Patent Application
20090078300
The present method and system relates to a solar photovoltaic generation system and more particularly relates to the control and management of electrical energy generated by solar photovoltaic generation system.
Solar arrays or panels generate electric power by converting solar energy into electrical energy. The power output of a solar array varies, among other factors, with the light intensity, the degree of insolation, the array voltage, and the array temperature.
A solar array consists of a collection of photovoltaic solar cells, and the array voltage of the solar array is determined by the number of photovoltaic solar cells connected in series and the cell voltage of each photovoltaic solar cell.
Solar systems equipped with maximum power point tracking (MPPT) capability track the output current-voltage and regulate the impedance at the terminals to extract maximum output power from the solar array. MPPT is particularly effective during cold weather, on cloudy or hazy days, or when the battery is deeply discharged. MPPT allows for driving a load at its maximum power by dynamically adjusting the impedance of the load to the operating condition of the solar array. For example, when an MPPT-capable solar system drives an electric motor directly from the solar array, the solar system can adjust the current draw of the solar array by varying the motor's speed so that the motor runs at its maximum power.
Solar cells producing lower cell voltage are serially connected in a string to produce a higher output voltage. The output voltage of a solar cell string consisting of multiple solar cells is the sum of the cell voltages of the individual solar cells, but the output current of the solar cell string is limited by the current of the least productive solar cell in the string.
Shading or partial illumination changes the output current-voltage characteristics of a solar array. The impedance of a shaded solar cell increases to the point where it generates little or no power. When a solar panel contains multiple solar cell strings connected in series including a shaded area, the high impedance of the shaded solar cells causes power dissipation instead of power generation, thus decreasing the output power of the entire solar panel even though the remaining solar cells continue to generate power. In such a case, a bypass diode is connected to the shaded solar cell in parallel so that the power dissipation caused by the shaded solar cell is minimized. The bypass diode reduces the voltage loss caused by the shaded solar cell, thus the local heating due to the power dissipation by the shaded solar cell is diminished. The current flowing through and the forward bias voltage of the bypass diode may still contribute to the power loss of the solar cell string, but the power loss by the bypass diode is significantly lower than the power loss caused by the shaded solar cell.
In order to efficiently bypass shaded solar cells and to minimize power loss caused by shading, bypass diodes are placed in parallel with each solar cell in the solar array. However, the parallel configuration of a bypass diode with each solar cell not only increases the total cost of the system, but also decreases the output power of each solar cell due to the forward bias voltage of the bypass diode. Therefore, the benefits of adding bypass diodes need to be well balanced with the power loss introduced by the bypass diodes.
Conventional MPPT systems run MPPT software algorithms using a microcontroller, a microprocessor, or a digital signal processor such that power draw from the attached solar array is continuously monitored and adjusted. One of drawbacks of such centrally controlled MPPT systems is that they may not well adapt to locally varying operating conditions, particularly when the system has a number of solar cells covering a wide area. For example, such MPPT systems may enter into a low-power mode even when the solar array is partially shaded. In such a case, substantially lower power is drawn from the solar array than the maximum power that the array is capable of generating.
From the foregoing, there is a need for a simple and efficient maximum power point tracking solar converter under varying operating conditions that uses cost-effective analog and digital, or mixed-signal circuit components in conjunction with a small number of solar cells in a group.
The present system and method provides a maximum power point tracking converter for use with a solar cell group in a distributed manner within a solar panel. According to one embodiment, one or more solar cells within a solar panel are grouped and coupled to a distributed converter that extracts maximum power from the coupled solar cell group.
The above and other preferred features described herein, including various novel details of implementation and combination of elements, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular methods and circuits embodying the invention are shown by way of illustration only and not as limitations of the invention. As will be understood by those skilled in the art, the principles and features of the teachings herein may be employed in various and numerous embodiments without departing from the scope of the claims.
The accompanying drawings, which are included as part of the present specification, illustrate the presently preferred embodiment of the present invention and together with the general description given above and the detailed description of the preferred embodiment given below serve to explain and teach the principles of the present invention.
It should be noted that the figures are not necessarily drawn to scale and that elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the figures. It also should be noted that the figures are only intended to facilitate the description of the various embodiments described herein. The figures do not describe every aspect of the teachings disclosed herein and do not limit the scope of the claims.
The present system and method provides maximum power point tracking (MPPT) for use with a solar cell group in a solar array. According to one embodiment, the distributed maximum power point tracking converter comprises a power sensing block for measuring a power signal associated with the power drawn from each solar cell group and a duty cycle adjust block for measuring and adjusting duty cycle for each solar cell group. A power comparator compares the power signal with previously measured power signal and generates a first logical signal. A duty cycle comparator compares the duty cycle with previously measured duty cycle and generates a second logical signal. A logic comparator provides a control signal for the duty cycle adjust block using the first logical signal from the power comparator and the second logical signal from the duty cycle comparator. The distributed maximum power point tracking converter, in integrated or discrete form, may be embedded within or outside of the solar panels and coupled to a central electrical bus to charge storage batteries or to deliver electrical energy to electrical loads such as an inverter tied to a power utility grid.
According to one embodiment, a maximum-power peak detection control is added to a switching converter that extract the maximum power from a single or a plurality of solar cells based on comparison of the present value of the current or voltage to the previous value of the current or voltage. If the present value of the current or voltage is larger than the previous value of the current of voltage, the duty cycle of the switching converter is adjusted such that it will provide a maximum power to any output load. The previous value of the current or voltage is stored or held in a resistor-capacitor storage circuit. According to one embodiment, the maximum power point tracking is implemented using analog and digital, or mixed-signal circuits without a need for a microcontroller, a microprocessor, or a digital signal processor.
In the following description, for purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that these specific details are not required in order to practice the present invention.
Each of the additional features and teachings disclosed below may be utilized separately or in conjunction with other features and teachings to provide distributed MPPT systems and methods for designing and using the same. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in combination, will now be described in further detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Therefore, combinations of features and steps disclosed in the following detail description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the present teachings.
Moreover, the various features of the representative examples and the dependent claims may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings. In addition, it is expressly noted that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter independent of the compositions of the features in the embodiments and/or the claims. It is also expressly noted that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure, as well as for the purpose of restricting the claimed subject matter.
It is expressly noted that the component values shown in the drawings are merely representative and may be changed as required to optimize the performance. It is expressly noted that the schematics themselves may be subject to variation as required by operational requirements.
According to one embodiment, controller 204 contains enhancements over the prior arts. For example, maximum power peak is detected for a single solar cell or each group of solar cells using analog/digital circuitries without a microcontroller, a microprocessor, or a digital signal processor, thus the MPPT capability is distributed to provide an improved efficiency.
According to one embodiment, logic comparator 252 receives signals from power comparator 251 as well as duty cycle comparator 254 and generates an output to duty cycle adjust block 253, which adjusts the duty cycle of converter 202. Controller 204 contains duty cycle limit block 255, which is an over-voltage protection circuit that limits the upper bound of duty cycle using duty cycle limit block 255. Controller 204 may be integrated into converter 202 or implemented as a separate controller coupled to converter 202.
According to another embodiment, output voltage 313 is obtained by sensing the output voltage of load 203. Alternatively, the combination of output current and output voltage of 203 might be used to obtain a power signal that represents the power extracted from solar array 201. Power comparator 251 continuously samples output current and/or output voltage and provides the power signal to logic comparator 252. According to one embodiment, power comparator 251 generates the power signal by comparing the present power signal with the rolling average of past sampled power signals.
According to one embodiment, power comparator 251 and duty cycle comparator 254 are analog comparators making use of resistor-capacitor network to retain previous signals at their inverting inputs. This configuration of analog resistor-capacitor network has greater cost and power advantages over software comparator algorithms implemented in a microcontroller, a microprocessor or a digital signal processor, and can be readily implemented in an integrated circuit form.
When controller 204 turns on, the voltage at duty cycle capacitor 261 is charged to an initial voltage level determined by the voltage divider 262. This initial value at duty cycle capacitor 261 is fed through a voltage follower 263 to serve as PWM CTRL signal 269, which is an input to duty cycle adjust block 253. Converter 202 draws power as determined by the duty cycle.
As the output power of converter 202 increases, current sensing circuit 303 senses the increase in load current delivered to load 203 and generate output voltage 313. Using difference amplifier 271, power comparator 251 compares the current value of output voltage 313, which is proportional to load current with the previously held output voltage 313 at its inverting input. The output of power comparator 251 is governed by output voltage 313 in comparison to its previous value; when the current value of output voltage 313 is greater than the previously held value, the output of power generator 251 is high, otherwise the output of power generator 251 is low.
The duty cycle of converter 202 is regulated to produce the maximum value of load current to achieve the maximum power point of operation of solar array 201. As the duty cycle increases, the power output from converter 202 increases, which extracts more power from solar array 201. Because of the current-voltage characteristics of solar cells as shown in
When both inputs to logic comparator 252 that are outputs of power comparator 251 and duty cycle comparator 254, are both high, the output of logic comparator 252 is high and causes amplifier 264 to generate a predetermined voltage output to further charge duty cycle capacitor 261. As such, the duty cycle of converter 202 increases, thus more power is drawn from solar array 201. Table 1 illustrates how the duty cycle is adjusted at logic comparator 252 based on logical inputs from power comparator 251 and duty cycle comparator 254.
As the duty cycle of converter 202 increases, thus more power is extracted, the output voltage from solar array 201 decreases due to the voltage-current characteristics of solar cells as shown in
According to one embodiment, duty cycle comparator 254 detects the change of the duty cycle by sensing the output of voltage follower 263 and changes its output accordingly. The output voltage of voltage follower 263 is used to generate PWM_CTRL signal 269. When both inputs to logic comparator 252 are low, the output of logic comparator 252 becomes high, thus the duty cycle capacitor 261 is charged. The measurements of both duty cycle and power draw at load 203 ensures that maximum power is extracted by solar array 201.
According to one embodiment, over-voltage protection is incorporated to prevent the output voltage of converter 262 from going over a threshold value. This maximum output voltage is set by voltage divider 266. When the sensed voltage 270 of load 203 is greater than the maximum voltage level set by voltage divider 266, over-voltage comparator 265 outputs low voltage, discharges duty cycle capacitor 261 and shuts down converter 202. The output voltage 313 of converter 202 drops until it goes below the set voltage by voltage divider 266. When the output voltage 313 drops below the set voltage, the output of over-voltage comparator 265 changes to high. The duty cycle capacitor 261 is charged through diode 267, and it restarts the MPPT process back to the normal operation.
Duty cycle model described herein provides current/voltage tracking with respect to the maximum power point. During a sampling period, duty cycle is increased or decreased using an observed signal so that the operating point is maintained at or near the maximum power point. In a preferred embodiment, the observed signal is output voltage signal 313 whose square is proportional to the output power. For a given configuration (e.g., the number of solar cells in the string or group in the solar panel 201) and operating condition (e.g., the degree of insolation and array temperature), the duty cycle of converter 202 is adjusted to achieve the maximum load current by maximum power tracking control signal 313. If the storage battery 203 is fully charged, constant voltage control signal 814 is used instead to provide a constant output voltage to trickle charge storage battery 203 such that storage battery 203 is maintained at full charge.
According to one embodiment, voltage control signal 614 is used together with PWM_CTRL signal 269 of MPPT controller 204 to generate PWM signal 314. In this case, constant voltage control circuit 601 replaces the test block 276 of
According to one embodiment, the circuit elements used in converter 202 and/or controller 203 are implemented with analog and digital, or mixed-signal components for minimal-delay MPPT control. The use of analog and digital or mixed-signal circuit components in the MPPT system is advantageous over microcontrollers, microprocessors, or digital signal processors for their lower cost and simplicity. Analog/digital circuit components also provide quick responses to the change in operating conditions such as insolation angle and array temperature that effect the operating point of solar array 201.
According to one embodiment, bypass diodes are integrated with a predetermined number of solar cells. The number of solar cells forming a group is determined based on various design factors such as the output voltage of the group, the size of solar cells, other electronics connected thereto. The duty cycle of converter 202 is adjusted to provide maximum load current by maximum power tracking control signal 313. When maximum power tracking control signal 313 fails to produce the maximum load current, constant voltage control signal 614 is used instead, and converter 202 stops charging storage batteries 203. The efficiency of the distributed solar panel is enhanced during shading over the conventional long-string approach (e.g., 18 solar cells in a string) because a smaller number of solar cells are grouped as compared to the long-string approach, and only the solar cells in the group containing a shaded solar cell are affected in the distributed approach.
When a solar cell 211 of solar array 201 is damaged or non-operational for whatever reasons, the string that contains the damaged or non-operational solar cell 211 is excluded from generating power to load 203 by the reverse biasing of diode 322 of converter 202. The rest of solar cells is still operational, even though the output power from solar array 201 might be slightly decreased due to the excluded string.
For a conventional solar array, a number of solar cells are coupled in a string or group. For example, three crystalline solar cells having an open circuit voltage of approximately 0.55 V are grouped to operate at 1.65 V. The number of solar cells grouped in an array (or a group) depends on the bias voltage of each solar cell which varies with the material used to construct the solar cells. According to one embodiment, the number of solar cells in a string is smaller than the typical number of solar cells in a string in conventional solar arrays. Therefore, the power reduction due to a damaged or non-operational solar cell of solar array 201 is minimized as compared to conventional solar arrays.
According to one embodiment, control loops may be added to perform additional functions. For example, a constant-current control loop or a constant-voltage control loop may be incorporated to draw constant current or constant voltage from solar array 201. Any or all of the these control loops may be incorporated into controller 204 and called upon to function and control the converter as determined by operating conditions.
When the output voltage from a solar cell group falls below a threshold voltage to operate the associated distributed MPPT converter, the solar cell group is isolated from other solar cell groups that produce sufficient output voltage. For example, the output voltage of solar cell group 201c may fall below the threshold voltage to generate any power by the shading effect or damaged solar cells contained therein. Because shaded or damaged solar cells present large impedance to the associated solar cell group, often drawing power rather than generating power, solar cell group 201c containing the shaded or damaged solar cells is automatically disabled by the reverse-biasing diode of distributed converter 202c. The rest of the distributed MPPT converters 202a, 202b, 202d, 202e, and 202f are still generating power to common charge bus 803.
According to one embodiment, multiple solar cell groups 201 are grouped to form solar array 801 to provide higher output power. Each solar cell group 201 having a dedicated distributed MPPT controller 202 may be connected directly to charge bus 802 without having a conventional maximum power point tracking converter for the entire solar array. Although
A method and system for providing a maximum power point tracking converter for use with one or more solar cells in a string or group in a distributed manner within a solar photovoltaic array is disclosed. Although various embodiments have been described with respect to specific examples and subsystems, it will be apparent to those of ordinary skill in the art that the concepts disclosed herein are not limited to these specific examples or subsystems but extends to other embodiments as well. Included within the scope of these concepts are all of these other embodiments as specified in the claims that follow.
The present application claims the benefit of and priority to U.S.Provisional Patent Application No. 60/971,421 filed on Sep. 11, 2007, entitled “A Distributed Maximum Power Point Tracker and Converter.” U.S. Provisional Patent Application 60/971,421 is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60971421 | Sep 2007 | US |
