Photovoltaic System Power Tracking Method

Abstract

A photovoltaic system including a photovoltaic cell, and an electronic module connected to the photovoltaic cell. The electronic module is adapted to produce at least one control signal indicative of electrical power being generated by the photovoltaic cells. A tracking controller is adapted to receive the control signal(s) and based on the control signal(s), the controller is adapted to control a tracking motor for adjusting the system so that electrical power generated by the photovoltaic cells is increased. The photovoltaic system may include an optical element, adapted for concentrating solar light onto the photovoltaic cells. The electronic module preferably performs direct current (DC) to direct current (DC) power conversion and maximum power point tracking by electrical power, current, or voltage at either their inputs or their outputs. Alternatively, the tracking controller is configured to also perform maximum power point tracking by increasing to a local maximum electrical power by varying at least one of (i) current or voltage output from the photovoltaic cell or (ii) current or voltage output from the electronic module.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application benefits from US60/992589 filed by the present inventors filed on Dec. 5, 2007.

TECHNICAL FIELD

The present invention relates to photovoltaic systems and more specifically to solar tracking and other opto-mechanical adjustments particularly applicable for optimizing performance of concentrated photovoltaic systems.

DESCRIPTION OF RELATED ART

In a concentrated photovoltaic (CPV) system, high-efficiency cells are typically used, on which concentrated sunlight is directed in order to increase the power production of the system. Therefore, in CPV systems it is important to track the sun so as to maintaining the concentrating sunlight focused on the solar cells. Solar concentrators employ reflectors, lenses, or a combination thereof to concentrate incident light on one or more photovoltaic cells or light receiver. The resulting irradiance pattern projected onto the receiver is generally non-uniform. The intensity in the central region of the receiver is generally much greater than the intensity at the periphery. This intensity variation can significantly impact the collection efficiency where the optical receiver consists of photovoltaic cells. Some concentrators use a secondary reflector to homogenize the light from the primary reflector. Although the secondary reflector can improve the uniformity of the pattern of light on the receiver cells, some non-uniformity generally persists and the additional reflections in the secondary generally result in some loss of light intensity. Additional loss of incident light may result from tracking errors that occur while the concentrator attempts to locate the sun or re-locate the sun after it reemerges from behind cloud cover, for example.

In a concentrated photovoltaic (CPV) system, the photovoltaic cells may be much more efficient than typical large area photovoltaic panels. Double- or triple junction cells are often used, in which a number of p-n junctions are constructed one on top of the other. Each junction absorbs light preferentially from different spectral bands allowing the non-absorbed light to be transmitted to another p-n junction. Thus, these cells attain a higher overall efficiency (with peak efficiencies of over 30%) compared to photovoltaic panels of a single junction. The power output of CPV systems depends upon accuracy of the tracking mechanism, light absorption of different spectral bands emitted by the sun, spatial distribution of solar energy on the optical receiver and temperature of the photovoltaic cells which is influenced mainly by the solar energy conversion efficiency and the efficacy of the cooling system in dissipating heat from the photovoltaic cells of the optical receiver.

In a CPV, the solar energy delivered to the concentrated photovoltaic cell is typically between 20 and 75 W/cm2. The energy that is not converted to electricity must be dissipated to prevent excessive cell heating and to maximize efficiency. Therefore, solar cell cooling is an integral part of the CPV design. The solar cell efficiency is a function of cell operating temperature and lower temperatures result in higher efficiencies. The solar cell must be kept below the melting point of the die and interconnect attach materials that are used to manufacture the multi-junction cell receiver package to prevent immediate cell failure. The reliability of the receiver is a function of the number of thermal cycles and the magnitude of the thermal excursion. Some experts claim that reliability or life expectancy is doubled for every ten-degree reduction in thermal excursion.

US20080143188 of the present inventors describes a system and method for combining power from DC power sources. Each power source is coupled to a converter. Each converter converts input power to output power by monitoring and maintaining the input power at a maximum power point. Substantially all input power is converted to the output power. The control is performed by allowing output voltage of the converter to vary. The converters are coupled in series. The series current and the output power of the converters, determine the output voltage at each converter.

BRIEF SUMMARY

The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

According to the present invention there is provided a photovoltaic system including a photovoltaic cell, and an electronic module connected respectively to the photovoltaic cell. The electronic module is adapted to produce at least one control signal indicative of electrical power being generated by the photovoltaic cells. A tracking controller is adapted to receive the control signal(s) and based on the control signal(s), the controller is adapted to control a tracking motor for adjusting the system so that electrical power generated by the photovoltaic cells is increased. The photovoltaic system may include an optical element, e.g. reflector, Fresnel lens) adapted for concentrating solar light onto the photovoltaic cells. The electronic module preferably performs direct current (DC) to direct current (DC) power conversion and maximum power point tracking by electrical power, current, or voltage at either their inputs or their outputs. Alternatively, the tracking controller is configured to also perform maximum power point tracking by increasing to a local maximum electrical power by varying at least one of (i) current or voltage output from the photovoltaic cell or (ii) current or voltage output from the electronic module. Typically, current from the photovoltaic cells varies due to factors other than variations of the solar light intensity. The factors include spatial distribution of the solar intensity over the photovoltaic cells, variations of temperature of the photovoltaic cells, and variations of electrical impedance of the electrical load receiving the electrical power.

According to the present invention there is provided a method for controlling a photovoltaic system including a photovoltaic cell and an electronic module connected to the photovoltaic cell. One or more control signals are produced indicative of electrical power being generated by the photovoltaic cell. The control signal(s) are received and based on the control signals, a tracking motor for adjusting the system is adjusted. Electrical power generated by the photovoltaic cell is thus increased and preferably maximized. Maximum power point power tracking may also be performed by maximizing electrical power individually from said photovoltaic cells at either respective the inputs or outputs of the electronic modules. The maximum power point tracking may be performed independently from the control of the tracking motor for adjusting the system by performing the maximum power point tracking and the opto-mechanical adjustments in either different time domains or different frequency domains.

In one embodiment, the MPPT control over the output voltage and current of the cells, and the tracker control, could be done by one integrated control unit. The adjustments preferably include orienting an optical component of the system in at least one of two angles toward the Sun. The opto-mechanical adjustments may include varying a distance of at least one optical component within the system such as a focus distance or a distance of an optical diffuser in the system. The control of the opto-mechanical adjustment may be performed based on a prior selection of one or more of the photovoltaic cells/signals. When there are multiple photovoltaic cells and electronic modules connected thereto, different photovoltaic cells with different control signals may be reselected periodically to maximize solar power on the different photovoltaic cell to increase reliability of the system. Alternatively, the control system may maximize the aggregate output power of all PV cells rather than focusing on some of them.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a system diagram showing the components of a solar tracking system, according to an embodiment of the present invention;

FIG. 1A is a system diagram showing the components of an alternative embodiment of the present invention using a single photovoltaic cell and a single controller for both optomechanical adjustments and MPPT tracking;

FIG. 2. is a system diagram showing the components of a solar tracking photovoltaic system, according to an embodiment of the present invention;

FIG. 3 is a system diagram showing the components of a solar tracking photovoltaic system, according to another embodiment of the present invention;

FIG. 4 is a plan view of the power module, according to embodiments of the present invention;

FIG. 5 is a flow diagram of a method for adjusting a photovoltaic system, based on electrical power generated, according to an embodiment of the present invention; and

FIG. 5A is flow diagram of a method for adjusting solar tracking photovoltaic system, according to an embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.

Implementation of the method and system of embodiments of the present invention involves performing or completing selected tasks or steps manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of preferred embodiments of the method and system of the present invention, several selected steps could be implemented by hardware or by software on any operating system of any firmware or a combination thereof. For example, as hardware, selected steps of the invention could be implemented as discrete integrated circuit, a system on chip an ASIC, or other circuit. Similar, control functions as described herein may be performed either using analog or digital circuitry or a combination thereof.

By way of introduction, according to aspects of the present invention, photovoltaic cells of concentrated photovoltaic systems are connected individually to electronic modules.

The term “electronic module” as used herein includes an electronic circuit which transfers power from a power generating device, e.g. photovoltaic cell and outputs at least one signal or monitor of the power generation and/or transfer. Such signals include a current (input to or output from) the electronic module, a voltage (input to or output from) the electronic module, electrical power, or another signal such as temperature of the photovoltaic cell.

Optionally, the electronic modules attached to the individual cells may include switching DC-DC power converters which perform maximum power point tracking (MPPT). The maximum power point (MPP) of photovoltaic cells varies with incident illumination and temperature of the cell. A maximum power point tracking (MPPT) system tracks the instantaneous power by continually sensing the voltage and current (and hence, power transfer), and uses this information to dynamically adjust the load so that maximum power is always transferred, regardless of the variation in solar energy effectively absorbed in the cell and cell temperature. Maximum power point tracking may be performed by maintaining a maximum electrical power at the outputs of the electronic modules. Alternatively, according to the teachings of US20080143188 maximum power point tracking is performed individually for each of the cells by maximizing the power output from the cells (power input to the electronic modules). Alternative functions of the electronic modules are DC/AC inverters (sometimes referred to as AC Modules), power monitoring equipment, and so forth.

It should be noted that although the embodiments shown herein mainly include concentrated photovoltaic systems, the present invention in other embodiments may be applied as well to photovoltaic panels which do not have concentrating optics.

The term “photovoltaic cell” includes a single PN junction, and may also include multiple junctions or cells connected in series (for increased voltage) and/or in parallel.

Reference is now made to FIG. 1 which illustrates a system diagram showing the components of a CPV power tracking system 100 according to an embodiment of the present invention. Direct light flux 18 from the sun is reflected from a reflector 16 (e.g. a paraboloid of revolution, a segment of a cylinder, and so forth) which places a focused or intensified reflected light flux 18a onto an optical surface of a CPV receiver 14. CPV receiver 14 is typically fixed relative to reflector 16. CPV cells 40 on the optical surface of receiver 14 absorb reflected light flux 18a to produce electrical power. A signal 19 proportional to electrical power produced by receiver 14 is sent to a tracking controller 12. Tracking controller 12 sends an orientation signal 17 to tracking servomotor 10, which may be a single-axis or dual-axis tracking system. The optimum orientation of reflector 16 is determined by tracking servomotor 10, receiver 14 and tracking controller 12 by forming a closed loop control system. Tracking servomotor 10 angularly orients parabolic reflector 16 so as to obtain a maximum of electrical power based on power signal 19.

Reference is now made to FIG. 1A a simplified system diagram showing the components of an alternative embodiment of the present invention using single photovoltaic cell 40 and single controller 12 adapted for both optomechanical tracking and maximum power point MPPT tracking. Controller 12 provides typically PWM signals 15 to a power converter, e.g. switched power converter 30. Control signal 19, which is preferably a single control signal indicative of electrical power generated in single CPV cell 40, or multiple control signals 19 one signal indicating current and another signal indicating voltage input or output to power converter 30. Control signals 19 are generated by a current sensor 11 and a voltage sensor 13 typically integrated into power converter 30. Control signals 19 are typically fed into controller 12 as analog signals and converted to digital signals in an analog/digital converter integrated within controller 12 or digital signals are input to controller 12 and the analog/digital conversion is performed externally by an A/D converter (not shown in FIG. 1A). Controller 12 also provides control signal 17, to tracking servomotor 10 for performing optomechanical adjustments which maximize electrical power generated by CPV cell 40.

Control by controller 12 of both optomechanical tracking and electrical MPPT tracking may be simultaneous and in parallel.

Reference is now made to FIG. 2 which is a simplified system diagram showing the components of a CPV receiver 14 according to an embodiment of the present invention. CPV cells 40/1, 40/2, 40/3 . . . 40/n are on the optical surface of receiver 14 (as shown in FIG. 1). The electrical power outputs of CPV cells 40/1, 40/2, 40/3 . . . 40/n, are fed into the inputs of electronic module 31/1, 31/2, 3113 . . . 311n respectively. The power outputs 32 of each electronic module 31/1, 31/2, 31/3 . . . 31/n are optionally combined into an electrical power combiner 34. Electronic modules 31 optionally include DC-DC switching converters which maximize power at their outputs. Power combiner 34 combines power inputs 32 (power outputs of electronic modules 31) to provide a combined power output to load 36. A power signal 19 proportional to power output 32 from one or more of electronic modules 31/1, 31/2, 31/3 . . . 31/n is a measure or indication of the electrical power being supplied by CPV cells 40/1, 40/2, 40/3 . . . 401n to load 36. Power signal 19 is typically produced by sensing current and voltage output of one or more cells 40/n and/or electrical power outputs 32 of electronic modules 31/n. In an embodiment of the present invention the sensing of current (I) and voltage (V) to produce power signal 19 from each electronic module 31/1, 31/2, 31/3 . . . 31/n is derived using the teachings of co-pending application entitled: “Current Sensing on a MOSFET”. Alternatively, a current/voltage sensing chip is included in electronic modules 31 (for example LTC4151, High Voltage I²C Current and Voltage Monitor, Linear Technology Corporation, Milpitas, Calif., USA). Electrical power signal 19 is supplied to an input of tracking controller 12.

Reference is now made to FIG. 3 which is a system diagram showing the components of a CPV receiver 14 according to another embodiment of the present invention. CPV cells 40/1, 40/2, 40/3 . . . 401n are on the optical surface of receiver 14 (as shown in FIG. 1). The electrical power output of CPV cells 40/1, 40/2, 40/3 . . . 401n, are fed into the inputs of power converter 30/1, 30/2, 30/3 . . . 301n respectively. A feedback loop 33 is provided for each power converter 30 such that the electrical power input to power converters 30/1, 30/2, 30/3 . . . 30/n is maximized according to the teachings of US20080143188. A typical example for power converter 30 is a DC to DC converter which has maximum power point tracking (MPPT) at its input. Note that also other forms of converters are relevant for this application, AC modules and micro-inverters for example. The embodiment of FIG. 3 also includes power outputs 32 connected to load 36 although these are not shown explicitly in the simplified block diagram. The power signals 19/1 . . . 19/n output by each power converter 30 is an indication of the electrical power being supplied by each CPV cell 40 to load 36. Power signals 19/n are input to tracking controller 12A which uses one or more of control signals 19/n for orienting CPV system 100 toward the Sun or otherwise opto-mechanical adjusting CPV system 100.

Reference is now made to FIG. 5, a flow diagram of an adjustment 50 of photovoltaic system 14, according to an embodiment of the present invention. In process 50, a control signal is produced indicative of electrical power being generated by photovoltaic system 14. The control signal is received (step 522). Photovoltaic system is adjusted (step 523), e.g. oriented angularly toward the Sun. In decision box 524, the controller applies a criterion to determine if a local maximum of electrical power generation has been reached. Such criteria are well known in the art of control engineering

Reference is now made to FIG. 5A according to an embodiment of the present invention. FIG. 5 shows a flow chart of a method for implementing an algorithm for adjusting system 100, e.g. angular orientation toward the Sun. In FIG. 5, tracking the sun begins with initializing (step 500) CPV power tracking system 100. Initialization (step 500) of power tracking system 100 occurs for example at the beginning of the day or otherwise when power tracking system 100 needs to be reset. Initialization (step 500) may include initially orienting system 100 toward the Sun based on global coordinates, time and stored information regarding the angular coordinates of the Sun. Initialization (step 500) may further include scanning with relatively large angular steps and then initially orienting system 100 based on a maximum electrical power output of a default selection, e.g. all, of photovoltaic cells 40. Photovoltaic cells 40 or corresponding control signals 19/n are optionally selected (step 502) for further use in the angular tracking algorithm. Reference is now also made to FIG. 4, a plan view of showing the CPV cells 40 arranged in a square matrix (5×5=25 CPV cells), arranged on the optical surface of receiver 14 by way of example. Four CPV cells 40A, 40B, 40C and 40D are located on the outer corners of receiver 14 and have been selected (step 502) by way of example only. Referring back to FIG. 5, after cells 40 are selected (step 502), system 100 is adjusted (step 50) in order to maximize electrical power as indicated by control signals 19/n of selected cells 40 for maximizing power by orienting system 100 in the direction of the Sun. In some embodiments of the present invention, adjustments other than angular orientation toward the Sun may also be performed such as slightly defocusing the optical system or adjusting a diffuser which may tend to distribute more evenly solar energy over more cells 40 and thereby increase system reliability without significantly reducing electrical power produced. After system 100 is adjusted (step 50), a time delay (step 506) optionally follows during which system performance results are optionally logged (step 508). If tracking is successful (decision box 510) then new cells 40/n (or control signals 19/n) are optionally selected (step 502) Changing selection (step 502) periodically tends to vary cells receiving peak solar energy and thereby tends to improve overall reliability of system 100. Otherwise, if tracking is not successful, e.g. because of cloud cover, then system 100 is optionally reoriented initially (step 500).

The articles “a”, “an”, as used hereinafter are intended to mean and be equivalent to “one or more” or “at least one”, For instance, “a photovoltaic cell” means “one or more photovoltaic cells”, and “an electronic module” means “one or more electronic modules”, and “a control signal” means “at least one control signal”.

The term “maximum” and “local maximum” are used herein interchangeably and refers to the largest amount of a desired quantity (e.g. electrical power) typically attainable with use of a control circuit.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

1-15. (canceled)

16. An apparatus comprising:

one or more photovoltaic cells configured to produce direct-current power;

a light diffuser; and

control circuitry configured to change a distance between the light diffuser and the one or more photovoltaic cells.

17. The apparatus of claim 16, further comprising a power converter configured to convert the direct-current power produced by the one or more photovoltaic cells.

18. The apparatus of claim 17, wherein the power converter is configured to perform direct-current to direct-current power conversion.

19. The apparatus of claim 16, wherein the control circuitry is configured to:

measure the direct-current power produced by the one or more photovoltaic cells; and

increase, based on the measure of the direct-current power, the direct-current power to a local maximum by varying at least one of current and voltage output from the one or more photovoltaic cells.

20. The apparatus of claim 16, wherein the control circuitry is configured to perform maximum power point tracking of the direct-current power produced by the one or more photovoltaic cells.

21. The apparatus of claim 20, wherein the control circuitry is configured to perform the maximum power point tracking independent from the change in the distance between the light diffuser and the one or more photovoltaic cells.

22. The apparatus of claim 16, further comprising an optical element configured to concentrate intensity of light onto the one or more photovoltaic cells.

23. The apparatus of claim 22, wherein the control circuitry is configured to increase evenness of the light distributed over the one or more photovoltaic cells by defocusing the optical element.

24. The apparatus of claim 22, wherein the control circuitry is configured to increase a number of the one or more photovoltaic cells over which the light is distributed by defocusing the optical element.

25. An apparatus comprising:

a plurality of photovoltaic cells configured to generate direct-current power;

an optical element configured to focus light onto the plurality of photovoltaic cells; and

control circuitry configured to increase a number of the plurality of photovoltaic cells over which the light is distributed by defocusing the optical element.

26. The apparatus of claim 25, wherein the control circuitry is configured to defocus the optical element by causing a distance between the optical element and the plurality of photovoltaic cells to change.

27. The apparatus of claim 25, wherein the control circuitry is configured to control the defocusing of the optical element such that electrical power produced by a selected subset of the plurality of photovoltaic cells is maximized.

28. The apparatus of claim 25, further comprising a motor configured to adjust a position of the optical element, wherein the control circuitry is configured to defocus the optical element by controlling the motor.

29. The apparatus of claim 25, wherein the control circuitry is configured to perform maximum power point tracking individually on each of the plurality of photovoltaic cells.

30. A method comprising:

combining electrical power produced by a plurality of photovoltaic cells arranged on an optical surface;

adjusting a position of the optical surface based on maximizing electrical power produced by a subset of the plurality of photovoltaic cells independently from power produced by all of the plurality of photovoltaic cells, wherein the subset of the plurality of photovoltaic cells is periodically changed to a different subset.

31. The method of claim 30, wherein the adjusting of the position of the optical surface comprises:

changing an angular orientation of the optical surface towards a sun.

32. The method of claim 30, wherein the adjusting of the position of the optical surface comprises:

adjusting a distance of the optical surface from an optical element that focuses light onto the plurality of photovoltaic cells.

33. The method of claim 30, wherein the adjusting of the position of the optical surface comprises:

defocusing light from an optical element onto the optical surface such that the electrical power produced by a subset of the plurality of photovoltaic cells is maximized.

34. The method of claim 30, wherein the adjusting of the position of the optical surface comprises:

adjusting a position of the optical surface with respect to a light diffuser that diffuses light onto the optical surface.

35. The method of claim 30, comprising:

performing maximum power point tracking of the plurality of photovoltaic cells by varying at least one of current and voltage output from the plurality of photovoltaic cells.