The distributed domestic photovoltaic introduced in this article refers to projects below 10kW that are installed on the roof of a home and are connected to the grid through 220V or 380V. Most of them use self-used spare capacity to access the Internet. The maximum installed capacity is 25% of the capacity of the next-level transformer.
1. Choosing the right installation site: The roof of the house is usually tiled and cemented. The installer must go to the site to investigate when selling PV or receiving user applications, because not every roof is suitable for installation of photovoltaics. First of all, we must determine whether the carrying capacity of the roof can meet the requirements. The requirements for the solar power station equipment on the roof are greater than 30kg/m2. Generally, the houses with cement structures built in the past five years can meet the requirements, and there are more than 10 years of brick and tile structure. Houses should be carefully examined; secondly, there should be no shadows around them. Even a few shadows will affect power generation, such as water heaters, utility poles, tall trees, etc. It will be contaminated, affecting the amount of power generated; the last thing to look at is the direction of the roof's orientation and inclination. The assembly will face the south and generate the highest amount of electricity at the best inclination angle. If it goes to the north, it will lose a lot of power. If it is not suitable to install photovoltaics, it must be rejected decisively. The need to influence the amount of power generated must be clearly stated by the owner in order to avoid subsequent follow-up.
2. Choosing the right photovoltaic module: PV modules include three technical lines: polysilicon, monocrystalline silicon, and thin film. Each technology has its advantages and disadvantages. Under the same conditions, the efficiency of the photovoltaic system is only related to the nominal power of the components. There is no direct relationship with the efficiency of the components. Component technology is mature. The quality of component manufacturers in the domestic first- and second-tier brands is relatively reliable. Customers need to choose to purchase from reliable sources. There are two types of photovoltaic modules: 60 cells and 72 cells. Distributed photovoltaics are generally small in size and difficult to install. Therefore, 60-cell modules are recommended. They are small, light, and easy to install.
According to the laws of the market, every year there will be a particularly large shipment of power components, the industry is called the mainstream components, the efficiency of the components are increasing every year, 2017 is polycrystalline 265W, single crystal 275W, this type of cost performance The highest, and relatively easy to buy, to 2018 is expected to be polycrystalline 270W, single crystal 280W cost-effective.
3. Select the appropriate bracket: According to the roof conditions, you can choose the aluminum bracket, C-beam, stainless steel and other brackets, taking into account the photovoltaic bracket strength, system cost, roof area utilization and other factors. Reduce the tilt angle of the PV array as much as possible to reduce the wind receiving surface while ensuring that the power generation of the system is not significantly reduced (by less than 1%), so as to increase the strength of the bracket, reduce the cost of the bracket, and increase the utilization of the limited space. . Leakage is a problem that needs attention in the process of installing photovoltaic power plants. Waterproof work is done well and photovoltaic power plants are safe. Photovoltaic stents are installed on the roof supporting the components and connected to the roof. Its design mostly uses the top-top method, which will not perforate and destroy the original waterproof of the roof. The pressure block adopts prefabricated components and does not need to be cast on site, thus avoiding the hard damage of the solar support installation to the waterproof layer of the roof.
4. Parallel design of PV arrays: In distributed photovoltaic systems, the solar cell modules are connected in series to form a series branch. The series connection is used to increase the DC voltage to the inverter voltage input range, and the solar cell module should not exceed the inverter voltage input range under various solar radiation illumination and various ambient temperature operating conditions.
The working voltage is about the rated operating voltage of the inverter, and the efficiency is the highest. The single-phase 220V inverter, the input voltage of the inverter is 360V, the three-phase 380V inverter, and the inverter input voltage is 650V. Such as 3kW inverter, with 260W components, working voltage 30.5V, with 12 pieces of work voltage 366V, power of 3.12kW is the best. The 10KW inverter is equipped with a 260W module, followed by 40 modules, each with 20 strings, a voltage of 610V, and a total power of 10.4kW.
5. Cable selection: In domestic photovoltaic systems, copper cables are recommended. Because the photovoltaic module MC4 connector, photovoltaic inverter output terminal, and grid connection switch terminal are all made of copper. If aluminum wire and copper and aluminum are directly connected, a chemical battery will be formed. corrosion. This will cause poor contact between copper and aluminum, increasing contact resistance. When there is a current passing through, the temperature of the joint will increase, and the increase in temperature will accelerate the corrosion of the joint, increase the contact resistance, and cause a vicious cycle until it burns. The output of the inverter is waterproof, and its wire diameter is also designed according to the copper wire. If aluminum wire is used, the wire of the type one is needed. Such as 30KW inverter, the design output uses 10 square copper wire, aluminum wire requires 16 square, the cable area increases, and the waterproof terminal area is limited, may not accommodate, some installers put the waterproof terminal removed Or damage, this will cause lax terminal protection, easy water, insulation is not good. Also choose multi-strand BVR soft copper wire, do not use BV hard copper wire, because the hard copper wire and the terminal block are easy to contact badly, there is stress in the turning place, it is easy to cause screw looseness and poor contact.
6. Common domestic photovoltaic system design: Common domestic photovoltaic systems, single phase is generally 3kW to 8kW, three-phase is generally 4-10kW, under the conditions permitting, it is recommended to use three-phase grid-connected, because under the same conditions, Three-phase grid-connected networks have less investment than single-phase grid-connected networks, and have high power generation. For example, in a 10kW system, two inverters are required for single-phase grid connection, four strings for DC input, and eight DC cables, and two switches are needed for AC. The phase-to-phase network requires only one inverter, two DC input strings and four DC cables, and the AC needs one switch. Compared with the single-phase current, the loss is less and less; the 380V grid-connected has less impact on the power grid and will not stop due to the increase of the grid voltage.
3kW home photovoltaic design
The plan needs about 30m2 of roof area, adopts 12 blocks of 265Wp photovoltaic modules and has a total power of 3.18KWp. The system adopts a 3KW photovoltaic inverter, accesses 220V lines and sends it to the homeowner's original indoor power distribution box, and then connects with the owner's indoor low-voltage distribution network via a 220V line to transmit electricity to the city power grid.
The maximum DC voltage (maximum array open circuit voltage) of the inverter is 550V, the maximum power voltage tracking range is 70~550V, and the number of MPPT channels is 1/1. The rated operating voltage of each solar cell module is 30.8V, the open circuit voltage is 38.3V, and the rated number of series branches of 12 solar cells in series is set at an ambient temperature of 25±2°C and a solar radiation illumination of 1000W/m2. The operating voltage is 369.6V, and the open circuit voltage is 459.6V. Both are within the allowable input range of the inverter and ensure normal operation.
When working conditions are changed, when the average extreme ambient temperature is -10°C, the maximum power point operating voltage of the solar battery module string is 12×30.8×(0.35%×35+1)=415V, which satisfies the maximum full load MPPT point of 550V. The input voltage requirement; when the extreme maximum ambient temperature is 42°C, the working voltage of the solar cell module is 12×31.2×(−0.35%×17+1)=352.1V, which satisfies the input voltage requirement of the minimum MPPT point of 70V.
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