Introduction to the key points of photovoltaic off-grid system configuration
Photovoltaic energy storage system generally refers to the application of photovoltaic modules, including energy storage batteries and other related equipment in the system. Depending on 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 on-grid/off-grid hybrid systems. Here, we mainly introduce some design considerations of photovoltaic off-grid systems.
Photovoltaic off-grid system composition:
Photovoltaic modules, off-grid inverters (including photovoltaic chargers/inverters), energy storage batteries (lead-acid/colloid/lead-carbon/lithium iron phosphate, etc.), photovoltaic brackets, cables, and distribution boxes are all important components of photovoltaic off-grid systems.
The biggest difference between off-grid systems and grid-connected systems is that the grid-connected system is based on investment returns, while the off-grid system is based on the basic demand of power supply, so they have different focuses when selecting components.
Solar Module:
At first, photovoltaic components were only used in some off-grid systems and small photovoltaic systems. Later, with the large-scale development of photovoltaic grid-connected applications and the annual update of photovoltaic component technology, the conversion efficiency of components has been greatly improved. In particular, some grid-connected power stations need more efficient components to improve the investment return ratio in order to make full use of site resources. Of course, since the general off-grid system has a relatively large site, it does not have too high requirements for component conversion efficiency, so conventional components are often the first consideration when selecting components during system design.
Off-grid inverter:
1. Consider AC load. Generally, loads are divided into three categories: resistive loads (electric lights, heaters, etc.), inductive loads (air conditioners, motors, etc.), and capacitive loads (computer host power supplies, etc.). Among them, since the current required for inductive loads to start is 3 to 5 times the rated current, and the short-term overload capacity of 150%-200% of general off-grid inverters cannot meet the requirements, the inductive load needs to consider the expansion design of the inverter (when the off-grid inverter is connected to the inductive load, the system design of at least 2 times the inductive load is required). For example, in the project where the off-grid inverter drives a 2P (2*750W) air conditioner, it is normal to select an inverter with a rated power of 3KVA or above. Of course, the three types of general loads exist at the same time, 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, which is generally available in two types: MPPT and PWM. With the update of technology, PWM chargers are gradually eliminated, and MPPT chargers 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, which will not be repeated 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 required energy storage battery discharge time;
3) Determine the charger power according to the local sunshine conditions and charging time requirements (for example, it needs to be fully charged within 1 day on average).
Energy storage battery:
1. Lead-acid/gel battery: 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 stability, safety and price. The general service life is about two to three years, resulting in high comprehensive use costs and poor experience.
2. Lead-carbon battery: A technology evolved from traditional lead-acid batteries. It can significantly increase the life of lead-acid batteries by adding activated carbon to the negative electrode of the lead-acid battery. However, as a technological update of lead-acid batteries, its cost is slightly higher;
3. Lithium iron phosphate battery: Compared with the above two types of energy storage batteries, lithium-ion batteries have the characteristics of higher power density, more charge and discharge cycles, and better discharge depth. However, due to the cost of lithium batteries and additional battery management technology (BMS), the system cost of lithium iron phosphate batteries is higher than that of lead-acid batteries. The normal service life is 5 years or more, the user experience is high, and the overall cost is low. It is the most popular battery on the market today. Universally installed energy storage batteries.
The above briefly introduces some basic applications of photovoltaic energy storage systems – photovoltaic off-grid systems, and gives some suggestions on the selection of basic equipment configurations, which can be used as a reference for photovoltaic industry personnel.