WO2009036502A1

WO2009036502A1 - Circuit for solar cell array

Info

Publication number: WO2009036502A1
Application number: PCT/AU2008/001380
Authority: WO
Inventors: John Anthony Maine
Original assignee: Queensland University Of Technology; Dyesol Ltd
Priority date: 2007-09-18
Filing date: 2008-09-18
Publication date: 2009-03-26

Abstract

A circuit for a solar cell array formed from clusters of serially connected photovoltaic cells that switch the output from a cluster to an output line depending on the level of illumination experienced by the cluster. The level of illumination may be detected by a photodetector or it may be estimated directly from each cluster by a current monitor circuit. The output from the cluster is switched to a highpower current line when the cluster experiences full or near full illumination or a low power current line when the cluster experiences below near full illumination. The switching level is determined relative to a threshold.

Description

TITLE CIRCUIT FOR SOLAR CELL ARRAY

FIELD OF THE INVENTION

The present invention relates to the field of solar cell arrays. More particularly, the invention relates to a circuit for a solar cell array to reduce power loss due to shading.

BACKGROUND TO THE INVENTION

Solar panels are comprised of arrays of photovoltaic cells. Each photovoltaic cell generates electricity when exposed to incident light, generally sunlight. A typical cell may generate 0.5 volts, so in order to generate useful voltages a number of cells are connected in series. The more cells that are connected in series the greater the voltage generated.

Similarly, the current generated by the array of cells can be increased by connecting cells or modules of cells, in parallel. The more parallel modules the greater the amperage of the array. A typical array may have hundreds of cells arranged into parallel connected modules of serially connected cells.

Each cell in a serially connected module is either a power producer or a power consumer depending on the level of illumination (or conversely the degree of shading). Different solar cell types demonstrate different behaviours when an array is shaded. A fully serially wired silicon array with no protection against shading requires uniform illumination to operate. Any substantial departure from this condition leads to voltage reversal on the shaded cell and failure of the cell if the array open circuit voltage exceeds the cell reverse breakdown voltage.

To prevent this potentially damaging situation various schemes employing diodes for protection have been suggested. Blocking diodes are used between parallel modules to prevent current flowing back into a shaded module. Bypass

285301 2.DOC diodes are used within modules to shunt current around shaded cells. These measures eliminate cells from the photovoltaic array and therefore reduce the maximum power that can be obtained from the array.

It is known that partial shading has a significant effect on array efficiency, especially larger arrays. Reference may be had to a review article on the subject, the content of which is incorporated herein by reference [A. Woyte, J. Nijs, R. Belmans, "Partial shadowing of photovoltaic arrays with different system configurations: literature review and field test results", Solar Energy 74 (2003) 217-233].

The inefficiency arising from shading can best be understood by considering that in a serially wired silicon module, of nominal power rating W-i, the current through the module is equal to the current through the shaded cells. If λ is the proportion of the module in full sun (so 0 < λ ≤ 1), then if any of the module is in a shadow (λ < 1) with an irradiance level k suns (0 < k < 1), and none of the cells avalanche, the power output WA from the module is approximated by the relationship:

WA = W^ (1)

The maximum possible power from the module WM, with all cells operating at their individual peak power points, would be the sum of the outputs of the unshaded part and the shaded part:

W_M = W₁ λ + W₁ k (1-λ) (2)

For modules employing some defence against shading, WA will be greater than the value stated in (1), although it could never exceed WM.

The ratio η_w = W_A/ WM for various proportions of shading λ and shade irradiance k is shown below:

285301 2.DOC

Table 1

Table 1 indicates that the power output of a shaded serially wired module is substantially less than the possible maximum, particularly for typical values of k (~ 0.2) and λ (~ 0.5), shown in bold type.

OBJECTS OF THE INVENTION

It is an object of the present invention to overcome or at least alleviate one or more of the above limitations including increasing the efficiency of photovoltaic arrays under partial shading conditions.

SUMMARY OF THE INVENTION

In one form, although it need not be the only or indeed the broadest form, the invention resides in an array of photovoltaic cells comprising a plurality of clusters of serially connected photovoltaic cells wherein each cluster is dynamically switched to a power line depending upon the level of illumination.

Suitably there is a low current power line and a high current power line. Clusters in full illumination or near full illumination are switched to the high current power line and clusters below near full illumination are switched to the low current power line.

The invention may further comprise an irradiance estimator associated with each cluster. The dynamic switching is suitably determined by the illumination level detected by the irradiance estimator.

285301 2.DOC In one form of the invention the irradiance estimator is a photodetector that measures the level of illumination at one point near each cluster of photovoltaic cells.

In an alternative form, the irradiance estimator is a circuit that estimates illumination level from current in each cluster of photovoltaic cells and the terminal voltage across each cluster of photovoltaic cells.

In a further form the invention resides in an array of photovoltaic cells comprising: at least two power lines; one or more clusters of serially connected photovoltaic cells; an irradiance estimator associated with each cluster; and a switching circuit associated with each cluster; wherein the switching circuit switches the output from the cluster to a power line depending on an illumination level detected by the irradiance estimator.

In a yet further form the invention resides in a method of operating an array of photovoltaic cells comprising a plurality of clusters of serially connected photovoltaic cells, by detecting an illumination level at each cluster and dynamically switching the cluster to a power line depending upon the level of illumination. Further features and advantages of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist in understanding the invention and to enable a person skilled in the art to put the invention into practical effect, preferred embodiments of the invention will be described by way of example only with reference to the accompanying drawings, in which:

FIG 1 is a plot of cell current vs power for a dye sensitised cell; FIG 2 is a plot of power output versus number of shaded cells;

285301 2.DOC FIG 3 is a schematic of a first embodiment of an array of photovoltaic cells incorporating dynamic switching; FIG 4 is a representative schematic diagram of the switching circuit of FIG 3; FIG 5 is a representative schematic diagram of an irradiance estimation circuit;

FIG 6 is a schematic of a modified input circuit of a peak power point tracker for the array of FIG 3; and

FIG 7 is plot of voltage across the inductor of FIG 6.

DETAILED DESCRIPTION QF THE INVENTION

Embodiments of the present invention reside primarily in configuration of clusters of photovoltaic cells and an associated switching circuit. Accordingly, the cells and circuits have been illustrated in concise schematic form in the drawings, showing only those specific details that are necessary for understanding the embodiments of the present invention, but so as not to obscure the disclosure with excessive detail that will be readily apparent to those of ordinary skill in the art having the benefit of the present description.

In this specification, adjectives such as first and second, left and right, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order. Words such as "comprises" or "includes" are intended to define a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed, including elements that are inherent to such a process, method, article, or apparatus.

Referring to FIG 1 there is shown a plot of the relationship between power output and cell current for a dye sensitised cell (DSC) at various illumination levels. The levels are conveniently considered as 1 sun (full sunlight illumination), 0.75 sun and 0.5 sun. The variation of voltage with irradiance is logarithmic while the variation of current with irradiance is linear, and therefore

285301 2.DOC peak voltage changes very little whereas, as can be seen in FIG 1 , peak current varies very substantially with irradiance. Because of this, it is much more important to protect series strings of cells against the effect of shading than parallel strings of cells.

The graph of FIG 2 demonstrates that the maximum power achievable from an array is when all cells are connected in parallel. FIG 2 shows the power output for 36 photovoltaic cells. A fully serially connected array (the series curve) has significant departure from maximum power. Close to the optimum power output can be obtained with an optimal series-parallel arrangement (6 parallel clusters of 6 serially connected cells per cluster), however this has not been achievable in practice as the cells that are receiving reduced illumination change dynamically with conditions (such as moving sun and moving vegetation). This problem is overcome by the arrangement shown in FIG 3.

FIG 3 shows an array 30 of twelve photovoltaic cells 31 arranged into two clusters 32, 33. For the purpose of description there is a shaded cluster 32 and a full sun cluster 33. It has been found that six cells is a suitable number for each cluster, however the invention is not limited to any particular number of cells per cluster. In fact, there could be a single cell in each cluster, however such an arrangement could defeat an economic benefit of the invention.

Each cluster 32, 33 is connected to either a low current power line 34 or a high current power line 35 by a switching circuit 36. There is a switching circuit 36 associated with each cluster 32, 33. The switching circuit 36 includes an irradiance estimator 37 that monitors the average light level at the cluster. The switching circuit compares the signal from the irradiance estimator against a threshold and switches the output from the cluster to the high power line or low power line accordingly.

In practice only two power levels are relevant, although there is no inherent reason for using only a high power line and a low power line. In certain situations it may be appropriate to have three or more separate power lines and switch clusters to the appropriate power line based on the level of illumination detected.

285301 2.DOC It was determined that two power lines would be appropriate in many situations by conducting tests whereby a small dye-sensitised cell (DSC) of area 0.88cm² was used to measure light intensity at various arbitrary points in the shadow of eucalypt trees in full sun. The tree branches were typically 4-5 metres above the cell, which was kept horizontal during the tests. The distribution of intensities was not strongly uniform and tended to be bimodal, with some points receiving full sun and others receiving sky dome levels or less. Although a DSC was used for the test the results are relevant for any photovoltaic cell.

A suitable switching circuit 36 is shown in FIG 4. The proposed circuit senses the ambient lighting level close to the cluster with irradiance estimator 37, which in one embodiment is a photodetector 38, and determines if it is near full sun or not. The photodetector only provides an irradiance estimate because it measures the level of illumination at a single point rather than across the entire cluster. In another embodiment there may be two or more photodetectors with the signals from the photodetectors being used to estimate the irradiance incident on the cells in the cluster.

The optimum value for the decision point to switch between circuits may be estimated by inspection of the power/current curves for a cell (as shown in FIG 1 for a DSC). It can be seen in FIG 1 that at the point where the power in a cell illuminated with 75% sun has dropped to zero the cell illuminated with 100% sun is producing close to maximum power. The voltage across the cell has dropped to zero because the internal photocurrent is insufficient to maintain any potential across the cell in the presence of an externally forced current equivalent to 100% sun. So when a shaded cell receives an irradiance of less than 75% full sun, while the remainder of the cells in its series receive 100% full sun, the power output from the shaded cell drops to zero. This suggests that the irradiance level at the decision point for a DSC should not be less than 75%. With this as a parameter, the circuit detects the irradiance and if it exceeds 75% of full sun level, the local cluster is connected into the high current line, if it is equal or less than 75% of full sun level, the cluster is connected to the low current line.

285301 2.DOC Persons skilled in the field will appreciate that the threshold level may be different for other photovoltaic cells. The most appropriate threshold value may generally be somewhere between 60% and 80% full sun.

Referring to FIG 4 in detail, in one embodiment of the invention the switching circuit 36 employs voltage driven switching elements. The output from the photodetector 38 is compared against a pre-set threshold 41 in a first OPAMP 42. The output 43 from the OPAMP 42 is connected to a pair of switching elements 44 which switch the output from the cluster 32 to the high power current line 35, and to a switching element 48 that closes the low power current line 34. The output 43 is also directed to one input of an inverting

OPAMP 45 that produces an inverted output 46. The output 46 from the inverting OPAMP 45 is connected to a pair of switching elements 47 which switch the output from the cluster 32 to the low power current line 34 and to a switching element 49 that closes the high power current line 35.

If the photodetector voltage exceeds the threshold, the OPAMP 42 outputs a positive voltage that closes switching element pair 44 to connect the cluster 32 to the high power current line 35 and closes switching element 48 in the low power current line 34, while the inverting OPAMP 45 opens switching element pair 47 and switching element 49. If the photodetector voltage drops below the threshold the output of OPAMP 42 drops to zero and opens the switching element pair 44 and the switching element 48 but the output of inverting OPAMP 45 closes the switching element pair 47 and switching element 49 to connect the cluster 32 to the low power current line 34.

Suitable switching elements include piezoelectric relays, transistors (such as MOSFETs) or other devices that will be known to persons skilled in the relevant art. The switching circuit 36 and photodetector 38 may also be incorporated into an application specific integrated circuit (ASIC).

Although a photodetector 38 is the most convenient and simplest irradiance estimator it may not be the most appropriate in some circumstances. If the scale of shading is similar to the size of the photovoltaic cells a single photodetector may provide a poor estimate of the level of shading across the

285301 2.D0C cells. An alternate embodiment is shown in FIG 5 that uses the photovoltaic cells to estimate the level of illumination.

The level of illumination across the photovoltaic cells can be estimated from the photocurrent since photocurrent is proportional to irradiance. The photocurrent can be determined by adding the external current (which can be measured) and the recombination current (which can be estimated). For an explanation of the recombination current refer Green, M.A. (1982) Solar Ceils - Operating Principles, Technology, and System Applications, Prentice-Hall, NJ., pp 50 et seq.

Referring to FIG 5 there is shown an irradiance estimator 37 which in this embodiment is a current monitor circuit 50 to estimate the external current and the recombination current. The external current is estimated by connecting the output of the cluster 32 to OPAMP 51. OPAMP 51 monitors the current flowing through cell 32 by measuring the voltage drop across resistor RL. Resistors R1 , R2, R3 and R4 complete a sub-circuit 52 that provides an output that is a measure of the external circuit current. Persons skilled in the field will be readily able to ascertain suitable values for the resistive elements.

The recombination current is estimated in sub-circuit 53 which includes OPAMP 54 connected to the output of the cluster 32. The output from OPAMP 54 is connected through a diode resistor combination to OPAMP 55. The diode resistor combination is selected depending on the electrical characteristics of the solar cells being controlled. Resistors R5, R6, R7, R8, R9 and R10 complete the recombination current sub-circuit 53. Persons skilled in the field will be readily able to ascertain suitable values for the resistive elements and the diode.

The output from the external current sub-circuit 52 and the recombination current sub-circuit 53 are potentials relative to zero and are summed (with the ratio set by R11 and R12) to provide an output 56 which is connected to the OPAMP 42 and compared with the threshold 41.

The threshold 41 may be a variable to account for variations in cell parameters and shading effect. With respect to the first embodiment employing a

285301 2.D0C single photodetector 38 it was sufficient to set an optimum threshold since the estimate of the level of illumination was taken at a single point near the cluster. In the case of the second embodiment employing the current monitor circuit 50 the effect of shading is measured across the entire cluster and may be more dynamic. It is envisaged that in some applications it will be appropriate to set the threshold at a fixed value. In other applications it may be appropriate to dynamically adjust the threshold with the level of illumination.

Summing the power contributions from the low power circuit and the high power circuit can be conveniently performed by adding an additional input to a conventional peak power point tracker for the low current circuit, as shown in FIG 6. A typical input arrangement for a tracker comprises a switching element 61 , an inductor 62, and an output diode 63, for the low current circuit 34, while a second, high current input circuit 35 of a switching element 64 and diode 65 can be easily included as shown in FIG 6. The proviso is only that both switches are not turned on at the same time. Note that this is only one of several possible configurations for summing the contributions from multiple power lines.

The switches 61 and 64 are switched at some tens of kHz under the control of a microprocessor system (not shown) whose function is to track the available power from the module and by adjusting the times for which the switch 61 and switch 64 are turned on, match the input impedances of the tracker to the output impedance of each circuit (high and low current) from the module. Operation was simulated in software using representative circuit values. At the commencement of a switching cycle, when the inductor current is zero, switch 64 closes. The voltage on the high current circuit 35 causes current to build up in the inductor 62. When it has reached the appropriate level, switch 64 opens and switch 61 closes. The voltage on the low current circuit 34 is now applied to the inductor 62, causing a further increase in current. Switch 61 then opens, the voltage across the inductor 62 reverses, and the current through the inductor 62 drops to zero as it discharges through the diode 66 and the external circuit. The voltage change across the inductor 62 during two cycles described above is shown in FIG 7. The switch 64 is closed at 71 ; the switch 61 is closed at 72 and the inductor 62 discharges at 73. Diode 66 carries a strongly discontinuous

285301 2.DOC current, pulsing from many times average value to zero. Capacitor 67 stores the pulses of current flowing through diode 66 such that a reasonably constant output voltage is presented to the load.

There are a number of advantages with the arrangement of FIG 3 in arrays of photovoltaic cells likely to be subject to shading. As described above, considerable power loss over and above that caused by simple loss of sunlight is expected due to operation of many cells far from their peak power point, even to the point of driving them into reverse bias. The distribution of the levels of irradiance on cells which are subject to shading tends to be bimodal with a peak at full sun and a secondary peak at the sky dome level, typically 20-40% of full sun. Therefore the cell clusters may be beneficially allocated to one of two series circuits according to the irradiance they experience, thus minimising the additional circuitry required. Each circuit may be connected to a separate input on a peak power point tracker, where one input is optimized for High Current operation and the other for Low Current operation. The circuitry for the low current input need only be about 20% of the capacity of high current circuit, further saving costs. The settings on the two circuits of the tracker will not be independent. When the low current circuit is handling maximum power, the high current circuit will be on minimum, and vice versa. This results in simplification of the design of the tracker circuitry and algorithms.

The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. Accordingly, this invention is intended to embrace all alternatives, modifications and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above described invention.

285301 2.DOC

Claims

1. An array of photovoltaic cells comprising a plurality of clusters of serially connected photovoltaic cells wherein each cluster is dynamically switched to a power line depending upon the level of illumination.

2. The array of claim 1 further comprising an irradiance estimator associated with each cluster for determining the level of illumination.

3. The array of claim 2 wherein the irradiance estimator is a photodetector that measures the level of illumination at one point near each cluster of photovoltaic cells.

4. The array of claim 2 wherein the irradiance estimator is a circuit that estimates the level of illumination from current in each cluster of photovoltaic cells and the terminal voltage across each cluster of photovoltaic cells.

5. An array of photovoltaic cells comprising: one or more clusters of serially connected photovoltaic cells; an irradiance estimator associated with each cluster; and at least two power lines; . wherein each cluster is dynamically switched to a power line depending upon a level of illumination detected by the irradiance estimator.

6. The array of claim 5 wherein the power lines comprise at least a high power line and a low power line.

7. The array of claim 5 further comprising a switching circuit that switches the output from the cluster to a power line depending on the level of illumination detected by the irradiance estimator.

8. The array of claim 7 wherein the switching circuit employs voltage driven switching elements.

285301 2.DOC

9. The array of claim 5 wherein the irradiance estimator is a photodetector that measures the level of illumination at one point near each cluster of photovoltaic cells.

10. The array of claim 5 wherein the irradiance estimator is a circuit that estimates the level of illumination from current in each cluster of photovoltaic cells and the terminal voltage across each cluster of photovoltaic cells.

11. The array of claim 10 further comprising an external current sub-circuit and a recombination current sub-circuit.

12. The array of claim 5 wherein the photovoltaic cells are dye sensitised cells.

13. The array of claim 7 wherein the switching circuit includes a comparator that compares the level of illumination to a threshold.

14. The array of claim 13 wherein the threshold is variable

15. The array of claim 13 wherein the threshold is a value equivalent to a range from 60% to 80% of full sun illumination.

16. The array of claim 13 wherein the threshold is a value equivalent to about 75% of full sun illumination.

17. A method of operating an array of photovoltaic cells comprising a plurality of clusters of serially connected photovoltaic cells, by detecting a level of illumination at each cluster and dynamically switching the cluster to a power line depending upon the level of illumination.

18. The method of claim 17 wherein the level of illumination experienced by a cluster is determined by a photodetector located near the cluster.

19. The method of claim 17 wherein the level of illumination experienced by a cluster is determined by estimating current passing through the cluster and the terminal voltage across each cluster of photovoltaic cells.

285301 2.DOC

20. The method of claim 17 wherein the level of illumination is determined from a summation of an output of a recombination current sub-circuit and a external current sub-circuit.

21. The method of claim 17 wherein clusters in full illumination or near full illumination are switched to a high current power line and clusters below near full illumination are switched to a low current power line.

22. The method of claim 17 wherein the step of dynamically switching includes comparing the level of illumination to a threshold and switching to the high current power line when the threshold is exceeded otherwise switching to the low current power line.

23. The method of claim 22 wherein the threshold is fixed.

24. The method of claim 22 wherein the threshold varies with the level of illumination.

285301 2.DOC