A screenshot from NREL’s PVWatts
Previously, doing a solar resource assessment involved setting up costly equipment onsite and waiting to see what happens. While this is still a relevant way of gathering information on large utility-scale sites (often because permitting already takes more than a year, and financers prefer high levels of certainty with detailed information), it doesn’t make much sense on commercial and residential projects under 10 MW.
“It would be a deal killer to sit and wait on development for a year just to collect ground level irradiance data,” said C.J. Colavito, director of engineering at Standard Solar.
There are more online tools and software options available today—many free thanks to DOE funding and NREL. These allow solar contractors to perform due diligence studies without ever leaving their desks.
Today, if Colavito is working on an individual project, he’ll look at local NREL TMY3 weather station data and compare it with other data sources. NREL offers data in typical meteorological year files, known as TMY3 files. These files are a mix of satellite and ground measurement data that show hourly values of solar radiation and meteorological elements for one year. Colavito downloads the files for free and imports them along with data from other sources into a custom excel-based analysis tool. He said these files are generally accepted as being fairly accurate for commercial and residential projects and can help contractors feel confident in guaranteeing production.
A screenshot from NREL’s Solar Prospector
Though the use of high-quality and accurate solar resource data is important for every project, Colavito said it’s less necessary to get third-party validated resource data on distributed generation projects. Many portfolios of commercial and residential projects are financed through project finance funds where projects are spread over a large geographic area and individual project risk is limited. Many large developers and EPCs have these funds, which can be backed by sources from banks to Google. Involved parties often like to see a solar resource assessment for multiple regions where projects will be located. Colavito said the analysis process is the same and just involves looking at data for more regions.
“For example, if we’re building projects in theDC metropolitan region, there’s three main TMY3 sites we’d look at: Baltimore, Reagan National Airport and Dulles International Airport,” he explained. “We would look at the local weather station data, plot it by year, graph it, compare it to yearly datasets from NREL and analyze it. Are we seeing a big difference between Baltimore and Reagan? If so, can we explain it or not? Doing this detailed solar resource data assessment helps us ensure data validity and confidently guarantee production.”
