Imec and its partners in the Smart PV project have developed a smart bypass for solar panels. Such a bypass minimizes power loss of the panel due to less performing cells. The eventual goal of the Smart PV project is to increase the energy yield of photovoltaic panels by transforming them into intelligent systems that can adapt to their environment.
Large-area PV panels consist of multiple individual cells, connected in series to deliver a large amount of electrical power to the load. By putting the cells in series, the energy transfer is accomplished at rather low current levels and a relatively high voltage, minimizing the power losses in the power lines. This system works well if all cells in a panel receive the same amount of light, so that they contribute equally to the overall power conversion. However, if one cell receives less light intensity because itís temporarily in the shade or because its surface got dirty, the current in the series connection of all cells will be limited by this failing cell. So not only the power of the failing cell itself is lost, but also the power generation of all well-illuminated cells is dramatically reduced. And in the limit, when the photocurrent of one cell drops to zero, the power conversion of the whole system is even stopped.
To work around this problem, some kind of electrical bypass must be placed in parallel to each cell (or in parallel to sub-strings, i.e. small chains of cells). This is commonly done by means of a silicon PN junction diode in anti-parallel to the PV cell. When the generated photocurrent of a particular cell becomes smaller than the photocurrent of all other cells in the panel, the load current will now circulate through the bypass diode in anti-parallel to this failing PV cell. So, even when its photocurrent drops to zero, there is still a conductive path for the circulation of the current generated by the other cells. Unfortunately, this situation is not yet ideal. In fact, the conducting bypass diode will dissipate an amount of power that is similar to the power generated by one of the other cells. As a result, we lose the power of the failing cell twice!
As a first step in the Smart PV project, we have replaced these anti-parallel diodes with a smart bypass to reduce the power loss during partial shading. This smart bypass senses when the current through the cell is not matched with the current of the other cells (reducing the total output power) and activates an internal switch to redirect the load current. The power dissipated in this switch will only be a fraction of the power lost in the traditionally used diode. The smart bypass is completely independent and is powered by the surrounding solar cells. Once the switch is activated, the smart bypass will periodically check to see whether the cell has regained full functionality. If this is the case, the smart bypass deactivates and the solar cell can again add power to the system. If the cell is still in the shade, the switch stays activated.
The Smart PV project is an IWT-SBO project, sponsored by the Flemish government. The project partners are imec, Ghent University, the University of Leuven and VITO.
Layout of the Smart Bypass ASIC. The chip was designed in the I3T50 smart-power technology of ON Semiconductor, a 50V extension of a standard 0.35Ķm CMOS process.About the authors:
Pieter Bauwens received his masterís degree and doctorate in electronic engineering from Ghent University in 2006 and 2010 respectively. He works at the Center for Microsystems Technology (Cmst), which is a research facility at Ghent University as well as an associated laboratory of imec. His main interest is the design of smart-power microsystems. He is the author or co-author of 11 papers in international technical journals and conference proceedings.
Jan Doutreloigne received his masterís degree and doctorate in electronic engineering from Ghent University in 1987 and 1992 respectively. He works at the Center for Microsystems Technology (Cmst), where he is in charge of a research team specialized in smart-power ASIC design and high-voltage device development. He is also a professor at Ghent University, teaching courses in the domain of microelectronics. He is the author or co-author of 120 papers in international technical journals and conference proceedings, and holds five patents.------------------------
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