ارزیابی قابلیت اطمینان از سیستم های خورشیدی فتوولتائیک با و بدون ذخیره سازی باتری Reliability Evaluation of a Solar Photovoltaic System with and without Battery Storage

The rapidly increasing energy demand and the concerns over environmental degradation resulted from use of conventional energy sources have opened the options for exploring more alternative sources for energy production. Solar energy is considered as the major renewable energy source and electricity generated through photovoltaic system involves zero greenhouse gas emission and zero dependence on fossil fuel. Declining production cost of photovoltaic modules coupled with economic incentives offered by governmental organizations will further increase the future installed capacity of solar power. However, the inherent uncertainty and viability associated with system components and input solar radiation pose serious challenges in designing such photovoltaic system. One way of nullifying the impact of intermittency is the inclusion of storage unit so that the surplus energy generated during periods of high solar insolation can be stored and utilized in later time. The reliability assessment of renewable energy based power system without or with energy storage need to be addressed differently from conventional power system since they are considered as variable capacity generation systems. The reported techniques in literature for reliability assessment can be broadly grouped under two categories: (i) analytical techniques (ii) Monte Carlo simulation. Analytical techniques model the system with mathematical equations and evaluate the desired reliability indices through direct numerical solution [1]. It fails to simulate the actual behaviors of the system which are stochastic in nature. This is overcome by Monte Carlo simulation by treating the problems as a series of real experiments and simulating the random behavior of system components [2]. Reliability evaluation of renewable energy based power system was started in 1980s. In [3], authors introduced new reliability concepts and terminology applicable to PV technology and applications. These new concepts accounts for the variability of input solar energy as well as unique characteristics of the PV array. Many researchers have made various contributions towards renewable energy sources modeling for reliability assessment [4] and [5]. Simulation based methods are adopted in [4] and [6] whereas analytical techniques are used by researchers in literatures found in [7] and [8]. Loss of power supply probability technique is used in [9] for designing stand-alone photovoltaic systems. In [10], chronological simulation technique is used for estimating the loss of load probability of stand-alone photovoltaic systems based on synthetic radiation sequences. A closed-form solution approach is developed in [11] for evaluation of the loss of power supply probability of stand-alone photovoltaic systems with battery storage. Reliability evaluation of renewable energy sources based systems is carried out in [12] using universal generating function. A novel approach for reliability assessment using probabilistic storage model is reported in [13] for an autonomous PV-wind-storage system. However, there are only very few literatures which model the hardware failure of the solar photovoltaic panels. In view of this, the current work develops a model for photovoltaic system taking into account variable behavior of solar resource and the outages due to hardware failure of panel. The developed model is applied for evaluating the reliability of a remote photovoltaic system identified with loss of load probability (LOLP) reliability index through Monte Carlo simulation. In the first case, a simple photovoltaic system without any storage is considered. A comparison is drawn between the calculated values of LOLP reliability index with photovoltaic output considering only variation in solar radiation and other considering both variation in solar radiation and hardware status of photovoltaic modules. Different ratings of photovoltaic such as 30kW, 40kW, 50kW and 60kW are used in order to account for the diversity in the number of photovoltaic modules. In the second case, battery storage is introduced into the previous system and numbers of battery strings are varied for each of the PV rating in order to achieve a LOLP value of 0.0001 while assuming battery and photovoltaic modules as components which is always available. With the numbers of battery strings obtained corresponding to LOLP value of 0.0001, each of the system is subjected to hardware availability of photovoltaic modules and variation in values of reliability index calculated are examined.