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Due to the impact that surface albedo has on PV yield calculations, particularly for bifacial systems, there has been an increased interest in learning about this parameter. These are the most typical questions we are receiving and their corresponding answers:
Bifacial PV modules is not a new technology, but it is gaining popularity amongst project developers. Bifacial modules offer several advantages in comparison with monofacial modules:
There are a few major factors that influence the bifacial gain. To get the most out of your bifacial modules you should consider:
Surface albedo (from the Latin word albus, which means white) is the measure of diffuse reflection of solar radiation from the ground back to space of incidence. It is dimensionless and measured on a scale from 0 (a black surface that absorbs all radiation) to 1 (a surface that reflects 100% of received radiation to space of incidence). So, for instance, a surface albedo value of 0.2 means that the surface reflects 20% of the received radiation.
This seemingly simple definition has relatively complex physical implications. Albedo is not a constant surface property of a particular surface. It is also dependent on atmospheric parameters and illuminating conditions. Albedo values vary in different temporal scales: minute, daily, seasonal and even interannual.
Typical daily variation of albedo (purple line) as measured at the clear-sky day of the 2nd of January 2011, in BSRN Bondville station, US:
Since albedo values are not constant, a typical range of values can only serve as a first preliminary estimate:
The historical values of surface albedo are a critical input for accurate yield simulation of bifacial PV projects. There are two main ways of obtaining historical albedo values at any location globally:
Weather events are often accompanied by clouds and satellites cannot "see" the Earth’s surface in the presence of clouds. In particular, data gaps due to snow events are especially challenging. We have observed under-detection and over-detection of snow albedo in all the sources we have checked. The reasons for this are:
To solve this, the strategy to generate (daily) albedo is based on a weighted average of 16-days time window centred on the day of interest. This approach, followed by satellite-based MODIS products, allows to partially fill the gaps.
However, in some regions and moments of the year, this is not possible to do because of the strong cloud persistence. In such cases, for running a complete gap-filling we need to consider additional sources such as:
When combining different sources, we want to reach an optimum balance between both accuracy and spatial representation.
Several years of data is required to capture the full temporal variability of the surface albedo of a particular site. Having information about the land use on that particular site is also of great interest to interpret any changes in the data and assess the representativeness of the area covered.
Surface albedo can be measured with an albedometer, a combination of two individual pyranometers -optimally identical- facing up to the sky in the horizontal position, as usual for measuring GHI (Global Horizontal Irradiance), and facing down to the ground for the GRI (Global Reflected Irradiance). The quotient of both magnitudes is the surface albedo.
Hukseflux SRA11 albedometer
Although installing the sensor in the POA (plane-of-array) is useful to know the GTI (Global Tilted Irradiance) that is received on the backside of the panels, surface albedo cannot be measured in such position.
Ground-based albedo measurements, covering only a few days is not enough for having a robust one-to-one comparison between albedometers and satellite-based data. To cover full seasonal behaviour, we recommend measuring albedo for at least one-year period. A slightly shorter measurement period can be considered depending on the location.
Yes, Solargis has prepared an albedo database that is tailored to the needs of the solar power industry. The database has been prepared by filtering available sources (coming from both satellite-based sources like MODIS and NWP like ERA5), properly combining them, applying downscaling methods, and running additional corrections. The data are available via Solargis products Prospect (as a new feature in the Professional Plan) and Evaluate (as an add-on to the Professional time series):
On the left, representation of surface albedo database in lower resolution with gaps. On the right, representation of Solargis albedo database:
Table with a description of the albedo data products offered by Solargis:
You can know more about technical specification of Albedo products in Solargis here: https://solargis.com/docs/methodology/albedo
The Solargis albedo database has been validated using ground measurements. At the date of publication of this text, we have validated the database at8 sites across the USA belonging to AMERIFLUX and SURFRAD ground stations networks. The period of evaluation we have used was five years, from 2011 to 2015.
After running the comparison, the validation results of Solargis albedo versus ground measured data are (in terms of the estimated uncertainty):
Solargis database obtained the lowest bias in comparison to other sources of data (some of them used as an input for the database). Additional validation and local adjustment of the values provided by the database can be done if there are sufficient ground measurements available at the project site.
The averages of the Solargis Albedo time series (Evaluate) may not be coincident with Solargis Albedo monthly averages (Prospect). The reasons for this are:
Representation of Solargis monthly averages of Albedo for a sample location in Plataforma Solar de Almería, Spain:
Representation of Solargis time series of Albedo for the same sample location in Plataforma Solar de Almería, Spain:
If you want to request your sample of Solargis albedo data, or you have any questions please contact us