How to reap the benefits of bifacial modules and design them into your next system
In the second Techie Top Tip of 2019, Jordan Wiley friendly technical department explains the benefits of bifacial PV systems.
What is bifacial?
The bifacial PV module offers greater power output when compared to conventional monofacial PV modules, due to its ability to harvest light that is reflected onto the rear side. The reflected light can come from a variety of sources, such as reflection from the ground or from a neighbouring row of PV modules.
Bifacial PV modules don’t have a white backsheet but instead are constructed with a transparent backsheet (or glass) on the rear. These types of modules are more durable because both sides are UV resistant. In addition, using glass back sheet and frameless design (certain modules only) dramatically reduces the PID (potential induced degradation) effect.
ITS Technologies range consists of framed Bifacial modules because of how easily they can be installed with any standard mounting solution we already stock. Despite the frameless modules offering even greater benefits, sometimes the cost and difficulty in sourcing bespoke mounting solutions can outweigh the advantages that offset the savings.
n theory there is nothing against putting bifacial modules on any site, even on standard tiled pitched roofs.
However, in practice, that might not be the most cost-effective solution and any potential advantages of bifacial modules will be lost. Since bifacial modules absorb light from the back as well as front, it’s important to use a mounting system that increases the so called ‘bifacial’ gain. Based on many trials, case studies and previous installations it has been established that bifacial modules offer highest increase in yield (in comparison to monofacial) on ground mount or flat roof installations.
The mounting system should be designed to cast as little shadow at the back of the modules as possible and ideally should be approved for bifacial modules.
What does bifacial gain depend on?
A bifacial PV Module’s performance depends on various conditions, such as system design, installation methods, location, etc. There are four main factors that have most effect on bifacial gain of the module.
1 - Module height above the ground
It's recommended that modules are fitted at least 20cm off the surface, but the larger the distance from the surface, the higher the bifacial gain will be. It’s also worth noting that this rule is only valid up to approx. 1m. Above 1m height, the bifacial gain increase is minimal.
2 - Module tilt angle
As with standard PV installations, the higher the tilt angle the better the bifacial gain. Modules with as little as 10° tilt can benefit from bifacial technology.
3 - Space between rows (the pitch)
Increasing row spacing allows more light to reach the ground and rebound to hit the modules backsheet.
4 - Surrounding ground - Albedo.
This term might be new to most installers. Albedo depends on reflectivity of the surface - the general rule is, the brighter the surface the more the surface reflects light, therefore higher albedo. The higher the albedo value the bigger the bifacial gain, so additional power can be extracted from the module. For that reason, the highest bifacial gain can be achieved on a white roof or snow-covered ground. Slightly lower, but still considerable gains, can be achieved on a light grey/beige roof. The lowest bifacial gain will be achieved on Asphalt or black covering, but because of potential considerable yield boost at only minimal price increase, bifacial should be considered for any flat roof systems.