This Week in Green Tech: Solar with the Lowest LCOE, Hydrogen Through Photocatalysis, and Engineering High-Power Solar Panels
Tom Lombardo posted on June 09, 2020 |
Bifacial solar with tracking gives the most bang for the buck, and more.

A study shows that bifacial solar modules with single-axis tracking give the most bang for the buck, a white paper discusses the engineering of high-power solar panels, and scientists extract hydrogen with near-perfect quantum efficiency.

Study Says Bifacial Solar with Single-Axis Tracking Delivers Lowest LCOE

Bifacial modules and tracking systems can both increase the production of a photovoltaic (PV) farm, but they also drive up the initial cost of the array, leaving designers wondering whether the added cost is worth the investment. A group of researchers decided to analyze the yield potential and cost-effectiveness of bifacial modules, fixed-tilt mounts, single-axis trackers, and dual-axis trackers to see which, if any, provides the most bang for the buck. In the electric utility world, the technical term for “bang for the buck” is “levelized cost of electricity (LCOE),” and in this context, it’s actually “bucks for the bang”—lower numbers are better. The researchers developed models of the various systems using actual data from a variety of locations, cross-referenced a multitude of factors to validate the models, and ran a slew of simulations.

Tracking Systems. (Image courtesy of Rodrıguez-Gallegos et al.)
Tracking Systems. (Image courtesy of Rodrıguez-Gallegos et al.)

The results, which were reported in the energy journal Joule, show that a combination of bifacial modules and dual-axis tracking delivers the most total energy. That’s not surprising, but it doesn’t take into account the added cost of the bifacial modules or the tracking systems. When those costs were factored in, the researchers discovered that for more than 90 percent of the analyzed land area (i.e., regions within 60 degrees of the equator), bifacial modules with single-axis tracking deliver the lowest LCOE. (In regions close to the equator, single-axis tracking with horizontal tilt is the best. Farther from the equator, single-axis tracking with the rows tilted toward the equator is preferable.)

Tracking is the single largest factor that affects yield, but in most cases, bifaciality makes a contribution as well. The following table shows the comparison between any two factors:

LCOE comparison (lower numbers are better). (Image courtesy of Rodrıguez-Gallegos et al.)
LCOE comparison (lower numbers are better). (Image courtesy of Rodrıguez-Gallegos et al.)

To interpret the table, note that each cell represents the ratio of the column (the “experimental” factor) with the row (the “comparison” factor). Looking at the Bifacial-1T column and the Monofacial-1T row, we see that the bifacial LCOE is 97 percent of the monofacial. In other words, for every million dollars spent per unit of energy from a monofacial system, you’ll only spend $970,000 to get the same amount of energy from the bifacial system. While not nearly as dramatic as the difference between tracking and fixed, it does offer an advantage worth considering. 

Note, however, that while the researchers had access to each site’s albedo (ground reflectivity) and did factor that into their energy calculations for each site, the table above shows overall averages. Albedo is the strongest influencer of bifacial gain, so a site with a high albedo may outperform the averages in the table, while one with low albedo won’t fare as well.

Engineering a 500-Watt Solar Panel

Trina Solar is known for its high-power photovoltaic (PV) panels. Ever wonder how they manage to eke out as much efficiency as they can from those modules? Well, you’re in luck, because the company released a white paper that sheds some light on the technology of its 500-watt Vertex solar panels. You can read the paper for all the details, but I’ll discuss one feature that lowers the cost and another that increases performance.

High-performance solar module. (Image courtesy of Trina Solar.)
High-performance solar module. (Image courtesy of Trina Solar.)

Silicon PV cells are made from the same material as the semiconductor chips found in nearly every electronic device, including the one you’re using to read this article. Purified silicon is rolled into a cylinder shape and sliced into ultrathin wafers. The larger the wafer diameter, the lower the cost per unit of area, since it takes the same number of steps to process a big wafer as it does to handle a small one. The industry is moving to wafers that are 210 mm (8.27 in) in diameter.

Wafers are then split into individual cells. The cutting process can cause defects in the silicon, which affects each cell’s mechanical strength as well as its electrical properties, so you might think larger cells are preferable. However, since the cell current is proportional to area, larger cells would produce high currents, resulting in the panels getting hotter and losing more power due to resistance in the wires. The ideal panel should produce high voltages and low currents.

In order to reduce cell current, PV cells are often cut in half (known as half-cell technology), which decreases power loss and reduces the required wire size. With even bigger wafers, Trina decided to go a step further: one-third cell technology. Why not one-fourth or even smaller? According to the white paper, “Trina Solar’s engineering team calculated the theoretical module power based on the different busbar numbers in combination with the different options to cut the cells in smaller pieces.” The results are shown in the following image:

(Image courtesy of Trina Solar.)
(Image courtesy of Trina Solar.)

As you can see, cells cut into fourths and fifths performed incrementally better, but after cells are cut into thirds, the law of diminishing returns makes it not worth pursuing at this time.

But there’s a trade-off: the more cuts made to a cell, the greater the potential for cell defects. Again, the white paper explains, “In order to overcome the risk, Trina Solar has adopted a non-destructive low-temperature cutting technology based on the principle of thermal expansion and contraction. Under heat stress, the wafer separates by itself. The cutting surface is very smooth without any micro-cracks. A [non-destructive cutting] NDC cell has a similar strength and mechanical robustness as a full cell and greatly surpasses that of the traditionally cut ones.”

A few other innovations include multi-busbar (MBB) technology, optimized cell layout, high-density cell interconnect technology, and hot-spot prevention. (See the white paper for details.)

Company white papers are generally used to plug their products, and this one does its share of corporate cheerleading. But regardless of that, I found it to be very informative about PV technology itself, independent of the brand. This paper would be great as a classroom/textbook supplement to discuss the pros and cons of modern PV technologies.

Water Splitting with Near-Perfect Efficiency

Hydrogen is nature’s near-perfect energy carrier and energy storage system—it’s light, clean-burning, and abundant. Unfortunately, the ability to extract pure hydrogen in a sustainable, efficient, and cost-effective manner has been “10 years away” for many decades. Researchers at Shinshu University recently made a breakthrough that could stop the perpetual shifting of that 10-year mark. Using a method called photocatalysis—employing light to split water molecules—the researchers experimented with the various arrangements of catalysts and found a combination that isolated hydrogen with 96 percent efficiency. Their work was published in Nature.

The study was limited to using a narrow UV portion of the solar spectrum, so more work needs to be done on using a bigger part of the solar resource, but the research lays a solid foundation for selecting and arranging materials that can absorb more of the solar spectrum. 

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