Sara MacAlpine

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.

Characterization of Power Optimizer Potential to Increase Energy Capture in Photovoltaic Systems Operating Under Nonuniform Conditions

Power optimizers, which perform power conversion and distributed maximum power point tracking (DMPPT) at the subarray level, are available to mitigate losses associated with nonuniform operating conditions in grid-tied photovoltaic (PV) arrays, yet there is not a good understanding of their potential to increase energy capture. This paper develops and demonstrates a methodology for the use of a detailed software tool that can accurately model both partial shading and electrical mismatch at the subpanel level in a PV array. Annual simulations are run to examine the device-independent opportunity for power recovery in arrays with light, moderate, and heavy shading, and subpanel electrical mismatch variations based on measurements from a monocrystalline silicon array. It is found that in unshaded arrays, the potential energy gain is <; 1% using power optimizers, but in shaded arrays it increases to 3-16% for panel-level DMPPT and 7-30% for cell-level DMPPT. In the set of simulated cases, panel-level power optimization recovers 34-42% of the energy that is lost to partial shading.

Module mismatch loss and recoverable power in unshaded PV installations

Distributed electronics which optimize power in PV systems have the potential to improve energy production even under unshaded conditions. This work investigates the extent to which mismatch in the unshaded electrical characteristics of PV panels causes system-level power losses, which can be recovered in arrays employing power optimizers. Of particular interest is how this potential for power recovery is affected by factors such as available light, cell temperature, panel technology, and field degradation. A system for simultaneous collection of panel-level I-V curves over an entire array is designed. This system is used to acquire high and low light module performance data for a variety of arrays at the National Renewable Energy Laboratory (NREL) test facility. The measured data show moderately low variation in module maximum power and maximum power producing current in all of the arrays. As a group, the tested arrays do not show any strong correlations between this variation and array age, technology type, or operating conditions. The measured data are used to create individual panel performance models for high and low light conditions. These models are then incorporated in annual hourly energy simulations for each array. Annual mismatch loss (and thus potential for increased energy capture using power optimizers) is found to be minimal, <;1 % for all of the sampled arrays. Due to the nature of the tested arrays, these results may or may not be indicative of typical PV array behavior; further investigation is planned over a larger group of PV installations to determine the general applicability of this studys results.

Use conditions and efficiency measurements of DC power optimizers for photovoltaic systems

No consensus standard exists for estimating annual conversion efficiency of DC-DC converters or power optimizers in photovoltaic (PV) applications. The performance benefits of PV power electronics including per-panel DC-DC converters depend in large part on the operating conditions of the PV system, along with the performance characteristics of the power optimizer itself. This work presents a case study of three system configurations that take advantage of the capabilities of DC power optimizers. Measured conversion efficiencies of DC-DC converters are applied to these scenarios to determine the annual weighted operating efficiency. A simplified general method of reporting weighted efficiency is given, based on the California Energy Commissions CEC efficiency rating and several input/output voltage ratios. Efficiency measurements of commercial power optimizer products are presented using the new performance metric, along with a description of the limitations of the approach.

Analysis of Potential for Mitigation of Building-Integrated PV Array Shading Losses Through Use of Distributed Power Converters

Partial shading of building-integrated photovoltaic (BIPV) arrays is very common, as they are limited by building geometry and most often installed in crowded urban or suburban environments. Power losses in shaded BIPV systems tend to be disproportionately large, due in large part to mismatches in operating conditions between panels. Maximum power point tracking at a modular level, which can be achieved through the use of module integrated dc-dc converters (MICs), may be used to mitigate some of these losses. This paper investigates the potential power gain provided by MICs for several representative partially shaded BIPV array scenarios. A flexible, comprehensive simulation model for BIPV systems is developed, which allows for variations in insolation and temperature at the PV cell level, while accurately modeling MICs and their effect on array performance. Shadows from nearby objects are mapped onto the modeled BIPV arrays and simulated on an annual, hourly basis, with varying array configuration as well as object size and placement. Results of these simulations show that the impact of MICs on system power output varies depending on factors such as radiation availability, time shaded throughout the year, shadow size and distribution on the array, and inverter design. Annual power gains of 3–30% are realized for a moderately shaded system with MICs when compared to conventional approaches. Further opportunities for increased energy capture in a BIPV system with MICs are identified and discussed.Copyright

Beyond the module model and into the array: Mismatch in series strings

Mismatch losses have the potential to significantly affect the performance of PV arrays, so it is important to model them as accurately as possible. There are many variables that affect mismatch in a system, and it is often not realistic for commercially-available tools to rigorously model the complex behavior of PV systems operating under non-uniform conditions. Though many tools have simplified methods for estimating mismatch loss, there are few general guidelines for how it should be characterized in PV systems. Various mismatch scenarios and resulting system-level power losses are modeled using a custom, comprehensive PV array simulation environment. Based on the results of these simulations, and the capabilities of current modeling tools, suggestions are made for accurate modeling of module mismatch, partial shading, and panel orientation differences in PV arrays. With improved understanding of mismatch-related PV system power loss, researchers and system designers will be better able to accurately assess and model its impact, enabling future PV research and maximizing the value of a diverse set of PV systems.

Effect of Distributed Power Conversion on the Annual Performance of Building-Integrated PV Arrays With Complex Roof Geometries

Building-integrated photovoltaic (BIPV) systems have gained greater popularity in recent years; however, their effectiveness is often limited by nonuniform operating conditions. To increase potential for energy capture in PV systems, particularly those with series string configurations, an improved module integrated dc-dc converter (MIC) with maximum power point tracking has been proposed. This paper investigates the potential power gain provided by these MICs in situations where the architecture or surroundings of a building necessitate that a PV array include panels with differing orientations, which can significantly reduce system efficiency. A flexible, comprehensive simulation model for BIPV systems is developed, which allows for variations in insolation and temperature at the PV cell level, while accurately modeling MICs and their effect on array performance. This model is used to simulate various directional array combinations in series string and parallel configurations for a representative set of climates around the US. Results of these simulations show power gains attributed to both the photovoltaic generator/converter portion of the system and to increased inverter efficiency arising from a constant, controlled string voltage. When differing panel orientations within an array are considered, there is potential for annual power output gains of over 10% for a system with MICs when compared to conventional approaches. Further opportunities for increased energy capture in a BIPV system with MICs are identified and discussed.

Photovoltaic module model accuracy at varying light levels and its effect on predicted annual energy output

Models to predict photovoltaic (PV) module power output are constantly improving, yet many are tuned to maximize accuracy at high irradiance, which has the potential to introduce errors when predicting energy output with other operating conditions. Correct power estimation under lower light conditions is very important, particularly in cloudy climates or when an array is non-optimally oriented, as is often the case with building-mounted or building-integrated PV systems. Newer CEC and IEC standards require manufacturers to provide data on module performance under a variety of operating conditions, and the purpose of this study is to explore the potential of these new data to improve model accuracy for different PV technologies. This study examines the accuracy of the commonly used, CEC-Wisconsin single diode five parameter PV model, comparing its predicted power output to measured and empirical model datasets over a range of incident irradiance levels and temperatures. The same comparisons are also performed for a modified, seven parameter version of this model, which includes the use of low irradiance data to assign values to the extra parameters. Results of this work further understanding of model misprediction at low irradiance and its impact on estimation of annual energy production. At medium-low irradiance (<500 W/m2), the original model is found to overpredict power output by as much as 15% for crystalline modules when compared with the datasets, and even more for thin film modules. The modified model brings the power overprediction down to ∼5% or less for most cases. Annual simulations are performed comparing the predicted energy output of the original and modified models to that of the datasets. The modified model shows a significant improvement in accuracy of annual energy prediction, especially for cloudy climates and situations where the array has a non-optimal tilt or orientation

Evaluation and field assessment of bifacial photovoltaic module power rating methodologies

1-sun power ratings for bifacial modules are currently undefined. This is partly because there is no standard definition of rear irradiance given 1000 Wm−2 on the front. Using field measurements and simulations, we evaluate multiple deployment scenarios for bifacial modules and provide details on the amount of irradiance that could be expected. A simplified case that represents a single module deployed under conditions consistent with existing 1-sun irradiance standards leads to a bifacial reference condition of 1000 Wm−2 Gfront and 130–140 Wm−2 Grear. For fielded systems of bifacial modules, Grear magnitude and spatial uniformity will be affected by self-shade from adjacent modules, varied ground cover, and ground-clearance height. A standard measurement procedure for bifacial modules is also currently undefined. A proposed international standard is under development, which provides the motivation for this work. Here, we compare outdoor field measurements of bifacial modules with irradiance on both sides with proposed indoor test methods where irradiance is only applied to one side at a time. The indoor method has multiple advantages, including controlled and repeatable irradiance and thermal environment, along with allowing the use of conventional single-sided flash test equipment. The comparison results are promising, showing that the indoor and outdoor methods agree within 1%–2% for multiple rear-irradiance conditions and bifacial module types.

Simulated Potential for Enhanced Performance of Mechanically Stacked Hybrid III-V/Si Tandem Photovoltaic Modules Using DC-DC Converters

Abstract. This work examines a tandem module design with GaInP2 mechanically stacked on top of crystalline Si, using a detailed photovoltaic (PV) system model to simulate four-terminal (4T) unconstrained and two-terminal voltage-matched (2T VM) parallel architectures. Module-level power electronics is proposed for the 2T VM module design to enhance its performance over the breadth of temperatures experienced by a typical PV installation. Annual, hourly simulations of various scenarios indicate that this design can reduce annual energy losses to ∼0.5% relative to the 4T module configuration. Consideration is given to both performance and practical design for building or ground mount installations, emphasizing compatibility with existing standard Si modules.