System memory effects in the sizing of stand‐alone PV systems

Models based on daily energy balance (or long‐term models) have been widely used as a tool in the stand‐alone photovoltaic (PV) system sizing, mainly with the purpose of obtaining analytical expressions of the relation between the generator size and the storage capacity of the battery. The system can then be designed to meet the reliability requirements of the specific case. However, such models represent the complex operation of a stand‐alone system in an oversimplified way. There is little research so far on the reliability and improvement of such models. Validation and possible modification of a long‐term system model requires comparison of the simulated state of charge (SOC) of the battery with that obtained from an experimental system. In this work, experimental data from a 6‐month operation of a basic stand‐alone PV system have been analysed and compared with modelling results. One obvious improvement that could be applied to the long‐term system model is to account for a charging efficiency of the battery, and this possibility is examined in the present work. However, comparison with the modelling results shows that the data cannot be fitted by simply taking into account battery inefficiency. A method to account for system memory effects in the increase of the battery SOC, imposed by the operation of the regulator, is necessary to accurately model the macroscopic diurnal charging/discharging process. Copyright © 2012 John Wiley & Sons, Ltd.     

Dependability analysis of stand‐alone photovoltaic systems

Long‐term performance of PV stand‐alone systems is analysed in this work in terms of dependability. On one side, the quality of a PV system, the energy service supplied to the users, depends on the initial design and sizing and on the component ageing that progressively decreases the availability of supply on demand (energy reliability). On the other side, technical failures lead to system stoppage until repairing is performed (technical reliability), which is crucial in real rural electrification applications. All those factors are analysed together with the basis of an extended field, laboratory and bibliographic review work. Copyright © 2006 John Wiley & Sons, Ltd.     

The present status and future direction of technology development for photovoltaic power generation in Japan

In 2004 NEDO established the PV Roadmap Toward 2030 PV2030 as a long‐term strategy for PV R&D. In this Roadmap, PV is expected by 2030 to supply approximately 50% of residential electricity consumption (cumulative installed capacity in the range of 100 GW). In terms of economic efficiency, electricity costs are targeted to equal commercial use, approximately 14 Yen/kW h, by 2020 and industrial use, approximately 7 Yen/kW h, by 2030. For future PV systems, it is essential to improve the stand‐alone capabilities of PV system with electricity storage and to develop community‐based PV systems using multi‐function inverters. Advanced technological innovations beyond the existing levels are also essential. Therefore, NEDO is undertaking 2‐year projects for preliminary research to make clear the next R&D of solar cells and PV system technology. Copyright © 2005 John Wiley & Sons, Ltd.     

Characterisation of charge voltage of lead‐acid batteries: application to the charge control strategy in photovoltaic systems

In stand‐alone photovoltaic (PV) systems, charge controllers prevent excessive battery overcharge by interrupting or limiting the current flow from the PV array to the battery when the battery becomes fully charged. Charge regulation is most often accomplished by limiting the battery voltage to a predetermined value or cut‐off voltage, higher than the gassing voltage. These regulation voltages are dependent on the temperature and battery charge current. An adequate selection of overcharge cut‐off voltage for each battery type and operating conditions would maintain the highest battery state of charge without causing significant overcharge thus improving battery performance and reliability. To perform this work, a sample of nine different lead‐acid batteries, typically used in stand‐alone PV systems including vented and sealed batteries with 2 V cells and monoblock configurations have been selected. This paper presents simple mathematical expressions fitting two charge characteristic voltages: the gassing voltage (V_g) and the end‐of charge voltage (V_fc) as function of charge current and temperature for the tested batteries. With these expressions, we have calculated V_g and V_fc at different current rates. An analysis of the different values obtained is presented here focusing in the implication in control strategies of batteries in stand‐alone PV systems. Copyright © 2006 John Wiley & Sons, Ltd.     

The battery voltage distribution: a possible tool for surveying the state of health of stand‐alone PV systems

Degradation phenomena can remain unnoticed over long periods in stand‐alone PV systems, before eventually causing service interruption. Problems include battery capacity reduction because of ageing, PV array capacity reduction because of wiring corrosion or excessive dirtiness, and mis‐adjustment of controller set‐points. This underlines the practical interest of an easily applicable method of early detection, so that preventive measures can be taken in good time. This paper proposes the use of the battery voltage distribution, as a tool for surveying the state of health of stand‐alone PV systems. Expected distributions can be derived by judicious modelling; while observed distributions can easily be obtained from systematic voltage measurements which are already made in many PV systems worldwide. Comparison between expected and observed distributions allows relatively simple diagnosis rules to be derived. Copyright © 2005 John Wiley & Sons, Ltd.     

Microcontroller‐based simple maximum power point tracking controller for single‐stage solar stand‐alone water pumping system

A simple microcontroller‐based maximum power point tracking controller is proposed for a single‐stage solar stand‐alone water pumping system for remote, isolated, and nonelectrified population, where less maintenance, low cost, and an efficient system is of prime interest. It consists of a photovoltaic (PV) module, a DC–AC converter utilizing space‐vector pulse‐width modulation, an induction motor coupled with a water pump, a voltage sensor, and a current sensor. A space‐vector pulse‐width modulation‐controlled DC–AC converter aided by a fast‐acting on–off supervisory controller with a modified perturb‐and‐observe algorithm performs both the functions of converting PV output voltage to a variable voltage, variable frequency output, as well as extracting the maximum power. A limited variable step size is preferred during transient state, and a steady frequency, which is calculated on the basis of steady‐state oscillation, is set during steady state. A fast‐acting on–off supervisory controller regulates DC link voltage during steady state and enables maximum power point tracking algorithm only during transient state to draw a new voltage reference. In the event of low voltage, the controller switches off the motor but continuously scans for an available PV voltage. The system is not protected against an overcurrent because the maximum current is equal to its short circuit current. The 16‐bit microcontroller dsPIC6010A (Microchip Technology, Inc., Chandler, AZ, USA) is used to implement the control functions. The proposed controller is verified through simulation as well as tested in the laboratory prototype model. The simulation and experimental results show good correlation. Copyright © 2011 John Wiley & Sons, Ltd.     

Method for photovoltaic parameter extraction according to a modified double‐diode model

The variables presented in the current–voltage equation of a photovoltaic (PV) device are usually called PV parameters. There are several different methods for PV parameter extraction from measured data according to different models. However, many of these methods provide results that do not represent I–V curves of thin films devices correctly. This can occur because either the applied model or the PV parameter extraction methods are not suitable. It is also possible that the extracted parameters provide a good mathematical representation of the curves but without physical meaning (e.g. negative series resistance). This work presents a method for PV parameter extraction based on a modified double‐diode model. In this model, the ideality factor related to the recombination of the charge carriers in the space‐charge region is assumed as a variable. This method has been tested for different I–V curves of different PV module technologies providing very good results and parameters with physical meaning in all the cases. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimal sizing of photovoltaic‐hydro power plant

The work presents a technological concept of energetically independent and ecologically sustainable system of electric energy production by joint operation of photovoltaic (PV) and hydro electric (HE) power plant as a unique technological system of solar hydroelectric (SHE) power plant. The sustainability of such system is based exclusively on the solar energy input, as the renewable and pure energy resource, and the use of hydro energy, due to the possibility of its continuous production of energy and its well‐known flexibility in covering the consumers' needs. For the purpose of connecting all relevant values into one integral SHE system, a mathematical model was developed for selecting the optimal size of the PV power plant as the key element for estimating the technological feasibility of the overall solution. The model was tested on electric energy supply from the island of Vis in Croatia. The obtained power of the PV power plant was 41 MW_p which corresponds to collector field of approximately 25 ha, while the estimated related storage was 20 hm³. The results show that the subject model describes the SHE very well and that the proposed concept of joint operation of PV and HE power plants is real and possible. The application of such sustainable SHE systems could significantly increase PV industry worldwide, i.e. the share of solar energy in energy balances of numerous countries. Proposed hybrid simulation‐dynamic programming model is suitable to optimize PV plant in accordance with system characteristics. Copyright © 2009 John Wiley & Sons, Ltd.     

A novel power benefit prediction approach for two‐axis sun‐tracking type photovoltaic systems based on semiconductor theory

Photovoltaic (PV) systems incorporated with sun‐tracking technology have been proposed and verified to effectively increase the power harvest. However, the actual power generated from a PV module has not been investigated and compared with that analyzed from theoretical models of the PV material. This study proposes a novel method for estimating the power benefit harvested by a two‐axis sun‐tracking type (STT) PV system. The method is based on semiconductor theory and the dynamic characteristics, including maximum power point tracking of PV modules that can be integrated with the database of annual solar incidences to predict the power harvested by any STT PV system. The increment of annual energy provided by an STT PV system installed at any arbitrary latitude, compared with that by a fixed‐type system, can be accurately estimated using the proposed method. To verify the feasibility and precision performance of this method, a fixed‐type and a two‐axis STT PV system were installed at 24.92° north latitude in northern Taiwan and tested through long‐term experiments. The experimental results show that the energy increments estimated by the theoretical model and actual measurement are 19.39% and 16.74%, respectively. The results demonstrate that the proposed method is capable of predicting the power benefit harvested by an STT PV system with high accuracy. Using our method, a PV system installer can evaluate beforehand the economic benefits of different types of PV systems while taking different construction locations into consideration, thereby obtaining a better installation strategy for PV systems. Copyright © 2012 John Wiley & Sons, Ltd.     

A study on the mismatch effect due to the use of different photovoltaic modules classes in large‐scale solar parks

In this paper, a study on the mismatch effect due to the use of different photovoltaic (PV) modules classes in large‐scale solar parks is presented. For this purpose, a new model for simulating current–voltage and power–voltage characteristics is introduced. The model is then applied for calculating mismatch losses in a number of case studies for a PV plant built in Bari, southern Italy. First, in order to test the effectiveness of the model, this is applied to homogeneous strings and field showing that the mismatch losses are zero. Subsequently, the use of inhomogeneous strings (i.e. made of modules belonging to different power classes) is investigated. Finally, the behaviour of 1 MW_p homogeneous and inhomogeneous PV fields is investigated, again with a focus on the mismatch effect. The operational conditions have been introduced starting from the definition of European efficiency. The use of standard test conditions can in fact lead to gross approximations because mismatch losses depend, as well as, on PV module characteristics, electrical connections and electrical architecture, also on the location of the PV system. The results presented in this work can be used both by PV system designers for carrying out yield calculations, and by operation and maintenance personnel for substituting modules during operation without compromising the productivity of the plant. Copyright © 2012 John Wiley & Sons, Ltd.     

A model for the simulation of energy gains when using distributed maximum power point tracking (DMPPT) in photovoltaic arrays

Over the past years, the photovoltaic (PV) market has been invaded with numerous power optimizers and micro‐inverters that claim large energy gains when used in PV generators with shading or module mismatch. These products provide distributed maximum power point tracking (DMPPT), normally at module level, allowing the maximum power to be extracted from each PV module. This topology can be beneficial in situations where the PV generator is shaded or when there is large module mismatch. However, it is not clear that this power gain will result in energy improvements over a whole year or the lifetime of the system. This paper presents a very detailed and precise model for simulating energy gains with DMPPT as well as its verification and simulation results with different shading profiles, showing the possible energy gain over a whole year. Simulation results show that the yearly energy gain is much lower than the maximum power gain. However, interesting yearly gains of up to 12% are obtained in one of the simulations. Copyright © 2013 John Wiley & Sons, Ltd.     

Optimal sizing and life cycle assessment of residential photovoltaic energy systems with battery storage

This paper presents the optimal sizing and life cycle assessment of residential photovoltaic (PV) energy systems. The system consists of PV modules as the main power producer, and lead–acid batteries as the medium of electricity storage, and other essential devices such as an inverter. Five‐parameter analytic PV cell model is used to calculate the energy production from the modules. Electrical needs for a family living under normal conditions of comfort are modelled and used within simulation of the system performance, with an average daily load of approximately 9·0 kWh. The system's performance simulations are carried out with typical yearly solar radiation and ambient temperature data from five different sites in Turkey. The typical years are selected from a total of 6 years data for each site. The life cycle cost of the PV system is analysed for various system configurations for a 20‐year system life. The role of the batteries in PV energy systems are analysed in terms of the cost and power loss. The system performance is analysed as a function of various parameters such as energy production and cost. It is shown that these change substantially for different system configurations and locations. The life cycle assessment of the energy system described was also carried out to determine the environmental impact. It was found that, with the conservative European average electricity mix, energy pay back time (EPBT) is 6·2 years and CO₂ pay back time is 4·6 years for the given system. Copyright © 2007 John Wiley & Sons, Ltd.     

PV pumping systems: a useful tool to check operational performance

From the daily water demand, total head and the daily average irradiation, is possible to determine the size of the PV generator for pumping systems. However, once the equipment is acquired, some tests are recommended, specially to verify its performance. One of the most relevant parameters to qualify a pumping system is the daily water delivered (m³/day) as a function of daily irradiation (Wh/m²). Facilities that fit different boundaries conditions, as for example constant total head (m) are not easily available, and just few laboratories have this capability. In this way, a simple instrumentation with the capability to determine the daily performance of PV pumping systems is presented. The proposed test tools use a hydraulic circuit with two motopumps, one connected to the PV system and the other to the electric grid. The total head is maintained constant by the variable‐speed drive connected to the grid. Copyright © 2006 John Wiley & Sons, Ltd.     

Technical and economic analysis of grid‐connected PV systems by means of simulation

Research on monitored grid‐connected PV systems can lead to an improved performance of PV systems. This view is based on monitoring results from PV systems in Western Europe which lag behind the expected values. However, current methods for analysing these systems do not allow to investigate the potential system efficiency improvement on the basis of field experience. Hence, we have developed a method for analysing monitored grid‐connected PV systems which meets this need. In this method the common technical approach to analysing PV systems is broadened with an economic assessment. First an energy loss analysis of the PV system is made using its monitored data. In our analysis the energy loss effects in the PV system are split up by simulation. This provides a profound insight into the actual performance of the system. Next, measures to enhance the performance of the system are identified. The costs involved to improve the performance are analysed. Finally, the cost‐effectiveness of the potential improvements is calculated. In this paper we will present our method TEAMS. Although we will not formulate strict rules, we will provide a well‐defined frame and structure for the application of the TEAMS method. It is shown that applying TEAMS contributes to improved transparency in the evaluation of monitored grid‐connected PV systems. Copyright © 1999 John Wiley & Sons, Ltd.     

Reactive power consumption in photovoltaic inverters: a novel configuration for voltage regulation in low‐voltage radial feeders with no need for central control

High photovoltaic (PV) system generation in low‐voltage feeders can cause voltage rise especially in low demand conditions. The conventional way of coping with voltage violation is disconnection of the PV systems or curtailment of the generated power. To address this issue, a novel configuration for voltage management in a radial feeder via regulated reactive power capability in PV inverters is presented. The novelty of the proposed configuration is based on the fact that all the PV inverters with the ability to consume reactive power are involved in voltage regulation without being centrally controlled. In order to apply the configuration, a reference voltage is initially estimated for each PV system, and the PV inverters are calibrated accordingly. These settings depend on the feeder topology and can be calculated by the distribution network operator with a simple power flow modelling tool. Finally, this paper presents a sensitivity analysis in order to examine how reactive power consumption in a single inverter influences PV penetration and inverter sizing at various PV topologies along the feeder. Copyright © 2014 John Wiley & Sons, Ltd.     

Test procedures for maximum power point tracking charge controllers characterization

Photovoltaic stand‐alone systems (PVSAS) are the most widespread technology for rural electrification of off‐grid areas, for communication systems, and for satellite applications. Such applications require high reliability to guarantee an adequate electrical supply. In these systems, charge controllers and inverters are used to fit the PV output power to the load. The power adaptation stage is essential to the proper performance of the PVSAS and to the optimization of the energy management of the system. However, there are no international standards yet approved that cover the quality requirements of this stage. This paper deals with the performance of those charge controllers that use maximum power point tracking algorithms. The aim of this work is to define the specific parameters that describe the real performance of these controllers, paying attention to their static and dynamic efficiency and establishing the acceptable minimum thresholds required for each one. A set of measurements of three commercial controllers was developed. Their behavior in real varying conditions was monitored along 1 year of operation. Results are presented here after being checked with repetitive indoor measurements using a PV array simulator. Main results of the operation in a PVSAS are included in the paper for selected representative days along the year. To perform the daily analysis, we have classified each day according to its weather conditions. The purpose is to analyze the relation between the performance of the controller and the weather variations. Within the conclusions of this work, some guidelines for the definition of test procedures for maximum power point tracking charge controllers are included. Copyright © 2011 John Wiley & Sons, Ltd.     

Fast and Efficient Method for Fire Detection Using Image Processing

Conventional fire detection systems use physical sensors to detect fire. Chemical properties of particles in the air are acquired by sensors and are used by conventional fire detection systems to raise an alarm. However, this can also cause false alarms; for example, a person smoking in a room may trigger a typical fire alarm system. In order to manage false alarms of conventional fire detection systems, a computer vision‐based fire detection algorithm is proposed in this paper. The proposed fire detection algorithm consists of two main parts: fire color modeling and motion detection. The algorithm can be used in parallel with conventional fire detection systems to reduce false alarms. It can also be deployed as a stand‐alone system to detect fire by using video frames acquired through a video acquisition device. A novel fire color model is developed in CIE L*a*b* color space to identify fire pixels. The proposed fire color model is tested with ten diverse video sequences including different types of fire. The experimental results are quite encouraging in terms of correctly classifying fire pixels according to color information only. The overall fire detection system's performance is tested over a benchmark fire video database, and its performance is compared with the state‐of‐the‐art fire detection method.     

A refrigeration facility for milk cooling powered by photovoltaic solar energy

The use of renewable energy sources is usually a reliable alternative in rural areas and developing countries, where the grid line does not exist or is at a great distance. In this work, the characteristics and working conditions of a refrigeration facility designed for cooling down an expected daily production of 150 l of milk are analyzed. The facility is a stand‐alone, direct‐coupled system where 20 photovoltaic modules, 120 W_p each, power two permanent magnet, direct current motors of 24 V, 650 W. Each motor drives a separate cooling system compressor, which provides the flexibility to operate the equipment with one or two motors and with various interconnections of the PV modules, depending on the available irradiance level and the thermodynamic state of the system. The photovoltaic energy obtained during daylight hours is stored in the form of sensible and latent heat of frozen water in a tank surrounding a milk container. Thermodynamic analysis of the system shows that the autonomy of the system is 2·5 consecutive cloudy days if the available stored ice energy is 80% of the nominal capacity of the water/ice tank. Results of the refrigeration efficiency are similar to those obtained by other commercial refrigeration facilities powered by a photovoltaic array, including batteries. Copyright © 2003 John Wiley & Sons, Ltd.     

A methodology for the design of photovoltaic water supply systems

Photovoltaic pumping systems are used nowadays as a valuable alternative to supply water to communities living in remote rural areas. Owing to the seasonal variation and the stochastic behavior of solar radiation, at certain times the supply of water may not be able to meet demand. A study has been made of the relationship between water pumping capacity, reservoir size and water demand, for a given water deficit. As a result, curves of equal water deficit (iso‐deficit lines) can be obtained for various combinations of PV pumping capacity and reservoir size. A methodology to generate those curves is described, using as its main tool the characteristic curve of the system, that is, the relationship between water flow and collected solar radiation. The characteristic curve represents the combined behavior of the water pumping system and the well. The influence of the minimum collected solar radiation level, necessary to start the system's operation (the critical radiation level I_C), is also analyzed. Results show that PV pumping systems with different characteristic curves, but with the same critical levels, yield the same set of iso‐deficit lines. This drastically reduces the number of necessary solutions to those corresponding to a few values of I_C. Iso‐deficit lines, calculated for the locality of Recife (PE), Brazil, are used to illustrate the sizing procedure PV water supply systems. Copyright © 2001 John Wiley & Sons, Ltd.     

Experience with PV‐diesel hybrid village power systems in Southern Morocco

In October 2002, under the auspices of Spanish Cooperation, a pilot electrification project put into operation two centralised PV‐diesel hybrid systems in two different Moroccan villages. These systems currently provide a full‐time energy service and supply electricity to more than a hundred of families, six community buildings, street lighting and one running water system. The appearance of the electricity service is very similar to an urban one: one phase AC supply (230 V/50 Hz) distributed up to each dwelling using a low‐voltage mini‐grid, which has been designed to be fully compatible with a future arrival of the utility grid. The management of this electricity service is based on a ‘fee‐for‐service’ scheme agreed between a local NGO, partner of the project, and electricity associations created in each village, which are in charge of, among other tasks, recording the daily energy production of systems and the monthly energy consumption of each house. This register of data allows a systematic evaluation of both the system performance and the energy consumption of users. Now, after 4 years of operation, this paper presents the experience of this pilot electrification project and draws lessons that can be useful for designing, managing and sizing this type of small village PV‐hybrid system. Copyright © 2007 John Wiley & Sons, Ltd.