The Rise of Bifacial Modules and Their Considerations in Utility-Scale Design

Bifacial solar modules have become the de facto choice for utility-scale PV ground mount projects in recent years. Unlike traditional modules, they capture light on both sides, offering production gains of 5-25% through their transparent back sheet design. While this newer technology is fairly straightforward, integrating bifacial modules effectively requires some considerations during the design phase – from racking selection to ground preparation. Understanding these nuances has become crucial as the technology moves from a more novel innovation to the industry standard.

Tune into this clip from HeatSpring instructor John Selby from the Utility-Scale Solar PV Design Applications course where he breaks down the basics of bifacial modules and the most important things to consider when incorporating them into your designs. 

​The bifacial module has just become almost ubiquitous for ground mount projects in my recent experience. So much so, that it seems like it’s almost a given. 

As you can see from this graph projection, the share of the market of installed power which is expected to be bifacial, is estimated to grow from 5% in 2017 to 90% by 2033. The adoption has been a drastic change to the market, but let’s spend a minute or two reviewing the difference anyways. 

While monofacial modules may have an opaque polymer or glass backsheet, as you can see here on the left, bifacial modules have a clear backside, which allows additional solar gain by exciting the silicon cells with rear side irradiance. This usually means that there can be a 5-25% increase in production over front-only modules. While the costs to manufacture bifacial modules are higher than monofacial modules, the increase to system output is worth it. 

It’s important to know what type of module will be used for a project, because it can influence the decisions that we’re about to make about racking style, ground spacing and other installation variables. There are also some design considerations to take into account, namely around ensuring that the rear side is exposed to as much rear-side light as possible. 

Here’s a nice diagram from NREL demonstrating all the different ways a bifacial module could receive that extra rear-side gain. 

The first is diffuse reflection, ambient light bouncing off objects and the ground and hitting the backside of the module. The other is beam reflection, or the main rays of the sun reflecting directly off the ground. The amount of light that bounces off the ground is determined by ground albedo and it’s expressed in a percentage of the light hitting the ground to the light which bounces off and gets to the back of the solar module. 

Even once you have the albedo factor established, the additional gain from your side of the solar module can be highly variable based on the azimuth, racking type, tilt, and the height off the ground.  

Here’s some albedo values for various types of ground surfaces which you might encounter for a solar project. As you can see, there’s a range for each type of ground cover. For instance, dry grass with a light brown color will reflect more light than a deep green grass. I won’t go into too much more detail here, but albedo is a key input factor for any system production modeling software, and can vary widely from site to site. 

Good weather data sets should also have hourly step albedo built into them so that the rear-side gain is accurately calculated, including days with high albedo due to snow cover for projects in colder climates.  

The last thing I’ll mention about bifacial modules is that the shading of the rear-side is a factor, just like it is on the front side. Ensuring that wiring, racking components, and other obstructions are minimized will help squeeze every bit of production out of the modules that you can get.  

Here we have two photos showing different styles of single axis trackers that have different rear-side shading factors from the torque tube and how that affects the energy production.