This work will demonstrate the performance of a battery energy storage system (BESS) designed for long duration energy storage thorough time-shifting photovoltaic (PV) power production in Nordic climate. The BESS has an energy capacity of 300kWh, a power capacity of 100kW, and the cells are of NMC-type in pouch-format. All components are installed in a temperature-controlled and insulated 20 feet container. Charging of the battery is controlled by real-time PV power production from the neighboring ground-mounted 24kWp PV-park. This implies that the battery capacity equals about 12 hours of peak PV power production. Discharging is set to occur some hours in the afternoon to support the grid during high-demand hours.
The performance is monitored by analyzing data from both battery, converter, auxiliary components, and local weather station to achieve a proper system evaluation covering both winter and summer conditions and the specifics of time-shifting PV-production in the Nordics. Periodic performance characterizations are conducted by full charging-discharging cycles to determine some of the key performance indicators described by the European Commission regulation for digital product passports. These include discharge capacity, round trip efficiency, and power tolerance at various power rates. Internal resistance should be included as one of the key performance metrics in the upcoming digital product passport. High-time resolution pulse tests are not possible to execute on system-level, so a battery quality analyzer is used to measure internal resistance on the individual modules using AC impedance spectroscopy at 1 kHz. To supplement the understanding of the performance fade during the demonstration period, battery module tests will be conducted in laboratory settings.
Initial results confirm that the control system operates as expected and data is being collected for upcoming data analyses. Due to low solar irradiance levels during mid-winter, the battery needs occasionally to be charged over the night to enable discharging during high-demand hours. The manual and occasional charge sessions could be avoided by updating the charge-discharge schedule. Historical PV production data from the site will be used to create statistically correct schedules related to both time and power-level thresholds. This will enable the PV-BESS power plant to adapt the usage to the distinct seasonal variations.