PV energy storage systems generally refer to applications with PV modules, including related equipment such as energy storage cells in the system. Depending on whether the stored energy needs to be sold grid-connected, PV energy storage systems can be divided into PV off-grid systems and hybrid PV grid-connected/off-grid systems. Here, we will focus on some design considerations for off-grid PV systems.
PV off-grid system components: PV modules, off-grid inverters (including PV chargers/inverters), energy storage batteries (lead-acid/colloid/lead-carbon/tertiary lithium/ lithium iron phosphate and other types), PV brackets, cables, and distribution boxes are all important components of the PV off-grid system.
The biggest difference between off-grid systems and grid-connected systems is that: grid-connected systems are based on the premise of investment income, while off-grid systems are based on the basic need for power supply, so they will have a different focus in the selection of components.
The earliest, photovoltaic components are only used in some off-grid systems and photovoltaic small systems. Later, along with the wide range of grid-connected PV applications, as well as PV module technology updates year by year, the component conversion efficiency has been greatly improved. Especially for grid-connected power plants, more efficient modules are needed to improve the ROI ratio because of the need to make full use of the site resources.
Of course, the general off-grid system because the site is relatively large, the component conversion efficiency is not too high requirements, so the conventional components are often the first consideration in the design of the system to choose components.
(1) AC load as a point of consideration. The general load is divided into three categories: resistive load (lights, heaters, etc.), inductive load (air conditioners, motors, etc.), capacitive load (computer host power, etc.).
Among them, since the current required for inductive loads to start is 3~5 times of the rated time, and the short-time overload capacity of 150%-200% of general off-grid inverters does not meet the requirements, inductive loads need special consideration for the expansion design of the inverter (when off-grid inverters are connected to inductive loads, at least 2 times of the system design of inductive loads is required).
(2) Take the DC side as a point of consideration. Off-grid inverters have built-in PV chargers, generally of two types, MPPT and PWM. With technological updates, PWM chargers are being phased out and MPPT chargers are becoming the preferred choice for off-grid inverters.
(1) lead-acid/colloid batteries: Energy storage systems generally choose maintenance-free sealed lead-acid batteries to reduce late maintenance. After 150 years of development, lead-acid batteries have significant advantages in stability, safety and price, both the highest percentage of the current energy storage battery application battery type, is also the first off-grid photovoltaic system energy storage battery type.
(2) Lead-carbon battery: a technology evolved from traditional lead-acid batteries, which can significantly improve the life of lead-acid batteries by adding activated carbon to the negative electrode of lead-acid batteries. However, as a technological update of the lead-acid battery, its cost is slightly higher.
(3) Ternary lithium / lithium iron phosphate batteries: compared with the above two types of energy storage batteries, lithium-ion batteries have higher power density, more charge and discharge cycles, better depth of discharge and other characteristics.
However, due to the need for additional battery management technology (BMS), the system cost of ternary lithium / lithium iron phosphate batteries, generally 2-3 times more than lead-acid batteries.
In addition, compared to lead-acid / lead carbon batteries, its thermal stability is also slightly less, so the application of off-grid PV systems in the proportion is not high. However, it is worth mentioning that, along with technological breakthroughs, the market share of ternary lithium / lithium iron phosphate batteries is also gradually increasing, which is a new application trend.