It is not uncommon for us to see in the solar parks which Greensolver manages that the actual energy yield deviates from the original forecast energy yield. However, a correct estimate of the yield on site is crucial for the correct design of the system and thereby the operational performance. That has a direct influence on the funding or value of the project.
The starting point for the design of every solar system is the calculation of the Global Horizontal Irradiation (GHI). GHI is a cumulative solar radiation over the year on a surface, usually expressed in kWh/m2. The GHI is primarily dependent on the location on the planet. The location on the planet and the light intensity over the year at the site. This can be modelled successfully on the basis of astronomical laws. However, the greatest uncertainty lies in the reduction of the solar radiation coming in by the atmosphere. This reduction is determined by location-specific, climatological and meteorological factors, such as:
- Absorption by the atmosphere (ozone, oxygen, water vapour etc.)
- Scattering in the atmosphere (Rayleigh / Mie scattering)
- Reflection on the surface (albedo value)
Modern solar PV design tools (such as PVSol and PVSyst) make estimates of these factors on the basis of satellite data, e.g. the PVGIS or NASA-SSE databases, which in turn are constructed from other datasets, including: GEOS-1, ISCCP D-1, GOES, POES, ISSCP, Meteostat, datasets. These datasets contain data from the period 1983 – 2005. Despite a relatively high reliability of the P50 and P90 values, the various tools often significantly give different outcomes. These differences are caused by the use of different datasets and algorithms.
In order to achieve a consistent and more reliable outcome, ‘long-term correlation’ is the buzzword. Additional correlation can be sought with other local sources, such as weather data from meteorological institutes, weather stations, etc.
Another reliable alternative is correlation and extrapolation on the base of actual measurements on site at the location. Correctly installed and calibrated pyranometers – the current standard in solar is ISO 9060:1990 allow the local conditions to be measured accurately. On-site measurements can be performed on a horizontal basis or even for a better approach, tilted to the panels’ inclination. With correct measurement, the margin can be reduced to 1% – 2%.
This enhanced reliability will lead to better P90 values for the project, which in turn will reduce project’s risk and therefore provide better funding terms. Naturally, the performance of the measurement and analysis of the findings have costs attached to them. A location-specific assessment will have to be made in order to determine whether the benefits outweigh the costs.