THOUGHT LEADERSHIP

Bifacial solar PV: shining light on all the angles

In the booming global solar industry, installation of bifacial panels has been rapidly overtaking conventional monofacial modules, particularly in utility-scale projects but increasingly at smaller scales (<5 MW) too. But are they the right technical investment for your solar project – and what do you need to consider?

We recommend getting to grips with the benefits, constraints and implications of bifacial modules as early in the development cycle of a project as possible. Here are some observations to get you started.

What are the advantages of bifacial solar PV?

Bifacial solar PV modules are solar panels capable of generating electric current from both sides of the panel, as opposed to monofacial panels, which generate from one side only. Sunlight can pass through a transparent top layer and be absorbed by the solar cells, while sunlight reflected off surfaces can be captured through the transparent bottom layer, increasing the overall power output and potential energy yield.

The advantages of bifacial solar modules include:

• enhanced energy yields (typically 5% and can be up to 10% when optimised at particular sites) with only minor differences in supply cost
• lower levelised cost of energy (LCOE) with greater return on investment (ROI)
• increased duration of maximised power export
• enhanced performance in diffuse light conditions, such as when it is cloudy, which can be beneficial for the stability of hybrid power systems
• greater power density achieved in space-constrained sites
• better end-of-life outcomes, as glass is more readily recyclable than plastic polymers used for the backsheet of monofacial modules
• some manufacturers also claim improved durability and longevity of panels due to double glass construction rather than the glass and polymer backsheet of monofacial modules. This is claimed to be more resistant to environmental factors such as moisture, humidity and fluctuations in temperature. It has also been anecdotally suggested that the glass backface increases protection from water ingress and resistance to corrosion.

Are there any potential downsides?

Bifacial modules typically have a front-side glass thickness of 2 mm with 2 mm on the rear side, compared to monofacial modules which have 4 mm on the front side only. This can increase susceptibility to hail damage, which may require further mitigation measures in hail-prone areas and could increase the cost of insurance.

What’s albedo and why does it matter?

The more reflective a site, the better its prospects for gaining the bifacial edge. Generally, there is a linear correlation between the ground reflectance conditions (albedo) at the site and the power gain from the backside of the bifacial panels. Albedo is also the single largest factor driving bifacial gain.

But a site’s ground conditions will change over time, so one of the most important considerations when calculating the possible benefits of deploying bifacial over monofacial solar modules is determining what the long-term average albedo is at the site. Many factors can play a part in the way the albedo is modelled – including the intended use of the site once the solar plant is built, revegetation strategies, grazing livestock, the frequency of droughts and flooding events, precipitation volume and water pooling, how green the grass is, and the colour of the earth. The highest albedo factors and bifacial gain will be in conditions such as frost or snow, with its high level of reflectance. The lowest albedo factors are achieved on surfaces such as dry asphalt or grasslands.

Is more height a good thing?

Another major factor driving bifacial power is the height of the installation. Bifacial power gain increases with installation height as a greater angle is available for reflection of direct and diffuse irradiation to the rear side of the modules. This gain is most prominent typically between the installation heights of 0.5 and 1 metre before levelling out above 2 metres. In areas prone to flooding, higher installation may also provide extra resilience to increasing weather extremes.

An important consideration here, however, is that although higher installation may increase energy yield and financial returns, there may be considerable additional capital costs and greater complexity of construction of the mounting infrastructure, particularly for longer piles.

What’s the right ground cover ratio?

When the percentage of area covered by PV modules increases, the bifacial gains decrease. If more ground is covered, more area is shaded, and there will be less reflection to the rear side of PV modules. Often there is an incentive for developers to maximise the solar DC power capacity of a given site to avoid costly additional land agreements and minimise the project footprint. However, this can result in a high ground cover ratio (GCR) which can cause shading between rows. This increase in ground shading reduces backside power and energy yield gains (although it can sometimes be mitigated by the ‘backtracking’ capability of single-axis trackers).

Recently, we have been seeing developers take a more conservative approach with this in mind, preferring a GCR below or approaching 0.30.

What about shade from the mounting structure and cables?

Increasingly, manufacturers of mounting structures are looking towards maintaining structural integrity of their equipment while also minimising shading. String cabling can also be a cause of rear shading, so they should be fixed underneath the torque tubes of single-axis trackers (SAT) or underneath the mounting structure supports to minimise any impact. We are noticing an increasing focus on consistency of construction in this regard and the inclusion of this check on installation test certificates as minor shading on one module has the cascading effects of derating the entire string of modules.

Could spikes fry the electricals?

Although asset owners are most interested in the potential for greater energy yield from bifacial modules, it is necessary to also assess the electrical maximum power point voltage and current limits caused by spikes during high irradiance events. These spikes can be caused by a range of environmental factors which may be specific to sites. These include early morning frost at low temperatures, increasing sunlight irradiance at the edge of lensing clouds (magnifying glass effect), snowfall or flooding/water pooling.

In some areas which experience high ground albedo in conjunction with technical designs for favourable backside power gain, the maximum instantaneous bifacial gain can be as much as 15 to 25% for some Australian contexts, which can impact the allowable number of modules in a string as well as the input parameters to combiner boxes, inverters and cables throughout a project.

What’s next under the sun?

Solar is an exciting sector of rapid, continuous innovation, so there will no doubt be ongoing technological evolution with new implications and applications to explore. Regardless of whether bifacial panels are right for your project at this stage, it’s worthwhile considering all the options that might work best for your site. In the transition to net zero, every solar installation has a crucial role to play. The better the yield and value that can be achieved from a solar project, so much the better for the developer, the community, our environment and the future.

If you need support to assess energy yield, design, and technical considerations for your solar project, please contact our business development managers, Patrick Pease (Australia) or Shekhar Prince (international).

About the author

Lachlan McKenna is a renewable energy engineer in Entura’s renewables development team. He works on solar, wind and BESS projects from concept and design through to operations and repowering in locations throughout Australia and the Indo-Pacific region. Prior to working for Entura, Lachlan gained experience in the commercial and industrial rooftop solar sector and European offshore wind industries.

See our previous articles on how to achieve solar success:

8 May, 2024