Site-Specific Information Critical in Yield Assessment for Solar Projects_ Report

The International Energy Agency (IEA), in its IEA Photovoltaic Power System (PVPS) Task 13 report, has said that the main challenge in long-term yield assessment for solar energy projects is the authenticity of site-specific information.

The report, which provides an insight into yield assessments and yield predictions in solar power projects, points out that the main source of uncertainty in yield assessment is the insolation estimation. Various factors need to be considered when assessing the insolation at a particular site. In yield assessment, the choice of the sources for long-term insolation data is the key. Insolation is a measurement of the cumulative energy measured over an area for a defined period.

The report states that irradiation data derived from satellite images are increasingly being used as input for long-term yield estimations. Solar irradiation at the earth’s surface is not stable over time for all locations. It may undergo significant long-term variations for particular regions, referred to as ‘global dimming and brightening.’

Annual Insolation Variability


A month or a year with less solar irradiation than the long-term average corresponds to a decrease of the solar project’s production for the considered period concerning the expected business model. Such variations over time are well known and should be accounted for in long-term yield assessment.

On the variation in irradiation over a period of time, the report adds that while 2016 was overall an average year in terms of solar resources for most European countries, 2017 capped off a good year for most of southern Europe with nearly 5 % more irradiation than the expected values.

However, 2017 was not good for northern European countries, with nearly 3 % to 5 % less than expected values. Interestingly, 2018 had the opposite behavior regarding solar irradiation, having high irradiation values for most northern European countries with over 8 % more irradiation than expected values.

Long-Term Trends

Long-term trends of solar irradiance might affect uncertainties of yield assessments. While this influence may be close to zero or even negative for some regions of the world, there are regions where a strong brightening can be observed.

Long-Term Solar Resource Uncertainties

According to the report, site adaptation techniques that combine short-term data and long-term satellite estimates can be used for yield assessments. Upon completing the measurement campaign, different methodologies can be applied to the target site’s measured data. However, there is a degree of uncertainty associated with the techniques.

Bifacial Irradiance

Emerging modules and systems have changed the solar landscape and promise to increase solar systems’ power output in various ways.

The report notes that in recent years, the problem of calculating bifacial irradiance has been addressed in many ways, and two main approaches that have emerged are ray tracing and view factors. Both methods have high accuracy potential if appropriately implemented.

Another factor that poses a challenge to yield assessment is installing solar power projects on sites characterized by uneven terrain. Depending on the chosen installation technique, it is not easy to ensure that all module mounting structures assume an optimal module tilt and orientation angle.

Uncertainty in Long-term Yield Prediction

A typical yield assessment refers to an initial year with an associated initial specific yield. The average annual specific year will include degradation. As the yield assessor is providing an average value, the interannual variability is not included.

“Knowing the time span of solar modules and systems or the remaining useful lifetime for operational systems is of great importance for making financial decisions and planning operation and maintenance (O&M) activities of solar systems,” the report added.

LCOE of Solar Projects

The levelized cost of energy (LCOE) includes all the costs and profit margins of the value chain, including manufacturing, installation, and O&M cost.

It also includes the cost of financing but excludes the profit margin of electricity sales. Thus, it represents the generation cost, not the electricity sales price, which can vary according to the market constraints.

The output of yield assessments can significantly impact the LCOE and the cash flow of business models.

In a report published in November last year, IEA said India would be the largest contributor to the renewable upswing in 2021. The country’s annual additions are expected to double in 2021 compared to 2020.

Earlier, in the ‘World Energy Outlook 2020’ report, IEA said solar was likely to be the new king in the world’s electricity market. India could be a leading market for utility-scale battery storage. The report focused on the pivotal period of the next ten years, exploring different pathways out of the Coronavirus crisis.