Pyranometers versus reference cells for solar monitoring

As the solar industry gains momentum, there is an increasing need for on-site
resource monitoring using pyranometers and reference cells. Peter N. Johnson of AWS Truepower
introduces the technical specifications and limitations of industry-standard pyranometers
and reference cells, and outlines best practices for monitoring with this equipment. How solar
measurements can be used to improve resource and energy estimates for utility-scale PV projects
is also discussed.

In recent years, an increasing number of large utility-scale PV projects have been developed, particularly in the desert south-west of the USA. The emphasis on larger projects has led to an increase in on-site solar monitoring. On-site monitoring reduces investment risk and helps to meet the requirements of power off-takers, utilities and jurisdictional authorities. The monitoring data are especially valuable for larger projects (>50MW) and for those in poorly-defined resource areas; in these cases, financial risks and grid-integration concerns may be greater.

On-site monitoring provides information about the solar resource and project performance that cannot be reliably obtained with modeled data sets alone: on-site measurements from pyranometers and reference cells can reduce uncertainty in energy estimates by 3–4% compared with modeled data. On-site measurements are also the most effective for characterizing seasonal trends, diurnal trends and short-term ramp events, all of which are critical for estimating time-of-day energy pricing and supporting grid integration. These data are used as inputs for pre-construction, operational and short-term energy forecasts, helping to inform financial models, and quantifying investment risk for developers and financiers.

Applications for pyranometers

For pre-construction applications, on-site pyranometer measurements can be used in combination with other regional data sources to accurately characterize a PV project’s long-term solar resource and energy potential. Because of inter-annual variability in the solar resource, shorter on-site measurement periods must be correlated to a high-quality longer-term data set of ten years or more. This combined approach leverages the complementary strengths of multiple data sets, taking account of high measurement accuracy as well as long-term regional trends. Long-term reference data sets may be modeled data or regional ground measurements from public meteorological networks. Reference data sets should be scrutinized to confirm accuracy, consistent trends, high data recovery, agreement with concurrent regional measurements and equipment maintenance when possible. Using less than a year of on-site measurements may result in a seasonal bias and is not recommended. The adoption of a blended approach leads to an overall solar resource uncertainty between 2 and 4%, depending on the pyranometer used, the regularity of site maintenance, the inter-annual variability, and the reference data set’s quality and recording period. (…)

You can download and read the rest of the article from the PV Tech website.

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