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Tracking Cloud Cover Effects on Large-Scale PV Plants

published: 2011-09-29 8:48

In late August 2011, the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) released the initial data from an ongoing study tracking how clouds can affect large-scale PV power plants in the 30 MW size range. The data produced from the NREL project gives an indication of what happens in a second-by-second time frame when clouds pass over a solar power installation and temporarily block the sun’s incoming radiation. It is hoped that the data models from the long-term monitoring by NREL’s Measurement and Instrumentation Data Center (MIDC) can be used by large-scale PV developers and utilities to develop technical strategies to better manage any solar power fluctuations.

Sunlight Fluctuations

Like wind farms that only produce power when the wind blows, generating electricity from the sun’s radiation happens only when that radiation is available. When clouds block that direct sunlight, there will be a decrease in power output from PV panels. Knowing to what degree there will be a power loss from a large-scale solar power installation means understanding the physical characteristics and impact of a cloud’s shadow as it passes over a large PV system. The more detailed that information is known ahead of time, the better energy managers can plan and control the power plants output. The key is being able to adequately verify what those changes will be and before the NREL study, no such data had existed.

What the NREL study has found so far is that apparently large-scale PV installations are not as quickly or dramatically as impacted by cloudiness as are smaller PV systems, such as those that are 1-MW or less. In fact, the larger the plant, the less the variability in power output due to cloud cover, according to Ben Kroposki, an NREL principal engineer and a leader of the project.

With a small 1-MW PV plant, the power fluctuation can go up or down very quickly since PVs respond rapidly to changes to the available sunlight. And that change can happen in seconds or tens-of-seconds. But with a larger, 30-MW installation, those fluctuations do not occur as rabidly, according to the study. This is particularly true with smaller clouds that simply do not affect the entire large-scale PV array at once.

The Oahu Solar Energy Study

The NREL study established 17 measurement stations near Hawaii’s Honolulu International Airport on the island of Oahu. NREL’s Solar Radiation Research Laboratory measured the impact of clouds with these sensor stations that collected data at 1-second intervals over the course of one year, producing 31.6 million seconds worth of data. Those time-synch measurements were all taken at exactly the same time and measured the level of solar radiation in the sun’s visible spectrum that reached a horizontal surface at ground level. The researchers also designed the equipment to incorporate a global positioning system to include 1-second measurements concurrently for each of the 17 sensor stations.

The collected data allowed NREL to set up a monitoring network that measured exactly how clouds would impact a large PV system through measuring the dips and jumps in PV power output based on changes in cloud cover. That data can then be used to predict what PV power output changes may occur at 1-second intervals for medium and large-scale PV power plants.

One interesting general conclusion is that with very large-scale PV arrays, there is a smoothing-out of the power fluctuations caused by cloud cover compared to the very sharp spikes and drops that a single PV panel or small rooftop PV array experiences when clouds pass.

The Hawaiian study comes in anticipation of what is expected to be the largest PV system for the islands. On March 24, 2011, Hawaii Electric Company had a ribbon cutting ceremony for its new 5-MW SunPower solar facility on the island of Oahu. Axio Power also plans to build a 5-MW solar facility on the island. These are the first two utility-scale projects as part of the state’s aggressive renewable energy plan, with another 20 to 30 MWs of large-scale solar projects in the pipeline for approval. The state already boasts the most installed solar kilowatts-per-person in the U.S., mostly from private residential installations.

Not the First Cloud Study

In a study published in early September 2010, the US Department of Energy's Lawrence Berkeley National Laboratory released a study, Implications of Wide-Area Geographic Diversity for Short-Term Variability of Solar Power, that refuted findings of earlier studies which had concluded that PV power plants could be greatly affected by short-term cloud cover that would limit their practicality as large-scale electric power sources. The Berkeley lab study, which measured PV output in Oklahoma and Kansas, concluded that the geographic diversity of large solar generating sites “are not substantially different from the costs for managing the short-term variability of similarly sited wind in this region.” In other words, it showed that the effect of cloud cover didn’t have as great an effect on PV arrays as formerly thought. The newly released NREL study appears to confirm those results.

The Significance

The significance of the current cloud cover study project is the potential to not only manage the output of large-scale PV farms, but to also contain those fluctuations so they do not adversely impact power flow to the grid system. The data will also be useful to those researchers at commercial labs and universities who are developing the next generation PVs to better understand how clouds impact large-scale solar plants.

Clouds can cause significant jumps or “ramps” over a very short period of time and those jumps can cause fluctuations in the grid with the potential to cause power fluctuations and even power surges. With such detailed statistical data about how certain cloud patterns impact PV output, researchers can design systems to minimize the impact of passing clouds and mitigate those fluctuations. Such systems may include storage of electricity generated by the PV array or infrastructure improvements and software to stabilize power fluctuations to the grid.

Sharing the Data

NREL says the data from Hawaii can also be used to predict the impact of clouds on solar projects in other areas that have a similar climate. For the U.S., this means areas such as found in Florida and the desert American Southwest, where large-scale solar projects are currently planned or being built. According to the NREL, another study is already planned in Florida using irradiance sensors to monitor the effects on a 300-MW solar power plant.

The DOE-funded study by NREL also included a partnership with General Electric, the Hawaiian Electric Company and the Hawaiian National Energy Institute. But NREL has been given the right to share all knowledge from the data set based on one-second intervals with other researchers, forecasters, utilities and solar developers globally. Currently, the one year’s worth of data can be found on NREL’s Measurement and Instrumentation Data Center website at //www.nrel.gov/midc.

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