A Photovoltaic Power Conversion System with Flat Efficiency Curve over a Wide Load Range Yantao Song Bingsen Wang Department of Electrical and Computer Engineering Michigan State University 2120 Engineering Building East Lansing, MI 48824, USA [email protected]; [email protected] Maximum power point tracking (MPPT) techniques based on Abstract-This paper proposes a modular power conditioning power electronic technology enable maximum power unit (PCU) that is intended to improve light load efficiency of extracted from solar panels. Many MPPT schemes, such as photovoltaic (PV) generation systems. The proposed PCU hill climbing, perturb-and-observe and incremental topology consists of many modules in parallel with lower power conductance methods, are proposed to further improve rating, rather than a single module with power rating matching with PV panels. The new structure features high efficiency over efficient utilization of solar cells [7-10]. a wide load range. Equalizing of utilization rate among PCU In PV generation systems, the power conditioning unit modules and maximum power point tracking control are (PCU), which converts electric power from solar panels to the presented. A case study of a 5 kW PV system demonstrates the form in parity with grid, typically contributes to energy loss superior efficiency performance of the proposed PCU structure and high energy cost of the systems. Therefore, improving of and verifies the control strategy. the overall conversion efficiency from the PV panel to loads or grids becomes important as evidenced by the research I. INTRODUCTION effort from both academic and industrial fields. Significant The continuously rising demand for electricity, in research attention has been devoted to the investigation of conjunction with the increasing cost of traditional fossil fuels novel topologies and control strategies aimed to improve the and environmental-safety concerns associated with energy peak efficiency of power electronic converters for consumptions, has favored the development of alternative photovoltaic application [11-14]. Nowadays the peak sources such as solar energy in recent years [1-2]. In efficiency of commercial power electronic systems for comparison with various traditional sources such as coal, oil, residential utility-interface application has already reached gas and hydro, the high energy production cost is the main 97%. In contrast, relatively less attention has been paid to the barrier that hinders the large-scale application of photovoltaic overall efficiency, in particular light-load efficiency. It is well (PV) generation systems. Besides reducing initial investment understood that converter efficiency normally reaches its cost, maximizing energy harvest in the service life of PV peak value around full power rating and drops quickly under systems as much as possible is an effective solution to partially loaded condition. It is a fact that PV panels more decrease their life-cycle energy cost since the abundant solar than often operate with output power being lower or much energy is free. In order to compensate for the high cost of lower than their rated peak power when power converters solar systems, many efforts are made to improve solar energy have lower-than-peak efficiency. This leads to deteriorated harvest of PV generation systems. utilization of solar generation systems and accordingly high PV modules are responsible for converting solar energy energy production cost. Improving efficiency of power into electricity power. Their low-conversion efficiency has converters under light load holds strong potential to increase contributed to high cost of PV-based solar energy. Many new energy harvest of photovoltaic generation systems. materials and technologies have been developed to improve This paper presents a new modular PCU architecture and the efficiency of solar cells [3-4]. However, single-junction corresponding control scheme that will enable a flat crystalline Si and GaAs solar cells are approaching their efficiency curve over a very wide load range. Thus the life- upper limits in terms of thermodynamic maximum efficiency. cycle energy production cost of PV generation systems can be Thin-film and other solar cells have disadvantages in large- reduced. The rest of the paper is organized as the following. scale applications. The conversion efficiency of solar cells Section II introduces the status of PCU efficiency and PV also depends on operating conditions such as temperature, module characteristics. The proposed PCU architecture and solar irradiation density and load conditions. Direct tracking control strategy are described in Section III. A case study is control techniques are developed to maintain sun radiation given in Section IV, followed by some discussions and perpendicular to surfaces of solar panels and maximize concluding remarks in Section V. performance by regulating angles of solar panels [5-6]. 978-1-61284-972-0/11/$26.00 ©2011 IEEE 1144 II. STATE OF THE ART PCU EFFICIENCY AND PV MODULE CHARACTERISTICS 1. PCU Efficiency The PCU in a PV generation system generally consists of DC/DC converters and DC/AC inverters. The main function of PCU is to convert electricity generated by PV panels into electric power that is compatible with utility grid. The power conversion process will result in energy loss, which exerts an important influence on the energy harvest of PV systems. Power losses of converters can be divided into constant losses and variable losses that are closely related to operating current or actual power of converters. The former mainly consist of driving losses of power semiconductors, controller losses and core losses of magnetic components. The latter (a) include conduction losses of semiconductors and copper losses of magnetic components, which follow a linear relation to the square of load current. In addition, the variable losses are associated with switching losses of semiconductor devices that vary in proportion with load current. Since these loss components can be described by different functions of load power, it is possible to shape the efficiency curve of converters with different design options. The resultant system may reach peak efficiency at full power, middle power or other loading conditions. Although the PCU design with efficiency optimized at middle power performs well according to CEC weighted average efficiency measurement because efficiency under three fourths of full rated power has (b) the highest weight, it renders thermal management a Fig. 2. Illustration of output characteristics of PV panels affected by (a) solar challenging task. insolation and (b) cell temperature. A typically designed power electronic converter reaches its peak efficiency at or slightly lower than full power. When The P-V curves and I-V curves are obtained from ideal model load power drops, constant losses maintain unchanged. As a of PV cells. The reverse saturation current is calculated based result, the converter has much lower efficiency than full load. on nominal parameters from the module datasheet such as Fig. 1 shows a typical efficiency curve versus changes of load open-circuit voltage and short-circuit current at 25 °C cell 2 power. Efficiency of this PCU is optimized under heavy load temperature and 1000 W/m insolation density [15]. The to mitigate pressure of thermal management. series and parallel resistors are neglected. As a result, the calculated peak powers deviate slightly from the measured 2. Output Characteristics of PV Modules values. At constant temperature, as insolation density drops The output characteristic of PV panels is strongly short-circuit current ISC drops proportionally. Open-circuit influenced by the solar irradiation density and the cell voltage VOC decreases modestly because it follows a temperature. Fig. 2 illustrates the calculated output logarithmic relationship with short-circuit current [16]. For characteristics of the Sanyo PV module HIT-Power-215A example, the peak output power of the PV module also under various insolation conditions and cell temperatures. decreases to a half if solar insolation density drops by one Efficiency Versus Load half as shown in Fig. 2(a). Likewise, in reference to Fig. 2(b), 0.97 cell temperature affects peak output power of the PV module. 0.96 0.95 For the temperature rise of 1 °C, ISC of this PV module 0.94 increases slightly by 0.035% while VOC decreases 0.93 0.92 approximately by 0.27%. It is apparent that the peak power 0.91 drops as the cell temperature rises. 0.9 0.89 The insolation density and temperature vary dramatically in 0.88 a very wide range during a day. Consequently the peak output 0.87 0.86 power of an installed PV module will fluctuate during the 0.85 daytime. Fig. 3(a) shows average solar insolation density of 10% 20% 40% 60% 80% 100% twelve days (twenty-first day of each month) in a year in a Fig. 1. Efficiency versus load for a 5 kW PV PCU. single-axis tracking system which is located in a place of the 1145 1. PCU Structure Insolation Density Versus Time 1100 Analysis of power losses of a power conditioning unit 1000 suggests that lower efficiency at light load is mainly caused 900 by constant portion of power losses. The reduction of 800 constant losses under partial load conditions will lead to the 700 improved PCU efficiency. This paper proposes a modular 600 PCU structure and control strategy to attain optimized 500 efficiency over a wide power range. The block diagram of the 400 proposed system is shown in Fig. 4. Rather than using a 300 single PCU that matches the full power rating of PV panels, 7 8 9 1011121314151617 the proposed structure employs N interleaved PCU modules with lower rated power to interface PV panels and grid. Each (a) PCU module has the same power rating Normalized Output Power Versus Time Pac 1.1 P (1) moduel N 1 where Pac is the total power rating of the PCUs, which 0.9 matches the total power rating of the PV panels.