Photovoltaic manufacturer Cogenra recently broke the record for power generated by a solar module made with single-junction monocrystalline silicon cells. Where other 72-cell modules max out at 350 Watts, Cogenra is making modules that deliver 400 Watts with the same size panel. I found two of their patent applications (string configuration and shingling) to learn how modules made with their Dense Cell Interconnect technology can deliver 15% more power with the same basic cells.
The picture above shows a standard PV panel on the left and Cogenra’s new panel on the right. On the standard panel, the silver grid pattern is the busbar, a set of wires that interconnect the cells. In this configuration, the cells are wired in series to produce 24 volts (nominal) or 36 volts (open circuit). Since the busbar carries current but doesn’t produce any power, its appearance on the panel takes up valuable space that could be used for more PV material. Also, the cells must be soldered to the busbar. According to a report by the National Renewable Energy Laboratory, solder joints and interconnections are the primary cause of module degradation and failure as solar panels age. Removing the busbars would improve a panel’s power output and increase its reliability. But how?
Shingles
Engineers at Cogenra may have worked their way through college as roofers, because they designed a solar panel that mimics the shingle structure on rooftops. Instead of the solar cells sitting side-by-side and connected with wires, the cells are arranged in a shingle fashion, with a solder-paste material making the inter-cell connections. (The angles in the following image are exaggerated. The components are just a few microns thick, so the cells are, for all practical purposes, flat.)
In effect, the busbars are hidden, so they don’t use valuable real-estate on the sun-facing surface of the panel. And keeping the solder joints out of the sunlight could reduce the degradation of the panels as they age. According to the patent application, “The lack of interconnection materials may remove some problems associated with uneven thermal expansion of conventional solar cells.” I’m a little concerned with the use of the word “may” in that statement, but I assume that they’ll perform some sort of accelerated testing to see the actual effect of repeated heat-cool cycles on the modules.
Cogenra claims that its manufacturing process is similar to that of conventional PV panels, so it wouldn’t take much to retool a factory to use the new method.
More Power
Independent testing by the Renewable Energy Test Center confirmed that Cogenra’s module produced 400 Watts, a new world record for a solar module made with single-junction monocrystalline silicon cells. (Cogenra’s process also works with the less expensive and less efficient polycrystalline cells, although such a module would only produce about 350 Watts instead of 400.)
A rooftop with a 70m2 unshaded southern exposure might fit about 30 PV panels on it. If those were standard panels, the array would produce about 10.5kW DC. With the Cogenra modules, the same roof would be able to produce closer to 12 kW DC. But because we typically size the array to meet the power needs, the original 10.5 kW could be achieved with just 26 Cogenra modules. Even if the modules were a little more expensive, using fewer panels reduces the balance of system and installation cost.
Shade-Tolerance
A photovoltaic cell is basically a light-controlled current source. When lit, each cell will produce about 0.6V, but its current will vary dramatically with the intensity of light. Since the cells in a panel are wired in series and all elements in series experience the same current, the panel’s output current is limited by the current produced by the weakest cell, much like a chain’s strength is limited by its weakest link. As a result, shading even a small fraction of a panel can reduce its total current output by 50% or more. Power is the product of voltage times current, so a little shade causes a big power loss.
To alleviate the shading problem, most PV modules come with bypass diodes that basically short circuit a group of poorly performing (i.e. shaded) cells. These diodes are soldered to the panel, increasing its complexity and cost, and decreasing its reliability. It’s common for a panel to have two to four bypass diodes - about one for every 18 cells. Cogenra’s patent shows a way to embed bypass diodes into the structure and suggests that they can be arranged to bypass any number of cells, making the module much less susceptible to shade related power loss.
Aesthetics
There are people who find solar panels aesthetically displeasing. (I’m not one of those people, but I’m an engineer - what do I know about aesthetics?) I suppose the busbar grid pattern isn’t the prettiest thing I’ve ever seen on a rooftop. The Cogenra modules are almost purely black with no visible busbars, making them blend in with the roof much better than standard PV modules.
Field Testing and Commercial Production
In October 2014, a one megawatt PV array consisting of Cogenra modules was installed at the University of Arizona’s Tech Park. It’s currently generating energy for Tucson Electric Power. Cogenra is increasing production at its California plant and expects to sell modules starting in April of 2015.
Is this a game-changing technology? Probably not. But anything that reduces the cost and increases the energy production of PV makes the future of solar just a little bit brighter. Now where did I put my sunglasses?
Images courtesy of Cogenra