In a recent pv magazine webinar, JinkoSolar’s Viaro presented the case for using bifacial modules in large-scale solar plants, and discussed the influencing factors and their impact on bifacial PV tracking. Jonathan Melendez , Sales Manager USA, Soltec also presented simulation results in terms of yield gain expectations. You can now listen to the webinar and download the presentations from both JinkoSolar and Soltec. Meanwhile, you will find answers to questions we did not have time to pose, from Viaro.
What is the bifacial factor?
Andrea Viaro: It is the rate between the peak power or nominal efficiency of the rear side of the PV module, and its front side.
If peak power is used to compare monofacial and bifacial modules and installations, is the power from the rear side taken into account to calculate the peak power, and to which percentage?
When comparing monofacial and bifacial PV arrays, the rear side Wp has to be clearly added on top of the front side power. The official standard for bifacial PV module power rating should be published shortly; and describes the procedure to measure both nominal Wp and bifacial factor.
What is the best inverter power when using bifacial modules? How should we consider the DC-to-AC-ratio?
DC/AC ratio depends mainly on the inverter characteristics and functioning windows for current and voltage. As a rule of thumb, when designing PV systems with bifacial modules, it has to be considered that the current increases almost linearly with the light absorbed by the cells. For instance, an albedo of 200W/m2 would generate an extra current of 20% through the bifacial factor.
What methods do you recommend to measure the power of bifacial cells and modules accurately?
The procedure defined by the new IEC standard for bifacial PV module power rating entails the measurement of the front side Wp at standard test conditions and the rear side power separately, so as to determine the bifacial factor. In this way it is then possible to flash-test bifacial modules in mass production similarly to monofacial ones, thus minimizing changes on the sun simulators currently used.
What is the increase in power rating for the Bai Cheng project comparing bifacial and monofacial in the same location, and how have you included the rear side effect in the power definition of the modules?
At the BaiCheng test site we evaluated only the performance of bifacial PV systems comparing different ground preparation. The characterization of the modules for their rear side nominal power was not necessary as the actual energy output was directly measured.
Is 90% bifaciality really achievable, and when?
90% bifaciality can be achieved and exceeded using n-type silicon technology. We at Jinko are evaluating such options at R&D level, although the cost is still significantly higher compared to the p-type used now in mass production. Nonetheless, we could rapidly switch our assembly lines if there were demand in the market for such product.
Jinko seems to recommend frameless modules for bifacial. Is this the best option when using bifacial, and how big is the difference to framed modules with respect to yield and costs?
We are actually developing both frameless and framed versions, as we see demand and applications for both solutions. In general, the most advantageous option is defined by the optimal balance between the cost of the module and components required for the mounting structure. In terms of yield, some optimization of the module design is still required, to minimize the possible shading induced by the frame on the rear of the PV module.
The clamps normally are designed for 6.8 mm thick modules, while bifacial glass-glass are thinner. Do you provide clamps for such thin frameless modules?
Jinko does not supply the clamps, but there are already some manufacturers in the market able to supply clamps for 6 mm thick modules. The number of solutions and products offered is plausibly going to increase with the expected market share growth of bifacial modules.
The product itself
We as project developers typically calculate a 0.8-0.9% annual degradation rate in our models. Why do you assume a degradation factor of 0.5% for your bifacial modules?
Jinko glass-backsheet modules are guaranteed for a maximum 0.7% annual degradation. Bifacial modules can achieve an even lower level of 0.5%, thanks to their glass-glass structure that enables to significantly reduce the encapsulant degradation due to moisture ingress, as well as the cell performance decrease caused by the development of microcracks induced by thermal and mechanical stress applied to the PV module in the field.
Which backsheet do you use for the bifacial module?
Our bifacial modules are laminated with a double glass structure, we are however studying already the alternative of transparent Tedlar-based backsheet, especially for the framed version.
Have you already tested the reliability and durability of the glass/glass lamination that the bifacial modules based of?
Jinko bifacial modules fulfill all IEC standard requirements, and even outperform the reliability level reachable by our standard glass-backsheet series in all accelerated aging tests, even beyond the normal standard protocols. In fact, our bifacial modules have the benefit of the already mature glass-glass technology launched in mass production since several years.
What is the bifaciality ratio of your 380 W module?
The bifacial Factor of all our p-type silicon series is 70%. We expect also to improve it with the further developments of mass production.