Silicon-based photovoltaic modules are currently the best tradeoff between cost, durability, and efficiency, in spite of the fact that for every six photons that hit the cell, only one is converted into usable electricity; the rest produce heat. Given that the fuel, sunlight, is free, that doesn't seem like such a bad deal. Land, on the other hand, is expensive, so it's in the industry's best interest to maximize the power-per-acre ratio. Recently, a few companies have experimented with bifacial solar modules, whose topsides work like conventional solar panels while their undersides convert diffuse and reflected light into electricity. Depending on the albedo (reflectivity) of the terrain, this could result in up to 30% more energy generated.
Laboratory Testing
In 2017, Sandia National Laboratories published a report showing the results of a year-long experiment comparing monofacial and bifacial modules. The results were promising - not only did the bifacial modules produce more energy overall, they also smoothed out the power curve. The bifacial gains were more prominent in the early mornings and late evenings, while midday gains were smaller.
Furthermore, bifacial modules facing west generated more total energy than monofacial modules facing due south, with the bifacial panels delivering more power during peak demand times. The experiment was limited, however, with just a few modules in various orientations.
Field Data
It's a good start when the folks in white lab coats publish favorable data, but a rooftop with thirty-two modules doesn't replicate the average PV farm. That's why JA Solar, a maker of monofacial and bifacial modules, is conducting field studies using both types of modules in the same location, allowing them to compare the results under real-world conditions. They're using the data to develop simulation tools that can model a system ahead of time to determine whether the additional up-front cost of the bifacial modules is justifiable in terms of levelized cost of energy (LCOE).
The first experiment is a relatively small 6.4 kW rooftop array at the Yangzhou R&D center in China, with modules facing due south at a fixed twenty-six-degree tilt angle. The bifacial panels generated almost eight percent more energy than monofacial modules over an eleven month period.
A more impressive 160 MW utility-scale array in China's Qinghai province featured a blend of monofacial and bifacial modules, mounted to a single-axis tracking system. The bifacial experimental modules outperformed the monofacial control group by over ten percent.
In addition to those arrays, the company conducted studies throughout Europe, experimenting with different tilt angles, albedos, and mounting elevations. If you're interested in the details, you can watch a webinar recording and download the slides at PVTech's site. (You'll need to register for a free account first.)
General Conclusions
Although bifacial modules are more costly to produce, increasing the capital expenditure of a PV farm, the operation and maintenance costs are on par with monofacial arrays under most conditions. In desert locations, however, module cleaning may be a bit more complex.
All other things being equal, increasing the height of the modules by an extra meter or so improves the bifacial gain. That bodes well for integrating PV farms into agricultural plots.
Because it takes fewer bifacial panels to generate the same amount of electricity, the balance-of-system cost, such as mounting hardware, wiring, and tracking devices, decreases. This partially offsets the higher module cost.
Modeling and simulation of bifacial arrays are still in their infancy, leaving plenty of room for applied researchers to conduct more studies. Because their performance varies with so many factors, it's likely that an optimal solar farm will consist of both types of modules, each in their ideal locations.
As wind farm designers are learning, one size does not fit all when it comes to sustainable energy. The bifacial PV module will soon find its place in the industry.
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