PV modules, off-grid inverters (including PV chargers/inverters), energy storage batteries (lead-acid/GEL/lead-carbon/tertiary lithium/LiFePo4, etc.), PV mounts, cables, and distribution boxes are all important components of off-grid PV systems.
The biggest difference between off-grid systems and on-grid systems is that on-grid systems take investment income as the premise of calculation, while off-grid systems take immediate power supply as the basic demand, so they will have a different focus in the selection of components.
1.Modules:
At the earliest, PV modules were only used in some off-grid systems and small PV systems. Later, along with the widespread development of on-grid PV applications and the yearly update of PV module technology, module conversion efficiency has been greatly improved. Of course, generally off-grid systems are not too demanding on module conversion efficiency due to the relatively large sites, so conventional modules are often the first consideration when choosing modules for system design.
2.Off-grid inverter:
a.Take AC load as the consideration point. Generally, there are three types of loads: group loads (lights, heaters, etc.), inductive loads (air conditioners, motors, etc.), and capacitive loads (computer mainframe power, etc.). Among them, since inductive loads require 3~5 times of rated current to start, and the short-time overload capacity of 150%-200% of general solar off-grid system inverters cannot meet the requirements, inductive loads require special consideration for inverter capacity expansion design (when off-grid inverters are connected to inductive loads, at least 2 times of inductive loads are required for system design).
b.Take the DC side as a point of consideration. Off-grid inverters have built-in PV chargers, generally of two types, MPPT and PWM. As technology updates, PWM chargers are being phased out and MPPT chargers are becoming the preferred choice for off-grid inverters.
3.Energy storage batteries:
a.Lead-acid/GEL batteries: Energy storage systems generally choose maintenance-free sealed lead-acid batteries to reduce later maintenance. After 150 years of development, lead-acid batteries have significant advantages in terms of stability, safety and price, and are both the battery type with the highest percentage in current energy storage battery applications and the preferred energy storage battery type for off-grid PV systems.
b.Lead carbon battery: a technology evolved from traditional lead-acid battery, which can significantly improve the life of lead-acid battery by adding activated carbon to the negative electrode of lead-acid battery. However, as a technological update of the lead-acid battery, its cost is slightly higher.
c.Trithium /LiFePo4 batteries: compared with the above two types of energy storage batteries, LiFePo4 batteries have higher power density, more charge and discharge cycles, better depth of discharge, etc. 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.