Inverters are a very important component of a PV system, requiring special optimization for the bifacial PV system configuration. Nowadays, inverters for PV applications come with a conversion efficiency of around 98 to 99%. Still, using the correct size of inverters is essential. Inverter oversizing leads to higher LCOE, and it might also lead to higher energy losses, as the inverter converts more energy at lower power ranges with lower efficiencies. On the other hand, undersized inverters lead to clipping losses, when PV-generated DC power exceeds the inverter's maximum input rating. In this case, the inverter trims the output to the maximum it can handle (see Maximizing Solar Power Efficiency).
When designing a PV system with bifacial modules, it is important to consider the expected bifacial gain at the DC/AC ratio – the main consideration for inverter sizing. The PV system's inverters shall be designed to be able to transmit the maximum output of the PV modules at all possible irradiation levels and ambient temperatures without power derating. Generally, DC/AC ratios vary between 1.1 and 2%. However, the DC/AC ratio should be calculated in a way that the clipping loss is minimized and below 2%. Furthermore, if the bifacial gain is not properly included in the design of the PV system and the inverter's limit is exceeded too often, the inverter warranty might be affected. Inverters with a high bandwidth of operating MPPT voltage range can properly utilize the bifacial module's power generation in low sunlight conditions like in the mornings, evenings, or cloudy days (See Albedo On-site Measurements).
Size matters: Proper inverter sizing is crucial to fully benefit from bifacial systems; undersized inverters can result in clipping losses. (Image Source: TaiyangNews Report)
The major inverter manufacturers have adapted their product portfolio to the needs of bifacial systems –from suppliers of central, string and optimizer based devices. However, taking into consideration that uneven irradiance at the rear side of the PV panel leads to higher mismatch losses than for monofacial modules, inverters with high granularity of maximum power point tracking (MPPT) are desirable. An optimum needs to be found, as inverter granularity also impacts the system's LCOE. Another important feature for bifacial inverters is their applicability in system configurations of negative string pole grounding to avoid PID. Here, due to safety reasons, automatic isolation resistance control should be possible, too.
The text is an excerpt from the TaiyangNews Bifacial Solar Systems 2024 Report, which can be downloaded for free here.