The purpose of this diploma thesis is described in the introduction. The increased awareness in regards with air pollution has affected the trends in the energy field, which now seeks to minimise the effects on the environment. The main focus is to minimise the greenhouse effects and to produce the electricity using renewable energy sources. One of the most advanced solutions for the above-described problem are the photovoltaic systems, which use the solar energy. Since the production of power using the photovoltaic modules minimise the greenhouse effects on the environment, the photovoltaic systems are becoming the primary and most commonly used source for the renewable energy source.
The main part of this diploma thesis presents the model of photovoltaic system using the battery storage tank. All system elements are described in separate chapters, including photovoltaic modules, DC/DC converters, the MPPT algorithm (maximum power point tracking), inverter inverter control unit, grid, the grid load and battery storage tank. Each system element and its functions are described in more detail using the mathematical background.
The additional chapter presents the performance measurements within the system, which serve as indicator of the system's operation. The ninth chapter is the most important part of this diploma thesis. It presents the simulations conducted under different conditions. The first, is the simulation of the system without additional load, and battery storage tank. It demonstrates the performance measurements of each element. First, it demonstrates the power changes throughout specific time span and then the voltage and current flow using the constant input conditions. Further on, it also demonstrates the elements using dynamic input conditions.
The results indicate that constant conditions, after the time it takes for the system to stabilise, do not change, therefore, the output conditions/measurements remain the same. However, the use of dynamic input conditions simultaneously changes the output conditions/measurements as well.
The output values further indicate that the system's output power and current depend on the temperature of the solar panels and solar radiation. If the solar radiation decreases, the output power and current decreases also. Moreover, if the temperature of the solar panels increases the output power and current decreases. To further test the system (with the constant input conditions) we gradually loaded the system with increased power load and we connected the battery storage tank to the system.
The presentation of the first simulation includes the results as indicated when adding the load at the output at a specific time. As expected the output power decreases exponentially, due to additional load. The second presentation includes the simulation of the conditions when the unloaded system in constant conditions is attached to the battery storage tank either when the battery is charging or when it is not. Therefore, we have completed two simulations.
It became evident that current at the input of the inverter power swings when we add the battery storage tank, which is also evident when measuring the output power. Thus, the output power of the system increases or decreases depending on whether the battery is charging or emitting energy to the system.
The next simulation was conducted using the dynamic load and different battery conditions. At this point we have been following the SOC (state of charge) and the quantity of the load attached to the system followed by specific measurements of the output values (power, voltage, electrical current) and the battery values.
The above-described system has been designed following the principle that if the battery is full enough, it emits the energy into the network, therefore, the SOC is higher than 75%. Hence, if we do not add the excessive load at the output and the appropriate amount of the solar radiation, the results indicate that there is also an excess of produced power up to the point the battery reaches SOC under 50%, when the battery charging starts. However, if we add extra load the charging stops. Similarly, if the system is loaded and the SOC is higher than 50%; the battery starts emitting the energy into the environment and thus, powers up the user. All above-described simulations are described in chapter 9.
The final conclusions indicate that changes on the DC side of the system affect the output sizes indicated that it takes time for the system to stabilize. However, when the DC voltage at the input of the inverter stabilizes, the three phase output voltage from the inverter also stabilizes.