How to reap the benefits of bifacial solar modules

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 from Segen UK’s friendly technical department explains the benefits of bifacial PV systems.

What is a bifacial module?

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.

Segen’s portfolio 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.

0Bills stocks Bifacial modules from LG, JA Solar & mounting from Renusol.

CAN BIFACIAL MODULES BE USED ON ALL SITES AND ALL ROOFS?

In 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. (Renusol do a flat-roof mounting system designed for bifacial modules – see it here).

It’s worth noting that bifacial modules will boost annual yield by a meaningful %, and cost not much more than a monofacial module of the same rating. They should therefore be considered for any flat roof system.

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.

Do they work with any inverter?

In most cases, yes it will, but as with any other modules, we recommend checking each string design to make sure the inverters can cope with the current and voltage.

Bifacial modules produce more power because the current produced from the rear is added to the current from the front. The voltage will stay the same, so inverter string design isn’t that different to monofacial. Bifacial module datasheets show total maximum voltage and current, considering bifacial gain from the rear side, in practice these current values might never be reached. In real life, the rear side is expected to reach peak power at different times of the day compared to peak power from the front side. Because of that, even if the maximum string current (including bifacial gain) exceeds the maximum allowed by inverter mppt, it will hardly ever be exceeded long term.

Most inverter manufacturers allow the maximum current per tracker to be exceeded for a short period of time, during these times the peak power will be clipped without damaging the inverter.

At this stage its worth mentioning that it makes even more sense than usual to optimise modules on bifacial arrays. This is the best way of extracting the most energy from both sides.

What else should be considered?

Like standard flat roof systems there are few compromises that installers might have to make to ensure the cost of the system stays at a reasonable level. In theory the best option for bifacial would be to keep the modules at least 1m from the surface of a roof covered with a white membrane and at 40° tilt using a specialist bifacial mounting kit. In practice, this would be costly due to required ballast or required space – it might not be possible to fit on the given roof at all.

For that reason, the installer will need to try to compromise between the 4 factors listed above (Module height, Module tilt, Space between rows and Albedo) and price of the system as well as site restrictions.

To Conclude…

Modules make up the largest portion of the equipment cost of a PV system, making choosing correct module, one of the most important decisions to be taken at the design stage. Also, over 50% of European commercial and industrial roof-top projects are limited by the available roof space, while at the same time the properties usually have high energy requirements, so would benefit from as much PV power as possible. For that reason, using high power modules, such as bifacial, nowadays makes more sense than ever. Minimal increase in cost of module allows installation of more kWp of PV on limited roof space, while the cost of the rest of hardware and labour remains unchanged.

Need any more technical information? Call the technical department on +44 (0) 333 7720526 and look out for next month’s TTT.