Technico-economic assessment of a lead-acid battery bank for standalone photovoltaic power plant

In this work, we will present and discuss the results of a long-term monitoring of a lead-acid battery bank, which is a part of a modular 7.2 kW(p) standalone photovoltaic power plant. Indeed, technical parameters such as the state of charge (SOC), battery voltage/temperature, and charging/discharging currents were monitored together with power flow into and out of the battery inverter. The incremental charge/discharge efficiency between two SOCs, the duration of charge (T-c) and discharge (T-d) cycles, Coulomb efficiency and the internal resistance of the battery bank, were evaluated from the measured data. Furthermore, a statistical analysis of the SOC was performed in order to specify the delivered energy from the battery bank. This allowed the assessment and a detailed understanding of the system operating performance from a technical and economical point of view. The average minimum and maximum SOC of the battery were found to be respectively, 34.0% and 87.45%, for the monitored period. The incremental cycle efficiency (Y-cd) was found to vary between 68 and 90%. A low efficiency is usually observed when either the charge or the discharge durations are small (or the corresponding currents are high). The Coulomb efficiency is marked by a quasi-constant degradation rate of 1.8% per year, and the internal resistance of the battery bank increased from 0.07 to 0.27 Ohms during the 4 year monitoring period. From the monitoring of these parameters, and by using real costs, the levelized cost of storage (LCOS) was determined for various discount and capacity degradation rates as well as battery bank replacement periods. The calculated LCOS was affected by the discount rate and the replacement period of the battery bank. Additionally, the LCOS analysis revealed that for PV standalone systems lead-acid battery is more cost effective than li-ion on the short term. The latter storage technology is expected to be cost effective for medium and long term.