SOME BASIC INFORMATION ABOUT BIFACIAL MODULES
Traditional PV modules are monofacial, meaning that their electrical power output is derived from the direct and diffused radiation captured on the front side of the cells only. Bifacial modules convert light captured on both the front AND back sides of the cells into electrical power. Bifaciality improves energy capture, very significantly in some cases, and alters conventional system design rules in interesting ways. Particularly in terms of costs and Return on Investment calculations. If correctly designed, Bifacial Systems are more powerful, and deliver more power per square meter installed.
A Brief History
Research on bifacial PV cells dates back to the very beginning of the PV industry. Japanese researcher H. Mori proposed a bifacial PV cell design as early as 1960 and had successfully developed a working prototype by 1966. Russian and Spanish researchers proposed uses for bifacial PV cells around the same time. It was the Russians, however, who first deployed bifacial PV modules in the 1970s, as part of their space program. A major milestone occurred in 1980, when Spanish Group of researchers documented the ability of light-colored surfaces to direct reflected light (albedo) to the back of a bifacial PV cell and increase its power output by 50%.
Bifacial PV cells currently make up an insignificant percentage of worldwide PV cell sales, but this is certain to increase in the future. The technology is a logical extension of standard monocrystalline silicon (mc-Si) cell technology, although the process requires some additional manufacturing steps compared to producing conventional monofacial cells. Most manufacturers are currently unable to produce this technology
In practice, more than 90% of the PV cells sold worldwide are based on a p-type architecture, while the vast majority of the bifacial cells are n-type devices. This underscores the fact that many n-type PV cells, which are primarily found in niche high-efficiency modules from companies such as LG and SunPower, are inherently bifacial. (The history of bifacial PV cells can be traced back to Bell Labs, as its first practical solar cell in 1954 was an n-type device.) P-type devices currently dominate the world market because they are cost-effective to fabricate at scale. While n-type bifacial cells can offer the higher efficiency, companies such as SolarWorld believe that p-type bifacial cells can provide a good sensible balance between performance and cost.
Regardless of the specific cell technology, the rear side of a bifacial PV cell needs to be able to act as a collector, which requires very advanced manufacturing techniques. Vast resources have been invested by leading companies in R&D and equipment, and this will prove to be very important in the future.
HOW BIFACIAL MODULES WORK
BIFACIAL VALUE PROPOSITION
The rapid growth of the solar industry in recent years has largely been due to significant equipment price reductions, especially lower costs for PV modules. Bifacial PV modules run counter to the grain in the market since they are more expensive than conventional monofacial modules. Fabricating bifacial PV cells requires not only high-quality mc-Si wafers, but also anywhere from two to six additional manufacturing steps compared to conventional cells. If you purchase a Bifacial module, it is likely to be very high quality
The basis of the bifacial value proposition is improved production and performance over the life of the system. This is a function of both bifacial energy gains and improved durability. Because bifacial modules offer high conversion efficiencies, they also have the potential to lower BOS costs, which make up an increasing percentage of up-front system costs. The ultimate goal, of course, is a lower levelized cost of energy (LCOE).
IN THE RIGHT CIRCUMSTANCES IT MAKES FINANCIAL SENSE TO USE BIFACIAL MODULES BECAUSE:
Increased energy generation – FASTER RETURN OF CAPITAL IN SAVINGS
Improved durability – GUARANTEED FOR 30 YEARS, AND LOW DEGRADATION
Reduced BOS – POTENTIALLY LOWER COST PER WATT PRODUCED
Lower LCOE – LOWER ENERGY COST OVER THE LONG TERM
The Best Bifacial Designs
As stated above, bifacial PV modules convert both front- and rear-side irradiance to electrical power. However, they put their best face forward, in the sense that front-side efficiencies are invariably higher than back-side efficiencies, whether due to semiconductor properties or the amount of back contact metallization. You can optimize bifacial PV systems by following a few simple guidelines: Basically, one should install bifacial arrays above surfaces that reflect as much light as possible, increase array height or tilt angle to collect more reflected light and avoid shading at the back side of the array. Canopies, carports, swimming pools and any elevated arrays are ideal. Even roof mounted systems in the traditional style work very well.
A bifacial PV system will generate more energy when installed over a light-colored rather than a dark-colored surface. This is because the former will reflect more light onto the back of the array, and the latter absorbs more of the incident irradiance. Albedo is a dimensionless quantity, usually expressed as a percentage that describes this ratio between light reflected off a surface and the original incident irradiance. Basically, the higher the albedo value, the higher the surface reflectivity.
The following Table provides representative albedo values for a variety of normal surface types. This is sourced from the SolarWorld white paper “How to Maximize Energy Yield with Bifacial Technology”. These values show that white roofing membranes, which reflect roughly 80% of the incident light when new and un-weathered, are an ideal surface under a bifacial PV array. At the low end, the measured albedo value for raw concrete is only 16%, but the albedo for concrete increases dramatically when it is simply painted white. SolarWorld’s research indicates that not all light-colored surfaces are created equal. White gravel, for example, has a low albedo due to an “open-pored structure that causes a large amount of light to be lost within the voids.”
Height and tilt angle.
The closer you install a bifacial array to the ground or roof surface, the more self-shading occurs.
SolarWorld simulations suggest that a significant bifacial energy boost is possible with just a modest height increase. Not only is the energy boost curve steepest between 0 and 0.2 meters (7.9 inches), but also the inflection point occurs somewhere around 0.5 meters (19.7 inches), after which point the curve begins to flatten out; the saturation point occurs around 1.0 meter (39.4 inches), meaning that additional energy gains are negligible above this height. These data suggest that bifacial modules are potentially well suited for just about any ground-mounted application, as the leading edge of these arrays is often 18 inches–36 inches above ground.
It is also possible to adapt conventional flat roof– mounting systems for use in bifacial applications.
To optimize bifacial energy gains, designs should, where possible, avoid shading the back side of the modules. Most racking systems have rails that run across the module’s backside, which an opaque white or black film usually covers. Support rails, are potential sources of shade in a bifacial system. As a result, mounting systems optimized for bifacial applications often locate mounting rails at the edge of the modules, orienting these in parallel with rather than perpendicular to the module frame or the edge of the glass.
Back-side shading is also a concern for bifacial module manufacturers. The junction box on many monofacial modules, for example, is located directly behind one or more PV cells. Bifacial modules normally have a low-profile junction box located at the perimeter of the module to minimize back-side cell shading. Though testing indicates that back-side shading from junction boxes or mounting structures will not damage a bifacial module, it does result in yield losses.
ACTUAL APPLICATIONS AND PERFORMANCE
Data from initial test beds and performance simulations suggest endless potential applications for bifacial PV systems. These include most conventional installations such as flat roofs and free fields, where installers currently deploy monofacial PV modules, as well as niche applications such as building-integrated PV (BIPV) carports and awnings, where typically early bifacial modules were used. With Glass-Glass modules, additional light is allowed to pass through the modules, creating more incident light, and also an effect that architects and designers often crave.
BIFACIAL modules – Back-side power collection rewrites the rules!
Solaren have data and performance relating to a selection of applications, plus an ample gallery and videos to demonstrate the uses of these modules. Performance is very, very impressive, so please watch this space for future developments.
OUR SOLARWORLD, GENUINE GERMAN MANUFACTURED BIFACIAL PANELS ARE GLASS-ON-GLASS, WITH 30 YEAR GUARANTEES. PLEASE ASK US FOR MORE INFORMATION