A bifacial solar panel can generate more than 100 additional watts!!
What are the technical touches to achieve this increase?
First, let's talk about bifacial panels in general, how to measure them, and then how to benefit from them.
Bifacial solar panels are panels designed to capture solar radiation from both the front and back sides.
The front side receives direct and diffused radiation, like conventional panels, while the back side benefits from radiation reflected from the ground or surrounding surfaces.
This design increases energy production and reduces the cost of electricity produced.
When discussing the performance of these panels, the concept of BNPI (Bifacial Nominal Power Index) comes to mind. Datasheets typically display two sets of values:
• STC (Standard Test Conditions): These are the standard test conditions for a front-only radiation of 1000 W/m², a cell temperature of 25°C, and a 1.5 AM spectrum.
• BNPI (Bifacial Nominal Power Index): This is a test condition for bifacial panels, where the test is performed at a frontal irradiance of 1000 W/m² + a rear irradiance of 135 W/m² (i.e., an approximate increase of 13.5%, representing an albedo of 0.15).
BNPI: This is a standard test condition for bifacial panels.
• It indicates the increase in:
• Short-circuit current (Isc)
• Current at maximum power (Impp)
• Maximum power (Pmpp)
• This is performed when there is back radiation in addition to front radiation.
• It is certified according to IEC TS 60904-1-2 (2019) standard for measuring current-voltage curves for bifacial photovoltaic panels.
Importance of BNPI:
• It provides a standardized means for comparing bifacial panels.
• It prevents confusion between the actual elevation gain at the location (based on albedo, elevation, and inclination angle) and the laboratory-measured elevation gain.
Its inclusion in the data sheets ensures data compliance with IEC standards.
The background radiation can be calculated according to the following equation:
Back radiation = Forward radiation × Albedo × Geometric coefficient (f)
For example, with an albedo of 0.3 and a geometric coefficient of f = 0.55, the value is:
1000 × 0.3 × 0.55 = 165 W/m²
Typical values for albedo:
• White soil or light sand ≈ 0.6
• Light concrete ≈ 0.3–0.35
• Brown soil or grass ≈ 0.2–0.25
• Black surfaces ≈ 0.05–0.1
The geometric coefficient (f) expresses the extent of the "visibility" of the back/back of the panel to the ground surface and obstructions (rails, pipes, adjacent rows). The greater the visibility, the fewer the obstructions and the higher the f.
• In the field, f often ranges between 0.3–0.65, depending on the design. Improving f is achieved by:
• Raising the panel ≥ 1 meter above the ground.
• Reducing shading between rows (lowering GCR).
• Moderate slope angle (10–20°).
• Reducing blocking from rails and cables (≤ 10% of the area).
• Using a single or dual tracker whenever possible.
Bifaciality Factor (BIF) is the ratio of the performance of the back face to the front face at the same irradiance. It varies depending on the technology:
• TOPCon: approximately 75–85%
• HJT: up to 90% or more
• Back Contact Bifacial: between 80–90%
Example:
We have a solar panel with a front power of 760 watts and a bifaciality of 80%:
• Theoretical power of the back face at the same front irradiance = 760 x 0.80 = 608 watts.
• At an albedo of 0.3 and a geometric factor of f = 0.55: Rear radiation = 1000 × 0.3 × 0.55 = 165 W/m².
• This is equivalent to 16.5% of the front.
• Added power from the rear = 608 × 0.165 ≈ 100 W.
• Expected total power = 760 + 100 = 860 W.
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