Bifacial Panels: What Is Under the Array Decides

A bifacial module converts light arriving at either face. The front behaves like any module. The rear converts whatever is reflected from the surface beneath and around the array, which means the module’s output is partly a property of the installation rather than of the module.

That is the useful way to think about them: the specification describes what the module can do, and the site decides how much of that is realised.

What Governs the Rear Contribution

Albedo. The fraction of light a surface reflects. A light, diffusely reflecting surface returns a great deal; a dark one returns little. This is the dominant factor.

Height above the surface. A module close to the ground illuminates a narrow band of it and receives reflection from a small area. Raising it lets light reflect from a wider area onto the whole rear face.

Tilt. Affects both how much of the sky the front sees and the geometry of reflection reaching the rear.

Rear-face shading. Rails, clamps, junction boxes and cable management all sit behind the module and block reflected light. Racking designed for bifacial modules minimises this, and racking that was not can remove much of the benefit.

Row spacing. In multi-row arrays, the rows shade the ground between them, reducing the area available to reflect.

Where They Make Sense

Ground-mounted arrays over reflective surfaces are the clear case. There is room to raise the modules, the surface beneath can be chosen or prepared, and row spacing can be set with reflection in mind.

Flat roofs with light-coloured membranes are a reasonable case, particularly with tilted racking that lifts the modules clear.

Elevated structures such as carports are interesting, because there is height and often a light-coloured surface beneath. See solar carports.

Vertical east-west installations are a distinct application rather than a variation. Modules mounted vertically with faces to the east and west produce in the morning and afternoon rather than at midday, which suits a load profile with peaks at both ends of the day and allows the land between rows to be used.

Flush-mounted residential roof arrays are the weakest case. The rear face sees a dark surface a few centimetres away.

Durability

Most bifacial modules use glass on both faces rather than a polymer backsheet. Glass-glass construction is more resistant to moisture ingress and to some degradation mechanisms, and it is heavier.

That weight matters for roof structures and for racking selection, and it is a consideration independent of the bifacial gain. See panel degradation.

Estimating Realistically

Manufacturer figures for bifacial gain are produced under defined conditions with a specified reflective surface. Those conditions describe a good case rather than a typical one.

For a real project, the gain should be modelled with the actual surface, height, tilt and row spacing. Modelling software handles this, and the difference between a modelled figure and a datasheet figure is frequently large.

Seasonal variation is real: a site with snow cover gains substantially in winter and much less in summer, and a site whose surface darkens with dirt or vegetation loses gain over time.

Electrical Consequences

A module producing from both faces produces more current than its front face alone would, and that has consequences beyond the energy figure.

Conductors, fuses and combiner equipment are sized from the module’s rated current with the code’s margins applied. Where rear-face contribution raises the actual current, that sizing has to account for it rather than being taken from the front-face rating.

The inverter sees a higher input under favourable conditions too, which interacts with how much module capacity is fitted relative to inverter capacity. A design that clips occasionally on a monofacial array may clip more on a bifacial one.

These are design questions for whoever specifies the system, and they are a reason bifacial modules are not a straight substitution into a design drawn for conventional ones.

Soiling and the Rear Face

The rear face collects dirt differently from the front. It is not rained on directly, so it does not receive the cleaning effect that keeps a front face reasonably clear.

On a ground mount, splash from rain hitting the surface below can throw soil onto the lower part of the rear face, and that band stays dirty. On a flat roof, dust accumulates.

Rear-face soiling is not usually addressed in cleaning routines, which are written for the front. Where rear contribution is a meaningful part of the expected output, it belongs in the maintenance plan; see panel cleaning.

What People Get Wrong About This

The first mistake is fitting bifacial modules flush to a sloped roof and expecting a meaningful rear contribution. There is nothing behind them to reflect.

The second is taking the datasheet gain as what the installation will produce. It describes test conditions.

A third is using racking that shades much of the rear face, which removes a large part of what was being paid for.

A fourth is ignoring the weight of glass-glass construction in the structural assessment.

And the last is assuming the surface stays as it was at commissioning. Gravel darkens, grass grows, membranes weather, and the rear contribution falls with them.

Related

Types of solar panels covers the wider range of module construction, and tilt and azimuth covers the geometry that governs the front face and interacts with the rear.