Stacked Controlled-Cell Power Conversion Architecture for Grid-Connected Photovoltaic Systems

Technology #14303

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FIG. 1 shows a schematic drawing of a conventional photovoltaic (PV) system 10. The system 100 includes a solar array 1, also commonly referred to as a “PV array.” The PV array 1 typically includes multiple PV modules electrically connected together in series. Each PV module in turn typically includes multiple PV cells (also electrically connected together in series). The DC output voltage 2 of the solar array 1 is controlled by a maximum power point tracking (MPPT) apparatus 3 to obtain optimum power extraction from the solar array 1. The maximum power point (MPP) of the array 1 is the operating point of output current and voltage at which the array produces the highest amount of power, and this point changes with temperature and irradiation; accordingly, the MPPT apparatus 3 dynamically adjusts the operating point of the solar array 1 to track these changes. The DC output voltage 4 of the MPPT typically is then fed to an inverter 5, which provides an AC voltage to the power grid 6. In some conventional implementations, the function of the MPPT apparatus 3 is integrated into the inverter 5. However, in other implementations an MPPT apparatus 3 may be implemented separately from the inverter 5.FIG. 2 shows an exemplary conventional PV module 31, which may include from thirty-six to seventy-two series-connected PV cells 32. The right side of FIG. 2 shows the respective circuit symbols for a PV module 31 and a PV cell 32. Because the cells 32 are all connected in series, the module output current is limited by the weakest cells. The output current of each cell 32 varies strongly with irradiation; FIGS. 3 a and 3 b respectively illustrate that the output power and current of a PC cell both are significantly higher when a PV cell receives full sunlight (e.g., 1 kW/m2) than when it receives 25% of full sunlight. The current also changes with manufacturing lot (sometimes also within a lot), temperature and age. The resulting problem, called cell-current mismatch, is a common phenomenon which reduces power yield in PV modules.FIG. 4 shows a schematic diagram illustrating the use of bypass diodes to prevent damage to shaded cells in conventional PV systems.FIGS. 5 a, 5 b, and 5 c respectively show schematic diagrams of conventional PV system architectures employing a central inverter, a string inverter, and a module inverter.FIGS. 5 a, 5 b, and 5 c respectively show schematic diagrams of conventional PV system architectures employing a central inverter, a string inverter, and a module inverter.FIGS. 5 a, 5 b, and 5 c respectively show schematic diagrams of conventional PV system architectures employing a central inverter, a string inverter, and a module inverter.
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Inventors
Professor David Perreault
Department of Electrical Engineering and Computer Science, MIT
External Link (www.rle.mit.edu)
Robert Pilawa-Podgurski
Department of Electrical Engineering and Computer Science, MIT
Alexander Latham
Dartmouth College
Charles Sullivan
Dartmouth College
External Link (www.dartmouth.edu)
Managed By
Christopher Noble
MIT Technology Licensing Officer - Clean and Renewable Energy
Patent Protection

Power Processing Methods and Apparatus for Photovoltaic Systems

US Patent Pending 2013-0221753
Publications
Pushing the Envelope in Power Electronics
ILP Institute Insider, August 4th, 2014

Applications

The price of the power electronics required to extract the maximum power of photovoltaic (PV) modules and to interface the PV system to the grid is becoming a larger part of the overall system cost. This technology is a design to minimize this cost and increase the power yield of PV systems.

Problem Addressed

The power extracted from maximum power point operation of a series- or parallel- connected set of PV cells may be substantially lower than the power that could be extracted if each cell were operated at its own individual maximum power point (MPP). In this architecture, an integrated circuit switched-mode MPP controller is connected to each cell, forming a “controlled cell”.

Technology

The MPP controllers are fabricated in a low-voltage complementary metal-oxide-semiconductor (CMOS) process, with power switches and control logic all integrated on a monolithic die. The controlled cells can be stacked in series to achieve a high output voltage, which enables the use of a high-power, highly efficient centralized inverter in grid-tied applications. In the new architecture, individual cells can each be operated at any current level below the current in the series controlled-cells string connections. By adjusting the duty ratio, the local MPP tracker can autonomously achieve MPP operation so long as the cell current at its MPP is equal to or less than that in the string. In this manner, the system level controller, implemented by the grid-interface inverter, can adjust the string current such that there is just sufficient string current available for the cell with the highest MPP current.

Advantages

  • Increases power yield
  • Reduces cost of manufacturing and installation
  • Improves reliability and lifetime