“Bifacial modules represent the largest step function improvement in project economics for minimal technology risk since the introduction of trackers.” This statement came from Jenya Meydbray, Vice President of Solar Technology at San Francisco based Cypress Creek Renewables, speaking at the U.S. National Renewable Energy Laboratory (NREL)’s BifiPV Workshop, where industry figures met in September 2018 to trade notes on bifacial performance.
And as bifacial modules roll off production lines and out into the field in ever expanding numbers, their potential for gains in energy output is quickly being proven, with increases typically anywhere between 5 and 20% being observed in projects.
But when it comes to system configuration in the field, there are many different factors affecting this performance boost at module, string and system level. And over a project’s lifetime these can add to millions of dollars in lost revenue. A recent study from Belgian research institute imec estimated that non-optimal system configuration in bifacial PV installations could result in as much as 40% in lost output, and the manufacturers of balance of systems components are responding with specialized offering designed to take advantage of the opportunity provided by bifacial module technology.
In 2019, installations featuring bifacial modules are beginning to appear at a large-scale. However, the technology is still held back by this difficulty in forecasting the output of a project – investors and project owners want a clear picture of how much energy their project will generate before committing to a technology. So, gaining a better understanding of the optimal locations and system configurations for bifacial installations will be key to their growth in the next few years.
Alongside the albedo of the surface below the modules, the angle and height of the modules, along with shading from system components and the distance between module rows, are among the key system configuration factors affecting the amount of light that reaches the rear side of a module, and therefore the energy gain, or ‘bifacial boost’. And this is where trackers come in.
With monofacial modules, trackers are a well-established technology known to be able to increase a project’s energy output by around 20%. It didn’t take the industry long to recognize the potential for trackers to be used in combination with bifacial modules for an even bigger gain. Tracker manufacturers are developing solutions tailored to bifacial modules and gathering data on the optimizations they can carry out to ensure that their systems get the most out of the modules. “A new technological race in the PV energy arena has begun,” as tracker manufacturer Soltec puts it in a recent white paper.
Spain headquartered tracker manufacturer Soltec is a leader in this research. In 2016, the company supplied trackers to one of the first large-scale PV installations to utilize bifacial modules, a project developed by Italian utility giant Enel Green Power to energize La Silla astronomical observatory in Chile’s Atacama Desert. Since its completion, data from this project shows an overall bifacial boost of 13%.
And in July 2018, Soltec inaugurated its Bifacial Tracker Evaluation Center (BiTEC), which aims to develop models that can forecast the energy output of a bifacial power plant.
BiTEC comprises 18 tracker installations with a total capacity of around 300 kW of modules. These are installed at varying heights and pitches, on a range of different colored surfaces. Soltec set up the center in collaboration with NREL, Renewable Energy Test Center and engineering firm Black & Veatch, one of the largest EPC firms operating in the United States. Leading module suppliers including Canadian Solar, Hanwha Q Cells, JinkoSolar, LG, Longi and REC have also participated in the center.
Mounting systems, and trackers in particular, provide plenty of opportunities for optimization. Many trackers on the market are designed with a torque tube in the center, covering part of the module’s rear side. Findings from BiTEC, which Soltec has now published in a white paper, entitled Bifacial Trackers, the Real Deal, showed that tube shading can result in a loss as high as 0.8% (both sides), and also that non-uniform irradiance across the module’s surface can result in non-uniform heating, which can damage performance in the long term.
To minimize shading from the torque tube, Soltec took a different approach with its SF7 bifacial tracker. Rather than mounting a single module with the torque tube at the center, the SF7 places two modules parallel to each other, in a portrait orientation, with a gap in the center below the tube. Calculations performed at BiTEC show that this reduces the rear shading factor – defined as the percentage of irradiance on the rear of a module lost due to rear shading (from 4.5% in a representative single module bifacial tracker, to 0.7% for the Soltec SF7 Bifacial). The design also allows for cabling to be run through the center of the torque tube, further reducing the potential for rear side shading.
On top of reducing rear side shading, Soltec’s SF7 Bifacial tracker also allows airflow between the modules, decreasing their operating temperature. Measurements from BiTEC show that modules installed in the ‘two in portrait’ set up of Soltec’s achieved a bifacial boost level 2.4% higher than a single module ‘one in portrait’ tracker type, 1.3% of this gain is attributed to the lower average module temperature.
Height and pitch
The height of modules, and the distance between rows (pitch) are also important factors when it comes to bifacial installations. A previous study carried out by TÜV Rheinland demonstrated that 2.5 meters is the optimal height for a bifacial module – below this height the bifacial boost is reduced, and there is no additional energy gain from positioning modules above this height – which presumably would bring more material costs for the mounting system as well.
Soltec has made similar findings when it comes to the pitch. Measurements performed at module level, on a system installed above a surface with 63% albedo, showed an additional 8.55% in energy output from modules placed 10 meters apart, compared with 8.7 meters. And this again increased a further 2.47% when the distance between rows is increased to 12 meters. This is attributed to the increase in reflective area below the modules.
The measurements taken at BiTEC clearly show the potential for bifacial technology to bring about significant gains in energy output, and the dependence of this on optimizations in installation and in trackers and other system components. Plenty more challenges lie ahead for bifacial PV, and the work being done at testing and research institutions such as BiTEC will be valuable in ensuring opportunities are not missed.