Sizing an energy storage to be used in parallel with a PV inverter to balance the fluctuations in output power from PV generator

Photovoltaic (PV) power systems are considered to be a significant part of future electricity generation infrastructure. Widespread implementation of PV power systems is necessary to establish a sustainable and environmentally friendly energy production and consumption system. However, the solar resource is inherently intermittent and the output power from PV generators closely follows the incident solar radiation. As a result, PV generated power can vary greatly and rapidly. Rapid changes in PV generated power can cause serious problems for relatively small power networks with high penetrations of PV generation. Traditional thermal power plants face difficulty to maintain the balance of power, when such rapid changes in power occur. Energy storage systems (ESS) are increasingly being used to integrate the PV power into the electricity grid. Energy storage systems can mitigate issues such as ramp rate deviations, frequency and voltage fluctuations etc. Power quality, network reliability and economic values of PV systems can be greatly enhanced by integrating energy storage systems with grid-connected PV power systems. In this thesis, an ideal energy storage system of undefined capacity, rating and technology is used to control the output power of a theoretical 1 kW rated PV generator. Solar irradiance data measured with the climatic and electric measurement systems in TUT solar PV research power plant are used to simulate the output power of the 1 kW rated PV generator. The ESS subtracts power from the PV generated power and adds to the PV generated power, whenever necessary, to limit the ramp rate under 10 % of the PV generator capacity per minute. Three main application scenarios of the energy storage system are considered in this thesis. Firstly, the ESS controls the rate of change of PV generator output power when the PV inverter is rated at 100 % of the PV generator capacity. Secondly, the ESS controls the rate of change of PV generator output power when the PV inverter is rated at 70 % of the PV generator capacity (power curtailment is active). Thirdly, the ESS automatically recharges itself using the excess energy when power curtailment is active (inverter operates at 70 % of the PV generator capacity) while controlling the rate of change of PV generator output power. The objective of this thesis is to analyze the operations of the energy storage system in order to size suitable energy storage systems for practical operations. Periodic (daily) and continuous operations of the ESS have been simulated and analyzed for each application scenario. For example, when the automatic recharging of the ESS is enabled during active power curtailment, the maximum energy stored in the ESS is found to be 1620 Wh during periodic operation. For the same application scenario, the maximum energy deficiency in the ESS is around 568 Wh. The energy balance in the ESS after one year of continuous operation is 77 kWh. The storage system operates for a total of 818 hours in one year. The maximum charging power of the ESS is 960 W and the maximum discharging power is 626 W