Q&A: JinkoSolar discusses half cut cells, multi-busbars and bifacial technology

pv magazine: By cutting cells in half and arranging them in a module in a standard formation, JinkoSolar’s new HC Series panels can increase power output by 5-10 W at little extra cost. Sounds simple, but is it? Can you explain how such a power boost is attained by half-cut cells?

Andrea Viaro: In fact, it looks like a rather simple solution, but with a significant impact on the performance of PV modules. The power increase is mainly due to the reduction on resistive losses, which is achieved by halving the cell current. Also the “water drop effect” plays an important role, meaning that the extra white space among cells enhances reflections within the laminate, thus solar cells’ light utilization. The result is that HC modules can achieve at least 1 higher power bin compared to the standard technology, and even 2 classes higher if combined to other add-on features, such as the Light-Reflecting Ribbon technology and the White EVA layer between cell and backsheet.

Aside from increased power output, what other advantages do HC modules offer? Are they more durable, more reliable, and more adaptable for various installations in differing environmental conditions?

Our Half-Cell Series fulfill all the highest quality standards of the entire Jinko production. Performance is further improved thanks to the better temperature coefficients, which allow Jinko's Half-Cell modules to produce more energy in hot environments, compared to our full-cell series, as well as the cell-string parallel configuration within the same module, which significantly mitigates mismatch losses, due to partial shading. Also reliability is positively affected by the fact that the lower cell-string current allows for slightly lower operating temperatures and, most important, about 20°C lower hot-spot levels, thanks to the lower energy dissipation in the instance of external effects, such as soiling or local shading.

JinkoSolar now has GW-scale mass-production capacities of HC modules. How much upgrade and upheaval was required in the company’s production facilities to achieve this?

Indeed our production is ramping up and we expect to reach more than 3 GW of capacity by the end of 2018. Our workshops in China, where HC are manufactured, are already operative since the end-of 2017, while our factory in Malaysia will complete this quarter, the transition on the lines dedicated for our HC products. The additional step where solar cells are laser-scribed and cut before stringing has been effectively deployed since last year. It required some fine-tuning of the completely automated processes and the implementation of additional quality controls, but it represents a rather smooth evolution of the existing production lines.

Moreover, the automated cell stringers that we employ in our workshops are equipped with the most advanced features in the market, which enable application of the so called light-reflecting ribbons which, combined with other add-on technologies, boosts the module power up to an additional power bin.

Other innovations we will discuss at the webinar include multi-busbar technology. Can you briefly explain JinkoSolar’s R&D in this area, and what the benefits of multi-busbar products are?

Multi-busbar, or “Multi-wire”, is the natural next evolution of the cell-stringing technology, which supports the constant increase in a PV module’s peak power; the same situation that we saw in the past years with the current main industrial technology, starting from the initial 2BB up to the 5BB of today. MBB tech. brings significant benefits at different levels: production, module efficiency and reliability. For instance, apart from the lower series resistance, the round-shaped busbar, a wire indeed, has an effect very similar to our Light-Reflecting technology for standard flat ribbons, meaning that it increases the scattering effect towards the cell surface for higher cell absorption; the result is that module power is boosted by about 3W. Another advantage in terms of reliability is the lower impact of possible microcracks that might occur during the modules’ lifetime, thanks to the shorter distance between the adjacent wires.

 ‘Multi’ can mean different amounts. What in your view is the optimal amount of busbars on a cell, based on current technology? And could this number grow higher using different materials?

Each technological improvement that we implement in production is the result of our R&D Team efforts to identify the most advantageous and cost effective solution. All the solar cells produced by JinkoSolar since 2017, for instance, adopt the 5BB configuration because, compared to 4BB, it allows for a still meaningful P-loss reduction without major changes in the stringing machines. Moving to 6BB would give just 50% of that improvement with some process adjustment headache.

On the other hand, from 5BB to MBB, the P-loss reduction is remarkable again and in this scenario the investment needed to jump directly to the 12-MBB (aka “Multi-wire”) is justified, which is the best balance between production cost and efficiency gain, given the technology currently available in the market.

A further increase in the number of wires would then not lead to significant module efficiency improvements, so it is rather reasonable to expect alternative solutions, such as Shingling or SmartWire technologies to be deployed in mass-production going ahead.

JinkoSolar has worked with TÜV Rheinland on bifacial module testing recently despite already producing N-type bifacial modules. Are there any inherent difficulties in testing bifacial modules? What have been the results of such testing?

The main challenge for the bifacial technology at present is the lack of an official standard for their power rating possibly. This circumstance, together with the still not perfectly optimized simulation tools, makes it rather complex for EPCs to accurately estimate PV plant yield in any conditions, consequently to obtain Investors acceptance and project bankability.

Fortunately, several pilot projects have already been developed to test the field performance of bifacial modules under different conditions, and the collected reference data are helping to validate the simulated results thus fine-tune the software.

According to our preliminary internal tests, the additional power given by our P-type bifacial modules ranges between 6-7% in case of grass fields, 9-10% for sand-desert environments and up to even 20% with white-painted ground or snow fields. Of course the BOS shall be adapted to maximize the plant performance, but we see that this transition is happening fast, as we are used to in the PV sector; specific trackers for instance are already commercially available and we can expect more developments will be implemented soon.

What drives the desire for JinkoSolar to continue pursuing new technological designs? Is it simply the demand for a lower LCOE, or do you view innovation in cell and module tech as something separate from that?

Our aim is always to provide our customers with the most cost-effective, bankable and reliable solution using the most cutting-edge technology available in the market. That’s the main reason for instance why Jinko heavily invested to enlarge mono PERC production capacity, given that the technology developments in the recent years have driven down manufacturing costs and PERC is widely recognized in the industry as possibly the main cell technology that currently allows for the lowest LCOE, as proved by the Sweihan Project milestone, as well as the large quantity of PV plants constructed with high efficiency modules under the Top Runner Program in China.

Finally, what other innovations are you working on at JinkoSolar, which will be discussed in more detail during the webinar?

JinkoSolar’s product portfolio is becoming more and more diversified thanks to the various additional features currently available at both the cell/wafer and module level; the common trend is obviously to go towards higher efficiencies, higher PV plant yields and improved PV module reliability.

In terms of new products, one of the main developments that is currently showing the highest potential growth is bifacial technology, which still represents a niche market, but it’s not excluded that it could become virtually mainstream for specific market segments in the next years.

Other add-on features, like Light-Reflecting Film (LRF) or Light-capture Ribbons (LCR), and White EVA encapsulant, play an important role in the reduction of the cell-to-module losses. Relatively big expectations come also from the large-cell concept that should be available by the end of 2018 and should increase the module Wp by up to 2 power bins.

In terms of reliability improvement, the White EVA solution for instance reduces by 10 times the UV Yellowing Index for higher long-term durability of the module encapsulating material. On the polycrystalline silicon side, the possibility to employ diamond-wire sawing processes, combined with metal-catalyst texturing improves the uniformity of wafers, reducing also the slicing cycle and waste material, and thus improving the overall environmental footprint.

Regarding mono PERC, the light-induced hydrogen passivation process (LiHP) against LID is now mature and effectively deployed in mass-production, which warrantees the highest module stability, therefore maximizing energy yield.