Welcome to the first edition of our OnePage Insight series! In these sessions, we’ll explore considerations in renewables, condensing them into easy-to-digest, one-page insights. This first insight focuses on cloud enhancement (CE) and its impact on modern solar farms, PV generators and inverters.
What is cloud enhancement (CE)?
CE occurs on partly cloudy days when irradiance temporarily exceeds clear-sky values due to scattering effects around clouds.[1]
The CE phenomenon is global, spanning the equator and polar regions, as well as lowlands and high-altitude areas. Many studies have reported extreme irradiance values:
- 1832 W/m2 in Ecuador at the altitude of 3400 m[2]
- 2000 W/m2 at 40 degrees Northern latitude at an altitude of 1829 m[3]
- 1600W/m2 at sea level at 60 degrees of Northern latitude[4]
The CE phenomenon has traditionally been attributed to reflections from cloud edges.[5] However, this explanation is overly simplistic and fails to capture the phenomenon's complexity. It has been suggested that the CE effect primarily arises from strong forward “Mie Scattering” within clouds. It occurs when light is scattered by spherical particles that are similar in size to, or larger than, the wavelength of the light. Unlike Rayleigh scattering, Mie scattering is not strongly dependent on wavelength, which is why clouds appear white as they scatter all visible wavelengths of light equally.
The most intense CE events occur when a narrow cloud gap is surrounded by thin clouds within 5° of the solar disk.[6]
Strong forward scattering by clouds can increase sunlight intensity by up to 1.8 times clear-sky levels for short periods.
CE is localised and transient, often lasting just a few seconds to minutes.
Impact on solar farms and PV generators:
Modern solar farms are generally designed with robust safety margins, so CE does not typically threaten day-to-day operations.
- PV generator output depends heavily on irradiance and temperature.
- Short-circuit current (Isc) rises almost linearly with irradiance. Open-circuit voltage (Voc) decreases slightly with temperature.
During CE events, irradiance can exceed standard test conditions (1000 W/m²), leading to temporary power output above nominal ratings, particularly in cold environments with high irradiance.
Knowing the magnitude of CE accurately helps to estimate real-world generator performance and help ensure reliability.
Impact on inverters:
Inverters have maximum power limits, and when PV output exceeds these limits, inverters operate in power-limiting mode.
CE events can cause temporary shifts in inverter operating points, sometimes preventing the full utilisation of the generator’s potential power.
Proper inverter sizing and configuration must account for rare but intense CE spikes to avoid efficiency losses or equipment stress.
Key takeaways:
- CE is a short-term, localised increase in solar irradiance.
- Modern PV farms are resilient, but CE can temporarily exceed their nominal ratings.
- This can put generators and inverters under transient stress, which highlights the importance of performance modelling and system design considerations.
- Understanding CE is crucial for the reliable and optimised operation of solar farms.
To learn more about Crawford’s Renewable Energy Practice, visit our Power and Renewable Energy page.
