Solar Performance Maps of the United States, recently released by Vaisala, reveal considerable solar irradiance variability during the summer months, even in desert locations, such as Nevada. Furthermore, these findings have, in turn, demonstrated the need to make more informed use of long-term data to guarantee the ongoing performance and profitability of sites. At the 392 MW Ivanpah Solar Power Facility site in Nevada, solar irradiance has been shown to have either exceeded or fallen short of expectations in 10 out of 15 summer months over the past five years — leading to annual performance fluctuations potentially worth $440,000.
Vaisala 3TIER Services measures and studies solar irradiance across the globe. Making use of this extensive dataset, over the past five years, the company has produced a Summer Solar Performance Map of the United States, analyzing solar irradiance variability during the three-month period when peak production is commonly expected.
These maps serve to graphically illustrate fluctuations in solar irradiance from long-term average levels, giving an indication of the inherent variability of the solar resource and demonstrating the influence of monthly weather patterns on project performance. Combined, they provide a tool developers can use to analyze and quantify the causes of over- or underperformance.
“The impact of solar variability on the balance sheet can and will make the difference between profitability and loss,” says Richard Pyle, Energy Segment director at Vaisala. “It therefore has a significant influence not only on individual project sites, but also on the international standing of solar energy as a viable investment proposition, particularly as the industry expands into emerging markets overseas.”
During the 2014 study, for analysis purposes, the area surrounding the Ivanpah Solar Power Facility in Nevada was chosen as a case study. While desert regions typically enjoy more consistent solar resources, this year’s figures indicate a clear fluctuation in solar irradiance — 5% above average levels in June, but 5 to 8% below average in July and August.
“It is critical that developers acknowledge that resource variability occurs not just from month to month, but also from year to year,” says Gwendalyn Bender, Energy Assessment product manager at Vaisala. “BrightSource Energy [the developer of Ivanpah] was ahead of its time when it first approached us in 2009 to help address this resource assessment issue. They took the long-view and saw a clear requirement to use more sophisticated long-term solar data and modeling techniques to guarantee ongoing site performance.”
Industry wide figures suggest that a 5% negative anomaly in solar irradiation during the summer months for a CSP or PV site can translate to a 1 to 1.5% decrease in annual revenue for a solar developer or operator, highlighting a clear industry requirement to take a more comprehensive approach to gathering long-term weather data.
Variations in solar irradiance caused by anomalous weather patterns are proven to have a substantial impact on the monthly and annual outputs of an asset and must be accounted for in early stage and ongoing performance estimates. However, the quantity and quality of irradiance data, traditionally gathered by developers for this purpose, has often proven inadequate.
It has become common practice to base long-term financial and operational decisions on one-year’s worth of TMY (Typical Meteorological Year) data. This strategy fails to account for inherent climatic variability over the project lifetime and puts assets at considerable risk of underperformance and subsequent loss of profitability in the long-term, especially considering that TMY data sets are based on a “normalized” year and actually screen out both extreme highs and extreme lows.
As the global PV and CSP industries mature and seek to become cost competitive with conventional energy sources, developers, operators, and lenders are increasingly required to make critical investment decisions which, ultimately, have a bearing on the future success of their expanding portfolios. Informed siting choices, resource assessment, and long-term energy modeling are a crucial part of ensuring profitability and return-on-investment.
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