Efficiency of maximum power point tracking in photovoltaic system under variable solar irradiance

Field conditions decrease the energy output of photovoltaic (PV) systems, mainly due to excessive temperatures. However, in regions with moderate ambient temperatures, as in Poland, solar energy is commonly delivered with highly fluctuating irradiance. This introduces yet another source of energy losses due to the non-ideal tracking of actual position of Maximum Power Point (MPP). Majority of PV-systems are equipped with DC/AC and grid-connected inverter. Since the solar energy flux is variable, an adequate MPP-tracking algorithm is required to handle a wide range of load levels and face rapid changes of input power. Along with the essential DC/AC conversion, the quality of MPP-tracking must also be taken into account in evaluation of inverter efficiency. The tracking in dynamic conditions has been addressed only recently. Several algorithms has been studied theoretically, experimentally or in laboratory conditions by applying artificial input test-patterns. This work takes the opposite approach by applying the recorded real-life solar irradiance and simulating the tracking behavior to study the problem for true field conditions in Poland. The simulation uses the unique high-quality irradiance data collected with 200 ms time resolution. The calculation of both static and dynamic MPP-tracking efficiency has been performed for representative variable-cloudy day, applying commonly used Perturb&Observe tracking algorithm

Simulation, Measurement, and Emulation of Photovoltaic Modules Using High Frequency and High Power Density Power Electronic Circuits

The number of solar photovoltaic (PV) installations is growing exponentially, and to improve the energy yield and the efficiency of PV systems, it is necessary to have correct methods for simulation, measurement, and emulation. PV systems can be simulated using PV models for different configurations and technologies of PV modules. Additionally, different environmental conditions of solar irradiance, temperature, and partial shading can be incorporated in the model to accurately simulate PV systems for any given condition. The electrical measurement of PV systems both prior to and after making electrical connections is important for attaining high efficiency and reliability. Measuring PV modules using a current-voltage (I-V) curve tracer allows the installer to know whether the PV modules are 100% operational. The installed modules can be properly matched to maximize performance. Once installed, the whole system needs to be characterized similarly to detect mismatches, partial shading, or installation damage before energizing the system. This will prevent any reliability issues from the onset and ensure the system efficiency will remain high. A capacitive load is implemented in making I-V curve measurements with the goal of minimizing the curve tracer volume and cost. Additionally, the increase of measurement resolution and accuracy is possible via the use of accurate voltage and current measurement methods and accurate PV models to translate the curves to standard testing conditions. A move from mechanical relays to solid-state MOSFETs improved system reliability while significantly reducing device volume and costs. Finally, emulating PV modules is necessary for testing electrical components of a PV system. PV emulation simplifies and standardizes the tests allowing for different irradiance, temperature and partial shading levels to be easily tested. Proper emulation of PV modules requires an accurate and mathematically simple PV model that incorporates all known system variables so that any PV module can be emulated as the design requires. A non-synchronous buck converter is proposed for the emulation of a single, high-power PV module using traditional silicon devices. With the proof-of-concept working and improvements in efficiency, power density and steady-state errors made, dynamic tests were performed using an inverter connected to the PV emulator. In order to improve the dynamic characteristics, a synchronous buck converter topology is proposed along with the use of advanced GaNFET devices which resulted in very high power efficiency and improved dynamic response characteristics when emulating PV modules

Evaluation of two prototype three phase photovoltaic water pumping systems

Bibliography: p. 221-223.Two prototype three phase AC photovoltaic pump systems (Solvo, ML T) and a DC PV pump (Miltek) were tested on a farm borehole in Namibia (latitude 21°6', longitude 17°6'). The PV array consisted of twelve modules (636Wpeak) mounted on a single-axis passive tracker. The depth of the water was 75m and a progressive cavity pump with a self-compensating stator was used in all the tests. Customised data acquisition was designed to measure performance characteristics through a range of operating conditions (mainly steady state); a secondary data acquisition system was used to capture samples of high frequency signals. The data allowed detailed analysis of system, subsystem and component performance, as well as performance evaluation over Standard Solar Days. The focus of the investigation was evaluation of the AC prototypes, in terms of performance, other technical factors, reliability and economic criteria. The analog-based DC system served as a basis for comparison. Both AC systems employed microprocessor control and PWM variable-frequency variable-voltage inversion. Efficiencies, optimality, stability, start-up behaviour, non-productive operating modes and protection were examined. A number of recommendations were proposed for improvements in the basic control algorithms, monitoring and managing non-productive modes, improved protection, layout and user diagnostic features

Integration of a lithium-ion battery in a micro-photovoltaic system: Passive versus active coupling architectures

A balcony photovoltaic (PV) system, also known as a micro-PV system, is a small PV system consisting of one or two solar modules with an output of 100–600 Wp and a corresponding inverter that uses standard plugs to feed the renewable energy into the house grid. In the present study we demonstrate the integration of a commercial lithium-ion battery into a commercial micro-PV system. We firstly show simulations over one year with one second time resolution which we use to assess the influence of battery and PV size on self-consumption, self-sufficiency and the annual cost savings. We then develop and operate experimental setups using two different architectures for integrating the battery into the micro-PV system. In the passive hybrid architecture, the battery is in parallel electrical connection to the PV module. In the active hybrid architecture, an additional DC-DC converter is used. Both architectures include measures to avoid maximum power point tracking of the battery by the module inverter. Resulting PV/battery/inverter systems with 300 Wp PV and 555 Wh battery were tested in continuous operation over three days under real solar irradiance conditions. Both architectures were able to maintain stable operation and demonstrate the shift of PV energy from the day into the night. System efficiencies were observed comparable to a reference system without battery. This study therefore demonstrates the feasibility of both active and passive coupling architectures

Evolution engine technology in exhaust gas recirculation for heavy-duty diesel engine

In this present year, engineers have been researching and inventing to get the optimum of less emission in every vehicle for a better environmental friendly. Diesel engines are known reusing of the exhaust gas in order to reduce the exhaust emissions such as NOx that contribute high factors in the pollution. In this paper, we have conducted a study that EGR instalment in the vehicle can be good as it helps to prevent highly amount of toxic gas formation, which NOx level can be lowered. But applying the EGR it can lead to more cooling and more space which will affect in terms of the costing. Throughout the research, fuelling in the engine affects the EGR producing less emission. Other than that, it contributes to the less of performance efficiency when vehicle load is less

Real-time Modelling, Diagnostics and Optimised MPPT for Residential PV Systems

The work documented in the thesis has been focused into two main sections. The first part is centred around Maximum Power Point Tracking (MPPT) techniques for photovoltaic arrays, optimised for fast-changing environmental conditions, and is described in Chapter 2. The second part is dedicated to diagnostic functions as an additional tool to maximise the energy yield of photovoltaic arrays (Chapter 4). Furthermore, mathematical models of PV panels and arrays have been developed and built (detailed in Chapter 3) for testing MPPT algorithms, and for diagnostic purposes.In Chapter 2 an overview of the today’s most popular MPPT algorithms is given, and, considering their difficulty in tracking under variable conditions, a simple technique is proposed to overcome this drawback. The method separates the MPPT perturbation effects from environmental changes and provides correct information to the tracker, which is therefore not affected by the environmental fluctuations. The method has been implemented based on the Perturb and Observe (P&O), and the experimental results demonstrate that it preserves the advantages of the existing tracker in being highly efficient during stable conditions, having a simple and generic nature, and has the benefit of also being efficient in fast-changing conditions. Furthermore, the algorithm has been successfully implemented on a commercial PV inverter, currently on the market. In Chapter 3, an overview of the existing mathematical models used to describe the electrical behaviour of PV panels is given, followed by the parameter determination for the five-parameter single-exponential model based on datasheet values, which has been used for the implementation of a PV simulator taking in account the shape, size ant intensity of partial shadow in respect to bypass diodes.In order to eliminate the iterative calculations for parameter determinations, a simplified three-parameter model is used throughout Chapter 4, dedicated to diagnostic functions of PV panels. Simple analytic expressions for the model important parameters, which could reflect deviations from the normal (e.g. from datasheet or reference measurement) I −V characteristic, is proposed.A considerable part of the thesis is dedicated to the diagnostic functions of crystalline photovoltaic panels, aimed to detect failures related to increased series resistance and partial shadowing, the two major factors responsible for yield-reduction of residential photovoltaic systems.Combining the model calculations with measurements, a method to detect changes in the panels’ series resistance based on the slope of the I − V curve in the vicinity of open-circuit conditions and scaled to Standard Test Conditions (STC) , is proposed. The results confirm the benefits of the proposed method in terms of robustness to irradiance changes and to partial shadows.In order to detect partial shadows on PV panels, a method based on equivalent thermal voltage (Vt) monitoring is proposed. Vt is calculated using the simplified three-parameter model, based on experimental curve. The main advantages of the method are the simple expression for Vt, high sensitivity to even a relatively small area of partial shadow and very good robustness against changes in series resistance.Finally, in order to quantify power losses due to different failures, e.g. partial shadows or increased series resistance, a model based approach has been proposed to estimate the panel rated power (in STC). Although it is known that the single-exponential model has low approximation precision at low irradiation conditions, using the previously determined parameters it was possible to achieve relatively good accuracy. The main advantage of the method is that it relies on already determined parameters (Rsm, Vt) based on measurements, therefore reducing the errors introduced by the limitation of the single-exponential model especially at low irradiation conditions

Extended Functionalities of Photovoltaic Systems with Flexible Power Point Tracking:Recent Advances

The power system is experiencing an ever-increasing integration of photovoltaic power plants (PVPPs), which leads demand on the power system operators to force new requirements to sustain with quality and reliability of the grid. Subsequently, a significant quantity of flexible power point tracking (FPPT) algorithms have been proposed in the literature to enhance functionalities PVPPs. The intention of FPPT algorithms is to regulate the PV power to a specific value imposed by the grid codes and operational conditions. This will inevitably interfere the maximum power point tracking (MPPT) operation of PV systems. Nevertheless, the FPPT control makes PVPPs much more grid-friendly. The main contribution of this paper is to comprehensively compare available FPPT algorithms in the literature from different aspects and provide a benchmark for researchers and engineers to select suitable FPPT algorithms for specific applications. A classification and short description of them are provided. The dynamic performances of the investigated algorithms are compared with experimental tests on a scaled-down prototype. Directions for future studies in this area are also presented.MOE (Min. of Education, S’pore)Accepted versio

Design process optimisation of solar photovoltaic systems

The design processes for solar photovoltaic (PV) systems is improved to achieve higher reliability and reduced levelised cost of energy (LCOE) throughout this thesis. The design processes currently used in the development of PV systems are reviewed. This review process included embedding the author in a project to deliver four rooftop PV systems which totalled a megawatt of installed generating capacity, which at the time represented very significant system sizes. The processes used in this are analysed to identify improvement potential. Shortcomings are identified in three main areas: safety assurance, design process integration and financial optimisation. Better design process integration is required because data is not readily exchanged between the industry standard software tools. There is also a lack of clarity about how to optimise design decisions with respect to factors such as shading and cable size. Financial optimisation is identified as a challenge because current software tools facilitate optimising for maximum output or minimum cost, but do not readily optimise for minimum levelised cost of energy which is the primary objective in striving for grid parity. To achieve improved design process integration and financial optimisation, a new modelling framework with the working title SolaSIM is conceived to accurately model the performance of solar photovoltaic systems. This framework is developed for grid connected systems operating in the UK climate, but it could readily be adapted for other climates with appropriate weather data. This software development was conducted using an overarching systems engineering approach from design and architecture through to verification and validation. Within this SolaSIM framework, the impact of shading on array and inverter efficiency is identified as a significant area of uncertainty. A novel method for the calculation of shaded irradiance on each cell of an array with high computational efficiency is presented. The shading sub-model is validated against outdoor measurements with a modelling accuracy within one percent. Final verification of the over-arching SolaSIM framework found that it satisfied the requirements which were identified and actioned. The author installed the new CREST outdoor measurement system version 4 (COMS4). COMS4 is a calibrated system which measures 26 PV devices simultaneously. Validation of SolaSIM models against COMS4 found the modelling error to be within the 4% accuracy target except two sub-systems which had electronic faults. The model is validated against PV systems and found to be within the specified limits

Small-signal modelling of maximum power point tracking for photovoltaic systems

In grid connected photovoltaic (PV) generation systems, inverters are used to convert the generated DC voltage to an AC voltage. An additional dc-dc converter is usually connected between the PV source and the inverter for Maximum Power Point Tracking (MPPT). An iterative MPPT algorithm searches for the optimum operating point of PV cells to maximise the output power under various atmospheric conditions. It is desirable to be able to represent the dynamics of the changing PV power yield within stability studies of the AC network. Unfortunately MPPT algorithms tend to be nonlinear and/or time-varying and cannot be easily combined with linear models of other system elements. In this work a new MPPT technique is developed in order to enable linear analysis of the PV system over reasonable time scales. The new MPPT method is based on interpolation and an emulated-load control technique. Numerical analysis and simulations are employed to develop and refine the MPPT. The small-signal modelling of the MPPT technique exploits the fact that the emulated-load control technique can be linearised and that short periods of interpolation can be neglected. A small-signal PV system model for variable irradiation conditions was developed. The PV system includes a PV module, a dc-dc boost converter, the proposed controller and a variety of possible loads. The new model was verified by component-level time-domain simulations. Be cause measured signals in PV systems contain noise, it is important to assess the impact of that noise on the MPPT and design an algorithm that operates effectively in pr esence of noise. For performance assessment of the new MPPT techniques, the efficiencies of various MPPT techniques in presence of noise were compared. This comparison showed superiority of the interpolation MPPT and led to conclusions about effective use of existing MPPT methods. The new MPPT method was also experimentally tested.Open Acces