations.

Claims (6)

1. Method for dimensioning and using a surface photovoltaic panel S intended for charging a very low voltage accumulator, of nominal value UA, characterized in that the panel is chosen so as to deliver, under nominal conditions of illumination, a short-circuit intensity as high as possible, provided that the no-load voltage, less than UA, remains above the operating threshold of a step-up converter interposed between said panel and said accumulator.  2. Method for dimensioning and using a surface photovoltaic panel S intended for charging a very low voltage accumulator, of nominal value UA, according to claim 1, characterized in that the panel is chosen so as to deliver , under nominal lighting conditions, a short-circuit current as high as possible, provided that the elbow voltage, less than UA, remains above the operating threshold of a step-up converter interposed between said panel and said accumulator.  3. Method for dimensioning and using a step-up converter intended for charging a very low voltage accumulator from a photovoltaic panel according to claim 2, characterized in that the panel comprises a single cell, or cells exclusively connected in parallel, and in that the converter has a minimum operating voltage lower than the no-load voltage of said panel.  4. Method for dimensioning and using a step-up converter intended for charging very low voltage accumulators from <Desc / Clms Page number 12>  of a photovoltaic panel according to claim 2, characterized in that the panel comprises only one cell, or cells exclusively connected in parallel, and in that the converter has a minimum operating voltage lower than the elbow voltage of said sign.  5. Device for implementing the method according to any one of the preceding claims, characterized in that the step-up converter uses a constant frequency.  6. Device according to claim 5, characterized in that the converter comprises active periods and inactive periods, which inactive periods are used to charge using the panel an upstream storage capacitor.