What are the components of a photovoltaic off-grid system? Design considerations

Photovoltaic energy storage systems generally refer to applications that use photovoltaic modules, such as energy storage batteries and other related equipment within the system. Depending on whether the stored energy must be connected to the grid for sale, photovoltaic energy storage systems can be divided into photovoltaic off-grid systems and photovoltaic on/off-grid hybrid systems.

We are one of the leading new energy application companies in China, providing many related products in the fields of solar power generation, wind power generation, tidal power generation, etc., as well as providing customers with design solutions for various aspects of power generation systems and energy storage systems. Today we are going to talk about Here are the design considerations for photovoltaic off-grid systems.

Photovoltaic off-grid system composition:

Photovoltaic off-grid systems are mainly photovoltaic modules, combiner boxes (usually not required for low-power products), off-grid inverters (including photovoltaic charge controllers/inverters), energy storage batteries (lead-acid)/colloid/lead-carbon/ 3-element lithium/lithium iron phosphate, etc.), photovoltaic supports, cables and junction boxes are key components of off-grid photovoltaic systems.

The main difference between an off-grid PV system and a grid-tied system is that in the grid, the grid-tied system is based on the calculation of ROI, while the off-grid system is based on the power required. Choose different components as needed.

Components:

Initially, PV modules were only used in some off-grid and small PV systems. Since then, the extensive development of grid-connected photovoltaic applications and the increasing updating of photovoltaic module technology have greatly improved the efficiency of module conversion. In particular, some grid-connected power plants make full use of their site resources. Therefore, more efficient components are needed to increase the return on investment. Of course, typical off-grid systems have relatively large locations, so the requirements for component conversion efficiency are not very high. Therefore, when selecting components during system design, legacy components should generally be considered first.

Off-grid inverter:

1. Consider the AC load. Common loads are divided into three categories: resistive loads (lamps, heaters, etc.), inductive loads (air conditioners, motors, etc.) and capacitive loads (computer host 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 150% to 200% of the ordinary off-grid inverter cannot meet the requirements. Careful consideration is required. Capacity expansion design (when the off-grid inverter is connected to the inductive load, the system design requires at least twice the inductive load). In projects where off-grid inverters drive 2P air conditioners (2*750W), an inverter with a rating of 3kVA or higher is the normal configuration. Of course, three types of loads are usually present at the same time, but the maximum load percentage can have a big impact on the inverter.

2. Consider the DC side. Off-grid inverters have built-in photovoltaic chargers and are generally divided into two types: MPPT and PWM. As technology has updated, PWM chargers have been phased out, making MPPT chargers the first choice for off-grid inverters.

3. Other options. In addition to the above two options, there are many formulas on the market, which will not be repeated 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 sunlight and charging time requirements (eg, must be fully charged within one day).

Energy storage battery:

1. Lead-acid/gel batteries: The energy storage system usually chooses maintenance-free sealed lead-acid batteries to reduce follow-up maintenance. After 150 years of development, lead-acid batteries have significant advantages in terms of stability, safety and price. At present, this type of battery has the highest proportion in secondary battery applications, even the first type of energy storage battery. Off-grid photovoltaic cells.

2. Lead-acid battery: a technology developed from traditional lead-acid batteries. Adding activated carbon to the negative electrode of a lead-acid battery can greatly extend the life of the lead-acid battery. However, as a technical update of lead-acid batteries, the cost is higher.

3. Lithium ternary lithium/iron phosphate battery: Compared with the above two energy storage batteries, lithium-ion batteries have higher power density, more charge and discharge cycles and better depth of discharge. Due to the need for additional battery management technology (BMS), the system cost of a lithium/iron phosphate ternary lithium battery is typically 2 to 3 times that of a lead-acid battery. In addition, compared with lead-acid batteries and lead-acid batteries, the thermal stability is somewhat insufficient, so the application rate of photovoltaic systems independent of the grid is not high. However, due to technological innovation, the market share of ternary lithium ternary/lithium iron batteries is gradually increasing, which is a trend of new applications.