Initially, PV modules will only be used in certain off-grid and small PV systems. Later, the extensive development of grid-connected photovoltaic applications and the update of photovoltaic module technology have greatly improved the efficiency of module conversion. In particular, some grid-connected power plants will make full use of their site resources. Therefore, more efficient components are needed to increase return on investment. Of course, typical off-grid systems have relatively large locations, so the requirement for component conversion efficiency is not very high. Therefore, when selecting components during system design, traditional components are usually considered first.
1. Consider AC load. Common loads fall into three categories: resistive loads (lamps, heaters, etc.), inductive loads (air conditioners, motors, etc.) and capacitive loads (computer mainframe power supplies, etc.). Among them, the inductive load is unique to the inverter, because the inductive load initially requires 3 to 5 times the rated current, and the short term overload capacity of ordinary off-grid inverters of 150% to 200% cannot meet the requirements. It needs careful consideration. Capacity expansion design (when an off-grid inverter is connected to an inductive load, the system design requires at least twice the inductive load). In projects where off-grid inverters drive 2P air conditioning (2*750W), inverters rated 3kVA or higher are the normal configuration. Of course, there are usually three types of loads at once, but the maximum load percentage can make a big difference to the inverter.
2. Consider the DC side. Off-grid inverters have built-in photovoltaic chargers and are usually divided into two types: MPPT and PWM. With the update of technology, PWM charger has been eliminated, making MPPT charger become the first choice of off-grid inverter.
3. Other options. In addition to the above two options, there are many formulas on the market, which will not be described here. However, the general instructions are as follows: 1) Determine the rated power of the off-grid inverter according to the size and type of the load. 2) Determine the kWh value of the energy storage battery according to the discharge time of the energy storage battery required by the load. 3) Determine the performance of the charger etc. according to local daylight and charging time requirements (e.g., it must be fully charged within a day).
Energy storage battery
1. Lead-acid/gel batteries: Energy storage systems usually choose maintenance-free sealed lead-acid batteries to reduce subsequent maintenance. After 150 years of development, lead-acid battery has a significant advantage in stability, safety and price. At present, this kind of battery accounts for the highest proportion of secondary battery applications, and even the first type of energy storage battery. Off-grid photovoltaic cells.
2. Lead-acid battery: A technology developed from the traditional lead-acid battery. The life of lead-acid batteries can be greatly extended by adding activated carbon to the negative electrode of lead-acid batteries. However, as a technical update to lead-acid batteries, the cost is higher.
3. Lithium terre lithium/iron phosphate battery: Compared with the above two kinds of energy storage batteries, lithium ion batteries have higher power density, more charging and discharging cycles and better discharge depth. Due to the additional battery management technology (BMS) required, the system cost of LI-ion/FE-phosphate terpolymer batteries is usually two to three times that of lead-acid batteries. In addition, compared with lead-acid batteries and lead-acid batteries, the thermal stability is somewhat less, so grid-independent photovoltaic systems are not used at a high rate. However, due to technological innovation, the market share of ternary Li-ternary/Li-iron batteries is gradually increasing, which is the trend of new applications.