- Satellite-derived measurement assesses ground reflection through varying satellite viewpoints over time, particularly via MODIS sensors on Terra and Aqua satellites since 2001
- BRDF Analysis utilizes multi-angle data from MODIS to determine the bidirectional reflectance distribution function (BRDF), describing surface reflectance changes with varying illumination and viewing angles
- Satellite data offer various albedo values, including black-sky and white-sky albedos, crucial for understanding surface reflectance under different lighting conditions
- Despite providing valuable insights, satellite-derived albedo values represent mean reflectance over sizable areas (approximately 4 km²), necessitating high-resolution data for precise analysis
For satellite-derived albedo values, the ground reflection is measured from a changing satellite viewpoint over several days. There are several satellite albedo data providers offering datasets with different resolution degrees. Most established are datasets using moderate resolution imaging spectroradiometer (MODIS) sensors, which have been onboard the Terra and Aqua satellites since 2001. Thereby, multi-angle data for clear skies are used to determine the bidirectional reflectance distribution function (BRDF), which mathematically describes the changes in reflectance observed when an illuminated surface is viewed from different angles (from one direction in the hemisphere into another direction in the hemisphere). The latest version of MODIS data contains daily albedo values differentiated into 7 spectral bands (0.47 μm, 0.55 μm, 0.67 μm, 0.86 μm, 1.24 μm, 1.64 μm, 2.1 μm) and 3 broadbands (short infrared, visible and near infrared) at a spatial resolution of 1 km (see Albedo On-site Measurements).
Additionally, different albedo values such as white and black-sky albedos are given (see graphic p. 20). Black-sky albedo is derived just by the reflection of direct sunlight, while white-sky albedo takes into consideration ideal diffuse radiation from skylight. The actual albedo, also called blue-sky albedo, is derived from the below mathematical expression using the black- and white-sky albedo values linked with the mixing coefficient λ.
Satellite data, however, report only an estimation of the surface albedo, since irradiance and the corresponding reflectance are not measured at the surface. In addition, each albedo data point or pixel in the satellite measurements depicts the mean albedo value in an approximately 4 km2 area. Given high variability of albedo, recent studies, as highlighted in the technical paper from Ortega et al., emphasize the need for the solar industry to use high-resolution albedo data with data for areas smaller than 100 m.
In summary, precise estimation of albedo values is important to estimate the financial feasibility of a bifacial system on the one hand. At the same time, the energy from the rear side in many cases is not too relevant, and the uncertainties obtained in the albedo value cause little variation in the total energy of the PV system. For the cases where it has high relevance, the research community has been developing models, such as the one described in the above-mentioned paper of Ortega et al., to estimate the albedo more precisely.
The text is an excerpt from the TaiyangNews Bifacial Solar Systems 2024 Report, which can be downloaded for free here.
