ISO advances modeling of solar, wind energy output

ISO New England recently gained new resources that will help more accurately predict when and how much electricity behind-the-meter photovoltaics (BTM PV) will produce throughout the day—and how that impacts the regional power grid.

When the sun is shining, BTM PV reduces the amount of electricity homes and businesses draw from the regional grid. When the sun isn’t shining, the grid needs more energy from other resources.

That’s where the ISO’s operational BTM PV forecast comes in. Based on historical data and predictions from a commercial weather vendor, the forecast attempts to quantify how much demand will fluctuate from hour to hour based on solar output. That system has been in place and undergoing refinement since 2019. Recently the ISO began incorporating data from two more sources in an effort to increase the forecast’s accuracy. ISO forecasters continually compare predictions with actual outcomes to refine their models, and apply their own expertise to guide decision making.

Most solar panels in New England are connected to local distribution systems, rather than the regional grid. This BTM PV adds up to more than 5,000 megawatts (MW) of nameplate capacity¹, and the ISO projects that will increase significantly in the coming years.

Challenges remain

As residential solar continues to grow, predicting its impact on the grid becomes more difficult. And while other aspects of the ISO’s weather forecasting are highly accurate, two factors crucial to PV production are difficult to model: cloud cover and snow cover.

Temperature, dew point, and wind speed can all be gauged accurately at ground level. Making observations about clouds in multiple layers of the atmosphere is more involved, and as a result there is less data to work with.

Depending on temperature and sunlight, snow can continue to cover solar panels for days after a snowstorm, meaning they won’t be producing energy under what could otherwise be ideal conditions. This can lead to a load forecast that’s thousands of megawatts off the mark. As of now, modeling of residual snow cover on solar panels is just being developed, and the ISO is working with commercial weather vendors to identify solutions.

A decade of experience forecasting renewables

As renewable resources grow in number in New England, so too do ISO New England’s forecasting abilities. In addition to refining its BTM PV forecast, the ISO is also developing tools to predict output from future offshore wind projects.

The ISO has been producing a seven-day wind power forecast since 2013. Each weekday, a CSV file providing an aggregate wind power projection for each hour for the next seven days is posted to the ISO website. This provides system operators with reliable information as they balance the region’s resource mix.

About 1,400 MW of onshore wind power generation are connected to the New England electric grid. Nearly 20,000 MW of wind power projects—most of them offshore—are in the ISO’s generation interconnection queue. That represents about 60% of all generation projects being proposed for development in the region. Typically about half of the projects that enter the queue end up being built.

The ISO is supporting a multi-year research project spearheaded by the Massachusetts-based Woods Hole Oceanographic Institution to improve predictions of output from offshore wind facilities. The study is aimed at creating a better understanding of atmospheric conditions affecting wind on the Northeast United States’ outer continental shelf.

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1. Nameplate capacity refers to the total amount of electricity a resource could produce running at 100% of its capability.