First, the choice of battery type
With the development of battery technology and the rapid decline in cost, lithium batteries have become the mainstream choice in household energy storage projects, and the market share of new chemical batteries has reached more than 95%.
Compared with lead-acid batteries, lithium batteries have the advantages of high efficiency, long cycle life, accurate battery data, and high consistency.
2. Four common misunderstandings in battery capacity design
1. Only select the battery capacity according to the load power and power consumption
In the design of battery capacity, the 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, the battery manual indicates the theoretical capacity of the battery, that is, under ideal conditions, the maximum power that the battery can release when the battery goes from SOC100% to SOC0%.
In practical applications, considering the battery life, it is not allowed to discharge to SOC0%, and the protection power will be set.
3. The larger the battery capacity, the better
In practical applications, battery usage should be considered. If the capacity of the photovoltaic system is small, or the load power consumption is large, the battery cannot be fully charged, which will cause waste.
4. The battery capacity design fits perfectly
Due to the process loss, the battery discharge capacity is less than the battery storage capacity, and the load power consumption is less than the battery discharge capacity. Neglecting efficiency losses is likely to result in insufficient battery power.
3. Battery capacity design in different application scenarios
This article mainly introduces the battery capacity design ideas in three common application scenarios: spontaneous self-consumption (high electricity cost or no subsidies), peak and valley electricity price, and backup power supply (the grid is unstable or has important loads).
1. "Spontaneous use"
Due to the high electricity price or low photovoltaic grid-connected subsidies (no subsidies), photovoltaic energy storage systems are installed to reduce electricity bills.
Assuming that the grid is stable, off-grid operation is not considered
Photovoltaic is only to reduce the electricity consumption of the grid
Generally, there is sufficient sunlight during the day
The ideal state is that the photovoltaic + energy storage system can completely cover household electricity. But this situation is difficult to achieve. Therefore, we comprehensively consider the input cost and electricity consumption, and can choose to choose the capacity of the battery according to the average daily electricity consumption (kWh) of the household (the default photovoltaic system has sufficient energy).
If the electricity consumption rules can be accurately collected, combined with the energy storage machine management settings, the system utilization rate can be improved as much as possible.
2. Peak and valley electricity price
The structure of peak and valley electricity price is roughly as shown in the figure below, 17:00-22:00 is the peak period of electricity consumption:
During the day, the power consumption is low (the photovoltaic system can basically cover it), and during the peak period of power consumption, it is necessary to ensure that at least half of the power is supplied by the battery to reduce the electricity bill.
Assume the average daily electricity consumption during peak period: 20kWh
Calculate the maximum demand value of battery capacity based on the total power consumption during the peak period. Then according to the capacity of the photovoltaic system and the benefit of the investment, an optimal battery power is found within this range.
3. Areas with unstable power grid - backup power supply
Mainly used in unstable power grid areas or situations with important loads. In early 2017, GoodWe once designed a project in Southeast Asia. The details are as follows:
Application site: chicken farm, considering the paved area of photovoltaic, it can install 5-8KW modules
Important load: 4* ventilation fans, the power of a single fan is 550W (if the ventilation fan does not work, the oxygen supply in the chicken shed is insufficient)
Power grid situation: the power grid is unstable, power outages are irregular, and 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 is powered off, the battery + photovoltaic ensures the normal operation of the important load (fan)
When choosing the battery capacity, what needs to be considered is the power required by the battery to supply the battery alone in the case of off-grid (assuming a power outage at night, no PV).
Among them, the total power consumption when off-grid and the estimated off-grid time are the most critical parameters. If there are other important loads in the system, you need to list them all (as in the example below), and then determine the required battery capacity based on the maximum load power and power consumption during the longest continuous power outage in the whole day.
Four, two important factors in battery capacity design
1. PV system capacity
Assume:
The battery is fully charged by photovoltaics
The maximum power of the energy storage machine to charge the battery is 5000W
The number of hours of sunshine per day is 4 hours
So:
①In the mode of battery as backup power supply, the battery with an effective capacity of 800Ah needs to be fully charged in an ideal state on average:
800Ah/100A/4h=2 days
②In the mode of spontaneous use, it is assumed that the system charges the battery with an average of 3000W within 4 hours a day. A fully charged battery with an effective capacity of 800Ah (without discharging) requires:
800Ah*50V/3000=13 days
Unable to meet the daily electricity consumption of the load. In a conventional self-consumption system, the battery cannot be fully charged.
2. Battery redundancy design
As mentioned in the three application scenarios mentioned above, due to the instability of photovoltaic power generation, line loss, invalid discharge, battery aging, etc., resulting in efficiency loss, it is necessary to reserve a certain margin when designing battery capacity.
The design of the remaining battery capacity is relatively free, and the designer can make a comprehensive judgment according to the actual situation of his own system design.