pv magazine test – February 2019 results

Of the 28 days in February 2019, 20 were hazy days in Xi’an, due to the Chinese New Year firework activity. The bifacial boost in February almost doubled with respect to the January value. (See table below left for detailed information). Haze reduces direct irradiation, which results in lower yield from the front side, whereas the diffuse irradiation has a much higher impact on the rear side yield. In hazy conditions the diffuse portion is higher, therefore the bifacial boost goes up.

Bifacial and diffuse irradiation

Bifacial boost and diffuse irradiation stayed in sync for most of the month, except on Feb. 10th and 18th, when bifacial boost spiked to 55% and 22% respectively (normal range is 5% -10%). Our investigation of the hourly energy yield data as well as the review of the CCTV footage identified that the root cause was snow coverage on the monofacial modules on both days, which brought down the mono-facial energy yield and resulted in unusually high bifacial boost. It is known that bifacial modules produce energy from the rear side even when covered, which results in faster snow shedding. This, together with the increased albedo of snowed grounds gives a big advantage to bifacial modules in snow. George Touloupas

Notes on the energy yield measurements:

• The energy yield is given in Wh/Wp and calculated by dividing the energy produced by the module by the Pmax at STC of the module. This Pmax is the maximum STC power after a process of stabilization.
• The results are grouped in categories, per module type.
• The bifacial boost depends on many parameters: the bifaciality factor, the installation geometry, the albedo of the ground, and also the sun angle and diffuse irradiance. The ground in this case is a plastic cover simulating green grass.