Differences between solar grid-connected and solar off-grid systems
From the current point of view, there are two typical applications of photovoltaics, grid-connected systems and off-grid systems. The grid-connected system relies on the grid, and adopts the working mode of "spontaneous self-use, surplus electricity connected to the grid" or "full grid connection". The off-grid system does not depend on the grid, but relies on the working mode of "storage while using" or "storage first and use later". For households in off-grid areas or areas with frequent power outages, off-grid systems are very practical.
Since off-grid is a rigid demand, the profit is relatively high. As a grid-connected installer, if you want to do off-grid projects, you need to change some of your thinking. If you continue to use the grid-connected idea to do off-grid projects, you may not be able to make deals or meet customer requirements.
1 Off-grid systems are not ROI
When we invest money in real estate, stocks, industries and other projects, we have to calculate how much money we can make each year and how many years it will take to pay back. There are state subsidies for grid-connected power generation. Building a power station is an investment behavior, and customers are most concerned about the return on investment. But when we buy a mobile phone, computer, clothes and other daily necessities, we don’t count how many years it will take to earn back the capital. The same is true for off-grid systems. To meet people's basic living electricity needs, if the same off-grid user calculates the return on investment, it may not be able to pay back the cost in 10 or even 15 years. Customers may cancel the idea of installing off-grid as soon as they hear it. Why is the off-grid system economically inferior to the grid-connected system, because the cost of the off-grid system is much higher than that of the grid-connected system.
2 The cost of off-grid system is high
The off-grid system consists of a photovoltaic array, a solar controller, an inverter, a battery pack, and a load. Compared with the grid-connected system, the addition of batteries accounts for 30-40% of the cost of the power generation system, which is almost the same as the components. Moreover, the service life of the battery is not long. The lead-acid battery is generally 3-5 years, and the lithium battery is generally 8-10 years, and needs to be replaced later.
For an inverter with the same power, the off-grid price is 1.5 to 3 times that of the grid-connected inverter. The structure of the off-grid inverter is more complicated than that of the grid-connected inverter. Grid inverters generally have a four-level structure, including controller, boost, inverter, and isolation, and the cost is about twice that of grid-connected inverters.
With the same power, the overload capacity of the off-grid inverter is more than 30% higher than that of the grid-connected inverter. The front-end of the grid-connected inverter is connected to the components, and the output is connected to the grid. Generally, the overload capacity is not needed, because few components have the output power More than the rated power, the output of the off-grid inverter is connected to the load, and many loads are inductive loads, and the starting power is 3-5 times the rated power, so the overload capacity of the off-grid inverter is a hard indicator. The power of the device will increase, which means that the cost will be high.
The output of off-grid inverters is low. At present, the photovoltaic grid-connected market share is about 98%, and the off-grid market share is about 2%.
3 Why should the off-grid system be equipped with energy storage batteries
In the photovoltaic off-grid system, energy storage batteries account for a large proportion, and the cost is similar to that of solar modules, but their lifespan is much shorter than that of solar modules. electricity. Off-grid systems must be equipped with energy storage batteries, why?
One is that the photovoltaic power generation time and the load power consumption time are not necessarily synchronized. In the photovoltaic off-grid system, the input is a component for power generation, and the output is connected to the load. Photovoltaics generate electricity during the day, only when there is sunlight, and the power generation is often the highest at noon, but at noon, the demand for electricity is not high. Many households use off-grid power stations at night, so what to do with the electricity generated during the day? Store energy first, and this power storage device is an energy storage battery. Wait for the peak of electricity consumption, such as seven or eight o'clock in the evening, and then release the electricity.
The second is that the power of photovoltaic power generation and load power are not necessarily the same. Photovoltaic power generation is not very stable due to the influence of radiation, and the load is also not stable. Like air conditioners and refrigerators, the starting power is very large, and the normal operating power is less. The load will cause the system to be unstable, and the voltage will fluctuate. The energy storage battery is a power balancing device. When the photovoltaic power is greater than the load power, the controller sends the excess energy to the battery pack for storage. .
In addition to the above three points, the design of the off-grid system is different from that of the grid-connected system. Components, inverters, and energy storage batteries must be customized according to the needs of users. Only by changing these thinking can the off-grid system be done well.
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