Patent Application
20110184583
The subject matter disclosed herein relates generally to photovoltaic power generation systems, and more particularly to models to estimate the power output of photovoltaic power generation systems.
Solar energy conversion systems are used to covert solar energy into electrical energy. The solar energy conversion system may include photovoltaic modules. Photovoltaic modules include a group of photovoltaic cells, also referred to as solar cells that are connected in series or in parallel and are encapsulated in an environmentally protective laminate. The photovoltaic cells that are the basic building blocks of photovoltaic modules, convert solar radiation to electrical energy. Conversion of solar energy into electrical energy includes reception of light, such as sunlight, absorption of sunlight into the solar cell, generation and separation of positive and negative charges creating a voltage in the solar cell, and collection and transfer of electrical charges through a terminal coupled to the solar cell.
Individual photovoltaic modules may generate only a small amount of power, usually much less power than is required by most applications. Desired voltage and current for practical applications is realized by interconnecting a plurality of photovoltaic modules in series and/or in parallel. This matrix is generally referred to as a photovoltaic array, and can be used to generate electrical energy from solar radiation for a variety of applications.
Large-scale photovoltaic systems may generate 10 MW or more. These large-scale photovoltaic systems may include at least 50,000 photovoltaic modules. Power losses may occur within these photovoltaic systems due to a variety of reasons such as shading of the cells, string failure, or open fuses. The large number of photovoltaic modules within these systems makes determining the source of the power loss difficult. Monitoring each component of these systems may be costly. Thus, there is a need for a cost-effective way to identify power losses within these large-scale photovoltaic systems.
In a first embodiment, a power management system is provided. The power management system includes a control system configured to monitor the actual power output of one or more arrays comprised of photovoltaic modules. The power management system further includes a processor-based device configured to receive input signals and to calculate an estimated power output of the one or more arrays of photovoltaic modules and the respective photovoltaic modules based on the input signals, wherein the processor-based device compares the estimated power output for the one or more arrays of photovoltaic modules to the actual power output measurement received from the control system.
In a second embodiment, a method for monitoring power generation is provided. The method includes obtaining input signals related to photovoltaic power generation and calculating an estimated power output for each photovoltaic module of a plurality of photovoltaic modules based on the obtained input signals, wherein the act of calculating is performed on a processor-based system. The method further includes calculating an estimated power output for the plurality of photovoltaic modules based on the estimated power output for each photovoltaic module of the plurality of photovoltaic modules and on the electrical connections between the photovoltaic modules, wherein the act of calculating is performed on the processor-based system.
In a third embodiment, a method for generating a model to estimate power generation is provided. The method includes identifying one or more photovoltaic module types to be modeled, incorporating the one or more photovoltaic module types into a model, identifying interconnections of a plurality of the photovoltaic modules to be modeled, incorporating the interconnections of the plurality of photovoltaic modules into the model, identifying internal operating conditions of the respective photovoltaic modules to be modeled, incorporating the internal operating conditions of the respective photovoltaic modules into the model, identifying external operating conditions of the respective photovoltaic modules to be modeled, and incorporating the external operating conditions of the respective photovoltaic modules into the model. The method further includes using the model to estimate the power output for the respective photovoltaic modules based on the respective photovoltaic module type, the interconnections of the photovoltaic module with other photovoltaic modules, and on one or more internal operating conditions and the external operating conditions applying to the respective photovoltaic modules.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Any examples of operating parameters and/or environmental conditions are not exclusive of other parameters/conditions of the disclosed embodiments.
Turning now to the figures,
The control system 12 may include power conditioning and control equipment. The power conditioning equipment may include circuitry to follow the maximum power point of each photovoltaic array 20. The control system 12 may also include an inverter. In large-scale photovoltaic systems 10 each photovoltaic array 20 may have an inverter. The inverter converts the DC output of the photovoltaic array 20 to an AC output required by any load. The inverter may also include circuitry to follow the maximum power point of each photovoltaic array 20.
The electrical power generated by the photovoltaic arrays 20 is fed into a power distribution hub 16. The power distribution hub 16 may be or may connect to a utility grid. In one embodiment, the utility grid may feed power loads to the photovoltaic power generation system 10.
The actual obtainable power for a photovoltaic module 14 may be less than the theoretical obtainable power derived from the sum of all the cells within the module 14. Within each photovoltaic module 14 the actual obtainable power is dependent on numerous factors such as the types of cells within the module 14, the number of cells, and the type of interconnections between the cells. If all the cells are interconnected in series, the open circuit voltage of the module 14 is the sum of the voltages from the cells, and the short circuit current for the module 14 is equivalent to lowest cell short circuit current. If one cell operates at a lower current level, then all of the cells within the same string will operate at the lower current level and these cells would not reach the maximum power point. If all the cells are interconnected in parallel, the open circuit voltage of the module 14 is equivalent to the lowest cell open circuit voltage, and the short circuit current for the module 14 is the sum of the currents from the cells. If one cell operates at a lower voltage level, then all of the cells will operate below their maximum voltage point and these cells would also not reach the maximum power point. Due to mismatches between cells within the module 14 and overall series resistance the photovoltaic module 14 may function less efficiently than the cells within the module 14.
The output and efficiency of a module 14 may also be affected by climatic conditions such as the amount of incident irradiation or the ambient temperature, which may affect the operating temperature of the module 14. As the operating temperature increases the voltage of the module 14 decreases while the current slightly increases, thus the efficiency of the module 14 decreases as the temperature rises. The impact of rising temperatures on module efficiency varies and depends on the cell types and structures within a photovoltaic module 14.
Similarly, a photovoltaic array 20 may operate less efficiently than individual modules 14 within the array 20 due to similar factors. To maximize the output and efficiency of each photovoltaic array 20 the respective photovoltaic modules 14 should be well matched. The photovoltaic modules 14 may be interconnected in parallel, in series, or both to form the photovoltaic arrays 20. Interconnecting the photovoltaic modules 14 in parallel increases the current level. Interconnecting the photovoltaic modules 14 in series increases the voltage level. The type of interconnection between the photovoltaic modules 14 will depend on the current and voltage requirements of the load circuitry within the power distribution hub 16.
Other factors may affect the overall efficiency of the photovoltaic power generation system 10 or the individual arrays 20 such as shading, string failure, or open fuses. Shading on any part of a photovoltaic array 20 will reduce the output of the array 20. For example, a decrease in light intensity to a single photovoltaic module 14 due to shading will reduce the output of the module 14 and may also affect the output of the other modules 14 even if the other modules 14 are unshaded. The effect of shading on the array 20 will depend on the configuration of the electrical connections between the photovoltaic modules 14. For a string 18 of modules 14 connected in series, shading of a single photovoltaic module 14 within the string 18 will act as a load on the string 18 which may damage the string 18 due to heating caused by the current within the string 18. If the shading is severe enough to affect multiple modules 14, the open circuit voltage of the string 18 may be below the operating point of the array 20, and as a result, the string 18 may not contribute to the output of the array 20. Thus shading may disrupt the output of an array 20 in multiple ways: reduction in output from the shaded cells and modules 14, reduction in output from the unshaded modules 14 within a shaded string 18 due to the failure of the string 18 to maintain an operating voltage, and reduction in output from the rest of the array 20 due to all of the individual strings 18 of the array 20 not operating at maximum power.
String failure within an array 20 may occur for a number of reasons. For example, in a short circuit failure, a module 14 within a string 18 may be defective or an interconnection on the module 14 may be defective, thus the module 14 contributes no power to the string 18. The string 18 may then not be able to contribute full power to the photovoltaic array 20. In the case of an open circuit failure, the breaking of a wire connecting modules 14 within a string 18 may result in the complete loss of power from the string 18.
An open fuse within a string 18 may also lead to the string 18 not contributing power to the photovoltaic array 20. If the current running through a string 18 of modules 14 exceeds the maximum tolerable current level of the fuse, an open fuse results.
Determining the source of power loss in these large photovoltaic power generation systems 10 may be difficult. Large-scale photovoltaic power generation systems 10 may generate 10 MW or more. These large-scale photovoltaic systems may include 50,000 or more photovoltaic modules 14, multiple strings 18, and multiple arrays 20. Pinpointing the source and cause of specific power losses may prove costly and/or time consuming.
The computer 22 may execute one or more algorithms pertaining to the model 20 to estimate the power output of a photovoltaic system 10. One such algorithm that may be implemented using computer 22 consists of steady state equations to take the inputs 24 and generate an estimated power output 48 for a single photovoltaic module 14. The estimated power output 48 of a single photovoltaic module 14 represents a theoretical local maximum power point within a photovoltaic system 10 based on the inputs 24. Additionally, the computer 22 may implement one or more other algorithms to generate an estimated power output 48 for the photovoltaic power generation system 10. Such a system level algorithm may take into account the estimated power outputs 48 for each module 14 and the overall structure of the photovoltaic system 10, among other factors. The estimated power output 48 for a photovoltaic system 10 represents a theoretical global maximum power point based on the inputs 24.
The actual power output measurement 28 of the photovoltaic system 10 (received form the control system 12) may be compared to the estimated power output 48 of the photovoltaic system 10, such as by computer 22. If the actual power output measurement 28 and the estimated power output 48 differ for the photovoltaic system 10, the computer 22 implements one or more additional algorithms to adjust the inputs 24, such as the degree of shading 38 or the open string fuses 46 with respect to one or more modules 14 or groups of modules 14. Based on the adjusted inputs 24 the algorithm generates an adjusted estimated power output 50 for the photovoltaic system 10 to match the actual power output measurement 28 of the photovoltaic system 10. The algorithm may generate multiple scenarios or instances of adjusted inputs 24 that substantially match the adjusted estimated power output 50 to the actual power output measurement 28 of the photovoltaic system 10.
Based on the scenarios generated by the model 20 sources for the power losses within the photovoltaic system 10 may be identified. For example, the model 20 may be used to identify an open string fuse within the photovoltaic system 10 by varying different values of the inputs 24 until an adjusted estimated power output 50 is modeled that corresponds to the actual power output 28. The inputs 24 that are varied to achieve an adjusted estimated power output 50 that corresponds to the actual power output 28 may then be determined and used to identify a power generation problem in a module 14, array 20, or system 10. The sources of the power loss may be confirmed by making physical adjustments to a module 14, string 18, or array 20 within the photovoltaic system 10, then monitoring the control system 12 for an improvement in actual power output 28 by the photovoltaic system 10. In large-scale photovoltaic systems 10, the model 20 may save money and reduce maintenance time.
The model 20 includes other benefits besides identifying potential sources of power loss within the photovoltaic system 10. The model 20 may also be useful in defining what the global maximum power point should be for the photovoltaic system 10 under particular climatic conditions and/or in the presence of multiple different local maximum power points identified within the system 10. For example, using inputs 24 such as the position of the sun 34, the time of the year 36, the degree of shading 38, the irradiation level 40, and the temperature 42, the model 20 may be used to identify the global maximum power point for the photovoltaic system 10 and/or to identify various local maximum power points within the system 10. In addition, the model 20 may be used to identify a global maximum power point when multiple local maximum power points are present that might otherwise make identification of the global maximum power point computationally difficult.
The processor-based system 22, as mentioned above, may include or execute algorithms to calculate the estimated power 60 and 62 of a photovoltaic module 14 or a photovoltaic system 10 and to compare the actual power output measurement 68 of the photovoltaic system 10 to the estimated power output 62. In addition, the processor-based system 22 may include or execute an algorithm to adjust the input signals 54 until the estimated power output 62 matches the actual power output measurement 68 of the plurality of photovoltaic modules 14 or photovoltaic system 10.
Technical effects of the above embodiments include the determination of the global maximum power point of a photovoltaic power generation system 10 under different climatic and operating conditions using a model 20. The model 20, which may include one or more algorithms, may be implemented in a processor-based system 22 where the processor-based system 22 receives data from the power system 10. In addition, the model 20 helps identify potential sources of power loss within large-scale photovoltaic systems 10. Use of the model 20 may save money and reduce maintenance time, and subsequently increase the annual energy production of the photovoltaic system 10.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
