Improved Energy Capture in Series String Photovoltaics via Smart Distributed Power Electronics
This paper proposes an improved module integrated converter to increase energy capture in the photovoltaic (PV) series string. Prototypes for self-powered, high efficiency dc-dc converters that operate with autonomous control for tracking the maximum power of solar panels locally and on a fine scale are simulated, built and tested. The resulting module is a low-cost, reliable smart PV panel that operates independently of the geometry and complexity of the surrounding system. The controller maximizes energy capture by selection of one of three possible modes: buck, boost and pass-through. Autonomous controllers achieve noninteracting maximum power point tracking and a constant string voltage. The proposed module-integrated converters are verified in simulation. Experimental results show that the converter prototype achieves efficiencies of over 95% for most of its operating range. A 3-module PV series string was tested under mismatched solar irradiation conditions and increases of up to 38% power capture were measured.
The growing nationwide interest in photovoltaic power systems has induced significant expansion and R&D efforts in the PV field. Drivers of this growth include proliferating consumer demands, environmental consciousness, new federal and state subsidies and mandates, and new government R&D programs. New game-changing technologies in low-cost thinfilm PV manufacturing (which provide both a path to costcompetitiveness without subsidies, and flexible or membrane structures), as well as in building-integrated photovoltaics (BIPV) (in which architects will value PV building materials that can function in more complex physical environments), provide exciting opportunities for new applications of power electronics in a rapidly growing and quickly changing field. This paper examines the benefits and challenges of the PV series string for BIPV and proposes the use of self-powered, high efficiency dc-dc converters that operate with autonomous control for tracking the maximum power of solar panels locally and on a fine scale [1-4]. Figure 1 shows a series string with converters integrated to each panel. The resulting system is a low-cost, reliable smart PV panel that operates independently of the geometry and complexity of the system around it. The converter studied here employs synchronous rectifiers to reduce conduction loss and (depending on the string’s behavior) can operate in any of three modes: buck, boost and pass-through. In the latter, the input of the converter is connected directly to the output, thereby obtaining minimum insertion loss under nominal conditions. Each controller is completely autonomous and the module maximum power point trackers are decoupled from each other, adding robustness and reliability. By using module integrated converters (MICs) it is possible to regulate the PV string voltage to a fixed value, giving rise to the possibility of adding more strings or batteries to the system. A constant string voltage is also beneficial because then it becomes possible to optimize the inverter design, size and cost. MIC architecture and high efficiency was demonstrated in simulation using a buck-boost converter averaged model [5] that includes common sources of loss for such a system. System simulation under different solar irradiance conditions shows that MICs produce a large percentage increase in captured output power, 30% for a single MIC per panel and 45% for two MICs per panel
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