New publication from the SUNLAB: Progress in Photovoltaics
Where simplifying assumptions break-down: spectral vs broadband albedo use in bifacial photovoltaic modelling & measurement
Researchers at SUNLAB have comprehensively studied the effects of spectral albedo on system-level model uncertainty and indoor photovoltaic (PV) device measurements. This analysis – led by Erin Tonita, a PhD candidate in the SUNLAB – characterizes the conditions under which the spectral nature of ground cover causes model and measurement uncertainty on the order of several percent.
The ground cover underneath a PV array is often characterized by a single-valued albedo, the broadband albedo, for use in common PV models. Broadband albedo is calculated by integrating the spectral albedo over the standard solar spectrum, AM1.5G, from 280 nm to 3000 nm. This simplifying assumption ignores the distribution of photon energy over the PV module technology absorption range, allowing for faster model computation and use with standard solar simulator filters which target the AM1.5G spectrum. However, a particular ground condition may preferentially reflect or absorb light in the PV module technology’s absorption range, enhancing or diminishing total incident irradiance on PV modules. This effect is not captured by broadband assumptions, and instead requires the use of spectral albedo.
In this paper, SUNLAB and Arizona State University researchers analyze the effects of spectral albedo for:
10 ground conditions, including grass and snow;
South-facing fixed-tilt photovoltaic arrays and E-W single-axis tracking arrays;
30 locations, spanning latitudes between 15-75°N;
7 PV device technologies, with an in-depth analysis for silicon heterojunction devices;
Monofacial vs bifacial PV arrays;
Solar simulator measurements of silicon heterojunction mini-modules.
The key take-aways
Researchers measured a short-circuit current variation of up to 2% by either including or omitting spectral albedo effects in bifacial device measurements. For PV system modelling, ground-reflected irradiance constitutes between 2% and 32% of all irradiance incident on PV modules, highlighting the importance of accurate ground modelling. Spectral effects caused up to a ±13% predicted rear irradiance uncertainty.
Overall, spectral albedo effects were found to be most significant for:
Fixed-tilt PV arrays at high latitude;
Wide band-gap technologies, such as perovskite and CdTe modules;
Albedos which vary steeply over the technology absorption range;
High albedo ground covers, like snow.
In these cases, spectral albedo effects cause model and measurement uncertainty on the order of several percent.
Including the spectral nature of albedo affects both photovoltaic energy conversion efficiency and photovoltaic irradiance modelling uncertainty
Click here for the full article.
E. M. Tonita, C. E. Valdivia, A. C. J. Russell, M. Martinez-Szewczyk, M. I. Bertoni, and K. Hinzer, Quantifying spectral albedo effects on bifacial photovoltaic module measurements and system model predictions, Prog. Photovolt. Res. Appl., 1-13 (2024). DOI: 10.1002/pip.3789