Bifacial solar panels ….simply the best module to use in a snow country ground mount array!
This test was conducted March 8th 2017, on the ice of a small central New Hampshire pond. The theory is that the sun’s rays will be very strong when reflected by snow or ice and provide a significant backside boost to a bifacial module. A second theory was that the gain would be especially noticable during the early and later hours of the day when the reflected angles would be most direct and the backside of the bifacial module fully illuminated. The results shown in the graph below confirmed my hopes in spades!
The Prism Solar Bi60-375 has a BSTC (Bifacial Standard Test Conditions) bifacial rating of 375 watts and a STC (Standardized Test Conditions) rating for the front side only of 295 watts. Many similarly sized 60 cell competitive single face panels can exceed this front face rating, but few can touch the bifacial wattage total.
Please note that the results in this test are based upon simple Voltage Open Current and I(amperage)Short Circuit readings without a proper resistance loading. These figures do come in a bit higher than the verified STC ratings used nearly universally by solar module manufacturers probably would. The combined BSTC rating for the Prism module is again 375 watts, but as you can see with the reflectivity of ice and the cool temperatures, the readings in bold face print far exceeded Prism’s BSTC rating during the midday hours! I’m certain that had I been set up earlier the 10 AM readings would have exceeded the BSTC ratings as well.
The bifacial total of the day’s incremental readings was 2,555 watts, while the total for the backside came to 326 watts. Thus the back side provided a 12.75% boost over the daily power harvest that would have been gathered from a comparable single face module.
Interestingly the backside boost did prove to be more significant at the edges of the day when the sun would be at a lower angle and providing better reflection angle.. At 3 pm for example, the backside was providing nearly 14.5 % of the panel’s total output. The results seem to prove that a bifacial module does provide both significantly more power, and for longerduring the day.
|Time & conditions||A. Full exposure of module
(Voc & Isc readings)
|B. Rear of module readings
(front surface masked)
|Front surface power
(A minus B)
The backside percentage gain would no doubt be lesser during the non-winter months due to the darker background of grass or soil, but it is safe to assume that there would still be some gain. Bifacial modules also do have a slightly higher initial cost, but that is very quickly offset by the extra power provided every day . Also significant for those of us in snow country, the greatest power boost comes during the short dark winter months when every extra watt can be most appreciated. From this point forward, New Hampshire Solar will be recommending the every fixed ground mount should be equipped bifacial solar modules!
February 2018 update. In November of 2017 we installed an array on a.perfectly oriented roof mount (194 degrees, 8/12 pitch) using 300 watt Hanwha panels. In December we installed a fixed ground mount with the same perfect solar orientation and Prism bi-facial panels rated at 292 watts on the front face. During the month of January the roof mounted Hanwhas produced a peak power of 238 watts per panel, while at the same time the Prism bifacial panels produced 287 watts per panel. The reflection of the snow onto the backside of the Prism panels produced a 17% boost in power over the conventional panels! Even more impressive the bifacials produced an average of 23.97 kilowatt hours per panel per day whereas the Hanwhas produced 18.8 kWh, that’s a whooping 22% more usable power! I would suspect that most of that gain came from the sunlight being reflected back up to the underside of the Prisms during the early morning and late afternoon hours. Extra power just when it is needed the most!
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