Cloud enhanced solar irradiance may overload photovoltaic systems

Under certain conditions, partly cloudy skies can have a larger solar irradiance than clear skies due to photons scattering off clouds near the sunbeams’ direct path. This process, called cloud enhancement (CE), can have detrimental effects on photovoltaic (PV) generators if the irradiance far exceeds the nominal operating condition. To determine how CE impacts PV performance and operation, Kari Lappalainen and Jan Kleissl measured parameters from real life events to feed into PV generator simulations.

The authors found the negative effects of CE events on PV systems reduce with increasing generator size. In general, normal CE events should not cause major problems for most utility-scale PV generators, but in cases of extreme climatic conditions, unexpected problems may arise.

“Strong CE events combined with high ambient temperature might cause problems for PV systems by increasing the temperature of system components, such as fuses, cables and PV modules,” Lappalainen said. “On the other hand, in cold climates, operating temperature can be much lower, especially if the generator has been shaded before the CE event, leading to higher power and voltage during the CE event.”

Using irradiance and cloud speed measurements, Lappalainen and Kleissl compared the measured irradiance on 62 partially cloudy days to the expected irradiance for clear sky conditions. For each CE event measured beyond a certain irradiance threshold, they conducted simulations of arrays of multicrystalline silicon PV modules to observe the energy loss and power curtailment repercussions.

Lappalainen noted future work will focus on considerably improving the accuracy of the model for a wider range of operating conditions and considering changes in irradiance and temperature with more detail.

Source: “Analysis of the cloud enhancement phenomenon and its effects on photovoltaic generators based on cloud speed sensor measurements,” by Kari Lappalainen and Jan Kleissl, Journal of Renewable and Sustainable Energy (2020). The article can be accessed at https://doi.org/10.1063/5.0007550 .