Evaluating the value of distributed photovoltaic generations inradial distribution systems

The impact of photovoltaic (PV) generations, when added to an existing rural utility's distribution system, is studied. The addition of PV is examined in the light of voltage support, loss reduction, and reduction in peak demand. Comparisons are made with the conventional and widely used methods for voltage control and loss minimization, such as the addition of series and shunt capacitors and voltage regulators. The economics of distributed PV systems in the context of conventional grid power purchases are also studied. Results of this study are meant to be used as general guidelines for evaluating the impact of significant PV penetration in any distribution system     

Effect of solar photovoltaic and various photovoltaic air thermal systems on hydrogen generation by water electrolysis

A novel photovoltaic thermal air (PVTa) system with semi length fins in the downstream portion of the air channel was tested experimentally for its performance capability for the generation of hydrogen in the present work. Fins are passive devices used to overcome the main detrimental effect of reduced power output due to photovoltaic panel heating. For this purpose, 2 semi length fin configurations namely longitudinal fin and wavy fin were placed in second half length of the channel in the direction of air flow. To compare the impact of PVTa systems with semi lengthened fins in hydrogen generation, the performance study of PV and PVTa system assisted hydrogen generation were also conducted. The experiments were conducted at the site of Tiruchirappalli district, Tamilnadu state, India having latitude and longitude of 10.82 and 78.70 respectively during March–June 2019 from 9 a.m. to 4 p.m. on clear and sunny days. The results indicated PVTa system with semi length wavy fins yielded maximum generation of hydrogen among the 4 cases of PV assisted hydrogen generation techniques considered. It was observed that downstream located longitudinal and wavy fins provided enhanced PV panel cooling which increased the current supply to the electrolyzer unit. The hydrogen generation rate was 13.5, 12.1, 9.5 and 7.8 ml/min for PVT with wavy fins, longitudinal fins, PVT and PV respectively. Keywords: Solar PV, fins, hydrogen, heat transfer.     

Influence of photovoltaic generation model and time resolution on the reliability evaluation of distribution systems

This paper aims to investigate the influence of photovoltaic (PV) generation on reliability evaluation of distribution systems. Two PV generation models are used to predict the output power injected into the grid, taking into account the main relevant environmental variables, the irradiance and ambient temperature. Issues that directly affect the output power, such as the spatial smoothing effect due to the plant size and the influence of the irradiance and temperature measurement interval are taken into consideration. Using measurement time series of irradiance and local temperature, the models are used to generate power series in 4‐minute and hourly resolutions. The generated power series are used in a reliability assessment model, with the objective of evaluating the impact of solar resource variability on the reliability indices of the system. Case studies on the IEEE RBTS‐Bus 2 and on the real distribution system of Fernando de Noronha in Brazil are presented and discussed, for power plants of different capacities, considering the effect of the PV generation models, the temporal resolution of the time series and the spatial smoothing of the power output fluctuations. The results show that the power time series in hourly resolution significantly underestimates the frequency of interruptions. For the real system, this index is underestimated at the system level (up to 43%) and at the load points (up to 72%). On the other hand, for the interruption duration index, the temporal aggregation results in a small underestimation (just 4%). The results also indicates that the smoothing effect is irrelevant for typical PV system sizes of distribution systems with discretization equal to or above 4 minutes.     

Thermodynamic assessment of photovoltaic systems

In this paper, an attempt is made to investigate the performance characteristics of a photovoltaic (PV) and photovoltaic-thermal (PV/T) system based on energy and exergy efficiencies, respectively. The PV system converts solar energy into DC electrical energy where as, the PV/T system also utilizes the thermal energy of the solar radiation along with electrical energy generation. Exergy efficiency for PV and PV/T systems is developed that is useful in studying the PV and PV/T performance and possible improvements. Exergy analysis is applied to a PV system and its components, in order to evaluate the exergy flow, losses and various efficiencies namely energy, exergy and power conversion efficiency. Energy efficiency of the system is calculated based on the first law of thermodynamics and the exergy efficiency, which incorporates the second law of thermodynamics and solar irradiation exergy values, is also calculated and found that the latter is lower for the electricity generation using the considered PV system. The values of “fill factor” are also determined for the system and the effect of the fill factor on the efficiencies is also evaluated. The experimental data for a typical day of March (27th March 2006) for New Delhi are used for the calculation of the energy and exergy efficiencies of the PV and PV/T systems. It is found that the energy efficiency varies from a minimum of 33% to a maximum of 45% respectively, the corresponding exergy efficiency (PV/T) varies from a minimum of 11.3% to a maximum of 16% and exergy efficiency (PV) varies from a minimum of 7.8% to a maximum of 13.8%, respectively.     

Reliability analysis of photovoltaic systems

Reliability is an important parameter for the user of photovoltaic (PV) power systems. A methodology for the analytical treatment of the reliability of PV systems is proposed in this paper. The method depends upon the logic of the fault-tree technique. The reliabilities of the different components of a PV system are used to predict the reliability of the overall system. Today's most commonly known systems are considered and a reliability formula is developed for each system. The methodology presented is appropriate for a wide range of applications and system types.     

Loss-of-load probabilities for stand-alone photovoltaic systems

A general method is presented for estimating the loss-of-load probability (LLP) of stand-alone photovoltaic systems. The method was developed by correlating simulation results. The simulations were driven with synthetic radiation sequences having the same statistical significance as available historical data. The method assumes a constant nighttime load and accounts for the distribution and persistence in daily solar radiation data. It is shown that the 10-year average performance of systems having loss-of-load probabilities less than about .01 can vary greatly from one 10-year period to the next and thereby cannot be considered realistic performance estimates of a system during its lifetime.     

A thermal model for photovoltaic systems

The energy balance of photovoltaic (PV) cells is modelled based on climate variables. Module temperature change is shown to be in a non-steady state with respect to time. Theoretical expressions model the energy transfer processes involved: short wave radiation, long wave radiation, convection and electrical energy production. The combined model is found to agree well with the response of the measured model temperature to transient changes in irradiance. It is found that the most precise fit to measured data is obtained by fitting the value of the forced convection coefficient for module convection. The fitted values of this coefficient were found to be within the range predicted by previous authors. Though the model is found to be accurate to within 6 K of measured temperature values 95% of the time in cloudy conditions, best accuracy is obtained in clear and overcast conditions when irradiance is subject to less fluctuation.     

Mathematic models of photovoltaic motor-pump systems

Photovoltaic (PV) pumping offers the possibility of supplying water to remote and desert regions for their daily needs. The sizing of the PV pumping systems is a very significant step in order to optimize the power peak of the PV array and to ensure the best choice of the motor, the pump and the inverter. Two mathematical models were proposed in this article to contribute in the studies of PV pumping sizing. These models link directly the operating electrical power to the water flow rate of the pump versus total head. These models are based essentially on the experimentation of pumps on CDER PV pumping test facility. Two pumping systems are tested: the first uses a centrifugal pump and the second uses a positive displacement pump. The results obtained by the models are very satisfactory. Also, the models enabled us to simulate the electrical and hydraulic performances of two tested pumps. The performances are calculated using the measured meteorological data of different sites located in Sahara and coastline regions of Algeria.     

A Fokker-Planck analysis of photovoltaic systems

The battery state-of-charge, S(t), of an arbitrary photovoltaic system is analyzed as a Markov process driven by random white Gaussian perturbations of periodic insolation and load-demand profiles. A Fokker-Planck equation for the probability density function of S(t) is derived, and S(t) minus its mean is recognized as a nonhomogeneous Wiener-Levy process. The Fokker-Planck equation is solved under conditions of no barriers, one absorbing barrier, and two absorbing barriers, and the resulting probability density functions are used to obtain bounds on the complementary cumulative distribution function for the first passage time, x(t)=P{T>t}, to the completely discharged or totally charged state. Limiting expressions for these bounds as t → 0 and t → ∞ are obtained, and their asymptotic values are compared. Finally, a simple system is analyzed to provide insight into the meaning of the equations developed.     

Hybrid photovoltaic/thermal solar systems

We present test results on hybrid solar systems, consisting of photovoltaic modules and thermal collectors (hybrid PV/T systems). The solar radiation increases the temperature of PV modules, resulting in a drop of their electrical efficiency. By proper circulation of a fluid with low inlet temperature, heat is extracted from the PV modules keeping the electrical efficiency at satisfactory values. The extracted thermal energy can be used in several ways, increasing the total energy output of the system. Hybrid PV/T systems can be applied mainly in buildings for the production of electricity and heat and are suitable for PV applications under high values of solar radiation and ambient temperature. Hybrid PV/T experimental models based on commercial PV modules of typical size are described and outdoor test results of the systems are presented and discussed. The results showed that PV cooling can increase the electrical efficiency of PV modules, increasing the total efficiency of the systems. Improvement of the system performance can be achieved by the use of an additional glazing to increase thermal output, a booster diffuse reflector to increase electrical and thermal output, or both, giving flexibility in system design.     

Reliability of large-scale grid-connected photovoltaic systems

This paper presents a method for assessing the reliability of large-scale grid-connected photovoltaic systems. Fault tree and probability analysis are used to compute the reliability equation and the developed model is applied on military-standard data and on data taken from scientific literature.The method provides a tool useful to single out the different impacts that the large number of components belonging to the photovoltaic field and the BOS (Balance of System) chain have on system overall reliability, hence granting the possibility to design and implement more effective monitoring/diagnostic strategies and maintenance plans.     

Parametric cost analysis of photovoltaic systems

A parametric cost analysis of photovoltaic systems based on different design philosophies has been carried out. The analysis takes into account the fixed costs involved in a photovoltaic installation (such as site preparation, array foundations and structure, inversion equipment, backup capacity, etc.) the cost of concentrators at different concentration ratios ranging from unity (i.e. flat plate array) to 10³, solar cell cost-per-unit area, system efficiency, etc. Resulting parametric curves allow the determination of the cost of electricity for a given set of system constraints. They also allow one to determine the optimum concentration ratio for a given solar cell cost per unit area. The analysis confirms many of the accepted conclusions, presents them in a concise form, and, in addition, allows one to see the relationship between the various system design regimes and the sensitivity of the total cost of electricity produced to the cost of the various system components.     

View factors of photovoltaic collector systems

The diffuse radiation is one component of the global radiation affecting the output of photovoltaic collectors. This radiation depends on the view factor between the collector and the sky. The view factor is 1 for a horizontal collector, and there is a simple expression for the view factor for a single inclined collector on a horizontal plane. However, for a photovoltaic system with a number of rows deployed on a horizontal or on an inclined plane, the view factor is more complicated to calculate because one collector row may obscure part of the sky for the other row. More complication is added when the azimuth of the collector rows do not coincides with the azimuth of the inclined plane. The present article deals with view factors of photovoltaic collectors deployed on inclined planes and oriented in any direction. A general mathematical expression for this view factor was developed and may be used to calculate the diffuse radiation reaching photovoltaic systems.     

Reliability improvement of isolated generating systems by photovoltaic ac fusion converters

In the case of isolated areas, power quality and service reliability are improved by fusion of the ac power generation with a PV system output. Such an arrangement decreases the operating costs considerably. The proposed model and the related reliability and performance indices illustrate the benefits of PV system fusion with ac generation.     

Predicting the energy production by solar photovoltaic systems in cold-climate regions

One challenge in designing a photovoltaic (PV) system is to predict its generation, given parameters such as location, meteorological conditions, and layout. A greater challenge is to predict the generation of such a system under snow-cover condition. Publicly available snowfall data provide records for horizontal surfaces. However, the effect of snow accumulated on a tilted PV module remains unknown. Hence, irradiance is insufficient for predicting the output of PV systems having any given layout configuration. The research in this paper aims to predict the daily generation of PV systems through the development of a predictive model flexible enough to accommodate different layout configurations based on long-term monitoring data collected from 85 sites. Snow coverage loss factors are derived empirically to enhance the performance of the model. A feed-forward artificial neural network model is developed and implemented with snow adjustments (snowfall data and snow coverage loss factors). Promising results are obtained and validated.     

State coordinated voltage control in an active distribution network with on-load tap changers and photovoltaic systems

Decreasing costs and favorable policies have resulted in increased penetration of solar photovoltaic (PV) power generation in distribution networks. As the PV systems penetration is likely to increase in the future, utilizing the reactive power capability of PV inverters to mitigate voltage deviations is being promoted. In recent years, droop control of inverter- based distributed energy resources has emerged as an essential tool for use in this study. The participation of PV systems in voltage regulation and its coordination with existing controllers, such as on-load tap changers, is paramount for controlling the voltage within specified limits. In this work, control strategies are presented that can be coordinated with the existing controls in a distributed manner. The effectiveness of the proposed method was demonstrated through simulation results on a distribution system.     

户用光伏接入对配电网电压影响及优化研究

户用光伏的随机性和波动性是制约网络消纳的关键因素,高比例的户用光伏并网将影响区域网络的静态电压稳定。文章通过户用光伏系统分析,研究了户用光伏接入对配电网电压波动的影响;并提出基于遗传算法的含户用光伏配电网无功优化方法;最后,利用IEEE33节点配电网模型,针对不同渗透率下户用光伏接入配电网进行仿真实验,验证了所提含户用光伏配电网无功优化方法的可行性与准确性,可有效解决配电网电压波动问题。     

Effects of large-scale photovoltaic power integration on electricity distribution networks

The public support in photovoltaic (PV) technologies and increasing markets have resulted in extensive applications of grid-connected PV, in particular in the consumer side and electricity distribution grid. In this paper, the effects of a high level of grid connected PV in the middle voltage distribution network have been analyzed. The emphasis is put on static phenomena, including voltage drop, network losses and grid benefits. A multi-purpose modeling tool is used for PV analysis in Lisbon and Helsinki climates. All network types studied can handle PV without problems with an amount of PV equaling at least up to the load (1 kW_p/household). The comb-type network showed the best performance. The PV is unable to shave the domestic load peak in the early evening hours but through orientating the PV panels both to east and west, the noon peak from PV can be reduced by 30%. PV integration reduces network losses positively up to a 1 kW_p/hh (100% of annual domestic load) level. For 2 kW_p/hh all but the comb-type networks demonstrate clear over-voltage situations and the annual network losses are much higher than without PV.     

Thermodynamic analysis of solar photovoltaic cell systems

The thermodynamic characteristics of solar photovoltaic (PV) cells are investigated from a perspective based on exergy. A new efficiency is developed that is useful in studying PV performance and possible improvements. Exergy analysis is applied to a PV system and its components, and exergy flows, losses and efficiencies are evaluated. Energy efficiency is seen to vary between 7% and 12% during the day. In contrast, exergy efficiencies, which incorporate the second law of thermodynamics and account for solar irradiation exergy values, are lower for electricity generation using the considered PV system, ranging from 2% to 8%. Values of “fill factors” are determined for the system and observed to be similar to values of exergy efficiency.     

Economic viability of photovoltaic water pumping systems

A comparison of the economic viability of photovoltaic and diesel water pumping systems is presented for system sizes in the range 2.8 kW_p to 15 kW_p. Actual performance data from installed systems are employed for the base case. Sensitivity analysis is carried out to generalize results for other locations and conditions. The effect of system oversizing due to mismatch of water supply and demand patterns on the economic viability of PV water pumping system is illustrated based on real data and three-year operational experience of eight installations. Investment prospects in PV water pumping applications for different selling price scenarios of water have been investigated.