Katherine A. Kim with adviser P. T. Krein
Photovoltaic (PV) cells connected in series experience internal and external mismatch that reduces output power. Differential power processing (DPP) architectures achieve high system efficiency by processing a fraction of the total power while maintaining PV maximum power point operation. The two main DPP architectures, PV-to-bus and PV-to-PV, exchange power with either the dc bus or neighboring PV elements, respectively. Simulations for both DPP architectures evaluate system performance over 25 years. Based on field study data, PV power variation in terms of coefficient of variation was determined for new (CV0) and 25-year-old (CV25) PV systems. To compensate for the PV variation expected over a typical system lifetime, 15-17% for PV-to-bus and 23-33% ratings for PV-to-PV converters are used for a 15-submodule system (five PV panels in series). Both DPP architectures deliver up to 2.8% more power (before losses) compared to a conventional series string architecture based on expected panel variation over 25 years.
DPP converter performance is also compared to dc optimizers, where each PV element has a dedicated converter rated at 100% PV power. Assume dc optimizers have 98% efficiency, while DPP converters have 90%. Performance is measured in terms of improvement figure of merit (IFoM), the ratio of energy produced to that of the conventional series string architecture. A simulation is run for both DPP converters and dc optimizers over a PV panel CV range of 0.01 to 0.20. Results for the dc optimizer, 17%-rated PV-to-bus converter, and 33%-rated PV-to-PV converter are shown in Fig. 16, with the values of CV0 and CV25. Lifetime performance is evaluated by integrating the IFoM between CV0 and CV25. Dc optimizers achieve a 0.8% lifetime energy capture increase, while PV-to-bus and PV-to-PV converters achieve a 2.4% and 2.1% increase, respectively. Thus, over 25 years, DPP converters are a better solution than dc optimizers because they yield higher power under nominal conditions and cost less. If DPP converters can be added at 2% of the system cost, the DPP system will pay for itself over time.
This research was supported by the Grainger Center for Electric Machinery and Electromechanics and the U.S. National Science Foundation through the Graduate Research Fellowship program. The information, data, or work presented herein was also funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000217.