Application of photovoltaic off-grid small system

Application of photovoltaic off-grid small system

After understanding the basic needs of customers, first, determine the selection of the leading equipment of the system and then confirm the scheme of the system. Photovoltaic off-grid systems are rigid requirements, and users rely heavily on electricity demand. Therefore, the system's reliability should be considered first in the design. Then different solutions should be provided according to the diverse needs of customers, on the premise of meeting customer needs, increasing power generation, and reducing system costs.

Low-cost small off-grid system solutions
Small off-grid system, the primary users are from impoverished areas without electricity, remote mountainous regions, herders and tourists, mainly to meet the needs of lighting, mobile phone charging, etc.; the system consumes less than 5 degrees of electricity per day, and the load power is less than 1kW; users The electricity demand is not very urgent, and the need for products is reliable and straightforward, and the price is low. Therefore, it is recommended to use a PWM controller and an inverter to correct the wave and integrate the controller, the inverter, and the battery. This method has a simple structure, high efficiency, convenient wiring, and low price. In addition, it can drive light bulbs, small TVs, small There is no problem with the fan.
Small and medium practical off-grid system solutions
The primary users of small and medium-sized off-grid systems come from relatively affluent power-deficient areas, such as herdsmen, island residents, medium-sized fishing boats, fairly remote scenic spots, and some communication and monitoring base stations. It mainly solves the basic needs of life such as lighting, TV sets, fans, and air conditioners; the daily electricity consumption of the system is below 50 kWh, and the total load power is below 20kW; users have specific needs for electricity consumption, and their demands for products are practical and reliable, Inexpensive. 
(1) If the user has few inductive loads, it is recommended to use the MPPT controller plus the high-frequency isolation inverter, which is light in weight and cheap; if the user has many inductive loads, it is recommended to use the MPPT controller to process the frequency isolation inverter. The solution is reliable in electricity consumption and can carry shock loads.
(2) If the user's load power is relatively small, but the electricity consumption time is very long, it is recommended to choose the controller and inverter split scheme, you can choose to use a more significant controller and a smaller inverter to increase the power generation, Reduce the system cost; if the user's load power is relatively large, but the electricity consumption time is not long, it is recommended to choose the integrated solution of the controller and the inverter, and the system wiring is simple.

Medium and large-scale reliable off-grid system solutions
Medium and large off-grid systems are mainly used in industrial and commercial areas, scenic areas, and other occasions where frequent power outages, high electricity prices, significant peak-to-valley price differences, and photovoltaics cannot be connected to the Internet. Main; the system load power is above 20kW and below 250kW, and the daily electricity consumption is below 500 kWh. There are various solutions for small and medium-sized off-grid systems.
For systems above 20kW and below 60kW, you can choose the scheme of connecting multiple single-phase small off-grid inverters in parallel. This scheme is more complicated in wiring and debugging, but the price is relatively low, and the flexibility is high. Furthermore, there is an inverter failure; the system can continue to run. You can also choose the controller and inverter split scheme and the controller and inverter integrated scheme, using a medium and large single inverter; the system wiring is simple, the debugging is convenient, and it can form a hybrid power supply system with the fuel generator set. Compared with pure off-grid photovoltaics, it can save a lot of expensive batteries, and the total power generation cost is low. For systems above 60kW, there are currently two topologies: DC coupling "DC Coupling" and AC coupling "AC Coupling," which can be selected according to the power consumption.

Large-scale multi-energy off-grid system solutions
Large-scale multi-energy off-grid systems are mainly used in remote mountainous areas, islands, tourist areas, and industrial and commercial places with high electricity prices without power grids, with a power of more than 250kW. In general, bidirectional energy storage converters are used, grid-connected inverters and batteries are combined into a micro-grid system. In addition to photovoltaics and energy storage, there are usually other power generation devices such as wind turbines and fuel-fired generators.
Most microgrids adopt AC-coupled topologies, using centralized inverters and bidirectional energy storage converters.
The microgrid can fully and effectively exert the potential of distributed clean energy, reduce unfavorable factors such as small capacity, unstable power generation, and low reliability of independent power supply, and ensure the safe operation of the system. The application of microgrids is flexible, and the scale can range from several kilowatts to tens of megawatts. Microgrids can be developed in factories, mines, hospitals, schools, and even small buildings.

Photovoltaic off-grid system composition:
Photovoltaic modules, off-grid inverters (including photovoltaic chargers/inverters), energy storage batteries (lead-acid/colloid/lead-carbon/ternary lithium/lithium iron phosphate, etc.), photovoltaic brackets, cables, and accessories Electric boxes, etc. are all critical components of photovoltaic off-grid systems.
The most significant difference between an off-grid system and a grid-connected system is based on investment income. In contrast, the off-grid system is based on a just-needed power supply, so they will focus differently when selecting components.

It may often happen that there is no mains access for planting or breeding on the mountain. At this time, we can install a photovoltaic energy storage system to design a reasonable photovoltaic energy storage system when there is no power grid supporting facilities in the area far away from the power grid. Can the system replace daily electricity needs?
The difference between the small off-grid photovoltaic energy storage system and the grid-connected system is that the off-grid system does not need to generate electricity and use itself through the grid itself. In contrast, the grid-connected system must usually be combined with the grid to work. As a result, an off-grid system is not as simple as a grid-connected system. For example, the power of the inverter and photovoltaic modules is similar, but an off-grid system is not.

What parameters need to be provided when designing an off-grid system?
1. The power of the electrical load equipment
2. The working time of the load = the actual number of total watts
3. Whether it is necessary to consider the number of rainy days (continuous power supply)
4. Light conditions of the installation site and installation inclination
Only by knowing these parameters can a set of the optimal photovoltaic off-grid system be reasonably designed. The energy storage battery stores the energy storage method of the off-grid system, and the off-grid inverter can output the power for use. The voltage matching of the off-grid system and the voltage of the grid-connected system (220V/380V) should reasonably correspond to the voltage of the grid-connected system. Generally, the voltage of the off-grid system is mainly boosted type and inverted by DC low voltage.The power of solar modules and inverters of off-grid systems is rarely the same. Each power demand site needs to be designed according to the actual power consumption, which is quite different from the grid-connected system. In general grid-connected systems, we usually directly say xx (kilowatt) KW. Off-grid systems are used now through DC inverter AC.If the design of the off-grid system is unreasonable, the power demand will not be met, and the hardware of the system components will be damaged.

What components do photovoltaic + energy storage off-grid systems need?
1. Photovoltaic modules
At the earliest, photovoltaic modules were only used in some off-grid and small photovoltaic systems. Later, with the large-scale development of photovoltaic grid-connected applications and the annual update of photovoltaic module technology, the conversion efficiency of modules has been dramatically improved. In particular, some grid-connected power stations need more efficient components to improve the investment income ratio due to the full use of site resources. Of course, the general off-grid system does not have high requirements on the conversion efficiency of components due to its relatively large site, so conventional components are often the first consideration when selecting components in system design.

2. Photovoltaic bracket
It would help if you were not unfamiliar with photovoltaic brackets. They are also used in grid-connected systems. There are two standard photovoltaic racks in the photovoltaic bracket market: aluminum alloy and hot-dip galvanized C-shaped steel. Whether the galvanized layer in the hot-dip galvanized, C-shaped steel bracket meets the standard means whether the service life meets the 20-year standard.

3. Off-grid switchgear
Control the entire circuit switch and lightning protection functions.

4. Energy storage battery
① Lead-acid/gel battery: The energy storage system generally chooses maintenance-free sealed lead-acid batteries to reduce later maintenance. After 150 years of development, lead-acid batteries have significant stability, safety, and price advantages. They are not only the battery type with the highest proportion of energy storage battery applications at present but also the first energy storage battery type for photovoltaic off-grid systems.
②Lead-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. But as a technical update of lead-acid batteries, its cost is slightly higher;
③Ternary lithium/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 need for additional battery management technology (BMS), the system cost of ternary lithium/lithium iron phosphate batteries is generally 2-3 times that of lead-acid batteries. In addition, compared with lead-acid/lead-carbon batteries, their thermal stability is also slightly insufficient, so the application ratio in photovoltaic off-grid systems is not high. But it is worth mentioning that, with technological breakthroughs, the market share of ternary lithium/lithium iron phosphate batteries is also gradually increasing, which is a new application trend.

5. Solar controller
The controller's primary function is to control the overshoot and over-discharge of the solar energy and the energy storage battery to protect the service life of the storm. Generally, the controller has the function of light control. During the day, the charging state automatically stops discharging, and when it is dark, it starts to release. This is why we usually see solar street lights why no one controls the automatic shutdown during the day and the automatic lighting at night. The maximum charging current of the controller is different for the solar modules equipped with it. For example, for a 48V30A controller, the charging current of the solar module must be lower than 30A. Otherwise, the controller will be damaged.

6. Photovoltaic cable
Photovoltaic cables have the advantages of high-temperature resistance (generally 120°C), anti-aging, anti-ultraviolet, anti-corrosion, etc., and can withstand harsh weather environments and mechanical shocks. In the outdoor environment, the service life of photovoltaic cables is eight times that of ordinary lines and 32 times that of PVC cables.

7. Off-grid inverter
①Take the AC load as the consideration point. General loads are divided into three categories: group loads (lights, heaters, etc.), inductive loads (air conditioners, motors, etc.), capacitive loads (computer host power supply, etc.). Since the current required by the inductive load to start is 3~5 times the rated time, and the short-time overload capacity of 150%-200% of the general off-grid inverter cannot meet the requirements, the inductive load needs special consideration of the inverter. (When the off-grid inverter is connected to an inductive load, a system design with at least twice the inductive load is required). For example, in a project where an off-grid inverter drives a 2P (2*750W) air conditioner, an inverter with a rated power of 3KVA and above is the standard configuration. Of course, three types of available loads exist simultaneously, but the load with the most significant proportion will have a significant impact on the inverter.
②Take the DC side as the consideration point. Off-grid inverters have built-in photovoltaic chargers, which generally have two types: MPPT and PWM. As technology is updated, PWM chargers are phased out, and MPPT chargers become the first choice for off-grid inverters.
③Other options. In addition to the above two selection methods, 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 size and type of the load; 2) Determine the kWh value of the energy storage battery pack according to the discharge time of the energy storage battery required by the load; 3) Determine the kWh value of the energy storage battery pack according to the local sunshine conditions and Charging time requirements (for example, it needs to be fully charged within one day on average), determine the charger power, etc.

(the image is only a reference)

Then a completely off-grid system needs to be equipped with the above materials. Of course, the inverter control integrated 

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