The complexity of matching inverters to bifacial panels arises from the dynamic and uneven light capture levels on the rear side of the panel over time. Different sources of reflected light on reflective surfaces with different albedos, or reflected light values, permutate to make bifacial light capture management no less than a symphonic performance for inverters. Huawei Technologies’ FusionSolar Smart PV Solution has refined inverter technology to address these issues, while providing substantial increases in bifacial system yields in the process.
The moving target of the energy yield gain from the rear of the bifacial module varies depending on the scenario, but the energy yield can increase anywhere from 5% to 39%, according to Huawei research. Beyond that base gain, the bifacial module can further increase the energy yield by 2% to 6% based on its performance in response to low light and low power loss under the working temperature, the researchers point out. Thus, the combined energy yield gains of a bifacial module can range from 7% to 45% when compared to a conventional monofacial poly-Si module, a goal the industry is eagerly pursuing.
Inverter load flexibility critical
Designing inverters for bifacial modules that are capable of managing a large and varying yield gain is a first basic task. If inverters are not upsized to efficiently match and adapt to the dynamically enhanced DC output from bifacial panels, then they will suffer from clipping and yield loss. Balancing the need for capability versus size is a delicate design problem.
Industry research on bifacial systems currently focuses on ray-tracing and view-factor models, based on 3D modeling, to narrow down the best targets for design optimization. While more details can be displayed by algorithms, computing is complex and time-consuming, which does not mesh well with improved inverter performance.
Huawei has simplified and optimized the two models and launched an industry-leading intelligent design tool for bifacial module systems based on the 2D physical model, according to Shawn Gu, the Chief Scientist for Huawei’s Smart PV Business. Huawei Smart Design is the online design platform for the company’s Smart PV solution.
Smart Design is used to assist in plant system design, including component selection and energy yield assessment. The tool can determine the balance point between calculation speed and design details, quickly and accurately calculating the optimal configuration of a bifacial module system.
Huawei’s 2D modeling method increases the overall energy yield from a bifacial PV system by more than 3% compared with solutions provided by other standard design methods, says Gu. This further compounds the bifacial boost.
Resolving overcurrent issues
Secure and reliable protection design is another key attribute of the next-gen bifacial inverter. “Every two strings of the Huawei FusionSolar Smart PV Solution string inverter form one MPPT circuit and have a fuseless security protection solution. The design ensures that no risk will occur at overcurrent condition,” says Gu. At the same time, security risks, frequent fuse replacement, and energy yield loss caused by fuse faults are avoided.
Inverter anticipation of a fault is another key capability for bifacial system design. Since frameless and glass-glass module designs suffer from a higher risk of cracking than modules with frames, the inspection of module fault is a topmost priority for O&M. “The complexity of the I-V curve of the bifacial module makes the intelligent diagnosis of string faults easy to misjudge,” notes Gu. Huawei’s latest Smart I-V Curve Diagnosis V3.0 uses a new, intelligent string diagnosis algorithm with a built-in database. This combination enables optimization of the input and output feature curves of various manufacturers’ PV modules, automatically filtering out the data “noise” that causes misjudgment.
MPPT granularity
Another necessary tool for bifacial inverters is more granular and dedicated MPPT units, industry experts broadly agree. “To prevent current mismatch within a string, there is a crucial need for dedicated strings supported by higher MPP granularity. Ideally this should take place at the individual string level,” says Shashwat Kumaria, Director of Engineering at Sunpreme.
The multiple sources of reflected light on the rear side of a bifacial panel, in wide variance during the day, necessitate far more detailed sensing by the inverter. With the mismatch of the bifacial module high, its I-V curve is more complex than that of a monofacial module, and its power-voltage curve will generate multiple peak values over time. This poses higher requirements on the detection precision and MPPT of inverters.
Thus, using more MPPT units per string was a beginning requirement for Huawei’s redesign of the optimal inverter for bifacial systems. “Our string inverter has multiple MPPT units, which can precisely track the maximum power point of every one or two PV strings, releasing the maximum output power of each PV module. This greatly reduces PV string mismatch caused by distance and shade between modules, along with other issues,” Gu says.
The degree of granularity improvement required will depend on the bifaciality factor of a given panel. “Since the overall output power of the [combined sides of the] PV module is different, the current discrete rate of the module is more than five percent. So, the MPPT granularity of inverters needs to be finer,” explains Gu. “Based on the Monte Carlo statistical method, the mismatch loss of a bifacial panel yield caused by inverters with one MPPT circuit per two strings is 1.1% lower than the loss caused by common inverters in a bifacial module system,” he adds. “In practice, the environment is not as ideal as expected in the simulation and therefore an even lower mismatch loss can be achieved by the multi-MPPT feature with two strings in one MPPT.”
AI algorithms crunch big data
Another key tool for advanced inverters serving bifacial systems is the use of AI and algorithms for analyzing the bifacial data that more granular inverters are reading. “Bifacial strings inherently result in more complex I-V curves so there is a definite need for more robust MPP tracking algorithms tailored to accurate MPP identification under steady-state and dynamic [partially cloudy] irradiance conditions,” Kumaria points out.
Huawei is prominent among the inverter suppliers pushing the algorithm envelope for bifacial applications, Gu asserts. “This year, our FusionSolar Smart PV Solution has further applied AI technology to PV plants. We have updated the traditional astronomical algorithm to achieve the integration of tracker control, power supply, and communication, thus maximizing energy yields.”
With bifacial projects, maintenance is a more complicated issue than for mono-facial systems, given the relative value of the boost components. “Big data analysis plus AI algorithms are oriented toward more refined component-level monitoring and management,” says Gu. “This can proactively identify a low-performance unit, in a revolutionary switch from passive maintenance to active, predictive maintenance.” Avoiding infinite calculations is a major contribution of AI, which eliminates much of the cumbersome rote method of data analysis.
“From the aspect of hardware, faster scanning of voltage and power must be supported, but frequent scanning of MPP will cause power loss. After introducing AI that includes high-speed memory and a tailored algorithm, the system gains the ‘experience’ to deal with all cases without frequently scanning,” Gu explains.
Such a melding of AI and algorithms can provide a system boost all of its own, a situation where the whole has more value than the sum of the parts alone.“An intelligent design tool for bifacial systems integrates full-scenario, adaptive, and self-learning intelligent control algorithms to accurately enable an optimal design solution,” Gu explains. “This increases the energy yield by more than three percent compared with solutions provided by other standard design methods.
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