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New Energy PV: From grid-connected to off-grid

At  present, there are two typical applications of PV, grid-connected systems and off-grid systems. The grid-connected system depends on the grid, and uses the “self-generation, surplus power online” or “full online” mode of operation. The off-grid system does not depend on the grid, and relies on the “store and use” or “store and use” mode of operation. Off-grid systems are highly practical for households in areas without a grid or in areas with frequent power outages.

Since off-grid is a rigid demand, the profit is relatively high. Installers who do grid-connected, want to do off-grid projects, which requires a partial change in thinking, if you continue to use the idea of grid-connected to do off-grid, it is possible that the deal will not be concluded or meet the requirements of customers.

1 off-grid system can not be considered a return on investment

We invest our money in real estate, stocks, industry and other projects, we have to calculate how much money we can make every year and how many years we can pay back. Grid-connected power generation is subsidized by the state, and building a power station is an investment, so customers are most concerned about the return on investment. But when we buy a cell phone, computer, clothes and other necessities, we do not calculate how many years we can earn back the capital, and the same goes for off-grid systems. The idea of off-grid installation is likely to be cancelled by the customer. The reason why off-grid systems are not economically viable is that they cost much more than grid-connected systems.

2 off-grid system cost is high

Off-grid systems consist of PV arrays, solar controllers, inverters, battery packs, loads, and so on. Compared with the grid-connected system, more batteries, accounting for 30-40% of the cost of the power generation system, and the components are almost the same. And the battery life is not long, lead-acid batteries are generally in 3-5 years, lithium batteries are generally in 8-10 years, after which they need to be replaced.

Off-grid inverters are 1.5 to 3 times more expensive than grid-connected inverters. Off-grid inverters have a more complex structure than grid-connected inverters; grid-connected inverters generally have a two-stage structure of booster and inverter, while off-grid inverters generally have a four-stage structure, including controller, booster, inverter, and isolation, and cost about two times more than grid-connected inverters.

The overload capacity of off-grid inverters is more than 30% higher than grid-connected inverters. Grid-connected inverters are connected to modules at the front stage and to the grid at the output, which generally do not need overload capacity because few modules have output power greater than the rated power, while off-grid inverters are connected to loads at the output, and many loads are inductive loads, and the starting power is 3-5 times the rated power.

Off-grid inverter production is low, the current PV grid-connected market share of about 98%, off-grid market share of about 2%, shipments are very low, can not be automated production, raw materials and production costs are much higher.

3 off-grid system why with energy storage battery

In the PV off-grid system, energy storage batteries account for a large proportion of the cost and solar modules, but the life is much shorter than the components, the task of energy storage batteries is energy storage, to ensure that the system power stability, at night or rainy days to ensure that the load power. Off-grid systems must be equipped with energy storage batteries, this is why?

One is the PV power generation time and load power time is not necessarily synchronized, PV off-grid system, the input is the module for power generation, the output connected to the load. Photovoltaic are daytime power generation, there is sunlight to generate electricity, often at noon the highest power generation, but at noon, the demand for electricity is not high, many household off-grid power station at night, then the electricity sent out during the day how to do, to be stored up, this storage device is energy storage batteries. Wait for the peak of electricity consumption such as seven or eight o’clock at night, and then release the power.

Second, photovoltaic power generation and load power is not necessarily the same, photovoltaic power generation is affected by radiance, not very stable, and the load is not stable, like air conditioners, refrigerators, start power is very large, usually running power is less, if photovoltaic direct with load, it will cause system instability, the voltage is high and low. Energy storage battery is a power balancing device, when the PV power is greater than the load power, the controller sends the excess energy to the battery bank storage, when the electricity generated by the PV can not meet the needs of the load, the controller and the battery power to the load.

In addition to the above three points, the design of off-grid systems and grid-connected systems are also different, components, inverters, storage batteries are customized according to the needs of users, only after these changes in thinking, in order to do a good job off-grid systems.

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Low power generation: what have you done to the inverter?

Overseas customer feedback, said the inverter output is limited, can not be effective output. After in-depth communication, we understand that there are problems with their inverter and module configurations: the modules are over-matched too much, resulting in excessive working current some of the time, inverter current-limited operation, and some power-limited phenomena.

In fact, this situation also happens in China from time to time, and many owners have done a lot of things that are unpleasant without understanding the operation mechanism of the inverter.

(1) Make a "shield" for the inverter

Mistake: Previously, when I did research, I found that customers installed shields to protect the safety of the inverter, but this not only did not protect the machine, but also affected the heat dissipation of the machine. Especially in the hot summer, the installation of the shield will easily cause the surrounding working environment to be too hot, resulting in the overheating and derating of the inverter, which will limit the output power and affect the power generation revenue.

Correct: String inverters are all IP65 or above protection level, and do not need additional overall protection. Therefore, we only suggest two points here: first, when string inverters are installed, you can install some protective canopy on its upper part to avoid direct rainfall; in addition, when installing multiple string inverters, you should pay attention to the inverter spacing to ensure enough space for heat dissipation and follow the installation manual recommendations as much as possible.

(2) Configure too many components, thinking that the more they are equipped, the more electricity they generate

There is a saying in the north that “the spoon is big, but the bowl is accurate”, which means that the bowl is only so big, and it can’t hold more rice. In fact, the same is true for inverter module configuration. For example, the maximum output power of Shengnengjie SE-5kw inverter is 5.5KVA, even if 6kw of power is input on the input side, its output will not reach 6kw, but only 5.5kw at most, which means the output power is limited.

Considering the impact of system loss, we usually recommend different over-provisioning ratio in different areas. Because, if the inverter over allocation is too high, it will not only waste the system cost, but also make the inverter overload for a long time, resulting in lower service life.

(3) Use DC sink box for convergence

As we all know, centralized inverters will add a sink box on the DC side, mainly to connect the components of the square array in series and parallel, and add lightning protection, fuses, DC switches and other protection devices, and then unified to the DC cabinet side of the inverter.

For string inverter, the DC sink box is directly omitted, and the lightning protection and overcurrent protection units are placed inside the machine, which greatly reduces the workload of construction and maintenance. However, we also found that some users are still using the old thinking of centralized configuration.

Wrong: The number of DC terminals on the input side of the inverter directly determines how many power components can be configured. Some customers ignore the current limit conditions on the input side and parallel-connect the modules before connecting to the inverter, resulting in excessive input current and burned fuses, or even damaged terminals, which affects the power generation revenue.

Correct: The power on the input side of the inverter, i.e. the current limit, needs to be fully considered. Generally, the maximum input current of the module is around 9-10A, and the double-sided module is slightly larger, but only around 11-12A, while the DC input side current of the inverter is mostly around 12.5A, so extra attention is needed when configuring.