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.     

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.     

On the role of solar photovoltaics in global energy transition scenarios

The global energy system has to be transformed towards high levels of sustainability in order to comply with the COP21 agreement. Solar photovoltaic (PV) offers excellent characteristics to play a major role in this energy transition. The key objective of this work is to investigate the role of PV in the global energy transition based on respective scenarios and a newly introduced energy transition model developed by the authors. A progressive group of energy transition scenarios present results of a fast growth of installed PV capacities and a high energy supply share of solar energy to the total primary energy demand in the world in the decades to come. These progressive energy transition scenarios can be confirmed. For the very first time, a full hourly modelling for an entire year is performed for the world, subdivided in 145 sub‐regions, which is required to reflect the intermittent character of the future energy system. The model derives total installed solar PV capacity requirements of 7.1–9.1 TWp for the electricity sector (as of the year 2015) and 27.4 TWp for the entire energy system in the mid‐term. The long‐term capacity is expected to be 42 TWp and, because of the ongoing cost reduction of PV and battery technologies, this value is found to be the lower limit for the installed capacities. Solar PV electricity is expected to be the largest, least cost and most relevant source of energy in the mid‐term to long‐term for the global energy supply. Copyright © 2017 John Wiley & Sons, Ltd.     

Relative performance of tracking versus fixed tilt photovoltaic systems in the USA

Tracking systems can increase the amount of electricity generated by photovoltaic (PV) modules, by actively orienting each module to intercept more solar energy. We find that horizontal one‐axis tracking systems can increase PV generation by 12–25% relative to south‐facing fixed mount PV systems with 25° tilts in the contiguous USA, and two‐axis tracking systems can increase PV generation by 30–45% relative to fixed mount systems. Tracking systems increase PV generation more significantly in arid regions such as the southwest USA than in humid regions with persistent cloud cover such as the Pacific Northwest and coastal Atlantic states. We find that fixed and tracking PV systems have similar interannual variability in their generation profiles, and this variability is primarily driven by project location. Tracking PV projects cost more than fixed tilt systems, per unit capacity, and we explore how much more tracking projects could cost while generating similar levelized costs of energy as fixed tilt systems. We define this as the breakeven additional cost of tracking and find that it is primarily driven by three factors: (i) regional tracking performance, (ii) fixed tilt system costs that tracking projects compete against, and (iii) additional tracking operation and maintenance costs. Using this framework, we explore the relative competitiveness of tracking systems for a range of fixed and tracking PV prices and evaluate how tracking competitiveness varies by region. Copyright © 2013 John Wiley & Sons, Ltd.     

Dust effects on PV array performance: in‐field observations with non‐uniform patterns

This paper presents the impact of non‐homogeneous deposits of dust on the performance of a PV array. The observations have been made in a 2‐MW PV park in the southeast region of Spain. The results are that inhomogeneous dust leads to more significant consequences than the mere short‐circuit current reduction resulting from transmittance losses. In particular, when the affected PV modules are part of a string together with other cleaned (or less dusty) ones, operation voltage losses arise. These voltage losses can be several times larger than the short‐circuit ones, leading to power losses that can be much larger than what measurements suggest when the PV modules are considered separately. Significant hot‐spot phenomena can also arise leading to cells exhibiting temperature differences of more than 20 degrees and thus representing a threat to the PV modules' lifetime. Copyright © 2013 John Wiley & Sons, Ltd.     

Exploiting existing dams for solar PV system installations

Recognizing the issues of land shortage and growing concerns for protecting natural lands, installers and project developers, with the help of scientists and engineers, continuously try to locate alternative spots for photovoltaic (PV) system installations. In the present paper a novel approach is suggested and analysed: installing solar PV systems on the downstream face of existing dams. This approach provides advantages that could favour even large‐scale systems with a capacity of several MWp. First, produced energy could cover water reservoirs' needs supporting energy‐intensive processes as water pumping and treatment in a sustainable manner. Moreover, energy provision to inhabited areas near the dams and the subsequent creation of independent mini grids could mitigate energy poverty. In the case of hydroelectric dams, the so‐created hybrid system (PV‐hydro) could become notably efficient, because the intermittent solar energy would be counterbalanced by the flexibility of hydropower. Finally, we found a notable number of existing water reservoirs in Africa that are either under‐utilized or non‐powered. That unexploited energy potential can also be amplified by PV‐system installation. The analysis included data collection from various sources. Datasets have been cross‐checked and extended in the newly created GIS‐based model, enabling the selection of the most suitable sites in South Africa, taken as case studies. Following their identification, the selected dams have been analysed using the PVGIS tool in order to estimate the annual energy production. The results have been very encouraging, indicating that PV systems on the face of dams are an advantageous option for renewable energy production. © 2015 The Authors. Progress in Photovoltaics: Research and Applications published by 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.     

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.     

Toward multiple maximum power point estimation of photovoltaic systems based on semiconductor theory

Environmental conditions, such as temperature, non‐uniform irradiation, and solar shading, deeply affect the characteristics of photovoltaic (PV) modules in PV‐assisted generation systems. Several local maximum power points (MPPs) are found in the power–voltage curve of PV systems constructed by series/parallel‐connected PV modules under partially shaded conditions. The characteristics of PV systems change unpredictably when multiple MPPs occur, so the actual MPP tracking (MPPT) becomes a difficult task. Conventional MPPT methods for the PV systems under partially shaded conditions cannot quickly find the actual MPP such that the optimal utilization of PV systems cannot be achieved. Based on the p–n junction semiconductor theory, we develop a multipoint direct‐estimation (MPDE) method to directly estimate the multiple MPPs of the PV systems under partially shaded conditions and to cope with the mentioned difficulties. Using the proposed MPDE method, the multiple MPPs of the PV systems under partially shaded conditions can be directly determined from their irradiated current–voltage and power–voltage characteristic curves. The performances of the proposed MPDE method are evaluated by examining the characteristics of multiple MPPs of PV systems with respect to different shading strengths and numbers of the shaded PV modules and also tested using the field data. The experimental results demonstrate that the proposed MPDE method can simply and accurately estimate the multiple MPPs of the PV systems under partially shaded conditions. The optimization of MPP control models and the MPPT for PV systems could be achieved promisingly by applying the proposed method. Copyright © 2014 John Wiley & Sons, Ltd.     

Five years of operating experience at a large,utility‐scale photovoltaic generating plant

Tucson Electric Power Company (TEP), headquartered in Tucson, AZ, currently has nearly 5·0 MWdc of utility‐scale grid‐connected photovoltaic (PV) systems installed in its service territory. These systems have been installed through a multiyear, pay‐as‐you‐go development of renewable energy, with kWhac energy production as a key program measurement. This PV capacity includes a total of 26 crystalline silicon collector systems, each rated at 135 kWdc for a total of 3·51 MWdc, that have been installed at the Springerville, AZ generating plant by TEP making this one of the largest PV plants in the world. This facility started operations in 2001 and recently passed the 5‐year milestone of continuous operations. These systems were installed in a standardized, cookie‐cutter approach whereby each uses the same array field design, mounting hardware, electrical interconnection, and inverter unit. This approach has allowed TEP to achieve a total installed system cost of $5·40/Wdc and a TEP‐calculated levelized energy cost of $0·062/kWhac for PV electrical generation. This paper presents an assessment of operating experience including performance, costs, maintenance, and plant operation over this 5‐year period making this one of the most detailed and complete databases of utility‐scale PV systems available to the US DOE Program. Published in 2007 by John Wiley & Sons, Ltd.     

Tracking US photovoltaic system prices 1998–2012: a rapidly changing market

The number of US photovoltaic (PV) systems has grown rapidly in recent years, driven by a wide array of government incentives and other supportive policies aimed, in part, at reducing PV system prices. This article draws from a sample of more than 200,000 US residential, commercial, and utility‐scale PV projects to summarize price trends for PV systems installed from 1998 through 2012. These data show that the average installed price of US residential and commercial PV systems declined 6–7% annually during 1998–2012, although the pace and source of price reductions varied. In 2012, the median reported installed price of residential and commercial PV systems was $5.3/W for systems of 10 kW or smaller, $4.9/W for systems of 10–100 kW, and $4.6/W for systems larger than 100 kW. The capacity‐weighted average installed price of crystalline, fixed‐tilt, utility‐scale PV systems (ground‐mounted systems at least 2 MW in size) declined from $6.2/W during 2007–2009 to $3.3/W in 2012. Recent price reductions are associated primarily with a precipitous drop in PV module prices, while non‐module costs have remained relatively stagnant since 2005. Further system price reductions will be needed to expand US PV deployment as incentive programs reduce their financial support. Because further module price reductions are limited, additional deep reductions in installed prices will require significant reductions in non‐module costs, particularly non‐hardware (“soft”) costs. Lower installed prices in international markets suggest that deep near‐term soft cost reductions in the USA are possible with the help of targeted policies. Copyright © 2014 John Wiley & Sons, Ltd.     

Photovoltaics in an architectural context

In well‐populated areas, such as western Europe, PV is often integrated into the building envelope. Despite the fact that there are many examples showing that PV can be an aesthetically neutral or visually attractive element in architecture, many BIPV systems display few architectural qualities. But if well applied, PV can increase a building's character and value. Within Task 7 of the IEA PVPS programme a team of experts with an architectural background studied which key requirements needed to be complied with (design criteria for good‐quality PV projects) in order to produce successful PV integration. These criteria are discussed in the article. PV is not automatically considered an indispensable material in architectural terms. This is why, no matter how well it is integrated, PV remains an ‘added’ element. Architects can take this as their starting point and can use one of the design approaches that are presented in the article. These criteria for incorporating PV in the building design and the design criteria for good‐quality PV projects are important to architects and architectural critics in determining why a BIPV project, be it their own design or that of a colleague, is or is not aesthetically pleasing. This offers learning opportunities and reasons for follow‐up or improvement options. Architects who apply PV in a well‐thought‐out way can make their clients very happy, and thereby contribute to a greater acceptance of PV technology. Copyright © 2004 John Wiley & Sons, Ltd.     

STC power for 15 MW of PV comparing nameplate,initial power and power after 4 years

To date, the majority of quality controls performed at PV plants are based on the measurement of a small sample of individual modules. Consequently, there is very little representative data on the real Standard Test Conditions (STC) power output values for PV generators. This paper presents the power output values for more than 1300 PV generators having a total installed power capacity of almost 15.3 MW. The values were obtained by the INGEPER‐UPNA group, in collaboration with the IES‐UPM, through a study to monitor the power output of a number of PV plants from 2006 to 2009. This work has made it possible to determine, amongst other things, the power dispersion that can be expected amongst generators made by different manufacturers, amongst generators made by the same manufacturer but comprising modules of different nameplate ratings and also amongst generators formed by modules with the same characteristics. The work also analyses the STC power output evolution over time in the course of this 4‐year study. The values presented here could be considered to be representative of generators with fault‐free modules. Copyright © 2012 John Wiley & Sons, Ltd.     

Smoothing of PV power fluctuations by geographical dispersion

The quality and the reliability of the power generated by large grid‐connected photovoltaic (PV) plants are negatively affected by the source characteristic variability. This paper deals with the smoothing of power fluctuations because of geographical dispersion of PV systems. The fluctuation frequency and the maximum fluctuation registered at a PV plant ensemble are analyzed to study these effects. We propose an empirical expression to compare the fluctuation attenuation because of both the size and the number of PV plants grouped. The convolution of single PV plants frequency distribution functions has turned out to be a successful tool to statistically describe the behavior of an ensemble of PV plants and determine their maximum output fluctuation. Our work is based on experimental 1‐s data collected throughout 2009 from seven PV plants, 20 MWp in total, separated between 6 and 360 km. Copyright © 2011 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.     

Performance,cost and life‐cycle assessment study of hybrid PVT/AIR solar systems

An alternative and cost‐effective solution to building integrated PV systems is to use hybrid photovoltaic/thermal (PV/T) solar systems. These systems consist of PV modules with an air channel at their rear surface, where ambient air is circulating in the channel for PV cooling and the extracted heat can be used for building thermal needs. To increase the system thermal efficiency, additional glazing is necessary, but this results in the decrease of the PV module electrical output from the additional optical losses of the solar radiation. PV/T solar systems with air heat extraction have been extensively studied at the University of Patras. Prototypes in their standard form and also with low‐cost modifications have been tested, aiming to achieve improved PV/T systems. An energetic and environmental assessment for the PV and PV/T systems tested has been performed by the University of Rome ‘La Sapienza’, implementing the specific software SimaPro 5·1 regarding the life‐cycle assessment (LCA) methodology applied. In this paper electrical and thermal energy output results for PV and PV/T systems are given, focusing on their performance improvements and environmental impact, considering their construction and operation requirements. The new outcome of the study was that the glazed type PV/T systems present optimum performance regarding energy, cost and LCA results. Copyright © 2005 John Wiley & Sons, Ltd.     

UPS电池系统的配置与维护探讨

UPS可单机运行，也可多机联合运行。多机联合工作又分为：并机、串机。并机既可主备并机，又可冗余并机。UPS中蓄电池容量的选配也至关重要。文中结合在实际工作中的维护经验，论述了UPS的选型和配置，UPS及其电池系统的维护检测方法，推荐一种安全的电池放电方法和一种简易的采用电池离线模拟放电的测量电池电压方法。     

Rural electrification using photovoltaic battery charging stations: a performance study in northern Thailand

The installed capacity of photovoltaic battery charging stations (PV BCS) in Thailand is more than 1·5 MW_p. A study on the performance and usefulness of ten PV BCS showed that all the observed systems still provide electricity to the villagers, underlining the long life and robust capabilities of the PV‐based electrification. Most users of the system have changed their lighting system from wood and kerosene to battery‐powered lamps, and find the system useful. The major malfunctioning component is the charge controller. The efficiency seems to be low, owing to the loss of opportunity of using small (6 V) batteries and not using the system throughout the daytime. Shallow‐discharge car/motor cycle batteries are used. Some do not use the system owing to specific reasons (cost, battery performance, distance of BCS from home, etc.). Training and an organisational set up to take care of the system is important if the performance of PV BCS is to be increased. Copyright © 2001 John Wiley & Sons, Ltd.