The National Renewable Energy Lab is building the first national database of albedo, or reflected light values, to help model the performance of bifacial solar panels, the highest-yield type of panel in the industry thus far. The database is an outgrowth of a large field test of bifacial panels at the lab, which includes a variety of fixed-mount and tracker systems and a mix of bifacial panels from different manufacturers.
The US Department of Energy-supported project will be a multi-year database of “measured and quality assessed albedo values with temporal resolution of one hour or less for numerous locations and types of ground surfaces,” according to Bill Marion, the Principal Engineer of the project.
Albedo is defined as the fraction of solar radiation that is reflected. The solar energy community defines albedo as the fraction of solar radiation that is reflected from the ground and ground cover — like grass or gravel, and bodies of water on the surface of the earth. Astronomers, meteorologists include reflectance by clouds and air, but tracker companies often refer to this segment of light capture as diffuse light. To reduce confusion, some solar researchers use the term ground reflectance for albedo, according to NREL.
Site meta data for each location will include pictures, instruments, and calibration history, soil and vegetation types, etc.. Site albedo statistics for each location will include diurnal profiles, monthly and annual means, and interannual variability.
The relative strength of the albedo and the ability of a bifacial panel to capture that light determines the “bifacial boost” in yield beyond a standard single-sided solar panel. Bench flash testing of bifacial panels suggest that different manufacturer’s panels could theoretically absorb up to 90+% of the albedo that reaches them.
However, because there are many variables involved in positioning the bifacial panels on a particular site to maximize the albedo absorption, the field testing of bifacial panels thus far shows low levels of absorption in the 10% to 15% range, generally.
Advanced testing of bifacial panels has returned boost levels of over 20%, and bifacial researchers suggest that a 30%+ boost will become the norm one day. That level of performance will require the mastering of half a dozen basic orientation factors, including ground cover, distance of the panel from the ground, angle of the panel, location of the torque tube (if there is one in the tracker design), location of wiring and microinverters or other electronics add-ons, and other factors.
The difference between the bench testing boost and the field test results is referred to as a mismatch. The NREL database will include “Analysis of the spectral mismatch for bifacial PV modules of the ground-reflected radiation for anticipated deployment surfaces (using both measured and modeled spectral albedo data).”
The NREL database will include data from the solar industry at large, where offered. “We are also asking the PV industry to provide their albedo measurements and take part in a cooperative effort to create an expansive and accurate data base of albedo values,” says Marion. “The albedo data should consist of measurements by two horizontal pyranometers, one facing the sky and the other inverted and facing the ground. Albedo data measured using reference cells is also sought,” he advises solar companies.
NREL also is asking the industry to share information on “Meta data related to the installation and equipment, as well as general maintenance information (instrument cleaning and calibration intervals, etc.).”
Meteorological data is also being requested by NREL. “Additional meteorological data for quality assessment and general PV performance modeling would also be very useful if available. This includes parameters such as direct normal and diffuse horizontal irradiance, ambient dry bulb temperature, relative humidity, wind speed and direction, snowfall, and rainfall (wet soils have lower albedo),” Marion advises.
The database also will compare the field test data with satellite data to correlate the accuracy of the two in use together. The database will also include “Comparisons of the measured albedos with available satellite-derived MODIS albedo products to validate the MODIS albedo products for estimating bifacial PV system performance,” Marion says. MODIS is NASA’s Moderate Resolution Imaging Spectroradiometer satellite system.
Finally, the database also will include data from the SURFRAD and Ameriflux measurement networks. SURFRAD is NOAA’s Surface Radiation Budget Network. Ameriflux is a “DOE network of PI-managed sites measuring ecosystem CO2, water, and energy fluxes in North, Central and South America. It was established to connect research on field sites representing major climate and ecological biomes, including tundra, grasslands, savanna, crops, and conifer, deciduous, and tropical forests,” the network states.
NREL will begin to populate the database in the coming months and the data and related products will then be available via the DuraMAT website.