Scientists at the Korea Institute of Civil Engineering and Building Technology analyzed different operating modes for integrating residential batteries with photovoltaic modules on the balconies of Korean apartment buildings.
In the proposed configuration, batteries are included in the hybrid system, but only to discharge household appliances connected to the load line. The loads are powered by balcony photovoltaic modules or batteries, with any remaining electricity stored in a storage system when needed.
The research team said that the battery should be charged with grid power when it is in a system check state or when the battery's (state of charge) level is in an emergency.
Researchers simulated the hybrid system in a demonstration house in Goyang, South Korea, using a 2.016 kWh prototype lithium iron phosphate battery with an efficiency of 93%. The system included a south-facing 1.2 kW balcony photovoltaic system, consisting of four 300-watt modules connected in series with an efficiency of 18%. The tilt angle was 70 degrees, matched with a 260-watt electric fan representing the connected load.
Researchers calculated the battery's storage capacity by considering the capacity of the photovoltaic system and the total electricity consumption of a household. They stated that the basic operating settings for the battery energy storage system are a batch charging voltage of 58.4V, a discharge cutoff voltage of 41V, and a charging voltage of 44V.
Researcher Ji-YoungEum said: "This system can be used anywhere, not just in South Korea, because of its small capacity."
Researchers evaluated the system's performance in four operating modes. They investigated a configuration where the photovoltaic system prioritizes supplying the load and uses only photovoltaic power to charge the battery, and a system that first supplies photovoltaic power to the load and then charges the battery using both photovoltaic and grid power. Furthermore, they considered a design where the battery is preferentially charged using photovoltaic power, uses only photovoltaic power, and a system that prioritizes charging the battery using photovoltaic power and then charges it using both photovoltaic and grid power.
The experiment lasted for about three months, from August 21, 2019 to November 8, 2019. Scientists said that the experiment was conducted on all days except rainy days and days when the instruments were being checked in order to examine the results of charging and discharging.
They found that in the first mode, approximately 92% of the battery capacity was discharged, and approximately 91% of the load was supplied to the battery. They also observed that the battery voltage reached the full discharge voltage range of 33V, therefore, the battery was not used because the battery voltage could not be maintained until the next photovoltaic charge.
They say that in the second mode, photovoltaic power is preferentially injected into the load, and if there is no remaining photovoltaic power after supplying the load, it is almost impossible to charge the battery. They claim that this makes it necessary to prevent battery discharge by reducing battery idle time and grid power usage.
The third operating mode ensures the battery is fully discharged, and the battery voltage is not maintained until the next day when it is recharged to supply photovoltaic power to the load and the grid. In the fourth configuration, due to grid charging, the battery has a high chance of not being fully discharged. When the battery voltage reaches the full discharge protection voltage, it is recharged by grid power.
The operating modes of the residential battery energy storage system and the balcony photovoltaic system were confirmed in individual apartments through experiments on each operating mode of the residential (battery energy storage system). The results showed that mode four was the most suitable of the four operating modes of the (battery energy storage system). Nevertheless, some additional features are still needed to apply this solution to individual apartments.
Researchers described their work in the operation mode of BESS (Balance of Surface Energy) and balcony photovoltaics in a South Korean apartment building, recently published in Sustainable Development.
