A study of ageing of photovoltaic system energy output in Malta

Abstract

As the ever-growing solar industry expands across the globe and Malta, it is important to understand all of the aspects that affect their performance. One of these is ageing, which is a long-term type of degradation that results in a reduction in energy output. The rate at which this ageing is experienced appears to vary in different climates and system to system and is dependent upon many aspects. The aim of this work is to find the degradation rate of four systems operating in Malta by direct power measurements. Energy output data for three of the four systems was obtained from the Sunny Portal website while the fourth system’s data was obtained from a private individual. Data from April to August was studied as this time period would increase the likelihood of fully sunny days which removes some of the variability of partially cloudy days. Two systems are polycrystalline while the other two are monocrystalline. Five methods were used in order to calculate the apparent degradation rates experienced by the systems. The first method involved plotting the entire data sets of energy production. The second method refined the first approach by plotting the individual months by year and the monthly averages. These approaches had very similar results but the first method understandably has a lot more noise. Method 3 involved the plotting of the average of the seven highest energy outputs from the months across the years. This ensured that only cloudless days were used. Method 4 incorporated weather data (temperature and irradiance) alongside energy output data, with days with the same conditions of individual months being plotted across the years. Method 5 took this approach even further, as specific hourly data was used for energy output, temperature and irradiance. This method was applied to three of the four systems, as one of the system’s hourly data was not available. Method 1 results were not taken into consideration as they were deemed less reliable due to the extensive variability over a whole year. Method 2 results ranged from -0.7% per year up to -1.5% per year. Method 3 results were between -0.4% and -1.0% annually. Method 4 results were calculated to be between -0.9% and -1.2% while method 5 results had the widest range from -0.1% to -1.5% per year. A clear decline is evident through methods 2 to 5 and also appears when looking at the annual production data for the systems. The actual degradation rate experienced by these systems is likely in the range of the values found in the methods 2 to 4 ranging from -0.4% to -1.5% per year, with method 5 results likely being too low for two of the systems. No appreciable difference in energy output decline between the monocrystalline and polycrystalline was observed.