Solar causes highest power peaks under mixed-cloud conditions

Dutch researchers have shown that power peaks caused by solar generation may be stronger under partial cloudiness than clear skies. According to their findings, mixed-cloud conditions can enhance PV power production due to light reflected off clouds, as well as their intermittent shadows on arrays, which reduce module temperatures.

July 14, 2020 Emiliano Bellini

Image: Myriams-Fotos, pixabay

Liander, a Dutch power and gas supplier, has revealed the results of a recent study in which it sought to assess the impact of solar energy fluctuations on the grid.

It conducted the study with several Dutch academic institutions, including the Royal Netherlands Meteorological Institute, Utrecht University, and the Meteorology and Air Quality Group. They have shown that the highest power peaks caused by strong solar generation do not occur on sunny days without clouds, as is commonly assumed, but under mixed-cloud conditions.

Partial cloudiness can actually amplify power peaks, the researchers claimed. “Clouds are white because they reflect a lot of light,” explained Frank Kreuvel, a Liander data scientist.

Kreuvel said that light reflection can create good conditions for solar generation. And when the clouds create shadows over the panels for short periods of time, they can help to reduce their temperature, while also increasing their power yield.

This favorable combination can occur on days when the sun is covered by fluffy cumulus clouds. “Most energy is generated on clear days, but the highest peaks are on cloudy days,” the researchers said.

They claimed that the classic 15-minute interval, which is typically used to analyze the power production of solar, poorly represents the behavior of PV systems at high temporal resolutions. “At small time scales, all systems under research show at highly variable days a bimodal probability density distribution of irradiance or PV power that vanishes under 15-minute averaging,” the academics said. Their measurements were obtained at three specific spatial scales: a single point pyranometer, two residential PV systems, and a commercial solar array.

The research team found that the largest peak power at high-time resolution was recorded for a residential PV system, coming in 22% higher than the 15-minute average. They also said that the related observed irradiance was also higher by around 18%.

“We conjecture that in addition to cloud enhancement, temperature related efficiency effects lead to the largest peak power observations of PV systems under mixed-cloud conditions,” the researchers explained, noting that the largest ramp rates were also found under mixed-cloud conditions.

They presented their findings in “Analysis of high frequency photovoltaic solar energy fluctuations“, published in Solar Energy and on the ScienceDirect website.

Liander expects strong PV growth over the next five years, and is taking measures to ensure that it will be sustainable. “A scenario that we are considering is that over the next five years the capacity of solar energy in Liander’s service area will grow to 6 GW, which is six times more than that currently connected to the grid,” the utility said in January 2019.

More recently, it installed two giant transformers to increase grid capacity in Gelderland province, a region afflicted by grid constraints. In March, the power supplier said it would apply congestion management to a bottleneck in the electricity grid that is jeopardizing the deployment of large-scale solar plants incentivized under the nation’s SDE+ program.

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