Management, optimal sizing and technical-economic analysis of batteries for constant production in photovoltaic systems

Developed countries promote the Photovoltaic (PV) energy production systems which are thriving. To improve their integration, it is recommended to control the energy production in order to increase economic viability, to decrease the impacts on the grids and potentially that can be traded on electricity markets like the conventional power plants/producers. An Energy Management Strategy (EMS) intended for a PV/Battery hybrid system is proposed in this paper. The EMS defines the constant maximum power injection into the grid by the hybrid system during all the day. An Off-Line Linear Programming (LP) optimization algorithm has been developed where the Daily Solar Energy Production (DSEP) is known. The operation of the algorithm under various values of its parameters has allowed to define generic rules valid at all times; regardless of the hybrid system size. So it results from the above that the optimal size of batteries and the maximum constant power are defined multiplying by 61% and 4.17% respectively, the daily solar energy produced by the PV (alone, i.e. without Energy Storage System - ESS). The impact of the minimum/maximum levels of battery State-of-Charge (SOC) were studied. The results showed that the decrease of the battery size has a linear impact on the energy efficiency and the value of the constant power injection into the grid; provided the capacity restriction is equitably distributed between the high and low thresholds of SOC. It is common practice in EMS to limit the battery charge/discharge rates to preserve its State-of-Health (SOH). However, an overly strict limitation degrades the performances of the system. So, the impact of thereof has been evaluated. Therefore, it is recommended to not limit the battery charge/discharge powers at a values less than 10% of the battery size. Two technical-economic analyzes were performed where Return On Investment (ROI) is evaluated by varying the annual PV energy production, the electricity feed in tariff, the battery price and size. ROI is 12 years if the financial compensation is 0.1S/kWh and the cost of the battery is 700$/kWh. Finally, the proposed tool is generic and aims to help R&D engineers in design step of a hybrid system. It will allow to choose the battery size, calculate the expected benefit and/or the ROI. During operation, it is useful to define the Daily Energy Production Plan (DEPP) of a PV/Battery hybrid system by determining the constant maximum power.