Photovoltaic panels are a crucial part of the world's effort to provide renewable energy and to reduce greenhouse gas emissions that cause global warming. Over 90% of deployed photovoltaics use silicon as the active material and employ a panel design that places a bypass diode in parallel with a group of roughly 20 cells in series to keep current levels manageable and to limit the maximum heating that can be experienced by a partially shaded cell. Although the hot spot problem has been known about for decades, and the community largely perceives that bypass diodes solves the problem, improvements in cell and panel efficiency call this perception into question.
In the summer of 2019, students Athena Li and Eric Chavez and I made measurements of hot spots on recently manufactured silicon solar panels, using conditions similar to those employed by the UL1703 safety test—which all three panels claimed to have passed. In these tests, the panel faces the Sun with its leads shorted together. One cell is then partially shaded and its temperature monitored with a thermal camera. We saw a maximum temperature of 187°C. This temperature is well in excess of the safe operating temperature of the backing material (ethylene vinyl acetate) used to seal out moisture. It was unclear to us how this panel could have passed the test, or any of the three, for that matter.
Before publishing these observations, it will be necessary to model the heating as a function of shading to present a thorough accounting of the range of situations in which hot spotting is a potentially serious problem. I am proposing to work with two students on this modeling. If circumstances permit students to return to campus, we may also make additional measurements on both the panels and on individual cells. In addition to modeling these conventional panels, the students and I will compare the performance of conventional panels to those operated using the HSP control mechanism patented by Kernahan under a variety of irradiance conditions. This technology forms the basis of the solar panel factory being developed in Pomona.