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Renewable energy sources (RES) continue to grow and gain increased relevance in modern electric power. The main driver of this growth was based on subsidies, typically, and feed-in tariffs that aim to reduce the air pollution through the replacement of fossil energy sources by clean and safe RES. Within the different types of RES, wind and solar photovoltaic (PV) are the most promising technologies to jointly produce a large share of renewable electricity needed to reach the EU’s 2030 ambitious targets.
The core parameter of a sound profitability forecast for a potential project site is the precise knowledge of the resource conditions (wind and accordingly solar irradiation).
We work to identify the optimal locations for resource measurement, select the appropriate sensors and equipment according to the respective requirements and international standards and guidelines, and monitor the measurement from installation to completion of the measurement campaign.
Accurate resource pre-assessment of a widely variable resource is the most critical feature for successful selecting the site of a future power plant.
Many years of experience in organizing, implementing and evaluating enable an exact determination of the site-specific energy potential.
Long term surface wind data is accessed from international sources for grid point nearest to the site. Data from Modern-Era Retrospective Analysis for Research and Applications (MERRA) we are using for the calculations. The data set is provided by National Aeronautics and Space Administration (NASA) and covers the modern era, from 1979 and onwards, during which remote sensing has been dominant when collecting data about the atmosphere. The development of MERRA was focused to improve the modelling of the water cycle.
MERRA data has been validated in different ways, for example by comparing with measured wind speed and temperature profiles. The temperature profiles are in good agreement while the wind profiles differ significantly. It is also shown that MERRA data is not internally consistent in the mountain range, causing a large uncertainty.
The recent MERRA-2 atmospheric reanalysis provides global, 1-hourly estimates of land surface conditions for 1980 to present at about 50-km spatial resolution. MERRA-2 replaces the MERRA data product, which was first published in 2010. Because of the precipitation corrections and because of improvements in the land surface model, the skill of the MERRA-2 land surface hydrology estimates (vs independent data) is generally (but not always) greater than that of MERRA estimates.
The solar radiation that reaches the top of the atmosphere on a perpendicular plane to the rays, known as solar constant, has an average value of 1361-1362 W/m2 which varies somewhat depending on the position of the Earth in its elliptical orbit.
As the solar radiation goes through the atmosphere it suffers different processes of absorption, dispersion or scattering that result in lower levels of solar radiation being received at the Earth's surface. These are due to the atmosphere components, such as ozone or CO2, and solid and liquid particles in suspension like aerosols or water vapour. However, the main source of attenuation is the cloud cover. Not only the broadband value is different, but also these processes of absorption and attenuation affect differently the wavelengths of solar radiation, so the spectral distribution of the solar radiation at ground level differs from the extraterrestrial one.
The solar radiation received at ground level, known as global radiation is sum of three components. The first one, named beam or direct radiation, is the fraction of the solar radiation that reaches the ground without being attenuated by the atmosphere and can be modelled as coming directly from the solar disc. The second part or diffuse is the solar radiation that reaches the ground after being reflected or scattered by the atmosphere and is considered to arrive from the whole sky dome. The third component, not always taken into account, is the reflected radiation from the ground surface or nearby obstacles. The beam component is only available when the solar disc is not blocked by clouds, while the diffuse component is always available, being the only radiation available whenever clouds block the solar disc.
Solar radiation under clear sky conditions (without clouds) and clean and dry atmosphere is a very important parameter as it provides information about the maximum radiation available at any location. This value is normally modelled and is used as input data for other models applied for the estimation of the solar radiation at normal atmospheric conditions.
We talk about solar radiation and use it in a generic way but we also use other terms that should be explained. Concepts like irradiance and irradiation should be clarified. Irradiance is the solar power falling into a surface per unit area and unit time. It is therefore expressed in W/m2. While irradiation is the amount of solar energy received per unit area during a period of time, therefore energy and it is expressed in Wh/m2.
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