A 24-h forecast of solar irradiance using artificial neural network: Application for performance prediction of a grid-connected PV plant at Trieste, Italy

Forecasting of solar irradiance is in general significant for planning the operations of power plants which convert renewable energies into electricity. In particular, the possibility to predict the solar irradiance (up to 24 h or even more) can became - with reference to the Grid Connected Photovoltaic Plants (GCPV) - fundamental in making power dispatching plans and - with reference to stand alone and hybrid systems - also a useful reference for improving the control algorithms of charge controllers. In this paper, a practical method for solar irradiance forecast using artificial neural network (ANN) is presented. The proposed Multilayer Perceptron MLP-model makes it possible to forecast the solar irradiance on a base of 24 h using the present values of the mean daily solar irradiance and air temperature. An experimental database of solar irradiance and air temperature data (from July 1st 2008 to May 23rd 2009 and from November 23rd 2009 to January 24th 2010) has been used. The database has been collected in Trieste (latitude 45°40′N, longitude 13°46′E), Italy. In order to check the generalization capability of the MLP-forecaster, a K-fold cross-validation was carried out. The results indicate that the proposed model performs well, while the correlation coefficient is in the range 98-99% for sunny days and 94-96% for cloudy days. As an application, the comparison between the forecasted one and the energy produced by the GCPV plant installed on the rooftop of the municipality of Trieste shows the goodness of the proposed model.     

Sunshine environment and spectrum analysis for concentrator PV systems in Japan

Normal direct irradiance levels for concentrator photovoltaic were investigated and anticipated in 26 sunshine regions in Japan. The spectrum and photon numbers were also examined. Normal direct photon number was assumed to be 92–60% of global the sunshine on sloped surfaces in all the districts in Japan.     

Estimation of hourly direct normal from measured global solar irradiance in Spain

The availability of a good data set, registered in six Spanish locations, including several radiometric variables, has been used to test different approaches for estimating hourly direct normal irradiance by decomposition models. Models proposed by different authors have been tested. Following this preliminary study, to improve the k_b–k_t correlations, another geometric variable has been used as a predictor of hourly beam transmittance, k_b, by means of piecewise correlations. The new beam transmittance correlations, which include additional geometric information, reduce the root mean square deviation. In addition, they show a better performance in terms of the determination coefficient of the regression analysis of measured vs calculated values, providing an improved capture of the real world effects than models that are function of the clearness index only. A new model that uses only two ranges of clearness index is proposed. The proposed model shows seasonal dependence and thus we have developed a seasonal version of it. However, the performance of the seasonal version has proved to be similar to the corresponding annual model.     

Prediction of daily global solar irradiance on horizontal surfaces based on neural-network techniques

In this study, a prediction model of global solar irradiance distribution on horizontal surfaces has been developed. The methodology is based on neural-network techniques and has been applied to the meteorological database of NTUA, Zografou Campus, Athens (37°58′26″N, 23°47′16″E). The investigation of the correlation between weather conditions, duration of daylight and the representative peak value of a Gaussian-type function plays an essential role in the development of the model. The weather conditions are categorized into six different states, whereas the daylight duration is obtained by familiar equations. Thereafter, a correction methodology for the Gaussian-type function—which stands for all six different states—is applied. Finally, the reliability of the developed model is investigated through a suitable validation procedure.     

Areal evolution of PV systems

If a large amount of the future penetration of PV systems is imaged, it is likely that they densely concentrate in a certain area. It may not be a simple aggregation of individually optimized system, but a different system image may result by the total optimization of an area. The author frames this new question as the problem of “Areal Evolution” or “PV-AREA”. The following two cases are dealt with and their concrete concepts are given: (1) PV systems for community, (2) global-scale utilization of PV systems.     

A techno-economic comparison of rural electrification based on solar home systems and PV microgrids

Solar home systems are typically used for providing basic electricity services to rural households that are not connected to electric grid. Off-grid PV power plants with their own distribution network (micro/minigrids) are also being considered for rural electrification. A techno-economic comparison of the two options to facilitate a choice between them is presented in this study on the basis of annualised life cycle costs (ALCC) for same type of loads and load patterns for varying number of households and varying length and costs of distribution network. The results highlight that microgrid is generally a more economic option for a village having a flat geographic terrain and more than 500 densely located households using 3–4 low power appliances (e.g. 9 W CFLs) for an average of 4 h daily. The study analyses the viability of the two options from the perspectives of the user, an energy service company and the society.     

组合跟踪的槽式聚焦集热器太阳辐照量的研究

在建立太阳直射辐射计算模型的基础上，分析了聚先反射抛物面镜面在各种跟踪方式下接受到的瞬时太阳直射辐射量，得出了晴天时昆明各月平均日辐照量的变化图。通过对所得数据度曲线图的比较和研究，获得了聚光反射抛物镜面两种较好的组合跟踪方式，效率分别达到双轴跟踪的98．9％和99．34％。可为太阳能申高温跟踪系统的发展和运用提供新的思路。     

Systematic error in the measurement of very high solar irradiance

The measurement of very high solar irradiance is required in an increasingly wide variety of technical applications. There is really only one commercial supplier of heat flux sensors that can be used for this purpose. These gages are calibrated using a black body as the radiant source. A systematic error has been detected when these heat flux sensors are later used to measure the irradiance from concentrated solar radiation; in this case, the calibration constants obtained for these gages in the laboratory must be systematically corrected.     

Improvement of performance in redox flow batteries for PV systems

A redox flow battery (RFB) is a new type battery. A new piping sytem to improve the performance of RFBs for PV systems is introduced. In this new system, the piping for the electrolytes also connects each stack in series just like the electric circuit. In this system, the leak current through the electrolytes in the piping between each stack is suppressed. This system is called multitank system because it has many minitanks between each stack. It is clarified from the experimental and simulation results of the multitank system that this system improves the performance of RFBs.     

Approaches for developing a sizing method for stand-alone PV systems with variable demand

Accurate sizing is one of the most important aspects to take into consideration when designing a stand-alone photovoltaic system (SAPV). Various methods, which differ in terms of their simplicity or reliability, have been developed for this purpose. Analytical methods, which seek functional relationships between variables of interest to the sizing problem, are one of these approaches.A series of rational considerations are presented in this paper with the aim of shedding light upon the basic principles and results of various sizing methods proposed by different authors. These considerations set the basis for a new analytical method that has been designed for systems with variable monthly energy demands.Following previous approaches, the method proposed is based on the concept of loss of load probability (LLP)—a parameter that is used to characterize system design. The method includes information on the standard deviation of loss of load probability (σ_LLP) and on two new parameters: annual number of system failures (f) and standard deviation of annual number of failures (σ_f).The method proves useful for sizing a PV system in a reliable manner and serves to explain the discrepancies found in the research on systems with LLP<10⁻². We demonstrate that reliability depends not only on the sizing variables and on the distribution function of solar radiation, but on the minimum value as well, which in a given location and with a monthly average clearness index, achieves total solar radiation on the receiver surface.     

Measures for diffusion of solar PV in selected African countries

This paper investigates how African governments are considering supporting and promoting the diffusion of solar PV. This issue is explored by examining so-called ‘technology action plans (TAPs)’, which were main outputs of the Technology Needs Assessment project implemented in 10 African countries from 2010 to 2013. The paper provides a review of three distinct but characteristic trajectories for PV market development in Kenya (private-led market for solar home systems), Morocco (utility-led fee-for service model) and Rwanda (donor-led market for institutional systems). The paper finds that governments’ strategies to promoting solar PV are moving from isolated projects towards frameworks for market development and that there are high expectations to upgrading in the PV value chain through local assembly of panels and local production of other system elements. Commonly identified measures include support to: local production; financing schemes; tax exemptions; establishment and reinforcement of standards; technical training; and research and development.     

Research on the influence of PV cell to thermal characteristics of photovoltaic/thermal solar system

In the paper, we analyzed internal thermal transmission characteristics of water‐heating photovoltaic/thermal (PV/T) solar collector covered by photovoltaic (PV) cell, established photothermal conversion model of PV/T solar system, and analyzed the influence of PV cell coverage to photothermal characteristics of PV/T solar system. Results show that the thermal efficiency of PV/T solar system by optimizing PV cells coverage can reach 68%. In addition, by designing four water‐heating PV/T solar system prototypes with PV cell coverage of 0.4, 0.56, 0.7, and 0.82, respectively, we conducted experimental researches for the four prototypes and found that the four prototypes can achieve thermal efficiencies of 58%, 51%, 64%, and 67%, respectively, in heating 250 L of water to 50°C. The experiment results are consistent with theoretical analysis results, indicating that it is feasible to improve thermal characteristics of PV/T solar system by optimizing PV cell coverage. Copyright © 2017 John Wiley & Sons, Ltd.     

Automatic fault detection in grid connected PV systems

This paper presents a detailed procedure for automatic supervision, fault detection, and diagnosis of possible failure sources leading to total or partial loss of productivity in grid connected PV systems. The diagnostic procedure is part of the monitoring system allowing, at the same time, modeling and simulation of the whole system and variables measurements in real time. The fault detection algorithm is based on the comparison of simulated and measured yields by analyzing the losses present in the system while the identification of the kind of fault is carried out by analyzing and comparing the amount of errors deviation of both DC current and voltage with respect to a set of errors thresholds evaluated on the basis of free fault system. The proposed method has been validated in with experimental data in a grid connected PV system in the Centre de Developpement des Energies Renouvelables (CDER) in Algeria.     

Utilisation of solar photovoltaic pumping for aeration systems in aquaculture ponds

This research pertains to solar photovoltaic (PV) pumping for aeration of aquaculture ponds and evaluates it economically. A stand-alone photovoltaic aeration system based on recycling of water at 10 cm under the water surface, at the surface (0 cm) and above the surface (30 cm) on a perforated tray was evaluated. The effect of pump discharges (35, 50, 65 L/min) on standard oxygen transfer rate (SOTR) and standard aeration efficiency (SAE) was investigated. Results revealed that the daily averages of PV energy output, PV efficiency and load energy were 0.844 kWh/d, 9.87% and 0.615 kWh/d, respectively, at 65 L/min and 42.7°C PV temperature. Water flow upward at 30 cm over the perforated tray in the radial direction proved to be better since it gave an opportunity for the exchange of gaseous between the water and the atmosphere. The SOTR and SAE for the third position were 0.025 kgO₂/h and 0.1 kgO₂/kW h, respectively.     

PV power conversion and short-term forecasting in a tropical,densely-built environment in Singapore

With the substantial growth of solar photovoltaic installations worldwide, forecasting irradiance becomes a critical step in providing a reliable integration of solar electricity into electric power grids. In Singapore, the number of PV installation has increased with a growth rate of 70% over the past 6 years. Within the next decade, solar power could represent up to 20% of the instant power generation. Challenges for PV grid integration in Singapore arise from the high variability in cloud movements and irradiance patterns due to the tropical climate. For a thorough analysis and modeling of the impact of an increasing share of variable PV power on the electric power system, it is indispensable (i) to have an accurate conversion model from irradiance to solar power generation, and (ii) to carry out irradiance forecasting on various time scales. In this work, we demonstrate how common assumptions and simplifications in PV power conversion methods negatively affect the output estimates of PV systems power in a tropical and densely-built environment such as in Singapore. In the second part, we propose and test a novel hybrid model for short-term irradiance forecasting for short-term intervals. The hybrid model outperforms the persistence forecast and other common statistical methods.     

Feasibility of solar tracking systems for PV panels in hot and cold regions

Solar tracking systems would probably increase the efficiency of a PV module, but when and where. There are many factors that affect the performance of PV panels, especially crystalline silicon panels, e.g. overheating due to excessive exposure to solar irradiance in a hot climate as in Sunbelt countries. So, it could be the case that a tracking system is not necessary for a Sunbelt country. The objective of this research is to determine mathematically the performance of a PV panel as a function of tracking the sun and the operating conditions. The used mathematical model is validated experimentally and then applied for several environments, i.e. hot as well as cold regions. It has been found that the gain in electrical energy from tracking the sun is about 39% in case of a cold city as Berlin, Germany. While the gain in energy does not exceed 8% in case of a hot city as Aswan, Egypt, due to overheating of the PV panels. However, if the energy needed for running the tracking system, which ranges from 5% to 10% of the energy generated, is included in this analysis then tracking the sun will not be feasible in hot countries.     

Hydrogen production by water electrolysis and off-grid solar PV

This work concerns a methodology for PV-H₂ hybrid system design that consider the weather data and the electrical variables of the subsystems to perform energy balances and to assess the systems in terms of the capacity and operation of the components, and the resulting costs.Two configurations (with and without batteries) and two locations (Madrid and Fisciano) were studied to find the best trade-off between the efficiency and sizes of the subsystems. Directly connected systems operate at intersection points between the PV output and electrolyzer (EL) input curves for different solar irradiance levels, while the battery assisted systems reduce the sizes of EL at the expense of higher energy loss and additional cost of batteries (B). It was found how is not convenient to operate the EL at fixed rate, resulting in high PV and B sizes, as well as power unbalances in winter and summer. Solutions are to run the EL at a minimal load at night and change the intensity of daytime operations to achieve null cumulative energy each season.The H₂ supplied by these systems has the merit of being sustainable (renewable) and autonomous (avoiding power constraints in off-grid locations), and the costs are around 6–7 €/kg_H2.     

Long-term field test of solar PV power generation using one-axis 3-position sun tracker

The 1 axis-3 position (1A-3P) sun tracking PV was built and tested to measure the daily and long-term power generation of the solar PV system. A comparative test using a fixed PV and a 1A-3P tracking PV was carried out with two identical stand-alone solar-powered LED lighting systems. The field test in the particular days shows that the 1A-3P tracking PV can generate 35.8% more electricity than the fixed PV in a partly-cloudy weather with daily-total solar irradiation H_T = 11.7 MJ/m² day, or 35.6% in clear weather with H_T = 18.5 MJ/m² day. This indicates that the present 1A-3P tracking PV can perform very close to a dual-axis continuous tracking PV (Kacira et al., 2004). The long-term outdoor test results have shown that the increase of daily power generation of 1A-3P tracking PV increases with increasing daily-total solar irradiation. The increase of monthly-total power generation for 1A-3P sun tracking PV is between 18.5-28.0%. The total power generation increase in the test period from March 1, 2010 to March 31, 2011, is 23.6% in Taipei (an area of low solar energy resource). The long-term performance of the present 1X-3P tracking PV is shown very close to the 1-axis continuous tracking PV in Taiwan (Chang, 2009). If the 1A-3P tracking PV is used in the area of high solar energy resource with yearly-average H_T > 17 MJ/m² day, the increase of total long-term power generation with respect to fixed PV will be higher than 37.5%. This is very close to that of dual-axis continuous tracking PV.The 1A-3P tracker can be easily mounted on the wall of a building. The cost of the whole tracker is about the same as the regular mounting cost of a conventional rooftop PV system. This means that there is no extra cost for 1A-3P PV mounted on buildings. The 1A-3P PV is quite suitable for building-integrated applications.