When designing the capacity configuration of energy storage equipment in a distributed power generation system, the main factors that need to be considered are: the self-supply power supply time of the energy storage equipment, the maximum energy storage requirements, the depth of discharge and the correction calculation of other influencing factors, etc. The following takes distributed photovoltaic power generation system as an example to configure energy storage equipment.
In the photovoltaic power generation system, the battery energy storage device is mainly used. The design of the energy storage system includes the calculation of the battery capacity and the design of the series-parallel combination of battery packs.
(1) The self-sufficiency time of energy storage equipment refers to the time during which the energy storage equipment itself can maintain the normal operation of the system and ensure the continuity of power supply when the external power supply does not have supplementary power. For example, the battery system of a photovoltaic power generation system should meet the requirement that users can still use electricity normally under the condition of insufficient sunlight. A meteorological condition parameter is quoted in the design: the maximum number of continuous cloudy and rainy days in the local area. When the load itself can accept a certain degree of insufficient power supply, the self-sufficiency time requirement of the energy storage device is relatively loose and can be slightly shorter. When the load is more important, the self-sufficiency time is longer. In addition, it is also necessary to consider the construction area of the distributed power generation system. If it is a remote independent power generation system, its system maintenance will take a certain period of time. Therefore, the design of the energy storage system must use a larger capacity battery to ensure the reliability of the system.
(2) Sometimes a single disturbance occurs in the distributed power generation system, and at this time, the energy storage unit needs to release a large amount of energy to support the normal operation of the system. For example, when a short-circuit fault occurs in the distributed power generation system, the node voltage drops, then the energy storage system should provide electrical energy to solve the voltage drop problem and maintain the stable operation of the system. Such a single event is an important basis for determining the maximum capacity of an energy storage system.
(3) For battery energy storage devices, excessive release of electrical energy will damage the battery, so the maximum allowable depth of discharge must be considered. The maximum allowable depth of discharge for deep-cycle batteries is 80%, and the maximum allowable depth of discharge for shallow-cycle batteries is only 50%. In practical applications, the maximum allowable depth of discharge can be appropriately reduced to increase the service life of the battery, reduce maintenance costs, and avoid affecting the efficiency of the energy storage system.
Battery equipped capacity = self-maintaining required energy or maximum energy storage / maximum allowable depth of discharge (1-1)
Take AC photovoltaic power generation system as an example: assuming that the AC load power consumption of the system is 10kWh/day, the efficiency of the photovoltaic power generation system inverter is 90%, and the input voltage is 24V, so the daily demand of the load is 10000Wh÷0.9÷24=462.96Ah. According to the user’s flexibility, choose 6 days for self-sufficiency, use deep-cycle batteries, and the discharge depth is about 75%, then the battery capacity is selected as 6×462.96÷0.75=3703.68Ah.
(4) In practical applications, the above formula should be revised in consideration of environmental factors and the performance parameters of energy storage equipment. For example, the capacity of a battery is related to the discharge rate. The discharge rate decreases and the battery capacity increases. When designing, you should check the capacity of this type of component under different discharge rates. Under normal circumstances, it can be roughly estimated that during slow discharge (50-200 hour rate), the capacity of the battery is increased by 5%-20% compared to its standard state, and the corresponding discharge correction coefficient is 0.95-0.8. The calculation formula for the average discharge rate of the photovoltaic system is
Average discharge rate = (number of consecutive cloudy and rainy days × load working time) / maximum allowable depth of discharge (1.2)
Load working time=∑(load power×single load working time)/∑load power (1.3)
According to the calculated average discharge rate of the photovoltaic system and the battery capacity of different types of batteries produced by the manufacturer at the corresponding discharge rate, the most suitable battery design capacity can be found.
The ambient temperature also has a certain influence on the battery capacity, the temperature decreases, the battery capacity decreases. The capacity of the lead-acid battery at 25°C is the nominal capacity; at 0°C, the capacity drops to 90% of the nominal capacity; at -20°C, the capacity drops to 80% of the rated capacity.
Actual configuration battery capacity = (load daily average power × system required self-sufficiency time × discharge rate correction coefficient) / (maximum discharge depth × temperature correction coefficient) (1.4)
After determining the capacity of the battery, it is necessary to design the battery pack in series and parallel.
The number of batteries in series = the working voltage of the system + the rated voltage of the battery (1.5)
The number of batteries in parallel = the total capacity of the battery group ÷ the rated capacity of the battery (1.6)
Assuming that the calculated capacity of the battery pack is 500Ah, a single 500Ah battery or two 250Ah batteries in parallel can be selected in the design. These choices are theoretically possible. In practical applications, considering that there may be an imbalance between the batteries connected in parallel, large-capacity batteries should be selected as far as possible to reduce the number of batteries in parallel.
Energy storage equipment design case: a DC load solar photovoltaic power generation system, the load working voltage is 24V, assuming that the power generation system has two sets of equipment loads, one set of equipment has a working current of 2A and works 24 hours a day; the working current of the other set of equipment is 5A, and the working time is 12h per day. It is known that the minimum temperature in this area is -20℃, and the maximum number of rainy days is 6 days. The use of deep-cycle batteries requires calculation of the capacity of the energy storage system battery pack and the number of series and parallel connections.
Solution: From the meaning of the question, the maximum allowable depth of discharge coefficient is 0.8, and the low temperature correction coefficient is 0.8.
Average load working time=[(2A×24h)+(5A×12h)]/(2A+5A)=15.4(h)
Average discharge rate=6×15.4/0.8=115 hour rate
The 115-hour rate is a slow discharge rate. According to the information provided by the manufacturer, the battery capacity of the battery at the 115-hour rate can be found to be corrected, or it can be estimated based on experience. The correction coefficient is 0.88, which is substituted into the formula calculation
Daily average power consumption of load=(2A×24h)+(5A×12h)=108(Ah)
The amount of battery pack = (108Ah × 6 × 0.88) / (0.8 × 0.8) = 891 (Ah)
Choose 2V/600Ah battery here, so
Number of batteries connected in series=24V/2V=12 (pcs)
The number of batteries in parallel=891Ah/600Ah=1.5≈2 (pcs)
Total number of battery packs=12×2=24 (pcs)
According to the above calculation, 24 pieces of 2V, 600Ah type batteries are needed in the design. After every 12 pieces of batteries are connected in series, 2 battery packs in series are connected in parallel.