An optimal sizing method for stand-alone photovoltaic power systems

The total life-cycle cost of stand-alone photovoltaic (SAPV) power systems is mathematically formulated. A new optimal sizing algorithm for the solar array and battery capacity is developed. The optimum value of a balancing parameter, M, for the optimal sizing of SAPV system components is derived. The proposed optimal sizing algorithm is used in an illustrative example, where a more economical life-cycle cost has been obtained. The question of cost versus reliability is briefly discussed.     

太阳能发电系统的最佳化设计

独立光伏发电系统需要进行最佳化设计。介绍了一种简明合理而又实用的最佳化设计方法。应用目前国外常用的倾斜面上太阳辐照量的计算公式，根据不同的蓄电池维持天数，应用能量平衡原理，得到相应的太阳电池方阵最佳倾角，然后通过循环计算，得出一系列太阳电池方阵和蓄电池容量的组合，再通过经济核算等，最后确定光伏系统的规模，编制了相应的计算机程序，并进行了实例计算。     

Priority load control algorithm for optimal energy management in stand-alone photovoltaic systems

In stand-alone PV System facilities no grid connection exists, therefore the solar generator and battery bank have to be carefully sized in order to supply the energy demand for a given period of time. Batteries are considered as a weak component of the system, comprising an important part of the total cost and are usually replaced one or two times during PV system lifetime. A priority load control algorithm has been developed in order to gain an optimal energy management over system loads and the battery storage, and therefore provides a better energy management efficiency and guarantee the energy supply for critical loads. This will increase the reliability of the system and the end-user satisfaction. This article describes a stand-alone PV system model used for the development of a priority load control algorithm and explains and implements the algorithm. The results of several test scenario simulations are shown and discussed.     

不停电独立光伏系统的优化设计

为零缺电率负载供电的独立光伏系统是对设计要求最高的光伏系统,必须慎重地进行优化设计。研究表明,如果采用一般的独立光伏系统优化设计程序来进行设计,只要用蓄电池维持天数等于零代入,即可确定太阳电池方阵的容量,同时可以用当地辐照量最低的月份得到最大辐照量所对应的倾角作为方阵的最佳倾角。确定蓄电池容量时应以当地最长连阴雨天数作为蓄电池维持天数的依据。通过实例分析,取得了良好的效果。     

Direct coupling photovoltaic power regulator for stand-alone power systems with hydrogen generation

This paper describes a photovoltaic power regulator aimed for photovoltaic stand-alone hydrogen-backup power systems. The main characteristics of the regulator are the following; it employs a modular approach where each power cell has three ports, one input and two outputs, the input port is connected to a photovoltaic source while the two output ports are connected to a battery and to an electrolyser, respectively. A second characteristic is that the proposed regulator is driven sequentially, minimising the regulator losses. The operation and features of the photovoltaic regulator are presented and analyzed. Design guidelines are suggested and experimental validation is also given for a 2 kW prototype.     

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.     

Multilevel PWM inverters suitable for the use of stand-alone photovoltaic power systems

This paper presents a new multilevel pulse width-modulation (PWM) inverter scheme for the use of stand-alone photovoltaic systems. It consists of a PWM inverter, an assembly of LEVEL inverters, generating staircase output voltages, and cascaded transformers. To produce high-quality output voltage waves, it synthesizes a large number of output voltage levels using cascaded transformers, which have a series-connected secondary. By a suitable selection of the secondary turn-ratio of the transformer, the amplitude of an output voltage appears at the rate of an integer to an input dc source. Operational principles and analysis are illustrated in depth. The validity of the proposed system is verified through computer-aided simulations and experimental results using prototypes generating output voltages of an 11 level and a 29 level, respectively, and their results are compared with conventional counterparts.     

Review of the maximum power point tracking algorithms for stand-alone photovoltaic systems

A survey of the algorithms for seeking the maximum power point (MPP) is proposed. As has been shown, there are many ways of distinguishing and grouping methods that seek the MPP from a photovoltaic (PV) generator. However, in this article they are grouped as either direct or nondirect methods. The indirect methods (“quasi seeks”) have the particular feature that the MPP is estimated from the measures of the PV generator's voltage and current PV, the irradiance, or using empiric data, by mathematical expressions of numerical approximations. Therefore, the estimation is carried out for a specific PV generator installed in the system. Thus, they do not obtain the maximum power for any irradiance or temperature and none of them are able to obtain the MPP exactly. Subsequently, they are known as “quasi seeks”.Nevertheless, the direct methods (“true seeking methods”) can also be distinguished. They offer the advantage that they obtain the actual maximum power from the measures of the PV generator's voltage and current PV. In that case, they are suitable for any irradiance and temperature. All algorithms, direct and indirect, can be included in some of the DC/DC converters, Maximum power point trackings (MPPTs), for the stand-alone systems.     

Microcontroller-based charge controller for stand-alone photovoltaic systems

Rural electrification using PV systems is widespread in the Third World. Charge controllers are needed to improve the efficiency of the system and to protect the storage batteries with special reference to automotive batteries. An intelligent charge controller, incorporating an SGS-Thompson microcontroller, ST62E20, has been designed with the object of prolonging battery life. The need for and advantages of such charge controllers are discussed. Tests have been performed on a working prototype which is versatile and has provision for more monitoring and control functions than are possible with the conventional logic or relay systems. A photovoltaic charge control algorithm is discussed.     

Sizing of the array for stand-alone photovoltaic systems

A brief critical review of stand-alone photovoltaic (SAPV) array sizing methods is presented, leading to the suggestion that a new class of methods is required in order to make the sizing exercise both accurate and accessible to a wide range of designers and users. The energy balance method is identified as an example of such a method. It is then used to obtain the array areas for a notional SAPV system situated in six widely-dispersed stations world-wide, with a total 24-hour load of 0.25, 2.5, or 25 kW h. Through correlational analysis of all the results and input data, it is recommended that the expression used to obtain array area is 1334Q^−2.272_TYL^0.986 with the terms and associated units being defined in the main body of the paper.     

Effect of different load profiles on the loss-of-load probability of stand-alone photovoltaic systems

As well as the technical design criteria, the performance of a stand-alone photovoltaic (PV) system depends on other variables, such as the solar radiation distribution and load profile. Different load profiles are encountered in stand-alone PV applications. Load profiles may vary from 24-h constant to only nighttime or oppositely only daytime load profiles. This article presents results of system performance simulations for analysing the effect of different load profiles on the system performance. The load demand used in this article is appropriate for an average residential application with an average 9.4 kWh of daily energy demand. The loss-of-load probability (LLP) of the PV system is simulated for five different weekly load profiles and the results are examined based on techno-economic parameters, including the total system cost or alternatively the cost of electricity per kWh for a 20-year system lifetime. The results are drawn based on 1-year long hourly time-series solar radiation and ambient temperature data.     

Comparing wind and photovoltaic stand-alone power systems used for the electrification of remote consumers

Wind power and photovoltaic driven stand-alone systems have turned into one of the most promising ways to handle the electrification requirements of numerous isolated consumers worldwide. In this context, the primary target of the present work is to estimate the appropriate dimensions of either a wind power or a photovoltaic stand-alone system that guarantees the energy autonomy of several typical remote consumers located in representative Greek territories. For all regions examined, long-term wind speed and solar radiation measurements as well as formal meteorological data are utilized. Accordingly, special emphasis is put on the detailed energy balance analysis of the proposed systems on an hourly basis, including also the battery bank depth of discharge time evolution. Finally, comparison is made between the wind and the solar based systems investigated, proving that in most Greek regions either a wind or photovoltaic driven stand-alone system is able to cover the electrification needs of remote consumers, at a moderate first installation cost, without any additional energy input.     

A microcontroller-based stand-alone photovoltaic power system for residential appliances

In this study, a stand-alone photovoltaic power system was designed and implemented to operate residential ac-powered appliances such as fluorescent lambs, fans etc. Sun-tracker is implemented for improved efficiency of the system by keeping the solar module perpendicular to the sun's incoming rays. The charge method is realized with closed-loop current control of buck-boost dc–dc converter. The proposed system also uses a voltage source type PWM inverter to convert DC voltage from battery storage to supply AC loads. In the PWM method used, selected harmonics are eliminated with the smallest number of switching and an improvement in the system efficiency by reducing switching losses and providing ease of filtering on the inverter output is obtained. Charge controller and PWM inverter systems have been realized by using PIC16F873 microcontrollers. An experimental system was implemented to demonstrate the effectiveness of the proposed system. Simulation and experimental results are given to verify the system's efficiency.     

Loss-of-load probability model for stand-alone photovoltaic systems in Europe

Sizing of stand-alone photovoltaic systems (SAPVS) requires knowledge of their reliability. Because of primary influence of the solar irradiance and meteorological conditions, simulations are the best way to compute an accurate reliability for a given location and fixed sizing parameters. These studies have been developed for more than two decades, but have had a narrow geographical applicability.In this paper, we perform a complete (in time and space) simulation of a standard SAPVS in Europe using 23 year radiation data corresponding to almost 2300 geographical points. At each point, the tilt angle that maximizes the energy reaching the PV array in December is estimated and the relation among sizing parameters and reliability is computed for wide ranges of values. Finally, multilayer perceptrons are trained for both computations, allowing (after their training) simple and fast estimations of the sizing parameters of this type of plants for any location in Europe.The procedure presented in this paper, although focused particularly in Europe, can be easily extended to almost any other region in the world.     

Maximum power point tracker for portable photovoltaic systems with resistive-like load

In this work we report on the design and realization of a maximum power point tracking (MPPT) circuit suitable for low power, portable applications with resistive load. The design rules included cost, size and power efficiency considerations. A novel scheme for the implementation of the control loop of the MPPT circuit is proposed, combining good performance with compact design. The operation and performances were simulated at circuit schematic level with simulation program with integrated circuit emphasis (SPICE). The improved operation of a PV system using our MPPT circuit was demonstrated using a purely resistive load.     

Dispatch strategy and model for hybrid photovoltaic and trigeneration power systems

The advent of small scale combined heat and power (CHP) systems has provided the opportunity for in-house power backup of residential-scale photovoltaic (PV) arrays. These hybrid systems enjoy a symbiotic relationship between components, but have large thermal energy wastes when operated to provide 100% of the electric load. In a novel hybrid system is proposed here of PV-trigeneration. In order to reduce waste from excess heat, an absorption chiller has been proposed to utilize the CHP-produced thermal energy for cooling of PV-CHP system. This complexity has brought forth entirely new levels of system dynamics and interaction that require numerical simulation in order to optimize system design. This paper introduces a dispatch strategy for such a system that accounts for electric, domestic hot water, space heating, and space cooling load categories. The dispatch strategy was simulated for a typical home in Vancouver and the results indicate an improvement in performance of over 50% available when a PV-CHP system also accounts for cooling. The dispatch strategy and simulation are to be used as a foundation for an optimization algorithm of such systems.     

A general multivariate qualitative model for sizing stand-alone photovoltaic systems

We considered a general model for sizing a stand-alone photovoltatic system, using as energy input data the information available in any irradiation atlas. The parameters of the model are estimated by multivariate linear regression. The results obtained from the numerical loss of load probability size method (LOLP) were used as initial input data to fit the mode. For this fit we have used daily global irradiation data taken from 222 US meteorological stations for the period 1961–1990. The expression proposed allows us to determine the photovoltatic array size, with a coefficient of determination to 0.96. This coefficient is independent of the used LOLP value. System parameters and mean monthly values for daily global irradiation on the modules surface are taken as independent variables in the model. It also shows that the proposed model can be used with the same accuracy for other locations not considered in the estimation of the model. We also propose a model which would allow us to calculate optimum tilts for the array surface taking the latitude into account as well as the variability of the incident irradiation.     

Energy pay-back period analysis of stand-alone photovoltaic systems

The exploitation of solar energy by autonomous, photovoltaic (PV) based systems offers the opportunity for satisfying the electrification needs of numerous remote consumers worldwide in an environmentally friendly way. On the other hand, the sustainable character of these systems is strongly questioned by the energy intensity of processes involved in the various life cycle (LC) stages of the system components. Although there are several studies concerned with the estimation of the energy pay-back period (EPBP) for grid-connected systems, the same is not valid for stand-alone configurations. In this context, an integrated methodology is currently developed in order to estimate the EPBP of PV-battery (PV-Bat) configurations ensuring 100% energy autonomy. The main scope of the proposed analysis is to determine the optimum size of a corresponding system, comprised of multi-crystalline (mc-Si) PV modules and lead-acid (PbA) batteries, based on the criterion of minimum embodied energy, i.e. minimum EPBP. For this purpose, a representative case study examined considers the electrification needs of a typical remote consumer on the Island of Rhodes, Greece. According to the results obtained, the autonomous energy character of the system is reflected by the comparatively higher EPBP in comparison with the corresponding grid-connected option, nevertheless the PV-Bat configurations analyzed clearly constitute sustainable energy solutions. Finally, in order to increase the reliability of the calculation results, a sensitivity analysis is carried out, based on the variation of the input energy content data.     

Sizing of a stand-alone photovoltaic power system at Dhaka

A stand-alone photovoltaic power system is designed to operate residential appliances such as fluorescent lamp, incandescent light and ceiling fan using standard methods. The total load is estimated for four hours of operation per day. The battery is sized considering different factors that affect battery efficiency to reliably operate the estimated loads during a sequence of below average insolation. The minimum battery size is obtained to be 128Ah @ 100 hr, 24V. The PV array is sized to operate the load on a daily basis based on average weather conditions using monthly average daily values of solar radiation data for 11 years. The array is sized to proper values in order to operate the estimated load reliably in the month of minimum insolation taking into account different types of power losses. The minimum array size is obtained as 6×47W_p.