Three-Port dc-dc Conversion in Light-to-Light Systems

Abstract

Renewable energies, like solar or wind, provide unlimited, clean and free energy that helps reducing (CO2) emissions, which alleviates global warming and greenhouse effects. Moreover, the ability to produce off-grid electricity allows local electric power generation. However, the main disadvantage of renewable sources is that they are strongly dependent on the weather conditions, and, therefore, intermittent and unpredictable. For this reason they need to be combined with other power sources, or energy storage elements, in order to ensure reliable and constant power to the load. Solar energy is one of the major renewable energies because the Sun is a vast, inexhaustible and clean resource. Photovoltaic cells transform sunlight into electrical energy and the generated power is proportional to the amount of solar irradiation. Light-emitting diodes (LEDs) present higher luminous efficiency and lifetime than conventional light sources based on heated filaments (incandescent and halogen) and gas discharge (fluorescent, sodium, etc). The rapid development of this technology makes it possible to replace the conventional technologies towards high brightness LED lighting systems. The combination of these technologies—solar cells, energy storage elements and LEDs—in a stand-alone solar powered LED system, can provide light where otherwise it would be cumbersome; in rural areas, where cabling can be challenging and expensive, and also in the urban environment, where the cost of digging and construction is very expensive. Solar powered systems are particularly challenging in locations far from the equator, where the solar resource is scarce, especially during winter, since the amount of solar irradiation is small and the length of the day is short. Therefore, these systems need to be optimized by maximizing the energy conversion efficiency under low irradiation conditions. This work is part of a Ph.D. research project to study the feasibility of implementing three-port converter (TPC) topologies in solar powered LED, light-to-light (LtL) systems. After the introduction in Chapter 1, an overview of the state-of-the art of solar cells, LED technology and energy storage elements, as well as a review of TPC topologies is given in Chapter 2. Following, the study of a low-power (10 - 50 W) stand-alone PV-LED system with aim on high efficiency energy conversion is presented in Chapter 3. The implemented power stage is based on component reutilization and is optimized at low irradiation conditions, achieving a peak efficiency of 98.9%. A discussion on the magnetic component design, semiconductor evaluation based on switching energy measurement and study of the loss distribution is presented. Scalability and implementation of multiple LED outputs on the proposed topology are discussed and demonstrated on a second prototype. A TPC for grid-tied light-to-light applications (100 - 1000 W) is studied in Chapter 4. Analysis, modeling and power flow regulation with two control variables are discussed in this section. The dynamic modeling and measurements prove that completely decoupled power flow regulation can be achieved. Finally, the conclusion and future work are presented in Chapter 5. Other research topics—not directly related with the project objectives—are shown in Chapter 6.