Supercapacitor assisted LED (SCALED) converter technique for solar powered DC-microgrids

Designing electronic products and systems with high energy efficiency is a major challenge for electronic power circuit designers. Almost all residential and industrial buildings are presently AC powered. However, they contain electronic equipment that is internally dependent on a bulk DC power rail with multiple DC-DC converters or DC-AC converters to supply motors controlled by variable speed drives. The overall efficiency of an electronic device with multiple voltage converters is determined by the product of the efficiencies of the individual conversion stages. Thus, overall efficiency drops drastically with the number of converter stages. However, with renewable energy sources such as photovoltaic solar cells generating DC electricity, we can eliminate the first AC-DC converter and reduce the number of converter stages, to achieve a signi ficant rise in end-to-end efficiency. This is the motivation for the DC-microgrid concept. DC-microgrids are local energy networks consisting of renewable energy sources and storage systems. Although power generation using solar energy is economical, designers have to cope with frequent fluctuations in irradiance and temperature and the non-availability of solar energy at night. Accordingly, there is a need for energy storage for reliable operation. The most common energy storage device used in solar power-based systems is the rechargeable battery pack, typically based on lead-acid and lithium-ion chemistries. However, all rechargeable batteries have limited charge-discharge life cycles as well as calendar lives, and are environmentally unfriendly. With the continuous developments of supercapacitor materials and manufacturing techniques over the last decade, it is now possible to adopt supercapacitors as short-term energy storage devices in solar powered DC-microgrids to replace electrochemical battery packs. When supercapacitors are used in solar power-based DC-microgrid environments, there is the possibility of directly operating \white goods" (such as washing machines, refrigerators, dishwashers and air conditioners) and lighting. However, if a supercapacitor bank is used as the sole energy storage, existing maximum power point tracking schemes are no longer appropriate. This is because the storage bank acts as a near-ideal capacitor, so its impedance depends on its state of charge, unlike a rechargeable battery pack with internal resistance. The supercapacitor assisted LED lighting (SCALED) converter is a new circuit topology that can be applied to solar powered DC-microgrids. This topology has been developed specially for low voltage LED lighting systems where a supercapacitor bank is used as an efficient energy storage device instead of a battery bank. The SCALED converter is another extension of the supercapacitor assisted loss management concept developed by the Power Electronics research team at the University of Waikato. This thesis presents details of this SCALED conversion system, developed for the Ports of Auckland DC-microgrid lighting systems, including theory and experimental efficiency measurements