1.1 type of battery
In the photovoltaic power generation system, the battery plays a role in storing and regulating the electric energy generated by the system. As the power output of the photovoltaic system changes every day, the battery can also provide relatively stable electric energy and maintain the stable electric energy output of the photovoltaic power station when there is little power generation in insufficient sunlight or the photovoltaic system needs to be repaired. When the sun is full, the battery can store excess electric energy. At present, lead-acid batteries, MH-Ni batteries, lithium-ion batteries, etc. are commonly used in the market. Table 1-1 shows the characteristics of several common batteries.
The development of photovoltaic industry has driven the rapid progress of the battery industry. Experts at home and abroad are working hard to develop new energy storage batteries. Considering that nickel chromium batteries and sodium sulfur batteries are in the research stage, and the cost of MH-Ni batteries is too high, photovoltaic power stations mainly use lead-acid batteries as energy storage components. As an energy storage battery, it should have the characteristics of high energy density, long cycle life and low price. Lead-acid battery has long service life, low cost and mature technology.
1.2 characteristics of lead-acid battery
The following briefly introduces several characteristic parameters of the battery that need to be considered in the energy storage system of the photovoltaic power station.
(1) Capacity of battery
DOD directly reflects the service life of battery and is an important reference quantity.
(2) Discharge rate of battery
Usually, the discharge rate is expressed by time rate or multiple rate, and the time rate refers to the discharge rate expressed by time; Magnification refers to the value of the discharge current expressed by the multiple of the capacity value. It is most commonly used to express the discharge rate by magnification.
(3) Self discharge phenomenon
Self discharge phenomenon is common and inevitable. It will not affect the service life of the battery, but it can be taken into account in the energy loss.
1.3 capacity matching of storage battery
In photovoltaic power stations, the capacity of the energy storage system needs to match the photovoltaic modules. When the external environment changes, the photovoltaic power generation power is less than its rated power. In order to meet the needs of the large power grid and load, the energy storage system is required to supplement the power generation reduction of photovoltaic cells. The calculation formula of battery capacity is as follows:
Where, q is the daily output of the battery; D is the number of days that the battery continuously provides energy (since the system charges the battery at night, D is taken as 1); η 1 is the efficiency of inverter and dc/dc circuit, taking 0.92; η 2: Is the discharge efficiency of the battery, taken as 0.85; K is the temperature correction coefficient, taken as 1.2; S is the discharge depth, taken as 0.8
The capacity selection of the battery should consider the continuous rainy weather, low solar irradiance, inoperative photovoltaic modules, and the rated power operation of the photovoltaic power station during the peak period of power consumption. Therefore, the capacity of the battery is c=0.92~1.23kw · H
1.4 energy flow model of pv-bess system
In the process of parallel off grid power generation of photovoltaic power stations, photovoltaic cells and batteries are mainly used to provide energy. Figure 1-1 shows several energy flow models of photovoltaic system. Figure 1-1 (a) shows that PV and battery supply power to the grid at the same time during the peak period of power consumption, and the PV Bess (photovoltaic battery energy storage system) system works normally to provide constant power for the large grid. Figure 1-1 (b) shows that during the low power consumption period, photovoltaic modules supply power to the energy storage battery and supplement the energy of the battery in real time. Figure 1-1 (c) shows that the solar irradiance is insufficient during the peak period of power consumption and rainy weather, and the photovoltaic cells cannot generate electricity normally, while the power grid is in a large number of power consumption periods. The battery supplies power to the large power grid, which plays the role of “peak shaving and valley filling”. Figure 1-1 (d) shows that during the low power consumption period at night, the large power grid charges the energy storage battery, effectively avoiding the under charged state of the battery during the next day’s power consumption period, and reasonably utilizing the excess power of the large power grid.
