Photovoltaic energy storage systems generally refer to applications with photovoltaic components, including energy storage batteries and other related equipment in the system. According to whether the stored energy needs to be connected to the grid for sale, photovoltaic energy storage systems can be divided into photovoltaic off-grid systems and photovoltaic parallel/off-grid hybrid systems. Here, we mainly introduce some design considerations for photovoltaic off-grid systems.
Composition of photovoltaic off-grid system:
Photovoltaic modules, off-grid inverters (including photovoltaic chargers/inverters), energy storage batteries (lead-acid/colloid/lead carbon/ternary lithium/lithium iron phosphate, etc.), photovoltaic brackets, cables, and accessories Electric boxes, etc., are an important part of photovoltaic off-grid systems.
The biggest difference between off-grid systems and grid-connected systems is that grid-connected systems take investment income as the calculation premise, while off-grid systems take just-needed power supply as the basic demand, so they will have different emphasis when selecting components.
At the earliest, photovoltaic modules were only used in some off-grid systems and small photovoltaic systems. Later, with the large-scale development of grid-connected photovoltaic applications and the annual update of photovoltaic module technology, module conversion efficiency has been greatly improved. Especially for some grid-connected power stations, due to the need to make full use of site resources, more efficient components are especially needed to increase the investment return ratio. Of course, the general off-grid system does not have too high requirements for component conversion efficiency due to the relatively large site, so conventional components are often the first consideration element when selecting components in system design.
1. Take AC load as the consideration point. General loads are divided into three categories: group loads (lights, heaters, etc.), inductive loads (air conditioners, motors, etc.), and capacitive loads (computer mainframe power supplies, etc.). Among them, since the inductive load requires 3~5 times the rated current when starting, and the short-time overload capacity of general off-grid inverters of 150%-200% cannot meet the requirements, so inductive loads need special consideration for the inverter. Capacity expansion design (when the off-grid inverter is connected to an inductive load, at least a system design that requires at least 2 times the inductive load). For example, in a project where an off-grid inverter drives a 2P (2*750W) air conditioner, an inverter with a rated power of 3KVA and above is the normal configuration. Of course, there are generally three types of loads, but the load with the largest proportion will have a major impact on the inverter.
2. Consider the DC side. The off-grid inverter has a built-in photovoltaic charger, generally two types: MPPT and PWM. With technological updates, PWM chargers are gradually eliminated, and MPPT chargers have become the first choice for off-grid inverters.
3. Other options. In addition to the above two options, there are many calculation formulas on the market, so I won’t repeat them here. But the general direction is: 1) Determine the rated power of the off-grid inverter according to the load size and type; 2) Determine the kWh value of the energy storage battery pack according to the load’s required energy storage battery discharge time; 3) According to the local sunshine and Charging time requirements (for example, it needs to be fully charged in 1 day per day), determine the charger power, etc.