0000000000015479
AUTHOR
Ole-morten Midtgard
Winding loss analysis and optimization of an AC inductor for a galvanically isolated PV inverter
This paper describes simple treatments for fringing fields of air gaps in the core and bypass fluxes in the winding window area of an existing high frequency AC inductor used by a commercial developer of PV inverters. For this purpose, Maxwell (ANSOFT) electromagnetic software package is used for winding eddy current loss analysis. It is displayed that air gaps cause high flux strength and, therefore, induce significantly high eddy currents to the surrounded windings. Proximity effect also causes non uniform current density in the winding. Altogether, the inductor is affected by fringing fields, and proximity effect produces a very high AC resistance, consequently resulting in undesirable h…
Optimum core dimension for minimizing proximity effect losses of an AC inductor for a galvanically isolated PV inverter
In this paper, it is shown how winding proximity effect losses in an existing AC inductor can be reduced without changing the actual winding. Instead, the core dimensions are changed with significant effects. For this purpose, Maxwell (ANSOFT) electromagnetic software package is used to investigate flux patterns and their influence on eddy current losses in the windings. Various core dimensions are analyzed with respect to flux patterns, and a design where the flux encloses a minimum number of winding layers is found. This significantly reduces the high frequency resistance of the windings up to 62.2% in comparison with the original design. The results are also explained in terms of the win…
Ideality factor behavior between the maximum power point and open circuit
The local ideality factor analysis of dark and light I-V curves has been used in the past to study various performance degradation effects in solar cells. Trapping, edge recombination and injection-level-dependent recombination are expressed as “lumps, humps and bumps” in the plots of the local ideality factor over cell voltage (m-V plots). Earlier applications of this differential technique did not correct the plots for the series resistance effect. Thus, the bumps at the higher voltages introduced by some mechanisms were more difficult to quantify. A possible solution is to analyze ISC-VOC curves, but their measurement is not always possible. We present a formula for calculation of the RS…
Analysis, design, and evaluation of a high frequency inductor to reduce manufacturing cost, and improve the efficiency of a PV inverter
Photovoltaic inverters are the major functional units of the photovoltaic systems. Therefore, efficiency and cost are vitally important in the design, and operation of the PV system. Magnetic components are the bulkiest component and highly affect the efficiency of the galvanically isolated PV inverter and design of such component generally involves a compromise between the reduction of core loss at the expense of increased winding loss or vice versa. The loss characteristics of the magnetic material itself present a fundamental limitation on core loss reduction, implying that the intrinsic reduction of core loss density depends on magnetic material improvements. Hence, the essential tradeo…
PV modules with variable ideality factors
The classical single-exponential I–V curve model for crystalline-Si PV devices assumes a constant ideality factor. The model is widely applied in e.g. modeling the power output of PV systems at different irradiances and temperatures, and also in calculating the Equivalent Cell Temperature (ECT) from the open-circuit voltage. The present paper presents results from several crystalline-Si (c-Si) module types suggesting that the ideality factor may change with irradiance. The possible implications of such variability are discussed, in particular the need for a new ECT calculation procedure as well as for revision of some PV performance models.
Review on up/down conversion materials for solar cell application
The present paper reviews the methods of photon up- and down conversion strategies for improving the efficiency of solar cells. Photons with a lower energy than the band gap will be lost in a normal solar cell. The principle of the up conversion technique is that two or more photons are converted into a photon with energy higher than the band gap energy. High energy photons will lose the energy above the band gap energy limit. Down conversion is a process where a high energy photon is converted into several lower energy photons with energies above the band gap. A description is given of the most common methods and materials for these conversions resulting in higher solar cell efficiencies.
Series resistance determination and further characterization of c-Si PV modules
Abstract This paper presents a new algorithm for determination of the series resistance of crystalline-Si PV modules from individual illuminated I–V curves. The ideality factor and the reverse saturation current are then extracted in the classic way. The approach is applied to in-situ measured data from modules based on two types of mc-Si feedstock. The results indicate that the method yields physically meaningful parameters. An improved definition of local ideality factor is suggested, resulting in m-V plots unaffected by the series resistance. In addition, m-I plots are introduced for the first time. The novel differential techniques reveal an unexpected rise of the ideality factor at ope…
100-millisecond Resolution for Accurate Overirradiance Measurements
Cloud enhancement of sunlight results in peak irradiance well exceeding extraterrestrial levels, even at high latitudes. Values above 1.8 kW/m2 are possible in the equatorial regions. Recently, we detected bursts over 1.5 kW/m2 in Northern Europe at latitude close to 60°N. Overirradiance events (intensities much higher than 1 sun) can last tens of minutes, as well as less than 1 s. They may have caused series arcing in photovoltaic modules, leading to fires and loss of property. The accurate measurement of short bursts requires sensors with response times on the order of milliseconds. The long response times of thermopile pyranometers smooth out important details of very short-lived peaks a…
Optimal temporal resolution for detailed studies of cloud-enhanced sunlight (Overirradiance)
Enhancement of sunlight by clouds can lead to irradiance peaks much exceeding the extraterrestrial levels - close to 2 suns near the Equator and at least 1.5 suns at latitudes of about 60°. Some extreme overirradiance events can last many minutes, but durations in the order of 1 second are also possible. The present paper reports the shortest bursts recorded in Southern Norway in the years 2012 and 2013. Our records of 10-millisecond resolution from 2012 show that the optimal instantaneous irradiance sampling interval is less than 0.15 s at the present test site, while the optimal averaging time is less than 0.13 s. We propose simple equations for deriving these times in an arbitrary geogra…
Electrostatic field theory and circuit analysis in the design of coalescers with pulsed dc voltage
This paper presents a mathematical analysis of the electric field and interfacial free charge experienced by the emulsion in an electrostatic coalescer that is subjected to a pulsed dc voltage. Previous work in this area is re-assessed, but the analysis is expanded. A novel static analysis of the system is presented, giving valuable physical insight. It becomes obvious that there must be a limited range of frequencies that are effective in a pulsed dc scheme. Thereafter, analytical expressions for the steady-state solution, where both the switching frequency and duty ratio (or mark space ratio) can be varied, are presented for the first time. These expressions are used to derive voltage and…
Test of the European Joint Research Centre performance model for c-Si PV modules
The performance of ten c-Si PV modules is studied at different levels of illumination and module temperatures and is modeled in terms of relative efficiency with respect to Standard Testing Conditions (STC). A recently proposed model is tested in Nordic conditions for generic mono- and mc-Si devices with screen-printed cells as well as for some high-efficiency cell designs. Two of the modules are field aged. Averaged model coefficients are derived and compared to values found in the literature. Irradiance and module temperatures measured in 2011 are then used to estimate the expected annual energy yields. The measured yields equal the predicted ones within 1.6% (mean bias error −1.0%) which…
Application of circuit theory for extraction of water from oil
This paper discusses optimization of the performance of a pulsed dc electrostatic coalescer, a device that is used in the oil industry to increase the speed of separation of water from a water-in-oil emulsion. A previously found criterion for optimum performance, the charge hypothesis, is reviewed. Analysis by circuit theory shows that the performance may be improved through selecting an optimum switching frequency and duty ratio, improving the design of the pulse train generator, and making physical modifications to the coalescer apparatus. The addition of an external charging circuit is beneficial. Experiments on the modified system could contribute to a better understanding of the physic…
Development of a test station for accurate in situ I-V curve measurements of photovoltaic modules in Southern Norway
The development of an outdoor test station for accurate in situ I-V curve measurements of photovoltaic (PV) modules is described. The modules are installed in an open-rack configuration at the University of Agder in Southern Norway. Seven new and three aged PV modules of different type and make are being tested, including mono-and multicrystalline silicon from differing manufacturing routes, triple-junction amorphous silicon, and CIS. Data acquisition is controlled with a multichannel electronic load system and LabVIEW software, recording high-resolution I-V curves at one-minute intervals. Between I-V curve sweeps, each module is operated at the maximum power point. Characteristic electrica…
Review of photovoltaic status in a European (EU) perspective
In the EU continent, a generation cost around 0.15€/kWh has been achieved [1]. Around the world, there is a mission to stimulate this green source of energy and it is believed that by 2030, PV will be able to deliver around 9 % of the world's electricity demands. According to an International Energy Agency (IEA) report [2], it is expected that PV will accomplish grid parity in at least 10% of the world by 2020. There was a significant 60 % annual growth in the grid connected PV systems in the span of 2004–2009 [3]. The present article will bring out the status & review of PV technology and the details of PV system standards as suggested by IEA. In the present paper, the main attention has b…
1.6 suns at 58°20′N - the solar resource in Southern Norway
We measured an extreme overirradiance event of 1.6 suns at latitude 58°20′N during a solar resource assessment in southern Norway. The burst occurred when the sun showed in a narrow gap between broken, optically thin altocumulus clouds. The enhancement was most intense within 3° around the solar disk. The annual solar irradiation in the plane of an optimally tilted PV array peaked at 1.30 MWh/m2 in the year 2013, and measured 1.20 MWh/m2 in the previous two years. These values exclude irradiance below 0.05 suns but include reflections from the sea surface and terrain which have not been quantified. The long-term average predicted by the Photovoltaic Geographical Information System (PVGIS) i…
Modeling and parameter identification of crystalline silicon photovoltaic devices
This paper tests the standard single-exponential model of the electrical characteristics of crystalline-Si photovoltaic devices, focusing on the (apparent) shunt current. Measured characteristics of illuminated polycrystalline-Si photovoltaic modules are modeled, and the apparent shunt current is analyzed. It is shown that an Ohmic-like behavior only takes place at voltages well below the maximum-power point. At higher voltages, the apparent shunt current quickly drops to negligible values. Modeling a crystalline-Si PV device with a fixed shunt resistance may therefore lead to underestimation of the maximum power exceeding 10% at certain irradiance levels.
Extreme overirradiance events in Norway: 1.6 suns measured close to 60°N
Abstract We report the most extreme overirradiance event of 1.6 kW/m 2 measured so far in Grimstad, southern Norway (latitude 58°20′), at altitude of only about 60 m a.s.l. Images of the sky conditions taken with a wide-dynamic-range camera suggest that this is not the ultimate value for our location, but that extremes as high as 1.7 kW/m 2 may be possible. The phenomenon was caused by strong forward scattering of sunlight within 3° around the solar disk in thin, broken altocumulus clouds. The normalized spectrum of sunlight during such extreme events is very similar to the normalized clear-sky spectrum measured at Air Mass 1.4 and irradiance of 1.0 kW/m 2 . We suggest that the values excee…
Equivalent cell temperature calculation for PV modules with variable ideality factors
Outdoor testing of PV modules involves monitoring their operating temperatures which have strong influence on the output power. Back-of-module sensing is associated with significant uncertainty due to temperature drop across the back encapsulation and also due to non-uniform temperature over the module's area. The international standard IEC 60904-5 defines a procedure for calculating the Equivalent Cell Temperature (ECT) of PV devices from their open-circuit voltage. This and other standards (as well as some module performance models) assume a single-exponential I-V curve model with a constant ideality factor. However, this parameter may vary with the illumination level and the operating te…
A new analytical solar cell I–V curve model
Abstract A simple mathematical equation that can represent empirical I–V curves of individual solar cells, systems of solar cells and modules has been found. The basic model is determined by four parameters: the open circuit voltage, the short circuit current and two shape parameters. With the four parameters determined, the complete current–voltage curve, the fill factor and the maximum power point are given by simple analytical functions. The model is valid both in the positive and the negative (dark condition) voltage range. Several simple examples demonstrate some of the potential of the model. Due to its mathematical simplicity, it is suggested that the model will be suitable for analy…
Extracting parameters from semi-log plots of polycrystalline silicon PV modules outdoor I–V data: Double-exponential model revisited
This paper presents a method for extracting physically meaningful parameters from measured I–V curves of PV modules. The 7-parameter double-exponential model is applied in the modeling. The method is based on linear fitting of semi-logarithmic plots. The paper demonstrates a new technique to estimate the series resistance of a module with high accuracy from such plots. As a result, also the reverse saturation current and the quality factor of the diffusion diode can be determined. The method is applied to outdoor I–V data from a test station with three similar, but not identical, polycrystalline-Si modules. The values of the series resistances found with this method deviate somewhat from th…
Modeling and parameter identification of crystalline silicon photovoltaic devices
In this paper the physical correctness of the standard single-exponential (one-diode) model of crystalline-Si photovoltaic devices is examined. In particular, we focus on the shunt current. I-V curves of in situ illuminated polycrystalline-Si photovoltaic modules are measured, and based on these measurements, we extract the shunt current. There is a certain voltage range in which the shunt current shows an Ohmic-like behavior, but the value of the resistance varies with irradiance and the quality of illumination. In addition, the Ohmic behavior takes place at voltages well below the maximum-power point (MPP). At higher voltages, the shunt current drops to negligible values. We conclude that…
Air gap fringing flux reduction in a high frequency inductor for a solar inverter
In a gapped inductor, air gap fringing flux induces eddy currents in conductors in the vicinity of the air gap producing unwanted power loss and heat in the coil. This paper presents a detailed analysis to evaluate the performance changes of a high frequency inductor used in a solar inverter by new arrangement scheme of the air gap locations. The effect of air gap positioning in the core and the ac-resistance and leakage inductance of the high frequency inductor used in a solar inverter is investigated by using the 2-D finite element analysis. The simulation results show that a significant improvement can be achieved with limited changes in the air gap locations.
Power loss analysis for single phase grid-connected PV inverters
This paper presents a method for power loss analysis applied on single-phase grid-connected PV inverter. The often neglected current ripple effects are included in power device switching and conduction losses. The relationships amongst component losses, output inductance, switching frequency and dc-link voltage are investigated. It is shown that current ripple effect is important to power loss analysis due to varying irradiation. The closed form solutions of component losses are proposed and verified with SPICE simulation results.
Overirradiance (Cloud Enhancement) Events at High Latitudes
Contrary to intuition, solar irradiance peaks at partially cloudy conditions. Clouds can boost sunlight by over 1.5 times, even at high latitudes. Depending on cloud velocity, the bursts last from seconds to minutes. Measuring irradiance on a tilted surface with 10-ms resolution allows for a detailed study of such events in Southern Norway, almost at sea level. All monthly maxima from April through September 2011 exceeded 1300 W/m2. The slow sensor registered an annual maximum of 1413 W/m2, while the fast sensor's range was found insufficient. A burst reaching 1528 W/m2 was registered in June 2012. Near the Equator, bursts exceeding 1800 W/m2 have been observed. These numbers are striking s…
Physically-consistent parameterization in the modeling of solar photovoltaic devices
This research tests the standard one-diode model of a crystalline-Si photovoltaic cell, focusing on the physical accuracy. In particular, the (apparent) shunt resistance and the diode ideality factor are studied. Current-voltage characteristics of illuminated crystalline-Si photovoltaic modules are analyzed, and some limits of applicability of the standard model are given. Typical values of the ideality factor for crystalline-Si devices are derived from own experimental data as well as from recently published literature. It is shown that the contribution of the apparent shunt resistance is only significant for cell voltages below about 0.45 V, and depends on irradiance. This result is consi…
Contributions to the knowledge base on PV performance: Evaluation of the operation of PV systems using different technologies installed in southern Norway
To assist in establishing an accepted knowledge base on PV-modules and systems performance using a representative range of technologies, devices have to be installed at diverse locations, covering a broad range of environmental conditions. For the example of a high latitude location, modules and systems are installed and under investigation in southern Norway (Kristiansand region) by the University of Agder in cooperation with industrial partners. This paper presents first results of the analysis of module performance. The operational behavior of the modules is used to derive a modeling scheme applicable for performance prediction. This use is demonstrated by giving the expected annual perf…
Outdoor Performance of 10 year Old a-Si and Poly-Si Modules in southern Norway conditions
The outdoor performance of 10 years modules has been measured in southern Norway conditions. For this study, Poly-Si and a-Si modules are selected and the measurements are carried out using a portable electronic load. The I–V characteristics and maximum power have been recorded for several modules of each technology. It is found that Poly-Si modules perform very well even after 10 years of span whereas some of a-Si modules have visual degradation which causes significant power losses. The experimentally outdoor monitored values of P mp , V mp , short circuit current (I sc ), Open circuit voltage (V oc ) and efficiency for different modules are compared with manufacturer's values.
A qualitative examination of performance and energy yield of photovoltaic modules in southern Norway
Three different, commercially available photovoltaic modules have been monitored outdoors in the town of Grimstad, Norway. The present paper describes the experimental setup that was implemented, in particular details of the low-cost electronic loads. Results compare measured performance with manufacturer's data, and temperature measurements enable a comparison with performance at standard test condition temperature. Overall, the monocrystalline module performed best both regarding maximum efficiency and overall energy production, whereas the module based on triple junction amorphous silicon technology had the worst performance considering these criteria. The gross numbers of energy yield c…
Power management of grid-connected photovoltaic inverter with storage battery
With increased penetration of PV generation in the power grid, it becomes necessary to control the PV generation similar to the conventional power sources. Grid-connected PV systems equipped with storage batteries are capable of active power control. PV power generation can be either fed into the grid or stored in the battery. This paper presents a deterministic power management algorithm that uses closed-form approximations derived from analytical loss models. It can be configured to maximize the efficiency or the revenue in a liberalized electricity market. The battery controller emulates an optimum load-line in which the PV power is autonomously shared between the battery and the feed-in…
Outdoor performance of north-facing multicrystalline modules in Southern Norway
The present paper deals with reports on an outdoor performance evaluation of multicrystalline modules facing north in Southern Norway. The relevance of this study is mainly for building integrated systems where some of the facades are in a far from optimum position. The impact of shading on the photovoltaic modules' performance is also evaluated. A dedicated experimental set up has been created to study the PV modules performance under the above mentioned conditions. The results indicate significant power extraction from the PV modules even in the north facing direction. Effects of shadowing are also considered.