TECHNOLOGYENERGY
One source of solar module failure is the bypass diode. There are no hard statistics for field failure rates due to diodes. Every manufacturer closely guards failure data for competitive reasons. Second, many failures may simply go unnoticed. There is some published data to suggest that there is a problem, though. It is worth taking a moment to understand the history of bypass diodes, why they are needed and, more importantly, why they are failing.
Typical failure mechanisms 22
Fig: 2 A bypass path has been the typical response
cost are still improving, but a new trend has recently emerged. With the advent of the Power Purchase Agreement (PPA) and a greater focus on ROI, a new acceptance metric of Operation and Maintenance cost (O&M) has arrived. To support this new acceptance metric while still honouring the classical metrics of efficiency, reliability, and cost of initial capital investment, the disruptive technology of Solar Module Monitoring is poised to shake up the solar industry. The implications and consequences of this are enormous.
Lacking effective means of monitoring, owners and/or operators of solar systems (especially residential) have been virtually blind to under- performance. This has inadvertently extended a false sense of security to module manufacturers in terms of warranties because end-users are simply unaware when a partial defect in one module arises. Think about it. Trying to make repeatable real-time power measurements on one module out of the string’s twenty, installed on a sloped roof, without a standardized “1-sun” calibrated energy source is almost impossible. Therefore, for the individual module, statistical methods of analysis have to be utilized. Now that these tools are entering commercial deployment, it is very likely that owners/operators will start noticing damage more frequently. Considering that it can cost $150/hour for a company to deploy field personnel to address maintenance, this may very well change the entire business model for module companies’ financial warranties.
Each solar string is typically comprised of 10 to 20 series-connected solar modules, each with 72 cells internally that, similarly, are all connected in series. Therefore, a typical solar string might have 1000 series-connected cells. Each of these cells produces current in direct proportion to sunlight intensity. If any of these cells become shaded, soiled or damaged, then the entire string current is limited to that of the weakest link. This, in itself, wouldn’t be so bad — it would just be a temporary loss of performance. However, the effect is much more sinister.
A typical silicon cell has a forward voltage of 0.5 volts when optimally loaded. If, for some reason (such as shading), a cell cannot produce as much current as the neighboring cells, then this same cell will now be forced into a reverse mode of operation where it now has a negative voltage of 5 to 30 volts. In truth, the solar cells are a little bit forgiving as to the mismatch. But, if enough mismatch is present, then the under-performing cell will be driven into the region of reverse breakdown. With 10 to 20 solar modules connected in series, the overall DC output can easily be 400 volts. Therefore, due to Kirchhoff’s Voltage Law, it is possible for the lone shaded solar cell to begin operating in reverse breakdown with 30 volts applied across it, while the remaining
Fig: 3 Evolution of bypass diode technology
www.solar-pv-management.com Issue VI 2010
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