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Wrenches, We have an off-grid customer with a system that has been growing since 1996; we have cared for it intermittently since 2000. The customer has two tracked arrays currently. One has ten Astropower AP120s for 1200 nominal watts, but that's not the topic today; it'll come up in 5 years or so. The other older array is 12 Solarex MSX77s, polycrystalline with 16.9Vmp, 21Voc (when new). The rest of the system is a SW4024, two Solar Boost 50s (one is currently failing) and two strings of 800 a/hr Rolls cells. The client wants to add 1,200 to 2,000 watts of additional PV. The problem is that we have two sets of existing buried conductors between arrays and BOS and don't want to dig up for more. Also, he wants to upgrade charge controllers, and has had past failures of the SW inverter - in fact, he keeps a spare put away for the next failure. I have suggested that by upgrading the inverter to a modern pure-sine 48V unit, we can accomplish the array upgrade using the existing input conductors. I can rewire the Solarexes from 6 24Vnom strings to two 72V strings and have plenty of copper to add more array without voltage drop issues. (The AP120s can be brought in as 2 60V strings to a separate FM60 or T80 or similar). This also lets me work with existing 60 or 80A charge controllers, which I can't do at 24V. The customer likes this approach, so here's my question: What modules would you suggest as having good compatibility with the existing Solarexes? I would assume that any polycrystalline 72-cell module would be fairly close but not ideal. I would especially expect the lower-rated ones within a particular size category to be better choices, as they would have lower Vmp to more closely match the existing. But would similar monocrystalline be close enough? Too high a voltage in the dominant (new) array drags the voltage above MPP, which reduces output considerably on the Solarexes. Also, I can't find the temperature coefficient for the old modules, and I suspect that degradation over 14 years has lowered the MPP of them anyway. I think of using 12 Kyocera KD135s Mitsubishi 125s or 130s; Solarworld's discontinued SW175 (mono), Sharp 123s, or possibly some Suntech monos. But I'm certainly open to advice and suggestions here. Also, while the existing arrays are on Wattsun DA trackers (customer's choice), the new array will be pole-top. I'm not too worried about tracking/fixed mismatch, as voltage quickly matches even with non-optimal orientation. But I'll take advice here too. Thanks. Allan --
Allan Sindelar |
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