1. Selection of battery type
With the development of battery technology and the rapid decline of costs, lithium batteries have become the mainstream choice in household energy storage projects due to their advantages such as high efficiency, long cycle life, accurate battery data, and high consistency.
2. Four common misunderstandings in battery capacity design
1. Select battery capacity only based on load power and power consumption
In battery capacity design, load condition is the most important reference factor. However, the charging and discharging capacity of the battery, the maximum power of the energy storage machine, and the power consumption period of the load cannot be ignored.
2. Theoretical capacity and actual capacity of the battery
Usually, what is marked on the battery manual is the theoretical capacity of the battery, which is the maximum power that the battery can release when the battery goes from SOC100% to SOC0% under ideal conditions.
In actual applications, considering battery life, it is generally not allowed to discharge to SOC0%, and a protective power level will be set.
3. The larger the battery capacity, the better.
Consider battery usage when using. If the capacity of the photovoltaic system is small, or the load power consumption is small, the battery cannot be fully charged, resulting in waste.
4. Battery capacity design perfectly matches
Due to process losses, the battery discharge capacity is less than the battery storage capacity, and the load power consumption is less than the battery discharge capacity. Ignoring efficiency loss is likely to cause insufficient battery power.
3. Battery capacity design in different application scenarios
This paper mainly introduces the battery capacity design ideas in three common application scenarios: self-consumption (high electricity bills or no subsidies), peak and valley electricity prices, and backup power supply (the power grid is unstable or has important loads).
1. "Spontaneous personal use"
Due to high electricity prices or low photovoltaic grid-connected subsidies (no subsidies), photovoltaic energy storage systems are installed to reduce electricity bills.
Assuming that the power grid is stable and off-grid operation is not considered, photovoltaics are only used to reduce grid power consumption, and generally there is sufficient light during the day.
The most ideal situation is that the photovoltaic + energy storage system can completely cover household electricity consumption. But this situation is difficult to achieve. Therefore, we comprehensively consider the input cost and electricity consumption, and can choose the battery capacity based on the average daily electricity consumption (kWh) of the household (the default photovoltaic system has sufficient energy). The design logic is as follows:
If the power consumption patterns can be accurately collected and combined with the energy storage machine management settings, the system utilization can be maximized.
2. Peak and valley electricity prices
The structure of peak and valley electricity prices is roughly 17:00-22:00, which is the peak period of electricity consumption:
Electricity consumption is low during the day (photovoltaic systems can basically cover it). During peak periods of electricity consumption, it is necessary to ensure that at least half of the electricity is powered by batteries to reduce electricity bills.
Assuming average daily electricity consumption during peak period: 20kWh
Its design ideas are as follows:
The maximum demand value of battery capacity is calculated based on the total power consumption during peak periods. Then find an optimal battery capacity within this range based on the capacity of the photovoltaic system and the return on investment.
3. Areas with unstable power grid - backup power supply
It is mainly used in areas with unstable power grids or situations with important loads.
For example: Application site: Approximately 5-8KW components can be installed
Important load: 4* ventilation fans, the power of a single fan is 550W
Power grid situation: The power grid is unstable and power outages occur from time to time. The longest power outage lasts 3 to 4 hours.
Application requirements: When the power grid is normal, the battery is charged first; when the power grid fails, the battery + photovoltaic ensure the normal operation of the important load (fan).
When selecting battery capacity, what needs to be considered is the power required by the battery to be supplied alone in an off-grid situation (assuming a power outage at night and no PV).
Among them, the total power consumption when off-grid and the estimated off-grid time are the most critical parameters. Calculated based on the expected longest power outage time of 4 hours, the design can refer to:
4. Two important factors in battery capacity design
1. Photovoltaic system capacity
Assume that the batteries are all charged by photovoltaics, the maximum power of the energy storage machine for charging the batteries is 5000W, and the number of sunshine hours per day is 4h.
So:
① When the battery is used as a backup power source, the average requirement for fully charging a battery with an effective capacity of 800Ah under ideal conditions is:
800Ah/100A/4h=2 days
2. Battery redundancy design
Due to the efficiency loss caused by instability, line loss, ineffective discharge, battery aging, etc. in photovoltaic power generation, a certain margin needs to be reserved when designing battery capacity.
The design of battery remaining capacity is relatively free and can be comprehensively determined based on the actual situation of your own system design.