Modeled solar radiation and meteorological parameters (C)


Accurate solar radiation datasets are required at all stages of development and operations of a solar generation facility. In most cases solar radiation measurements using ground-based instruments are only available on-site for about a year before a project receives financing. As solar resource varies significantly on an annual basis, understanding the inter-annual variability and collecting data for multiple years is important since the variability could range in the order of 5%-10% for Global Horizontal Irradiance (GHI) and up to 15% for Direct Normal Irradiance (DNI)(Gueymard & Wilcox, 2011).

Various stage of solar energy projects as shown in Figure 2 have different data requirements. As can be seen in this figure from the Solar Resource Handbook (Sengupta et al., 2017) all stages of a project require modeled solar resource information.

(C) Due diligence

At this point in the process there is a need for highly accurate ground measurements mostly measuring resource every minute coupled with modeled data that covers the period during which the ground data was collected. Normally, over a year of ground data is required. This ground data is then used for site-adaptation of the modeled data that captures seasonal and interannual variability. The expectation is that the site adapted modeled data is more accurate and can be used for developing the final design of the project and conducting a detail assessment of the financials of the project (see Chapter 8 of (Sengupta et al., 2017)).

Minimum or recommended values for the relevant specification items:

  • Spatial resolution: < 5km.
  • Region covered: power plant or nearby location .
  • Solar radiation variables included: GHI, DNI.
  • Accuracy / uncertainty of the solar radiation: <5%.
  • Time resolution: 15 minutes.
  • Time period: >10 years.
  • Extended variables: temperature, relative humidity, wind speed and direction.
TABLE 1. LOOK UP TABLE OF CATEGORIE AND USESGrey cells in Table 1 are later described. Cells width N.A. legend, means that no relevant applications are showed later.
Proyect developers needs at diferent plant stages Other users
Category of product service (A) Pre-feasibility (B) Feasibility & Design (C) Due Diligence Financing (D) Plant Accep-tance Test (E) Systems or Plant Operations (F) Grid operators (G) Policy makers (H) Education / Outreach

Modeled Solar Radiation series

It is known that the ground measurement stations are scarce, on the other hand, geostationary meteorological satellites have been available for most locations on the globe for periods of over 10 and sometimes 20 years (Figure 6). As these satellites have the necessary channels to sense and measure cloud properties and other meteorological datasets, they have been used extensively to model solar radiation over the long-term. These satellites provide data at high temporal (5 minutes to hourly) and spatial (2-5 km) and complement the short-term solar measurements taken on-site. Similarly, the ground solar measurement data can be used for site adaptation to remove biases in the satellite data. Details about the ground measurements and satellite derived dataset is described in Chapter 6 of (Sengupta et al., 2017).

Location of Geosatélites
Figure 6. Location of the current geostationary satellites that provide coverage around theglobe. Image from NASA

While solar radiation is the most important variable information, additional meteorological variables are required to model electric generation using various technologies such as photovoltaics and solar thermal technologies. Some of the meteorological parameters such as temperature, humidity, pressure, wind-speed and surface albedo are important. As these parameters are not regularly measured on a long-term basis at prospective sites modeled datasets form an important component of the requirements for solar generation modeling. These modeled data can derived from reanalysis datasets such as Modern-Era Retrospective Analysis for Research and Applications, version-2 (MERRA-2) from NASA(Gelaro et al., 2017); North American Regional Reanalysis (NARR)(Mesinger et al., 2006); Climate Forecast System Reanalysis from the National Center from Environmental Prediction (NCEP) and ERA-5 from the European Center for Medium Range Weather Forecasting (ECMWF).

The relevant features for the modeled solar radiation and meteorological parameters are:

  1. Spatial resolution. It is related with the grid size. Depends mainly on the data source for the estimation.
  2. Region covered. Usually, cover continents, countries, or specific zones.
  3. Solar radiation variables included.
  4. Accuracy / uncertainty of the solar radiation.
  5. Time resolution.
  6. Time period.
  7. Extended variables.