I. selection of battery type

With the rapid development of battery technology and cost reduction, lithium battery has become the mainstream choice in household energy storage projects, and the market share of new chemical battery is more than 95%.

Compared with lead-acid battery, lithium battery has the advantages of high efficiency, long cycle life, accurate battery data and high consistency.

II. Four common misunderstandings in battery capacity design

1. Select battery capacity only according to load power and power consumption

In the design of battery capacity, load is the most important reference factor. However, the capacity of battery charge and discharge, the maximum power of energy storage machine, and the power consumption period of load can not be ignored.

2. Theoretical capacity and actual capacity of battery

Generally, the battery manual indicates the theoretical capacity of the battery, that is, the maximum amount of power that the battery can release when the battery is from soc100% to soc0%.

In the practical application, 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 is, the better

In practical application, the battery utilization should be considered. If the capacity of photovoltaic system is small, or the load power consumption is large, the battery cannot be fully charged, which will cause waste.

4. Perfect fit of battery capacity design

Due to the process loss, the discharge capacity of the battery is less than that of the battery, and the load consumption is less than that of the battery. Neglecting efficiency loss is likely to cause the phenomenon of insufficient battery power supply.

III. battery capacity design in different application scenarios

This paper mainly introduces three common application scenarios of battery capacity design ideas: spontaneous self use (high or no subsidy), peak valley price, standby power (unstable grid or important load).

1. "Self use"

Due to the high electricity price or low subsidy (no subsidy) for photovoltaic grid connection, photovoltaic energy storage system shall be installed to reduce the electricity cost.

Assuming the power grid is stable, the off grid operation is not considered

Photovoltaic is just to reduce the power consumption of the grid

Generally, there is enough light in the daytime

Ideally, PV + energy storage system can completely cover household electricity. But this is hard to achieve. Therefore, considering the input cost and power consumption, we can choose the capacity of the battery according to the average household daily power consumption (KWH) (the default photovoltaic system has sufficient energy).

If we can accurately collect the law of power consumption, combined with the energy storage management settings, we can try to improve the system utilization.

2. Peak valley price

The structure of peak 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 small (the photovoltaic system can basically cover), during the peak period of power consumption, it is necessary to ensure that at least half of the power is supplied by the battery, and reduce the electricity expense.

Assuming average daily power consumption in peak period: 20kwh

The maximum demand value of battery capacity is calculated based on the total power consumption in peak period. Then, according to the capacity of photovoltaic system and the benefit of investment, we can find the best battery quantity in this range.

3. Unstable area of power grid - standby power supply

It is mainly used in the unstable area of power grid or the situation with important load. In early 2017, goodway designed a project in Southeast Asia, as follows:

Application site: chicken farm, considering the laying area of photovoltaic, about 5-8kw modules can be installed

Important load: 4 * ventilation fan, the power of a single fan is 550W (if the ventilation fan does not work, there is insufficient oxygen supply in the chicken house)

Power grid situation: the power grid is unstable, and the power is cut off irregularly. The longest time of power cut is 3-4 hours

Application requirements: under the normal condition of power grid, the battery shall be charged first; in case of power grid outage, the battery + photovoltaic shall ensure the normal operation of important load (fan)

When selecting the capacity of the battery, it is necessary to consider that the battery can supply the required power independently in the case of off grid (assuming power failure at night, no PV).

The total power consumption and the estimated time of off grid are the most critical parameters. If there are other important loads in the system, it is necessary to list them all (as shown in the following example) and then determine the required battery capacity according to the maximum load power and power consumption within the longest continuous outage duration of the whole day.

IV. two important factors in battery capacity design

1. Photovoltaic system capacity

Assume:

All batteries are charged by photovoltaic

The maximum power of the accumulator to charge the battery is 5000W

4 hours of sunshine per day

Then:

① in the mode of battery as standby power supply, the average demand for full charge of the battery with the effective capacity of 800ah in the ideal state:

800ah / 100A / 4H = 2 days

② in the mode of self use, it is assumed that the system charges the battery with 3000W on average within 4 hours every day. A battery with an effective capacity of 800ah needs to be fully charged (without discharging):

800ah * 50V / 3000 = 13 days

Unable to meet the daily power consumption of load. However, in the conventional self use system, the battery cannot be fully charged.

2. Battery redundancy design

In the three application scenarios mentioned above, it is mentioned that due to the instability, line loss, invalid discharge, and aging of the battery caused by photovoltaic power generation, a certain margin needs to be reserved in the battery capacity design.

The design of battery margin is relatively free, and the designer can make a comprehensive judgment according to the actual situation of his own system design.