Chris Deline

Partially shaded operation of a grid-tied PV system

Partial shading of a PV installation has a disproportionate impact on its power production. This paper presents background and experimental results from a single string grid-tied PV system, operated under a variety of shading conditions. In this configuration a shadow can represent a reduction in power over 30 times its physical size. Results are presented relating size and positions of shading to power reduction of the PV system. A simulation method is also described that provides an accurate description of shade based on a single site survey. This process can provide the basis for an accurate simulation of power reduction in a partially shaded PV system.

Performance of Power-Limited Differential Power Processing Architectures in Mismatched PV Systems

Differential power processing (DPP) architectures employ distributed, low power processing, submodule-integrated converters to mitigate mismatches in photovoltaic (PV) power systems, while introducing no insertion losses. This paper evaluates the effects of the simple voltage-balancing DPP control approach on the submodule-level maximum power point (MPP) efficiency. It is shown that the submodule MPP efficiency of voltage-balancing DPP converters exceeds 98% in the presence of worst-case MPP voltage variations due to irradiance or temperature mismatches. Furthermore, the effects of reduced converter power rating in the isolated-port DPP architecture are investigated by long-term, high-granularity simulations of five representative PV system scenarios. For partially shaded systems, it is shown that the isolated-port DPP architecture offers about two times larger energy yield improvements compared to full power processing (FPP) module-level converters, and that it outperforms module-level FPP approaches even when the power rating of DPP converters is only 20-30% of the PV system peak power. In the cases of aging-related mismatches, more than 90% of the energy yield improvements are obtained with DPP converters rated at only 10% of the PV peak power.

Performance of Mismatched PV Systems With Submodule Integrated Converters

Mismatch power losses in photovoltaic (PV) systems can be reduced by the use of distributed power electronics at the module or submodule level. This paper presents an experimentally validated numerical model that can be used to predict power production with distributed maximum power point tracking (DMPPT) down to the cell level. The model allows the investigations of different DMPPT architectures, as well as the impact of conversion efficiencies and power constraints. Results are presented for annual simulations of three representative partial shading scenarios and two scenarios where mismatches are due to aging over a period of 25 years. It is shown that DMPPT solutions that are based on submodule integrated converters offer 6.9-11.1% improvements in annual energy yield relative to a baseline centralized MPPT scenario.

Partially shaded operation of multi-string photovoltaic systems

New shade mitigation technologies are available claiming improved performance under shaded conditions. A typical multi-string photovoltaic (PV) system was operated under partial shading conditions with and without these DC-DC converters. Power loss was attributed to low irradiance on shaded modules, current mismatch within shaded PV strings, and voltage mismatch between parallel strings. Indirect loss from voltage mismatch contributed up to 40% of the total power loss from shading. A representative residential solar installation was evaluated for potential benefit from DC-DC equipment. Detailed shading site survey information was collected and used to modify a PV production model. Using various assumptions on the severity of shade in this PV model, the application of DC-DC converters improved annual modeled power production by 5%–10% and recovered 24%–48% of the power lost due to shading.

Plume detachment from a magnetic nozzle

High-powered electric propulsion thrusters utilizing a magnetized plasma require that plasma exhaust detach from the applied magnetic field in order to produce thrust. This paper presents experimental results demonstrating that a sufficiently energetic and flowing plasma can indeed detach from a magnetic nozzle. Microwave interferometer and probe measurements provide plume density, electron temperature, and ion flux measurements in the nozzle region. Measurements of ion flux show a low-beta plasma plume which follows applied magnetic field lines until the plasma kinetic pressure reaches the magnetic pressure and a high-beta plume expanding ballistically afterward. Several magnetic configurations were tested including a reversed field nozzle configuration. Despite the dramatic change in magnetic field profile, the reversed field configuration yielded little measurable change in plume trajectory, demonstrating the plume is detached. Numerical simulations yield density profiles in agreement with the experimental results.

Partial-Shading Assessment of Photovoltaic Installations via Module-Level Monitoring

Distributed maximum power point tracking (DMPPT) is a topic of much interest in improving photovoltaic (PV) system performance. This study uses measured performance data at the module level for 542 PV systems to estimate lost system performance due to partial shade. Because each of the monitored systems is equipped with module-level dc power optimizers, an estimate is made of the overall system shading loss and the performance improvement that the system has received from this use of DMPPT. The estimate of shade extent and performance improvement predicted by this approach is verified experimentally against a system that has site survey images, and measured production with and without module-level electronics. Summary data for this analysis across 542 systems find an average power loss of 8.3% due to partial shading, which would have increased to 13% were the systems not equipped with panel-level optimizers. It is estimated that on average, 36% of the power lost from partial shading has been recovered through use of module-level dc power electronics.

Performance and Reliability Implications of Two-Dimensional Shading in Monolithic Thin-Film Photovoltaic Modules

We analyze the problem of partial shading in monolithically integrated thin-film photovoltaic (TFPV) modules, and explore how the shape and size of the shadows dictate their performance and reliability. We focus on the aspects of the shading problem unique to monolithic TFPV, arising from thin long rectangular series-connected cells, with partial shadows covering only a fraction of the cell area. Using calibrated 2-D circuit simulations, we show that due to the cell shape, the unshaded portion of partially shaded cell experiences higher heat dissipation due to redistribution of voltages and currents across the cells. We then use thermal imaging techniques to compare our results with module behavior under shade in realistic situations. We also analyze the effect of shadow size and orientation by considering several possible shading scenarios. We find that thin edge shadows can cause potentially catastrophic reverse bias damage, depending on their orientation. Finally, we show that external bypass diodes cannot protect the individual cells from shadow-induced reverse stress, but can limit the string output power loss for larger shadows.

Striving for a standard protocol for preconditioning or stabilization of polycrystalline thin film photovoltaic modules

Polycrystalline photovoltaic (PV) modules containing cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS) thin film materials can exhibit substantial transient or metastable current-voltage (I-V) characteristics depending on prior exposure history. Transient I-V phenomena confound the accurate determination of module performance, their reliability, and their measured temperature coefficients, which can introduce error in energy ratings models or servicelifetime predictions. Indeed, for either of these two technologies, a unique performance metric may be illusory without first specifying recent exposure or state—even at standard test conditions. The current standard preconditioning procedure for thin-film PV modules was designed for amorphous silicon (a-Si), and is likely inadequate for CdTe and CIGS. For a-Si, the Staebler-Wronski effect is known to result from defects, created via breaking of weak silicon bonds or light-activated trapping at the device junction, occurring rapidly upon light-exposure. For CdTe and CIGS devices, there is less agreement on the causes of metastable behavior. The data suggests that either deep-trapping of charge carriers, or the migration and/or electronic activation of copper may be responsible. Because these are quite disparate mechanisms, we suspect that there may be a more practical preconditioning procedure that can be employed prior to accurate performance testing for CdTe and CIGS modules. We devise a test plan to examine and compare the effects of light soaking versus forward-biased dark exposure at elevated temperatures, as parallel strategies to determine a feasible standard protocol for preconditioning and stabilizing these polycrystalline PV technologies, and report on the results of our tests.

Determining Outdoor CPV Cell Temperature

An accurate method is needed for determining cell temperature when measuring CPV modules outdoors. It has been suggested that cell temperature can be calculated through a procedure that shutters sunlight to the cells while measuring the transients in open‐circuit voltage (Voc) and heat sink temperature. This paper documents application of this shutter procedure to multiple CPV modules at NREL. The challenges and limitations are presented along with an alternate approach to measuring CPV cell operating temperature.

Thermal and Electrical Effects of Partial Shade in Monolithic Thin-Film Photovoltaic Modules

Photovoltaic cells can be damaged by reverse bias stress, which arises during service when a monolithically integrated thin-film module is partially shaded. We introduce a model for describing a modules internal thermal and electrical state, which cannot normally be measured. Using this model and experimental measurements, we present several results with relevance for reliability testing and module engineering: Modules with a small breakdown voltage experience less stress than those with a large breakdown voltage, with some exceptions for modules having light-enhanced reverse breakdown. Masks leaving a small part of the masked cells illuminated can lead to very high temperature and current density compared to masks covering entire cells.